Optical parametric amplifier for multi-channel communications

Abstract

A system and methods for low noise multi-channel optical signal amplification using an optical parametric amplifier are described. A waveguide etched into a ϰ(2) nonlinear optical material, such as thin film lithium niobate (TFLN), provides confined optical parametric amplification using a pump laser to amplify multi-channel optical signals of lower energy. Noise and crosstalk between channels within the OPA waveguide amplifier can be reduced if the OPA waveguide physical dimensions, along with the optical properties of the waveguide material, are adjusted to create a group velocity mismatch (GVM) between the propagating pump light and the amplified signal channels. OPA waveguide designs with larger GVM reduces crosstalk between modes in the waveguide, reducing noise, as measured by the error vector magnitude (EVM), and improving signal fidelity for the amplified optical signals.

Description

OPTICAL PARAMETRIC AMPLIFIERFOR MULTI-CHANNEL COMMUNICATIONSInventors: Timothy MCKENNA Edwin NG Marc JANKOWSKICROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Patent ApplicationNo. 63 / 720,500, filed 14 November 2024, which is hereby incorporated by reference in its entirety for all purposes.FIELD OF THE TECHNOLOGY DISCLOSED

[0002] The technology disclosed relates to the transmission of optical communication signals, and in particular, amplifying signals propagating within waveguides for use in long distance communication by using optical parametric amplification.BACKGROUND

[0003] The subject matter discussed in this section should not be assumed to be prior art merely as a result of its mention in this section. Similarly, a problem mentioned in this section or associated with the subject matter provided as background should not be assumed to have been previously recognized in the prior art. The subject matter in this section merely represents different approaches, which in and of themselves may also correspond to implementations of the claimed technology.

[0004] An optical parametric amplifier (OP A) is a nonlinear optical device that amplifies a weak input light signal using a parametric amplification process. Such a process relies on a nonlinear interaction between light and a medium with / (2) nonlinear optical properties. In an optical parametric amplifier, as illustrated in FIGs. 1 A and IB, the energy hcopof a photon of a pump beam having frequency copmay be converted into two photons, which can be called signal photons and idler photons, having frequencies a)sand a)i and withhcop= ha>s + ha)i . [Eqn. 1] Generated frequencies a)sand a)i may vary, as long as the terms of Eqn. 1 are met.{00989007.DOCX } 1

[0005] If, however, the conversion occurs in the presence of an incoming light with photons of energy hasi, the emitted photons at energy ha>smay occur at ha)siand coherently amplify the incoming light. If the conversion occurs in the presence of multiple incoming photon streams with different energies ha)si, ha)s2, ha>S3, hcoSn, each stream may be amplified using the pump photons as long as the correct conditions for interaction through the / (2)nonlinear coefficients of the material are met.

[0006] Optical parametric amplifiers have the potential to offer significantly higher gain than erbium-doped fiber amplifiers (EDFAs). While typical EDFAs can produce approximately 32 dB of gain, the ability to provide gain is limited by the number of dopant atoms available to form population inversions, while in an OP A, every incoming photon has the potential to be converted to amplify signals. Up to 100 dB of gain may therefore be possible.

[0007] If the pump and incoming signals are at constant intensity, the conversion of photon energy from hcopto multiple photon energies ha)si, h )-,2, ha)s3, ... , ha>snwill be relatively constant as well, with only the incoming noise on the inputs being reproduced in the outputs. However, if the various incoming photon streams are modulated, as may be the case with wavelengthdivision multiplexed (WDM) communication signals, the relative amplification of different incoming channels may vary significantly. The conversion of the pump to signals is no longer steady-state, and significant intensity variations can be introduced as the depletion of the pump varies with the varying energy in the signal channels. This introduces crosstalk between signal channels, and therefore an increase in the error vector magnitude (EVM) of the signals.

[0008] There is therefore a need for an OPA design that can reduce added noise and cross-talk between channels when amplifying multiple channels.

[0009] Note: when stating “free space wavelength” in this disclosure, the term is used to represent the wavelength in vacuum corresponding to the various photon energies hcop, ha)si, ha)S2, etc. The wavelength for a light within a material with a refractive index n greater than 1, such as lithium niobate (n 2.2), is multiplied by the refractive index.{00989007.DOCX } 2BRIEF SUMMARY

[0010] The technology disclosed provides system and methods for low noise multi-channel optical signal amplification using an optical parametric amplifier. In particular, a waveguide etched into a / (2) nonlinear optical material, such as thin film lithium niobate, provides a confined nonlinear optical environment for optical parametric amplification using a pump laser to amplify optical signals of lower energy.

[0011] In some embodiments the waveguide facilitating the parametric amplification is fabricated using periodic poling in the nonlinear material to facilitate phase matching and therefore allowing amplification over distances as long as several centimeters within the waveguide. In some embodiments, noise and crosstalk between channels can be reduced if the waveguide physical dimensions, along with the optical properties of the waveguide material, are adjusted to create a group velocity mismatch (GVM) between the propagating pump light and the amplified signal channels amplified by the nonlinear interactions in the waveguide.

[0012] Particular aspects of the technology disclosed are described in the claims, specification, and drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The disclosure will be understood more fully from the detailed description given below and from the accompanying figures of embodiments of the disclosure. The figures are used to provide knowledge and understanding of embodiments of the disclosure and do not limit the scope of the disclosure to these specific embodiments. Unless otherwise indicated, the drawings provided throughout the disclosure should not be interpreted as to-scale drawings.

[0014] FIG. 1 A illustrates an energy level diagram for optical parametric amplification.

[0015] FIG. IB illustrates an example schematic of an amplifier for optical parametric amplification.

[0016] FIG. 2 illustrates multi-channel signals that may be input to an optical parametric amplifier.

[0017] FIG. 3 illustrates the simulated WDM quasi-noise profile for a single channel and for multiple channels.

[0018] FIGS. 4 A and 4B illustrate a schematic perspective view of a waveguide with an optical parametric amplifier according to an embodiment of the invention.

[0019] FIG. 5 illustrates propagation of a pulse in signal intensity and the corresponding pump power with GVM equal to 0.{00989007.DOCX } 3

[0020] FIG. 6 illustrates propagation of a pulse in signal intensity and the corresponding pump power with GVM not equal to 0.

[0021] FIG. 7 illustrates a cross section of a waveguide as used in embodiments of the invention.

[0022] FIG. 8 illustrates a flow chart for a method of designing and optimizing an OPA waveguide.

[0023] FIG. 9A illustrates a contour plot of error vector magnitude (EVM) for various pump and signal powers for GVM = 0.

[0024] FIG. 9B illustrates a contour plot of EVM for various pump and signal powers for GVM = 1000 fs / mm.

[0025] FIG. 10 illustrates EVM vs. signal power with constant input pump power for a variety of GVM values within the OPA.

