Side Mode Suppression for Extended C-Band Tunable Lasers

Abstract

To provide a method of improving a wideband waveform variable laser.SOLUTION: In a method of improving a wideband waveform variable laser, it is included that a gain region between a first facet and a second facet is structured to a transverse direction to a PN junction having an active layer between a p-type clad layer and an N-type clad layer. The method further includes a step of passing it via the first facet to a waveform adjuster structured by being excited in the active layer, coupling the light partially reflected from the second facet, and generating a joint interference spectrum having a multimode mode separated by a joint free spectrum range (JFSR). Additionally, the method contains a step of structuring the second facet so as to have a reflection rate reduced for increasing a wavelength. Further, the method contains a step of guiding a gain loss on the wavelength shorter than the longest wavelength related to a short wavelength side mode by re-structuring a gain chip having an absorption layer near the active layer.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to optical communication techniques. More specifically, the present invention provides a reflective semiconductor optical amplifier having an extended C-band, a gain chip of a reflective semiconductor optical amplifier of a variable laser via an extended C-band having side mode suppression, and a method for suppressing side modes for a wavelength variable laser based on the wavelength variable laser having the same.

Background Art

[0002] In the past few years, the use of communication networks has become widespread. In the dawn of the Internet, the main uses were limited to email, bulletin boards, and generally text-based web page surfing for information, and the amount of data transferred was usually relatively small. Nowadays, the Internet and mobile applications require a large amount of bandwidth to transfer photos, videos, music, and other multimedia files. For example, social networks such as Facebook process more than 500TB of data every day. Due to such high demands for data and data transfer, existing data communication systems need to be improved to address these needs.

[0003] 40 Gbit / s and 100 Gbit / s data rate wideband DWDM (Dense Wavelength Division Multiplexing) optical transmission, replacing existing single-mode fibers, is a goal of next-generation optical fiber communication networks. Chip-scale wideband wavelength-variable lasers have been useful for many applications such as wideband DWDM communication and wavelength-steering optical detection and ranging (LIDAR) sensing. More recently, photonic components have been integrated on silicon (Si) substrates to fabricate large-scale optical integrated circuits that coexist with microelectronic chips. Photonic components, including all of filters, multiplexers (demultiplexers), splitters, modulators, and photodetectors, have generally been demonstrated in silicon-on-insulator (SOI) platforms. The SOI platform is particularly suitable for the standard WDM communication bands of 1300 nm and 1550 nm. This is because both silicon (n = 3.48) and its oxide SiO2 (n = 1.44) are transparent and form high-refractive-index contrast, high-confinement waveguides that are ideally suitable as the medium for high-integration silicon photonic integrated circuits (SPICs).

[0004] Wavelength-variable semiconductor lasers in silicon photonic platforms have been implemented as a key element for many applications of wideband optical fiber communication with increased spectral efficiency. Various spectral efficiency modulation formats such as phase-shift keying (PSK) and quadrature phase-amplitude modulation (QAM) without using more complex optical phase-locked loops. However, there are technical challenges such as creating Vernier burning of small-size variable filters under a silicon photonic platform to expand the free spectral range with sufficient side-mode suppression ratio in extended wideband wavelength-variable lasers based on reflective semiconductor optical amplifiers. Therefore, improved techniques and methods are desired. SUMMARY OF THE INVENTION

[0005] The present invention relates to optical communication techniques. More specifically, the present invention provides a method for suppressing side modes of the interference spectrum of a wavelength-variable laser in the extended C band, a gain chip of a reflective semiconductor optical amplifier configured to direct laser light onto a high reflectivity (HR) facet having a modified gain curve with reduced reflectivity for suppressing long wavelength side modes and excess loss for suppressing short wavelength side modes, and a wavelength-variable laser having a Vernier reflector adjuster for adjusting a laser emitted from the same gain chip having an optical wavelength in the extended C band, although other applications are possible.

[0006] In an embodiment, the present invention provides a method for improving a wideband wavelength-variable laser. The method comprises the step of configuring a gain chip longitudinally in a gain region between a first facet and a second facet and transversely with respect to a PN junction having an active layer between a P-type clad layer and an N-type clad layer. The method further includes passing the first facet through a wavelength adjuster configured to generate a joint interference spectrum having multiple modes at isolated spectral peaks separated by a joint free spectral range (JFSR) by coupling light excited in the active layer and at least partially reflected from the second facet. Additionally, the method includes configuring the second facet to have a reduced optical reflectivity that increases the wavelength from a fundamental mode JFSR peak to a long wavelength side mode JFSR peak. Further, the method includes reconfiguring the gain chip having an absorption layer disposed in the N-type clad layer near the active layer to induce a gain loss for wavelengths shorter than the longest wavelength associated with the short wavelength side mode JFSR peak. Further, the method includes amplifying light at the fundamental mode JFSR peak in the gain chip.

[0007] In an alternative embodiment, the present invention provides a gain chip of a reflective semiconductor optical amplifier for a wideband wavelength-variable laser. The gain chip includes a gain region configured longitudinally as a straight ridge waveguide between a first facet and a second facet and transversely as a PN junction of a P-type cladding layer and an N-type cladding layer. The gain chip further includes an active layer formed between the P-type cladding layer and the N-type cladding layer and configured to excite light. Additionally, the gain chip includes an absorption layer in the N-type cladding layer near the active layer having an overlapping energy distribution to cause excessive loss in the optical gain profile for wavelengths shorter than a predetermined value. The gain chip further includes antireflection optical characteristics configured at the first facet. Further, the gain chip includes partial reflection optical characteristics configured at the second facet. Further, the light excited in the active layer is partially reflected at the second facet and passed through the first facet into a wavelength adjuster that generates a joint interference spectrum having multiple modes at isolated spectral peaks separated by a joint free spectral range (JFSR). The fundamental mode JFSR peak of the multiple modes is adjusted by the wavelength adjuster at a wideband wavelength, amplified in the gain region before being emitted as laser light, while the long-wavelength side mode JFSR peak and the short-wavelength JFSR peak are suppressed.

