The development of all-solid-state, diode-laser pumped neodymium (Nd) lasers and optical parametric oscillators (OPOs) is described, which realise practical sources of coherent radiation with a high degree of frequency agility, are efficient, reliable and potentially compact. A comparison of various neodymium doped host materials reveals yttrium lithium fluoride (YLF) to be an appropriate replacement for the more widely known host yttrium aluminium garnet (YAG) in diode-laser pumped devices. The development of an end-pumped Nd:YLF laser that utilises a 12-mJ, 60W, quasi-CW diode-laser bar is initially described. Multilongitudinal-mode, TEM00 pulse energies of greater than 2 mJ have been observed, with corresponding peak output powers in excess of 118 kW. The incorporation of a novel pre-lase Q-switching technique has realised single-longitudinal-mode peak powers in excess of 90 kW continuing to be achieved. Further, the development of a more powerful end- pumped Nd:YLF laser utilising 2, 3-bar diode-laser arrays, each providing 72-mJ of pump energy is described. In this case, Q-switched, multilongitudinal-mode, TEM00 pulse energies of greater than 11 mJ are reported, with the clear potential for increasing this to greater than 20 mJ, based on measured fixed-Q pulse energies of greater than 30 mJ. Complementing the development of these diode-laser pumped solid-state lasers is the development of optical parametric oscillators based on the nonlinear materials lithium triborate (LBO) and beta-barium borate (?-BBO). Pumped by the frequency up-converted (third harmonic) output of the mid laser, such optical parametric oscillators introduce extensive frequency agility spanning a spectral range from the deep blue (0.4 mum) to the mid-infrared (2.5 mum). Initially, the development of an LBO based device is reported, which in a type I critical phase- match (CPM) geometry has a measured oscillation threshold of < 0.3 mJ, when pumped by the frequency tripled output of the 144-mJ diode-laser pumped Nd:YLF laser at 0.349 mum. Observed pump depletions are as high as 35%. A similar CPM geometry is reported in beta-BBO, in this case pumped by the frequency tripled and amplified output of a diode-laser pumped Nd:YAG laser at 0.355 mum. This is a more energetic device with thresholds of >5 mJ, but through the introduction of interferometric, dispersive and injection seeding techniques made to operate on a single axial mode. Near transform limited linewidths are reported in devices which continue to have modest pump thresholds and broad tunability. The parametric generation of broad spectral bandwidths (polychromatic) by the use of suitable phase-matching geometries is also reported. Greater than 100 nm simultaneous bandwidth in the visible spectrum is generated in a collimated signal-wave from a novel, noncollinear phase-matching geometry in a beta-BBO optical parametric oscillator, which is pumped by the collimated output of frequency tripled diode-laser pumped Nd:YAG laser. The device is demonstrated to be efficient, having a similar pump threshold and efficiency to that of the well known collinear phase-matched tunable device, and to continue to encompass a degree of tunability allowing the large simultaneous bandwidth to be tuned across the entire visible spectrum. Dispersive cavity tuning of the optical parametric oscillator by the use of a Littrow-mounted grating or acousto-optic tuning filter, with a static crystal and pump configuration, is also described.

The work presented in this thesis describes the design, configuration and operation of femtosecond optical parametric oscillators based on the materials KTiOPO4 (KTP) and RbTiOAsO4 (RTA) and pumped by a self- modelocked Ti:sapphire laser. The alignment of the pump laser is detailed and thermal effects in the Ti:sapphire rod are examined in the context of a general technique which optimises modelocked performance at any pump power. A KTP-based femtosecond parametric oscillator is described which produces 400-fs-duration signal pulses at an average output power of 150 mW when operated in the absence of group-velocity dispersion- compensation. With intracavity dispersion-compensation, the oscillator produces 40-fs-duration pulses with an average power of 50 mW. Tuning is demonstrated from 1.12 - 1.25 mum in the signal wave and from 2.5 - 3.0 mum in the idler wave by changing only the pump-laser wavelength. Using a novel idler-feedback arrangement, reductions in the oscillation threshold and increases in the signal output power of 10 % are described. Soliton generation in the oscillator is achieved when the net cavity dispersion is positive and results show good agreement with theory. An oscillator using RTA is demonstrated which achieves conversion efficiencies exceeding 30 % and has an operating threshold of only 50 mW. Average signal powers of 100 mW and 185 mW are extracted from the oscillators with and without dispersion-compensation respectively. The corresponding pulse durations are 67 fs and 980 fs and tunability in the signal and idler waves from 1.23 - 1.34 mum and 2.10 - 2.43 mum is demonstrated. Visible output from 620 - 660 nm is obtained by intracavity- doubling and powers of up to 170 mW are measured. These results suggest that RTA has a higher nonlinear coefficient than KTP.