[0026] FIG. 11 illustrates the walk-off associated with GVM for a range of physical dimensions for a waveguide as illustrated in FIG. 7.

[0027] FIG. 12 illustrates modeling results for the magnetic (H) field in x and y for the pump, signal, and idler modes when operating a waveguide with the optimum GVM identified from the plot of FIG. 11.

[0028] FIG. 13 illustrates a plot of the GVM for various signal wavelengths when operating a waveguide with the optimum parameters identified from the plot of FIG. 11.

[0029] FIG. 14 illustrates a flow chart for a method of amplifying multiple communication signals with reduced noise.

[0030] FIG. 15 A illustrates a schematic view of an alternative arrangement for an OPA device.

[0031] FIG. 15B illustrates a schematic view of an alternative arrangement for an OPA device.

[0032] FIG. 16 illustrates a schematic view of an alternative structure to achieve low noise amplification using delay lines along with group velocity mismatch.

[0033] FIG. 17 illustrates a schematic view of an alternative structure to achieve low noise amplification using photonic crystals along with group velocity mismatch.

[0034] FIG. 18 illustrates a schematic view of an alternative structure to achieve low noise amplification using photonic crystals along with group velocity mismatch.

[0035] FIG. 19 illustrates a schematic view of an alternative structure to achieve low noise amplification using group velocity mismatch with a circulating pump.{00989007.DOCX } 4DETAILED DESCRIPTION

[0036] The following detailed description is made with reference to FIGs. 1 - 19. Exemplary implementations are described to illustrate the technology disclosed, not to limit its scope, which is defined by the claims. Those of ordinary skill in the art will recognize a variety of equivalent variations on the description that follows.Optical Parametric Amplification for Communication Signals

[0037] Optical parametric amplification is a well-known technology in nonlinear optics involving the conversion of a pump photon into two photons of lesser energy. The interaction is facilitated by x(2) nonlinearities in materials such as lithium niobate or lithium tantalite. To take advantage of these nonlinearities, laser light with high electric fields is used, with the high field obtained either from high peak powers during pulsed operation, or from confinement within a waveguide.

[0038] FIGS. 1 A and IB present simple illustrations of the process used in an optical parametric amplifier. As described above, in a nonlinear material 40, incoming pump photons 10 of energy ha)pare converted into signal photons 55 and idler photons 25 with respective energies ha>s and hcoi. The exact values of the signal and idler energies are dictated by the phase matching conditions within the nonlinear crystal. When also in the presence of incoming signal photons 50 of energy ha)si, the OPA signal photon energy ha>s= ha)siand amplification of the incoming signal can occur.

[0039] For this conversion of pump power to signal power to be efficient, it is desirable for the nonlinear interaction to occur over relatively long interaction distances (e.g. mm to cm) with both high peak power and phase matching maintained. Confinement of the photons in a waveguide of nonlinear material, such as lithium niobate, allows high continuous wave (CW) peak power to be maintained, and the use of periodic poling of the nonlinear material allows phase matching to be maintained over long distances.

[0040] When used with CW steady-state pump and signal inputs, the pump is monotonically depleted and the signal monotonically amplified as they pass through the nonlinear material. No new noise is introduced. However, when used with an incoming communication signal, such as an incoming signal with phase and amplitude modulation, power in the signal can vary significantly, and the pump is not depleted uniformly over time. With multiple communication channels, the amplification can be significantly more complex. In a wavelength-division multiplexing (WDM) system, in which multiple channels, each with independent modulation, propagate together as a single beam, with each channel having a slightly different wavelength,{00989007.DOCX } 5the large number of independent channels being simultaneously modulated with communication information begin to give the appearance of noise.

[0041] FIG. 2 illustrates the frequency spectrum for an example of a dense WDM system. Modulation amplitudes can approach 100% in various channels, and there is significant variation between wavelength channels being simultaneously transmitted. This leads to signals that, while deterministic, appear as noise, and have the statistical properties resembling noise.

[0042] FIG. 3 illustrates the variation properties of ensemble signals as a complementary cumulative distribution function (CCDF). For various wavetrains, for a given value of the signal above average, the probability of a wavetrain being at or above that value is plotted in the chart of FIG. 3. For a single WDM channel, with no other channels active, the probability of a heightened transient power level is low. But when multiple channels, each with their own modulation patterns are introduced, the probability for high intensity transients to occur increases, and continues to increase as more channels are added until, with approximately 51 channels, the noise distribution approaches that of Gaussian noise.

[0043] These communication systems perform better with a signal that has a low error vector magnitude (EVM). In cases where an amplifier may be employed, the noise and distortion generated through use of the amplifier typically increases the error vector magnitude of the communication signals.

[0044] The effect this can have in a waveguide optical parametric amplifier can be significant. However, certain design considerations according to the technology disclosed herein can mitigate the risk of noise amplification and propagation.OPA Waveguide Architecture

[0045] FIG. 4 A shows an example of a system 100 comprising a waveguide to facilitate the amplification of optical signals using optical parametric amplification. FIG. 4A illustrates a perspective schematic view of an OPA waveguide 140 and FIG. 4B illustrates a cross section of the OPA waveguide 140. The cross section in this non-limiting example is in the form of a ridge waveguide. Such a waveguide has well defined modes for the propagation of light that meet the electromagnetic boundary conditions of the waveguide, including transverse electric (TE) modes, transverse magnetic (TM) modes, and transverse electromagnetic (TEM) modes based on the orientation of the electric and magnetic fields and the direction of propagation along the z-axis of the waveguide. In an ideal OPA waveguide, the waveguide will support both one or more modes for the propagation of the pump light that overlap with one or more modes for the propagation of the signal and idler light.{00989007.DOCX } 6

[0046] For the example of FIG. 4B, a substrate 120 comprising material such as silicon dioxide (SiCh, including materials such as quartz or glass) or sapphire (AI2O3) is coated with a thin film of nonlinear optical material 130, such as lithium niobate. Thin film lithium niobate (TFLN) products on glass or sapphire substrates 150 mm or 200 mm in diameter with lithium niobate thicknesses ranging from 300 nm to over 1000 nm thick are available and fabrication techniques are known to those skilled in the art. For the purposes of this disclosure, a “thin film” of nonlinear material such as lithium niobate (LN) will be any planar material with a thickness of 1.2 micrometer (1200 nm) or less.

[0047] The use of thin film materials allows for tight confinement of light in the waveguide of the optical parametric amplifier, which boosts the overall gain of the amplifier. The increased concentration of light in the waveguide can increase the gain of the optical parametric amplifier lOOx or more compared to OP As based on bulk materials. Furthermore, tightly confined waveguides made from thin-film materials allow for engineering of the speed of light and its derivatives to achieve bandwidths greater than, for example, 20 THz. By being able to support large bandwidth applications (e.g., around 20 THz), the OPA waveguide 140 may enable fiber links with higher throughput compared to the current state of the art.

