CN104810719A - IG (Ince-Gaussian) mode controllable neodymium-doped yttrium vanadate and chromium-doped yttrium aluminium garnet composite microchip laser - Google Patents

Abstract

The IG mode controllable neodymium-doped yttrium vanadate composite chromium-doped yttrium aluminum garnet microchip laser relates to a microchip laser capable of producing Innes-Gaussian mode. It is equipped with a pumping source, a first cylindrical lens, a second cylindrical lens, a laser working medium Nd:YVO ₄ /Cr ⁴⁺ :YAG composite crystal, and an output coupling mirror. The pump source, the first lenticular lens, the second lenticular lens, the laser working medium Nd:YVO ₄ /Cr ⁴⁺ :YAG composite crystal and the output coupling mirror are arranged in sequence from front to back and are located on the same optical axis; the laser working medium The back surface of Nd:YVO ₄ /Cr ⁴⁺ :YAG composite crystal is coated with high reflection film and anti-reflection film as the back cavity mirror of the laser cavity, and the back surface of the output coupling mirror is coated with reflective film as the front cavity mirror of the laser cavity. It is a controllable neodymium-doped yttrium vanadate composite chromium-doped yttrium aluminum garnet microchip laser that can directly generate high-order IG modes.

Description

IG mode controllable neodymium-doped yttrium vanadate composite chromium-doped yttrium aluminum garnet microchip laser

technical field

The invention relates to a microchip laser capable of producing Innes-Gauss (IG) mode, in particular to an IG mode controllable neodymium-doped yttrium vanadate composite chromium-doped yttrium aluminum garnet microchip laser, the laser working substance is made of gain medium Nd Composite crystal composed of :YVO ₄ and saturable absorber Cr ⁴⁺ :YAG.

Background technique

Semiconductor laser diode end-pumped all-solid-state lasers have become a research hotspot in recent years because of their ability to output high-quality laser beams. The high beam quality, high peak power, and high repetition rate of passive Q-switched lasers make them have very broad application prospects in the fields of laser radar, laser ranging, remote sensing and remote control, and micromachining. Nd:YVO ₄ crystal is one of the crystals suitable for laser diode end-pumped miniaturized solid-state lasers. It has a relatively large absorption cross section and a relatively wide absorption bandwidth. The principle of a saturable absorber used as a passive Q-switching switch is based on the nonlinear saturable absorption effect, that is, at a low energy density, the saturable absorber has a large absorption, which suppresses the oscillation of the laser, and can be regarded as a laser A loss of the cavity; at high energy densities, the saturable absorber is "bleached", does not absorb light, and is "completely" transparent. Currently, Cr ⁴⁺ ion-doped crystals and ceramics such as Cr ⁴⁺ : YAG, semiconductor saturable absorber mirror (SESAM), organic dyes, and color center crystals are mainly used as passive Q-switching switches. Saturable absorbers doped with Cr ⁴⁺ ions have been widely used in passively Q-switched solid-state lasers due to their advantages such as high damage threshold, large absorption cross section, low saturation light intensity, low cost and simple structure. Cr ⁴⁺ : YAG crystal is a new type of near-infrared broadband tunable solid-state laser material that appeared in 1988. It has good thermal conductivity, stable physical and chemical properties, and high damage resistance threshold. It is used in long-distance sensing, optical Communication, medical treatment, laser technology and other fields have broad application prospects, and have become research hotspots in recent years.

