Q-switched pulsed lasers

With the invention of the laser in 1960 and the subsequent development of pulsed lasers using Q-switching (Chapter 1), monochromatic and highly-collimated light sources became available with pulse durations in the nanosecond timescale. These Q-switched pulsed lasers allow the study of photo-induced processes that occur some 103 times faster than events measured by flash lamp-based flash photolysis. 

Nonexpendable light sources, such as a Q-switched pulsed laser can be protected. from destructive forces encountered in the photography of explosive material by piping the light through fiber optics, to the experimental zone. Occasionally lens systems are used to relay the light from mirrors located near a protective barrier shielding the laser (Ref 16)... 

With the photographic flash lamp the light pulse has a duration of several microseconds at best. The Q-switched pulsed laser provides pulses some thousand times faster, and the kinetic detection technique remains similar since photomultiplier tubes and oscilloscopes operate adequately on this time-scale. The situation is different with the spectrographic technique electronic delay units must be replaced by optical delay lines, a technique used mostly in picosecond spectroscopy. This is discussed in Chapter 8. 

So far, I have outlined the deployment of our initial research with respect to the applications of liquid crystal PCs. They are described in detail in references [127-130]. Starting from there, the progress of research related to liquid crystal lasers has advanced remarkably. In the early studies, it was necessary to excite samples with large Q-switched pulse lasers, but the research results led to a cmistant improvement of materials and device structures that reduced lasing threshold and improved slope efficiency [131-136]. This made it possible to excite those PCs with semiconductor lasers. Furthermore, it has even come possible to excite with a CW laser also shown in [137]. In addition, control of the lasing wavelengths by modulation of the helical pitch (by temperature and electric field) in the early days of development led to effective refractive index modulation and fast stability control [138]. In 2000 it was quite a surprise that laser actimis occur even in a system containing fluctuation such as liquid crystals, but by now, it can be said that... 

Fig. 4. Temporal pulse characteristics of lasers (a) millisecond laser pulse (b) relaxation oscillations (c) Q-switched pulse (d) mode-locked train of pulses, where Fis the distance between mirrors and i is the velocity of light for L = 37.5 cm, 2L j c = 2.5 ns (e) ultrafast (femtosecond or picosecond) pulse.

A series of papers by. Menichelli Yang (Refs 82, 84 86) showed that Q-switched ruby lasers could initiate steady detonation in PETN (and RDX or Tetryl) in <0.5 psec when a lOOOA-thick Al layer was deposited on the face of the sample, and subsequently exposed to laser radiation of 0.5 to 4.2 J with a pulse width of 25nanosec... 

Karlov etal. investigated inversion kinetics in a pulsed CO2 laser, which was Q-switched with a rotating mirror, by shifting the time delay between excitation and Q-switch pulses and measuring the laser intensity as a function of delay time and discharge conditions 377) Similar experiments were performed by Lee era/.378). 

Short energetic laser pulses, low rep rate (ex. dye, Nd YAG, Q-switched ruby lasers) Yes No Yes... 

We have undertaken an experiment to try to improve the performance of pulse amplifier experiments. The system is shown schematically in figure 2. It consisted of a continuous-wave C102 dye laser amplified in three stages by a frequency tripled Q-switched NdtYAG laser. The output energy was approximately 2.0 mJ in a 150 MHz linewidth and was up-shifted from the continuous-wave laser by 60 MHz caused by the frequency chirp. This light was then spectrally filtered in a confocal interferometer with a finesse of 40 and a free spectral range of 300 MHz. The linewidth of the filtered radiation was approximately 16 MHz. 

The photoisomerization is induced by irradiation with green light (532 nm) from either a continuous laser or pulsed laser (frequency doubled Nd.-YAG Q-switched laser operated in single pulse mode). The second harmonic is generated by a pulsed optical source such as produced by NdrYLF (1053 nm) or Nd YAG (1064 nm) Q-switched nanosecond laser. The probe... 

Q-switched Ruby Laser 694.3 nm Pulse energy 2-8 Pulse duration 20-30... 

Because THG only measures the electronic contribution to the pulse width it is not as relevant as in other type of experiments. Usually a Q-switched pulsed Nd + Yag laser with nanosecond pulses is used, at low repetition rate (10-30 Hz). The selection of the wavelength is important in THG, especially when organic materials have to be characterized since organic materials generally absorb in the UV spectral range. For this reason, the fundamental output of the Nd + Yag laser is usually shifted to a longer wavelength. 

Fig. 10.13. Random and channeling backscattering spectra of as-implanted Si (100) showing the annealing results from pulsed laser irradiation (Q-switched ruby laser 1.6 J cnT2, 75As 100 keV, 1.4 x 10ls in (100) Si, [110] aligned spectrum) (after White et al. 1979)...

For the first time range, ultrashort laser pulses generated by mode-locked lasers in the femto- to picosecond range (Chap. 6) are needed, whereas for the second class of experiments pulsed lasers in the nanosecond to microsecond range (Q-switched CO2 lasers, excimers, or dye lasers) can be employed. Most experiments performed until now have used pulsed CO2 lasers, chemical lasers, or excimer lasers. For femtosecond pulses Kerr-lens mode-locked Tiisapphire lasers with subsequent amplifier stages are available (see Sect. 6.1). 

For the first time range ultrashort laser pulses, generated by mode-locked lasers (Chap. 11), are needed, whereas for the second class of experiments in the nanosecond to microsecond range pulsed lasers (Q-switched CO2 lasers. 

Structural interconversions of dichloro-l,l,7,7-tetraethyldiethylenetriamine-nickel(n) (NiLClg) in acetonitrile have been studied by relaxation methods in which the equilibria were suddenly perturbed by an electric-field jump or by means of a pulse of radiation from a Q-switched neodymium laser. The results are interpreted in terms of the mechanism... 

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