The type II non-critical phase match condition in LBO corresponds to the case where 9 = 0, (j) = 0. The phase matching geometry is illustrated in figure (5-2), where crystal dimensions as used in this experiment are 3 X 3 X 16 mm^ in the x, y and z
directions respectively. The two optical faces were not coated. In this type II scheme, the pump radiation at 355 nm or 266 nm is linearly polarised along the x axis of the crystal, and the signal and idler wave generated are linearly polarised along the y-axis
and the x-axis respectively. We have carried out phase match calculations for this geometry using the recently revised Sellmeier equations due to Chen[7], and the
predicted tuning curves for the signal and idler wave are presented in Appendix III, where angle tuning by rotation about both the x-axis and the y-axis are displayed. As described in the Chapter 3, LBO also exhibits temperature tuning, so it is possible to tune the signal and idler wavelength by this means while still maintaining non-critical phase matching.
355nm
i
1350nm480.6nm
Fig. 5-2 The type II non-critical phase matching geometry in LBO for optical parametric oscillator.
It is well known that non-critical phase matching geometries have some advantages compared to the critically phase matched situations. Firstly, the Poynting vector walkoff is eliminated in this geometry. Secondly, this geometry has a large angular acceptance band which tends to infinity. Thirdly, the effective nonlinear coefficients reaches its maximum values (deff = 0.89 pm/V). To sum up the advantages above it is clear that this non-critical geometry allows tight focusing of the pump light in the
nonlinear crystal, so enabling high energy densities to be obtained at modest pulse energies. In this way it is possible to operate an optical parametric oscillator based on this material at many times threshold, and hence obtain high conversion efficiencies, with pulse energies of the order of 1 mJ.
5.3 Cavity
The OPO cavities used in this experiment were of two types; the simple standing wave cavity formed by plane mirrors and the mode matching cavity formed by curved mirrors. The former one has been used in the investigation of general performance
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CHAPTER 5 NCPM LBO OPO pumped at 355 nm and 266 nm
characteristics such as OPO tuning range, linewidth, and output characteristics. The
latter one has been used to investigate the optimum geometry of the OPO cavity when related to the optimum focussing of the pump wave. The Boyd and Kleinman theory
has been used through out the analysis[8] (Please see Chapter 2). 5.3.1 Cavity with plane-parallel geometry
The cavity of the optical parametric oscillator in this normal scheme is a simple standing wave cavity formed by two mirrors with reflectivity 95 % at 480 nm (the total output coupling is then 10 % per round trip) which are generally spaced by 18 mm, just slightly longer than the crystal for minimising the pulse build-up time effects. The
mirrors are highly transmitting at the pump wavelength of 355 nm, and have only a low residual reflectivity at the idler wavelength of 1.35 p,m. The optical parametric oscillator
hence operates in singly resonant mode for the signal wave. The radiation generated at 355 nm by the frequency tripling configuration described previously was used to pump the optical parametric oscillator directly without any ancillary focusing optic being introduced. The radius of the 355 nm pump beam at the LBO crystal was measured to be 350 |im in the flash-lamp pumped system, and 280 jim in the diode pumped system.
5.3.2 Cavity with tightly focused pump
As we have described in Chapter 2, for given conditions, including the crystal length, and focused pump spot size, we can calculate the optimum signal spot size (singly resonated OPO) using the theory of Guha et al[9], and from this parameter we can identify the optimum radii of curvature of the cavity mirrors. Also we know from theoretical considerations that for non-critical geometries (B=0), optimum focusing conditions are the same in type I and type II geometries. Therefore, the previously calculated results (which were for a type I geometry) can be used in this analysis.
The curved mirrors used in the experiments were coated the same as the plane mirrors, and among them the smallest radius of curvature of the mirror was 100 mm. The cavity length was generally set up as short as possible to avoid the difficulty of
aligning a close to confocal cavity and in order to minimise the pulse build-up loss[10]. The relation between the radius of curvature of the cavity mirrors and the oscillation threshold are discussed in section (5.5).