Polarization-insensitive integrated wavelength converter

A polarization-insensitive integrated wavelength converter system includes polarization-separating, polarization-rotating and wavelength-converter structures integrated into a monolithic optical structure. In one embodiment, a lithium niobate waveguide structure includes an integrated polarization separator which comprises two coupled Zinc-diffused and annealed-proton-exchanged waveguides, an electro-optic quarter-wave retarder, a mirror structure and a quasi-phasematching structure. In another embodiment, an electro-optic half-wave retarder and bent waveguide are used in place of the electro-optic quarter-wave retarder and mirror structure. In a further embodiment, an optical circulator is used in conjunction with the waveguide structure in order to discriminate between input and output optical signals.

BACKGROUND OF THE INVENTION

Many integrated optical processors, such as wavelength converters based on waveguides in PPLN (Periodically-Poled Lithium Niobate), are sensitive to polarization modes of the interacting optical signals. These PPLN wavelength converters can receive one or more input signals at different frequencies fs, and convert these signals to different frequencies fcwith the help of energizing pump signals at frequencies fp. The frequency fc, of a converted signal is related to the frequency f, of its corresponding input signal and the frequency fpof its pump signal to provide a mechanism to define the frequencies of the converted signals from their input signals. However, PPLN wavelength converters are polarization sensitive and various physical effects in the processor can cause undesired results on an input signal of mixed polarization modes. These effects include dependence of the nonlinear coefficient and waveguide loss on the polarization of the interacting optical beams.

Optical systems designed to compensate for polarization sensitivity (e.g., systems that achieve “polarization diversity”) are known in the art. However, many of these systems utilize multiple discrete optical components that are not suitable for practical integration into monolithic optical processing systems.

On the other hand, the present invention provides for which polarization-insensitive integrated wavelength converters with components and designs which can be efficiently integrated into monolithic optical systems. Costs can be reduced and reliability can be increased, among other improvements.

SUMMARY OF THE INVENTION

According to a further aspect of the invention, a polarizing beam splitter is utilized to couple different polarization modes of an input signal into an optical processing element and a bypass path.

According to another aspect of the invention, a waveguide polarization separator and converter, which utilizes a junction between two coupled waveguides, wherein each of the coupled waveguides have preferential transmission properties for TE and TM polarization modes thereby serving to separate the polarization modes of the component beams into two separate arms of a waveguide structure, and a wavelength converter used for performing wavelength conversion on at least one of the component beams.

According to another aspect of the invention, a component beam is processed, wherein the processing includes wavelength conversion by transmission of the component beam through a PPLN structure.

According to another aspect of the invention, a component beam is processed, wherein the processing includes amplification by transmission of the component beam through a PPLN structure.

According to another aspect of the invention, an optical filter serves to discriminate between converted and non-converted optical signals.

According to another aspect of the invention, an interchannel crosstalk modulator serves to detect and reduce the level of interchannel crosstalk between converted optical signals.

According to another aspect of the invention, a mirror and quarter-wave plate are used to change polarization modes of the component beams so that both component beams are processed and then recombined to form a processed output beam.

According to another aspect of the invention, a half-wave plate and bent waveguide are used to change polarization modes of the component beams so that both component beams are processed and then recombined to form a processed output beam.

According to another aspect of the invention, an waveguide electrooptic wave plate is used to change the polarization mode of the component beams so that both component beams are processed.

According to another aspect of the invention, a waveguide electro-optic mirror is used to reflect a portion of the component beams into a polarization rotator and PPLN structure so that both component beams are processed.

According to another aspect of the invention, a waveguide electro-optic phase modulator is used to adjust the phase of at least one of the component beams so that both component beams are processed.

According to another aspect of the invention, a twisted fiber is utilized to change the polarization mode of the component beams so that each component is processed.

According to another aspect of the invention, techniques for eliminating polarization sensitivity are integrated in an optical frequency synthesizer and filter.

According to another aspect of the invention, multiple channels comprising polarization-insensitive integrated wavelength converters are integrated into a single monolithic system.

Other features and advantages of the invention will be apparent in view of the following detailed description.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

First, it should be noted that the present invention is described with reference to the drawings with the same or similar elements given the same reference numbers.

