Side mode suppression for extended c-band tunable lasers

Abstract

Embodiments of the present disclosure relate to extending the side mode suppression of C-band tunable lasers. A method for improving a wideband wavelength tunable laser. The method includes configuring a gain region between a first facet and a second facet and a PN junction with an active layer laterally between a P-type cladding layer and an N-type cladding layer. The method also includes coupling light excited in the active layer and partially reflected from the second facet to pass through the first facet to a wavelength tuner configured to generate a joint interference spectrum with multiple modes separated by a joint free spectral range (JFSR). Further, the method includes configuring the second facet to have a reduced reflectivity for increasing wavelengths. Further, the method includes reconfiguring the gain chip to have an absorption layer near the active layer to introduce a gain loss for short wavelengths relative to a longest wavelength associated with a short wavelength side mode. Further, the method includes outputting the amplified light in a fundamental mode via the second facet.

Description

Technical Field

[0001] This invention relates to optical communication technology. More specifically, the invention provides a method for suppressing side modes of a wavelength-tunable laser based on a reflective semiconductor optical amplifier with an extended C-band, a gain chip for a reflective semiconductor optical amplifier for a tunable laser with side mode suppression in the extended C-band, and a wavelength-tunable laser having the gain chip. Background Technology

[0002] Over the past few decades, the use of communication networks has exploded. In the early days of the internet, popular applications were limited to email, bulletin boards, and were mostly informational and text-based web browsing, with relatively small amounts of data transmitted. Today, the internet and mobile applications require massive amounts of bandwidth to transmit photos, videos, music, and other multimedia files. For example, social networks like Facebook process over 500TB of data daily. This high demand for data and data transmission necessitates improvements to existing data communication systems to meet these needs.

[0003] Broadband DWDM (Dense Wavelength Division Multiplexing) optical transmission with data rates of 40-Gbit / s and then 100-Gbit / s over existing single-mode fiber is the goal of next-generation fiber optic communication networks. Chip-scale broadband wavelength-tunable lasers have attracted considerable attention in many applications such as broadband DWDM communication and wavelength-controlled optical detection and ranging (LIDAR) sensing. Recently, optical components have been integrated on silicon (Si) substrates for the fabrication of large-scale photonic integrated circuits that coexist with microelectronic chips. A full range of photonic components, including filters, (de)multiplexers, splitters, modulators, and photodetectors, have been demonstrated, primarily in silicon-on-insulator (SOI) platforms. SOI platforms are particularly suitable for the standard WDM communication bands of 1300nm and 1550nm because silicon (n=3.48) and its oxide SiO2 (n=1.44) are both transmissive and form high-index contrast, high-confined waveguides that are well-suited for medium- to high-integration silicon photonic integrated circuits (SPICs).

[0004] Wavelength-tunable semiconductor lasers in silicon photonics platforms have been realized as key components for many applications in broadband fiber optic communications, where spectral efficiency has been improved. Various spectrally efficient modulation schemes, such as phase-shift keying (PSK) and quadrature amplitude modulation (QAM), do not rely on rather complex optical phase-locked loops. However, technical challenges exist, such as fabricating small-sized Vernier loops for tunable filters in silicon photonics platforms to extend the free spectral range and achieve sufficient side-mode suppression ratios in extended broadband wavelength-tunable lasers based on reflective semiconductor optical amplifiers. Therefore, improved techniques and methods are needed. Summary of the Invention

[0005] This invention relates to optical communication technology. More specifically, the invention provides a method for suppressing side-mode interference spectra of a wavelength-tunable laser in the extended C-band; a gain chip of a reflective semiconductor optical amplifier configured to emit laser light from a high reflectivity (HR) facet, having a modified gain curve with additional loss for suppressing reduced reflectivity for long-wavelength side-modes and for suppressing short-wavelength side-modes; and a wavelength-tunable laser having a Vernier ring reflector tuner for tuning laser light emitted from the same gain chip with optical wavelengths in the extended C-band, although other applications are also possible.

[0006] In one embodiment, the present invention provides a method for improving a broadband wavelength-tunable laser. The method includes configuring a gain chip longitudinally about a gain region between a first facet and a second facet and laterally about a PN junction having an active layer between a P-type cladding layer and an N-type cladding layer. The method also includes coupling light excited in the active layer and at least partially reflected from the second facet to pass through the first facet to a wavelength tuner configured to generate a joint interference spectrum having multiple modes in isolated spectral peaks separated by the joint free spectral range (JFSR). Furthermore, the method includes configuring the second facet to have reduced optical reflectivity for wavelengths increasing from the fundamental mode JFSR peak to the long-wavelength side-mode JFSR peak. Additionally, the method includes reconfiguring the gain chip to have an absorption layer to introduce gain loss for wavelengths shorter than the longest wavelength associated with the short-wavelength side-mode JFSR peak, the absorption layer being disposed in the N-type cladding layer near the active layer. Furthermore, the method includes amplifying light in the gain chip at the fundamental mode JFSR peak.

[0007] In an alternative embodiment, the present invention provides a gain chip for a reflective semiconductor optical amplifier for a broadband wavelength-tunable laser. The gain chip includes a gain region longitudinally configured as a linear ridge waveguide between a first and a second facet and laterally configured as a PN junction of a P-type cladding layer and an N-type cladding layer. The gain chip also includes an active layer formed between the P-type and N-type cladding layers and configured to excite light. Furthermore, the gain chip includes an absorption layer within the N-type cladding layer, located near the active layer, and having an overlapping energy distribution to introduce additional loss in the gain profile of the light for wavelengths shorter than a predetermined value. The gain chip also includes an anti-reflective optical feature configured at the first facet. Additionally, the gain chip includes a partially reflective optical feature configured at the second facet. Furthermore, light excited in the active layer is partially reflected at the second facet and passes through the first facet into a wavelength tuner, which generates a joint interference spectrum with multiple modes in isolated spectral peaks separated by the Joint Free Spectral Range (JFSR). The fundamental mode JFSR peaks of multiple modes are tuned at a wide wavelength by a wavelength tuner and amplified in the gain region before leaving as laser light, while the long-wavelength side-mode JFSR peaks and short-wavelength JFSR peaks are suppressed.

