Device and method for generating high-power Raman structured light field

Abstract

The present invention discloses a device and method for generating a high-power Raman structured optical field, where the pump light output by a pump source with a central wavelength of λ p is incident on a Raman resonator mounted on a two-dimensional movable platform after passing through a coupling system composed of a telescope system, a polarizer, a half-wave plate, and a focusing lens; in the resonator, the pump light with a central wavelength of λ p undergoes stimulated Raman scattering with a Raman crystal to generate Stokes signal light with a central wavelength of λ s . By adjusting the offset of the two-dimensional movable platform and the pitch and yaw angles of the output mirror, the optical field structure of the Stokes signal light is regulated, and a high-power Raman structured optical field with extended wavelength is output; the pump light is filtered out by a long-pass filter to obtain pure Stokes structured light. The present invention can enable the output optical field of a solid-state structured light laser to have a certain complex structure while extending the output wavelength of the structured light and solving the problem of power limitation.

Description

Technical Field

[0001] The present invention relates to the field of intracavity optical field regulation, and particularly to a device and method for generating a high-power Raman structured optical field. Background Art

[0002] In recent years, structured light, especially vortex beams with orbital angular momentum, can be widely applied in the fields of optical communication, optical trapping, laser processing, rotational speed measurement, etc. However, the preparation of lasers that can output high power and have a complex optical field structure has become a major problem, restricting the application of structured light. The generation methods of solid-state structured light sources are divided into two types: extracavity and intracavity. Among them, extracavity generation mainly relies on device modulation, such as: spiral phase plates, Q plates, spatial light modulators, metasurfaces, etc. The obtained structured light wavelength cannot be extended and the power is low. Generating a structured optical field intracavity is relatively convenient, with high purity of the output structured light, adjustable optical field structure, wide wavelength and time-domain magnitude range, which is the main method for preparing solid-state structured light sources. Non-collinear pumping is the main means for preparing solid-state structured light sources based on intracavity mode selection, with the advantages of controllable optical field structure, complex optical field structure, and convenient and controllable adjustment.

[0003] However, currently, such structured light sources cannot ensure both the advantages of outputting a complex optical field structure and high power. Microchip lasers have great potential for generating a highly complex optical field structure, but the output power is limited; for general lasers, high-power output can be ensured, but the complexity of the optical field structure is very limited. Summary of the Invention

[0004] The present invention provides a device and method for generating a high-power Raman structured optical field. The present invention can enable the output optical field of a solid-state structured light laser to have a certain complex structure while extending the output wavelength of the structured light and solving the problem of limited power, as described in detail below:

[0005] A device for generating a high-power Raman structured optical field, the device comprising: the pump light output by a pump source with a central wavelength of λ p after passing through a coupling system composed of a telescope system, a polarizer, a half-wave plate, and a focusing lens, is incident on a Raman resonator mounted on a two-dimensional movable platform;

[0006] Stimulated Raman scattering occurs between the pump light with a central wavelength of λ p in the resonator and the Raman crystal, generating Stokes signal light with a central wavelength of λ s Adjust the offset of the two-dimensional movable platform and the pitch and yaw angles of the output mirror to control the optical field structure of the Stokes signal light, and output a high-power Raman structured optical field with an extended wavelength; the pump light is filtered out by a long-pass filter to obtain pure Stokes structured light.

[0007] Among them, the Raman resonator is composed of an input mirror, an output mirror and a Raman crystal. The input mirror is coated to have high transmittance for pump light with a central wavelength of λ p and high reflectance for Stokes light with a central wavelength of λ s . The output mirror is coated to have high reflectance for pump light with a central wavelength of λ p ;

[0008] Partially reflect the Stokes light with a central wavelength of λ s to output the Stokes light. The two end faces of the Raman crystal are coated to have high transmittance for pump light with a central wavelength of λ p and Stokes light with a central wavelength of λ s . The Raman crystal is placed in a constant temperature heat sink.

[0009] Furthermore, the polarizer and half-wave plate in the coupling system transform the passing high-power pump light into a beam with a controllable linear polarization state and match the gain characteristics of the laser.

[0010] The radius of curvature R1 and R2 of the two cavity mirrors of the resonator, and the optical length L of the resonator C satisfy the following relationships:

[0011] k = R2 / R1, and R1 ≤ R2

[0012] L c = n*(R1 + R2), and n ≤ 1

[0013]

[0014] Among them, n represents the ratio of the radius of curvature R1 and R2 of the two cavity mirrors to the optical length L of the resonator c . Ω<1 / 2 represents the ratio of the transverse mode interval to the longitudinal mode interval, expressed as a proper fraction P / Q, that is, P and Q are relatively prime; when given Ω and k, two n values are solved to improve the quality factor Q of the resonator.