Illustrative of the elements and their functions of the present invention, the general organization of one embodiment of the present invention is depicted in FIG.1A. The polarization-insensitive integrated wavelength converter includes an optical circulator12, a polarization beam splitter (PBS)13, a difference frequency generator (DFG) in the form of a PPLN (Periodically-Poled Lithium Niobate) waveguide10, an optical bypass path14and a half-wave plate16.

The PPLN may utilize quasi-phasematching or other nonlinear optical phasematching processes in order to achieve three-wave mixing. While DFG is one nonlinear interaction enabled by three-wave mixing, other interactions, such as second harmonic generation (SHG), optical parametric amplification (OPA), optical parametric generation (OPG), and sum frequency generation (SFG) are also forms of wavelength conversion and as such may also be performed by the DFG. The PPLN may be comprised of ferroelectric, semiconductor, polymer, organic or other materials known to perform nonlinear optical frequency mixing. Other ferroelectric materials include congruent lithium niobate, congruent magnesium-doped lithium niobate, congruent lithium tantalate, congruent magnesium-doped lithium tantalate, stoichiometric lithium tantalate, stoichiometric magnesium-doped lithium tantalate, stoichiometric lithium niobate, stoichiometric magnesium-doped lithium niobate, KTP, BBO, and other known optical materials. Quasi-phasematching may be enabled in the PPLN by use of electric-field poling or other techniques for achieving periodic inversion of domains. In this fashion, phasematching between the pump, signal and converted signals may be achieved, and energy may be exchanged between the three waves, thereby enabling optical frequency conversion, or “wavelength conversion”.

The optical circulator12has three ports so that optical signals received at port1are passed to port2, optical signals received at port2are passed to port3. The first port1of the circulator12receives one or more input signals at frequency fs, and one or more energizing pump signals at frequency fp. The first end of the PPLN waveguide10, i.e., the DFG, is coupled to the second port2of the optical circulator through the PBS13. The third port3is the output port of the polarization-insensitive integrated wavelength converter. Stated generally, the input signals and pump signals from port1of the circulator12are passed to the PPLN waveguide10which mixes the input signals and pump signals to generate converted signals. The PBS13, the optical by-pass path14, and the half-wave plate16operate on the polarization states of the optical signals for maximum efficiency of the integrated wavelength converter, and the resulting converted signals, input signals and pump signals are passed back through the PBS13to port2of the optical circulator12, which passes all the optical signals out through port3. This is indicated by the “fp, Fs, Fc” in the drawings.

The polarizing beam splitter (PBS)13couples a component of light signals in a first polarization mode, in this example, the transverse magnetic (TM) mode, into the DFG waveguide10from port2and couples a component of light signals in the second polarization mode, in this example, transverse electric (TE), into the bypass path14. Various types of PBS structures, such a proton exchange type or titanium (Ti) intersecting waveguides, are known in the art and are described in greater detail below. The bypass path14couples the second end of the DFG PPLN waveguide10to the PBS13and changes the polarization mode of an optical signal that traverses it. In this embodiment, the bypass path14includes a half-wave plate16which, as is well known in the art, rotates linearly polarized light 90° to convert light from one linear polarization mode to the other. The DFG waveguide10performs second-order mixing to generate converted light signals having frequencies fcequal to linear combinations of the input signal frequency fs, and pump frequency fp.

Operationally, the pump and input signals received by first port1of the optical circulator12are coupled to the PBS13through second port2. The first signal component (TM) of these signals is coupled into the DFG waveguide10at its first end and the second signal component (TE) of the signals is coupled into the bypass path14. The first component pump and input signals are mixed in the DFG waveguide10to form a first component (TM) of a converted signal which is coupled into the bypass path14at the second end of the waveguide10to be propagated back to the PBS13. When the first signal component propagates through the half-wave plate16, it is switched from the TM to the TE polarization mode. The second signal component (TE) from the PBS13has its polarization mode is switched from TE to TM in the bypass path14by the half-wave plate16and is then coupled into the DFG waveguide10at its second end to traverse the waveguide10in the opposite direction of the first signal component. The DFG PPLN waveguide10mixes the second signal component of the input signal and pump signal to form a second component of the converted signal.