The research presented in this thesis is concerned with the generation and characterisation of femtosecond pulses in the near and mid-infrared spectral regions. The three optical parametric oscillators which were constructed were synchronously- pumped by a self-modelocked femtosecond Ti:sapphire laser. Noncollinear critical birefringent phasematching was used in an oscillator based on KTiOAsO4, which was tunable from 1.03 to 1.2 mum and 2.51 to 4.1 mum by varying the crystal angle. The mid-infrared pulses were sub-100 fs, and essentially free from frequency chirp. With appropriate dispersion compensation the near-infrared signal pulses were temporally compressed to 69 fs. Theoretical models of noncollinear phasematching were derived and the results agreed closely with experiment. A novel optical parametric oscillator design based on a semi-monolithic noncritically phasematched RbTiOAsO4 crystal was implemented. This unique cavity configuration allowed independent focussing of the pump and signal beams within the crystal. It facilitated a reduction in cavity length to bring the signal pulse repetition rate into synchronism with the second (172 MHz) and fourth (344 MHz) harmonics of the pump pulse repetition frequency. Extraction efficiencies as high as 55% were observed. Quasi-phasematched femtosecond optical parametric oscillation was demonstrated in periodically poled lithium niobate. This device offered extensive tunability, covering 0.975 to 1.54 mum in the signal branch and 1.67 to 4.55 mum in the idler branch, from a combination of grating, pump wavelength and cavity length tuning. A theoretical model indicated that a very broad gain bandwidth allowed the wide tuning range. An attractively low oscillation threshold of 45 mW was recorded, and a visible output of 70 mW at 540 nm was observed, caused by simultaneously phasematched frequency-doubling of the signal output. The pulses from the Ti:sapphire laser and from the optical parametric oscillators were characterised by autocorrelation and frequency-resolved optical gating techniques. A highly advantageous autocorrelator arrangement based on quadratic nonlinearity in light-emitting diodes and photodiodes was demonstrated, and a novel second harmonic generation frequency-resolved optical gating system allowed real-time monitoring of pulsed outputs and complete characterisation of the intensity and phase of pulses.

This thesis describes the design, configuration and operation of picosecond optical parametric oscillators (OPOs) tunable from the visible to mid infrared. These systems were based on the materials LiB3O5 (LBO) and KTiOAsO4 (KTA), and were pumped by a self-mode-locked Ti:sapphire laser at a repetition rate of 81 MHz. The initial design of the picosecond parametric oscillator was based on a 16 mm long crystal of LBO. This system produces transform-limited signal pulses with durations of -720 fs. Total average output powers of up to 90 mW over a signal (idler) tuning range of 1.374-1.530 mum (1.676-1.828 mum) have been generated at 1.3 times the 900 mW threshold. The system performance was improved by the use of a new LBO crystal of length 30 mm. This system was continuously tunable from 1.160 to 2.185 mum. Up to 690 mW of output power has been generated for 2 W of input pump power at 5 times threshold. For this output power a depletion of 52 % was achieved with a corresponding external extraction efficiency of 34.5 %. Picosecond pulse generation in the visible by external single-pass frequency- doubling of the LBO OPO to provide picosecond pulses in the 584-771 nm range has been demonstrated. Conversion efficiencies as high as 18 % have been demonstrated, with output powers in excess of 65 mW being measured, when utilising a combination of type I and type II temperature-tuned non-critical phase-matching in LBO. The pulse width of the second harmonic was in the region of 840-880 fs. A further new source of tunable high-repetition-rate picosecond pulses for the visible has also been demonstrated, which is based on an internally-doubled, Ti:sapphire-pumped OPO that uses temperature-tuned LBO both as the OPO and SHG crystal. Oscillation has been obtained for an input pump power of 700 mW with output powers in excess of 320 mW being generated, representing conversion efficiencies of as much as 16 %. The system is continuously tunable from 584 to 771 nm and can provide transform-limited visible pulses with durations of 840-880 fs across the available range. The ability to tune beyond wavelengths of 2.5 mum was also required. To this end a new source of tunable picosecond pulses for the near - to mid - infrared has been developed which is based on the material KTA. Oscillation has been obtained for input pump powers as low as 230 mW. The system produces total output powers in excess of 403 mW with conversion efficiencies of 31 % at 5.2 times threshold. Transform-limited signal (idler) pulses of 1.02 (2.9) ps have been generated over the tuning range 1.139-1.281 (2.377-3.160) mum.

