Background of the Invention The increasing demand for high-speed voice and data communications has led to an increased reliance on optical communications, especially optical fiber communications. The use of optical signals as a vehicle to carry channeled information at high speed is preferred in many instances to carrying channeled information at other electromagnetic wavelengths/frequencies in media such as microwave transmission lines, coaxial cable lines, and twisted copper pair transmission lines.

Advantages of optical media include higher channel capacities (bandwidth), greater immunity to electromagnetic interference, and lower propagation loss. In fact, it is common for high-speed optical systems to have signal rates in the range of approximately several megabits per second (Mbit/s) to approximately several tens of gigabits per second (Gbit/s), and greater.

However, as the communication capacity is further increased to transmit greater amounts of information at greater rates over fiber, maintaining signal integrity can be exceedingly challenging.

One way to more efficiently use available resources in the quest for high-speed information transmission is known as multiplexing, in which a plurality of channels are transmitted along an optical waveguide (e. g. an optical fiber). One particular type of multiplexing is wavelength division multiplexing (WDM). In WDM, each high-speed information channel has a center wavelength and prescribed bandwidth. At the receiver end, the interleaved channels are the optical signals are selectively separated and may be further processed by electronics. (By convention, when the number of channels transmitted by such a multiplexing technique exceeds approximately four, the technique is referred to as dense WDM or DWDM).

While transmission of information via an optical medium has offered significant improvements in information transmission, increased demand for capacity may still adversely impact signal quality during transmission. For example, the number of channels that can be carried in a single optical fiber is limited by cross-talk, narrow operation bandwidth of optical amplifiers, and optical fiber non-linearities.

Current wavelength channel center wavelengths, channel bandwidths and spacing between interleaved channels corresponding wavelengths preferably conform to an International Telecommunication Union (ITU) channel grid. For example, one ITU channel grid has a channel spacing requirement of 100 GHz. In this case, the channel spacing is referenced in terms of a frequency spacing, which corresponds to a channel center wavelength spacing of 0.8 nm.

With 100 GHz channel spacing, channel"n"would have a center frequency 100 GHz less than channel"n+1" (or channel"n"would have a center wavelength 0.8 nm greater than the center wavelength of channel"n+1").

As can be appreciated, the more the information that is sent over a particular medium, the greater the number of channels that are needed. It follows, that due to bandwidth considerations, the larger the number of channels, the closer the separation between channels. Among other difficulties, the decrease in channel spacing makes separating the optical channels more challenging. For example, in order to preserve the integrity of the signal at the receiver end of the communication link, cross-talk in the form of received channel overlap must be minimized.

As can be appreciated, meeting these performance requirements of ever-increasing demand poses technical and practical challenges.

The technical and practical challenges described above are further exacerbated by environmental factors. These environmental factors can adversely impact the performance of the devices. One deleterious environmental factor is the ambient temperature. For example, the change in the ambient temperature can create temperature induced wavelength drift of the WDM.

This wavelength drift can cause wavelength channel overlap. In the closely spaced channels of ITU grids discussed above, optical system performance may be adversely impacted.

Accordingly, there is often a need to compensate for temperature fluctuations in WDM systems. While it may be possible to control the ambient temperature surrounding the WDM device, this generally requires rather elaborate climate control devices, which can be relatively complex and expensive. Moreover, these devices do not ensure the particular elements of a WDM are immune to temperature fluctuations. As such, in addition to adding complexity and expense, known active temperature control schemes may be unreliable.

Accordingly, what is needed is an optical interleaver/deinterleaver that caters to immediate and future needs for high speed optical networks without the disadvantages associated with current components and approaches.

Summary of the Invention The present invention relates to a method and apparatus for interleaving/deinterleaving optical signals having a passive thermal compensation component.

In accordance with an exemplary embodiment of the present invention, an optical device includes an interleaver/deinterleaver, which includes a passive thermal compensator, wherein an optical signal, which traverses the optical device, undergoes substantially no temperature induced frequency drift over a desired temperature range.

Brief Description of the Drawings The invention is based understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features in the drawing figures may not necessarily be drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or decreased for clarity of discussion.

Fig. 1 is a schematic view of an interleaver according to an exemplary embodiment of the present application.

Fig. 2 is a schematic view of an interleaver according to another exemplary embodiment of the present invention.