[0008] In yet another alternative embodiment, the present invention provides a wavelength tunable semiconductor laser. The wavelength tunable semiconductor laser is longitudinally configured as a straight waveguide between a first facet having an antireflection characteristic that decreases optical reflectivity for longer wavelengths and a second facet having a low reflectivity characteristic, and includes a gain chip including a gain region transversely configured as a PN junction including an active layer for exciting light between a P-type clad layer and an N-type clad layer. The wavelength tunable semiconductor laser further includes an absorption layer formed near the active layer in the N-type clad layer to cause excess loss in the optical gain profile in the gain region. Additionally, the wavelength tunable semiconductor laser is formed on a silicon photonics substrate and includes a wavelength adjuster coupled to the first facet to receive light partially reflected from the second facet. Further, the wavelength tunable semiconductor laser includes a reflector coupled to the wavelength adjuster to reflect light into the gain region in an extended cavity to generate a joint interference spectrum having multiple modes separated by a joint free spectral range (JFSR). Further, the light in the multiple modes of the JFSR peak includes a fundamental mode amplified in the extended cavity before applying the laser light through the second facet, which is adjustable from about 1526 nm to about 1568 nm by the wavelength adjuster in the extended C band, while the high wavelength side modes are suppressed by the low reflectivity characteristic having a decreased reflectivity for longer wavelengths provided by the second facet, and the short wavelength side modes are suppressed by the excess loss in the gain profile caused by the absorption layer.

[0009] In yet another alternative embodiment, the present invention provides a wavelength tunable semiconductor laser. The wavelength tunable semiconductor laser is longitudinally configured as a straight waveguide between a first facet having an anti-reflection characteristic and a second facet having a high reflectivity characteristic but with a reduced optical reflectivity for a specific wavelength longer than the high end of the extended C band, and is transversely configured as a PN junction including an active layer for exciting light between a P-type cladding layer and an N-type cladding layer, and includes a first gain chip including a first gain region. The wavelength tunable semiconductor laser further includes an absorption layer formed near the active layer in the N-type cladding layer to cause excessive loss in the gain profile of light in the first gain region. Additionally, the wavelength tunable semiconductor laser is formed on a silicon photonics substrate and includes a wavelength tuner coupled to the first facet for receiving light reflected from the second facet, forming an extended cavity at the second facet in the first gain region, and generating a joint interference spectrum having multiple modes separated by a joint free spectral range (JFSR). The wavelength tuner is configured to suppress high wavelength side modes by a low reflectivity characteristic having a reduced reflectivity for wavelengths longer than those provided by the second facet, and to suppress short wavelength side modes by excessive loss in the gain profile caused by the absorption layer while adjusting the wavelength of the fundamental mode in the multiple modes of the JFSR peak. Further, the wavelength tunable semiconductor laser receives light having a wavelength of the fundamental mode adjusted by the wavelength tuner in the extended C band from about 1526 nm to about 1568 nm, and includes a wavelength locker coupled to the wavelength tuner for finely adjusting and locking the wavelength. Further, the wavelength tunable semiconductor laser includes a laser light exit coupled to a wavelength locker more distal from the first gain chip for laser irradiating light having the wavelength. Optionally, the laser light exit is coupled to a facet of a second gain chip of a semiconductor optical amplifier coupled to the wavelength locker.

[0010] The present invention realizes these benefits and others in the context of known art of wavelength-variable lasers having a wavelength adjuster and optionally including a reflector, a wavelength locker, and a semiconductor optical amplifier. However, a further understanding of the nature and features of the present invention can be realized by referring to the latter part of the specification and the accompanying drawings.

Brief Description of the Drawings

[0011] The following figures are merely examples and should not unduly limit the claims of this specification. Those skilled in the art will recognize many other variations, modifications, and alternative forms. Also, the examples and embodiments described herein are for illustrative purposes only, and various modifications or changes based thereon are suggested to those skilled in the art and are to be included within the spirit and scope of the appended claims of this process and scope.

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DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention relates to optical communication techniques. More specifically, the present invention provides a method for suppressing side modes of the interference spectrum of a wavelength-variable laser in the extended C band, a gain chip of a reflective semiconductor optical amplifier configured to apply laser light from a high-reflectivity (HR) facet having a modified gain curve with reduced reflectivity for suppressing long-wavelength side modes and excess loss for suppressing short-wavelength side modes, and a wavelength-variable laser having a Vernier ring reflector adjuster for adjusting a laser emitted from the same gain chip having an optical wavelength in the extended C band, although other applications are possible.

[0021] The following description is presented to enable a person skilled in the art to make and use the invention and to incorporate it into a particular application context. Various modifications and various uses in different applications will be readily apparent to those skilled in the art, and the general principles defined herein apply to a wide range of embodiments. Accordingly, the present invention is not limited to the embodiments presented, but rather conforms to the broadest scope consistent with the principles and novel mechanisms disclosed herein.

[0022] To provide a more complete understanding of the present invention, many specific details are set forth in the following detailed description. However, it will be apparent to those skilled in the art that the present invention may be practiced without necessarily being limited to these specific details. In other instances, well-known structures and devices are shown in block diagram form rather than in detail to avoid obscuring the present invention.

[0023] The reader's attention is directed to all papers and documents that are filed herewith and that are publicly accessible with this specification, and the contents of all such papers and documents are incorporated herein by reference. All of the mechanisms disclosed in this specification (including any appended claims, abstract, and drawings) may be replaced by alternative mechanisms that perform the same, equivalent, or similar function, unless expressly stated otherwise. Accordingly, unless expressly stated otherwise, each mechanism disclosed is only an example of a general series of equivalent or similar mechanisms.

[0024] Furthermore, any element in a claim that does not expressly recite a "means" for performing a specified function or a "step" for performing a particular function should not be construed as a "means" or "step" clause as recited in 35 U.S.C. § 112, ¶ 6. In particular, the use of "step" or "act" in the claims of this specification is not intended to invoke the provisions of 35 U.S.C. § 112, ¶ 6.

[0025] Note that labels such as inside, outside, left, right, front, rear, top, bottom, last, forward, reverse, clockwise, and counterclockwise, when used, are used for convenience only and are not intended to imply any particular fixed direction. Rather, they are used to reflect the relative position and / or direction between various parts of an object.

[0026] In one aspect, the present disclosure provides a method for improving wavelength-variable laser performance over an extended wideband for various communication applications. In the embodiment shown in FIG. 1, the variable laser 10 is provided with not only a wavelength adjuster 400, but also a laser flip chip 100 coupled to a reflector 410 + wavelength locker 300. The laser flip chip 100 includes a gain chip 111 having a gain region 112 with a transverse PN junction diode in a longitudinal straight waveguide configured as a reflective semiconductor optical amplifier (RSOA), which is coupled to a silicon photonics substrate. In particular, the gain region 112 of the RSOA includes a laser optical cavity along a straight waveguide between an antireflection (AR) facet 102 and a high reflectivity (HR) facet 101. Light can be excited from the active layer in the PN junction. In an embodiment, the light excited in the active layer can be amplified in the high reflectivity laser optical cavity at the HR facet 101 before exiting from the AR facet 102. For a variable laser having a normal gain chip in an RSOA configuration, the light exiting from the AR facet 102 enters the wavelength adjuster 400 + wavelength locker 300. The laser having the adjusted wavelength finally exits from an exit port that is more distal to the HR facet 101. In an alternative embodiment shown in FIG. 1, different from a normal gain chip in an RSOA configuration mainly used as a reflector, the gain chip 111 in the RSOA+ configuration is designed to configure the HR facet 101 to function as a laser exit port after the light is adjusted by a wavelength adjuster 400 coupled to the AR facet 102 so as to be partially reflective and partially transmissive. The HR facet is named from the conventional term for the high reflectivity facet of the RSOA. On the other hand, the HR facet 101 for the gain chip 111 in the RSOA+ configuration can be set to a relatively low (less than 30%) value.

