Thin-film filters for tunable lasers

Abstract

This application relates to thin-film filters for tunable lasers. A thin-film device for a wavelength-tunable semiconductor laser is also provided. The device includes a cavity located between a high-reflectivity surface and an anti-reflectivity surface, the cavity being designed to emit laser light with wavelengths within a tunable range defined by two vernier ring resonators having a joint free spectral range between a first wavelength and a second wavelength. The device also includes a film comprising multiple pairs of first and second layers, the first and second layers being stacked sequentially on the outer side of the high-reflectivity surface. Each layer in each pair has a respective optical thickness of one unit, except for one of the first or second layers in a pair having a larger optical thickness. The film is configured to produce an internal reflectivity of at least greater than 90% of the laser light from the high-reflectivity surface for wavelengths within the tunable range starting from the first wavelength, but for wavelengths within a 25 nm range surrounding the second wavelength, the internal reflectivity of the laser light is at least less than 50%.

Description

Technical Field

[0001] This invention relates to optical communication technology. More specifically, this invention provides a thin-film filter for a wavelength-tunable laser gain chip based on silicon photonics, a tunable filter having the thin-film filter, and a silicon photonics system having the thin-film filter. Background Technology

[0002] Over the past few decades, the use of communication networks has surged. In the early days of the internet, popular applications were limited to email, bulletin boards, and primarily text-based and text-based web browsing, with relatively small amounts of data typically 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 at data rates of 40 Gbit / s and 100 Gbit / s over existing single-mode fibers is the goal of next-generation fiber optic communication networks. For many applications, such as broadband DWDM communication and wavelength-controlled optical detection and ranging (LIDAR) sensing, chip-scale, widely tunable lasers have attracted considerable interest. Recently, optical components have been integrated onto silicon (Si) substrates to fabricate large-scale photonic integrated circuits that coexist with microelectronic chips. It has been demonstrated that entire photonic components, including filters, demultiplexers, splitters, modulators, and photodetectors, are primarily housed on silicon-on-insulator (SOI) platforms. SOI platforms are particularly well-suited for the standard DWDM communication bands of 1300 nm and 1550 nm because silicon (n = 3.48) and its oxide SiO2 (n = 1.44) are both transparent and form high-refractive-index, high-confined waveguides ideally suited for medium-to-high integration planar integrated circuits (SPICs).

[0004] Wavelength-tunable semiconductor lasers in silicon photonics platforms have been implemented as fundamental components in many fiber optic communication applications, including coherent optical transmission with improved spectral efficiency. Various spectrally efficient modulation formats, such as M-phase shift keying (PSK) and quadrature amplitude modulation (QAM), are available without relying on rather complex optical phase-locked loops. Furthermore, since phase information is retained after detection, electrical post-processing functions, such as dispersion compensation and polarization mode dispersion compensation, can be implemented in the digital domain. However, several technical challenges remain, such as fabricating small-sized vernier rings on silicon photonic substrates to extend the free spectral range and achieving sufficient side-mode suppression ratios for fabricating polarization-independent wavelength-tunable lasers and integrating them into compact silicon photonics platforms to form coherent systems. Therefore, improved techniques and methods are needed. Summary of the Invention

[0005] This invention relates to optical communication technology. More specifically, this invention provides a thin-film filter with a high-reflectivity surface for use in a silicon photonics-based tunable laser gain chip. Tunable filters have similar applications in silicon photonic integrated circuits, although other applications are possible.

[0006] In one embodiment, the present invention provides a thin-film device for a wavelength-tunable semiconductor laser. The thin-film device includes a cavity located between a high-reflectivity surface and an anti-reflectivity surface, the cavity being designed to emit laser light with wavelengths within a tunable range defined by two vernier ring resonators having a joint free spectral range between a first wavelength and a second wavelength. The thin-film device also includes a film comprising multiple pairs of layers, each pair comprising a first layer and a second layer, the first and second layers being stacked sequentially on the outer side of the high-reflectivity surface. Each of the first and second layers in each pair has a respective optical thickness of one unit, except for one of the first or second layers in a pair having a larger optical thickness. The film is configured to produce an internal reflectivity of at least greater than 90% of the laser light from the high-reflectivity surface for wavelengths within the tunable range starting from the first wavelength, but for wavelengths within a 25 nm range surrounding the second wavelength, the internal reflectivity of the laser light is at least less than 50%.

[0007] In an alternative embodiment, the present invention provides a gain chip for a wavelength-tunable semiconductor laser. The gain chip includes a laser cavity comprising an active region between a high-reflectivity surface and an anti-reflection surface. The laser cavity is designed to emit laser light with wavelengths within a tunable range defined by two vernier ring resonators having a joint free spectral range between a first wavelength and a second wavelength. The gain chip also includes a laser cavity film comprising multiple pairs of layers, each pair comprising a first layer and a second layer, the first and second layers being stacked sequentially on the outer side of the high-reflectivity surface. Each of the first and second layers in each pair has a respective optical thickness of one unit, except for one of the first or second layers in a pair having a larger optical thickness. The film is configured to produce an internal reflectivity of at least greater than 90% of the laser light from the high-reflectivity surface for wavelengths within the tunable range starting from the first wavelength, but for wavelengths within a 25 nm range surrounding the second wavelength, the internal reflectivity of the laser light is at least less than 50%.

