Single-longitudinal-mode fiber laser based on linear main cavity combined with traveling-wave auxiliary cavity

Abstract

The application provides a single-longitudinal-mode fiber laser based on a linear main cavity combined with a traveling wave auxiliary cavity, and relates to the technical field of laser equipment, and the laser comprises a first reflecting device, a gain medium, a composite sub-cavity, a second reflecting device and a pump source, the composite sub-cavity comprises a first circulator, a second circulator, a first beam splitter and a second beam splitter, the first circulator and the second circulator are connected into a resonant cavity, the first circulator is connected with the first beam splitter and the second beam splitter respectively, and the transmission directions of the connecting light paths between the first circulator and the first beam splitter and the second beam splitter are unidirectional; the second circulator is connected with the first beam splitter and the second beam splitter respectively, and the transmission directions of the connecting light paths between the second circulator and the first beam splitter and the second beam splitter are unidirectional; and a unidirectional circulating light path is formed between the first beam splitter and the second beam splitter.

Description

Technical Field

[0001] This invention relates to the field of laser equipment technology, and in particular to a single longitudinal mode fiber laser based on a linear main cavity combined with a traveling wave auxiliary cavity. Background Technology

[0002] Single-longitudinal-mode fiber lasers can be classified into three types according to their structure and operating principle: Distributed Bragg Reflector (DBR), Distributed Feedback (DFB), and Traveling Wave Cavity (TWCC). Short-cavity DBRs require gain fiber lengths in the centimeter range, but due to the low gain coefficient of doped silica fiber, the output power is only in the hundreds of microwatts. If high-gain phosphate-doped fiber is used, the large difference in melting point compared to ordinary fiber leads to difficulties in splicing, high loss, or poor environmental reliability. DFB lasers utilize π-phase-shift gratings for feedback and mode selection, exhibiting excellent mode selectivity. However, similar to DBRs, their standing wave cavity structure suffers from a "spatial hole burning effect," which easily leads to multimode oscillations under strong pump excitation, hindering single-longitudinal-mode output. Traveling-wave cavities employ ring resonators, with the laser propagating in a traveling-wave manner. While this eliminates the "spatial hole burning effect," the relatively long cavity length, greater number of longitudinal modes, and smaller mode spacing make achieving long-term stable single-mode output challenging. This typically requires additional frequency selection techniques such as composite cavities, polarization control, unsaturated absorbers, or external cavity injection locking. In summary, existing single-mode fiber lasers still face technical bottlenecks in terms of output power, environmental stability, mode selection reliability, and structural complexity. Summary of the Invention

[0003] The purpose of this invention is to provide a single longitudinal mode fiber laser based on a linear main cavity combined with a traveling wave auxiliary cavity, so as to alleviate the technical problems of existing lasers with small longitudinal mode spacing in long cavities, susceptibility to environmental disturbances, mode competition caused by the "spatial hole burning effect", and complex mode selection structure.

[0004] The present invention provides a single longitudinal mode fiber laser based on a linear main cavity combined with a traveling wave auxiliary cavity, comprising: a first reflecting device, a gain medium, a composite sub-cavity, a second reflecting device and a pump source, wherein the first reflecting device, the gain medium, the composite sub-cavity and the second reflecting device are sequentially connected to form a resonant cavity; The pump source can couple the pump light it generates into the resonant cavity; The composite subcavity includes a first circulator, a second circulator, a first beam splitter, and a second beam splitter. The first circulator and the second circulator are connected to the resonant cavity. Light in the resonant cavity is input to and output from the composite subcavity through the first circulator, and light in the resonant cavity is input to and output from the composite subcavity through the second circulator. The first circulator is connected to the first beam splitter and the second beam splitter respectively, and the transmission direction of the optical path connecting the first circulator to the first beam splitter and the second beam splitter is unidirectional. The second circulator is connected to the first beam splitter and the second beam splitter respectively, and the transmission direction of the optical path connecting the second circulator to the first beam splitter and the second beam splitter is unidirectional. A unidirectional circular optical path is formed between the first beam splitter and the second beam splitter.

[0005] Furthermore, the laser forms a forward optical path from the first reflective device to the second reflective device; In the forward optical path, the light reflected by the first reflector passes through the gain medium and enters the composite sub-cavity through the first circulator. Then, after being split by the first beam splitter, it is transmitted to the second beam splitter and the second circulator respectively. A portion of the light emitted from the second beam splitter returns to the first beam splitter, while the other portion enters the first circulator and is transmitted to the first reflector. The light transmitted from the first beam splitter to the second circulator is then transmitted to the second reflector after passing through the second circulator. The laser forms a reverse optical path from the second reflector to the first reflector. In the reverse optical path, the light reflected by the second reflector enters the composite cavity through the second circulator, and then is split by the second beam splitter and transmitted to the first circulator and the first beam splitter respectively. A portion of the light emitted from the first beam splitter returns to the second beam splitter, and the other portion enters the second circulator and is transmitted to the second reflector. The light transmitted from the second beam splitter to the first circulator is transmitted to the first reflector after passing through the first circulator.

