Mode-locked laser and method for stabilizing repetition frequency of mode-locked laser

Abstract

The present application discloses a mode-locked laser and a method for stabilizing the repetition frequency of the mode-locked laser, and belongs to the field of laser technology. The mode-locked laser includes a first fiber collimator, a first quarter-wave plate, a first half-wave plate, an optical isolator, a second quarter-wave plate, a second fiber collimator, and an optical fiber, which are optically connected in sequence to form an optical ring cavity. The optical ring cavity is provided with a grating pair for generating negative dispersion between the second quarter-wave plate and the second fiber collimator, and the length of the optical fiber is configured to a first preset length so that the change in wavelength of the mode-locked laser during thermal expansion can compensate for the change in repetition frequency. The mode-locked laser provided in the present application, by setting an appropriate optical fiber length, enables the change in wavelength of the mode-locked laser to compensate for the reduction in repetition frequency, thereby improving the stability of the repetition frequency of the mode-locked laser, and the implementation method is simple, without the need to add additional repetition frequency control devices and control systems.

Description

Technical Field

[0001] The present application relates to the field of laser technology, and in particular to a mode-locked laser and a method for stabilizing the repetition frequency of the mode-locked laser. Background Art

[0002] Mode-locked lasers generally use passive mode locking, and their pulse repetition frequency is affected by the cavity length. Generally speaking, the repetition frequency is the equivalent optical path divided by twice the cavity length (linear cavity) or the optical path divided by the cavity length (ring cavity). When the ambient temperature changes, the laser material deforms due to the temperature, and the cavity length will change accordingly, causing the pulse repetition frequency to change, which reduces the stability of the repetition frequency. In the field of laser measurement, such as lidar and optical frequency combs, the stability of the repetition frequency is closely related to the measurement accuracy and is a key indicator. Reducing the impact of the ambient temperature on the repetition frequency stability can effectively expand the use environment of the laser and improve measurement accuracy.

[0003] The existing method for stabilizing the pulse repetition rate of a mode-locked laser generally involves adding a piezoelectric ceramic (PZT) to the cavity, using the PZT to change the cavity length and thus control the repetition rate. However, this method requires monitoring the repetition rate changes and controlling the PZT to form feedback, which requires the addition of additional PZT components and the design of a complex control system. Summary of the Invention

[0004] In view of this, the purpose of this application is to overcome the deficiencies in the prior art and provide a mode-locked laser and a method for stabilizing the repetition frequency of a mode-locked laser, so as to solve the technical problem that the means for controlling the repetition frequency in the prior art are relatively complicated.

[0005] To solve the above technical problems, this application provides:

[0006] A mode-locked laser comprises a first fiber collimator, a second fiber collimator, an optical isolator, a first quarter-wave plate, a second quarter-wave plate, and a first half-wave plate, wherein the first fiber collimator and the second fiber collimator are connected via an optical fiber, and the first fiber collimator, the first quarter-wave plate, the first half-wave plate, the optical isolator, the second quarter-wave plate, the second fiber collimator, and the optical fiber are optically connected in sequence to form an optical ring cavity, wherein a grating pair is provided in the optical ring cavity between the second quarter-wave plate and the second fiber collimator, and the grating pair is used to generate negative dispersion;

[0007] The length of the optical fiber is configured as a first preset length so that a change in wavelength of the mode-locked laser during thermal expansion can compensate for a change in repetition frequency.

[0008] In a possible embodiment, the optical isolator includes a first polarization beam splitter, a rotator, a second half-wave plate, and a second polarization beam splitter, and the first half-wave plate, the first polarization beam splitter, the rotator, the second half-wave plate, and the second polarization beam splitter are optically connected in sequence to form a part of the optical ring cavity.

[0009] In a possible embodiment, the optical rotator is a Faraday rotator crystal.

[0010] In a possible embodiment, the first fiber collimator and the second fiber collimator are configured with the same parameters to couple the optical fibers.

[0011] In a possible embodiment, the grating pair includes two blazed gratings, and the two blazed gratings are configured with the same parameters.

[0012] In a possible embodiment, the optical fiber is an active optical fiber, and the active optical fiber is used to provide a stimulated emission gain medium.