The first and second components of the converted signal are recombined at the PBS13and coupled into the second port of the optical circulator12and are then passed to the third port as output, along with any remaining input signals and pump signals.

The optical path length of the bypass path14(including the half-wave plate16) is set to be equal to the optical path length of the DFG waveguide10to assure that the recombined components are in phase. This can be done by controlling the index of refraction and physical length of the path14, as is well-known in the art.

Only one polarization mode is passed through the DFG waveguide10, in this case TM, so that the DFG is completely insensitive to the initial polarization states of the input signal and pump signals. A variation of the polarization-insensitive integrated wavelength converter places the half-wave plate16at the second end of the DFG waveguide10, rather than separated in the by-pass path14as shown in FIG.1A.

FIG. 1Bdepicts a multi-channel system similar to that disclosed in FIG.1A. Three optical circulators12a,12band12creceive three sets of input signals and pump signals. The PBS13is coupled to the second port2of the optical circulators12a,12band12cto create a plurality of channels, which is each coupled to a DFG waveguide10a,10band10cin a single PPLN structure (in this embodiment)11for generating converted signals from the input and pump signals of each channel. A plurality of bypass paths14a,14band14cand a half-wave plate16connect, respectively, the output of the DFG waveguides10a,10band10cto the PBS13. The converted signals (and input and pump signals) of each channel are passed to the second port2and out the third port3of one of the optical circulators12a,12band12c.

The DFG can be either pumped at the pump wavelength, which is typically around 775 nm for performing wavelength conversion in the WDM C-band. Alternatively, the DFG can operate in a cascaded second-harmonic-generation-difference frequency-generation (SHG:DFG) process. In this fashion, the DFG is pumped at the subharmonic (or fundamental) of the pump wavelength, typically around 1550 nm for wavelength conversion in the WDM C-band, wherein the DFG serves to both produce the 775 nm pump light by second harmonic generation (SHG) as well as wavelength conversion by difference frequency generation. The structure depicted inFIG. 1Bis well-suited to integration into a monolithic substrate so that multiple optical signals can be converted in parallel without regard to the polarization states of the input signal and pump.

Another embodiment of the invention is depicted inFIG. 2A, the structure of which is similar to the embodiment depicted in FIG.1A. However, in this embodiment the bypass path14maintains the polarization mode of the light signal it propagates. Polarization mode changing is done by a quarter-wave plate15and a mirror17. The PBS13couples a first signal component (TM) to the first end of the DFG waveguide19. The second signal component (TE) is transmitted through the quarter-wave plate15and is reflected back by the mirror17through the quarter-wave plate15, and coupled into the bypass path14. The two traversals of the quarter-wave plate15are equivalent to one traversal through a half-wave plate so that the polarization state of the second signal component is switched from TE to TM. The second signal component is then coupled into the second end of the DFG waveguide10so only TM components are converted in the DFG waveguide10, as explained with respect toFIG. 1A, and the system is insensitive to the polarization modes of the initial pump and signal.

Another embodiment similar to the embodiment ofFIG. 1Ais depicted in FIG.2B. The bypass path is formed of an optical fiber18which is twisted by 90° to switch the polarization mode of an optical signal propagating in the bypass path. A polarization-maintaining fiber can be used for the optical fiber18. Substrate-to-fiber couplers19connect the two ends of the of the fiber18to the second end of the DFG waveguide10and the PBS13respectively.

It is known in the art that PPLN wavelength converters enable the simultaneous conversion of multiple channels (or “signals”). However, depletion of pump energy may result in interchannel cross-talk between converted channels. Therefore, a pump and signal monitor module can be utilized to monitor power levels of the pump, input and converted channels. Amplifiers and or modulators can be also utilized to control power levels of the pump and/or channels to reduce interchannel or other forms of crosstalk and signal degradation. Further, a feedback loop and module may be utilized to monitor and manage the power control process. Furthermore, the gain of the PPLN wavelength converter may be adjusted to control crosstalk. In this fashion, the PPLN crystal temperature, quasi-phasematching grating period, angle, and other properties of the PPLN crystal may be modified in order to control the nonlinear optical gain of the PPLN. Also, an electric field, applied to electrodes in the region of the waveguide, may be utilized to control the gain in this regard.

