An intracavity-pumped diamond mid-infrared Raman laser generating device and method

Abstract

The present invention provides an intracavity-pumped diamond mid-infrared Raman laser generating device and method. The device: a high-brightness laser pump source, a reflective laser beam expander-focusing device, a resonator input mirror, a laser gain medium, and a resonator output mirror are sequentially arranged at intervals to form a laser generating device. The method: according to the target laser wavelength, the optimal pump wavelength is calculated by the first-order and second-order Raman frequency shift formulas; according to the Raman gain relationship between the pump wavelength and the diamond crystal, the optimal values of the pump spot size, the mode field diameter of the Raman signal laser, and the diamond crystal length are designed; the resonator input mirror and the resonator output mirror are precisely adjusted to obtain the required Raman signal laser output; the switching of the output laser wavelength is achieved through the type of dielectric film of the resonator output mirror; continuous tunability of any wavelength from 2500 nm to 3780 nm of the output laser is realized. The device and method can achieve multi-wavelength output and can effectively output mid-infrared lasers in the 2 μm - 5 μm band.

Description

Technical Field

[0001] The present invention relates to an intracavity-pumped diamond mid-infrared Raman laser generating device and method, belonging to the technical field of lasers. Background Art

[0002] The mid-infrared laser with a wavelength of 2-5 μm is restricted by the laser gain material, resulting in low laser generation efficiency, low power, and a limited laser wavelength range in the existing methods, which has become a difficult problem for generating high-power mid-infrared lasers. Diamond has the following advantages as a Raman gain medium: wide light transmission range (225 nm - 20 μm), large Raman frequency shift (1332 cm -1 ), large Raman gain coefficient (the value at 1 μm wavelength is 10 cm / GW), high thermal conductivity (2000 W / m·K), and small thermal expansion coefficient (1.1×10 -6 / K), as specifically shown in Figure 1。In the field of diamond Raman laser generation, theoretically, directly pumping diamond with lasers in the range of 2.1 μm to 2.5 μm can achieve mid-infrared laser output in the range of 2.5 μm to 3.8 μm through the first-order stimulated Raman process. However, in this wavelength band, the three-phonon intrinsic absorption effect of diamond is significant, and multi-phonon absorption leads to very serious laser loss in the 2.5 μm to 6.5 μm band, and high-power lasers cannot be directly generated between 2.3 μm and 3.8 μm. Therefore, traditional technical solutions avoid the high-loss region in the diamond Raman gain region and attempt to obtain mid-infrared laser output for two low-loss windows. In 2014, Alexander Sabella et al. (Opt. Lett., 39(13):4037-4040(2014), Proc SPIE 8959(0B-1)2014) adopted a direct pumping method and used a wavelength-tunable, pulsed-output optical parametric oscillator laser as a pump source to achieve laser output with a wavelength of 3.5 μm in the low-loss region of 3.4 μm to 3.5 μm in diamond. However, the output wavelength of this technology is only limited to around 3.5 μm and cannot be extended to other wide wavelength bands. In 2019, GIORGOS DEMETRIOU et al. (Opt Express, 27(7):10296-10303(2019)) and Shao Zhenhua et al. from Fudan University (Laser Physics Letters, 2021, 18(7), 075001) proposed directly pumping a diamond crystal with a 1.89 μm laser to generate a high first-order Raman signal laser of 2.52 μm near another low-loss region of 2.5 μm. However, this method can only generate Raman signal lasers with a wavelength of 2.5 μm and also cannot be extended to other wide ranges of wavelength bands in the diamond gain region. In short, existing technical solutions only attempt to obtain mid-infrared lasers for two low-loss windows in the vast diamond gain region and cannot solve the problem of efficiently and directly generating lasers in other large ranges of the mid-infrared band, seriously hindering the technical development and application of mid-infrared lasers. Summary of the Invention

[0003] Aiming at the problems existing in the above-mentioned prior art, the present invention provides an intracavity-pumped diamond mid-infrared Raman laser generation device and method. The device has a simple structure, low loss, low threshold, good stability, and can achieve multi-wavelength output. It can effectively output mid-infrared lasers in the 2 μm to 5 μm band; the method has simple steps and is easy to implement, and can stably generate lasers in the 2.55 μm to 3.80 μm band through the Raman nonlinear process.