[0048] A waveguide OPA may be formed in the TFLN layer 130 by removing lithium niobate material in a pattern corresponding to the desired waveguide. In FIG. 4B, the OPA waveguide 140 is a linear waveguide of width w etched to a depth d in the lithium niobate 130. The patterning of the waveguide may be carried out using other lithographic techniques, either using UV optical lithography or electron beam lithography to pattern a photoresist layer, and then selectively etching or otherwise removing the material not protected by the photoresist. For some fabrication approaches, electron beam lithography and dry etching using Ar+ions can produce low loss waveguides. Other fabrication and patterning methods will be known to those skilled in the art.

[0049] The waveguide 140 as illustrated is shown having at least three sections including an input portion 142, an amplification portion 144, and an output portion 146.

[0050] As inputs to the input portion 142, the system 100 shown in FIG. 4 A has a pump light source 110 that produces pump light 111 that enters the OPA waveguide 140. The pump light source 110 generating the pump light 111 may be provided by a laser, such as a diode laser, operating, in some embodiments, in the near infrared with a free space wavelength in the range from 950 nm to 990 nm. Other embodiments may use other sources of pump light with other free-space wavelengths, such as those operating in the deep blue in a range of 400-450 nm, those operating in the near infrared in a range of 750-800 nm, or those operating in the near infrared in{00989007.DOCX } 7a range of 1030-1064 nm. Other wavelength ranges for pump light beyond these mentioned may be used by those skilled in the art.

[0051] The pump light source 110 as illustrated is shown for convenience as being positioned on the nonlinear optical material 130 in close proximity to the entrance of the OP A waveguide 140, and the pump light 111 enters the OP A waveguide 140 through free space propagation at pump coupling 118. However, in some embodiments, the pump light source 110 may be positioned elsewhere away from the OPA waveguide 140, and coupled to the input portion 142 of the OPA waveguide 140 over some distance using fiber couplers or other means known to those skilled in the art. The OPA waveguide 140 can be configured to allow the photons of the pump light 111, once coupled into the input portion 142 of the OPA waveguide 140, to propagate down the waveguide in a low loss waveguide mode.

[0052] As another set of inputs to the input portion 142, the system 100 shown in FIG. 4 A illustrates one or more optical signals 151, 152, 153, ... , 15« that enter a signal waveguide 150 through a signal coupling 158. The optical signals 151 ... 15w may represent optical communication channels carrying information. In some embodiments, the channels may be modulated in amplitude, in some embodiments modulated in phase, and in some embodiments in both phase and amplitude. In some embodiments, the channels may be modulated according to a 16-QAM (Quadrature Amplitude Modulation) format. In some embodiments, the channels may use protocols of wavelength division multiplexing (WDM) to allow each channel to operate at a different wavelength, with each wavelength carrying a different stream of information.

[0053] Light sources for these channels, in some embodiments, may be diode lasers operating in the infrared within a free space wavelength within one of the commonly designated optical communication bands. The ranges for several optical communication bands are specified in Table I.Table I: Optical Communication Bands{00989007.DOCX } 8

[0054] Long distance telecommunications systems often use lasers operating with free-space wavelengths in the range from 1530 nm to 1565 nm, also known as the optical C-Band. Modern data centers with optical links between servers often use lasers operating with free-space wavelengths in the range from 1300 to 1310 nm, within the optical O-band. Other wavelength ranges for signal light beyond these mentioned may be used by those skilled in the art.

[0055] The optical signals 151 ... 15 / 7 propagate through the signal waveguide 150 and couple into the input portion 142 of the OPA waveguide 140 through an input coupler 180. The OPA waveguide 140 can be configured to allow the photons of the optical signals 151 ... 15«, once coupled into the input portion 142 of the OPA waveguide 140, to propagate down the waveguide in low loss waveguide modes.

[0056] Both the pump light 111 and the optical signals 151 ... 15 / 7 propagate into the next portion of the OPA waveguide 140, the amplification portion 144, where optical parametric amplification takes place. Pump light 111 enters the waveguide modes corresponding to the pump wavelengths, and the optical signals enter waveguide modes corresponding to the respective wavelengths of the optical signals 151 ... 15«. The nonlinear interaction between the pump mode and the optical signal modes is facilitated by x(2) nonlinearities in the waveguide 140. However, to facilitate amplification and greater amplification efficiency in converting pump photons to signal photons, interaction over long distances, such as millimeters or even centimeters, in the waveguide are preferred.

[0057] However, as is well known in the art, because of differing refractive indices for the pump wavelength and signal wavelengths, phase matching between the pump wavelengths and over long distances in nonlinear crystals is not usually possible, and the energy transfer will be diminished. The lack of long-distance phase matching can be addressed using techniques known as quasi-phase matching, in which alternating orientations for crystal poling are induced in the nonlinear material. Alternating poling, sometimes called periodic poling if the poling periods are uniform, allows phase drift between pump and signal wavelengths in one period to be reversed for the next period, maintaining phase matching over longer interaction lengths.

[0058] For the system 100 illustrated in FIGS. 4A and 4B, the amplification portion 144 is shown with a region of periodically poled lithium niobate (PPLN). Although only a few periods are shown in the illustration, actual waveguide devices may have periods with widths on the order of approximately 1 pm to 20 pm, and there can be as many as a hundred or even a thousand or more periods over the interaction length of the amplification portion 144 of the waveguide 140, which may be typically millimeters or even centimeters long.

[0059] Periodically poled regions can be fabricated using other lithography techniques.Patterns for the periods of the regions to be poled may be formed in photoresist coated onto the {00989007.DOCX } 9TFLN, and used to selectively deposit periodic metal electrodes onto the surface. This may be done on un-etched lithium niobate, while the surface is still planar. The electrodes are then used to apply a high voltage, producing a large electric field in the material and reversing the poling of the nonlinear material under the electrodes. Once the poling is completed, the electrodes are removed, and the thin film lithium niobate may then be patterned to produce the various waveguide structures used for parametric amplification, as well as any other structures on the wafers needed for device formation.

[0060] The above description is a non-limiting example of an approach to creating waveguides with sections having PPLN; other methods may be known to those skilled in the art.

[0061] After propagating through the amplification portion 144 of the waveguide 140, power has been transferred from the pump mode of the waveguide to amplify photons in the signal modes for the optical signals 151 ... 15 / / . Both the pump mode and signal modes then propagate into the output portion 146 of the waveguide 140.

