Development of versatile high power bounce geometry lasers

This thesis details an investigation into the development of bounce geometry lasers to achieve a more versatile range of laser characteristics. The bounce geometry has matured in recent years into a useful solid-state pumping scheme, but its performance has to date been limited by a number of factors, as well as largely restricted to neodymium systems. For real-world application, a more versatile range of laser characteristics would be desirable. A new design for a bounce geometry amplifier is presented that achieves a symmetric gain profile and thermal lens by control of the amplifier dimensions. The laser produces a circular stigmatic TEM00 (M2 < 1:11) beam with 14 W power. When Q-switched, the design permits versatile control over the repetition rate (single-shot to 480 kHz) with pulse energies up to 0.45 mJ. The stigmatic design also allows the direct generation of a Laguerre-Gaussian `vortex' beam, and proves favourable for modelocking with the nonlinear mirror method. Several designs are investigated to study power scaling in a master oscillator power amplifier (MOPA) configuration, including a stigmatic MOPA based on the amplifier described above, and a chain of multiple power amplifiers. A folded dual-pumped amplifier design is also demonstrated, which reduces the size and complexity of a multi-stage amplifier and allows power scaling to the 100 W level. Pulse amplification is also investigated, and a MOPA is optimised for energy extraction by a Q-switched oscillator. Finally a 3-micron bounce laser is presented using an erbium-doped YSGG gain medium. Different cavity designs are investigated, and a simple compact cavity is found to be optimum. Thermal effects are investigated and found to be a limiting factor on the laser's performance. Quasi-continuous wave pulse energies of up to 15 mJ are demonstrated, with an average power of up to 430 mW

Optics and Quantum Electronics

Contains table of contents for Section 3 and reports on twenty-one research projects.Joint Services Electronics Program Contract DAAL03-89-C-0001Joint Services Electronics Program Contract DAAL03-92-C-0001U.S. Air Force - Office of Scientific Research Contract F49620-91-C-0091Charles S. Draper Laboratories Contract DL-H-441629MIT Lincoln LaboratoryCharles S. Draper Laboratories, Inc. Contract DL-H-418478Fujitsu LaboratoriesNational Science Foundation Grant ECS 90-12787National Center for Integrated PhotonicsNational Science Foundation Grant EET 88-15834National Science Foundation Grant ECS 85-52701U.S. Air Force - Office of Scientific Research Contract F49620-88-C-0089U.S. Navy - Office of Naval Research Contract N00014-91-C-0084U.S. Navy - Office of Naval Research Grant N00014-91-J-1956Johnson and Johnson Research GrantNational Institutes of Health Contract 2-R01-GM35459U.S. Department of Energy Grant DE-FG02-89 ER14012-A00

High power modelocking using a nonlinear mirror

This thesis presents work on the high average power operation of pulsed diode-pumped solid-state lasers by using a laser configuration known as the bounce geometry. The bounce geometry has previously produced efficient, high power and high spatial quality laser outputs in continuous-wave, Q-switched and modelocked regimes. This thesis explores the capabilities of the bounce geometry for power scaling, shown using Nd:YVO4 and Nd:GdVO4 in both a passively Q-switched laser system and a variety of nonlinear mirror modelocked systems. The high gain experienced by Nd-doped gain media pumped at 808 nm has traditionally posed difficulties in producing stable passive Q-switching with Cr4+:YAG. By using a novel stigmatic design of the bounce geometry that experiences lower gain, but highly circular output, passive Q-switching with > 11 W of average power is produced, at a pulse repetition rate of 190 kHz. This is the highest output power ever achieved from a passively Q-switched Nd-doped vanadate laser to date. Nonlinear mirror modelocking is a passive modelocking technique that employs a χ(2) nonlinear medium in combination with a dichroic output coupler. The first nonlinear mirror modelocking of a bounce geometry laser is presented, obtaining 11.3 W of average power and 57 ps pulse duration using a type-II phase-matched KTP nonlinear crystal. Using type-I phase-matched BiBO, shorter pulses of 5.7 ps in duration are obtained at an average power of 6.1 W. The nonlinear mirror modelocking technique is then applied to the stigmatic bounce geometry laser, obtaining a highly stable train of modelocked pulses with pulse duration 14 ps and an average power of 12 W, with high spatial quality output. Mixed vanadate lasers offer customisation of the laser fluorescence spectrum, but tend to experience lower gain than single vanadates. Using the mixed vanadate combination Nd:Gd0.6Y0.4YVO4 in the bounce geometry, 27.5 W of average power in continuous-wave operation is shown. This is the highest power of any mixed vanadate laser ever reported. By then applying the nonlinear mirror modelocking technique to the mixed vanadate system, 16.8 W of average modelocked output power and a pulse duration of 12.7 ps is obtained. This is simultaneously the first time that the nonlinear mirror technique has been applied to mixed vanadate gain media and the highest power of any modelocked mixed vanadate laser to date. Finally, power scaling of a nonlinear mirror modelocked Nd:GdVO4 laser in the bounce geometry is achieved through use of the double bounce geometry design and through use of a high power pump diode. The system employing the high power pumping produced > 30 W of average power — world record power using the nonlinear mirror technique

