Integrated optical amplification with continuous-wave light injection

Abstract

Described herein are optical systems that reduce channel crosstalk in semiconductor optical amplifiers (SOAs) without sacrificing suppression of noise due to amplified spontaneous emission, thereby overcoming trade-offs existing in conventional amplifiers. These schemes involve injection of continuous wave (CW) light into the SOA. The CW assist light may be provided at a wavelength within the transparency region of an SOA. Injecting CW assist light in the transparency region results in a speed-up of the carrier lifetime and gain recovery. Application of CW assist light in the transparency region establishes a feedback mechanism by which the more carriers are depleted, the faster the carrier recovery. Furthermore, the injection of CW light results in pinning of the carrier density and constant gain. CW light injection can be used in SOAs that use bulk, quantum-well, and quantum dot gain material.

Description

INTEGRATED OPTICAL AMPLIFICATION WITH CONTINUOUS-WAVE LIGHT INJECTIONCROSS-REFFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application Serial No. 63 / 712,278, filed on October 25, 2024, under Attorney Docket No. L0858.70102US00 and entitled “INTEGRATED OPTICAL AMPLIFICATION WITH CONTINUOUS-WAVE LIGHT INJECTION,” which is hereby incorporated herein by reference in its entirety.BACKGROUND

[0002] Optical amplifiers play an important role in wavelength division multiplexing (WDM) systems because they enable the systems to overcome inherent signal absorption due to lossy components. Semiconductor optical amplifiers (SOA) in particular have great potential for use in data center fabric due to their low cost, small size, chip-level integration, and low power consumption compared to fiber amplifiers.

[0003] Due to wide amplification bandwidths, a single SOA can be used to amplify multiple WDM wavelengths. A WDM system may, for example, include multiple channels (e.g., wavelengths) permitting transmission of high-speed data over an optical path (e.g., fiber, waveguides, free space, etc.). In one example, each channel may carry high-speed data at 56Gb / s using non-return-to-zero (NRZ encoding) or 112Gb / s using pulse-amplitude modulation 4-level (PAM-4) encoding.BRIEF SUMMARY

[0004] In some aspects, the techniques described herein relate to an optical system, including: a continuous wave (CW) source configured to emit CW assist light, wherein the CW source is disposed on a substrate; and a semiconductor optical amplifier (SOA) disposed on the substrate and having an input waveguide, an output waveguide and an active medium coupled between the input waveguide and the output waveguide, wherein the SOA is configured to: receive a first wavelength division multiplexing (WDM) signal from the input waveguide, and in response to receiving the CW assist light either from the input waveguide or from the output waveguide, generating an amplified first WDM signal at the output waveguide by amplifying the first WDM signal.

[0005] In some aspects, the techniques described herein relate to an optical system, wherein the CW assist light has a wavelength lying within a transparency region of the SOA.

[0006] In some aspects, the techniques described herein relate to an optical system, wherein the transparency region is a blue transparency region among a plurality of transparency regions of the SOA.

[0007] In some aspects, the techniques described herein relate to an optical system, wherein the wavelength of CW assist light is between 1240 nm and 1260 nm, and the first WDM signal has a wavelength between 1300 nm and 1320 nm.

[0008] In some aspects, the techniques described herein relate to an optical system, wherein the CW assist light has a wavelength lying within a gain region of the SOA.

[0009] In some aspects, the techniques described herein relate to an optical system, further including a WDM source configured to generate the first WDM signal, wherein the WDM source is disposed on the substrate.

[0010] In some aspects, the techniques described herein relate to an optical system, further including an optical coupler having: an output waveguide coupled to the input waveguide of the SOA; a first input waveguide configured to provide the CW assist light to the output waveguide of the optical coupler; and a second input waveguide configured to provide the first WDM signal to the output waveguide of the optical coupler.

[0011] In some aspects, the techniques described herein relate to an optical system, further including an optical multiplexer having: a plurality of input waveguides configured to receive a plurality of WDM signals including the first WDM signal; and an output coupled to the second input waveguide of the optical coupler.