[0008] In another alternative embodiment, the present invention provides a wavelength-tunable semiconductor laser. The wavelength-tunable semiconductor laser includes a vernier tuner having at least two ring resonators formed in a silicon photonic substrate. The at least two ring resonators are configured to generate a joint free spectral range between a first wavelength and a second wavelength. The wavelength-tunable semiconductor laser also includes a phase-matching device having a waveguide formed in the silicon photonic substrate and coupled to the at least two ring resonators. Additionally, the wavelength-tunable semiconductor laser includes a gain chip including a laser cavity having an active region with an exit located between a high-reflectivity surface and an anti-reflection surface. The gain chip is flip-chip-mounted onto the silicon photonic substrate such that the exit is coupled to the waveguide of the phase-matching device. The laser cavity is designed to emit laser light from the exit within a tunable range of wavelengths from the first wavelength to the midpoint of the joint free spectral range. Furthermore, the wavelength-tunable semiconductor laser includes a film made from multiple pairs of layers, each pair comprising a first layer and a second layer, the first and second layers being stacked sequentially on the outside of the high-reflectivity surface. Each of the first and second layers in each pair has a respective optical thickness of one unit corresponding to a quarter of the reference wavelength, except for the first or second layer in the pair that has the larger optical thickness. The film is configured to produce an internal reflectivity of at least greater than 90% for wavelengths within a tunable range from the high reflectivity surface, but for wavelengths within a 25 nm range around the second wavelength, an internal reflectivity of at least less than 50%.

[0009] This invention achieves these and other advantages within the context of known waveguide laser modulation techniques. However, a further understanding of the nature and advantages of this invention can be achieved by referring to the latter part of the specification and the accompanying drawings. Attached Figure Description

[0010] The accompanying drawings are merely examples and should not unduly limit the scope of the claims. Those skilled in the art will recognize many other variations, modifications, and substitutions. It should also be understood that the examples and embodiments described herein are for illustrative purposes only, and various modifications or alterations thereto will be suggested to those skilled in the art and will be included within the spirit and scope of the process and the appended claims.

[0011] Figure 1 This is a schematic diagram of a tunable laser with a vernier reflector tuner and a phase shifter according to an embodiment of the present invention.

[0012] Figure 2 This is an example diagram of the individual and combined reflection spectra of two vernier rings from a tunable laser according to an embodiment of the present invention.

[0013] Figure 3This is an example diagram showing the gain of a tunable laser for suppressing side-mode lasers according to an embodiment of the present invention, and the reflectivity at the high reflectivity (HR) surface of its gain chip.

[0014] Figure 4 This is a simplified side view of a thin-film filter according to an embodiment of the present invention, which is proposed to... Figure 3 The long wave troughs are used to achieve HR reflectivity.

[0015] Figure 5 This is a graph showing the reflectivity of two model filters, one with and one without phase shift, over a wide wavelength range according to an embodiment of the present invention.

[0016] Figure 6 This is a graph of the reflectance of a model filter defined in the illustrated table, according to a specific embodiment of the present invention.

[0017] Figure 7 This is a graph of the reflectance of the model filter defined in the illustrated table, according to another specific embodiment of the present invention.

[0018] Figure 8 According to an embodiment of the present invention, based on Figure 7 The graph shows the reflectance of the model filter proposed in the paper, derived from the sample filter.

[0019] Figure 9 This is a schematic diagram of the gain chip of a tunable laser according to an embodiment of the present invention, which has a thin-film filter formed on its high reflectivity surface, the thin-film filter being designed to suppress side-mode laser.

[0020] Figure 10 This is a schematic diagram of a wavelength-tunable semiconductor laser according to an embodiment of the present invention, which has a thin-film filter on the high-reflectivity surface of a gain chip of a reflective semiconductor optical amplifier added therein. Detailed Implementation

[0021] This invention relates to optical communication technology. More specifically, this invention provides a thin-film filter with a high-reflectivity surface for use in a silicon photonics-based tunable laser gain chip. Tunable filters have similar applications in silicon photonic integrated circuits, although other applications are possible.

[0022] The following description is intended to enable those skilled in the art to make and use the invention and incorporate it into specific application environments. Various modifications and uses in different applications will be apparent to those skilled in the art, and the general principles defined herein are applicable to a wide range of implementations. Therefore, the invention is not limited to the presented embodiments but is accorded the widest scope consistent with the principles and novel features disclosed herein.

[0023] In the following detailed description, numerous specific details are set forth in order 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.

[0024] The reader's attention is focused on all documents and files submitted concurrently with this specification, which are publicly available for examination of this specification, and the contents of all such documents and files 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 by alternative features serving the same, equivalent, or similar purpose. Therefore, unless otherwise expressly stated, each disclosed feature is merely one example from a series of generally equivalent or similar features.