[0006] Furthermore, the first circulator includes port A1, port A2 and port A3, and the optical path from port A1 to port A2 is open, while the optical path from port A2 to port A1 is closed; the optical path from port A2 to port A3 is open, while the optical path from port A3 to port A2 is closed. The second circulator includes ports B1, B2, and B3. The optical path from port B1 to port B2 is open, while the optical path from port B2 to port B1 is closed. The optical path from port B2 to port B3 is open, while the optical path from port B3 to port B2 is closed. The first beam splitter includes inlet C1, inlet C2, outlet C3 and outlet C4, and inlet C1 and inlet C2 are respectively connected to the optical paths of outlet C3 and outlet C4; the second beam splitter includes inlet D1, inlet D2, outlet D3 and outlet D4, and inlet D1 and inlet D2 are respectively connected to the optical paths of outlet D3 and outlet D4. Port A3 is connected to inlet C1; outlet C3 is connected to port B1; port B3 is connected to inlet D2; outlet D4 is connected to port A1; outlet D3 is connected to inlet C2; outlet C4 is connected to inlet D1. Port A2 is connected to the gain medium; port B2 is connected to the second reflector.

[0007] Furthermore, the composite subcavity also includes n third beam splitters, where n is an even number, and the n third beam splitters are all sequentially arranged between the first beam splitter and the second beam splitter. The third beam splitter includes inlet E1, inlet E2, outlet E3 and outlet E4, and inlet E1 and inlet E2 are respectively connected to the optical paths of outlet E3 and outlet E4. The n third beam splitters are labeled k1, k2...k n In this configuration, the inlet E1 of the third beam splitter, numbered k1, is connected to the inlet C4; the outlet E3 of the third beam splitter, numbered k1, is connected to the inlet C2. The inlet E2 of the third beam splitter numbered k1 is connected to the outlet E3 of the third beam splitter numbered k2; the outlet E4 of the third beam splitter numbered k1 is connected to the inlet E1 of the third beam splitter numbered k2. And so on; Number k n-1 The third beam splitter's outlet E4 and the one numbered k n The third beam splitter's inlet E1 connection; numbered k n-1 The third beam splitter's inlet E2 and the one numbered k n The third beam splitter's output E3 connection; Number k n The outlet E4 of the third beam splitter is connected to the inlet D1; it is numbered k. n The inlet E2 of the third beam splitter is connected to the outlet D3.

[0008] Furthermore, the laser also includes an optical wavelength division multiplexer connected between the gain medium and the composite subcavity. The pump source is connected to the optical wavelength division multiplexer, which is used to couple the pump light generated by the pump source into the resonant cavity.

[0009] Furthermore, the first reflecting device is a 90° Faraday rotating mirror.

[0010] Furthermore, the pump source is a polarization-maintaining pumped laser; The gain medium is a polarization-maintaining highly doped optical fiber; The first reflecting device is a 90° polarization-maintaining Faraday rotating mirror; The optical wavelength division multiplexer is a polarization-maintaining optical wavelength division multiplexer; The first circulator is a single-polarization circulator. Ports A1 to A2 allow slow-axis light to pass through while blocking fast-axis light. Ports A2 to A3 allow both slow-axis and fast-axis light to pass through, and the output axis of port A3 is coupled to the slow-axis output at 90°. Port A1 and outlet D4 are connected via a single-polarization fiber. Port A3 and inlet C1 are connected via a single-polarization fiber. Port A2 is connected to the optical wavelength division multiplexer via a polarization-maintaining fiber. Both the first and second beam splitters are single-polarization beam splitters, so that slow-axis light can pass through between each inlet and outlet, while fast-axis light is blocked. The second circulator is a single-polarization circulator, and ports B1 to B2 allow slow-axis light to pass through while blocking fast-axis light; ports B2 to B3 allow slow-axis light to pass through while blocking fast-axis light.

[0011] Furthermore, the second reflecting device is a single-polarization narrowband grating; An unpumped, single-polarization, low-doped fiber is connected between the composite subcavity and the second reflector, with the single-polarization, low-doped fiber acting as a saturable absorber.

[0012] Furthermore, the laser also includes a self-injection structure connected to the second reflector. The self-injection structure is used to return a portion of the light emitted from the second reflector back into the resonant cavity via the second reflector.

[0013] Furthermore, the self-injection structure includes a third circulator, a tail divider, and an output head; The third circulator includes ports F1, F2, and F3. The optical path from port F1 to port F2 is open, while the optical path from port F2 to port F1 is closed. The optical path from port F2 to port F3 is open, while the optical path from port F3 to port F2 is closed. The tail beam splitter includes an inlet G1, an outlet G2, and an outlet G3, and the optical paths of the inlet G1 are connected to those of the outlet G2 and the outlet G3, respectively. Port F2 is connected to the second reflector; port F3 is connected to inlet G1; outlet G2 is connected to the output head; outlet G3 is connected to port F1.

[0014] Furthermore, outlet G3 is connected to port F1 via a single-polarization fiber extension line.

[0015] Furthermore, both the third circulator and the tail beam splitter are single-polarization beam splitters to allow slow-axis light to pass through between each inlet and outlet while blocking fast-axis light. The splitting ratio of export G2 and export G3 is 90 / 10.

