Highly Stable Self-Mode-Locked Fiber Laser Based on Phase Modulation

Abstract

The present invention discloses a highly stable self - mode - locking fiber laser based on phase modulation, which is composed of an optical pump source, a wavelength - division multiplexer, an isolator, a coupler, a polarization beam splitter, a 45° Faraday rotator mirror, a 90° Faraday rotation reflector mirror, a non - polarization - maintaining single - mode fiber, a polarization - maintaining single - mode fiber, and an erbium - doped fiber. The wavelength - division multiplexer, the isolator, and the coupler can be integrated into a three - in - one device or placed separately. The intracavity pigtail and the output end are both set as polarization - maintaining single - mode fibers and are connected to the pump source; the polarization - maintaining single - mode fibers are set at both the combining end and the splitting end of the polarization beam splitter; in the 45° Faraday rotator mirror, one side of the pigtail is set as a non - polarization - maintaining single - mode fiber and the other side is set as a polarization - maintaining single - mode fiber. The present invention utilizes the polarization - state change related to the light intensity when the light field propagates in the fiber to achieve the saturable absorption effect. However, different from the traditional NPR, the overall structure does not require the interference of a polarization controller, and only by controlling the input light intensity can the expected effect be achieved.

Description

Technical Field

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

[0002] In recent years, the rapid development of fiber laser technology has attracted wide attention due to its many advantages. Its advantages such as stability, high energy, high peak power, and ultrashort pulse width have important applications in fields such as scientific research, biomedical imaging, micro-machining, and optical measurement. Among them, passive mode-locking fiber lasers, due to their characteristics such as compact structure, firmness, versatility, high integration, and large degrees of freedom, can be regarded as ideal stable ultrashort pulse sources. Compared with fiber lasers, solid-state lasers can also generate ultrashort pulses, but their disadvantages such as complex structure, large volume, and high cost cannot be ignored.

[0003] With the increasing application requirements of passive mode-locking fiber lasers year by year, achieving low system loss, high output energy efficiency, simple debugging, steadily improving the reliability of the system, and making the overall structure more perfect have always been the key points of research and breakthrough in the field of fiber lasers.

[0004] There are two principles for passive mode-locking fiber lasers: the true saturable absorption mechanism and the quasi-saturable absorption mechanism. When developing commercial mode-locking light sources, people often use real passive saturable absorbers such as semiconductor saturable absorption mirrors (SESAMs), single-walled carbon nanotubes (SWNTs), etc. because of the unstable environments of nonlinear polarization rotation mode-locking (NPR) and nonlinear amplifying loop mirror (NALM). However, such materials often lead to non-saturable losses and short lifetimes. If high-quality materials are selected, there will be the shortcoming of high cost. Therefore, the true saturable absorption mechanism also has great limitations.

[0005] For the quasi-saturable absorption mechanism, among the currently reported passive mode-locking technologies, mode-locking lasers based on NALM are set as the "figure-eight" type or the "figure-nine" type. They have poor anti-interference ability against the outside world. Many mainstream "figure-nine cavities" add free-space structures, which increases the overall instability of the system. Therefore, obtaining an all-fiber resonator, having many tunable parameters in the cavity, low cost, and self-starting ultrashort pulse fiber lasers is the current focus of research. Summary of the Invention

[0006] Based on the deficiencies of the above-mentioned existing technologies, the present invention provides a highly stable self-mode-locking fiber laser based on phase modulation, aiming to achieve a mode-locking laser that is all-fiber, low-cost, highly stable, easy to tune, and self-starting.

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

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

[0009] The operation process of the laser is as follows: The optical pump source provides light, which enters the laser cavity through the wavelength - division multiplexer. The gain optical amplification is realized through the erbium - doped fiber to generate signal light. The isolator inside the unidirectional ring cavity forces the signal light to transmit only in one direction. After passing through the polarization beam splitter, it passes through the 45° Faraday rotator and is reflected by the 90° Faraday rotation reflector mirror and returns to the ring cavity through the reverse path, thus obtaining a complete intracavity cycle. In this process, the mode - locking pulse is obtained by using the nonlinear polarization rotation characteristic, and finally the pulsed light is emitted from the output end.

