WO2016122464A1

WO2016122464A1 - Tunable microring resonator switches

Info

Publication number: WO2016122464A1
Application number: PCT/US2015/013125
Authority: WO
Inventors: Sagi Varghese MATHAI; Michael Renne Ty Tan; Wayne V SORIN; Joaquin MATRES
Original assignee: Hewlett Packard Enterprise Development Lp
Priority date: 2015-01-27
Filing date: 2015-01-27
Publication date: 2016-08-04

Abstract

In the examples provided herein, a method transmits a signal at a first wavelength in a first direction along a first waveguide. The first waveguide crosses a second waveguide at a crossing point, and four microring resonators are positioned near the crossing point with one resonator in each quadrant formed by the waveguides. The method tunes a first and a fourth microring resonator in a first and fourth quadrant, respectively, such that an effect on the signal is selectable from: 1) the signal is dropped to a third direction along the second waveguide, 2) the signal is dropped to a fourth direction along the second waveguide that is opposite to the third direction, 3) a first portion of the signal is dropped to the third direction and a second portion of the signal is dropped to the fourth direction, and 4) the signal continues in the first direction.

Description

TUNABLE MICRORING RESONATOR SWITCHES

BACKGROUND

[0001 J A microring resonator is a waveguide formed in a dosed loop. Light can be evanescentiy coupled from a second waveguide placed close to the microring resonator. At resonant wavelengths of the resonator, opticai power from the second waveguide develops as a traveling wave in the resonator. However, light propagating at non-resonant wavelengths in the second waveguide continues to propagate with no coupling effect to the resonator. The resonant wavelength of the resonator can be tuned by changing the effective refractive index of the waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

[00023 The accompanying drawings illustrate various examples of the principles described below. The examples and drawings are illustrative rather than limiting.

|0003] FIG. 1A depicts a diagram of an example switch building block that includes a set of four microring resonators. FIG. 1B depicts example spectral responses of signals transmitted to and dropped from a port of the switch building block.

[00043 !G. 2A depicts a diagram of example parallel cascaded resonators. FIG. 2B depicis example optical filter responses of parallel cascaded resonators

200,

[00063 FIG. 3 depicts a diagram of example series cascaded resonators.

[00063 ^G- depicts a diagram of two serially cascaded switch building blocks. FIG 4B depicts example input spectra and signals dropped to the output ports of two serially cascaded switch building blocks. FIG. 4C depicts a diagram of four serially cascaded switch building blocks. FIG 4D depicis example input spectra and waveiength signals dropped from the output ports of four serially cascaded switch building blocks.

[0007] FIG, 5A depicts an example 2x2 cyclic crossbar switch, FIG. 5B depicts an example 4x4 cyclic crossbar switch. fOOOS] FIG. 6A depicts a diagram of an example switch building block with photodetectors that tap Sight power from microring resonators. FIG. 8B depicts a diagram of an example switch building block with integrated photodetectors.

{00093 FIGS. 7A and 7B depict a 2x2 cyclic crossbar switch with example time- shared photodetectors,

[0010] FIG. 8 depicts a flow diagram illustrating an example process of tuning microring resonators of a switch building block to transmit and drop a first signal.

[0011] FIGS. 9A and 9B depict a flow diagram illustrating an example process of tuning microring resonators of a switch building block to transmit and drop a first and a second signal, and to calibrate the microring resonators.

[0012] FIG. 10 depicts a flow diagram: illustrating an example process of using a switch building block to transmit an additional signal in a waveguide with the first signal.

[0013] FIGS. 1 1A and 1 1 B depict a flow diagram illustrating an example process of using a 2x2 cyclic crossbar switch.

[0014] FIG, 12 depicts nomenclature for directions of signals propagating through a 2x2 cyclic crossbar switch.

DETAILED DESCRIPTION

[0015] In a wavelength-division multiplexing (WDM) optical system, multiple signals at different wavelengths are joined and transmitted along an optical waveguide to increase the transmission capacity of the system. However, the joined signals may not have the same destination in the optical system, and some signals at certain wavelengths may be switched to another optical waveguide with a different destination, it would be beneficia! to be able to select signals at particular wavelengths for switching. The techniques below describe a microring resonator- based optical switch that can selectively switch wavelengths between waveguides.

[0016] FIG. 1A depicts a diagram of an example switch building block 100 that includes a set of four microring resonators 101-104. The switch building block 100 is a 4-port device that includes a first waveguide 1 10 and a second waveguide 11 1 that crosses the first waveguide at a first crossing point 15, The first crossing point 115 is where the first and second waveguides 1 10, 11 1 intersect. This region may be designed to minimize losses experienced by signals that propagate through the intersection. Although the first waveguide 1 10 and the second waveguide 1 11 are shown to intersect at right angles in the figures, the waveguides may intersect at any angle.

[00173 T^ne switch building block 100 also includes a first set of four tunable microring resonators 101 -104 positioned near the first crossing point 1 15. For clarity, compass directional points may be used to identify directions relative to the first waveguide 1 10 and the second waveguide 11 1 , although the switch building block 100 may be oriented in any direction. In the example of FIG. 1A, a first signal at a first wavelength λ-s propagating from east to west in the first waveguide 110 may be referred to as propagating in a first direction. A second signal at a second wavelength Xi propagating from west to east in the first waveguide 110 may be referred to as propagating in a second direction. Signals dropped from the first waveguide 1 10 to the second waveguide 1 1 that propagate from the south toward the north may be referred to as propagating in a third direction, and dropped signals that propagate from the north toward the south may be referred to as propagating in a fourth direction.

[0018] Signals may be input from either end of the first waveguide 110. The end of the first waveguide 1 10 in the west may be referred to as the west input port, and the end of the first waveguide 1 10 in the east may be referred to as the east input port. Also, the end of second waveguide 111 in the north may be referred to as the north output port, and the end of the second waveguide 111 in the south may be referred to as the south output port.

{0019| One microring resonator is positioned near the first crossing point 1 15 in each quadrant formed b the first and second waveguides 110, 11 1 such that light at resonant wavelengths of the resonator in the waveguides 110, 1 1 1 may couple to and from the resonator- In the northeast quadrant, referred to as the first quadrant, microring resonator 1 101 is coupled to the first and second waveguides 110, 111 ; in the northwest quadrant, referred to as the second quadrant, microring resonator 2 102 is coupled to the first and second waveguides 1 0, 1 1 ; in the southwest quadrant, referred to as the third quadrant, microring resonator 3 103 is coupled to the first and second waveguides 1 10, 11 1 ; and in the southeast quadrant, referred to as the fourth quadrant, microring resonator 4 104 is coupled to the first and second waveguides 1 10, 1 1 1.

(0020J A microring resonator may be referred to as being on when the resonator's resonant wavelength is tuned to the wavelength of a signal of interest, and the resonator may be referred to as being off when the resonator's resonant wavelength is tuned away from the wavelength of the signal of interest. For example, if a signal having a wavelength λ_¾ is transmitted in the first direction from east to west in the first waveguide 1 10, and resonator 1 101 and resonator 4 104 are both off relative to wavelength ι, the signal propagates unaffected through the first waveguide 110 to exit at the west port of the first waveguide 1 10. However, if resonator 1 101 is on relative to wavelength λι, and resonator 4 104 is off, the signal in the first waveguide 1 10 couples to resonator 1 101 and is dropped in the third direction toward the north in the second waveguide 1 1 . Similarly, if resonator 4 104 is on relative to wavelength λι, and resonator 1 01 Is off, the signal couples to resonator 4 104 and is dropped in the fourth direction toward the south in the second waveguide 11 1. If both resonator 1 101 and resonator 4 104 are on relative to wavelength λι, the signal is split and one portion is dropped in the third direction, and another portion is dropped in the fourth direction. The power splitting ratio between the third and fourth directions may be controlled by resonator 1 101 and resonator 4 104. i0021| ^!f1 some implementations, the resonant wavelengths of resonator 1 101 and resonator 4 104 may be different. For example, if the resonant wavelength of resonator 1 101 is A, and the resonant waveiength of resonator 4 104 Is B, a signal at wavelength A propagating in the first direction is dropped in the third direction, while a signal at waveiength B propagating in the first direction is dropped in the fourth direction.

