Laser Q-switching

PMMA finds ordn usage in several areas in ballistic or impact shields for missiles or airplanes also as windows, windshields or canopies in aircraft (Refs 7 and 22) as a Laser Q switch host using an organic Ni complex dye (Ref 22) and in proplnts as fuel (with A1 and NG as cofuels — Ref 20) and Amm perchlorate or K perchlorate as oxidizers (Refs 2, 4, 8—11, 13,14 16—20). Also see under Aeroplex Propellants in Vol 1, A108-R and under Composite Propellants in Vol 3, C464-L to C474-L Refs 1) Beil 2, [398] and (1279 1283 ... 

NIR region (see Chapter 9.13), complexes of 1,2-dithiolene (DT) and related ligands have attracted considerable attention for their (largely cubic) NLO properties. The complex (156) (a.k.a. BDN) is a highly photochemically stable, saturable absorber and has hence found extensive applications in laser Q-switching. The cubic NLO properties of (156) have been studied by DFWM148,403-407 and more recently, Z-scan.408 Time-resolved DFWM has been applied to square planar Co, Ni, Cu, or Pt complexes of 1,2-benzenedithiolate (BDT) or 1,2-aminobenzenethiolate ligands by Lindle and co-workers.409,410... 

Fig. 7. Experimental arrangement of a giant-pulse laser (Q-switching by dye solution). AM, active material (e.g. ruby crystal rod), F, flashlamp, Mj, 2, resonator mirrors, DC, dye cell...

As early as 1982, a diode laser-pumped miniature NdtYAG laser with a linewidlh of less lhan 10 kHz. was demonstrated. The research in this area continued apace at Stanford University and by a numher of commercial electronics limis. w ith emphasis placed on ihe development of three-level lasers. Q-switched and mode-locked operation, single-frequency operation (monolithic nonplanar ring oscillator), visible radiation by harmonic generation, and array-pumped solid-slate lasers. See Fig. ft. 

Two classes of material will be described here - the metal dithiolenes and rare earth metallocenes. In the metal dithiolenes a strong, low energy pi-pi transistion occurs in the near IR (9.10). This can be tuned from about 700 nm to 1400 nm by altering the metal ion, substituents or charge state of the dithiolene. The dithiolenes are particularly attractive because of their optical stability which has been exploited in their use as laser Q-switch materials. In the rare earth complexes the near IR band is provided by/-/transistions of the rare earth ion rather than the cyclopentadienyl ring structure various nonlinear optical phenomena have been observed in glasses incorporating similar ions. Previous studies have shown that dicyclopentadienyl complexes such as ferrocene have off-resonant nonlinearities similar to nitrobenzene or carbon disulphide (11-13)... 

Lentigines and keratoses should be treated by another method in parallel with AHA peels. There are other, more efficient, techniques to treat these problems liquid nitrogen, dry ice, Only Touch peel, intense pulsed light (IPL), laser, etc. Dr Robert Vergereau (France) compared the use of dry ice, Erbium laser, Q-switched laser, coagulation and Only Touch peel. He concluded that If all these methods are satisfactory, in my opinion, the technique using trichloroacetic acid is the most beneficial . 

O. Kafri, S. Speiser, S. Kimel, Doppler effect mechanism for laser Q-switching with a rotating mirror. IEEE J. Quantum Electron. 7, 122 (1971)... 

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. 

So far, we have considered only experiments with continuous-wave lasers under steady state conditions. With time-resolved experiments, on the other hand, energy transfer rates and transition probabilities can be obtained. Such measurements were carried out by mechanically chopping the laser beam directed into an external absorption cell together with the microwave radiation. Later, Levy et at reported time-resolved infrared-microwave experiments with an N2O laser Q-switched with a rotating mirror to produce pulses less than 1 /tsec in duration. They observed a transient nutation of the inversion levels of the molecule following the infrared laser pulse. Based on the Bloch equations, the observed phenomena could be explained quantitatively. From the decay envelope of the oscillations a value for the transverse relaxation time T2 was determined. Similar effects were produced by rapidly switching a Stark field which brings the molecules into resonance with the cw microwave radiation. 

A typical schematic diagram of the measuring system for TOP-NIRS is illustrated in Pigure 4.2.1. The system consists of an exciter laser (Q-switched Nd YAG laser), an... 

Figure 7 Hyper-Raman spectra of CeHeexcited with a Nd YAG laser (Aq = 1.064 nm) Q-switched at 1 kHz (A) and of CgDe in the lower spectrum with the laser Q-switched at 6 kHz (B). Reproduced by permission of Elsevier Science from Acker WP, Leach DH and Chang RK (1989) Stokes and anti-Stokes hyper Raman scattering from benzene, deuterated benzene, and carbon tetrachloride. Chemical Physics Letters 155 491-495.

In order to achieve a reasonable signal strength from the nonlinear response of approximately one atomic monolayer at an interface, a laser source with high peak power is generally required. Conuuon sources include Q-switched ( 10 ns pulsewidth) and mode-locked ( 100 ps) Nd YAG lasers, and mode-locked ( 10 fs-1 ps) Ti sapphire lasers. Broadly tunable sources have traditionally been based on dye lasers. More recently, optical parametric oscillator/amplifier (OPO/OPA) systems are coming into widespread use for tunable sources of both visible and infrared radiation. 

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.

Metal-free, chloroaluminum phthalocyanine [14154-42-8] vanadyl phthalocyanine [13930-88-6], or magnesium phthalocyanines are sufficiently soluble in organic solvents and show enough bleachable absorption at 694.3 nm to serve as repeated Q-switching elements for mby lasers (qv) (180). Phthalocyanines have been used in other lasers as weU (181). 

A Q-switched, frequency-quadrupled Nd—YAG laser (X, = 266 nm) and its accompanying optical components produce and focus the laser pulse onto the sample surface. The typical laser spot size in this instrument is approximately 2 pm. A He-Ne pilot laser, coaxial with the UV laser, enables the desired area to be located. A calibrated photodiode for the measurement of laser energy levels is also present... 

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... 

A Q-switched Nd YAG laser (7 ns pulse duration, Quanta-Ray DRC-1A) operated at 10 Hz was used as a light source. The 1064 nm fundamental was frequency doubled to 532 nm for some experiments. In all experiments reported here a geometry was used which focused the laser beam in front of the entrance window of the sample cell such that the laser beam was diverging as it passed through the sample cell. In this geometry the laser beam was about 3 mm in diameter at the region viewed by the light detection system. 

Figure 19. A set of 6 frequency-doubled, Q-switched Nd YAG lasers are used to pump the DM0, preamplifier and power amplifier.

The 6 Nd YAG lasers pump the DM0, preamplifier and power amplifier (Fig. 19, Friedman et al., 1998). The YAG lasers are built from commercially available flashlamp/laser rod assemblies, acousto-optic Q-switches and frequency doubling crystals (LBO and KTP). Most of the mirror mounts and crystal holders are commercial. Nd YAGs are frequency doubled to 532 nm using a nonlinear crystal. The Nd YAG rod and nonlinear crystal are both in the pump laser cavity to provide efficient frequency conversion. The 532 nm light is coupled out through a dichroic and fed to multimode fibers which transport the light to the DM0 and amplifier dye cells. 

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