A monolithic integrated multi-section cascaded optical frequency comb and a chip thereof

Abstract

A monolithic integrated multi-section cascaded optical frequency comb and a chip thereof belong to the technical field of sensing detection, quantum information and optical communication technology. The monolithic integrated multi-section cascaded optical frequency comb comprises a first semiconductor passive mode-locked laser (MLL1), a semiconductor optical amplifier (003) and a second semiconductor passive mode-locked laser (MLL2) which are sequentially integrated and connected. The first semiconductor passive mode-locked laser (MLL1) comprises a first reverse-biased absorption region (001) integrated with an optical current distribution gate (OG) and a first gain cavity length extender (002), and the first gain cavity length extender (002) can be coupled multiple rings or multiple disks. The second semiconductor passive mode-locked laser (MLL2) comprises a second reverse-biased absorption region (005) and a second gain cavity length extender (004), and the second gain cavity length extender (004) can be coupled multiple rings or multiple disks. Each structure is connected through an electrically isolated trench, thereby realizing an optical frequency comb which integrates small size, high efficiency, tunability, fundamental mode, high power, narrow pulse width, wide spectrum and flexible control.

Description

Technical Field

[0001] This invention relates to the fields of sensing and detection, quantum information and optical communication technologies, and more specifically to a monolithically integrated multi-segment cascaded optical frequency comb and its chip. Background Technology

[0002] An optical frequency comb (OFC) is a type of optical frequency comb. Each "tooth" of an optical frequency comb is equivalent to a monochromatic laser beam. These monochromatic lasers have extremely high coherence with each other. OFC can directly convert light waves to microwave frequencies. By utilizing the nonlinear advantages of ultrashort pulses, OFC can achieve different frequencies in various spectral regions.

[0003] Since the advent of optical frequency combs (OFCs), OFCs based on traditional fiber lasers, solid-state lasers, and microcavity structures have suffered from problems such as large size, numerous components, low repetition frequency, susceptibility to external environmental influences, and separation of the light source and control ends. These issues have affected the performance of optical combs and limited their application and development.

[0004] Furthermore, traditional semiconductor mode-locked lasers require short cavity lengths (within 1000 μm) to achieve a wide wavelength range optical frequency comb. Such a cavity length results in very low output power, and the reverse bias voltage applied in the reverse bias region leads to significant light absorption, making it difficult to achieve a wide-spectrum optical frequency comb output, let alone high-power output.

[0005] Therefore, how to reduce the size of optical frequency combs while ensuring high-power, wide-spectrum output is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0006] In view of this, the present invention provides a monolithically integrated multi-segment cascaded optical frequency comb and its chip, which is used to solve the problem of realizing a small size, high efficiency, tunability, fundamental mode, high power, narrow pulse width, wide spectrum and flexible controllability optical frequency comb.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] First, this invention discloses a monolithically integrated multi-segment cascaded optical frequency comb, comprising:

[0009] A first semiconductor passively mode-locked laser, a semiconductor optical amplifier, and a second semiconductor passively mode-locked laser;

[0010] The first semiconductor passive mode-locked laser includes a first reverse-biased absorption region and a first gain cavity length extender integrated with a photocurrent distribution grid; the first reverse-biased absorption region is connected to one end of the first gain cavity length extender through a first electrical isolation trench, and the other end of the first gain cavity length extender is connected to the semiconductor optical amplifier through a second electrical isolation trench.

[0011] The second semiconductor passive mode-locked laser includes a second gain cavity length extender and a second reverse-biased absorption region. One end of the second gain cavity length extender is connected to the semiconductor optical amplifier through a third electrical isolation trench, and the other end of the second gain cavity length extender is connected to the second reverse-biased absorption region through a fourth electrical isolation trench.

[0012] Preferably, the etching depth of the photocurrent distribution grating extends to one-hundredth to one-half of the thickness downwards from the top edge of the upper waveguide layer of the monolithically integrated multi-segment cascaded optical frequency comb, and the length of the photocurrent distribution grating accounts for one-third to four-fifths of the overall length of the first reverse-bias absorption region; the photocurrent distribution grating includes a narrow-spectrum optical compression grating.

[0013] Preferably, both the first gain cavity length extender and the second gain cavity length extender include multiple mutually coupled disk cavities. Simultaneously, this coupled multi-disc structure gain cavity length extender also functions as a beam splitter and optical feedback unit.

[0014] Preferably, the diameter of each disk cavity is 5-100 μm, the height of the disk surface etched by each disk cavity is 0.5-10 μm, and the shortest distance between the outer edges of two adjacent disk cavities is -20 μm to 200 nm.

