High-stability self-mode-locking fiber laser based on phase modulation

Abstract

The invention discloses a high-stability self-mode-locking optical fiber laser based on phase modulation. The high-stability self-mode-locking optical fiber laser is composed of an optical pumping source, a wavelength division multiplexer, an isolator, a coupler, a polarization beam splitter, a 45-degree Faraday rotating mirror, a 90-degree Faraday rotating mirror, a non-polarization-maintaining single-mode optical fiber, a polarization-maintaining single-mode optical fiber and an erbium-doped optical fiber. The wavelength division multiplexer, the isolator and the coupler can be integrated into a three-in-one device and can also be placed separately. The intra-cavity tail fiber and the output end are arranged to be polarization-maintaining single-mode optical fibers and are connected with a pumping source; polarization-maintaining single-mode optical fibers are arranged at the beam combining end and the beam splitting end of the polarization beam splitter; in the 45-degree Faraday rotating mirror, a tail fiber on one side is arranged to be a non-polarization-maintaining single-mode fiber, and a polarization-maintaining single-mode fiber is arranged on the other side. The saturable absorption effect is achieved through polarization state changes related to light intensity when a light field is transmitted in an optical fiber, but different from a traditional NPR, the whole structure does not need interference of a polarization controller, and the expected effect can be achieved only by controlling the input light intensity.

Description

Technical Field

[0001] The invention belongs to the technical field of lasers, and in particular relates to a highly stable self-mode-locked optical fiber laser based on phase modulation. Background Art

[0002] In recent years, the rapid development of fiber laser technology has attracted widespread attention for its many advantages. Its advantages such as stability, high energy, high peak power, and ultra-short pulse width have important applications in scientific research, biomedical imaging, micromachining, optical measurement, etc. Among them, passively mode-locked fiber lasers can be used as ideal and stable ultra-short pulse sources due to their compact structure, sturdiness, versatility, high integration, and high degree of freedom. Compared with fiber lasers, solid-state lasers can also produce ultra-short pulses, but their complex structure, large size, and high cost are disadvantages that cannot be ignored.

[0003] As the application demand for passively mode-locked fiber lasers increases year by year, achieving low system loss, high output energy efficiency, simple debugging, steadily improving system reliability, and making the overall structure more perfect have always been the focus of research and breakthroughs in the field of fiber lasers.

[0004] There are two principles for passive mode-locked fiber lasers: true saturable absorption mechanism and quasi-saturable absorption mechanism. When developing commercial mode-locked light sources, people often use true passive saturable absorbers, such as semiconductor saturable absorber mirrors (SESAMs) and single-walled carbon nanotubes (SWNTs), because of the unstable environment of nonlinear polarization rotation mode locking (NPR) and nonlinear amplified ambient mode locking (NALM). However, such materials often lead to non-saturated loss and short life. If high-quality materials are selected, there will be a shortcoming of high cost, so the true saturable absorption mechanism is also greatly limited.

[0005] For the quasi-saturable absorption mechanism, in the passive mode-locking technology that has been reported so far, the NALM-based mode-locked laser will be set to the "8-figure" type or the "9-figure" type. They have poor ability to resist external interference. Many mainstream "9-figure cavities" will add free space structures, which increases the overall instability of the system. Therefore, obtaining an ultrashort pulse fiber laser with all-fiber resonance, many adjustable parameters in the cavity, low cost, and self-starting is the current research focus. Summary of the invention

[0006] Based on the above-mentioned shortcomings of the prior art, the present invention provides a highly stable self-mode-locked fiber laser based on phase modulation, aiming to realize an all-fiber, low-cost, high-stability, easy-to-tune, and self-starting mode-locked laser.

[0007] A highly stable self-locked mode fiber laser based on phase modulation, which consists of an optical pump source, a wavelength division multiplexer, an isolator, a coupler, a polarization beam splitter, a 45° Faraday rotation mirror, a 90° Faraday rotation reflector, a non-polarization-maintaining single-mode fiber, a polarization-maintaining single-mode fiber and an erbium-doped fiber, wherein the wavelength division multiplexers are all 980 / 1550 wavelength division multiplexers.

