Polarization multiplexed dual comb laser with online polarization control and mode locking

Abstract

The polarization multiplexing double-frequency comb laser with online polarization and mode locking belongs to the field of double-frequency comb lasers. The problems of large preparation difficulty of double-frequency comb optical system, instability of double-frequency comb output light source and uncontrollable power loss are solved. The passive mode locking is realized through the mode locker, and the circulating oscillation of the set direction of the resonant cavity is realized through the direction control component (polarization insensitive isolator or ring). Since the mode locker and the online polarizer are prepared by the side polished fiber, the preparation difficulty is small, the power loss is controllable, and the online polarizer is introduced to replace the traditional unstable polarization control. The online polarizer introduced makes the double-frequency comb more stable output. The present application is mainly used for generating stable double-frequency comb light.

Description

Technical Field

[0001] This invention relates to the field of dual-frequency comb lasers. Background Technology

[0002] An optical frequency comb (or simply optical frequency comb) is a spectrum consisting of a series of uniformly spaced frequency components with a coherent and stable phase relationship. Due to its extremely high measurement accuracy and resolution, the advent of optical frequency combs has led to unprecedented advancements in high-resolution spectroscopy. Compared to other precision spectroscopic techniques related to optical frequency combs, dual-frequency comb spectroscopy has been extensively studied because it can fully utilize the high resolution and high accuracy of optical frequency combs.

[0003] The core of dual-frequency comb spectral measurement technology lies in the dual-frequency comb light source. Single-cavity dual-frequency combs inherit the advantages of high output quality and stability from mode-locked laser combs, effectively avoiding the limitations of electro-optic modulated combs (limited spectral range) and Kerr combs (poor stability). Furthermore, since both combs originate from the same laser, common-mode noise is suppressed; and the resulting dual combs exhibit high coherence, eliminating the need for complex phase-locking systems. However, the traditional single-cavity dual-frequency comb faces two main challenges: firstly, the optical path system requires expensive and difficult-to-fabricate components; and secondly, the instability of the dual-frequency comb output light source and the uncontrollable power loss.

[0004] The unique structure of D-type optical fiber, combined with the properties of two-dimensional materials (2D), gives it broad application potential in various optical devices. Li W, Yi L, Zheng R, et al. Fabrication and application of agraphene polarizer with strong saturable absorption[J]. Photonics Research, 2016, 4(02): 41-44. This paper discloses a method for fabricating a polarizer and mode-locked device with an extinction ratio exceeding 24 dB by depositing graphene and gold films on D-type optical fiber. However, its loss exceeds 3 dB, and the operation of transferring the graphene film is complex and difficult to implement.

[0005] The unique structure of D-type optical fiber, combined with the properties of two-dimensional materials (2D), gives it broad application potential in various optical devices. Li W., Yi L., Zheng R., et al. Fabrication and application of agraphene polarizer with strong saturable absorption[J]. Photonics Research, 2016, 4(02): 41-44. This paper discloses a method for fabricating a polarizer and mode-locked device with an extinction ratio exceeding 24 dB by depositing graphene and gold films on D-type optical fiber. However, its loss exceeds 3 dB, and the operation of transferring the graphene film is complex and difficult to implement.

[0006] Therefore, reducing the difficulty of fabricating dual-frequency comb output light sources in dual-frequency comb optical path systems and improving the stability and reliability of dual-frequency comb light sources are technical problems that urgently need to be solved by those skilled in the art. Summary of the Invention

[0007] The purpose of this invention is to solve the problems of high fabrication difficulty in dual-frequency comb optical path systems, as well as the instability and uncontrollable power loss of the output light source within the dual-frequency comb. This invention provides a polarization-multiplexed dual-frequency comb laser with online polarization and mode-locking, and this online polarization-multiplexed dual-frequency comb laser has the following three structures, specifically:

[0008] First structure:

[0009] A polarization-multiplexed dual-frequency comb laser with online polarization and mode-locking includes a pump laser source, a polarization beam splitter, and a ring resonant cavity composed of a wavelength division multiplexer, an optical coupler, an online polarizer, a polarization-maintaining fiber, a through-type mode-locker, a polarization-insensitive isolator, and an erbium-doped fiber arranged in a counterclockwise direction; both the online polarizer and the through-type mode-locker are made of side-polished fiber.

[0010] The laser emitted from the pump laser source is amplified by the wavelength division multiplexer and then incident on the erbium-doped fiber. The resulting laser is incident on the polarization-insensitive isolator. After being isolated and reflected by the polarization-insensitive isolator, it returns to the wavelength division multiplexer along the original path. The laser emitted from the wavelength division multiplexer is split by the optical coupler. The laser output from the first output end of the optical coupler oscillates repeatedly in a counterclockwise direction within its ring resonant cavity, achieving stable mode locking.