In the embodiment depicted inFIG. 3, the capability of the present invention for integration is better illustrated. The optical circulator12is coupled to the input of a wavelength converter structure30which has a Ti waveguide32and a DFG formed of an Annealed Proton Exchanged (APE) waveguide34in an LiNbO3substrate36. Ti-diffused LiNbO3waveguides are widely used because of their good optical properties and are formed by diffusing titanium (Ti) into a lithium niobate (LiNbO3) substrate. Ti waveguides can be designed to propagate both modes of polarization. APE waveguides are formed by masking the surface of a LiNbO3substrate to define a waveguide area, immersing the waveguide in a bath of liquid acid that provides a source of protons to be exchanged with lithium ions to create a thin layer of protons at the surface, and annealing the structure to redistribute proton and lithium ions to form a stable waveguide structure. The APE waveguide can be designed to propagate only the TM mode of polarization.

The APE waveguide34includes PPLN structures35for mixing the pump and input signal to form a converted signal. Couplers are formed at each end of the APE waveguide; in this embodiment the Ti and APE waveguides32and34are brought close together to form first and second junctions38and40which effect evanescent coupling between the waveguides32and34. The Ti waveguide32is terminated at one end by a polarization switching structure42, which in this embodiment is a quarter-wave plate25and a mirror27. The other end of the Ti waveguide32has an enlarged coupling region31to facilitate the coupling between the second port2of the circulator12and the waveguide32.

The Ti waveguide can alternatively utilize any waveguide material, instead of titanium, that is generally known to support both TM and TE modes of polarization; for example, zinc (Zn), and other metals diffused into lithium niobate support such modes. Likewise, the APE waveguide may utilize other materials that support a single, TE or TM, polarization mode. As is known in the art, APE waveguides only support propagation of light signals in the TM mode. In the operation of theFIG. 3wavelength converter, the input and pump signals are coupled into the first end of the Ti waveguide32by port2of the optical circulator12. At the first junction38a first signal component in a TM polarization mode is coupled into the APE waveguide43and a second component in a TE polarization mode propagates through the Ti waveguide32. In the APE waveguide34, the pump and input signals are mixed to form a first component of the converted signal and the first component of the converted signal is coupled back into the Ti waveguide32at the second junction40. Both signals have their polarization modes switched by the mirror27and quarter-wave plate25and are then coupled back into the second end of the Ti waveguide32to propagate in the opposite direction.

The second component, now in a TM polarization mode, is coupled into the APE waveguide32at the second junction40and the input and pump signals are mixed to form a second component of the converted signal. The second component of the converted signal is coupled back into the Ti waveguide32at the first junction38and both components of the converted signal are output to the second port2of the optical circulator12to be passed to the output port3. Because both components of the signal and pump are converted in the TM mode in the APE waveguide DFG structure35, the conversion process is independent of the polarization modes of the initial pump and signal.

Additionally, an optical circulator may be integrated into the structure along with the wavelength converter, polarization rotator, polarization separator, and other elements. In this fashion, an integrated circulator may comprise a polarization separator based on waveguides that discriminate TM and TE polarization modes, as described above, and a magneto-optic or other nonreciprocal optical element, such as a Faraday rotator, that rotates polarization in a direction dependent upon the direction of propagation of light through the element.

FIG. 4Adepicts a variation of theFIG. 3wavelength converter where a different technique are utilized to change the polarization mode of the signal components. InFIG. 4a simple mirror37terminates one end of the Ti waveguide32. Additionally, a half-wave electro-optic region50is disposed along the Ti waveguide32between the first and second coupling junctions38and40and two electrodes51on the surface of the substrate36and on either side of the Ti waveguide32. The two electrodes51are connected to a voltage source52. As is known in the art, the application of voltage to the half-wave electro-optic region50causes the switching of the polarization mode of an optical signal propagating through the region. The changing of the polarization mode in the electro-optical region50is controlled by length and composition of the region along the waveguide32with well-known techniques.FIG. 4Billustrates the cross-sectional view of the half-wave electro-optic region50and the electric fields (represented by dotted lines53) created in the substrate36and the Ti waveguide32by the electrodes51. The mirror37and half-wave electro-optic region50operate as the polarization switching structure42in FIG.3.

