WO2002035261A2

WO2002035261A2 - Method and apparatus for thermally compensating a birefringent optical element

Info

Publication number: WO2002035261A2
Application number: PCT/US2001/051107
Authority: WO
Inventors: Xiaofeng Han; Zhicheng Yang
Original assignee: Adc Telecommunications, Inc.
Priority date: 2000-10-23
Filing date: 2001-10-23
Publication date: 2002-05-02
Also published as: WO2002035261A9; AU2002231364A1; WO2002035261A3

Abstract

In a multi-segmented optical device, the lengths and thermal properties of the segments are selected so that thermally induced changes in the optical path through the device are compensated. In particular, a birefringent optical device that includes at least two birefringent segments formed from different birefringent material on an optical path. The lengths of the segments are selected so that thermal effects on the birefingence of the optical path through the first birefringent segment are substantially compensated by thermal effects on the birefringence of the optical path through the other birefringent segments.

Description

METHOD AND APPARATUS FOR THERMALLY COMPENSATING A

BIREFRINGENT OPTICAL ELEMENT

Field of the Invention

The present invention is directed generally to optical devices, and more particularly to optical devices requiring high precision in the path length of an optical element over a range of temperatures.

Background

Some optical elements require that the length of the optical path through the element be very precise. For example, the thickness of a retardation wave plate should be precise in order to impose the desired degree of retardation at the wavelength of interest. Manufacturing a retardation plate to a precise thickness is commonplace for zero or low order waveplates, since they are so thin. However, manufacturing a high order retardation plate, having a thickness of several mm, to a high tolerance in length is more difficult, which leads to increased costs. Furthermore, once an optical element is installed in an optical system, the optical path length through the element changes due to temperature fluctuations. This can cause the inefficiencies in the optical system the optical path length of the device is critical. Therefore, there is a need to reduce the effect of temperature changes on an optical element, and to increase the manufacturability.

Summary of the Invention

Generally, the present invention relates to a multi-segmented optical device, the lengths and thermal properties of the segments being selected so that thermally induced cήanges in the optical path through the device are compensated.

A first embodiment of the invention is directed to a birefringent optical device that includes at least one first birefringent segment formed from a first birefringent material disposed on an optical path. The device also includes at least one second birefringent segment disposed on the beam path. The at least one second birefringent segment is formed from birefringent material different from the first birefringent material, and segment lengths of the at least one first birefringent segment and the at least one second birefringent segment are selected so that thermal effects on the birefringence of the optical path through the at least one first birefringent segment are substantially compensated by thermal effects on the birefringence of the optical path through the at least one second birefringent segment. Another embodiment of the invention is directed to a method for compensating thermal path length effects in a birefringent optical element, the method includes providing the birefringent optical element as at least two segments having an optical beam passing therethrough, at least one of the segments being formed a first birefringent material and at least another of the segments being formed from a second birefringent material different from the first birefringent material. The method also includes setting lengths of the at least two segments so that thermal effects on the birefringence of the optical path through the at least one of the segments formed from the first birefringent material are substantially compensated by thermal effects on the optical path through the other segments.

Another embodiment of the invention is directed to an optical element with a thermally compensated optical path length. The optical element includes at least two optical segments disposed along an optical path. At least one of the segments is formed from a first material and at least another of the segments is formed from a second material different from the first material. The lengths of the at least two segments are selected so that thermal effects on the optical path' through the at least one of the segments formed from the first material are substantially compensated by thermal effects on the optical path through the other segments.

Another embodiment of the invention is directed to a birefringent device that includes a first birefringent segment having a first birefringence. The first birefringent segment is disposed on an optical path to rotate polarization of light propagating along the optical path. The device also includes at least one additional birefringent segment having a birefringence different from the first birefringence. The at least one additional birefringent segment is disposed on the optical path to rotate polarization of light propagating along the optical path. The lengths of the first and additional birefringent segments are selected so that the device rotates polarization of a set of odd WDM channels to a first selected angle and rotates polarization of a set of even WDM channels to a second selected angle different from the first selected angle by about 90°.

The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments.