[0027] In a particular embodiment of the variable laser 10 based on flip-chip of RSOA+100, the HR facet 101 is composed of a reflectivity of only about 5% on average in the extended C band from about 1526 nm to 1568 nm, but is composed of a relatively high transmittance in order to function as an output port for outputting laser light. The AR facet 102 also enables a high light transmittance of 99.99%, so that the light excited in the active region 112 can be easily coupled to the waveguide 120 to the wavelength adjuster 400 formed on the silicon photonics substrate via the coupler 202. In a particular embodiment, the wavelength adjuster 400 is provided as a Vernier ring adjuster including at least two ring-shaped waveguides, for example, the ring resonator R1 and the ring resonator R2 formed in the silicon photonics substrate. Each ring resonator R1 or R2 in the extended cavity related to the reflector 410 provides a reflection spectrum having a plurality of resonance peaks (shown in the upper part of FIG. 2) in a wide wavelength range of 1520 nm to 1620 nm in the extended cavity between the reflector 410 and the HR facet 101 of the gain chip 111. Optionally, the reflector 410 is a reflective waveguide mirror provided with a reflectivity higher than 95% for wideband light. The wavelength interval between the plurality of resonance peaks depends on the respective ring diameters and the optical index of the medium material used to shape the ring-shaped waveguide. In the extended cavity related to the combination of the wavelength adjuster 400 and the reflector 410, the two reflection spectra generate a joint interference spectrum (referred to as R1*R2) in the same wavelength range as shown in the lower part of FIG. 2. The joint interference spectrum is characterized by a so-called mode having a JFSR as the wavelength interval between a plurality of resonance peaks or between two adjacent main interference joint free spectral range (JFSR) peaks. The value of the JFSR depends on the ring diameters of the two ring-shaped waveguides.

[0028] In an embodiment, the wavelength tuner 400 for a wideband tunable laser is preferably designed to have a JFSR much larger than the desired tunable range in order to allow passing the fundamental mode JFSR peak while suppressing the nearest side mode JFSR peak. In an example, the JFSR is sized 85 nm for the proposed wavelength tuning in the extended C-band of 1526 nm to 1568 nm. When the JFSR is set to 85 nm, for the fundamental mode JFSR peak at the first wavelength of about 1526 nm, the corresponding nearest long wavelength side mode JFSR peak is seen at the second wavelength of about 1611 nm. For the fundamental mode JFSR peak at the first wavelength of about 1568 nm, the corresponding nearest short wavelength side mode JFSR peak may be seen at the second wavelength of about 1483 nm. Both the long wavelength side mode and the short wavelength side mode are preferably substantially suppressed to improve the performance of the wideband tunable laser 10. Optionally, the wavelength tuner 400 can be provided as a dual etalon filter that passes only the fundamental mode JFSR peak and uses a reflector to reflect it to the gain chip.

[0029] In principle, the wavelength adjuster + reflector configuration operates as a wavelength selection filter or a wavelength adjuster to introduce adjuster transmissivity in the return direction to the gain chip 111 having a joint interference spectrum. The return direction adjuster transmissivity, including the base mode JFSR peak and all side mode JFSR peaks, can be deformed in the gain chip 111 to remove or at least suppress all side modes. In an embodiment, a wavelength tunable laser in the extended C band is configured by implementing the gain chip 111 in an RSOA+ configuration where the laser emits from the HR facet. The fundamental mode JFSR peak can be adjusted from a low-end wavelength, C_low = 1526 nm, to a high-end wavelength, C_high = 1568 nm. When the fundamental mode is adjusted to C_low, the long wavelength side mode of the JFSR peak at approximately 1611 nm should generally be suppressed during return transmission to avoid being included in the emitted laser. While the fundamental mode is adjusted to C_high, the short wavelength side mode of the JFSR peak should generally be suppressed to improve the performance of the tunable laser. When the side mode laser light is properly suppressed, the fundamental mode JFSR peak deforms the center or the optimal position of the gain profile and determines the laser emission with a shaped peak having a single wavelength. On the other hand, the wavelength corresponding to the fundamental mode JFSR peak can be adjustable based on the design of the wavelength adjuster + reflector configuration by changing the optical index of each waveguide by changing the temperature. For example, the fundamental mode JFSR peak is initially set to a specific wavelength by setting the optimal temperature using the pre-calibrated voltages supplied to the resistive heaters associated with the two resonators R1 and R2. As shown in FIG. 1, the pre-calibrated voltages VR1 and VR2 can be stored in a look-up table of memory that can be read each time a silicon photonics-based tunable laser device is activated. Coarse wavelength adjustment can be achieved by changing the temperature around the two ring resonators R1 and R2 to adjust the wavelength in an extended adjustable range around the optimal gain profile position. The temperature change is controlled using an electric heater (not shown) driven by the applied voltages VR1 and VR2.Additionally, fine wavelength adjustment can be performed by changing the temperature around the wavelength locker 300.

[0030] FIG. 3 is an exemplary diagram of a laser spectrum output by a tunable laser with an adjusted laser wavelength according to an embodiment of the present invention. As shown, the laser emission wavelength is given by the peak position of a spectrum that is deformed by superimposing the fundamental mode JFSR peak on the gain profile of the laser emission. In the example, the laser emission wavelength is adjusted from 1555 nm to 1535 nm. For wideband wavelength tuning, it is preferable to have a JFSR designed for the widest possible tunable laser in order to achieve better mode selectivity and have strong single-wavelength emission in the fundamental mode while minimizing interference in the side modes. However, to achieve a wide JFSR in Vernier burning, a very small diameter of the ring resonator and a more complex waveguide structure formed in a silicon photonics substrate to achieve mode selection / suppression and stability are required. Designing and constructing a ring resonator with a very small diameter or adding additional elements on a silicon photonics substrate to improve mode selection is technically difficult and not economically preferable in several aspects.