(0O22| Thus, a first micronng resonator in a first quadrant and a fourth micronng resonator in a fourth quadrant may be tuned to resonance at a first wavelength to selectively switch signals at the first wavelength from the first waveguide to the second waveguide. Additionally, a second microring resonator in a second quadrant and a third microring resonator in a third quadrant may be tuned to resonance at a second wavelength to selectively switch signals at the second wavelength from the first waveguide to the second waveguide,

IOO233 As another example, if a signal having a wavelength ?„2 is transmitted in the second direction from west to east in the first waveguide 110, and resonator 2

102 and resonator 3 103 are off relative to wavelength λ₂, the signal propagates unaffected through the first waveguide 1 10 to the east port of the first waveguide 110. However, if resonator 2 102 is on relative to wavelength hi, and resonator 3

103 is off, the signal couples to resonator 2 102 and is dropped in the third direction toward the north in the second waveguide 1 1 1. Similarly, if resonator 3 103 is on relative to wavelength X2, and resonator 2 102 is off, the signal couples to resonator 3 103 and is dropped In the fourth direction toward the south in the second waveguide 1 1 1 .

[0024] Each microring resonator 101-104 in the set of four resonators has a periodic spectral response and should have the same free spectral range (FSR). FIG. 1B depicts example speciral responses of signals transmitted to and dropped from a port of the switch building block. In the top graph in FIG. 1 B, the example through port spectrum for a signai traveling in the first direction has notch-shaped characteristics at the wavelength λ_Α. corresponding to the resonant wavelength of resonator 1 101 , and at the wavelength AB, corresponding to the resonant wavelength of resonator 4 104. Notch-shaped characteristics are also located at free spectra! range intervals of these wavelengths. As shown in the middle graph of FIG. I B, the drop port spectrum for signals propagating toward the north is compiementary to the through port spectrum at the resonant wavelength Λ,Α and at FSR intervals of λ_/ And the bottom graph of FIG.. 1 B shows the drop port spectrum for signals propagating toward the south, complementary to the through port spectrum at the resonant wavelength XB and at FSR intervals of B.

[0025] A set of four microring resonators in a switch building block configuration may be tuned to 16 different states. The example where resonator 1 101 and resonator 4 104 have the same resonant wavelength λε, and resonator 2 102 and resonator 3 103 have the same resonant wavelength λ¾ν is discussed here. When ali four resonators are off, propagating signals in either direction of the first waveguide 110 pass through unaffected by the resonators. When the four resonators are on, portions of signals propagating in the first direction at wavelength λ_·Ε are split between the north port of the second waveguide 11 1 in the third direction and the south port of the second waveguide 1 1 1 in the fourth direction. Similarly, portions of signals propagating in the second direction at wavelength A_w are split between the north port of the second waveguide 1 1 1 in the third direction and the south port of the second waveguide 1 11 in the fourth direction. Thus, the switch building block 100 may multiplex and/or broadcast signals from the first and second directions into the third and fourth directions. 0026] For the case where one of the four resonators is on, the signals propagating in the direction of the first waveguide 110 that initially encounters two resonators in the off state propagate unaffected, while the signals propagating in the other direction of the first waveguide 1 10 are dropped to the port nearest the resonator that is on.

[0027] The case where two of the four resonators are on is discussed next. When resonator 2 102 and resonator 1 101 are on, signals propagating in the first and second directions in the first waveguide 1 10 are multiplexed to the north output port, and when resonator 3 103 and resonator 4 104 are on, signals propagating in the first and second directions in the first waveguide 1 10 are multiplexed to the south output port. When resonator 2 102 and resonator 3 103 are on, signals propagating in the second direction in the first waveguide 110 are split, or broadcast, to the north and south output ports, while signals propagating in the first direction in the fsrst waveguide 1 10 are transmitted unaffected to the west port. When resonator 1 101 and resonator 4 104 are on, signals propagating in the first direction in the first waveguide 110 are multicast to the north and south output ports, while signals propagating in the second direction in the first waveguide 110 are transmitted unaffected to the east port. When resonator 2 102 and resonator 4 104 are on, signals propagating in the first direction are dropped to the south output port, and signals propagating in the second direction are dropped to the north output port. When resonator 1 101 and resonator 3 103 are on, signals propagating in the first direction are dropped to the north output port, and signals propagating in the second direction are dropped to the south output port.

[00283 The case where three of the four resonators are on is discussed next. When resonator 1 101 is off, signals propagating in the first direction are dropped to the south output port, and signals propagating in the second direction are broadcast to the north and south output ports. When resonator 2 102 is off, signals propagating in the first direction are broadcast to the north and south output ports, and signals propagating in the second direction are dropped to the south output port. When resonator 3 103 is off, signals propagating in the first direction are broadcast to the north and south output ports, and signals propagating in the second direction are dropped to the north output port. When resonato 4 104 is off, signals propagating in the first direction are dropped to the north output port, and signals propagating in the second direction are broadcast to the north and south output ports. i0029| Thus, when the first rnicrortng resonator 101 is resonant at the first wavelength λι, at least a portion of a signal at the first wavelength ι propagating in a first direction in the first waveguide 110 is coupled to a third direction in the second waveguide 111. Further, when the fourth rnicrortng resonator 104 is resonant at the first wavelength ι_¾ at least a portion of a signal at the first wavelength X% propagating in the first direction in the first waveguide 10 is coupled to a fourth direction opposite to the third direction in the second waveguide 111, When the first microring resonator and the fourth microring resonator are not resonant at the first wavelength, the signal at the first wavelength continues propagating in the first direction. Additionally, when the second microring resonator 102 is resonant at the second wavelength X_2t at least a portion of a signal at the second wavelength λ₂ propagating in a second direction opposite to the first direction in the first waveguide

110 is coupled to the third direction in the second waveguide 111. And aiso, when the third microring resonator 103 is resonant at the second wavelength λ₂, at least a portion of a signal at the second wavelength 2 propagating in the second direction in the first waveguide 110 is coupled to the fourth direction in the second waveguide

111 When the second microring resonator and the third microring resonator are not resonant at th second wavelength, the signal at the second wavelength continues propagating in the second direction.

[0030] The switch building block is not limited to a single microring resonator per quadrant as shown in the example of FIG.. 1A. Multiple microring resonators may be cascaded in parallel in each quadrant to achieve arbitrarily shaped optical filter responses. FIG. 2A depicts a diagram of example parallel cascaded resonators 200. As depicted in FIG. 2A, a first set of four microring resonators 101-104 are positioned closest to the first crossing point 115, wit one resonator in each quadrant. At Ieast one additional set of four microring resonators 211-214 is used in the parallel cascaded resonator 200. A first micronng resonator of the additional set 21 1 is positioned in the first quadrant along a diagonal, for example, between the first and second waveguides 1 10, 11 adjacent and coupled fo the first microring resonator of the first set 101 , and a fourth microring resonator of the additional set 214 is positioned in the fourth quadrant along a diagonal, for example, between the first and second waveguides 1 10, 11 1 adjacent and coupled io the fourth microring resonator of the first set 104, The first and fourth additional microring resonators 211, 214 are resonant at the first wavelength. In some implementations, the set of parallel cascaded resonators in each quadrant may have different resonant frequencies and/or free spectral range, and be placed in a non-diagonal arrangement. 00313 A second microring resonator of the additional set 212 is positioned in the second quadrant adjacent to the second microring resonator of the first set 102, for example, along a diagonal, for example, between the first and second waveguides 110, 1 1 1 . Also, a third microring resonator of the additionai set 213 is positioned in the third quadrant adjacent to the third microring resonator of the first set 213, for example, along a diagonal, for example, between the first and second waveguides 1 10, 1 1 1. The second and third microring resonators of the additional set 213, 214 are resonant at the second wavelength. In some implementations, the set of parallel cascaded resonators in each quadrant may have different resonant frequencies and/or free spectral range, and may be placed in a non-diagonal arrangement.