[0015] The semiconductor optical amplifier has a total length of 400-5000μm, a ridge waveguide width of 2-30μm, and a ridge waveguide height of 0.5-6μm.

[0016] In the first, second, third and fourth electrical isolation trenches, the trench width is 5-20μm, the corrosion depth is 0.3-0.7μm, and the length is 300-500μm.

[0017] Preferably, both the first gain cavity length extender and the second gain cavity length extender include multiple mutually coupled ring cavities. Simultaneously, this coupled multi-ring structure gain cavity length extender also functions as a beam splitter and optical feedback unit.

[0018] Preferably, the outer diameter of each annular cavity is 5-100 μm, the height of the ridge waveguide formed by etching in each annular cavity is 0.5-10 μm, the ring width of each annular cavity is 100 nm-20 μm, and the shortest distance between the outer edges of two adjacent annular cavities is -20 μm-200 nm.

[0019] The semiconductor optical amplifier has a total length of 400-5000μm, a ridge waveguide width of 2-30μm, and a ridge waveguide height of 0.5-6μm.

[0020] In the first, second, third and fourth electrical isolation trenches, the trench width is 10-20μm, the corrosion depth is 0.3-0.7μm, and the length is 300-500μm.

[0021] Secondly, the present invention also discloses a monolithically integrated multi-segment cascaded optical frequency comb chip, wherein the monolithically integrated multi-segment cascaded optical frequency comb chip includes the monolithically integrated multi-segment cascaded optical frequency comb described in any one of the above claims of the present invention.

[0022] Preferably, the monolithically integrated multi-segment cascaded optical frequency comb chip includes an integrated structure formed on a semiconductor laser epitaxial wafer, wherein the semiconductor laser epitaxial wafer comprises, from bottom to top: a substrate, a transition layer, a lower confinement layer, a lower waveguide layer, an active layer, an upper waveguide layer, an upper confinement layer, and a highly doped layer.

[0023] Preferably, the active layer includes a single quantum well active layer, a multi-quantum well active layer, a quantum dot active layer, or an active layer with a combination of quantum dots and quantum wells.

[0024] Preferably, the device further includes a lower electrode fabricated beneath the substrate and an upper electrode fabricated above the heavily doped layer. For bipolar devices, electrons are injected from the lower electrode and flow out from the upper electrode. For unipolar devices, electrons are injected from the upper electrode and flow out from the lower electrode.

[0025] As can be seen from the above technical solution, compared with the prior art, the present invention discloses a monolithically integrated multi-segment cascaded optical frequency comb and its chip, which has the following beneficial effects:

[0026] This invention proposes a multi-segment cascaded monolithic integrated optical frequency comb and its chip, which can realize an optical frequency comb that integrates small size, high efficiency, tunability, fundamental mode, high power, narrow pulse width and flexible control. It has important practical significance for the integrated application and research of optical comb chips. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0028] Figure 1 This is a schematic diagram of a monolithic integrated multi-segment cascaded optical frequency comb structure with a dual-disk structure provided in an embodiment of the present invention;

[0029] Figure 2 A schematic diagram of a monolithic integrated multi-segment cascaded optical frequency comb structure with a dual-ring structure provided in an embodiment of the present invention;

[0030] Figure 3 This is a schematic diagram of the end face structure of a monolithically integrated multi-segment cascaded optical frequency comb chip provided in an embodiment of the present invention;

[0031] Figure 4 The spectral output characteristics of a GaSb-based ~2μm semiconductor laser epitaxial wafer provided in an embodiment of the present invention are shown.

[0032] Figure 5 The spectral output characteristics of the InP-based 1653nm semiconductor laser epitaxial wafer provided in this embodiment of the invention.

[0033] In the figure, 001-first reverse-bias absorption section with integrated photocurrent distribution grid; 002-first gain cavity length extender section; 003-semiconductor optical amplifier section; 004-second gain cavity length extender section; 005-second reverse-bias absorption section; the sections are connected by electrically insulating isolation trenches; 1-substrate 1; 2-transition layer; 3-lower confinement layer; 4-lower waveguide layer; 5-active layer; 6-upper waveguide layer; 7-upper confinement layer 7; 8-highly doped layer; 01-lower electrode; 02-upper electrode; 03-SiO2 film layer. Detailed Implementation

[0034] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] In the description of this application, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are used only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as limiting the scope of protection of this application. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, features defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this application, unless otherwise stated, "a plurality of" means two or more.

