WO2008055390A1

WO2008055390A1 - Third harmonic ultraviolet laser of semiconductor double end face pumping

Info

Publication number: WO2008055390A1
Application number: PCT/CN2006/003334
Authority: WO
Inventors: Yunfeng Gao; Fengping Lv; Junhui Zheng; Yao Wu; Shuzhen Ma
Original assignee: Shenzhen Han's Laser Technology Co., Limited
Priority date: 2006-11-09
Filing date: 2006-12-08
Publication date: 2008-05-15
Also published as: CN101179176A

Abstract

A third harmonic ultraviolet laser of semiconductor double end face pumping comprises a semiconductor pumping module (1), an optical coupling system (3), fundamental gain-medium crystals (5,6), a second harmonic nonlinear crystal (10), a third harmonic nonlinear crystal (13), a wave plate (11), a modulation device (7), laser resonant cavity mirrors (4,9,14), and an ultraviolet laser reflecting mirror (12). The pump-light emitted by the semiconductor pumping module is transmitted into the optical coupling system, then collimated and focused by the optical coupling system, and incidents directly on the end faces of the fundamental gain-medium crystals. The light emitted by the excited fundamental gain-medium crystals forms fundamental frequency light beam via mode selection of the laser resonant cavity mirrors, and the modulated laser beam is obtained by the modulation function of the modulation device. The modulated fundamental frequency laser beam is converted to green laser beam via the second harmonic nonlinear crystal and is polarized by the wave plate. The fundamental frequency laser beam and green laser beam which are the same polarization are incident into the third harmonic nonlinear crystal, thereby performing sum-frequency and generating third harmonic ultraviolet laserbeam.

Description

Semiconductor double-end pumped third harmonic ultraviolet laser

The invention belongs to the field of laser technology and relates to a semiconductor laser diode double-pumped three-harmonic ultraviolet laser. Background technique

The third harmonic ultraviolet laser has always been a technical difficulty in the field of solid-state lasers, but because it has a wide range of applications in many practical applications, the 'third harmonic ultraviolet laser has become a hot spot of research. The process of processing materials using a third-harmonic ultraviolet laser is called the “photo-etching” effect. High-energy photons directly destroy the chemical bond of the material into a “cold” process, and the heat-affected area is minimal. In contrast, visible and infrared lasers use heat that is focused to the processing site to melt the material, which conducts heat to the surrounding material, creating a harmful heat-affected zone. At the same time, because the third harmonic UV laser is focused, focusing The dots can be as small as submicron, which makes the metal and polymer micro-processing more advantageous. It can process small parts and achieve higher energy density even at low pulse energy levels. Material processing. Therefore, the third harmonic UV laser has good "cold processing" and "focusing" properties, which combine to make it possible to process extremely small parts. Moreover, most materials can effectively absorb ultraviolet lasers, thus The third harmonic UV laser has greater flexibility and a wider range of applications and can be used to process materials that cannot be processed by infrared and visible lasers.

Compared with the deep ultraviolet laser with a wavelength of 266 nm, the third harmonic ultraviolet laser has more mature technology, and its performance is more stable. It can output higher laser power and laser peak power, which can be very good on various processed materials. The processing effect; its characteristic size is also very small, any metal with a thickness less than 125 microns can be used for rapid cutting, and the cutting is small, which has a good application prospect.

In the method of generating the third harmonic ultraviolet laser, the method of laser cavity mixing outside the cavity is often used. This method is difficult to obtain a high-power, high-efficiency triple harmonic output. In order to improve the conversion efficiency of the extra-harmonic harmonics, lens focusing is usually used to enhance the power density of the fundamental wave, but the focused beam is easily nonlinear. The destruction of the crystal film layer and the crystal itself affects the output of the total harmonic power; meanwhile, the out-of-cavity mixing is a one-way behavior, and the fundamental wave and the second harmonic pass through the triple frequency crystal at one time, and the conversion efficiency is low.

