CA2367162A1 - Virtual waveplate - Google Patents

Abstract

A U-shaped virtual waveplate is useful for example in an interleaver design.
The virtual waveplate has to polarizing beamsplitters, two mirrors, and a combination of optical paths defined in glass and air, or glass and another medium/material which is selected such that the assembly functions as a temperature insensitive waveplate of very high order.

Description

Doc. No. 10-421 CA Patent VIRTUAL WAVEPLATE
Field of the Invention [0001] This invention relates generally to a device which is capable of providing the functionality of a birefringent waveplate and which is substantially less temperature sensitive than a birefringent waveplate.
Background of the Invention to [0002] Optical wavelength division multiplexing (WDM) has gradually become the standard backbone network for fiber optic communication systems. WDM systems employ signals consisting of a number of different wavelength optical signals, known as carrier signals or channels, to transmit information on optical fibers. Each carrier signal is modulated by one or more information signals. As a result, a significant number of channels may be transmitted over 15 a single optical fiber using WDM technology.
[0003] Despite the substantially higher fiber bandwidth utilization provided by WDM
technology, a number of serious problems must be overcome, for example, multiplexing, demultiplexing, and routing optical signals, if these systems are to become commercially viable.
2o The addition of the wavelength domain increases the complexity for network management because processing now involves both filtering and routing. Multiplexing involves the process of combining multiple channels (each defined by its own frequency spectrum) into a single WDM
signal.
25 [0004] Demultiplexing is the opposite process in which a single multiplexed signal is decomposed into individual channels. The individual channels are spatially separated and coupled to specific output ports.
[0005] U.S. patent number 4,685,773 in the name of Carlson assigned to GTE
Laboratories and 3o U.S. patent number 4,744,075 in the name of Buhrer assigned to GTE
Laboratories incorporated herein by reference disclose multiplexing/de-multiplexing optical circuits dependent upon ' CA 02367162 2002-O1-10 Doc. No. 10-421 CA Patent birefringent waveplates which are known to have a periodic wavelength vs.
polarization output response; for example when a series of consecutive evenly spaced linearly polarized channels are passed through a birefringent waveplate at a predetermined orientation the output light will be polarized orthogonally for even and odd channels respectively; A polarizer or polarization discriminator can then be used to separate even and odd channels in dependence upon their polarization. Several other waveplate-based demultiplexer/multiplexer circuits have been disclosed recently in US patent application numbers 09/476,034 filed 12/31/99, 09/476,611 filed 12/31/99, 09/476,609 filed 12/31/99, 09/517,640 filed 3/3/00, and 09/626,698 filed 7/27/00 assigned to E-tek Dynamics.
to [0006] Although these interleaver/deinterleaver multiplexer/demultiplexer circuits perform their intended function, in some instances it is desired to have devices which perform the same function and which do not require active temperature control circuitry.
[0007] Furthermore, it is desired to have a device wherein the temperature sensitivity of the waveplate is relatively low.
[0008] It is an object of this invention to provide an optical device that will yield a retardance between orthogonal polarization components for an input beam of light and wherein the device is 2o substantially temperature stable.
[0009] It is an object of this invention to provide a virtual waveplate which provides the advantages of a waveplate and which lessens disadvantages associated with using a birefringent crystal waveplate.
3o Summary of the Invention

