Dual-gain series-coupled dual-cavity solid-state laser with continuously adjustable pulse width and working method thereof

Abstract

This invention discloses a continuously adjustable pulse width dual-gain tandem dual-cavity solid-state laser and its operating method, relating to the field of laser technology. The laser includes a main resonant cavity and a sub-resonant cavity. Along the optical path, the main resonant cavity sequentially comprises a total reflection mirror, a first gain medium, a Q-switching switch, and a main output mirror. The sub-resonant cavity is located along the output optical path of the main resonant cavity, and along this path, sequentially comprises a second gain medium, a secondary output mirror, and a displacement driving device. The secondary output mirror is mounted on the displacement driving device. After the output pulse from the main resonant cavity couples into the sub-resonant cavity, it undergoes multiple round-trip reflections to form multiple delayed pulse components, which are then superimposed in the time domain to output a broadened pulse. The output pulse width is continuously adjustable by adjusting the optical cavity length of the sub-resonant cavity through the displacement driving device.

Description

Technical Field

[0001] This invention relates to the field of laser technology, specifically to a pulse-width-continuously-tunable dual-gain tandem dual-cavity solid-state laser and its operating method. Background Technology

[0002] Solid-state pulsed lasers have important applications in industrial micromachining, medical surgery, and scientific research. The pulse width of the output laser is a core performance parameter that directly affects the processing effect, the size of the heat-affected zone, and the physical mechanism of the process. In certain specific applications, such as precision drilling (to reduce taper) or photoacoustic imaging (to improve signal-to-noise ratio), pulsed lasers with stable output pulse widths (tens to hundreds of nanoseconds) and excellent beam quality are required.

[0003] However, the output pulse width of traditional single-cavity Q-switched solid-state lasers is mainly limited by the cavity gain and the switching speed of the Q-switching switch, resulting in a limited adjustment range. To obtain a wider tunable pulse width, existing technologies mainly employ the following approaches:

[0004] Cavity length switching scheme: This scheme switches between two different resonant cavity lengths via a mechanical device, with the pulse width varying according to the cavity length. The disadvantages of this scheme are the presence of moving mechanical parts, which affects stability and reliability; and the sudden changes in cavity length can easily lead to laser mode instability and power fluctuations.

[0005] Extracavity electro-optic modulation scheme: This scheme uses an independent electro-optic modulator at the laser output end to perform time-domain shaping and broadening of the output pulse. This scheme requires high-precision external circuit control, making the system complex; the electro-optic modulator and its driver source are expensive; and it introduces additional insertion loss, reducing the overall system efficiency.

[0006] Dual-cavity coupling scheme: This scheme employs a dual-cavity structure to adjust the pulse width. For example, patent application CN119134018A discloses a dual-cavity Q-switched laser, which uses a rotatable half-wave plate to form two resonant cavities sharing a laser crystal and a Q-switching switch but with different cavity lengths. By rotating the half-wave plate to change the polarization state of the light, the light is switched between resonant cavities of different lengths, thereby achieving pulse width variation. However, while this actively switching dual-cavity structure can change the pulse width to some extent, pulse width adjustment is usually step-like, relying on a limited number of discrete resonant cavity modes, making continuous and fine tuning difficult to achieve.

[0007] While expanding the pulse width, the existing technologies mentioned above have failed to effectively solve the problem of balancing output stability and pulse width continuous adjustability. Summary of the Invention

[0008] The purpose of this invention is to solve the problems of discontinuous pulse width adjustment and poor system stability in existing dual-cavity structures mentioned in the background art, and to propose a dual-gain tandem dual-cavity solid-state laser with continuously adjustable pulse width.

[0009] A first aspect of the present invention provides a pulse-width-continuously-tunable dual-gain tandem dual-cavity solid-state laser, the laser comprising:

[0010] The main resonant cavity is provided with a total reflection mirror, a first gain medium, a Q-switching switch and a main output mirror arranged sequentially along the optical path.

[0011] A sub-resonant cavity is disposed in the output optical path of the main resonant cavity. A second gain medium, a sub-output mirror, and a displacement driving device are sequentially disposed along the optical path. The sub-output mirror is mounted on the displacement driving device. The displacement driving device is used to adjust the distance between the sub-output mirror and the main output mirror to change the optical cavity length of the sub-resonant cavity.

[0012] Wherein, the gain of the main resonant cavity is greater than the gain of the sub-resonant cavity; and the net gain of the sub-resonant cavity is lower than the self-excited oscillation threshold.

[0013] Optionally, the ratio of the stimulated emission cross section of the first gain medium to the stimulated emission cross section of the second gain medium is greater than 2.