Fig. 3 is a schematic representation of an interleaver according to another exemplary embodiment of the present invention.

Detailed Description The invention will now be described more fully with reference to the accompanying drawing figures, in which exemplary embodiments are shown. In the following detailed description, for purposes of explanation and not limitation, exemplary embodiments disclosing specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure, that the present invention may be practiced in other embodiments that depart from the specific details disclosed herein. Moreover, descriptions of well-known devices, methods and materials may be omitted so as to not obscure the description of the present invention.

As used herein"interleaving"refers to combining two or more streams of optical signals, wherein each stream contains a plurality of optical channels; and"de-interleaving"refers to separating an optical signal, which contains a plurality of optical channels, into two or more streams of optical signals, each of which contains a subset of the plurality of optical channels.

Generally interleaving decreases the channel spacing between adjacent channels, and de- interleaving increases the channel spacing between adjacent channels.

Moreover, the illustrative embodiments herein describe the de-interleaving function. Of course, from the reciprocity principle of optics, the methods and apparati of the exemplary embodiments present invention described herein may be used to achieve an interleaving function. In such a case, the interleaved optical signal would have channel spacing that is an even fraction (e. g. one-half) of the channel spacing of the two input optical signals. It is noted that in the exemplary embodiments described herein, the deinterleaved optical signals could be input to concatenated interleaver/deinterleavers, similar to those described herein, and the channel spacing could be increased (e. g., by a factor of two). Of course, further interleavers/deinterleavers could be concatenated, with each increasing the channel spacing. It is further noted that the deinterleaved optical signals could also be selectively coupled to other devices such as demultiplexers and optical add/drops.

Briefly, the invention is drawn to a method and apparatus for interleaving/deinterleaving optical signals, with a passive thermal compensator included which enables the interleaver/deinterleaver to operate over a predetermined temperature range with substantially no temperature induced frequency (and, thereby, wavelength) drift of the output signal.

Turning initially to Fig. 1, an optical device 100 in accordance with an illustrative embodiment of the present invention is shown. An input signal 105 is incident on the interleaver/deinterleaver 101 (hereinafter referred to as interleaver 101). The input signal 105 is a multiplexed optical signal illustratively having channels 1, 2,... n, with respective channel center wavelengths of X 2,. -; lwn-Output signals 103, 104 are deinterleaved, and have a channel spacing that is an integral multiple of the channel spacing of the input signal 105.

In the exemplary embodiment of Fig. 1, the interleaver 101 includes at least one birefringent element, which is used to separate the polarization states of the input signal 105 into orthogonal polarization components. As is described in greater detail herein, ambient temperature affects both the optical path length and the birefringence of birefringent elements.

Accordingly, it is desirable to compensate for changes in the ambient temperature, which can adversely impact the output signal, and to do so in a passive manner.

A passive thermal compensator 102 according to an illustrative embodiment of the present invention compensates for thermal effects (e. g., ambient thermal effects) so that the output signals 103,104 of the optical device 100 experience substantially no temperature-induced frequency/wavelength drift compared to the input optical signal 105. According to an illustrative embodiment of the present invention, the passive thermal compensator 102 includes at least one

birefringent element, which compensates for temperature-induced frequency or wavelength shift of the input signal 105 that is caused by thermal influences on the interleaver 101.

Accordingly, the output signals 103 and 104 (deinterleaved signals in this illustrative embodiment) are substantially unaffected by variations in the ambient temperature over a predetermined range. As such, fluctuations in the optical network ambient temperature, which may result from internal and external components of the optical network, are compensated for by virtue of the passive thermal compensator 102 without the necessity for ambient temperature controls, such as cooling elements. Moreover, as will become clearer as the present description proceeds, the passive thermal compensator 102 in accordance with the present exemplary embodiment compensates/corrects for thermal effects rather than attempts to prevent them as an active ambient temperature controller would.

Fig. 2 shows another illustrative embodiment of the invention of the present disclosure.

An interleaver/deinterleaver 200 (hereinafter referred to as interleaver 200) is illustratively based on polarization interferometry using birefringent materials and includes a passive thermal compensator. An input signal 201 is incident perpendicularly to the end face of the polarization splitter 202, which illustratively includes a birefringent material such as rutile, calcite or yttrium vanadate (YV04). The polarization splitter 202 fosters polarization diversity, and ultimately enables the interleaver 200 to function independently of the polarization of the input signal 201.