[0025] Furthermore, any element in the claims that does not expressly specify a "means" or "step" for performing a particular function should not be construed as a "means" or "step" as defined in paragraph 6 of section 112 of 35 U.S.SC. Specifically, the use of "step" or "action" in the claims herein does not invoke the provisions of paragraph 6 of section 112 of 35 U.S.SC.

[0026] Note that if used, the labels left, right, front, back, top, bottom, forward, reverse, clockwise, and counterclockwise are for convenience only and do not imply any particular fixed direction. Rather, they are used to reflect the relative position and / or orientation between different parts of an object.

[0027] On one hand, this disclosure provides a thin-film device arranged in the laser cavity of a wavelength-tunable semiconductor laser based on a silicon photonics platform for supporting mode selection / suppression and stability. For example... Figure 1As shown, a tunable laser typically provides a gain chip 100 coupled to one or more vernier ring resonators 300 via a phase matcher 200. The gain chip 100 includes an active region in a PN junction diode within a linear waveguide disposed between an antireflective (FR) surface and a high reflectivity (HR) surface in a laser cavity. The laser cavity is designed to induce laser excitation emitted from the active region and resonating between the FR and HR surfaces. The vernier tuner 300 is a ring waveguide structure formed in a silicon photonic substrate, providing an extended cavity to generate a specific free spectral range (FSR) in the interference spectrum and providing tunability of the laser wavelength emitted from the gain chip 100 based on the laser cavity. The phase matcher 200 is a linearly shaped waveguide formed in a silicon photonic substrate with thermal phase control, used to further provide better tunability to determine the final wavelength of the laser emitted from the laser cavity.

[0028] In one embodiment, the vernier tuner 300 includes at least two ring resonators R1 and R2, formed in a silicon photonic substrate. Figure 1 (Not explicitly shown in the text). Each ring resonator R1 or R2 provides a reflection spectrum in the extended cavity, such as... Figure 2 As shown in the upper part, this occurs over a wide wavelength range from 1520 nm to 1620 nm. The specific positions of multiple constructive interference peaks in each reflection spectrum are also contributed by adjusting the phase shift of the phase matcher 200. Figure 2 The bottom of the image shows the joint interference spectrum (denoted as R1*R2) of two reflection spectra within the same wavelength range. It can be seen that the joint spectrum is characterized by the joint free spectral range (JFSR), i.e., the wavelength interval between the two main interference peaks. In this example, the JFSR is 85 nm, ranging from a first wavelength of 1526 nm at the JFSR peak of the fundamental mode to a second wavelength of 1611 nm at the JFSR peak of the higher-order or side-mode. The laser emission wavelength can be tuned by adjusting the vernier tuner 300 and the phase matcher 200. For tuning the laser emission wavelength in broadband DWDM applications, a tunable laser with a wider FSR is preferred to achieve better laser wavelength selectivity. However, to obtain a wider FSR for better wavelength tunability, a very small diameter ring resonator needs to be formed on the silicon photonic substrate. Additionally, an element needs to be added in the extended cavity to achieve mode selection / suppression and stability. Designing and constructing a ring resonator with an ultra-small diameter or adding an additional element on the silicon photonic substrate to improve mode selection is technically challenging and economically disadvantageous.

[0029] Figure 3This is an example diagram showing the gain of a tunable laser for suppressing side-mode lasing according to an embodiment of the present invention and the reflectivity at the high-reflectivity surface of its gain chip. As shown, in the current vernier tuner design, the gain curve of laser emission from the gain chip 100 has a width greater than or at least close to the JFSR. However, in a vernier tuner design with two ring resonators, if the gain chip 100 has flat reflectivity at its high-reflectivity (HR) surface, the laser cavity can emit laser light at both the fundamental mode JFSR peak position (e.g., 1526 nm) and the side-mode JFSR peak position (e.g., 1611 nm). Therefore, it is also preferable to change the reflectivity of the high-reflectivity surface of the gain chip 100 with a sufficiently large side-mode suppression ratio (SMSR) to effectively suppress lasing at the side-mode JFSR peak. For a tunable laser with the desired tunability range in the extended C-band from 1526 nm to 1568 nm, under a vernier tuner design with a JFSR of 85 nm, one objective is to maintain a high HR reflectivity (>90%) within the tunable range, while simultaneously reducing the side-mode JFSR peak at approximately 1610–1620 nm with at least a 3 dB loss. Figure 3 As shown, the exemplary HR reflectivity is designed to produce a drop or large loss (almost to 0%) around the long wavelength side of the JFSR at 1610 nm, while maintaining high (>90%) reflectivity in the lower part of the JFSR over a wide wavelength range (from 1580 nm to 1520 nm) to maintain wavelength tuning capabilities for extended C-band.