[0012] In some aspects, the techniques described herein relate to an optical system, further including an optical coupler having: a first waveguide coupled to the output waveguide of the SOA; a second waveguide configured to provide the CW assist light to the output waveguide of the SOA; and a third waveguide configured to receive the amplified first WDM signal from the output waveguide of the SOA.

[0013] In some aspects, the techniques described herein relate to an optical system, wherein the optical coupler includes a circulator.

[0014] In some aspects, the techniques described herein relate to an optical system, wherein the SOA is bidirectional such that the first WDM signal traverses the SOA in afirst direction and a second WDM signal traverses the SOA in a second direction that is opposite the first direction.

[0015] In some aspects, the techniques described herein relate to an optical system, including: a continuous wave (CW) source configured to emit CW assist light, wherein the CW source is disposed on a substrate; a 1-M coupler configured to receive the CW assist light emitted by the CW source; a plurality of 2-1 couplers, wherein each of the plurality of 2-1 couplers is configured to receive, via a first input waveguide, the CW assist light from the 1-M coupler and to receive, via a second input waveguide, a WDM signal; a plurality of semiconductor optical amplifiers (SOAs) disposed on the substrate, wherein each of the plurality of SOAs is configured to receive the CW assist light and the WDM signal from a respective one of the plurality of 2-1 couplers; and a plurality of output waveguides, wherein each of the plurality of output waveguides is configured to receive an amplified WDM signal from a respective SOA of the plurality of SOAs.

[0016] In some aspects, the techniques described herein relate to an optical system, wherein the CW assist light has a wavelength lying within a transparency region of a first SOA of the plurality of SOA.

[0017] In some aspects, the techniques described herein relate to an optical system, wherein the transparency region is a blue transparency region among a plurality of transparency regions of the first SOA.

[0018] In some aspects, the techniques described herein relate to an optical system, wherein the wavelength of CW assist light is between 1240 nm and 1260 nm, and the WDM signals have wavelengths between 1300 nm and 1320 nm.

[0019] In some aspects, the techniques described herein relate to an optical system, wherein the CW source, the 1-M coupler, the plurality of 1-2 couplers, and the plurality of SOAs are integrated on a common photonic integrated circuit (PIC).

[0020] In some aspects, the techniques described herein relate to a method for controlling a semiconductor optical amplifier (SOA) having an input waveguide, an output waveguide and an active medium coupled between the input waveguide and the output waveguide, the method including: receiving a first wavelength division multiplexing (WDM) signal from the input waveguide, receiving, either from the input waveguide or from the output waveguide, continuous wave (CW) assist light emitted by a CW source; and in response to receiving the CW assist light, generating an amplified first WDM signal at the output waveguide by amplifying the first WDM signal.

[0021] In some aspects, the techniques described herein relate to a method, wherein the CW assist light has a wavelength lying within a transparency region of the SOA.

[0022] In some aspects, the techniques described herein relate to a method, wherein: receiving the first WDM signal includes receiving the first WDM signal from an output of an optical coupler, and receiving the CW assist light includes receiving the CW assist light from the output of the optical coupler.

[0023] In some aspects, the techniques described herein relate to a method, wherein receiving the CW assist light includes receiving the CW assist light from the output of the SOA.BRIEF DESCRIPTION OF DRAWINGS

[0024] Various aspects and embodiments of the application will be described with reference to the following figures. It should be appreciated that the figures are not necessarily drawn to scale. Items appearing in multiple figures are indicated by the same reference number in the figures in which they appear.

[0025] FIG. 1 is a plot illustrating the gain region, loss region and transparency region of a semiconductor optical amplifier (SOA), in accordance with some embodiments.

[0026] FIG. 2A is a block diagram illustrating an example SOA used in a unidirectional configuration where continuous wave (CW) assist light enters the SOA in the same direction as the WDM signals, in accordance with some embodiments.