[0004] To achieve the above object, the present invention provides an intracavity-pumped diamond mid-infrared Raman laser generation device, including a high-brightness laser pump source, a laser gain medium, a reflective laser beam expander-focusing device, a resonator input mirror, and a resonator output mirror;

[0005] The high-brightness laser pumping source, the reflective laser beam expander-focusing device, the resonator input mirror, the laser gain medium, and the resonator output mirror are sequentially arranged at intervals to form a laser generating device;

[0006] The high-brightness laser pumping source is a pumping source with any wavelength or tunable wavelength within the range of 1500 nm to 1880 nm;

[0007] The laser beam expander-focusing device includes optical lenses, which are geometric curved mirrors, and the specific surface shape is a spherical surface or an off-axis paraboloid; the ratio of the curvature radii R1 and R2 of the optical lenses is 2:1, and specifically, they can be 200 mm and 100 mm or 100 mm and 50 mm respectively. The distance L between the optical lenses satisfies: L = (R1 + R2) / 2. The spatial angular positions of the reflection curved surface centers of the optical lenses are parallel or perpendicular. The surfaces of the optical lenses are coated with broadband high-reflection films, and the reflectivity is between 96% and 99.6% between 1500 nm and 1880 nm;

[0008] The coating of the resonator input mirror is a multi-color film system, which is highly transmissive to the pumping laser of 1500 - 1880 nm, and its transmittance > 98%. It is highly reflective to the laser of 1900 - 2500 nm, and the reflectivity > 99.6%. It is highly reflective to the laser of 2600 - 3780 nm, and the reflectivity > 99.6%;

[0009] The coating of the resonator output mirror satisfies that it is highly reflective to the pumping laser of 1500 nm to 1880 nm, and the reflectivity > 99%. It is highly reflective or partially reflective to the first-order Raman signal laser of 1900 nm to 2500 nm, and the reflectivity > 99.6%. It has a high transmittance to the second-order Raman signal laser of 2600 nm to 3780 nm, and the numerical value of its output transmittance T satisfies: 95% < T < 100%;

[0010] The resonator input mirror and the resonator output mirror form a 1.91 μm - 2.51 μm laser resonator; the geometric surface shapes of the resonator input mirror and the resonator output mirror are spherical surfaces, and the ratio of their curvature radii is 1:1, specifically 50 mm and 50 mm or 75 mm and 75 mm respectively, and their positions are at the confocal position, or their geometric surface shapes are both flat surfaces;

[0011] The laser gain medium is a diamond crystal, and its two light-transmitting surfaces are cut at Brewster angles or perpendicularly, and it is arranged on the optical path between the resonator input mirror and the resonator output mirror, and generates Raman laser within the range of 2.55 μm - 3.80 μm after receiving Raman laser within the range of 1.91 μm - 2.51 μm.

[0012] As a preference, the diamond crystal is b-cut, with a length between 3 and 5 mm, and its Raman gain coefficient has a value > 10 cm / GW in the wavelength range of 1500 nm to 1880 nm.

[0013] As a preference, in order to avoid excessive material intrinsic absorption loss, the substrate material of the resonator input mirror is fused silica or calcium fluoride material with high transparency in the 1.5 μm to 1.88 μm band, and the substrate material of the resonator output mirror is calcium fluoride or zinc selenide with high transparency to mid-infrared band lasers, and the thickness of its light-transmitting center should be between 1 mm and 2 mm.

[0014] Furthermore, in order to improve the laser generation efficiency, the high-brightness laser pump source is a high-power erbium-doped or thulium-doped single-mode fiber laser, or an Er:Y2O3 sesquioxide ceramic or Er:Lu2O3 ceramic or Er:YAG ceramic solid laser with low phonon energy, or a high-beam-quality quantum well semiconductor laser.

[0015] In the present invention, a 1.91 μm - 2.51 μm laser resonator is formed by using a resonator input mirror and a resonator output mirror. This resonator has high reflectivity for the first-order Raman light and partially outputs the second-order Raman light, thereby enabling the formation of high-power-density intracavity oscillation of the first-order Raman and facilitating the excitation of the second-order Raman. A reflective laser beam expander - focusing device is arranged between the high-brightness laser pump source and the laser resonator, eliminating the change in focal length caused by the dispersion of different wavelengths in the traditional lens system. For a wavelength-tunable pump laser source, automatic and precise matching of the modes of the pump light and the generated Raman signal laser can be achieved under wavelength tuning, significantly improving the stability of the wavelength-tunable generation of the Raman signal and conveniently realizing the efficient output of tunable mid-infrared laser. Using a diamond crystal as the laser gain medium and pumping the diamond Raman medium with a high-brightness laser pump source in the range of 1500 nm - 1880 nm, the diamond can be pumped in the first-order Raman intracavity without phonon absorption loss, and then Raman laser of 2.55 μm - 3.80 μm can be generated. In this way, the Raman laser threshold can be effectively reduced, and the effective output of mid-infrared laser in the 2 μm - 5 μm band can be achieved. In this device, through the intracavity oscillation of the first-order Raman and in cooperation with phonon-free absorption loss, it can be ensured that the output transmittance of the second-order Raman is higher, approaching 100%. During the output process of the second-order Raman laser, the number of round trips of photons in the cavity is small, the target wavelength (second-order Raman, 2.55 μm - 3.80 μm) is close to passing through the diamond Raman gain medium in a single pass, the phonon absorption loss is low, and the problem of large multi-phonon absorption loss caused by the reflection of the output mirror is avoided, greatly enhancing the power of mid-infrared laser above 2.5 microns wavelength. At the same time, the power density of the first-order Raman intracavity as the pump light is high. Because the resonator mirror has high reflectivity or very low output transmittance for the first-order Raman, the intracavity power density can be higher than the incident pump light. The high intracavity power density of the first-order Raman can effectively reduce the generation threshold of the second-order Raman (band III) laser and improve the conversion rate from the first-order Raman signal laser to the second-order Raman signal laser, significantly enhancing the generation efficiency of mid-infrared laser in the high-phonon absorption region of diamond. This device has a simple structure, low loss, low threshold, good stability, can achieve multi-wavelength output, can effectively improve the generation efficiency of Raman laser in the ~3 μm band, and at the same time, does not increase the complexity of the system design.

