Quasi-continuous-wave (quasi-CW) operation of a laser means that its pump source is switched on only for certain time intervals, which are short enough to reduce thermal effects significantly, but still long enough that the laser process is close to its steady state, i.e. the laser is optically in the state of continuous-wave operation. The duty cycle may be, e.g., a few percent, thus strongly reducing the heating and all the related thermal effects, such as thermal lensing and damage through overheating. Therefore, quasi-CW operation allows the operation with higher output peak powers at the expense of a lower average power.
A quasi-continuous-wave operation is most often used with diode bars and diode stacks. Such devices are sometimes even designed specifically for quasi-CW operation: their cooling arrangement is designed for a smaller heat load, and the emitters can be more closely packed in order to obtain a higher brightness and beam quality. Compared with an ordinary continuous-wave operation, additional lifetime issues can result from the quasi-CW operation, related e.g. to higher optical peak intensities or to frequent temperature changes. Some doped-insulator solid-state lasers are also operated in quasi-cow operation. Such lasers are sometimes called heat capacity lasers.
Vacuum-ultraviolet (VUV) coherent light has been proposed for different applications, as optical data storage, metrology, biomedical application, fundamental spectroscopic research, and laser lithography. Conventional coherent VUV laser sources at this wavelength are excimer lasers. These lasers generate a high output power of more than 100 W, however, their structures are huge and complex. Moreover, high manufacturing and maintenance costs are required. The low repetition rate (usually several kHz) of such excimer laser systems also restricts the applicability of many shot statistics during data acquisition in spectroscopic measurements. The development of coherent radiation with high-repetition-rate (quasi-continuous wave) or continuous wave (CW) in VUV has to be based on the frequency conversion or exploitation of new laser materials. At present, the later is a more challenging topic and the former is the unique way to generate VUV laser light with the high-repetition-rate or continuous wave. Generally, techniques for optical frequency conversion for generating short wavelength light are the second harmonic generation (SHG) and sum frequency generation (SFG) process.
