Significance Ultrafast lasers with pulse durations on the orders of picosecond and femtosecond are widely used in various fields, such as supercontinuum generation, photoelectron microscopy, and material micromachining. The traditional high-power ultrafast lasers with repetition rates of kHz-MHz exhibit a large heat-affected zone during material micromachining, leading to unpleasant damage to the materials. The emergence of pulse lasers with ~GHz repetition rate can effectively solve this problem. Combining the very high repetition rate of ~GHz and novel burst mode processing technique, the GHz “burst-mode” femtosecond lasers have been used to ablate the target materials before the residual heat deposited by previous pulses diffuses away from the processing region, which can not only improve the ablation efficiency, but also ensure excellent processing quality.
Due to its short wavelength, high resolution, and high photon energy, deep ultraviolet (DUV) lasers are widely used in chip defect detection and photoelectron spectroscopy experiments. In order to obtain DUV lasers with high beam quality, high coherence and high repetition rate, near-infrared all-solid-state lasers are usually used as the fundamental drivers to DUV lasers through the nonlinear optical crystals-based multi-stage frequency conversion technique. Due to the high peak powers and high wavelength conversion efficiencies of the near-infrared pulsed lasers with repetition rates of kHz and MHz, it is easy to obtain high-power DUV lasers for lasers with those repetition rates. At present, the repetition rates of industrial high-power ultraviolet lasers are usually in kHz and MHz range. There are very few research results on DUV lasers with ~GHz repetition rate, which greatly limits the application potential of DUV lasers in the above aspects.
In recent years, various methods have been proposed to achieve DUV laser pulses with repetition rates of ~GHz. However, these methods still face a series of challenges. Therefore, it is necessary to summarize recent development tendency of technology of high repetition rate ultrashort laser pulse generation and frequency conversion.
Progress There are many methods for producing GHz bursts of laser pulses. Femtosecond pulses at multi-GHz repetition rates can be obtained directly from the oscillators with harmonic mode-locking technique, semiconductor saturable absorber mirror and Kerr lens based passive mode-locking techniques. Typical pulse repetition rates of pulse trains generated by mode-locked fiber oscillators are in the range from tens up to hundreds of MHz. The GHz pulses can be obtained through repetition rate multiplication techniques. In this study, we briefly illustrate their pros and cons and review their recent developments. The emergence of multi-stage amplification systems has increased the average power of ~GHz femtosecond pulses in the near-infrared band to the order of hundreds of watts (
There are many methods for producing DUV lasers. For the method of nonlinear crystal frequency conversion, the research of 266/258 nm DUV nanosecond lasers (
For high power GHz repetition rate near-infrared femtosecond pulse lasers, the difficulty lies in the generation of GHz seed. For GHz repetition rate amplifier, it is relatively easy to obtain higher average powers due to low single pulse energy and low peak power. For GHz repetition rate DUV femtosecond pulse laser source, the difficulty is not in the generation of the fundamental frequency laser, but in the low peak power of the fundamental frequency laser and the thin nonlinear medium used, which leads to low nonlinear frequency conversion efficiency, and it is difficult to obtain GHz femtosecond pulse laser in the DUV band (
Conclusion and Prospect In recent years, the French company Amplitude has put forward the idea of “GHz Revolution”, which mainly refers to the development of ultra-short pulse laser sources with pulse repetition rate in GHz. The emergence of multi-stage amplification systems has increased the average power of GHz femtosecond pulses in the near-infrared band to the order of hundreds of watts, which successfully solves the problem of the GHz pulse in industrial processing. Therefore, the development of high-power near-infrared band GHz repetition rate pulse lasers is relatively mature at present. Coupled with the continuous improvement of nonlinear frequency conversion technology, DUV laser repetition rate has entered the GHz. Although the industrialization and commercialization of DUV laser techniques still face some problems, such as easily damaged crystal coating, low wavelength conversion efficiency of DUV lasers, and long-term unstable operation of high-power DUV lasers, these problems have been gradually solved in practice. With the further maturity of frequency conversion and power amplification techniques, perhaps kilowatt-level DUV lasers will appear in the next 5-10 years, all of which will certainly make a breakthrough in the secondary laser source based on ultraviolet laser and DUV laser.