[0036] Example 1

[0037] This invention discloses a monolithically integrated multi-segment cascaded optical frequency comb, comprising a first semiconductor passively mode-locked laser, a semiconductor optical amplifier, and a second semiconductor passively mode-locked laser. The first semiconductor passively mode-locked laser includes a first reverse-biased absorption region and a first gain cavity length extender integrated with a photocurrent distribution grid. The first reverse-biased absorption region is connected to one end of the first gain cavity length extender via a first electrical isolation trench, and the other end of the first gain cavity length extender is connected to the semiconductor optical amplifier via a second electrical isolation trench. The second semiconductor passively mode-locked laser includes a second gain cavity length extender and a second reverse-biased absorption region. One end of the second gain cavity length extender is connected to the semiconductor optical amplifier via a third electrical isolation trench, and the other end of the second gain cavity length extender is connected to the second reverse-biased absorption region via a fourth electrical isolation trench.

[0038] like Figure 1 As shown, in this embodiment, the first semiconductor passive mode-locked laser MLL1 is composed of a photocurrent distribution grating OG (SA1+OG segment 001) integrated in the first reverse polarization absorption region SA1, and a first gain cavity length extender segment 002 integrated in the gain region of MLL1. Specifically, in the first semiconductor passive mode-locked laser MLL1, the photocurrent distribution grating OG can adopt a spectral optical compression grating structure. The second semiconductor passive mode-locked laser MLL2 is composed of a second gain cavity length extender segment 004 and a second reverse polarization absorption segment 005. The difference between the second semiconductor passive mode-locked laser MLL2 and the first semiconductor passive mode-locked laser MLL1 is that the reverse polarization region of the second semiconductor passive mode-locked laser MLL2 does not have a grating structure. The first semiconductor passive mode-locked laser MLL1 and the second semiconductor passive mode-locked laser MLL2 are directly connected by a semiconductor optical amplifier (SOA) segment 003 through an electrically isolated trench.

[0039] In this embodiment, both the first gain cavity length extender section 002 and the second gain cavity length extender section 004 include two mutually coupled disk-shaped cavities.

[0040] In Example 1, the monolithically integrated multi-segment cascaded optical frequency comb consists of five functional segments. Although pre-chirped managed pulse amplification (PCMA) can use gratings to precisely control the negative chirp of the pulse, the spatial structure of the gratings can disrupt the all-optical waveguide structure, which is detrimental to the stability of the amplification system. The integrated multi-segment cascaded optical frequency comb structure based on a semiconductor laser epitaxial material platform proposed in this example does not have this problem.

[0041] SA1+OG section 001 integrates a photocurrent grating structure on the reverse bias waveguide. The photocurrent grating generates photocurrent through reverse bias of the working voltage. The photocurrent is distributed through the grating electrode to form a refractive index periodic structure to achieve spectral linewidth compression and wavelength control of the light transmitted in the waveguide. In this embodiment, the grating etch cutoff position is more than two-thirds of the thickness of the upper waveguide, and the length of the entire grating accounts for two-thirds of the length of the first reverse bias region.

[0042] The first gain cavity length extender section 002 is the controllable modulation feedback region of light. Together with the other second gain cavity length extender section 004 and the two end cavities of the optical frequency comb OFC, it forms a multi-cavity structure. The generated Kerr effect and the multi-periodicity of light in the two dual disks enrich the "tooth" of the optical comb. The two dual disks also have the function of linewidth modulation. The semiconductor optical amplifier section 003 is a straight ridge waveguide structure with forward current drive, which plays the role of bidirectional optical gain amplification and spectral broadening. The second reverse bias absorption section 005 is the reverse bias region at the other end of the optical frequency comb OFC, which can realize the assimilation or differentiation control with the structure and modulation of the other end.

[0043] In this embodiment, the monolithically integrated multi-segment cascaded optical frequency comb combines chirped pulse amplification (CPA), waveguide dissipation, four-wave mixing (FWMF), Kerr effect, and passive mode-locking technology during operation. Multi-segment cascading is achieved through a semiconductor laser epitaxial structure platform. Specifically, the optical pulses generated by the second semiconductor passive mode-locked laser MLL2 are partially broadened by a semiconductor optical amplifier (SOA), then compressed by the anti-polarization absorption region SA1 of the first semiconductor passive mode-locked laser MLL1 with a photocurrent distribution grating structure. The feedback light then passes through a dual-disk structure—the first gain cavity length extender DD1 and the second gain cavity length extender DD2—to achieve dense multi-comb teeth. Mode locking in the mode-locked region ultimately realizes the output of the monolithic optical comb light source.