[0016] The present invention also provides a method for generating intracavity-pumped diamond mid-infrared Raman laser, including the following steps:

[0017] Step 1: According to the target laser wavelength, calculate the optimal pump wavelength by using the Raman frequency shift formulas of the first order and the second order;

[0018] Step 2: According to the pump wavelength and the Raman gain relationship of the diamond crystal, obtain the optimal values of the pump spot size, the mode field diameter of the Raman signal laser, and the length of the diamond crystal according to formula (1);

[0019]

[0020] In the formula, I i is the pump and the intensities of the Stokes lights of all orders, where i = p, 1, 2, 3, and β i is the absorption coefficient of the Stokes light of each order, and g i = g0ω i / ω p is the Raman gain coefficient of the i-th order Stokes light, ω i and ω p are the angular frequencies of the Stokes lights of all orders and the pump light respectively, and g0 is the Raman gain coefficient of the pump wavelength;

[0021] Step Three: Precisely adjust the input mirror and the output mirror of the resonator to obtain the required Raman signal laser output;

[0022] Step Four: Realize the switching of the output laser wavelength through the type of the dielectric film of the output mirror of the resonator;

[0023] S1: Make the dielectric film of the output mirror of the resonator used be highly reflective in both the range of 1500 nm to 1880 nm and the range of 1900 nm to 2500 nm, and the reflectivity is greater than 99.6%, and output single-wavelength Raman second-order laser with a wavelength between 2600 nm and 3780 nm;

[0024] S2: Make the dielectric film of the output mirror of the resonator used be highly reflective in the range of 1500 nm to 1880 nm and partially reflective with a reflectivity between 96% and 99.6% in the range of 1900 nm to 2500 nm, and output dual-wavelength laser of the first-order and second-order Raman signals, and their wavelengths are respectively between 1900 nm and 2500 nm and between 2600 nm and 3780 nm;

[0025] S3: Make the dielectric film of the output mirror of the resonator used show partial high reflectivity in both the range of 1500 nm to 1880 nm and the range of 1900 nm to 2500 nm, and the reflectivity is between 96% and 99.6%, and output triple-wavelength laser of the pump light, the first-order Raman signal laser and the second-order Raman signal laser, and their wavelengths are respectively between 1500 nm and 1880 nm, between 1900 nm and 2500 nm and between 2600 nm and 3780 nm;

[0026] Step Five: By tuning the wavelength of the high-brightness laser pump source to any wavelength or continuously changing within the wavelength range of 1500 nm to 1880 nm, and keeping the pump coupling system, the laser gain medium, the input mirror of the resonator and the output mirror of the resonator of the original device unchanged, realize the continuous tunability of the output laser from any wavelength between 2500 nm and 3780 nm, and its tuning range and wavelength shift direction are consistent with the tuning method of the pump laser.