[0008] In another alternative embodiment, the present invention provides a wavelength-tunable semiconductor laser. The wavelength-tunable semiconductor laser includes a gain chip comprising a gain region longitudinally configured as a linear waveguide between a first facet having anti-reflection characteristics and a second facet having low-reflection characteristics and reduced light reflectivity for longer wavelengths, and laterally configured as a PN junction comprising an active layer for excitation light between a P-type cladding layer and an N-type cladding layer. The wavelength-tunable semiconductor laser also includes an absorption layer formed in the N-type cladding layer near the active layer to introduce additional losses in the gain profile of the light in the gain region. Furthermore, the wavelength-tunable semiconductor laser includes a wavelength tuner formed in a silicon photonic substrate and coupled to the first facet to receive light partially reflected from the second facet. Additionally, the wavelength-tunable semiconductor laser includes a reflector coupled to the wavelength tuner to reflect light back 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). Furthermore, the light in multiple modes of the JFSR peak includes the fundamental mode, which is tunable in the extended C-band from about 1526 nm to about 1568 nm by a wavelength tuner and amplified in the extended cavity before being emitted via the second facet, while the long-wavelength side modes are suppressed by having a low-reflectivity characteristic that reduces reflectivity for the longer wavelengths provided at the second facet, and the short-wavelength side modes are suppressed by additional losses in the gain curve introduced by the absorption layer.

[0009] In another alternative embodiment, the present invention provides a wavelength-tunable semiconductor laser. The wavelength-tunable semiconductor laser includes a first gain chip comprising a first gain region, the first gain region being longitudinally configured as a linear waveguide between a first facet having anti-reflective characteristics and a second facet having high reflectivity but decreasing optical reflectivity for a specific wavelength longer than the upper end of the extended C-band, and laterally configured as a PN junction comprising an active layer for excitation light between a P-type cladding layer and an N-type cladding layer. The wavelength-tunable semiconductor laser also includes an absorption layer formed in the N-type cladding layer near the active layer to introduce additional losses in the gain profile of light in the first gain region. Furthermore, the wavelength-tunable semiconductor laser includes a wavelength tuner formed in a silicon photonic substrate and coupled to the first facet to receive light reflected from the second facet, forming an extended cavity in the first gain region and the second facet to generate a joint interference spectrum having multiple modes separated by a joint free spectral range (JFSR). The wavelength tuner is configured to tune the wavelength of the fundamental mode in multiple modes at the JFSR peak, while long-wavelength sidemodes are suppressed by low-reflectivity characteristics with reduced reflectivity for the longer wavelengths provided at the second facet, and short-wavelength sidemodes are suppressed by additional losses in the gain profile introduced by the absorption layer. Furthermore, the wavelength-tunable semiconductor laser includes a wavelength lock coupled to the wavelength tuner to receive light of the wavelength of the fundamental mode tuned by the wavelength tuner in the extended C-band from approximately 1526 nm to approximately 1568 nm, and to fine-tune and lock this wavelength. Additionally, the wavelength-tunable semiconductor laser includes a laser exit port coupled to the wavelength lockr further away from the first gain chip to emit light of that wavelength. Optionally, the laser exit port is coupled to a facet of a second gain chip of a semiconductor optical amplifier, which is coupled to the wavelength lockr.

[0010] This invention achieves these and other advantages within the context of known art of wavelength-tunable lasers with wavelength tuners, which optionally include reflectors, wavelength lockers, and semiconductor optical amplifiers. However, a further understanding of the nature and advantages of the invention can be achieved by referring to the latter part of the specification and the accompanying drawings. Attached Figure Description

[0011] The following illustrations are merely examples and should not be used to unduly limit the scope of the claims herein. Those skilled in the art will recognize many other variations, modifications, and alternatives. It should also be understood that the examples and embodiments described herein are for illustrative purposes only, and various modifications or variations proposed by those skilled in the art will be included within the spirit and scope of this method and the appended claims.

[0012] Figure 1This is a schematic diagram of a tunable laser based on a reflective semiconductor optical amplifier coupled to a wavelength tuner and a reflector, according to an embodiment of the present invention.

[0013] Figure 2 Figures of individual and joint reflection spectra from a wavelength tuner based on two Vernier rings coupled to a reflector, according to an embodiment of the present invention, are shown.

[0014] Figure 3 This is an example diagram of a laser spectrum having a tunable wavelength in the C-band according to an embodiment of the present invention;

[0015] Figure 4 Simplified views of (A) a top view of a gain region having a high reflectance (HR) facet and an antireflection (AR) facet according to an embodiment of the present invention, and (B) a cross-sectional view of a gain region having an active layer and an absorption layer;

[0016] Figure 5 According to embodiment (A) of the present invention. Figure 4 (A) A cross-sectional view of the spatial distribution of energy density around the active layer in the gain region and (B) a vertical distribution of energy density along the Y direction intersecting with the active layer in the gain region;

[0017] Figure 6 The following diagram illustrates a specific embodiment of the invention: 1) a fundamental mode having a tuner transmittance of JFSR = 85 nm and a low end of the C-band (approximately 1526 nm), 2) a gain curve of a laser in the gain region of RSOA+, and 3) an example diagram of a reflectivity curve generated by an HR faceted coating.

[0018] Figure 7 The following diagram illustrates another specific embodiment of the invention: 1) a tuner transmittance of JFSR = -85 nm and a fundamental mode at the high end of the C-band (approximately 1568 nm); 2) a gain curve of a laser in the gain region of RSOA+ with / without absorptivity at wavelengths <1490 nm; and 3) an example diagram of a reflectivity curve generated by an HR faceted coating; and

[0019] Figure 8 This is a schematic diagram of a tunable laser based on a reflective semiconductor optical amplifier having a wavelength tuner and optionally a semiconductor optical amplifier, according to another embodiment of the present invention. Detailed Implementation

[0020] This invention relates to optical communication technology. More specifically, the invention provides a method for suppressing side-mode interference spectra of a wavelength-tunable laser in the extended C-band; a gain chip of a reflective semiconductor optical amplifier configured to emit laser light from a high reflectivity (HR) facet, having a modified gain curve with additional loss for suppressing reduced reflectivity for long-wavelength side-modes and for suppressing short-wavelength side-modes; and a wavelength-tunable laser having a Vernier ring reflector tuner for tuning laser light emitted from the same gain chip with optical wavelengths in the extended C-band, although other applications are also possible.

[0021] The following description is provided to enable those skilled in the art to make and use the invention, and is incorporated in the context of a particular application. Various modifications and uses in different applications will be apparent to those skilled in the art, and the general principles defined herein can be applied to various embodiments. Therefore, the invention is not intended to be limited to the presented embodiments, but is accorded the widest scope consistent with the principles and novel features disclosed herein.

[0022] In the following detailed description, numerous specific details are set forth to provide a more thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention can be practiced without 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 invention.

[0023] The reader's attention is drawn to all papers and documents submitted concurrently with and made publicly available with this specification, the contents of which are incorporated herein by reference. Unless otherwise expressly stated, all features disclosed in this specification (including any appended claims, abstracts, and drawings) may be replaced with alternative features for the same, equivalent, or similar purposes. Therefore, unless otherwise expressly stated, each disclosed feature is merely one example of a general series of equivalent or similar features.

