WO2002098038A1

WO2002098038A1 - Integrated optical add-drop multiplexer (oadm) using optical waveguide mirrors and multiplexer/demultiplexer

Info

Publication number: WO2002098038A1
Application number: PCT/CA2002/000781
Authority: WO
Inventors: Siegfried Janz; Dan-Xia Xu; Pavel Cheben; Andre Delage; Lynden Erickson; Boris Lamontagne; Sylvain Charbonneau
Original assignee: Lnl Technologies Canada Inc.
Priority date: 2001-05-28
Filing date: 2002-05-28
Publication date: 2002-12-05
Also published as: CA2349029A1

Abstract

Disclosed is an optical add-drop multiplexer comprising a first and a second multiplexer/demultiplexer, each connected to optical fibres via a first and second terminal, respectively, and processing elements integrated on one chip. A wavelength division multiplexed (WDM) signal is demultiplexed by the first multiplexer/demultiplexer. The wavelengths to be dropped are multiplexed by the second multiplexer/demultiplexer and transmitted to an optical fibre via the second terminal. The wavelengths not be dropped are transmitted back to the first multiplexer/demultiplexer by the processing element and are multiplexed by the first multiplexer/demultiplexer with wavelengths to be added, which were transmitted from an optical fibre to the first multiplexer/demultiplexer via the second terminal. This multiplexed signal is transmitted to an optical fibre via the first terminal. The OADM requires only two connection terminals.

Description

Integrated Optical Add-Drop Multiplexer (OADM) Using Optical Waveguide Mirrors

And Multiplexer/ Demultiplexer

Field of the Invention

[0001] This invention relates to the field of photonics, and in particular to an integrated optical add-drop multiplexer (OADM) using optical waveguide mirrors and a multiplexer/ demultiplexer, a method of adding and dropping optical channels using an optical add-drop multiplexer, as well as to an optical telecommunications system comprising at least one optical add-drop multiplexer.

Background of the Invention

[0002] Accompanied with the recent advanced developments and intricacies in telecommunications technology (including voice, video and data signals), wavelength division multiplexed (WDM) transmission has been proposed as a way to transmit large amounts of information on optical fibres. WDM refers to sending a WDM signal, comprising multiple wavelengths, down the same fibre, in order to multiply the capacity of an individual fibre. Wavelength selective couplers distribute signals according to their wavelengths and are useful in order to separate and route WDM signals to their proper destinations. One type of wavelength selective coupler is an add-drop multiplexer, which separates a single wavelengths in order to route the wavelength to a particular destination, while sending other wavelengths elsewhere in the network; they also may add a new signal at that wavelength. Dropping and adding wavelengths is an important routing function at a node in an optical telecommunications network based on WDM. This capability is necessary for directing incoming signals to the correct downstream node or to users, and for adding signals coming upstream (e.g. from users) onto the network.

[0003] Figure 1 schematically represents the operation of an example WDM telecommunications network. One or more pairs of optical fibres are provided for upward and downward communication lines as transmission paths. Optical transmitting terminal stations 10, 11, 12 transmits a of WDM signal, comprising a different wavelengths, to one of the optical fibres 18, 19, 20, respectively. The transmitting terminal stations 10, 11, 12 typically consist of a plurality of optical transmitters (which may be semiconductor lasers) (not shown), and an optical wavelength multiplexer (not shown) which combines all optical wavelengths into a WDM signal before it is launched over the optical fibres 18, 19, 20. Each transmitting terminal station 10, 11, 12 operates at a different wavelength and is modulated with a different data signal.

[0004] The add-drop multiplexer 24 distributes the WDM signal to transmission paths per a wavelength to receiver terminal stations 14, 15, 16, which selects and detects wavelengths of the WDM signal. At the receiver terminal stations 14, 15, 16, an optical wavelength demultiplexer (not shown) separates the light received over the fibres 18, 21, 20 respectively, according to the wavelength.