[0027] This method has simple steps and is easy to implement. It pumps a diamond Raman gain medium with a pump source of 1.52 μm - 1.88 μm, and uses the diamond phonon - free absorption loss to pump in the first - order Raman cavity, so as to stably generate laser in the 2.55 μm - 3.80 μm band through the Raman nonlinear process. At the same time, this method can achieve the simultaneous output of pump light in band I, first - order Raman in band II, and second - order Raman in band III with double - wavelength or triple - wavelength by simply switching output cavity mirrors of different color systems, greatly enriching the wavelength types of lasers in the mid - infrared region. Brief Description of the Drawings

[0028] Figure 1 It is a phonon absorption band of diamond crystal and a curve graph of existing Raman laser emission wavelengths;

[0029] Figure 2 It is a schematic diagram of the device for generating Raman - signal laser by intracavity cascaded pumping proposed by the present invention;

[0030] Figure 3 It is a polarized Raman gain curve graph of the diamond crystal adopted by the present invention;

[0031] Figure 4 It is a Raman frequency - shift relationship graph of the diamond crystal used in the present invention obtained through calculation;

[0032] Figure 5 It is a comparison effect diagram of the intracavity power density distribution of generating first - order Raman - signal laser by the existing direct - pumping scheme and generating diamond Raman - pumped laser and first - order and second - order signal lasers by the intracavity cascaded - pumping scheme of the present invention: (a) effect of the intracavity cascaded - pumping scheme, (b) existing direct - pumping scheme;

[0033] Figure 6 It is a schematic diagram of the device for the scheme of generating Raman - signal laser with a wide wavelength range by cascaded intracavity pumping of diamond crystal in the present invention;

[0034] Figure 7 It is an output - wavelength curve graph of the second - order Raman - signal laser generated by the intracavity cascaded - pumping scheme of the present invention;

[0035] Figure 8 It is a comparison graph of the threshold effects of direct pumping and cascaded intracavity pumping proposed by the present invention;

[0036] Figure 9 It is an output - pulse - energy graph of the second - order Raman - signal laser corresponding to the cascaded - pumping scheme;

[0037] Figure 10 It is a schematic diagram of the beam quality and distribution of the 2.9 - μm - wavelength laser of the second - order Raman - signal obtained by the present invention.

[0038] In the figure: 1. High-brightness laser pumping source; 2. Reflective laser beam expander-focusing device; 3. Resonator input mirror; 4. Laser gain medium; 5. Resonator output mirror. Specific embodiments

[0039] The following further describes the present invention in conjunction with embodiments and the attached Figures 1 to 10 drawings.

[0040] As Figure 2 shown, the present invention provides an intracavity-pumped mid-infrared Raman laser generating device, which includes a high-brightness laser pumping source 1, a laser gain medium 4, a reflective laser beam expander-focusing device 2, a resonator input mirror 3, and a resonator output mirror 5;

[0041] The high-brightness laser pumping source 1, the reflective laser beam expander-focusing device 2, the resonator input mirror 3, the laser gain medium 4, and the resonator output mirror 5 are sequentially arranged at intervals to form a laser generating device;

[0042] The high-brightness laser pumping source 1 is a pumping source with an arbitrary wavelength or a tunable wavelength within the range of 1500 nm to 1880 nm;

[0043] The laser beam expander-focusing device 2 includes optical lenses 2-1 and 2-2, which are geometric curved mirrors, and the specific surface shape is a spherical surface or an off-axis paraboloid; the ratio of the curvature radii R1 and R2 of the optical lenses 2-1 and 2-2 is 2:1, and specifically, they can be 200 mm and 100 mm or 100 mm and 50 mm respectively. The distance L between the optical lenses 2-1 and 2-2 satisfies: L = (R1 + R2) / 2. The spatial angular positions of the central tangent planes of the reflective surfaces of the optical lenses 2-1 and 2-2 are placed parallel or perpendicular. The surfaces of the optical lenses 2-1 and 2-2 are coated with broadband high-reflection films, and the reflectivity is between 96% and 99.6% between 1500 nm and 1880 nm;

[0044] The resonator input mirror 3 is coated with a multi-color film system, which is highly transmissive to the pumping laser of 1500 - 1880 nm, and its transmittance > 98%. It is highly reflective to the laser of 1900 - 2500 nm (first-order Raman), and the reflectivity > 99.6%. It is highly reflective to the laser of 2600 - 3780 nm (second-order Raman), and the reflectivity > 99.6%;

[0045] The coating of the output mirror 5 of the resonant cavity satisfies high reflectivity for the pump laser in the range of 1500 nm to 1880 nm, with a reflectivity > 99%, high reflectivity or partial reflection for the first-order Raman signal laser in the range of 1900 nm to 2500 nm, with a reflectivity > 99.6%, and high transmittance for the second-order Raman signal laser in the range of 2600 nm to 3780 nm. The value of its output transmittance T satisfies: 95% < T < 100%;

[0046] The input mirror 3 and the output mirror 5 of the resonant cavity form a laser resonant cavity in the range of 1.91 μm to 2.51 μm; the geometric surface shapes of the input mirror 3 and the output mirror 5 of the resonant cavity are spherical, and the ratio of their curvature radii is 1:1, specifically 50 mm and 50 mm or 75 mm and 75 mm respectively, and their positions are at the confocal position, or their geometric surface shapes are both flat;

[0047] The laser gain medium 4 is a diamond crystal, and its two light-transmitting surfaces are cut at the Brewster angle (specific angle is 54.7 degrees, as Figure 3 shown) or vertically cut (0 degrees), and it is arranged on the optical path between the input mirror 3 and the output mirror 5 of the resonant cavity, and generates Raman laser in the range of 2.55 μm to 3.80 μm after receiving Raman laser in the range of 1.91 μm to 2.51 μm.