Nonlinear frequency conversion of a continuous-wave, laser diode-pumped Nd:YLF laser

A continuous-wave, frequency-doubled, diode-pumped Nd:YLF laser, capable of generating 1-W of single-frequency radiation at 523.5 nm, with a linewidth of 10 kHz and frequency tunability of 9 GHz has been developed. By using the tunable green laser as pump source, 4.5 GHz continuous smooth tuning in the range of 1020 - 1070 nm has been demonstrated in a low threshold doubly-resonant optical parametric oscillator. The investigation of thermal effect in the end-pump Nd:YLF laser crystal and the consideration of diode array end-pump geometry have led to an optimum folded-cavity design. Such an optical resonator can eliminate the astigmatism in the laser output, which is induced by the cavity folding on a curved mirror and the anisotropic thermal effect in Nd:YLF, resulting in a circular fundamental laser mode. In addition, a tightly focused beam waist is produced inside the cavity so that efficient intracavity SHG can be achieved. Over 30% optical conversion efficiency from diode to TEM00 1047 nm laser output and 10% conversion efficiency from diode to single-frequency SHG green radiation has been demonstrated. A novel intracavity birefringent filter frequency selection technique has been applied in the standing-wave laser resonator to achieve the single-frequency operation. The performances of the laser in the fundamental wave and in second harmonic generation are investigated in detail in this thesis. The tuning behaviour and stability requirements of type-I and type-II CW doubly-resonant OPO have been compared by using the above green laser as the pump source and LBO, KTP as nonlinear crystals. Through utilisation of a simple cavity length servo, type-II phase matching allows single signal-idler mode-pair operation. By means of pump frequency tuning, and cavity length servo control, the output of signal and idler frequency can be tuned continuously over the KTP crystal phase-matching range. Theoretical analyses on diode-end-pumped Nd:YLF lasers, intracavity frequency doubling, the novel intracavity birefringent filter, and the timing behaviour and stability requirements of single cavity DRO are also presented in this work

Compact, low-threshold femtosecond lasers

This thesis is concerned with the design and development of compact, all-solid-state femtosecond pulse lasers with low pump power requirements. A number of directly-diode- pumped laser systems based on the gain materials Cr3:LiSrGaF6 (chromium-doped lithium strontium aluminium fluoride) and Cr3:LiSrGaF6 (chromium-doped lithium strontium gallium fluoride) pumped with AlGaInP laser diodes are described. The motivation behind this work was the development of portable, low-noise and lower cost ultrashort pulse lasers for a number of low-power applications such as the characterisation of electron-optical streak camera systems. The investigation into the modelocking of lasers with modest intracavity powers was also an important challenge. The achievement of a battery-powered, compact and efficient laser system represents an excellent outcome for this research programme. Major consideration is given to the key factors that determine both the cw and modelocking thresholds of an ultrashort-pulse laser. In particular, the reduction of intracavity optical losses by designing the laser to operate with fewer cavity elements, the optimisation of second-order and higher-order dispersion for efficient modelocked operation and the inclusion of a semiconductor saturable absorber mirror for increased stability are discussed. This has enabled pump thresholds to be reduced to a level permitting, for the first time, the use of diffraction-limited, narrow-stripe laser diodes for efficient, low-power optical pumping. A number of laser oscillators with novel cavity designs and progressively lower pump thresholds are described. Pulses as short as 57 fs and average output powers as high as 9 mW for only 80 mW of incident pump power are reported for a battery powered femtosecond Cr:LiSAF laser. This represents an overall electrical-to-optical conversion efficiency of approximately 1% which is excellent for a femtosecond pulse laser system. In addition, the amplitude and phase noise performance is shown to be exceptionally good and is believed to be the best yet reported for this type of ultrashort pulse laser. The design and demonstration of highly compact, ultrashort-pulse lasers incorporating novel resonator configurations and simplified dispersion compensation schemes are then described. These lasers produced sub-ps pulses at cavity frequencies as high as 450 MHz