[0032] Yet further additional sets of four microring resonators may be used in the parallel cascaded resonator 200, where the further additionai sets of resonators are positioned, one in each quadrant, adjacent to the corresponding resonator from the additional set of resonators 21 1-214 in that quadrant, as shown in the example of FIG, 2A.

[0033] FIG. 2B depicts example optical filter responses of parallel cascaded resonators 200. So one example, signals propagating in the first direction may include wavelengths AEI, AEZ AES. and λεβ, where wavelengths & and λέ$ are separated by a free spectral range (FSR), and wavelengths e and AES are also separated by a FSR. Also, resonator 1 101 has a resonant wavelength ει , and resonator 4 104 has a resonant wavelength A . Also, signals propagating in the second direction may include wavelengths νι, X . Xm_, and X , where m and -WS are separated by a free spectral range (FSR), and wavelengths A_W2 and A_W6 are also separated b a FSR Additionally, resonator 2 102 has a resonant wavelength λνν , and resonator 3 103 has a resonant wavelength X-m. The top graph In FIG. 2B shows an example filter response for signals dropped to the north output port, and the bottom graph shows an example filter response for signals dropped to the south output port. Note that the filter response for each dropped wavelength is wider than the notch-shaped filter responses shown in FIG. 1 B associated with the switch building block of FIG. 1A that has a single resonator in each quadrant. The width of the filter response can be made wider with the use of more microring resonators cascaded in parallel in each quadrant of the device shown in FIG. 2A. For example, four cascaded resonators in a quadrant produce a correspondingly wider filter response than three cascaded resonators.

[0034] Th resonator closest to the first crossing point 1 15 functions like a gat to allow light into and out of the cascaded resonators in that quadrant. For example, if in quadrant 1 , resonator 1 101 is turned on, the light in the first waveguide 10 interacts with every resonator in the first quadrant. However, if resonator 101 is turned off, because the light in the first waveguide 1 10 does not interact with resonator 1 101 , the light has no way to interact with the rest of the resonators in the first quadrant. Also note that the bandwidth of the drop port may be dynamically adjusted by turning on/off one or more resonators in the parallel cascaded array of resonators.

[0035] FIG. 3 depicts a diagram of example series cascaded resonators 300. The series cascaded resonators 300 includes a first waveguide 110, and a second waveguide 1 1 1 where the first and second waveguides are crossed waveguides. The series cascaded resonators 300 also include a plurality of microring resonators positioned in each quadrant formed by the first and second waveguides 110, 1 11. The plurality of microring resonators in each quadrant are arrayed between the first waveguide 1 10 and the second waveguide 111. For example, In quadrant 1 , the microring resonators 301 , 321 , 31 1 form a line between the first waveguide 1 0 and the second waveguide 1 11 ; In quadrant 2, the microring resonators 302, 322, 312 form a line between the first waveguide 1 10 and the second waveguide 1 11 ; in quadrant 3, the microring resonators 303, 323, 313 form a line between the first waveguide 1 10 and the second waveguide 1 1 1 ; and in quadrant 4, the microring resonators 304, 324, 314 form a line between the first waveguide 1 10 and the second waveguide 111 , While the resonators in each quadrant shown in FIG, 3 are formed in a line, the resonators in each quadrant may be arrayed in a nan-coSiinear arrangement.

[0036J Each microring resonator 301 , 311 in the first quadrant adjacent to the first and second waveguides 1 10, 11 1 , and each microring resonator 304, 314 in the fourth quadrant adjacent to the first and second waveguides 110, 111 may be tunable to a first resonant wavelength. Also, each microring resonator 302, 312 in the second quadrant adjacent to the first and second waveguides 110, 1 1 1 , and each microring resonator 303, 313 in the third quadrant adjacent to the first and second waveguides 1 10, 1 1 1 may be tunable to a second resonant wavelength. Additionally, the first resonant wavelength is different from the second resonant wavelength.

[0037] The resonators closest to the first and second waveguides 1 10, 1 11 may be tuned together, on or off, depending on the direction a signal should be switched. For example, in quadrant 1 , if resonator 301 and resonator 31 1 are turned on for a particular wavelength, light propagating in the first waveguide at that particular wavelength also interacts with resonator 321. However, if resonator 301 and resonator 311 are turned off for that particular wavelength, light propagating in the first waveguide 1 10 at that particular wavelength cannot interact with resonator 321. In this case, the signal is isolated to resonator 321 and not allowed to proceed from the east port to the west port. In some implementations, the set of parallel cascaded resonators in each quadrant may have different resonant frequencies and/or free spectral range, and/or be placed in a non-colSinear arrangement. Also note that the bandwidth of the drop port can be dynamically adjusted by turning on/off one or more resonators in the parallel cascaded array of resonators.

[003B3 FIG, 4A depicts a diagram of two serially cascaded switch building blocks 400A. Starting with the switch building block configuration, for example, as discussed above and shown in FIG. 1A, a third waveguide 412 and a second set of four tunable microring resonators 401-404 are also included as part of the serially cascaded switch 400A. The first and third waveguides 1 10, 412 cross at a second crossing point 416, and one microring resonator of the second set is positioned near the second crossing point 418 in each quadrant formed by the first and third waveguides 1 10, 412.

(0O39J In some implementations, a first microring resonator of the second set 401 in a first quadrant of the second crossing point and a fourth microring resonator of the second set 404 in a fourth quadrant of the second crossing point may be tunable to resonance at a third wavelength to switch signals at the third wavelength from the first waveguide 110 to the third waveguide 412. Further, a second microring resonator of the second set 402 in a second quadrant of the second crossing point and a third microring resonator of the second set 403 in a third quadrant of the second crossing point may be tunable to resonance at a fourth wavelength to switch signals at the fourth wavelength from the first waveguide 1 10 to the third waveguide 412. The third wavelength may be different from the first wavelength and not an integer multiple of the free spectral range from the first wavelength. Also, the fourth wavelength may be different from the second wavelength and not an integer multiple of the free spectral range from the second wavelength. [0040| FI G 4B depicts example input spectra and signals dropped to the output ports of two serially cascaded switch building blocks. The top graph shows signals propagating in the first direction at four wavelengths, λει λε2, XES, and λεβ. The difference in wavelength between λει and is a FSR, and the difference in wavelength between and >,E6 is also a FSR. The second graph shows signals propagating in the second direction at four wavelengths, wi, Xw2, X , and .we. The difference in wavelength between λν ι and Xws is a FSR, and the difference in wavelength between λ¾¾ an Xws ts also a FSR.

[0041] The third graph shows the signals dropped to the south output ports of the second waveguide 1 1 1 and the third waveguide 412. The solid lines correspond to the signals dropped to the south output port of the second waveguide 1 11 , and the dotted lines correspond to the signals dropped to the south output port of the third waveguide 412 when resonator 103 is tuned to resonance at wavelength λννι . and resonator 403 is tuned to resonance at wavelength _¾¾. Because wavelength Xm is a FSR from wavelength m, resonator 03 is also tuned to resonance at wavelength λνγ_¾; these wavelengths are indicated within resonator 103 in FIG. 4A. Thus, both wavelength λ_\,νι and ws are dropped from the south output port of the second waveguide 1 1 1 via resonator 103. Similarly, wavelengths /„W2 and ½e are dropped to the south output port of the third waveguide 412 via resonator 403,

[0042] The bottom graph shows the signals dropped to the north output ports of the second waveguide 1 1 1 and the third waveguide 412. The solid lines correspond to the signals dropped to the north port of the second waveguide 1 1 1 , and the dotted lines correspond to the signals dropped to the north port of the third waveguide 412 when resonator 101 is tuned to resonance at wavelength λ._Ει _, and resonator 401 is tuned to resonance at wavelength X I. Because wavelength X S is a FSR from wavelength λ_{Ε ,} resonator 101 is also tuned to resonance at wavelength XES I these wavelengths are indicated within resonator 101 in FIG. 4A. Thus, both wavelengths ει and .ES are dropped to the north port of the second waveguide 1 11 via resonator 101. Similarly, wavelengths ΛΕΞ and λ~_¾ are dropped to the north port of the third waveguide 412 via resonator 401, Labeled arrows corresponding to each of the signals at the various wavelengths are shown at both input ports and output ports of FIG. 4A.