[0014] Optionally, the optical cavity length of the sub-resonant cavity is greater than the optical cavity length of the main resonant cavity.

[0015] Optionally, the ratio of the optical cavity length of the sub-resonant cavity to the optical cavity length of the main resonant cavity is not less than 1.5:1.

[0016] Optionally, the displacement driving device is any one of a piezoelectric ceramic driver, a voice coil motor, or a stepper motor driving guide.

[0017] Optionally, the sub-resonator is further provided with a beam quality control element for filtering out higher-order transverse modes and optimizing the output beam quality.

[0018] Optionally, the beam quality control element is a soft-edge aperture.

[0019] Optionally, the sub-resonant cavity is further provided with a frequency doubling crystal and a beam splitter for achieving intracavity frequency doubling.

[0020] A second aspect of this invention provides a method for operating a pulse-width-continuously-tunable dual-gain tandem dual-cavity solid-state laser, the method comprising:

[0021] S1: Generate an initial laser pulse in the main resonant cavity;

[0022] S2: Based on the transmittance of the main output mirror, a portion of the initial laser pulse is coupled into the sub-resonant cavity through the main output mirror;

[0023] S3: The optical pulse coupled into the sub-resonant cavity undergoes multiple round-trip reflections between the main output mirror and the secondary output mirror within the sub-resonant cavity. Each reflection receives a fixed time delay and gain compensation provided by the second gain medium.

[0024] S4: The pulse portions with different time delays generated by multiple round-trip reflections are superimposed in the time domain, and a pulse with a broadened envelope is output from the secondary output mirror;

[0025] The output pulse width can be continuously adjusted by adjusting the optical cavity length of the sub-resonant cavity.

[0026] The beneficial effects of this invention are:

[0027] 1. By adopting a structure of main resonant cavity connected in series with sub-resonant cavity, and by setting a second gain medium, a secondary output mirror and a displacement driving device, the output pulse width is continuously adjustable, without the need for a complex main cavity mechanical switching mechanism or a high-cost external high-speed electro-optic modulation system.

[0028] 2. By adopting a gain management method of "high-gain main cavity + low-gain sub-cavity", the pulse width adjustment is mainly completed in the sub-cavity, while the main cavity maintains relatively stable operation, thereby improving the output stability and consistency.

[0029] 3. The second gain medium can compensate for the loss of the pulse during multiple round trips in the sub-resonant cavity, so as to maintain high output energy and system efficiency while achieving pulse width broadening; and by controlling the net gain of the sub-resonant cavity below the self-excited oscillation threshold, parasitic oscillations and additional noise pulses can be effectively suppressed, and the stability, predictability and repeatability of the output pulse waveform can be improved. Attached Figure Description

[0030] Figure 1 A schematic diagram of a pulse-width-adjustable dual-gain tandem dual-cavity solid-state laser provided in an embodiment of the present invention;

[0031] Figure 2 This is a schematic diagram of the structure of an end-pumped dual-gain tandem dual-cavity solid-state laser provided in an embodiment of the present invention;

[0032] Figure 3 This is a schematic diagram of a dual-gain tandem dual-cavity solid-state laser with a quality control element provided in an embodiment of the present invention;

[0033] Figure 4 This is a schematic diagram of a dual-gain tandem dual-cavity solid-state laser with intracavity frequency doubling function, provided as an embodiment of the present invention.

[0034] The labels in the diagram mean the following: 1-Main resonant cavity; 101-Total reflection mirror; 102-First gain medium; 103-Q-switching switch; 104-Main output mirror; 2-Sub-resonant cavity; 201-Secondary output mirror; 202-Second gain medium; 203-Displacement drive device; 204-Beam quality control element; 205-Frequency doubling crystal; 206-Beam splitter; 301-Main resonant cavity pump source; 302-Sub-resonant cavity pump source; 303-Dichromatic reflector. Detailed Implementation

[0035] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features and effects of the present invention is provided in conjunction with the accompanying drawings and preferred embodiments.

[0036] This invention provides a pulse-width-continuously-tunable dual-gain tandem dual-cavity solid-state laser. See also... Figure 1 , Figure 1 This is a schematic diagram of a dual-gain, tandem, dual-cavity solid-state laser with continuously adjustable pulse width, provided as an embodiment of the present invention. The laser mainly consists of a main resonant cavity 1 for increasing gain and a sub-resonant cavity 2 for pulse envelope broadening, connected in series. Wherein:

[0037] The main resonant cavity 1 is provided with a total reflection mirror 101, a first gain medium 102, a Q-switching switch 103 and a main output mirror 104 arranged sequentially along the optical path.