[0030] This disclosure provides a thin-film device attached to a gain chip for altering the reflectivity characteristics of its HR surface, thereby generating a decrease in reflectivity on the long-wavelength side of the JFSR while maintaining high reflectivity at the lower part of the JFSR. Figure 4 This is a simplified side view of a thin-film filter according to an embodiment of the present invention, the thin-film filter being used to implement... Figure 3 The figure shows a long-wavelength decrease in HR reflectivity. This figure is merely an example and should not unduly limit the scope of the claims. Many variations, substitutions, and modifications will be recognized by those skilled in the art. As shown, a substrate 100 with surface O is provided. Optionally, substrate 100 is part of a gain chip including a laser cavity, and surface O serves as its high reflectivity surface. A thin-film device 101 is attached to the outside of surface O and configured with a multilayer structure 101 forming a bandpass filter to provide high internal reflectivity (~100%) flatness for a wide wavelength range greater than or close to JFSR. Reference Figure 4 The multilayer structure 101 includes layers 10 to 1, which are stacked sequentially on the outside of a surface 0 that has high reflectivity over a wide wavelength range, while having a reflectivity tilt on the long wavelength side.

[0031] In one embodiment, the multilayer structure 101 is configured as multiple pairs of layers stacked together. Each pair of layers of the multilayer structure 101 includes a first layer with a thickness h1 of a first material having a high refractive index n1 and a second layer with a thickness h2 of a second material having a low refractive index n2, where n2 < n1. In Figure 4 the example, the first pair of layers includes layer 1 as the first layer and layer 2 as the second layer. The second pair of layers includes layer 3 as the first layer and layer 4 as the second layer, and so on. Additionally, in one embodiment, each of the first and second layers in each pair of layers is configured to have an optical thickness of one unit, n1×h1 = n2×h2, which is set to a quarter of the reference wavelength. The multilayer structure 101 of this embodiment provides a thin-film filter that has a long-wavelength decrease in reflectivity, which is Figure 3 the desired characteristic of an HR reflectivity with a long-wavelength decrease in Figure 4 . Optionally, in another embodiment, all pairs except one pair include a first or second layer having a thickness h3 greater than h1 or h2. In

[0032] the example, the first layer of the third pair of layers, i.e., layer 5, is a layer with a greater thickness h3. Compared with the high-reflectivity flat portion over a wide range in the lower part of the JFSR, the greater thickness h3 provides an additional phase shift to generate a greater SMSR, which has a decrease in reflectivity within a narrow wavelength range around the side-mode JFSR peak. Figure 4 Optionally, the number of pairs of layers is 5, 6 or more. In one embodiment, it is preferred to make the total thickness of the multilayer structure 101 thinner while still achieving the desired HR reflectivity characteristics. Another factor to consider in making the multilayer structure 101 thinner is to select the smallest possible reference wavelength, provided that the optical thickness unit of each layer in each pair of layers (except for the optical thickness unit in the pair with an optional phase shift) is set to a quarter of the reference wavelength. Thus, the multilayer structure 101 can be optimized by coherently tuning all of the following factors, including: the thickness of each layer, the reference wavelength, the number of pairs of layers, the selection of materials with different refractive indices, and the amount of phase shift added to a pair of layers with a greater thickness. Preferably, it is desired to Figure 4 optimize the multilayer structure 101 shown to have a thin total thickness and be optically configured to have a reflectivity of at least greater than 90% over a tunable range of laser emission designed from the lower end of the JFSR to the midpoint of the JFSR, while having a reflectivity of at least less than 50% around the upper end of the JFSR.

[0033] In some embodiments, the multilayer structure 101 is formed on surface 0 by sputtering deposition, which provides excellent thickness control for many metal, semiconductor, or oxide materials and produces a dense bulk layer with high quality and stable optical properties. In one example, the first layer with a high refractive index is sputtered silicon with n1 = 3.82, and the second layer with a low refractive index is silicon oxide (SiO2) with n2 = 1.42 or aluminum oxide (Al2O3) with n2 = 1.63. In another example, the first layer with a high refractive index is sputtered tantalum oxide (Ta2O5) with n1 = 2.12, and the second layer with a low refractive index is silicon oxide (SiO2) with n2 = 1.42. i O2) or aluminum oxide (Al2O3) having n2 = 1.63. The substrate 100 may optionally be a material with a refractive index n s The indium phosphide material has a strength of 3.185 to 3.20 (used to form the gain chip of the cavity laser). The medium outside the thin-film filter 101 is air. Reflectivity refers to the internal reflectivity of the laser light reflected from surface O back to the substrate 100 (e.g., an indium phosphide-based gain chip) through the thin-film filter 101.

[0034] Figure 5 This is a graph showing the reflectivity of two model filters, one with and one without phase shift, over a wide wavelength range according to an embodiment of the present invention. (Reference) Figure 5 The first model filter provides 5 pairs of SiO2 / S i Layers, stacked on a substrate, each set to an optical thickness (n×h) equal to one-quarter of the reference wavelength, without any phase shift. Reflectance curves for the first model filter were obtained via computer simulations, showing that reflectance drops to <80% at 1560 nm and gradually decreases to around 5% near 1610 nm before rising again. It fails to maintain reflectance greater than 90% within 1568 nm (the upper limit of the extended C-band) or the desired tunable range. Between 1580 nm and 1610 nm, the reflectance decreases slowly, also exhibiting relatively poor contrast.