[0027] FIG. 2B is a block diagram illustrating a possible implementation of the configuration of FIG. 2A, in accordance with some embodiments.

[0028] FIG. 3A is a block diagram illustrating an example SOA used in a unidirectional configuration where a CW assist light enters the SOA in the opposite direction from the WDM signals, in accordance with some embodiments.

[0029] FIG. 3B is a block diagram illustrating a possible implementation of the configuration of FIG. 3A, in accordance with some embodiments.

[0030] FIG. 4A is a block diagram illustrating an example SOA that can be used either in a unidirectional configuration or in a bidirectional configuration, in accordance with some embodiments.

[0031] FIG. 4B is a block diagram illustrating a possible implementation of the configuration of FIG. 4A, in accordance with some embodiments.DETAILED DESCRIPTION

[0032] The inventors have recognized and appreciated that propagating multiple highspeed, wavelength multiplexing (WDM) signals through a semiconductor optical amplifier (SOA) results in crosstalk between adjacent channels. Furthermore, the performance of each channel tends to degrade due to the presence of amplified spontaneous emission (ASE) noise. These issues result in the degradation of the bit error rate (BER) and the noise floor.

[0033] The saturation power (referred to as Psat) of an SOA represents the input optical power level at which the amplifier’s gain drops by 3 dB from the unsaturated level. When the input power associated with a WDM channel is lower than the saturation power of the SOA, the signal-to-noise ratio (SNR) of the amplified signals is severely degraded due to the ASE noise being dominant. While increasing the input power of a WDM signal closer to Psat can suppress this noise, it results in crosstalk between the WDM signals. Crosstalk among different channels is intensified when the amplifier is operated near or above Psat since carrier depletion by one channel reduces the gain for another channel. Thus, there is a trade-off between ASE noise suppression and reduction of channel crosstalk.

[0034] Described herein are optical systems that reduce channel crosstalk in SOAs without sacrificing ASE noise suppression, thereby overcoming the trade-off described above. The optical systems developed by the inventors involve injection of continuous wave (CW) light into the SOA. This light is also referred to herein as “CW assist light.” The term “CW” indicates that the light is unmodulated (or at most, is modulated at a low frequency, such as less than 10 MHz) while the term “assist” indicates that the light provides assistance to the amplification process via gain clamping and accelerated carrier recovery. In some embodiments, the CW assist light may be provided at a wavelength within the transparency region of an SOA. Injecting CW assist light in the transparency region results in a speed-up of the carrier lifetime and gain recovery. Furthermore, the injection of CW light results in pinning of the carrier density and constant gain. CW light injection can be used in SOAs that use bulk, quantum- well, and quantum dot gain material.

[0035] CW assist light may also be applied in the gain region, in addition to or instead of being applied in the transparency region. CW light in the gain region reduces crosstalk among channels by speeding up carrier lifetime (and consequently, gain recovery) due toincreased stimulated emission. It also reduces noise as it depletes carriers available to create ASE. However, since CW assist light injected in the gain region competes with input signals for gain, the power and wavelength of the CW assist light should be carefully optimized to ensure acceptable performance.

[0036] The inventors have further recognized and appreciated that it is advantageous to heterogeneously integrate a WDM light source, a CW light source, and a SOA into a single substrate (e.g., a photonic integrated circuit (PIC)). Co-integrating these devices together results in a system that has low manufacturing costs, a reduced footprint, a high coupling efficiency, a high wall-plug-efficiency (WPE) and a reduced number of components (e.g., eliminating the need for an isolator in some embodiments).

