This thesis describes the design and implementation of several novel, nanosecondpulsed, intracavity optical parametric oscillators for the generation of terahertzradiation. The application of the intracavity approach in the context of terahertzoptical parametric oscillators has been demonstrated for the first time, and the pumpwave energy required was thereby reduced by an order of magnitude. The terahertzwave was tunable from under 1THz up to 3THz with a free running linewidth of~50GHz and pulse energies up to ~20nJ (pulses were a few nanoseconds in duration).The terahertz beam profile was of Gaussian shape and could be focussed down to 2.3times the diffraction limited spot size (M² values of 2.3 and 6.7 in the components ofthe beam parallel and perpendicular to the silicon prism array output couplerrespectively).Developments of this intracavity source with regard to the linewidth are also reported.Implementation of etalons in the optical (laser and OPO) cavities was shown to be apromising technique that brings the terahertz linewidth down below 1GHz (close tothe transform limit of nanosecond pulses) while retaining the tuning range and beamcharacteristics of the free running system. Close to Fourier transform limited pulseswere obtained (<100MHz linewidth) via an injection seeding technique, although withsignificantly increased system complexity. A deleterious effect caused by the modebeating of a multimode host laser was also discovered, in that sidebands were inducedon the seeded downconverted wave. This has wider implications in the field ofintracavity OPOs.Finally, quasi-phasematching techniques implementing periodically poled lithiumniobate were investigated as a way to lower the downconversion threshold energyrequirement (by collinear propagation of the optical waves), and also to extract theterahertz wave rapidly from the (highly absorbing in the terahertz region) lithiumniobate crystal. The existence of two phasematching solutions arising from thebidirectionality of the grating vector was identified as a serious design constraint inthe context of an OPO where either solution can build up from noise photons, and soprefers the solution with the lowest walkoff of the downconverted waves - possiblyresulting in unextractable terahertz radiation. Quasi-phasematching with anorthogonal grating vector (with identical but opposite phasematching solutions) was demonstrated and cascaded downconversion processes observed and characterised.These cascaded processes are permitted by the collinearality of the optical waves andmay allow efficiency improvements through overcoming the quantum defect limit.This research has resulted in four peer reviewed papers in respected journals, and theintracavity terahertz OPO has been licensed to a company who have commercialisedthe technology (M Squared Lasers, Glasgow).
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