Brief Description of the Drawings

The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:

FIG. 1 schematically illustrates a wavelength division-multiplexed (WDM) fiber optics communications system;

FIGs. 2A and 2C schematically illustrate one particular embodiment of a birefringent interleaver according to the present invention;

FIGs. 2B and 2D illustrate polarization states of light propagating through the interleavers of FIGS. 2A and 2C respectively; FIG. 3 schematically^'illustra^'tes an embodiment of a single -segment optical element;

FIG. 4 schematically illustrates an embodiment of a two -segment optical element according to the present invention; and FIG. 5 schematically illustrates an embodiment of a three-segment optical element according to the present invention.

While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

Detailed Description WDM systems include several channels of light at different optical frequencies. According to the ITU standards, the channels are evenly spaced by frequency. Thus, the mth channel has a frequency given by v₀ + mΔv, where v₀ is a lowest channel frequency, Δv is the channel separation and m is an integer value ranging from 0 to mo, the upper value. According to commonly used ITU standards, the channel separation, Δv, is 100 GHz or 50 GHz. Those channels whose frequencies correspond to the even values of m (m = 0, 2, 4... etc.) are typically referred to as the even channels, while those channels whose frequencies correspond to the odd values of m (m = 1 , 3, 5.... etc.) are referred to as the odd channels. Interleaving is the operation of mixing two signals, one containing the even channels with the containing the odd channels, to produce a signal containing both the even and odd channels. De-interleaving is the operation of separating a signal containing odd and even channels into a first signal containing the even channels and a second signal containing the odd channels. Many devices used for interleaving may also be used in reverse for de-interleaving. Consequently, the term "interleaving" is often used to denote the operations of interleaving and de-interleaving.

The following discussion describes the different WDM channels in terms of both frequency and wavelength. It will be appreciated that each channel has a unique wavelength and frequency given through the relationship vm.λm = c, where vm and λm are, respectively, the frequency and wavelength of the mth channel, and c is the speed of light.

One particular embodiment of a WDM optical communications system is illustrated in schematic form in FIG. 1. A WDM transmitter 102 directs a WDM signal having m₀+1 channels through a fiber communications link 104 to a WDM receiver 106.

This particular embodiment of WDM transmitter 102 includes a number of light sources 108a - 108c that generate light at different wavelengths, λO, λ2 and λrτio-1 , corresponding to the even optical channels. The light output from the light sources 108a-108c is combined in a first WDM combiner 110a, to produce a first output 112a. The light in the first output 112a from the first WDM combiner 110a includes light at the wavelengths λO, λ2 and λmo-1.

The WDM transmitter 102 also includes other light sources 108d - 108f that generate light at a different set of wavelengths, λ1 , λ3 and λm₀ respectively, corresponding to the odd optical channels. The light output from the light sources 108d-108f is combined in a second WDM combiner 110b to produce a second output 112b. The light in the second output 112b from the second WDM combiner 110b includes light at the wavelengths λ1 , λ3 and λm₀. The channel spacing in each of the first and second outputs 112a and 112 b is 2Δv.

The light of the first and second outputs 112a and 112b is combined in the interleaver 114 to produce an interleaved output containing λO, λ1 , λ2

....λm₀, a a channel separation of Δv. The interleaved output is launched into the fiber communications link 104 for propagation to the WDM receiver

106. Light sources 108a-l'08f may be modulated laser sources, or laser sources whose output is externally modulated, or the like. It will be appreciated that the WDM transmitter 102 may be configured in many different ways to produce the first and second outputs 112a and 112b that are input to the interleaver 114. For example, other types of coupler may be employed to combine the outputs from light sources than a WDM coupler. Furthermore, the WDM transmitter 102 may be equipped with any suitable number of light sources for generating the required number of optical channels. For example, there may be twenty, forty or eighty optical channels. The WDM transmitter 102 may also be redundantly equipped with additional light sources to replace failed light sources.