[0037] FIG. 1 is a plot illustrating the gain curve of an example SOA, in accordance with some embodiments. The curve exhibits a gain region, a loss region and a transparency region. The gain region represents the optical band over which light injected into the SOA is amplified (where gain is positive). In WDM applications, wavelength carriers are typically injected in this band. On the other hand, the loss region represents the optical band over which light injected into the SOA is attenuated (where gain is negative). The transparency region represents the optical band at the transition from the gain region to the loss region. The term “transparency” indicates that light injected in this band is neither amplified nor attenuated. It should be noted that the transparency region is not necessarily limited to the wavelength at which the gain curve is strictly equal to zero, but it may slightly extend into the gain region and into the loss region. For example, the transparency region may cover the portion of the gain region where gain is less than IdB and may cover the portion of the loss region where loss is less than IdB. In this interval, gain / loss is so small that the corresponding variation in carrier density is negligible.

[0038] In the example of FIG. 1, the wavelength at which the gain curve is equal to 0 is defined as I2. The dashed line curve 10 represents the gain curve of an SOA when no signals having wavelengths in the gain region are injected. When a WDM signal having a wavelength in the gain region (referred to as i) is injected, the signal is amplified and the SOA gain is saturated, resulting in a change in the gain curve from the dashed line curve 10 to the solid line curve 20. The saturation effect occurs because injection of a signal in the gain region triggers stimulated emission. If the power of the signal is sufficiently large, carriers are depleted faster than they can be replenished. As a result, the carrier density (and consequently the gain) drops. As can be appreciated from FIG. 1,the gain spectrum of the SOA is shifted to longer wavelengths following the amplification of the signal at i. Notably, this shift in the gain curve also shifts the transparency point of the gain curve. Thus, light injected at I2 - which is neither amplified nor attenuated when dashed line curve 10 represents the SOA’s gain curve -lies in the loss region when the gain curve shifts to the solid line curve 20.

[0039] The inventors have recognized and appreciated that this effect can be exploited to reduce channel crosstalk without sacrificing ASE noise suppression, thereby overcoming the trade-off described above. By injecting CW assist light in the transparency region corresponding to the dashed line curve 10, the CW assist light lies in the loss region when a signal is injected in the gain region (e.g., i). Consequently, following the injection at i, application of CW assist light results in accelerated carrier recovery, thereby enhancing gain recovery. It should be noted that the same feedback mechanism can be established by injecting CW assist light in the gain region, so long as the CW assist light lies in the loss region when a signal at i is amplified.

[0040] In some embodiments, there may be more than one transparency point in the gain curve. For example, a gain curve may exhibit a second transparency point at a wavelength greater than the gain region. This additional transparency point, referred to as a red transparency point, also exhibits a gain equal to 0. This point is referred to as “red” to indicate that it has a higher wavelength. Vice versa, the transparency point lying at the lower wavelength is referred to as the “blue” transparency point. (It should be noted that the terms red and blue color do not indicate the color of the light, but indicate their relative placement in the electromagnetic spectrum). CW assist light can be applied either in the red transparency region, in the blue transparency regions (or both).

[0041] The techniques described herein may be applied to unidirectional configurations - in which data is passed through an SOA only in one direction - or to bidirectional configurations - in which data is passed through an SOA in two opposite directions. Additionally or alternatively, CW assist light may be injected in the same direction as the WDM signals, or in the opposite direction. The case in which CW assist and WDM signals are injected in the same direction is referred to as co-propagating injection; the case in which CW assist and WDM signals are injected in opposite directions is referred to as counter-propagating injection. FIGs 2A-2B illustrate examples of SOAs undergoing co-propagating injection; FIGs 3A-3B illustrate examples of SOAs undergoing counter-propagating injection; FIGs 4A-4B illustrate examples of SOAs undergoing both copropagating injection and counter-propagating injection.

[0042] Referring first to FIG. 2 A, a semiconductor optical amplifier 110 is depicted. SOA 110 may be implemented to include an active medium made from a direct bandgap semiconductor (e.g., InGaAsP, AlGalnAs, InGaAs or GaAs on a silicon substrate). The bandgap is engineered so that the energy of the photons corresponding to the desired WDM signals roughly matches the energy difference between the conduction band and the valence band. Electrical injection of carriers achieves population inversion, allowing incoming photons to stimulate emission of additional photons at the same wavelength. To suppress lasing, the facets of the SOA may be coated with anti-reflection films. The active medium is coupled between the input waveguide of the SOA and the output waveguide of the SOA.

