Monolithic integrated multi-section cascaded optical frequency comb and chip comprising same

Abstract

A monolithic integrated multi-section cascaded optical frequency comb and a chip comprising same, relating to the technical field of sensing detection, quantum information, and optical communications. The monolithic integrated multi-section cascaded optical frequency comb comprises a first semiconductor passive mode-locking laser MLL1, a semiconductor optical amplifier (003), and a second semiconductor passive mode-locking laser MLL2 which are sequentially integrated and connected; the first semiconductor passive mode-locking laser MLL1 comprises: a first reverse bias absorption area (001) integrated with an optogalvanic distribution grating OG, and a first gain cavity length extender (002), wherein the first gain cavity length extender (002) may be of a coupled multi-ring or multi-disc structure; and the second semiconductor passive mode-locking laser MLL2 comprises a second reverse bias absorption area (005) and a second gain cavity length extender (004), wherein the second gain cavity length extender (004) may be of a coupled multi-ring or multi-disc structure. Structures are connected to each other by means of electrical isolation trenches, thereby realizing an optical frequency comb having a small size, high efficiency, tunability, a fundamental mode, a high power, narrow pulse width, a wide spectrum, and flexible controllability.

Description

A monolithic integrated multi-segment cascade optical frequency comb and its chip Technical Field

[0001] The present invention relates to the technical fields of sensing detection, quantum information and optical communication technology, and more particularly to a monolithic integrated multi-segment cascade optical frequency comb and a chip thereof. Background Art

[0002] An optical frequency comb (OFC), also known as an optical frequency comb or optical comb, consists of a single "tooth" that acts as a monochromatic laser beam. These monochromatic lasers are highly coherent with each other, and OFC can directly convert light waves into microwave frequencies. By leveraging the nonlinearity of ultrashort pulses, OFC can be achieved in various spectral regions and at different frequency resolutions.

[0003] Since the birth of optical frequency combs (OFCs), OFCs based on traditional fiber lasers, solid-state laser technology, and microcavity structures have had problems such as large size, many components, low repetition frequency, susceptibility to external environmental influences, and separation of the light source and control ends, which have affected the performance of the optical comb and limited its application and development.

[0004] Furthermore, conventional semiconductor mode-locked lasers require a short cavity length, under 1000 μm, to achieve optical frequency combs across a wide wavelength range. This cavity length results in very low output power, and the reverse bias region, where reverse bias is applied, significantly absorbs light, making it difficult to achieve wide-spectrum optical frequency comb output, let alone high-power output.

[0005] Therefore, how to reduce the size of the optical frequency comb while ensuring high-power and wide-spectral-range optical frequency comb output is an urgent problem that technicians in this field need to solve.

[0006] Summary of the Invention

[0007] In view of this, the present invention provides a monolithic integrated multi-segment cascaded optical frequency comb and its chip, which are used to solve the problem of achieving a small size, high efficiency, tunable, fundamental mode, high power, narrow pulse width, wide spectrum and flexible controllable optical frequency comb.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] First, the present invention discloses a monolithic integrated multi-segment cascaded optical frequency comb, comprising:

[0010] A first semiconductor passively mode-locked laser, a semiconductor optical amplifier, and a second semiconductor passively mode-locked laser;

[0011] The first semiconductor passively mode-locked laser comprises a first reverse-biased absorption region integrated with a photocurrent distribution grid and a first gain cavity length extender; 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;

[0012] 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 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.

[0013] Preferably, the etching depth of the photocurrent distribution gate is from one percent to one half of the thickness downward from the upper edge of the upper waveguide layer of the monolithic integrated multi-segment cascaded optical frequency comb as the starting point, and the length of the photocurrent distribution gate accounts for one third to four fifths of the overall length of the first reverse bias absorption region; the photocurrent distribution gate includes a narrow spectrum light compression grating.

[0014] Preferably, the first gain cavity length extender and the second gain cavity length extender each comprise a plurality of mutually coupled disk-shaped cavities. Simultaneously, the coupled multi-disk structure gain cavity length extender also functions as a light splitter and optical feedback device.