Upon reaching the WDM receiver 106, the interleaved signal is passed through a de-interleaver 116, which separates the interleaved signal into an even channel signal 118a, containing the even channels, and an odd channel signal 118b, containing the odd channels. The even channel signal 118a is passed into a first wavelength division demultiplexer (WDDM) unit 120a which separates the even channels into individual channels that are directed to respective detectors 122a-122c. Likewise, the odd channel signal 118b is passed into a second WDDM unit 120b that separates the odd channels into individual channels that are directed to respective detectors 122d-122f.

The exemplary WDM transmitter and receiver architecture illustrated in FIG. 1 permits the user to employ relatively straightforward WDM components for all multiplexing and demultiplexing operations except for interleaving and de-interleaving. This is advantageous in that the component costs for the transmitter 102 and receiver 106 may be kept low, since only the interleaver and de-interleaver have the requirement of operating at dense multiplexing, at the channel separation Δv, while the other components in the transmitter 102 and receiver 106 typically operate with less dense channel separation. One particular embodiment' of a birefringent interleaver is schematically illustrated in FIG. 2A. The interleaver 200 includes a birefringent polarization rotating crystal 202 and a polarization-sensitive beam splitting element 204. The polarization-sensitive beam splitting element 204 may be any suitable element that splits an incoming light beam into beams of orthogonal polarizations, such as a polarizing beamsplitter or a birefringent splitting crystal. A birefringent splitting crystal is particularly advantageous for maintaining small size in devices compatible with fiber optical components. The interleaver 200 may be used to de-interleave a dense multiplexed signal into two less densely multiplexed signals. De-interleaving with the interleaver 200 is described with reference to FIG. 2B, which illustrates the polarization state and lateral position of the light beam passing through the interleaver 200 at various positions along the interleaver 200. FIG. 2B schematically represents the cross-section of the interleaver 200 as viewed in a direction along the z-axis.

A first optical unit 206 delivers a polarized light beam 208, containing both the even and odd channels, to the interleaver 200, as illustrated for position z1. The even and odd channels are indicated as λe and λo respectively.

The birefringent polarization rotating crystal 202 is oriented so that its optical axis lies in the x-y plane, the plane perpendicular to the direction that light propagates within the crystal 202. Furthermore, the optical axis of the birefringent polarization rotating crystal 202 lies at 45° to the y axis, the axis along which the light entering the polarization crystal 202 is polarized. As a result of the particular orientation of the polarization rotating crystal relative to the z-axis, the propagation direction, the polarization of the light beam 208 is rotated by the polarization rotating crystal 202.

The length and birefringence of the polarization rotating crystal 202 are selected so that, after passing through the polarization rotating crystal

202, the polarizations of the even channels are each effectively rotated to the same angle. Likewise, the polarizations of the odd channels are each effectively rotated to the same angle. However, the angle through which the even channels are rotated differs from the angle through which the odd channels are rotated by approximately 90°. Consequently, at the output of the polarization rotating crystal 202, position z2, the even channels are polarized parallel to each other and are orthogonal to the polarization of the odd channels.

Although the illustration in FIG. 2B shows that the polarization rotating crystal 202 effectively rotates the polarization of the odd channels through 90° while effectively not rotating the polarization of the even channels, it will be appreciated that this need not be the case, and other configurations are possible. For example, the polarization of the even channels might be rotated through 90°, while the polarization of the odd channels is effectively unrotated. The length, L, of the polarization rotating crystal 202 that is required to effectively rotate the odd channels through an angle 90° different from the even channels is given by:

L = c/[2(n_e - n„) Δv] (1)

where c is the speed of light, (n_e-n₀) is the difference between the ordinary and extraordinary refractive indices for the crystal, also known as the birefringence, and Δv is the spacing between odd and even channels.

Thus, where the polarization rotating crystal 202 is formed from ortho- vanadate (YVO₄), having a birefringence of 0.2039, and the channel separation is 50 GHz, then the length of the polarization rotating crystal 202 is approximately 14.7 mm. It will be appreciated that any suitable birefringent material may be used, for example lithium niobate. However, YVO₄ is particularly advantageous since its birefringence is high, which reduces the length of crystal required for the polarization rotating crystal 202, thus making the overall length of the interleaver 200 shorter. After leaving the polarization rotating crystal 202, the polarization rotated beam 210 enters the polarization-sensitive beam splitting element 204, where the two polarizations are split from each other. In the particular embodiment illustrated, the polarization-sensitive beam splitting element 204 is a birefringent splitting crystal, where the entering beam 210 is split into an ordinary ray 212 and an extraordinary ray 214. At the output from the birefringent splitting crystal 204, the odd channels, propagating as the extraordinary ray 214, have been separated from the even channels, propagating as the ordinary ray 212, due to birefringent walk-off, as shown for position z3. The two beams 212 and 214 from the birefringent splitting crystal 204 may then be directed to two different output fibers 220 by the second optical unit 216.