In 2000, Yu Jin ([1] Yu Jin. Quantum well diode-pumped Nd:YVO ₄ /Cr ⁴⁺ :YAG high repetition rate passive Q-switched laser. Optical Precision Engineering, 2000, 8(1): 23-25 .) In a quantum well diode end-pumped Nd:YVO ₄ laser using Cr ⁴⁺ :YAG as a saturable absorber, a high repetition rate of 1.06 μm was obtained in a simple linear planar-cavity structure by end-pumping rate passively Q-switched pulsed laser output. When the absorbed pump power is 528.3mW, the output pulse energy is 0.19μJ, the pulse width is 32ns, and the pulse repetition rate is 158.7kHz. Before inserting the Cr ⁴⁺ :YAG saturable absorber, the 1.06μm continuous laser output was obtained; after inserting the Cr ⁴⁺ :YAG wafer into the resonator, the Nd:YVO ₄ / Cr ⁴⁺ : YAG high repetition rate passive Q-switched pulsed laser output. In 2004, Li Jian et al. ([2]JianLi, Cuiling Wang, Yanzi Wang, etc.High peak power laser-diode-pumped passivelyQ-switched Nd:YVO ₄ /Cr ⁴⁺ :YAG laser.Chinese Optics Letters, 2004, 2(10):590-591.) studied the average output power, pulse repetition frequency, pulse width and peak power of LD-pumped passively Q-switched Nd:YVO ₄ /Cr ⁴⁺ :YAG lasers under different pump powers. Changes. When the pump power is 21.2W, an average output power of 4.23W and a pulse peak power of 18kW can be obtained. In 2009, Chen Zemin et al. ([3] Chen Zemin, Li Liwei, Min Dayong et al. High-power LD-pumped Cr ⁴⁺ :YAG passively Q-switched Nd:YVO ₄ laser. Optics and Optoelectronics Technology, 2009, 7(2): 82-85.) The effects of pump power, cavity length, output mirror transmittance on the output power, and the changes of pulse repetition frequency and pulse width under different pump powers were studied. When the cavity length is 8cm and the pump power is 27W, a Q-switched pulse sequence with a repetition frequency of 37.8kHz and an average output power of 3.5W is obtained. The single pulse energy is 93μJ, the pulse width is 24nm, and the peak power is 3.9kW.

The application of high-order IG transverse modes generated by all-solid-state lasers pumped by semiconductor laser diodes in chaos research, laser cooling, optical trapping, and fluid physics has also received more and more attention. In 2004, Schwarz et al. ([4] Ulrich T. Schwarz, Miguel A. Bandres and Julio C. Gutiérrez-Vega. Observation of Ince-Gaussian modes in stable resonators. Optics Letters, 2004, 29(16): 1870-1872. ) conducted a study on the generation of high-quality high-order IG modes using laser diode-pumped Nd:YVO ₄ lasers. In the experiment, a semiconductor laser diode pumped at 808nm was used as the pump source, Nd:YVO ₄ was used as the laser working material, and the structure of the flat-concave cavity was adopted. The output coupling mirror was moved obliquely for a small distance to destroy the laser resonator. Symmetry, so as to obtain the output of IG mode. As for the principle of obtaining the IG mode, at present, in addition to the well-known mechanism of breaking the symmetry of the resonator, Cr, Nd:YAG self-Q-switching laser crystals are recently used as the working material, and the incident angle of the pump beam is inclined at The high-order IG mode laser output was directly generated in the Cr,Nd:YAG microchip laser ([5]Dong J.,Ma J.,Ren Y.,XuG.,Kaminskii AAGeneration of Ince-Gaussian beams in highly efficient,nanosecond Cr, Nd: YAG microchip lasers. Laser Physics Letters, 2013, 10(8): 085803). The research results show that the non-uniform distribution of dopant ion density in laser crystal leads to different distribution of excited state ions is a possible reason for the direct generation of IG mode.

Contents of the invention

The object of the present invention is to provide an IG mode controllable neodymium-doped yttrium vanadate compound chromium-doped yttrium aluminum garnet microchip laser.

The invention is provided with a pumping source, a first cylindrical lens, a second cylindrical lens, a laser working medium Nd:YVO ₄ /Cr ⁴⁺ :YAG composite crystal, and an output coupling mirror.

