Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/26—Optical coupling means
  • G02B6/27—Optical coupling means with polarisation selective and adjusting means
  • G02B6/2706—Optical coupling means with polarisation selective and adjusting means as bulk elements, i.e. free space arrangements external to a light guide, e.g. polarising beam splitters
  • G02B6/2713—Optical coupling means with polarisation selective and adjusting means as bulk elements, i.e. free space arrangements external to a light guide, e.g. polarising beam splitters cascade of polarisation selective or adjusting operations
  • G02B6/272—Optical coupling means with polarisation selective and adjusting means as bulk elements, i.e. free space arrangements external to a light guide, e.g. polarising beam splitters cascade of polarisation selective or adjusting operations comprising polarisation means for beam splitting and combining
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/26—Optical coupling means
  • G02B6/27—Optical coupling means with polarisation selective and adjusting means
  • G02B6/2753—Optical coupling means with polarisation selective and adjusting means characterised by their function or use, i.e. of the complete device
  • G02B6/2773—Polarisation splitting or combining
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/26—Optical coupling means
  • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
  • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
  • G02B6/29379—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device
  • G02B6/2938—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device for multiplexing or demultiplexing, i.e. combining or separating wavelengths, e.g. 1xN, NxM
  • G02B6/29386—Interleaving or deinterleaving, i.e. separating or mixing subsets of optical signals, e.g. combining even and odd channels into a single optical signal
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/26—Optical coupling means
  • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
  • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
  • G02B6/29379—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device
  • G02B6/29398—Temperature insensitivity
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/26—Optical coupling means
  • G02B6/27—Optical coupling means with polarisation selective and adjusting means
  • G02B6/2746—Optical coupling means with polarisation selective and adjusting means comprising non-reciprocal devices, e.g. isolators, FRM, circulators, quasi-isolators