[0015] A method for generating a high-power Raman structured light field, the method includes:

[0016] The pump light output by the pump source is incident into the Raman resonator through the coupling system, and after adjusting the cavity mirror for collimation, a fundamental mode Gaussian spot is obtained;

[0017] Adjust the two-dimensional movable platform to form an offset between the overall Raman resonator and the optical axis of the pump light, forming transverse and longitudinal pump off-axis, and the off-axis amounts are Δx and Δy respectively. Gradually increase the off-axis amount to excite the structured light field S (n,m) mode;

[0018] S (n,m) The excitation threshold power of the mode is expressed as:

[0019]

[0020] Among them, T is the transmittance of the output mirror, σ is the theoretical maximum slope efficiency, and G (n,m) is the Raman power gain; when the off-axis amount is constant, a specific S (n,m) mode reaches the lowest threshold oscillation, and the theoretical maximum slope efficiency σ is expressed as:

[0021] σ = T·λ p / λ s / (T + 2αl)

[0022] Among them, l is the length of the Raman crystal, α is the absorption coefficient of the Raman crystal, and λ p , λ s are the central wavelengths of the pump light and the Stokes light, respectively;

[0023] The Raman power gain G (n,m) is expressed as:

[0024]

[0025] Among them, g s is the Raman gain coefficient, w p , w s are the beam sizes of the pump light and the Stokes light in the Raman crystal, respectively, and Δx, Δy

[0026] are the off-axis amounts in the transverse and longitudinal directions. N n,m is related to the mode order (n, m) and is expressed as:

[0027]

[0028] Among them, F n (w p , w s , Δx) characterizes the overlap integral of the pump light and the Stokes light under unidirectional off-axis:

[0029]

[0030] The beneficial effects of the technical solution provided by the present invention are:

[0031] 1. The structured light laser provided by the present invention has the characteristics of stability and strong controllability, and the operation is relatively simple;

[0032] 2. The external cavity Raman laser resonator provided by the present invention can greatly enrich the structure of the output optical field. The obtained laser has excellent frequency-domain oscillation characteristics. By selecting a Raman crystal, such as diamond, the wavelength of the structured optical field can be extended and the corresponding output power can be increased. Placing the external cavity Raman laser on a two-dimensional adjustable displacement platform simplifies the optical field regulation operation and has excellent stability;

[0033] 3. The laser provided by the present invention has a compact structure and good stability, and can be used in fields such as free space optical communication, laser processing, and laser remote sensing systems. Brief Description of the Drawings

[0034] Figure 1 It is a schematic diagram of the principle of a device for generating a high-power Raman structured optical field;

[0035] Figure 2 It is a schematic diagram of the beam transmission of the pump light coupling system;

[0036] Figure 3 It is a schematic diagram of the optical path transmission inside the collinear pump Raman resonator;

[0037] Figure 4 It is a schematic diagram of the principle of off-axis pumping to generate a Raman structured optical field;

[0038] Figure 5 It is a schematic diagram of the principle of regulating the optical field structure by the deflection angle of the output mirror. Detailed Embodiments

[0039] To make the objectives, technical solutions, and advantages of the present invention clearer, the following further describes the embodiments of the present invention in detail.

[0040] The external cavity Raman laser based on the stimulated Raman scattering of the third-order nonlinear optical effect has no population inversion, which can reduce the accumulation of thermal effects and increase the output power to a certain extent. A typical Raman crystal, such as diamond, has a wide transmission spectrum, excellent thermal conductivity, high Raman gain coefficient, and large Raman frequency shift. In recent years, using the excellent properties of Raman crystals to prepare external cavity Raman lasers has become an effective means to obtain wavelength-extended and high-power laser output. Combining with the in-cavity generation technology of structured optical fields, it becomes possible to prepare a structured light laser that can output high power, and a high-power, high-purity, high-structural complexity, simple and compact structured light output with extended wavelength can be obtained. Such a laser can, to a certain extent, break the limitations of power and wavelength on the application of structured light.

[0041] A device for generating a high-power Raman structured optical field, see Figure 1 and Figure 2 , the device includes: the central wavelength is λ pThe pump light 1 output by the pump source passes through the coupling system 2 composed of a telescope system, a polarizer, a half-wave plate and a focusing lens, and then enters the Raman resonator 3 installed on the two-dimensional movable platform. The central wavelength in the resonator 3 is λ p Stimulated Raman scattering occurs between the pump light with the central wavelength of λ and the Raman crystal, thereby generating Stokes signal light with the central wavelength of λ s By adjusting the offset of the two-dimensional movable platform and the pitch and yaw angles of the output mirror, the light field structure of the Stokes signal light is regulated, so as to output a high-power Raman structured light field with wavelength expansion; the pump light is filtered by a long-pass filter to obtain pure Stokes structured light.