[0044] In this embodiment, each segment is separated by an electrically isolated band, providing greater freedom, richer controllability, and diverse selectivity for the optical comb output. The photocurrent distribution grating structure in the reverse-biased saturation region of the first semiconductor passive mode-locked laser, MLL1, not only provides pulse compression but also performs wavelength tuning through electrical injection. The gain cavity length extender, through two mutually coupled disk cavities, effectively extends optical gain and cavity length, reducing waveguide loss and adjusting and controlling the optical comb tooth spacing. The semiconductor optical amplifier (SOA) segment performs pulse broadening and optical intensity amplification, equivalent to the broadening grating in CPA technology. The integration of the two passively mode-locked lasers further enhances the adjustment of the optical comb tooth density, controlling the optical comb. The entire structure facilitates the realization of wide-wavelength range optical modulation output.

[0045] refer to Figure 1In this embodiment, the total length of the first semiconductor passive mode-locked laser MLL1 (the sum of SA1+OG section 001 and the first gain cavity length extender section 002) is 1500 μm, the ridge waveguide width is 3 μm, and the ridge waveguide height is 3 μm. The length of SA1+OG section 001 is 100 μm. The photocurrent grating OG period and its constant are designed according to the modulated wavelength, satisfying the following Bragg equation based on the center wavelength: λ Bragg =2nΛ, where n is the refractive index and Λ is the grating period. Since parameters n and Λ are affected by temperature and strain, the center wavelength of the Bragg reflected light also changes with temperature or strain, or both. The grating etching cutoff position is more than two-thirds of the thickness of the upper waveguide, and the entire grating length accounts for two-thirds of the length of the first anti-bias region; in the first gain cavity length extender section 002, the straight ridge waveguide length is 2000μm, and the ridge waveguide width is 3μm; the diameter of each disk cavity is 10μm, and the height of each disk cavity mesa formed by the etching process is 3μm, with a distance of 200nm between the outer edges of the two disks; semiconductor optical amplifier section 003... The total length is 3000μm, the ridge waveguide width is 3μm, and the ridge waveguide height is 3μm; the total length of the second semiconductor passive mode-locked laser MLL2 (the sum of the second gain cavity length extender section 004 and the second reverse bias absorption section 005) is 2000μm, the ridge waveguide width is 3μm, and the ridge waveguide height is 3μm, wherein the length of the second reverse bias absorption section 005 is 100μm; in the second gain cavity length extender section 004, the diameter of each disk cavity is 10μm, the height of each disk cavity mesa formed by the etching process is 3μm, and the distance between the outer edges of the two disks is 200nm.

[0046] Electrically isolated trench parameters: trench width 10μm, corrosion depth 0.5μm, length 500μm.

[0047] Example 2

[0048] like Figure 2 As shown, the difference between Embodiment 2 and Embodiment 1 is that both the first gain cavity length extender section 002 and the second gain cavity length extender section 004 include two mutually coupled annular cavities.

[0049] refer to Figure 2 In this embodiment, the first semiconductor passive mode-locked laser MLL1 (the sum of SA1+OG section 001 and the first gain cavity length extender section 002) has a total length of 1500 μm, a ridge waveguide width of 5 μm, and a ridge waveguide height of 2 μm. The SA1+OG section 001 has a length of 300 μm. The photocurrent grating OG period and its constant are designed according to the modulated wavelength, satisfying the following Bragg equation based on the center wavelength: λ Bragg= 2nΛ, where n is the refractive index and Λ is the grating period. Since parameters n and Λ are affected by temperature and strain, the center wavelength of the Bragg reflected light will also change with temperature or strain, or both. The grating etching cutoff position is more than half the thickness of the upper waveguide, and the entire grating length accounts for one-third to four-fifths of the length of the first anti-bias region. The straight ridge waveguide length of the first gain cavity length extender section 002 is 1800μm, the ridge waveguide width is 5μm, the diameter of each annular cavity DR1 is 30μm, and the ring width is 2μm. Each annular cavity DR1 is etched... The ring waveguide ridge structure formed by the process has a height of 2 μm, and the outer edges of the two rings are -2 μm apart; the total length of the semiconductor optical amplifier SOA section 003 is 2000 μm, the width of the ridge waveguide is 5 μm, and the height of the ridge waveguide is 2 μm; the total length of the second semiconductor passive mode-locked laser MLL2 (the sum of the second gain cavity length extender section 004 and the second reverse bias absorption section 005) is 2000 μm, the width of the ridge waveguide is 5 μm, and the height of the ridge waveguide is 2 μm, of which the length of the second reverse bias absorption section 005 is 200 μm.

