Device and method for narrowing line width of frequency-doubled laser

Abstract

The invention provides a device and a method for narrowing the line width of a frequency-doubled laser. The device comprises a pumping module and a laser resonant cavity, the pumping module is used for providing linearly polarized light in a specific polarization direction to a light path; the laser resonant cavity is internally provided with a gain medium, at least one birefringence optical filter and a frequency doubling crystal which are sequentially arranged along an optical path, and the at least one birefringence optical filter is inserted into the optical path at a Brewster angle; wherein the linearly polarized light is focused in the gain medium, the central wavelength of the linearly polarized light is the same as the absorption peak wavelength of the gain medium, and the polarization direction of the linearly polarized light is the same as the maximum absorption direction of the gain medium. According to the invention, through specific adjustment of the birefringence optical filter (group) inserted into the laser resonant cavity, narrow-band filtering can be conveniently and rapidly realized in various intracavity frequency doubling lasers, and compared with the prior art, the power loss introduced by the application is relatively small.

Description

Technical Field

[0001] The present disclosure relates to the field of optoelectronic technology, and in particular to a device and method for narrowing the width of a frequency-doubled laser spectral line. Background Art

[0002] All-solid-state continuous narrow linewidth lasers have the advantages of good monochromaticity, low noise, high beam quality, and high stability, and are widely used in precision measurement, ultrafine spectroscopy, cold atomic physics, quantum communication, lidar, laser remote sensing, and material processing. Output linewidth is one of the important parameters for measuring the performance of solid-state lasers, which directly affects the coherence of the laser and its performance in specific applications.

[0003] With the development of science and technology and the deepening of basic research, people have put forward higher requirements for the output spectral linewidth of all-solid-state lasers. The main factors affecting the output linewidth of solid-state lasers are the characteristics of the gain medium, the design of the resonant cavity, the stability of the pump source, external environmental factors, etc. However, among these decisive factors, it is difficult to make major changes after the optical path of the laser is built, so other optical elements must be used to achieve the compression of the output spectral linewidth. As mentioned above, for all-solid-state lasers, most of the factors that determine the output laser linewidth have been determined at the beginning of the design of the laser itself, so it is necessary to find suitable optical elements to insert into the optical path of the laser to achieve the purpose of linewidth compression.

[0004] Most of the existing all-solid-state lasers do not have linewidth compression elements added, because the insertion of additional optical elements in the laser cavity will introduce large power loss and reduce the output power. For lasers without linewidth compression, the full width at half maximum (FWHM) of the output spectrum is mostly in the nanometer level, with low spectral purity, poor monochromaticity, low spectral peak density, and low signal-to-noise ratio, which may not meet the requirements of certain specific applications.

[0005] Therefore, how to perform narrow-band filtering on the output spectrum in a simple and convenient manner while introducing less power loss, thereby outputting a laser with a narrow linewidth, is a technical problem that urgently needs to be solved in this field. Summary of the invention

[0006] The purpose of the present application is to provide a device and method for narrowing the width of a frequency-doubled laser spectral line.

[0007] The first aspect of the present application provides a device for narrowing the width of a frequency-doubled laser line, comprising:

[0008] A pump module, used to provide linear polarized light with a specific polarization direction into the optical path;

[0009] A laser resonant cavity, wherein a gain medium, at least one birefringent filter and a frequency doubling crystal are sequentially arranged along an optical path, wherein the at least one birefringent filter is inserted into the optical path at a Brewster angle;

[0010] The linearly polarized light is focused in the gain medium, the central wavelength of the linearly polarized light is the same as the absorption peak wavelength of the gain medium, and the polarization direction of the linearly polarized light is the same as the maximum absorption direction of the gain medium.

[0011] In some implementations of the present application, the number of the birefringent filters is greater than one, and the thicknesses of all the birefringent filters are in a preset ratio.

