Q-switched nd:yag laser single / dual longitudinal mode switchable output control method

Abstract

The present application relates to a kind of Q-switching Nd:YAG laser single-double longitudinal mode switchable output control method, belong to the technical field of pulsed solid laser.The present application adopts F-P etalon and torsional module component to form composite mode selection structure, and suppresses spatial hole burning effect by torsional mode and the position of adjusting crystal dual mechanism.On this basis, two independent switching paths are provided: one is fixed pump power adjustment F-P etalon inclination angle;Second is fixed F-P etalon angle adjustment pump power, both can realize mode switching.The present application can stably output high-purity single longitudinal mode and stable beat frequency double longitudinal mode pulse in the same 100 Hz repetition frequency electro-optic Q-switching laser, single longitudinal mode linewidth 883.9 MHz, double longitudinal mode stable beat frequency 487 MHz, solve the technical problems that traditional laser single-double longitudinal mode cannot be flexibly switched, mode purity is poor, double longitudinal mode intensity fluctuation is big, can be widely applied in spectral detection, laser radar, precision ranging and other fields.

Description

Technical Field

[0001] This invention relates to the field of pulsed solid-state laser technology, specifically to a longitudinal mode control method for an electro-optically Q-switched Nd:YAG laser, which enables flexible switching between single and dual longitudinal mode outputs and is suitable for applications such as spectral detection, lidar, and precision ranging. Background Technology

[0002] Q-switched single-mode lasers, with their narrow linewidth and high peak power, have significant application value in fields such as spectral detection and lidar. Dual-mode lasers, with their dual-frequency output, can utilize the beat frequency effect, offering unique technological advantages in precision ranging. Achieving flexible switching between single and dual-mode output would allow a single Q-switched laser to be adapted to various application scenarios, significantly improving the equipment's versatility and practicality. However, existing Q-switched laser longitudinal mode control technologies have the following drawbacks: First, single-mode and dual-mode lasers typically employ independent optical path designs, resulting in poor equipment versatility and an inability to meet the needs of multiple scenarios with a single device. Furthermore, traditional standing-wave cavity solid-state lasers suffer from severe spatial hole-burning effects. In single-mode output, the unconsumed high-gain regions at the standing wave nodes lead to edge longitudinal mode oscillation, resulting in low single-mode purity. In dual-mode output, the overlapping hole-burning regions of the two longitudinal modes cause intense gain competition, leading to continuous fluctuations in output intensity and an inability to form a stable beat frequency signal. Third, existing longitudinal mode switching methods are limited, often relying solely on cavity length adjustment or etalon angle adjustment. These methods require high precision, have complex structures, and cannot provide multiple independent switching paths to adapt to different application scenarios. Furthermore, when only FP etalon selection is used without space burn-in suppression measures, the mode stability after switching is poor, with large fluctuations in pulse energy and linewidth, making long-term stable operation difficult. Therefore, there is an urgent need to develop a control method that can effectively suppress space burn-in effects, provides multiple switching paths, and allows for stable output in both single and dual longitudinal modes. Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of existing Q-switched lasers, such as the inability to flexibly switch between single and dual longitudinal modes, poor mode purity due to spatial hole burning, large intensity fluctuations in dual longitudinal modes, and a single switching method. This invention provides a single / dual longitudinal mode switchable output control method for Q-switched Nd:YAG lasers. By combining a composite mode selection structure and a dual spatial hole burning suppression mechanism with two independent tuning paths, the same laser can achieve stable output of high-purity single longitudinal mode and stable beat frequency dual longitudinal mode pulses respectively.

[0004] To achieve the above objectives, a single- and dual-longitudinal-mode switchable output control method for Q-switched Nd:YAG lasers is proposed. The core of the method includes three parts: spatial hole burning suppression optical path construction, two independent mode switching paths, and Q-switching control. The specific technical solution is as follows.

