Polarization multiplexing double-frequency comb laser for online polarization and mode locking

Abstract

The invention discloses a polarization multiplexing double-frequency comb laser for online polarization and mode locking, and belongs to the field of double-frequency comb laser. The problems that a double-frequency comb light path system is high in preparation difficulty, and a double-frequency comb output light source in the double-frequency comb light path system is unstable and uncontrollable in power loss are solved. Passive mode locking is achieved through the mode locker, circulating oscillation in the set direction of the resonant cavity is achieved through the direction control component (the polarization insensitive isolator or the circulator), the mode locker and the on-line polarizer are both prepared from side-throwing optical fibers, the preparation difficulty is small, power loss is controllable, the on-line polarizer is introduced, and the power consumption is low. The traditional unstable polarization control is replaced, and the introduced online polarizer enables the output of the double-frequency comb to be more stable. The invention is mainly used for generating stable double-frequency comb light.

Description

Technical Field

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

[0002] An optical frequency comb (OFC) is a spectrum consisting of a series of frequency components that are evenly spaced and have a coherent and stable phase relationship. Due to the extremely high measurement accuracy and resolution of the OFC, the emergence of the OFC has enabled unprecedented development of high-resolution spectroscopy. Compared with other precision spectroscopy techniques related to the OFC, the dual-frequency comb spectroscopy measurement technique has been widely studied because it can fully utilize the high resolution and high precision of the OFC.

[0003] The core of dual-frequency comb spectroscopy measurement technology lies in the dual-frequency comb light source. The single-cavity dual-frequency comb inherits the advantages of good output quality and high stability of the mode-locked laser light comb, and can effectively avoid the defects of the small spectral operating range of the electro-optical modulated light comb and the poor stability of the Kerr light comb. At the same time, because the two light combs come from the same laser, the common-mode noise can be suppressed; and the generated dual light combs have high coherence, without the need for a complex phase-locking system. However, the production of traditional single-cavity dual-frequency combs faces two major problems: one is that the components required for the optical path system are expensive and difficult to prepare; the other is the instability of the dual-frequency comb output light source and the uncontrollability of power loss.

[0004] The unique structure of D-type optical fiber combined with the characteristics of two-dimensional materials (2D) makes it show a wide range of 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. A polarizer and a mode-locking device with an extinction ratio exceeding 24 dB are prepared by coating a graphene film and a gold film on a D-type optical fiber. However, the loss exceeds 3 dB, and the operation is complicated and difficult to achieve when transferring the graphene film.

[0005] The unique structure of D-type optical fiber combined with the characteristics of two-dimensional materials (2D) makes it show a wide range of 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. A polarizer and a mode-locking device with an extinction ratio exceeding 24 dB are prepared by coating a graphene film and a gold film on a D-type optical fiber. However, the loss exceeds 3 dB, and the operation of transferring the graphene film is complicated and difficult to achieve.

[0006] Therefore, reducing the difficulty of preparing a dual-frequency comb output light source in a dual-frequency comb optical path system and improving the stability and reliability of the dual-frequency comb light source are technical problems that need to be urgently solved by technical personnel in this field. Summary of the invention

[0007] The purpose of the present invention is to solve the problem that the dual-frequency comb optical path system is difficult to prepare and the instability and power loss of the dual-frequency comb output light source therein are uncontrollable. The present invention provides an online polarization multiplexing dual-frequency comb laser with mode locking, and the online polarization multiplexing dual-frequency comb laser with mode locking has the following three structures, specifically:

[0008] The first structure:

[0009] A polarization multiplexing dual-frequency comb laser with online polarization and mode locking, comprising a pump laser source, a polarization beam splitter, and a ring resonator composed of a wavelength division multiplexer, an optical coupler, an online polarizer, a polarization-maintaining optical fiber, a direct mode locker, a polarization-insensitive isolator and an erbium-doped optical fiber arranged in sequence in a counterclockwise direction; the online polarizer and the direct mode locker are both made of side-thrown optical fiber;

[0010] The laser emitted by the pump laser source is incident on the erbium-doped optical fiber through the wavelength division multiplexer for amplification, and then the generated 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. After the laser emitted by the wavelength division multiplexer is split by the optical coupler, the laser output from the first output end of the optical coupler oscillates back and forth in the counterclockwise direction for multiple times in the ring resonant cavity where it is located, so as to achieve stable mode locking;

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

[0012] In the ring resonator, an online polarizer and a polarization-maintaining fiber form a filter, and the two are used for polarizing and maintaining the polarization of the running laser respectively; a straight-through mode locker is used for mode locking the running laser.