[0062] In the output portion 146, an output coupler 280 is positioned to allow photons of the amplified signal modes of the waveguide to transfer into a signal output waveguide 250. These signal photons then propagate as amplified signals 251, 252, 253, ..., 25«, and can be coupled out of the output waveguide 250 to a communication channel for further transmission.Noise Under Amplification

[0063] For a single signal channel, such an amplifier as shown in FIGS. 4A and 4B may be well behaved and not add significant noise. When the signal channel amplitude is large, larger pump depletion occurs, but when the signal channel amplitude is smaller, less depletion occurs. The transfer of energy from pump to signal can therefore be relatively well behaved.

[0064] However, as shown above in FIGs. 2 and 3, when many channels are simultaneously propagated through an amplifier as illustrated in FIGS. 4A and 4B, and many signal channels, each having its own modulation pattern, are superimposed within the waveguide, the combined intensity may have large peaks and troughs, and the signal variation begins to approximate that of a Gaussian noise distribution. Certain channels may simultaneously have greater amplitudes, and cause greater gain for some channels, but also greater pump depletion for other channels, leading to time-based irregularities in the depletion of the pump, and therefore irregular amplification of the signal modes, cross talk between modes, and increased noise.

[0065] If the waveguide is sufficiently long, and the waveguide dimensions and dispersion relationships are such that the pump and signal have the same group velocity, these irregular intensities in the pump will propagate down the waveguide in synchrony with the irregular{00989007.DOCX } 10amplification. The irregularities are therefore amplified, not reduced, as the pump and signals propagate, and the noise consequences can be significant.

[0066] This is illustrated in FIG. 5. At an initial position zi in the waveguide, the channel power is shown to have a transient spike at a time ti, which provides a transient depletion of the pump gain at that time. If the group velocity Vg of the pump mode and signal modes are the same, that signal transient propagates down the waveguide at the same speed as the pump depletion, so at a later position Z2 that transient (now at time t2 = ti + (z2 - zi) / Vg) has been amplified by the energy acquired from the pump, while the depletion has continued to synchronously diminish the pump. This depletion starves other signal modes from receiving gain from the pump, and the irregular amplification of the various signal modes increases the noise profile of the various signals.

[0067] On the other hand, this noise amplification can be mitigated if the waveguide is designed so that the group velocity mismatch between the pump mode and the signal modes is large. With a large group velocity mismatch (GVM), noise performance of the amplifier can be improved.

[0068] Group Velocity Mismatch (GVM) is not simply the difference in speeds for two group velocities, but is conventionally presented as a value in femtoseconds per millimeter that quantifies how much a pulse with one wavelength is delayed relative to another as they both travel through the waveguide. For example, a GVM of 200 fs / mm means that one pulse is delayed by 200 femtoseconds for every millimeter of travel relative to the other pulse.

[0069] This is illustrated in FIG. 6. At the initial position zi in the waveguide, the channel power has the same transient spike at a time ti, as was shown in FIG. 5. As before, at this moment, a transient depletion of the pump gain is observed. However, if the group velocity of the pump mode and signal modes are different, the transient reduction in pump depletion propagates down the waveguide at a different speed than the signal transient. Therefore, at the later position Z2, while that transient has still been amplified somewhat by the energy acquired from the pump, the depletion of the pump has propagated at a different velocity than the transient, and has therefore been distributed in time, broadening the depletion, and the contribution to the noise or crosstalk between channels is diminished.Quantification of Group Velocity Mismatch

[0070] To estimate the contribution of GVM to a nonlinear parametric amplifier as disclosed above, determination of the commonly used error vector magnitude (EVM) for communication signals, expressed in percent, can provide a useful metric.{00989007.DOCX } 11

[0071] FIG. 7 illustrates a cross section view the dimensions of an example of an OPA Waveguide 140 with a ridge waveguide design, showing the definitions of some variables used to parameterize the waveguide. In this example, the nonlinear material 130 is thin film lithium niobite (LN) on a substrate 120 of silicon dioxide. The original thickness of the LN has been lithographically patterned and etched to a depth d to form the waveguide 140 with top width w, leaving a thin lithium niobate layer of thickness t underneath.

[0072] FIG. 8 illustrates a flow chart for a non-limiting method for designing a waveguide of the type illustrated in FIG. 7.

[0073] In the initial operation 1100, the parameter ranges of the waveguide, such as w, t / , and / , as well as waveguide interaction length / ., are determined, and may be dictated by practical fabrication limitations. Optical properties, such as refractive index spectra and dispersion relations, for the waveguide material and the substrate may also be retrieved from published reference materials, or measured from the waveguide itself.

[0074] In the next operation 1200, the pump and signal photon energies and corresponding free space wavelengths are determined. This allows the refractive index information to be selected for use in the subsequent computations.

[0075] In the next operation 1300, group velocities for waves propagating at the pump and signal photon energies are determined. As the group velocity also depends on the mode profile in the waveguide and not merely the bulk refractive index and dispersion properties, computation of the group velocities takes the waveguide parameters into account, and compute group velocities for ranges of the waveguide parameters.

[0076] In the next operation 1400, the group velocity mismatch (GVM) for these various parameter ranges can be calculated. Ideally, these are also computed using various pump and signal powers, as the noise and crosstalk introduced can be predicted. In some embodiments, the EVM may be computed.

[0077] In the next operation 1500, the optimum parameters that lead to the greatest GVM and corresponding reduction in introduced EVM may be selected.

[0078] FIGS. 9 A and 9B illustrate shaded contour plots of EVM for a nonlinear waveguide with length L = 20 mm used for amplifying 51 16-QAM optical communication channels. The x and y axes are, respectively, the magnitude of the signal channel power and the magnitude of the pump light power, represented in dBm. Specifically, the plot of FIG. 9A corresponds to GVM = 0 fs / mm (in other words, no group velocity mismatch) and the plot of FIG. 9B corresponds to GVM = 1000 fs / mm. Darker tones represent higher relative noise as a percentage of the signal, while lighter values represent smaller (lower noise) values of EVM.{00989007.DOCX } 12

[0079] Referring to the GVM = 0 plot (FIG. 9A), for very weak signals, no matter what the pump strength, EVM is large, dominated by quantum fluctuations in the waveguide modes. This is not a concern, as such weak signal powers will not be used in communication channels. As the signal power increases, relative noise is reduced, achieving a minimum at around 0 dBm.However, for the higher signal powers, noise begins to increase again, and if the pump power also is higher, the noise increases further as well. These higher values of introduced EVM at high signal and pump powers are indicative of the increased crosstalk between channels, as described above.

[0080] Referring to the GVM = 1000 fs / mm plot (FIG. 9B), for higher signal powers, even with high pump power, the increase in noise is relatively minor, and is significantly improved over the case where GVM = 0.