[0043] In the scheme of FIG. 2A, an input set of WDM signals is injected in SOA 110. The output set represents the WDM signals exiting SOA 110 upon experiencing optical amplification. Each WDM signal has a carrier wavelength lying in a corresponding WDM channel. WDM channels define wavelength intervals used to perform optical communication consistent with WDM techniques. Each WDM channel is characterized by a corresponding carrier wavelength. A carrier wavelength of a WDM channel may be the wavelength positioned in the middle of the wavelength interval of a WDM channel. Alternatively or additionally, a carrier wavelength of a WDM channel may be the wavelength that exhibits the absolute peak intensity within the wavelength interval of a WDM channel. Alternatively or additionally, a carrier wavelength of a WDM channel may be the nominal wavelength of emission of an optical source. The wavelength of emission may be “nominal” in that the optical source may emit a finite spectrum of wavelengths around the nominal wavelength due to spectral broadening effects.

[0044] In some embodiments, the WDM signals have carrier wavelengths in the O-band. Adjacent carriers may be spaced from one another by a wavelength separation in the range of 1.72 nm - 2.86 nm in some embodiments. This wavelength separation, corresponding to a frequency separation of about 300 GHz - 500 GHz in the O-band, provides sufficient room to prevent adjacent channels from interfering with each other in the presence of spectral broadening. Spectral broadening is a phenomenon that occurs, among other potential causes, as a result of modulation. When the output of a monochromatic (or quasi-monochromatic) optical source is modulated with data, thespectrum of the signal broadens because the modulation introduces sidebands and additional frequency components. For example, amplitude modulation creates sidebands above and below a carrier wavelength.

[0045] In FIG. 2A, CW assist light is injected in the same direction as the input WDM signals. As a result, the CW assist light and the input WDM signals travel along the SOA’s active medium in the same direction. The CW assist light is preferably injected at a wavelength that lies in the transparency region (more specifically, the transparency region of the SOA’s gain curve when no signals are injected into the gain region). As a result, a feedback mechanism is established by which the more carriers are depleted due to stimulated emission in the gain region, the faster the carrier recovery. Alternatively, the CW assist light is injected at a wavelength that lies in the gain region.

[0046] FIG. 2B is a block diagram illustrating a possible implementation of the scheme of FIG. 2A, in accordance with some embodiments. In this implementation, a WDM source 100, a CW source 106 and an SOA 110 may be disposed on the same substrate 50, (e.g., a photonic integrated circuit (PIC)). The substrate may be a silicon substrate or an indium phosphide substrate, for example. These components may be monolithically defined on the same substrate, or may be part of distinct chips bonded to a common substrate (an arrangement referred to as heterogeneous integration). In either scenario, WDM source 100, CW source 106 and SOA 110 are said to be co-integrated on the same PIC.

[0047] WDM source 100 is configured to emit light at multiple discrete carrier wavelengths, where each carrier wavelength is intended to support a respective WDM channel. WDM source 100 may be implemented, for example, using a bank of lasers emitting light at different carrier wavelengths. In some embodiments, WDM source 100 produces carrier wavelengths in the O-band, as noted above. In some embodiments, CW light emitted by WDM source 100 may undergo optical modulation, thereby imparting data on the WDM signals. This is performed using optical modulators (“Mod”) 10 li, 1012...101N. In other embodiments, however, the modulators may be omitted, and WDM signals may be injected in SOA 110 without first undergoing optical modulation. A multiplexer (“Mux”) 104 combines the WDM signals on a common waveguide.