[0008] The wavelength division multiplexer, isolator and coupler can be integrated into a three-in-one device or placed separately. The pigtail and output end of the cavity are both set as polarization-maintaining single-mode optical fibers, and the pump end connected to the HI1060 optical fiber is connected to the pump source; the beam combining end and the beam splitting end of the polarization beam splitter are both set as polarization-maintaining single-mode optical fibers; in the 45° Faraday rotator, one side of the pigtail is set as a non-polarization-maintaining single-mode optical fiber, and the other side is set as a polarization-maintaining single-mode optical fiber.

[0009] The operation process of the laser is as follows: the optical pump source provides a light source that enters the laser cavity through a wavelength division multiplexer, achieves gain light amplification through erbium-doped optical fiber, generates signal light, and the isolator inside the unidirectional ring cavity forces the signal light to be transmitted in one direction only. After passing through the polarization beam splitter, it passes through a 45° Faraday rotator, is reflected by a 90° Faraday rotation mirror, and returns to the ring cavity through a reverse path, thereby obtaining a complete intracavity cycle. In this process, the nonlinear polarization rotation characteristics are used to obtain a mode-locked pulse, and finally a pulse light is emitted from the output end.

[0010] The mode-locking principle of the laser is as follows: the horizontal linear polarized light emitted by the polarization beam splitter undergoes a 90° rotation and two 45° rotations, and finally returns to the horizontal polarization direction; if the nonlinear effect is not considered, the horizontal polarized light cannot enter the polarization beam splitter again, because the transmission direction is vertical, and the final transmittance is 0; if the nonlinear effect is considered, the nonlinear phase shift allows the polarized light at this time to be decomposed into a horizontal polarization component E and a vertical polarization component E. Since the transmission direction of the polarization beam splitter is vertical, only the vertical polarization component E can enter the ring cavity to achieve gain amplification. Due to the twisted optical fiber, the slow axis directions at both ends match the two output directions of the polarization beam splitter, and finally the polarization beam splitter is emitted as horizontal polarized light.

[0011] To sum up, the greater the light intensity, the stronger the nonlinear effect, the easier it is for light to pass through, weak light is continuously weakened, and strong light is continuously amplified, satisfying the saturable absorption mechanism.

[0012] The specific light field changes are as follows:

[0013] 1. After the light has run in the cavity for many times, the signal light emitted from the beam combining end of the polarization beam splitter is used as the initial light field. Since the transmission direction is horizontal, it is assumed to be horizontally polarized light with an amplitude of E0.

[0014] 2. The horizontally polarized light keeps its polarization state unchanged after passing through the polarization-maintaining fiber. After reaching the 45° Faraday rotator, its direction is rotated 45° in a fixed direction. After that, it runs for a certain distance on the single-mode fiber, during which polarization rotation occurs. When it reaches the 90° Faraday rotator, it may evolve into other polarization states. Due to the rotation of the mirror, it rotates 90° in the same direction again and passes through the same length of single-mode fiber path in the opposite direction. The linear phase shift accumulated in the single-mode fiber is finally offset. After being rotated 45° in the same direction again, the light field is left with only nonlinear phase shift and linear phase shift of π / 2. The polarization direction is vertical and the amplitude is in is the nonlinear phase shift accumulated by the non-polarization-maintaining single-mode fiber; it enters the ring cavity again through the polarization beam splitter, accumulates part of the nonlinear phase shift and then emits to the starting light field, and the final amplitude is Where G is the magnification, It is the accumulated nonlinear phase shift of polarization-maintaining single-mode fiber, and the polarization direction is horizontal.

[0015] 3. After a complete cycle, the transmittance function is obtained Where G is the magnification, It is the overall nonlinear phase shift of the light field passing through the polarization-maintaining and non-polarization-maintaining optical fibers.

[0016] 4. It is proportional to the light intensity. The transmittance obtained at this time is a periodic function of the incident light power P. The transmittance increases with the increase of the incident light intensity, which makes it easy for the pulse to work in the saturable absorption region.

[0017] Furthermore, the continuously sharpened pulses are subject to positive feedback when passing through the gain fiber, the laser is continuously amplified, and the entire fiber loop is equivalent to a saturable absorber.

[0018] Furthermore, the mode locking effect of the fiber laser and the quality of the output pulse laser are determined by the size of the pump light, the length of the gain fiber and the output coupling ratio.