[0035] Refer again Figure 5 The second model filter is configured with 5 pairs of SiO2 / S iA phase-shifting layer, stacked on the substrate, is set to a unit optical thickness (n×h), equal to one-quarter of the reference wavelength, but with a phase shift in the fifth Si layer, which is thicker than the Si layers in the remaining layer pairs. As shown in the figure, the reflectivity curve obtained by computer simulation based on the second model filter has a high reflectivity of >90% from 1520nm to 1580nm, and then drops rapidly to 0% at 1610nm. This results in a significant contraction of the reflectivity variation from the extended C-band tunable range to 1610nm, maintaining a high-quality broadband laser tunable range, but substantially suppressing the side-mode JFSR laser.

[0036] Figure 6 This is a graph of the reflectance of a model filter defined in the illustrated table, according to a specific embodiment of the present invention. (Reference) Figure 6 This is a model filter defined using the 10-layer structure or 5 pairs of SiO2 / Si phase-shifted layers shown in the insert table, stacked on the HR plane of a tunable laser gain chip. The Si layer, the first layer in each pair, is sputter-deposited silicon without phase shift, with a refractive index of 3.82, a physical thickness of 105.36 nm, and an optical thickness of 402.5 nm, which is one unit optical thickness equal to 1 / 4 of the 1610 nm reference wavelength. The SiO2 layer, the second layer in each pair, has a refractive index of 1.456, a physical thickness of 276.31 nm, and an optical thickness of 402.5 nm, which is also equal to one unit optical thickness equal to 1 / 4 of the 1610 nm reference wavelength. The fifth layer, for example, is a Si layer with phase shift, having a physical thickness of 210.73 nm, which is twice the thickness of the first Si layer, making the phase shift accurate to 1 / 4 of the reference wavelength. Optionally, the phase-shifted layer can be one of the second layers or in alternating layers. Figure 6 The 10-layer thin-film filter results in a sharp drop in reflectivity to about 5% at 1610 nm, and near-perfectly high reflectivity for wavelengths below 1580 nm down to 1500 nm. It does produce very high SMSR, but the total thickness of the 10-layer structure is greater than 2 μm.

[0037] Figure 7 This is a graph of the reflectance of the model filter defined in the illustrated table, according to another specific embodiment of the present invention. (Reference) Figure 7 This is a model filter defined using the 10-layer structure or 5 pairs of SiO2 / Si phase-shifted layers shown in the insert table, stacked on the HR plane of a tunable laser gain chip. The Si layer, the first layer in each pair, is unshifted sputtered-deposited silicon with a refractive index of 3.82, a physical thickness of 77.22 nm, and an optical thickness of 295 nm, which is one unit of optical thickness equal to 1 / 4 of the 1180 nm reference wavelength. The reference wavelength for this multilayer structure is chosen to be 1180 nm, much smaller than the wavelength used for... Figure 6 The previous module filter has a wavelength of 1610 nm. The second SiO2 layer in each pair has a refractive index of 1.456, a physical thickness of 202.52 nm, and an optical thickness of 295 nm, which is also equal to one unit of optical thickness, equal to 1 / 4 of the 1180 nm reference wavelength. The fifth layer, for example, is a Si layer with a phase shift, having a physical thickness of 253.68 nm, which is 3.285 times thicker than the first Si layer, thus producing a larger phase shift. Optionally, the layer with the phase shift can be one of the second layers or in alternating layers. Based on... Figure 7 The 10-layer thin-film filter structure results in a perfect drop in reflectivity to 0% at 1610 nm, and high reflectivity >90% for wavelengths below 1580 nm down to 1500 nm. It also produces very high SMSR, making it suitable as a gain-sensitive HR filter for chips. Furthermore, due to the choice of a smaller reference wavelength of 1180 nm, the total thickness of the 10-layer structure is only slightly greater than 1.5 μm. Thinner thin-film filters offer several advantages in manufacturing, namely shorter coating times and better mechanical properties in terms of film stress.

[0038] Figure 8 According to an embodiment of the present invention, based on Figure 7 The graph shows the reflectance of the sample filter from the proposed model filter. (Reference) Figure 8 ,based on Figure 7 A sample filter was fabricated based on the model filter proposed in the paper. By controlling the film thickness, mechanical properties, and optical properties of each layer in the 10-layer structure, and considering wavelength-dependent variations and numerous manufacturing process variations, the reflectance curves for wavelengths of 1500–1650 nm obtained through computer simulation perfectly matched the reflectance curves of the model filter. As shown in the figure, the reflectance is at least 90% or higher from 1500 nm to 1565 nm, and for wavelengths above 1580 nm, the reflectance drops rapidly, giving a minimum of approximately 10% at 1617 nm. The reflectance is at least <30% at a side-mode JFSR wavelength of 1610 nm. This is based on... Figure 4 A multilayer HR filter with 5 pairs of SiO2 / Si with phase shift, which satisfies the following conditions: Figure 3 The design goals for reflectivity characteristics are shown. Of course, those skilled in the art will recognize that the fewer variations, alternatives, and modifications there are in the number of multiple layers, the choice of materials for the first / second layer, the method of forming each layer, the phase shift applied to a selected pair of multiple layers, and the selected reference wavelength, the better.