[0013] Preferably, the straight-through mode locker includes a first side-polished optical fiber arranged 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 multiplexing dual-frequency comb laser with online polarization and mode locking comprises a pump laser source, a polarization beam splitter, and a resonant cavity composed of a linear cavity and a ring cavity; wherein the linear cavity is composed of a circulator and a reflective mode locker arranged in sequence from right to left, and the ring cavity is composed of a wavelength division multiplexer, an optical coupler, an online polarizer, a polarization-maintaining optical fiber, a circulator, and an erbium-doped optical fiber arranged in sequence in a counterclockwise direction; the circulator is shared by the linear cavity and the ring cavity; the online polarizer and the reflective mode locker are both made of side-thrown optical fiber;

[0017] The laser emitted by the pump laser source is incident on the erbium-doped fiber through the wavelength division multiplexer to generate laser after amplification. After being isolated by the No. 3 port of the circulator, it returns to the wavelength division multiplexer along the original route, and then is incident on the optical coupler through the wavelength division multiplexer for beam splitting. The laser output from the first output end of the optical coupler passes through the online polarizer and the polarization-maintaining fiber in turn and is incident on the No. 1 port of the circulator. It is output from the No. 2 port of the circulator and then incident on the reflective mode locker. After being mode-locked and reflected by the reflective mode locker, it is output from the No. 3 port of the circulator and oscillates back and forth for multiple times along the transmission link in the resonant cavity to achieve stable mode locking.

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

[0019] In the ring resonator, an online polarizer and a polarization-maintaining fiber form a filter, and the two are used for polarizing and maintaining the polarization of the running laser, respectively.

[0020] The third structure:

[0021] A polarization multiplexing dual-frequency comb laser with online polarization and mode locking comprises a pump laser source, a polarization beam splitter, and a resonant cavity composed of a linear cavity and a ring cavity, wherein the linear cavity is composed of a reflective mode locker, a wavelength division multiplexer, an erbium-doped fiber, an optical coupler, and a circulator arranged in sequence from right to left, and the ring cavity is composed of a circulator, a polarization-maintaining fiber, and an online polarizer arranged in sequence in a counterclockwise direction; the circulator is shared by the linear cavity and the ring cavity; the online polarizer and the reflective mode locker are both made of side-thrown optical fiber;

[0022] The laser emitted by the pump laser source is incident on the erbium-doped optical fiber through the wavelength division multiplexer for amplification, and the generated laser is split through the optical coupler. The laser output from the first output end of the optical coupler is incident on the No. 2 port of the circulator, and is output through its No. 3 port and then incident on the linear polarizer, the polarization-maintaining optical fiber and the No. 1 port of the circulator in sequence. After being output from the No. 2 port of the circulator, it returns to the wavelength division multiplexer along the original path, and is incident on the reflective mode locker through the wavelength division multiplexer for mode locking and reflection, and then oscillates back and forth multiple times along the resonant cavity where it is located to achieve stable mode locking;

[0023] After the laser output from the second output end of the optical coupler is incident on the polarization beam splitter, a dual-frequency comb light composed of s-polarized light and p-polarized light is output; in the ring resonator, an online polarizer and a polarization-maintaining fiber form 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 locker includes a second side-polished optical fiber arranged in a second quartz glass groove, and a nanoscale metal film is plated on one end face of the second quartz glass groove, and the end face 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:

[0026] The online polarizer comprises a third side-polished optical fiber arranged in a third quartz glass groove, and a layer of two-dimensional material is firstly drop-coated in the polishing groove of the third side-polished optical fiber, and then a nano-scale metal film is plated 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 light intensity ratio of the laser output from the first output end and the second output end of the optical coupler is 9:1.

[0030] Advantages of the present invention:

[0031] The polarization multiplexing dual-frequency comb laser with online polarization and mode locking of the present invention realizes passive mode locking through a mode locker and realizes cyclic oscillation in a set direction of the resonant cavity through a direction control component (polarization insensitive isolator or circulator), thereby obtaining the output of the dual-frequency comb from the same resonant cavity. Since the mode locker and the online polarizer are both prepared by side-throwing optical fiber, the dual-frequency comb laser of the present invention is easy to prepare and the power loss is controllable. In addition, the present invention improves the optical path structure of the traditional dual-frequency comb optical path system and introduces an online polarizer to replace the traditional unstable polarization control. The introduced online polarizer makes the dual-frequency comb output more stable.