[0048] As a preference, the diamond crystal is b-cut, its length is between 3 and 5 mm, and the value of its Raman gain coefficient in the wavelength range of 1500 nm to 1880 nm is > 10 cm / GW.

[0049] To avoid excessive material intrinsic absorption loss, the substrate material of the input mirror 3 of the resonant cavity is fused silica or calcium fluoride material with high transparency in the 1.5 μm to 1.88 μm band, and the substrate material of the output mirror 5 of the resonant cavity is calcium fluoride or zinc selenide with high transparency for mid-infrared band lasers, and the thickness of its light-transmitting center should be between 1 mm and 2 mm.

[0050] To improve the laser generation efficiency, the high-brightness laser pump source 1 is a high-power erbium-doped ion (Er 3+ ) or thulium-doped ion (Tm 3+ ) single-mode fiber laser, or an Er:Y2O3 sesquioxide ceramic or Er:Lu2O3 ceramic or Er:YAG ceramic solid laser with low phonon energy or a quantum well semiconductor laser with high beam quality.

[0051] In the present invention, a laser resonator operating at 1.91 μm to 2.51 μm is formed by using an input mirror of the resonator and an output mirror of the resonator. This resonator has high reflectivity for the first-order Raman light and partially outputs the second-order Raman light, thereby enabling high-power-density intracavity oscillation of the first-order Raman and facilitating the excitation of the second-order Raman. A reflective laser beam expander-focusing device is arranged between the high-brightness laser pump source and the laser resonator, eliminating the change in focal length caused by the dispersion of different wavelengths in the traditional lens system. For a wavelength-tunable pump laser source, automatic and precise matching of the modes of the pump light and the generated Raman signal laser under wavelength tuning can be achieved, significantly improving the stability of the wavelength-tunable generation of the Raman signal and conveniently realizing the efficient output of tunable mid-infrared laser. Using a diamond crystal as the laser gain medium and pumping the diamond Raman medium with a high-brightness laser pump source in the range of 1500 nm to 1880 nm, the diamond can be pumped in the first-order Raman intracavity with phononless absorption loss, and then Raman laser in the range of 2.55 μm to 3.80 μm can be generated. In this way, the Raman laser threshold can be effectively reduced, and the effective output of mid-infrared laser in the 2 μm to 5 μm band can be realized. In this device, through the intracavity oscillation of the first-order Raman and in cooperation with phononless absorption loss, it can ensure a higher transmittance of the second-order Raman output, which can be close to 100%. During the output process of the second-order Raman laser, the number of round trips of photons in the cavity is small, the target wavelength (second-order Raman, 2.55 μm to 3.80 μm) is close to passing through the diamond Raman gain medium in a single pass, the phonon absorption loss is low, and the problem of large multi-phonon absorption loss caused by the reflection of the output mirror is avoided, greatly improving the power of mid-infrared laser above 2.5 microns wavelength. At the same time, the power density of the first-order Raman intracavity as the pump light is high. Because the resonator mirror has high reflectivity or very low output transmittance for the first-order Raman, the intracavity power density can be higher than the incident pump light. The high intracavity power density of the first-order Raman can effectively reduce the generation threshold of the second-order Raman (band III) laser and improve the conversion rate from the first-order Raman signal laser to the second-order Raman signal laser, significantly enhancing the generation efficiency of mid-infrared laser in the high phonon absorption region of diamond. This device has a simple structure, low loss, low threshold, good stability, and can achieve multi-wavelength output. It can effectively improve the generation efficiency of Raman laser in the ~3 μm band, and at the same time, does not increase the complexity of the system design.

[0052] The present invention also provides a method for generating intracavity-pumped diamond mid-infrared Raman laser, including the following steps:

[0053] Step 1: According to the target laser wavelength, calculate the optimal pump wavelength from the Raman shift formulas of the first order and the second order, as Figure 4 shown;

[0054] Step 2: According to the pump wavelength and the Raman gain relationship of the diamond crystal, design the optimal values of the pump spot, the mode field diameter of the Raman signal laser, and the length of the diamond crystal.