[0012] In some embodiments of the present application, the gain medium is any one of the following solid laser crystals:

[0013] Nd:YVO4, Nd:YAG, Nd:YLF, Yb:YAG, Tm:YAG, Er:YAG and Cr 3+ :BeAl2O4.

[0014] In some embodiments of the present application, the frequency doubling crystal is any one of the following:

[0015] KDP crystal, LBO crystal and BBO crystal.

[0016] In some embodiments of the present application, the focusing position of the linearly polarized light is at the center of the gain medium.

[0017] In some embodiments of the present application, the type of the laser resonant cavity is any one of the following:

[0018] Plane-to-plane critical cavity, plane-to-concave cavity, annular cavity, X-shaped cavity and V-shaped cavity.

[0019] In some implementations of the present application, the pumping mode of the pump module is end pumping.

[0020] A second aspect of the present application provides a method for narrowing the width of a frequency-doubled laser spectral line, based on the device for narrowing the width of a frequency-doubled laser spectral line described in the first aspect, the method comprising:

[0021] Move the birefringent filter and frequency doubling crystal in the laser resonant cavity out of the optical path;

[0022] Controlling the pump module to provide linear polarized light with a specific polarization direction into the optical path;

[0023] Adjusting the gain medium so that the power of the fundamental frequency laser output by the laser resonant cavity reaches the highest;

[0024] Inserting the birefringent filter into the optical path at the Brewster angle, and adjusting the birefringent filter to select the wavelength of the output fundamental frequency laser;

[0025] Inserting the frequency doubling crystal into the optical path, adjusting the frequency doubling crystal to achieve a phase matching angle for generating a second harmonic, and generating a frequency doubling laser;

[0026] The birefringent filter and the frequency doubling crystal are fine-tuned again to change the transmittance peak position of the birefringent filter for the fundamental frequency laser and the frequency doubling laser, so that the birefringent filter can simultaneously narrow the spectral line width of the fundamental frequency laser and the frequency doubling laser to obtain a frequency doubling laser with a narrowed spectral line width.

[0027] Compared with the prior art, the device for narrowing the line width of a frequency-doubled laser provided in the present application includes a pump module and a laser resonant cavity; the pump module is used to provide linearly polarized light of a specific polarization direction in an optical path; the laser resonant cavity has a gain medium, at least one birefringent filter and a frequency-doubled crystal arranged in sequence along the optical path in the laser resonant cavity, and the at least one birefringent filter is inserted into the optical path at a Brewster angle; wherein the linearly polarized light is focused in the gain medium, the central wavelength of the linearly polarized light is the same as the absorption peak wavelength of the gain medium, and the polarization direction of the linearly polarized light is the same as the maximum absorption direction of the gain medium. The present application can make specific adjustments to the birefringent filter (group) inserted into the laser resonant cavity, so that the birefringent filter (group) does not completely correspond to the transmittance peaks of the fundamental frequency light and the frequency-doubled light in the resonant cavity, so that the fundamental frequency laser that has undergone a line width compression is converted into a second harmonic and the spectral line width is compressed again by the same birefringent filter (group), and finally a frequency-doubled laser with a large degree of spectral line width narrowing can be obtained. The present application can achieve a better line width narrowing effect by using fewer filtering elements, and can conveniently and quickly implement narrowband filtering in a variety of intracavity frequency-doubled lasers. Compared with the prior art, the power loss introduced by the present application is smaller. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present disclosure. Also, the same reference symbols are used throughout the accompanying drawings to represent the same components. In the accompanying drawings:

[0029] Figure 1 A schematic diagram showing the structure of a device for narrowing the width of a frequency-doubled laser line provided by the present disclosure is shown;

[0030] Figure 2 A schematic structural diagram of another device for narrowing the width of a frequency-doubled laser line provided by the present disclosure is shown;

[0031] Figure 3 A flow chart of a method for narrowing the width of a frequency-doubled laser line provided by the present disclosure is shown;

[0032] Figure 4 A schematic diagram showing the spectral line width obtained without using the device of the present application;

[0033] Figure 5 A schematic diagram showing the spectral line width obtained using the device of the present application. DETAILED DESCRIPTION

[0034] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the present disclosure. In addition, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessary confusion of the concepts of the present disclosure.