2 Doc. No. 10-421 CA Patent [00010] In accordance with the invention there is provided a virtual waveplate for providing a relative group delay (Optical Path Difference, or OPD) between orthogonal polarization components of a beam of light propagated therethrough, the delay substantially equal to L~n~ap, the virtual waveplate comprising:
a polarization beam splitter for receiving an input beam and for splitting and directing the beam into two orthogonally polarized sub-beams along first and second separate paths respectively;
a polarization beam combiner optically coupled with the polarization beam splitter for combining two sub-beams received from the polarization beam sputter along the first and second paths;
the first path being defined through a light transmissive material (LTM) having a group to refractive index nL-,~,t and the first path having a length Lt + L3 and an optical path length (OPL) of LInL~ + L3nL~, the second path defined at least partially through a light transmissive material having a group refractive index nL~, said second path having an optical path length of L2nL~
+ Ltngap + L3nL~
wherein Lt is a length of a gap defining a section of the second path and where ngaP is the group refractive index of air or another gap medium or material having a refractive index which is substantially unchanged by a change in temperature or wavelength, the length L, not varying substantially with temperature, LZ is a length of a portion of the second path through the light transmissive medium, and L3 is a length through the light transmissive material which is common to the first and second paths, where in general the group refractive index n8 of a material is related to the more common phase refractive index nr according to n$ = n~(1-~,(dn,Jd~,)).
[00011] In accordance with the invention, a virtual waveplate is provided having two r optically coupled polarization beam splitter/combiners and two paths defined therebetween having first and second path lengths respectively, a retardance induced between two orthogonal polarization components of a beam of light launched therein being substantially equal to ~OPL, a difference in the first and second optical path lengths which is equal to LtngaP wherein ngap is the group refractive index of the gap medium/material, e.g. air and Lt is a physical length of the gap which only one of the two orthogonal components of the beam traverses along only one of 3o the first and second paths.

Doc. No. 10-421 CA Patent [00012] In accordance with another aspect of the invention there is provided, a comb filter comprising;
means for providing one or more polarized sub-beams of light from a beam having an unknown polarization state;
a virtual waveplate as defined above, the waveplate being oriented such that the one or more linearly polarized sub-beams of light is launched therein such that the polarization of the one or more beams is offset relative to both principal axes of the virtual waveplate defined by one of the beamsplitter combiners; and, a polarizer for receiving one or more output beams from the virtual waveplate for filtering and directing predetermined periodic polarization components of the one or more output beams to a first output port and for directing other periodic polarization components to a second output port.
[00013] In accordance with another aspect of'the invention, there is provided a virtual waveplate for providing a relative group delay between orthogonal polarization components of a beam of light propagated therethrough substantially equal to L2n~omp, the virtual waveplate comprising:
a polarization beam sputter for receiving an input beam and for splitting and directing the beam into two orthogonally polarized sub-beams along first and second separate paths 2o respectively;
a polarization beam combiner optically coupled with the polarization beam sputter for combining two sub-beams received from the polarization beam sputter along the first and second paths;
the first path being defined through a light transmissive material (LTM) having a group refractive index n~TM and having an optical path length of L~nL~ + L3nLTM, the second path defined at least partially through a light transmissive material having a group refractive index nLTT,,, said second path having an optical path length of LZn~,~ + Llri~omp +
L3nL~ wherein LZ is a length of a compensating section defining a part of the second path, and where n~omP is the refractive index of air or another medium/material of the compensating section wherein the optical path length of the first path and the optical path length of the second path have a substantially constant difference as temperature varies.