[0031] In one aspect, the present invention provides a method for improving the fundamental mode wavelength selection of tunable laser emission from an extended C-band from a gain chip of an RSOA coupled to a wavelength tuner + reflector by suppressing both short wavelength and long wavelength side mode JFSR peaks. The method is based on reconfiguring the gain chip by modifying the optical reflectivity at the HR facet and modifying the optical absorption. FIG. 4 is a schematic top view of a gain region (A) having an HR facet and an AR facet and a cross-sectional view of a gain region (B) having an active layer and an absorption layer, according to an embodiment of the present invention. This figure is merely an example and should not unduly limit the claims. Those skilled in the art will recognize many variations, alternatives, and modifications. Referring to part (A) of FIG. 4 and also referring to FIG. 1, the gain region 112 of the gain chip 111 of the RSOA + 100 is configured in the longitudinal direction (Z direction) of a straight waveguide having an HR facet 101 and an AR facet 102. That is, the AR facet 102 has antireflection optical properties with an optical transmittance set to 99.99% or more for the passage of light in a wide wavelength range. In particular, the light excited in the gain region easily passes through the AR facet at the end of the gain chip 111 and the coupler 202 to the waveguide 120 in the silicon photonics substrate (see FIG. 1). Further, the light in the waveguide leads to a configuration of a wavelength tuner + reflector formed on the same silicon photonics substrate. In an embodiment, the HR facet 101 is provided with partial reflection optical properties. On the other hand, the partial reflection optical properties are optional and provide a low reflectivity (less than 30%) to reflect the light excited in the gain chip towards the AR facet in the RSOA + configuration. Optionally, since a variable laser having a tunable wavelength in the C-band is applied to the gain chip of the RSOA +, the low reflectivity is set to less than 10%. On the other hand, the partial reflection optical properties provide a high transmission rate for emitting the laser generated in the extended cavity of the RSOA + associated with a wavelength tuner and reflector whose wavelength is locked by a wavelength locker.

[0032] Additionally, the partial reflection optical characteristics include a wavelength-dependent mechanism of light reflectivity. In a preferred embodiment, the light reflectivity is configured to monotonically decrease with an increase in wavelength. In an example, the light reflectivity monotonically decreases from about 10% at about 1483 nm to less than 4% at about 1568 nm. Optionally, the tilted AR coating applied to the HR facet can realize this mechanism. Optionally, a coating of a single-layer or three-layer structure is made from those selected from Al2O3, Ta2O5, Si, SiO2, or a combination of these materials. As the joint interference spectrum is generated in the extended cavity of the RSOA+ associated with the wavelength adjuster and the reflector, the lower reflectivity in the HR facet of the longer wavelength light provides a natural suppression of the long wavelength side mode JFSR peak in the joint interference spectrum returning to the gain chip. The long wavelength side mode JFSR suppression is particularly beneficial for adjusting the wavelength variable laser to the low end of the extended C band of the fundamental mode.

[0033] Optionally, the HR facet is configured to provide partial reflection optical characteristics of high reflectivity for light of wavelengths in the C band and a reduction in reflectivity (less than 50%) around the wavelength of the long wavelength side mode JFSR peak. This HR facet configuration of the gain chip of the RSOA can be implemented in a wavelength variable laser having a laser output port at the other end of the wavelength adjuster, which is more distal to the HR facet of the gain chip of the RSOA. Optionally, the output port is on one facet of another semiconductor optical amplifier (SOA). Further details can be described in FIG. 8 and the related description below.

[0034] Referring to part (B) of FIG. 4, it is a schematic cross-sectional view of a gain region 112 configured in a transverse direction with respect to a PN junction diode in the XY plane, where an active layer 1120 is sandwiched between a P-type semiconductor material and an N-type semiconductor material. Optionally, both the P-type semiconductor material and the N-type semiconductor material are indium phosphide (InP) materials having different electrical dopants. The cross-section of the PN junction is provided as a ridge structure on a wider base. The P-type semiconductor material forms a P-type cladding layer on top of the N-type semiconductor material as an N-type cladding layer in the ridge structure along the vertical Y direction. The N-type cladding layer has a base portion wider than the ridge portion in the horizontal X direction. Optionally, the active layer 1120 is a strained layer quantum well structure made of InGaAs or InGaAsP or AlGaInAs. Generally, the active layer 1120 is responsible for inducing a laser having amplified optical energy in a laser light cavity of the gain region 112 having a gain value corresponding to a specific wavelength by causing light emission within a limited space of the PN junction. For a variable laser having an optical wavelength tuned across the extended C band, the normal gain profile is obtained by plotting the gain value as a function of the laser wavelength.

[0035] In an embodiment, the gain chip is reconfigured by disposing an absorption layer 1121 near the active layer 1120 to provide a specified excess loss to at least a portion of the normal gain profile. In particular, the absorption layer 1121 is intended to introduce loss into the gain profile for short wavelengths in order to suppress the short wavelength side mode JFSR peak and enhance the laser light at the fundamental mode JFSR peak. In particular, when the fundamental mode wavelength is adjusted to the high end of the extended C band by a wavelength tuner, the nearest short wavelength side mode JFSR peak is quite close to the low end of the extended C band and should be appropriately suppressed. Optionally, the absorption layer 1121 is disposed in the N-type cladding layer near the active layer 1120. This is because the N-type side of the PN junction has lower free carrier absorption loss. Optionally, the absorption layer 1121 is configured to absorb light at wavelengths shorter than a predetermined value (e.g., 1490 nm, or shorter) that can be the longest wavelength associated with the short wavelength side mode JFSR peak. Optionally, the absorption layer 1121 is made of a layer of GaInAs or AlGaInAs semiconductor material having a bandgap lower than the wavelength of the short wavelength side mode JFSR peak (about 1483 nm), and is intended to suppress it by filtering the short wavelength side mode JFSR peak and shorter wavelengths in the joint interference spectrum.

[0036] Optionally, the gain chip reconfiguration to provide an absorption layer for introducing excess gain loss for a specified short wavelength is applicable to the gain chip in an RSOA+ configuration for a wavelength tunable laser having laser light emission at the HR facet. Optionally, the above gain chip reconfiguration is also applicable to the gain chip in an RSOA configuration for a wavelength tunable laser having a laser light exit at the other end of the wavelength tuner, which is distal to the gain chip of the RSOA. Optionally, the laser light exit is associated with another gain chip in a semiconductor optical amplifier (SOA) configuration added to the other end of the wavelength tuner and a wavelength locker (see FIG. 8).