[0048] CW source 106 is configured to emit light at a wavelength that lies in the transparency region or in the gain region of SOA 110, as described in connection with FIG. 1. In one example, CW source 106 is configured to emit light at a wavelengthbetween 1240 nm and 1260 nm (e.g., between 1249 nm and 1251 nm). A 2-1 optical coupler 108 combines the input WDM signals and the CW assist light on a common waveguide, the input waveguide of SOA 110. The output set represents the WDM signals exiting SOA 110 upon experiencing optical amplification.

[0049] The scheme of FIG. 3 A is similar to the scheme of FIG. 2A in that SOA 110 amplifies input WDM signals to produce output WDM signals while CW assist light is injected in the SOA. However, in the scheme of FIG. 3A, injection is performed in accordance with a counter-propagating scheme - CW assist light is injected in the opposite direction relative to the input WDM signals. For example, CW assist light may be injected from the output waveguide of SOA 110, the same waveguide from which the output WDM signals exit the SOA. As in the scheme of FIG. 2A, a feedback mechanism is established by which the more carriers are depleted due to stimulated emission in the gain region, the faster the carrier recovery.

[0050] FIG. 3B is a block diagram illustrating a possible implementation of the scheme of FIG. 3A, in accordance with some embodiments. As in the implementation of FIG. 2B, WDM source 100, CW source 106 and SOA 110 may be disposed on the same substrate 51 (e.g., on the same PIC). Coupler 208 permits injection of CW assist light on the output waveguide of SOA 110. In some embodiments, coupler 208 is implemented as a circulator. As such, coupler 208 permits propagation of light having wavelengths corresponding to the WDM channels only in one direction (from left to right in FIG. 3B) and permits propagation of CW assist light only in the opposite direction (from right to left in FIG. 3B).

[0051] Accordingly, some embodiments relate to an optical system comprising a CW source and an SOA. The CW source is configured to emit CW assist light. The CW assist light has a wavelength lying within a transparency region or a gain region of the SOA. The SOA has an input waveguide, an output waveguide and an active medium coupled between the input waveguide and the output waveguide. The SOA receives a first WDM signal from the input waveguide. Further, the SOA receives the CW assist light emitted by the CW source, either from the input waveguide or from the output waveguide (the expression “either from the input waveguide or from the output waveguide” includes the scenario in which the SOA receives the CW assist light both from the input waveguide and from the output waveguide). The case in which the SOA receives the CW assist light from the input waveguide represents the co-propagating scheme of FIG. 2A, forexample. The case in which the SOA receives the CW assist light from the output waveguide represents the counter-propagating scheme of FIG. 3A, for example. Upon receiving the CW assist light, the SOA accelerates carrier recovery. As a result, the SOA generates an amplified first WDM signal at the output waveguide by amplifying the first WDM signal.

[0052] The schemes of FIGs. 2A and 3A are unidirectional in that WDM signals are injected only from one side of an SOA. By contrast, the scheme of FIG. 4A is bidirectional - WDM signals are injected from both sides of an SOA. In one direction, first output WDM signals result from the amplification of first input WDM signals; in the opposite direction, second output WDM signals result from the amplification of second input WDM signals. CW assist light is injected in either direction (or, optionally, in both directions). This configuration may be useful in transceivers feeding bidirectional fiber links - e.g., in configurations in which a fiber supports propagation of a first WDM set in a one direction and propagation of a second, non-overlapping WDM set in the opposite direction. In one example, a WDM configuration may define the following carrier wavelengths: 1301.47 nm, 1302.6 nm, 1303.73 nm, 1304.87 nm, 1306.01 nm, 1307.14 nm, 1308.28 nm and 1309.43 nm. In this example, the carrier wavelengths 1301.47 nm, 1303.73 nm, 1306.01 nm and 1308.28 nm may be injected from one side of SOA 110 and the carrier wavelengths 1302.6 nm, 1304.87 nm, 1307.14 nm, and 1309.43 nm may be injected from the opposite side of SOA 110. In this example, the carrier wavelengths of the first set are interleaved with the carrier wavelengths of the second set. In another example, carrier wavelengths 1301.47 nm, 1302.6 nm, 1303.73 nm, 1304.87 nm may be injected from one side of SOA 110 and carrier wavelengths 1306.01 nm, 1307.14 nm, 1308.28 nm and 1309.43 nm may be injected from the opposite side of SOA 110. In this other example, the carrier wavelengths of each set are contiguous.

