GB2076994A

GB2076994A - Polarization independent optical switch

Info

Publication number: GB2076994A
Application number: GB8116860A
Authority: GB
Original assignee: Western Electric Co Inc
Current assignee: AT&T Corp
Priority date: 1980-06-03
Filing date: 1981-06-02
Publication date: 1981-12-09
Also published as: JPS5726824A; SE8103251L; NL8102680A

Abstract

An input beam of radiation (100) incident upon a polarization independent optical switch is resolved into two component beams (110 and 120) having orthogonal linear polarizations. The component beams both pass through a polarization rotator (20), which rotator transmits the component beams without attenuation. The two component beams are then recombined to provide the output beam of radiation (130 or 102) which emerges from the device. When the polarization rotator cell is configured to rotate the polarization of the component beams by 0 DEG the output beam of radiation travels in a first direction (130) and when the polarization rotator is configured to rotate the polarization by 90 DEG the output beam of radiation is switched to a second direction (102). In one embodiment the polarization rotator is a liquid crystal twist cell. <IMAGE>

Description

SPECIFICATION Polarization independent optical switch This invention pertains to the field of optical switches, and more particularly to the field of polarization independent optical switches.

The use of optical switches is becoming an important element in optical communication systems. It is desirable to have an optical switch that is both polarization and wavelength independent. It is also desirable that the switch have a good crosstalk ratio and use as small an amount of energy as possible.

According to the present invention there is provided an optical device comprising means for splitting an incident beam of radiation into two component beams of radiation, said component beams being substantially orthogonally polarized, rotator means disposed in the path of said two component beams, for rotating the polarization of each of said component beams, combiner means disposed in the path of the two component beams with rotated polarizations, for combining the two component beams into outgoing radiation, and means for adjusting the rotator means to adjust the rotation of the polarization of the component beams so that when the polarization of the beams is rotated by substantially 0 or substantially 90 the outgoing radiation is switched from one direction to another direction, or so that when the polarization is variably rotated the outgoing radiation is split into two beams in two directions, the power of the radiation in each of the two beams being selectably adjusted.

An input beam of radiation incident upon a polarization independent optical switch constructed according to an embodiment of the present invention is resolved into two component beams having orthogonal linear polarizations. The component beams both pass through a polarization rotator, which rotator transmits the component beams without attenuation. The two component beams are then recombined to provide the output beam of radiation which emerges from the device. When the polarization rotator is configured to rotate the polarization of the component beams by substantially 0" the output beam of radiation travels in a first direction and when the polarization rotator is configured to rotate the polarization by substantially 90 the output beam of radiation is switched to a second direction.In this embodiment the polarization rotator is a liquid crystal twist cell.

A complete understanding of the present invention may be gained from a consideration of the detailed description presented hereinbelow in connection with the accompanying drawings in which: Figure 1 shows, in pictorial form, an embodiment of the present invention with the output beam of radiation traveling in a first direction; Figure 2 shows, in pictorial form, an embodiment of the present invention where the output beam is switched to travel in a second direction; Figure 3 shows a graph of the cross-talk ratio of the embodiment of the present invention shown in Figs. 1 and 2 as a function of polarization rotation; Figure 4 shows an embodiment of the present invention utilizing optical fibers to couple an input beam of radiation into a polarization independent switch by means of GRIN-rod lenses; and Figure 5 shows, in pictorial form, an embodiment of the present invention which switches a multiplicity of input beams simultaneously.

Fig. 1 shows an embodiment of a polarization independent optical switch for switching a collimated, unpolarized, incoherent input beam of radiation; e.g., input beam of radiation 100 is assumed to be collimated and have an arbitrary polarization state. Input beam 1 00, which impinges upon polarization beam splitter 10, is resolved into component beam 11 0, having a polarization perpendicular to the plane of the drawing, and component beam 120, having a polarization lying in the plane of the drawing, i.e., polarization beam splitter 10 resolves input beam 100 into two component beams having orthogonal linear polarizations.

Component beam 1 10 is reflected by mirror 11 to pass through polarization rotator 20.