The pump source, the first lenticular lens, the second lenticular lens, the laser working medium Nd:YVO ₄ /Cr ⁴⁺ :YAG composite crystal and the output coupling mirror are arranged in sequence from front to back and are located on the same optical axis; the laser working medium The back surface of Nd:YVO ₄ /Cr ⁴⁺ :YAG composite crystal is coated with high reflection film and anti-reflection film as the back cavity mirror of the laser cavity, and the back surface of the output coupling mirror is coated with reflective film as the front cavity mirror of the laser cavity.

If the focal lengths of the first lenticular lens and the second lenticular lens are f ₁ and f ₂ respectively, and the thickness of the laser gain medium Nd:YVO ₄ is d, then the distance from the pump source to the first lenticular lens is f ₁ , The distance from the first lenticular lens to the second lenticular lens is adjustable from 0 to f ₁ + f ₂ , and the distance from the second lenticular lens to the back surface of the laser working medium Nd:YVO ₄ /Cr ⁴⁺ :YAG composite crystal is f ₂ -d～f ₂ +d can be adjusted. The laser working medium Nd:YVO ₄ /Cr ⁴⁺ :YAG composite crystal and the output coupling mirror are tightly pressed together to form a microsheet structure.

The pumping source can adopt a high-brightness 808nm single-tube high-brightness semiconductor laser, and the rear surface of the Nd:YVO ₄ /Cr ⁴⁺ :YAG composite crystal (that is, the Nd:YVO ₄ end face facing the pumping source) is plated with an 808nm Antireflection coating and 1064nm high reflection coating, the rear surface of the output coupling mirror is coated with 1064nm reflection coating, and the coating reflectance R _oc ranges from 40% to 98%.

The invention is a controllable neodymium-doped yttrium vanadate compound chromium-doped yttrium aluminum garnet microchip laser that can directly generate high-order IG modes.

The present invention is characterized in that:

(1) A microchip laser composed of a laser working medium Nd:YVO ₄ /Cr ⁴⁺ :YAG composite crystal. The two surfaces of the composite crystal are parallel planes, and the laser cavity formed is a Fabry-Perot cavity ( FP cavity), which has the advantages of excellent beam directionality, small divergence angle, large mode volume, relatively easy to obtain single longitudinal mode oscillation, and the microchip laser structure can output pulses with high brightness, high peak power, and high beam quality laser.

(2) Use 808nm single-tube high-brightness semiconductor laser as the pump source, which has high brightness and small volume. The indirect pumping method is that in the process of excitation and laser generation, the ground state particles are first excited by the pump system to an intermediate state energy level, and then transferred to the upper energy level of the laser.

(3) The laser gain medium Nd:YVO ₄ and the saturable absorber Cr ⁴⁺ :YAG are directly bonded to form a composite crystal through thermal bonding technology, eliminating the gap between the Nd:YVO ₄ crystal and the Cr ⁴⁺ :YAG crystal The optical loss formed by the interface can improve the laser thermal performance and beam quality, and also facilitate the direct formation of parallel planar cavities to obtain high-efficiency laser output.

(4) Utilize a camera-based beam profiler to provide more details and true 2D analysis of the beam power density distribution. This allows complex mode patterns to be identified when optimizing laser systems. In the experiment, the IG high-order transverse mode generated by the microchip laser is received by the beam quality analyzer, and the 2D/3D high-order transverse mode pattern is observed by connecting with the computer and using Thorlabs Beam software.

The advantages of the present invention are:

(1) The microchip laser of the present invention can obtain optical pulse laser output with different repetition frequencies within a relatively large pump power range, and the peak power of the laser pulse is high, and the single pulse width is narrow.

(2) Using a high-brightness single-tube semiconductor laser as a pumping source can make the whole set of laser devices more miniaturized, integrated and practical. Compared with the direct pumping method, the indirect pumping method has the following advantages: First, the intermediate energy level has a lifetime much longer than that of the upper energy level of the laser, and can be an energy band formed by many energy levels, so it is easy to accumulate a large amount of energy in the intermediate energy level. Second, in some cases, the probability of exciting a particle from the ground state to an intermediate energy level is much greater than the probability of directly exciting it to an upper energy level, which reduces the requirement for pumping. Therefore, the indirect pumping method can obtain more efficient laser output.