[0005] First, a basic optical path for the resonant cavity to suppress the spatial hole burning effect is constructed. The total cavity length is 255 mm, with a 1064 nm concave high-reflectivity mirror with a radius of curvature of 2000 mm at one end and an output coupling mirror with 10% transmittance at the other end. Two polarization beam-splitters are placed inside the cavity to ensure that the laser operates in a linear polarization state. A λ / 4 waveplate is placed on each side of the Nd:YAG crystal, with the optical axes of the two waveplates orthogonal to each other at 45°. Together with the polarization beam-splitters, they form a torsional mode structure, causing the laser to alternately form left-handed and right-handed circularly polarized light as it travels back and forth within the crystal, breaking the spatial symmetry of the standing wave field and uniformly consuming the inverted particles. The Nd:YAG crystal is precisely positioned at the geometric center of the resonant cavity. Utilizing the spatial distribution characteristics of the longitudinal modes at the cavity center, the torsional mode structure works in conjunction with the longitudinal mode structure to eliminate the longitudinal mode gain competition caused by the spatial hole burning effect. An uncoated FP etalon with a thickness of 9 mm and a reflectivity of R=4% is inserted into the cavity and mounted on a precision rotating mirror frame for mode selection. A KDP crystal is used as the electro-optic Q-switching element. A nm semiconductor laser pumps an Nd:YAG crystal via a coupling lens array. The crystal is temperature-controlled by constant-temperature water cooling at 25℃.

[0006] Based on the above optical path, two independent single / dual longitudinal mode switching paths are provided.

[0007] Path 1: FP etalon angle tuning switching. Maintaining a constant pump power of 10 W and a Q-switching repetition frequency of 100 Hz, the tilt angle of the FP etalon is changed by precisely rotating the mirror mount: when the etalon tilt angle is 0.493°, its transmission spectrum bandwidth only covers the single longitudinal mode at the center of the resonant cavity; the remaining longitudinal modes are suppressed because the loss introduced by the FP is higher than the oscillation threshold, and the laser stably outputs a single longitudinal mode Q-switched pulse; when the etalon tilt angle is adjusted to 0.393°, its transmission spectrum bandwidth simultaneously covers two adjacent longitudinal modes, and the gain and loss of the two longitudinal modes are matched, forming a stable dual-longitudinal-mode oscillation output.

[0008] Path 2: Pump power tuning switching. The FP etalon tilt angle is fixed at 0.41°, maintaining a constant 100 Hz Q-switching repetition frequency. Only the 808 nm pump power is adjusted: When the pump power is reduced to 9 W, the inverted particle number density in the gain medium only meets the oscillation threshold requirement of the central longitudinal mode. Adjacent longitudinal modes cannot oscillate due to their higher thresholds, and the laser outputs a single longitudinal mode pulse. When the pump power is increased to 10 W, the inverted particle number density simultaneously reaches the oscillation threshold of two adjacent longitudinal modes. Under the condition that spatial hole burning is effectively suppressed, there is no severe gain competition between the two longitudinal modes, forming a stable dual-longitudinal-mode oscillation output. When the pump power is further increased to 12 W, the dual-longitudinal-mode intensity ratio and beat frequency remain stable.

[0009] The electro-optic Q-switched crystal is always loaded with a quarter-wavelength Q-switched high voltage at a fixed repetition frequency of 100 Hz, ensuring that the output of a time-stable narrow pulse laser is maintained under both switching paths.

[0010] Compared with existing technologies, the beneficial effects of this invention include the following: First, dual independent switching paths provide strong adaptability: it simultaneously provides two switching methods, FP etalon angle tuning and pump power electrical tuning. Angle tuning is suitable for optical path switching under fixed pump conditions, while power tuning is suitable for one-button electrical control switching under fixed optical paths, without requiring modification of the resonant cavity structure, significantly improving the equipment's versatility. Second, dual spatial hole burning suppression ensures good mode stability: the torsional mode structure uniformly consumes inverted particles at the polarization state level, and the crystal cavity center arrangement eliminates longitudinal mode competition at the spatial gain distribution level. Single longitudinal mode output has no extraneous mode oscillation and high purity, while dual longitudinal mode output has no intensity jitter and long-term stable beat frequency. Third, the structure is simple and easy to engineer. It only requires adding an FP etalon and a pair of λ / 4 waveplates to a traditional electro-optic Q-switched laser. Mode switching can be completed by rotating the mirror mount or adjusting the pump current, making debugging difficult and allowing integration into miniaturized laser equipment. Attached Figure Description

[0011] Figure 1 Schematic diagram of the optical path structure of the Q-switched Nd:YAG laser of this invention; Explanation of reference numerals: 1. Concave high-reflection mirror; 2. First polarizing beam splitter prism; 3. FP etalon; 4. First λ / 4 waveplate; 5. Dichroic mirror; 6. Nd:YAG gain crystal; 7. Second λ / 4 waveplate; 8. Second polarizing beam splitter prism; 9. Electro-optic Q-switching crystal; 10. Output coupling mirror; 11. Pump coupling lens group f1, f2; 12. 808 nm pump source.