Component beam 1 20 also passes through polarization rotator 20. After passage through polarization rotator 20, beam 1 20 is reflected from mirror 1 2 toward polarization beam splitter 1 3. Component beam 1 20 passes through polarization beam splitter 1 3 and emerges as part of output beam 1 30. Component beam 1 10 is reflected in polarization beam splitter 1 3 and combines with component beam 1 20 to form output beam 1 30. In Fig. 1 polarization rotator 20 rotates the polarization of component beams 110 or 1 20 by 0 If both component beams 1 10 and 1 20 are transmitted without attenuation, the power of output beam 1 30 is equal to the power of input beam 100.

Fig. 2 shows the same embodiment of the polarization-independent switch when polarization rotator 20 rotates the polarization of component beams 1 10 and 1 20 by 90 . The change in rotation from 0" to 90 is activated by apparatus 50 which changes the amount of polarization rotation of rotator 20. Here, component beam 1 20 is reflected by polarization beam splitter 1 3 to become part of output beam 140, where output beam 1 40 travels in a direction perpendicular to the direction shown- for output beam 1 30 in Fig. 1.Component beam 110 passes through polarization beam splitter 1 3 and combines with component beam 1 20 to form output beam 140. As above, if both component beams 110 and 1 20 are transmitted without attenuation the power of output beam 140 is equal to the power of input beam 1 00.

In general the directions of output beams 1 30 and 1 40 need not be perpendicular and are shown this way for ease of understanding the operation of the apparatus. For example, if input beam 100 were nowt perpendicularly incident upon surface 200 of polarization beam splitter 10 then output beams 1 30 and 140 would not be perpendicular. Furthermore, if surfaces 210 and 211 were oriented at a different angle inside polarization splitter 10, output beams 1 30 and 1 40 would also not be perpendicular.

As noted above, if both component beams 110 and 1 20 are transmitted without attenuation, the power of output beams 1 30 or 1 40 are equal to the power of input beam 1 00. However, if there are relative phase shifts between component beams 110 and 1 20 after passing through rotator 20, then the polarization state of output beams 1 30 and 140 could differ from the polarization state of input beam 100, even though the transmitted power would be unaffected.

It is also noted that the spectral characteristics of the output beam, the speed of response and the electrical requirements for the switch depend on the characteristics of rotator 20. It should also be clear that there may be some spectral dependence on the characteristics of the polarization splitters 10 and 13.

Component beams 11 0 and 120 do not interfere in the output regardless of the polarization rotation applied by polarization rotator 20 because polarization splitter 1 3 isolates the two orthogonal polarization components. Thus, output beam 1 30 or output beam 1 40 is the incoherent superposition of the two component beams 110 and 120 and the power of the output beam is equal, in principle, to the power of input beam 1 00. Note that since the switch does not rely on the coherence of input beam 110 or interference effects, interferometric alignment of the parts is not necessary.

The Fig. 1 embodiment utilizes one input beam. However, the switch shown in Fig. 1 can switch two beams input from independent directions as well. A second input beam of radiation, incident upon polarization beam splitter 10 along the direction of arrow 101, would have an output beam in the direction of arrow 1 02 when input beam 100 has its output beam in the direction shown by beam 130. When polarization rotator 20 rotates polarizations by 90 , the direction of the output beam for a second input would be along the direction shown by beam 130.

Further note that polarization rotator 20 provides a controlled amount of polarization rotation for component beams 110 and 120. This amount of rotation determines whether all the radiation in input beam 100 exits the switch in beam 1 30 or in beam 1 40 or is divided between them.

For lossless, perfectly poiarizing components, the fraction of optical power f1 output along a first direction, e.g., along beam 130, and the fraction f2 output along a second direction, e.g., along arrow 102, depend only on the polarization rotation angle 0 provided by polarizat(on rotator 20: f, = cos20 (1) f2=sin20 (2) EGGS. 1 and 2 indicate that switching between directions along beam 1 30 and along arrow 1 02 is possible with any material that produces a switchable rotation of substantially 90 .