[0010] The mode - locking principle of the laser is as follows: The horizontally polarized light emitted by the polarization beam splitter undergoes one 90° rotation and two 45° rotations and finally returns to the horizontal polarization direction. Without considering the nonlinear effect, the horizontally polarized light cannot enter the polarization beam splitter again because the transmission direction is the vertical direction and the final transmittance is 0. If the nonlinear effect is considered, the nonlinear phase shift makes the polarized light at this time can be decomposed into a horizontal polarization component E and a vertical polarization component E. Since the transmission direction of the polarization beam splitter is the vertical direction, only the vertical polarization component E can enter the ring cavity to realize gain amplification. Due to the twisted fiber making the slow - axis directions at both ends match the two output directions of the polarization beam splitter, finally, the horizontally polarized light is emitted from the polarization beam splitter.

[0011] To sum up, the greater the light intensity, the stronger the nonlinear effect, the easier the light passes through, the weak light is continuously weakened, and the strong light is continuously amplified, meeting the saturable absorption mechanism.

[0012] The specific change of the optical field is as follows:

[0013] 1. After the light runs in the cavity for many times, taking the signal light emitted from the combining end of the polarization beam splitter as the initial optical field, since the transmission direction is the horizontal direction, let it be horizontally polarized light with an amplitude of E0.

[0014] 2. The horizontally polarized light passes through the polarization-maintaining fiber and keeps its polarization state unchanged. After reaching the 45° Faraday rotator mirror, its direction is rotated by 45° in a fixed direction. Then it travels a certain distance in the single-mode fiber, during which polarization rotation occurs. When it reaches the 90° Faraday rotator reflector, it may evolve into other polarization states. Due to the rotation of the reflector, it is rotated by 90° again in the same direction. After passing through the single-mode fiber path of the same length in the reverse direction, the linear phase shift accumulated in the single-mode fiber is finally cancelled out. After being rotated by 45° again in the same direction, only the non-linear phase shift and the linear phase shift of π / 2 remain in the optical field, the polarization direction is the vertical direction, and the amplitude size is where is the magnitude of the non-linear phase shift accumulated in the non-polarization-maintaining single-mode fiber; it enters the ring cavity again through the polarization beam splitter, accumulates part of the non-linear phase shift and then exits to the initial optical field. Finally, the amplitude size is where G is the amplification factor, is the magnitude of the non-linear phase shift accumulated in the polarization-maintaining single-mode fiber, and the polarization direction is the horizontal direction.

[0015] 3. After a complete round trip, the transmittance function is obtained, where G is the amplification factor, is the overall non-linear phase shift of the optical field passing through the polarization-maintaining and non-polarization-maintaining fibers.

[0016] 4. is proportional to the light intensity. At this time, the obtained transmittance is a periodic function of the incident optical power P. The size of the transmittance increases with the increase of the incident light intensity, which easily makes the pulse work in the saturable absorption region.

[0017] Furthermore, the continuously sharpened pulse will be subject to positive feedback when passing through the gain fiber, and the laser is continuously amplified. 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 pulsed laser are determined by the pump light size, the gain fiber length, and the output coupling ratio.

[0019] Furthermore, since the overall laser is set in the shape of a "9", in addition to the reflector being able to limit the pulsed light to circulate in the cavity, the ring cavity can also act as a 90° Faraday rotator reflector and can also accumulate non-linear phase shift.

[0020] The design of this instrument fully utilizes the non-linear 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 reflected in:

[0022] 1. The present invention utilizes the polarization state change related to the light intensity during the transmission of the optical field in the optical fiber to achieve the saturable absorption effect. Different from the traditional NPR, the overall structure does not require the interference of a polarization controller, and only by controlling the input light intensity can the expected effect be achieved;

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

[0024] 3. Compared with the "8"-shaped cavity and the ring cavity laser, the present invention can greatly reduce the overall cavity length, which is beneficial to generating a laser output with a higher repetition frequency, is not easy to lose lock, has a simple structure and is convenient to build. Brief Description of the Drawings

[0025] Figure 1 Schematic diagram of the main structure of the fiber laser disclosed in the present invention Figure 1 ;

[0026] Figure 2 Schematic diagram of the structure of the fiber laser disclosed in Embodiment 1;

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

[0028] Figure 4 Schematic diagram of the polarization evolution of the signal light of the fiber laser disclosed in Embodiment 1;

[0029] Figure 5 Schematic diagram of the structure of the fiber laser disclosed in Embodiment 2;

[0030] Figure 6 Schematic diagram of integrating the wavelength division multiplexer, isolator, and coupler into a three-in-one device based on Embodiment 2;

[0031] Figure 7 Graph of the change of light transmittance with the non-linear phase shift;

[0032] Figure 8 Schematic diagram of the main structure of the fiber laser disclosed in the present invention Figure 2 ;

[0033] Figure 9 Schematic diagram of the polarization-maintaining unidirectional ring structure and working principle of the polarization-maintaining circulator;

[0034] Figure 10 Schematic diagram of the structure of the fiber laser disclosed in Embodiment 3;

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

[0036] Reference numerals 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 implementation manners

[0037] The present invention will be further described in detail below with reference to the accompanying drawings and specific implementation manners. The embodiments of the present invention are given for purposes of illustration and description, and are not exhaustive or limit the present invention to the disclosed form. Many modifications and variations are obvious to those of ordinary skill in the art. The embodiments are selected and described to better illustrate the principles and practical applications of the present invention, and enable those of ordinary skill in the art to understand the present invention and thus design various embodiments with various modifications suitable for specific purposes.