[0081] FIG. 10 represents another view of EVM values from the plots of FIG. 9 A and 9B, but for a variation in signal input power with a constant value of the pump power (in other words, a horizontal cross-section through the plots of FIGs. 9 A and 9B) and for waveguide designs with various values of GVM. The minimum intrinsic noise floor which may be considered a first EVM for the system, in the plot is -1%, and with no cross talk at all, the EVM would be expected approach this first EVM value at the higher input powers.

[0082] As can be seen from the plot, GVM = 0 has significant noise introduced for higher signal power, with EVM reaching approximately 25% at a power of 15 dBm. This is a noise value comparable to the quantum noise seen at extremely low powers, and represents an introduced, or second, EVM of -24.3% over the noise floor, or first EVM. However, with progressively higher values of GVM, the value for the introduced EVM at high signal powers steadily drops, until at 1000 fs / mm, the introduced EVM over the noise floor for higher input powers has a maximum of only -5.5 %. This represents a reduction in introduced EVM by nearly a factor of 4. And even for more modest values of GVM, such as GVM = 200 fs / mm, the introduced EVM is still smaller by a factor of approximately -1.4 for these higher signal powers.

[0083] FIG. 11 illustrates an example of a computation for an exemplary OPA waveguide. The waveguide of this example is designed for pump light having a free-space wavelength of 980 nm and signal channels with light having a free space wavelength of -1560 nm, in the optical C-Band. The fixed dimensions of the waveguide for this calculation are d= 500 nm and t = 500 nm, and the top width of the waveguide is varied to optimize the GVM.

[0084] As shown in FIG. 11, the GVM in fs / mm is near 0 when the top width is near or smaller than the free-space wavelength of the signal at 1560 nm. However, when the top width is increased, a sudden, dramatic increase in the walk-off is observed, peaking at a value of w = 1984 nm.{00989007.DOCX } 13

[0085] FIG. 12 illustrates the mode patterns within the waveguide for this optimal value of w = 1984 nm. The greyscale plots within the waveguide represents Hyand Hxfield magnitude for the pump mode (plots labeled (a) & (b)), the signal mode (plots labeled (c) and (d)), and the idler mode (plots labeled (e) and (f)). As can be seen from the plots, the pump is not polarized, while the signal and idler are both polarized.

[0086] FIG. 13 illustrates the resulting GVM using this optimal value of w = 1984 nm for various signals over a range of free space wavelengths within the optical C-band. As can be seen from the plot, the GVM is relatively constant regardless of the / .signal used, meaning that all communications channels in this range should experience a noise reduction by an approximate factor of 2, as presented in FIG. 10.Low Noise Amplification Method using an OPA Waveguide

[0087] FIG. 14 illustrates a flow chart for a method 2000 to produce low noise amplification of signals in an OPA waveguide such as that illustrated in the non-limiting example of FIG. 4.

[0088] In the initial operation 2100, pump light is received into the waveguide. The free space signal wavelength may be any number of wavelengths, but in some embodiments, the pump light will be from a laser operating at a free space wavelength of 980 nm.

[0089] In the next operation 2200, modulated light from one or more signal channels is received into the signal modes of the waveguide. The signal light wavelength may be any number of wavelengths, but in some embodiments, the signal light will be from one or more lasers operating in the optical C-band, having free-space wavelengths from 1530 to 1565 nm, and in some embodiments, may be modulated according to the 16-QAM format.

[0090] In the next operation 2300, the pump mode and the one or more signal modes propagate through a nonlinear optical parametric amplifier portion of the waveguide, comprising periodic poling to preserve phase matching between the pump and signal modes (which, in some embodiments, may be achieved using a waveguide formed in thin film lithium niobate and using periodic poling to achieve phase matching). The dimensions and dispersion of the waveguide may be optimized in some embodiments to achieve a pump-signal group velocity mismatch (GVM) of 500 fs / mm or more.

[0091] In the next operation 2400, the amplified one or more signal modes are decoupled into a separate output channel, from which they may be further coupled into a fiber optic cable or other communication link.

[0092] The operations of the method 2000 may be practiced once over a limited time for a particular input signal, or operated continuously as streams of signals are transmitted, as may be commonly found in a communications link such as an undersea cable.{00989007.DOCX } 14Particular Implementations

[0093] Exemplary thin film nonlinear materials may include ferroelectric and semiconductor materials such as lithium niobate, lithium tantalate, aluminum nitride, gallium arsenide, gallium phosphide, indium phosphide and the like. In some embodiments, the nonlinear material may be doped with magnesium oxide (MgO).

[0094] Waveguides may be fabricated on various insulating substrates such as silicon dioxide (SiCh), including materials such as quartz, silica, or glass, other materials such as sapphire (AI2O3). In some embodiments, silicon (Si) wafers may be used as the substrate. When silicon wafers are used as the substrate, additional electronic circuitry or integrated circuits may be placed on the silicon as well. This electronic circuitry may be integrated into modulators or other electro-optic devices that may be integrated with the functioning of the OP A, or otherwise used for the adjustment of the properties of the light transiting the waveguides.

[0095] Likewise, the pump laser in some embodiments may be positioned on and attached to the planar surface comprising the nonlinear material, such as lithium niobite. Such a system is illustrated in FIG. 4. In some embodiments, the pump laser may be positioned on the substrate supporting the lithium niobate. In some embodiments, the pump laser may be positioned entirely off the nonlinear material and substrate. Coupling to the OPA waveguide may be made through free-space coupling, fiber coupling, or other coupling methods that will be known to those skilled in the art.

[0096] Although FIGs. 4 and 7 illustrate a simple ridge waveguide, other dispersive structures with more complex waveguide cross sections may be used. For example, the OPA waveguide may have an architecture comprising a slotted waveguide. Other waveguide architectures will be known to those skilled in the art.

[0097] FIG. 15A illustrates an exemplary device 400 that includes an optical parametric amplifier 420, according to example embodiments. As shown, device 400 may be representative of a fiber coupled inline amplifier. Device 400 may include optical parametric amplifier 420 disposed therein. Optical parametric amplifier 420 may be representative of one or more improved optical parametric amplifiers disclosed herein that are formed from thin film nonlinear materials.

[0098] In some embodiments, device 400 may include a pump laser 440. In some embodiments, pump laser 440 may be disposed in device 400. In some embodiments, pump laser may be external to device 400. More generally, pump laser 440 may be coupled with optical parametric amplifier 420. For example, pump laser could be coupled to the chip containing{00989007.DOCX } 15optical parametric amplifier 420 or die bonded to the chip containing optical parametric amplifier 420.

[0099] FIG. 15B illustrates an exemplary device 500 that includes an optical parametric amplifier, according to example embodiments. As shown, device 500 may be representative of an amplifier heterogeneously integrated with another material (e.g., silicon). As shown, device 500 may include a substrate 520 (e.g., silicon chip) and an amplifier chip 540 coupled thereto. Substrate 520 may be designed as a transmitter or receiver of the signal. Amplifier chip 540 may include an optical parametric amplifier, such as those described herein. Amplifier chip 540 may be configured to function as an output amplifier.