[0015] Preferably, the diameter of each disk-shaped cavity is 5-100 μm, the height of the disk surface etched by each disk-shaped cavity is 0.5-10 μm, and the shortest distance between the outer edges of two adjacent disk-shaped cavities is -20 μm-200 nm;

[0016] The semiconductor optical amplifier has a total length of 400-5000 μm, a ridge waveguide width of 2-30 μm, and a ridge waveguide height of 0.5-6 μm;

[0017] In the first, second, third and fourth electrical isolation trenches, the trench width is 5-20 μm, the etching depth is 0.3-0.7 μm, and the length is 300-500 μm.

[0018] Preferably, the first gain cavity length extender and the second gain cavity length extender each comprise a plurality of mutually coupled ring cavities. Simultaneously, such coupled multi-ring structure gain cavity length extenders also function as light splitters and optical feedback devices.

[0019] Preferably, the outer diameter of each annular cavity is 5-100 μm, the height of the ridge waveguide formed by etching of 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;

[0020] The semiconductor optical amplifier has a total length of 400-5000 μm, a ridge waveguide width of 2-30 μm, and a ridge waveguide height of 0.5-6 μm;

[0021] In the first, second, third and fourth electrical isolation trenches, the trench width is 10-20 μm, the etching depth is 0.3-0.7 μm, and the length is 300-500 μm.

[0022] Secondly, the present invention also discloses a monolithic integrated multi-segment cascaded optical frequency comb chip, wherein the monolithic integrated multi-segment cascaded optical frequency comb chip includes the monolithic integrated multi-segment cascaded optical frequency comb described in any one of the above items of the present invention.

[0023] Preferably, the monolithic integrated multi-segment cascaded optical frequency comb chip includes an integrated structure formed on a semiconductor laser epitaxial wafer, and 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.

[0024] 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 combined structure of quantum dots and quantum wells.

[0025] Preferably, the device further comprises a lower electrode formed below the substrate and an upper electrode formed above the highly doped layer. For a bipolar device, electrons are injected from the lower electrode and discharged from the upper electrode. For a unipolar device, electrons are injected from the upper electrode and discharged from the lower electrode.

[0026] As can be seen from the above technical solution, compared with the prior art, the present invention discloses a monolithic integrated multi-segment cascaded optical frequency comb and its chip, which has the following beneficial effects:

[0027] The present invention proposes a multi-segment cascaded monolithic integrated optical frequency comb and its chip, which can realize an optical frequency comb that combines small size, high efficiency, tunability, fundamental mode, high power, narrow pulse width and flexible controllability. It has important practical significance for the integrated application and research of optical comb chips. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0029] FIG1 is a schematic diagram of a monolithically integrated multi-segment cascaded optical frequency comb structure with a dual-disk structure according to an embodiment of the present invention;

[0030] FIG2 is a schematic diagram of a monolithically integrated multi-segment cascaded optical frequency comb structure with a dual-ring structure according to an embodiment of the present invention;

[0031] FIG3 is a schematic diagram of the end face structure of a monolithically integrated multi-segment cascaded optical frequency comb chip provided by an embodiment of the present invention;

[0032] FIG4 is a graph showing the spectral output characteristics of a GaSb-based ~2 μm semiconductor laser epitaxial wafer provided by an embodiment of the present invention;

[0033] FIG5 is a graph showing the spectral output characteristics of an InP-based 1653 nm semiconductor laser epitaxial wafer provided by an embodiment of the present invention.

[0034] In the figure, 001-the first reverse-biased absorption section with an integrated photocurrent distribution grid; 002-the first gain cavity length extender section; 003-the semiconductor optical amplifier section; 004-the second gain cavity length extender section; 005-the second reverse-biased absorption section; each section is connected by an electrically insulating isolation groove; 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 DESCRIPTION

[0035] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0036] In the description of the present application, it should be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the scope of protection of the present application. In addition, the terms "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, features defined as "first", "second", etc. may explicitly or implicitly include one or more of the features. In the description created by the present application, unless otherwise specified, "multiple" means two or more.

[0037] Example 1

[0038] An embodiment of the present 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; wherein the first semiconductor passively mode-locked laser comprises a first reverse-biased absorption region integrated with a photocurrent distribution grid and a first gain cavity length extender; 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 comprises 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.