[0038] The sub-resonant cavity 2 is disposed in the output optical path of the main resonant cavity 1. A second gain medium 202, a secondary output mirror 201, and a displacement driving device 203 are arranged sequentially along the optical path. The secondary output mirror 201 is mounted on the displacement driving device 203. The displacement driving device 203 is used to adjust the distance between the secondary output mirror 201 and the main output mirror 104 to change the optical cavity length of the sub-resonant cavity 2.

[0039] Among them, the gain of the main resonant cavity 1 is greater than the gain of the sub-resonant cavity 2; and the net gain of the sub-resonant cavity 2 is lower than the self-excited oscillation threshold.

[0040] This invention provides a pulse-width-continuously-tunable dual-gain tandem dual-cavity solid-state laser. By tandemly configuring the main resonant cavity and sub-resonant cavity, employing a combination of high-gain and low-gain media, and configuring the net gain of the sub-resonant cavity below the self-oscillation threshold, multipath delay superposition and broadening of the short output pulse from the main cavity are achieved. The secondary output mirror is mounted on a displacement drive device. By continuously adjusting the optical cavity length of the sub-resonant cavity, continuous and precise adjustment of the output pulse width can be achieved, avoiding mode instability and power fluctuation problems caused by traditional mechanical switching structures. Simultaneously, since the sub-resonant cavity cannot self-oscillate, parasitic oscillations and mode competition are effectively suppressed, ensuring the stability and consistency of the output beam.

[0041] In one embodiment, an end-pump scheme is implemented by introducing a deflected optical path into the pump optical coupling before the second gain medium 202. See also Figure 2 , Figure 2 This is a schematic diagram of an end-pumped, dual-gain, tandem, dual-cavity solid-state laser according to an embodiment of the present invention. The laser also includes a main resonant cavity pump source 301, a sub-resonant cavity pump source 302, and a dichroic mirror 303. Wherein:

[0042] The total reflection mirror 101 is a bicolor total reflection mirror for pumping. The main resonant cavity pump source 301 pumps the first gain medium 102 through the total reflection mirror 101.

[0043] A bicolor reflector 303 is positioned between the main output mirror 104 and the second gain medium 202. The sub-resonant cavity pump source 302 pumps the second gain medium 202 through the bicolor reflector 303.

[0044] In this embodiment, the main resonant cavity and the sub-resonant cavity are each equipped with an independent pump source, which allows the gain of the main cavity and the sub-cavity to be adjusted independently, further improving the system's flexibility and controllability in gain matching.

[0045] In one implementation, a gain management strategy of "high gain (main cavity) + low gain (sub-cavity)" is adopted, where the ratio of the gain of the main resonant cavity to the gain of the sub-cavity is greater than 2. The main resonant cavity 1 uses a high emission cross-section medium, such as a highly doped or longer Nd:YAG (neodymium-doped yttrium aluminum garnet) crystal, to ensure high conversion efficiency and generate the initial pulse. The sub-cavity 2 uses a low emission cross-section medium, such as a lightly doped or shorter Nd:YAG crystal, with a stimulated emission cross-section less than 2.0 × 10⁻⁶. -19 cm 2 Its low gain coefficient means that the net gain of the sub-cavity itself is insufficient to generate self-excited oscillations, effectively suppressing mode competition and instability. Calculations show that when the ratio of the stimulated emission cross-section of the main cavity (high emission cross-section medium) to that of the sub-cavity (low emission cross-section medium) is greater than 2.0, self-excited oscillations can be effectively suppressed, and the probability of self-excited oscillations can be reduced to below 1%.

[0046] In one implementation, the pulse width broadening process of the laser includes:

[0047] S1: Generates an initial laser pulse in the main resonant cavity 1. When the Q-switching switch 103 is running, it can generate short pulses on the order of nanoseconds in the main resonant cavity 1.

[0048] S2: Based on the transmittance of the main output mirror 104, the initial laser pulse portion is coupled into the sub-resonant cavity 2 through the main output mirror 104.

[0049] S3: The optical pulse coupled into the sub-resonant cavity 2 undergoes multiple round-trip reflections between the main output mirror 104 and the secondary output mirror 201 within the sub-resonant cavity 2, with each reflection obtaining a fixed time delay and gain compensation.

[0050] S4: The pulse portions with different time delays generated by multiple round trip reflections are superimposed in the time domain, and a pulse with a broadened envelope is output from the secondary output mirror 201.

[0051] The output pulse width can be continuously adjusted by adjusting the optical cavity length of the sub-resonant cavity 2.