The three-harmonic ultraviolet lasers that are commonly used today can be classified into gas, lamp-pumped, side-pumped, and end-pumped. Gas UV lasers are bulky, inefficient, and equipment is too complex to maintain; lamp-pumped UV lasers are less efficient and less reliable; side-pumped and end-pumped Ultraviolet lasers are similar in size and operation, but in contrast, the beam quality and conversion efficiency of end-pumped UV lasers are unmatched by side-pumped UV lasers.

In addition, the traditional semiconductor-pumped three-harmonic ultraviolet laser has certain defects in technical design, which makes the stability and beam quality of the UV laser a difficult problem to be solved. For example, in the cavity design of the laser, in order to obtain better optical quality, it is usually necessary to use a convex mirror as a cavity mirror of the laser cavity, as shown in Chinese Patent Application No. 200410073574. 8, but the laser formed by the convex mirror The resonant cavity is sensitive, and the precision of the mechanical design is difficult to achieve. There is a possibility that the laser power or the quality of the laser beam is degraded due to vibration or deformation during the handling process, so it is not suitable for productization.

At the same time, in the three-harmonic ultraviolet laser with double-end pumping, the pumping method for generating the fundamental light is mostly the structure of the double-pumped monolithic gain medium crystal, as shown in US Pat. No. 6,587,487. However, the thermal stress generated by the thermal effect in the crystal of the gain medium cannot exceed the limitation of the fracture stress of the crystal. The maximum pump power per unit area of the gain medium crystal and the output power are limited ("Power scaling of diode-pumped Nd:" YV04 Lasers, IEEE J Quantum Electronics, 2002, 38 (9): 129Γ1299), so it is difficult to obtain high UV laser output. Summary of the invention

The technical problem to be solved by the present invention is to provide a semiconductor double-end pumped third harmonic ultraviolet laser which utilizes the strong fundamental wave light in the cavity to obtain high efficiency and high beam quality.

The technical solution adopted by the invention is: a semiconductor double-end pumped third harmonic ultraviolet laser, comprising a semiconductor pump module, an optical coupling system, a fundamental gain medium crystal, a second harmonic nonlinear crystal, and a third harmonic non- The linear crystal, the wave plate, the modulation device, the laser cavity mirror, the ultraviolet laser mirror, and the pump light output from the semiconductor pump module are transmitted to the optical coupling system, and are collimated by the optical coupling system and coupled to the fundamental gain medium crystal. The end face, the "C" axis direction of the fundamental gain medium crystal is placed vertically upward; the fundamental gain medium crystal absorbs the pump light energy to generate stimulated emission, and the emitted light is selected by the laser cavity mirror in the laser cavity. The fundamental frequency beam that forms a high beam quality is modulated by the modulation device to obtain a modulated laser with high peak power. The modulated fundamental laser passes through the second harmonic nonlinear crystal to obtain the green laser output and is polarized by the wave plate. Rotation; the fundamental polarization laser and the green laser of the same polarization state are injected into the third harmonic nonlinear crystal for mixing, and three are obtained. Harmonic ultraviolet laser output; the remaining green laser passes through the third harmonic nonlinear crystal again under the reflection of the resonant cavity mirror; the third harmonic ultraviolet laser that is converted into the first time is reflected by the resonant cavity mirror and the second conversion The third harmonic ultraviolet laser is outputted together with the ultraviolet laser mirror to output the laser cavity outside the cavity, and the third harmonic violet is obtained. External laser output.

Two fundamental gain pump crystals are separately pumped by two semiconductor pump modules.

The center wavelength of the semiconductor pump module is 808 nm or 880 nm, and the semiconductor pump module of other wavelengths may be selected according to the selected fundamental frequency gain medium crystal.

The fundamental gain medium crystal is Nd : YV04, or is Nd : YLF, Nd : YAG, Nd : Glass, Yb : YAG, Er : YAG crystal.

The "C" axis direction of the fundamental gain medium crystal used is placed vertically upwards, or it can be rotated 90° in the "C" axis direction, that is, horizontally.