' ' CA 02367162 2002-O1-10 Doc. No. 10-421 CA Patent Brief Description of the Drawings [00014] Exemplary embodiments of the invention will now be described in conjunction with the drawings in which:
Fig. 1 is a block diagram illustrating a side view of a U-shaped virtual waveplate in accordance with an embodiment of the invention, to Fig. 2 is a block diagram similar to that of Fig. 1 illustrating the two principal paths through the device used in a single pass operation, Fig. 3 is a top view of an embodiment of the invention wherein retro-reflectors are disposed at ends to allow multiple passes through the virtual waveplate, Fig. 4a is an isometric view of an interleaver/deinterleaver circuit in accordance with an embodiment of the invention wherein two virtual waveplate blocks of different path lengths are utilized, 2o Fig. 4b is a top view of the interleaver/deinterleaver shown in Fig. 4a;
Fig. 4c is a side view of the interleaver/deinterleaver shown in Fig. 4a;
Fig. 5a is a side view of an arrangement for splitting the input beam into two sub-beams with a birefringent crystal, and Fig. 5b is a side view of an arrangement for recombining the two sub-beams into a single output beam.
3o Detailed Description Doc. No. 10-421 CA Patent [00015] The term virtual waveplate referred to in this specification denotes a device providing a retardance between orthogonal polarization components of light launched therein, and having less temperature dependence than a conventional birefringent waveplate.
[00016] Referring now to Fig. 1, a U-shaped virtual waveplate is shown having two upstanding end blocks 10a and 10b, each comprising a polarizes 12a and 12b respectively and a reflector 14a and 14b respectively. A block 16 is shown disposed between 10a and lOb bridging the blocks. The blocks are dimensioned such that a beam B;n launched into the device from the left side into block 10a is split by the polarizes 12a into two orthogonally polarized sub-beams; a first beam S ~ follows a straight-through path through block 16 and onward to block 10b; a to second sub-beam S2 is directed upward though a portion of block 10a until it is redirected by the reflector 14a out of the block 10a and into free-space through the air gap between blocks 10a and 10b. The second sub-beam SZ then enters the block lOb and is directed by the reflector 14b downward toward the polarizes 12b. The first and second sub-beams are then combined into a single beam Bo"c where they are output from the right side of block 10b. As can be seen in Fig.
1, LZ is the sum of two path lengths L2/2 in the block 10a and lOb respectively, and the same applies to path length L3 as shown in Fig. 2. If the path lengths are even i.e. Lt = L2, then the combined length of the vertical paths marked by up and down arrows defining part of the path of the second beam is equal to the length Lt of the portion of the path followed by the first beam and the short additional sections through the blocks are such that the total distance traversed 2o through the light transmissive blocks are the same along both paths, and the only difference in optical path length between the path traversed by the first sub-beam and the second sub-beam is the length of the air-gap traversed by the second sub-beam. This is advantageous since if the glass material is chosen to have very low thermal expansion coefficient, then the length Lt is substantially constant, and the optical path length of the air gap is substantially constant and is for all intents and purposes constant.
[00017] Preferably the blocks are affixed to one another by optically contacting the blocks after preparing the contacting surfaces accordingly.
[00018] As it will be understood, this design has significant advantages. The only substantial difference in optical path length between two paths followed by the two sub-beams is ~ CA 02367162 2002-O1-10 Doc. No. 10-421 CA Patent due to the distance across the air gap traversed by the second sub-beam. This is equivalent to passing light through a very thick waveplate. The dimensions of the glass portions in the two paths are selected such that they are substantially the same for both paths.
[001119] Alternatively, other materials can be used for the twa paths of the U-waveplate instead of fused silica and air, provided that the materials have the same coefficient of thermal expansion to prevent thermal stresses but different temperature dependence of refractive index.
For example, the high temperature dependence of refractive index of silicon would allow a small piece of silicon in the connector block to compensate for the temperature dependence of the to material in the U-arm, enabling a device to be made with no air path.
[00020] If the thermal expansion coefficient of the glass material is significant, or the refractive index of the air gap changes significantly with temperature, the following discussion explains how to select the values of Ll and La obtain temperature independence of OPD.
[00021] Turning now to Fig. 2, the second path is shown in dashed lines and the first path is shown indicated by a solid line. The second path including the air gap consists of L2nL~,, +
LingaP + L3n~TM and the first path consists of L~n~~, + L3n~~.
[00022] The optical path difference (OPD) between path 1 and path 2 is given by OPD = Ltn~P+ (L2-Lt)nL~ (1) [00023] As temperature changes by a small amount 0T, the changes in L~, L2, n~TT,r, and ngap are given by dL,= aL~ L, OT
OLz = aL~rn~ L2 4T
tlnL~ _ (dnL~/dT) ~T
~ln~P = (dn~p/dT) AT