[0012] Figure 2 The transmittance curve of the FP standard used in this invention at a tilt angle of 0.493°.

[0013] Figure 3 The transmittance curve of the FP standard used in this invention at a tilt angle of 0.393°.

[0014] Figure 4 This invention provides a single-longitudinal-mode output pulse waveform under an angle-tuning path.

[0015] Figure 5 This invention provides a dual longitudinal mode output pulse waveform under an angle-tuned path.

[0016] Figure 6 The present invention presents the output pulse waveforms of different pump powers under the power tuning path. (a) Single-mode pulse time waveform under 9 W pump power and (b) Dual-mode pulse time waveform under 10 W pump power, with the inset showing frequency difference information. Detailed Implementation

[0017] To further understand the content of this invention, the invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0018] Example 1: This example implements single / dual longitudinal mode switching based on the FP etalon angle tuning path. The specific implementation steps are as follows: 1. Optical path setup according to Figure 1 The resonant cavity is constructed as shown: the total cavity length is precisely adjusted to 255 mm; a concave high-reflectivity mirror 1 with a curvature radius of 2000 mm is installed at the left end, coated with a 1064 nm high-reflectivity film, with a reflectivity >99.9%; an output coupling mirror 10 with a 1064 nm transmittance of 10% is installed at the right end; from left to right, the following are installed in the cavity: a first polarizing beam splitter 2, an FP standard etalon 3 mounted on a precision rotating mirror frame, a first λ / 4 waveplate 4, a dichroic mirror 5 placed at 45°, an Nd:YAG gain crystal 6, a second λ / 4 waveplate 7, a second polarizing beam splitter 8, and a KDP electro-optic Q-switching crystal 9; wherein the optical axes of the first λ / 4 waveplate 4 and the second λ / 4 waveplate 7 are adjusted to be orthogonal at 45°, and the Nd:YAG crystal 6 has dimensions of Φ3×3×10 mm and a doping concentration of 1. The at.%, precisely placed at the geometric center of the resonant cavity, is installed in a 25℃ constant temperature water-cooled fixture, with temperature control via a semiconductor cooling chip; the FP etalon 3 is made of uncoated K9 glass with a thickness of 9 mm and a reflectivity R=4%; the 808 nm pump source 12 is pumped by the Nd:YAG crystal 6 through the end face of the dichroic mirror 5 via a coupling lens group 11 with a focal length of 18 mm and 100 mm; the KDP electro-optic Q-switching crystal 9 is connected to the Q-switching drive power supply, with a repetition frequency of 100 Hz and a quarter-wavelength voltage of 2.8 kV.

[0019] 2. Dual Longitudinal Mode Output Adjustment: Turn on the pump source and set the pump power to 10 W. After the crystal temperature stabilizes, precisely rotate the FP etalon mount to adjust the etalon tilt angle to 0.393°. Use a fast photodetector with a rise time of 125 ps to receive the output laser. Connect a 2 GHz bandwidth oscilloscope to observe the pulse waveform. At this time, the output pulse width is 42 ns, and the pulse period is 10 ms, which is consistent with the Q-switched repetition frequency. The average output power is measured to be 57.9 mW, corresponding to a single pulse energy of 579 μJ. The longitudinal mode characteristics are measured using the FP interferometer method to obtain a stable dual longitudinal mode output. The frequency difference between the two longitudinal modes is 487 MHz, which is consistent with the longitudinal mode spacing corresponding to a cavity length of 255 mm. The bus width is 1.446 GHz, the pulse intensity has no obvious fluctuations, and the beat frequency signal is stable.