[0039] As shown in Figure 1, 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-biased 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 light 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-biased absorption segment 005. The difference from the first semiconductor passive mode-locked laser MLL1 is that the reverse-biased 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 to each other by a semiconductor optical amplifier (SOA) segment 003 through an electrical isolation trench.

[0040] In this embodiment, the first gain cavity length extender section 002 and the second gain cavity length extender section 004 both include two mutually coupled disk-shaped cavities.

[0041] In Example 1, the monolithically integrated multi-segment cascaded optical frequency comb consists of five functional segments. Although pre-chirp managed pulse amplification (PCMA) technology can precisely control the amount of pulse negative chirp using grating pairs, the spatial structure of the gratings can disrupt the all-optical waveguide structure, hindering the stability of the amplification system. However, the integrated multi-segment cascaded optical frequency comb structure proposed in this example, based on a semiconductor laser epitaxial material platform, does not suffer from this problem.

[0042] The SA1+OG segment 001 integrates a photocurrent grating structure on the reverse-biased waveguide. The photocurrent grating generates photocurrent by reverse-biasing the operating voltage. The photocurrent is distributed through the grating electrode, forming a periodic refractive index structure to achieve spectral linewidth compression and wavelength control of the light transmitted in the waveguide. In this embodiment, the grating corrosion cutoff position is more than two-thirds of the upper waveguide thickness, and the entire grating length accounts for two-thirds of the length of the first reverse-biased region.

[0043] The first gain cavity length extender section 002 is a controllable modulation feedback zone for light, forming a multi-cavity structure with the second gain cavity length extender section 004 and the two end cavity surfaces of the optical frequency comb OFC. The Kerr effect generated inside the two sections and the multi-periodicity of light in the two double disks enrich the "comb teeth" of the optical comb, and the two double disks also have a linewidth control function; the semiconductor optical amplifier section 003 is a straight-ridge waveguide structure with forward current drive, which plays a role in 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 achieve assimilation or differentiation control of the structure and modulation at the other end.

[0044] In this embodiment, a monolithic integrated multi-segment cascaded optical frequency comb combines chirped pulse amplification (CPA), waveguide dissipation, four-wave mixing (FWMF), the Kerr effect, and passive mode locking technologies in its operation. Multi-segment cascading is achieved through a semiconductor laser epitaxial structure platform. Part of the optical pulse generated by the second semiconductor passive mode-locked laser MLL2 passes through a semiconductor optical amplifier SOA for pulse broadening, and then passes through the reverse-biased absorption region SA1 of the first semiconductor passive mode-locked laser MLL1 with a photocurrent distribution grating structure to achieve optical pulse compression. The feedback light then passes through the first gain cavity length extender DD1 and the second gain cavity length extender DD2 of the double-disk structure to achieve dense multi-comb teeth. Through mode locking in the mode-locked region, the output of a single-chip optical comb light source is finally achieved.

[0045] In this embodiment, each segment is separated by an electrical isolation band, providing greater degrees of freedom, rich controllability, and diverse selectivity for the optical comb output. The photocurrent distribution grating structure in the reverse-biased saturation region of the first semiconductor passively mode-locked laser MLL1 not only provides pulse compression but also enables wavelength tuning through electrical injection. The gain cavity length extender uses two mutually coupled disk cavities to achieve optical gain and equivalent cavity length extension, reducing waveguide losses and regulating and controlling the optical comb tooth spacing. The semiconductor optical amplifier (SOA) segment provides pulse stretching and light intensity amplification, equivalent to the stretching grating in CPA technology. The integration of two passively mode-locked lasers further enhances the optical comb tooth density regulation and control of the optical comb. The entire structure facilitates optical modulation output over a wide wavelength range.