[0053] FIG. 4B is a block diagram illustrating a possible implementation of the scheme of FIG. 4A, in accordance with some embodiments. As in the implementation of FIGs.2B and 3B, WDM source 100 (not shown in FIG. 4B), CW source 106 and multiple SOAs 110 may be disposed on the same substrate 52 (e.g., on the PIC). To increase data throughput, multiple channels are defined, each channel having a corresponding SOA. In this example, an optical bandpass filter 402 is inserted downstream from CW source 106. The filter can prevent back reflections from other light sources and filter out ASE from the SOAs. In this way, isolators may be omitted. In one example, optical bandpass filter402 has a bandwidth of 2 nm or less (e.g., permitting passage of wavelengths between 1249 nm and 1251 nm, in one example). Although not illustrated, a similar optical bandpass filter may be used in the schemes of FIGs. 2B and 3B. A 1-M coupler 404 splits the CW assist light into a set of M branches to create M separate optical paths. Each of these paths is combined with a respective bidirectional (“BiDi”) WDM data path at a respective 2-1 coupler 406. The WDM signals may be defined at wavelengths between 1300-1320 nm in one example, and there may be as many as four, eight, sixteen, thirty-two, sixty-four, etc. different WDM channels in each direction. The output of each coupler 406 is input into a respective SOA 110. Amplified WDM signals exit the SOA from the right-hand side of the SOA. Simultaneously, an additional set of WDM channels are injected in the SOA in the counter-propagating direction. These counterpropagating WDM channels are also amplified by the SOA and are then guided to the 2-1 couplers 406, where the amplified counterpropagating WDM channels exit the same waveguide from which the first set of WDM channels enters.

[0054] Embodiments above have shown arrangements with single SOAs associated with each waveguides. However, the techniques described herein can also apply to cascaded SOAs, where multiple SOAs (made from different or the same material) in series amplify WDM signals.

[0055] Having thus described several aspects and embodiments of the technology of this application, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those of ordinary skill in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the technology described in the application. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive embodiments may be practiced otherwise than as specifically described. In addition, any combination of two or more features, systems, articles, materials, and / or methods described herein, if such features, systems, articles, materials, and / or methods are not mutually inconsistent, is included within the scope of the present disclosure.

[0056] Also, as described, some aspects may be embodied as one or more methods. The acts performed as part of the method may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in an order different thandescribed, which may include performing some acts simultaneously, even though shown as sequential acts in illustrative embodiments.

[0057] All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and / or ordinary meanings of the defined terms.

[0058] The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.”

[0059] The phrase “and / or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases.

[0060] As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified.

[0061] The terms “approximately” and “about” may be used to mean within ±20% of a target value in some embodiments, within ±10% of a target value in some embodiments, within ±5% of a target value in some embodiments, and yet within ±2% of a target value in some embodiments. The terms “approximately” and “about” may include the target value.

Claims

CLAIMSWhat is claimed is:

1. An optical system, comprising:a continuous wave (CW) source configured to emit CW assist light, wherein the CW source is disposed on a substrate; anda semiconductor optical amplifier (SOA) disposed on the substrate and having an input waveguide, an output waveguide and an active medium coupled between the input waveguide and the output waveguide, wherein the SOA is configured to:receive a first wavelength division multiplexing (WDM) signal from the input waveguide, andin response to receiving the CW assist light either from the input waveguide or from the output waveguide, generating an amplified first WDM signal at the output waveguide by amplifying the first WDM signal.

2. The optical system of claim 1, wherein the CW assist light has a wavelength lying within a transparency region of the SOA.

