High-energy thulium pump holmium laser oscillation amplifier

Abstract

The invention provides a high-energy thulium pump holmium laser oscillation amplifier, and belongs to the technical field of solid lasers, and the high-energy thulium pump holmium laser oscillation amplifier comprises a pumping source which comprises a thulium-doped yttrium aluminum garnet crystal and is used for outputting pumping laser of a first wavelength; the oscillator comprises a holmium-doped yttrium aluminum garnet crystal and is used for generating seed laser with a second wavelength by taking the pump laser as pump light; the amplifier comprises a holmium-doped yttrium aluminum garnet crystal and is used for amplifying the seed laser by taking the pump laser as pump light; wherein the first wavelength is not located on an absorption main peak of the holmium-doped yttrium aluminum garnet crystal. According to the invention, high-energy narrow-pulse two-micron laser output is realized, and meanwhile, good thermal management performance and system stability are achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of solid-state lasers, and particularly relates to a large-energy thulium-pumped holmium laser oscillator amplifier. BACKGROUND

[0002] Large-energy two-micron laser is in the atmospheric transmission window, and has important applications in photoelectric countermeasures, gas detection, laser radar and the like, especially as an ideal pump source for generating mid-infrared laser through optical parametric means. According to the pumping method, there are currently four main ways to generate large-energy two-micron laser: the first way is to directly pump a thulium-doped yttrium aluminum garnet laser gain medium with a semiconductor laser; the second way is to pump a holmium-doped yttrium aluminum garnet or a holmium-doped yttrium lithium fluoride crystal with a thulium-doped yttrium lithium fluoride solid-state laser or a thulium fiber laser; the third way is to pump a holmium-doped yttrium lithium fluoride or a holmium-doped yttrium aluminum crystal with a thulium-doped yttrium aluminum solid-state laser; and the fourth way is to pump a holmium-doped lithium lutetium fluoride crystal with a thulium fiber laser.

[0003] The foregoing several pumping methods mainly face the following problems: high-energy and narrow pulse width of the output two-micron nanosecond pulse laser cannot be achieved simultaneously; the thulium fiber laser is expensive and has a short service life, and is difficult to work for a long time; the thulium-doped yttrium lithium fluoride solid-state laser needs to accurately control the output laser wavelength because the output laser wavelength is in the water absorption peak, and the structure is relatively complex; and the holmium-doped yttrium lithium fluoride and the holmium-doped lithium lutetium fluoride crystals are difficult to grow, and it is difficult to obtain large-size high-quality crystals. Therefore, there is an urgent need for a large-energy two-micron laser generation scheme that can output larger energy, effectively control thermal effects, and have a stable and reliable structure. SUMMARY

[0004] The present application provides a large-energy thulium-pumped holmium laser oscillator amplifier, which realizes high-energy narrow pulse two-micron laser output, and has good thermal management performance and system stability.

[0005] The present application provides a large-energy thulium-pumped holmium laser oscillator amplifier, which realizes high-energy narrow pulse two-micron laser output, and has good thermal management performance and system stability. The pumping source includes a thulium-doped yttrium aluminum garnet crystal, and is used for outputting pump laser of a first wavelength; The oscillator includes a holmium-doped yttrium aluminum garnet crystal, and is used for generating seed laser of a second wavelength by taking the pump laser as pump light; The amplifier includes a holmium-doped yttrium aluminum garnet crystal, and is used for amplifying the seed laser by taking the pump laser as pump light; The first wavelength is not on the main absorption peak of the holmium-doped yttrium aluminum garnet crystal.

[0006] According to the large-energy thulium-pumped holmium laser oscillator amplifier provided by the present application, the pumping source further includes: a laser diode, which is used to pump the thulium-doped yttrium aluminum garnet crystal by side-pumping or end-pumping; a first resonant cavity, in which the thulium-doped yttrium aluminum garnet crystal is arranged, and the first resonant cavity is a short cavity structure.

[0007] According to the application, the thulium-doped yttrium aluminum garnet crystal is a bonded crystal, which is bonded by at least one pure yttrium aluminum garnet crystal and at least one thulium-doped yttrium aluminum garnet crystal along the optical axis.

[0008] According to the application, the oscillator further comprises: a second resonant cavity, in which the holmium-doped yttrium aluminum garnet crystal in the oscillator is arranged, and the second resonant cavity is a flat concave cavity or a double concave cavity structure; a Q-switching device, which is arranged in the second resonant cavity.

[0009] According to the application, the Q-switching device is an acousto-optic modulator or an electro-optic modulator.

[0010] According to the application, the amplifier further comprises: a first dichroic mirror and a second dichroic mirror, which are arranged on opposite sides of the holmium-doped yttrium aluminum garnet crystal in the amplifier, the pump laser enters the amplifier after being transmitted by the first dichroic mirror, and the seed laser enters the amplifier after being reflected by the first dichroic mirror.

[0011] According to the application, the amplifier is a single-end pumping or double-end pumping structure, and the number of the holmium-doped yttrium aluminum garnet crystals in the amplifier is one or more.

[0012] According to the application, the amplifier further comprises: a beam shaping module, which comprises a first shaping lens group arranged between the pump source and the oscillator, and a second shaping lens group arranged between the oscillator and the amplifier.

[0013] According to the application, the amplifier further comprises: a cooling device, which is thermally coupled with the thulium-doped yttrium aluminum garnet crystal, the holmium-doped yttrium aluminum garnet crystal in the oscillator, and the holmium-doped yttrium aluminum garnet crystal in the amplifier.

[0014] According to the application, a large-energy thulium-pumped holmium laser oscillator-amplifier is provided, wherein the length of the holmium-doped yttrium aluminum garnet crystal in the oscillator is greater than or equal to 100 mm, and the doping concentration ranges from 0.5% to 1%; and / or, the length of the holmium-doped yttrium aluminum garnet crystal in the amplifier ranges from 100 mm to 160 mm, and the doping concentration ranges from 0.5% to 1%.