The polarization splitter 202 splits the input signal 201 into two beams 203. These two beams 203 have orthogonal polarization states, which are often referred to as s and p polarization states having a relative phase that is determined by the thickness and material properties (birefringence) of the polarization splitter 202. The input optical signal 201 is multiplexed having channels 1,2,... n, and the channels have respective channel center wavelengths A 2,

... X,. The polarization splitter 202 merely splits the electric field vector of input signal 201 into its orthogonal components, the polarization states of beams 203. It follows, therefore that each of the beams 203 contain all the channels.

Next, beams 203 are incident upon polarization transforming (PT) element 204. In the illustrative embodiment, element 204 is a birefringent crystal, such as calcite, rutile, or (YV04).

The physical properties that are desirable in PT element 204 are its optical anisotropies for effecting the polarization transformation of beams 203. Ultimately, the optical properties of element 204 are useful in deinterleaving the input signal 201 into output optical signals having a channel spacing that is twice that of the channel spacing of the input signal 201.

Accordingly, while illustratively a birefringent crystal is used for element 204, it is clear that other optically anisotropic materials could be used as element 204. These include, but are not limited to, known birefringent elements such as birefringent optical fiber. Moreover, known phase retarders may be used for PT element 204.

It is noted that in the illustrative embodiment shown in Fig. 2, PT element 204 is a birefringent crystal having its principle section and c-axis oriented diagonally (at a 45° angle) relative to end phases of the crystal. As such, the c-axis is at 45° angle relative to the plane of polarization of the beams 203.

In the illustrative embodiment shown in Fig. 2, the channel separation that ultimately enables the deinterleaving of the input optical signal 201, exploits the anisotropic optical properties of element 204. Moreover, the PT element 204 is illustratively a birefringent crystal having ordinary and extraordinary axes. In the exemplary embodiment of Fig. 2, the polarization vectors of beams 203 are in mutually orthogonal polarization states. The c-axis of birefringent PT element 204 is oriented at 45° relative to each of these polarization states. As such, it can be

shown that for beams 203 having the polarization state described above, the transmittance of beam 207 (odd channels) is given by: where v is the relative optical frequency of a particular channel, 00 is a phase constant, and T iS the temporal delay between the ordinary and extraordinary polarization vectors.

(Moreover, 1-T is transmitted by beam 210 (even channels) after its combination by elements 206,208 and 209). The temporal delay, x, is given by : where L is the length of the birefringent material (such as PT element 204), c is the speed of light in vacuum, ne is the index of refraction along the extraordinary axis, no is index of refraction along the extraordinary axis, and Ang is the group index of refraction difference between the ordinary and extraordinary index of refraction for the center wavelength of the particular channel.

The temporal delay, T, between the extraordinary and ordinary beams is exploited to effect the desired channel spacing in the deinterleaving of the input signal 201. However, as mentioned briefly above, the temperature of birefringent elements (such as element 204) may affect both the length and indices of refraction of the birefringent element. Ultimately, because the channel spacing is related to the temporal delay, the channel spacing may also be adversely impacted by the affect of temperature on the optical path length and birefringence.

Quantitatively, the temperature induced frequency shift of element 204 is given by: where, <BR> <BR> <BR> <BR> <BR> <BR> d#n dne dno<BR> <BR> <BR> = - eqn.(4),<BR> <BR> dT dT dT α is the thermal expansion coefficient of element 204, # is the wavelength and f is the relative optical frequency.

Illustratively, using known material properties of YV04, the temperature induced wavelength drift is on the order of approximately-4.2 GHz/°C. This is, unfortunately, on the order 5 times the temperature drift of an uncompensated fiber Bragg grating, which may be used in the deinterleaving of a WDM/DWDM signal. In order for WDM/DWDM devices based on anisotropic optical elements such as birefringent crystals to be practical in deployed optical systems, it is important to reduce the thermal drift either by active temperature control (such as climate control elements) or by passive compensation. As discussed above, active temperature control techniques have certain drawbacks making the passive compensation technique of the present invention an attractive alternative to active control of the ambient temperature.

