Intracavity terahertz optical parametric oscillators
Abstract
This thesis describes the design and implementation of several novel, nanosecond
pulsed, intracavity optical parametric oscillators for the generation of terahertz
radiation. The application of the intracavity approach in the context of terahertz
optical parametric oscillators has been demonstrated for the first time, and the pump
wave energy required was thereby reduced by an order of magnitude. The terahertz
wave 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 2.3
times the diffraction limited spot size (M² values of 2.3 and 6.7 in the components of
the beam parallel and perpendicular to the silicon prism array output coupler
respectively).
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 a
promising technique that brings the terahertz linewidth down below 1GHz (close to
the transform limit of nanosecond pulses) while retaining the tuning range and beam
characteristics of the free running system. Close to Fourier transform limited pulses
were obtained (<100MHz linewidth) via an injection seeding technique, although with
significantly increased system complexity. A deleterious effect caused by the mode
beating of a multimode host laser was also discovered, in that sidebands were induced
on the seeded downconverted wave. This has wider implications in the field of
intracavity OPOs.
Finally, quasi-phasematching techniques implementing periodically poled lithium
niobate were investigated as a way to lower the downconversion threshold energy
requirement (by collinear propagation of the optical waves), and also to extract the
terahertz wave rapidly from the (highly absorbing in the terahertz region) lithium
niobate crystal. The existence of two phasematching solutions arising from the
bidirectionality of the grating vector was identified as a serious design constraint in
the context of an OPO where either solution can build up from noise photons, and so
prefers the solution with the lowest walkoff of the downconverted waves - possibly
resulting in unextractable terahertz radiation. Quasi-phasematching with an
orthogonal 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 and
may allow efficiency improvements through overcoming the quantum defect limit.
This research has resulted in four peer reviewed papers in respected journals, and the
intracavity terahertz OPO has been licensed to a company who have commercialised
the technology (M Squared Lasers, Glasgow).
Type
Thesis, PhD Doctor of Philosophy
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