[0013] In a possible embodiment, the mode-locked laser further includes a pump laser and a wavelength division multiplexer, the wavelength division multiplexer is connected to the active optical fiber, and the pump laser is connected to the wavelength division multiplexer as a pump source.

[0014] In addition, the present application also provides a method for stabilizing the repetition frequency of a mode-locked laser, comprising the following steps:

[0015] S1, based on an optical fiber of a second preset length, obtaining a repetition frequency of a mode-locked laser corresponding to each preset temperature within a preset temperature range;

[0016] S2, calculating a change in the repetition frequency within the preset temperature range based on the obtained multiple repetition frequencies;

[0017] S3, obtaining an equivalent optical path of the mode-locked laser optical ring cavity, and calculating a change in the equivalent optical path within the preset temperature range based on the obtained repetition frequency and the equivalent optical path;

[0018] S4, calculating the change in the equivalent optical path per meter of optical fiber per degree Celsius based on the change in the equivalent optical path;

[0019] S5, respectively obtaining the material coefficient of the optical fiber and the change in the wavelength of the mode-locked laser within the preset temperature range;

[0020] S6, calculating the change of the refractive index per degree Celsius based on the obtained material coefficient of the optical fiber and the change of the wavelength;

[0021] S7. Calculate a first preset length of the optical fiber according to the change in the refractive index per degree Celsius.

[0022] In a possible embodiment, the second preset length is set to 10 cm, the preset temperature range is set to 19° to 29°, and the repetition frequency is obtained every time the preset temperature changes by 2°.

[0023] In a possible embodiment, the repetition frequency is a central repetition frequency, and the wavelength is a central wavelength.

[0024] Beneficial effects of this application:

[0025] The present application proposes a mode-locked laser, comprising a first fiber collimator, a second fiber collimator, an optical isolator, a first quarter-wave plate, a second quarter-wave plate, and a first half-wave plate.

[0026] The first fiber collimator and the second fiber collimator are connected through an optical fiber, and the first fiber collimator, the first quarter wave plate, the first half wave plate, the optical isolator, the second quarter wave plate, the second fiber collimator and the optical fiber are optically connected in sequence to form an optical ring cavity. The optical ring cavity is provided with a grating pair for generating negative dispersion between the second quarter wave plate and the second fiber collimator, and the length of the optical fiber is configured to be a first preset length so that a change in wavelength of the mode-locked laser during thermal expansion can compensate for a change in repetition frequency.

[0027] The mode-locked laser provided in the present application, by setting an appropriate optical fiber length, enables the change in the wavelength of the mode-locked laser to compensate for the reduction in the repetition frequency, thereby improving the stability of the repetition frequency of the mode-locked laser. The implementation is simple and does not require the addition of additional repetition frequency control devices and control systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without creative work.

[0029] Figure 1 A schematic diagram of the connection structure of the optical ring cavity of the mode-locked laser in some embodiments of the present application is shown;

[0030] Figure 2 A schematic diagram of spatial chirping of a grating pair of a mode-locked laser optical ring cavity in some embodiments of the present application is shown;

[0031] Figure 3A flow chart of a method for stabilizing the repetition frequency of a mode-locked laser in some embodiments of the present application is shown;

[0032] Figure 4 A graph showing the relationship between the central repetition frequency of a mode-locked laser and the ambient temperature in some embodiments of the present application is shown;

[0033] Figure 5 Shows a spectrum test diagram of the repetition frequency laser generated by the mode-locked laser in some embodiments of the present application;

[0034] Figure 6 A graph showing the relationship between the refractive index and the central wavelength of the mode-locked laser in some embodiments of the present application is shown.

[0035] Description of main component symbols:

[0036] 100-mode-locked laser; 1-first fiber collimator; 2-first quarter-wave plate; 3-first half-wave plate; 4-optical isolator; 41-first polarization splitter; 42-optical rotator; 43-second half-wave plate; 44-second polarization splitter; 5-second quarter-wave plate; 6-grating pair; 61-first blazed grating; 62-second blazed grating; 7-second fiber collimator; 8-optical fiber; 9-wavelength division multiplexer; 10-pump laser. DETAILED DESCRIPTION

[0037] The following describes in detail embodiments of the present application. Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present application and are not to be construed as limiting the present application.