As stated above, the thermal effects on the birefringent elements such as element 204 are generally manifest in a change in the length of the element 204 as well as a change in indices of refraction along the ordinary and extraordinary axes. These thermal effects must be compensated for, as a temperature induced change in these parameters ultimately affects the temporal delay, the free spectral range (FSR), and the channel spacing. According to illustrative embodiments of the present invention, the thermal compensation is effected passively, incorporating a passive thermal compensating (PTC) element 205 to effect temperature compensation. Illustratively, PTC element 205 is an anisotropic optical element.

Moreover, PTC element 205 is adjacent PT element 204, and PT element 204 and PTC element 205 are illustratively birefringent materials, which are adhered to one another by suitable adhesive. As such, the PT element 204 and PTC element 205 behave substantially as one optical element.

As stated, an object of the present invention is to minimize the temperature induced wavelength drift. Accordingly, one objective of the present invention is that the temperature induced frequency shift of element 204 is nullified. Quantitatively, this means that eqn. (3) is zero: <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> #f<BR> <BR> = O eqn.(5)<BR> <BR> <BR> <BR> #T In the exemplary embodiment of Fig. 2, where a compensating element such as PTC element 205 is used to achieve passive thermal compensation, thermal compensation requires <BR> <BR> <BR> <BR> <BR> <BR> <BR> that:<BR> d#n1 d#n2 L1 + L2 + #n1L1α1 + #n2L2α2 = O eqn.(6) dT dT where L I is the length of element 204, L2 is the length of element 205, An, is the birefringence of element 204, An2 is the birefringence of element 205, (xl is the expansion coefficient of element 204 and a2 is the expansion coefficient for element 205. This is referred to as the condition of athermalization.

Other objectives of the passive thermal compensation of the present invention are to maintain the channel spacing, the temporal delay T, and free spectral range (FSR) across a

particular temperature range. The temporal delay,, of birefringent elements 204 and 205 are additive. As such : T = T1 + T2 eqn. (7) so # = L1(#n1)+L2(#n2) eqn.(8) c c where Tl and T2 are the temporal delays resulting from birefringent elements 204 and 205, respectively.

Because the temperature dependence of the indices of refraction of the birefringent elements, as well as the expansion coefficients al, ai of elements 204 and 205, respectively, are known, in order to satisfy the above relations of eqn. (6) and (8), it is necessary to determine the suitable lengths Li and L2 for the particular materials chosen for birefringent elements 204 and 205. Practically, this requires determining the ratio of L'from eqn. (6), the condition for zu athermalization. Accordingly, once this ratio is determined, the interleaver 200 is passively athermalized for a variety of free spectral ranges (FSR), and thereby channel spacings. Stated differently, when the ratio of the lengths of elements 204 and 205 is determined from eqn. (6), the interleaver 200 according to an illustrative embodiment of the invention of the present disclosure is athermalized for free spectral ranges and channel spacings of interest. It follows of course that if it is desired to have an athermalized interleaver with a particular free spectral range/channel spacing, it is necessary to determine the appropriate ratio of the lengths of elements 205 and 205. Illustratively, the interleaver 200 may interleave/deinterleaver optical

signals having free spectral ranges (and channel spacings) of 400GHz, 200GHz, 100 GHz, 50 GHz and 25GHz, and 12.5 may be realized.

It is noted that according to the presently described exemplary embodiment, the fast axes of PT element 204 and PTC element 205 should be either perpendicular or parallel to one another. If the fast axis of PT element 204 were oriented at an angle between 0° and 90°, and multiples thereof, the output would not be the same as the ideal situation where there is no need to compensate for thermal effects with element 205. To this end, if the fast axes of the PT element 204 and PTC element 205 are not oriented parallel or perpendicular to one another, the phase delay between the extraordinary and ordinary rays will result in harmonics and the output will not be the desired output, which is illustratively the deinterleaved output signal described in the parent application.

According to an illustrative embodiment of the present invention, the crystal that may be used for PTC element 205 has a fast axis which is parallel to the fast axis of birefringent element 204. Moreover, the crystal should be chosen so that dAn, and dAn'have opposite signs. Since dT dT -- is usually the dominating term, it is possible to cancel the thermal affect while the dT birefringence of the second crystal is merely additive to that of the first crystal. In this particular embodiment, the length of the PT element 204 (also referred to as the dominating element) is shorter than the birefringent element of the uncompensated interleaver, as described in the parent application. Again, this is because the birefringence is additive in this particular embodiment.