[0039] On the other hand, this disclosure also provides a gain chip for wavelength-tunable semiconductor lasers. As described above, the thin-film filter provided in this disclosure is applied to the high-reflectivity surface of the gain chip for wavelength-tunable lasers, but other applications are also possible. Figure 9 A schematic diagram of a gain chip for a tunable laser according to an embodiment of the present invention is shown, the gain chip having a thin-film filter formed on its high reflectivity surface, which is designed to suppress side-mode lasing. This diagram is merely an example and should not unduly limit the scope of the claims. Many variations, substitutions, and modifications will be recognized by those skilled in the art. As shown, the gain chip 100 is provided as a diode structure based on InP material, with an active region 100-A sandwiched between a p-type region 100-P and an n-type region 100-N formed along its length, and terminating at two end faces. One end face is configured as an anti-reflective (AR) end face, while the other end face is configured as a high reflectivity (HR) end face. Utilizing the AR and HR end faces, the gain chip 100 is configured as a laser cavity, and the active region 100-A is driven by a bias current applied to the p-type region 100-P and the n-type region 100-N to emit light that is further excited and amplified within the laser cavity to form a laser beam. Due to the HR filter 101 attached at the HR surface, the laser reflected from the HR surface each time is typically configured to have high reflectivity (>90%).

[0040] In this disclosure, gain chip 100 is applied in a reflective semiconductor optical amplifier (RSOA) of a vernier-tuned tunable laser having at least two ring resonators formed in a silicon photonic substrate. The vernier tuner is used to tune wavelengths over a wide range based on a joint free spectral range (JFSR) between a first and a second wavelength, determined by appropriate design of the ring resonators. It is assumed that the laser cavity primarily outputs laser emission at the fundamental mode JFSR peak at the first wavelength, while including laser emission at the side-mode JFSR peak at the second wavelength. The wavelength tunable range can be provided by at least the lower half of the JFSR from the first wavelength to the midpoint of the JFSR. As described in a previous section of the specification, HR filter 101 is reconfigured to provide high (>90%) reflectivity for wavelengths within the wavelength tunable range, while introducing a narrow range (approximately 25 nm) of reflectivity reduction (<50%) around the side-mode JFSR peak at the second wavelength. HR filter 101 in… Figure 3 It basically has the characteristics and is designed to be Figure 4 The multi-layer structure 101 shown. Figure 6 and Figure 7 Several model filters were proposed and described in the paper. For example... Figure 8 As shown, the manufactured sample filter basically meets the design requirements.

[0041] In another aspect, this disclosure provides a broadband wavelength-tunable semiconductor laser based on silicon photonics, which includes a gain chip having the HR filter described herein. Figure 10 This is a schematic diagram of a wavelength-tunable semiconductor laser according to an embodiment of the present invention, having a thin-film filter on the high-reflectivity surface of a gain chip of a reflective semiconductor optical amplifier added thereto. This diagram is merely an example and should not unduly limit the scope of the claims. Many variations, substitutions, and modifications will be recognized by those skilled in the art. As shown, the tunable laser 1000 includes a first laser diode chip 430 bonded to a first chip location 410 of a silicon photonic substrate 120, a second laser diode chip 440 bonded to a second chip location 420 of the silicon photonic substrate 120, and a tunable filter (i.e., a vernier tuner) including two ring resonators 471 and 472 and a phase matcher 475, one end of which is coupled to a first gain region 435 of the first laser diode chip 430 via a first waveguide 491 connected to a first integrated coupler 451. The tunable filter at the other end is coupled to a second gain region 445 of the second laser diode chip 440 via a second waveguide 492 connected to a second integrated coupler 460. The second gain region 445 of the second laser diode chip 440 has a laser cavity disposed between the antireflective (AR) surface 441 and the high reflectivity (HR) surface 442, and serves as a reflective semiconductor optical amplifier (RSOA). Laser light can be generated in the laser cavity of the second gain region 445. The laser light is reflected from the HR surface 442 and amplified as reflected light from the AR surface 441 before being emitted to the tunable filter via the second integrated coupler 460. Optionally, the tunable laser 1000 includes a wavelength lock (WLL) 480, which is coupled to the tunable filter via a waveguide 490 and to the first gain chip 435 via a first waveguide 491. The wavelength lock 480 is configured to lock the wavelength of the reflected light from the tunable filter using a waveguide-based delay line interferometer. The reflected light having a wavelength tuned by a vernier tuner and locked by the wavelength lock 480 is guided into the first gain region 435 of the first laser diode chip 430 via the first integrated coupler 451. The first gain region has a laser cavity disposed between two anti-reflective (AR) surfaces 431 and 432 and serves as a semiconductor optical amplifier (SOA) to amplify the laser therein. When reflected light is guided into the first gain region 435, it is amplified therein before being output to the waveguide in the silicon photonic substrate 120 via the third integrated coupler 452.