[0016] Furthermore, the reflectivity of the first reflective device is greater than 99%, and a filter is integrated within the first reflective device to filter out spontaneous radiation.

[0017] Furthermore, the reflectivity of the second reflective device is 80% to 90%, and the reflective bandwidth is no greater than 0.1 nm.

[0018] This invention provides a single-longitudinal-mode fiber laser based on a linear main cavity combined with a traveling-wave auxiliary cavity, comprising: a first reflecting device, a gain medium, a composite sub-cavity, a second reflecting device, and a pump source. The first reflecting device, gain medium, composite sub-cavity, and second reflecting device are sequentially connected to form a resonant cavity. The pump source can couple its generated pump light into the resonant cavity. The composite sub-cavity includes a first circulator, a second circulator, a first beam splitter, and a second beam splitter. The first circulator and the second circulator are connected to the resonant cavity. Light in the resonant cavity is input to and output from the composite sub-cavity through the first circulator, and light in the resonant cavity is input to and output from the composite sub-cavity through the second circulator. The first circulator is connected to the first beam splitter and the second beam splitter, and the transmission direction of the connecting optical paths between the first circulator and the first and second beam splitters is unidirectional. The second circulator is connected to the first and second beam splitters, and the transmission direction of the connecting optical paths between the second circulator and the first and second beam splitters is unidirectional. A unidirectional circulating optical path is formed between the first and second beam splitters.

[0019] This scheme forms a composite cavity optical path by combining a main resonant cavity with composite sub-cavities. The composite sub-cavities include a first circulator, a second circulator, a first beamsplitter, and a second beamsplitter. Multiple sub-cavities exist within the composite sub-cavities, each with a longitudinal mode spacing and a periodic transmission spectrum. A single longitudinal mode is selected through the "Vernier effect" of the composite cavity. Specifically, the multiple sub-cavities have similar but slightly different free spectral ranges (FSRs). When these periodic transmission spectra are superimposed, the effective free spectral range is greatly expanded. Only at the point where the transmission peaks of all sub-cavities overlap can the lowest loss be obtained, and the remaining longitudinal modes are significantly suppressed, thereby increasing the longitudinal mode spacing. This ensures that only a single longitudinal mode can oscillate within the effective gain bandwidth, thus increasing the longitudinal mode spacing and eliminating mode competition. Furthermore, the light propagation direction within each sub-cavity is unidirectional, forming multiple strictly closed-loop resonant composite sub-cavities. The traveling wave cavity ensures unidirectional transmission of optical signals, effectively avoiding standing wave generation and mode aliasing, thus guaranteeing high signal purity and stability. Compared with the traditional full beam splitter combination, the optimized combination of circulator and beam splitter forms a highly directional and low-loss optical transmission path, effectively improving the system's optical conversion efficiency. Attached Figure Description

[0020] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of a single longitudinal mode fiber laser based on a linear main cavity combined with a traveling wave auxiliary cavity, provided in Embodiment 1 of the present invention. Figure 2 This is a schematic diagram of the composite sub-cavity in a single longitudinal mode fiber laser based on a linear main cavity combined with a traveling wave auxiliary cavity, as provided in Embodiment 1 of the present invention. Figure 3 This is a schematic diagram of the composite sub-cavity in a single longitudinal mode fiber laser based on a linear main cavity combined with a traveling wave auxiliary cavity, as provided in Embodiment 2 of the present invention.

[0022] Icons: 1-First reflective device; 2-Gain medium; 3-Optical wavelength division multiplexer; 4-Pump source; 5-Combined sub-cavity; 51-First circulator; 52-Second circulator; 53-First beam splitter; 54-Second beam splitter; 6-Second reflective device; 7-Single-polarization low-doped fiber; 81-Third circulator; 82-Tail beam splitter; 83-Output head; 84-Single-polarization fiber extension line. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, 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. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0024] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0025] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0026] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of this invention is in use. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention. In addition, the terms "first," "second," "third," etc., are only used to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0027] Furthermore, terms such as "horizontal" and "vertical" do not imply that components must be absolutely horizontal or suspended, but rather that they can be slightly tilted. For example, "horizontal" simply means that its direction is more horizontal than "vertical," and does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0028] In the description of this invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0029] like Figure 1 As shown, the single longitudinal mode fiber laser based on a linear main cavity combined with a traveling wave auxiliary cavity provided by this invention adopts a single polarization optical path structure with a linear main cavity combined with a traveling wave auxiliary cavity. It uses a novel composite sub-cavity 5 structure to increase the longitudinal mode spacing, uses an unpumped single polarization low-doped fiber 7 combined with a single polarization narrowband grating, uses the "spatial hole burning effect" to realize a dynamic gain grating for filtering and mode selection, and finally uses a single polarization self-injection locking technique to obtain single longitudinal mode laser output. Its output wavelength can cover laser bands such as 1μm, 1.5μm and 2μm.