[0005] Figure 2 illustrates the basic structure of a typical add-drop multiplexer 24

(ADM). The ADM 24 demultiplexes an optical signal received via optical fibre 18 with demultiplexer 25. The ADM 24 drops selected wavelengths to optical fibre 21, and adds a wavelength input from the transmission fibre 19. Then, using multiplexer 27, the remaining wavelengths and the added wavelengths are multiplexed in order to output the combined optical signal downstream the optical fibre 18.

[0006] One approach of adding and dropping wavelengths is conducted electronically by optoelectronic (OE) conversion of all signals to electrical form, sorting the wavelengths electronically, and then retransmitting onto the next fibre span. Figures 3 a and 3b illustrate how the add-drop function can be done optically for a single optical fibre 18 using a narrow band filter with a transmission notch at the drop wavelength, combined with two circulators. Referring to Figure 3a, the incoming data stream comprising wavelengths λ_l5 λ₂ and λ₃, passes through the first circulator 30 and through the filter 32. The wavelengths λ₂, corresponding to the filter transmission resonance, is transmitted through to the second circulator 34 and directed to the drop port 36. Referring to Figure 3b, the add signal, λ₂ is fed into the add port 38 of the circulator 34 and is directed back through the filter 32 to be combined with the remaining wavelengths λi, λ₃ at the first circulator 30. The filter 32 used in this configuration can be a multilayer dielectric or fibre Bragg grating (FBG) filter as known in the art.

[0007] Adding and dropping signals electronically by OE conversion is expensive, due to the large number of active components required (i.e. ITU (International Telecommunications Union) grid high speed lasers, high speed photodetectors), as well as the electronic processing involved. The scheme shown in Figures 3 a and 3b is practical only when the appropriate filter is available for the required add-drop wavelenghts. As network wavelength separation decreases to 50 GHz, it becomes more difficult to obtain filters with the required performance. If more than one wavelength is to be dropped, and the dropped wavelengths are selected arbitrarily from the spectrum, unique custom notch or band filters are required for every configuration. In addition, the configuration in Figures 3a and 3b cannot be easily extended to a wavelength selectable OADM device, except in the simplest case of shifting a single drop wavelength by temperature tuning the filter or FBG. Finally schemes requiring the combination of large numbers of separate components connected by fibre jumpers involve enormous packaging and assembly costs.

[0008] Other schemes for adding and dropping wavelengths using discrete demultiplexers, multiplexers and switching networks (for wavelength selectable add-drop multiplexers) have been proposed. Such schemes require a great deal of packaging of discrete components and connecting fibre.

[0009] Therefore, an all-optical solution which can be transparent to bit rate and data format is very attractive. There are adding and dropping devices that are completely optical; these devices do however have a few limitations. For example, if several nodes in a network are coupled together in cascade, the spontaneous emission from optical amplifiers that are required at each node tends to get high and become a serious problem.

[0010] Therefore, there is a need in the art for an optical add-drop multiplexer that mitigates or obviates the problems associated with prior art teachings.

Summary of the Invention

[0011] This invention contemplates integrating optical planar waveguide demultiplexers and waveguide mirrors on a single chip to fabricate an optical add-drop multiplexer (OADM).

[0012] According to one aspect, the invention provides an optical add-drop multiplexer

(OADM) for adding and dropping wavelengths to and from a wavelength division multiplexed (WDM) signal, comprising on a single chip: a first terminal for receiving a multiplexed WDM signal from an optical fibre, and for transmitting a WDM signal to an optical fibre, a second terminal for receiving one or more wavelengths to be added to said multiplexed WDM signal and selectively dropping one or more wavelengths from said multiplexed WDM signal, a first multiplexer/demultiplexer associated with said first terminal for demultiplexing said received WDM signal into a plurality of separate wavelengths and multiplexing separate WDM wavelengths to be transmitted to an optical fibre, a second multiplexer/demultiplexer associated with said second terminal for multiplexing said separate WDM wavelengths to be dropped by said second terminal and demultiplexing said one or more wavelengths to be added to said multiplexed WDM signal, a plurality of channels for carrying the separate wavelength between the first and second multiplexer/demultiplexers, and a processing element in each of the channels between the first and second multiplexer/demultiplexer to selectively transmit or reflect signals.