[0037] FIG. 5 is a cross-sectional view of the spatial distribution of the energy density around the active layer in the gain region of FIG. 4, and a plot of the vertical distribution of the energy density along the Y direction across the active layer, according to one embodiment of the present invention. This figure is merely a plot of the energy distribution of an exemplary design of the reconstructed gain chip. As shown in part (A) of FIG. 5, the central plane of the distribution is along the active layer 1120 of the gain region. The refractive index of the active layer 1120 is always much higher than that of the surrounding cladding layer in the InP material, and due to the optical focusing effect, the fundamental mode of the light emitted from the active layer is limited to those with high energy density. As shown in part (A) of FIG. 5, the high energy density is mainly distributed around the central plane of the active layer 1120 along the X direction, and extends in a narrow range of about ±2 μm above or below the active layer 1120 in the Y direction. Referring to part (A) of FIG. 5, in the embodiment, the absorption layer 1121 is disposed near the active layer 1120 and effectively causes a modification to the energy distribution as described above. In a particular embodiment, the absorption layer 1121 is disposed in the N-type cladding layer near the active layer 1120 within a 2-μm range in the Y direction. The absorption layer 1121 needs to significantly overlap with the fundamental mode of the active layer 1120 in order to effectively play the role of modifying the gain profile. Optionally, the absorption layer 1121 is designed to have a preferred mode overlap of the confinement factor in the range of about 20-30% of the quantum wells associated with the active layer 1120. As a result, the spatial distribution of the energy density in the gain region is changed to be less symmetric compared to the central plane around the periphery of the active layer.

[0038] Furthermore, as shown in part (B) of FIG. 5, the sharp and large peak in the vertical distribution of the energy density is due to the optical focusing effect caused by the high refractive index of the active layer compared to the low refractive indices of the upper or lower cladding layers, indicating that the fundamental mode of the light emission from the active layer in the gain region is mainly limited to around the central plane along the X direction. However, the absorption layer, which also has a relatively high refractive index compared to the nearby InP material cladding layer, also generates a small peak that overlaps within the tail portion of the large peak. Therefore, the absorption layer 1121 disposed so that the active layer and the energy distribution significantly overlap is sufficiently exposed to the light emission from the active layer, and as a result, a sufficient amount of light can be absorbed by the absorption layer. Additionally, the nature of the light absorption depends on the bandgap designed for the absorption layer. In an embodiment, the bandgap of the absorption layer is set lower than a predetermined wavelength, and as a result, any light with a wavelength shorter than the predetermined wavelength value (e.g., 1490 nm) can generally be absorbed by the absorption layer, modifying the gain profile of the gain chip of the RSOA+ in the corresponding wavelength range.

[0039] FIG. 6 shows an exemplary plot of 1) the modulator transmittance of the fundamental mode at the low end of the C-band (about 1526 nm) with a JFSR = 85 nm, 2) the gain profile of the laser emission of the RSOA+, and 3) the reflectivity curve generated by the HR facet coating, according to a particular embodiment of the present invention. This figure is for example only and should not unduly limit the claims. Those skilled in the art will recognize many variations, alternatives, and modifications. In the example, under a particular wavelength modulator design having a joint free spectral range (JFSR) of the joint interference spectrum provided at 85 nm, the joint interference spectrum can include an adjustable fundamental mode JFSR peak at the extended C-band and multiple side mode JFSR peaks of perception. Since the fundamental mode JFSR peak is adjusted to 1526 nm around the low end (C_low) of the extended C-band, referring to FIG. 6, the long wavelength side mode JFSR peak is seen at about 1611 nm. The gain profile (represented by the dotted line) in the same wavelength range for the exemplary gain chip design has a fairly flat and wide gain range across the extended C-band, with a peak around 1540 nm, as well as a relatively reduced (but still greater than 50% of the peak value) gain value in the direction of the fundamental mode JFSR peak at about 1526 nm and the long wavelength side mode JFSR peak at about 1611 nm. This means that the gain chip can potentially be illuminated with laser light at both the preferred fundamental mode JFSR peak (about 1526 nm) and the unwanted long wavelength side mode JFSR peak (about 1611 nm). In this case, a reconfigured gain chip of the RSOA+ having a mechanism for suppressing the unwanted laser light at the wavelength of the long wavelength side mode of the JFSR peak is desirable.

[0040] In an embodiment of the gain chip of RSOA+ shown in FIG. 4(A), the gain chip in the present invention is configured to have an HR facet that provides an optical characteristic in which the optical reflectivity monotonically decreases as the wavelength increases. Referring to FIG. 6, the optical reflectivity monotonically decreases at least from the wavelength of the fundamental mode JFSR peak position to the wavelength of the long-wavelength side mode JFSR peak position. In the example shown in FIG. 6, the reflectivity curve (represented by a single dotted line) shows that the optical reflectivity R = 6.2% at the fundamental mode position of about 1526 nm is reduced to 2.1% at the long-wavelength side mode position. This results in a side mode suppression ratio (SMSR) of about 3 times, enables sufficient suppression of the long-wavelength side mode JFSR peak, and removes unwanted laser light from the gain chip of RSOA+. For the gain chip of RSOA having a high reflectivity facet configured as a simple reflector, the high reflectivity facet provides high reflectivity to light in the C band, but can be configured to reduce the reflectivity for light having a wavelength around the long-wavelength side mode JFSR peak at about 1611 nm and suppress unwanted side mode laser light.

[0041] FIG. 7 shows an exemplary plot of 1) the modulator transmittance of the fundamental mode with JFSR = -85 nm at the high end (about 1568 nm) of the C-band, 2) the gain profile of the lasing of an RSOA+ with / without absorption at optical wavelengths less than 1490 nm, and 3) the reflectivity curve generated by an HR facet coating, according to another specific embodiment of the present invention. This figure is for example only and should not unduly limit the claims. Those skilled in the art will recognize many variations, alternatives, and modifications. In the example, under the same wavelength modulator design with a JFSR given at 85 nm, the joint interference spectrum generated in the extended cavity associated with the wavelength modulator and the gain chip includes a fundamental mode JFSR peak adjustable up to about 1568 nm at the high end of the extended C-band (C_high). Referring to FIG. 7, when the fundamental mode JFSR peak is set at about 1568 nm, the short-wavelength side mode JFSR peak closest to the side mode compared to the extended C-band is about 1483 nm. Also, as shown in FIG. 7, the gain profile (represented by the dotted line) over the same wavelength range is fairly flat, having a peak at about 1530 nm, as well as a fundamental mode JFSR peak at about 1568 nm, and a relatively low gain value (but still greater than 60% of the peak value) in the direction of the short-wavelength side mode JFSR peak at about 1483 nm. The gain chip can potentially illuminate laser light at both the preferred fundamental mode JFSR peak (about 1568 nm) and the undesirable short-wavelength side mode JFSR peak (about 1483 nm). In this case, the gain chip of the RSOA+ is reconfigured to provide a mechanism for removing the side mode laser light, or specifically, it is desirable to suppress the short-wavelength side mode of the JFSR peak. Here, the reflectivity curve (represented by the dash-dot line) from the low-reflectivity coating on the HR facet is plotted, but there is no effect of suppressing the short-wavelength side mode. This is because the reflectivity is actually higher at shorter wavelengths.

