The biggest advantage of end-pumped mode is that it is easy to obtain good beam quality and can realize high brightness solid-state laser. Therefore, the attempt of end-pumping has never stopped. In this system, 8W semiconductor laser is used as the pump source, and the output is shaped by prism group, and the beam divergence angle is compressed and focused, then it is input into the laser crystal. The end face of the laser crystal near the pump source is coated with an antireflection film of 808nm and a highly reflective film of 1064nm. The anti-reflection film of 808nm minimizes the loss of the pumped laser with 808nm wavelength before it enters the laser crystal, while the high reflection film of 1064nm is combined with the output mirror coated with 1064nm partial reflection film to form a resonant cavity, so that the laser of 1064nm can be oscillated and amplified. In this structure, the volume of crystal mode activated by pump light is small, which is generally used in low power occasions. For example, the laser designed by ACI Company is used in 3W laser marking machine system. The advantage of end-pumping is that the output laser mode is good, which is convenient to realize TEM00 output, and it is very practical in some occasions where power requirements are not high and collimation is needed. Such as laser ranging and marking of electronic components. Bilateral pumped solid-state laser
Researchers at Hughes Aerospace Laboratory obtained a high power output of 0.95KW by side pumping rod-shaped Yb:YAG crystal. This is the maximum power output obtained by pumping a single Yb:YAG with a semiconductor laser at present. The laser head of a side-pumped solid-state laser is a pump source composed of three diode pump modules in a circle, and each pump module is composed of three diode linear arrays with microlenses. The average output power of each linear array is 20W, and the output wavelength is 808nm. The device skillfully designs the pump cavity and refrigeration channel by using the glass tube. Most of the surface of the glass tube is coated with 808nm high reflection film, and the rest is coated with three layers of 808nm antireflection film at 120 to form a pump cavity. The light emitted by the diode pump source is converged to three long and narrow areas coated with antireflection films through three pairs of beam shaping lenses, and then absorbed by the crystal through the glass tube wall. Because most areas of the glass tube are coated with highly reflective films, the pump light will be reflected back and forth in the pump cavity until it is completely absorbed by the crystal, forming a uniform gain distribution on the cross section of the crystal. Meanwhile, the glass tube
It can also be used for refrigeration, and the cooling water passing at high speed can quickly take away the generated heat. The crystal is a Nd:YAG rod with composite structure, the effective size is j3*63mm, and the doping concentration is 1.5at.%. When the pump power is 180W, the laser output is 72 W, and the optical-optical conversion efficiency is as high as 40%.
3 thin disc pump
Thin-plate laser is a new design scheme of solid-state laser, which combines the advantages of end pumping and side pumping. It was first proposed by researchers from the Institute of Technical Physics of the German Aerospace Research Institute. The basic concept is to end-pump an extremely thin crystal with a fiber-coupled semiconductor laser as a pump source, so that the pump light passes through the crystal sheet of several hundred microns for many times, and the distribution direction of thermal gradient is the same as the propagation direction of laser beam. In the new pump design, a parabolic imaging mirror is used instead of the original four-sided spherical imaging mirror, so that the number of times the pump light passes through the crystal increases from the original 8 times to 16 times. With an improved pumping structure, at room temperature, pumped by a 24W CW laser and using a j3*0.2 Nd:YAG crystal plate, the continuous light output of TEM00 of 10W was obtained, and the light efficiency was 4 1.7%. This thin-plate laser has the characteristic of proportional power amplification. By cascading multiple thin-plate crystals on the same heat sink, it is expected to obtain a kilowatt-class all-solid-state laser with high efficiency close to the diffraction limit. The output optical quality of this kind of laser is between end pumping and side pumping, and higher output power and better optical mode can be obtained. However, it is difficult to design and debug this kind of laser, so most laser companies do not use it.
4 fiber laser
Fiber laser evolved from optical amplifier in optical communication industry in recent years. As soon as it was introduced, it caused a shock in the industry. Its good optical quality, high output power, long service life and maintenance-free features have attracted the attention of many companies. Strictly speaking, it belongs to a kind of end pumping. The pumping source of modern high-power fiber laser is a high-power multimode diode, which is realized by double cladding around the single-mode fiber core.
In the 1970s, the idea of replacing the multimode emission output of solid-state lasers or broadband semiconductor laser diodes with single-mode fiber lasers was first put forward. In a simple double-clad fiber structure, the axial single-mode glass fiber core is doped with required laser ions, such as rubidium, erbium, ytterbium and thulium. The core is surrounded by undoped glass cladding several times its diameter, and the cladding has a low refractive index. Next, the inner pump cladding is covered by the outer undoped glass cladding, which also has a low refractive index. In this fiber structure, the multimode diode pump light enters the pump cladding through the end face of the composite fiber, propagates in the fiber structure, periodically passes through the doped single-mode fiber core, and produces population inversion in the core fiber.
IPG Laser Department (a branch of IPG photonics) has developed a more advanced fully enhanced side-pumped fiber laser. It includes an active optical fiber with polyhedral structure, which can be freely fused with other optical elements or gain stages, thus making it possible to inject pump light into the cladding from multiple points. In this way, simple proportional control of optical fiber output power becomes feasible. Other side pumping techniques include V-groove coupling. 1996, IPG Photonics Company introduced the industrial-grade cladding pumped fiber laser with diffraction limit of 10W to the market. Polaroid Company (Cambridge, Massachusetts), Spectral Diode Laboratory (JDS Single Phase) and Spectral Physics Company will soon launch similar lasers.
Coupling the output power of multiple 100 watt fiber lasers can improve the output power of fiber lasers to a higher level. For example, the beams output by 7100W fiber lasers are transmitted over 30 meters through 7 single-mode fibers, and then combined in a multi-core fiber beam coupler to output beams with a diameter of 80 microns and a divergence angle of less than 40 mrad. This is equivalent to an output beam parameter:10mmrad, and the output power density is only 50 times that of fiber laser. The size of the 700-watt fiber laser is 55×60×95cm3, and its weight is 120 kg. This type of laser can lengthen the optical fiber according to the required power, so it can achieve high power. However, it has a fatal weakness, that is, the single pulse energy is not high, which makes the application field of fiber laser limited to some extent. How to improve the single pulse energy of fiber laser is a key research and development topic all over the world.
summary
In this paper, the technical characteristics of laser head, the core component of several semiconductor pumped solid-state lasers, are expounded from the perspective of experimental equipment and principle. High-power and high-brightness DPSSL has always been a frontier topic in the laser field at home and abroad. There have been many reports of kilowatt DPSSL systems abroad, and Japan is also expected to realize a high-power all-solid-state laser with average output power ≥ 10KW, electro-optical efficiency ≥20% and laser head size ≤0.05m3 in 2005. The development of DPSSL in China is relatively backward, and the manufacturing technology of high-power LD and LD array in China is gradually mature, and DPSSL will surely develop more vigorously.