[0011] The laser output from the second output end of the optical coupler is incident on the polarization beam splitter and outputs a dual-frequency comb consisting of s-polarized light and p-polarized light.

[0012] In the ring resonant cavity, the online polarizer and the polarization-maintaining fiber constitute a filter, and the two are used to polarize and maintain the running laser, respectively; the through-type mode lock is used to mode lock the running laser.

[0013] Preferably, the through-type mold locking device includes a first side-polished optical fiber disposed in a first quartz glass groove, and a layer of two-dimensional material is drop-coated in the polishing groove of the first side-polished optical fiber.

[0014] Preferably, the two-dimensional material is a black phosphorus dispersion, a graphene oxide dispersion, or a graphene dispersion.

[0015] The second structure:

[0016] A polarization-multiplexed dual-frequency comb laser with online polarization and mode-locking includes a pump laser source, a polarization beam splitter, and a resonant cavity composed of a linear cavity and a ring cavity. The linear cavity consists of a circulator and a reflective mode-locking device arranged from right to left. The ring cavity consists of a wavelength division multiplexer, an optical coupler, an online polarizer, a polarization-maintaining fiber, another circulator, and an erbium-doped fiber arranged counterclockwise. The circulator is shared by both the linear cavity and the ring cavity. Both the online polarizer and the reflective mode-locking device are made of side-polished fiber.

[0017] The laser emitted from the pump laser source is amplified by an erbium-doped fiber after being incident on a wavelength division multiplexer. After being isolated by port 3 of the circulator, it returns to the wavelength division multiplexer along the original path. After passing through the wavelength division multiplexer, it is incident on an optical coupler for beam splitting. The laser output from the first output end of the optical coupler passes through an online polarizer and a polarization-maintaining fiber in sequence and is incident on port 1 of the circulator. After being output from port 2 of the circulator, it is incident on a reflective mode lock. After being mode-locked and reflected by the reflective mode lock, it is output from port 3 of the circulator and oscillates repeatedly along the transmission link in the resonant cavity to achieve stable mode locking.

[0018] The laser output from the second output end of the optical coupler is incident on the polarization beam splitter and outputs a dual-frequency comb consisting of s-polarized light and p-polarized light.

[0019] In the ring resonant cavity, the online polarizer and the polarization-maintaining fiber constitute a filter, and the two are used to polarize and maintain the polarization of the running laser, respectively.

[0020] The third structure:

[0021] A polarization-multiplexed dual-frequency comb laser with online polarization and mode-locking includes a pump laser source, a polarization beam splitter, and a resonant cavity composed of a linear cavity and a ring cavity. The linear cavity consists of a reflective mode-locking device, a wavelength division multiplexer, an erbium-doped fiber, an optical coupler, and a circulator arranged from right to left. The ring cavity consists of a circulator, a polarization-maintaining fiber, and an online polarizer arranged counterclockwise. The circulator is shared by both the linear cavity and the ring cavity. Both the online polarizer and the reflective mode-locking device are fabricated from side-polished fiber.

[0022] The laser emitted from the pump laser source is amplified by the wavelength division multiplexer and then incident on the erbium-doped fiber. The resulting laser beam is split by the optical coupler. The laser output from the first output end of the optical coupler is incident on port 2 of the circulator, and then output from port 3. It is then incident sequentially on the linear polarizer, the polarization-maintaining fiber, and port 1 of the circulator. After being output from port 2 of the circulator, it returns to the wavelength division multiplexer along the original path. After being incident on the reflective mode locker through the wavelength division multiplexer, it is reflected and then oscillates repeatedly in the resonant cavity to achieve stable mode locking.

[0023] The laser output from the second output end of the optical coupler is incident on the polarization beam splitter and outputs a dual-frequency comb consisting of s-polarized light and p-polarized light. In the ring resonant cavity, the online polarizer and the polarization-maintaining fiber constitute a filter, and the two are used to polarize and maintain the polarization of the running laser, respectively.

[0024] Preferably, in the second and third structures, the reflective mode lock includes a second side-polished optical fiber disposed in a second quartz glass trough, and a nanoscale metal film is deposited on one end face of the second quartz glass trough, which is perpendicular to the propagation direction of the second side-polished optical fiber, and a layer of two-dimensional material is drop-coated in the polishing groove of the second side-polished optical fiber.

[0025] Preferably, in the first to third structures, the structure of the online polarizer is as follows:

[0026] The online polarizer includes a third side-polished optical fiber disposed in a third quartz glass tank. A layer of two-dimensional material is first drop-coated in the polishing tank of the third side-polished optical fiber, and then a nanoscale metal film is deposited by ion sputtering.

[0027] Preferably, the thickness of the nanoscale metal film is 40 to 80 nanometers.