[0050] Electrically isolated trench parameters: trench width 10μm, corrosion depth 0.7μm, length 400μm.

[0051] Example 3

[0052] This embodiment discloses a chip comprising any of the above-mentioned monolithically integrated multi-segment cascaded optical frequency combs, based on Embodiment 1 or Embodiment 2. For example... Figure 3 As shown, the epitaxial structure of the monolithically integrated multi-segment cascaded optical frequency comb chip, from bottom to top, consists of: substrate 1, transition layer 2, lower confinement layer 3, lower waveguide layer 4, active layer 5, upper waveguide layer 6, upper confinement layer 7, and highly doped layer 8.

[0053] In this embodiment, the active layer 5 can specifically be: a single quantum well active layer, a multi-quantum well active layer, a quantum dot active layer, or an active layer with a combination of quantum dots and quantum wells.

[0054] like Figure 3 As shown, the chip integrating a multi-segment cascaded optical frequency comb on a single chip also includes a lower electrode 01 fabricated below the substrate 1 and an upper electrode 02 fabricated above the highly doped layer 8. In addition, a SiO2 film layer 03 is fabricated between the upper electrode 02 and the upper waveguide layer 6.

[0055] Typically, the lower electrode uses an N-face electrode, and the upper electrode uses a P-face electrode.

[0056] In this embodiment, a high-reflectivity film and an anti-reflection film can be deposited at both ends of the chip's output light, respectively.

[0057] In this embodiment, the integrated multi-segment cascaded optical frequency comb chip suppresses thermal refractive index noise through a dual passive mode-locked semiconductor laser structure to obtain the optical comb output, realizing a single-chip integrated optical comb that combines active and passive functions. The structure sequentially includes: a first reverse-bias absorption region SA1 with an integrated photocurrent distribution grid OG, a first gain cavity length extender (dual-disk DD1 / dual-ring DR1 modulation region structure), a semiconductor optical amplifier region structure SOA, a second gain cavity length extender (dual-disk DD2 / dual-ring DR2 modulation region structure), and a second reverse-bias absorption region SA2 structure. Each segment is connected by an electrically isolated trench strip structure.

[0058] Example 4

[0059] This embodiment discloses a GaSb-based ~2μm semiconductor laser epitaxial wafer, and the materials and dimensions of each layer in the epitaxial structure are shown in Table 1.

[0060] Table 1

[0061]

[0062]

[0063] In this embodiment, the gain cavity length extender uses two mutually coupled disk-shaped cavities, and the dimensions of each section are as follows: First reverse bias absorption region L SA1 = 302.08μm, waveguide width 2μm; grating etching depth 2200nm, total grating length 230μm; length L of the straight waveguide coupled to the disk cavity in the first gain long extender. DD1 =3171.8μm, single disk cavity radius R=20μm, waveguide width 2μm; semiconductor optical amplifier section L SOA =800μm, waveguide width 2μm; second reverse polarization absorption region L SA2 =302.08μm waveguide width 2μm; length L of the straight waveguide coupled to the disk cavity in the second gain long extender DD2 =3171.8μm, single disk cavity radius R=20μm, waveguide width 2μm; the distance between the two disks in the first and second gain long extenders is 50nm, waveguide width 2μm; the ridge height formed by etching process for each disk cavity and ridge waveguide is 3μm; the distance between the two disks and the straight waveguide is 50nm; the width of the electrical isolation trench is 10μm, the etching depth is 0.5μm, and the length of the electrical isolation trench is 500μm; at room temperature, the reverse bias voltage V in the reverse bias absorption region is... a -1V, current I g Under 300mA conditions, the spectral output characteristics of this embodiment are shown in the figure below. Figure 4 As shown.

[0064] In this embodiment, the reverse bias voltage applied to the first reverse bias absorption region and the reverse bias voltage applied to the second reverse bias absorption region are both -1V. In other embodiments, the reverse bias voltage of the first reverse bias absorption region and the reverse bias voltage of the second reverse bias absorption region may also use different voltage values.

[0065] Example 5

[0066] This embodiment discloses an InP-based 1653nm semiconductor laser epitaxial wafer, and the materials and dimensions of each layer in the epitaxial structure are shown in Table 2.