[0024] Furthermore, any element in the claims that does not expressly specify "means for performing a particular function" or "steps for performing a particular function" should not be construed as a "means" or "steps" clause, as provided in Section 112(6) of Chapter 35 of the United States Code. In particular, the use of "steps for performing a particular function" or "actions for performing a particular function" in the claims herein is not intended to invoke Section 112(6) of Chapter 35 of the United States Code.

[0025] Please note that if used, the labels inner, outer, left, right, front, back, top, bottom, end, forward, reverse, clockwise, and counterclockwise are for convenience only and are not intended to suggest any particular fixed direction. Rather, they are used to reflect the relative position and / or orientation between different parts of an object.

[0026] In one aspect, this disclosure provides a method for improving the performance of wavelength-tunable lasers over extended broadband for various communication applications. Figure 1 In the illustrated embodiment, the tunable laser 10 is provided with a laser flip chip 100 coupled to a wavelength tuner 400, along with a reflector 410 and a wavelength lockout 300. The laser flip chip 100 is bonded to a silicon photonics substrate and includes a gain chip 111 having a gain region 112 that laterally has a PN junction diode within a linear waveguide configured longitudinally as a reflective semiconductor optical amplifier (RSOA). Specifically, the RSOA gain region 112 includes a laser cavity along the linear waveguide between an anti-reflection (AR) facet 102 and a high-reflectivity (HR) facet 101. Light can be excited from an active layer in the PN junction. In one embodiment, light excited in the active layer can be amplified with high reflectivity in the laser cavity at the HR facet 101 before exiting the AR facet 102. For a tunable laser with a normal gain chip in an RSOA configuration, light exiting from the AR facet 102 enters the wavelength tuner 400 and wavelength locker 300. The laser with the tuned wavelength eventually exits from the exit port further away from the HR facet 101. Figure 1 In the alternative embodiment shown, unlike the normal gain chip in the RSOA configuration, which is primarily used as a reflector, the RSOA+ configuration gain chip 111 is designed to configure the HR facet 101 as partially reflective and partially transmissive, and to serve as a laser exit port after the light is coupled to the wavelength tuner 400 of the AR facet 102 for tuning. The HR facet is named using conventional terminology related to the high reflectivity of RSOA. The HR facet 101 of the RSOA+ configuration gain chip 111 can be set to a relatively low (<30%) value.

[0027] In one specific embodiment of the tunable laser 10 based on RSOA+ flip chip 100, the HR facet 101 is configured to have an average reflectivity of only about 5% over the extended C-band from about 1526 nm to 1568 nm, while having relatively high transmittance for use as an exit port for the output laser. The AR facet 102 still allows for a high transmittance of 99.99%, enabling light excited in the active region 112 to be easily coupled via coupler 202 into waveguide 120 to enter wavelength tuner 400 formed in a silicon photonics substrate. In a specific embodiment, wavelength tuner 400 is provided as a Vernier ring tuner, which includes at least two ring waveguides formed in the silicon photonics substrate, such as ring resonators R1 and R2. Each ring resonator R1 or R2 in the extended cavity associated with reflector 410 provides a reflection spectrum with multiple resonant peaks over a wide wavelength range from 1520 nm to 1620 nm (e.g., ...). Figure 2 (As shown in the upper part). Optionally, reflector 410 is a reflective waveguide mirror that provides >95% reflectivity for light across a wide wavelength range. The wavelength spacing between the multiple resonant peaks depends on the corresponding ring diameter and optical index of the dielectric material used to form the ring waveguide. In the extended cavity associated with the combination of wavelength tuner 400 and reflector 410, the two reflection spectra generate a joint interference spectrum (referred to as R1*R2) within the same wavelength range, as shown in the upper part. Figure 2 As shown at the bottom. The joint interference spectrum is characterized by multiple resonant peaks, or so-called modes, with a joint free spectral range (JFSR) as the wavelength interval between two adjacent principal interference JFSR peaks. The value of the JFSR depends on the ring diameters of the two ring waveguides.

[0028] In one embodiment, the wavelength tuner 400 for the broadband tunable laser is preferably designed to make the JFSR much larger than the desired tunable range to allow the fundamental mode JFSR peak to pass while suppressing the nearest sidemode JFSR peak. In one example, the JFSR is made as large as 85 nm for the proposed wavelength tuning in the extended C-band from 1526 nm to 1568 nm. With the JFSR set to 85 nm, for the fundamental mode JFSR peak at a first wavelength of approximately 1526 nm, a corresponding nearest long-wavelength sidemode JFSR peak will be found at a second wavelength of approximately 1611 nm. For the fundamental mode JFSR peak at a first wavelength of approximately 1568 nm, a corresponding nearest short-wavelength sidemode JFSR peak can be found at a second wavelength of approximately 1483 nm. Both long-wavelength sidemodes and short-wavelength sidemodes are preferably sufficiently suppressed to improve the performance of the broadband tunable laser 10. Alternatively, the wavelength tuner 400 can be provided as a dual-etalon filter to allow only the fundamental mode JFSR peak to pass through and to reflect it back to the gain chip using a reflector.

[0029] In principle, the wavelength tuner plus reflector configuration acts as a wavelength selective filter or wavelength tuner to introduce the returned tuner transmission along with the joint interference spectrum back to the gain chip 111. The returned tuner transmission contains the fundamental mode JFSR peak, and all side-mode JFSR peaks can be shaped in the gain chip 111 to eliminate or at least suppress all side-modes. In one embodiment, the wavelength-tunable laser for extended C-band is configured by implementing a gain chip 111 in an RSOA+ configuration, where the laser exits from the HR facet. The fundamental mode JFSR peak can be tuned from a low-end wavelength C_low = 1526 nm to a high-end wavelength C_high = 1568 nm. When the fundamental mode is tuned to C_low, most of the long-wavelength side-modes of the JFSR peak at approximately 1611 nm should be suppressed in the returned transmission to avoid them being included in the emitted laser. When the fundamental mode is tuned to C_high, most of the short-wavelength side-modes of the JFSR peak should be suppressed to improve the performance of the tunable laser. If side-mode lasers are properly suppressed, the fundamental mode JFSR peak can be shaped to the center or optimal position of the gain curve to determine laser emission with a single-wavelength spike. The wavelength corresponding to the fundamental mode JFSR peak can be tuned based on the design of the wavelength tuner plus reflector configuration, by changing the temperature to alter the optical indices of each waveguide. For example, by setting the optimal temperature using a pre-calibration voltage supplied to the resistive heaters associated with the two resonators R1 and R2, the fundamental mode JFSR peak is initially set to a certain wavelength. Figure 1As shown, the pre-calibration voltages VR1 and VR2 can be stored in a lookup table in memory, which can be read each time to initialize the silicon photonics-based tunable laser device. Coarse wavelength tuning can be achieved by varying the temperature around the two ring resonators R1 and R2 to tune the wavelength within an extended tunable range near the optimal gain curve position. Temperature variations are controlled using an electric heater (not shown) driven by the applied voltages VR1 and VR2. Furthermore, fine wavelength tuning can be performed by varying the temperature around the wavelength lockout 300.