[0028] Preferably, in the first to third structures, the slow axis of the polarization-maintaining fiber is fused with the core of the third side-polished fiber in the online polarizer, and the angle between the slow axis of the polarization-maintaining fiber and the extension direction of the polishing groove of the third side-polished fiber is 45°.

[0029] Preferably, in the first to third structures, the ratio of the light intensity of the laser output from the first output end and the second output end of the optical coupler is 9:1.

[0030] Advantages of this invention:

[0031] The polarization-multiplexed dual-frequency comb laser of this invention, characterized by online polarization and mode-locking, achieves passive mode-locking through a mode-locker and cyclic oscillation of the resonant cavity in a set direction through a direction control component (polarization-insensitive isolator or circulator), thereby obtaining dual-frequency comb output from the same resonant cavity. Since both the mode-locker and the online polarizer are fabricated from side-thrown fiber, the fabrication of this dual-frequency comb laser is simple, and the power loss is controllable. Furthermore, this invention improves upon the optical path structure of traditional dual-frequency comb optical systems by introducing an online polarizer, replacing the traditional unstable polarization control, resulting in a more stable dual-frequency comb output.

[0032] This invention uses side-projected optical fiber to fabricate an online polarizer and mode locker. The composite structure of two-dimensional material dispersion and nano-metal film achieves an extremely low loss of 0.2dB while ensuring polarization and mode locker effects, further improving the stability of the dual-frequency comb laser.

[0033] This invention employs a common cavity design for polarized beams, which features common-mode noise suppression, significantly improving the coherence and stability of the light source, thereby effectively reducing the complexity and overall cost of dual-frequency comb lasers. Attached Figure Description

[0034] Figure 1 This is a frame diagram of the polarization multiplexing dual-frequency comb laser with online polarization and mode-locking as described in this invention;

[0035] Figure 2 This is a schematic diagram of the specific structure of the polarization multiplexing dual-frequency comb laser with online polarization and mode-locking as described in Specific Implementation Method 1;

[0036] Figure 3 This is a schematic diagram of the specific structure of the polarization multiplexing dual-frequency comb laser with online polarization and mode-locking as described in Specific Implementation Method 2;

[0037] Figure 4 This is a schematic diagram of the specific structure of the polarization multiplexing dual-frequency comb laser with online polarization and mode-locking as described in Specific Implementation Method 3;

[0038] Figure 5 This is a schematic diagram of the straight-through mold clamp 7;

[0039] Figure 6 This is a schematic diagram of the reflective mold clamp 10;

[0040] Figure 7 This is a schematic diagram of the online polarizer 5;

[0041] Figure 8This is a main sectional view of the third side-sprayed optical fiber 5-2; wherein, the attached figure label 5-2-1 is the core of the third side-sprayed optical fiber 5-2. Detailed Implementation

[0042] 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.

[0043] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0044] See Figure 1 Under the same inventive concept, this invention provides three different structures for online polarization-multiplexed dual-frequency comb lasers with polarization-locking and mode-locking capabilities. The first structure mainly consists of a through-type mode-locker and a polarization-insensitive isolator. The second and third structures mainly consist of a reflective mode-locker and a circulator. The first to third structures are described in detail in specific embodiments one to three, as follows:

[0045] Specific Implementation Method 1: Combination Figure 2 This embodiment describes a first structure of a polarization-multiplexed dual-frequency comb laser with online polarization and mode-locking, comprising a pump laser source 1, a polarization beam splitter 9, and a ring resonant cavity composed of a wavelength division multiplexer 2, an optical coupler 4, an online polarizer 5, a polarization-maintaining fiber 6, a through-type mode-locker 7, a polarization-insensitive isolator 8, and an erbium-doped fiber 3 arranged sequentially in a counterclockwise direction; both the online polarizer 5 and the through-type mode-locker 7 are made of side-polished fiber.

[0046] The laser emitted from the pump laser source 1 is amplified by the erbium-doped fiber 3 after passing through the wavelength division multiplexer 2. The resulting laser is then incident on the polarization-insensitive isolator 8. After being isolated and reflected by the polarization-insensitive isolator 8, the laser returns to the wavelength division multiplexer 2 along the original path. The laser emitted from the wavelength division multiplexer 2 is split by the optical coupler 4. The laser output from the first output end of the optical coupler 4 oscillates repeatedly in a counterclockwise direction within its ring resonant cavity, achieving stable mode locking.

[0047] The laser output from the second output end of the optical coupler 4 is incident on the polarization beam splitter 9 and outputs a dual-frequency comb consisting of s-polarized light and p-polarized light.

[0048] In the ring resonant cavity, the online polarizer 5 and the polarization-maintaining fiber 6 constitute a filter, and the two are used to polarize and maintain the running laser, respectively; the through-type mode lock 7 is used to mode lock the running laser.