3. The optical system of claim 2, wherein the transparency region is a blue transparency region among a plurality of transparency regions of the SOA.

4. The optical system of claim 2, wherein the wavelength of CW assist light is between 1240 nm and 1260 nm, and the first WDM signal has a wavelength between 1300 nm and 1320 nm.

5. The optical system of claim 1, wherein the CW assist light has a wavelength lying within a gain region of the SOA.

6. The optical system of claim 5, further comprising a WDM source configured to generate the first WDM signal, wherein the WDM source is disposed on the substrate.

7. The optical system of claim 1, further comprising an optical coupler having: an output waveguide coupled to the input waveguide of the SOA;a first input waveguide configured to provide the CW assist light to the output waveguide of the optical coupler; anda second input waveguide configured to provide the first WDM signal to the output waveguide of the optical coupler.

8. The optical system of claim 7, further comprising an optical multiplexer having:a plurality of input waveguides configured to receive a plurality of WDM signals including the first WDM signal; andan output coupled to the second input waveguide of the optical coupler.

9. The optical system of claim 1, further comprising an optical coupler having: a first waveguide coupled to the output waveguide of the SOA;a second waveguide configured to provide the CW assist light to the output waveguide of the SOA; anda third waveguide configured to receive the amplified first WDM signal from the output waveguide of the SOA.

10. The optical system of claim 9, wherein the optical coupler comprises a circulator.

11. The optical system of claim 1, wherein the SOA is bidirectional such that the first WDM signal traverses the SOA in a first direction and a second WDM signal traverses the SOA in a second direction that is opposite the first direction.

12. An optical system, comprising:a continuous wave (CW) source configured to emit CW assist light, wherein the CW source is disposed on a substrate;a 1-M coupler configured to receive the CW assist light emitted by the CW source;a plurality of 2- 1 couplers, wherein each of the plurality of 2- 1 couplers is configured to receive, via a first input waveguide, the CW assist light from the 1-M coupler and to receive, via a second input waveguide, a WDM signal;a plurality of semiconductor optical amplifiers (SOAs) disposed on the substrate, wherein each of the plurality of SOAs is configured to receive the CW assist light and the WDM signal from a respective one of the plurality of 2-1 couplers; anda plurality of output waveguides, wherein each of the plurality of output waveguides is configured to receive an amplified WDM signal from a respective SOA of the plurality of SOAs.

13. The optical system of claim 12, wherein the CW assist light has a wavelength lying within a transparency region of a first SOA of the plurality of SOA.

14. The optical system of claim 13, wherein the transparency region is a blue transparency region among a plurality of transparency regions of the first SOA.

15. The optical system of claim 13, wherein the wavelength of CW assist light is between 1240 nm and 1260 nm, and the WDM signals have wavelengths between 1300 nm and 1320 nm.

16. The optical system of claim 12, wherein the CW source, the 1-M coupler, the plurality of 1-2 couplers, and the plurality of SOAs are integrated on a common photonic integrated circuit (PIC).

17. A method for controlling a semiconductor optical amplifier (SOA) having an input waveguide, an output waveguide and an active medium coupled between the input waveguide and the output waveguide, the method comprising:receiving a first wavelength division multiplexing (WDM) signal from the input waveguide,receiving, either from the input waveguide or from the output waveguide, continuous wave (CW) assist light emitted by a CW source; andin response to receiving the CW assist light, generating an amplified first WDM signal at the output waveguide by amplifying the first WDM signal.

18. The method of claim 17, wherein the CW assist light has a wavelength lying within a transparency region of the SOA.

19. The method of claim 17, wherein:receiving the first WDM signal comprises receiving the first WDM signal from an output of an optical coupler, andreceiving the CW assist light comprises receiving the CW assist light from the output of the optical coupler.

20. The method of claim 17, wherein receiving the CW assist light comprises receiving the CW assist light from the output of the SOA.

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