One particular embodiment of birefringent splitting crystal 204 has its optical axis at -45° to the z-axis in the x-z plane. As is the case with the polarization rotating crystal 202, the birefringent splitting crystal 204 may be formed from any suitable birefringent material, such as lithium niobate or ortho-vanadate. However, since the walk-off angle between the ordinary and extraordinary rays is dependent on the magnitude of the birefringence, a highly birefringent material, such as ortho-vanadate, is advantageous since it reduces the length of the crystal required to obtain separation between the ordinary and extraordinary beams 212 and 214.

The first optical unit 206 may be coupled to receive input light from an external optical fiber 218. The first optical unit 206 may also include one or more collimating lenses to collimate the light from the fiber 218 before passage through the interleaver 200. The first optical unit 206 may also be provided with optical elements to produce the polarized beam 208. For example, if the output from the fiber 218 is unpolarized, then the first optical unit 206 may include a polarizer to polarize the output from the fiber 218. Furthermore, the first optical unit may produce more than one polarized beam 208 for propagation through the interleaver, as is discussed in U.S. Patent Application Serial No. 09/694,150, filed on October 23, 2000, having the title "BIREFRINGENT INTERLEAVER FOR WDM FIBER OPTIC COMMUNICATIONS" by B. Barry Zhang and Zhicheng Yang, having an attorney reference number 980.1071 US01 , which is incorporated herein by reference. The output from the fiber 218 may be polarized, for example if the fiber 218 is a polarization maintaining fiber, in which case the first optical unit need not include a polarizer to produce the polarized beam 208.

The second optical unit 216 may be coupled to output fibers 220 and may include a light focusing system (not shown) to direct the separated beams 212 and 214 into respective fibers 220. The light focusing system may include separate lenses for each beam 212 and 214, or may include a lens system that operates on both beams 212 and 214. Where the interleaver produces more than two output beams, as is discussed below, the second optical unit 216 may be provided with combining optics to combine two or more of the output beams into a single output beam before transmitting the single output beam into the respective optical fiber 220. The birefringent interleaver 200 is able to perform a de-interleaving operation, as has just been described, in other words it separates the odd channels from the even channels. It will be appreciated that the interleaver may also perform an interleaving operation, in other words combining a beam that includes odd channels with a beam that includes oven channels, to produce a single beam that includes both odd and even channels.

An interleaving operation may be achieved by passing light through the interleaver 200 in the backwards direction, as is now discussed with reference to FIGs. 2C and 2D. Two orthogonally polarized beams 230 and 232 are directed at the birefringent splitting crystal 204 from the second optical unit 216. The first polarized beam 230 contains the even channels, while the second polarized beam 232 contains the odd channels. The beams 230 and 232 are separate upon entering the birefringent splitting crystal 204. One of the beams 230 and 232, in this case the second beam 232, enters the birefringent splitting crystal 204 as an extraordinary beam and the other beam, in this case beam 230, enters as an ordinary beam, as shown for position z3. Passage through the birefringent splitting crystal 204 in the reverse direction results in the extraordinary beam and ordinary beam combining into a single beam 234 at position z2. The single beam 234 contains the odd channels having one polarization and the even channels having the orthogonal polarization, as shown for position z2.

The single beam 234 then passes through the polarization rotating crystal 202. The polarization rotating crystal 202 effectively rotates the polarization of the odd channels through a first angle and the polarization of the even channels through a second angle different from the first angle by approximately 90°. Consequently, after propagating through the polarization rotating crystal 202, the beam 236 is polarized and contains all the even and odd channels. The beam 236 may then pass through the first optical unit 206 to the fiber 218. Thus, it will be appreciated that the interleaver 200 may be operated to interleave odd and even channels when the light is passed therethrough in one direction and as a de-interleaver when the light passes through the interleaver 200 in the opposite direction.