(3) The thermal bonding technology is used to bond Nd:YVO ₄ and the saturable absorber Cr ⁴⁺ :YAG into a composite crystal, which can effectively shorten the cavity length, compress the pulse width, and increase the pulse peak power to obtain a high peak value. Power, miniaturization and integration of solid-state laser light sources.

(4) The saturable absorber Cr ⁴⁺ :YAG is used as the Q-switching element to directly generate the IG mode, which facilitates the optimal design of the laser, reduces the difficulty of laser development and maintenance, and is convenient for operation and research.

(5) The Fabry-Perot cavity parallel to the plane is used in the work, and the end-pumped single-tube diode→composite crystal is on the same straight line, which greatly shortens the cavity length, simplifies the structure, and makes the design portable and miniaturized products made possible.

(6) The laser uses few optical components, low production cost, simple and compact structure, and is convenient for production, installation and operation by non-professionals.

Description of drawings

Fig. 1 is a structural composition diagram of an embodiment of the present invention.

Fig. 2 is the experimental result of Nd:YVO ₄ /Cr ⁴⁺ :YAG passively Q-switched microchip laser directly generating IG _0,0 mode laser beam output.

Fig. 3 is the experimental result of Nd:YVO ₄ /Cr ⁴⁺ :YAG passively Q-switched microchip laser directly generating IG ⁰ _3,1 mode laser beam output.

Fig. 4 is the experimental result of the Nd:YVO ₄ /Cr ⁴⁺ :YAG passively Q-switched microchip laser directly generating the IG ⁰ _5,1 mode laser beam output.

Fig. 5 shows the experimental results of the Nd:YVO ₄ /Cr ⁴⁺ :YAG passively Q-switched microchip laser directly generating the IG ^e _5,1 mode laser beam output.

Fig. 6 is the experimental result of the Nd:YVO ₄ /Cr ⁴⁺ :YAG passively Q-switched microchip laser directly generating the IG ^e _6,2 mode laser beam output.

Detailed ways

The following embodiments will further illustrate the present invention in conjunction with the accompanying drawings.

Referring to Fig. 1, the embodiment of the present invention is provided with a pumping source 1, a first lenticular lens 2, a second lenticular lens 3, a laser working substance Nd:YVO ₄ /Cr ⁴⁺ :YAG composite crystal 4, a first coating 5, a second 2 coating 6, output coupling mirror 7 and beam quality analyzer 8; selected pump source 1, first lenticular lens 2, second lenticular lens 3, laser working medium Nd:YVO ₄ /Cr ⁴⁺ :YAG composite crystal 4 , output coupling mirror 7 and beam quality analyzer 8 are arranged sequentially from front to back and are located on the same optical axis; the rear surface of the laser working medium Nd:YVO ₄ /Cr ⁴⁺ :YAG composite crystal 4 is provided with a first coating 5, The rear surface of the output coupling mirror 7 is provided with a second coating 6 to form the front and rear cavity mirrors of the laser cavity.