[0019] Furthermore, since the overall laser is arranged in a "9" shape, in addition to the reflector that can confine the pulse light to circulate in the cavity, the ring cavity can also act as a 90° Faraday rotation mirror and accumulate nonlinear phase shift.

[0020] The design of the instrument makes full use of the nonlinear polarization rotation effect, and the overall design is ingenious and flexible.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] 1. The present invention uses the polarization state change related to the light intensity when the light field is transmitted in the optical fiber to achieve the saturable absorption effect. However, unlike the traditional NPR, the overall structure does not require the intervention of the polarization controller, and only needs to control the input light intensity to achieve the expected effect;

[0023] 2. Compared with the resonant cavity with a spatial coupling structure, the all-fiber resonant cavity provided in the present invention is easy to package, simple in structure, compact, maintenance-free, and has extremely high coupling efficiency and high quality of the output laser beam, which greatly improves the stability and anti-interference ability of the laser;

[0024] 3. Compared with the "8" cavity and ring cavity laser, the present invention can greatly reduce the overall cavity length, which is conducive to generating laser output with higher repetition frequency, is not easy to lose lock, has a simple structure and is easy to build. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The main structure of the fiber laser disclosed in the present invention is shown in FIG. Figure 1 ;

[0026] Figure 2 This is a schematic diagram of the structure of the fiber laser disclosed in Example 1;

[0027] Figure 3 Schematic diagram of the polarization-maintaining unidirectional ring structure and working principle of the polarization beam splitter;

[0028] Figure 4 A schematic diagram of the polarization evolution of the signal light of the optical fiber laser disclosed in Example 1;

[0029] Figure 5 This is a schematic diagram of the structure of the fiber laser disclosed in Example 2;

[0030] Figure 6 This is a schematic diagram of integrating a wavelength division multiplexer, an isolator, and a coupler into a three-in-one device based on Example 2;

[0031] Figure 7 is a curve diagram of light transmittance changing with nonlinear phase shift;

[0032] Figure 8 The main structure of the fiber laser disclosed in the present invention is shown in FIG. Figure 2 ;

[0033] Fig. 9 This is a schematic diagram of the polarization-maintaining unidirectional ring structure and working principle of the polarization-maintaining circulator;

[0034] Fig.10 This is a schematic diagram of the structure of the fiber laser disclosed in Example 3;

[0035] Fig.11Schematic diagram of the polarization evolution of the signal light of the fiber laser disclosed in Example 1.

[0036] Numbers in the figure: 1-isolator, 2-polarization beam splitter, 3-45° Faraday rotator, 4-90° Faraday rotation mirror, 5-polarization-maintaining single-mode fiber, 6-non-polarization-maintaining single-mode fiber, 7-polarization-maintaining circulator, 8-optical pump source, 9-wavelength division multiplexer, 10-erbium-doped fiber, 11-coupler, 12-pump protector. DETAILED DESCRIPTION

[0037] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. The embodiments of the present invention are provided for the purpose of illustration and description, and are not intended to be exhaustive or to limit the present invention to the disclosed forms. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiments are selected and described in order to better illustrate the principles and practical applications of the present invention, and to enable those of ordinary skill in the art to understand the present invention and thereby design various embodiments with various modifications suitable for specific uses.

[0038] Example 1

[0039] Highly stable self-mode-locked fiber lasers based on phase modulation, such as Figure 1 As shown, it includes a laser cavity, which includes a unidirectional ring cavity and a linear cavity, and the unidirectional ring cavity and the linear cavity are connected through a polarization beam splitter 2. The unidirectional ring cavity is composed of polarization-maintaining single-mode optical fibers 5 connected end to end, and the polarization-maintaining single-mode optical fibers 5 are provided with an isolator 1.

[0040] The linear cavity comprises a 45° Faraday rotator 3 and a 90° Faraday rotating mirror 4. The 45° Faraday rotator 3 is connected to the polarization beam splitter 2 via a polarization-maintaining single-mode optical fiber 5. The 45° Faraday rotator 3 is connected to the 90° Faraday rotating mirror 4 via a non-polarization-maintaining single-mode optical fiber 6.