[0035] Various structural schematic diagrams according to embodiments of the present disclosure are shown in the accompanying drawings. These figures are not drawn to scale, and some details are magnified and some details may be omitted for the purpose of clear expression. The shapes of various regions and layers shown in the figures and the relative sizes and positional relationships therebetween are only exemplary, and may deviate in practice due to manufacturing tolerances or technical limitations, and those skilled in the art may further design regions / layers with different shapes, sizes, and relative positions according to actual needs.

[0036] In the context of the present disclosure, when a layer / element is referred to as being "on" another layer / element, the layer / element may be directly on the other layer / element or an intervening layer / element may exist therebetween. In addition, if a layer / element is "on" another layer / element in one orientation, the layer / element may be "below" the other layer / element when the orientation is reversed.

[0037] Since most of the existing all-solid-state lasers do not have optical elements for line width compression, there are two reasons: first, it is inconvenient to adjust the general line width narrowing device, which makes the whole system more complicated and increases the instability of the system; second, the insertion of line width compression elements will increase power loss and cause the output power to drop. Therefore, it is necessary to find a suitable method to effectively narrow the output laser line width while minimizing the above disadvantages.

[0038] In view of this, embodiments of the present application provide a device for narrowing the width of a frequency-doubled laser spectral line and a method for narrowing the width of a frequency-doubled laser spectral line, which are described below in conjunction with the accompanying drawings.

[0039] Please refer to Figure 1 , which shows a schematic diagram of the structure of a device for narrowing the width of a frequency-doubled laser line provided in an embodiment of the present application, such as Figure 1As shown, the device includes: a pump module 100 and a laser resonant cavity 200 .

[0040] The pump module 100 can provide linear polarized light with a specific polarization direction in the optical path. The pumping mode of the pump module 100 can be end pumping or side pumping. Different pumping modes can be selected as long as the design is reasonable.

[0041] The type of the laser resonant cavity 200 can be a plane-plane critical cavity, a plane-concave cavity, or a cavity type composed of more reflectors such as a ring cavity, an X-shaped cavity, a V-shaped cavity, etc. that meets specific needs.

[0042] like Figure 1 As shown, the laser resonant cavity 200 is composed of a reflector 201 and a reflector 202. In the laser resonant cavity 200, there are a gain medium 300, at least one birefringent filter 400 and a frequency doubling crystal 500 arranged in sequence along the optical path. The at least one birefringent filter 400 is inserted into the optical path at the Brewster angle.

[0043] The linearly polarized light provided by the pump module 100 is focused in the gain medium 300 , and specifically its focusing position can be at the center of the gain medium 300 . The central wavelength of the linearly polarized light is the same as the absorption peak wavelength of the gain medium 300 , and the polarization direction of the linearly polarized light is the same as the maximum absorption direction of the gain medium 300 .

[0044] Specifically, the gain medium 300 may be Nd:YVO4, Nd:YAG, Nd:YLF, Yb:YAG, Tm:YAG, Er:YAG, and Cr 3+ :BeAl2O4 and other solid laser crystals.

[0045] Specifically, the frequency doubling crystal 500 may be a KDP crystal (potassium dihydrogen phosphate crystal), an LBO crystal (lithium triborate crystal), or a BBO crystal (barium metaborate crystal).

[0046] like Figure 1 The working principle of the device for narrowing the frequency-doubled laser line width is as follows:

[0047] The principle of birefringent filter narrowing line width is based on the birefringence effect. When linearly polarized light passes through, the o-light and the e-light produce a certain phase difference δ, which can be expressed as:

[0048]

[0049] Among them, n e and n oThey represent the refractive index of e-light and o-light respectively, d is the thickness of the birefringent filter, γ is the angle between the optical axis and the ordinary light wave vector, λ is the laser wavelength, and θ represents the incidence at the Brewster angle.