[0043} FIG. 4C depicts a diagram of an example of four serially cascaded switch building blocks 400C. There is a first waveguide 1 10 with west and east input ports, and four waveguides 441-444 that cross the first waveguide 1 10 with north and south output ports. There are four microring resonators in each of the four quadrants of the four intersections formed by the waveguides.

[0044] As an illustrative example, there may be signals with eight different wavelengths ει - λεβ. with the wavelength separated by Λλ, propagating in the first direction from east to west in the first waveguide 1 10, and there may be signals with eight different wavelengths ι - . with the wavelength separated by AX, propagating in the second direction from west to east in the first waveguide 1 10. In this example, the signals in the first waveguide have different wavelengths. Wavelengths ε and X&s are a FSR apart; wavelengths XE2 and Xm are a FSR apart; wavelengths and EI are a FSR apart; and wavelengths λ¾ and ES are a FSR apart. Also, in this example, wavelengths km and Xws are a FSR apart; wavelengths Xw2 and λννβ are a FSR apart; wavelengths Xwz and Xwr are a FSR apart; and wavelengths m and Xws are a FSR apart. The FSR should be greater than the wavelength difference between eight times the wavelength separation AX between the wavelength signals to provide some manufacturing tolerance in fabricating the devices.

[0045] In this example, there are two resonators in each group of four resonators near each crossing point 451-454 thai are tuned to a different one of the wavelengths propagating in the first waveguide 10. In the first quadrant of the crossing points 451 -454, resonator 461 is tuned to resonance at wavelength λει; resonator 471 is tuned to resonance at wavelength λ ί resonator 481 is tuned to resonance at wavelength , and resonator 491 is tuned to resonance at wavelength A_E4. In the third quadrant of the crossing points 451 -454, resonator 463 is tuned to resonance at wavelength AWI; resonator 473 is tuned to resonance at wavelength k^; resonator 483 is tuned to resonance at wavelength WS; and resonator 493 is tuned to resonance at wavelength

100463 Of the eight signals propagating in the first direction, signals with wavelengths E and Xes are dropped via resonator 491 to the north output port of waveguide 444, while the remaining signals are transmitted. At the next crossing point 453, wavelengths XE3 and ,E? are dropped via resonator 481 to the north output port of waveguide 443, while the remaining signals are transmitted. At crossing point 452, wavelengths ?,E2 and Aee are dropped via resonator 497 to the north output port of waveguide 442, while the remaining two signals are transmitted. And at crossing point 451 , the remaining wavelengths λει and AES are dropped via resonator 461 to the north output port of waveguide 441

[0047] For the eight signals propagating in the second direction, signals with wavelengths wi and Xws are dropped via resonator 483 to the south output port of waveguide 441 , while the remaining signals ar transmitted. At the next crossing point 452, wavelengths Xw2 and .we are dropped via resonator 473 to the south output port of waveguide 442, while the remaining signals are transmitted. At the next crossing point 453, wavelengths ws and AW? are dropped via resonator 483 to the south output port of waveguide 443, while the remaining signals ar transmitted. And at the last crossing point 454_s the two remaining wavelengths ,_W4 and A_WS are dropped via resonator 493 to the south output port of waveguide 444.

[0048] FIG 4D depicts example input spectra and wavelength signals dropped to the output ports of the four waveguides 441-444. The top graph shows the input spectra for signals propagating in the first direction from east to west, and the next graph down shows the input spectra for signals propagating in the second direction from west to east. [00493 The third graph from the top in FIG. 4D shows the signals dropped to the south output ports of the four waveguides 441-444. The solid tines correspond to signals dropped to the south output port of the waveguide 441; the dashed Sines correspond to signals dropped to the south output port of the waveguide 442; the dotted Sines correspond to signals dropped to the south output port of the waveguide 443; and the dash-dotted Hnes correspond to signals dropped to the south output port of the waveguide 444. The bottom graph in FIG. 4D shows the signals dropped to the north output ports of the four waveguides 441-444. The solid Sines correspond to signals dropped to the north output port of the waveguide 441; the dashed Sines correspond to signals dropped to the north output port of the waveguide 442; the dotted lines correspond to signals dropped to the north output port of the waveguide 443; and the dash-dotted lines correspond to signals dropped to the north output port of the waveguide 444. 0050| FIG. 5A depicts an example 2x2 cyclic crossbar switch 500A comprising four waveguides, a first waveguide 1 10, a second waveguide 111 , a third waveguide 512, and a fourth waveguide 513. The first waveguide 110 and the second waveguide intersect at a first crossing point 501 ; the first waveguide 1 10 and the third waveguide 512 intersect at a second crossing point 502; the fourth waveguide 513 and the second waveguide 1 11 intersect at a third crossing point 503; and the fourth waveguide 513 and the third waveguide 512 intersect at the fourth crossing point 504, Light may be coupled into and out of the 2x2 cyclic crossbar switch using grating couplers located at the east, west, north, south ports.

[0051} A first set of four tunable microring resonators are positioned near the first crossing point 501 in each quadrant formed by the first and second waveguides 1 1 , 1 11 ; a second set of four tunable microring resonators are positioned near the second crossing point 502 in each quadrant formed by th first and third waveguides 110, 512; a third set of four tunable microring resonators are positioned near the third crossing point 503 in each quadrant formed by the fourth and second waveguides 513, 1 1 1 ; and a fourth set of four tunable microring resonators are positioned near the fourth crossing point 604 in each quadrant formed by the fourth and third waveguides 513, 512.

[0052] in an example, a first microring resonator of the first set of resonators in a first quadrant of the first crossing point 501 and a fourth microhng resonator of the first set in a fourth quadrant of the first crossing point 501 are tunable to resonance at a first wavelength to switch signals at the first wavelength from the first waveguide 110 to the second waveguide 111. A second microring resonator of the first set in a second quadrant of the first crossing point 501 and a third microring resonator of the first set in a third quadrant of the first crossing point 501 are tunable to resonance at a second wavelength to switch signals at the second wavelength from the fsrst waveguide 1 10 to the second waveguide 1 11 .

[0053] In the example, a first microring resonator of the second set in a first quadrant of the second crossing point 502 and a fourth microring resonator of the second set in a fourth quadrant of the second crossing point 502 are tunable to resonance at a third wavelength to switch signals at the third wavelength from the first waveguide 1 10 to the third waveguide 512. A second microring resonator of the second set in a second quadrant of the second crossing point 502 and a third microring resonator of the second set in a third quadrant of the second crossing point 502 are tunable to resonance at a fourth wavelength to switch signals at the fourth wavelength from the first waveguide 110 to the third waveguide 512. The third wavelength is different from the first wavelength and not an integer muStipie of the free spectral range from the first wavelength, and the fourth wavelength is different from the second wavelength and not an integer multiple of the free spectra! range from the second wavelength.