[0049] In this preferred embodiment, a filter consisting of an online polarizer 5 and a polarization-maintaining fiber 6 is introduced into the ring resonant cavity to adjust the polarization state so that the laser output from the polarization-maintaining fiber 6 is orthogonally polarized light; a direct-through mode-locker 7 performs passive mode-locking; an erbium-doped fiber 3 serves as a reverse pump gain medium to pump and amplify the laser emitted from the pump laser source 1; a polarization-insensitive isolator 8 serves as a direction control component to control the laser transmission direction; and an all-fiber mode-locked laser is constructed under the action of the direct-through mode-locker 7, based on the cavity length, the dispersion within the cavity, and the length of the erbium-doped fiber 3.

[0050] This invention achieves passive mode-locking through a through-type mode-locking device 7 and cyclic oscillation of the resonant cavity in a set direction through a direction control component (polarization-insensitive isolator or circulator), thereby obtaining dual-frequency comb output from the same resonant cavity. Since both the mode-locking device and the online polarizer are made of side-thrown fiber, the fabrication of the dual-frequency comb laser of this invention is less difficult, and the power loss is controllable. Furthermore, this invention improves the optical path structure of traditional dual-frequency comb optical systems by introducing an online polarizer, replacing the traditional unstable polarization control. The introduced online polarizer makes the dual-frequency comb output more stable.

[0051] In practical applications, the ratio of the light intensity of the laser output from the first output terminal and the second output terminal of the optical coupler 4 is 9:1.

[0052] See Figure 5 The through-type mold clamping device 7 includes a first side-polished optical fiber 7-2 disposed within a first quartz glass tank 7-1, and a layer of two-dimensional material 12 is drop-coated into the polishing tank of the first side-polished optical fiber 7-2. As an example, the two-dimensional material 12 is a black phosphorus dispersion, a graphene oxide dispersion, or a graphene dispersion.

[0053] In practical applications, the first side-polished fiber 7-2 is implemented using a D-type fiber with a polishing depth of 4μm-6μm from the fiber core center.

[0054] In this preferred embodiment, a straight-through mode-locking device 7 is prepared by side-polishing optical fiber, and a layer of two-dimensional material is drop-coated in the polishing tank to make its surface smoother, thereby achieving low-loss mode-locking and making its power loss controllable.

[0055] See Figure 7 and Figure 8 The structure of the online polarizer 5 is as follows:

[0056] The online polarizer 5 includes a third side-polished optical fiber 5-2 disposed in a third quartz glass tank 5-1. A layer of two-dimensional material 12 is first drop-coated in the polishing tank of the third side-polished optical fiber 5-2, and then a nanoscale metal film 13 is deposited by ion sputtering.

[0057] In this preferred embodiment, two layers of coatings of different materials are drop-coated into the polishing groove of the online polarizer 5. The bottom layer is a two-dimensional material 12 to make the surface of the polishing groove smoother, and the top layer is a nanoscale metal film 13, with an optimal thickness of 40 to 80 nanometers. The metal film is made of materials including, but not limited to, gold, copper, and aluminum. The metal film covers the surface of the polishing fiber, and through the interaction with the evanescent field leaked from the polishing fiber, a surface plasmon resonance is formed. This mode can absorb TM polarization, while polarization states perpendicular to it can pass through, thereby achieving the polarization effect. The specific process for preparing the second structure of the online polarizer 5 is as follows:

[0058] A D-shaped optical fiber with a core center distance of 4 μm, based on single-mode fiber, is ground and polished. This fiber is then fixed in a quartz glass trough using high-temperature resistant optical UV adhesive. A certain amount of two-dimensional material dispersion is then drop-coated onto the surface of the fixed D-shaped fiber. After multiple drop-coating and drying processes, a nano-metal film is deposited. Specifically, the reflective metal film includes, but is not limited to, gold, copper, and aluminum.

[0059] Furthermore, the slow axis of the polarization-maintaining fiber 6 is fused with the core of the third side-polished fiber 5-2 within the online polarizer 5, and the angle between the slow axis of the polarization-maintaining fiber 6 and the extension direction of the polishing groove of the third side-polished fiber 5-2 is 45°. This 45° fusion splicing method serves a filtering function, and the interaction between the two is not easily altered, resulting in high stability and achieving stable output signal from the dual-frequency comb.