[0046] Referring to FIG1 , in this embodiment, the total length of the first semiconductor passively mode-locked laser MLL1 (the sum of the 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 the SA1+OG section 001 is 100 μm. The photocurrent grating OG period and its constant are designed according to the modulated wavelength, and the following Bragg equation is satisfied based on the central wavelength: λ Bragg =2nΛ, where n is the refractive index and Λ is the grating period. Because the parameters n and Λ are affected by temperature and strain, the central wavelength of the Bragg reflected light will also change with changes in temperature or strain, or both. The grating etching cutoff position is more than two-thirds of the upper waveguide thickness, and the entire grating length accounts for two-thirds of the length of the first reverse bias region. The straight ridge waveguide in the first gain cavity length extender section 002 is 2000μm long and 3μm wide. The diameter of each disk cavity is 10μm, and the height of each disk cavity mesa formed by the etching process is 3μm. The outer edges of the two disks are 200nm apart. 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 passively mode-locked laser MLL2 (the sum of the second gain cavity length extender section 004 and the second reverse polarization absorption section 005) is 2000μm, the ridge waveguide width is 3μm, and the ridge waveguide height is 3μm, of which the length of the second reverse polarization absorption section 005 is 100μm; in the second gain cavity length extender section 004, the diameter of each disk-shaped cavity is 10μm, the height of each disk-shaped cavity table formed by the etching process is 3μm, and the outer edges of the two disks are 200nm apart.

[0047] Electrical isolation trench parameters: trench width 10 μm, etching depth 0.5 μm, length 500 μm.

[0048] Example 2

[0049] As shown in FIG2 , the difference between Example 2 and Example 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 ring cavities.

[0050] Referring to FIG2 , in this embodiment, the first semiconductor passively mode-locked laser MLL1 (the sum of the 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 length of the SA1+OG section 001 is 300 μm, and the photocurrent grating OG period and its constant are designed according to the modulated wavelength, satisfying the following Bragg equation based on the central wavelength: λ Bragg=2nΛ, where n is the refractive index and Λ is the grating period. Because the parameters n and Λ are affected by temperature and strain, the central wavelength of the Bragg reflected light will also change with changes in temperature or strain, or both. The grating etching cutoff position is more than half of the upper waveguide thickness, and the entire grating length accounts for one-third to four-fifths of the length of the first reverse 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 ring cavity DR1 is 30μm, and the ring width is 2μm. Each ring cavity DR1 is etched The ring waveguide ridge structure formed by the process has a height of 2um, and the outer edges of the two rings are -2μm apart; the total length of the semiconductor optical amplifier SOA segment 003 is 2000μm, the ridge waveguide width is 5μm, and the ridge waveguide height is 2μm; the total length of the second semiconductor passively mode-locked laser MLL2 (the sum of the second gain cavity length extender segment 004 and the second reverse polarization absorption segment 005) is 2000μm, the ridge waveguide width is 5μm, and the ridge waveguide height is 2μm, of which the length of the second reverse polarization absorption segment 005 is 200μm.

[0051] Electrical isolation trench parameters: trench width 10 μm, etching depth 0.7 μm, length 400 μm.

[0052] Example 3

[0053] This embodiment, based on either Example 1 or Example 2, discloses a chip comprising either of the aforementioned monolithically integrated multi-segment cascaded optical frequency combs. As shown in Figure 3, the epitaxial structure of the monolithically integrated multi-segment cascaded optical frequency comb chip comprises, from bottom to top: 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.

[0054] 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 combined structure of quantum dots and quantum wells.

[0055] As shown in Figure 3, the monolithically integrated multi-segment cascaded optical frequency comb chip also includes a lower electrode 01 formed below substrate 1 and an upper electrode 02 formed above highly doped layer 8. Furthermore, a SiO2 film layer 03 is formed between upper electrode 02 and upper waveguide layer 6.

[0056] Typically, the lower electrode uses an N-side electrode and the upper electrode uses a P-side electrode.

[0057] In this embodiment, both ends of the chip output light can be coated with a high-reflection film and an anti-reflection film respectively.

[0058] In this embodiment, an integrated multi-segment cascaded optical frequency comb chip utilizes a dual passively mode-locked semiconductor laser structure to suppress thermal refractive index noise, generating an optical comb output, thereby achieving a single-chip integrated optical comb with both active and passive functions. The structure comprises: a first reverse-biased absorption region SA1 with an integrated photocurrent distribution gate (OG); a first gain cavity length extender (a dual-disk DD1 / dual-ring DR1 modulation region structure); a semiconductor optical amplifier region structure SOA; a second gain cavity length extender (a dual-disk DD2 / dual-ring DR2 modulation region structure); and a second reverse-biased absorption region SA2 structure. Each segment is connected via an electrically isolated trench strip structure.