[0013] According to another aspect, the invention provides a method of adding and dropping wavelengths to and from a wavelength division multiplexed (WDM) signal, the method comprising the steps of: inputting the WDM signal from an optical fibre into an integrated optical add-drop multiplexer (OADM) via a first terminal, demultiplexing the WDM signal into individual wavelengths, multiplexing wavelengths to be dropped and transmitting the multiplexed wavelengths to be dropped via a second terminal, transmitting wavelengths to be added via the second terminal to the OADM and multiplexing the added wavelengths with the wavelengths not to be dropped, and transmitting the multiplexed signal comprising the added wavelengths and wavelengths not to be dropped to the optical fibre via the first terminal.

[0014] According to another aspect, the invention provides an optical transmission system comprising an optical fibre installed between a transmitting terminal station and a receiver terminal station, and an integrated optical add-drop multiplexer (OADM) being positioned between the transmitting terminal station and the receiver terminal station, the OADM comprising only a first and second terminal for connection to the optical fibre.

[0015] The chip requires only one input/output fibre connection, and one add/drop fibre connection, independent of the number or arrangement of add/drop wavelengths. The OADM can be implemented as a fixed wavelenght passive device, or as an active programmable OADM if switchable mirrors (e.g. based on microelectromechanical (MEM) or other switching technology, such as Mach-Zehnder or digital optical switches) are used.

Brief Description of the Drawings

[0016] The invention will now be described in more detail, by way of example, only with reference to the accompanying drawings, in which:

Figure 1 illustrates an example of an optical network;

Figure 2 is a schematic diagram illustrating the general operation of an add-drop multiplexer device;

Figure 3 A and 3B illustrates a the operation of a conventional filter based optical add-drop multiplexer;

Figures 4A and 4B illustrate an integrated add-drop multiplexer in accordance with the invention; and

Figure 5 illustrates the operations of a switching network.

[0017] This invention will now be described in detail with respect to certain specific representative embodiments thereof, the materials, apparatus and process steps being understood as examples that are intended to be illustrative only. In particular, the invention is not intended to be limited to the methods, materials, conditions, process parameters, apparatus and the like specifically recited herein.

Detailed Description of the Preferred Embodiments

[0018] An object of the present invention is to switch or re-route an optical signal to and from an optical fibre in a WDM transmission system using an optical add-drop multiplexer (OADM).

[0019] The WDM transmission system of Figure 1 is constructed such that each optical fibre 18, 19, 20 has a single direction optical transmission connecting optical transmitting terminal stations 10, 12 and receiver terminal stations 14, 16 to transmit and receive WDM signals. For the sake of simplicity, only signals transmitted in one direction (optical fibre 18) are considered. One normally skilled in the art will understand the operation of a bidirectional network. Also for simplicity, the invention is described in terms of a receiver and transmitter, whereas a pair of transceivers could optionally be used.

[0020] In order to accomplish the aforementioned object, the integrated OADM 40 in accordance with the invention, which is shown schematically in Figures 4A and 4B, is used. An optical signal consisting of many different wavelengths λ_ls λ₂, λ₃, is directed to the chip 40 by a first switching circuit comprising an optical circulator 30. The optical circulator 30, shown in Figure 5, has three terminals TI, T2, T3, and transmits WDM signals input from one terminal to an adjacent terminal in a direction shown by the arrow. The optical circulator 30 could have a different number of terminals. The terminal TI is connected to the input side of the optical fibre 18, the terminal T2 is connected to the terminal of the OADM 40, and the terminal T3 is connected to the output side of the optical fibre 18. When the optical signal is input from the optical fibre 18 to terminal TI, the circulator 30 guides the optical signal in the direction shown by the arrow and outputs the optical signal from terminal T2 to the OADM 40. That is, terminal T2 is the adjacent terminal to terminal TI. The signal is coupled from the fibre 18 to the input waveguide 44 through the single input/output fibre comiection at T2. The wavelengths are separated out and directed into corresponding connecting waveguides 46 by an echelle grating 48. A processing element 50 can be a simple mirror that reflects the wavelengths not dropped λ_1} back to the input waveguide 44 through T2 of the optical circulator 30, and downstream optical fibre 18. The dropped wavelengths λ₂, λ , pass through their respective connecting waveguides 46 and are combined onto the output waveguide 52 by the final echelle grating 54.