[0038] Embodiment 1

[0039] A highly stable self-mode-locked fiber laser based on phase modulation, as Figure 1 shown, includes a laser cavity, which includes a unidirectional ring cavity and a linear cavity. The unidirectional ring cavity and the linear cavity are connected by a polarization beam splitter 2. The unidirectional ring cavity is formed by connecting the polarization-maintaining single-mode fiber 5 end to end, and the polarization-maintaining single-mode fiber 5 is provided with an isolator 1.

[0040] The linear cavity includes a 45° Faraday rotator 3 and a 90° Faraday rotation mirror 4. The 45° Faraday rotator 3 is connected to the polarization beam splitter 2 by a polarization-maintaining single-mode fiber 5, and the 45° Faraday rotator 3 and the 90° Faraday rotation mirror 4 are connected by a non-polarization-maintaining single-mode fiber 6.

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

[0042] During the transmission of the non-polarization-maintaining single-mode fiber 6, the phase shift generated by the linear polarization rotation is cancelled. Due to the existence of non-linear polarization rotation, the polarization state of the optical field will not return to the initial state and will have a projection component in the transmission direction of the polarization beam splitter 2. This part returns to the operation of the unidirectional ring cavity, obtaining a complete intracavity circulation. The greater the initial light intensity, the greater the projection component, and the transmittance is prone to increase with the light intensity, so a saturable absorption effect is formed, and the mode-locked pulse obtained is output by the coupler.

[0043] Refer to Figure 2 , the pump and gain components are set in the linear cavity part, and the output component is set 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 for introducing 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 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 provided in the unidirectional ring cavity, and the coupler 11 is used to output observable optical pulses.

[0044] The wavelength division multiplexer 9 is used to introduce 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 optical pulse does not damage the optical pump source 8, a pump protector 12 is also provided on the input path.

[0045] Due to the fact that the linear cavity structure on the outside cancels the linear polarization rotation of the signal light on the non-polarization-maintaining single-mode fiber 6, the two splitting ends of the polarization beam splitter 2 are fused together to form a ring circuit, which is equivalent to rotating the optical polarization direction by 90° and then emitting it, perpendicular to the transmission direction of the combined end entering the 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] Refer to Figure 3 , the prism is the polarization beam splitter 2, the combined end is the polarization-maintaining single-mode fiber 5 with biaxial operation, the splitting end is the polarization-maintaining single-mode fiber 5 with slow-axis operation, and the optical polarization states of the splitting ends are aligned with the slow-axis direction of the polarization-maintaining fiber, and then the fast and slow axes of the two polarization-maintaining fibers after splitting are aligned and fused. Taking vertically polarized light as an example, after entering the polarization beam splitter 2 from the combined end, it can only all flow into the vertical splitting end, and the horizontal component is 0. Due to the limitation of polarization-maintaining unidirectional transmission, only linearly polarized light can be emitted in the horizontal direction, and the emitted linearly polarized light works on the fast axis of the combined end. If the initial optical field is in other polarization states, it is transmitted with the incident projection component.

[0047] Refer to Figure 4, starting from the linearly polarized light emitted from the combining end of the polarization beam splitter 2, if the influence of nonlinearity is ignored, in the ideal state, the polarization direction of the signal light in the cavity rotates linearly multiple times, including two 45° rotations and one 90° rotation, and finally returns to the initial polarization direction. In this state, the polarized light 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 change continuously, and it 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 a partially polarized light to continue to evolve. There is also nonlinear polarization rotation in the polarization-maintaining fiber, and then it exits the polarization beam splitter 2 again. The greater the previous light intensity, the greater the projection component. It can be seen from this 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 the 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 the trimming of the pigtail of the wavelength division multiplexer being too short may cause inconvenience to the fusion splicing. At the same time, due to the different types of erbium-doped fiber and polarization-maintaining single-mode fiber, the light needs to pass through the cavity length twice when circulating in the linear cavity once, and it is difficult to accurately grasp the overall dispersion value of the laser. At the same time, since there is no isolator in the linear cavity, a pump protector is required to isolate the harm caused by the reflected light, and 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 example, the pump and gain components are arranged in the unidirectional ring cavity part, and the output component is arranged in the linear cavity part, and other structures are the same as those in Example 1.