[0059] Example 4

[0060] This embodiment discloses a GaSb-based ~2μm semiconductor laser epitaxial wafer. The materials and dimensions of each layer in the epitaxial structure are shown in Table 1.

[0061] Table 1

[0062] In this embodiment, the gain cavity length extender uses two mutually coupled disc-shaped cavities, and the size specifications of each section are as follows: the first reverse bias absorption zone L SA1 =302.08μm, waveguide width 2μm; grating etching depth 2200nm, total grating length 230μm; the straight waveguide length L coupled to the disc cavity in the first gain extender DD1 =3171.8μm, single disk cavity radius R = 20μm, waveguide width 2um; semiconductor optical amplifier section L SOA =800μm, waveguide width 2μm; second reverse bias absorption region L SA2 =302.08μm waveguide width 2μm; the straight waveguide length L coupled to the disc cavity in the second gain extender DD2 =3171.8μm, the radius of a single disk cavity R = 20μm, the waveguide width is 2μm; the distance between the two disks in the first gain extender and the second gain extender is 50nm, and the waveguide width is 2μm; the ridge height formed by the etching process of 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 groove is 10μm, the etching depth is 0.5μm, and the length of the electrical isolation groove is 500μm; at room temperature, the reverse bias voltage V a is -1V, current I g FIG4 shows a graph of the spectral output characteristics of the embodiment under the condition of 300 mA.

[0063] 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 applied to the first reverse bias absorption region and the reverse bias voltage applied to the second reverse bias absorption region may also adopt different voltage values.

[0064] Example 5

[0065] This embodiment discloses an InP-based 1653nm semiconductor laser epitaxial wafer. The materials and dimensions of each layer in the epitaxial structure are shown in Table 2.

[0066] Table 2

[0067] In this embodiment, the gain cavity length extender uses two mutually coupled ring cavities, and the size specifications of each section are as follows: the first reverse bias absorption zone L SA1 =300μm; the entire grating length is 200μm; the straight waveguide length L coupled to the ring cavity in the first gain extender DD1 =3000μm, single ring cavity radius R = 25μm (half of the outer diameter of the ring cavity), waveguide width 3μm; semiconductor optical amplifier section L SOA =5000μm waveguide width 3μm; second reverse bias absorption region L SA2 =300μm waveguide width 3μm; the straight waveguide length L coupled to the ring cavity in the second gain extender DD1 =3000μm, waveguide width 3μm; the distance between the two rings in the first gain extender and the second gain extender is 50nm, the waveguide width is 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 width of the electrical isolation groove is 10μm, the etching depth is 0.6μm, and the length of the electrical isolation groove is 500μm; the spectral output characteristic diagram of this embodiment is shown in Figure 5.

[0068] Each segment in the chip of the monolithically integrated multi-segment cascaded optical frequency comb in an embodiment of the present invention will be further described below.

[0069] 1) The first reverse bias absorption region SA1 integrated with the photocurrent distribution gate OG

[0070] In the first reverse-bias absorption region SA1, the length of the reverse-bias region determines the peak intensity and spectral width of the optical pulse. A photocurrent-modulated grating structure is integrated on the straight ridges of this region. The photocurrent intensity distribution generated in this region induces a grating-like periodic variation in the current-gradient refractive index. This structured grating compresses the optical pulse, achieving spectral width compression and modulation.

[0071] 2) First gain cavity length extender (double-disk DD1 / double-ring DR1 modulation area structure)

[0072] The embodiments of the present invention transcend the limitations of traditional single rings or disks by fully utilizing the dual-disk / ring modulation region to perform optical feedback, optical modulation, and cavity extension. Through the multi-order or high-order nonlinear effects of the dual-disk / ring structure, nonlinear proportional modulation of light from the waveguide region is achieved. The disk structure achieves higher light transmission efficiency, while the ring structure facilitates optical modulation. The dual-disk / ring's function is determined not only by its own parameters (radius and curvature), but also by its coupling with the waveguide, which determines its modulation efficiency and effectiveness.

[0073] 3) Semiconductor Optical Amplifier (SOA)

[0074] The optical gain amplification region achieves both optical gain amplification and spectral broadening of the transmitted light. Its structural parameters are directly related to the optical amplification capability. To facilitate integration, the width can be determined by taking into account the waveguide parameters of other sections under high-power single-mode operation. Furthermore, specific etching parameters and sidewall smoothness are required. Excessively deep waveguide etching is detrimental to stable optical amplification and increases waveguide losses during light propagation. A moderate etching depth and excellent sidewall smoothness are beneficial for achieving high gain.

