The OPO consisted of a single urea crystal enclosed between a pair of dichroic mirrors, forming the optical cavity. The crystal was located
Chapter 5: Urea Optical Parametric Oscillator 132
inside a cell containing -hexane as index-matching fluid (IMP). The choice of this particular IMP was governed by its high UV transparency (which we measured to be k 85% @ 308nm for a 10mm fluid path length), its relative chemical inertness, and a comparable index of refraction (n ~ 1.4) to that of ui'ea. Several OPO cells were used during these experiments, as were several urea crystals. The cells were fabricated of nylon or perspex, and incorporated uncoated quartz windows (thickness = 2 mm, diameter = 1", ^ 2 minutes of arc wedge) . 0 -rings were used to seal the
structure. Nylon and perspex were favoured as the cell material, because experiments with cells manufactured of some metals (aluminium, brass)
revealed unacceptable levels of IMP contamination over relatively short periods (a few hours) of IMP enclosure within the cell. In particular, in
the case of brass, this was found to lead to a serious degradation in the UV transparency of the IMP, resulting in strong absorption of the pump light (at 308 nm) before it was incident on the urea crystal. The OPO cavity was of a simple plane-parallel design, and was formed by two flat mirrors. The mirrors were both highly reflecting at the wavelength of the signal wave to
be resonated, and had high transmittance at the idler wavelength. They were also highly transmitting at the pump wavelength and were mostly AR-coated on their rear surfaces at 308 nm. The high pump transmission of the OPO mirrors is particularly important, since any absorption of the pump light by the mirrors could lead to serious optical damage to the coatings, given the high pump power levels involved, and prevent OPO
operation. The rear surfaces of the mirrors were also wedged to avoid competing cavity effects. All mirrors were 1" in diameter, with 1/4" thickness, and were placed inside adjustable gimbals to enable cavity alignment. The OPO cavity length was maintained as short as possible to provide a maximum number of round-trips of the resonated signal wave
Chapter 5: Urea Optical Parametric Oscillator 133
parametric gain to reach oscillation threshold. The cell structures were also designed with minimum thickness to accommodate this short cavity length. An XY-translation stage was used for precise location of the urea crystal within the OPO cavity and a miniature rotation mount was used to allow angle-timing of the crystal as well as to enable accurate measure ments of the phase-match angle 0. Location along the axis of the optical
system (i.e. along z) could be adjusted by translation of the entire OPO assembly along the optical bench, A He-Ne laser beam propagated through pinholes, along the pump beam direction, was used for alignment of the OPO cavity.