~ CA 02367162 2002-O1-10 Doc. No. 10-421 CA Patent where aL~ is the thermal expansion coefficient of the light transmissive material.
Then the change in OPD caused by a change in temperature is approximately given by (ignoring second order terms) d(OPD) ~ (L~dn~p/dT + aLTMLtngap+L2dnL~/dT + aL~LZnL~ - LldnL~/dT-aL~L,nL~) ~T= 0 (for no temperature dependence) [00024] This can be rewritten as n ~y dd~p + (.4 LTM n.enP
LZ= L, 1- (2) d ~T + aLTM nLTM
[40025] Replacing equation (2) into equation (1), OPD = Lln~p - LInL~ [ ~dnBaP l dT ~+ aLTM ngw~ ]
dnLTM I dT )+ aLTM nLTM
[00026] which can be rearranged to obtain OPD
ddT + a, n Ban ng~ - nLTM
dnLrM
dT + aLTM nLTM
[00027] Given a desired OPD and the material properties of the LTM and the gap, equation (3) can be used to determine the required value of L~. Subsequently equation (2) can be used to determine L2. This provides substantially stable temperature performance such that d(OPD)/dT=0, even if aL~ ~ 0 or dn~p/dT ~ 0. Therefore, while it is advantageous to use a material with low thermal expansion such as fused silica for the light transmissive material and a material with low dn/dT such as air for the gap material, a temperature stable virtual waveplate s Doc. No. 10-421 CA Patent can in principle be constructed from almost any two different materials. The additional constraint if a solid material is chosen for the gap is that the thermal expansion coefficients of the two materials should be approximately equal.
[00028] The change in OPD with wavelength ~, can be obtained by differentiating equation ( 1 ) to obtain d (OPD) _ dn8~ + L _ dnLr~r 4 d~, L' d~, ~ 2 ~ d ( ) [00029] Since it is desirable to keep d(OPD)/d~, small in an interleaver or deinterleaver 1o application, it is logical to select materials such that d(OPD)/d7~ is at a minimum. Fortunately, one such solution is to choose air for the gap material so that dnga~/d~, ~ 0 and dnga~/dT = 0, and choose a material with low thermal expansion coefficient such as fused silica for the light transmissive material so that aL~ = 0, then by equation (2), L2 = Ll and by equation (4), d(OPD)/d~, = 0.
[00030] Turning now to Fig. 3, a top view of an alternative embodiment of a virtual waveplate is shown. In addition to the embodiment shown in Fig. 1, retro-reflectors 22a and 22b are added to direct a beam of light launched into the waveplate to pass multiple times through the device. By so doing, a first pass having a retardance R can be realized, and the subsequent two 2o passes provide a retardance of 2R. Hence three passes through one U-shaped virtual waveplate provide a necessary retardance to achieve interleaving and deinterleaving functionality.
[00031] In addition, another advantage is afforded by the mufti-pass design shown in Fig.