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

[0033] The present invention performs a common cavity design for polarized light beams and has a common mode noise suppression function, so that the coherence and stability of the light source are greatly improved, thereby effectively reducing the complexity and overall cost of the dual-frequency comb laser. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a framework diagram of the polarization multiplexing dual-frequency comb laser with online polarization and mode locking according to the present invention;

[0035] Figure 2 It is a schematic diagram of the specific structure of the polarization multiplexing dual-frequency comb laser with online polarization and mode locking described in the first specific implementation mode;

[0036] Figure 3 It is a schematic diagram of the specific structure of the polarization multiplexing dual-frequency comb laser with online polarization and mode locking described in the second specific implementation mode;

[0037] Figure 4 It is a schematic diagram of the specific structure of the polarization multiplexing dual-frequency comb laser with online polarization and mode locking described in the third specific implementation mode;

[0038] Figure 5 It is a structural schematic diagram of a straight-through mode locker 7;

[0039] Figure 6 is a schematic structural diagram of a reflective mode locker 10;

[0040] Figure 7 It is a schematic structural diagram of an online polarizer 5;

[0041] Figure 8It is a main cross-sectional view of the third side-cast optical fiber 5-2; wherein, reference numeral 5-2-1 is the core of the third side-cast optical fiber 5-2. DETAILED DESCRIPTION

[0042] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. 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 creative work are within the scope of protection of the present invention.

[0043] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0044] See also Figure 1 Under the same inventive concept, the present invention provides three different structures of online polarization and mode-locked polarization multiplexing dual-frequency comb lasers. The first structure is mainly composed of a straight-through mode locker and a polarization-insensitive isolator, and the second and third structures are mainly composed of a reflective mode locker and a circulator. The first to third structures refer to specific embodiments 1 to 3, respectively, as follows:

[0045] Specific implementation method 1. Combination Figure 2 The present embodiment is described. The first structure of the polarization multiplexing dual-frequency comb laser with online polarization and mode locking described in the present embodiment comprises a pump laser source 1, a polarization beam splitter 9, and a ring resonator composed of a wavelength division multiplexer 2, an optical coupler 4, an online polarizer 5, a polarization-maintaining optical fiber 6, a direct mode locker 7, a polarization-insensitive isolator 8 and an erbium-doped optical fiber 3 arranged in sequence in a counterclockwise direction; the online polarizer 5 and the direct mode locker 7 are both made of side-thrown optical fiber;

[0046] The laser emitted by the pump laser source 1 is incident on the erbium-doped optical fiber 3 through the wavelength division multiplexer 2 for amplification, and then the generated laser is 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. After the laser emitted by 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 back and forth in the counterclockwise direction for multiple times in the ring resonant cavity where it is located, thereby realizing stable mode locking.

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

[0048] In the ring resonator, the online polarizer 5 and the polarization-maintaining optical fiber 6 form a filter, and the two are used to polarize and maintain the polarization of the running laser respectively; the straight-through mode locker 7 is used to mode-lock the running laser.

[0049] In this preferred embodiment, a filter composed of an online polarizer 5 and a polarization-maintaining fiber 6 is introduced into the ring resonator to adjust the polarization state so that the laser output by the polarization-maintaining fiber 6 is orthogonal polarized light; the straight-through mode locker 7 performs passive mode locking; the erbium-doped fiber 3 is used as a reverse pump gain medium to pump and amplify the laser emitted by the pump laser source 1, and the polarization-insensitive isolator 8 is used as a direction control component to control the laser transmission direction. Under the action of the straight-through mode locker 7, and according to the cavity length of the resonant cavity, the dispersion amount in the resonant cavity, and the length of the erbium-doped fiber 3, an all-fiber mode-locked laser is constructed.

[0050] The present invention realizes passive mode locking through a straight-through mode locker 7 and realizes cyclic oscillation of a set direction of a resonant cavity through a direction control component (polarization-insensitive isolator or circulator), thereby obtaining the output of a dual-frequency comb from the same resonant cavity. Since the mode locker and the online polarizer are both prepared by side-throwing optical fiber, the dual-frequency comb laser of the present invention is easy to prepare and the power loss is controllable. In addition, the present invention improves the optical path structure of a traditional dual-frequency comb optical path system and introduces an online polarizer to replace the traditional unstable polarization control. The introduced online polarizer makes the dual-frequency comb output more stable.