This thesis describes the development of singly-resonant optical parametric oscillators (OPOs) based on the nonlinear material KTP (potassium titanyl phosphate), and used to provide tunable light in the infrared, with low oscillation threshold and high efficiency. Further, the generation of tunable red light by the frequency mixing of the signal wave from the OPO with the pump wave in a non-critical temperature phase-matched lithium triborate crystal (LBO) is reported. We believe this is the first demonstration of such an application of LBO. Two diode-pumped solid-state lasers were used as the pump sources. One was an electro-optically Q-switched Nd:YLF laser which provided high peak power (~600 kW) pulses; and the other one was an acousto-optically Q-switched slab-geometry Nd:YLF laser which provided high repetition rate (1~10 kHz) a.nd low peak power(< 30 kW) pulses. A second version of the acousto-optically Q-switched slab-geometry Nd:YLF laser was designed and constructed with improvements in the pump module and cooling system so as to be much more compact and easier to control. In the first stage of this work, two theoretical models were constructed. One was a model for pump threshold of singly-resonant OPOs for the case of focused Gaussian beams, and was based on Guha's theory. The second one was a model for conversion efficiency of singly-resonant OPOs, for the case of plane waves with pump depletion, and was derived from the coupled wave equations. In the second stage of this work, the effects of beam focusing and Poynting vector walk-off on pump threshold and conversion efficiency for OPOs were extensively studied theoretically and experimentally. Experimental results were found to be in good agreement with theory. The high pump threshold of the critically phase-matched KTP OPO led to several other pump configurations being considered, including intracavity OPOs, cylindrical focusing, and donble-pass of the pump. As a result of the KTP OPO study, very low pump thresholds were achieved in both non-critical phase-matched (NCPM) and critical phase-matched (CPM) KTP OPOs by using long crystal in both intracavity OPOs and the double-pass-pump configuration. Maximum external conversion efficiency from pump to signal was demonstrated to be 37% for the NCPM OPO and 40% for the CPM OPO. The signal wavelength tuning ranges were observed to be 1.54-1.56 mum from the NCPM KTP OPO, and 1.58-1.8 mum from the CPM KTP OPO. In the final stage of this work, the temperature phase-matching properties of LBO were investigated with the use of our measured thermo-optical coefficients of LBO. A particularly interesting result of the investigation is the possibility of sum-frequency generation in non-critically phase-matched LBO with temperature tuning giving considerable wavelength ranges for both the type I and type II geometries. Experimentally, we demonstrated tunalile red light generation by sum-frequency mixing of the 1 mum pump wave and the signal wave of the KTP OPO with an over all conversion efficiency of more than 13%. The wavelength tuning range was observed to 0.62-0.65 mum. The effects of beam focusing on the conversion efficiency for sum-frequency generation were analysed theoretically, and several opportunities for further improvement were shown clearly from this analysis.