3. By providing a wedge-shaped mid section 16, the retardance can be tuned during manufacture and adjusted accordingly if the optical path length need be adjusted to achieve a desired retardance. For this to occur the wedge shaped block is moved upward or downward in the direction of the arrows shown.
[00032] The basic interleaving and deinterleaving principles are not different from those 3o shown in United States Patents 4,685,773, 4,744,075, or co-pending U.S.
patent applications in Doc. No. 10-421 CA Patent the name of E-tek Dynamics disclosed heretofore herein by reference; however the novel virtual waveplate is believed to be advantageous.
[00033] Figs 4a through 4c illustrate an interleaver/deinterleaver that utilizes the virtual waveplate disclosed herein. Turning to Fig. 4c, a beam of light is launched into the left side of the device into birefringent crystal 40a where it is split into orthogonally polarized sub-beams S 1 and S2 that follow two substantially similar parallel paths. Each of the sub-beams is further divided into two further orthogonally polarized sub-beams 512, S11, 522, S21 by the first virtual waveplate 50 which induces a predetermined phase retardance between each of the wo further orthogonally polarized beams prior to recombining them into S 1R and S2R.
[034] A similar process occurs as sub-beams S 1R and S2R are launched into virtual waveplate 52 that induces a retardance that is twice of the retardance induced in the virtual waveplate 50. A rotator in the form of a half waveplate 47 is disposed between virtual waveplates 50 and 52 to ensure that the beams S1R and S2R are appropriately aligned to the virtual waveplate 52 to achieve a desired deinterleaving at the right side output end.
Nevertheless, it should be noted that this device can function as an interleaver or deinterleaver depending upon the required use. After passing through the two virtual waveplates, a polarization beamsplitter combiner block 48 provides the necessary polarization discrimination and combining so as to provide two deinterleaved streams of even and odd channels.
[00035] Referring now to Fig. 4a, a reflective interleaver/deinterleaver is shown having a centrally disposed input port 1 and having two output ports 2 and 3 for launching or receiving even and odd channels respectively.
[00036] The interleaver/deinterleaver is comprised of two U-shaped virtual waveplates 50, 52 in series. Each virtual waveplate provides the functionality of a conventional waveplate of the type used in interleaver optical circuits. Each of the virtual waveplates includes a polarization beam splitter (PBS) at each end and each of the waveplates provides substantially temperature-3o stable operation. A first of the two U-shaped waveplates 50 is of a length L having PBS 42a and 44a, and the second U-shaped waveplate 52 is of a length 2L having PBS 45a and 46a.
1o Doc. No. 10-421 CA Patent A thin glass plate 70 is shown supported by a block 71 and inserted into the gap of each virtual waveplate 50 and 51. This thin glass plate can be tilted to make fine adjustments to the delay of the virtual waveplate, then the block 71 can be fixed in position using an adhesive.
[00(i37] In operation, an input beam S1 is launched into port 1 at an end face of a graded-index (GRIN) lens 39b and is collimated. The collimated beam is then separated into two orthogonal linear polarized sub-beams SB~ after passing through a birefringent crystal 40b, for example a ruble or a YV04 crystal. Two wave-plates in the form of a waveplate pair 41a and 41b each in the path of the each sub-beam following the crystal 40b rotate the two orthogonally to polarized sub-beams in the same polarization direction. By so doing no light is lost and both sub-beams travel through together passing through substantially similar paths until they are combined at the output after varying their relative polarization states.
[00038] Figure Sa shows the details of the splitting of the input beam by the birefringent crystal 40b into two sub-beams S 1 and S2 and the action of the half waveplates 41 a and 41b in rotating the polarization of the two sub-beams such that they have the same polarization orientation. To obtain proper interleaver operation it is desired to have the polarization of each sub-beam oriented at 45 degrees. A similar arrangement is shown in Fig. 5b for recombining the two sub-beams into a single output beam.
[00039] For ease of explanation and understanding, the circuit will hereafter be explained as if it had only one of the two (same) parallel beams traversing the circuit.
Hence, although the input beam is divided into two orthogonal beams, at the input, only one of these sub-beams will be referred hereafter for ease of understanding.
[00040) As the sub-beam SB1 traverses the first U-shaped waveplate 50 comprising elements 42a, 45a, and 44a, its two polarization components are relatively retarded as they travel paths S 1 ~ and S 12. The components are then re-combined into SB 1 however this beam has now had its polarization components relatively retarded by a predetermined amount.
This process is 3o repeated after the beam SBA has been rotated by a half waveplate 47 disposed between the two U-shaped virtual waveplates. The beam is then separated into orthogonal components and is Doc. No. 10-421 CA Patent relatively retarded a second time by twice the relative delay of the first virtual waveplate. The combined output beams are then rotated by a second half waveplate 54 prior to being separated by the polarization beam sputter 48 which is perpendicular to the other beam sputters incorporated within the U-shaped virtual waveplates.
[00041] Although the output signals at the output of 48 would provide deinterleaved substantially flat top channels, it is preferable to route the output back through the same components in order to increase isolation and to compensate for any dispersion. Thus a quarter waveplate 58 and a mirror 60 are disposed at the rightmost end of the device shown for l0 providing two passes through 58 and for directing the beams back through the device. These two beams follow parallel paths to the forward propagating beams. At the output ports, the crystals 40a and 40c then combine the two beams.

