WO2008017214A1

WO2008017214A1 - A method for generating a fourth harmonic solid laser

Info

Publication number: WO2008017214A1
Application number: PCT/CN2006/001978
Authority: WO
Inventors: Shuzhen Ma; Liying Chen; Yunfeng Gao
Original assignee: Shenzhen Han's Laser Technology Co., Limited
Priority date: 2006-08-04
Filing date: 2006-08-04
Publication date: 2008-02-14

Abstract

A method for generating a fourth harmonic solid laser, generating a second harmonic by interacting a high power fundamental wavelength with a second harmonic nonlinear optical crystal (9), generating a fourth harmonic by entering the second harmonic into a fourth harmonic nonlinear optical crystal (12). A method is used that the second harmonic is reflected time after time in a cavity or out a cavity so as to generate an accumulative fourth harmonic. Therefore, the method can attain high efficiency and the stabilizing output frequency. The nonlinear optical crystal has long life.

Description

Fourth harmonic solid laser generation method

The invention relates to a fourth harmonic solid laser generating method. Background technique

In recent years, the international laser precision processing has developed extremely rapidly. Its market output exceeds laser marking, which has become the main laser industry after laser cutting and welding. The short-wavelength high-power ultraviolet laser has high resolution and high absorption. Features are an important development direction of laser fine processing.

Coherent Corporation of the United States has developed a laser average power of 1- 3wM: YAG Q-quad four-frequency laser, mainly used for laser marking and TFT cutting; developed a laser power of 200 to earn continuous quadruple-frequency laser, mainly used in semiconductor silicon wafers Quality inspection and ^: lithography.

Spectra-Physics Inc. of the United States has developed a laser average power of l-2w Q-switched Nd: Υ Ο lw continuous quadratic laser, which can be used for wafer cutting, gem substrate dicing, drilling microvia, FBG manufacturing and DVD disc engraving. system.

In order to generate high-power fourth-order harmonic lasers, the harmonics are usually focused outside the laser cavity to a quadruple-frequency nonlinear crystal, and sufficient secondary harmonic power density is obtained to improve the frequency multiplication efficiency, such as the US patent of Sony Corporation. U.S. Patent No. 6,249,371 and Acuhiglir, U.S. Patent No. 6,716,620 B2, which is simple and stable, is suitable for generating medium and small power, but is liable to cause damage to nonlinear media by high power lasers. In addition, the second harmonic that is not converted to the fourth harmonic will be wasted through the nonlinear crystal. Therefore, the conversion efficiency of the fourth harmonic laser is also limited. In order to overcome the problems of frequency-doubled crystal destruction and low efficiency, Spectra-Physics and Coherent use a moving nonlinear crystal method to scan the incident second harmonic light in a two-dimensional point-by-point manner on the fourth harmonic crystal. High power, high efficiency fourth harmonic laser output is available. However, the point-by-point scanning has a problem of nonlinear crystal temperature matching, and the crystal shift instantaneously causes a laser abrupt change to cause the fourth-order harmonic laser power to form a transient power drop sharply.

Another laser Q-switched fourth harmonic generation method is a harmonic wave series method in a fundamental wave cavity. U.S. Patent No. 6,697,391 B2 to Lightwave Electronics and U.S. Patent No. 6,229,829 to Photonics Indus tr. International Corporation. This method utilizes the high power density of the fundamental wave in the cavity to improve the conversion efficiency of the harmonics, in order to avoid the ultraviolet light to the cavity.

1

Confirmation Damage to the internal components, using a prism or a non-linear crystal end face to extract the fourth harmonic laser out of the cavity. The method is based on one-way harmonic action, and there is a second harmonic laser that is not converted to fourth harmonic. It is still wasted, limiting the fourth harmonic output power. In order to increase efficiency and achieve high power output,

Intracavity sub-cavity and multiple reflection methods are proposed by U.S. Patent No. 5,278,852, issued to Kigre, and U.S. Patent No. 5,943,351, issued to U.S. Pat. Using the intracavity second harmonic frequency doubling light to multiply multiple times on the fourth harmonic crystal to produce high efficiency, high power fourth harmonic laser output. The problem of this method is that the ultraviolet laser output is double beam and ultraviolet. Damage to the components within the cavity by light limits both output power and lifetime.