[0030] Specifically, the laser includes a first reflective device 1, a gain medium 2, an optical wavelength division multiplexer 3, a pump source 4, a composite sub-cavity 5, a single-polarization low-doped fiber 7, a second reflective device 6, and a self-injection structure. The first reflective device 1, gain medium 2, optical wavelength division multiplexer 3, composite sub-cavity 5, single-polarization low-doped fiber 7, and second reflective device 6 are sequentially connected to form a resonant cavity. Light moves repeatedly within the resonant cavity. The first reflective device 1 is a total reflection device (reflectivity greater than 99%), while the second reflective device 6 is a partial reflection device. The linear main cavity (resonant cavity) includes a gain-torsional mode cavity and a non-gain standing wave cavity. The traveling wave auxiliary cavity (composite sub-cavity 5) is a traveling wave cavity. The two can be connected by a single-polarization coupling element to form a composite resonant cavity structure.

[0031] The first reflecting device 1 can be a 90° polarization-maintaining Faraday rotator. The reflectivity of the first reflecting device 1 is greater than 99%. The first reflecting device 1 operates on two axes, and the coupling fiber is a polarization-maintaining fiber. The first reflecting device 1 also integrates a filter to filter out spontaneous emission. The first reflecting device 1 and the second reflecting device 6 constitute the resonant cavity of the laser. After passing through the 90° polarization-maintaining Faraday rotator, the slow-axis light (solid arrow) is converted into fast-axis light (hollow arrow). There is an alternation of fast and slow axes between the first reflecting device 1 and the composite sub-cavity 5. To address the mode competition problem caused by the standing wave field in the pump gain fiber under linear cavity conditions, a gain torsion cavity is formed using an alternating fast and slow axis gain mechanism to eliminate the standing wave field distribution, thereby eliminating the "spatial hole burning effect" under pump gain.

[0032] Gain medium 2 can be a polarization-maintaining highly doped fiber, and the dopant ions in the polarization-maintaining highly doped fiber include Yb 3+ Er 3+ Tm 3+ Ho 3+ At least one of the following. The gain medium 2 of the laser is a rare-earth ion-doped fiber, including but not limited to ytterbium-doped fiber, erbium-doped fiber, or thulium-doped fiber, corresponding to single-frequency output in the 1μm, 1.5μm, and 2μm bands, respectively.

[0033] The optical wavelength division multiplexer 3 can be a polarization-maintaining filter wavelength division multiplexer, operating on two axes. The coupling fiber is a polarization-maintaining fiber, and an integrated filter is used to filter out spontaneous emission and couple the laser from the pump source 4 into the resonant cavity.

[0034] Pump source 4 can be a polarization-maintaining pumped laser, and the coupling fiber is a polarization-maintaining fiber. The main pump wavelengths are 793nm, 915nm, 940nm, 976nm and 980nm, which are selected according to the gain medium 2.

[0035] The single-polarization optical path employs polarization-maintaining fiber and a single-polarization fiber system to resist the effects of environmental disturbances such as temperature and vibration on the stability of laser polarization state and frequency.

[0036] like Figure 2 As shown, the composite sub-cavity 5 includes a first circulator 51, a second circulator 52, a first beam splitter 53, and a second beam splitter 54. The first circulator 51 and the second circulator 52 are connected inside the resonant cavity. Light transmitted from the first reflecting device 1 toward the second reflecting device 6 is input into the composite sub-cavity 5 through the first circulator 51 and output from the composite sub-cavity 5 through the second circulator 52. Correspondingly, light transmitted from the second reflecting device 6 toward the second reflecting device 6 is input into the composite sub-cavity 5 through the second circulator 52 and output from the composite sub-cavity 5 through the first circulator 51.

[0037] The first circulator 51 is connected to the first beam splitter 53 and the second beam splitter 54 respectively, and the transmission direction of the optical path connecting the first circulator 51 to the first beam splitter 53 and the second beam splitter 54 is unidirectional; the second circulator 52 is connected to the first beam splitter 53 and the second beam splitter 54 respectively, and the transmission direction of the optical path connecting the second circulator 52 to the first beam splitter 53 and the second beam splitter 54 is unidirectional; a unidirectional circulating optical path is formed between the first beam splitter 53 and the second beam splitter 54.

[0038] The laser forms a forward optical path from the first reflecting device 1 to the second reflecting device 6. In the forward optical path, the light reflected by the first reflecting device 1 passes through the gain medium 2 and then enters the composite sub-cavity 5 through the first circulator 51. It is then split by the first beam splitter 53 and transmitted to the second beam splitter 54 and the second circulator 52. A portion of the light emitted from the second beam splitter 54 returns to the first beam splitter 53, while the other portion enters the first circulator 51 and is transmitted to the first reflecting device 1. The light transmitted from the first beam splitter 53 to the second circulator 52 is then transmitted to the second reflecting device 6 after passing through the second circulator 52. Under the reflection of the second reflecting device 6, a portion of the light is transmitted toward the first reflecting device 1. Specifically, the laser forms a reverse optical path from the second reflecting device 6 to the first reflecting device 1. In the reverse optical path, the light reflected by the second reflecting device 6 enters the composite sub-cavity 5 through the second circulator 52, and then is split by the second beam splitter 54 and transmitted to the first circulator 51 and the first beam splitter 53 respectively. A portion of the light emitted from the first beam splitter 53 returns to the second beam splitter 54, and another portion enters the second circulator 52 and is transmitted to the second reflecting device 6. The light transmitted from the second beam splitter 54 to the first circulator 51 is transmitted to the first reflecting device 1 after passing through the first circulator 51.