The second harmonic nonlinear crystal is a class I LB0, and may also be a class I BB0, a class I CLB0 or a class I nonlinear crystal.

The third harmonic nonlinear crystal is a class I LB0, and may also be a class I BB0, a class I CLB0 or a class I nonlinear crystal.

The end face of the fundamental gain medium crystal is plated with an anti-reflection film for pump light.

The modulation device is an acousto-optic modulation device, and may also be an electro-optic modulation device or an absorption passive Q-switch.

The laser cavity mirrors are all plane mirrors, so that the laser cavity forms a flat cavity, and the laser cavity can also adopt a cavity structure composed of a double cavity, a flat cavity or other lenses.

The laser cavity structure is an "L" cavity structure, and a "V" angle folding cavity structure may also be used.

The laser further includes an optical fiber that transmits the pump light output from the semiconductor laser diode to the optical coupling system, and the pump light output from the semiconductor laser diode can also be directly transmitted to the optical coupling system without passing through the optical fiber.

The laser further includes a prism positioned on the optical path to separate the third harmonic ultraviolet laser from the other light to obtain a third harmonic ultraviolet laser output.

The beneficial effects achieved by the invention are as follows: Two laser diodes are respectively used to pump two fundamental gain medium crystals, which reduces the pump power of each crystal and avoids the high power density on a single crystal. The problem of damage; At the same time, within the threshold of the fundamental wave gain medium crystal damage threshold, the pump power of the semiconductor pump module can be appropriately increased to obtain a higher ultraviolet laser output; the bases under different pump powers are calculated and measured. The thermal lens effect of the wave gain medium crystal, the spatial distribution of the Gaussian mode transfer in the cavity is calculated by the optical matrix method, and the laser cavity mirror is designed to ensure that the fundamental frequency laser can maintain stable oscillation under the large range of thermal lens. The intracavity frequency doubling and mixing method are utilized to make full use of the characteristics of the strong fundamental wave in the cavity, thereby obtaining the ultraviolet laser output with high conversion efficiency; The length of the crystal makes the unconverted fundamental frequency optical power and the converted green laser power maintain a reasonable ratio, so that the laser in the cavity is fully converted into an ultraviolet laser, which improves the conversion efficiency of the ultraviolet laser; In the third harmonic nonlinear crystal, the conversion efficiency of the third harmonic is further improved; under the condition of ensuring high efficiency and high beam quality of the ultraviolet laser, the cavity mirror constituting the laser cavity is designed as a plane mirror, avoiding the use of the convex mirror The instability factor is brought about, and the mechanical design difficulty is reduced. DRAWINGS

The present invention will be further described below in conjunction with the embodiments with reference to the accompanying drawings.

1 is a schematic view showing the structure of a semiconductor double-end pumped third harmonic ultraviolet laser according to the present invention.

Fig. 2 is a theoretical calculation result of the change of the thermal lens when the fundamental wave gain medium crystal of the present invention absorbs different pump powers.

Fig. 3 is a graph showing the variation of the ultraviolet laser power with current of a semiconductor double-end pumped third harmonic ultraviolet laser of the present invention.

4 is a single laser pulse waveform measured when the semiconductor double-end pumped third harmonic ultraviolet laser has a power of 6 W and a Q-switched frequency of 25 KHz.

Fig. 5 is a view showing a plurality of laser pulse waveforms measured by a semiconductor double-end pumped third harmonic ultraviolet laser having a power of 6 W and a Q-switched frequency of 25 kHz.

Fig. 6 is a graph showing the variation of the laser power over time at 5 W of the semiconductor double-end pumped third harmonic ultraviolet laser of the invention. detailed description

As shown in FIG. 1, the semiconductor laser diode double-end pumped third harmonic ultraviolet laser of the present invention comprises: a semiconductor pumping module 1 (2 in total), an optical fiber 2 (2 in total), and an optical coupling system 3 (2 sets in total) ), laser cavity 16 (composed of mirrors 4, 9 and 14), fundamental gain dielectric crystals 5 and 6 (2 in total), modulation device 7, aperture 8, second harmonic nonlinear crystal 10, wave Sheet 11 (WP: λ @ 1064 nm & A /2 i532 nm), ultraviolet laser mirror 12, third harmonic nonlinear crystal 13, and triangular prism 15.

