Output w avelength

The output signal wavelength for the NCPM geometry used here, which is shown

schematically in fig. 5.19, with a pump wavelength of 1.047 |4,m was measured to be

1.539 pm. The corresponding idler wavelength, from energy conservation, is therefore 3.276 pm. The accuracy of this measurement is estimated to be ± 0.5 nm due to the calibration of the monochromator with a Rb lamp. These are exactly the wavelengths predicted by the Sellmeier equations of Vanherzeele et al [11], validating the choice of this set of Sellmeier equations in chapter 3. It was also found that the signal wavelength

for NCPM when pumping with 1.064 pm from the Nd:YAG laser was 1.571 pm, again in agreement with these Sellmeier equations. The NCPM geometry results in a more efficient way of generating nanosecond pulses at the eye-safe wavelength of 1.54 pm than has been achieved with the Er:glass laser. High energy pulses of 1 J have been achieved for flashlamp pumping of the Er lasers, but at an efficiency of less than 0.5 %

[12]. Although the efficiency for diode pumping is improved by using Yb as a sensitiser, the low gain obtained results in a poor Q-switched performance (0.3 mJ for 240 mJ pump [13]). b A Ep pump A Es signal idler RiX

fig. 5.19 Schematic diagram of NCPM geometry and polarisations

Angle tuning brought about by tilting the crystal was found to be small, approximately 1 nm deviation from the value for propagation along the a-axis. A similar level of tuning could be obtained by tilting the cavity while keeping the crystal and beam alignment fixed. This corresponds to the limits of non-collinear phase matching.

The large birefringence of KTP accompanied by the NCPM operation both result in a narrow (for OPOs) bandwidth. The bandwidth is determined by restrictions on the maximum value of Ak which the system will tolerate. The expression for the signal bandwidth for the type II o-oe geometry can be obtained from the more general equation presented in chapter 2. By assuming, for calculation purposes, single

frequency pump and idler, the expression for the signal bandwidth is

-1

Ch.5 : KTP OPO

For the case considered here this is AXg^l.l nm (FWHM). The finite linewidth of the pump, ~ few GHz, will widen this slightly but not noticeably so. This pump linewidth is well within the calculated pump spectral acceptance of ~ 0.28 nm, which is calculated in a similar fashion to the signal bandwidth above by assuming single frequency signal and idler. As mentioned above, the tuning corresponded effectively to tuning within this signal bandwidth. More will be said about this in the following section which discusses the detailed spectral properties of the output.

It is thought that the main reason that larger tuning was not achieved is probably due to the cavity misalignment when the crystal is rotated. Readjustment of the mirrors was unable to compensate for this, mainly due to the curvature of the mirrors which is itself responsible for the attainment of low thresholds. Although the walk-off angle increases dramatically as the crystal is rotated away from the a-axis, the threshold modelling presented in chapter 2 showed that for the values of angle and walk-off concerned the increase in threshold was not sufficient to explain why operation could not be obtained away from NCPM.

This points towards potential coarse tunability from this device, while maintaining low thresholds, by using a tunable pump laser, with fine tuning within the gain bandwidth controlled by cavity adjustments. This scheme has already been demonstrated by Kato and Masutani [11] who achieved tuning over the ranges 1.04-1.38 and 2.15-3.09 [im when pumping with several dye lasers. Tuning could be achieved over a much larger range, 1-3.3 |im, with an all solid state device if the pump source was a Tiisapphire laser. Tiisapphire lasers can produce nanosecond pulses in all-solid-state configuration

3.0 2.5 o_wave e_wave 2.0 1.5 1.0 0.7 0.8 0.9 1.0 1.1 pump wavelength / pm

Ch.5 : KTP OPO

when pumped by a doubled diode-pumped Nd:YAG or YLF laser [14]. Fig. 5.20 shows the potential tuning that could be obtained with this scheme.