It will be appreciated that many different embodiments of the interleaver may be used. Some of these embodiments are discussed in U.S. Patent Application Serial No. 09/694,150, filed on October 23, 2000, having the title "BIREFRINGENT INTERLEAVER FOR WDM FIBER OPTIC COMMUNICATIONS" by B. Barry Zhang and Zhicheng Yang, having an attorney reference number 980.1071 US01.

The rotation of the polarization of the odd and even channels in the polarization rotating crystal 202 is required to be precise. For an 80 channel WDM signal, if the difference in the angle of rotation for adjacent channels is 1/10°, then the angle of rotation of the last channel may be 8° different from that of the first channel. Therefore, in order to keep the odd and even channels substantially separated by the polarization separator splitting element 204, and also to maintain uniform power across the comb of interleaved channels, the optical path length through the polarization rotating crystal 202 also has to be precise. The tolerance in the length of the polarization rotating crystal 202 may be smaller than the wavelength. Such a high level of tolerance is difficult to achieve in a single-segmented element, especially where the length of the polarization rotating crystal 202 is several millimeters or more. Approaches to achieving such high tolerances using a multi-segmented polarization crystal are discussed in U.S. Patent Application Serial No. 09/694,691 , filed on October 23, 2000, having the title "METHOD AND APPARATUS FOR ADJUSTING AN OPTICAL ELEMENT TO ACHIEVE A PRECISE LENGTH" by Xiaofeng Han and B. Barry Zhang, having an attorney reference number 980.1074US01 , incorporated herein by reference.

Furthermore, the path length through the polarization rotating crystal 202 should be constant over a large range of operating temperatures so that the interleaver 200 is operable in a wide range of environmental conditions. The present invention is directed to an approach for reducing the temperature dependence of the optical path length through an optical element, such as the polarization rotating crystal 202, and is based on the use of a multi-segmented optical element.

A simple optical element 300 formed from a single segment 302 of material is illustrated in FIG. 3. The element 300 has a length L in the direction of the optical beam 304 that passes through the element 300. The refractive index of the element 300 is n.

The optical path, L_opt, through the element 100 is given by the expression:

Lopt = n L (2)

The change in optical path length due to a change in temperature may be expressed as:

3L_opt/3T = L. dn/dl + n. 5L/3T (3) where T is temperature.

It is difficult to find an optical material whose temperature dependence of refractive index, 3n/3T, and thermal expansion coefficient, 3L/ST, are such that the thermal change in optical path length can be made to be zero, especially without constricting the length, L. This is particularly the case for the polarization rotating crystal 202, where the length must be selected according to the channel spacing, given by expression (1) above.

In many applications, multi-segmented elements may be used, such as the two-segment optical element 400 illustrated in FIG. 4. The element 400 is formed from two segments 402 and 404, each having respective refractive index ni and n₂. The length of the first segment 402 is Li and the length of the second segment 404 is L₂. The optical path length through the optical element 400 is given by the expression:

The thermal effect on the optical path length due to thermal expansion and thermal change in the refractive index is given by the expression:

dLopt/ST = U dm/ST + m d løT + L₂5n₂/3T + n₂3L₂/δT (5)

Since this expression provides more degrees of freedom for an optical designer, it is easier to design an optical element, of a given length, whose thermal effect on the optical path length is small, or even zero, by judiciously selecting the materials from which the different segments are formed, and the length of each segment. It will be appreciated that expressions (4) and (5) may be generalized for larger numbers of segments. For example, equation (5) may be generalized for a multi-segmented element having m₀ segments as: m₀

∑ (L_m n_m/aτ + n_m dL dJ) = 0 (6) m

where L_m is the length of the mth segment, and n_m is the refractive index of the mth segment.