[0060] Specific Implementation Method Two: Combination Figure 3 This embodiment describes a second structure of a polarization-multiplexed dual-frequency comb laser with online polarization and mode-locking, comprising a pump laser source 1, a polarization beam splitter 9, and a resonant cavity composed of a linear cavity and a ring cavity. The linear cavity is composed of a circulator 11 and a reflective mode-locking device 10 arranged sequentially from right to left. The ring cavity is composed of a wavelength division multiplexer 2, an optical coupler 4, an online polarizer 5, a polarization-maintaining fiber 6, the circulator 11, and an erbium-doped fiber 3 arranged sequentially in a counter-clockwise direction. The circulator 11 is shared by the linear cavity and the ring cavity. Both the online polarizer 5 and the reflective mode-locking device 10 are made of side-polished fiber.

[0061] The laser emitted from the pump laser source 1 is amplified by the erbium-doped fiber 3 after being incident on the wavelength division multiplexer 2. After being isolated by port 3 of the circulator 11, it returns to the wavelength division multiplexer 2 along the original path. After being incident on the optical coupler 4 through the wavelength division multiplexer 2, the laser beam is split. The laser output from the first output end of the optical coupler 4 passes through the online polarizer 5 and the polarization-maintaining fiber 6 in sequence and is incident on port 1 of the circulator 11. After being output from port 2 of the circulator 11, it is incident on the reflective mode lock 10. After being mode-locked and reflected by the reflective mode lock 10, the laser output from port 3 of the circulator 11 oscillates repeatedly along the transmission link in the resonant cavity to achieve stable mode locking.

[0062] The laser output from the second output end of the optical coupler 4 is incident on the polarization beam splitter 9 and outputs a dual-frequency comb consisting of s-polarized light and p-polarized light.

[0063] In the ring resonant cavity, the online polarizer 5 and the polarization-maintaining fiber 6 constitute a filter, and the two are used to polarize and maintain the polarization of the running laser, respectively.

[0064] In this embodiment, passive mode-locking is achieved through a reflective mode-locking device 10, and cyclic oscillation of the resonant cavity in a set direction is achieved through a circulator, a direction control component, thereby obtaining the output of a dual-frequency comb from the same resonant cavity. Since both the mode-locking device and the online polarizer are made of side-thrown fiber, the fabrication of the dual-frequency comb laser of this invention is less difficult, and the power loss is controllable. Furthermore, this invention improves the optical path structure of a traditional dual-frequency comb optical system by introducing an online polarizer, replacing the traditional unstable polarization control. The introduced online polarizer makes the dual-frequency comb output more stable.

[0065] Specifically, the ratio of the light intensity of the laser output from the first output terminal and the second output terminal of the optical coupler 4 is 9:1.

[0066] See Figure 6 The reflective mode-locking device 10 includes a second side-polished optical fiber 10-2 disposed within a second quartz glass tank 10-1, and a nanoscale metal film 13 deposited on one end face of the second quartz glass tank 10-1, the end face being perpendicular to the propagation direction of the second side-polished optical fiber 10-2. A layer of two-dimensional material 12 is drop-coated into the polishing groove of the second side-polished optical fiber 10-2. Specifically, the two-dimensional material 12 is a black phosphorus dispersion, a graphene oxide dispersion, or a graphene dispersion. The optimal thickness of the nanoscale metal film 13 is 40 to 80 nanometers.

[0067] In this preferred embodiment, the reflective mode lock 10 is coated with a nanoscale metal film 13 on one end face of the second quartz glass groove 10-1. This end face is perpendicular to the propagation direction of the second side-sprayed optical fiber 10-2, which makes its optical path shorter and the alignment requirements lower, reducing losses and making adjustment more convenient.

[0068] See Figure 7 and Figure 8 The structure of the online polarizer 5 is as follows:

[0069] The online polarizer 5 includes a third side-polished optical fiber 5-2 disposed in a third quartz glass tank 5-1. A layer of two-dimensional material 12 is first drop-coated in the polishing tank of the third side-polished optical fiber 5-2, and then a nanoscale metal film 13 is deposited by ion sputtering.

[0070] In this preferred embodiment, two layers of coatings of different materials are drop-coated into the polishing groove of the online polarizer 5. The bottom layer is a two-dimensional material 12 to make the surface of the polishing groove smoother, and the top layer is a nanoscale metal film 13, with an optimal thickness of 40 to 80 nanometers. The metal film is made of materials including, but not limited to, gold, copper, and aluminum. The metal film covers the surface of the polishing fiber, and through the interaction with the evanescent field leaked from the polishing fiber, a surface plasmon resonance is formed. This mode can absorb TM polarization, while polarization states perpendicular to it can pass through, thereby achieving the effect of polarization.

[0071] Furthermore, the slow axis of the polarization-maintaining fiber 6 is fused with the core of the third side-polished fiber 5-2 inside the online polarizer 5, and the angle between the slow axis of the polarization-maintaining fiber 6 and the extension direction of the polishing groove of the third side-polished fiber 5-2 is 45°.