[0050] When the phase difference satisfies δ, which is an integer multiple of 2π, the birefringent filter is equivalent to a full-wave plate. The laser that satisfies this condition can be output losslessly. At this time, the transmission peak wavelength λ is:

[0051]

[0052] Where k is the interference order. For fundamental frequency light and double frequency light, when fundamental frequency light can be completely transmitted, double frequency light can also be completely transmitted in theory, but the interference order k is twice. However, in actual situations, since the refractive index of light in quartz crystal changes with wavelength, that is, quartz crystal has material dispersion effect, it can be expressed as follows:

[0053] n 2 =a0+a1·λ 2 +a2·λ -2 +a3·λ -4 +a4·λ -6 +a5·λ -8 ;

[0054] This results in that the transmittance peaks of the fundamental frequency light and the doubled frequency light do not completely correspond, but will be staggered to a certain extent. The line width compressed by the birefringent filter will be compressed again after frequency doubling, so that the birefringent filter 400 can simultaneously compress the spectral line widths of the fundamental frequency laser and the doubled frequency laser. The laser spectrum finally output by the entire device should be jointly affected and generated by various components such as the laser crystal, the birefringent filter, the doubled frequency crystal, and the reflector. Fine-tune the birefringent filter, the smaller the overlap between the main peak of the fundamental frequency light transmittance and the main peak of the doubled frequency light transmittance, the narrower the line width of the ultraviolet laser output can be produced. And after theoretical and experimental verification, the birefringent filter 400 inserted at the Brewster angle can greatly reduce the reflection of the fundamental frequency p-polarized light, and the reflectivity of the doubled frequency s-polarized light is also low (about 0.14), so the loss introduced by the narrowing of the doubled frequency laser line width in this application is small.

[0055] When the designed birefringent filter 400 is placed in the optical path at the Brewster angle and the birefringent filter is rotated around the surface normal, the position of the transmittance peak can be adjusted, thereby playing the role of narrowing the laser line width in a wide range of wavelengths.

[0056] Specifically, the birefringent filter 400 may be a single birefringent filter of a certain thickness, or a combination of multiple birefringent filters of a certain thickness ratio, such as 1:2:4 or 1:2:5:9. All the birefringent filters are inserted into the laser resonant cavity at the Brewster angle in a manner that the surfaces and the optical axes are parallel to each other. This can further narrow the output laser line width, but at the same time there will be greater power loss. The pros and cons can be weighed to select and design according to specific needs.

[0057] One side of the laser resonant cavity reflector 201 is coated with an anti-reflection film for the pump light band, and the other side is coated with a high-reflection film for the laser band; the reflector 202 is coated with a high-reflection film for the fundamental light band and an anti-reflection film for the frequency doubling light band. After the frequency doubling crystal 500 is correctly placed in the cavity, the birefringent filter 400 and the frequency doubling crystal 500 are repeatedly adjusted to output a narrow linewidth frequency doubling laser.

[0058] The present application conveniently and quickly realizes narrowband filtering in various intracavity frequency-doubled lasers by making specific adjustments to the inserted birefringent filter (group), and can tune the central wavelength of the output laser. When the designed birefringent filter is inserted into the optical path, the phase delay effect generated by the birefringence effect makes the birefringent filter have a special transmittance curve for light of each wavelength band, and the position of the transmittance peak can be adjusted when the birefringent filter is rotated around the surface normal, thereby playing the role of narrowing and tuning the laser line width in a wide range of wavelength bands.

[0059] For ease of understanding, this application also provides Figure 2 Another device for narrowing the line width of a frequency-doubled laser is shown, in which the laser resonant cavity adopts a V-shaped folded cavity.