[0051] In a specific application, the light intensity ratio of the laser output from the first output end and the second output end of the optical coupler 4 is 9:1.

[0052] See also Figure 5 The straight-through mode locker 7 includes a first side-polished fiber 7-2 disposed in a first quartz glass groove 7-1, and a layer of two-dimensional material 12 is drop-coated in the polishing groove of the first side-polished 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 specific applications, the first side-polished optical fiber 7-2 is implemented using a D-type optical fiber with a polishing depth of 4μm-6μm from the center of the fiber core.

[0054] In this preferred embodiment, a straight-through mode locker 7 is prepared by side-casting an optical fiber, and a layer of two-dimensional material is drop-coated in a polishing tank to make its surface smoother so as to perform low-loss mode locking and make its power loss controllable.

[0055] See also Figure 7 and Figure 8 , the structure of the online polarizer 5 is:

[0056] The online polarizer 5 includes a third side-polished optical fiber 5-2 arranged in a third quartz glass groove 5-1, and a layer of two-dimensional material 12 is firstly drop-coated in the polishing groove of the third side-polished optical fiber 5-2, and then a nano-scale metal film 13 is plated by ion sputtering.

[0057] In this preferred embodiment, two layers of coatings of different materials are dripped 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, and the optimal thickness of the nanoscale metal film 13 is 40 to 80 nanometers. The material of the metal film includes but is not limited to gold, copper and aluminum. The metal film is covered on the surface of the polished optical fiber, and the evanescent field leaked from the polished optical fiber interacts to form a surface plasma resonance. This mode can absorb TM polarization, while the polarization state perpendicular to it can pass through, thereby achieving the effect of polarization. The specific process of preparing the second structure of the online polarizer 5 is:

[0058] Based on the single-mode optical fiber, a D-type optical fiber with a distance of 4 μm from the core center is polished, fixed in a quartz glass tank using high-temperature resistant optical UV glue, and a certain amount of two-dimensional material dispersion is dripped on the surface of the fixed D-type optical fiber. After multiple drip coatings and drying, a nano-metal film is plated. Specifically, the reflective material 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 in 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°. The fusion method at a 45° angle plays a filtering role, and the interaction between the two is not easy to change and has high stability, thereby achieving the stability of the output signal of the dual-frequency comb.

[0060] Specific implementation method 2: Combination Figure 3 The present embodiment is described. The second structure of the polarization multiplexing dual-frequency comb laser with online polarization and mode locking described in the present embodiment comprises 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 locker 10 arranged in sequence 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 sequence 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 locker 10 are both made of side-thrown optical fiber;

[0061] The laser emitted by the pump laser source 1 is incident on the erbium-doped optical fiber 3 through the wavelength division multiplexer 2 to generate laser after amplification. After being isolated by the port No. 3 of the circulator 11, it returns to the wavelength division multiplexer 2 along the original route, and then is incident on the optical coupler 4 through the wavelength division multiplexer 2 for beam splitting. The laser output from the first output end of the optical coupler 4 is incident on the port No. 1 of the circulator 11 through the online polarizer 5 and the polarization-maintaining optical fiber 6 in sequence, and is output from the port No. 2 of the circulator 11 and then incident on the reflective mode locker 10. After being mode-locked and reflected by the reflective mode locker 10, it is output from the port No. 3 of the circulator 11 and oscillates back and forth for multiple times along the transmission link in the resonant cavity to achieve stable mode locking.

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

[0063] In the ring resonator, the online polarizer 5 and the polarization-maintaining optical fiber 6 form a filter, and the two are used for polarizing and maintaining the polarization of the running laser, respectively.

[0064] In this embodiment, passive mode locking is achieved through the reflective mode locker 10, and the circulator of the resonant cavity in the set direction is achieved through the direction control component circulator, so as to obtain the output of the dual-frequency comb from the same resonant cavity. Since the mode locker and the online polarizer are both prepared by side-throwing optical fiber, the dual-frequency comb laser of the present invention is easy to prepare and the power loss is controllable. In addition, the present invention improves the optical path structure of the traditional dual-frequency comb optical path system, introduces an online polarizer, and replaces the traditional unstable polarization control. The introduced linear polarizer makes the dual-frequency comb output more stable.