[0038] In the description of the present application, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present application.

[0039] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referred to. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. Throughout the description of this application, "plurality" means two or more, unless otherwise specifically defined.

[0040] In this application, unless otherwise expressly specified or limited, terms such as "mounted," "connected," "connect," and "fixed" should be understood in a broad sense. For example, they may refer to fixed connections, detachable connections, or integration; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; and internal connections between two components or interactions between two components. Those skilled in the art will understand the specific meanings of these terms in this application based on specific circumstances.

[0041] In this application, unless otherwise expressly specified or limited, when a first feature is "above" or "below" a second feature, it may mean that the first and second features are in direct contact, or the first and second features are in indirect contact through an intermediate medium. Furthermore, when a first feature is "above," "above," or "above" a second feature, it may mean that the first feature is directly above or diagonally above the second feature, or simply means that the first feature is at a higher level than the second feature. When a first feature is "below," "below," or "below" a second feature, it may mean that the first feature is directly below or diagonally below the second feature, or simply means that the first feature is at a lower level than the second feature.

[0042] Example 1

[0043] See Figure 1 Embodiment 1 of the present application proposes a mode-locked laser 100, which belongs to the field of laser technology. Mode-locking technology uses a specific modulation method to achieve a certain phase relationship between the longitudinal modes of laser oscillation at different frequencies. That is, the intervals between adjacent longitudinal modes are equal and fixed. Therefore, mode locking is also called phase locking.

[0044] See Figure 1 and Figure 2 The mode-locked laser 100 proposed in this embodiment may include: a first fiber collimator 1, a second fiber collimator 7, an optical isolator 4, a first quarter-wave plate 2, a second quarter-wave plate 5 and a first half-wave plate 3.

[0045] Among them, the first fiber collimator 1 and the second fiber collimator 7 are connected by an optical fiber 8. The first fiber collimator 1, the first quarter wave plate 2, the first half wave plate 3, the optical isolator 4, the second quarter wave plate 5, the second fiber collimator 7 and the optical fiber 8 are optically connected in sequence to form an optical ring cavity. The optical ring cavity is provided with a grating pair 6 for generating negative dispersion between the second quarter wave plate 5 and the second fiber collimator 7. The length of the optical fiber 8 is configured to a first preset length so that the change in wavelength of the mode-locked laser 100 during thermal expansion can compensate for the change in repetition frequency.

[0046] In this embodiment, specifically, the optical fiber 8 is an active optical fiber, which is used to provide a stimulated emission gain medium. The mode-locked laser 100 also includes a pump laser 10 and a wavelength division multiplexer 9. The wavelength division multiplexer 9 is connected to the active optical fiber 8, and the pump laser 10 is connected to the wavelength division multiplexer 9 as a pump source.

[0047] The mode-locking principle of the mode-locked laser 100 of this embodiment is based on NPR (nonlinear polarization rotation). The basic principle of NPR lasers is that when light is transmitted in the optical fiber 8, due to the nonlinear effect, the polarization state is a quantity related to the light intensity. By selecting the first quarter-wave plate 2, the first half-wave plate 3, and the second quarter-wave plate 5 (adjusting the polarization state within the loop), the polarization state that can pass through the isolator is exactly corresponding to the strongest light intensity. At this time, the loss of strong light intensity is small, while the loss of weak light intensity is large. An equivalent saturable absorber is formed in the loop, thereby achieving mode-locked output.

[0048] In this embodiment, an adjustable grating pair 6 is added to the NPR laser. Figure 2 After the non-monochromatic light passes through the grating pair 6, spatial chirp will be generated. This spatial wavelength arrangement, after coupling into the second fiber collimator 7, is equivalent to passing through a filter, and the wavelength of the filter can be changed by the position of the first fiber collimator 1 and the second fiber collimator 7.