The pumping source 1 is an 808nm single-tube high-brightness semiconductor laser with a laser power of 10W; its emission cross-sectional area is 1 μm×50 μm. Then after the optical coupling and focusing of the first lenticular lens 2 and the second lenticular lens 3 with two focal lengths of 8 mm, a laser working material Nd:YVO ₄ /Cr ⁴⁺ :YAG composite crystal is formed on the incident surface An incident light spot with an area of (80 μm×80 μm); the laser gain medium used in the present invention is a piece of Nd:YVO ₄ laser crystal cut along the <111> direction. The doping concentration of Nd ³⁺ ions is 1 at.%, and the doping concentration of Cr ⁴⁺ ions is 0.01 at.%. Nd:YVO ₄ crystals and Cr ⁴⁺ :YAG crystals and dimensions are both 10mm×10mm in cross-section and 1mm in thickness. The end face of the Nd:YVO ₄ crystal facing the pump source is coated with an 808nm anti-reflection coating and a 1064nm high reflection coating, and the end face of the output coupling mirror is coated with a 1064nm reflective film. The reflectivity R _oc ranges from 40% to 98%. The Nd:YVO ₄ /Cr ⁴⁺ :YAG composite crystal was placed on the experimental platform using an optical fixture and worked at room temperature.

To measure the output 1064nm laser performance, a laser power meter can be used to measure the average output power of the laser. The test of output laser pulse characteristics (including output laser pulse waveform, repetition frequency characteristics and pulse width characteristics) can be tested by a high-sensitivity, high-bandwidth digital oscilloscope. The mode and beam quality characteristics of the output laser are monitored and recorded by a beam profiler. The spectrum analyzer monitors the number of longitudinal modes of oscillation in the laser cavity and their competition in real time.

The main mechanism for the laser to generate the IG mode in the present invention is: after the pump light of the large pump spot is incident on the laser crystal inclined at a slight angle (about 5°) perpendicular to the horizontal axis, the particles generated inside the laser crystal The uneven distribution of the number of inversions realizes the laser output of different IG modes. The 808nm single-tube diode used in the experiment is used as the pump source, which is incident on the Nd:YVO ₄ /Cr ⁴⁺ :YAG composite crystal to produce a near-infrared IG mode laser output of 1064nm.

The core key technology of the present invention is the use of Nd:YVO ₄ /Cr ⁴⁺ :YAG composite crystal. The preparation process is through the thermal bonding technology (Thermal bonding), firstly two pieces of Nd:YVO ₄ crystals and Cr ⁴⁺ :YAG crystals that have been precisely processed are closely bonded together at room temperature, relying on the contact between the two surfaces Forces hold the two together. After the surface of the crystal is treated, a hydrophilic surface can be obtained, and two hydrophilic surfaces with high processing precision and fine roughness can attract each other at room temperature by virtue of the hydrogen bond to form a photoresist. As the temperature increases, the diffusion of ions and holes on the interface gradually intensifies, and because there are many dangling bonds on the surface, a stable structure can be formed after a certain period of lattice adjustment and reconstruction. ([6] Lv Jingshu, Yan Ping, Gong Mali, etc. Thermal Bonding Technology and Its Application in Laser. Optical Technology. 2002, 28: 355-359.).

When the laser is working, only part of the energy of the pump light is converted into laser light, and the remaining energy is consumed by other competing mechanisms, such as non-radiative transition, spontaneous emission, matrix absorption, etc. These processes will generate heat in the laser medium . Generally, high-power systems must add a cooling system, but the cooling system will cause uneven temperature distribution inside the laser medium, making the inner temperature higher than the edge. Since the refractive index of the crystal changes with temperature, thermal lensing will occur, and stress birefringence will also occur after the crystal is heated, which will seriously affect the beam quality. The microchip laser technology adopted in the present invention enables the heat generated by the pump light in the crystal to be effectively dissipated through the large-area contact between the laser crystal and the aluminum substrate with excellent thermal conductivity, so that the cooling efficiency of the laser at room temperature is high. End pumping makes the heat conduction direction and temperature gradient of the crystal mainly along the axial direction, so that the temperature distribution of the crystal is very uniform and the wavefront deformation is very small. At the same time, the end-pumping method is used to make the heat conduction direction parallel to the direction of laser beam propagation (that is, the axial direction), and the radial temperature gradient is very small, thus avoiding the inherent limitations of traditional rod crystal lasers, such as thermal lens effect at high power, laser Crystal deformation, birefringence effects, etc. Therefore, microchip solid-state lasers can easily obtain very high output power without affecting its excellent optical performance, and generally can obtain high beam quality laser output close to the diffraction limit (M ² <1.1).