[0020] 3. Switch to single-longitudinal-mode output while maintaining a pump power of 10 W and a Q-switching frequency of 100 Hz. Continue to precisely rotate the FP etalon mount to adjust the tilt angle to 0.493°. At this point, the oscilloscope observes an output pulse width of 48 ns, with a smooth waveform and no modulation. The power meter measures an average output power of 49.5 mW, corresponding to a single pulse energy of 495 μJ. The FP interferometer shows a single-longitudinal-mode output with a linewidth of 883.9 MHz, no extraneous mode oscillation, and high single-longitudinal-mode purity.

[0021] Example 2: This example implements single-mode / dual-mode switching based on the pump power tuning path. The optical path structure is exactly the same as in Example 1. The specific implementation steps are as follows: 1. Initial setup: Fix the tilt angle of FP standard etalon 3 to 0.41° and lock the frame. KDP electro-optic Q-switched crystal maintains a repetition frequency of 100 Hz and Nd:YAG crystal temperature is stabilized at 25℃.

[0022] 2. Adjusting the pump source drive current for single-mode output reduced the 808 nm pump power to 9 W; the oscilloscope observed an output pulse width of 48 ns and a stable pulse period of 10 ms; the power meter measured an average output power of 21.4 mW, corresponding to a single pulse energy of 214 μJ; FP interferometer measurements confirmed that this was a high-purity single-mode output.

[0023] 3. Switch to dual longitudinal mode output while keeping the FP etalon angle unchanged, and increase the pump power to 10 W by electronic control; the oscilloscope observes that the pulse waveform shows stable beat frequency modulation with a pulse width of 48.4 ns; the power meter measures an average output power of 46.7mW, corresponding to a single pulse energy of 467 μJ; the spectrum measurement shows that the beat frequency of the two longitudinal modes is 487 MHz, and the dual longitudinal mode output is stable.

[0024] The present invention and its embodiments have been described above illustratively. This description is not restrictive, and actual implementations are not limited to the above-described methods. Those skilled in the art, upon learning of the present invention, can make several simple deductions, substitutions, or modifications without departing from the spirit of the invention, all of which should fall within the protection scope of the present invention.

Claims

1. A method for switchable single / dual longitudinal mode output control of a Q-switched Nd:YAG laser, characterized in that, The laser resonant cavity is equipped with a torsional mode polarization component, a FP etalon, and an electro-optic Q-switched crystal, with an Nd:YAG gain crystal positioned at the geometric center of the resonant cavity; the control method includes the following steps: S1. Constructing a resonant cavity optical path to suppress the spatial hole burning effect: A pair of λ / 4 waveplates with optical axes orthogonal at 45° are set at both ends of the Nd:YAG crystal. Together with two polarizing beam splitters in the cavity, they form a torsional mode structure, so that when the laser propagates back and forth in the gain crystal, it alternately forms left-handed and right-handed circularly polarized light, uniformly consuming the inverted particles in the crystal; The Nd:YAG crystal is fixed at the geometric center of the resonant cavity to eliminate the longitudinal mode gain competition caused by the spatial hole burning of the standing wave field; S2, First Mode Switching Path: FP Ether Angle Tuning Switch S21, Single Longitudinal Mode Output Control: Maintain constant 808nm end-face pump power, precisely rotate the FP etalon to the first tilt angle, so that the FP etalon transmission spectrum bandwidth only covers the single central longitudinal mode of the resonant cavity, and the other longitudinal modes are suppressed because the cavity loss is higher than the oscillation threshold, and the laser outputs a stable single longitudinal mode Q-switched pulse. S22, Dual Longitudinal Mode Output Control: Keeping the pump power constant, rotate the FP etalon to the second tilt angle so that the transmission spectrum bandwidth of the FP etalon simultaneously covers the two adjacent longitudinal modes of the resonant cavity. The gain of the two longitudinal modes is matched with the cavity loss, and the laser outputs a stable dual longitudinal mode Q-switched pulse. S3, Second Mode Switching Path: Pump Power Tuning Switching S31, with the FP standard etalon tilt angle unchanged, the 808nm end-face pump power is reduced to the first power threshold. The inverted particle number density in the gain medium only satisfies the single central longitudinal mode oscillation condition, and the adjacent longitudinal modes cannot reach the oscillation threshold. The laser outputs a single longitudinal mode Q-switched pulse. S32, the 808nm end-face pump power is increased to the second power threshold, the inverted particle number density simultaneously meets the oscillation requirements of the two adjacent longitudinal modes, the two longitudinal modes oscillate synchronously and stably under the condition that the spatial hole burning is suppressed, and the laser switches to dual longitudinal mode Q-switched pulse output; S4, the electro-optic Q-switched crystal is loaded with a Q-switching high voltage at a fixed repetition frequency, and outputs a regular narrow pulse laser in both switching paths.