[0015] The application utilizes a thulium-doped yttrium aluminum garnet pump source to pump a holmium-doped yttrium aluminum garnet crystal, generates a seed laser, and amplifies the energy of the seed laser through an amplifier to realize large-energy two-micron laser output. First, the thulium-doped yttrium aluminum garnet crystal has high thermal conductivity, allowing high pumping power and easy high-power laser output. Second, the thulium-doped yttrium aluminum garnet pump source outputs laser at a wavelength not in the water absorption peak, so that special control of the wavelength is not needed, simplifying the structure of the pump source and improving the reliability of the pump source. Finally, the thulium-doped yttrium aluminum garnet laser wavelength is not on the main absorption peak of the holmium-doped yttrium aluminum garnet crystal, so that, in the end-pumping mode, the thermal effect of the holmium-doped yttrium aluminum garnet crystal end face and internal can be effectively alleviated. Under the same thermal focal length condition, compared with the thulium-doped yttrium lithium fluoride laser and the thulium fiber laser pump source, the holmium-doped yttrium aluminum garnet crystal can withstand higher-power thulium-doped yttrium aluminum garnet laser pumping, thereby realizing higher-power or energy seed laser and amplified laser output. BRIEF DESCRIPTION OF DRAWINGS

[0016] In order to more clearly illustrate the technical solutions in the application or the prior art, the following will briefly introduce the drawings needed in the embodiments or prior art description. Obviously, the drawings in the following description are some embodiments of the application, and for those skilled in the art, other drawings can also be obtained without creative labor.

[0017] Figure 1 Fig. 1 is a structural schematic diagram of a large-energy thulium-pumped holmium laser oscillator-amplifier provided by the application.

[0018] Figure 2 Fig. 2 is a structural schematic diagram of a laser diode side-pumped thulium-doped yttrium aluminum garnet crystal provided by the application.

[0019] Figure 3 Fig. 3 is a structural schematic diagram of a bonded thulium-doped yttrium aluminum garnet crystal provided by the application. DETAILED DESCRIPTION

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0021] Figure 1 This is a schematic diagram of the structure of a high-energy thulium-pumped holmium laser oscillator amplifier provided by the present invention. Figure 1 As shown, the high-energy thulium-pumped holmium laser oscillator amplifier includes a pump source, an oscillator, and an amplifier. The pump source includes a thulium-doped yttrium aluminum garnet crystal 2-1, which is used to output a pump laser of the first wavelength. The oscillator includes a holmium-doped yttrium aluminum garnet crystal 6-1, which is used to generate a seed laser of the second wavelength using the pump laser as the pump light. The amplifier includes a holmium-doped yttrium aluminum garnet crystal 6-2, which is used to amplify the seed laser using the pump laser as the pump light. The first wavelength is not located at the main absorption peak of the holmium-doped yttrium aluminum garnet crystal.

[0022] Specifically, thulium-doped yttrium aluminum garnet crystal 2-1 refers to a laser gain medium using yttrium aluminum garnet as a matrix and doped with thulium ions, with the chemical formula Tm:YAG. When pumped by a laser diode at a wavelength of approximately 785 nm, it can produce a first wavelength, i.e., a laser output of approximately 2.01 micrometers. Holmium-doped yttrium aluminum garnet crystal 6-1 refers to a laser gain medium using yttrium aluminum garnet as a matrix and doped with holmium ions, with the chemical formula Ho:YAG. When pumped by light at a wavelength of approximately 2.01 micrometers, it can produce a second wavelength, i.e., a laser output of approximately 2.09 micrometers. The absorption peak refers to the wavelength position where the absorption coefficient of a substance reaches its local maximum for light radiation. For holmium-doped yttrium aluminum garnet crystals, its main absorption peak for pump light is located in the approximately 1.9 micrometer band.

[0023] First, a pump source with a first wavelength of approximately 2.01 micrometers is generated by a pump source centered on a thulium-doped yttrium aluminum garnet crystal 2-1. This pump laser is simultaneously used as the energy source for two independent optical processes. First, it is injected into an oscillator centered on a holmium-doped yttrium aluminum garnet crystal 6-1, pumping the crystal to induce population inversion and generate stimulated emission oscillations within a second resonant cavity, ultimately outputting a second wavelength seed laser of approximately 2.09 micrometers. Second, it is injected into an amplifier centered on another holmium-doped yttrium aluminum garnet crystal 6-2, while the seed laser output from the oscillator is also guided to this amplifier. Inside the amplifier, the seed laser passes through the holmium-doped yttrium aluminum garnet crystal 6-2, which is in a population-inverted state, and extracts the energy stored in the crystal through stimulated emission, thereby amplifying its own optical power or pulse energy. In addition, the first wavelength, about 2.01 micrometers, was intentionally chosen not to be at the main absorption peak of the holmium-doped yttrium aluminum garnet crystal, about 1.9 micrometers, so that the absorption coefficient of the pump light in the holmium-doped yttrium aluminum garnet crystal is relatively low.

[0024] Therefore, in this embodiment of the invention, because the pump wavelength deviates from the absorption peak, the absorption of pump light by the holmium-doped yttrium aluminum garnet crystal becomes relatively mild. Compared to pumping at the absorption peak, the pump power density absorbed per unit volume of crystal is significantly reduced. Specifically, with the absorption power distributed more uniformly due to the reduced absorption coefficient and the lower peak value, the distribution of waste heat is also more uniform, and the peak heat load is reduced, effectively mitigating the thermal lensing effect and thermal stress birefringence effect caused by the temperature gradient. Thus, the holmium-doped yttrium aluminum garnet crystal can withstand higher total power pumping without failing due to thermal damage or severe beam quality degradation, laying the physical foundation for achieving high-energy laser output.