[0041] The gain, pump, and output components of the fiber laser can be placed in a unidirectional ring cavity or a linear cavity. The optical operation process is as follows: the light source is provided by an optical pump source 8, enters the laser cavity through a wavelength division multiplexer 9, and is amplified by the erbium-doped fiber 10 to generate a signal light, which passes through a 45° Faraday rotator and a 90° Faraday rotating mirror to accumulate a phase difference of π.

[0042] The phase shift caused by the linear polarization rotation during the transmission of the non-polarization-maintaining single-mode optical fiber 6 is offset. Due to the existence of nonlinear polarization rotation, the polarization state of the light field will not return to the initial state, and there will be a projection component in the transmission direction of the polarization beam splitter 2. This part returns to the unidirectional ring cavity to operate and obtain a complete intra-cavity cycle. The greater the initial light intensity, the greater the projection component will be, and the transmittance tends to increase with the light intensity, thus forming a saturable absorption effect, and the obtained mode-locked pulse is output by the coupler.

[0043] Reference Figure 2 , the pump and gain components are arranged in the linear cavity part, and the output components are arranged in the unidirectional ring cavity part. The wavelength division multiplexer 9 is located between the 45° Faraday rotator 3 and the 90° Faraday rotating reflector 4, and is used to introduce the pump light into the cavity; the incident end of the wavelength division multiplexer 9 is provided with an optical pump source 8, and an erbium-doped optical fiber 10 is arranged between the wavelength division multiplexer 9 and the 90° Faraday rotating reflector 4 to achieve gain amplification; the unidirectional ring cavity is provided with a coupler 11 and an isolator 1, and the coupler 11 is used to output observable optical pulses.

[0044] The wavelength division multiplexer 9 is used to guide the pump light into the cavity and achieve gain amplification through the erbium-doped fiber 10. Since the signal light will continuously travel back and forth in the cavity, in order to ensure that the reflected light pulse does not damage the optical pump source 8, a pump protector 12 is also provided on the input path.

[0045] Since the linear cavity structure on the outside cancels out the linear polarization rotation of the signal light on the non-polarization-maintaining single-mode fiber 6, the two side beam splitting ends of the polarization beam splitter 2 are fused together to form a ring loop, which is equivalent to rotating the polarization direction of the light by 90° before emitting it, which is perpendicular to the transmission direction of the beam combining end entering the beam splitting end. The isolator 1 ensures the unidirectional transmission of the signal light in the polarization-maintaining ring cavity part; the coupler 11 is used to output observable optical pulses.

[0046] Reference Figure 3 , the prism is the polarization beam splitter 2, the beam combining end is the polarization-maintaining single-mode optical fiber 5 working in dual axes, the beam splitting end is the polarization-maintaining single-mode optical fiber 5 working in the slow axis, the polarization states of the respective light at the beam splitting end are aligned with the slow axis direction of the polarization-maintaining optical fiber, and then the fast and slow axes of the two polarization-maintaining optical fibers of the split beam are aligned and fused. Taking vertical linear polarized light as an example, after entering the polarization beam splitter 2 from the beam combining end, it can only flow into the vertical direction of the beam splitting end, and the horizontal component is 0. Due to the limitation of polarization-maintaining unidirectional transmission, linear polarized light can only be emitted from the horizontal direction, and the emitted linear polarized light works on the fast axis of the beam combining end. If the initial light field is in other polarization states, it is transmitted as the incident projection component.

[0047] Reference Figure 4, taking the linear polarized light emitted from the beam combining end of the polarization beam splitter 2 as the starting point, if the influence of nonlinearity is ignored, under ideal conditions, the polarization direction of the signal light in the linear cavity undergoes multiple linear rotations, including two 45° and one 90°, and finally returns to the initial polarized light direction. The polarized light in this state cannot pass through the polarization beam splitter 2, resulting in a transmittance of 0; if the influence of nonlinearity is considered, the Kerr effect will cause the polarization state of the light field to change randomly, and the horizontal and vertical components will continue to change, and may exist in an elliptical polarization state. When entering the polarization beam splitter 2, the light field will have a projection component in the transmission direction, and enter the ring cavity as partially polarized light to continue evolving. There is also nonlinear polarization rotation in the polarization-maintaining fiber, and then it is emitted from the polarization beam splitter 2 again. The greater the initial light intensity, the greater the projection component. From this, it can be seen that the transmittance tends to increase with the light intensity.