Claims (25)

WE CLAIM:

1. A virtual waveplate for providing a relative group delay between orthogonal polarization components of a beam of light propagated therethrough, the delay substantially equal to L1n gap, the virtual waveplate comprising:
a polarization beam splitter for receiving an input beam and for splitting and directing the beam into two orthogonally polarized sub-beams along first and second separate paths respectively;
a polarization beam combiner optically coupled with the polarization beam splitter for combining two sub-beams received from the polarization beam splitter along the first and second paths;
the first path being defined through a light transmissive material (LTM) having a group refractive index nLTM and the first path having an optical path length of L1nLTM + L3n LTM, the second path defined at least partially through a light transmissive material having a group refractive index nLTM, said second path having an optical path length of L2n LTM + L1n gap +
L3n LTM, wherein L1 is a length of a gap defining a section of the second path and where n gap is the group refractive index of the material within the gap, L2 is the length of a section of the second path which is through the LTM, and L3 is a length of path through the LTM which is common to both the first and second paths.

2. A virtual waveplate as defined in claim 1, wherein L1 and L2 are such as to minimize temperature dependence of OPD by meeting conditions of equation (2) and equation (3).
3. A virtual waveplate as defined in claim 2, where the gap material is air or vacuum and the LTM is a material with low thermal expansion coefficient, such that the change in OPD with wavelength as defined in equation (4) is minimized.

3. A virtual waveplate as defined in claim 1 including a wedge shaped block for adjusting the relative group delay during manufacture thereof.

4. A virtual waveplate as defined in claim 1 including a glass plate within the gap which can be tilted to adjust the relative group delay during manufacture thereof.

5. A virtual waveplate as defined in claim 1 wherein the second optical path is comprised of first, second and third serial portions, and wherein the first and third portions are a material having a refractive index nLTM and wherein the second portion is disposed between the first and third portion and has a refractive index n gap which is different than nLTM
such that a sub-beam traversing the second path passes through the second portion after traversing only one of the first and third portions.

6. A virtual waveplate as defined in claim 1 further comprising retro-reflectors to direct a beam of light launched therein therethrough a plurality of times to provide multiple retardances.

7. A virtual waveplate as defined in claim 5 wherein the first optical path is defined through a light transmissive material having a refractive index n LTM.

8. A virtual waveplate as defined in claim 5, wherein the waveplate is a substantially U-shaped structure having two side sections and a mid-section comprised of a same material having the refractive index n LTM.

9. A virtual waveplate as defined in claim 8 wherein one of the two side sections include the polarization beam splitter and the other of the two sides includes the polarization beam combiner.

10. A virtual waveplate as defined in claim 9 wherein the polarization beam splitter functions as a polarization beam combiner and wherein the polarization beam combiner functions as a polarization beam splitter.

11. A virtual waveplate as defined in claim 8 wherein one of the two side sections include the polarization beam splitter and a reflector and the other of the two sides includes the polarization beam combiner and another reflector and wherein the first optical path is a straight through path and wherein the second optical path is a U-shaped path and wherein the first and second optical paths together form a rectangle.

12. A virtual waveplate having two optically coupled polarization beam splitter/combiners and two paths defined therebetween having first and second path lengths respectively, a retardance induced between two orthogonal polarization components of a beam of light launched therein being substantially equal to .DELTA.OPL, a difference in the first and second optical path lengths which is equal to Ln air wherein n air is the refractive index of air and L is a physical length of an air gap which only one of the two orthogonal components of the beam traverses along only one of the first and second paths.