Definitions

• the present invention relates generally to optical communications, and particularly to an optical interleaver/deinterleaver having passive thermal compensation.
• 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.
• optical media include higher channel capacities (bandwidth), greater immunity to electromagnetic interference, and lower propagation loss.
• Mbit/s megabits per second
• Gbit/s gigabits per second
• WDM wavelength division multiplexing
• each high-speed information channel has a center wavelength and prescribed bandwidth.
• 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.
• 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.
• ITU International Telecommunication Union
• one ITU channel grid has a channel spacing requirement of 100 GHz.
• the channel spacing is referenced in terms of a frequency spacing, which corresponds to a channel center wavelength spacing of 0.8 nm.
• 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").
• the more the information that is sent over a particular medium the greater the number of channels that are needed.
• 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.
• the change in the ambient temperature can create temperature induced wavelength drift of the WDM. This wavelength drift can cause wavelength channel overlap.
• optical system performance may be adversely impacted.
• the present invention relates to a method and apparatus for interleaving/deinterleaving optical signals having a passive thermal compensation component.
• 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.
• 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.
• 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.
• interleaving decreases the channel spacing between adjacent channels, and de- interleaving increases the channel spacing between adjacent channels.
• the illustrative embodiments herein describe the de-interleaving function.
• the methods and apparati of the exemplary embodiments present invention described herein may be used to achieve an interleaving function.
• 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.
• 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).
• interleavers/deinterleavers could be concatenated, with each increasing the channel spacing.
• deinterleaved optical signals could also be selectively coupled to other devices such as demultiplexers and optical add/drops.
• 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.
• 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 i multiplexed optical signal illustratively having channels 1, 2, ...n, with respective channel center
• 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.
• 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.
• 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 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.
• 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.
• the output signals 103 and 104 are substantially unaffected by variations in the ambient temperature over a predetermined range.
• 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.
• 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 (YVO ).
• 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 ⁇ l5 ⁇ 2 , ... ⁇ n .
• the polarization splitter 202 merely splits the electric field vector of input signal 201 into
• each of the beams 203 contain all the channels.
• PT element 204 is a birefringent crystal, such as calcite, rutile, or (YYO 4 ).
• the physical properties that are desirable in PT element 204 are its optical anisotropies for effecting the polarization transformation of beams 203.
• 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.
• birefringent crystal is used for element 204
• 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.
• known phase retarders may be used for PT element 204.
• 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.
• the channel separation that ultimately enables the deinterleaving of the input optical signal 201 exploits the anisotropic optical properties of element 204.
• the PT element 204 is illustratively a birefringent crystal having ordinary and extraordinary axes.
• 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.
• ⁇ is the relative optical frequency of a particular channel
• ⁇ Q is a phase constant
• T is the temporal delay between the ordinary and extraordinary polarization vectors.
• 1-T is transmitted by beam 210 (even channels) after its combination by elements 206, 208 and 209).
• the temporal delay, ⁇ is given by:
• L is the length of the birefringent material (such as PT element 204), c is the speed of light in vacuum, n e is the index of refraction along the extraordinary axis, n 0 is index of refraction along the extraordinary axis, and ⁇ n g is the group index of refraction difference between the ordinary and extraordinary index of refraction for the center wavelength of the particular channel.
• the temperature of birefringent elements may affect both the length and indices of refraction of the birefringent element.
• the channel spacing is related to the temporal delay ⁇ , the channel spacing may also be adversely
• dT dT dT a is the thermal expansion coefficient of element 204
• ⁇ is the wavelength and / is the relative
• wavelength drift is on the order of approximately - 4.2 GHz/°C. This is, unfortunately, on the
• 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
• the thermal compensation is effected passively, incorporating a passive thermal compensating (PTC) element 205 to effect temperature compensation.
• PTC element 205 is an anisotropic optical element.
• 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.
• 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:
• thermal compensation requires that:
• Li is the length of element 204
• L is the length of element 205
• An is the
• the temporal delay, ⁇ , of birefringent elements 204 and 205 are
• the interleaver 200 is passively athermalized for a variety of free spectral ranges (FSR), and thereby channel spacings.
• FSR free spectral ranges
• 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.
• 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.
• the fast axes of PT element 204 and PTC element 205 should be either perpendicular or parallel to one
• the crystal that may be used for PTC element 205 has a fast axis which is parallel to the fast axis of birefringent element
• the crystal should be chosen so that -and L have opposite signs. Since dT dT
• 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.
• the above described embodiment enables the passive compensation for thermal effects by determination of the lengths Li and L 2 which satisfy the relation of eqn. (6).
• 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 (YVO 4 ) is used for PT element 204
• the PTC element 205 compensates for the the ⁇ nal 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.
• the ratio of the lengths of PT element 204 to the PTC element 205 may be determined.
• the lengths Lj and L 2 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 (LiNbO ) may be used for element 205 in this illustrative embodiment, with YVO 4 illustratively being used as the material for element 204.
• the materials used for elements 202, 204, 205 and 206 could be other anisotropic optical elements, illustratively birefringent optical fiber.
• the embodiment shown in Fig. 2 provides a passive thermal compensation to the to a WDM optical system.
• 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.
• FBG fiber Bragg grating
• Fig. 3 Such an element is shown in Fig. 3 at 301.
• 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.
• the interleaver 200 shown generically in Fig. 2 is a very good temperature sensor.
• a polarization independent interleaver 200 which contains at least one birefringence element, higher order temperature coefficients can be derived.
• the second order elements can be used in combination with the first order elements and the basic
• L 3 , An 3 , a 3 are the length, birefringence, and expansion coefficient, respectively,
• 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.

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• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)

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• 2002
  • 2002-03-28 AU AU2002303178A patent/AU2002303178A1/en not_active Abandoned
  • 2002-03-28 EP EP02731186A patent/EP1393119A2/de not_active Withdrawn
  • 2002-03-28 WO PCT/US2002/009733 patent/WO2002079817A2/en not_active Application Discontinuation

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