[0048] Example 2

[0049] In Example 1, the gain fiber and the wavelength division multiplexer 9 are arranged on a non-polarization-maintaining path. The advantages are low cost, simple manufacturing process, and easy production. The disadvantages are that it is difficult to control the cavity length, and cutting the wavelength division multiplexer pigtail too short may cause inconvenience to welding. At the same time, due to the different types of erbium-doped fiber and polarization-maintaining single-mode fiber, light needs to pass through the cavity length twice in one circulation in the linear cavity, and the overall dispersion value of the laser is difficult to accurately grasp. At the same time, since no isolator is set in the linear cavity, a pump protector is required to isolate the hazards caused by the reflected light. The addition of the pump protector increases the complexity of the overall laser.

[0050] In order to avoid the above defects, some changes are made to the laser. Different from Example 1, in this embodiment, the pump and gain components are arranged in the unidirectional ring cavity part, the output component is arranged in the linear cavity part, and the other structures are the same as Example 1.

[0051] Reference Figure 5 The functions of the optical fiber components in this structure are the same as those in Example 1, and the polarization evolution is also the same as Figure 4 The same as shown in the figure also utilizes the characteristics of nonlinear polarization rotation. All devices work together to obtain a saturable absorption effect and obtain an ultrashort pulse output.

[0052] In this embodiment, the erbium-doped fiber 10 and the wavelength division multiplexer 9 are arranged on the polarization-maintaining path. After reducing the length of the ring cavity, the ring cavity part can be temporarily fixed, and the control of the remaining cavity length can be adjusted by the external line cavity, which greatly reduces the workload of modification and optimization. At the same time, the control of dispersion can be achieved by the dispersion-shifted fiber. With the combined effect of the dispersion-shifted fiber and the single-mode fiber, the dispersion value can be adjusted to the size we need. However, due to the large number of polarization-maintaining devices, the cost of this embodiment is relatively high.

[0053] If you want to reduce the complexity of the laser and shorten the cavity length, you can integrate the isolator, wavelength division multiplexer, and coupler into a three-in-one device, such as Figure 6 As shown; if you want to flexibly adjust the laser quality, you can also place them separately.

[0054] Figure 7 This is the light transmittance curve of Example 2 with and without the addition of the 45° Faraday rotator 3. When the addition is performed, the transmittance function is: When no addition is made, the transmittance function is T2= The horizontal axis is the magnitude of the nonlinear phase shift, and the vertical axis is the magnitude of the transmittance.

[0055] The two curves show how the transmittance changes with light intensity (assuming G=1): the dotted line is the situation when a 45° Faraday rotator is not added. The transmittance is the largest in the absence of light, and the transmittance decreases as the light intensity increases. When a certain intensity is reached, the transmittance is zero, and then it rises again. The back-and-forth fluctuation trend changes with a cosine trend; the solid line is the situation when a 45° Faraday rotator is added. Contrary to the above changes, the transmittance is the smallest in the absence of light, and the transmittance increases with the light intensity. It also changes with a cosine trend, and is easy to act on the saturable absorption region to achieve the locking condition.

[0056] It can be seen that the addition of the 45° Faraday rotator 3 makes the overall design structure more ingenious, makes the operating principle of the laser more understandable, and achieves the ideal effect. Similarly, other angle rotators can also be tried to translate the transmittance curve.

[0057] Example 3

[0058] Reference Figure 8 , the 45° Faraday rotator 3 and the polarization beam splitter 2 in embodiments 1 and 2 can be replaced by a polarization-maintaining circulator 7, and the polarization-maintaining fiber in the linear cavity is fused with the non-polarization-maintaining fiber. The optical operation process is as follows: the linear phase shift of the gain-amplified signal light generated during the single-mode fiber transmission process is offset, leaving a 90° rotation effect. Due to the existence of nonlinear polarization rotation, the polarization direction of the light field will not be completely rotated by 90°, and there will be a projection component in the transmission direction of the polarization-maintaining circulator. Similarly, the transmittance tends to increase with the light intensity, which is similar to Figure 1 The device has the same effect.