[0052] The pulse width broadening principle of this method is as follows: the light pulse entering the sub-resonant cavity 2 does not form a new independent oscillation within the cavity, but rather causes multi-beam interference between the two mirrors of the sub-resonant cavity 2. This principle is similar to the Fabry-Perot etalon. Each time a portion of the light reflected back to the main resonant cavity 1 from the sub-output mirror 201 has a delayed superposition effect with the remaining gain in the main resonant cavity 1, which is in a de-excited or relaxed state. This superposition of multiple delayed feedbacks forms a significantly lengthened and smoothed pulse envelope in the time domain, which is ultimately output from the sub-output mirror 201.

[0053] In one implementation, the second gain medium 202 may not be an active gain medium, but rather a nonlinear medium, such as a stimulated Brillouin scattering cell or other nonlinear medium, which uses nonlinear effects (such as stimulated Brillouin scattering mirrors) to simulate part of the feedback and delay, but does not self-oscillate.

[0054] In one implementation, the displacement driving device 203 can be a piezoelectric ceramic driver, a voice coil motor, or a stepper motor driving rail, etc. The secondary output mirror 201 is mounted on the mirror frame of the displacement driving device 203. By precisely controlling the displacement driving device 203, the physical length (L) of the sub-cavity can be finely adjusted. Changes in the cavity length of the sub-cavity directly alter the time delay period t of the pulse envelope superposition (t=2L / c, where c is the speed of light), thereby continuously and in real-time adjusting the width of the output pulse. The output pulse width has an approximately linear relationship with the sub-cavity length; for example, for every 10 cm increase in the sub-cavity length, the output pulse width increases by approximately 0.67 nanoseconds.

[0055] To achieve a significant stretching effect, the optical length of the subcavity should be greater than the length of the main cavity, with a recommended ratio of 1.5:1 or higher.

[0056] In one implementation, the transmittance of the main output mirror 104 is approximately 30%, forming a conventional plano-concave or plano-plano cavity with the total reflection mirror 101, creating a stable laser oscillation environment. The secondary output mirror 201 is a high-reflection mirror with a transmittance controlled between 5% and 10%, ensuring envelope superposition and controlling energy coupling. The specific transmittance value of the secondary output mirror 201 needs to be jointly optimized with the transmittance of the main output mirror 104 (e.g., 30%) and the pump power to balance output energy and broadening effect.

[0057] In one implementation, after optimizing the transmittance parameters, the additional system loss introduced can be controlled within 5%, while the electro-optical conversion efficiency can still be maintained at a relatively high level of over 15%.

[0058] One implementation proposes a beam quality control scheme. See [link to implementation details]. Figure 3 , Figure 3 This is a schematic diagram of a dual-gain tandem dual-cavity solid-state laser with a quality control element provided in an embodiment of the present invention. A beam quality control element 204, such as a low-loss soft-edge aperture, is also disposed within the sub-resonant cavity 2 to filter out higher-order transverse modes and optimize the output beam quality. The beam quality control element 204 can be located in front of or behind the second gain medium 202, that is, it can be located between the second gain medium 202 and the sub-output mirror 201, or between the second gain medium 202 and the main output mirror 104.

[0059] One implementation proposes an intracavity frequency doubling scheme. See [link to implementation details]. Figure 4 , Figure 4 This is a schematic diagram of a dual-gain tandem dual-cavity solid-state laser with intracavity frequency doubling capability, provided in an embodiment of the present invention. A frequency doubling crystal 205 and a beam splitter 206 are also disposed within the sub-resonant cavity 2. The frequency doubling crystal 205 can be LBO (lithium triborate), BBO (barium metaborate), etc., to achieve intracavity frequency doubling. The frequency doubling crystal 205 is located between the second gain medium 202 and the sub-output mirror 201. The beam splitter 206 is located between the second gain medium 202 and the frequency doubling crystal 205, and is used to guide the frequency-doubled light. At this time, the sub-output mirror 201 is configured as a total reflection mirror, reflecting both the fundamental frequency light and the frequency-doubled light. The beam splitter 206 has high transmittance for the fundamental frequency light and total reflection for the frequency-doubled light.

[0060] This invention provides a solution for obtaining tunable pulse widths in solid-state lasers. One of the core concepts of this solution is to utilize a sub-cavity constructed with a gain medium having a low gain coefficient (or a small stimulated emission cross-section) to perform multipath delay superposition on short pulses from a high-gain main cavity, thereby achieving pulse width broadening while effectively suppressing self-oscillations within the cavity. This ensures output stability while achieving continuous pulse width tuning. The "dual-gain medium (high / low gain combination)" and "dual-output mirror tandem cavity" are preferred structural forms for realizing the above concept. Replacing the sub-cavity gain medium with other low-gain media (such as Nd:YVO4, Yb:YAG, etc.) or adjusting the curvature of the cavity mirrors to achieve a stable mode should be considered as conventional modifications to the specific implementation under the core technical concept of this invention, rather than alternative solutions.