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

[0066] See also Figure 6 The reflective mode locker 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 plated on one end face of the second quartz glass groove 10-1, the end face is perpendicular to the propagation direction of the second side-polished optical fiber 10-2, and a layer of two-dimensional material 12 is drop-coated in 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 locker 10 is plated with a nanoscale metal film 13 on one end face of the second quartz glass groove 10-1, and the end face is perpendicular to the propagation direction of the second side-cast optical fiber 10-2, which makes its optical path shorter and has lower alignment requirements to reduce losses and is more convenient to adjust.

[0068] See also Figure 7 and Figure 8 , the structure of the online polarizer 5 is:

[0069] The online polarizer 5 includes a third side-polished optical fiber 5-2 arranged in a third quartz glass groove 5-1, and a layer of two-dimensional material 12 is firstly drop-coated in the polishing groove of the third side-polished optical fiber 5-2, and then a nano-scale metal film 13 is plated by ion sputtering.

[0070] In this preferred embodiment, two layers of coatings of different materials are dripped 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 nano-scale metal film 13, and the optimal thickness of the nano-scale metal film 13 is 40 to 80 nanometers. The material of the metal film includes but is not limited to gold, copper and aluminum. The metal film covers the surface of the polished optical fiber, and interacts with the evanescent field leaked from the polished optical fiber to form a surface plasma resonance. This mode can absorb TM polarization, while the polarization state perpendicular to it can pass through, thereby achieving the effect of polarization.

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

[0072] The fusion splicing at a 45° angle plays a filtering role, and the interaction between the two is not easy to change and has high stability, thereby achieving the stability of the output signal of the dual-frequency comb.

[0073] Specific implementation method 3. Participation Figure 4 The third structure of the polarization multiplexing dual-frequency comb laser with online polarization and mode locking described in this embodiment comprises 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 reflective mode locker 10, a wavelength division multiplexer 2, an erbium-doped fiber 3, an optical coupler 4, and a circulator 11 arranged in sequence from right to left, and the ring cavity is composed of a circulator 11, a polarization-maintaining fiber 6, and an online polarizer 5 arranged in sequence 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 locker 10 are both made of side-thrown optical fiber;

[0074] The laser emitted by the pump laser source 1 is incident on the erbium-doped optical fiber 3 through the wavelength division multiplexer 2 for amplification, and the generated laser is split through the optical coupler 4. The laser output from the first output end of the optical coupler 4 is incident on the No. 2 port of the circulator 11, and is output through its No. 3 port and then incident on the linear polarizer 5, the polarization-maintaining optical fiber 6 and the No. 1 port of the circulator 11 in sequence, and is output from the No. 2 port of the circulator 11 and then returns to the wavelength division multiplexer 2 along the original path, and is incident on the reflective mode locker 10 through the wavelength division multiplexer 2 for mode locking and reflection, and then oscillates back and forth for multiple times along the resonant cavity where it is located to achieve stable mode locking;

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

[0076] In this embodiment, passive mode locking is achieved through the reflective mode locker 10, and the circulator of the resonant cavity in the set direction is achieved through the direction control component circulator, so as to obtain the output of the dual-frequency comb from the same resonant cavity. Since the mode locker and the online polarizer are both prepared by side-throwing optical fiber, the dual-frequency comb laser of the present invention is easy to prepare and the power loss is controllable. In addition, the present invention improves the optical path structure of the traditional dual-frequency comb optical path system, introduces an online polarizer, and replaces the traditional unstable polarization control. The introduced linear polarizer makes the dual-frequency comb output more stable.

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

[0078] See also Figure 6 The reflective mode locker 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 plated on one end face of the second quartz glass groove 10-1, the end face is perpendicular to the propagation direction of the second side-polished optical fiber 10-2, and a layer of two-dimensional material 12 is drop-coated in 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 locker 10 is plated with a nanoscale metal film 13 on one end face of the second quartz glass groove 10-1, and the end face is perpendicular to the propagation direction of the second side-cast optical fiber 10-2, which makes its optical path shorter and has lower alignment requirements to reduce losses and is more convenient to adjust.

[0080] See also Figure 7 and Figure 8 , the structure of the online polarizer 5 is:

[0081] The online polarizer 5 includes a third side-polished optical fiber 5-2 arranged in a third quartz glass groove 5-1, and a layer of two-dimensional material 12 is firstly drop-coated in the polishing groove of the third side-polished optical fiber 5-2, and then a layer of nano-scale metal film 13 is drop-coated.