[0042] In the embodiment shown in FIG. 4(B), the gain chip of the RSOA in the present invention is reconfigured such that an absorption layer is added in the N-type clad layer near the active layer in order to absorb light having a wavelength shorter than the longest wavelength associated with the short-wavelength side-mode JFSR peak in joint interference transmission. Referring to FIG. 6, the gain profile with the added absorption layer causes a significant loss of the gain value at wavelengths shorter than 1490 nm, effectively suppressing the short-wavelength side-mode JFSR peak at about 1483 nm and removing the corresponding side-mode laser light.

[0043] In another aspect, the present disclosure also provides a gain chip in a reflective semiconductor optical amplifier (RSOA) for a wavelength tunable laser. Optionally, the gain chip emits laser light from a high reflectivity (HR) facet having partial reflection and partial transmission characteristics, and is designed to pass light through an anti-reflection (AR) facet having a substantially high transmittance to adjust the wavelength in the extended C band in an RSOA+ configuration. Optionally, the wavelength tuner is provided as a Vernier ring tuner having two resonant ring waveguides formed on a silicon photonics substrate. Optionally, the wavelength tuner is provided as a dual etalon filter. Optionally, a reflector coupled to the wavelength tuner is included to generate an extended cavity having the HR facet of the gain chip. Optionally, a wavelength locker is added between the wavelength tuner and the gain chip. As described above, the gain chip of the RSOA+ includes a gain region configured as a longitudinal ridge waveguide between the AR facet and the HR facet used to emit a laser. The gain region is configured as a PN junction diode that is transverse to the active layer in the central plane between the P-type cladding layer and the N-type cladding layer. The AR facet includes an anti-reflection coating to allow light emitted from the active layer in the gain region to pass through the AR facet with a transmittance of 99.99% or more. The HR facet, unlike the normal high reflectivity (>90%) coating in the gain chip of a normal RSOA configuration, provides a low reflectivity (<10%) to light emitted from the active layer having a wavelength in the extended C band, and includes a partial reflection coating to provide a partial transmittance to allow the laser light amplified by the RSOA+ to be emitted. Additionally, the gain chip of the RSOA+ includes an absorption layer inserted into the N-type cladding layer of the PN junction diode in the gain region at an overlapping position near the quantum well of the active layer. Both the low reflectivity coating in the HR facet and the absorption layer near the active layer are utilized to modify the gain chip of the RSOA+ to suppress unwanted side mode laser light while allowing desirable fundamental mode laser light having an adjustable wavelength in the extended C band.

[0044] In an embodiment, the partial reflection coating on the HR facet is configured to be an inclined antireflection coating whose reflectivity monotonically decreases with an increase in wavelength. Since the wavelength adjuster adjusts the wavelength of the light that is partially reflected from the HR facet and passes through the AR facet, the joint interference spectrum generated in the extended cavity associated with the wavelength adjuster, the reflector, and the gain chip is directly affected by the decreasing reflectivity of the coating at the HR facet. The higher wavelength JFSR peaks that appear in the joint interference spectrum are subject to suppression by the decreasing reflectivity. For example, for a Vernier ring reflector adjuster designed with JFSR = 85 nm, when the fundamental mode JFSR peak is tuned in the C band and amplified as laser light in the extended cavity, the long wavelength side mode (the nearest one at about 1611 nm) can be similarly suppressed in the laser from the RSOA+.

[0045] In an embodiment, an absorption layer inserted into a PN junction diode near an active layer, within a range of about 4 μm and having a confinement factor of the active layer quantum well of 20 to 30%, is responsible for exposing sufficient optical energy of laser emission induced by the active layer and inducing optical absorption for at least a corresponding wavelength range. At the same time, the absorption layer is designed with a bandgap smaller than the longest JFSR wavelength that needs to be filtered in relation to the short-wavelength side-mode JFSR peak. Thus, the absorption layer provides excess loss to the gain profile by absorbing light having a wavelength shorter than the longest wavelength associated with the short-wavelength side-mode JFSR peak in the joint interference spectrum. Optionally, the change in the gain profile causes a decrease close to 50% of the gain value at wavelengths shorter than 1490 nm, and substantially suppresses the short-wavelength side-mode JFSR peak at about 1483 nm in the laser from the HR facet RSOA+. Optionally, a similar change to the gain profile causes a decrease close to 50% of the gain value for wavelengths shorter than 1490 nm, and substantially suppresses the short-wavelength side-mode JFSR peak at about 1483 nm in the laser from the output port associated with the gain chip of a semiconductor optical amplifier (SOA) located at the other end of a wavelength adjuster further distal from the gain chip of the RSOA+.

[0046] In yet another aspect, the present disclosure provides a wideband wavelength tunable laser based on a gain chip in a reflective semiconductor optical amplifier (RSOA+) configuration described herein. The gain chip of the RSOA+ has an antireflection facet coupled to a wavelength adjuster + reflector in a silicon photonics platform to generate a joint interference spectrum having multiple modes of JFSR peaks separated by a joint free spectral range (JFSR). Optionally, the wavelength adjuster is provided as a Vernier tuning regulator configured to tune the fundamental mode JFSR peak in a wide wavelength range such as the extended C band. Optionally, the wavelength adjuster is provided as a dual etalon filter configured to pass only the fundamental mode. The gain chip of the RSOA+ also has a high reflectivity facet configured as an output port for a laser having a wavelength tuned by the wavelength adjuster in the extended C band. The gain chip of the RSOA+ is described throughout the specification, and low reflectivity optical properties are provided at the high reflectivity facet, and as the wavelength increases, the reflectivity decreases, thereby suppressing the nearest long wavelength side mode JFSR peak in the joint interference spectrum in the extended cavity provided between the reflector and the high reflectivity facet. In an example where the JFSR of the wavelength adjuster is set to 85 nm, since the fundamental mode JFSR peak is set at the low end of the C band at about 1526 nm, the nearest long wavelength side mode JFSR peak is located at about 1611 nm. The optical properties at the high reflectivity facet are configured with a reduced reflectivity of about 2.1% at about 1611 nm compared to about 6.2% at about 1568 nm, and the long wavelength side mode JFSR peak at about 1611 nm can be effectively suppressed by at least 3 times the side mode suppression ratio (SMSR). Additionally, the gain chip is reconfigured with an absorption layer disposed to partially overlap the active layer quantum well to change the gain profile and suppress the short wavelength side mode JFSR peak by providing excess loss in the short wavelength range.In the example of JFSR = 85 nm, since the fundamental mode JFSR peak is set at the high end of the C band at about 1568 nm, the nearest short-wavelength side mode JFSR peak is about 1483 nm. The absorption layer is configured to have a bandgap smaller than about 1490 nm, which is a predetermined wavelength value, and the gain for light excited by the active layer having a wavelength shorter than about 1490 nm can be reduced by half due to light absorption by the absorption layer. Therefore, this short-wavelength side mode JFSR peak with a wavelength around about 1483 nm < 1490 nm can be substantially suppressed. Therefore, the wavelength tunable laser is directed only at the laser having a single wavelength at the fundamental mode JFSR peak that can be adjusted in the C band of 1526 nm to 1568 nm from the high reflectivity facet of the RSOA+.