[0072] The fusion splicing method at a 45° angle serves as a filter, and the interaction between the two is not easily changed, resulting in high stability and achieving stability of the output signal of the dual-frequency comb.

[0073] Detailed Implementation Method Three: Participation Figure 4 This embodiment describes a third structure of a polarization-multiplexed dual-frequency comb laser with online polarization and mode-locking. The structure includes a pump laser source 1, a polarization beam splitter 9, and a resonant cavity composed of a linear cavity and a ring cavity. The linear cavity is composed of a reflective mode-locking device 10, a wavelength division multiplexer 2, an erbium-doped fiber 3, an optical coupler 4, and a circulator 11, arranged sequentially from right to left. The ring cavity is composed of a circulator 11, a polarization-maintaining fiber 6, and an online polarizer 5, arranged sequentially in a counter-clockwise direction. The circulator 11 is shared by both the linear cavity and the ring cavity. Both the online polarizer 5 and the reflective mode-locking device 10 are made of side-polished fiber.

[0074] The laser emitted from the pump laser source 1 is amplified by the wavelength division multiplexer 2 and then amplified by the erbium-doped fiber 3. The resulting laser beam is split by the optical coupler 4. The laser output from the first output end of the optical coupler 4 is incident on port 2 of the circulator 11 and output from port 3. It is then sequentially incident on the linear polarizer 5, the polarization-maintaining fiber 6, and port 1 of the circulator 11. After being output from port 2 of the circulator 11, it returns to the wavelength division multiplexer 2 along the original path. It is then incident on the reflective mode lock 10 for mode locking and reflection. After reflection, it oscillates repeatedly in the resonant cavity where it is located to achieve stable mode locking.

[0075] The laser output from the second output end of the optical coupler 4 is incident on the polarization beam splitter 9 and outputs a dual-frequency comb consisting of s-polarized light and p-polarized light. In the ring resonant cavity, the online polarizer 5 and the polarization-maintaining fiber 6 constitute a filter, and the two are used to polarize and maintain the polarization of the running laser, respectively.

[0076] In this embodiment, passive mode-locking is achieved through a reflective mode-locking device 10, and cyclic oscillation of the resonant cavity in a set direction is achieved through a circulator, a direction control component, thereby obtaining the output of a dual-frequency comb from the same resonant cavity. Since both the mode-locking device and the online polarizer are made of side-thrown fiber, the fabrication of the dual-frequency comb laser of this invention is less difficult, and the power loss is controllable. Furthermore, this invention improves the optical path structure of a traditional dual-frequency comb optical system by introducing an online polarizer, replacing the traditional unstable polarization control. The introduced online polarizer makes the dual-frequency comb output more stable.

[0077] Specifically, the ratio of the light intensity of the laser output from the first output terminal and the second output terminal of the optical coupler 4 is 9:1.

[0078] See Figure 6 The reflective mode-locking device 10 includes a second side-polished optical fiber 10-2 disposed within a second quartz glass tank 10-1, and a nanoscale metal film 13 deposited on one end face of the second quartz glass tank 10-1, the end face being perpendicular to the propagation direction of the second side-polished optical fiber 10-2. A layer of two-dimensional material 12 is drop-coated into the polishing groove of the second side-polished optical fiber 10-2. Specifically, the two-dimensional material 12 is a black phosphorus dispersion, a graphene oxide dispersion, or a graphene dispersion. The optimal thickness of the nanoscale metal film 13 is 40 to 80 nanometers.

[0079] In this preferred embodiment, the reflective mode lock 10 is coated with a nanoscale metal film 13 on one end face of the second quartz glass groove 10-1. This end face is perpendicular to the propagation direction of the second side-sprayed optical fiber 10-2, which makes its optical path shorter and the alignment requirements lower, reducing losses and making adjustment more convenient.

[0080] See Figure 7 and Figure 8 The structure of the online polarizer 5 is as follows:

[0081] The online polarizer 5 includes a third side-polished optical fiber 5-2 disposed in a third quartz glass tank 5-1, and a two-dimensional material 12 is first deposited in the polishing tank of the third side-polished optical fiber 5-2, and then a nanoscale metal film 13 is deposited.

[0082] In this preferred embodiment, two layers of coatings of different materials are drop-coated into the polishing groove of the online polarizer 5. The bottom layer is a two-dimensional material 12 to make the surface of the polishing groove smoother, and the top layer is a nanoscale metal film 13, with an optimal thickness of 40 to 80 nanometers. The metal film is made of materials including, but not limited to, gold, copper, and aluminum. The metal film covers the surface of the polishing fiber, and through the interaction with the evanescent field leaked from the polishing fiber, a surface plasmon resonance is formed. This mode can absorb TM polarization, while polarization states perpendicular to it can pass through, thereby achieving the effect of polarization.