[0067] Table 2

[0068]

[0069]

[0070] In this embodiment, the gain cavity length extender uses two mutually coupled annular cavities, and the dimensions of each section are as follows: First reverse bias absorption region L SA1 = 300μm; total grating length 200μm; length L of the straight waveguide coupled to the ring cavity in the first gain extender. DD1 =3000μm, single ring cavity radius R = 25μm (half the outer diameter of the ring cavity), waveguide width 3μm; semiconductor optical amplifier section L SOA =5000μm waveguide width 3μm; second reverse polarization absorption region L SA2 =300μm waveguide width 3μm; the length L of the straight waveguide coupled to the ring cavity in the second gain long extender DD1 =3000μm, waveguide width 3μm; the two rings in the first and second gain long extenders are spaced 50nm apart, waveguide width 3μm, and the etching depth of each ring cavity waveguide is 2μm; the distance between the two rings and the straight waveguide is 50nm; the electrical isolation trench is 10μm wide, etched to a depth of 0.6μm, and 500μm long; the spectral output characteristics of this embodiment are shown in the figure. Figure 5 As shown.

[0071] The following is a further explanation of each segment in the chip of the monolithically integrated multi-segment cascaded optical frequency comb in the embodiments of the present invention.

[0072] 1) The first reverse-biased absorption region SA1 integrated with the photocurrent distribution gate OG

[0073] In the first reverse-biased absorption region SA1, the length of the reverse-biased region determines the peak intensity and spectral width of the optical pulse. A photocurrent modulation grating structure is integrated on the straight ridge of this region. The current gradient refractive index caused by the photocurrent intensity distribution generated in this region changes periodically in a grating shape. By using a grating with a specific structure to compress the optical pulse, the spectral width can be compressed and modulated.

[0074] 2) First gain cavity length extender (dual-disk DD1 / dual-ring DR1 modulation region structure)

[0075] This invention breaks through the limitations of traditional single-ring or disk structures, fully utilizing the dual-disk / ring modulation region to perform optical feedback, optical modulation, and optical cavity extension. Through the multi-order or higher-order nonlinear effects of the dual-disk / ring structure, nonlinear proportional modulation of light from the waveguide region is achieved. Disk structures can achieve higher optical transmission efficiency, while ring structures facilitate optical modulation more easily. Besides being determined by its own parameters (radius, curvature), the coupling between the dual-disk / ring and the waveguide determines its modulation efficiency and effect.

[0076] 3) Semiconductor Optical Amplifier (SOA)

[0077] The optical gain amplification region can both amplify the optical gain and broaden the spectral width of the transmitted light; its structural parameters are directly related to the optical amplification capability. For ease of integration, the width can be determined by considering the waveguide parameters of other sections under high-power single-mode operation. Simultaneously, specific etching parameters and sidewall smoothness are required. Excessive waveguide etching is detrimental to stable optical amplification and increases waveguide loss during light propagation. Appropriate etching depth and excellent sidewall smoothness are beneficial for achieving high gain.

[0078] 4) Second gain cavity length extender (dual-disk DD2 / dual-ring DR2 modulation region structure)

[0079] In the second gain cavity length extender, the dual-disk / ring modulation region, besides sharing the same structure and function as the dual-disk / ring of the first gain cavity length extender mentioned above, can also achieve the multifunctionality of the OFC chip through electrical modulation of this region, in conjunction with the first gain cavity length extender. This is achieved through the vernier effect of the two electrically injected dual-disk / rings, the splitting ratio, gain amplification, interlocking, and self-locking. This includes output parameters such as four-wave mixing (FWM) level, wavelength position, linewidth, and OFC width, and even soliton characteristics. Research on its position and structural parameters helps to achieve broadband, high-power, and stable output characteristics of the optical comb.

[0080] 5) Second reverse-bias absorption region SA2 structure

[0081] In the second reverse-biased absorption region SA2, this section does not integrate a photocurrent distribution gate structure. This region, through reverse current bias and adjacent surface-mount optical feedback, provides phase-locked, feedback light, and broadened OFC output, and is part of the integrated passively mode-locked semiconductor mode-locked laser. Its length determines the pulse width and intensity of the feedback light and the spectral width of the OFC. Therefore, to ensure high power and single-mode operation, its relevant structural parameters need to be matched with its output characteristics.