[0047] In an alternative embodiment, the present disclosure provides a silicon photonics-based wideband wavelength tunable laser including a gain chip in a reflective semiconductor optical amplifier (RSOA) described herein. FIG. 8 is a diagram of a wavelength tunable semiconductor laser module having a thin film filter added to a high reflectivity facet of a gain chip of a reflective semiconductor optical amplifier according to an embodiment of the present invention. This figure is merely an example and should not unduly limit the scope of the claims. Those skilled in the art will recognize many variations, alternatives, and modifications. As shown, the tunable laser module 20 includes a flip of a first laser diode chip 410 coupled to a silicon photonics substrate. Chip 410 has a gain region 415 sandwiched between a first facet 401 and a second facet 402. The first facet 401 is configured to be a high reflectivity facet that operates as a reflector for light excited in the gain region 415. The second facet 402 is configured to have an antireflection property that allows light excited in the gain region 415 and reflected by the first facet 401 to pass through. The second facet 402 is coupled to a first waveguide 491 formed in the silicon photonics substrate via an edge coupler 430. The first waveguide 491 leads to a wavelength adjuster 470, i.e., a variable filter, configured to form an extended cavity with the first facet 401 (as a reflector) in the gain region 415, where an interference spectrum is generated. The interference spectrum is characterized by multiple modes of JFSR peaks separated by a joint free spectral range (JFSR). Optionally, the wavelength adjuster 470 is provided as a Vernier tuning regulator (shown in FIG. 8) configured to adjust the fundamental mode JFSR peak in a wide wavelength range such as the extended C band. Optionally, the wavelength adjuster 470 is provided as a dual etalon filter configured to pass only the fundamental mode. Optionally, the light of the wavelength adjusted by the wavelength adjuster 470 is output to a second waveguide 492 or, optionally, first coupled to a wavelength locker 480 for fine adjustment and locking to a specific wavelength within the extended C band. Optionally, the light of the wavelength selected by the wavelength locker 480 is laser irradiated.Optionally, a second laser chip 460 configured in a semiconductor optical amplifier (SOA) is added to couple to the second waveguide 492 in front of the laser output port 440. The SOA provides a gain region 465 and further amplifies the gain of the laser light before emission through the laser output port 440.

[0048] Similarly, the interference spectrum generated in the extended cavity associated with the wavelength adjuster 470 and the gain region 415 includes a plurality of side-mode JFSR peaks. For example, for the fundamental mode set at the low end of the C band at about 1526 nm with a JFSR set at 85 nm, the long-wavelength side-mode JFSR peak appears at about 1611 nm, and for the fundamental mode set at the high end of the C band at about 1568 nm, the short-wavelength side-mode JFSR peak appears at about 1483 nm. These side modes are in an adjustable range that can become potential but undesirable laser light modes of the wavelength-variable laser 20, i.e., near the extended C band. In an embodiment, the first facet 401 of the chip of the RSOA 410 is configured to add a coating to cause a decrease in reflectivity around the long-wavelength side-mode JFSR peak at about 1611 nm while maintaining a high reflectivity (at least >90%) for wavelengths shorter than 1580 nm. For example, the coating causes the optical reflectivity to decrease by 50% at about 1611 nm compared to the C band. As a result, the long-wavelength side-mode JFSR peak in the interference spectrum is effectively suppressed. Additionally, in an embodiment, the gain region 415 of the chip of the RSOA 410 having a PN junction including an active layer as a normal laser diode can be reconfigured to create an absorption layer in the N-type cladding near the active layer (see, for example, FIG. 4). The position of the absorption layer is set in the N-type cladding with a high free carrier density to help reduce absorption loss. The position of the absorption layer is set near the active layer so as to significantly overlap with the high-energy distribution region of the light excited in the active layer. For example, the active layer is formed as a strained-layer quantum well structure in the PN junction, and an absorption layer that is 20 - 30% of the quantum confinement coefficient of the active layer quantum well is disposed. Further, the absorption layer is configured to be a semiconductor material having a bandgap set smaller than a predetermined value such that optical absorption occurs only for wavelengths shorter than a specific value, for example, shorter than about 1490 nm. For example, an absorption layer made of GaInAsP or AlGaInA having an appropriately designed bandgap can provide an additional 50% loss to the gain profile for wavelengths shorter than 1490 nm.When the absorption layer described in this specification is added to the gain region 415, short-wavelength side-mode JFSR peaks with wavelengths around about 1483 nm < 1490 nm can be substantially suppressed. Therefore, the wavelength-variable laser 20 hits only a single-wavelength laser that is adjustable in the C band of 1526 nm to 1568 nm at the fundamental-mode JFSR peak.

[0049] The above description is a complete description of a particular embodiment, but various modified forms, alternative structures, and equivalents may be used. Therefore, the above description and examples should not be construed as limiting the scope of the invention defined by the appended claims.

Claims

1. A method for improving a wideband wavelength tunable laser, comprising: forming a gain chip that is longitudinal with respect to a gain region between a first facet and a second facet and transverse with respect to a PN junction having an active layer between a P-type clad layer and an N-type clad layer; passing the first facet through a wavelength adjuster configured to couple light excited in the active layer and at least partially reflected from the second facet to generate a joint interference spectrum having multiple modes at isolated spectral peaks separated by a joint free spectral range (JFSR); forming the second facet to have a reduced optical reflectivity for wavelengths increasing from a fundamental mode JFSR peak to a long wavelength side mode JFSR peak; reconfiguring the gain chip having an absorption layer disposed in the N-type clad layer near the active layer to induce gain loss for wavelengths shorter than the longest wavelength associated with a short wavelength side mode JFSR peak; amplifying light at the fundamental mode JFSR peak in the gain chip; and a method comprising the steps of:

2. The step of forming a gain chip includes forming a reflective semiconductor optical amplifier based on the gain region, making the first facet an antireflection facet with a transmittance higher than 99.99% to pass light from the gain region to the wavelength adjuster, and making the second facet have a reflectivity of less than 10% and be partially transmissive for light having a wavelength in an extended C band from about 1526 nm to about 1568 nm, so as to function as an output port for outputting a laser having a wavelength in the extended C band adjusted by the wavelength adjuster. The method according to claim 1.