In the present invention, Nd:YVO ₄ crystals are bonded with Cr ⁴⁺ :YAG crystals to form an integrated laser system, which realizes miniaturization and integration of the system. The adhesive surfaces of the discrete crystals are in close contact to form a whole, which can reduce the difficulty of adjustment and is also conducive to the design of the clamping structure.

808nm single-tube laser diode directly end-pumped Nd:YVO ₄ /Cr ⁴⁺ :YAG discrete crystal microchip laser, mainly involving Nd:YVO ₄ crystal as laser gain medium and Cr ⁴⁺ :YAG crystal as passive Q-switching switch, By controlling Cr ⁴⁺ :YAG crystals with different initial transmittances and Nd:YVO ₄ crystals with different doping concentrations and thicknesses, a miniaturized microchip laser with high peak power and short pulse high-order IG mode laser output can be realized.

Figures 2 to 6 are Nd:YVO ₄ /Cr ⁴⁺ :YAG passively Q-switched microchip lasers that directly generate IG modes (IG _0,0 IG ⁰ _3,1 IG ⁰ _5,1 IG ^e _5,1 IG ^e _6,2 ) Experimental results of laser beam output.

Claims (4)

1. IG mode controllable neodymium-doped yttrium vanadate composite chromium-doped yttrium aluminum garnet microchip laser, which is characterized in that it is equipped with a pump source, a first cylindrical lens, a second cylindrical lens, and a laser working medium Nd:YVO ₄ /Cr ⁴⁺ : YAG composite crystal, output coupling mirror; The pump source, the first lenticular lens, the second lenticular lens, the laser working medium Nd:YVO ₄ /Cr ⁴⁺ :YAG composite crystal and the output coupling mirror are arranged in sequence from front to back and are located on the same optical axis; the laser working medium The back surface of Nd:YVO ₄ /Cr ⁴⁺ :YAG composite crystal is coated with high reflection film and anti-reflection film as the back cavity mirror of the laser cavity, and the back surface of the output coupling mirror is coated with reflective film as the front cavity mirror of the laser cavity. 2. IG mode controllable neodymium-doped yttrium vanadate composite chromium-doped yttrium aluminum garnet microchip laser as claimed in claim 1, is characterized in that if the focal length of the first cylindrical lens is marked as f ₁ , the focal length of the second cylindrical lens is marked as is f ₂ , the thickness of the laser gain medium Nd:YVO ₄ is recorded as d, then the distance from the pump source to the first lenticular lens is f ₁ , and the distance from the first lenticular lens to the second lenticular lens is 0～(f ₁ + Adjustable between f ₂ ), the distance between the second cylindrical lens and the back surface of the laser working medium Nd:YVO ₄ /Cr ⁴⁺ :YAG composite crystal is adjustable between (f ₂ -d)～(f ₂ +d) ; The laser working medium Nd:YVO ₄ /Cr ⁴⁺ :YAG composite crystal and the output coupling mirror are tightly pressed together to form a microchip structure. 3. The IG mode controllable neodymium-doped yttrium vanadate composite chromium-doped yttrium aluminum garnet microchip laser according to claim 1, characterized in that the pump source is a high-brightness 808nm single-tube high-brightness semiconductor laser. 4. IG mode controllable neodymium-doped yttrium vanadate composite chromium-doped yttrium aluminum garnet microchip laser as claimed in claim 1, is characterized in that described Nd: YVO ₄ /Cr ⁴⁺ : the back surface of YAG composite crystal, namely The end face of the Nd:YVO ₄ facing the pump source is coated with an 808nm antireflection coating and a 1064nm high reflection coating, and the rear surface of the output coupling mirror is coated with a 1064nm reflection coating, and the coating reflectivity R _oc ranges from 40% to 98%.