FIGS. 5A-5Cillustrate further polarization-independent converters in which still more techniques are used to change the polarization mode of the signal components. InFIG. 5Aa quarter-wave electro-optic region50is disposed between the second coupling junction40and the mirror37. Both components propagate twice through the quarter-wave electro-optic region50so that their polarization modes are switched.

The wavelength converter inFIG. 5Buses a twisted fiber in a manner like the wavelength converter illustrated in FIG.2B. Instead of a mirror at the second ends of the Ti and APE waveguides32and34, the second ends are coupled by an optical fiber60twisted by 90° to induce a change in the polarization modes of the signal components. Substrate-to-fiber couplers61connect the two ends of the fiber60to the substrate36and to the second ends of the Ti waveguide32and APE waveguide34.

InFIG. 5Cthe Ti waveguide32is coupled to the APE waveguide34at a first junction38and an intersection70. As before, the APE waveguide34includes a PPLN section35for difference frequency generation (DFG) and extends around to the intersection70. The Ti waveguide32include a half-wave electro-optic region50disposed before the intersection70with the APE waveguide34. In operation, the input signal and pump signal are coupled into the Ti waveguide32at the enlarged coupler section31and first components of the input signal and pump (TM) are coupled into the APE waveguide34at the first junction38. The first components are frequency mixed, exit the APE waveguide34into the Ti waveguide32at the intersection70, and are polarization mode switched to TE when they propagate through the electrooptic region50. In the opposite direction, the second components of the input signal and pump signals are polarization switched to TM when they propagate through the electro-optic region50and are coupled into the APE waveguide34at the intersection70. The second components of the signal and pump are frequency mixed by the PPLN section35and are coupled to the Ti waveguide32at the first junction38before being sent back to the second port2of the circulator12.

In all of the embodiments inFIGS. 3,4,5A-5C, the optical path lengths of the waveguides are adjusted with techniques known in the art so that the first and second components of the signals are in phase when recombined.

Another embodiment of the present invention is illustrated inFIG. 6in which a wavelength converter structure is defined in a substrate86. The structure has an APE Y-junction90includes first and second legs92and94joined into a single waveguide96which includes a PPLN section95for performing DFG. A Ti waveguide98is disposed near a curve in the first leg92of the APE Y-junction to form a first coupling junction100and terminates at an intersection101with the second leg94of the APE Y-junction90. The Ti waveguide98also includes a electro-optic region102for changing the polarization mode of a light signal propagating in the Ti waveguide. The electro-optic region102is formed from electrodes and a connected voltage source, as described previously for other wavelength converter structures.

Operationally, the pump and input signal (an optical circulator is not shown) are coupled into one end of the Ti waveguide98and the first component of these signals (TM) is coupled into the first leg92of the APE Y-junction90at the first coupling junction100. The second signal component (TE) has its polarization mode switched by the voltage electrodes in the polarization mode switching section102of the Ti waveguide98so that the second component is switched to the TM mode. The second component then enters the second leg94of the APE Y-junction90through the intersection101in the TM mode. Both components are then joined by the Y-junction90and propagate through the mixing PPLN region95in the TM state. Thus, the conversion process does not depend on the polarization modes of the initial signal and pump. Since it is critical that the signal components be in phase when joined by the Y-junction90, the APE leg92includes a phase matching section104(the techniques of which are well known in the field) to adjust the phase of the first component to take into account phase changes induced in the second component as it propagates through the Ti waveguide.