[0030] Figure 3 This is an example diagram of the laser spectrum output by a tunable laser whose laser wavelength is tuned according to an embodiment of the present invention. As shown, the laser emission wavelength is given by the peak position of the spectrum shaped by superimposing the fundamental mode JFSR peak with the laser emission gain curve. In this example, the laser emission wavelength is tuned from 1555 nm to 1535 nm. For broadband wavelength tuning, it is preferable to make the JFSR designed for the tunable laser as wide as possible to achieve better mode selection, thereby having strong single-wavelength light emission at the fundamental mode while minimizing interference at the side modes. However, to achieve a wide JFSR in a Vernier ring, a very small ring resonator diameter is required, and a more complex waveguide structure needs to be formed in the silicon photonic substrate to achieve mode selection / suppression and stability. Designing and constructing ring resonators with ultra-small diameters or adding additional components to improve mode selection on silicon photonic substrates is technically challenging and, in many ways, economically disadvantageous.

[0031] In one aspect, the present invention provides a method for improving the selection of the fundamental mode wavelength from a gain chip coupled to a wavelength tuner and reflector of an RSOA (Resonance Radiation Optimizer) outside the extended C-band by suppressing both short-wavelength and long-wavelength side-mode JFSR peaks. This method is based on reconfiguring the gain chip by modifying the optical reflectivity at the HR (High Reflectance) facet and altering the optical absorption. Figure 4 Simplified views of (A) a top view of a gain region having HR and AR facets and (B) a cross-sectional view of a gain region having an active layer and an absorber layer, according to embodiments of the present invention. These figures are merely illustrative and should not be construed as limiting the scope of the claims undue. Many variations, alternatives, and modifications will be recognized by those skilled in the art. References Figure 4 Part (A) and also refer to Figure 1The gain region 112 of the RSOA+100 gain chip 111 is configured longitudinally (in the Z direction) as a linear waveguide with an HR facet 101 and an AR facet 102. Specifically, the AR facet 102 is fabricated with anti-reflective optical features, where the transmittance is set to 99.99% or higher to allow light over a wide wavelength range to pass through. In particular, light excited in the gain region readily passes through the AR facet and the coupler 202 at the edge of the gain chip 111 into the waveguide 120 in the silicon photonic substrate (see [link to original text]). Figure 1 Furthermore, light in the waveguide is guided to a wavelength tuner plus reflector configuration formed in the same silicon photonic substrate. In one embodiment, the HR facet 101 is provided with partially reflective optical features. On one hand, the partially reflective optical features optionally provide low reflectivity (<30%) to reflect light excited in the gain chip to the AR facet of the RSOA+ configuration. Optionally, for the RSOA+ gain chip used in a wavelength-tunable laser applied to the C-band, the low reflectivity is set to <10%. On the other hand, the partially reflective optical features provide high transmittance for exciting lasers generated in the extended cavity of the RSOA+ associated with the wavelength tuner and reflector at a wavelength locked by the wavelength locker.

[0032] Furthermore, some reflective optical features include a wavelength-dependent characteristic of optical reflectivity. In a preferred embodiment, optical reflectivity is configured to decrease monotonically with increasing wavelength. In one example, optical reflectivity monotonically decreases from about 10% at about 1483 nm to <4% at about 1568 nm. Optionally, this feature can be achieved by applying a tilted AR coating to the HR facet. Optionally, the coating comprises a single-layer or three-layer structure made of a material selected from Al2O3, Ta2O5, Si, SiO2, or combinations thereof. Due to the joint interference spectrum generated in the RSOA+ extended cavity associated with the wavelength tuner and reflector, the lower reflectivity at the HR facet for longer wavelengths naturally suppresses long-wavelength side-mode JFSR peaks in the joint interference spectrum returning to the gain chip. Long-wavelength side-mode JFSR suppression is particularly beneficial for wavelength-tunable lasers to tune the fundamental mode to the lower end of the extended C-band.

[0033] Optionally, the HR facet is configured to provide partially reflective optical characteristics with high reflectivity for wavelengths in the C-band while exhibiting reduced reflectivity (<50%) for wavelengths near the JFSR peak of long-wavelength sidemodes. This HR facet configuration of the RSOA's gain chip can be implemented in a wavelength-tunable laser, where the laser exit port is located at the other end of the HR facet of the RSOA's gain chip, further away from the wavelength tuner. Optionally, the exit port is located at a facet of another semiconductor optical amplifier (SOA). More details can be found on [the relevant page / link]. Figure 8 It can be found in the related descriptions below.

[0034] refer to Figure 4 Part (B) is a schematic cross-sectional view of the gain region 112, which is laterally configured as a PN junction diode in the XY plane, wherein the active layer 1120 is sandwiched between a P-type semiconductor material and an N-type semiconductor material. Optionally, both the P-type and N-type semiconductor materials are indium phosphide (InP) materials with 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, which serves as an N-type cladding layer, in the ridge structure along the vertical Y direction. The N-type cladding layer has a base wider than the ridge in the horizontal X direction. Optionally, the active layer 1120 is a strained layer quantum well structure made of InGaAs, InGaAsP, or AlGaInAs. Generally, the active layer 1120 is specified to induce light emission within the confined space of the PN junction to induce the light energy of the laser to be amplified in the laser cavity of the gain region 112 with a gain value corresponding to a specific wavelength. For a tunable laser whose wavelength is tuned in the extended C-band, a normal gain curve is obtained by plotting the gain value as a function of the laser wavelength.

[0035] In this embodiment, the gain chip is reconfigured by placing the absorption layer 1121 near the active layer 1120 to provide specified additional loss to at least a portion of the normal gain profile. Specifically, the absorption layer 1121 is designed to introduce gain profile loss for short wavelengths to suppress short-wavelength side-mode JFSR peaks, thereby enhancing laser emission at the fundamental mode JFSR peak. In particular, when the wavelength tuner tunes the fundamental mode wavelength to the high end of the extended C-band, 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 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 with wavelengths shorter than a predetermined value (e.g., 1490 nm or less), which may be the longest wavelength associated with the short-wavelength side-mode JFSR peak. Optionally, the absorption layer 1121 is made of a GaInAsP or AlGaInAs semiconductor material layer, the band gap of which is smaller than the wavelength of the short-wavelength side-mode JFSR peak (about 1483 nm), with the aim of suppressing the short-wavelength side-mode JFSR peak by filtering it and the shorter wavelength in the joint interference spectrum.