[0042] The Raman resonator 3 is composed of an input mirror, an output mirror and a Raman crystal. The coating of the input mirror should ensure high transmittance for the pump light with the central wavelength of λ p and high reflectivity for the Stokes light with the central wavelength of λ s The coating of the output mirror should ensure high reflectivity for the pump light with the central wavelength of λ p to form a double-pass pump structure to further improve the conversion efficiency. At the same time, the Stokes light with the central wavelength of λ s is partially reflected to output the Stokes light. The coatings on both end faces of the Raman crystal should ensure high transmittance for both the pump light with the central wavelength of λ p and the Stokes light with the central wavelength of λ s The Raman crystal is placed in a constant-temperature heat sink to reduce the influence brought by the thermal effect.

[0043] The polarizer and half-wave plate in the coupling system 2 can transform the passed high-power pump light into a beam with a controllable linear polarization state and match the gain characteristics of the laser.

[0044] The telescope system in the coupling system 2 can magnify the beam size of the passed high-power pump light by a certain multiple, and with the matching focusing lens, high-power pump light can be tightly focused to stimulate stimulated Raman scattering and amplify the oscillating signal light.

[0045] A method for generating a high-power Raman structured light field, see Figures 3 - 5 , and this method includes the following steps:

[0046] Step 101: The pump light output by the pump source 1 passes through the coupling system 2 and enters the Raman resonator 3. The cavity mirrors 3-1 and 3-2 are adjusted to obtain a fundamental-mode Gaussian spot;

[0047] Step 102: Adjust the two-dimensional movable platform 3-4 so that the whole Raman resonator 3 forms an offset with the optical axis of the pump light, thereby forming transverse and longitudinal pump off-axis, and the off-axis amounts are Δx and Δy respectively. Gradually increase the off-axis amount to stimulate a structured light field;

[0048] Step 103: When the off-axis amount is constant, the pitch angle and yaw angle of the output mirror 3-2 can be adjusted to further adjust the light field structure.

[0049] Combine Figure 3 To illustrate this embodiment, the Raman resonant cavity 3 in step 101 is composed of an input mirror 3-1 with a curvature radius of R1, an output mirror 3-2 with a curvature radius of R2, and a Raman crystal 3-3. The Raman resonant cavity 3 has an optical length L C , it should be possible to form an intrinsic beam waist that matches the size of the tightly focused pump beam and to couple the frequencies of the transverse mode and the longitudinal mode. That is, the resonant cavity is a degenerate cavity with transverse and longitudinal frequency coupling. The curvature radii R1 and R2 of the two cavity mirrors and the optical length L C The three should satisfy the following relationship:

[0050] k=R2 / R1, and R1≤R2 (1)

[0051] L c =n*(R1+R2), and n≤1 (2)

[0052]

[0053] In the above formula, n represents the curvature radius of the two cavity mirrors R1 and R2 and the optical length of the resonant cavity L c Ω < 1 / 2 represents the ratio of the transverse mode spacing to the longitudinal mode spacing, and should be expressible as a proper fraction P / Q, meaning that P and Q are coprime. Using the above equation, given Ω and k, two n values can be obtained. Smaller values should be chosen to increase the resonant cavity's quality parameter, the Q factor, to ensure the complexity of the light field structure generated within the cavity.

[0054] Example 1

[0055] Combine Figure 1 and Figure 2 This embodiment describes a device for generating a high-power Raman structured light field, including: a pump source 1, a coupling system 2, a Raman resonant cavity 3, and a long-pass filter 4;

[0056] The central wavelength is λ p The pump light output by the pump source 1 passes through the coupling system 2 composed of the telescope system 2-1, the polarizer 2-2, the half-wave plate 2-3 and the focusing lens 2-4, and is incident on the Raman resonator 3. Stimulated Raman scattering occurs in the cavity and amplifies the central wavelength λ s The Stokes signal light is obtained by filtering the pump light through the long-pass filter 4 and obtaining the pure Stokes structured light.

[0057] The Raman resonator 3 is composed of an input mirror, an output mirror, and a Raman crystal. The coating of the input mirror should ensure high transmittance for the pump light with a central wavelength of λ p and high reflectance for the Stokes light with a central wavelength of λ s . The coating of the output mirror should ensure high reflectance for the pump light with a central wavelength of λ p to form a double-pass pump structure to further improve the conversion efficiency. At the same time, partial reflection of the Stokes light with a central wavelength of λ s is used to output the Stokes light. The coatings on both end faces of the Raman crystal should ensure high transmittance for both the pump light with a central wavelength of λ p and the Stokes light with a central wavelength of λ s . The Raman crystal is placed in a constant-temperature heat sink to reduce the influence of thermal effects.