[0021] The second switching circuit comprises optical output circulator 34, which directs the dropped channels to their destination, via drop port 36. The optical circulator 34 has three terminals and operates in the same manner as described above with regard to the optical circulator 30. That is, an optical signal enters the optical circulator at one terminal, and is moved in the direction shown by the arrow, and exits the optical circulator at the next adjacent terminal through the single add/drop fibre connection. At the same time, optical circulator 34 directs the added wavelengths, λ₂, λ₃, into the OADM 40 through the same add/drop fibre connection. Optical circulator 34 receives optical signals, λ₂, λ₃ via add port 38. The optical signals are applied to the terminal of the optical circulator 34 and output to another terminal having an optical fibre connected thereto. Since the path of the added wavelengths λ₂, λ₃, is exactly the reverse of the dropped wavelengths λ₂, λ₃, the added channels will pass through the chip unhindered and join the wavelength, λj, directed downstream by circulator 30.

[0022] In an active wavelength selectable OADM, the mirror 50 is replaced by a switchable mirror, which can either reflect the given wavelength channel back upstream, or allow it to pass unhindered into the drop stream. Such switching could be achieved using microelectromechanical (MEM) based mirrors or a cantilevered waveguide, or by any 1X2 optical waveguide switch where one arm terminates in a passive mirror while the other provides a through path to the drop output.

[0023] The forgoing specific description has related exclusively to OADMs employing optical waveguide gratings, but it should be clearly understood that the invention is not limited exclusively to the use of this particular type of optical grating, but is applicable to OADMs employing any optical detractive element. Thus, either an echelle grating based device, or an arrayed waveguide grating (AWG) device can be used as a waveguide based multiplexing/demultiplexing device. The echelle demultiplexer is preferred since the demultiplexer footprint is much smaller than that for an AWG. For a discussion on each technology, see the White Paper prepared by the Applicant entitled "Silicon-based Echelle Grating Technology For Metropolitan and Long-Haul DWDM Applications", 2001, which is incorporated herein by reference.

[0024] The passive OADM waveguide mirrors require vertical etches (to within one degree or less) in the material system used. High reflectivity can be achieved using metal or multilayer dielectric coatings. In the case of high refractive index waveguides such as SOI, silicon oxynitride or hiGaAsP, high reflectivity can be achieved by terminating the waveguides with right angle corner reflectors. Total internal reflection at the waveguide/air interface should in theory give 100% reflectivity. To achieve customisable OADM manufacturing, a mirror design should be used that can be implemented as a last step on preexisting wafers. In particular, a critical issue for etched grating demultiplexers is the verticality and smoothness of the deeply etched grating facets. In silica-based materials, the technique used to fabricate the waveguides and grating is reactive ion etching. Using this technique, grating wall verticality better than 89.8° with a RMS roughness better than 30 nm over 30 microns can be achieved on a production tool. The reliability and reproducibility of the fabrication process for vertical facets in silica-based planar waveguide eliminates the main disadvantage of echelle grating demultiplexers.

[0025] With the first switching unit, the second switching unit, the demultiplexing/multiplexing devices and the processing element, the OADM can add or drop optical signals. An advantage of this OADM configuration is the reduction of required assembly. There are only two fibre-to-waveguide junctions required, for any number of add- drop wavelength. This will lead to a reduction in assembled device cost. Separate optical circulators are required to separate the up and downstream paths, but connectorized circulators are readily available with very good performance at a small relative cost.