The capability of the switch to extinguish the outputs in one or the other direction may be limited by the accuracy of the polarization rotation. The required accuracy of rotation for a given level of extinction is given by EQ. 1 and 2 to be ~ + 5" for a - 20 dB extinction. Such rotation is regarded as being substantially 0" and substantially 90 .

The ability of the switch to extinguish the outputs may also be limited by beam splitter quality. A poor quality polarization beam splitter could include some amount of the incorrect polarization in each of the two component beams 110 and 1 20. When account is taken of partial polarization due to polarization beam splitter imperfections, EQS. 1 and 2 are rewritten as: F1 =(1 - 6)cos28 + asian28 (3) F2 = (1 - 6)sin28 + 6cos28 (4) where S, the measure of the polarization beam splitter imperfection, is given by:: 8 = (e51 + epr + eS2 + rep2)/2 (5) and eS1 and ep1 represent the fractions of s- and p- polarizations that are incorrectly admitted by polarization beam splitter 10, and s2 and p2 represent the corresponding quantities for polarization beam splitter 1 3. The quantity 8 represents the quality of the be.m splitter pair and is typically about 10-2 for dielectric film polarizers.

EQS. 3 and 4 neglect interference effects which are present for partially-polarized beams. This is valid however for incoherent sources where the coherence length of the input beam is less than the optical path difference for the two paths. Furthermore, interference effects are significant with coherent sources only for the extinguished outputs and not for the principle outputs.

An important measure of the polarization division switch is the crosstalk ratio f2/f1, as determined from EGGS. 3 and 4. For a single input beam, such as beam 100 in Fig. 1, this ratio represents the relative strength of the two output beams 1 30 and 1 02. For two input beams, such as 100 and 101, the ratio represents the crosstalk between the two outputs in a single location such as is given by output 1 30. Fig. 3 illustrates graphically how the crosstalk ratio f2/f1 is influenced by beam splitter imperfections 8 and polarization rotation 0.Fig. 3 shows a family of crosstalk curves for various values of 8 which indicates that a crosstalk level of - 20 dB requires a beam splitter pair with 8 < 0.007 and polarization rotation accuracy of about + 3 .

Such rotation is also regarded as being substantially 0" or substantially 90 .

Switching action can be obtained by using a liquid crystal twist cell as polarization rotator 20.

In one embodiment, a cell 6ym thick was filled with a nematic liquid crystal material having a large positive dielectric anisotropy. Table 1 summarizes the liquid crystal twist cell characteristics. In this embodiment apparatus 50 supplies a 1 kHz signal that can be switched between 0.8 and 2.5 volts rms to actuate the liquid crystal cell and provide switching. For the switch embodiment having a liquid crystal twist cell polarization rotator, a typical insertion loss, which neglects reflections and prism coating absorption, is 0.4 dB with an extinction ratio of about - 20 dB.

Table 2 summarizes the insertion loss and extinction ratio measurements for the switch with liquid crystal twist cell rotator. These measurements include reflection and absorption losses which amount to 1.6 dB for the twist cell rotator ON or OFF, but which can be largely eliminated as described hereinbelow. The measured insertion losses exceeded 1.6 dB by only 0.3-0.4 dB for the twist cell ON and by only 0. 1-0.2 dB for the cell OFF. This remainder is probably due to scattering in the cell.

These measurements were made at a wavelength of 633 nm by using an incoherent, unpolarized input beam from a Xe-arc source with a 10 nm bandpass interference filter. The switch has similar loss and crosstalk for light from a HeNe laser. The major contributions to the insertion loss are reflections from the component surfaces and absorption in the mirror coatings, as summarized in Table 3. These can be largely eliminated by using uncoated prisms and by cementing the pieces together. The crosstalk level of - 20 dB is limited by the quality of the polarization beam splitters. The polarization beam splitters consist of a multilayer dielectric film 210 sandwiched between fused silica prisms 220 and 221. These polarization beam splitters were designed for operation at 633 nm and have a measured 6-0.01.

The switch also has the capability for multimode operation because small changes in the incidence angle of the input beam do not effect the switch performance significantly. For example, a t 2 change produces a negligible increase in insertion loss and only a measured degradation of about 1.0 dB in crosstalk capabilities. As is discussed hereinbelow this is sufficient for multimode optical fiber applications if lenses are used to collimate the optical fiber output.

