Air-cooled tunable single-frequency pulse slab laser

Abstract

The invention discloses an air-cooled tunable single-frequency pulse slab laser, and belongs to the technical field of solid laser. According to the method, a diode stack array, a pumping shaping system, a coupling cavity mirror, a slab laser crystal, a first reflecting mirror and a thin film polaroid are included, and the diode stack array serves as a pumping source; the pumping shaping system is used for eliminating the non-uniformity in the slow axis direction of the diode stack array and directly pumping the slab laser crystal based on the end face of the coupling cavity mirror; the coupling cavity mirror, the first reflecting mirror and the thin film polaroid jointly form an annular resonant cavity, oscillation laser passes through the lath laser crystal back and forth in the annular resonant cavity for multiple times, and finally single-frequency pulse laser is output through the thin film polaroid. Continuous seed modulation or seed injection locking is not needed, the system is simple and reliable, the cost is low, and the large-energy single-frequency pulse laser light source can be used for atmospheric exploration.

Description

Technical Field

[0001] The invention belongs to the technical field of solid lasers, and in particular relates to an air-cooled tunable single-frequency pulse slab laser. Background Art

[0002] Single-frequency lasers have the advantages of narrow laser linewidth and long coherence length. In recent years, they have been widely used in laser wind radar and atmospheric detection. In order to achieve spatial resolution, laser radars are often based on single-frequency pulsed lasers as the transmitting light source.

[0003] In order to achieve long-distance wind measurement, LiDAR atmospheric detection requires a high-energy single-frequency pulsed laser light source. The double frequency of 532nm, which is the fundamental frequency of 1064nm, is often used for detection. Assuming the wind speed is 100m / s, the Doppler frequency shift is only 0.354pm, and the corresponding frequency is only 376MHz. Generally, the emission wavelength of 532nm is locked to the edge of the iodine molecule absorption line, and the Doppler frequency shift is converted into the signal intensity change. If the wind measurement accuracy is required to be 0.6m / s, the emission wavelength frequency stability is required to be less than 1MHz, and the single pulse energy is often required to be tens or even tens of millijoules. However, the conventional solid-state laser gain linewidth is very wide, and the linewidth often reaches tens of GHz without taking frequency control measures. The frequency also drifts due to environmental disturbances, and the maximum can also reach tens of GHz, which cannot be directly applied to wind measurement LiDAR.

[0004] Existing high-energy single-frequency pulse laser systems often use two schemes as follows: one is to use continuous single-frequency seeds for fiber pulse modulation. Since the duty cycle is often low, the power loss of continuous seed laser is very large. The energy of a single pulse after modulation is only in the nJ order. It must undergo multi-stage fiber pre-amplification before being injected into the subsequent solid-state amplifier. The laser system is often more complex and costly. The second is to inject a continuous single-frequency seed source into the resonant cavity, and at the same time detect the resonance signal generated by the seed laser injected into the resonant cavity. The cavity length of the injected resonant cavity and the frequency of the injected continuous seed laser are matched and locked based on piezoelectric elements, electro-optical or acousto-optic devices. After locking, Q-switching is completed to enable the seed laser to achieve multi-pass amplification, suppress quantum noise, and achieve high-energy single-frequency pulse laser output. However, the locking technology is often very complex and costly. Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides an air-cooled tunable single-frequency pulse slab laser, which directly realizes large single pulse energy single-frequency pulse laser output based on slab laser crystal, ring cavity technology, etalon filtering technology and high-precision temperature control.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0007] An air-cooled tunable single-frequency pulse slab laser comprises a diode stack, a pump shaping system, a coupling cavity mirror, a slab laser crystal, a first reflector, and a thin film polarizer, wherein:

[0008] The diode stack array is used as a pump source;

[0009] The pump shaping system is used to eliminate the non-uniformity in the slow axis direction of the diode stack array, and directly pumps the slab laser crystal with part of the end face based on the coupling cavity mirror;

[0010] The coupling cavity mirror, the first reflecting mirror and the thin-film polarizer together form a ring resonant cavity, and the oscillating laser passes through the slab laser crystal back and forth for many times in the ring resonant cavity, and finally outputs a single-frequency pulse laser through the thin-film polarizer.

[0011] The beneficial effects of the present invention are:

[0012] The present invention realizes higher pump power injection, the slab laser crystal eliminates thermally induced depolarization and thermally induced birefringence effects, which is beneficial to high beam quality and large single pulse energy output, the annular cavity overcomes the spatial hole burning effect, and can realize single-frequency pulse laser output, combined with high-precision temperature control, it can overcome mode hopping, and the standard tool can realize wavelength tuning without continuous seed modulation or seed injection locking. The system is simple and reliable with low cost, and can be used as a large-energy single-frequency pulse laser light source required for atmospheric detection, and can be used as an excellent single-frequency seed source for cascaded solid-state amplifiers to further improve the single pulse energy. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is a structural diagram of an air-cooled tunable single-frequency pulse slab laser according to the present invention.