[0058] The polarizer 2-2 and the half-wave plate 2-3 in the coupling system 2 can transform the passed high-power pump light into a beam with a controllable linear polarization state and match the gain characteristics of the laser.

[0059] The telescope system 2-1 in the coupling system 2 can magnify the beam size of the passed high-power pump light by a certain multiple. With the collimating lens 2-4, tight focusing of the high-power pump light can be achieved, that is, a smaller focused beam size, so as to obtain a higher pump power density to excite stimulated Raman scattering and amplify the oscillating signal light.

[0060] Embodiment 2

[0061] Combined with Figure 3 , Figure 4 and Figure 5 to illustrate this embodiment. The Raman cavity 3 has an optical length L C and is composed of an input mirror 3-1 with a radius of curvature R1, an output mirror 3-2 with a radius of curvature R, and a Raman crystal 3-3.

[0062] The entire Raman resonator 3 is installed on a two-dimensional movable platform 3-4. By adjusting the two-dimensional movable platform 3-4, the off-axis offsets Δx and Δy between the signal light and the pump light in the cavity are formed, so as to excite higher-order eigenmodes to form a complex structured light field. By adjusting the pitch angle and yaw angle of the output mirror 3-2, the distribution of the signal light in the cavity is further adjusted, the light field structure of the Stokes signal light is controlled, and a high-power Raman structured light field with extended wavelength is output.

[0063] Embodiment 3

[0064] The high-power pump 1 selects a high-power Nd:YAG laser, and the corresponding laser output parameters are: the central wavelength λ p is 1064 nm, and the beam quality M 2<1.1. The optical size diameter is 2 mm; the coupling system 2 consists of a telescope system 2-1, a polarizer 2-2, a half-wave plate 2-3, and a focusing lens 2-4. The coupling system 2 is coated with an antireflection film for 1064 nm laser. The telescope system 2-1 includes two lenses with focal lengths of 50 mm and 200 mm respectively, separated by 250 mm. The focal length of the focusing lens 2-4 is selected to be 250 mm, and the waist size radius of the tightly focused pump beam generated thereby is approximately 0.06 mm; the Raman resonator 3 consists of a spherical input mirror 3-1 with a radius of curvature R1 = 50 mm, a spherical output mirror 3-2 with a radius of curvature R2 = 100 mm, and a Raman crystal 3-3 with dimensions of 3*3*4 mm 3 The Raman crystal 3-3 is made of diamond, and the center wavelength λ s of the corresponding Stokes signal light is 1240 nm. The input mirror 3-1 is coated with an antireflection film for 1064 nm and a high-reflection film for 1240 nm. The output mirror has a reflectivity of 97% for 1240 nm Stokes light and highly reflects 1064 nm light. Both ends of the diamond are coated with an antireflection film for 1064 nm and an antireflection film for 1240 nm.

[0065] Among them, the optical length L C of the Raman resonator 3 is confirmed by applying Equation (1), Equation (2), and Equation (3): According to Equation (2), k = 1.5 is obtained; according to Equation (2) and Equation (3), when Ω = 1 / 4 is specified, n = 0.86, 0.15 can be solved, and n = 0.15 is selected, that is, the optical length of the resonator is determined to be 22.5 mm; at this time, the Raman resonator 3 is in a degenerate cavity with transverse and longitudinal frequency coupling.

[0066] The aperture of the diamond Raman crystal 3-3 is 3*3 mm 2 , which allows the order of the fundamental transverse mode that can oscillate in the cavity to reach 200; the selected two-dimensional adjustable displacement platform 3-4 has a regulation accuracy of 0.01 mm and a regulation range of at least 3 mm, which can make the Raman resonator 3 have a large enough off-axis distance from the tightly focused high-power pump to excite the ultra-high-order fundamental transverse mode; after the cavity reaches the "aligned" state, a circular light spot, that is, a fundamental mode Gaussian beam, should be output. By respectively regulating the transverse off-axis amount Δx and the longitudinal off-axis amount Δy to 0.4 mm, it can be observed that the light field structure gradually changes to high-order Hermite-Gaussian modes HG 0,2 , HG 0,4 , HG 0,7 and other various independent high-order transverse modes, as well as coherent superposition modes HG 0,1 +i HG 1,0 , HG 1,2 +i HG 2,1 , HG 1,1 +i HG 1,2 +i HG 2,1etc.; adjusting the pitch and yaw angles of the output mirror can assist off-axis pumping to regulate the optical field structure and generate a vortex lattice containing multiple singularities; increasing the off-axis amount can result in a more complex structured optical field, such as: ultra-high-order elliptical or hyperbolic eigen optical fields, or even ray duality modes, all with high output power.