[0039] The specific connection structure within the composite sub-cavity 5 is as follows: The first circulator 51 includes ports A1, A2, and A3, with the optical path from port A1 to port A2 being open and the optical path from port A2 to port A1 being closed; the optical path from port A2 to port A3 is open and the optical path from port A3 to port A2 is closed. Light input from port A1 can only be output from port A2; light input from port A2 can only be output from port A3.

[0040] The second circulator 52 includes ports B1, B2, and B3. The optical path from port B1 to port B2 is open, while the optical path from port B2 to port B1 is closed. The optical path from port B2 to port B3 is open, while the optical path from port B3 to port B2 is closed. Light input from port B1 can only be output from port B2; light input from port B2 can only be output from port B3.

[0041] The first beam splitter 53 includes an inlet C1, an inlet C2, an outlet C3, and an outlet C4, with inlet C1 and inlet C2 being optically connected to outlet C3 and outlet C4, respectively. Light entering from inlet C1 can be output from outlet C3 and outlet C4, and similarly, light entering from inlet C2 can be output from outlet C3 and outlet C4.

[0042] The second beam splitter 54 includes an inlet D1, an inlet D2, an outlet D3, and an outlet D4, with inlet D1 and inlet D2 being optically connected to outlet D3 and outlet D4, respectively. Light entering from inlet D1 can be output from outlet D3 and outlet D4, and similarly, light entering from inlet D2 can be output from outlet D3 and outlet D4.

[0043] The first circulator 51 is a single-polarization circulator. Ports A1 to A2 allow slow-axis light to pass through while blocking fast-axis light; ports A2 to A3 allow both slow-axis and fast-axis light to pass through, and the output axis of port A3 is coupled 90° to the slow-axis output. Port A1 and outlet D4 are connected via single-polarization fiber; port A3 and inlet C1 are connected via single-polarization fiber; port A2 is connected to the optical wavelength division multiplexer 3 via polarization-maintaining fiber. The first beamsplitter 53 and the second beamsplitter 54 are both single-polarization beamsplitters, with strong resistance to environmental disturbances to ensure that slow-axis light can pass through both inlets and outlets while blocking fast-axis light. The second circulator 52 is a single-polarization circulator. Ports B1 to B2 allow slow-axis light to pass through while blocking fast-axis light; ports B2 to B3 allow slow-axis light to pass through while blocking fast-axis light.

[0044] Port A3 is connected to inlet C1, outlet C3 is connected to port B1, port B3 is connected to inlet D2, outlet D4 is connected to port A1, outlet D3 is connected to inlet C2, outlet C4 is connected to inlet D1, and port A2 is connected to gain medium 2 via optical wavelength division multiplexer 3. Port B2 is connected to the second reflector 6. The direction of light propagation between the two connected optical elements within the composite subcavity 5 is as follows: Figure 2 As indicated by the arrows, the composite sub-cavity 5 contains multiple sub-cavities (for example, the loop optical path between the first beamsplitter 53 and the second beamsplitter 54 forms a sub-cavity). Each sub-cavity has a longitudinal mode spacing and a periodic transmission spectrum. A single longitudinal mode is selected through the "Vernier effect" of the composite cavity. Specifically, the multiple sub-cavities have similar but slightly different free spectral ranges (FSRs). After these periodic transmission spectra are superimposed, the effective free spectral range is greatly expanded. Only the position where the transmission peaks of all sub-cavities overlap can achieve the lowest loss, and the remaining longitudinal modes are significantly suppressed. This increases the longitudinal mode spacing, suppresses mode competition of non-target longitudinal modes, and ensures that only a single longitudinal mode can oscillate within the effective gain bandwidth. Therefore, increasing the longitudinal mode spacing eliminates mode competition.

[0045] Instead of using only beamsplitters, a composite subcavity 5 is formed by utilizing a first beamsplitter 53, a second beamsplitter 54, a first circulator 51, and a third beamsplitter 53, thereby reducing energy loss. Furthermore, the light propagation within the composite subcavity 5 is unidirectional, ensuring that the light propagation direction within each subcavity is unidirectional, thus forming multiple strictly closed-loop traveling-wave resonant composite subcavities. This cavity ensures unidirectional transmission of the optical signal, effectively avoiding standing wave generation and mode aliasing, thereby guaranteeing high signal purity and stability. Compared to the traditional all-beamsplitter combination, the optimized combination of the circulator and beamsplitter forms a highly directional, low-loss optical transmission path, effectively improving the system's optical conversion efficiency.

[0046] The second reflective device 6 can be a single-polarization narrowband grating, which has strong resistance to environmental disturbances. The reflectivity of the second reflective device 6 is 80% to 90%, and the reflection bandwidth is no more than 0.1 nm. It plays the role of resonant cavity feedback, output, and coarsely limiting the laser center wavelength range.