[0075] 4) Second gain cavity length extender (double-disk DD2 / double-ring DR2 modulation area structure)

[0076] In the second-gain cavity extender, the dual-disk / ring modulation region shares the same structure and functionality as the dual-disk / ring region in the first-gain cavity extender. This region's electrical modulation, combined with the first-gain cavity extender, allows for the OFC chip's multifunctionality through the Vernier effect of the electrically injected dual-disk / ring, splitting ratio, gain amplification, interlocking, and self-locking. This includes output metrics such as four-wave mixing (FWM) level, wavelength position, linewidth, OFC width, and even soliton characteristics. Studying its position and structural parameters helps achieve broad spectral, high-power, and stable optical comb output characteristics.

[0077] 5) Second reverse bias absorption region SA2 structure

[0078] The second reverse-biased absorption region SA2 does not integrate a photocurrent distribution gate structure. This region, through reverse current bias and adjacent cavity surface light feedback, provides phase locking, feedback light, and a broadened OFC output, forming part of the integrated passively mode-locked semiconductor mode-locked laser. Its length determines the pulse width and intensity of the feedback light, as well as the OFC spectral width. Therefore, while ensuring high-power, single-mode operation, its structural parameters must be aligned with the output characteristics.

[0079] Electrical isolation strip structure between sections

[0080] Each segment in this embodiment's integrated multi-segment cascaded optical frequency comb chip has a distinct function, with significant differences between reverse and forward biases. To prevent adjacent segments from influencing each other's electrical properties, sufficient electrical isolation width and etching depth are required between segments. An excessively wide isolation band increases internal light losses within the band, affecting light extraction efficiency and degrading the chip's output characteristics. An excessively narrow isolation band can easily cause electrical interference between adjacent segments, impacting their functionality. An excessively deep isolation band weakens light transmission and increases internal losses, while an excessively shallow isolation band can lead to electrical crosstalk between adjacent segments, impacting device output characteristics.

[0081] OFC chip preparation process

[0082] In this embodiment, the chip with a monolithic integrated multi-segment cascaded optical frequency comb can be studied using a combination of wet and dry etching processes. The structure involves the coupling method between the grating, double disk / ring, and straight ridge waveguide. The prepared structure and morphology directly determine the optical output characteristics of the device. Dry etching can achieve high-precision control, especially the control of the coupling distance and coupling method between the grating, double disk / ring, and straight ridge waveguide, which is a key factor in determining the optical output characteristics of the OFC. Wet etching has a smooth surface morphology, which is conducive to forming a low-loss interface. The combination of dry and wet methods can achieve excellent device structure.

[0083] The segmented process can achieve the optimal preparation of the structure of each segment and avoid the impact of device performance caused by differences in process parameters between segments.

[0084] The dual disk / ring used in the gain cavity length extender in this embodiment has multiple functions. First, it functions as a cavity facet. Feedback from the cavity facet end creates a passively mode-locked semiconductor laser structure that is mode-locked with the reverse-biased region adjacent to the cavity facet end. The optical path length differences of the different components of light rotating through the dual disk / ring form the individual teeth of the optical comb. Two passively mode-locked semiconductor laser structures with dual disks / rings, through cavity facet feedback at the output end, form a rich pattern of optical teeth.

[0085] Secondly, it acts as a gain zone together with the optical amplifier (SOA). Through the intracavity oscillation between the two dual disks / rings, strong optical gain amplification is generated, providing sufficiently high light intensity and spectral broadening for the mode-locked zone. Thirdly, the external strong light pulse induces optical nonlinear effects within it. If the external light pulse intensity is sufficient, the external strong light pulse can induce nonlinear effects within the dual disk / ring itself, generating four-wave mixing and Kerr effect, which is equivalent to an active micro-disk / ring structure. Each dual disk / ring can form bidirectional feedback. The nonlinear effects of the disk / ring region are closely related to the light intensity of the external straight waveguide region and the structure within the disk / ring active waveguide. The parameters of the disk / ring affect its own optical loss.