[0100] FIG. 16 illustrates a structure 600 for achieving group velocity mismatch, according to exemplary embodiments. As shown, one way to achieve group velocity mismatch is to manufacture structure 600 that includes delay sections 620 interspersed between amplification sections 640. In some embodiments, a delay section 620 may be implemented using a dichroic coupler. Each amplification section may comprise an improved optical parametric amplifier formed from thin film materials as described above. Delay sections 620 may be placed throughout structure 600 to achieve a pattern of amplification, delay, amplification, delay etc. The time delay from delay sections 620 may de-correlate the input from the gain transients, thus preventing large gain transients from building up, minimizing the opportunity for channel crosstalk. In some embodiments, delay sections 620 may only couple signal and idler, while pump avoids delay sections 620 and only passes through amplification sections 640. In some embodiments, rather than delay the signal, an operator may choose to delay the pump by sending the pump through delay sections 620 and passing the signal directly through amplification sections 640.

[0101] FIG. 17 illustrates a top-down view of a structure 700 for achieving group velocity mismatch, according to exemplary embodiments. Structure 700 may achieve group velocity mismatch by being formed to slow down input light. As shown, structure 700 may include slow light structures 720 interspersed with amplifiers 740. Each amplifier 740 may comprise an improved optical parametric amplifier formed from thin film materials as described above. In some embodiments, slow light structures 720 may be implemented using, for example, photonic crystals. Slow light structures may be configured to create a bandgap, which modifies the group velocities of wavelengths near the bandgap. Use of slow light structures 720 may cause large group velocity differences between pump and signal wavelengths. In some embodiments, the period of the photonic crystal may be adjusted to place the bandgap near the pump wavelength. In some embodiments, slow light structure 720 may be periodically poled. In some embodiments, structure 700 may alternate between slow light structures 720 and amplifiers 740. {00989007.DOCX } 16

[0102] FIG. 18 illustrates a top-down view of a structure 800 for achieving group velocity mismatch, according to exemplary embodiments. Structure 800 may achieve group velocity mismatch by being formed to slow down input light. As shown, structure 800 may include slow light structures 820 interspersed with amplifiers 840. Each amplifier 840 may comprise an improved optical parametric amplifier formed from thin film materials as described above. Slow light structures 820 may be configured to create a bandgap, which modifies the group velocities of wavelengths near the bandgap. Use of slow light structures 820 may cause large group velocity differences between pump and signal wavelengths. In some embodiments, the period of the photonic crystal may be adjusted to place the bandgap near the pump wavelength. In some embodiments, slow light structure 820 may be periodically poled. In some embodiments, structure 800 may alternate between slow light structures 820 and amplifiers 840.

[0103] FIG. 19 illustrates an exemplary structure 900 for reducing transient gain effects, according to example embodiments. As shown, exemplary structure 900 may comprise a recirculating pump in a resonator. Pump amplitude fluctuations add noise and cross talk to the amplified signal because the amplitude modulates the gain. The lifetime of the resonator averages out pump amplitude fluctuations. The effect is to low-pass-filter pump fluctuations with a Lorentzian line-shape of the resonator response.Other Variations

[0104] The technology disclosed can be practiced as a system, method, or article of manufacture. One or more features of an implementation can be combined with the base implementation. Implementations that are not mutually exclusive are taught to be combinable. One or more features of an implementation can be combined with other implementations. This disclosure periodically reminds the user of these options. Omission from some implementations of recitations that repeat these options should not be taken as limiting the combinations taught in the preceding sections, as these recitations are hereby incorporated forward by reference into each of the following implementations. Other implementations may include systems that may incorporate a non-transitory computer readable storage medium storing instructions executable by a processor to perform functions described above. Yet another implementation may include a method performing the functions described above in a system.

[0105] The terms and expressions employed herein are used as terms and expressions of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof. In addition, having described certain implementations of the technology disclosed, it will be apparent to those of ordinary skill in the art that other implementations incorporating the {00989007.DOCX } 17concepts disclosed herein can be used without departing from the spirit and scope of the technology disclosed. Accordingly, the described implementations are to be considered in all respects as only illustrative and not restrictive.

[0106] A number of flowcharts are described herein. The logic within these flowcharts can be implemented using processors programmed using computer programs stored in memory accessible to the computer systems and executable by the processors, by dedicated logic hardware, including field programmable integrated circuits, and by combinations of dedicated logic hardware and computer programs. With all flowcharts herein, it will be appreciated that many of the operations can be combined, performed in parallel or performed in a different sequence without affecting the functions achieved. In some cases, a re-arrangement of operations will achieve the same results only if certain other changes are made as well. In other cases, a rearrangement of operations will achieve the same results only if certain conditions are satisfied. Furthermore, it will be appreciated that the flowcharts herein show only operations that are pertinent to an understanding of the disclosed technology, and it will be understood that numerous additional operations for accomplishing other functions can be performed before, after and between those shown.

[0107] One or more implementations of the technology disclosed or elements thereof can be implemented in the form of a computer product, including a non-transitory computer readable storage medium with computer usable program code for performing the method steps and operations indicated. Furthermore, one or more implementations of the technology disclosed or elements thereof can be implemented in the form of an apparatus including a memory and at least one processor that is coupled to the memory and operative to perform exemplary method operations. Yet further, in another aspect, one or more implementations of the technology disclosed or elements thereof can be implemented in the form of means for carrying out one or more of the method operations described herein; the means can include (i) hardware module(s), (ii) software module(s) executing on one or more hardware processors, or (iii) a combination of hardware and software modules; any of (i)-(iii) implement the specific techniques set forth herein, and the software modules are stored in a computer readable storage medium (or multiple such media).

[0108] It should be noted that the terms “including” and “comprising” should be interpreted as meaning “including, but not limited to”. If not already set forth explicitly in the claims, the term “a” should be interpreted as “at least one” and “the”, “said”, etc. should be interpreted as “the at least one”, “said at least one”, etc. Furthermore, it is the Applicant’s intent that only claims that include the express language “means for” or “step for” be interpreted under 35 U.S.C. 112(f).{00989007.DOCX } 18Claims that do not expressly include the phrase “means for” or “step for” are not to be interpreted under 35 U.S. C. 112(f).