The above-mentioned commonly used fourth harmonic crystal is BB0, and the second harmonic laser is multiplied by BB0 to obtain the ultraviolet band. US Patent No. 6,249,371 of the sony company uses a fundamental wave and a third harmonic to obtain a fourth harmonic laser output by mixing on the LB0 crystal. In addition, the new nonlinear crystal CLB0 has high harmonic conversion efficiency and excellent deep ultraviolet transmission. Optical performance, Japan Research Deve lopment used CLB0 to generate fourth harmonic and fifth harmonic deep ultraviolet laser. However, CLB0 is prone to deliquescence and the material is soft and brittle, and needs to be gradually improved to be used in industrial products.

Ultraviolet continuous laser generation uses a second harmonic single-frequency laser input to the external cavity resonator, and uses a strong focusing method to increase the spatial power density to produce an ultraviolet fourth harmonic laser. Coherent used X-type and Cfra-Phys i cs to obtain Q. 2-2 W fourth-order harmonic laser output using a Δ-type cavity. However, this external resonant cavity is strictly required by the environment. It must be equipped with an automatic optical path compensation follow-up device and an optical mechanical automatic tracking device. It requires high mechanical and temperature stability and is usually used for scientific research of experiments. In industrial production.

The domestic fourth-order harmonic ultraviolet laser is still in the basic research stage. The joint research of Nanjing University and Shandong Normal University has obtained 63 bribe/266 ship laser output by the out-of-cavity focusing frequency multiplication method (《Journal of Physics》, He Jingliang, et al., Volume 49 , No. 10, 2000, pp. 2106-2108). A continuous 266 mn laser signal was detected by the external cavity resonance method used by Xi'an Opto-mechanics (Journal of Photonics, Chen Guofu et al., Vol. 28, No. 8, 1999, pp. 684-687). Tsinghua University and Peking University jointly studied the use of CLB0 nonlinear crystal frequency doubling to get 78 丽/266 let laser (Journal of Synthetic Crystals, Sun Tongqing et al., Vol. 33, No. 2, 2004, pp. 133-135) ).

Summary of the invention

The object of the present invention is to overcome the shortcomings of the prior art and provide a fourth harmonic which can obtain a stable fourth harmonic solid laser output and has high light conversion efficiency and long service life of a nonlinear crystal. Solid laser generation method.

The technical solution adopted by the present invention to solve the technical problem is: The fourth harmonic solid laser generating method applies a high power density fundamental wave to a second frequency nonlinear crystal to generate a second harmonic, and a second harmonic injection On a quadruple-frequency nonlinear crystal, a multi-integrated fourth-order harmonic solid-state laser output is generated by intracavity-cavity multiple reflection and non-focusing beams, and the fundamental and second harmonic polarized light are folded at a small angle to form an optical path. Low-loss polarization coupling, the infrared fundamental resonator is designed to balance the high power density stable cavity structure with the laser medium thermal lens, and fold the optical path with multiple small angles of total internal reflection.

The innovation of the present invention over the prior art is: In the fourth harmonic solid laser generation method of the present invention, the fundamental wave and the second harmonic polarized light are folded at a small angle to form a low loss polarization coupling, and the infrared fundamental wave resonator It is designed to balance the high power density stable cavity structure with the laser medium thermal lens, and fold the optical path with multiple small angles of internal total reflection to form a high linear polarization laser oscillation. Under these two conditions, a high power density fundamental wave is generated.