[0054] So the example, a first microring resonator of the third set in a first quadrant of the third crossing point 503 and a fourth microring resonator in a fourth quadrant of the third crossing point 503 are tunabie to resonance at the third wavelength to switch signals at the third vvaveiength from the fourth waveguide 513 to the second waveguide 111. A second microhng resonator in a second quadrant of the third crossing point 503 and a third microring resonator in a third quadrant of the third crossing point 503 are tunable to resonance at the fourth wavelength to switch signals at the fourth wavelength from the fourth waveguide 5 3 to the second waveguide 1 1 1 , The four microring resonators of th third set have a same free spectral range,

[0055| ^Sn ^^e example, a first microring resonator of the fourth set in a first quadrant of the fourth crossing point 504 and a fourth microring resonator of the fourth set in a fourth quadrant of the fourth crossing point 504 are tunabie to resonance at a first wavelength to switch signals at the first wavelength from the fourth waveguide 513 to the third waveguide 512, A second microring resonator in a second quadrant of the fourth crossing point 504 and a third microring resonator in a third quadrant of the fourth crossing point 504 are tunable to resonance at a second wavelength to switch signals at the second wavelength from the fourth waveguide 513 to the third waveguide 512. The four microring resonators of the fourth set have a same free spectral range.

[OOS83 As shown in the example of FIG, 5A, the resonators in the first and third quadrants of the crossing points 501-504 are labeled with the respective wavelengths at which the resonator is resonant. The resonators in the second and fourth quadrants are off or non-resonant. Each of the input ports E1 , E2 from the east for signals propagating in the first direction has signals at the same two wavelengths, } g and ¼ Each of the input ports W1 , W2 from the west for signals propagating in the second direction has signals at the same two wavelengths, λι and λ The signals with the same wavelengths that are transmitted via different input ports should not be dropped to the same output port to prevent collisions of signals having the same wavelength but carrying different data. Thus, a signal with wavelength λ₅ from input port El is dropped to the north output port of the second waveguide 1 1 1, while a signal with wavelength AS from input port E2 is dropped to the north output port of the third waveguide 512 to prevent collisions. Also, a signal with wavelength λ₆ from input port E1 is dropped to the north output port of the third waveguide 512, while a signal with wavelength λβ from input port E2 is dropped to the north output port of the second waveguide 111 to prevent collisions. Similarly, a signal with waveiength λι from input port W1 is dropped to the south output port of the second waveguide 111, while a signal with wavelength ι from input port W2 is dropped to the south output port of the third waveguide 512 to prevent collisions. Also, a signal with waveiength ),₂ from input port W1 is dropped to the output south port of the third waveguide 512, while a signal with wavelength X₂from input port W2 is dropped to the south output port of the second waveguide 111 to prevent collisions.

[0057] FIG. 5B depicts an example 4x4 cyclic crossbar switch 5008 comprising four east-west waveguides 501-504 and four north-south waveguides with 16 crossing points. The resonators in the first and third quadrants of each crossing point are labeled with the respective wavelengths at which the resonator is resonant. The resonators in the second and fourth quadrants are off or non-resonant in the example of FIG, 5B, Signals at four different wavelengths λ₆, λ?, λβ are transmitted in the first direction from east to west in each east-west waveguide 501-504, and signals at four other different wavelengths _ί( λ2, λ₃. λ are transmitted in the second direction from west to east in each east-west waveguide 501-504, As with the 2x2 cyclic crossbar switch, signals having the same wavelength that are transmitted from a different input port should not be dropped to the same output port. Thus, a cyclic pattern may be adopted for distributing signals having the same wavelengths and propagating from different input ports to different waveguide output ports. For example, the resonators in the first quadrant of the crossing points of the top waveguide 501 in the switch are resonant at the wavelengths In the following sequence: .■_>. X_6r λ_?, λβ, from left to right in the switch. Thus these resonators drop signals having the respective resonant wavelengths to the corresponding north output ports in the different waveguides. However, the resonators in the first quadrant of the crossing points of the next waveguide down 502 in the switch are resonant at the wavelengths in the following sequence: kg, Xs, λδ, , from left to right in the switch. Thus, the resonances have been cycled one resonator to the right, and the Sast resonant wavelength wraps back around to the leftmost resonator. Similarly, for the resonators in the first quadrant of the crossing points of the next waveguide down 503 in the switch are resonant at the wavelengths in the following sequence: λ_7; β_, ks_, β, from left to right in the switch, and the resonators in the first quadrant of the crossing points of the bottom waveguide 504 in the switch are resonant at the wavelengths in the following sequence: λβ, λ _, λ_8, As_? from left to right in the switch. The same type of cyclic pattern occurs for the input signals from the west input ports.

[0058] To calibrate the microring resonators for determining the on and off states, feedback signals from the resonators may be used. FIG. 6A depicts a diagram of an example switch building block with four photodetectors that tap light from the respective microring resonators. Light may be tapped from the microring resonators by using a waveguide 611-614 coupled to each resonator as the resonator control signal is swept across an appropriate range. Then photodetectors 601-804 may receive the tapped off Sight power to measure the spectral response of the light for use as a feedback signal to calibrate the corresponding resonator. Alternatively, a photodetector 821-624 may be integrated within each of the microring resonators, as shown in FIG. 6B.

[0059] Once a switch using microring resonators is initially set up in a network, a broad wavelength range or several wavelength signals may be transmitted from a source, and the refractive index of each microring resonator may be swept across a range of refractive indices while monitoring the power detected by the photodetector. The refractive index of a microring resonator may be changed in a number of ways, such as using micro-heaters and the thermo-optic effect to temperature tune the resonators, applying an electric field to semiconductor materials and polymers of the resonators and using the bulk electro-optic effect, and moving a mechanical feature close to the resonator structure to impact its refractive index or moving a lossy material to the resonator so no energy is permitted to build up in the resonator. [00803 When sweeping the refractive index of the resonators, at the resonant frequency, the detector shou!d detect a peak signal power level, and at non-resonani frequencies, the detector should not, in the ideal case, detect any signal The control signal at which a resonator is on, or resonant, for a particular wavelength and the control signal at which the resonator is off, or non-resonant, should be stored in a memory for subsequent tuning of each resonator. Then the management layer of the network may use this data for tuning the resonators as needed for dropping wavelength signals to appropriate drop ports.

|0061] in some implementations, time-shared photodetectors may be used to calibrate the microring resonators, rather than having a dedicated photodetector for each resonator, FIG, 7 A depicts a 2x2 cyclic crossbar switch 700 with example shared photodetectors 701-704. The east input ports A, B may use the photodetector 702 near north output port E to calibrate the resonators in the first quadrant of the crossing points 731 , 733 formed by the intersection of the waveguide 713 with the waveguides 711 , 712. The west input ports C, D may also use the photodetector 702 near the north output port E to calibrate the resonators in the second quadrant of the crossing points 731, 733. Although FIGS, 7A and 7B show the photodetectors integrated with the waveguides 71 -714, they may be implemented off-chip as a separate component or components.

[0062] The east input ports A, B may use the photodetector 701 near north output port F to calibrate the resonators in the first quadrant of the crossing points 732, 734 formed by the intersection of the waveguide 714 with the waveguides 711 , 712. The west input ports C, D may also use the photodetector 701 near north output port F to calibrate the resonators in the second quadrant of the crossing points 732, 734. Similarly shared photodetectors 703, 704 near the south output ports F, G may be used to calibrate resonators that drop signals to the south output ports.

[0063| Alternatively, as shown in FIG. 7B, the time-shared photodetectors 721- 724 may be positioned to measure signals at the east and west input ports, rather than the north and south output ports. However, in this case, a through measurement of signals transmitted to the west port from the east port should not be performed simultaneously with a through measurement of signals transmitted to the east port from the west port to distinguish resonators dropping coupled signals.