[0055] Specifically, based on the rate equation, the intracavity power distribution of the second-order Raman signal laser generated by intracavity-pumped diamond is calculated. The numerical model used is shown in formula (1):

[0056]

[0057] In the formula, I i is the pump and the intensities of the Stokes lights of each order, where i = p, 1, 2, 3, and β i is the absorption coefficient of the Stokes light of each order, and g i = g0ω i / ω p is the Raman gain coefficient of the i-th order Stokes light. ω i and ω p are the angular frequencies of the Stokes lights of each order and the pump light respectively, and g0 is the Raman gain coefficient at the pump wavelength;

[0058] The calculated results show that the intracavity laser power density of the 2.1 μm wavelength (first-order Raman signal laser) generated by pumping a diamond crystal with a 1.6 μm laser is much higher than that of the traditional scheme using a 2 μm laser for direct external cavity pumping. Therefore, the threshold pump power of the 2.9 μm signal laser (the laser at the target wavelength) can be effectively reduced, and the Raman signal power of the output 2.9 μm wavelength is also much greater than that of the 2.9 μm signal laser generated by direct pumping in the traditional scheme (specifically as Figure 5 shown, the 2 μm laser power density obtained by intracavity pumping reaches 300 MW / cm 2 , while the 2 μm laser power density that can be achieved by direct pumping with the same power is only 200 MW / cm 2 , indicating that the scheme of the present invention reduces the threshold of generating the 2.9 μm Raman signal laser by 40%, and the output power of the target Raman signal laser is doubled).

[0059] Step 3: Precisely adjust the resonator input mirror 3 and the resonator output mirror 5 to obtain the required Raman signal laser output;

[0060] Step 4: Achieve the switching of the output laser wavelength through the type of the dielectric film of the resonator output mirror 5;

[0061] S1: Make the dielectric film of the resonator output mirror 5 used be highly reflective in both the range of 1500 nm to 1880 nm and the range of 1900 nm to 2500 nm, and the reflectivity is greater than 99.6%, and output a single-wavelength Raman second-order laser with a wavelength between 2600 nm and 3780;

[0062] S2: Make the dielectric film of the resonant cavity output mirror 5 adopted have high reflectivity in the range of 1500 nm to 1880 nm, and have partial reflectivity with a reflectivity between 96% and 99.6% in the range of 1900 nm to 2500 nm, output dual-wavelength lasers of the first-order and second-order Raman signals, and their wavelengths are respectively in the ranges of 1900 nm to 2500 nm and 2600 nm to 3780 nm;

[0063] S3: Make the dielectric film of the resonant cavity output mirror 5 adopted show partial high reflectivity both in the range of 1500 nm to 1880 nm and in the range of 1900 nm to 2500 nm, and the reflectivity is between 96% and 99.6%, output triple-wavelength lasers of pump light, first-order Raman signal laser and second-order Raman signal laser, and their wavelengths are respectively in the ranges of 1500 nm to 1880 nm, 1900 nm to 2500 nm and 2600 nm to 3780 nm;

[0064] Step Five: By tuning the wavelength of the high-brightness laser pump source 1 to any wavelength or continuously changing in the wavelength range of 1500 nm to 1880 nm, keeping the pump coupling system, the laser gain medium 4, the resonant cavity input mirror 3 and the resonant cavity output mirror 5 of the original device unchanged, realize the continuous tunability of the output laser at any wavelength from 2500 nm to 3780 nm, and its tuning range and wavelength moving direction are consistent with the tuning method of the pump laser.

[0065] The steps of this method are simple and easy to implement. Pump the diamond Raman gain medium with a 1.52 μm to 1.88 μm pump source, and use the diamond phononless absorption to pump in the first-order Raman cavity, so as to stably generate lasers in the 2.55 μm to 3.80 μm band through the Raman nonlinear process. At the same time, this method can realize the simultaneous output of the pump light in Band I, the first-order Raman in Band II, and the second-order Raman in Band III with dual-wavelength or triple-wavelength only by simply switching the output cavity mirrors of different color systems, greatly enriching the wavelength types of lasers in the mid-infrared region.