2. The method for switchable single / dual longitudinal mode output control of a Q-switched Nd:YAG laser according to claim 1, characterized in that, The FP etalon is an uncoated optical etalon with a reflectivity R=4% and a thickness of 9mm; the first tilt angle in step S21 is 0.493°, and the second tilt angle in step S22 is 0.393°.

3. The method for switchable single / dual longitudinal mode output control of a Q-switched Nd:YAG laser according to claim 1, characterized in that, In step S31, the first power threshold is 9W, and in step S32, the second power threshold is 10W; when executing the second mode switching path, the tilt angle of the FP etalon is fixed at 0.41°.

4. The method for switchable single / dual longitudinal mode output control of a Q-switched Nd:YAG laser according to claim 1, characterized in that, The torsional mode polarization assembly includes a first polarizing beam splitter, a second polarizing beam splitter, a first λ / 4 waveplate, and a second λ / 4 waveplate; the first λ / 4 waveplate and the second λ / 4 waveplate are respectively arranged on both sides of the Nd:YAG crystal, and the optical axes of the two waveplates are placed orthogonally at an angle of 45°.

5. The method for switchable single / dual longitudinal mode output control of a Q-switched Nd:YAG laser according to claim 1, characterized in that, The total cavity length of the resonant cavity is 255mm; one end of the resonant cavity is a concave high-reflectivity mirror with a radius of curvature of 2000mm, coated with a 1064nm high-reflectivity film, with a reflectivity >99.9%; the other end is an output coupling mirror with a 1064nm wavelength transmittance of 10%.

6. The method for switchable single / dual longitudinal mode output control of a Q-switched Nd:YAG laser according to claim 1, characterized in that, The Nd:YAG crystal has a size of Φ3×3×10mm and a doping concentration of 1at.%. It is temperature controlled by a combination of 25℃ constant temperature water cooling and a semiconductor refrigeration chip. The electro-optic Q-switched crystal is a KDP crystal with a fixed Q-switching repetition frequency of 100Hz and a quarter-wavelength voltage of 2.8kV.

7. The method for switchable single / dual longitudinal mode output control of a Q-switched Nd:YAG laser according to claim 1, characterized in that, An 808nm pump light is pumped onto an Nd:YAG crystal via a coupling lens group with focal lengths f1=18mm and f2=100mm. A dichroic mirror is placed at 45° inside the cavity, with an 808nm high-transmittance and 1064nm high-reflection coating deposited on the surface facing the cavity and an 808nm high-transmittance coating deposited on the surface facing the pump source.

8. The method for switchable single / dual longitudinal mode output control of a Q-switched Nd:YAG laser according to claim 1, characterized in that, When executing the first mode switching path, the pump power is constant at 10W; the single longitudinal mode output pulse width is 48ns, the average power is 49.5mW, the single pulse energy is 495μJ, and the output linewidth is 883.9MHz; the dual longitudinal mode output pulse width is 42ns, the average power is 57.9mW, the single pulse energy is 579μJ, the beat frequency of the two longitudinal modes is 487MHz, and the output linewidth is 1.446GHz.

9. The method for switchable single / dual longitudinal mode output control of a Q-switched Nd:YAG laser according to claim 1, characterized in that, When executing the second mode switching path, the single longitudinal mode output pulse width is 48ns, the average power is 21.4mW, and the single pulse energy is 214μJ under a 9W pump power; the dual longitudinal mode output pulse width is 48.4ns, the average power is 46.7mW, the single pulse energy is 467μJ, and the beat frequency of the two longitudinal modes is 487MHz under a 10W pump power.

10. The method for switchable single / dual longitudinal mode output control of a Q-switched Nd:YAG laser according to claim 1, characterized in that, When the pump power exceeds 10W and continues to increase, the pulse width of the dual longitudinal mode output no longer narrows significantly; only the pulse energy increases, while the intensity ratio of the two longitudinal modes and the beat frequency remain stable.

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