[0025] Figure 2 This is a schematic diagram of a laser diode side-pumped thulium-doped yttrium aluminum garnet crystal provided by the present invention. (Combined with...) Figure 1 and Figure 2 The pump source also includes a laser diode (LD) and a first resonant cavity. The laser diode (LD) employs... Figure 2 The thulium-doped yttrium aluminum garnet crystal 2-1 is pumped by the side pumping method shown, or the end face pumping method can also be used. The thulium-doped yttrium aluminum garnet crystal 2-1 is rod-shaped or plate-shaped. The thulium-doped yttrium aluminum garnet crystal 2-1 is disposed in the first resonant cavity, which is a short cavity structure.

[0026] Specifically, a laser diode (LD) emits laser light of a specific wavelength when energized, with an emission wavelength of approximately 785 nanometers, used to pump a thulium-doped yttrium aluminum garnet crystal 2-1. Side pumping is a pumping method where the pump light is incident from the side of the gain medium, i.e., the thulium-doped yttrium aluminum garnet crystal 2-1, and injected into the crystal, such as... Figure 2As shown. End-face pumping is a pumping method in which pump light is incident from the end face of the gain medium, i.e., the thulium-doped yttrium aluminum garnet crystal 2-1, and injected into the crystal. The first resonant cavity is composed of at least two optical mirrors, allowing light to propagate back and forth within it and interfere, forming a stable oscillating optical structure. Short cavity structure refers to a laser resonant cavity with a relatively short optical length, much smaller than that of common laser resonant cavities, characterized by a large longitudinal mode spacing and a compact structure. The end face refers to the two end surfaces of the gain medium along its optical axis. For a cylindrical rod-shaped crystal, the two end faces are parallel circular planes; for a slab-shaped crystal, they are two larger rectangular planes. End-face pumping means that the main propagation direction of the pump laser beam is parallel or substantially parallel to the optical axis of the crystal, and it is directly incident into the crystal interior from one or two end faces perpendicularly or nearly perpendicularly. The side faces refer to all cylindrical or side surfaces of the gain medium other than the two end faces. For a cylindrical rod-shaped crystal, the side faces are curved surfaces; for a slab-shaped crystal, the side faces are narrow, elongated planes. Side pumping refers to the pump laser beam being incident from the side of the crystal, with its main propagation direction being basically perpendicular to or at a certain angle to the optical axis of the crystal. The pump light enters the crystal from the side and is absorbed in a cross section perpendicular to the optical axis.

[0027] A laser diode (LD) serves as the initial pump source, emitting a 785 nm wavelength laser. This laser is injected into a thulium-doped yttrium aluminum garnet (YAG) crystal 2-1 via a side or end face, exciting the thulium ions within it to a high-energy state. The excited YAG crystal 2-1 serves as the gain medium and is placed within the first resonant cavity. The first resonant cavity is a short-cavity structure, which can be constructed by placing a total reflection mirror and an output mirror opposite to each other, with the YAG crystal 2-1 positioned between the two mirrors. The light travels back and forth through the gain medium within the short cavity, continuously amplified, and finally coupled out from the output mirror to output the first wavelength pump laser. The short-cavity structure means that the one-way transit time of the light within the cavity is short.

[0028] Therefore, in this embodiment of the invention, a laser diode (LD) is used to directly pump a thulium-doped yttrium aluminum garnet crystal 2-1, resulting in a reliable and highly efficient light source. Placing the thulium-doped yttrium aluminum garnet crystal 2-1 within a short-cavity structure, due to the small physical size and high mechanical stability of the short-cavity structure itself, contributes to the long-term stable operation of the pump source. It should be noted that the first resonant cavity structure of the pump source is not limited to a short-cavity structure; multiple gain media can be placed within the cavity.

[0029] Figure 3 This is a schematic diagram of the structure of a bonded thulium-doped yttrium aluminum garnet crystal provided by the present invention. Figures 1 to 3 2-1 is a bonded crystal composed of at least one section of pure yttrium aluminum garnet crystal and at least one section of thulium-doped yttrium aluminum garnet crystal bonded along the optical axis.

[0030] Specifically, bonded crystals refer to composite crystal structures in which two or more crystals of the same or different materials are firmly bonded together at the interface through processes such as optical contact and thermal diffusion. Pure yttrium aluminum garnet crystals refer to yttrium aluminum garnet crystals without any doping with activating ions such as thulium or holmium, possessing excellent thermal conductivity and optical transmittance. In laser systems, the optical axis refers to the direction of the central axis of laser beam propagation, which is also the direction of the axis of the resonant cavity.

[0031] The thulium-doped yttrium aluminum garnet crystal 2-1 is not a single, uniformly doped crystal, but a composite crystal manufactured using bonding technology. Its structure, along the optical axis (i.e., the laser propagation direction), sequentially comprises at least one segment of pure yttrium aluminum garnet crystal and at least one segment of thulium-doped yttrium aluminum garnet crystal 2-1. For example, as... Figure 3 As shown, for example, the thulium-doped yttrium aluminum garnet crystal 2-1 can be configured as a three-segment bonded crystal composed of a pure yttrium aluminum garnet crystal, a thulium-doped yttrium aluminum garnet crystal, and a pure yttrium aluminum garnet crystal bonded sequentially, i.e., a three-segment bonded crystal composed of YAG, Tm:YAG, and YAG. For the end-pumping scheme, the 785 nm pump light first passes through a pure yttrium aluminum garnet crystal segment, then enters the thulium-doped yttrium aluminum garnet crystal segment where it is absorbed and gains are generated. The resulting 2.01 μm laser light is then output through the other end of the pure yttrium aluminum garnet crystal.