[0067] Among them, the long-pass filter 4 only allows the Stokes light with a central wavelength of 1240 nm to be highly transmitted. That is, only pure Stokes structured light passes through the long-pass filter 4 after the beam output by the Raman resonator 3, and it has high output power and a complex optical field structure.

[0068] In summary, the embodiments of the present invention provide a method and device for generating a high-power Raman structured optical field applicable to fields such as optical communication, laser processing, and optical manipulation, which can effectively generate a stable high-power structured light output with a complex optical field structure, and at the same time, the wavelength can be correspondingly extended, thus greatly expanding its application scope.

[0069] In the embodiments of the present invention, except for those with special specifications for the models of each device, the models of other devices are not limited, as long as they can perform the above functions.

[0070] Those skilled in the art can understand that the drawings are only schematic diagrams of a preferred embodiment, and the serial numbers of the above embodiments of the present invention are only for description and do not represent the superiority or inferiority of the embodiments.

[0071] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A generating device for a high-power Raman structured optical field, characterized in that, The device includes: After the pump light output by a pump source with a central wavelength of passes through a coupling system composed of a telescope system, a polarizer, a half-wave plate and a focusing lens, it is incident on a Raman resonator mounted on a two-dimensional movable platform; The pump light with a central wavelength in the resonant cavity undergoes stimulated Raman scattering with the Raman crystal, generating Stokes signal light with a central wavelength of ; the offset of the two-dimensional movable platform and the pitch and yaw angles of the output mirror are adjusted to control the optical field structure of the Stokes signal light, and a high-power Raman structured optical field with extended wavelength is output; the pump light is filtered out by a long-pass filter to obtain pure Stokes structured light; Among them, the Raman resonator is composed of an input mirror, an output mirror, and a Raman crystal. The input mirror is coated to have high transmittance for pump light with a central wavelength of and high reflectance for Stokes light with a central wavelength of . The output mirror is coated to have high reflectance for pump light with a central wavelength of ; Partially reflect the Stokes light with a central wavelength of to output the Stokes light. The two end faces of the Raman crystal are coated to have high transmittance for the pump light with a central wavelength of and the Stokes light with a central wavelength of . The Raman crystal is placed in a constant-temperature heat sink block; The radius of curvature of the two mirrors of the resonant cavity , , and the optical length of the resonant cavity The three satisfy the following relationship: ; Among them, represents the radius of curvature of the two endoscopes , and the ratio of the optical length of the resonant cavity . <1 / 2 represents the ratio of the transverse mode interval to the longitudinal mode interval, expressed as a proper fraction P / Q, that is, P and Q are co-prime numbers; when given and k, two values of n are obtained to improve the quality factor Q of the resonant cavity.

2. The generating device for a high-power Raman structured optical field according to claim 1, characterized in that, The polarizer and half-wave plate in the coupling system transform the incident high-power pump light into a beam with a controllable linear polarization state and match the gain characteristics of the laser.

3. A method for generating a high-power Raman structured optical field, characterized in that, The method includes: The pump light output by the pump source is incident on the Raman resonant cavity through the coupling system, and after adjusting the cavity mirror for collimation, a fundamental mode Gaussian spot is obtained. Adjust the two-dimensional movable platform to form an offset between the overall Raman resonator and the optical axis of the pump light, resulting in transverse and longitudinal pump off-axis, with the off-axis amounts being and , respectively. Gradually increase the off-axis amount to excite the structured optical field mode; The excitation threshold power of the mode is expressed as: ； where T is the transmittance of the output mirror, is the theoretical maximum slope efficiency, is the Raman power gain; when the off-axis amount is constant, a specific mode reaches the lowest threshold oscillation, and the theoretical maximum slope efficiency is expressed as: ； where, l is the length of the Raman crystal, is the absorption coefficient of the Raman crystal, , are the central wavelengths of the pump light and the Stokes light, respectively; Raman power gain Expressed as: ； Among them, is the Raman gain coefficient, , are the beam sizes of the pump light and the Stokes light in the Raman crystal respectively, , are the off-axis amounts in the transverse and longitudinal directions, related to the mode order is expressed as: ； Among them, characterized by the overlapping integral of the unidirectional off-axis downward pump light and the Stokes light: ; 。

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