[0109] Various aspects (X1-X30) of different embodiments of the present invention are also expressed below:

[0110] XI . A waveguide comprising:an input portion configured to receive (i) pump laser light and (ii) light including a plurality of optical signals;an amplification portion having a waveguide width and a waveguide depth that supports (i) at least one pump mode and (ii) optical signal modes, wherein the amplification portion is configured to:receive the pump laser light from the input portion;receive the light including the plurality of optical signals from the input portion;perform optical parametric amplification of the optical signal modes by the at least one pump mode over an interaction length, the optical signal modes having a first error vector magnitude (EVM), wherein the amplification results in an introduced second EVM due to crosstalk between signal modes; andpropagate the at least one pump mode at a first group velocity and propagate the optical signal modes at a second group velocity, such that a group velocity mismatch between the first group velocity and the second group velocity is at least a predetermined value, thereby reducing the second EVM, wherein the predetermined value is based, at least in part, on one or more of the waveguide width, the waveguide depth, and the interaction length; and an output portion configured to extract amplified optical signals from the optical signal modes of the waveguide.[oni] X2. The waveguide of aspect XI, whereinthe amplification portion of the waveguide is fabricated, at least in part, from a planar layer of a nonlinear optical material as a linear ridge structure having a height and a width.

[0112] X3. The waveguide of aspect X2, whereinthe planar layer of nonlinear optical material comprises thin film lithium niobate.

[0113] X4. The waveguide of aspect X2 or X3, whereinthe planar layer of nonlinear optical material comprises at least one of lithium niobate, lithium tantalate, aluminum nitride, gallium arsenide, gallium phosphide, and indium phosphide.

[0114] X5. The waveguide of aspect X2 through X4, whereinthe planar layer of nonlinear optical material is supported by a substrate.

[0115] X6. The waveguide of aspect X5, whereinthe substrate comprises at least one of silicon dioxide, quartz, sapphire, and silicon.{00989007.DOCX } 19

[0116] X7. The waveguide of any of the preceding aspects, whereinthe amplification portion of the waveguide comprises a sequence of alternately poled sections.

[0117] X8. The waveguide of any of the preceding aspects, whereinthe amplification portion of the waveguide comprises a sequence of periodically poled lithium niobate sections.

[0118] X9. The waveguide of any of the preceding aspects, whereinthe linear ridge structure is fabricated using lithographic patterning and selective etching of the nonlinear optical material.

[0119] XI 0. The waveguide of any of the preceding aspects, whereinthe nonlinear optical material is configured for optical parametric amplification of light in the one or more signal modes based on light in the at least one pump mode.

[0120] XI 1. The waveguide of any of the preceding aspects, additionally comprising:a pump coupler configured to provide the pump laser light into the input portion of the waveguide; andan input coupler configured to couple the light carrying the plurality of optical signals into the input portion of the waveguide,wherein the input portion of the waveguide is configured to transmit the pump laser light into the at least one pump mode of the amplification portion of the waveguide, andwherein the input portion of the waveguide is configured to transmit the light including the plurality of optical signals into one or more of the optical signal modes of the amplification portion of the waveguide.

[0121] X12. The waveguide of any of the preceding aspects, whereinthe at least one pump mode corresponds to light having a free space wavelength between 950 and 990 nm.

[0122] XI 3. The waveguide of any of the preceding aspects, whereinone or more of the optical signal modes correspond to light having a free space wavelength within an optical band, wherein the band is at least one of the optical O-band, the optical E- band, the optical S-band, the optical C-band, the optical L-band, and the optical U-band.

[0123] X14. The waveguide of any of the preceding aspects, whereinthe predetermined value is 200 fs / mm, thereby reducing the second EVM due to crosstalk between signal modes by at least a factor of 1.4.

[0124] XI 5. The waveguide of any of the preceding aspects, whereinthe predetermined value is 1000 fs / mm, thereby reducing the second EVM due to crosstalk between signal modes by at least a factor of 4.

[0125] XI 6. The waveguide of any of the preceding aspects, wherein{00989007.DOCX } 20the predetermined value is at least 500 fs / mm.

[0126] XI 7. A system for amplification of optical signals, comprising:a waveguide comprising an input portion, an amplification portion, and an output portion, the waveguide being configured for light to propagate from the input portion through the amplification portion into the output portion;a pump coupler configured to receive light from one or more pump lasers into the input portion of the waveguide; andan input coupler configured to couple light including a plurality of optical signals into the input portion of the waveguide,wherein the amplification portion of the waveguide:(a) supports (i) at least one pump mode and (ii) one or more optical signal modes;(b) comprises a nonlinear optical material configured for optical parametric amplification of light in the one or more optical signal modes using light in the at least one pump mode; and (c) has (i) a first group velocity corresponding to the at least one pump mode, and (ii) a second group velocity corresponding to the one or more optical signal modes, and has a group velocity mismatch between the first group velocity and the second group velocity that is at least a predetermined value.

[0127] X18. The system of aspect X17, whereinthe amplification portion of the waveguide comprises a sequence of periodically poled lithium niobate sections.

[0128] XI 9. The system of aspect XI 7 or XI 8, whereinthe light from one or more pump lasers has a free space wavelength between 950 and 990 nm, andthe light including a plurality of optical signals has at least one free space wavelength between 1530 and 1565 nm.

[0129] X20. The system of aspects XI 7 through XI 9, whereinthe predetermined value is at least 200 fs / mm.

[0130] X21. The system of aspect XI 7 through X20, whereinthe predetermined value is 1000 fs / mm, thereby reducing introduced EVM due to crosstalk between signal modes by at least a factor of 4.

[0131] X22. A method for amplifying optical communication signals, comprising: receiving pump light into a pump mode of waveguide, the pump mode having a first group velocity within the waveguide;receiving signal light carrying one or more signals into one or more optical signal modes of the waveguide, the one or more optical signal modes having a second group velocity within the {00989007.DOCX } 21waveguide, wherein a group velocity mismatch between the first group velocity and the second group velocity is greater than 200 fs / mm, thereby reducing introduced EVM due to crosstalk between signal modes by at least a factor of 1.4; andpropagating the pump light and the signal light through a portion of the waveguide comprising periodically poled lithium niobate, oriented so energy from the pump mode is transferred to the one or more optical signal modes through optical parametric amplification.

[0132] X23. The method of aspect X22, whereinthe pump mode corresponds to light having a free space wavelength between 950 and 990 nm.

[0133] X24. The method of aspect X22 or X23, whereinone or more of the optical signal modes correspond to light having a free space wavelength between 1530 and 1565 nm.

[0134] X25. The method of aspect X22 or X23, whereinone or more of the optical signal modes correspond to light having a free space wavelength within an optical band, wherein the band is at least one of the optical O-band, the optical E- band, the optical S-band, the optical C-band, the optical L-band, and the optical U-band.

[0135] X26. The method of aspect X22 through X25, whereinthe predetermined value is greater than 500 fs / mm.

[0136] X27. The method of aspect X22 through X26, whereinthe predetermined value is 1000 fs / mm, thereby reducing introduced EVM due to crosstalk between signal modes by at least a factor of 4.