[0032] Therefore, in this embodiment of the invention, the pure yttrium aluminum garnet crystal segments have high thermal conductivity and do not generate heat themselves. They are bonded to both ends of the heat-generating thulium-doped yttrium aluminum garnet crystal, effectively adding an efficient heat sink or heat dissipation channel to the heat source. Heat can be rapidly conducted from the doped segments to the pure crystal segments and diffused to a larger volume, thereby significantly reducing the peak temperature of the active region of the thulium-doped yttrium aluminum garnet crystal, especially its end face. This effectively reduces the end face thermal lensing effect and thermal stress, improves the crystal's thermal damage resistance threshold, and allows for the injection of higher pump power. Simultaneously, the pure crystal segments also protect the optical thin film deposited on the end face, keeping it away from high-temperature regions and improving the reliability and lifespan of the film.

[0033] In some embodiments, such as Figure 1 As shown, the oscillator also includes a second resonant cavity and a Q-switching device 9. The holmium-doped yttrium aluminum garnet crystal 6-1 in the oscillator is located in the second resonant cavity. The second resonant cavity is a plano-concave cavity or a double-concave cavity structure. The Q-switching device 9 is located in the second resonant cavity.

[0034] Specifically, the second resonant cavity refers to the optical resonant cavity constructed around the holmium-doped yttrium aluminum garnet crystal 6-1 in the oscillator, used to generate and maintain seed laser oscillation. A plano-concave cavity is formed by placing a plane mirror and a concave mirror opposite each other, while a double-concave cavity is formed by placing two concave mirrors opposite each other. The Q-switching device 9 is a device capable of rapidly changing the losses within the resonant cavity, used to compress the energy of continuous or long-pulse lasers into short-duration, high-peak-power giant pulses.

[0035] The oscillator section, besides the gain medium (holmium-doped yttrium aluminum garnet crystal 6-1), includes a second resonant cavity and a Q-switching device 9. The second resonant cavity is a plano-concave or bi-concave cavity structure, meaning at least one mirror constituting the second resonant cavity is a concave mirror. The holmium-doped yttrium aluminum garnet crystal 6-1 is located within the second resonant cavity. A 2.01-micron pump light from the pump source is injected into the holmium-doped yttrium aluminum garnet crystal 6-1, causing population inversion. The Q-switching device 9 is also located within this second resonant cavity. When the Q-switching device 9 is not activated, it keeps the resonant cavity in a high-loss state, suppressing laser oscillation and allowing holmium ions to accumulate a large amount of energy in the upper energy level. When population inversion reaches its peak, the Q-switching device 9 quickly switches to a low-loss state, rapidly establishing extremely strong laser oscillation within the cavity. Within a very short time, the stored energy is released in the form of light pulses, thus outputting a high-energy nanosecond seed laser pulse.

[0036] Therefore, embodiments of the present invention employ a plano-concave cavity or a double-concave cavity as the resonant cavity of the oscillator. The concave mirror provides focusing capability, which helps to achieve good matching between the pump light mode field and the laser oscillation mode field within the gain medium, thereby improving pump efficiency. Simultaneously, this type of cavity design has a certain compensation capability for the thermal lensing effect of the gain medium, helping to maintain stable laser mode output at higher pump power. Furthermore, the introduction of the Q-switching device 9 is a key technology for achieving nanosecond-level narrow pulse output. The Q-switching device 9 compresses the continuous or quasi-continuous pump energy in time, resulting in a seed laser with not only high energy but also extremely high peak power, which is crucial for the efficiency of the subsequent amplification process.

[0037] In some embodiments, the Q-switching device 9 is an acousto-optic modulator or an electro-optic modulator.

[0038] Specifically, an acousto-optic modulator is a device that uses the periodic refractive index change generated when sound waves propagate in a medium to diffract and modulate a light beam. In Q-switching applications, an acoustic field is generated by applying a radio frequency signal, causing the laser to deflect and thus generating intracavity loss. An electro-optic modulator is a modulator that utilizes electro-optic effects, such as the Pockels effect. In Q-switching applications, a high-voltage electric pulse is applied to the electro-optic crystal to change its refractive index, thereby changing the polarization state of the laser passing through the crystal. Combined with polarization elements, this achieves rapid switching of intracavity loss.

[0039] When using an acousto-optic modulator, an radio frequency (RF) signal is applied during the energy storage phase. The acoustic wave forms a diffraction grating in the crystal, deflecting the laser traveling inside the cavity and causing it to escape, resulting in high loss. When an output pulse is needed, the RF signal is removed, the diffraction grating disappears, the cavity loss drops sharply, and the laser pulse is established and output. When using an electro-optic modulator, it can be used in conjunction with a polarizer. During the energy storage phase, a half-wave voltage is applied to the electro-optic crystal, causing the polarization direction of the laser passing through the crystal to rotate by 90 degrees, thus being blocked by the polarizer and resulting in high loss. When an output pulse is needed, the voltage is removed, the laser polarization state remains unchanged, and it can pass through the polarizer without loss, allowing rapid oscillation within the cavity.

[0040] Therefore, in this embodiment of the invention, acousto-optic Q-switching technology is mature, has a low driving voltage, and is easy to achieve high repetition frequency pulse output, but its switching speed is relatively slow, making it suitable for scenarios with medium peak power requirements. Electro-optic Q-switching has an extremely fast switching speed and can generate narrower pulses, thereby obtaining higher peak power, making it a preferred solution for achieving extremely high energy density output, but it requires a high-voltage driving pulse of several kilovolts, which can be flexibly configured according to different output pulse width and repetition frequency requirements.

[0041] In some embodiments, such as Figure 1 As shown, the amplifier also includes: a first dichroic mirror 5-2 and a second dichroic mirror 7-2 disposed on opposite sides of the holmium-doped yttrium aluminum garnet crystal 6-2 in the amplifier. The pump laser enters the amplifier after being transmitted through the first dichroic mirror 5-2, and the seed laser enters the amplifier after being reflected by the first dichroic mirror 5-2.