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

[0052] In this example, the erbium-doped fiber 10 and the wavelength division multiplexer 9 are arranged on the polarization-maintaining path. After reducing the ring cavity length, the ring cavity part can be temporarily fixed, and the remaining cavity length control can be handed over to the external linear cavity for adjustment, 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 action 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 example is relatively high.

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

[0054] Figure 7 For Example 2, the light transmittance curves in two cases of adding a 45° Faraday rotator 3 and not adding a 45° Faraday rotator 3 are shown. When adding, the transmittance function is When not adding, the transmittance function is The abscissa is the magnitude of the nonlinear phase shift, and the ordinate is the magnitude of the transmittance.

[0055] It can be seen from the two curves the variation of the respective transmittance with the light intensity (assuming G = 1): the dotted line is the case without adding a 45° Faraday rotator. The transmittance is the largest in the absence of light, and the transmittance becomes smaller as the light intensity increases. When it reaches a certain intensity, the transmittance is zero, and then it rises again, with the trend of rising and falling back and forth changing cosine-like. The solid line is the case with a 45° Faraday rotator added. Contrary to the above changes, the transmittance is the smallest in the absence of light, and the transmittance increases as the light intensity increases, also changing in a cosine trend, and it is easy to act on the saturable absorption region to achieve the mode-locking condition.

[0056] Thus, 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, achieving the ideal effect. Similarly, other angle rotators can also be tried to shift the transmittance curve.

[0057] Example 3

[0058] Refer to Figure 8 , in Examples 1 and 2, the 45° Faraday rotator 3 and the polarization beam splitter 2 can be replaced by a polarization-maintaining circulator 7. The polarization-maintaining fiber and the non-polarization-maintaining fiber in the linear cavity are fusion-spliced. The optical operation process is as follows: the linear phase shift generated during the transmission of the signal light amplified by gain in the single-mode fiber is canceled, leaving a 90° rotation effect. Due to the existence of non-linear polarization rotation, the polarization direction of the optical field will not rotate exactly 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, having the same effect as Figure 1 the device.

[0059] Refer to Figure 9 , the polarization-maintaining circulator 7 has three ports, namely port a, port b, and port c, all set as polarization-maintaining single-mode fibers operating on the slow axis. The signal light entering from port a can only all exit from port b, the signal light entering from port b can only exit from all port c, and the signal light entering from port c can only all exit from port a. Connect port b and port c, then the signal light enters from port a and exits from port a again. When fusion-splicing, align the polarization-maintaining fast and slow axes, then the horizontally polarized light is reflected back with the same horizontal polarization direction.

[0060] Reference Figure 10 , in this embodiment, the polarization-maintaining circulator 7 is used to achieve the same mode-locking effect as in Embodiments 1 and 2. The functions of the optical fiber devices are the same as those in Embodiments 1 and 2. In addition, the isolator 1 is not added to the unidirectional ring cavity because the unidirectional transmission characteristic of the polarization-maintaining circulator 7 has the same effect. In this embodiment, the pump and gain components are both placed on the non-polarization-maintaining path. Different from the case where an isolation device must be added to the polarization beam splitter 2, there is only optical fiber connection in the unidirectional ring cavity, which can minimize the ring cavity length and increase the pulse repetition frequency. The polarization-maintaining circulator 7 in this embodiment is equivalent to the combined action of the polarization beam splitter 2 and the 45° Faraday rotator in Structure II, which is to maintain the polarization direction unchanged and reflect and accumulate the nonlinear phase shift.

[0061] Reference Figure 11 , taking the linearly polarized light emitted from port a in Figure 9 as the starting point, if the influence of nonlinearity is ignored, in the ideal state, the polarization direction of the signal light in the optical cavity undergoes a 90° linear rotation once, and the linear phase shift during the transmission of the single-mode optical fiber is cancelled out. Finally, the polarization direction is perpendicular to the initial direction. In this state, the polarized light 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. When entering the polarization-maintaining circulator, there will be a projection component of the optical field in the transmission direction, and the light field enters the ring cavity as partially polarized light for another nonlinear polarization rotation, and part of it exits the polarization-maintaining circulator.

[0062] Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art and related fields without creative efforts shall fall within the protection scope of the present invention.