[0082] Inter-segment electrical isolation strip structure

[0083] In this embodiment, each segment of the integrated multi-segment cascaded optical frequency comb chip has a different function, with significant differences between reverse and forward bias. To avoid mutual electrical interference between adjacent segments, sufficient electrical isolation width and etching depth are necessary between segments. Excessively wide isolation bands increase internal light loss within the isolation band, affecting light extraction efficiency and reducing the chip's output characteristics; excessively narrow isolation bands can easily cause electrical interference between adjacent segments, affecting their respective functionality. Excessively deep isolation bands weaken light transmission and increase internal loss; excessively shallow isolation bands lead to electrical crosstalk in adjacent areas, affecting the device's output characteristics.

[0084] OFC chip fabrication process

[0085] In this embodiment, the chip integrating a multi-segment cascaded optical frequency comb on a single chip can be studied using a combination of wet and dry etching processes. The structure involves the coupling methods of the grating, dual disks / rings, and straight-ridge waveguides. The fabricated structure and morphology directly determine the optical output characteristics of the device. Dry etching allows for high-precision control, especially the control of the coupling distance and coupling method of the grating, dual disks / rings, and straight-ridge waveguides, which is a crucial factor determining the optical output characteristics of OFC. Wet etching produces a smooth surface morphology, which is beneficial for forming low-loss interfaces. The combination of wet and dry etching can achieve an excellent device structure.

[0086] Segmented fabrication can achieve optimal fabrication of each segment structure, avoiding the impact on device performance caused by differences in process parameters between segments.

[0087] In this embodiment, the dual-disk / ring used in the gain cavity length extender has multiple functions. First, it serves as a cavity surface function. The light fed back from the cavity surface end forms a mode-locked passive mode-locked semiconductor laser structure with the anti-bias region adjacent to the cavity surface end. The optical path difference of 0 to n times as different components of light rotate in the dual-disk / ring forms the comb teeth for each frequency. The two passive mode-locked semiconductor laser structures with dual-disk / rings form rich optical teeth through cavity surface feedback at the output end.

[0088] Secondly, it serves as the gain region together with the optical amplification area (SOA). Strong optical gain amplification is achieved through intracavity oscillation between the two formed dual disks / rings, providing sufficiently high light intensity and spectral broadening for the mode-locked region. Thirdly, external strong light pulses induce internal optical nonlinear effects. With sufficient external light pulse intensity, the dual disks / rings themselves can generate nonlinear effects through these pulses, producing four-wave mixing and Kerr effects, equivalent to an active microdisk / ring structure. Each dual disk / ring can form bidirectional feedback. The nonlinear effects in the disk / ring region are closely related to the light intensity of the external straight waveguide region and the structure within the active waveguide of the disk / ring. The parameters of the disk / ring affect its own optical loss.

[0089] Finally, the radii of the two disks / rings determine the tunable bandwidth and the comb tooth spacing. The selection and position adjustment of the comb teeth are achieved through the vernier effect. A Ti / Au thin film is fabricated on the straight ridge waveguide coupled to the two disks / rings as an external modulation electrode, replacing the simple Al / TiN heating electrode. This allows for control of the optical feedback of the two-disk / ring resonator, provides a certain optical gain, and reduces optical loss. Optimal modulation of the light within the waveguide can be achieved through two disks / rings. Furthermore, the coupling method and related parameters between the two disks / rings and the straight waveguide affect the output optical comb performance, significantly influencing the optical feedback efficiency and tunable wavelength bandwidth.

[0090] In multi-segment OFC light source chips, the influence of higher-order Kerr effects and free-wave dynamic range (FWM) in each segment is closely related to the structural parameters of each segment. Small structural parameters, high light intensity, and large gain characteristics will inevitably make it easier to induce nonlinear optical effects and FWM within the device, and to utilize these effects and principles to generate meaningful device output characteristics.

[0091] In this embodiment, the waveguide of the entire optical comb chip is epitaxially mounted on a unified material platform with an uninterrupted structure, and the chip structure and process do not compromise the integrity of the platform. Furthermore, the two dual-disk / ring structures can serve as cavity surfaces for each other, providing both containment and selectivity for various types of light. Therefore, the stability of the amplification system is not affected.

[0092] Compared to traditional passive OFC structures with only a single-ring microcavity and fiber ring, the multi-segment cascaded integrated active controllable OFC light source chip structure disclosed in this invention has the following advantages: small system size, enabling integrated control of active and passive OFC components on a single chip; good system stability, requiring no complex or special external temperature control conditions; wide applicable wavelength range, with adjustable linewidth and wavelength range to some extent; strong light output capability and high peak pulse power; fabrication process integrated with semiconductor laser fabrication process, easy to implement, suitable for mass production, and reducing costs; independent control of each segment, laying the structural foundation for achieving fundamental mode, high power, and wide spectrum output of the overall integrated device. Compared to the single-ring passive microcavity OFC structure, it has significant advantages.