A high power density fundamental wave semiconductor pumped solid-state laser cavity and efficient harmonic conversion efficiency are designed using matrix optics and nonlinear programming. The high-power density fundamental wave is applied to the second-frequency nonlinear crystal to generate the second harmonic, and the second harmonic is injected into the quadruple-frequency nonlinear crystal, and the second harmonic cavity-in-cavity multiple reflection method is adopted. The second harmonic forms a closed circuit multiple total reflection, and generates a quadratic harmonic solid laser output that is accumulated multiple times. The second harmonic that is not converted into the fourth harmonic is multiplied by the nonlinear crystal multiple times, and fully utilized. The second harmonic power is used to achieve high conversion efficiency; the non-focusing beam method is adopted to avoid the second harmonic of high power density incident on the quadruple frequency nonlinear crystal to destroy the surface film layer and prolong the service life of the nonlinear crystal. In addition, the fourth harmonic solid laser generating method of the present invention does not need to move the nonlinear crystal, and avoids the sudden change of the laser power caused by the temperature mismatch caused by the point-by-point scanning, thereby ensuring the stability of the output laser. DRAWINGS

The invention will now be further described with reference to the drawings and embodiments.

Figure la is a schematic diagram of the principle of the fourth harmonic solid laser generation method of the present invention.

Figure lb is a schematic diagram of the principle of the fourth harmonic solid laser generating method of the present invention.

2a is a simulation result of stability of a laser cavity in a preferred mode of the fourth-order harmonic solid-state laser generating method of the present invention, wherein the abscissa glaze unit is: mm; the thermal lens focal length: f _th =300 ram; cavity stability range : f _th =214-1000瞧.

2b is a preferred mode of the fourth harmonic solid laser generating method of the present invention, in a laser cavity Gaussian beam oscillation trajectory simulation calculation results, where the abscissa axis unit is: leg; the ordinate axis unit is: μιη; light waist: ωοι = ω ₀₂ = 0. 32 mm; light divergence: Q _} = θ ₂ = 2.13 mrad; activation area: d _A = 0. 94 mm.

Fig. 3 is a graph showing the results of calculation of the efficiency and power accumulation of the fourth harmonic solid laser generated in the preferred mode of the fourth harmonic solid laser generating method of the present invention.

4 is a power density calculation result of the second harmonic and the fourth harmonic operation in the cavity in the preferred mode of the fourth harmonic solid laser generating method of the present invention.

Fig. 5 is a graph showing the calculation results of the variation curves of the fourth harmonic solid laser output power and the double frequency to quadruple frequency conversion efficiency corresponding to different repetition frequencies in the preferred mode of the fourth harmonic solid laser generation method of the present invention.

Fig. 6 is a graph showing the experimental results of the fourth-order harmonic solid laser power and the pulse width variation curve corresponding to different input currents in the fourth harmonic solid-state laser generating method of the present invention (F = l 0KHZ ).

Fig. 7 is a pulse waveform diagram of the fourth harmonic solid laser generated in the actual inspection of the fourth harmonic solid laser generating method of the present invention (F = 10 KHZ, I = 60 A).

8 is an experimental result data (Ι=55Α) of the power of the fourth harmonic laser generated in the experiment of the fourth harmonic solid laser generating method of the present invention, wherein the abscissa axis is time, and the unit is: hour. . detailed description

The principle of the fourth harmonic solid laser generating method of the present invention is shown in FIG. 1a, and a fundamental wave oscillator is used, which includes a high power laser diode array side pumping module 7, a Q switch 3, a horizontal linear polarizing plate 6, and a limit. Die hole 2, end total mirror (1, 10, 13), nonlinear crystal (9, 12) and angle mirror (4, 5, 8, 11). The angle mirrors (4, 5, 8, and 11) are small angle mirrors in the cavity. By calculating and measuring the thermal lens effect of the pump module 7 at different pump powers, and calculating the spatial distribution of Gaussian mode transfer in the cavity by optical matrix method, the cavity length and the curvature of the end face mirror are designed to make the infrared laser cavity hot. The lens still maintains stable oscillation under a wide range of changes, and the calculation results are shown in Figures 2a and 2b. Under high power continuous LD pumping, the equivalent thermal lens focal length of pump module 7 is Fth = 300fflm (I = 50A), and the laser cavity maintains stable oscillation to allow the thermal lens to vary from Fth = 214 - 1000 faces. The mode limiting aperture 2 is placed near the optical waist of the end total reflection mirror 1 to control the fundamental laser to oscillate in a single mode or a low order mode. At the other optical waist close to the end total reflection mirror 10, a double frequency nonlinear crystal is placed. Because the infrared laser cavity is closed or internally totally reflected, and is balanced with the thermal lens effect of high power pumping, This achieves a high intracavity power density.