[0066] Example 1: Generate 2.9 μm signal laser based on intracavity cascaded pumping of diamond;

[0067] For the target laser wavelength of 2.9 μm, according to the specific steps of a method for generating mid-infrared Raman lasers by intracavity pumping of diamond, design the specific parameters of diamond, resonant cavity mirrors and pump coupling systems, and build as Figure 6The laser device shown: A pump source is used to pump a diamond Raman gain medium with a 1645 nm pulsed laser to generate a first-order Raman signal in the 2.1 μm region of the second region with a high power density inside the cavity, and then cascaded pumping is used to generate a third-stage Raman signal in the 2.9 μm band. The 1645 nm pulsed laser is generated by pumping an Er:YAG crystal and an acousto-optic modulator with a 1532 nm high-power Er:Yb co-doped single-mode fiber laser made in the laboratory to generate a Q-switched pulse with a wavelength of 1645 nm. The length of the diamond crystal used is 51 mm, and the light-passing surfaces at both ends are cut at the Brewster angle uniformly. The energy of the input pump pulse is 4 mJ, the pump pulse width is 30 ns, the input coupling system uses a reflective beam-expanding and focusing system, and the spot diameters of the pump light and the lasers of the first-order and second-order Raman signals are both 100 μm, and the output coupling mirror T oc = 95%, and 2.9 μm laser can be efficiently generated through cascaded pumping, and its proof is the second-order Raman signal ( Figure 7 ). The comparison scheme is the 2 μm direct pumping scheme: the pump laser is 2.1 μm, and other parameters of the system such as the spot diameter, crystal length, and cutting method are the same as those of the cascaded pumping method. The experimental results show that as Figure 8 and 9 shown, the threshold of the intracavity cascaded pumping scheme of the present invention is lower and the output power is greatly improved, while ensuring good beam quality of the output Raman signal laser ( Figure 10 ). Therefore, the scheme of the present invention has a very significant technological progress compared with the traditional direct pumping scheme.

[0068] Example 2: Based on a wavelength-tunable cavity laser in the range of 1550 nm to 1880 nm, intracavity cascaded pumping of diamond is used to generate signal lasers with any wavelength in the 2.55 to 3.8 μm band;

[0069] The specific method is the same as that of Example 1. Replace the pump source in Example 1 with a wavelength-tunable laser with a wavelength in the range of 1550 nm - 1880 nm, and the other specific structures and adjustment methods are the same as those of Example 1, and signal lasers with any wavelength in the range of 2.55 μm - 3.8 μm or a continuously wavelength-tunable state can be obtained. The specific methods and details will not be elaborated.

[0070] The two embodiments involved in the present invention show feasible schemes for generating 2.55 μm - 3.8 μm Raman lasers using a diamond Raman gain medium. In actual use, the cavity type can be designed according to actual needs. The present invention should by no means be limited to the specific embodiments described above.

Claims

1. An intracavity-pumped mid-infrared Raman laser generation device in diamond, comprising a high-brightness laser pump source (1) and a laser gain medium (4), characterized in that, It further includes a reflective laser beam expander - focusing device (2), a resonator input mirror (3), and a resonator output mirror (5); The high - brightness laser pump source (1), the reflective laser beam expander - focusing device (2), the resonator input mirror (3), the laser gain medium (4), and the resonator output mirror (5) are arranged at intervals in sequence to form a laser generating device; The high - brightness laser pump source (1) is a pump source with an arbitrary wavelength or a tunable wavelength within the range of 1500 nm to 1880 nm; The laser beam expander - focusing device (2) includes optical lenses (2 - 1) and optical lenses (2 - 2), which are geometric curved mirrors, and the specific surface shape is a spherical surface or an off - axis paraboloid; the ratio of the curvature radii R1 and R2 of the optical lenses (2 - 1) and optical lenses (2 - 2) is 2:1, specifically 200 mm and 100 mm or 100 mm and 50 mm respectively. The distance L between the optical lenses (2 - 1) and optical lenses (2 - 2) satisfies: L=(R1 + R2) / 2. The spatial angular position of the central tangent plane of the reflective surfaces of the optical lenses (2 - 1) and optical lenses (2 - 2) is parallel or perpendicular. The surfaces of the optical lenses (2 - 1) and optical lenses (2 - 2) are coated with broadband high - reflection films, and the reflectivity is between 96% and 99.6% between 1500 nm and 1880 nm; The coating of the resonator input mirror (3) is a multi - color film system, highly transmissive to the pump laser of 1500 - 1880 nm, with a transmittance > 98%, highly reflective to the laser of 1900 - 2500 nm, with a reflectivity > 99.6%, and highly reflective to the laser of 2600 - 3780 nm, with a reflectivity > 99.6%; The coating of the resonator output mirror (5) satisfies being highly reflective to the pump laser of 1500 nm to 1880 nm, with a reflectivity > 99%, highly reflective or partially reflective to the first - order Raman signal laser of 1900 nm to 2500 nm, with a reflectivity > 99.6%, and highly transmissive to the second - order Raman signal laser of 2600 nm to 3780 nm, and the value of its output transmittance T satisfies: 95% < T < 100%; The resonator input mirror (3) and the resonator output mirror (5) form a 1.91 μm - 2.51 μm laser resonator; the geometric surface shapes of the resonator input mirror (3) and the resonator output mirror (5) are spherical surfaces, and the ratio of their curvature radii is 1:1, specifically 50 mm and 50 mm or 75 mm and 75 mm respectively, and the positions of the resonator input mirror (3) and the resonator output mirror (5) are at the co - focal position, or the geometric surface shapes of the resonator input mirror (3) and the resonator output mirror (5) are both planar; The laser gain medium (4) is a diamond crystal, and its two light - passing surfaces are cut at Brewster's angle or perpendicularly, and it is arranged on the optical path between the resonator input mirror (3) and the resonator output mirror (5), and generates Raman laser within the range of 2.55 μm - 3.80 μm after receiving Raman laser within the range of 1.91 μm - 2.51 μm.