[0060] like Figure 2 The V-shaped folded cavity alexandrite intracavity frequency-doubled ultraviolet laser shown uses a monolithic birefringent filter to narrow the output laser line width. The entire device includes a pump module 100, a pump light collimation system 110, half-wave plates 121 and 122, a polarization beam splitter prism 130, a focusing lens 140, laser resonant cavity reflectors 601, 602 and 603, a gain medium 300, a monolithic birefringent filter 400 and a frequency-doubled crystal 500.

[0061] Among them, the pump module 100 is composed of a 638nm red light LD (laser diode, with a maximum output power of up to 40W) and a coupling optical fiber. After the pump light is collimated by the collimation system 110, it is adjusted by the half-wave plate 121 so that the horizontal polarization component is dominant, and then after passing through the polarization beam splitter prism 130, the remaining light is almost all horizontally polarized linear polarized light. The role of the half-wave plate 122 is to fine-tune the polarization direction again so that the polarization direction of the pump light is parallel to the maximum absorption axis of the subsequent gain medium, thereby making the overall efficiency higher. The focusing mirror 140 is a lens with a focal length of f = 50mm, which focuses the pump light. Reflectors 601, 602 and 603 together form a V-shaped folded cavity: the front side of reflector 601 is coated with a 638nm anti-reflection film, and the back side is coated with a high-reflection film of the 756nm band, and its reflectivity can reach 99.9%; the plano-concave reflector 602 is a laser output coupling mirror, and its concave surface is coated with a high-transmission film of the 378nm band and a high-reflection film of the 756nm band when the incident angle is 10°, and its curvature radius RC = 150mm, and the plane is coated with an anti-reflection film of the 378nm band, so that the doubled frequency ultraviolet laser can be output here; reflector 603 has a high reflectivity of both 756nm and 378nm. 400 is alexandrite crystal, i.e. Cr 3+ :BeAl2O4 crystal is used as the gain medium of the laser. It is connected to a temperature control device to ensure the stability of the working state. Its b-axis is placed horizontally, and the focusing mirror 140 is adjusted to allow the linearly polarized pump light to irradiate the center of the gain medium to achieve the highest absorption efficiency. 400 is a single birefringent filter with a certain thickness, fixed on a wave plate frame, and placed in the optical path at the Brewster angle. Its thickness can be adjusted according to the line width required. Generally, a thickness of 2 mm can be selected. The birefringent filter 400 is inserted between the reflector 602 and the frequency doubling crystal 500 at the Brewster angle. The frequency doubling crystal 500 uses a lithium triborate crystal (LBO crystal), and the cutting angle corresponds to the type I phase matching angle of the 756nm photon frequency doubling, and is connected to a water cooling device to ensure the stability of the work.

[0062] First, adjust the built laser so that the fundamental frequency light power at the reflector 603 can reach the highest, and then add the birefringent filter 400 and the frequency doubling crystal 500 in sequence. First, place the birefringent filter 400, adjust the position and pitch angle, and rotate it to select the wavelength of the output fundamental frequency light. At this time, the spectral line width of the fundamental frequency light has been narrowed by the birefringent filter, and the monochromaticity of the fundamental frequency photon is relatively high. Then put the frequency doubling crystal 500 into the optical path, connect the frequency doubling crystal to the temperature control device to ensure the temperature stability, and then adjust its position and angle to achieve the phase matching angle for generating the second harmonic. The second harmonic generated at the frequency doubling crystal will pass through the birefringent filter 400 again after being reflected by the reflector 603.

[0063] It can be known from the above principle that the frequency-doubled spectrum at this time will be compressed again by the birefringent filter 400, and finally the frequency-doubled ultraviolet laser with narrow line width is output at the reflector 602, and the output spectrum pattern can be tested by a spectrometer. Finally, according to the output spectrum, the birefringent filter 400 and the frequency-doubled crystal 500 are repeatedly adjusted to obtain a narrower frequency-doubled laser spectrum output. The present application can make specific adjustments to the birefringent filter (group) inserted into the laser resonant cavity, so that the birefringent filter (group) does not completely correspond to the transmittance peaks of the fundamental frequency light and the frequency-doubled light in the resonant cavity, so that the fundamental frequency laser that has undergone a line width compression is converted into a second harmonic and the spectrum line width will be compressed again by the same birefringent filter (group), and finally a frequency-doubled laser with a large degree of spectrum line width compression can be obtained.