[0059] Reference Fig. 9 The polarization-maintaining circulator 7 has three ports, namely port a, port b, and port c, all of which are set as polarization-maintaining single-mode optical fibers working on the slow axis. The signal light entering port a can only be emitted from port b, the signal light entering port b can only be emitted from all ports c, and the signal light entering port c can only be emitted from port a. When port b is connected to port c, the signal light enters from port a and then all exits from port a. When the polarization-maintaining fast and slow axes are aligned during the fusion, the horizontally polarized light reflected back is still in the horizontal polarization direction.

[0060] Reference Fig.10 In this embodiment, a polarization-maintaining circulator 7 is used to achieve the same mode-locking effect as in embodiments 1 and 2. The function of the optical fiber device is the same as in embodiments 1 and 2. In addition, the isolator 1 is not added to the unidirectional ring cavity. This is because the unidirectional transmission characteristics of the polarization-maintaining circulator 7 have the same effect. In this embodiment, the pump and gain components are placed on the non-polarization-maintaining path. Unlike the polarization beam splitter 2, which must add an isolation device, there is only an optical fiber connection in the unidirectional ring cavity. This can minimize the length of the ring cavity and increase the pulse repetition frequency. The joint action of the polarization beam splitter 2 and the 45° Faraday rotator in the equivalent structure 2 of the polarization-maintaining circulator 7 of this embodiment is to maintain the polarization direction unchanged reflection and accumulated nonlinear phase shift.

[0061] Reference Fig.11 ,by Fig. 9 Taking the linearly polarized light emitted from the middle port a as the starting point, if the influence of nonlinearity is ignored, in an ideal state, the polarization direction of the signal light in the linear cavity undergoes a 90° linear rotation, and the linear phase shift during the single-mode optical fiber transmission is offset, and the final polarization direction is perpendicular to the initial direction. The polarized light in this state cannot pass through the polarization-maintaining circulator, resulting in a transmittance of 0; if the influence of nonlinearity is considered, the polarization state changes randomly, and the light field will have a projection component in the transmission direction when entering the polarization-maintaining circulator, so that part of the polarized light enters the ring cavity and undergoes nonlinear polarization rotation again, and part of it exits the polarization-maintaining circulator.

[0062] Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field and related fields without creative work should fall within the scope of protection of the present invention.

Claims

1. Highly stable self-mode-locked fiber laser based on phase modulation, characterized by: It comprises a laser cavity, which comprises a unidirectional ring cavity and a linear cavity, wherein the unidirectional ring cavity and the linear cavity are connected via a polarization beam splitter (2); The unidirectional ring cavity is composed of polarization-maintaining single-mode optical fibers (5) connected end to end, and is provided with an isolator (1) for ensuring unidirectional transmission of signal light; The linear cavity comprises a 45° Faraday rotator (3) and a 90° Faraday rotating reflector (4); the 45° Faraday rotator (3) and the polarization beam splitter (2) are connected via a polarization-maintaining single-mode optical fiber (5); and the 45° Faraday rotator (3) and the 90° Faraday rotating reflector (4) are connected via a non-polarization-maintaining single-mode optical fiber (6); The pump, gain and output components are arranged in the linear cavity part / unidirectional ring cavity part, and the optical operation process is as follows: an optical pump source (8) is arranged to provide a light source, which enters the laser cavity through a wavelength division multiplexer (9), and achieves gain light amplification through an erbium-doped optical fiber (10), generates signal light, passes through a 45° Faraday rotator (3) and a 90° Faraday rotating reflector (4), and accumulates a phase difference of π; The phase shift caused by the linear polarization rotation during the transmission of the non-polarization-maintaining single-mode optical fiber (6) is offset. Due to the existence of nonlinear polarization rotation, the polarization state of the light field will not be restored to the initial state, and there will be a projection component in the transmission direction of the polarization beam splitter (2). The projection component returns to the unidirectional ring cavity to operate, and a complete intra-cavity cycle is obtained. The greater the initial light intensity, the greater the projection component will be, and the transmittance tends to increase with the light intensity, so a saturable absorption effect is formed, and the obtained mode-locked pulse is output by the coupler.