If the polarization rotating crystal 202 is formed from two segments, for example as illustrated in FIG. 4, then we may rewrite expression (1) as:

(n_eι - n₀ι) + L₂ (n_e2 - n_o2) = c/(2Δv) (7)

where Li and L₂ are the lengths of the first and second segments respectively, n_eι and n₀₁ are respectively the extraordinary and ordinary refractive indices for the first segment, and n_e2 and n₀₂ are respectively the extraordinary and ordinary refractive indices for the second segment.

For zero thermal effect, the expression on the left side of equation (7) has no net temperature sensitivity, i.e.

Li 3(n_θι - n„ι)/dT + (n_βι - n₀-ι)3Lι/3T + (8) L₂d(n_β2 - n₀2)/5T + (n_e2 - n₀2)dL₂/3T = 0

Equations (7) and (8) may be generalized for a multi-segmented element, having mo segments where m₀ is greater than 1. For example, the condition (7) on a multi-segmented polarization rotating crystal 202 may be generalized to:

mo

∑ L_m (n_em - n_om) = c/(2Δv) (9) m

and the temperature differential condition (8) may be generalized to: m₀

∑ (L_md(n_em ' n_om)/3T + (n_em - n_om)dL dT) = 0 (10) m

where L_m is the length of the mth segment, and (n_em - n_om) is the birefringence of the mth segment. In each case, the sum is taken over the mo segments.

In an example of a two-segmented element, ortho-vanadate (YVO₄) is selected as the material of the first segment and lithium niobate (LiNbO₃) is selected as the material of the second segment. The properties of these two materials are listed in Table I.

Table I. Optical and Thermal Characteristics of YVO₄ and LiNbO₃

Substituting the values from Table I into expression (8) results in a ratio of segment lengths that produces zero thermal effect: L1/L2 = 1.65. Substituting this ratio back into equation (7) yields the result:

L₂ = 7.32 mm Therefore, a thermally compensated, polarization rotating crystal 202 may be fabricated from two different types of materials, such as vanadate and lithium niobate. The overall length of such a polarization rotating crystal 202 is approximately 5 mm longer than had the polarization rotating crystal 202 been fabricated from vanadate alone.

Experiments have demonstrated that the transmission peaks of a birefringent interleaver, using a single birefringent crystal without temperature compensation, drift with temperature change. A drift of up to in excess of one GHz K^"1 has been observed for an interleaver having a 50 GHz spacing. Therefore temperature compensation of the birefringent polarization rotating crystal is important.

In addition to providing temperature compensation, a multi- segmented birefringent polarization rotating crystal may also be advantageous in setting the pass wavelengths to those of the ITU standards. The tolerance in the length of the polarization rotating crystal in the birefringent interleaver necessary to set the pass wavelengths equal to those of the ITU standards is relatively high. The tolerance in length, ΔL, is given by:

ΔL = m Δλ/[ n_e - n₀] (11)

where m is an integer value indicating the order of rotation of the particular wavelength. For example, with a value of Δλ « 0.04 nm (Δv = 50 GHz), the value of ΔL for a YVO₄ crystal is less than 400 nm. Higher birefringence leads to overall shorter lengths, but the length tolerance also decreases. Accordingly, use of a second segment, of a lower birefringence, relaxes the length tolerance required to set the pass wavelengths to the ITU standards. Therefore, it is advantageous to select crystal lengths to lock to the

ITU wavelengths at the same time as providing temperature compensation. If one assumes that only two different birefringent materials are used, the birefringence of the low birefringence crystal may be selected to be significantly smaller than that of the high birefringence crystal, and to have a temperature dependence that has a sign opposite that of the high birefringence crystal. Such a combination of materials may be used to compensate thermal changes in the optical path length at the same time as setting the pass wavelengths. For example, where LiNbO₃ is used along with YVO₄, the length tolerance of the LiNbO₃ crystal, ΔL, may be larger than 1 μm, which is a significantly easier tolerance to achieve than 400 nm. If the birefringence of the temperature compensating crystal is less than 0.01 , then the length tolerance required to lock to an ITU wavelength may be higher than 4 μm.