[0047] An unpumped, single-polarized, low-doped fiber 7 is connected between the composite subcavity 5 and the second reflector 6. This single-polarization fiber exhibits strong resistance to environmental disturbances and acts as a saturable absorber. The dopant ions in the single-polarized, low-doped fiber 7 are the same as those in the gain medium 2. The single-polarized, low-doped fiber 7 is a section of unpumped doped fiber that acts as a saturable absorber. One end is connected to the composite subcavity 5, and the other end is connected to the second reflector 6 as a mirror. The reflected light and incident light form a standing wave interference field in the saturable absorber, creating a dynamic gain grating that stabilizes the longitudinal modes and narrows the laser linewidth. Simultaneously, the back-reflected light forms a self-injection structure, further narrowing the laser linewidth. By setting an appropriate length of unpumped, single-polarized, low-doped fiber 7, while maintaining the polarization state, the "spatial hole-burning effect" of the non-gain standing wave field allows the unpumped single-polarized, low-doped fiber 7 to form a dynamic gain grating. Combined with a single-polarization narrowband grating, this achieves the filtering and mode selection function.

[0048] The self-injection structure is connected to the second reflector 6. The self-injection structure is used to return a portion of the light emitted from the second reflector 6 back into the resonant cavity. Specifically, the self-injection structure includes a third circulator 81, a tail beam splitter 82, and an output head 83. The third circulator 81 includes ports F1, F2, and F3, with the optical path from port F1 to port F2 being open, the optical path from port F2 to port F1 being closed, the optical path from port F2 to port F3 being open, and the optical path from port F3 to port F2 being closed. The tail beam splitter 82 includes an inlet G1, an outlet G2, and an outlet G3, with the inlet G1 being open to both outlet G2 and outlet G3. Port F2 is connected to the second reflector 6; port F3 is connected to the inlet G1; outlet G2 is connected to the output head 83; and outlet G3 is connected to port F1.

[0049] The outlet G3 and port F1 are connected by a single-polarization fiber extension line 84, which has strong resistance to environmental disturbances.

[0050] Both the third circulator 81 and the tail beam splitter 82 are single-polarization beam splitters, exhibiting strong resistance to environmental disturbances. This allows slow-axis light to pass through both inlets and outlets while blocking fast-axis light. The splitting ratio at outlets G2 and G3 is 90 / 10. A 10% output end, coupled with a suitable-length single-polarization fiber extension line 84, helps increase the photon lifetime of the external cavity. Coupled into the resonant cavity via the single-polarization circulator B, it compresses the laser linewidth within the resonant cavity. Simultaneously, the third circulator 81 also isolates the output. Finally, the 90% output end is fused with a suitable-length FC / APC single-polarization output head 83 to output the laser. Single-polarization self-injection locking technology is used to feed back a portion of the laser output from the main cavity, achieving linewidth self-compression and frequency self-locking.

[0051] like Figure 3As shown, the laser also includes n third beam splitters, where n is an even number, for example, n is 2 or 4, thereby increasing the number of subcavities. The composite subcavities 5 induce a "vernier effect" that can further increase the longitudinal mode spacing.

[0052] n third beam splitters are sequentially arranged between the first beam splitter 53 and the second beam splitter 54. Specifically, each third beam splitter includes an inlet E1, an inlet E2, an outlet E3, and an outlet E4, with inlets E1 and E2 respectively connected to the optical paths of outlets E3 and E4. That is, light entering from inlet E1 can exit from outlets E3 and E4, and similarly, light entering from inlet E2 can exit from outlets E3 and E4.

[0053] The n third beam splitters are labeled k1, k2...k n In this configuration, the inlet E1 of the third beam splitter (numbered k1) is connected to the inlet C4; the outlet E3 of the third beam splitter (numbered k1) is connected to the inlet C2. The inlet E2 of the third beam splitter (numbered k1) is connected to the outlet E3 of the third beam splitter (numbered k2); the outlet E4 of the third beam splitter (numbered k1) is connected to the inlet E1 of the third beam splitter (numbered k2), and so on. n-1 The third beam splitter's outlet E4 and the one numbered k n The third beam splitter's inlet E1 connection; numbered k n-1 The third beam splitter's inlet E2 and the one numbered k n The third beam splitter's output E3 is connected. Numbered k n The outlet E4 of the third beam splitter is connected to the inlet D1; it is numbered k. n The inlet E2 of the third beam splitter is connected to the outlet D3.

[0054] This invention has at least the following advantages or beneficial effects: It employs a single-polarization fiber system to construct a linear main cavity and a traveling-wave auxiliary cavity, effectively resisting environmental disturbances such as temperature and vibration; it utilizes alternating fast and slow axis gain to eliminate the "spatial hole burning effect"; it increases the longitudinal mode spacing through the vernier effect of the composite sub-cavity 5; it uses unpumped single-polarization low-doped fiber 7 to form a dynamic gain grating and combines it with a single-polarization narrowband grating to achieve filtering and mode selection; and it uses single-polarization self-injection locking technology to compress the linewidth and lock the frequency, thereby obtaining a stable and reliable single longitudinal mode output. Simultaneously, it is compatible with various rare-earth-doped fibers, covering 1μm, 1.5μm, and 2μm bands, and has the advantages of being fully fiber-based, highly integrated, and easy to apply in engineering.