[0082] In this preferred embodiment, two layers of coatings of different materials are dripped 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 nano-scale metal film 13, and the optimal thickness of the nano-scale metal film 13 is 40 to 80 nanometers. The material of the metal film includes but is not limited to gold, copper and aluminum. The metal film covers the surface of the polished optical fiber, and interacts with the evanescent field leaked from the polished optical fiber to form a surface plasma resonance. This mode can absorb TM polarization, while the polarization state 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 in 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°. The fusion method at a 45° angle plays a filtering role, and the interaction between the two is not easy to change and has high stability, thereby achieving the stability of the output signal of the dual-frequency comb.

[0084] Principle analysis: The present invention discloses three structures of polarization multiplexing dual-frequency comb lasers with online polarization and mode locking. The dual-frequency comb lasers of the three structures are all prepared with online polarization and mode locking devices through low-loss side-thrown optical fibers. Then, a pump laser source is used to inject the pump laser into the resonant cavity, and the resonance direction is controlled by a direction control component. Passive mode locking is achieved according to the corresponding structural mode locking device. A filtering effect is introduced by combining an online polarizer prepared by low-loss side-thrown optical fibers and a polarization-maintaining optical fiber, so that when the nonlinear interaction between orthogonal polarization modes and the group velocity mismatch caused by birefringence cannot be balanced with each other, a stable dual-frequency comb with unidirectional propagation is obtained.

[0085] Although the present invention is described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the present invention. It should therefore be understood that many modifications may be made to the exemplary embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims. It should be understood that the various dependent claims and features described herein may be combined in a manner different from that described in the original claims. It should also be understood that features described in conjunction with individual embodiments may be used in other described embodiments.

Claims

1. Polarization multiplexing dual-frequency comb laser with online polarization and mode locking, characterized in that: The invention comprises 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 optical fiber (6), a direct-type mode locker (7), a polarization-insensitive isolator (8) and an erbium-doped optical fiber (3) arranged in sequence in a counterclockwise direction; the online polarizer (5) and the direct-type mode locker (7) are both made of side-thrown optical fiber; The laser light emitted by the pump laser source (1) is incident on the erbium-doped optical fiber (3) through the wavelength division multiplexer (2) for amplification, and then is incident on the polarization-insensitive isolator (8). After being isolated and reflected by the polarization-insensitive isolator (8), the laser light is returned to the wavelength division multiplexer (2) along the original path. The laser light emitted by the wavelength division multiplexer (2) is split by the optical coupler (4). The laser light output from the first output end of the optical coupler (4) oscillates back and forth multiple times in the counterclockwise direction in the ring resonant cavity where the laser light is located, thereby achieving stable mode locking. After the laser output from the second output end of the optical coupler (4) is incident on the polarization beam splitter (9), a dual-frequency comb light consisting of s-polarized light and p-polarized light is output; In the ring resonant cavity, an online polarizer (5) and a polarization-maintaining optical fiber (6) form a filter, and the two are used to polarize and maintain the polarization of the running laser respectively; a straight-through mode locker (7) is used to mode lock the running laser.

2. The polarization multiplexing dual-frequency comb laser with online polarization and mode locking according to claim 1, characterized in that: The straight-through mode locker (7) comprises a first side-polished optical fiber (7-2) arranged in a first quartz glass groove (7-1), and a layer of two-dimensional material (12) is drop-coated in the polishing groove of the first side-polished optical fiber (7-2).