[0042] Specifically, the first dichroic mirror 5-2 and the second dichroic mirror 7-2 refer to optical lenses that have selective transmission or reflection functions for specific wavelengths. Specifically, they can be lenses that have high transmission for pump light wavelengths and high reflection for seed laser and amplified laser wavelengths.

[0043] The amplifier section includes a holmium-doped yttrium aluminum garnet crystal 6-2 and a first dichroic mirror 5-2 and a second dichroic mirror 7-2 positioned on opposite sides of it. The first and second dichroic mirrors 5-2 and 7-2 have high reflectivity for the seed laser; therefore, they and the space between them together constitute the amplifier for the amplified laser. The pump light can pass through the first dichroic mirror 5-2 with almost no loss, enter the region defined by these two mirrors, and illuminate the holmium-doped yttrium aluminum garnet crystal 6-2, providing it with energy and putting the crystal in a gain state. After being output from the oscillator, the seed laser is reflected by the first dichroic mirror 5-2, changes direction, enters the region, and passes through the crystal in the gain state. During this single passage through the crystal, the seed laser extracts the pump energy stored in the crystal through stimulated emission, thereby achieving amplification. The amplified laser is reflected and output upon reaching the second dichroic mirror 7-2.

[0044] Therefore, this invention ingeniously combines the pump light injection and laser amplification processes spatially. The dichroic mirror enables the combining and splitting of the pump light path, the seed laser, and the amplified laser light path, resulting in a compact system structure. The dichroic mirror constituting the amplifier can provide some feedback to the amplified laser. In a multi-pass amplification structure, it can increase the interaction length between the seed laser and the gain medium, improve the energy extraction efficiency of the single-stage amplifier, and facilitate the realization of an effective optical configuration for high-efficiency, high-gain amplification.

[0045] In some embodiments, the amplifier is a single-ended pumped or double-ended pumped structure, and the number of holmium-doped yttrium aluminum garnet crystals 6-2 in the amplifier is one or more.

[0046] Specifically, single-ended pumping refers to pump light being injected from only one end of the gain medium, i.e., the holmium-doped yttrium aluminum garnet crystal 6-2; double-ended pumping refers to pump light being injected from both ends of the gain medium, i.e., the holmium-doped yttrium aluminum garnet crystal 6-2. The term "one or more" indicates that the holmium-doped yttrium aluminum garnet crystal 6-2 used in the amplifier can be one piece, or two or more pieces used in series.

[0047] In a single-ended pump structure, all pump light is incident from one end of the crystal, for example, the end closest to the first dichroic mirror 5-2. Figure 1 As shown. In the dual-pump structure, the pump light is split into two beams, which are incident from opposite ends of the crystal. This requires a pump light injection optical system to be set on the other side opposite the first dichroic mirror 5-2, which may include another pump source, a shaping lens, and another dichroic mirror with high transmission to the pump light. In addition, a single crystal constitutes a single-stage amplifier, such as... Figure 1 As shown. Alternatively, multiple crystals can be connected in series to form a multi-stage amplifier. The output laser of the previous stage amplifier, after being isolated or shaped, serves as the input seed light for the next stage amplifier. At the same time, each stage is pumped by independent or split-beam pump light.

[0048] Therefore, in this embodiment of the invention, dual-end pumping can make the pump light energy more uniformly distributed along the length of the crystal, avoiding the severely uneven axial thermal load distribution caused by single-end pumping, thereby further improving the thermal effect and allowing a single crystal to withstand higher total pump power. Using multiple crystals to form a multi-stage amplifier is an effective way to break through the energy storage limit of a single crystal and achieve energy amplification step by step. Each stage amplifier can independently optimize its pump power and gain, thereby achieving higher extraction efficiency and better output beam quality overall.

[0049] In some embodiments, the high-energy thulium-pumped holmium laser oscillator amplifier further includes a beam shaping module, comprising a first shaping lens group 4-1 disposed between the pump source and the oscillator, and a second shaping lens group 11 disposed between the oscillator and the amplifier.

[0050] Specifically, the beam shaping module consists of a series of optical lenses used to change the diameter, divergence angle, wavefront shape, or intensity distribution of the laser beam. The first shaping lens group 4-1 refers to the lens group positioned between the pump source and the oscillator, used to shape the pump laser. The second shaping lens group 11 refers to the lens group positioned between the oscillator and the amplifier, used to shape the seed laser. The first shaping lens group 4-1, located after the pump source output and before the oscillator, collimates, focuses, or modulates the pump laser output from the pump source, which has a certain divergence angle and spot pattern, to achieve optimal matching between its spot size, divergence angle, and intensity distribution and the mode field characteristics of the holmium-doped yttrium aluminum garnet crystal 6-1 within the oscillator, as well as the requirements of the pump optical path, ensuring that the pump light is efficiently absorbed by the crystal and forms a uniform gain distribution. The second shaping lens group 11, located after the oscillator output and before the amplifier, performs similar shaping on the seed laser output from the oscillator. For example, the seed laser needs to be collimated and its spot size adjusted to match the optimal energy extraction mode field in the amplifier crystal. At the same time, the wavefront aberration of the seed laser may also need to be corrected to ensure that it can be coupled into the amplifier with high quality and extract energy efficiently.

[0051] Therefore, this embodiment of the invention, through precise beam shaping, can maximize the coupling efficiency of the pump light to the oscillator gain medium, reducing unnecessary reflection and scattering losses. Simultaneously, good mode matching is a prerequisite for obtaining high-efficiency laser oscillation and high-beam-quality seed light. For amplifiers, shaping the seed laser to a mode ideally matched to the amplifier crystal gain region can maximize energy extraction while avoiding the excitation of higher-order modes, ensuring the beam quality of the amplified laser, and achieving the coexistence of high energy and high beam quality.