The element generating the second harmonic includes a nonlinear crystal 9, an end total reflection mirror 10, an end total reflection mirror 13, an angle mirror 8 and an angle mirror 11, wherein the nonlinear crystal 9 is a double frequency crystal. The Brewster mirror is set in an orientation suitable for horizontally polarized transmission and vertically polarized high reflection angle. Angle mirror 8 plated fundamental wave P-direction high transmission and second harmonic S-direction high-reflection two-color film, angle mirror 11 plated second harmonic S-direction high reflection, fourth harmonic P-direction high transmission two-color film. The second harmonic constitutes closed low-loss polarization coupled total reflection, and the formation process is as follows: horizontally polarized fundamental wave light from the direction of the end total reflection mirror 10, and vertically polarized second harmonic light is generated in the double frequency crystal 9 The harmonic light is applied to the quadruple frequency crystal 12 via the angled mirror 8 and 11 polarization-coupling total reflection, and is then totally reflected from the end total reflection mirror 13 to return from the original optical path.

The components for generating the fourth harmonic include a nonlinear crystal 12, a polarization coupling angle mirror 11 and an end total reflection mirror 13, which are single-ended open reflection cavity structures with low loss and single beam output characteristics, wherein the nonlinear crystal 12 is used as Quadruple frequency crystal. The fourth harmonic generation process is as follows: the vertically polarized second harmonic light from the angle mirror 8 is reflected by the angle mirror 11 and then incident on the quadruple frequency crystal 12 to generate a horizontally polarized fourth harmonic laser, the fourth The double frequency light is reflected back from the original path by the end total reflection mirror 13 and output through the angle mirror 11. The second harmonic which is not converted is again composed of the end total reflection mirror 10, the angle mirror 8, the angle mirror 11, and the end total reflection mirror 13 again. After the reflection, the quadruple frequency crystal 12 is applied, and the regenerated fourth harmonic light is coupled and output through the angle mirror 11, so that multiple round trips are multiplied in the quadruple frequency crystal 12, thereby generating a highly efficient fourth harmonic laser. Output.

The present invention can also employ a vertical linear polarizing plate 6, in which the process of generating the second harmonic and the fourth harmonic is as shown in Fig. lb:

The element generating the second harmonic includes a nonlinear crystal 9, an end total reflection mirror 10, an end total reflection mirror 13, an angle mirror 8 and an angle mirror 11, wherein the nonlinear crystal 9 is a double frequency crystal. Angle mirror 11 plated second harmonic P-direction high transmission, fourth harmonic S-direction high reflection two-color film. The second harmonic constitutes closed low-loss polarization coupled total reflection, and the formation process is as follows: vertical polarization fundamental wave light from the direction of the end total reflection mirror 10, horizontally polarized second harmonic light is generated in the double frequency crystal 9 The harmonic light is totally reflected by the angle mirror 8, and the angle mirror 11 is totally transmitted to the quadruple frequency crystal 12, and then totally reflected from the end total reflection mirror 13 to return from the original optical path.