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

4. Polarization multiplexing dual-frequency comb laser with online polarization and mode locking, characterized in that: The invention comprises 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 locker (10) arranged in sequence 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 optical fiber (6), a circulator (11), and an erbium-doped optical fiber (3) arranged in sequence in a counterclockwise direction; the circulator (11) is shared by the linear cavity and the ring cavity; and the online polarizer (5) and the reflective mode locker (10) are both made of side-cast optical fiber; The laser light emitted by the pump laser source (1) is incident on the erbium-doped optical fiber (3) through the wavelength division multiplexer (2) to generate laser light after amplification. After being isolated by the No. 3 port of the circulator (11), the laser light is returned to the wavelength division multiplexer (2) along the original path. After being incident on the optical coupler (4) through the wavelength division multiplexer (2) for beam splitting, the laser light outputted from the first output end of the optical coupler (4) is incident on the No. 1 port of the circulator (11) through the online polarizer (5) and the polarization-maintaining optical fiber (6) in sequence. After being outputted from the No. 2 port of the circulator (11), the laser light is incident on the reflective mode locker (10). After being mode locked and reflected by the reflective mode locker (10), the laser light is outputted from the No. 3 port of the circulator (11) and oscillates back and forth for multiple times along the transmission link in the resonant cavity to achieve stable mode locking. After the laser output from the second output end of the optical coupler (4) is incident on the polarization beam splitter (9), a dual-frequency comb light consisting of s-polarized light and p-polarized light is output; In the ring resonant cavity, an online polarizer (5) and a polarization-maintaining optical fiber (6) form a filter, and the two are used for polarizing and maintaining the polarization of the running laser respectively.

5. Polarization multiplexing dual frequency comb laser with online polarization and mode locking, characterized in that: The invention comprises 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 reflective mode locker (10), a wavelength division multiplexer (2), an erbium-doped optical fiber (3), an optical coupler (4), and a circulator (11) arranged in sequence from right to left, and the ring cavity is composed of a circulator (11), a polarization-maintaining optical fiber (6), and an online polarizer (5) arranged in sequence in a counterclockwise direction; the circulator (11) is shared by the linear cavity and the ring cavity; and the online polarizer (5) and the reflective mode locker (10) are both made of side-cast optical fiber. The laser light emitted by the pump laser source (1) is incident on the erbium-doped optical fiber (3) through the wavelength division multiplexer (2) for amplification, and then the generated laser light is split through the optical coupler (4). The laser light outputted from the first output end of the optical coupler (4) is incident on the No. 2 port of the circulator (11), and then outputted through the No. 3 port thereof, and then incident on the linear polarizer (5), the polarization-maintaining optical fiber (6) and the No. 1 port of the circulator (11) in sequence. After being outputted from the No. 2 port of the circulator (11), the laser light returns to the wavelength division multiplexer (2) along the original path, and then is incident on the reflective mode locker (10) through the wavelength division multiplexer (2) for mode locking and reflection, and then oscillates back and forth multiple times along the resonant cavity where the laser light is located, so as to achieve stable mode locking. After the laser output from the second output end of the optical coupler (4) is incident on the polarization beam splitter (9), dual-frequency comb light consisting of s-polarized light and p-polarized light is output; in the ring resonant cavity, an online polarizer (5) and a polarization-maintaining optical fiber (6) form a filter, and the two are used to polarize and maintain the polarization of the running laser, respectively.

6. The polarization multiplexing dual-frequency comb laser with online polarization and mode locking according to claim 4 or 5, characterized in that: The reflective mode locker (10) comprises a second side-polished optical fiber (10-2) arranged in a second quartz glass groove (10-1), and a nano-scale metal film (13) is plated on one end face of the second quartz glass groove (10-1), the end face is perpendicular to the propagation direction of the second side-polished optical fiber (10-2), and a layer of two-dimensional material (12) is drop-coated in the polishing groove of the second side-polished optical fiber (10-2).

7. The polarization multiplexing dual-frequency comb laser with online polarization and mode locking according to claim 1, 4 or 5, characterized in that: The structure of the online polarizer (5) is: The online polarizer (5) comprises a third side-polished optical fiber (5-2) arranged in a third quartz glass groove (5-1), and a layer of two-dimensional material (12) is firstly drop-coated in the polishing groove of the third side-polished optical fiber (5-2), and then a nano-scale metal film (13) is plated by ion sputtering.

8. The polarization multiplexing dual-frequency comb laser with online polarization and mode locking according to claim 7, characterized in that: The thickness of the nano-scale metal film (13) is 40 to 80 nanometers.

9. The polarization multiplexing dual-frequency comb laser with online polarization and mode locking according to claim 7, characterized in that: The slow axis of the polarization-maintaining optical fiber (6) is fused with the core of the third side-polished optical fiber (5-2) in the online polarizer (5), and the angle between the slow axis of the polarization-maintaining optical fiber (6) and the extension direction of the polishing groove of the third side-polished optical fiber (5-2) is 45°.

10. The polarization multiplexing dual-frequency comb laser with online polarization and mode locking according to claim 1, 4 or 5, characterized in that: The light intensity ratio 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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