2. The highly stable self-mode-locked fiber laser based on phase modulation according to claim 1, characterized in that: The pump and gain components are arranged in the linear cavity part, and the output component is arranged in the unidirectional ring cavity part; The wavelength division multiplexer (9) is located between the 45° Faraday rotator (3) and the 90° Faraday rotation mirror (4) and is used to guide the pump light into the cavity; an optical pump source (8) is provided at the incident end of the wavelength division multiplexer (9), and an erbium-doped optical fiber (10) is provided between the wavelength division multiplexer (9) and the 90° Faraday rotation mirror (4) to achieve gain amplification; A coupler (11) and an isolator (1) are arranged in the unidirectional ring cavity, and the coupler (11) is used to output observable optical pulses.

3. The highly stable self-mode-locked fiber laser based on phase modulation according to claim 1, characterized in that: The pump and gain components are arranged in the unidirectional ring cavity part, and the output component is arranged in the linear cavity part; The wavelength division multiplexer (9) and erbium-doped optical fiber (10) of the laser are serially connected to the polarization-maintaining single-mode optical fiber (5) of the unidirectional ring cavity, and an optical pump source (8) is arranged at the incident end of the wavelength division multiplexer (9); The coupler (11) is serially connected to a non-polarization-maintaining single-mode optical fiber (6) between a 90° Faraday rotation mirror (4) and a 45° Faraday rotator (3), and by utilizing the characteristics of nonlinear polarization rotation, all devices work together to obtain a saturable absorption effect, thereby obtaining an ultrashort pulse output.

4. The highly stable self-mode-locked fiber laser based on phase modulation according to claim 1, characterized in that: The isolator (1), the wavelength division multiplexer (9) and the coupler (11) are integrated into a three-in-one device, and are all located in the unidirectional ring cavity part.

5. The highly stable self-mode-locked fiber laser based on phase modulation according to claim 1, characterized in that: A pump protector (12) is provided on the input path between the optical pump source (8) and the wavelength division multiplexer (9).

6. The highly stable self-mode-locked fiber laser based on phase modulation according to claim 1, characterized in that: The polarization beam splitter (2) is a prism, the beam combining end is a polarization-maintaining single-mode optical fiber working in dual axes, the beam splitting end is a polarization-maintaining single-mode optical fiber working in slow axis, the polarization states of the respective light at the beam splitting end are aligned with the slow axis direction of the polarization-maintaining optical fiber, and then the fast and slow axes of the two polarization-maintaining optical fibers of the split beams are aligned and fused, after the vertical linear polarization light enters the polarization beam splitter from the beam combining end, it can only flow into the beam splitting end in the vertical direction, and the horizontal component is 0. Due to the limitation of polarization-maintaining unidirectional transmission, the linear polarization light can only be emitted in the horizontal direction, and the emitted linear polarization light works on the fast axis of the beam combining end. If the initial light field is in other polarization states, it is transmitted as the incident projection component.

7. Highly stable self-mode-locked fiber laser based on phase modulation, characterized by: The 45° Faraday rotator (3) and the polarization beam splitter (2) in any one of claims 1 to 5 are replaced by a polarization-maintaining circulator (7), and there is no isolator (1) in the unidirectional ring cavity; A wavelength division multiplexer (9), an erbium-doped optical fiber (10) and a coupler (11) are sequentially connected in series between the polarization-maintaining circulator (7) and the 90° Faraday rotation mirror (4); an optical pump source (8) is installed at the incident end of the wavelength division multiplexer (9); the connection ends of the unidirectional ring cavity and the linear cavity corresponding to the polarization-maintaining circulator (7) are both polarization-maintaining single-mode optical fibers (5); and the linear cavity adopts a non-polarization-maintaining single-mode optical fiber (6).

8. The highly stable self-mode-locked fiber laser based on phase modulation according to claim 7, characterized in that: The polarization-maintaining circulator (7) has three ports, namely port a, port b, and port c, all of which are polarization-maintaining optical fibers operating on a slow axis. Signal light entering port a can only be emitted from port b, signal light entering port b can only be emitted from port c, and signal light entering port c can only be emitted from port a. When port b is connected to port c, signal light enters from port a and then all exits from port a. When the polarization-maintaining fast and slow axes are aligned during fusion splicing, the horizontally polarized light reflected back is still in the horizontal polarization direction.

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