[0064] Therefore, based on the device for narrowing the width of a frequency-doubled laser line provided in the above embodiment, the present application also provides a method for narrowing the width of a frequency-doubled laser line, please refer to Figure 3 , which shows a flow chart of a method for narrowing the width of a frequency-doubled laser line provided in an embodiment of the present application. As shown in the figure, the method includes:

[0065] Step S101: moving the birefringent filter and the frequency doubling crystal in the laser resonant cavity out of the optical path;

[0066] Step S102: controlling the pump module to provide linearly polarized light with a specific polarization direction into the optical path;

[0067] Step S103: adjusting the gain medium so that the power of the fundamental frequency laser output by the laser resonant cavity reaches the maximum;

[0068] Step S104: inserting the birefringent filter into the optical path at the Brewster angle, and adjusting the birefringent filter to select the wavelength of the output fundamental frequency laser;

[0069] Step S105: inserting the frequency doubling crystal into the optical path, adjusting the frequency doubling crystal to achieve a phase matching angle for generating a second harmonic, and generating a frequency doubling laser;

[0070] Step S106: fine-tune the birefringent filter and the frequency doubling crystal again to change the transmittance peak position of the birefringent filter for the fundamental laser and the frequency doubling laser, so that the birefringent filter can simultaneously narrow the spectral line width of the fundamental laser and the frequency doubling laser to obtain a frequency doubling laser with a narrowed spectral line width.

[0071] Figure 4 is the spectral line width obtained without using the device of the present application, Figure 5 The spectral line width obtained using the device of this application is Figure 4 and Figure 5Compared with the spectral line width effect of the spectral line width of the laser, the line width (FWHM) is reduced from 479.9pm to 7.599pm, which is narrowed to 1 / 63 of the original. It can be seen that the device and method for narrowing the output laser line width provided by the present application are effective.

[0072] The present application achieves the effect of narrowing the output laser line width by inserting a designed birefringent filter at the Brewster angle into the resonant cavity. The birefringent filter itself is made of quartz crystal and has the following advantages: low cost and easy processing; good stability and not easy to deliquesce; high damage threshold and can be inserted into a higher power laser cavity; quartz crystal has high light transmittance in the near-ultraviolet, visible and infrared bands, and as an optical element itself has a small insertion loss, etc. This method of narrowing the line width is relatively simple, uses a small number of components, is easy to operate, and can be applied to various common lasers to achieve a good line width compression effect.

[0073] It should be noted that the flowcharts and block diagrams in the accompanying drawings show the possible architecture, functions and operations of the systems, methods and computer program products according to multiple embodiments of the present application. In this regard, each box in the flowchart or block diagram can represent a module, a program segment or a part of a code, and the module, a program segment or a part of a code contains one or more executable instructions for implementing a specified logical function. It should also be noted that in some alternative implementations, the functions marked in the box can also occur in a different order from the order marked in the accompanying drawings. For example, two consecutive boxes can actually be executed substantially in parallel, and they can sometimes be executed in the opposite order, depending on the functions involved. It should also be noted that each box in the block diagram and / or flowchart, and the combination of the boxes in the block diagram and / or flowchart can be implemented with a dedicated hardware-based system that performs a specified function or action, or can be implemented with a combination of dedicated hardware and computer instructions.

[0074] Those skilled in the art can clearly understand that, for the convenience and brevity of description, the specific working processes of the systems, devices and units described above can refer to the corresponding processes in the aforementioned method embodiments and will not be repeated here.

[0075] In the several embodiments provided in the present application, it should be understood that the disclosed devices and methods can be implemented in other ways. The device embodiments described above are merely schematic. For example, the division of the units is only a logical function division. There may be other division methods in actual implementation. For example, multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be through some communication interfaces, and the indirect coupling or communication connection of devices or units can be electrical, mechanical or other forms.