3. The step of coupling the light includes connecting the wavelength adjuster formed in a silicon photonics substrate to the first facet via an edge coupler to receive the light from the gain chip. The method according to claim 2.

4. The step of coupling the light further includes connecting the wavelength adjuster in the silicon photonics substrate to a reflector via a waveguide to reflect the light that interferes with the light received from the gain chip, thereby generating the joint interference spectrum, according to the method of claim 3.

5. In addition to the reflector, the wavelength adjuster is designed to have the JFSR equal to about 85 nm between the fundamental mode and the nearest side mode, configured to adjust the fundamental mode in the extended C band, and generate the long wavelength side mode at about 1611 nm and the short wavelength side mode at about 1483 nm, according to the method of claim 4.

6. The step of forming the second facet includes forming a low reflectivity coating characterized by a light reflectivity of less than 10% for the extended C band from about 1526 nm to about 1568 nm, where the reflectivity decreases monotonically with increasing wavelength, to suppress the long wavelength side mode JFSR peak at about 1611 nm with a side mode suppression ratio (SMSR) of about three times compared to the reflectivity at the fundamental mode JFSR peak at about 1526 nm, according to the method of any one of claims 2 to 5.

7. The low reflectivity coating is Al ２ O ３ , Ta ２ O ５ , Si, SiO ２ The method according to claim 6, comprising a single layer or a three-layer structure made of a material selected from or a combination of these materials.

8. The active layer includes a strained layer quantum well structure made of InGaAs or InGaAsP or AlGaInAs, configured to excite the light having an energy distribution restricted within a 4-μm range, according to the method of any one of claims 1 to 7.

9. The step of reconfiguring the gain chip includes forming the absorption layer near the active layer within a 2-μm range in the N-type cladding layer, according to the method of claim 8.

10. The absorption layer is formed with a confinement factor of 20 to 30% of the strained layer quantum well structure of the active layer, according to the method of claim 9.

11. The absorption layer includes a GaInAsP or AlGaInAs semiconductor material having a bandgap smaller than the longest wavelength associated with the short wavelength side mode JFSR peak at about 1490 nm, according to the method of claim 9 or 10.

12. The step of forming the gain chip includes: forming a reflective semiconductor optical amplifier based on the gain region; making the second facet a high-reflectivity facet having a reflectivity higher than 90% for wavelengths in the extended C band from about 1526 nm to about 1568 nm while having a reflectivity drop near the wavelengths around the long-wavelength side-mode JFSR peak; making the first facet an antireflection facet having a transmittance higher than 99.99% in order to pass light from the gain region to the wavelength adjuster and further to an output port for outputting a laser having a wavelength in the extended C band adjusted by the wavelength adjuster. The method according to any one of claims 1 to 11.

13. A gain chip of a reflective semiconductor optical amplifier for a wideband wavelength-variable laser, comprising: A gain region configured longitudinally as a straight ridge waveguide between a first facet and a second facet and transversely as a PN junction of a P-type cladding layer and an N-type cladding layer; An active layer formed between the P-type cladding layer and the N-type cladding layer and configured to excite light; An absorption layer in the N-type cladding layer near the active layer, having an overlapping energy distribution to cause excess loss in the gain profile of the light for wavelengths shorter than a predetermined value; Antireflection optical characteristics configured at the first facet; Partial reflection optical characteristics configured at the second facet The light excited in the active layer is partially reflected at the second facet and passed through the first facet into a wavelength adjuster that generates a joint interference spectrum having multiple modes at isolated spectral peaks separated by a joint free spectral range (JFSR). The fundamental mode JFSR peak of the multiple modes is adjusted by the wavelength adjuster at a wideband wavelength and amplified in the gain region before being output as laser light, while the long-wavelength side-mode JFSR peak and the short-wavelength JFSR peak are suppressed. A gain chip.

14. The active layer is composed of a strained layer quantum well structure made of InGaAs or InGaAsP or AlGaInAs, configured to excite the light having a spatially restricted energy distribution within a range of 4 μm, the gain chip according to claim 13.

15. The absorption layer includes a position in the N-type cladding layer near the overlapping active layer, having a confinement coefficient of 20% to 30% of the strained layer quantum well structure, the gain chip according to claim 14.

16. The antireflection optical property in the first facet includes an antireflection coating having a light transmittance higher than 99.99%, enabling the light excited in the active layer to pass into the wavelength adjuster, the gain chip according to any one of claims 13 to 15.

17. The partial reflection optical property in the second facet includes a low reflectance coating having a light reflectance lower than 10% and partial transmissivity for light having a wavelength in the extended C band from about 1526 nm to about 1568 nm, in order to configure the second facet as a laser output port, the gain chip according to any one of claims 13 to 16.

18. The partial reflection optical property in the second facet includes a high reflectance coating having a light reflectance greater than 90% for light having a wavelength in the extended C band from about 1526 nm to about 1568 nm, in order to configure the second facet as a reflector for reflecting light through the wavelength adjuster coupled to another laser output port or associated with another semiconductor optical amplifier, the gain chip according to any one of claims 13 to 16.

19. The wavelength adjuster is designed to be able to adjust the fundamental mode JFSR peak over the entire extended C band from about 1526 nm to about 1568 nm having the JFSR of about 85 nm in size, and the joint interference spectrum includes the nearest long wavelength side mode JFSR peak at about 1611 nm corresponding to the fundamental mode JFSR peak at the low end of the extended C band, and the nearest short wavelength side mode JFSR peak at about 1483 nm corresponding to the fundamental mode JFSR peak at the high end of the extended C band, the gain chip according to any one of claims 13 to 18.

20. The absorption layer includes a GaInAs or AlGaInAs semiconductor material having a bandgap smaller than a predetermined value (= 1490 nm) in order to suppress the nearest short-wavelength side mode JFSR peak at about 1483 nm, the gain chip according to claim 19.

21. The partial reflection optical property in the second facet is configured to provide a light reflectivity that monotonically decreases with an increase in wavelength in order to suppress the nearest long-wavelength side mode JFSR peak at about 1611 nm, and is Al ２ O ３ , Ta ２ O ５ , Si, SiO ２ The gain chip according to claim 19 or 20, comprising a coating of a single-layer or three-layer structure made of a material selected from or a combination of those materials.

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