Claims

1. A highly stable self - mode - locked fiber laser based on phase modulation, characterized in that: It includes a laser cavity, which consists of a unidirectional ring cavity and a linear cavity. The unidirectional ring cavity and the linear cavity are connected by a polarization beam splitter (2). The unidirectional ring cavity is formed by connecting polarization-maintaining single-mode fibers (5) head to tail, and an isolator (1) is provided to ensure the unidirectional transmission of the signal light. The linear cavity includes a 45° Faraday rotator (3) and a 90° Faraday rotator mirror (4). The 45° Faraday rotator (3) is connected to the polarization beam splitter (2) by a polarization-maintaining single-mode fiber (5), and the 45° Faraday rotator (3) is connected to the 90° Faraday rotator mirror (4) by a non-polarization-maintaining single-mode fiber (6). The pump, gain, and output components are arranged in the linear cavity part / unidirectional ring cavity part. Its optical operation process is as follows: An optical pump source (8) is set to provide a light source, which enters the laser cavity through a wavelength division multiplexer (9), and the gain light amplification is achieved through an erbium-doped fiber (10) to generate signal light. After passing through the 45° Faraday rotator (3) and the 90° Faraday rotator mirror (4), a phase difference of π is accumulated. During the transmission of the non-polarization-maintaining single-mode fiber (6), the phase shift generated by the linear polarization rotation is cancelled out. Due to the existence of non-linear polarization rotation, the polarization state of the optical 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 projection component returns to the unidirectional ring cavity for operation and obtains a complete intracavity circulation. The greater the initial light intensity, the greater the projection component, and the transmittance is prone to increase with the light intensity, so a saturable absorption effect is formed, and the mode-locked pulse obtained is output by the coupler.

2. The high-stability self-mode-locked fiber laser based on phase modulation according to claim 1, wherein: 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 rotator mirror (4) and is used to introduce 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 fiber (10) is arranged between the wavelength division multiplexer (9) and the 90° Faraday rotator mirror (4) to achieve gain amplification. An isolator (1) and a coupler (11) are arranged in the unidirectional ring cavity, and the coupler (11) is used to output observable optical pulses.

3. The high-stability 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 the erbium-doped fiber (10) of the laser are connected in series on the polarization-maintaining single-mode fiber (5) of the unidirectional ring cavity, and an optical pump source (8) is provided at the incident end of the wavelength division multiplexer (9). The coupler (11) is connected in series on the non-polarization-maintaining single-mode fiber (6) between the 90° Faraday rotator mirror (4) and the 45° Faraday rotator (3). Utilizing the characteristics of non-linear polarization rotation, all devices work together to obtain a saturable absorption effect and obtain an ultrashort pulse output.

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

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

6. The high-stability 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 combined beam end is a polarization-maintaining single-mode fiber operating in a biaxial manner, and the split beam ends are polarization-maintaining single-mode fibers operating in the slow axis direction. The optical polarization states at the split beam ends are aligned with the slow axis direction of the polarization-maintaining fiber. Then, the fast and slow axes of the two split polarization-maintaining fibers are aligned and fusion spliced. After the vertically polarized light enters the polarization beam splitter from the combined beam end, it can only all flow into the split beam end in the vertical direction, and the horizontal component is 0. Due to the limitation of the unidirectional transmission of polarization-maintaining, only linearly polarized light can be emitted from the horizontal direction. The linearly polarized light after emission operates in the fast axis at the combined beam end. If the initial optical field is in other polarization states, it is transmitted with the incident projection component.

7. A highly stable self - mode - locked fiber laser based on phase modulation, characterized in that: The 45° Faraday rotator (3) and the polarization beam splitter (2) in any one of claims 1-5 are replaced by a polarization-maintaining circulator (7), and there is no isolator (1) in the unidirectional ring cavity; A wavelength division multiplexer (9) and an erbium-doped fiber (10) are connected in series in sequence between the polarization-maintaining circulator (7) and the 90° Faraday rotator 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 fibers (5), and the linear cavity uses a non-polarization-maintaining single-mode fiber (6).

8. The high-stability 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 set as polarization-maintaining fibers operating in the slow axis direction. The signal light entering from port a can only all be emitted from port b. The signal light entering from port b can only be emitted from all port c. The signal light entering from port c can only all be emitted from port a. If port b and port c are connected, the signal light enters from port a and then all is emitted from port a. When fusion splicing, the fast and slow axes of the polarization-maintaining are aligned, so the horizontally polarized light is still horizontally polarized when reflected back.

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