[0076] The units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed on multiple network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0077] In addition, each functional unit in each embodiment of the present application may be integrated into one processing unit, or each unit may exist physically separately, or two or more units may be integrated into one unit.

[0078] If the functions are implemented in the form of software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present application, or the part that contributes to the prior art or the part of the technical solution, can be embodied in the form of a software product, which is stored in a storage medium and includes several instructions for a computer device (which can be a personal computer, a server, or a network device, etc.) to perform all or part of the steps of the methods described in each embodiment of the present application. The aforementioned storage medium includes: various media that can store program codes, such as a USB flash drive, a mobile hard disk, a read-only memory (ROM), a random access memory (RAM), a magnetic disk or an optical disk.

[0079] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit it. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein by equivalents. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present application, and they should all be included in the scope of the claims and specification of the present application.

Claims

1. A device for narrowing the line width of a frequency-doubled laser, characterized in that: include: A pump module, used to provide linear polarized light with a specific polarization direction into the optical path; A laser resonant cavity, wherein a gain medium, at least one birefringent filter and a frequency doubling crystal are sequentially arranged along an optical path in the laser resonant cavity, wherein the at least one birefringent filter is inserted into the optical path at a Brewster angle; The linearly polarized light is focused in the gain medium, the central wavelength of the linearly polarized light is the same as the absorption peak wavelength of the gain medium, and the polarization direction of the linearly polarized light is the same as the maximum absorption direction of the gain medium.

2. The device for narrowing the frequency-doubled laser line width according to claim 1, characterized in that: The number of the birefringent filters is greater than one, and the thicknesses of all the birefringent filters are in a preset ratio.

3. The device for narrowing the line width of a frequency-doubled laser according to claim 1, characterized in that: The gain medium is any one of the following solid laser crystals: Nd:YVO4, Nd:YAG, Nd:YLF, Yb:YAG, Tm:YAG, Er:YAG, and Cr 3+ :BeAl2O4.

4. The device for narrowing the line width of a frequency-doubled laser according to claim 1, characterized in that: The frequency doubling crystal is any one of the following: KDP crystal, LBO crystal and BBO crystal.

5. The device for narrowing the line width of a frequency-doubled laser according to claim 1, characterized in that: The focusing position of the linearly polarized light is at the center of the gain medium.

6. The device for narrowing the line width of a frequency-doubled laser according to claim 1, characterized in that: The type of the laser resonant cavity is any one of the following: Plane-to-plane critical cavity, plane-to-concave cavity, annular cavity, X-shaped cavity and V-shaped cavity.

7. The device for narrowing the line width of a frequency-doubled laser according to claim 1, characterized in that: The pumping mode of the pump module is end pumping.

8. A method for narrowing the width of a frequency-doubled laser spectral line, based on the device for narrowing the width of a frequency-doubled laser spectral line as claimed in any one of claims 1 to 7, characterized in that: The method comprises: Move the birefringent filter and frequency doubling crystal in the laser resonant cavity out of the optical path; Controlling the pump module to provide linear polarized light with a specific polarization direction into the optical path; Adjusting the gain medium so that the power of the fundamental frequency laser output by the laser resonant cavity reaches the highest; Inserting the birefringent filter into the optical path at the Brewster angle, and adjusting the birefringent filter to select the wavelength of the output fundamental frequency laser; Inserting the frequency doubling crystal into the optical path, adjusting the frequency doubling crystal to achieve a phase matching angle for generating a second harmonic, and generating a frequency doubling laser; The birefringent filter and the frequency doubling crystal are fine-tuned again to change the transmittance peak position of the birefringent filter for the fundamental frequency laser and the frequency doubling laser, so that the birefringent filter can simultaneously narrow the spectral line width of the fundamental frequency laser and the frequency doubling laser to obtain a frequency doubling laser with a narrowed spectral line width.

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