[0036] Optionally, a gain chip reconfiguration is provided to the RSOA+ configuration, which introduces additional gain loss for a specified short wavelength, for a wavelength-tunable laser with the laser exit at the HR facet. Alternatively, the aforementioned gain chip reconfiguration also applies to the RSOA configuration gain chip for a wavelength-tunable laser with the laser exit at the other end of the wavelength tuner, further from the RSOA gain chip. Optionally, the laser exit port is added to the other end of the wavelength tuner and a wavelength lockout (see...). Figure 8 It is associated with another gain chip in the configuration of a semiconductor optical amplifier (SOA).

[0037] Figure 5 According to embodiment (A) of the present invention. Figure 4 A cross-sectional view of the spatial distribution of energy density around the active layer in the gain region, and a vertical distribution of energy density along the Y-direction intersecting the active layer. This diagram is merely an energy distribution diagram of an exemplary design for a reconfigured gain chip. Figure 5 As shown in section (A), the central plane of the distribution runs along the active layer 1120 of the gain region. Since the refractive index of the active layer 1120 is always much higher than the refractive index around the InP cladding layer, an optical focusing effect causes the fundamental mode of light emitted from the active layer to be confined therewith at a high energy density. As... Figure 5 As shown in section (A), the high energy density is primarily distributed along the X-direction around the central plane of the active layer 1120, and extends in the Y-direction to a narrow range of approximately ±2 μm above or below the active layer 1120. (Reference) Figure 5 In part (A), in one embodiment, the absorber layer 1121 is disposed near the active layer 1120 to effectively induce the change in energy distribution as described above. In a specific embodiment, the absorber layer 1121 is disposed in the N-type cladding layer within a 2 μm radius near the active layer 1120 in the Y direction. The absorber layer 1121 needs to significantly overlap with the fundamental mode of the active layer 1120 to effectively modify the gain profile. Optionally, the absorber layer 1121 is designed to be within a preferred mode overlap range of approximately 20-30% confinement factor of the quantum well associated with the active layer 1120. As a result, the spatial distribution of energy density in the gain region is changed to be less symmetrical relative to the central plane surrounding the active layer.

[0038] Further as Figure 5As shown in section (B), the sharp main peak in the vertical energy density distribution indicates that, due to the optical focusing effect caused by the high refractive index of the active layer relative to the low refractive index of the upper or lower cladding layer, the fundamental mode of light emission from the active layer in the gain region is mainly confined to the area around the central plane along the X direction. However, the absorption layer, which also has a relatively high refractive index compared to the nearby cladding layer with InP material, also generates overlapping small peaks at the tail of the main peak. Therefore, the absorption layer 1121, with its significant energy distribution overlapping with the active layer, is sufficiently exposed to light emission from the active layer, allowing a sufficient amount of light to be absorbed by the absorption layer. Furthermore, the nature of the light absorption depends on the band gap designed for the absorption layer. In one embodiment, the band gap of the absorption layer is set below a predetermined wavelength, such that any light with a wavelength shorter than the predetermined wavelength value (e.g., 1490 nm) can be largely absorbed by the absorption layer to modify the gain curve of the gain chip of RSOA+ in the corresponding wavelength range.

[0039] Figure 6 An example diagram is shown according to a specific embodiment of the invention: 1) a fundamental mode with a tuner transmittance of JFSR = 85 nm and the low end of the C-band (approximately 1526 nm); 2) a gain curve of laser emission from RSOA+; and 3) a reflectivity curve generated by an HR faceted coating. This diagram is merely illustrative and should not be overly limiting of the scope of the claims. Many variations, alternatives, and modifications will be recognized by those skilled in the art. In this example, under a specific wavelength tuner design with a joint free spectral range (JFSR) providing a joint interference spectrum at 85 nm, the joint interference spectrum may include a fundamental mode JFSR peak tunable in the extended C-band and several nearby side-mode JFSR peaks. Since the fundamental mode JFSR peak is tuned to 1526 nm near the low end of the extended C-band (C_low), reference... Figure 6 Therefore, the long-wavelength side-mode JFSR peak will appear at approximately 1611 nm. For the exemplary gain chip design, the gain curve (represented by the dashed line) within the same wavelength range has a fairly flat and wide gain range in the extended C-band, with a peak around 1540 nm, and the gain value decreases relatively (but is still greater than 50% of the peak) towards the fundamental mode JFSR peak at approximately 1526 nm and the long-wavelength side-mode JFSR peak at approximately 1611 nm. This means that the gain chip may generate lasing at both the fundamental mode JFSR peak (approximately 1526 nm) (which is preferred) and the long-wavelength side-mode JFSR peak (approximately 1611 nm) (which is undesirable). In this case, a reconfigured gain chip of RSOA+ is needed, which features unwanted lasing at the wavelengths of the long-wavelength side-modes used to suppress the JFSR peaks.

[0040] exist Figure 4In the embodiment of the RSOA+ gain chip shown in (A), the gain chip of the present invention is configured such that the HR facet has an optical feature in which the light reflectivity monotonically decreases with increasing wavelength. Reference Figure 6 The light reflectivity monotonically decreases from the wavelength at the peak position of the fundamental mode JFSR to the wavelength at the peak position of the longer-wavelength side-mode JFSR. Figure 6 In the example shown, the reflectivity curves (represented by the dotted lines) indicate that the light reflectivity R = 6.2% at the fundamental mode location (approximately 1526 nm) decreases to 2.1% at the long-wavelength sidemode location. This translates to approximately a 3x sidemode suppression ratio (SMSR) to adequately suppress the long-wavelength sidemode JFSR peak, thereby eliminating unwanted laser emission in the RSOA+ gain chip. For RSOA gain chips where the high-reflectivity facet is configured as a pure reflector, the high-reflectivity facet can be configured to provide high reflectivity for C-band light but reduce reflectivity for light near the long-wavelength sidemode JFSR peak at approximately 1611 nm to suppress unwanted sidemode lasing.

[0041] Figure 7 An example diagram is shown based on another specific embodiment of the invention: 1) a tuner transmittance with a JFSR of -85 nm and a fundamental mode at the high end of the C-band (approximately 1568 nm); 2) a gain curve for laser emission with / without absorptivity at wavelengths <1490 nm for RSOA+; and 3) an example diagram of reflectivity generated by an HR faceted coating. This diagram is merely illustrative and should not be overly limiting of the scope of the claims. Many variations, alternatives, and modifications will be recognized by those skilled in the art. In this example, with the same wavelength tuner design, the JFSR is given at 85 nm, and the joint interference spectrum generated in the extended cavity associated with the wavelength tuner and gain chip includes a fundamental mode JFSR peak tunable up to approximately 1568 nm at the high end of the extended C-band (C_high). Reference Figure 7 When the fundamental mode JFSR peak is set at approximately 1568 nm, the short-wavelength sidemode JFSR peak, which is closest to the extended C-band sidemode, is approximately 1483 nm. Similarly... Figure 7As shown, the gain curves (represented by dashed lines) within the same wavelength range are fairly flat, with a peak around 1530 nm. The gain decreases relatively towards the fundamental mode JFSR peak at approximately 1568 nm and the short-wavelength side-mode JFSR peak at approximately 1483 nm (but is still greater than 60% of the peak). The gain chip may generate lasing at both the fundamental mode JFSR peak (approximately 1568 nm) (preferred) and the short-wavelength side-mode JFSR peak (approximately 1483 nm) (undesirable). In this case, a reconfigured gain chip from RSOA+ is needed to provide characteristics for eliminating side-mode lasing or specifically suppressing short-wavelength side-modes at JFSR peaks. The reflectivity curve (represented by dashed lines) from the low-reflectivity coating at the HR facet is plotted here, but it has no effect on suppressing short-wavelength side-modes because its reflectivity is actually higher at shorter wavelengths.