[0055] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A single-longitudinal-mode fiber laser based on a linear main cavity combined with a traveling-wave auxiliary cavity, characterized in that, include: The first reflective device (1), the gain medium (2), the composite sub-cavity (5), the second reflective device (6) and the pump source (4) are connected in sequence to form a resonant cavity; The pump source (4) can couple the pump light it generates into the resonant cavity; The composite sub-cavity (5) includes a first circulator (51), a second circulator (52), a first beam splitter (53), and a second beam splitter (54). The first circulator (51) and the second circulator (52) are connected to the resonant cavity. Light in the resonant cavity is input to and output to the composite sub-cavity (5) through the first circulator (51), and light in the resonant cavity is input to and output to the composite sub-cavity (5) through the second circulator (52). The first circulator (51) is connected to the first beam splitter (53) and the second beam splitter (54) respectively, and the transmission direction of the optical path connecting the first circulator (51) to the first beam splitter (53) and the second beam splitter (54) is unidirectional. The second circulator (52) is connected to the first beam splitter (53) and the second beam splitter (54) respectively, and the transmission direction of the optical path connecting the second circulator (52) to the first beam splitter (53) and the second beam splitter (54) is unidirectional. A unidirectional circular optical path is formed between the first beam splitter (53) and the second beam splitter (54).

2. The single-longitudinal-mode fiber laser based on a linear main cavity combined with a traveling-wave auxiliary cavity according to claim 1, characterized in that, The laser forms a forward optical path from the first reflective device (1) to the second reflective device (6); In the forward optical path, the light reflected by the first reflective device (1) passes through the gain medium (2) and then enters the composite sub-cavity (5) through the first circulator (51). After being split by the first beam splitter (53), the light is transmitted to the second beam splitter (54) and the second circulator (52). A portion of the light emitted from the second beam splitter (54) returns to the first beam splitter (53), while the other portion enters the first circulator (51) and is transmitted to the first reflective device (1). The light transmitted from the first beam splitter (53) to the second circulator (52) is transmitted to the second reflective device (6) after passing through the second circulator (52). The laser forms a reverse optical path from the second reflective device (6) to the first reflective device (1). In the reverse optical path, the light reflected by the second reflective device (6) enters the composite sub-cavity (5) through the second circulator (52), and then is split by the second beam splitter (54) and transmitted to the first circulator (51) and the first beam splitter (53) respectively. A portion of the light emitted from the first beam splitter (53) returns to the second beam splitter (54), and another portion enters the second circulator (52) and is transmitted to the second reflective device (6). The light transmitted from the second beam splitter (54) to the first circulator (51) is transmitted to the first reflective device (1) after passing through the first circulator (51).

3. The single-longitudinal-mode fiber laser based on a linear main cavity combined with a traveling-wave auxiliary cavity according to claim 2, characterized in that, The first circulator (51) includes port A1, port A2 and port A3, and the optical path from port A1 to port A2 is open, while the optical path from port A2 to port A1 is closed; the optical path from port A2 to port A3 is open, while the optical path from port A3 to port A2 is closed. The second circulator (52) includes port B1, port B2 and port B3, and the optical path from port B1 to port B2 is open, while the optical path from port B2 to port B1 is closed; the optical path from port B2 to port B3 is open, while the optical path from port B3 to port B2 is closed. The first beam splitter (53) includes an inlet C1, an inlet C2, an outlet C3, and an outlet C4, and the inlet C1 and the inlet C2 are optically connected to the outlet C3 and the outlet C4, respectively; the second beam splitter (54) includes an inlet D1, an inlet D2, an outlet D3, and an outlet D4, and the inlet D1 and the inlet D2 are optically connected to the outlet D3 and the outlet D4, respectively; Port A3 is connected to inlet C1; outlet C3 is connected to port B1; port B3 is connected to inlet D2; outlet D4 is connected to port A1; outlet D3 is connected to inlet C2; outlet C4 is connected to inlet D1. Port A2 is connected to the gain medium (2); port B2 is connected to the second reflective device (6).

4. The single-longitudinal-mode fiber laser based on a linear main cavity combined with a traveling-wave auxiliary cavity according to claim 3, characterized in that, The composite subcavity (5) also includes n third beam splitters, where n is an even number, and the n third beam splitters are arranged sequentially between the first beam splitter (53) and the second beam splitter (54). The third beam splitter includes inlet E1, inlet E2, outlet E3 and outlet E4, and inlet E1 and inlet E2 are respectively connected to the optical paths of outlet E3 and outlet E4. The n third beam splitters are respectively labeled k1, k2...k n In this configuration, the inlet E1 of the third beam splitter, numbered k1, is connected to the inlet C4; the outlet E3 of the third beam splitter, numbered k1, is connected to the inlet C2. The inlet E2 of the third beam splitter numbered k1 is connected to the outlet E3 of the third beam splitter numbered k2; the outlet E4 of the third beam splitter numbered k1 is connected to the inlet E1 of the third beam splitter numbered k2. And so on; Number k n-1 The third beam splitter's outlet E4 and the one numbered k n The third beam splitter's inlet E1 connection; numbered k n-1 The third beam splitter's inlet E2 and the one numbered k n The third beam splitter's output E3 is connected; Number k n The outlet E4 of the third beam splitter is connected to the inlet D1; it is numbered k. n The inlet E2 of the third beam splitter is connected to the outlet D3.