[0086] Finally, the radius of each dual disk / ring determines the tunable bandwidth and comb tooth spacing. Comb tooth selection and position adjustment are achieved through a vernier effect. Ti / Au thin films are fabricated on the dual disk / ring and its coupled straight-ridge waveguide as external modulation electrodes, replacing the simple Al / TiN heating electrodes. This allows for control of the optical feedback of the dual disk / ring resonator, providing a certain amount of optical gain and reducing optical losses. The two dual disks / rings achieve optimal modulation of the light within the waveguide. Furthermore, the coupling method and related parameters of the dual disk / ring and the straight waveguide influence the output optical comb performance, significantly impacting the optical feedback efficiency and tunable wavelength bandwidth.

[0087] In multi-segment OFC light source chips, the impact of the high-order Kerr effect and FWMF within each segment is closely related to the structural parameters of each segment. Small structural parameters, high light intensity, and high gain characteristics undoubtedly increase the likelihood of generating optical nonlinear effects and FWMF within the device. These effects and principles can be exploited to produce meaningful device output characteristics.

[0088] In this embodiment, the waveguides of the entire optical comb chip are constructed on a uniform, epitaxially consistent platform with uninterrupted structure. The chip's structure and process do not compromise the platform's integrity. Furthermore, the two dual-disk / ring structures serve as cavity facets for each other, ensuring both tolerance and selectivity for a wide range of light sources. Consequently, this does not compromise the stability of the amplification system.

[0089] Compared to traditional passive OFC structures with only a single-ring microcavity and fiber ring structure, the multi-segment cascaded integrated active and controllable OFC light source chip structure disclosed in this invention offers the following advantages: a small system size, enabling integrated OFC chip control of both active and passive components; excellent system operational stability, eliminating the need for complex and specialized temperature control conditions; a wide applicable wavelength range, with adjustable linewidth and wavelength range to a certain extent; strong light output capability and high peak pulse power; a fabrication process that integrates semiconductor laser fabrication, making it easy to implement and suitable for mass production, reducing costs; and independent control of each segment, laying the structural foundation for achieving fundamental mode, high power, and wide-spectrum output in the integrated device. This presents significant advantages over single-ring passive microcavity OFC structures.

[0090] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. Reference can be made to the common and similar parts between the various embodiments. For the devices disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple, and the relevant parts can be referred to the method description.

[0091] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one 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 present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

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

2. The monolithic integrated multi-segment cascaded optical frequency comb according to claim 1, characterized in that: The etching depth of the photocurrent distribution gate is from one percent to one half of the thickness downward from the upper edge of the upper waveguide layer of the monolithic integrated multi-segment cascade optical frequency comb as the starting point, and the length of the photocurrent distribution gate accounts for one third to four fifths of the overall length of the first reverse bias absorption region; the photocurrent distribution gate includes a narrow spectrum light compression grating.

3. The monolithic integrated multi-segment cascaded optical frequency comb according to claim 1, characterized in that: The first gain cavity length extender and the second gain cavity length extender each include a plurality of disk-shaped cavities coupled to each other.

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

5. The monolithic integrated multi-segment cascaded optical frequency comb according to claim 1, characterized in that: The first gain cavity length extender and the second gain cavity length extender each include a plurality of mutually coupled annular cavities.

6. The monolithic 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 of 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 400-5000 μm, a ridge waveguide width of 2-30 μm, and a ridge waveguide height of 0.5-6 μm; In the first, second, third and fourth electrical isolation grooves, the groove width is 10-20 μm, the etching depth is 0.3-0.7 μm, and the length is 300-500 μm.

7. A monolithically integrated multi-segment cascaded optical frequency comb chip, characterized in that: A monolithic integrated multi-segment cascaded optical frequency comb comprising 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 monolithic integrated multi-segment cascade optical frequency comb chip includes an integrated structure formed on a semiconductor laser epitaxial wafer, and the semiconductor laser epitaxial wafer is composed of: 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 from bottom to top.

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 of a combination structure of quantum dots and quantum wells.

10. The monolithically integrated multi-segment cascaded optical frequency comb chip according to claim 8, characterized in that: The invention also includes a lower electrode prepared below the substrate and an upper electrode prepared above the high-doping layer.

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