Table 1 Liquid Crystal Twist Cell Characteristics Cell Material Properties: Nematic Liquid Crystal Merck E7 Nematic Range 10 to 60.5"C Dielectric Anisotropy (## - #|) 13.8 Effective Elastic Modulus (k) 1.09 X 10-6 dynes Viscosity (rl) 0.38 poise Cell Construction: Thickness 6 lim' Electrodes ln2O3 Surface Treatment Vacuum Evaporated SiO, Cell Performance:: Threshold Voltage 1 volt rms Rotation (2.5V rms) 2.7 Rotation (0.8V rms) 87.1 Rise time (0.8-2.5 volt transition) 80 msec Decay Time (2.5-0.8 volt transition) 140 msec Resistance 16M ohms Capacitance (Cll) cstimated 3 X 10-9 farads/cm2 Power Dissipation estimated 0.3 microwatts/cm2 Table 2 Summary of Loss and Crosstalk Measurements yA=633nm; er= 0.01J Insertion Source Rotator State Rotation Loss Crosstalk Xe-Arc Twist Cell ON 2.7 2.0 dB - 18.8 dB OFF 87.1' 1.8dB -16.8dB HeNe Laser Twist Cell ON 2.7 1.9 dB - 18.8 dB OFF 87.1' 1.7dB -19.4dB Measured reflection and prism coating losses amount to 1.6 dB for the Twist Cell ON or OFF (see Table 3).

Table 3 Measured Insertion Loss Contributions Reflections- Beamsplitters (AR-coated) 0.1 dB Prisms 0.4 dB Rotator 0.4 dB Absorption-Prism Coating 0.7 dB 1.6 dB The liquid crystal twist cell rotator provides broadband switching at integrated circuit (IC) compatible voltage and power levels. Above the threshold voltage, which can be as small as one volt or less, the twist cell provides a 90 rotation for all visible and near-infrared wavelengths.

The liquid crystal cell is compact and the liquid crystal materials have an index of refraction similar to that of glass, which fact can be used to minimize reflections.

In other embodiments of an optical switch, different polarization rotators may be used, for example, Pockels electro-optic effect devices, Kerr cells or Faraday effect devices. For apparatus constructed using the above-mentioned polarization rotators, the amount of rotation provided by a given device depends upon the wavelength of the radiation passing therethrough. This has the effect of restricting the usable wavelength range of a switch which uses these rotator devices.

Also, the Pockels effect is weak and requires large operating voltages and bulky devices to provide a 90 rotation. For example, the half-wave voltage in a KD"P Pockels cell is typically 3000 v at 633 nm. Furthermore, the Pockels cell may be several centimeters long, A Faraday rotator is functionally different from either the liquid crystal twist cell or the Pockels cell described hereinabove. The use of a Faraday rotator provides a circulator device because the polarization rotation of radiation is nonreciprocal with respect to the direction of propagation of radiation passing therethrough.The orientation of the magnetic field applied to the Faraday material determines the order of circulation among the input and output beams of radiation and the order of circulation can be switched by reversing the direction of this magnetic field. It should be clear to those skilled in the art as to how the magnetic field may be switched. For example, apparatus 50 may be provided by surrounding the Faraday material with a magnet and a coil. An apparatus which reverses the direction of the current in the coil changes the direction of the field passing through the material.

At wavelengths above 1 Ism, where YIG crystals are transparent, compact Faraday rotators can be built because the rotational power of YIG is about 20"/mum at the saturation magnetization.

At wavelengths below 1mum, however, the strongest transparent Faraday materials require large propagation distances.

It should also be clear to those skilled in the art that apparatus 50 may comprise mechanical means for changing the polarization rotation, either by inserting different polarization rotators into the position occupied by polarization rotate 20 in Fig. 1, or by rotating a halfwave plate in place. In one embodiment rotation may be changed by using different thickness waveplates affixed to a rotatable wheel.

