An overview of some of the materials suitable for constructing an OPO pumped at 1.064 |im was presented in section 3 of chapter 3. In this appendix a more detailed discussion
of the materials is presented. No further discussion of KTP is presented here as it has already been presented in detail in chapter 3.
A,2 Lithium Niobate (LiNbOs) and MgOiLiNbOa
LiNbOs is a negative uniaxial crystal of point group 3m [2.1]. It possesses a large non- linearity which is often characterised by the coefficient d i5 (= dgi) = 5.95 pm/V [2.2], though the known variation of the non-linearity with composition may explain a recent measurement for 5% MgOtLiNbOs of d3i = 4.7 pm/V [2.3].
It also possesses a relatively high surface damage threshold, given by one manufacturer
as 350 MWcm"2 for 25 ns pulses at 1.064 jim (no rep rate) [2.4]. An intensive study of damage mechanisms by Brosnan and Byer [2.5] found a surface damage fluence of 2.7 J cm"2 for pulses at 1.06 fim in the 10-30 ns range for a bare surface, which increases to 11 J cm"^ when the spot size is reduced below the mean defect spacing of 60 -70 jim. The damage threshold was also seen to increase when the crystal was in an O2 atmosphere or
AR coated with Si02. The damage fluence pulsewidth scaling was found to be (t:)0-5 for pulses short enough to not completely heat the surface, and t for longer pulses.
However, early samples were found to suffer photorefractive damage at much lower intensities, < 50 MWcm'^ [2.6]. The damage is thought to arise due to the presence of impurities in the crystal (possibly Fe^+ [2.7]). As the impurity level is already low, improvement of this would likely add substantially to the cost of fabrication. The most successful improvement technique to date has been that of doping the congruent melt with approx. 5 % MgO which has increased the photorefractive damage level by a factor of a hundred due to an increase in the photoconductivity [2.8]. Another technique involved
growth of crystals with an adjusted Li/Nb ratio which raised the SHG temperature for 1.064 jim from 4 to 238 °C, which, being above the annealing temperature, meant that any damage was annealed out [2.9].
LiN bO ] possesses a large negative birefringence which varies from « 0.1 to 0.06 throughout its transparency range of 0.4 to 5 jim [2.1], with a refractive index of ~ 2.2 at
Appendix A : Non-linear Optical Materials
1.06 |im. The large birefringence facilitates a large tuning range with degeneracy for 1.06 pumping at » 44 0, but also results in a relatively large walk-off 36 mrad. The tuning curve and variation of walk-off and deff calculated from the Sellmeier eqns of Hobden and Warner [2.10] are shown in figs. a.l (a) and (b). This critical phase match geometry results in the angular and spectral acceptances for a signal and idler wavelength pair of
5 4 t 3 2 44 46 48 50 52 9 /d eg
fig.a.l (a) tuning curve for e-oo phase matching in the ac-plane at 22®C pumped at 1.064 pm (from
Sellmeier eqns of [2.8]) 36.0 6.0 35. 35.6 "Q 5.8 35.4 > 35.2 a, pump_walkoff deff 5.6 35.0 34.: 5.4 34.6 2.0 signal wavelength / pm
fig. a.l (b) (b) walk-off and deff curves for e-oo phase matching in the ac-plane at 22®C pumped at 1.064 pm (from Sellmeier eqns of [2.8])
1.6 and 3.2 pm being ~ 1.4 mrad cm in, and 9.8 mrad cm^-^orthogonal to, the tuning plane and 1.5 nm respectively. Although the calculated temperature for a 1.06 pm
Appendix A : Non-linear Optical Materials
has been demonstrated in MgOiLiNbOg at 107 ®C [2.11]. The calculated temperature acceptance for a 1.6 and 3.2 pm pair is 2.7 ®C. Note that the acceptance parameters quoted here and for all other materials are FSVHM, and are as defined in chapter 2.
After much investigation into its growth properties, LiNbOg can be grown with high
optical quality and large dimensions. High optical quality crystals have been grown as large as 50 mm long by 15 mm diameter [2.12], and the loss of high quality MgOrLiNbOs at 1.064 pm has been measured as less than 0.003 cm"l. These properties along with those already mentioned have ensured its success as a non-ünear material. Parametric oscillation has been demonstrated in LiNbOg with both pulsed [2.14,15] and cw [2.16] pump lasers, and with both angle [2.15] and temperature tuning [2.14,16]. Although temperature tuning is normally employed with 532 nm pumping for NCPM, LiN bO ] has also been temperature tuned when pumped at 1.064 pm [2.17]. The improved photorefractive properties of MgOiLiNbO] have been utilised as an excellent material for resonant doubling of 1 pm light [2.18], and for cw [2.11] and Q-switched, mode-locked [2.19] operation of OPOs.
LiNbOg has also been used in fibre and waveguide geometries. Periodic domain reversal in waveguides facilitates quasi-phase matching which allows use of the large dgg coefficient [2.20]. The photorefractive effect in LiNbOg has actually been productively utilised in an OPO by creating a dynamic diffraction grating in the material for linewidth control [2.21].