It will be appreciated that more than two birefringent materials may be used to produce an interleaver polarization rotator that has a thermally compensated path length. One particular embodiment of such a multi- segmented device is illustrated in FIG. 5. The birefringent device 500, which may be a polarization rotator for use in a birefringent interleaver, includes a first segment 502 formed from a material of high birefringence, Δn-i, a second segment 504 formed from a material of intermediate birefringence, Δn₂, and a third segment 506 formed from a material of low birefringence, Δn₃. An advantage of such an approach is that the high birefringence segment 502 may be used to produce the bulk of the polarization rotation, in order to reduce the overall length of the device. The intermediate birefringence segment 504, typically having a shorter length than the high birefringence segment 502, may be used to provide thermal compensation. The low birefringence segment 506, typically shorter than the other segments 502 and 504, may be used to set the overall path length for locking to the ITU wavelengths.

Other approaches to setting path lengths is to use crystals having non-parallel faces, for example as is discussed in U.S. Patent Application Serial No. 09/694,691 , filed on October 23, 2000, having the title "METHOD AND APPARATUS FOR ADJUSTING AN OPTICAL ELEMENT TO ACHIEVE A PRECISE LENGTH" by Xiaofeng Han and B. Barry Zhang, having an attorney reference number 980.1074US01. As noted above, the present invention is applicable to optical systems and is believed to be particularly useful for providing an optical element whose optical path length remains substantially independent of temperature. The present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices.

Claims

WE CLAIM:

1. A birefringent optical device, comprising: at least one first birefringent segment formed from a first birefringent material and being disposed on an optical path; and at least one second birefringent segment disposed on the beam path, the at least one second birefringent segment being formed from birefringent material different from the first birefringent material, segment lengths of the at least one first birefringent segment and the at least one second birefringent segment being selected so that thermal effects on the birefringence of the optical path through the at least one first birefringent segment are substantially compensated by thermal effects on the birefringence of the optical path through the at least one second birefringent segment.

2. An optical device as recited in claim 1 , wherein the at least one first birefringent segment has a first crystal optical axis oriented approximately perpendicular to the optical path, and the at least one second birefringent segment has a second crystal optical axis oriented approximately perpendicular to the optical path.

3. An optical device as recited in claim 1 , wherein the segment lengths of the at least one first birefringent segment and the at least one second birefringent segment are selected so that polarizations of selected wavelengths of light are rotated through selected angles upon passage through the device.

4. An optical device as recited in claim 3, wherein the selected wavelengths are in accordance with International Telecommunications

Union standards for wavelength division multiplexing (WDM) optical communications.

5. An optical device as recited in claim 1, further comprising an optical communications transmitter transmitting a plurality of optical channels propagating along the optical path, wherein lengths of the at least one first birefringent segment and the at least one second birefringent segment are selected so that the device effectively rotates polarization of odd channels of the plurality of optical channels to a first angle and effectively rotates polarization of even channels of the plurality of optical channels to a second angle different from the first angle by about 90°.

6. An optical device as recited in claim 1 , wherein optical device has mo segments, mo being greater than 1 , and the segment lengths of the at least one first birefringent segment and the at last one second birefringent segment substantially satisfy the following equation:

mo (L_m (nem - n_om)/ T + (n_em - n₀m) L_m/ T) = 0 m where L_m is the segment length for the mth segment, n_em and n_om are respectively the extraordinary and ordinary refractive indices for the mth segment and T is temperature.

7. An optical device as recited in claim 1 , wherein the optical device has mo segments, mo being greater than 1 , and the segment lengths of the at least one first birefringent segment and the at last one second birefringent segment substantially satisfy the following equation:

m₀

L_m (n_em - n_om) = c/(2Δv) m where L_m is the segment length for the mth segment, n_em and n_om are respectively the extraordinary and ordinary refractive indices for the mth segment, c is the speed of light and Δv is a desired channel separation.

8. An optical device as recited in claim 1, wherein at least one of the first and second birefringent segments is formed from vanadate.

9. A method for compensating thermal path length effects in a birefringent optical element, comprising: providing the birefringent optical element as at least two segments having an optical beam passing therethrough, at least one of the segments being formed a first birefringent material and at least another of the segments being formed from a second birefringent material different from the first birefringent material; and setting lengths of the at least two segments so that thermal effects on the birefringence of the optical path through the at least one of the segments formed from the first birefringent material are substantially compensated by thermal effects on the optical path through the other segments.