The components for generating the fourth harmonic include a nonlinear crystal 12, a polarization combining angle mirror 11 and an end total reflection mirror 13, which are single-ended open-type reflective cavity structures with low loss and single beam output characteristics, wherein the nonlinear crystal 12 As a quadruple frequency crystal. The fourth harmonic generation process is as follows: the horizontally polarized second harmonic light from the angle mirror 8 is transmitted through the angle mirror 11 and then incident on the quadruple frequency crystal 12, causing the vertical a linearly polarized fourth harmonic laser that is reflected back from the original path by the end total reflection mirror 13 and reflected by the angle mirror 11 to be outputted. The second harmonic that is not converted is provided by the end total reflection mirror 10 and the angle mirror. 8. The angle mirror 11 and the end total reflection mirror 13 are again reflected to act on the quadruple frequency crystal 12, and the regenerated fourth harmonic light is reflected and output through the angle mirror 11, so that multiple round trips are used in the quadruple frequency crystal 12 Therefore, a very efficient fourth harmonic laser output is produced.

In order to verify the fourth harmonic solid laser generation method of the present invention, a preferred mode was selected for calculation. The efficiency and power of the fourth harmonic laser are calculated by the NL0 program and multiple times of frequency-doubled light superposition method. The calculation results are shown in Fig. 3. Calculate the power density changes of the second harmonic and the fourth harmonic when successively and stepwisely generate the double frequency and quadruple frequency. The calculation result is shown in Fig. 4. The power component of the multiplier is a nonlinear decrementing curve, and the cumulative power of the successive multiplier is 2.98 times that of the single mixing power, which is 1.75 times of the two mixing powers. The conversion efficiency of the fundamental wave to the double frequency is 80%, the conversion efficiency of the second frequency to the quadruple frequency is 20%, the power output of the fourth harmonic is 2. 4W, and the beam quality is M ² 4. Fig. 5 is a graph showing the calculation results of the variation curves of the fourth harmonic laser output power and the double frequency to quadruple frequency conversion efficiency corresponding to different repetition frequencies.

According to the fourth harmonic solid laser generating method of the present invention, a high power diode laser array side pumped laser cavity four-frequency frequency experimental device is used for the test. Fig. 6 is the experimental result of the fourth harmonic laser power and pulse width variation curve corresponding to different input currents (F=10KHZ), and Fig. 7 is the pulse waveform diagram of the fourth harmonic laser (F=10KHZ, Ι=60Α) Figure 8 shows the experimental results of the fourth-order harmonic laser power long-term stability (I = 55A). The maximum fourth harmonic output power P = 2. 4W, pulse width T = 76ns. The fundamental to second harmonic conversion efficiency η = 80%. When the laser operation repetition frequency f=7-8 ,, the harmonic power conversion is most effective, and the fourth harmonic efficiency is 20%. The fourth-order harmonic solid-state laser output has a diameter d = 1, 5 faces, a beam divergence Θ =1. 6 mrad, and a beam quality M ² 4 . The optical pulse transient peak fluctuation Δ T = 8% PP, long-term operating power fluctuation Δ Τ = 6% / 40 hours. It can be seen that the experimental results are basically consistent with the theoretical calculations.

In the fourth harmonic solid laser generating method of the present invention, the fundamental wave and the second harmonic polarized light are folded at a small angle to form a low loss polarization coupling, and the infrared fundamental wave resonator is designed to balance the high power density stable cavity with the laser medium thermal lens. The structure, and the use of multiple small angles of total internal reflection to fold the optical path, forms a high linear polarization laser oscillation, under these two conditions, produces a high power density fundamental wave.

The semiconductor pumped solid-state laser cavity with high power density fundamental wave and high efficiency harmonic conversion efficiency are designed by matrix optics and nonlinear programming. The high-power density fundamental wave is applied to the second-frequency nonlinear crystal to generate the second harmonic, and the second harmonic is injected into the quadruple-frequency nonlinear crystal, and the second harmonic cavity-in-cavity multiple reflection method is adopted. Making the second harmonic constitute a closed circuit multiple times total reflection, resulting in multiple accumulation of four Subharmonic solid-state laser output, the second harmonic that is not converted to the fourth harmonic is multiplied by the nonlinear crystal multiple times, making full use of the second harmonic power to achieve high conversion efficiency; using the unfocused beam method Avoid the second harmonic of high power density incident on the quadruple-frequency nonlinear crystal to destroy the surface film layer and prolong the service life of the nonlinear crystal. In addition, the fourth harmonic solid laser generating method of the invention does not need to move the nonlinear crystal, and avoids the sudden change of the laser power caused by the temperature mismatch caused by the point-by-point scanning, thereby ensuring the stability of the output laser.