FIGS. 7A-7Cdepict the application of the polarization-insensitive integrated wavelength converters in a larger system, in this example, a filter system which provides separate outputs for the unconverted signal, i.e., the input signal, and the converted signal. A detailed description of such a filter is included in the co-pending, commonly assigned U.S. patent application Ser. No. 09/991,272, entitled “Optical Signal Converter with Filtered Output,” and filed November 16, 2001 by Mark Sher and Robert Batchko. Generally stated, the described filter uses interferometric devices to separate alternating channel wavelengths, such as defined by the ITU (International Telecommunications Union), from each other. For convenience purposes, one set of communication channel wavelengths (or frequencies) is termed “even” and the other termed “odd.” By properly selecting the frequency fpof the pump signal for DFG conversion, the input signals in one set of communication channels can be converted to output signals in another set of communication channels and an even/odd separator can be used to separate the input signals from the converted signals. More details can be found in the above-cited patent application. The present invention improves the efficiency of this filter system.

In the exemplary system shown inFIG. 7A, the even and odd input signals to an input even/odd separator120are optically coupled to first and second polarization-insensitive integrated wavelength converters130aand130bdelineated by dotted lines. The arrangement ofFIG. 1Ais used as a representative of the polarization-insensitive integrated wavelength converters of the present invention. The outputs of these polarization-insensitive optical frequency synthesizers are respectively coupled to first and second output even/odd separators122and124. The outputs of the first and second even/odd separators122and124are coupled so that the unconverted signal and converted signal are provided at separate output ports. Hence independent of their polarization, the input signals are converted to different wavelength channels. The converted signals at frequencies fcare also separated from the input signals at frequencies fs.

FIGS. 7B and 7Cillustrate different arrangements of the polarization-insensitive integrated wavelength converters of the present invention in the described filter system.

FIG. 8shows an alternative polarization-insensitive integrated wavelength converter in a single waveguide chip. As previously described, the phases of the two light components when combined at the output should be matched. In this case the polarization components are split into two waveguide arms, the incorrect polarization rotated by a half-wave plate, wavelength conversion is performed in at least one arm of the split waveguides, then the two components are recombined in a modified Mach-Zehnder structure (MZ). One arm of the MZ structure may include a high-voltage electrode. The electrode applies a refractive-index-changing electric field thereby controllably varying the optical path difference between the two arms of the MZ, serving to maintain in-phase constructive recombination of the fields in the two arms. An error signal may be tapped off of the orthogonal polarization for a feedback control. The orthogonal polarization is minimized to optimize the correct polarization. Since the telecom signals are very much faster than the variation of polarization in the system, a simple slow detector can be utilized to average out the variations due to signal rather than actual polarization change. This arrangement, as well as the other embodiments and pluralities thereof, can also be integrated onto the same chip as arrays of waveguides and wavelength converters.

The present invention provides for the integration polarization-insensitive wavelength converters. As described earlier, the wavelength converter has a first waveguide may support both TE and TM polarization modes, and a second waveguide supports substantially only the TM polarization mode. Fabrication processes utilizing titanium and annealed-proton-exchanged (APE) waveguides for supporting orthogonal polarization modes have been described in the literature. Further, zinc-diffused waveguides, or other waveguide materials including metals as known in the art, may be employed. The polarization beam splitter then separates un-polarized light into orthogonal TE and TM components.

For the half-wave plate, a cut may be introduced into the waveguide, or into both the waveguide and the substrate, into which a snug-fitting half-wave plate may be introduced. In this fashion, the half-wave plate rotates the polarization of the undesirable component by 90° to match the desirable polarization of the wavelength converter structure. One or more wavelength converter structures, i.e., the PPLN regions, may be employed in either or both of the waveguides. The two waveguides are then recombined to produce a single output. If the waveplate material is the same as that of the substrate (e.g., lithium niobate), then the optical path lengths of the two waveguides should be substantially identical and the recombined beams may add in-phase with minimal loss. A small correction for phase can be introduced by adding a voltage across a portion of one waveguide to vary the refractive index and hence the path length, as described with respect to FIG.8. Alternatively, a second half-wave plate may be introduced into either of the waveguides downstream of the wavelength converter structure.

FIG. 9illustrates another integrated approach is shown which utilizes a polarization beam splitter (PBS). As in the previous embodiments of the present invention, the polarization splitter may be comprised of differing waveguide materials that support orthogonal polarization modes. Additionally, a GRIN lens130may be configured to reflect and interchange the beams exiting each waveguide at the opposite end of the chip. A quarter-wave plate132rotates the reflected light by 90° (via two passes) and the GRIN lens130ensures the rotated components enter the appropriate waveguide which supports that polarization. At least one waveguide arm contains a wavelength converter structure. The reflected beams are then recombined at the input of the chip and may exit an input fiber via a circulator, as previously described.