This thesis gives a description of studies relating to the development of continuous-wave (cw) optical parametric oscillators (OPOs) and their application to schemes that require high-precision, narrow-linewidth, and frequency-tunable radiation. There are three separate aims to the work presented within this thesis. First, the requirements on pump sources, nonlinear materials, OPO cavity resonances and phase-matching geometries are analysed with a view to operating cw OPOs with stability above threshold. Second, the results of four distinct experiments are outlined, compared to theory, and discussed within the general context of cw OPO development. Third, this thesis is the first comprehensive review of the above-mentioned performance characteristics of cw OPOs, and focuses on their role as optical frequency dividers within frequency synthesis chains spanning the optical spectrum. The modelling sections highlight the importance of cavity resonances in cw OPOs when evaluating pump power thresholds, conversion efficiencies, and mode-selection properties. Simultaneous signal and idler cavity resonances are shown to be critical when relying upon cw laser sources to reach OPO threshold powers. Such arrangements require the use of stable pump lasers and servo-locked OPO cavity lengths to maintain this doubleresonance condition. There is an in-depth analysis of OPO cavity geometries that can generate frequency-stable and continuously-tunable outputs. The selection of nonlinear materials for cw OPOs is also considered with regard to providing signal and idler frequencies in integral-related frequency ratios, thereby satisfying an important requirement within optical frequency division techniques. Four specific experiments were designed to address many of the issues raised within the modelling sections. These experiments realized the following novel arrangements; the use of lithium triborate as a gain material within cw OPOs; a cw OPO which used a pump source operating in the ultraviolet spectral region; the highest frequency output from a cw OPO; a cw OPO which used a tunable pump source operating in the near infra-red spectral region; multiple parameter pump / OPO coarse frequency tuning; a non- degenerate type II cw OPO phase-matching geometry; and a dual-cavity doubly-resonant cw OPO. Other notable features of the experimental designs included stabilized single-frequency output from a single-cavity cw OPO geometry, continuous frequency tuning from a dual-cavity, doubly-resonant OPO resonator, and the general characteristics of low pump power thresholds and moderate conversion efficiencies. An important feature discussed in detail throughout the thesis is the comparison between type I and type II phase-matching geometries. These two cases give rise to different polarization states for the signal and idler fields within OPO cavities. Type II phase-matching geometries are shown, both in theory and experiments, to be preferable to equivalent type I geometries, when considering stable OPO operation, fine frequency tuning, and multiple cavity oscillators. This is so because type II phase-matching geometries, in general, provide significantly different signal and idler refractive indices which in turn yield a considerable mis-match in the signal and idler free spectral ranges. Subsequently this relaxes the stability requirements within single-cavity doubly-resonant OPOs, and allows for polarization separation to form dual-cavity resonators which are vital to the effective operation of cw OPOs within metrology and spectroscopy. The work contained in this thesis forms an integral part of current research in cw OPOs, a field presently enjoying its most productive and prosperous period. The potential incorporation of cw OPOs within frequency synthesis chains is shown to be dependent on the further development of pump lasers and nonlinear materials. In the short term, the actual use of cw OPOs is assessed in relation to more convenient and widespread techniques for converting, comparing, and measuring absolute frequencies.

No description available.

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).