Accordingly, the above described embodiment enables the passive compensation for thermal effects by determination of the lengths Li and L2 which satisfy the relation of eqn. (6).

Alternatively, the fast axes of PT element 204 and PTC element 205 may be orthogonal.

Accordingly, the fast axis of PT element 204 will be parallel to the slow axis of PTC element 205. In this case, and particularly when yttrium vanadate (YV04) is used for PT element 204 and PTC element 205, dAn2 has the same sign as As such, not only does the PTC element 205 compensate for the thermal effect, but it also reduces the amount of birefringence provided by the first element 204. This partial cancellation of the birefringence of the PT element 204 by the PTC element 205 necessitates, of course, that the first element 204 has a greater length (Li) than in the case in which there is no compensation for thermal effects.

By solving the condition for athermalization as set forth in eqn. (6), the ratio of the lengths of PT element 204 to the PTC element 205 may be determined. Moreover, by solving eqn. (6) and eqn. (8) the lengths Li and L2 for PT element 204 and PTC element 205, respectively, for particular materials, temporal delay, FSR and channel spacing may be determined in absolute value. Suitable materials such as lithium niobate (LiNbO3) may be used for element 205 in this illustrative embodiment, with YV04 illustratively being used as the material for element 204. Of course, the materials used for elements 202,204,205 and 206 could be other anisotropic optical elements, illustratively birefringent optical fiber.

As can be readily appreciated, the embodiment shown in Fig. 2 provides a passive thermal compensation to the to a WDM optical system. Depending upon applications, residual temperature drift in WDM optical systems based on polarization interferometry may be required to be less than approximately 1 GHz over a temperature range of approximately-5°C to approximately +70°C. This is an improvement on the order of approximately a factor of 170

compared to polarization interferometry based WDM that is not thermally compensated, and is comparable to the performance of a fiber Bragg grating (FBG) based interleaver.

In some circumstances, it may be desirable to compensate for second order temperature effects. According to an exemplary embodiment, this may be carried out using an additional PTC element. Such an element is shown in Fig. 3 at 301. To this end, the element 300 shown in Fig. 3 includes PT element 204, and PTC element 205 of Fig. 2 as well as another PTC element shown at 301. The basic teachings of Fig. 2 apply to Fig. 3, and the other elements of the interleaver 200 of Fig. 2 have been forgone in Fig. 3 in the interest of clarity of discussion. (Of course, element 300 would be located between element 202 and element 206 in interleaver 200).

For precise passive thermal compensation a detailed knowledge of the thermal properties of elements 204,205 and 301 is useful. Fortunately, the interleaver 200 shown generically in Fig. 2 is a very good temperature sensor. By measuring the temperature dependence of the frequency (wavelength) on shift in a polarization independent interleaver 200, which contains at least one birefringence element, higher order temperature coefficients can be derived. As such, the second order elements can be used in combination with the first order elements and the basic equation for the temporal delay, T in order to determine the suitable ratios for the lengths (Li, L2, L3) of elements 204,205 and 301, respectively, for various materials used in these capacities.

Quantitatively, it can be shown that: # = L1/C(#n1)+L2/C(#n2)+L3/C(#n3) eqn.(9) d(#n1α1) d(#n2α2) d(#n3α3)<BR> <BR> L1 + L2 + L3 = O eqn.(10)<BR> dT dT dT d2 d2 d2 <BR> <BR> <BR> <BR> <BR> L1 (#n1α1) + L2 (#n2α2) + L3 (#n3α3) = O eqn.(11),<BR> <BR> <BR> <BR> dT² dT² dT² where L3, An3, a3 are the length, birefringence, and expansion coefficient, respectively, of the third element 301. Finally, the orientation of the fast axes of elements 204,205 and 301 described above are illustrative. As would readily be appreciated by one of ordinary skill in the art having had the benefit of the present disclosure, the orientation of the fast axes of elements 204,205 and 301 can be a variety of permutations of parallel and perpendicular orientations.

While not particularly spelled out in the present invention, these are, of course, within the scope of the present invention.

The invention having been described in detail in connection through a discussion of exemplary embodiments, it is clear that modifications of the invention will be apparent to on of ordinary skill in the art, having had the benefit of the present disclosure. Such variations and modifications are included in the scope of the appended claims.