Passively Q-switched nanosecond pulse-train Nd:YAG laser system

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Fig. 1. Experimental setup ofthe pulse-train laser system: Ml, M2, resonator mirrors (spherical and flat, respectively); LH, Nd:YAG
laser head; DP, dielectric polarizers; SA, saturable absorber (AR coated Cr4:YAG crystal); A, aperture; T, telescope; AMP,
Nd:YAG amplifier; 2/4, retardation plates; HR, high-reflective mirrors; L, AR coated bi-convex lens; SBS, stimulated
Brillouin scattering cell.
and a plane output mirror M2 was passively Q-switched by a Cr4:YAG saturable absorber with an initial transmission of
25%. After oscillator a Galilean telescope was used. The telescope has two functions: changes the beam diameter by a
factor M=3 and allows for control ofthe laser beam divergence. Laser head with 5 mm x 3.5" Nd:YAG laser rod pumped
by a linear Xe flashlamp was used as a double-pass amplifier. A polarization switch consisting of 2/4 retardation plate
and dielectric polarizer was used to extract the laser pulse after second pass through the amplifier. After amplification a
laser pulses were directed to the SBS compressor through 2/2 retardation plate and polarization switch. The beam was
focused with a lens of 70 cm focal length into SBS cell. The SBS cell was a glass tube of 150 cm length containing
perfluorohexane (C6F14) or carbon tetrachioride (Cd4) as reference medium. Laser power supplies with high-current
insulated gate bipolar transistors were used for direct control of the flashlamp current in a microsecond time scale.
Rectangular pumping pulses with variable pulse width up to 2 ms were used. The laser system was operated with
repetition rate 1-10 Hz.
The laser pulse energy diagnostics were incorporated into the laser system using uncoated BK-7 beam splitters
and calibrated dual channel energy meter (Laser Precision RJ-7620) with the pyroelectric detectors (RJP-735). The
temporal profiles of the laser pulses were recorded using a large area vacuum photodiodes (ITL TF 1 850) coupled to the
digital oscilloscope (Tektronix TDS 620). The time integrated spatial profiles of the laser pulses were imaged onto a
CCD camera (Pulnix TM-745) and recorded using laser beam analizer (Spiricon LBA 100-A)
3. RESULTS
Temporally smooth, nearly Gaussian-shaped Q-switched pulses were achieved owing to the longitudinal-mode selection
properties of the saturable absorber. Typical temporal shape of the laser pulse is presented in Fig. 2. Very stable 13 ns
laser pulses were obtained using short cavity configuration and moderate pumping level to avoid depolarization and
thermal effects. The oscillator was operated at the fundamental transversal mode and near field spatial distribution of the
laser beam is presented in Fig. 3. The output laser beam was intracavity polarized by a dielectric polarizer for proper
operation of the polarization switches used to extract laser pulses at the amplifier and SBS pulse compressor systems. A
telescope of magnification M=3 was set up for normal adjustment at 632.8 nm with an autocollimator. A correction was
then made for 1064 nm to minimize of a laser beam divergence for selected pump and repetition rate using laser beam
analizer. The magnification of the telescope was chosen to match the beam profile to the double pass amplifier aperture
and avoid diffraction effects.
Proc. of SPIE Vol. 5707 349