[0052] In some embodiments, the high-energy thulium-pumped holmium laser oscillator amplifier further includes a cooling device thermally coupled to the thulium-doped yttrium aluminum garnet crystal 2-1, the holmium-doped yttrium aluminum garnet crystal 6-1 in the oscillator, and the holmium-doped yttrium aluminum garnet crystal 6-2 in the amplifier.

[0053] Specifically, the cooling device refers to the equipment used to remove the waste heat generated during the operation of the thulium-doped yttrium aluminum garnet crystal 2-1, the holmium-doped yttrium aluminum garnet crystal 6-1 in the oscillator, and the holmium-doped yttrium aluminum garnet crystal 6-2 in the amplifier, as well as the laser diode LD, maintaining the devices at a suitable operating temperature. This can be a water-cooling system or a semiconductor thermoelectric cooler. Thermal coupling refers to a good heat conduction path between the cooling device and the object to be cooled, ensuring that heat can be effectively transferred and removed.

[0054] In high-energy laser systems, laser diodes (LDs), thulium-doped yttrium aluminum garnet (YAG) crystals, and holmium-doped YAG crystals generate significant waste heat during operation due to quantum defects and non-radiative relaxation. Cooling devices, such as water-cooled jackets with internal cooling water flow, are in physical contact with these heat-generating components. For example, the crystals can be tightly encased within the water-cooled jacket to achieve thermal coupling. The cooling medium flows through the cooling device, continuously removing the heat generated by the devices through convective heat transfer, thus maintaining the temperature of the crystals and laser diodes (LDs) within a set range.

[0055] Therefore, in this embodiment, effective cooling is fundamental to the stable and long-term operation of the entire system. First, cooling maintains the stable operating temperature of the gain medium and the laser diode (LD). Temperature changes cause variations in parameters such as the crystal's refractive index, emission cross-section, and absorption cross-section, leading to laser output wavelength drift, power fluctuations, and a decrease in beam quality. Stable temperature is a prerequisite for stable output performance. Second, cooling capacity directly determines the upper limit of the average thermal load the system can handle, thus determining the maximum usable pump power and output power. Without sufficient cooling, all the high-energy output designs described in the above embodiments would be impossible to achieve due to thermal damage.

[0056] In some embodiments, the length of the holmium-doped yttrium aluminum garnet crystal 6-1 in the oscillator is greater than or equal to 100 mm, and the doping concentration ranges from 0.5% to 1%; and / or, the length of the holmium-doped yttrium aluminum garnet crystal 6-2 in the amplifier ranges from 100 mm to 160 mm, and the doping concentration ranges from 0.5% to 1%.

[0057] Specifically, length refers to the dimension of the gain medium along the laser propagation direction, i.e., the optical axis. Doping concentration refers to the atomic percentage concentration of active ions, i.e., holmium ions, replacing matrix ions in the crystal matrix. For holmium-doped yttrium aluminum garnet (YAG) crystals 6-1 in oscillators, with a length greater than or equal to 100 mm and a doping concentration between 0.5% and 1%, longer YAG crystals 6-1 can increase the absorption length of the pump light, ensuring efficient absorption even with a lower absorption coefficient. Appropriate doping concentration balances gain and quenching effects. For YAG crystals 6-1 in amplifiers, with a length range of 100 mm to 160 mm, a larger energy storage volume is provided. Even longer YAG crystals 6-2 mean more pump energy can be stored, providing a larger energy reservoir for the amplification process. Similarly, a doping concentration of 0.5% to 1% ensures sufficient gain while avoiding losses such as energy transfer upconversion caused by excessive concentration.

[0058] Therefore, this embodiment of the invention sets a lower limit for the oscillator crystal length and a suitable doping concentration, ensuring sufficient pump absorption and effective population inversion even under non-peak pumping conditions, thereby reliably generating high-energy seed pulses. This is a necessary physical condition for realizing a high-energy seed source. Furthermore, by setting the range of amplifier crystal length and matching the doping concentration, considerable energy storage capacity is provided for a single-stage amplifier. Increasing the length increases the maximum energy that a single crystal can store, providing a material basis for achieving high-energy output through single-stage or multi-stage amplification.

[0059] The following is based on Figure 1 For example, combined with Figure 2 and Figure 3 This paper provides a detailed description of each component in a high-energy thulium-pumped holmium laser oscillator amplifier: The pump source total reflection mirror 1-1 can be a plano-plano mirror or a plano-convex mirror. The plano-convex mirror has a radius of curvature of 800 mm. The convex surface is coated with a 1900 nm to 2100 nm total reflection film at 0°, with a reflectivity greater than 99.5%. The convex surface faces to the right. Together with the pump source output mirror 3-1, it forms a plano-convex short cavity structure to provide optical feedback and is a component of the first resonant cavity. The plano-convex structure is beneficial for mode field matching within the cavity, and the 800 mm radius of curvature provides a larger mode volume.

[0060] Thulium-doped yttrium aluminum garnet crystal 2-1 can be a three-segment bonded crystal consisting of pure yttrium aluminum garnet crystal, thulium-doped yttrium aluminum garnet crystal, and pure yttrium aluminum garnet crystal. The pure yttrium aluminum garnet crystal can be 20 mm long, and the thulium-doped yttrium aluminum garnet crystal 2-1 can be 68 mm long. The doping concentration ranges from 2% to 3.5%, and the overall diameter is 3 mm. Double-sided antireflection coatings of 1900 nm to 2100 nm can be deposited, with a transmittance greater than 99.5%. Figure 2 As shown, a five-way array of laser diodes (LDs) can be set to surround the crystal, or a three-way array of laser diodes (LDs) can be set to surround the crystal. The wavelength of the laser diodes (LDs) is 785nm, and they operate in quasi-continuous mode with a repetition frequency of 100Hz and a pulse width of 1.5ms.