As described hereinabove, the polarization independent optical switch operates best with collimated light. However, the optical switch may also be used with optical sources and receptors such as lasers, light emitting diodes, photodiodes and multimode optical fibers by using lenses to collimate the light. In the discussion that follows I will use the example of optical fibers for the sake of ease of understanding. For example, a 3 mm focal length lens used with a 501lm core diameter optical fiber is sufficient to produce a beam which is well enough collimated for satisfactory switch operation, that is, the beam of radiation has a beam spread of less than 1'.

Fig. 4 shows an individual two beam switch where the four optical fibers 200-203 utilize lenses 210-213. All the parts shown in Fig. 4 can be cemented together to form a single rigid assembly. In Fig. 4 the lenses may be GRIN-rod lenses having a quarter-period. The fibers are shown to be affixed at the surface of one of the faces of the lenses at the point where the optical axis of a lens intersects that face.

In order to provide low losses in the switch, the alignment between the optical fibers, the lenses and the switch parts must be accurately maintained. The two recombined component beams must be parallel in order to produce spots that coincide on the surface of the lens to which the output fiber is affixed. This requires that the component beams must be parallel to within about 3 minutes of arc for a 3ym spot misalignment and a 3 mm focal length lens. This tolerance is within commercial fabrication tolerances for prisms and polarization beam splitters to such an extent that the switch parts and the lenses shown in Fig. 4 can be cemented together without further adjustment.However, the alignment of the fibers shown in Fig. 4 relative to the lenses has tolerances similar to that for fiber-to-fiber butt joints, which tolerances require microminipulation of the individual fibers.

When the parts are aligned and cemented together reflection losses are generally eliminated, but coupling losses due to lens aberrations and diffraction remain. These, however, can be as low as 0.8 dB for a pair of GRIN-rod lenses, even when the two lenses are separated by the distance needed for the switch parts. In a further embodiment of the switch shown in Fig. 4, one may use spherical-rod lenses. These lenses have coupling losses of 0.8 dB, but have the advantage that larger lens separations can be tolerated without increasing coupling loss. It should also be clear to those skilled in the art that equivalent lens structures may also be used to fabricate further embodiments of the present invention.

In a further embodiment of the present invention, shown in Fig. 5, the polarization independent optical switch is used in an array configuration where the polarization rotator contains individually controlled segments 300-303 controlled by apparatus 320. The provision of individually controlled segments shown as 300-303 on rotator 310 in Fig. 5 are commonly provided in liquid crystal twist cells. This is normally provided by utilizing individually segmented electrodes on a single cell. These segments can be a few miilimeters in size and can be spaced at distances as small as 200m to provide a very compact array. To utilize a compact array with optical fibers, one would normally use an array of input and output lenses with fibers attached thereto in a manner similar to that shown in Fig. 4.For this particular embodiment, the number of elements in an array is limited by polarization beam splitter size, which size determines the separation between input and output lenses. Because beam spreading occurs, this separation determines the required lens diameter and thereby the number of lenses that can be packed into the beam splitter clear aperture. For example, a 25 mm wide beam splitter may be used to build 100 independent switches, using 3 mm focal length lenses placed in a 10 X 10 matrix array and spaced by 2 mm in the array.

As described hereinabove in relation to the discussion regarding EQS. 1 and 2, the fraction of optical output along a first direction, e.g., along output beam 1 30 in Fig. 1, and the fraction of optical output along a second direction, e.g., along arrow 102 in Fig. 1 depend only on the polarization rotation angle 0 provided by polarization rotator 20. This points out another aspect of the present invention, that aspect being an adjustable beam splitter. The ratio of the power split in the two directions is determined in accordance with the polarization rotation angle. This aspect is also illustrated in Fig. 3. However, it is noted that the- liquid crystal twist-cell rotator operates in such a fashion that linearly polarized radiation is transmitted therethrough when the rotation is 0 or 90 . At intermediate rotation angles the radiation transmitted may be elliptically polarized. In such a case the analysis discussed in accordance with Fig. 3 is not appropriate.

Nevertheless, by varying th.e voltage across the cell the radiation can be apportioned between the two output directions.

