History and Status of Ultrashort Pulse Laser Technology

In lasers, the generation of ultra-short pulsed lasers is important because pulsed light can be generated by controlling the coherent light waves of the laser, the time width of which is beyond the control of electronics. In a broad sense, ultrashort pulsed light refers to pulsed light less than 1 ns. In the mid-1960s, scientists carried out experimental research on mode-locked ultrashort pulses of light generated by ruby ​​lasers and Nd-doped lasers pulsed by flash lamps. Since then, the generation technology of short pulse light has stepped from mode-locked sub-picosecond pulses to femtosecond pulses. In recent years, ultrashort pulse laser technology has been popularized, and various tunable ultrashort pulse mode-locked solid-state lasers have been practical since the 1990s. A tunable laser is a photon-terminated laser (Photon-terminated laser) whose energy level is in a vibrational excitation state under the laser, which broadens the oscillation frequency band. Typical Ti:sapphire lasers operate stably, realizing ultrashort (shortest about 5 fs) pulsed light with an average output power of 1 W. If a laser crystal doped with Yb ions is used, sub-picosecond pulse output with higher average output power can be obtained. Semiconductor lasers have the characteristics of fast relaxation and high-speed modulation of the pump (current), so even without mode locking, ultrashort pulses in the picosecond region (10-10~10-12 s) can be generated by using the gain transition phenomenon. Light.
The recent development of small-scale picosecond and femtosecond pulsed lasers has enabled the development of ultrashort pulsed light sources. Considering the requirements for ultra-short pulse light sources from the perspective of light utilization, whether to effectively utilize the characteristics of the time domain (ultra-high speed) or to utilize the high peak intensity of concentrated light energy in a short time are the two major research directions. In practical applications, these two directions are closely related. From the above viewpoints, maximizing the pursuit of light source performance to achieve the generation of shorter pulsed light and higher peak intensity is the driving force behind the development of this technology. In addition, improving the performance of new light sources, popularizing new functions or new phenomena discovered and making them practical; improving the reliability, stability and life of light sources and reducing costs are also the keys to technology development. In addition to improving the pulse width and pulse energy, improving the beam quality is also an extremely important research topic. This has enormous implications for developments in this technical field, as is the case with maximizing coherence both in time and space.
People have accumulated a lot of experience in the development of ultrashort pulse laser technology, such as effectively generating high-intensity pulses and obtaining high-energy pulses as much as possible in the pulse generation stage; various attempts have been made to directly generate high-intensity ultrashort pulses. , and obtained research results that contributed to the development of this field. However, in the process of generating and utilizing high-intensity pulses, problems such as the coherence of optical pulses or the repeatability and reliability of waveforms, wavelengths, etc., are not ideal. Therefore, the high-repetition pulse output of the selected mode-locked laser oscillator and the way of high-magnification amplification have become the mainstream. Although the energy of each pulse is small, the pulse generation source can easily obtain pulses with good coherence by using a continuous oscillation mode-locked laser.