Microchip Laser Research Group
.
The Q-switched laser developed at St Andrews for the ELVIS Project. The brass tubes at the left hold the lenses that collimate and focus the pump lght. The gain crystal is in the left hand mirror mount, and the mirror is visible on the right hand mount. In between is the electro-optic deflector. The deflector, which was developed and fabricated at Leysop, is shown in the picture below.
The application of the ELVIS laser required active Q-switching to produce pulses in the nanosecond range. This leads to a demand for a short laser, and a concomitantly short and low loss Q-switch. The quadrupole electro-optic deflector was chosen for this. The quadrupole deflector works by using an electrode geometry that provides a linear variation in transverse electric field with distance up the active aperture. When the drive voltage is applied (~900 V), the refractive index gradient that is produced leads to a deflection of the laser beam that passes through the device. The deflector acts much like a prism in this regard. When the voltage is removed, the beam can pass straight through. The deflection is sufficient to prevent laser oscillation, and so the device can act as a Q-switch. There is no need for additional polarisers, and the deflection is effectively independent of temperature. This means that the Q-switch is compact, low loss, and relatively insensitive to environmental parameters.
This novel Q-switch was used in a short laser cavity, as shown schematically above. This consisted of a 1mm thick slice of 1%-doped Nd:YVO4, which was pumped with 1.2 W from a fibre-coupled diode laser. The 50% output coupler was placed close to the end of the deflector, which resulted in a cavity of length approximately 12 mm. At a repetition rate of 2 kHz, when pumped with 1.2 W, this laser produced pulses of ~1.3 ns full width half maximum and peak power 14 kW. At 20 kHz the pulses were 1.5 ns long and of 6.8 kW peak power. These infra-red pulses were then passed through a KTP crystal outside the cavity. At 20 kHz this resulted in a 42% conversion efficiency to the green, generating 1.1 ns pulses of 3.9 kW peak power. The timing jitter was better than 0.5 ns, and the amplitude jitter was better than 8%. These characteristics made this laser appropriate for the imaging applications of the ELVIS system.
This work was supported in part by the DTI through the LINK project ELVIS. The project collaborators are GEC Marconi Ltd, Elforlight Ltd, Leysop Ltd, Tritech International Ltd, and the University of St Andrews. Support was also provided through EPSRC grants GR/K14766 and GR/L27345.
For more information please contact one of
Bruce Sinclair, School of Physics and Astronomy, University of St
Andrews, St Andrews, Fife, Scotland, KY16 9SS,
email b.d.sinclair@st-andrews.ac.uk
OR
Dr Leslie Laycock, GEC Marconi Research Centre, Great Baddow,
England,
email leslie.laycock@gecm.com