[0042] refer to Figure 10The tunable filter of the tunable laser 1000 further includes: a first heater (ring 1_HTR) having a resistive film covering the first ring resonator 471; a second heater (ring 2_HTR) having a resistive film covering the second ring resonator 472; and a third heater (phase_HTR) having a resistive film covering a matching unit 475 connected to the second waveguide 492. These heaters are configured to change the temperature to cause a change in the transmission spectrum of light passing through the respective ring resonators. Each transmission spectrum of the ring resonator has multiple resonance peaks (see...). Figure 2 In this implementation, the two ring resonators 471 and 472 are provided with slightly different radii, resulting in an offset between the two transmission spectra when they overlap (see...). Figure 2 The first and second heaters controllably alter the temperatures of their respective first and second ring resonators to shift the corresponding resonance peaks, providing an extended tunable range of wavelengths for those peaks. After the initial excitation light, passing through the tunable filter, is reflected back by the HR surface 442 of the second gain region 445, the joint reflectivity spectrum yields a strong central peak (see [link to relevant documentation]). Figure 2 The basic mode JFSR peak in the spectrum can be better tuned by changing the temperature of the phase matcher 475 using a third heater. Simultaneously, the joint reflectance spectrum also introduces a second peak at a larger wavelength (distant from the joint free spectral range or JFSR), which is... Figure 2 The side-mode JFSR peak in the laser can be amplified to output laser light if the reflectivity curve of the thin-film filter 101 attached to the HR surface 442 is flat across all wavelengths.

[0043] To obtain single-wavelength laser light from a tunable laser module, the reflectivity profile can be improved by reducing reflectivity over longer wavelengths. Based on the thin-film filter 101 provided in this disclosure and described in a previous section of the specification, the reflectivity profile of the HR surface 442 is modified to provide at least 90% high reflectivity (e.g., 1526 nm to 1568 nm) over a nominal tunable range from the fundamental mode JFSR peak to the midpoint of the JFSR, but at least less than 50% high contrast reflectivity reduction over a narrow range near the side-mode JFSR peak (e.g., a 25 nm range near 1610 nm). Therefore, the side-mode laser light of the tunable laser module 1000 is substantially suppressed.

[0044] The wavelength-tunable semiconductor lasers described herein can be applied to integrated coherent optical transceivers based on silicon photonics platforms. Detailed information on other applications of the coherent optical transceiver chip and the wavelength-tunable semiconductor laser can be found in patent application No. 16 / 357,095, filed March 18, 2019, and jointly assigned to Inphi Corp of Santa Clara, California. Depending on the application, the integrated coherent transceiver package can be configured with a compact form factor to accommodate any system design used for coherent optical communications.

[0045] While the foregoing is a complete description of specific embodiments, various modifications, alternative structures, 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 thin-film device for a wavelength-tunable semiconductor laser, comprising: A cavity located between a high-reflectivity surface and an anti-reflection surface, the cavity being designed to emit laser light with wavelengths within a tunable range defined by two vernier ring resonators having a joint free spectral range between a first wavelength and a second wavelength; The film comprises multiple pairs of layers, each pair containing a first layer and a second layer, the first layer and the second layer being stacked sequentially on the outside of the high reflectivity surface, each of the first layer and the second layer in each pair having a respective optical thickness of one unit, except for one of the first or second layers in a pair having a larger optical thickness. The film is configured to produce an internal reflectivity of at least 90% for wavelengths within the tunable range starting from the first wavelength, but an internal reflectivity of at least 50% for wavelengths within a 25 nm range surrounding the second wavelength.

2. The thin film device according to claim 1, wherein, The cavity is part of a gain chip, which includes an indium phosphide PN junction in a linear waveguide configuration.

3. The thin film device according to claim 1, wherein, The first layer of each of the plurality of pairs of layers comprises a high refractive index material, and the second layer comprises a low refractive index material.

4. The thin film device according to claim 3, wherein, The high refractive index material is selected from one of the Si layers, and the low refractive index material is selected from one of the SiO2 layer and the Al2O3 layer.

5. The thin film device according to claim 3, wherein, In the multiple pairs of layers, each first layer and each second layer, except for one of the pairs, each has an optical thickness that is substantially equal to one-quarter of the reference wavelength.

6. The thin film apparatus according to claim 5, wherein, The reference wavelength is selected from wavelengths that are substantially smaller than the second wavelength.

7. The thin film device according to claim 1, wherein, The plurality of layers includes at least five pairs of layers, the at least five pairs of layers including the pair having the larger optical thickness in the first layer or the second layer, so as to produce at least 3 dB of reflectivity loss around the second wavelength relative to a reflectivity greater than 90% at wavelengths within the tunable range.

8. The thin film apparatus according to claim 7, wherein, The pair is arranged in the middle of the at least five pairs of layers, and the larger optical thickness in the first or second layer provides additional phase shift to achieve maximum reflectivity loss around the second wavelength.