The selected semiconductor pump module 1 is a semiconductor laser diode with a center wavelength of 808 ran or 880 nm, and may be other center wavelength semiconductor pump laser diodes selected according to the selected fundamental gain medium crystal.

The fundamental laser is a 1064 nm oscillator, and the pump light output from the semiconductor pump module 1 is transmitted from the optical fiber 2 to the optical coupling system 3, and is directly coupled to the fundamental gain medium crystals 5 and 6 by the optical affinity system 3. End face; the "C" axis of the fundamental gain medium crystals 5 and 6 are placed vertically to ensure their generation The polarization direction of the fundamental laser is vertical; the fundamental gain medium crystals 5 and 6 absorb the pump light energy to generate stimulated emission, and the emitted light in the laser cavity 16 is selected to form a high beam quality fundamental frequency. The beam, under the modulation of the modulation device 7, produces a modulated laser of high peak power. In order to ensure the stable operation of the fundamental laser in a wide range, the thermal lens effect of the fundamental gain medium crystals 5 and 6 under different pump powers is calculated and measured in detail, and the intracavity Gaussian mode transfer is calculated by the optical matrix method. The spatial distribution, the cavity mirror of the laser cavity of the laser is designed to ensure that the fundamental frequency laser can maintain stable oscillation under the wide range of thermal lens.

Generation of green laser: The modulated fundamental laser is subjected to frequency doubling of the second harmonic nonlinear crystal 10 to obtain a green laser output whose polarization direction is horizontal; the horizontally polarized green laser is rotated 90 by the wave plate 11 After °, it becomes vertically polarized light in the same direction as the polarization state of the fundamental light; Reasonably designing the length of the secondary nonlinear crystal 10, so that the unconverted fundamental frequency optical power and the converted green laser power are kept reasonable. The ratio is such that the laser in the cavity is fully converted into an ultraviolet laser, which improves the conversion efficiency of the ultraviolet laser.

Generation of ultraviolet laser: The fundamental frequency laser and the green laser which are vertically polarized are mixed by the third harmonic nonlinear crystal 13 to obtain a third harmonic ultraviolet laser; the remaining green laser is again reflected by the reflection of the plane mirror 14 Through the third harmonic nonlinear crystal 13, the green laser acts twice on the third harmonic nonlinear crystal 13 to further improve the conversion efficiency of the third harmonic; the third harmonic ultraviolet laser that is first converted into the planar cavity mirror 14 The reflection effect is outputted to the laser cavity 16 by the reflection of the ultraviolet laser mirror 12 together with the third harmonic ultraviolet laser converted into the second time. Under the separation of the prism 15, a third harmonic ultraviolet laser output is obtained.

Wherein, the fundamental wave gain dielectric crystals 5 and 6 are plated with an antireflection coating for pumping light and 1064 nm laser to increase the absorption of the pump light; the fundamental gain medium used is Nd: YV04, It is also possible to use a fundamental gain medium crystal such as Nd:YLF, Nd:YAG, Nd: Glass, Yb:YAG, Er:YAG; the both ends of the modulation device 7 are plated with a 1064 ηηι antireflection film, and the modulation device used is sound and light. The modulation device may also be an electro-optic modulation device and an absorbing passive Q-switch; the second harmonic nonlinear crystal 10 is a Class I LB0 crystal, and both ends are plated with a 532 nm and 1064 nm two-color anti-reflection film, or may be a Class I BB0, Class I CLB0 or other Class I nonlinear crystals, such as LiNb304 crystals; Third harmonic nonlinear crystals 13 are Class I LB0 crystals, both ends are plated with 355nm, 532nm and 1064nm three-color antireflection coatings, or Class I BB0 , Class I CLB0 or other Class I nonlinear crystals; Wave plates can also be removed, using nonlinear crystals such as LB0 of II.