[0014] Reference numerals:

[0015] 1. Diode stack; 2. Pump shaping system; 3. Coupled cavity mirror; 4. Slab laser crystal; 5. Slab Faraday rotator crystal; 6. Passive Q-switched crystal; 7. λ / 2 wave plate; 8. First reflector; 9. Thin-film polarizer; 10. Etalon; 11. Second reflector; 12. Third reflector. DETAILED DESCRIPTION

[0016] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0017] The invention provides an air-cooled tunable single-frequency pulse slab laser. The slab laser crystal, the ring cavity technology, the etalon filtering technology and the high-precision temperature control are used to directly realize the single-frequency pulse laser output with large single pulse energy. The slab laser crystal adopts partial end-face pumping, and the pump light field is flat-topped in the narrow and long direction of the slab crystal, so that the temperature field of the slab laser crystal is one-dimensionally distributed, and the problems of thermal depolarization and thermal birefringence under high-power pumping are eliminated. The ring cavity is a traveling wave resonant cavity, which overcomes the mode competition caused by the standing wave and thus greatly suppresses the spatial hole burning effect, and can realize single-frequency pulse laser output. However, due to the long cavity length of the resonant cavity, the longitudinal mode frequency interval is relatively small. Based on the high-precision temperature control to overcome the mode hopping problem, the etalon is a frequency-selective device to realize wavelength tuning. Based on the above design, high beam quality and large single pulse energy direct output can be realized, and continuous seed modulation or seed injection locking is not required. The laser light source with large energy and single frequency pulse required for atmospheric detection can be used.

[0018] Specifically, Figure 1 As shown, the light-emitting cross-section of the diode stack 1 is rectangular, and the pump shaping system 2 is used to eliminate the non-uniformity of the diode stack 1 in the slow axis direction. Based on the coupling cavity mirror 3, the slab laser crystal 4 is directly partially end-face pumped. The temperature field inside the slab laser crystal 4 is a one-dimensional distribution. The laser is a multi-pass optical path folding structure. The oscillating laser passes through the slab laser crystal 4 for many times. The length of the resonant cavity and the cavity mode size are effectively improved. Since the thermal lens effect only exists in the thickness direction of the slab laser crystal 4, the oscillating laser is an elliptical spot. The second reflector 11 and the third reflector 12 are placed at a small angle to eliminate parasitic oscillations of the resonant cavity under high-power pumping. The coupling cavity mirror 3, the first reflector 8 and the thin-film polarizer 9 together constitute the resonant cavity mirror. The resonant cavity as a whole is a ring cavity design to achieve single-frequency pulse laser output. The etalon 10 can achieve wavelength tuning. Combined with high-precision temperature control, mode hopping can be overcome without continuous seed modulation or seed injection locking.

[0019] The laser has a multi-pass optical path folding structure, which folds back and forth through the slab laser crystal 4 many times. Since the thermal lens only exists in the direction of the slab thickness, the cavity mode sizes of the resonant cavity in the orthogonal direction of the slab laser crystal 4 are inconsistent, and the resonant cavity oscillating laser is an elliptical spot. The oscillating laser is output based on the thin film polarizer 9. However, since the thickness of the slab laser crystal 4 is only 1 mm, the output laser is easily circularly symmetric based on cylindrical shaping and is a Gaussian base mode, and the beam quality is very high.

[0020] The diode stack 1 is a vertical stack that can be flexibly stacked according to the single pulse energy output requirements. It can work in either pulse pumping or continuous pumping systems. The light-emitting cross-section is rectangular, but since the slow axis direction is the superposition of multiple diode single-tube light-emitting areas, the beam quality is poor and the pump distribution often appears sawtooth-shaped. The pump shaping system 2 is often based on the beam integration concept such as rectangular waveguides or microlenses to perform homogenization and shaping in the slow axis direction of the diode stack 1, so that the long strip direction of the slab laser crystal 4 has a one-dimensional temperature field distribution, which is used to eliminate the problem of thermally induced birefringence of rod-shaped crystals under high-power pumping.

[0021] The coupling cavity mirror 3 is used to couple the pump energy and also serves as a folding cavity mirror of the laser resonant cavity. Considering that spontaneous radiation loss may occur in the energy storage stage under high-power pumping, the second reflector 11 and the third reflector 12 adopt a discrete structure with a gap between them. Spontaneous radiation can leak through the gap without parasitic oscillation, thereby wasting pump energy storage and reducing single pulse energy output and system efficiency.

[0022] The slab laser crystal 4 can be expanded in width according to the single pulse energy output, while the thickness direction remains unchanged to achieve efficient heat dissipation. Two larger heat dissipation surfaces are welded to the heat sink to achieve efficient and uniform heat dissipation. The laser oscillation reciprocating transmission can almost fill the slab laser crystal, thereby efficiently extracting all stored energy.

[0023] In order to ensure that the oscillating laser only runs in one direction to eliminate the spatial hole burning effect and realize single-frequency pulses, the Faraday rotator crystal 5 is a slat structure. The rotation of the polarization direction of the polarized laser depends only on the direction of the applied magnetic field. Since the oscillating laser passes through the slat Faraday rotator crystal 5 many times, relative to the conventional Faraday rotator, this means that the applied magnetic field strength can be reduced by 1 / N, where N is the number of times the oscillating laser passes back and forth, which is beneficial to the magnetic shielding design of the laser as a whole.