[0083] Furthermore, the slow axis of the polarization-maintaining fiber 6 is fused with the core of the third side-polished fiber 5-2 within the online polarizer 5, and the angle between the slow axis of the polarization-maintaining fiber 6 and the extension direction of the polishing groove of the third side-polished fiber 5-2 is 45°. This 45° fusion splicing method serves a filtering function, and the interaction between the two is not easily altered, resulting in high stability and achieving stable output signal from the dual-frequency comb.

[0084] Principle Analysis: This invention discloses three structures of polarization-multiplexed dual-frequency comb lasers with online polarization and mode-locking. All three structures utilize online polarization and mode-locking devices fabricated from low-loss side-projected fiber. A pump laser source is then injected into the resonant cavity, and the resonant direction is controlled by a direction control component. Passive mode-locking is achieved according to the corresponding structural mode-locking devices. The combination of the online polarizer fabricated from low-loss side-projected fiber and the polarization-maintaining fiber introduces a filtering effect, enabling stable dual-frequency comb propagation in one direction when the nonlinear interaction between orthogonal polarization modes and the group velocity mismatch caused by birefringence cannot be balanced.

[0085] While the invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the invention. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that different dependent claims and features described herein can be combined in ways different from those described in the original claims. It is also understood that features described in conjunction with individual embodiments can be used in other described embodiments.

Claims

1. A polarization-multiplexed dual-frequency comb laser with online polarization and mode-locking, characterized in that, The device includes a pump laser source (1), a polarization beam splitter (9), and a ring resonant cavity consisting of a wavelength division multiplexer (2), an optical coupler (4), an online polarizer (5), a polarization-maintaining fiber (6), a through-type mode lock (7), a polarization-insensitive isolator (8), and an erbium-doped fiber (3) arranged in a counterclockwise direction; the online polarizer (5) and the through-type mode lock (7) are both made of side-polished fiber; The laser emitted from the pump laser source (1) is amplified by the erbium-doped fiber (3) after being incident on the wavelength division multiplexer (2). The laser then enters the polarization-insensitive isolator (8), and after being isolated and reflected by the polarization-insensitive isolator (8), it returns to the wavelength division multiplexer (2) along the original path. The laser emitted from the wavelength division multiplexer (2) is split by the optical coupler (4). The laser output from the first output end of the optical coupler (4) oscillates repeatedly in the counterclockwise direction in the ring resonant cavity where it is located, thereby achieving stable mode locking. The laser output from the second output end of the optical coupler (4) is incident on the polarization beam splitter (9) and outputs a dual-frequency comb consisting of s-polarized light and p-polarized light. In the ring resonant cavity, the online polarizer (5) and the polarization-maintaining fiber (6) constitute a filter, and the two are used to polarize and maintain the running laser, respectively; the through-type mode lock (7) is used to mode lock the running laser. The through-type mold locking device (7) includes a first side-polished optical fiber (7-2) disposed in a first quartz glass tank (7-1), and a layer of two-dimensional material (12) is drop-coated in the polishing tank of the first side-polished optical fiber (7-2). The structure of the online polarizer (5) is as follows: The online polarizer (5) includes a third side-polished optical fiber (5-2) disposed in a third quartz glass tank (5-1), and a layer of two-dimensional material (12) is first drop-coated in the polishing tank of the third side-polished optical fiber (5-2), and then a nanoscale metal film (13) is deposited by ion sputtering.

2. The polarization-multiplexed dual-frequency comb laser with online polarization and mode-locking according to claim 1, characterized in that, The two-dimensional material (12) is a black phosphorus dispersion, a graphene oxide dispersion, or a graphene dispersion.

