Experime nts

The basic experimental set-up that was used in this work is shown schematically in fig. 5.5. Isolation between the pump laser and the OPO was provided by a Faraday rotator (O FR10-5-YAG) positioned between two calcite polarisers. Isolation was provided for two reasons. Firstly, the work of Smith [3] showed that feedback into the laser can cause instability of the laser output, and hence instability of the OPO. This is perhaps of more relevance to doubly resonant oscillators where a single frequency pump is required. It was found that isolation was required in any case to prevent damage to the pump laser due to on-axis reflections from the OPO. When operating without isolation, the laser polariser was seen to damage due to a reflection from extra-cavity optics. No damage occurred when isolation was employed. The zero-order XJ2 plate combined with the first polariser additionally acted as a variable attenuator. A zero-order wave- plate was required to provide sufficient bandwidth for operation at both 1.047 and

c/l J ; KTP OPO

î

I

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1

5

iil

t

I

i

Ch5 : KTP OPO 1.064 p.m.

With the zero-power mirrors, mode matching was achieved with a single 30 cm fused silica lens, AR coated for the pump. A fortunate coincidence was the fact that for the pump spot sizes of interest, this spherical lens produced pump focal spots which, although they were elliptical in the case of the YLF 2 laser, possessed little astigmatism. This is despite the fairly large (-10 cm) astigmatism present in the laser. All measurements of the OPO output were based on the signal wave. Although the pyroelectric detectors (Molectron J25 and J3 probes) which were used to measure the signal and pump beams were capable of measuring the idler output at 3.3 jxm, most of the idler was absorbed in the crystal, the mirror substrates and signal steering mirrors.

In addition, no detectors were available for temporal or spatial measurements of the idler.

For signal wave measurements the pump was blocked with two silicon filters, and the

signal was tapped off from the remaining beam with signal steering mirrors, Ms. These mirrors were highly reflecting (98 %) at the signal wavelength, and highly transmitting at pump and idler wavelengths. The signal energy was measured directly with the J3 probe. The measurements were then corrected for the measured signal transmission of the pump blocking filters and the collimating lens, and also for the small losses at the signal steering mirrors. The output profile of the signal was measured with a pinholed

germanium photodiode in the focal plane of the signal collimating lens. Temporal profiles were measured with a fast germanium photodiode (Germanium Power Devices GM3) which had a bandwidth > 1 GHz. The active area was only 100 }im diameter, therefore tight focusing into the photodiode had to be employed to obtain the fast time response.

The internal conversion to both signal and idler was measured via pump depletion. The pump depletion measurement was made by removing the pump filters and detecting the beam with signal removed from behind the first signal mirror. The transmission of the idler to the detector for these measurements was measured to be only ~ 2 % of the pump due to mirror substrate and crystal absorption, and was therefore not expected to affect the pump depletion measurements significantly.

The output spectrum at signal wavelengths was observed by using a Czemy-Tumer 1 m monochromator (Monospec 1000) in conjunction with an IR vidicon (Hamamatsu Beam Finder II, C3283). The wavelength of the monochromator was calibrated with the 780.02 and 794.76 nm lines of Rb in second order. To monitor the pulse to pulse spectral variation the output slits were opened up (~ 4 mm) and imaged onto the IR

vidicon with a 2.5 cm lens. The dispersion of the Czemy-Tumer monochromator can be expressed as

ChJS : KTP OPO

dx ^ nFcos{<t>l2) dX dcos{<p-e)

where x is the spatial co-ordinate at the output slit, n is the diffraction order, F is the focal length (1 m), ^ is the blaze angle (18.43° for a 600 lines/mm grating blazed for 1 pm), d is the grating spacing, and 0 is the rotation angle of the grating. This gives the dispersion at the output slit. To get the magnification of this dispersion at the vidicon, the separation of the Na D-lines was measured in first and second order. This measurement allowed the vidicon output to be calibrated in terms of the dispersion for 1.54 pm. Thus the spectrum of the OPO signal could be monitored, while accurately knowing the width of any features in the spectrum. The output from the vidicon was input to an in-house frame grabber, with the images stored in a BBC Archimedes computer. Thus the output spectrum, in terms of the time averaged spectrum throughout an entire pulse, could be examined on a pulse to pulse basis. No information, however, was obtained from this about the dynamics of any mode jumps of the signal throughout an individual pulse.

The spectral content of the pump laser was monitored by passing part of the beam through a 10 GHz étalon and monitoring the fringes with a Si CCD camera (EEV photon). This allowed for any correlation between the pump mode spectrum and the

signal spectrum to be identified.