9. The thin film apparatus according to claim 7, wherein, The tunable range is given from the extended C-band, which extends from the lower end of the joint free spectral range to the midpoint of the joint free spectral range, the lower end of which is at the first wavelength of 1526 nm, and the midpoint of which is at 1568 nm. Wherein, at the high end of the joint free spectral range, the film at the high reflectivity surface produces at least 50% reflectivity loss, the high end of the joint free spectral range being at the second wavelength of 1610 nm.

10. A gain chip for a wavelength-tunable semiconductor laser, comprising: A laser cavity, comprising an active region between a highly reflective surface and an antireflective surface, is designed to emit laser light with wavelengths within a tunable range defined by two vernier ring resonators having a joint free spectral range between a first wavelength and a second wavelength; The film comprises multiple pairs of layers, each pair containing a first layer and a second layer, the first layer and the second layer being stacked sequentially on the outside of the high reflectivity surface, each of the first layer and the second layer in each pair having a respective optical thickness of one unit, except for one of the first or second layers in a pair having a larger optical thickness. The film is configured to produce an internal reflectivity of at least 90% for wavelengths within the tunable range starting from the first wavelength, but an internal reflectivity of at least 50% for wavelengths within a 25 nm range surrounding the second wavelength.

11. The gain chip according to claim 10, wherein, The active region includes an indium phosphide PN junction in a linear waveguide configuration to emit laser excitation between the high reflectivity surface and the antireflection surface.

12. The gain chip according to claim 10, wherein, The first layer of each of the plurality of pairs of layers comprises a high refractive index material, and the second layer comprises a low refractive index material.

13. The gain chip according to claim 12, wherein, The high refractive index material is selected from one of the Si layers, and the low refractive index material is selected from one of the SiO2 layer and the Al2O3 layer.

14. The gain chip according to claim 12, wherein, In the plurality of pairs of layers, each first layer and each second layer, except for one of the pairs, each has an optical thickness that is substantially equal to one-quarter of the reference wavelength, which is substantially smaller than the second wavelength.

15. The gain chip according to claim 10, wherein, The plurality of layers includes at least five pairs of layers, the at least five pairs of layers including the pair having the larger optical thickness in the first layer or the second layer, so as to produce at least 3 dB of reflectivity loss around the second wavelength relative to a reflectivity greater than 90% at wavelengths within the tunable range.

16. The gain chip according to claim 15, wherein, The pair is arranged in the middle of the at least five pairs of layers, and the larger optical thickness in the first or second layer is tuned to produce an additional phase shift to maximize reflectivity loss around the second wavelength to filter out laser light associated with higher-order joint free spectral ranges.

17. The gain chip according to claim 15, wherein, The tunable range is given from the lower end of the joint free spectral range to the midpoint of the joint free spectral range, the lower end of the joint free spectral range being at the first wavelength of 1526 nm, and the midpoint of the joint free spectral range being at 1568 nm. Wherein, at the high end of the joint free spectral range, the film at the high reflectivity surface produces at least 50% reflectivity loss, the high end of the joint free spectral range being at the second wavelength of 1610 nm.

18. The gain chip according to claim 10 is configured in the reflective semiconductor optical amplifier of the wavelength-tunable semiconductor laser.

19. A wavelength-tunable semiconductor laser, comprising: A vernier tuner includes at least two ring resonators formed in a silicon photonic substrate, the at least two ring resonators being configured to generate a joint free spectral range between a first wavelength and a second wavelength; A phase matcher, comprising a waveguide formed in the silicon photonic substrate and coupled to the at least two ring resonators; A gain chip, including a laser cavity, has an active region with an exit located between a high reflectivity surface and an antireflection surface. The gain chip is flip-chip mounted on the silicon photonic substrate such that the exit is coupled to the waveguide of the phase matcher. The laser cavity is designed to emit laser light from the exit in a tunable range with wavelengths from a first wavelength to the midpoint of the joint free spectral range. The film comprises multiple pairs of layers, each pair containing a first layer and a second layer, the first layer and the second layer being stacked sequentially on the outside of the high reflectivity surface, each of the first layer and the second layer in each pair having a respective optical thickness of one unit corresponding to a quarter of the reference wavelength, except for one of the first or second layers in a pair having a larger optical thickness. The film is configured to produce an internal reflectivity of at least 90% for wavelengths within the tunable range from the high reflectivity surface, but for wavelengths within a 25 nm range surrounding the second wavelength, the internal reflectivity of the laser is at least less than 50%.

20. The wavelength-tunable semiconductor laser according to claim 19, wherein, The gain chip is configured in a reflective semiconductor optical amplifier to amplify the laser while reflecting the laser from the high reflectivity surface to the vernier tuner via the phase matcher. The wavelength-tunable semiconductor laser also includes a wavelength lockout coupled to the vernier tuner and a semiconductor optical amplifier having a second gain chip configured to amplify the laser having a single wavelength within the tunable range tuned by the vernier tuner and locked by the wavelength lockout before outputting the laser having a single wavelength substantially unaffected by the second wavelength.

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