[0061] The pump source output mirror 3-1 can be a flat mirror coated with a reflective film with a transmittance of 15% and a wavelength of 1900 nm to 2100 nm. Partial reflection maintains the oscillation within the cavity, while partial transmission outputs pump light of approximately 2010 nm.

[0062] The first shaping lens group 4-1 consists of one or more lenses. The anti-reflection coating of 1900 nm to 2100 nm with 0° on both sides converts the multimode laser output from the pump source into a light spot that matches the mode field of the holmium-doped yttrium aluminum garnet crystal 6-1 in the oscillator. Specifically, it can include beam diameter transformation, light intensity distribution homogenization, collimation or focusing adjustment.

[0063] The pump light anti-reflection laser high-reflection dichroic mirror 5-1 is placed at a 45° angle and coated with a film system that enhances the transmission of 2010 nm pump light with a transmittance greater than 99% and provides high reflectivity of 2090 nm laser light with a reflectivity greater than 99.5%. It plays the role of transmitting pump light into the resonant cavity and reflecting oscillating laser light to maintain oscillation. It is the input optical element of the second resonant cavity and realizes the spatial beam combining of the pump light path and the laser oscillation light path.

[0064] The holmium-doped yttrium aluminum garnet crystal 6-1 in the oscillator can be a rod-shaped crystal with a diameter of 4 mm and a length of 100 mm. The doping concentration can be 0.5% to 1%, and the end face is coated with an antireflection film of 1900 nm to 2100 nm. It is placed in the second resonant cavity of the oscillator to absorb pump light and generate population inversion. The 100 mm length ensures sufficient absorption of 2010 nm pump light, while the long size is beneficial for reducing power density and uniform heat generation.

[0065] The pump light anti-reflection laser high-reflection dichroic mirror 7-1 is placed at a 45° angle. Its coating is the same as that of the pump light anti-reflection laser high-reflection dichroic mirror 5-1. It is symmetrically arranged on the other side of the holmium-doped yttrium aluminum garnet crystal 6-1. It plays the role of drawing the remaining pump light out of the cavity and preventing it from stray reflection in the cavity. It is a component of the second resonant cavity of the oscillator.

[0066] The Q-switching device 9, which can be an acousto-optic modulator or an electro-optic modulator, is placed in the second resonant cavity of the oscillator near the end of the total reflection mirror 10. It plays the role of accumulating energy and then rapidly releasing it to generate nanosecond pulses by periodically controlling the loss inside the cavity.

[0067] The total reflection mirror 10 can be a 0° plano-concave mirror or a plano-flat mirror. The radius of curvature of the plano-concave mirror can be 500 mm. The concave surface is coated with a 1900 nm to 2100 nm high-reflectivity film. The concave surface faces into the second resonant cavity, and together with the oscillator output mirror 8, it forms a double-concave cavity structure, i.e., the second resonant cavity. It provides optical feedback and is a component of the oscillator's second resonant cavity. The concave design helps to compensate for the thermal lensing effect of the gain medium.

[0068] The oscillator output mirror 8 can be a 0° plano-concave mirror or a plano-flat mirror. The radius of curvature of the plano-concave mirror can be 500 mm, and the concave surface is coated with a 1900 nm to 2100 nm partial reflective film. The transmittance can be selected between 25% and 50%. It is placed opposite the total reflection mirror 10 to form a double concave cavity structure, i.e., the second resonant cavity. It serves to couple and output the seed laser. The transmittance is adjustable to optimize the output energy and efficiency. The oscillator is a symmetrical cavity with a resonant cavity length of approximately 700 mm, capable of outputting a 2.09-micron laser with a single pulse energy greater than or equal to 50 mJ, a repetition frequency of 100 Hz, and a pulse width of less than 40 ns.

[0069] The second shaping lens group 11, which can be composed of lens groups, has double-sided 1900 nm to 2100 nm anti-reflection coatings on both sides and is placed after the oscillator output mirror 8. It serves to collimate the seed laser, match the spot size, and correct wavefront aberrations to ensure that the seed light is injected into the subsequent amplifier with high quality.

[0070] Corresponding to the pump source total reflection mirror 1-1, thulium-doped yttrium aluminum garnet crystal 2-1, pump source output mirror 3-1, and first shaping lens group 4-1, another identical structure is symmetrically arranged, namely the pump source total reflection mirror 1-2, thulium-doped yttrium aluminum garnet crystal 2-2, pump source output mirror 3-2, and first shaping lens group 4-2. The pump source total reflection mirror 1-1, thulium-doped yttrium aluminum garnet crystal 2-1, pump source output mirror 3-1, and first shaping lens group 4-1 are used to provide pump light to the oscillator, while the pump source total reflection mirror 1-2, thulium-doped yttrium aluminum garnet crystal 2-2, pump source output mirror 3-2, and first shaping lens group 4-2 are used to provide pump light to the amplifier. The pump source total reflection mirror 1-2, the thulium-doped yttrium aluminum garnet crystal 2-2, the pump source output mirror 3-2, and the first shaping lens group 4-2 are configured the same as the pump source total reflection mirror 1-1, the thulium-doped yttrium aluminum garnet crystal 2-1, the pump source output mirror 3-1, and the first shaping lens group 4-1, respectively, and will not be described in detail here.

[0071] The first dichroic mirror 5-2, placed at a 45° angle, is coated with a film that enhances the transmittance of the 2010 nm pump light and provides high reflectivity for the 2090 nm seed laser. It serves to reflect the seed light and transmit the pump light into the amplifier's gain medium, namely the holmium-doped yttrium aluminum garnet crystal 6-2, thus achieving spatial beam combining of the seed light and pump light at the amplifier's input.

