CO MPRO BA CIÓ N DE H IPÓ TESIS

The flash lamp pumped Nd:YAG laser used in these investigations is a Quantel YG 481 laser t It was used only as a simulator pump source at 1.064 pm before the diode pumped Nd:YAG laser was constructed. By theoretical calculations we estimated that the output energy from the diode pumped Nd:YAG laser would be around 10 mJ, so initially the output energy of the flash-lamp pumped Nd:YAG laser was limited to this level.

The cavity configuration of the Quantel YG 481 laser is shown in figure (4-1), and is a plano-concave geometry. The radius of curvature of the curved mirror is 3 m and both mirrors were coated for high reflection at 1.064 pm and were physically separated by around 1.2 m.

3 4 8 9 10

—c J—

He-Ne Laser

11 12 13

Fig. 4-1 Schematic diagram of flash-lamp pumped Quantel YG481 Nd:YAG laser. (1) HR Mirror, (2) KD*P EO-Q-switch, (3) Quarter-

wave plate, (4) Aperture, (5) Glan-Thomson polarizer, (6) Laser head, (7) Aperture, (8) Aperture, (9) Quarter-wave plate, (10) HR Mirror, (11) HR prism, (12) Half wave plate, (13) Glass plate with Brewster

angle.

t Please see Synopsis book of Quantel Model YG 481 laser, March, 1981

The oscillator cavity contains a laser head, a KD*P Pockels cell, two quarter-wave

plates, a Glan-Thomson type polarizer, and two apertures to limit high order transverse mode oscillation.

The laser head contains one Nd:YAG rod which is doped to about 1% with

neodymium ions, has a diameter of 9 mm, a length of 115 mm and its two optical faces ( cut to a 2° angle from the transverse plane) are anti-reflection coated for 1.064 pm wavelength. The laser head also contains two high pressure Krypton flash-lamps for pumping. The length of the discharge gap in there is 101.6 mm (4 inch) and the tube

inner diameter is 5 mm. The pump reflector cavity of the laser head is a diffusing reflection glass cavity. The laser head is cooled by de-ionized water with closed-circuit flow.

The Pockels cell is fitted with a KD*P crystal, the optic axis of which is parallel to the light propagation direction as well as to the longitudinal electric field. The optical windows closing the cell are externally anti-reflection coated. There is a liquid for index matching between the faces of the crystal and the internal faces of the windows.

The Glan-Thomson type polarizer was anti-reflection coated at 1.064 pm on all of its optical faces.

The combination of the Pockels cell with the polarizer, a quarter wave plate and the curved cavity mirror formed a switch unit. As long as there is no voltage across the Pockels cell the laser cannot oscillate. The laser cavity is closed by turning the quarter

wave plate placed between the Pockels cell and the polarizer so as to form in effect a half wave plate. As soon as the appropriate high voltage (a quarter-wave voltage at 3.3 KV) was applied to the Pockels cell, the laser oscillated and the power extracted from the cavity was optimised by rotating another quarter wave plate placed on the other side of the polarizer.

The output characteristics of this flash-lamp pumped NdrYAG laser are listed table (4-1).

The output beam profile was measured using a pin-hole-scanning method. The results clearly showed that the output laser beam was not exactly a Gaussian distribution and hence was not a TEMqomode. This is detrimental to the frequency

CHAPTER 4 Pump sources

measured to be 12 ns (FWHM), when the maximum output peak power was IMW (12mJ, 12 ns).

Table 4-1 The output characteristics of the flash-lamp pumped NdrYAG laser.

Wavelength 1.064 pm

Output energy 12 mJ

Pulse width 12 ns

Energy stability ±3.5 %

Repetitions 1,5, 10 Hz

Mode structure Lower order mode ((

Beam divergence >0.7 mrad

Line width >0.7 cm'^