The mirrors 4 and 9 constituting the calender resonator 16 are plated with a high-transparent film for pumping light, and a high-reflection film for 1064 rai laser; the mirror 14 is plated with a three-color high-reflection film for 1064 nm, 532 nm, and 355 nm; The mirrors 4, 9, and 14 are all plane mirrors, so the laser cavity is a flat cavity; it is also possible to use a cavity with a double cavity or a flat cavity. The laser cavity structure used in the present invention is a cavity structure, and is also suitable for a "V" cavity or other angle folding cavity structure.

As used fundamental wave gain medium crystal Nd: "C" axis of YV04 vertically upward, which also "C" axis of rotation may be placed horizontally i.e., 90 ^fl, and the nonlinear crystal and the second harmonic three The direction of the harmonic nonlinear crystal is rotated by 90 ^Q.

The fundamental gain medium crystal and the harmonic nonlinear crystal (including the second harmonic and the third harmonic nonlinear crystal) are wrapped in indium foil and placed in a water-cooled heat sink crystal holder.

Without changing the internal structure of the laser, the pump power of the laser diode can be increased within the threshold range of the fundamental gain medium crystal damage, and the power density of the fundamental frequency laser in the cavity can be further increased, thereby obtaining a higher power ultraviolet laser. Output.

The semiconductor double-end pumped third harmonic ultraviolet laser of the invention has the characteristics

The thermal lens effect of fundamental wave gain dielectric crystals 5 and 6 at different pump powers is calculated and measured. The spatial distribution of intracavity Gaussian mode transfer is calculated by optical matrix method. The laser cavity mirror is designed to ensure the basis. The frequency laser can maintain stable oscillation under the wide range of thermal lens;

2. Two laser diodes are used to pump two fundamental gain medium crystals 5 and 6, respectively, which reduces the pump power of each crystal and avoids the problem of being easily damaged due to excessive power density on a single crystal. ;

3. Without changing the internal structure of the laser, the pump power of the laser diode can be increased within the threshold of the fundamental wave gain medium crystal damage threshold, and the power density of the fundamental frequency laser in the cavity can be further increased, thereby obtaining higher power. Ultraviolet laser output;

4. Using intracavity frequency doubling and mixing, fully utilizing the characteristics of strong fundamental wave in the cavity to obtain high conversion efficiency of ultraviolet laser output;

5. Reasonably design the length of the secondary nonlinear crystal 10 to maintain a reasonable ratio of the unconverted fundamental frequency optical power and the converted green laser power, thereby fully converting the laser in the cavity into an ultraviolet laser, and improving the ultraviolet laser. Conversion efficiency

6. The green laser acts twice on the third harmonic nonlinear crystal 13, further improving the conversion efficiency of the third harmonic;

7. Under the condition of ensuring high efficiency and high beam quality of ultraviolet laser, the mirrors 4, 9 and 14 constituting the laser cavity 16 are designed as plane mirrors, which avoids the instability factor caused by the use of the convex mirror, and simultaneously reduces Small mechanical design difficulty.

The experimental results obtained according to the above technical solutions are as follows:

Double-end pumped third harmonic ultraviolet laser of semiconductor pump module established according to the above technical solution The triple-frequency ultraviolet laser power is greater than 6W, the conversion efficiency of the fundamental laser to the second harmonic laser is 87%, the conversion efficiency of the second harmonic laser to the third harmonic laser is 72%, and the minimum laser pulse width is less than 15ns. , Light velocity divergence angle <21111^(1, M2<2 without adding light, long-term stability less than 5%, compact structure, high conversion efficiency, good stability, etc. Figure 2 is the basis of the present invention The theoretical calculation results of the thermal lens change when the wave gain medium crystal absorbs different pump powers. Figure 3 is a graph showing the variation of the ultraviolet laser power with current of the semiconductor double-end pumped third harmonic ultraviolet laser according to the present invention. The single-end pumped three-harmonic ultraviolet laser has a power of 6W and a Q-switched frequency of 25KHz. Figure 5 is a semiconductor double-end pumped third harmonic UV laser with a power of 6W and Q-switched. A plurality of laser pulse waveforms measured at a frequency of 25 KHz. Figure 6 is a graph showing the variation of laser power with time at 5 W of a semiconductor double-end pumped third harmonic ultraviolet laser of the present invention.