An alternative polarization diversity scheme is shown inFIG. 10. Aquarter-wave plate is created, preferably in lithium niobate or the same material as the wavelength converter (WC), with its axis oriented 45 degrees to the preferred polarization direction of the WC. The waveguide material is chosen to support both TE and TM polarization modes. For example, metals such as titanium- or zinc-diffused waveguides in lithium niobate, as well as others known in the art, may be used. Since the two materials are identical, they can be butted together without index matching or subsequent losses. Furthermore, the waveguide can be fabricated into the combined substrate (i.e., WC and waveplate) so as to eliminate losses. The exterior surface of the waveplate is coated with a dielectric mirror coating reflecting the pump and signal wavelengths; this coating could also transmit the spectral region into which the converted output will be transmitted, thus eliminating the out-of-band amplified spontaneous emission (ASE) noise from an erbium doped fiber amplifier (EDFA). The waveplate causes a 90 degree rotation of the input signals, so that the polarization component that was optimal for the forward pass is flipped 90 degrees to the non-converting orientation, and vice versa. The pump is frequency-doubled within the waveguide, and the waveplate is fabricated to be quarter-wave in the 1550-nm band and half-wave at the frequency-doubled band (˜780 nm). Thus the polarization of the frequency-doubled light is oriented at its original orientation on the roundtrip and can still pump the flipped polarization component of the signal light on the return pass.

Additionally, the pump source may be chosen to be a single-longitudinal-mode source with high phase correlation. The nonlinear gain for the WC is greatly reduced for signals with randomly varying phases; thus a single longitudinal mode ITU grid signal still converts with a high nonlinear gain, but random noise converts with a reduced gain. The advantage of this scheme is that it eliminates the fiber-to-chip coupling losses of other schemes and simplifies packaging into a single compact and monolithic unit. Ideally, if a WC is employed in every EDFA in the network, then the need for optical-electrical-optical (OEO) regenerators may be eliminated or restricted to ultra-long fiber spans. To this end, there is another noise enhancing characteristic utilizing spectral inversion.

FIG. 11shows a polarization diversity scheme in a polarization splitter is provided by two waveguides supporting two orthogonal polarization modes. In this fashion, a first waveguide may support both TE and TM polarization modes, and a second waveguide supports substantially only the TM polarization mode. A fabrication process utilizing Titanium and annealed-proton-exchanged (APE) waveguides for supporting orthogonal polarization modes is described by UTP. Further, Zinc-diffused waveguides, or other waveguide materials including metals as known in the art, may be employed. The polarization splitter then separates un-polarized light into orthogonal TE and TM components. A cut may be introduced into the waveguide, or into both the waveguide and the substrate, into which a snug-fitting half-wave plate may be introduced. In this fashion, the half-wave plate rotates the polarization of the undesirable component by 90 degrees to match the desirable polarization of the wavelength converter structure. One or more wavelength converter (WC) structures may be employed in either or both of the waveguides. The two waveguides are then recombined to produce a single output. If the waveplate material is the same as that of the substrate (e.g., lithium niobate), then the optical path lengths of the two waveguides will be substantially identical and the recombined beams may add in-phase with minimal loss. A small correction for phase can be introduced by adding a voltage across a portion of one waveguide to vary the refractive index and hence the path length. Alternatively, a second half-wave plate may be introduced into either of the waveguides downstream of the WC structure.

The invention has now been described with reference to the preferred embodiments. Alternatives and substitutions will now be apparent to persons of skill in the art. For example, in the above-described embodiments the optical processor is a difference frequency generator. However, various active and passive optical devices, such as an optical amplifier, can be substituted for the DFG. Additionally, in several embodiments particular polarization modes are described by way of example, not limitation. The invention is not dependent on particular polarization modes of the signal components. Accordingly, it is not intended to limit the invention except as provided by the appended claims.