[0137] X28. A method for designing an OPA waveguide comprising:determining OPA waveguide physical parameter ranges;determining OPA waveguide materials properties;determining energies and corresponding wavelengths for a pump laser and at least one optical signal;computing a first group velocity for the determined pump laser wavelengths and a second group velocity for the determined optical signal wavelengths over a range of the OPA waveguide parameters;computing at least one group velocity mismatch (GVM) based, at least in part, on the first group velocity and the second group velocity; anddetermining waveguide physical parameter settings corresponding to a selected value of GVM.

[0138] X29. The method of aspect X28, whereinthe OPA waveguide physical parameter ranges include at least one of width, etch depth, base thickness, and interaction length.{00989007.DOCX } 22

[0139] X30. The method of aspect X28 or X29, whereinOPA waveguide materials properties include refractive index and dispersion over a first wavelength range and nonlinear optical coefficients over a second wavelength range.

[0140] While the technology disclosed is disclosed by reference to the preferred implementations and examples detailed above, it is to be understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will readily occur to those skilled in the art, which modifications and combinations will be within the spirit of the innovation and the scope of the following claims.{00989007.DOCX } 23

Claims

CLAIMSWhat is claimed is:

1. A waveguide comprising:an input portion configured to receive (i) pump laser light and (ii) light including a plurality of optical signals;an amplification portion having a waveguide width and a waveguide depth that supports (i) at least one pump mode and (ii) optical signal modes, wherein the amplification portion is configured to:receive the pump laser light from the input portion;receive the light including the plurality of optical signals from the input portion; perform optical parametric amplification of the optical signal modes by the at least one pump mode over an interaction length, the optical signal modes having a first error vector magnitude (EVM), wherein the amplification results in an introduced second EVM due to crosstalk between signal modes; andpropagate the at least one pump mode at a first group velocity and propagate the optical signal modes at a second group velocity, such that a group velocity mismatch between the first group velocity and the second group velocity is at least a predetermined value, thereby reducing the second EVM, wherein the predetermined value is based, at least in part, on one or more of the waveguide width, the waveguide depth, and the interaction length; andan output portion configured to extract amplified optical signals from the optical signal modes of the waveguide.

2. The waveguide of claim 1, whereinthe amplification portion of the waveguide is fabricated, at least in part, from a planar layer of a nonlinear optical material as a linear ridge structure having a ridge height and a ridge width.{00989007.DOCX } 243. The waveguide of claim 2, whereinthe planar layer of nonlinear optical material comprises thin film lithium niobate.

4. The waveguide of claim 2, whereinthe planar layer of nonlinear optical material comprises at least one of lithium niobate, lithium tantalate, aluminum nitride, gallium arsenide, gallium phosphide, and indium phosphide.

5. The waveguide of claim 2, whereinthe planar layer of nonlinear optical material is supported by a substrate.

6. The waveguide of claim 5, whereinthe substrate comprises at least one of silicon dioxide, quartz, sapphire, and silicon.

7. The waveguide of claim 1, whereinthe amplification portion of the waveguide comprises a sequence of alternately poled sections.

8. The waveguide of claim 1, whereinthe amplification portion of the waveguide comprises a sequence of periodically poled lithium niobate sections.

9. The waveguide of claim 2 whereinthe linear ridge structure is fabricated using lithographic patterning and selective etching of the nonlinear optical material.

10. The waveguide of claim 2 whereinthe nonlinear optical material is configured for optical parametric amplification of light in one or more of the optical signal modes based on light in the at least one pump mode.{00989007.DOCX } 2511. The waveguide of claim 1, additionally comprising:a pump coupler configured to provide the pump laser light into the input portion of the waveguide; andan input coupler configured to couple the light carrying the plurality of optical signals into the input portion of the waveguide,wherein the input portion of the waveguide is configured to transmit the pump laser light into the at least one pump mode of the amplification portion of the waveguide, and wherein the input portion of the waveguide is configured to transmit the light including the plurality of optical signals into one or more of the optical signal modes of the amplification portion of the waveguide.

12. The waveguide of claim 1, whereinthe at least one pump mode corresponds to light having a free space wavelength between 950 and 990 nm.

13. The waveguide of claim 1, whereinone or more of the optical signal modes correspond to light having a free space wavelength within an optical band, wherein the band is at least one of the optical O-band, the optical E-band, the optical S-band, the optical C-band, the optical L-band, and the optical U-band.

14. The waveguide of claim 1, whereinthe predetermined value is 200 fs / mm, thereby reducing second EVM due to crosstalk between signal modes by at least a factor of 1.4.

15. The waveguide of claim 1, whereinthe predetermined value is 1000 fs / mm, thereby reducing the second EVM due to crosstalk between signal modes by at least a factor of 4.{00989007.DOCX } 2616. A system for amplification of optical signals, comprising:a waveguide comprising an input portion, an amplification portion, and an output portion, the waveguide being configured for light to propagate from the input portion through the amplification portion into the output portion;a pump coupler configured to receive light from one or more pump lasers into the input portion of the waveguide; andan input coupler configured to couple light including a plurality of optical signals into the input portion of the waveguide,wherein the amplification portion of the waveguide:(a) supports (i) at least one pump mode and (ii) one or more optical signal modes;(b) comprises a nonlinear optical material configured for optical parametric amplification of light in the one or more optical signal modes using light in the at least one pump mode; and (c) has (i) a first group velocity corresponding to the at least one pump mode, and (ii) a second group velocity corresponding to the one or more optical signal modes, and has a group velocity mismatch between the first group velocity and the second group velocity that is at least a predetermined value.

17. The system of claim 16, whereinthe amplification portion of the waveguide comprises a sequence of periodically poled lithium niobate sections.

18. The system of claim 16, whereinthe light from one or more pump lasers has a free space wavelength between 950 and 990 nm, andthe light including a plurality of optical signals has at least one free space wavelength between 1530 and 1565 nm.

19. The system of claim 16, whereinthe predetermined value is 1000 fs / mm, thereby reducing introduced EVM due to crosstalk between signal modes by at least a factor of 4.

20. A method for amplifying optical communication signals, comprising:receiving pump light into a pump mode of waveguide, the pump mode having a first group velocity within the waveguide;{00989007.DOCX } 27receiving signal light carrying one or more signals into one or more optical signal modes of the waveguide, the one or more optical signal modes having a second group velocity within the waveguide, wherein a group velocity mismatch between the first group velocity and the second group velocity is greater than 200 fs / mm, thereby reducing introduced EVM due to crosstalk between signal modes by at least a factor of 1.4; and propagating the pump light and the signal light through a portion of the waveguide comprising periodically poled lithium niobate, oriented so energy from the pump mode is transferred to the one or more optical signal modes through optical parametric amplification.{00989007.DOCX } 28

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