[0042] exist Figure 4 In the embodiment shown in (B), the gain chip of the RSOA of the present invention is reconfigured to have an absorption layer added in an N-type cladding layer near the active layer to absorb wavelengths smaller than the longest wavelength associated with the short-wavelength sidemode JFSR peak in joint interferometric transmission. Reference Figure 6 The gain curve with the added absorption layer generates significant gain loss at wavelengths shorter than 1490nm, effectively suppressing the short-wavelength side-mode JFSR peak at approximately 1483nm to eliminate the corresponding side-mode lasing.

[0043] On the other hand, this disclosure also provides a gain chip in a reflective semiconductor optical amplifier (RSOA) for a wavelength-tunable laser. Optionally, the gain chip is configured in an RSOA+ configuration, designed to emit laser light from a highly reflective (HR) facet having partially reflective and partially transmissive characteristics, and to transmit the light via an antireflective (AR) facet having substantially high transmittance to a wavelength tuner to tune an extended C-band wavelength. Optionally, the wavelength tuner is provided as a Vernier ring tuner having two resonant ring waveguides formed in a silicon photonic substrate. Optionally, the wavelength tuner is provided as a bietalon filter. Optionally, a reflector coupled to the wavelength tuner is included to generate an extended cavity with the HR facet of the gain chip. Optionally, a wavelength lock is added between the wavelength tuner and the gain chip. As described above, the gain chip of the RSOA+ includes a gain region longitudinally configured as a ridge waveguide between the AR and HR facets for emitting laser light. The gain region is laterally configured as a PN junction diode with an active layer in the central plane between a P-type cladding layer and an N-type cladding layer. The AR facet includes an anti-reflective coating to allow light emitted from the active layer in the gain region to pass through the AR facet with 99.99% or higher transmittance. The HR facet includes a partially reflective coating, which, unlike the conventional high-reflectivity (>90%) coating in the gain chip of a normal RSOA configuration, provides low reflectivity (<10%) for light emitted from the active layer at wavelengths in the extended C-band, and provides partial transmittance to allow laser amplification through the RSOA+. Additionally, the RSOA+ gain chip includes an absorption layer inserted into the N-type cladding layer of the PN junction diode in the gain region at an overlap location near the quantum well of the active layer. Both the low-reflectivity coating of the HR facet and the absorption layer near the active layer are used to modify the RSOA+ gain chip to suppress unwanted side-mode lasers while allowing the desired fundamental mode laser to be tunable in the extended C-band.

[0044] In one embodiment, the partially reflective coating at the HR facet is configured as a tilted antireflective coating with a monotonically decreasing reflectivity as wavelength increases. Since the wavelength tuner tunes the wavelength of light partially reflected from the HR facet and passing through the AR facet, the joint interference spectrum generated in the extended cavity associated with the wavelength tuner, reflector, and gain chip is directly affected by the reduced reflectivity of the coating at the HR facet. JFSR peaks appearing in the joint interference spectrum at longer wavelengths are suppressed by the reduced reflectivity. For example, for a Vernier ring reflector tuner with a JFSR of 85 nm, when the fundamental mode JFSR peak is tuned in the C-band and used for laser amplification in the extended cavity, long-wavelength side modes (the most recent one at approximately 1611 nm) can be correspondingly suppressed in lasers from RSOA+.

[0045] In one embodiment, an absorption layer is inserted into the PN junction diode. A confinement factor of approximately 20-30% is specified in the vicinity of the active layer and the quantum well of the active layer to expose sufficient light energy for laser emission induced by the active layer and to induce light absorption at least within the corresponding wavelength range. Simultaneously, the bandgap of the absorption layer is designed to be smaller than the longest JFSR wavelength that needs to be filtered in relation to the short-wavelength side-mode JFSR peaks. Therefore, the absorption layer provides additional loss to the gain profile by absorbing light with wavelengths shorter than the longest wavelength associated with the short-wavelength side-mode JFSR peaks in the joint interference spectrum. Optionally, the change in the gain profile results in a gain value decrease of nearly 50% at wavelengths less than 1490 nm, thereby significantly suppressing the short-wavelength side-mode JFSR peaks at approximately 1483 nm in lasers outside the HR facet RSOA+. Alternatively, a similar change to the gain profile results in a gain value decrease of nearly 50% at wavelengths less than 1490 nm, which greatly suppresses the short-wavelength sidemode JFSR peak at approximately 1483 nm in the laser coming from the exit port of the semiconductor optical amplifier (SOA) associated with the gain chip located at the other end of the wavelength tuner further away from the gain chip of RSOA+.

[0046] In another aspect, this disclosure provides a broadband wavelength-tunable laser based on a gain chip configured as a reflective semiconductor optical amplifier (RSOA+) as described herein. The RSOA+ gain chip has an anti-reflection facet coupled to a wavelength tuner plus a reflector in a silicon photonics platform to generate a joint interference spectrum having JFSR peaks of multiple modes separated by the joint free spectral range (JFSR). Optionally, the wavelength tuner is provided as a Vernier ring tuner configured to tune the fundamental mode JFSR peak over a wide wavelength range, such as the extended C-band. Optionally, the wavelength tuner is provided as a bieta filter configured to allow only the fundamental mode to pass through. The RSOA+ gain chip also has a high reflectivity facet configured as an exit port for a laser whose wavelength is tuned by the wavelength tuner in the extended C-band. As described throughout the specification, the RSOA+ gain chip provides low-reflectivity optical characteristics at the high-reflectivity facet, with reflectivity decreasing as wavelength increases, to suppress the nearest long-wavelength side-mode JFSR peak in the joint interference spectrum within the extended cavity positioned between the reflector and the high-reflectivity facet. In an example where the wavelength tuner's JFSR is set to 85 nm, the nearest long-wavelength side-mode JFSR peak is located at approximately 1611 nm, since the fundamental mode JFSR peak is set at the lower end of the C-band at approximately 1526 nm. Because the optical characteristics of the high-reflectivity facet are configured to reduce reflectivity by approximately 2.1% at approximately 1611 nm and by approximately 6.2% at approximately 1568 nm, the long-wavelength side-mode JFSR peak at approximately 1611 nm can be effectively suppressed with a side-mode suppression ratio (SMSR) of at least 3X. Furthermore, the gain chip is reconfigured where the absorption layer is set to partially overlap with the active layer quantum well, thereby altering the gain profile by providing additional losses over the short wavelength range, thus suppressing short-wavelength side-mode JFSR peaks. In the example with JFSR = 85 nm, the most recent short-wavelength sidemode JFSR peak is at approximately 1483 nm, because the fundamental mode JFSR peak is set at the high end of the C-band at approximately 1568 nm. Since the absorption layer is configured with a bandgap of a predetermined wavelength value less than approximately 1490 nm, the gain of light excited by the active layer at wavelengths less than approximately 1490 nm can be reduced by half due to light absorption by the absorption layer. Therefore, this short-wavelength sidemode JFSR peak (with wavelengths of approximately 1483 nm < 1490 nm) can be sufficiently suppressed. Thus, the wavelength-tunable laser emits laser light with a single wavelength only at the fundamental mode JFSR peak, which is tunable in the C-band from 1526 nm to 1568 nm outside the high reflectivity facet of RSOA+.