Claims

CLAIMS-

1. An optical device comprising means for splitting an incident beam of radiation into two component beams of radiation, said component beams being substantially orthogonally polarized, rotator means disposed in the path of said two component bearns, for rotating the polarization- of each of said component beams, combiner means disposed in the path of the two component beams with rotated polarization, for combining the two component beams into outgoing radiation, and means for adjusting the rotator means to adjust the rotation of the polarization of the component beams so that when the polarization of the beams is rotated by substantially 0" or substantially 90 the outgoing radiation is switched from one direction to another direction, or so that when the polarization is variably rotated the outgoing radiation is split into two beams in two directions, the power of the radiation in each of the two beams being selectabiy adjusted.

2. A device in accordance with claim 1, wherein said rotator means is a liquid crystal twist cell.

3. A device in accordance with claim 1, wherein said rotator means is a Pockels electro-optic cell.

4. A device in accordance with claim 1, wherein said rotator means is a Kerr cell.

5. A device in accordance with claim 1, wherein said rotator means is a Faraday rotator.

6. A device in accordance with claim 5, wherein said Faraday rotator is a YIG cell.

7. A device in accordance with claim 1, when used for coupling radiation from optical source means, into optical receptor means, which device further comprises first lens means disposed between the optical source means and said splitter means for coupling radiation from said optical source means to said splitter means; and second lens means disposed between said combiner means and the optical receptor means for coupling radiation from said combiner means to said optical receptor means.

8. A device in accordance with claim 7, wherein said optical source means and said optical receptor means are optical fibers.

9. A device in accordance with claim 7, wherein said first lens means and second lens means are GRIN-rod lenses.

10. A device in accordance with claim 7, wherein said first lens means and said second lens means are spherical-rod lenses.

11. A device in accordance with claim 7, wherein said rotator means is a liquid crystal twist cell.

1 2. A device in accordance with claim 7, wherein said rotator means is a Pockels electrooptic cell.

1 3. A device in accordance with claim 7, wherein said rotator means is a Kerr cell.

14. A device in accordance with claim 7, wherein said rotator means is a Faraday rotator.

1 5. A device in accordance with claim 14, wherein said Faraday rotator is a YIG cell.

16. A device according-to any one of claims 1 to 15, wherein the splitter means serves to split each of a multiplicity of incident beams of radiation into a pair of component beams of radiation, said component beams of each pair being substantially orthogonally polarized; wherein the rotator means is disposed in the path of each component beam of each of said pairs of component beams, for rotating the polarization of each component beam; wherein the combiner means is disposed in the path of each component beam of each of said pairs of component beams with rotated polarizations, for combining each pair of said component beams into an outgoing beam; and wherein the means for adjusting said rotator means serves to independently rotate the polarization of each pair of said pairs of component beams by substantially 0" or substantially 90 , whereby each of said outgoing beams is independently switched from one direction to another direction.

1 7. A device in accordance with claim 1 6 as appendant to claim 2, wherein the liquid crystal twist cell serves to independently rotate the polarization of said component beams.

1 8. A device according to claim 1, wherein the splitter means serves to split each of a multipicity of incident beams of radiation into a pair of component beams of radiation, said component beams of each pair being substantially orthogonally polarized; wherein the rotator means is disposed in the pathof each component beam of each of said pairs of component beams, for rotating the polarization of each component beam; wherein the combiner means is disposed in the path of each component beam of each of said pairs of component beams with rotated polarizations, for combining each pair of said component beams into a multiplicity of outgoing radiation; and wherein the means for selectably adjusting said rotator means serves to independently and variably rotate the polarization of each pair of said pairs of component beams, whereby each of said outgoing radiation is split into two directions, the power of the radiation in each of said multiplicity of two directions being independently selectably adjusted when said rotator means is independently selectably adjusted to variably rotate the polarization of said component beams.

19. A device according to claim 18, wherein said rotator means comprises a liquid crystal twist cell for independently rotating the polarization of said component beams.

20. An optical device substantially as hereinbefore described with reference to Figs. 1, 2 and 3, or Fig. 4 or 5 of the accompanying drawings.