5. The single-longitudinal-mode fiber laser based on a linear main cavity combined with a traveling-wave auxiliary cavity according to claim 3, characterized in that, The laser also includes an optical wavelength division multiplexer (3), which is connected between the gain medium (2) and the composite subcavity (5). The pump source (4) is connected to the optical wavelength division multiplexer (3), which is used to couple the pump light generated by the pump source (4) into the resonant cavity.

6. The single-longitudinal-mode fiber laser based on a linear main cavity combined with a traveling-wave auxiliary cavity according to claim 5, characterized in that, The first reflective device (1) is a 90° Faraday rotating reflector.

7. The single-longitudinal-mode fiber laser based on a linear main cavity combined with a traveling-wave auxiliary cavity according to claim 5, characterized in that, The pump source (4) is a polarization-maintaining pump laser; The gain medium (2) is a polarization-maintaining highly doped optical fiber; The first reflecting device (1) is a 90° polarization-maintaining Faraday rotation mirror; The optical wavelength division multiplexer (3) is a polarization-maintaining optical wavelength division multiplexer; The first circulator (51) is a single-polarization circulator. Ports A1 to A2 allow slow-axis light to pass through while blocking fast-axis light. Ports A2 to A3 allow both slow-axis and fast-axis light to pass through, and the output axis of port A3 is coupled to the slow-axis output at 90°. Port A1 and outlet D4 are connected by a single-polarization fiber. Port A3 and inlet C1 are connected by a single-polarization fiber. Port A2 is connected to the optical wavelength division multiplexer (3) by a polarization-maintaining fiber. The first beam splitter (53) and the second beam splitter (54) are both single-polarization beam splitters, so as to allow slow-axis light to pass through between each inlet and outlet, and block fast-axis light; The second circulator (52) is a single polarization circulator, and ports B1 to B2 allow slow-axis light to pass through while blocking fast-axis light; ports B2 to B3 allow slow-axis light to pass through while blocking fast-axis light.

8. The single-longitudinal-mode fiber laser based on a linear main cavity combined with a traveling-wave auxiliary cavity according to claim 1, characterized in that, The second reflective device (6) is a single-polarization narrowband grating; An unpumped, single-polarization, low-doped fiber (7) is connected between the composite subcavity (5) and the second reflective device (6), and the single-polarization, low-doped fiber (7) serves as a saturable absorber.

9. The single-longitudinal-mode fiber laser based on a linear main cavity combined with a traveling-wave auxiliary cavity according to claim 3, characterized in that, The laser also includes a self-injection structure connected to the second reflective device (6), which is used to return a portion of the light emitted from the second reflective device (6) back into the resonant cavity.

10. The single-longitudinal-mode fiber laser based on a linear main cavity combined with a traveling-wave auxiliary cavity according to claim 9, characterized in that, The self-injection structure includes a third circulator (81), a tail divider (82), and an output head (83). The third circulator (81) includes port F1, port F2 and port F3, and the optical path from port F1 to port F2 is open, while the optical path from port F2 to port F1 is closed; the optical path from port F2 to port F3 is open, while the optical path from port F3 to port F2 is closed. The tail beam splitter (82) includes an inlet G1, an outlet G2 and an outlet G3, and the inlet G1 is optically connected to the outlet G2 and the outlet G3 respectively; The port F2 is connected to the second reflector (6); the port F3 is connected to the inlet G1; the outlet G2 is connected to the output head (83); and the outlet G3 is connected to the port F1.

11. The single-longitudinal-mode fiber laser based on a linear main cavity combined with a traveling-wave auxiliary cavity according to claim 10, characterized in that, The outlet G3 and the port F1 are connected by a single polarization fiber extension line (84).

12. The single-longitudinal-mode fiber laser based on a linear main cavity combined with a traveling-wave auxiliary cavity according to claim 10, characterized in that, The third circulator (81) and the tail beam splitter (82) are both single-polarization beam splitters, so as to allow slow-axis light to pass through between each inlet and outlet, and block fast-axis light; The splitting ratio of outlet G2 and outlet G3 is 90 / 10.

13. The single-longitudinal-mode fiber laser based on a linear main cavity combined with a traveling-wave auxiliary cavity according to claim 1, characterized in that, The first reflective device (1) has a reflectivity greater than 99%, and the first reflective device (1) integrates a filter for filtering out spontaneous radiation.

14. The single-longitudinal-mode fiber laser based on a linear main cavity combined with a traveling-wave auxiliary cavity according to claim 1, characterized in that, The second reflective device (6) has a reflectivity of 80% to 90% and a reflectivity bandwidth of no more than 0.1 nm.

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