13. A virtual waveplate as defined in claim 12 wherein the second path is a substantially U-shaped path including the air gap of length L and wherein the first path is a substantially straight line.

14. A virtual waveplate as defined in claim 13 wherein the first path is defined through blocks of a light transmissive material having a refractive index n LTM.

15. A virtual waveplate as defined in claim 13 wherein the second path is defined through blocks of a light transmissive material having a refractive index n LTM and wherein the blocks are separated by the air gap of length L.

16. A virtual waveplate as defined in claim 13 wherein the first optical path includes a wedge shaped tuning block.

17. A virtual waveplate as defined in claim 13 wherein the second optical path includes a tiltable glass plate.

18. A virtual waveplate as defined in claim 12 wherein the first path and the second path each include sections of light transmissive material having an optical path length substantially equal to Ln LTM between the beamsplitter/combiners and wherein the total length of the first path is Ln LTM
and wherein the total length of the second optical path is Ln LTM + Ln air.

19. A comb filter comprising;

a virtual waveplate as defined in claim 12, the virtual waveplate being oriented such that a linearly polarized beam of light is launched therein such that its polarization is offset to both of two principle axis of the virtual waveplate defined by one of the beamsplitter combiners; and, a polarizer for receiving an output beam from the virtual waveplate for filtering and directing predetermined periodic polarization components of the output beam to an output port and preventing other polarization components within the output beam from propagating to said output port.

20. A comb filter comprising;
means for providing one or more polarized sub-beams of light from a beam having an unknown polarization state;
a virtual waveplate as defined in claim 12, the virtual waveplate being oriented such that the one ore more linearly polarized sub-beams of light is launched therein such that the polarization of the one or more beams is offset to both of two principle axis of the virtual waveplate defined by one of the beamsplitter combiners; and, a polarizer for receiving one or more output beams from the virtual waveplate for filtering and directing predetermined periodic polarization components of the one or more output beams to a first output port and for directing other periodic polarization components to a second output port.

21. A comb filter as defined in claim 20 wherein the means for providing one or more polarized sub-beams of light is a polarization beam splitter.

22. A comb filter as defined in claim 20 wherein the polarizer is a polarization beam combiner.

23. A virtual waveplate for providing a relative group delay between orthogonal polarization components of a beam of light propagated therethrough substantially equal to L1n gap, the virtual waveplate comprising:
a polarization beam splitter for receiving an input beam and for splitting and directing the beam into two orthogonally polarized sub-beams along first and second separate paths respectively;

a polarization beam combiner optically coupled with the polarization beam splitter for combining two sub-beams received from the polarization beam splitter along the first and second paths;
the first path being defined through a light transmissive material (LTM) having a refractive index n LTM and the first path having an optical path length of L1n LTM + L3n LTM, the second path defined at least partially through a light transmissive material having a refractive index n LTM, said second path having an optical path length of L2n LTM + L1n gap + L3n LTM
wherein L2 is a length of a compensating section defining a section of the second path, L3 is a length common to the first and second paths, L1 is a length of a gap defining a section of the second path and where n gap is the refractive index of air or another material, wherein the optical path length of the first path and the optical path length of the second path have a substantially constant difference as temperature varies.

24. A deinterleaver circuit comprising:
a) the virtual waveplate as defined in claim 23;
b) means for splitting an input beam of light into two orthogonally polarized beams to be launched into the virtual waveplate;
c) means for combining polarization components from a first beam output from the virtual waveplate with polarization components with a second beam output from the virtual waveplate to yield a first stream of periodic channels corresponding to a first group of wavelengths.

25. A deinterleaver circuit as defined in claim 24 wherein the means for combining further combines other polarization components of the first beam and the second beam to yield another stream of periodic channels corresponding to a different group of wavelengths.