[0049] However, the first fiber collimator 1 and the second fiber collimator 7 of the mode-locked laser 100 are generally fixed to the metal structure of the mode-locked laser 100, which is not limited to a metal housing or a metal mounting base. As the ambient temperature rises, the metal structure expands due to thermal expansion, causing the length of the optical ring cavity of the mode-locked laser 100 to increase. Simultaneously, as the metal structure expands due to thermal expansion, the first fiber collimator 1 and the second fiber collimator 7 shift, causing the wavelength to change, moving toward longer wavelengths. However, since the optical fiber 8 is made of conventional dispersive material, its refractive index decreases with wavelength changes, reducing the equivalent optical path length of the optical ring cavity. Furthermore, because the thermal expansion coefficient of the optical fiber 8 is relatively small, its expansion cannot exceed the equivalent optical path length reduction caused by wavelength changes. The longer the optical fiber 8, the greater the equivalent optical path length reduction caused by wavelength changes. Therefore, selecting an appropriate length of optical fiber 8 can offset the equivalent optical path length change caused by thermal expansion of the metal structure, reducing the degree to which the equivalent optical path length within the optical ring cavity is affected by temperature, thereby stabilizing the pulse repetition frequency of the mode-locked laser 100 when the ambient temperature changes.

[0050] The mode-locked laser 100 provided in this embodiment, by providing an optical fiber 8 of a suitable length, i.e., an optical fiber 8 of a first preset length, ensures that the change in the wavelength of the mode-locked laser 100 can just compensate for the change in the pulse repetition frequency, thereby improving the stability of the repetition frequency of the mode-locked laser 100. This eliminates the need for additional repetition frequency control devices and control systems, resulting in a simple implementation and low cost.

[0051] Continue reading Figure 1 In some embodiments of the present application, optionally, the optical isolator 4 includes a first polarization beam splitter 41, a rotator 42, a second half-wave plate 43 and a second polarization beam splitter 44. The first half-wave plate 3, the first polarization beam splitter 41, the rotator 42, the second half-wave plate 43 and the second polarization beam splitter 44 are optically connected in sequence to form a part of the optical ring cavity.

[0052] In this embodiment, specifically, the first polarization beam splitter 41, the optical rotator 42, the second half-wave plate 43, and the second polarization beam splitter 44 together constitute a polarization-dependent optical isolator 4, which ensures that light propagates in one direction within the loop, while limiting the passage of a single polarization and losing other polarizations to achieve mode locking.

[0053] In the embodiment of the optical isolator 4 described above, the optical rotator 42 may optionally be a Faraday rotator crystal. A Faraday rotator crystal is a tool that utilizes the Faraday effect. By utilizing the non-reciprocity of magneto-optical materials, the Faraday rotator crystal can rotate the polarization planes of both forward incident light and reverse incident light of the same wavelength in the same direction and by the same angle, regardless of the propagation direction of the light beam.

[0054] In some embodiments of the present application, optionally, the first fiber collimator 1 and the second fiber collimator 7 are configured with the same parameters to achieve coupling of the optical fiber 8 and form a loop of a space-fiber structure.

[0055] See Figure 2 In some embodiments of the present application, the grating pair 6 includes two blazed gratings, and the two blazed gratings are configured with the same parameters.

[0056] In this embodiment, specifically, the grating pair 6 includes a first blazed grating 61 and a second blazed grating 62. The parameters of the first blazed grating 61 and the second blazed grating 62 are the same, which can ensure high diffraction efficiency while generating negative dispersion, balancing the positive dispersion of the optical fiber 8 in the loop, adjusting the output parameters and ensuring that the output pulse is narrow enough.

[0057] Example 2

[0058] The second embodiment of the present application proposes a method for stabilizing the repetition frequency of a mode-locked laser 100, comprising the following steps:

[0059] S1 , based on the optical fiber 8 of the second preset length, obtaining the repetition frequency of the mode-locked laser 100 corresponding to each preset temperature within a preset temperature range.

[0060] Optionally, the preset temperature range is set to 19° to 29°, the second preset length is set to 10 cm, and a 10 cm long optical fiber 8 is selected to evaluate the thermal expansion characteristics of the metal structure of the mode-locked laser 100, see Figure 4 ,The temperature interval between two adjacent preset temperatures is 2°, and the repetition frequency is obtained once for each preset temperature. Within the preset temperature range of 19° to 29°, a total of 6 sets of data are obtained.