[0093] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to the method section.

[0094] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A monolithically integrated multi-segment cascaded optical frequency comb, characterized in that, include: A first semiconductor passively mode-locked laser, a semiconductor optical amplifier, and a second semiconductor passively mode-locked laser; The first semiconductor passive mode-locked laser includes a first reverse-biased absorption region and a first gain cavity length extender integrated with a photocurrent distribution grid; the first reverse-biased absorption region is connected to one end of the first gain cavity length extender through a first electrical isolation trench, and the other end of the first gain cavity length extender is connected to the semiconductor optical amplifier through a second electrical isolation trench. The second semiconductor passive mode-locked laser includes a second gain cavity length extender and a second reverse-biased absorption region. One end of the second gain cavity length extender is connected to the semiconductor optical amplifier through a third electrical isolation trench, and the other end of the second gain cavity length extender is connected to the second reverse-biased absorption region through a fourth electrical isolation trench.

2. The monolithically integrated multi-segment cascaded optical frequency comb according to claim 1, characterized in that, The etching depth of the photocurrent distribution grating extends to one-hundredth to one-half of the thickness downwards from the top edge of the upper waveguide layer of the monolithically integrated multi-segment cascaded optical frequency comb. The length of the photocurrent distribution grating accounts for one-third to four-fifths of the overall length of the first reverse-bias absorption region. The photocurrent distribution grating includes a narrow-spectrum optical compression grating.

3. The monolithically integrated multi-segment cascaded optical frequency comb according to claim 1, characterized in that, Both the first gain cavity length extender and the second gain cavity length extender include multiple disk-shaped cavities that are coupled to each other.

4. The monolithically integrated multi-segment cascaded optical frequency comb according to claim 3, characterized in that, The diameter of each of the disk cavities is 5-100 μm, the height of the disk surface etched by each of the disk cavities is 0.5-10 μm, and the shortest distance between the outer edges of two disk cavities is -20 μm to 200 nm. The semiconductor optical amplifier has a total length of 400-5000μm, a ridge waveguide width of 2-30μm, and a ridge waveguide height of 0.5-6μm. In the first, second, third and fourth electrical isolation trenches, the trench width is 5-20μm, the corrosion depth is 0.3-0.7μm, and the length is 300-500μm.

5. The monolithically integrated multi-segment cascaded optical frequency comb according to claim 1, characterized in that, Both the first gain cavity length extender and the second gain cavity length extender include multiple annular cavities that are coupled to each other.

6. The monolithically integrated multi-segment cascaded optical frequency comb according to claim 5, characterized in that, The outer diameter of each annular cavity is 5-100 μm, the height of the ridge waveguide formed by etching in each annular cavity is 0.5-10 μm, the ring width of each annular cavity is 100 nm-20 μm, and the shortest distance between the outer edges of two adjacent annular cavities is -20 μm-200 nm. The semiconductor optical amplifier has a total length of 400-5000μm, a ridge waveguide width of 2-30μm, and a ridge waveguide height of 0.5-6μm. In the first, second, third and fourth electrical isolation trenches, the trench width is 10-20μm, the corrosion depth is 0.3-0.7μm, and the length is 300-500μm.

7. A monolithically integrated multi-segment cascaded optical frequency comb chip, characterized in that, Includes the monolithic integrated multi-segment cascaded optical frequency comb as described in any one of claims 1-6.

8. The monolithically integrated multi-segment cascaded optical frequency comb chip according to claim 7, characterized in that, The monolithically integrated multi-segment cascaded optical frequency comb chip includes an integrated structure formed on a semiconductor laser epitaxial wafer, wherein the semiconductor laser epitaxial wafer consists of, from bottom to top: a substrate, a transition layer, a lower confinement layer, a lower waveguide layer, an active layer, an upper waveguide layer, an upper confinement layer, and a highly doped layer.

9. The monolithically integrated multi-segment cascaded optical frequency comb chip according to claim 8, characterized in that, The active layer includes a single quantum well active layer, a multi-quantum well active layer, a quantum dot active layer, or an active layer with a combination of quantum dots and quantum wells.

10. The monolithically integrated multi-segment cascaded optical frequency comb chip according to claim 8, characterized in that, It also includes a lower electrode fabricated below the substrate and an upper electrode fabricated above the highly doped layer.

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