[0064] Thus, the photodetectors for a 2x2 cyclic crossbar switch may include a first set of two shared photodetectors 721, 722 to detect light in a first waveguide 71 1 and a fourth waveguide 712 at a first set of input ports to a first side of the four crossing points 731-734, and a second set of two shared photodetectors 723, 724 to detect light in the first waveguide 1 1 and the fourth waveguide 712 at a second set of input ports at a second side opposite from the first side of the four crossing points 731-734, Alternatively, the photodetectors for a 2x2 cyclic crossbar switch may include a third set of two shared photodetectors 701 ₍ 702 to detect light in the second waveguide 713 and the third waveguide 714 at a first set of output ports to a third side of the four crossing points 731-734, and a fourth set of two shared photodetectors 703, 704 to detect light In the second waveguide 713 and the third waveguide 714 at a second set of output ports at a fourth side opposite from the third side of the four crossing points 731-734, The shared photodetectors 701-704 may be operated with reverse bias when calibrating resonators. However, calibration is not performed when the network switches are operational because the signals used for calibration would interfere with the data signals. During an operational state, the photodetectors 701-704 may be operated with forward bias to function as an optical amplifier to amplify light signals transmiited through the photodetectors and cyclic crossbar switch to overcome some of the switch optical losses.

[0085] FIGS. 8-11 B depict flow diagrams illustrating example processes performed with respect to switch building blocks and 2x2 cyclic crossbar switches. FIG. 12 depicts nomenclature for directions of signals propagating through a 2x2 cyclic crossbar switch, as referenced in the flow diagrams of FIGS. 8-1 I B. [0086| F^. 8 depicts a flow diagram illustrating an example process 800 of tuning microring resonators of a switch building block to transmit and drop a first signai,

100673 The process begins at biock 805, where a first signal may be transmitted at a first wavelength in a first direction along a first waveguide. The first waveguide crosses a second waveguide at a first crossing point, and a first set of four microring resonators are positioned near the first crossing point with one microring resonator in each quadrant formed b the first and second waveguides,

[0068] At block 810, a first microring resonator of the first set in a first quadrant formed by the first and second waveguides may be tuned. At block 815, a fourth microring resonator of the first set in a fourth quadrant formed by the first and second waveguides may be tuned. The first and fourth microring resonators of the first set may be tuned such that an effect on the first signai is selectable from one of the following: 1} the first signal is dropped to a third direction along the second waveguide, 2) the first signal is dropped to a fourth direction along the second waveguide that is opposite to the third direction, 3} a first portion of the first signal is dropped to the third direction and a second portion of the first signal is dropped to the fourth direction, and 4) the first signal continues in the first direction.

[0069] FIGS. 9A and 9B depict a flow diagram illustrating an example process 900 of tuning microring resonators of a switch building block to transmit and drop a first and a second signal, and to calibrate the microring resonators.

10070] The process begins at block 905 which may he similar to block 805 described with respect to the process 800 of FIG. 8. Blocks 910 and 915 may also be similar to blocks 810 and 815, respectively, of FIG. 8,

[0071 J At block 920, a second signal may be transmitted at a second wavelength in a second direction opposite the first direction along the first waveguide. The second wavelength is different from the first wavelength, and the four microring resonators of the first set have a same fre spectral range. Also, the second wavelength is not a multiple of the free spectra! range from the first wavelength.

[0072] At block 925, a second microring resonator of the first set in a second quadrant formed by the first and second waveguides may be tuned.

[0073] At block 930, a third microring resonato of the first set in a third quadrant formed by the first and second waveguides may be tuned. The second and third microring resonators of the first set may be tuned such that an effect on the second signal is selectable from one of the following: 1 ) the second signal is dropped to the third direction along the second waveguide, 2} the second signal is dropped to the fourth direction along the second waveguide, 3) a first portion of the second signal is dropped to the third direction and a second portion of the second signal is dropped to the fourth direction, and 4) the second signal continues in the second direction

[0074] The microring resonators may be calibrated using photodeiectors positioned in different locations. In one configuration of the photodeiectors, a first photodetector detects power from a first position along the second waveguide beyond the first and second microring resonators of the first set from the first crossing point, and a second photodetector detects power from a second position along the second waveguide beyond the third and fourth microring resonators of the first set from the first crossing point. Thus, the photodetectors may detect power from the north output port and south output port of the second waveguide, where the positional labels shown in F!G. 1A are referenced here.

[0075J At block 932, calibration signals may be transmitted simultaneously or sequentially in the first direction and the second direction along the first waveguide.

[0076] At block 934, the refractive indices of th resonators may be swept, and the first and second photodetectors may be operated in reverse bias to calibrate the microring resonators of the first set. [00773 *^π another configuration of the photodetectors, a third photodetector detects power from a third position along the first waveguide beyond the first and fourth microring resonators of the first set from the first crossing point, and a fourth photodetector detects power from a fourth position aiong the first waveguide beyond the second and third microring resonators of the first set from the first crossing point. Thus, the photodetectors ma detect power from the east input port and west input port of the first waveguide, referencing the positional labels shown in FIG. 1 A.

[0078] At b!ock 942, calibration signals may be transmitted sequentially in the third and fourth directions along the second waveguide.

[0079] At block 944, the refractive indices of the resonators may be swept, and the third and fourth photodetectors may be operated in reverse bias to calibrate the microring resonators of the first set.

[0080] At block 950, the first and second photodetectors may be operated with forward bias to amplify signals propagating in the third and fourth directions, and the third and fourth photodetectors may be operated with forward bias to amplify signals propagating in the first and second directions.

[0081] FIG. 10 depicts a flow diagram illustrating an example process 1000 of using a switch building block to transmit an additional signal in a waveguide with the first signal.

[0082] The process begins at block 1005 which may again be similar to block 805 described with respect to the process 800 of FIG. 8. Blocks 1010 and 1015 may also be similar to blocks 810 and 815, respectively, of FIG. 8.

[0083] At block 1020, an additional signal at an additional wavelength may be transmitted in the first direction along the first waveguide. The additional wavelength is different from th first wavelength. Also, each microring resonator of the first set of four microring resonators has a same free spectral range, and the additional wavelength is a multiple of the free spectral from the first wavelength. Additionally,

^■■".iS""" tuning the first and fourth microring resonators of the first set has a same effect on the additional signai as on the first signal.

[0084] FIGS, 11A and 11B depict a flow diagram illustrating an example process 1100 of using a 2x2 cyclic crossbar switch.

[0085] The process begins at block 1105 which may yet again be similar to block 805 described with respect to the process 800 of FIG. 8. Blocks 1110 and 1115 may also be similar to blocks 810 and 815, respectively, of FIG. 8.

[0086] At block 1120, a third signal at a third wavelength may be transmitted in the first direction along the first waveguide. The third wavelength is different from the first wavelength, and the third wavelength is not a multiple of the free spectra! range from the first wavelength. Also, the first and fourth microring resonators of the first set have no effect on the third signal near the first crossing point, and wherein a third waveguide crosses the first waveguide at a second crossing point, and a second set of four microring resonators are positioned near the second crossing point with one microring resonator of the second set in each quadrant formed by the first and third waveguides.