3. A polarization-multiplexed dual-frequency comb laser with online polarization and mode-locking, characterized in that, It includes a pump laser source (1), a polarization beam splitter (9), and a resonant cavity composed of a linear cavity and a ring cavity; wherein, the linear cavity is composed of a circulator (11) and a reflective mode lock (10) arranged from right to left, and the ring cavity is composed of a wavelength division multiplexer (2), an optical coupler (4), an online polarizer (5), a polarization-maintaining fiber (6), a circulator (11), and an erbium-doped fiber (3) arranged in a counterclockwise direction; the circulator (11) is shared by the linear cavity and the ring cavity; the online polarizer (5) and the reflective mode lock (10) are both made of side-polished fiber; The laser emitted from the pump laser source (1) is amplified by the erbium-doped fiber (3) after being incident on the wavelength division multiplexer (2). After being isolated by port 3 of the circulator (11), it returns to the wavelength division multiplexer (2) along the original path. After being incident on the optical coupler (4) through the wavelength division multiplexer (2), the laser is split. The laser output from the first output end of the optical coupler (4) passes through the online polarizer (5) and the polarization-maintaining fiber (6) in sequence and is incident on port 1 of the circulator (11). After being output from port 2 of the circulator (11), it is incident on the reflective mode lock (10). After being mode-locked and reflected by the reflective mode lock (10), it is output from port 3 of the circulator (11) and oscillates repeatedly along the transmission link in the resonant cavity to achieve stable mode-locking. The laser output from the second output end of the optical coupler (4) is incident on the polarization beam splitter (9) and outputs a dual-frequency comb consisting of s-polarized light and p-polarized light. In the ring cavity, the online polarizer (5) and the polarization-maintaining fiber (6) constitute a filter, and the two are used to polarize and maintain the polarization of the running laser, respectively. The reflective mode lock device (10) includes a second side-polished optical fiber (10-2) disposed in a second quartz glass groove (10-1), and a nanoscale metal film (13) is deposited on one end face of the second quartz glass groove (10-1), which is perpendicular to the propagation direction of the second side-polished optical fiber (10-2). A layer of two-dimensional material (12) is drop-coated in the polishing groove of the second side-polished optical fiber (10-2). The structure of the online polarizer (5) is as follows: The online polarizer (5) includes a third side-polished optical fiber (5-2) disposed in a third quartz glass tank (5-1), and a layer of two-dimensional material (12) is first drop-coated in the polishing tank of the third side-polished optical fiber (5-2), and then a nanoscale metal film (13) is deposited by ion sputtering.

4. A polarization-multiplexed dual-frequency comb laser with online polarization and mode-locking, characterized in that, The system includes a pump laser source (1), a polarization beam splitter (9), and a resonant cavity composed of a linear cavity and a ring cavity. The linear cavity is composed of a reflective mode-locked device (10), a wavelength division multiplexer (2), an erbium-doped fiber (3), an optical coupler (4), and a circulator (11) arranged from right to left. The ring cavity is composed of a circulator (11), a polarization-maintaining fiber (6), and an online polarizer (5) arranged in a counterclockwise direction. The circulator (11) is shared by the linear cavity and the ring cavity. The online polarizer (5) and the reflective mode-locked device (10) are both made of side-polished fiber. The laser emitted from the pump laser source (1) is amplified by the wavelength division multiplexer (2) and then amplified by the erbium-doped fiber (3). The laser beam is split by the optical coupler (4). The laser output from the first output end of the optical coupler (4) is incident on port 2 of the circulator (11), and after being output from port 3, it is sequentially incident on the line polarizer (5), the polarization-maintaining fiber (6), and port 1 of the circulator (11). After being output from port 2 of the circulator (11), it returns to the wavelength division multiplexer (2) along the original path. After being incident on the reflective mode lock device (10) through the wavelength division multiplexer (2) for mode locking and reflection, it oscillates repeatedly in the resonant cavity where it is located to achieve stable mode locking. The laser output from the second output end of the optical coupler (4) is incident on the polarization beam splitter (9) and outputs a dual-frequency comb consisting of s-polarized light and p-polarized light. In the ring cavity, the online polarizer (5) and the polarization-maintaining fiber (6) constitute a filter, and the two are used to polarize and maintain the polarization of the running laser, respectively. The reflective mode lock device (10) includes a second side-polished optical fiber (10-2) disposed in a second quartz glass groove (10-1), and a nanoscale metal film (13) is deposited on one end face of the second quartz glass groove (10-1), which is perpendicular to the propagation direction of the second side-polished optical fiber (10-2). A layer of two-dimensional material (12) is drop-coated in the polishing groove of the second side-polished optical fiber (10-2). The structure of the online polarizer (5) is as follows: The online polarizer (5) includes a third side-polished optical fiber (5-2) disposed in a third quartz glass tank (5-1), and a layer of two-dimensional material (12) is first drop-coated in the polishing tank of the third side-polished optical fiber (5-2), and then a nanoscale metal film (13) is deposited by ion sputtering.

5. The polarization-multiplexed dual-frequency comb laser with online polarization and mode-locking according to claim 1, 3, or 4, characterized in that, The thickness of the nanoscale metal film (13) is 40 to 80 nanometers.

6. The polarization-multiplexed dual-frequency comb laser with online polarization and mode-locking according to claim 1, 3, or 4, characterized in that, The slow axis of the polarization-maintaining fiber (6) is fused with the core of the third side-polished fiber (5-2) inside the online polarizer (5), and the angle between the slow axis of the polarization-maintaining fiber (6) and the extension direction of the polishing groove of the third side-polished fiber (5-2) is 45°.

7. The polarization-multiplexed dual-frequency comb laser with online polarization and mode-locking according to claim 1, 3, or 4, characterized in that, The ratio of the light intensity of the laser output from the first output end and the second output end of the optical coupler (4) is 9:1.

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