10. A method as recited in claim 9, further comprising directing a communications signal having a plurality of optical channels propagating along the optical path, and setting lengths of the at least two segments also includes selecting the segment lengths so as to effectively rotate polarization of odd channels of the plurality of optical channels to a first angle and to effectively rotate polarization of even channels of the plurality of optical channels to a second angle different from the first angle by about 90°.

11. A method as recited in claim 10, wherein the optical element has mo segments, mo being greater than 1 , and setting the lengths of the at least two segments includes setting the segment lengths to substantially satisfy the following equation: mo

m where L_m is the segment length for the mth segment, n_e and n_om are respectively extraordinary and ordinary refractive indices for the mth segment , c is the speed of light and Δv is a channel separation between adjacent odd and even channels.

12. A method as recited in claim 9, wherein the optical element has mo segments, m₀ being greater than 1 , and setting the lengths of the at least two segments includes setting the segment lengths to substantially satisfy the following equation:

mo (L_m (n_em - n₀m)/ T + (n_em - n_om) L_m/ T) = 0 m wherein L_m is the segment length for the mth segment, n_em and n_om are respectively the extraordinary and ordinary refractive indices for the mth segment and T is temperature.

13. An optical element with a thermally compensated optical path length, comprising: at least two optical segments disposed along an optical path, at least one of the segments being formed from a first material and at least another of the segments being formed from a second material different from the first material, lengths of the at least two segments being selected so that thermal effects on the optical path through the at least one of the segments formed from the first material are substantially compensated by thermal effects on the optical path through the other segments.

14. An optical device as recited in claim 13, wherein the optical device has mo segments, mo being greater than 1 , and the segment lengths of the at least two optical segments substantially satisfy the following equation:

mo

(Lm n_m/ T + n_m L_m/ T) = 0 m wherein L_m is the segment length for the mth segment, n_m is the refractive index for the mth segment and T is temperature.

15. A birefringent device, comprising: a first birefringent segment having a first birefringence and disposed on an optical path to rotate polarization of light propagating along the optical path; and at least one additional birefringent segment having a birefringence different from the first birefringence and disposed on the optical path to rotate polarization of light propagating along the optical path, lengths of the first and additional birefringent segments being selected so that the device rotates polarization of a set of odd WDM channels to a first selected angle and rotates polarization of a set of even WDM channels to a second selected angle different from the first selected angle by about 90°.

16. A device as recited in claim 15, wherein the lengths of the first and additional birefringent elements are selected to substantially compensate temperature dependence of device birefringence so that an amount of polarization rotation imparted by the device is substantially independent of temperature.

17. A device as reeited in claim 15, wherein the birefringence of the first birefringent segment is greater than the birefringence of the at least one additional birefringent segment and the first birefringent segment has a first segment length greater than a length of any other birefringent segment in the device.

18. A device as recited in claim 17, wherein the at least one additional birefringent segment includes a second birefringent segment having a second birefringence and a second segment length and a third birefringent segment having a third birefringence and a third segment length, the second birefringence being greater than the third birefringence and the second length being greater than the third length.

19. An optical device as recited in claim 15, wherein the optical device has m₀ segments, m₀ being greater than 1 , and the segment lengths of the first and at least one additional birefringent segments substantially satisfy the following equation:

m₀

(L_m (n_em - n₀m)/ T + (n_em - n_om) L_m/ T) = 0 m where L_m is the segment length for the mth segment, n_em and n_om are respectively the extraordinary and ordinary refractive indices for the mth segment and T is temperature.

20. An optical device as recited in claim 15, wherein the optical device has mo segments, m₀ being greater than 1 , and the segment lengths of the first and at least one additional birefringent segments substantially satisfy the following equation:

m₀

m where L_m is the segment length for the mth segment, n_em and n_om are respectively the extraordinary and ordinary refractive indices for the mth segment, c is the speed of light and Δv is a desired channel separation.