Claims

1. A semiconductor double-end pumped third harmonic ultraviolet laser, including a semiconductor pump module, an optical coupling system, a fundamental gain medium crystal, a second harmonic nonlinear crystal, a third harmonic nonlinear crystal, a wave plate, and a modulation The device, the laser cavity mirror, and the ultraviolet laser mirror are characterized in that: the pump light outputted by the semiconductor pumping module is transmitted to the optical coupling system, and is collimated by the optical coupling system and coupled to the end face of the fundamental wave gain medium crystal. The "C" axis direction of the wave gain dielectric crystal is placed vertically upward; the fundamental gain medium crystal absorbs the pump light energy to generate stimulated emission, and the emitted light is formed in the laser cavity by the selection of the laser cavity mirror. The fundamental frequency of the beam quality, the modulation of the modulation device

Next, a modulated laser with high peak power is obtained; the modulated fundamental laser passes through the second harmonic nonlinear crystal to obtain a green laser output, and then is rotated by the polarization of the wave plate; the fundamental laser and the green laser are injected in the same polarization state. The third harmonic ultraviolet laser output is obtained by mixing in the third harmonic nonlinear crystal; the remaining green laser passes through the third harmonic nonlinear crystal again under the reflection of the resonant cavity mirror; the third harmonic ultraviolet light is converted into the first time. The reflection of the laser through the cavity mirror is outputted by the ultraviolet laser mirror together with the third harmonic ultraviolet laser converted into the laser cavity, and the three-order harmonic ultraviolet laser output is obtained.

2. The semiconductor double-end pumped third harmonic ultraviolet laser of claim 1, wherein: the two semiconductor pump modules are used to pump two fundamental gain medium crystals respectively.

3. The semiconductor double-end pumped third harmonic ultraviolet laser of claim 2, wherein: the semiconductor pump module has a center wavelength of 808 nm or 880 nm.

4. The semiconductor double-end pumped third harmonic ultraviolet laser according to claim 1, wherein: said fundamental gain medium crystal is Nd:YV04, or Nd:YLF, Nd:YAG, Nd: Glass, Yb: YAG, Er: YAG crystal.

5. The semiconductor double-end pumped third harmonic ultraviolet laser according to claim 1, wherein: the "C" axis direction of the fundamental wave gain medium crystal used is vertically placed upward, and "C" can also be used. The axis direction is rotated by 90°, that is, placed horizontally.

6. The semiconductor double-end pumped third harmonic ultraviolet laser according to claim 1, wherein: said second or third harmonic nonlinear crystal is a class I LB0, and may also be a class I BB0, a class I CLB0. Or a class I nonlinear crystal.

7. The semiconductor double-end pumped cubic i-wave ultraviolet laser according to claim 1, wherein: an end surface of said fundamental gain medium crystal is plated with an antireflection film for pump light.

8. The semiconductor double-end pumped third harmonic ultraviolet laser according to claim 1, wherein: said modulation device is an acousto-optic modulation device, or an electro-optic modulation device or an absorbing passive Q-switching device. Shaoguan.

9. The semiconductor double-end pumped third harmonic ultraviolet laser according to claim 2, wherein: the laser cavity mirrors are plane mirrors, so that the laser cavity forms a flat cavity, and the laser cavity can also be double concave. A cavity structure consisting of a cavity, a flat cavity or other lens.

10. The semiconductor double-end pumped third harmonic ultraviolet laser according to claim 9, wherein: the laser cavity structure is an "L" cavity structure, and a "V" angle folding cavity structure can also be used.