According to the principle of the fourth harmonic solid laser generating method of the present invention, it is not difficult to infer that the double frequency nonlinear crystal used may be a class I LBO, a class I BB0 or a class I CLB0 nonlinear crystal; The nonlinear crystal may be a class I LBO, a class I BB0 or a class I CLB0 nonlinear crystal; the fundamental solid laser medium used may be: M: YAG, Nd: YV04, Nd: YLF, Nd: Glass, Yb: YAG Or Er: YAG; The Q switch used can be an acousto-optic switch, an electro-optic switch or a saturation-activated passive Q-switch. In addition, the pumping source used may be a side pump of a high power semiconductor laser diode, a longitudinal pumping of the diode end face, or a side pump of a xenon lamp or a xenon lamp. A small angle folding cavity structure, a 45° angle folded cavity structure or a Brewster angle folded cavity structure can be used.

Claims

Rights request

A fourth-order harmonic solid-state laser generating method, comprising a pumping module, a Q-switch, a plurality of angle mirrors and an end mirror, wherein: the pumping module pump generates a fundamental light, and the Q-switch and the angle mirror The high power density fundamental wave light generated by the end mirror acts on the double frequency nonlinear crystal to produce a second harmonic laser, the second harmonic laser is reflected by the end mirror and the angle mirror is reflected or transmitted. On the quadruple-frequency nonlinear crystal, the intra-cavity-external multiple reflection method is used to generate multiple cumulative fourth-order harmonic solid-state laser output, and the fundamental wave and the second harmonic polarized light are folded at a small angle to form a low-loss Polarized hybrid, the infrared fundamental resonator is designed to balance the high power density stable cavity structure with the laser medium thermal lens, and fold the optical path with multiple small angles of total internal reflection.

2. The fourth harmonic solid laser generating method according to claim 1, wherein: the double frequency nonlinear crystal used may be a class I LB0, a class I BB0 or a class I CLB0 nonlinear crystal.

3. The fourth harmonic solid-state laser generating method according to claim 1, wherein: the quadruple-frequency nonlinear crystal used may be a class I LB0, a class I BB0 or a class I CLB0 nonlinear crystal.

4. The fourth harmonic solid laser generating method according to claim 1, wherein: the fundamental solid laser medium used can be: Nd: YAG, Nd: YV04, Nd: YLF, Nd: Glass, Yb : YAG or Er: YAG.

The fourth harmonic solid laser generating method according to claim 1, wherein the Q switch can be an acousto-optic switch, an electro-optical switch or a saturation-excited passive Q-switch.

6. The fourth harmonic solid laser generating method according to claim 1, wherein: the pump module used is a side pump of a high power semiconductor laser diode, a longitudinal pumping of the diode end face, or a xenon lamp or a xenon lamp. Side pumping.

7. The fourth harmonic solid-state laser generating method according to claim 1, wherein: the laser cavity is a small-angle folded cavity structure, a 45° angle folded cavity structure or a Brewster angle folded cavity structure. '

8. The fourth harmonic solid-state laser generating method according to claim 1, further comprising: a mode limiting aperture disposed at a waist close to the end mirror.

9. The method of producing a fourth harmonic solid laser according to claim 1, wherein: said double frequency nonlinear crystal is placed near the waist of the end mirror.

10. The fourth harmonic solid laser generating method according to claim 1, further comprising: a vertical linear polarizing film disposed on the optical path of the laser cavity.