[0047] In an alternative embodiment, this disclosure provides a silicon photonics-based broadband wavelength-tunable laser that includes a gain chip in a reflective semiconductor optical amplifier (RSOA) as described herein. Figure 8 This is a schematic diagram of a wavelength-tunable semiconductor laser module with 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 diagram is merely illustrative and should not be overly limiting of the scope of the claims. Many variations, alternatives, and modifications will be recognized by those skilled in the art. As shown, the tunable laser module 20 includes a first laser diode chip 410 flip-chip bonded to a silicon photonic substrate. The chip 410 has a gain region 415 defined between a first facet 401 and a second facet 402. The first facet 401 is configured as a high-reflectivity facet, which serves as a reflector for light excited in the gain region 415. The second facet 402 is configured with anti-reflective features to allow light excited in the gain region 415 and reflected by the first facet 401 to pass through. The second facet 402 is coupled via an edge coupler 430 to a first waveguide 491 formed in the silicon photonic substrate. A first waveguide 491 is guided to a wavelength tuner 470, i.e., a tunable filter, which is configured to generate an extended cavity with a first facet 401 (as a reflector) in a gain region 415 to generate an interference spectrum therein. The interference spectrum is characterized by multiple modes of the JFSR peak being separated by a joint free spectral range (JFSR). Optionally, the wavelength tuner 470 is provided as a Vernier ring tuner (e.g., configured to tune the fundamental mode JFSR peak over a wide wavelength range, such as the extended C-band) Figure 8 (As shown). Optionally, wavelength tuner 470 is provided as a bietallar filter configured to allow only the fundamental mode to pass through. Optionally, light of a wavelength tuned by wavelength tuner 470 will be output to second waveguide 492, or optionally first coupled to wavelength locker 480 for fine tuning and locked to a specific wavelength within the extended C-band. Optionally, light of wavelength selected by wavelength locker 480 will be emitted by laser. Optionally, a second laser chip 460 coupled to second waveguide 492 and configured in a semiconductor optical amplifier (SOA) is added before laser exit port 440. SOA provides gain region 465 to further amplify the laser gain before emission via laser exit port 440.

[0048] Similarly, the interference spectrum generated in the extended cavity associated with wavelength tuner 470 and gain region 415 includes multiple side-mode JFSR peaks. For example, with the JFSR set to 85 nm, for the fundamental mode set at the lower end of the C-band at approximately 1526 nm, a long-wavelength side-mode JFSR peak appears at approximately 1611 nm; for the fundamental mode set at the higher end of the C-band at approximately 1568 nm, a short-wavelength side-mode JFSR peak appears at approximately 1483 nm. These side modes approach the tunable range, i.e., the extended C-band, becoming potential, but undesired, laser modes for the wavelength-tunable laser 20. In this embodiment, the first facet 401 of the RSOA 410 chip is configured to add a coating to introduce a decrease in reflectivity around the long-wavelength side-mode JFSR peak at approximately 1611 nm, while retaining high reflectivity (at least >90%) for wavelengths shorter than 1580 nm. For example, the coating results in a 50% decrease in light reflectivity at approximately 1611 nm compared to the C-band. As a result, the long-wavelength side-mode JFSR peaks in the interference spectrum were effectively suppressed. Furthermore, in this embodiment, the gain region 415 of the RSOA 410 chip, which has a PN junction and an active layer as a conventional laser diode, can be reconfigured to introduce an absorption layer near the active layer in the N-type cladding layer (e.g., see...). Figure 4 The absorption layer is positioned within an N-type cladding layer with a high free carrier density to help reduce absorption losses. The absorption layer is positioned near the active layer such that it significantly overlaps with the high-energy distribution region of the light excited in the active layer. For example, the active layer is formed as a strained quantum well structure in the PN junction, and the absorption layer is configured to have a quantum confinement factor of 20-30% of the active layer quantum well. Furthermore, the absorption layer is configured with a semiconductor material having a bandgap set to a value less than a predetermined value, such that light absorption occurs only for wavelengths shorter than a specific value (e.g., about 1490 nm). For example, using an absorption layer made of GaInAsP or AlGaInAs with a properly designed bandgap can provide an additional 50% loss for gain curves with wavelengths shorter than 1490 nm. By adding the absorption layer described herein to gain region 415, short-wavelength sidemode JFSR peaks with wavelengths from about 1483 nm to 1490 nm can be substantially suppressed. Therefore, the wavelength-tunable laser 20 emits a single-wavelength laser only at the peak of the fundamental mode JFSR, which is tunable in the C-band from 1526 nm to 1568 nm.

[0049] While the foregoing is a complete description of specific embodiments, various modifications, alternative constructions, and equivalents may be used. Therefore, the foregoing description and illustrations should not be construed as limiting the scope of the invention as defined by the appended claims.

Claims

1. A method for improving a broadband wavelength tunable laser, comprising: The gain chip is configured longitudinally about the gain region between the first and second facets and laterally about the PN junction with an active layer between the P-type and N-type cladding layers; Light excited in the active layer and at least partially reflected from the second facet to pass through the first facet is coupled to a wavelength tuner configured to generate a joint interference spectrum with multiple modes in isolated spectral peaks separated by the Joint Free Spectral Range (JFSR). The second facet is configured to have reduced optical reflectivity for wavelengths increasing from the fundamental mode JFSR peak to the long-wavelength side-mode JFSR peak; The gain chip is reconfigured to have an absorption layer to introduce gain loss for wavelengths shorter than the longest wavelength, which is associated with the short-wavelength side-mode JFSR peak. The absorption layer is disposed in the N-type cladding layer near the active layer. as well as The light at the JFSR peak of the fundamental mode is amplified in the gain chip. The second facet has an inclined anti-reflective coating, the light reflectivity of which decreases with increasing wavelength.