[0087] At block 1125, a first microring resonato of the second set in a first quadrant formed by the first and third waveguides may be tuned, and at block 1130, a fourth microring resonator of the second set in a fourth quadrant formed by the first and third waveguides may be tuned. The first and fourth microring resonators of the second set ma be tuned such that an effect on the third signal is selectable from one of the following: 1 ) the third signal is dropped to a fifth direction along the third waveguide, 2) the third signal is dropped to a sixth direction along the third waveguide thai is opposite the fifth direction, 3) a first portion of the third signal is dropped to the fifth direction and a second portion of the third signal is dropped to the sixth direction, and 4) the third signal continues in the first direction. The first and fourth microring resonators of the second set each have a resonant wavelength different from the first wavelength and have no effect on the first signai [0088| At block 1 135, a fifth signal may be transmitted at the first wavelength in a seventh direction along a fourth waveguide. The fourth waveguide crosses the second waveguide at a third crossing point and crosses the third waveguide at a fourth crossing point. A third set of four microring resonators are positioned near the third crossing with one microring resonator of the third set in each quadrant formed by the fourth and second waveguides. Also, a fourth set of four microring resonators are positioned near the fourth crossing with one microring resonator of the fourth set in each quadrant formed by the fourth and third waveguides. A second microring resonator of the third set in a second quadrant formed by the fourth and second waveguides, and a third microring resonator of the third set in a third quadrant formed by the fourth and second waveguides each have a resonant wavelength that is different from the first wavelength and have no effect on the fifth signal

[0089} At block 1140, a first microring resonator of the fourth set in a first quadrant formed by the fourth and third waveguides may be tuned. At block 1 145, a fourth microring resonator of the fourth set in a fourth quadrant formed by the fourth and third waveguides may be tuned. The first and fourth microring resonators of the fourth set may be tuned such that the effect on the fifth signal is selectable from one of the following: 1 ) the fifth signal is dropped to the fifth direction along the third waveguide, 2} the fifth signal is dropped to the sixth direction opposite the fifth direction aiong the third waveguide, 3) a first portion of the fifth signal is dropped to the fifth direction and a second portion of the fifth signal is dropped to the sixth direction, and 4) the fifth signal continues in the seventh direction.

[0090] At block 1150, a seventh signal may b transmitted at the third wavelength in the seventh direction along the fourth waveguide.

[00913 At biock 1 155, the first microring resonator of the third set in a first quadrant formed by the fourth and second waveguides may be tuned. And at block 1 160, the fourth microring resonator of the third set in a fourth quadrant formed by the fourth and second waveguides. The first and fourth microring resonators of the third set may be tuned such that the effect on the seventh signal is selectable from one of the following: 1} the seventh signal is dropped to a third direction along the second waveguide, 2} the seventh signal is dropped to a fourth direction opposite the third direction along the second waveguide, 3) a first portion of the seventh signal is dropped to the third direction and a second portion of the seventh signal is dropped to the fourth direction, and 4) the seventh signal continues in the seventh direction. The first microring resonator of the fourth set in a first quadrant formed by the fourth and third waveguides, and the fourth microring resonator of the fourth set in the fourth quadrant formed by the fourth and third waveguides have no effect on the seventh signal.

[0092] Not all of the steps or features presented above are used in each implementation of the presented techniques. Steps may be performed in a different order than presented.

Claims

CLAIMS What Is claimed is:

1. A system comprising:

a first waveguide;

a second waveguide crossing the first waveguide at a first crossing point; and

a first set of four tunable microring resonators, wherein one microring resonator is positioned near the first crossing point in each quadrant formed by the first and second waveguides, and the four microring resonators have a same free spectral range,

wherein a first microring resonator in a first quadrant and a fourth microring resonator in a fourth quadrant are tunable to resonance at a first wavelength to selectively switch signals at the first wavelength from the first waveguide to the second waveguide, wherein a second microring resonator in a second quadrant and a third microring resonator in a third quadrant are tunable to resonance at a second wavelength to selectively switch signals at the second wavelength from the first waveguide to the second waveguide, and

further wherein the first wavelength is different from the second wavelength and not an integer multiple of the free spectra! range from the second wavelength,

2. The system of claim 1 ,

wherein when the first microring resonator is resonant at the first wavelength, at least a portion of a signal at the first wavelength propagating in a first direction in the first waveguide is coupled to a third direction in the second waveguide, wherein when the fourth microring resonator is resonant at the first wavelength, at least a portion of a signal at the first wavelength propagating in the first direction in the first waveguide is coupled to a fourth direction opposite to th third direction in the second waveguide,

wherein when the first microring resonator and the fourth microring resonator are not resonant at the first wavelength, the signal at the first wavelength continues propagating in the first direction, wherein when the second microring resonator is resonant at the second wavelength, at least a portion of a signal at the second wavelength propagating in a second direction opposite to the first direction in the first waveguide is coupled to the third direction in the second waveguide,

wherein when the third microring resonator is resonant at the second wavelength, at least a portion of a signal at the second wavelength propagating in the second direction in the first waveguide is cou ied to the fourth direction in the second waveguide,

wherein when the second microring resonator and the third microring resonator are not resonant at the second wavelength, the signal at the second wavelength continues propagating in the second direction.

3. The system of claim 1 , further comprising:

multiple photodetectors, wherein each one of the photodetectors measures a spectrai response of a different microring resonator for use as a feedback signal to calibrate the corresponding microring resonator, wherein the multiple photodetectors are each integrated within a ring of the corresponding microring resonator or receive Sight power tapped off from the corresponding microring resonator.

The system of claim 1 , further comprising:

an additionai set of at least four microring resonators,

wherein a first microring resonator of the additionai set is positioned in the first quadrant between the first and second waveguides adjacent to the first microring resonator of the first set, and a fourth microring resonator of the additionai set is positioned in the fourth quadrant between the first and second waveguides adjacent to the fourth microring resonator,

wherein a second microring resonator of the additional set is positioned in the second quadrant between the first and second waveguides adjacent to the second microring resonator of the first set, and a third microring resonator of the additional set is positioned in the third quadrant along a diagonal between the first and second waveguides adjacent to the third microring resonator of the first set.

The system of claim 1, further comprising:

a third waveguide, wherein the first and third waveguides cross at a second crossing point; and

a second set of four tunable microring resonators, wherein one microring resonato of the second set is positioned near the second crossing point in each quadrant formed by the first and third waveguides,

wherein a first microring resonator of the second set in a first quadrant of the second crossing point and a fourth microring resonator of the second set in a fourth quadrant of the second crossing point are tunable to resonance at a third wavelength to switch signals at the third wavelength from the first waveguide to the third waveguide, and

wherein a second microring resonator of the second set in a second quadrant of the second crossing point and a third microring resonator of the second set in a third quadrant of the second crossing point are tunab!e to resonance at a fourth wavelength to switch signals at the fourth wavelength from the first waveguide to the third waveguide,

wherein the third wavelength is different from the first wavelength and not an integer multiple of the free spectral range from the first wavelength, and

wherein the fourth wavelength is different from the second wavelength and not an integer multiple of the free spectral range from the second wavelength.

The system of claim 5, further comprising:

a fourth waveguide, wherein the fourth and second waveguides cross at a third crossing point, and wherein the fourth and third waveguides cross at a fourth crossing point;

a third set of four tunable microring resonators, wherein one microring resonato of the third set is positioned near the third crossing point in each quadrant formed by the fourth and second waveguides, and further wherein the four microring resonators of the third set have a same free spectral range,

wherein a first microring resonator of the third set in a first quadrant of the third crossing point and a fourth microring resonator in a fourth quadrant of the third crossing point are tunable to resonance at the third wavelength to switch signals at the third wavelength from the fourth waveguid to the second waveguide, wherein a second microring resonator in a second quadrant of the third crossing point and a third microring resonator in a third quadrant of the third crossi g point are tunable to resonance at the fourth wavelength to switch signals at the fourth wavelengt from the fourth waveguide to the second waveguide; and

a fourth set of four tunable microring resonators, wherein one microring resonator of the fourth set is positioned near the fourth crossing point in each quadrant formed by the fourth and third waveguides, and further wherein the four microring resonators of the fourth set have a same free spectral range,

wherein a first microring resonator of the fourth set in a first quadrant of the fourth crossing point and a fourth microring resonator in a fourth quadrant of the fourth crossing point are tunable to resonance at a first wavelength to switch signals at the first wavelength from the fourth waveguide to the third waveguide,

wherein a second microring resonator of the fourth set in a second quadrant of the fourth crossing point and a third microring resonator in a third quadrant of the fourth crossing point are tunable to resonance at a second wavelength to switch signals at the second wavelength from the fourth waveguide to the third waveguide.