The advent of new quasi phase matched materials and high spectral/spatial quality pump sources has led to a renaissance in the development of continuous-wave optical parametric oscillators for the coherent generation of broadly tunable light in the mid infrared spectral region. This thesis describes a novel technique which overcomes the threshold constraints of the singly resonant oscillator (SRO) and stability constraints of the doubly resonant oscillator (DRO) traditionally associated with these devices by placing a singly resonant optical parametric oscillator in the high circulating field found within the cavity of a laser: the intracavity optical parametric oscillator. An SRO based upon the nonlinear material periodically poled LiNb03 (PPLN) operating internal to an all solid state, 1W diode pumped Nd:YV04 mini-laser is demonstrated and characterised. This system exhibits SRO threshold at only 330mW of external diode pump power, and produced a total of 70mW of extractable idler at 1W diode pump power. Through multi-parameter tuning of the poled nonlinear material we demonstrate broad tuning of the non-resonant idler through the spectrally important range 3.1 - 4μm. Novel cavity design desensitises the system to the effects of thermal lensing in the nonlinear medium, resulting in stable spatial and mean power outputs. The short term pump field stability is characterised by intensity modulation brought about by the onset of relaxation oscillations ; a consequence of placing the SRO within the cavity of the pump laser. A comparative study of SRO's based upon PPLN and the new material periodically poled RbTi0As04 (PPRTA) pumped internal to a high power fibre coupled diode pumped Nd:YV04 laser cavity is undertaken and presented. We see that although the nonlinearity and interaction length of the PPRTA is smaller than that of PPLN, the system based upon PPRTA outperforms or is at least comparable with that based upon PPLN in every respect with the exception of idler tuning range. We attribute this to the reduced sensitivity of this material to the effect of thermal lensing. Up to 440mW of extracted idler was produced by each system. The reduction of interferometric feedback of the pump field by the signal cavity mirror was found to eliminate the onset of relaxation oscillations in the case of PPRTA but not PPLN, due to thermal air currents dominating the triggering process in iii this system. Recently published Sellmeier equations and temperature derivatives for PPRTA are compared with the experimentally observed temperature tuning behaviour. The inclusion of an optical parametric oscillator within the cavity of the pump laser impacts significantly upon the transient dynamics of the pump laser in which it resides. We show experimental evidence of this effect and outline a strategy to minimise the effects of relaxation oscillations in the context of a simple numerical model which shall be derived. Possible future avenues of research are discussed in the context of the results and conclusions obtained over the course of this research program.

This thesis describes research on near degenerate quasi phase-matched opticalparametric oscillators (OPO) where volume Bragg gratings (VBG) are used toproduce narrow oscillation bandwidth. These OPOs are then used to pump a secondOPO to generate mid-infrared radiation. The atmospheric transmission windows in the 3.5 to 5 μm wavelength region areused for seekers on infrared homing missiles. These missiles are available to guerrillaand terrorist groups and have been used in a number of attacks on military and civilianaircraft. Laser sources at the same wavelengths are an important component incountermeasure systems for aircraft self-protection. Similar laser sources also haveapplications in laser surgery. At wavelengths longer than 4 μm crystal materials for multi-Watt level averagepower nonlinear devices is a problem. The best solution so far is to use ZnGeP2(ZGP). ZGP and the available alternatives all have a problem of near-infraredabsorption, and a mid-infrared OPO thus has to use a pump wavelength near 2 μm.This pump source can be a neodymium laser at 1.06 μm with a near degenerate OPO. Nonlinear devices for low to medium pulse energies are dominated by quasi phasematchedmaterials because of their higher effective nonlinearities and lack of walkoff.In addition they allow type I interaction where signal and idler from the OPOhave the same polarization, which has the advantage that both waves can be used topump the ZGP OPO. The drawback of this is that the near-degenerate interaction hasvery wide gain bandwidth. Efficient pumping of the second OPO demands narrowbandwidth output from the first OPO.Volume Bragg gratings that are glass materials with a periodic refractive indexmodulation have emerged as high quality narrow bandwidth reflectors. By using aVBG as one cavity mirror in an OPO the feedback bandwidth and hence the OPOoscillation bandwidth can be kept very narrow. Signal and idler bandwidths of 10 and20 GHz (FWHM) at 2122 and 2135 nm, respectively, have been demonstrated. Thisshould be compared to the several hundred nanometre bandwidth from an OPO usingdielectric mirrors. Very narrow bandwidth operation has been achieved so close todegeneracy that the signal and idler are not resolvable. The total output energy generated in the PPKTP OPO (signal and idler together)has been used to pump a ZGP OPO that produced mid-IR radiation. Tuning of thesignal from a ZGP OPO from 2.9 μm to degeneracy at 4.3 μm has been shown, with acorresponding idler wavelength tuneable up to 8 μm. The highest conversionefficiency that has been reached from 1.06 μm to the mid-IR was 12 %. This setupused a PPKTP OPO with 30 % conversion efficiency and 13 nm separation of signaland idler (2122 and 2135 nm). The pulse repetition frequency was 20 kHz and thegenerated output power in the mid-IR was 3.2 W. / QC 20100517