[0061] Methods for achieving low-gain subcavities are not limited to low-gain materials, but should also include, but are not limited to:

[0062] Reduce doping concentration: Use the same material as the main cavity (e.g., Nd:YAG), but with a doping concentration much lower than that of the main cavity medium.

[0063] Shorten the gain length: Use a shorter gain medium to reduce the overall gain.

[0064] Introducing controllable losses: Actively introducing an adjustable attenuator (such as an acousto-optic modulator) into the sub-cavity to consume the gain, so that its net gain is lower than the threshold.

[0065] These should all be regarded as routine changes to specific implementations under the core technical ideas of this invention, rather than alternatives.

[0066] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention are within the scope of the claims of the present invention.

Claims

1. A pulse-width-continuously-tunable dual-gain tandem dual-cavity solid-state laser, characterized in that, The laser includes: The main resonant cavity (1) is provided with a total reflection mirror (101), a first gain medium (102), a Q-switching switch (103) and a main output mirror (104) in sequence along the optical path. A sub-resonant cavity (2) is disposed on the output optical path of the main resonant cavity (1). A second gain medium (202), a sub-output mirror (201), and a displacement driving device (203) are sequentially disposed along the optical path. The sub-output mirror (201) is mounted on the displacement driving device (203). The displacement driving device (203) is used to adjust the distance between the sub-output mirror (201) and the main output mirror (104) to change the optical cavity length of the sub-resonant cavity (2). The gain of the main resonant cavity (1) is greater than the gain of the sub-resonant cavity (2); and the net gain of the sub-resonant cavity (2) is lower than the self-excited oscillation threshold.

2. The pulse width-adjustable dual-gain tandem dual-cavity solid-state laser according to claim 1, characterized in that, The ratio of the stimulated emission cross section of the first gain medium (102) to the stimulated emission cross section of the second gain medium (202) is greater than 2.

3. The pulse width-continuously adjustable dual-gain tandem dual-cavity solid-state laser according to claim 1, characterized in that, The optical cavity length of the sub-resonant cavity (2) is greater than the optical cavity length of the main resonant cavity (1).

4. A pulse-width-continuously-tunable dual-gain tandem dual-cavity solid-state laser according to claim 3, characterized in that, The ratio of the optical cavity length of the sub-resonant cavity (2) to the optical cavity length of the main resonant cavity (1) is not less than 1.5:

1.

5. A pulse-width-continuously-tunable dual-gain tandem dual-cavity solid-state laser according to claim 1, characterized in that, The displacement driving device (203) is any one of a piezoelectric ceramic driver, a voice coil motor, or a stepper motor driving rail.

6. A pulse-width-continuously-tunable dual-gain tandem dual-cavity solid-state laser according to claim 1, characterized in that, The sub-resonator (2) is also provided with a beam quality control element (204) for filtering out higher-order transverse modes and optimizing the output beam quality.

7. A pulse-width-continuously-tunable dual-gain tandem dual-cavity solid-state laser according to claim 6, characterized in that, The beam quality control element (204) is a soft-edge aperture.

8. A pulse-width-continuously-tunable dual-gain tandem dual-cavity solid-state laser according to claim 1, characterized in that, The sub-resonant cavity (2) is also provided with a frequency doubling crystal (205) and a beam splitter (206) for realizing intracavity frequency doubling.

9. A method for operating a pulse-width-continuously-tunable dual-gain tandem dual-cavity solid-state laser based on any one of claims 1 to 8, characterized in that, The method includes: S1: Generate an initial laser pulse in the main resonant cavity (1); S2: Based on the transmittance of the main output mirror (104), the initial laser pulse portion is coupled into the sub-resonant cavity (2) through the main output mirror (104); S3: The light pulse coupled into the sub-resonant cavity (2) undergoes multiple round-trip reflections between the main output mirror (104) and the sub-output mirror (201) within the sub-resonant cavity (2), with each reflection receiving a fixed time delay and gain compensation provided by the second gain medium (202); S4: The pulse portions with different time delays generated by multiple round trip reflections are superimposed in the time domain, and a pulse with a broadened envelope is output from the sub-output mirror (201); The output pulse width can be continuously adjusted by adjusting the optical cavity length of the sub-resonant cavity (2).

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