[0024] The slat Faraday rotator crystal 5, the λ / 2 wave plate 7 and the thin-film polarizer 9 together constitute an optical unidirectional diode, which only allows the oscillating laser to propagate in one direction along the ring cavity, eliminating the spatial hole burning effect caused by the standing wave cavity. The ratio of p-polarization and s-polarization can be flexibly achieved by tuning the rotation angle of the λ / 2 wave plate 7. The s-polarization oscillates in the resonant cavity, and the p-polarization is output by the thin-film polarizer 9. No separate output cavity mirror is required, and the effective output reflectivity can be flexibly adjusted according to the pump design and the single pulse energy requirement. The Jones matrix is ​​often used for calculation, and the equivalent reflectivity of the resonant cavity can achieve 100%-40% continuous tuning, which is sufficient to cope with the conventional resonant cavity laser pulse output.

[0025] The passive Q-switched crystal 6 is used to generate Q-switched pulses. During the pump energy storage stage, the oscillating laser cannot oscillate back and forth, and the slab laser crystal forms an effective particle number inversion density storage. Once the oscillation threshold is reached, the passive Q-switched crystal 6 is highly transparent to the oscillating laser, causing the laser to oscillate rapidly to form a pulse output.

[0026] The etalon 10 is inserted into the resonant cavity at a small tilted angle. The coupling cavity mirror 3, the first reflector 8 and the thin film polarizer 9 together constitute the resonant cavity mirror. If the surface of the etalon 10 is parallel to the resonant cavity mirror, the etalon 10 will form a composite cavity with the original resonant cavity, and the resonant frequency in the cavity will change. In addition, the relative jitter between the etalon 10 and the resonant cavity is extremely sensitive, and a stable single-frequency laser output cannot be obtained. The etalon 10 is placed tilted to eliminate the coupled resonance between the end face and the resonant cavity mirror, and the resonant frequency is no longer sensitive to the relative movement between the etalon 10 and the resonant cavity mirror. The etalon 10 only acts as a bandpass filter and selects a large number of frequencies in the resonant cavity. In addition, in order to reduce insertion loss, the FP insertion position is generally selected at a position with a larger resonant cavity mode size.

[0027] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. An air-cooled tunable single-frequency pulsed slab laser, characterized in that: It includes a diode stack, a pump shaping system, a coupling cavity mirror, a slab laser crystal, a first reflector, and a thin film polarizer, wherein: The diode stack array is used as a pump source; The pump shaping system is used to eliminate the non-uniformity in the slow axis direction of the diode stack array, and directly pumps the slab laser crystal with part of the end face based on the coupling cavity mirror; The coupling cavity mirror, the first reflecting mirror and the thin-film polarizer together form a ring resonant cavity, and the oscillating laser passes through the slab laser crystal back and forth for many times in the ring resonant cavity, and finally outputs a single-frequency pulse laser through the thin-film polarizer.

2. The air-cooled tunable single-frequency pulsed slab laser according to claim 1, characterized in that: The rear end of the slab laser crystal is connected to a slab Faraday rotator crystal, and the slab Faraday rotator crystal is a slab structure.

3. The air-cooled tunable single-frequency pulsed slab laser according to claim 1, characterized in that: The cavity mode sizes of the resonant cavity in the orthogonal directions of the slab laser crystal are inconsistent, and the oscillating laser in the ring resonant cavity is an elliptical spot.

4. The air-cooled tunable single-frequency pulsed slab laser according to claim 1, characterized in that: The diode stack is a vertical stack, and the light emitting cross section is a rectangle.

5. The air-cooled tunable single-frequency pulsed slab laser according to claim 1, characterized in that: The pump shaping system performs homogenization shaping in the slow axis direction of the diode stack based on the beam integration concept, so that the long strip direction of the slab laser crystal has a one-dimensional temperature field distribution.

6. The air-cooled tunable single-frequency pulsed slab laser according to claim 2, characterized in that: A second reflector and a third reflector are arranged longitudinally at the rear end of the slab Faraday rotator crystal, and a gap is provided between the second reflector and the third reflector.

7. The air-cooled tunable single-frequency pulsed slab laser according to claim 1, characterized in that: A λ / 2 wave plate is also provided at the front end of the first reflector. The ratio of p-polarization and s-polarization is achieved by tuning the rotation angle of the λ / 2 wave plate. The s-polarization oscillates in the ring resonant cavity, and the p-polarization is output by the thin-film polarizer.

8. The air-cooled tunable single-frequency pulsed slab laser according to claim 1, characterized in that: The front end of the thin film polarizer is also provided with an etalon.

9. The air-cooled tunable single-frequency pulsed slab laser according to claim 7, characterized in that: A passive Q-switched crystal is also provided at the front end of the λ / 2 wave plate to achieve narrow pulse width pulse laser output.

10. The air-cooled tunable single-frequency pulsed slab laser according to claim 8, characterized in that: The etalon is inserted into the ring resonator at an angle.

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