2. The method according to claim 1, wherein configuring the gain chip comprises: A reflective semiconductor optical amplifier is formed based on the gain region, such that the first facet is an anti-reflective facet with >99.99% transmittance for transmitting light from the gain region to the wavelength tuner, and the second facet has <10% reflectivity and partial transmittance for light in the extended C-band with wavelengths in the range of 1526 nm to 1568 nm, to serve as an exit port for outputting laser light with wavelengths in the extended C-band tuned by the wavelength tuner.

3. The method of claim 2, wherein coupling the light comprises: The wavelength tuner formed in the silicon photonic substrate is connected to the first facet via an edge coupler to receive the light from the gain chip.

4. The method of claim 3, wherein coupling the light further comprises: The wavelength tuner in the silicon photonic substrate is connected to a reflector via a waveguide to reflect light that interferes with the light received from the gain chip, thereby generating the joint interference spectrum.

5. The method of claim 4, wherein the wavelength tuner plus the reflector is designed to have a JFSR of 85 nm between the fundamental mode and the nearest side mode, and is configured to tune the fundamental mode in the extended C band to produce the long wavelength side mode at 1611 nm and the short wavelength side mode at 1483 nm.

6. The method of claim 2, wherein configuring the second facet comprises: A low-reflectivity coating is formed, characterized in that the light reflectivity for the extended C-band from 1526 nm to 1568 nm is <10%, and the light reflectivity decreases monotonically with increasing wavelength, so as to suppress the long-wavelength side-mode JFSR peak at 1611 nm with a side-mode suppression ratio (SMSR) of 3 times compared to the reflectivity of the fundamental mode JFSR peak at 1526 nm.

7. The method of claim 6, wherein the low reflectivity coating comprises a one-layer or three-layer structure made of a material selected from Al2O3, Ta2O5, Si, SiO2, or combinations thereof.

8. The method of claim 1, wherein the active layer comprises a strained layer quantum well structure made of InGaAs, or InGaAsP, or AlGaInAs, the strained layer quantum well structure being configured to excite the light using an energy distribution confined to its 4 μm range.

9. The method of claim 8, wherein reconfiguring the gain chip comprises: The absorption layer is formed in the N-type coating layer within a 2 μm range near the active layer.

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

11. The method of claim 9, wherein the absorption layer comprises a GaInAsP or AlGaInAs semiconductor material having a band gap smaller than the longest wavelength associated with the short-wavelength sidemode JFSR peak at 1490 nm.

12. The method of claim 1, wherein configuring the gain chip comprises: A reflective semiconductor optical amplifier is formed based on the gain region such that the second facet is a high-reflectivity facet with >90% reflectivity for wavelengths in the extended C-band from 1526 nm to 1568 nm, and a decrease in reflectivity near the wavelength around the JFSR peak of the long-wavelength sidemode, and the first facet is an anti-reflective facet with >99.99% transmittance for transmitting light from the gain region to the wavelength tuner and further to an exit port for outputting a laser with wavelengths in the extended C-band tuned by the wavelength tuner.

13. A gain chip for a reflective semiconductor optical amplifier used in a broadband wavelength tunable laser, comprising: The gain region is configured longitudinally as a linear ridge waveguide between the first and second facets and laterally as a PN junction with a P-type cladding layer and an N-type cladding layer. An active layer is formed between the P-type cladding layer and the N-type cladding layer and is configured to excite light; An absorption layer, located near the active layer in the N-type cladding layer, and having an overlapping energy distribution, is used to introduce additional losses in the gain curve of the light for wavelengths shorter than a predetermined value. Anti-reflective optical features are disposed at the first facet; and Partial reflective optical features are disposed at the second facet; The light excited in the active layer is partially reflected at the second facet and passes through the first facet into a wavelength tuner, which generates a joint interference spectrum with multiple modes in isolated spectral peaks separated by the joint free spectral range JFSR; wherein the fundamental mode JFSR peaks of the multiple modes are tuned at a broadband wavelength by the wavelength tuner and amplified in the gain region before leaving as laser light, while long-wavelength side-mode JFSR peaks and short-wavelength JFSR peaks are suppressed. The second facet has an inclined anti-reflective coating, the light reflectivity of which decreases with increasing wavelength.

14. The gain chip of claim 13, wherein the active layer comprises a strained layer quantum well structure made of InGaAs, or InGaAsP, or AlGaInAs, the strained layer quantum well structure being configured to excite the light using an energy distribution spatially confined within its 4 μm range.

15. The gain chip of claim 14, wherein the absorption layer comprises a position in the N-type cladding layer near the active layer that overlaps with a 20-30% confinement factor of the strained layer quantum well structure.

16. The gain chip of claim 13, wherein the anti-reflective optical feature at the first facet includes an anti-reflective coating having >99.99% transmittance for allowing light excited in the active layer to enter the wavelength tuner.

17. The gain chip of claim 13, wherein the partial reflective optical feature at the second facet comprises a low reflectivity coating having <10% optical reflectivity and partial transmittance for light in the extended C-band from 1526 nm to 1568 nm, for configuring the second facet as a laser exit port.

18. The gain chip of claim 13, wherein the partial reflective optical feature at the second facet includes a high reflectivity coating having >90% reflectivity for light in the extended C-band from 1526 nm to 1568 nm, for configuring the second facet as a reflector for reflecting light passing through the wavelength tuner, the reflector being coupled to another laser exit port itself or associated with another semiconductor optical amplifier.

19. The gain chip of claim 13, wherein the wavelength tuner is designed to make the fundamental mode JFSR peak tunable over the entire extended C-band from 1526 nm to 1568 nm, wherein the JFSR is 85 nm, such that the joint interference spectrum includes: The most recent long-wavelength side-mode JFSR peak is located at 1611 nm, corresponding to the fundamental mode JFSR peak at the lower end of the extended C-band; and the most recent short-wavelength side-mode JFSR peak is located at 1483 nm, corresponding to the fundamental mode JFSR at the upper end of the extended C-band.

20. The gain chip of claim 19, wherein the absorption layer comprises a GaInAsP or AlGaInAs semiconductor material with a band gap less than a predetermined value of 1490 nm to suppress the nearest short-wavelength sidemode JFSR peak at 1483 nm.

21. The gain chip of claim 19, wherein the partial reflective optical feature at the second facet comprises a one- or three-layer coating made of a material selected from Al2O3, Ta2O5, Si, SiO2, or a combination thereof, the coating being configured to provide a light reflectance that monotonically decreases with increasing wavelength to suppress the nearest long-wavelength sidemode JFSR peak at 1611 nm.

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