7. The system of claim 6, further comprising:

a first set of shared photodetectors to deiect light in the first waveguide and the fourth waveguide at a first set of input ports to a first side of the four crossing points, and a second set of shared photodetectors to detect light in the first waveguide and the fourth waveguide at a second set of input ports at a second side opposite from the first side of the four crossing points; or a third set of shared photodetectors to detect light in the second waveguide and the third waveguide at a first set of output ports to a third side of the four crossing points, and a fourth set of shared photodetectors to detect light in the second waveguide and the third waveguide at a second set of output ports at a fourth side opposite from the third side of the four crossing points .

A system comprising:

a first waveguide;

a second waveguide, wherein the first and second waveguides are crossed waveguides;

a piuralii of microring resonators positioned in each quadrant formed by the first and second waveguides,

wherein the plurality of microring resonators in each quadrant are arrayed between the first waveguide and the second waveguide, wherein each microring resonator adjacent to the first and second waveguides in a first quadrant, and each microring resonator adjacent to the first and second waveguides in a fourth quadrant are tunable to a first resonant wavelength,

wherein each microring resonator adjacent to the first and second waveguides in a second quadrant, and each microring resonator adjacent to the first and second waveguides in a third quadrant are tunable to a second resonant wavelength, and

further wherein the first resonant wavelength is different from the second resonant wavelength.

A method comprising: transmitting a first signal at a first wavelength in a first direction along a first waveguide,

wherein the first waveguide crosses a second waveguide at a first crossing point, and a first set of four microring resonators are positioned near the first crossing point with one microring resonator in each quadrant formed by the first and second waveguides;

tuning a first microring resonator of the first set in a first quadrant formed by the first and second waveguides; and

tuning a fourth microring resonator of the first set in a fourth quadrant formed by the first and second waveguides,

wherein the first and fourth microring resonators of the first set are tuned such that an effect on the first signal is selectable from one of the following I 1) the first signal is dropped to a third direction along the second waveguide, 2) the first signal is dropped to a fourth direction along the second waveguide that is opposite to the third direction, 3) a first portion of the first signal is dropped to the third direction and a second portion of the first signal is dropped to the fourth direction, and 4) the first signal continues in the first direction.

The method of claim 9, further comprising:

transmitting a second signal at a second wavelength in a second direction opposite the first direction along the first waveguide, wherein the second wavelength is different from the first wavelength, wherein the four microring resonators of the first set have a same free spectral range, and further wherein the second wavelength is not a multiple of the free spectral range from the first wavelength; tuning a second microring resonator of the first set in a second quadrant formed by the first and second waveguides; and tuning a third microring resonator of the first set in a third quadrant formed by the first and second waveguides,

wherein the second and third microring resonators of the first set are tuned such that an effect on the second signai is selectable from one of the following: 1} the second signal is dropped to the third direction along the second waveguide, 2) the first signai is dropped to the fourth direction aiong the second waveguide, 3} a first portion of the second signal is dropped to the third direction and a second portion of the second signai is dropped to the fourth direction, and 4) the second signal continues in the second direction.

11. The method of claim 10, further comprising:

for a first configuration wherein a first photodetector detects power from a first position along the second waveguide beyond the first and second microring resonators of the first set from the first crossing point, and a second photodetector detects power from a second position along the second waveguide beyond the third and fourth microring resonators of the first set from the first crossing point, transmitting calibration signals in the first direction and the second direction along the first waveguide; and

operating the first and second photodetectors in reverse bias to calibrate the microring resonators of the first set; or for a second configuration wherein a third photodetector detects power from a third position along the first waveguide beyond the first and fourth microring resonators of the first set from the first crossing point, and a fourth photodetector detects power from a fourth position along the first waveguide beyond the second and third micron ng resonators of the first set from the first crossing point, transmitting caiibration signals in the third and fourth directions along the second waveguide;

operating the third and fourth photodetectors in reverse bias to caSibrate the microring resonators of the first set.

The method of claim 11 , further comprising:

operating the first and second photodetectors with forward bias to amplify signals propagating in the third and fourth directions, and operating the third and fourth photodetectors with forward bias to amplify signals propagating in the first and second directions.

The method of claim 9, further comprising:

transmitting an additional signal at an additional wavelength in the first direction along the first waveguide,

wherein the additional wavelength is different from the first wavelength, and

further wherein each microring resonator of the first set of four microring resonators has a same free spectral range, and the additional wavelength is a multiple of the free spectral from the first wavelength;

wherein tuning the first and fourth microring resonators of the first set has a same effect on the additional signal as on the first signal.

The method of claim 9, further comprising;

transmitting a third signal at a third wavelength in the first direction along the first waveguide, wherein the third wavelength is different from the first wavelength, wherein the third wavelength is not a multiple of the free spectra! range from the first wavelength, and further wherein the first and fourth microring resonators of the first set have no effect on the third signal near the first crossing point, and wherein a third waveguide crosses the first waveguide at a second crossing point, and a second set of four microring resonators are positioned near the second crossing point with one microring resonator of the second set in each quadrant formed by the first and third waveguides- tuning a first microring resonator of the second set in a first quadrant formed by the first and third waveguides; and

tuning a fourth microring resonator of the second set in a fourth quadrant formed fay the first and third waveguides,

wherein the first and fourth microring resonators of the second set are tuned such that an effect on the third signal is selectable from one of the following: 1) the third signal is dropped to a fifth direction along the third waveguide, 2} the third signal is dropped to a sixth direction along the third waveguide that is opposite the fifth direction, 3} a first portion of the third signai is dropped to the fifth direction and a second portion of the third signal is dropped to the sixth direction, and 4} the third signal continues in the first direction, and

further wherein the first and fourth microring resonators of the second set each have a resonant wavelength different from the first wavelength and have no effect on th first signal.

The method of claim 14, further comprising: transmitting a fifth signal at the first wavelength in a seventh direction along a fourth waveguide,

wherein the fourth waveguide crosses the second waveguide at a third crossing point and crosses the third waveguide at a fourth crossing point, and

wherein a third set of four microring resonators are positioned near the third crossing with one microring resonator of the third set in each quadrant formed by the fourth and second waveguides, and

further wherein a fourth set of four microring resonators are positioned near the fourth crossing with one microring resonator of the fourth set in each quadrant formed by the fourth and third waveguides,

wherein a second microring resonator of the third set in a second quadrant formed by the fourth and second waveguides, and a third microring resonator of the third set in a third quadrant formed by the fourth and second waveguides each have a resonant wavelength that is different from the first waveiength and have no effect on the fifth signal;

tuning a first microring resonator of the fourth set in a first quadrant formed by the fourth and third waveguides; and

tuning a fourth microring resonator of the fourth set in a fourth quadrant formed by the fourth and third waveguides,

wherein the first and fourth microring resonators of the fourth set are tuned such that the effect on the fifth signal is selectable from one of the following; 1 ) the fifth signal is dropped to the fifth direction along the third waveguide, 2) the fifth signal is dropped to the sixth direction opposite the fifth direction along the third waveguide, 3} a first portion of the fifth signal is dropped to the fifth direction and a second portion of the fifth signal is dropped to the sixth direction, and 4) the fifth signal continues in the seventh direction; and

transmitting a seventh signal at the third wavelength in the seventh direction along the fourth waveguide;

tuning the first micronng resonator of the third set in a first quadrant formed by the fourth and second waveguides; and

tuning the fourth microring resonator of the third set in a fourth quadrant formed by the fourth and second waveguides,

wherein the first and fourth microring resonators of the third set are tuned such that the effect on the seventh signal is adjustable to one of the following: 1 ) the seventh signal is dropped to a third direction along the second waveguide, 2) the sevent signal is dropped to a fourth direction opposite the third direction along the second waveguide, 3} a first portion of the seventh signal is dropped to the third direction and a second portion of the seventh signal is dropped to the fourth direction, and 4} the seventh signal continues in the seventh direction,

wherein the first microring resonator of the fourth set in a first quadrant formed by the fourth and third waveguides, and the fourth microring resonator of the fourth set in the fourth quadrant formed by the fourth and third waveguides have no effect on the seventh signal.