[0072] The holmium-doped yttrium aluminum garnet crystal 6-2 in the amplifier can be a rod-shaped crystal with a diameter of 4 mm and a length selectable between 100 mm and 160 mm. The doping concentration can be between 0.5% and 1%, and the end face is coated with an antireflection film of 1900 nm to 2100 nm. It is placed between the first dichroic mirror 5-2 and the second dichroic mirror 7-2 to store pump energy and amplify the passing seed laser. The longer crystal provides a larger energy storage volume.

[0073] The second dichroic mirror 7-2, placed at a 45° angle, has the same coating as 5-2 and is symmetrically arranged on the other side of the holmium-doped yttrium aluminum garnet crystal 6-2 in the amplifier. It serves to reflect the amplified laser output and transmit the remaining pump light. Combined with the first dichroic mirror 5-2, it can form different amplifier pump structures, such as single-ended or double-ended pumping. The amplifier can achieve higher energy 2.09-micron laser output through multi-stage amplification; for example, three-stage amplification can achieve a single-pulse energy greater than or equal to 300 mJ of 2.09-micron laser output.

[0074] In summary, the high-energy thulium-pumped holmium laser oscillator amplifier provided in this embodiment of the invention uses a pump source with thulium-doped yttrium aluminum garnet (YAG) crystal as the gain medium. It has advantages such as simple structure, high output power, stable long-term operation, and continuous or quasi-continuous operation. The gain medium used is either thulium-doped YAG or holmium-doped YAG crystal. Both crystal growth technologies are mature, making it easy to obtain large-size, high-optical-quality crystals, thus reducing laser costs. Using a pump source with thulium-doped YAG crystal as the gain medium to pump the holmium-doped YAG crystal results in an output laser wavelength far from the water absorption peak, eliminating the need for precise wavelength control and simplifying the structure. Furthermore, the output laser wavelength of the thulium-doped YAG crystal pump source is not located at the absorption peak of the holmium-doped YAG crystal, which helps alleviate crystal thermal effects and achieves high-power or high-energy 2.09-micron laser output while maintaining good beam quality. Therefore, the high-energy thulium-pumped holmium laser oscillator amplifier can achieve high-energy nanosecond 2.09-micron seed laser output, thereby realizing high-energy amplified laser output, and can also achieve high beam quality laser amplification. Moreover, it has a simple, compact structure and good stability.

[0075] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A high-energy thulium-pumped holmium laser oscillator amplifier, characterized in that, include: The pump source, comprising a thulium-doped yttrium aluminum garnet crystal, is used to output the pump laser of the first wavelength; An oscillator, comprising a holmium-doped yttrium aluminum garnet crystal, is used to generate a seed laser of a second wavelength using the pump laser as the pump light; An amplifier, comprising a holmium-doped yttrium aluminum garnet crystal, is used to amplify the seed laser using the pump laser as pump light; Wherein, the first wavelength is not located at the main absorption peak of the holmium-doped yttrium aluminum garnet crystal.

2. The high-energy thulium-pumped holmium laser oscillator amplifier according to claim 1, characterized in that, The pump source also includes: A laser diode is used to pump the thulium-doped yttrium aluminum garnet crystal using either side-pumping or end-pumping methods. The first resonant cavity is provided with the thulium-doped yttrium aluminum garnet crystal disposed within the first resonant cavity, which is a short cavity structure.

3. The high-energy thulium-pumped holmium laser oscillator amplifier according to claim 1, characterized in that, The thulium-doped yttrium aluminum garnet crystal is a bonded crystal, formed by bonding at least one section of pure yttrium aluminum garnet crystal and at least one section of thulium-doped yttrium aluminum garnet crystal along the optical axis.

4. The high-energy thulium-pumped holmium laser oscillator amplifier according to claim 1, characterized in that, The oscillator also includes: The second resonant cavity is located within the holmium-doped yttrium aluminum garnet crystal in the oscillator. The second resonant cavity is a plano-concave cavity or a double-concave cavity structure. The Q-switching device is located inside the second resonant cavity.

5. The high-energy thulium-pumped holmium laser oscillation amplifier according to claim 4, characterized in that, The Q-switching device is an acousto-optic modulator or an electro-optic modulator.

6. The high-energy thulium-pumped holmium laser oscillator amplifier according to claim 1, characterized in that, The amplifier also includes: A first dichroic mirror and a second dichroic mirror are disposed on opposite sides of the holmium-doped yttrium aluminum garnet crystal in the amplifier. The pump laser is transmitted through the first dichroic mirror and enters the amplifier, and the seed laser is reflected by the first dichroic mirror and enters the amplifier.

7. The high-energy thulium-pumped holmium laser oscillator amplifier according to claim 6, characterized in that, The amplifier has a single-ended pumping or double-ended pumping structure, and the number of holmium-doped yttrium aluminum garnet crystals in the amplifier is one or more.

8. The high-energy thulium-pumped holmium laser oscillator amplifier according to any one of claims 1-7, characterized in that, Also includes: The beam shaping module includes a first shaping lens group disposed between the pump source and the oscillator, and a second shaping lens group disposed between the oscillator and the amplifier.

9. The high-energy thulium-pumped holmium laser oscillator amplifier according to any one of claims 1-7, characterized in that, Also includes: The cooling device is thermally coupled to the thulium-doped yttrium aluminum garnet crystal, the holmium-doped yttrium aluminum garnet crystal in the oscillator, and the holmium-doped yttrium aluminum garnet crystal in the amplifier.

10. The high-energy thulium-pumped holmium laser oscillator amplifier according to any one of claims 1-7, characterized in that, The holmium-doped yttrium aluminum garnet crystal in the oscillator has a length greater than or equal to 100 mm and a doping concentration ranging from 0.5% to 1%; and / or, the holmium-doped yttrium aluminum garnet crystal in the amplifier has a length ranging from 100 mm to 160 mm and a doping concentration ranging from 0.5% to 1%.

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