[0061] S2, calculating a change in the repetition frequency within a preset temperature range based on the obtained multiple repetition frequencies.

[0062] See Figure 4 Specifically, within the preset temperature range of 19° to 29°, the repetition frequency of the mode-locked laser 100 changes from 840.6406 MHz to 840.6106 MHz, a difference of 30 kHz, that is, within the preset temperature range of 19° to 29°, the change in repetition frequency is 30 kHz.

[0063] S3, obtaining the equivalent optical path of the optical ring cavity of the mode-locked laser 100, and calculating the change of the equivalent optical path within a preset temperature range based on the obtained repetition frequency and the equivalent optical path.

[0064] Specifically, according to the relationship between the repetition frequency and the equivalent optical path of the optical ring cavity: f = c / L (L is the equivalent optical path of the optical ring cavity of the mode-locked laser 100, also called the cavity length of the resonant cavity; c is the speed of light; f is the repetition frequency), the equivalent optical path corresponding to each repetition frequency is calculated, and the change in the equivalent optical path is calculated to be 12.7 μm.

[0065] See Figure 4 It can be seen that the change in the equivalent optical path is close to linear with temperature, that is, the change in the equivalent optical path per degree Celsius is about 1.27um, or 1.27um / ℃.

[0066] S4, calculating the change in the equivalent optical path per meter of optical fiber per degree Celsius based on the change in the equivalent optical path.

[0067] Specifically, according to experimental tests, when only the temperature of the optical fiber 8 is considered, the change in repetition frequency within the preset temperature range of 19° to 29° is 1 kHz (one thirtieth of the test result 30 kHz in S2). It is calculated that the equivalent optical path change of 10 cm optical fiber 8 is approximately 0.4 um, that is, the equivalent optical path change is approximately 0.4 um per meter of optical fiber per degree Celsius, that is, 0.4 um / m℃.

[0068] S5 , respectively obtaining the material coefficient of the optical fiber 8 and the wavelength variation of the mode-locked laser 100 within a preset temperature range.

[0069] S6. Calculate the change in the refractive index per degree Celsius based on the obtained material coefficient of the optical fiber 8 and the change in the wavelength.

[0070] For details, see Figure 5 In the spectrum test chart, the wavelength change corresponding to one cell in the horizontal axis direction is 6nm. According to the spectrum test chart, the change of the center wavelength in the preset temperature range of 19° to 29° is about two-thirds of the cell (the peak moves toward the shortwave direction by about two-thirds of the length of the cell), that is, the change of the center wavelength is 4nm.

[0071] According to the relationship between the core refractive index (n) and wavelength (λ) of optical fiber 8:

[0072]

[0073] Wherein, L1, L2, L3 and b1, b2, b3 are the material coefficients of the optical fiber 8; λ is the wavelength; and n is the refractive index.

[0074] The change in wavelength λ is 4 nm. The material coefficient values of the optical fiber 8 obtained from experimental measurements are: L1 = 68.4043; L2 = 116.2414; L3 = 9896.161; b1 = 0.6961663; b2 = 0.4079426; and b3 = 0.897479.

[0075] See Figure 6 In the central wavelength range of the mode-locked laser 100 from 1030 nm to 1060 nm, the refractive index changes nearly linearly. It is calculated that the change in refractive index per degree Celsius corresponding to the change in wavelength is Δn=4.86*10^(-6).

[0076] S7, calculating a first preset length of the optical fiber 8 according to the change in the refractive index per degree Celsius.

[0077] Specifically, the change in equivalent optical path caused by the change in refractive index due to temperature change is Δn*s*10^6 (s is in m), and the change in equivalent optical path caused by the expansion of optical fiber 8 due to temperature change is 0.4*s (s is in m), and the change in optical path caused by mechanical expansion due to temperature change is 1.27um / ℃. Let the change in equivalent optical path caused by the change in refractive index due to temperature change be equal to the sum of the change in equivalent optical path caused by the expansion of optical fiber 8 due to temperature change and the change in optical path caused by mechanical expansion due to temperature change, that is, the total optical path remains unchanged, and the relationship is: Δn*s*10^6=0.4*s+1.27 (Δn is the change in refractive index; s is the optical fiber length, in m).