2. The intracavity-pumped diamond mid-infrared Raman laser generating device according to claim 1, wherein The diamond crystal is b-cut, with a length between 3 and 5 mm, and its Raman gain coefficient has a value > 10 cm / GW in the wavelength range of 1500 nm to 1880 nm.

3. The intracavity pumped diamond mid-infrared Raman laser generating device according to claim 1 or 2, characterized in that, The substrate material of the input mirror (3) of the resonator is fused silica or calcium fluoride material that is highly transparent in the 1.5 μm to 1.88 μm band. The substrate material of the output mirror (5) of the resonator is calcium fluoride or zinc selenide that is highly transparent to mid-infrared laser, and the thickness of its optical center should be between 1 mm and 2 mm.

4. The intracavity-pumped diamond mid-infrared Raman laser generating device according to claim 3, wherein, The high-brightness laser pump source (1) is a high-power erbium-doped or thulium-doped single-mode fiber laser, or a low-phonon-energy Er:Y2O3 sesquioxide ceramic or Er:Lu2O3 ceramic or Er:YAG ceramic solid laser or a high-beam-quality quantum well semiconductor laser.

5. A method for generating intracavity-pumped diamond mid-infrared Raman laser, which uses an intracavity-pumped diamond mid-infrared Raman laser generating device according to any one of claims 1 to 4, characterized in that, It includes the following steps: Step 1: According to the target laser wavelength, calculate the optimal pump wavelength using the first-order and second-order Raman shift formulas. Step 2: According to the Raman gain relationship between the pump wavelength and the diamond crystal, obtain the optimal values of the mode field diameters of the pump spot and the Raman signal laser and the length of the diamond crystal according to formula (1). Where I i is the pump and the intensities of the Stokes lights of each order, i = p, 1, 2, 3, β i is the absorption coefficient of the Stokes lights of each order, g i = g0ω i / ω p is the Raman gain coefficient of the i-th order Stokes light, ω i and ω p are the angular frequencies of the Stokes lights of each order and the pump light respectively, and g0 is the Raman gain coefficient at the pump wavelength; Step 3: Precisely adjust the input mirror (3) and the output mirror (5) of the resonator to obtain the required output of the Raman signal laser. Step 4: Achieve the switching of the output laser wavelength through the type of dielectric film of the output mirror (5) of the resonator. S1: Make the dielectric film of the output mirror (5) of the resonator used be highly reflective in both the 1500 nm to 1880 nm range and the 1900 nm to 2500 nm range, and the reflectivity is greater than 99.6%, and output single-wavelength Raman second-order laser with a wavelength between 2600 nm and 3780 nm. S2: Make the dielectric film of the output mirror (5) of the resonator used be highly reflective in the 1500 nm to 1880 nm range and partially reflective with a reflectivity between 96% and 99.6% in the 1900 nm to 2500 nm range, and output dual-wavelength laser of the first-order and second-order Raman signals, and their wavelengths are respectively between 1900 nm and 2500 nm and between 2600 nm and 3780 nm. S3: Make the dielectric film of the output mirror (5) of the resonator used show partial high reflectivity in both the 1500 nm to 1880 nm range and the 1900 nm to 2500 nm range, and the reflectivity is between 96% and 99.6%, and output triple-wavelength laser of the pump light, the first-order Raman signal laser and the second-order Raman signal laser, and their wavelengths are respectively between 1500 nm and 1880 nm, between 1900 nm and 2500 nm and between 2600 nm and 3780 nm. Step 5: By tuning the wavelength of the high-brightness laser pump source (1) to any wavelength or continuously changing in the wavelength range of 1500 nm to 1880 nm, keeping the pump coupling system, the laser gain medium (4), the input mirror (3) and the output mirror (5) of the resonator of the original device unchanged, realize the continuous tunability of the output laser to any wavelength from 2500 nm to 3780 nm, and its tuning range and wavelength shift direction are consistent with the tuning method of the pump laser.

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