[0026] In the invention, a waveguide demultiplexer and multiplexer are connected by simple waveguide on a single chip. Each connecting waveguide incorporates a simple processing element that deteπnines the wavelengths to be dropped and added. One overall layout is the basis for an OADM for any number and arrangement of add/drop wavelengths. For a passive waveguide OADM, the insertion of reflecting elements into the connecting waveguides could be done as a final customisable manufacturing step; this would allow rapid customisation of OADM components to customer requirements, while only maintaining an inventory of standardized OADM wafers.

[0027] The channel spacing and density of this OADM configuration are set only by the demultiplexer. Given the present echelle grating technology, it could be scalable to 50 GHz, 80 channel systems and upwards. Any number and combination of wavelength channels can be programmed in as the add-drop channels.

[0028] Numerous modifications maybe made without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

What is claimed is:

1. An optical add-drop multiplexer (OADM) for adding and dropping wavelengths to and from a wavelength division multiplexed (WDM) signal, comprising on a single chip: a first terminal for receiving a multiplexed WDM signal from an optical fibre, and for transmitting a WDM signal to an optical fibre; a second terminal for receiving one or more wavelengths to be added to said multiplexed WDM signal and selectively dropping one or more wavelengths from said multiplexed WDM signal; a first multiplexer/demultiplexer associated with said first terminal for demultiplexing said received WDM signal into a plurality of separate wavelengths and multiplexing separate WDM wavelengths to be transmitted to an optical fibre; a second multiplexer/demultiplexer associated with said second terminal for multiplexing said separate WDM wavelengths to be dropped by said second terminal and demultiplexing said one or more wavelengths to be added to said multiplexed WDM signal; a plurality of channels for carrying the separate wavelength between the first and second multiplexer/demultiplexers; and a processing element in each of the channels between the first and second multiplexer/demultiplexer to selectively transmit or reflect signals.

2. The OADM of claim 1, wherein the processing element is a reflective mirror.

3. The OADM of claim 1, wherein the processing element is a switchable mirror.

4. The OADM of claim 3, wherein the switchable mirror is based on microelectromechanical switching technology.

5. The OADM of claim 3, wherein the switchable mirror is based on Mach-Zehnder switches.

6. The OADM of claim 3, wherein the switchable mirror is based on digital optical switches.

7. The OADM of claim 1, further comprising: an input waveguide for guiding the WDM signal between the first terminal and the first multiplexer/demultiplexer.

8. The OADM of claim 1, wherein the first and second terminals are part a first and second switching circuit, respectively.

9. The OADM of claim 8, wherein each switching circuit is an optical circulator.

10. The OADM of claim 1 , wherein the first and second multiplexer/demultiplexers are echelle gratings.

11. The OADM of claim 1 , wherein the first and second multiplexer/demultiplexers are arrayed waveguide gratings.

12. A method of adding and dropping wavelengths to and from a wavelength division multiplexed (WDM) signal, the method comprising the steps of: inputting the WDM signal from an optical fibre into an integrated optical add-drop multiplexer (OADM) via a first terminal; demultiplexing the WDM signal into individual wavelengths; multiplexing wavelengths to be dropped and transmitting the multiplexed wavelengths to be dropped via a second terminal; transmitting wavelengths to be added via the second terminal to the OADM and multiplexing the added wavelengths with the wavelengths not to be dropped; and transmitting the multiplexed signal comprising the added wavelengths and wavelengths not to be dropped to the optical fibre via the first terminal.

13. An optical transmission system comprising: an optical fibre installed between a transmitting terminal station and a receiver terminal station; and an integrated optical add-drop multiplexer (OADM) being positioned between the transmitting terminal station and the receiver terminal station, the OADM comprising only a first and a second tenninal for connection to the optical fibre.

14. The optical transmission system of claim 13, further comprising a plurality of optical fibres, each being installed between a corresponding transmitting terminal station and a corresponding receiver terminal station, and an integrated OADM being positioned between each pair of stations.