Ultrafast lasers are not just fast - TRUMPF Scientific Lasers' ultra-high energy ultrafast laser system
Continued from the above----Ultrafast lasers are no longer just fast - AMPHOS's ultra-high average power ultrafast laser system
There are three main types of high repetition frequency ultrafast lasers based on Yb-doped media: fiber ultrafast lasers, slab ultrafast lasers and sheet ultrafast lasers. In the previous issue, the working principle and typical characteristics of slab lasers were introduced using the products of German AMPHOS company as an example. This article continues to introduce high-power, high-energy thin-film ultrafast lasers.
Thin sheet laser, also known as disc or disk laser, as the name suggests, its main feature is that the gain medium is in the shape of a sheet, its thickness is generally between 100μm-400μm, and its diameter is in the order of millimeters. The front side of the sheet is plated with a pair of pump lasers (generally choose 940nm or 969nm semiconductor laser (generally 1030nm) laser) and the amplified laser (generally 1030nm). The back is coated with a high-reflection film for the wavelength of the pump light and gain laser, and a special process is used to tightly bond the back of the sheet to the heat sink. The water flow can efficiently dissipate heat to the sheet through the heat sink on the back. The schematic diagram of the sheet structure is shown in Figure 1.
Figure 1 Schematic diagram of the sheet structure
The medium of the sheet structure can withstand very high pump power and the crystal itself will not have too high temperature rise and thermal deformation. At the same time, because the thickness of the sheet is thin, the one-way absorption of pump light and laser is small. The laser can pass through the sheet multiple times to obtain huge gain amplification, so it is very suitable for the gain medium structure of the amplifier. Regenerative amplifiers based on thin-sheet gain medium structures can amplify the energy and average power of ultrafast seed lasers by several orders of magnitude while maintaining excellent beam quality and conversion efficiency. They are by far the most suitable technology for ultrafast lasers of hundreds of millijoules and kilowatts, and can be maturely used in industrial fields. Thin slice amplifiers have absolute advantages over lath structures, fiber amplifiers, etc. The output laser can be both high power and high energy, and maintain a beam quality close to that of a seed laser.
In sheet lasers, the regenerative amplifier also plays a vital role. The regenerative amplifier itself is an oscillator that can generate laser light. It is mainly composed of an optical switch, a gain medium, an excitation source and a resonant cavity, also called a regenerative cavity. Its working principle is to select a pulse from the pulse sequence of the ultrafast seed laser after the pulse width has been broadened, and inject it into the regeneration cavity. Under the pumping of the pump laser, the pulse passes through the gain medium multiple times for amplification. When the energy is amplified to the required value, the time for the optical switch to apply high voltage is changed, and then it is exported from the cavity. Injection and export are usually completed through polarizers, Faraday rotators and half-wave plates. Figure 2 is a schematic diagram of the optical path of a regenerative amplifier based on a thin sheet structure.
Figure 2 Schematic diagram of the optical path of a regenerative amplifier based on a thin sheet structure
Thin sheet laser gain modules generally use fiber-coupled semiconductor laser end-pump structures. The pump light is output by the fiber and reflected by a relatively large parabolic mirror onto the thin sheet crystal. The structure is simple and the pumping efficiency is high. At present, the development of 940nm fiber-coupled semiconductor lasers has been very mature and can provide sufficiently high pumping capabilities. Figure 3 is a schematic diagram of the transmission of pump laser and gain laser in thin sheet media.
Figure 3 Optical path diagram of sheet laser gain
Industrial disc lasers were first developed by TRUMPF Group for laser cutting and laser welding. A single sheet laser can obtain an output power of more than 12,000 watts. TRUMPF has always been the leader in sheet lasers. As the market's requirements for laser peak power are increasing day by day, the single pulse energy of ultrafast lasers needs to be increased, and the management of thermal deposition and nonlinearity has become a huge challenge. Nowadays, the application of thin sheet structure and regenerative amplification technology to ultrafast lasers has led to new breakthroughs in ultrafast lasers. TRUMPF Scientific Lasers (TSL) was established in 2012 and is a wholly-owned subsidiary of TRUMPF Group. The company mainly develops and manufactures high-end thin-film ultrashort pulse lasers for scientific research, and provides customization and joint innovation services. Currently, TSL can provide laser output of <2ps, hundreds of millijoules, and kilowatts. In the case of non-full energy injection into the compressor, the minimum pulse width can be compressed to <600fs. The detailed parameters are shown in Table 1.
Table 1 Product Parameters of TSL Company
Compared with the slab structure, the main advantage of thin-film ultrafast lasers is that they can achieve single pulse energy up to several hundred millijoules, have ultra-high power characteristics, and maintain beam quality close to that of seed lasers. This has irreplaceable advantages in certain application fields, such as inverse Compton scattering, atmospheric filamentation, OPCPA, etc. Figure 4 is the actual measurement results of typical parameters of TSL laser.
Figure 4 Typical parameter measurement of TSL laser
As the exclusive agent of TRUMPF Scientific Lasers in China, Waveform Technology has been focusing on ultrafast laser technology and services for many years. It joins hands with the world's top ultrafast laser brands and the most advanced laser technology to provide industrial and scientific research customers with ultrafast laser systems with both high average power and high peak power. Combined with Waveform Technology's rich independent research and development capabilities, it also provides customers with comprehensive solutions for ultrafast laser products and applications. We warmly welcome customers to call us!
Next issue’s preview: Laser lightning triggering for high-energy thin-sheet ultrafast laser applications, so stay tuned!