[0078] Therefore, the equivalent optical path (cavity length of the resonant cavity) of the optical ring cavity of the mode-locked laser 100 remains unchanged, and the frequency remains unchanged, achieving a stable frequency effect. The calculation results show that s = 0.285m, that is, the first preset length of the optical fiber 8 is 0.285m. The structure of the mode-locked laser 100 exemplified in this application uses an optical fiber 8 of this first preset length, which can just compensate for the decrease in pulse repetition frequency caused by thermal expansion by the change in the wavelength of the mode-locked laser 100. At this time, the pulse repetition frequency stability is greatly improved, and no additional adjustment device is required.

[0079] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification and features of different embodiments or examples without contradiction.

[0080] Although the embodiments of the present application have been shown and described above, it can be understood that the above embodiments are exemplary and cannot be understood as limitations on the present application. Ordinary technicians in this field can change, modify, replace and modify the above embodiments within the scope of the present application.

Claims

1. A mode-locked laser, characterized in that: The optical fiber collimator comprises a first optical fiber collimator, a second optical fiber collimator, an optical isolator, a first quarter-wave plate, a second quarter-wave plate, and a first half-wave plate, wherein the first optical fiber collimator and the second optical fiber collimator are connected via an optical fiber, and the first optical fiber collimator, the first quarter-wave plate, the first half-wave plate, the optical isolator, the second quarter-wave plate, the second optical fiber collimator, and the optical fiber are optically connected in sequence to form an optical ring cavity, wherein a grating pair is provided in the optical ring cavity between the second quarter-wave plate and the second optical fiber collimator, and the grating pair is used to generate negative dispersion; wherein the length of the optical fiber is configured to be a first preset length so that a change in the wavelength of the mode-locked laser during thermal expansion can compensate for a change in the repetition frequency; Based on the optical fiber of a second preset length, obtaining a repetition frequency of the mode-locked laser corresponding to each preset temperature within a preset temperature range; Calculating a change in the repetition frequency within the preset temperature range based on the obtained multiple repetition frequencies; Obtaining an equivalent optical path of the optical ring cavity of the mode-locked laser, and calculating a change in the equivalent optical path within the preset temperature range based on the obtained repetition frequency and the equivalent optical path; Calculating the change in the equivalent optical path per meter of optical fiber per degree Celsius based on the change in the equivalent optical path generated when only the temperature change of the optical fiber is considered; respectively obtaining a material coefficient of the optical fiber and a change in the wavelength of the mode-locked laser within the preset temperature range; Calculating a change in refractive index per degree Celsius based on the obtained material coefficient of the optical fiber and the change in the wavelength; The first preset length of the optical fiber is calculated according to the change in the refractive index per degree Celsius.

2. The mode-locked laser according to claim 1, characterized in that The optical isolator includes a first polarization beam splitter, a rotator, a second half-wave plate and a second polarization beam splitter. The first half-wave plate, the first polarization beam splitter, the rotator, the second half-wave plate and the second polarization beam splitter are optically connected in sequence to form a part of the optical ring cavity.

3. The mode-locked laser according to claim 2, characterized in that The optical rotator is a Faraday rotator crystal.

4. The mode-locked laser according to claim 1, wherein The first fiber collimator and the second fiber collimator are configured with the same parameters to couple the optical fibers.

5. The mode-locked laser according to claim 1, characterized in that The grating pair includes two blazed gratings, and the two blazed gratings are configured with the same parameters.

6. The mode-locked laser according to claim 1, characterized in that The optical fiber is an active optical fiber, and the active optical fiber is used to provide a stimulated emission gain medium.

7. The mode-locked laser according to claim 6, characterized in that The mode-locked laser further includes a pump laser and a wavelength division multiplexer. The wavelength division multiplexer is connected to the active optical fiber, and the pump laser is connected to the wavelength division multiplexer as a pump source.

8. The mode-locked laser according to claim 1, wherein The second preset length is set to 10 cm, the preset temperature range is set to 19° to 29°, and the repetition frequency is obtained every time the preset temperature changes by 2°.

9. The mode-locked laser according to claim 1, characterized in that The repetition frequency is the central repetition frequency, and the wavelength is the central wavelength.

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