Development of all-solid-state modelocked laser sources at 1.55 μM

This thesis concerns the generation of tunable ultrashort pulses near the 1.55 mum telecommunications window. Two principal laser systems are considered: i) the NaCl:OH colour-centre laser, which employs the technique of synchronously-pumped modelocking to generate tunable picosecond pulses and ii) the self-modelocked Cr4:YAG laser to generate femtosecond pulses tunable from 1.5-1.56 mum. Details are given for an all-solid-state cw and cw-modelocked pump source for Cr4:YAG and colour-centre lasers based on Nd:YAG. Fibre-coupled AlGaAs laser diodes are employed as the solid-state pump source to this laser. When operated cw, up to 8.5 W of linearly polarised output power in a TEM00 beam is obtained. A compact cw actively-modelocked Nd:YAG laser is described having a pulse repetition rate of 194 MHz. Pulse durations down to 34 ps and output powers up to 6.0 W are obtained from this system. An 82 MHz Nd:YVO4 laser is also detailed producing pulsewidths down to 75 ps and average output powers up to 3.5 W. The intrinsic noise source associated with the synchronous modelocking technique is discussed and a simple passive stabilisation scheme, coherent-photon-seeding (CPS), is described and applied to the synchronously-modelocked NaCl:OH laser. Results of a simulation of this laser are reported and a comparison is made with the practical observations of the stabilised laser. For the first time, theoretical and experimental evidence for the presence of high frequency pulse jitter in synchronously-pumped- modelocked (SPML) lasers is presented and the coherent photon seeding technique is shown to eliminate this noise. Details are also given for the construction of a compact, all-solid-state, femtosecond Cr4+:YAG laser. A design prescription for laser resonators having a high propensity for self-modelocking is presented and an unconventional 3-mirror resonator is adopted for optimised self-modelocked operation. Using this design, modelocked output powers up to 300 mW with 120 fs pulses from a compact, regeneratively initiated laser having a pulse repetition rate of 320 MHz is reported for 4.7W incident pump power. Self- modelocking is demonstrated for pump powers down to ~1W with this cavity design. A compact cavity design for self-modelocking is also assessed, with a footprint of just 20 X 25 cm, which places a prism in each cavity arm. 470 fs pulses at 220 mW average output power are reported

Optimisation of a colliding-pulse modelocked dye laser

The work presented in this thesis describes the operation, characterisation and optimisation of a colliding-pulse modelocked (CPM) dye laser. A method of pulse analysis has been developed which is capable of determining the shape and chirp of the output pulses to a first approximation. It involves an iterative pulse-fitting to intensity autocorrelation, interferometric autocorrelation and spectral measurements. The use of a four-prism sequence for intracavity dispersion compensation in a CPM dye laser resulted in pulse durations of 40-50fs. However, operating the laser close to the instability regime so as to obtain strong focusing in the absorber dye jet enabled pulse durations as short as 19fs to be obtained. A detailed empirical study of the dispersion- compensated laser, together with a theoretical and experimental chirp analysis, indicated the presence of strong phase shaping arising from a net positive self-phase modulation, which was attributed to the optical Kerr effect occurring in the absorber dye solvent. Various modes of operation were observed, including unidirectional lasing and a higher- order solitonlike regime. The results of pulse-fitting were found to yield strong evidence for pulse asymmetry, the pulse profiles corresponding closely to an asymmetric sech2 pulse function with a longer leading edge. A computer simulation of the CPM dye laser provided a comprehensive understanding of the underlying pulse shaping dynamics of this system, elucidating fully the experimental behaviours observed, as well as providing a clear strategy for further optimisation of the laser. In particular, optimal performance was found to depend on strong amplitude and strong phase shaping, minimal spectral filtering, the control of higher-order dispersion and the provision of extracavity dispersion compensation. An experimental study of Gires-Tournois interferometers (GTI's) for intracavity cubic phase compensation identified the key requirements for cubic phase control in the CPM dye laser, while highlighting the limitations of utilising conventional GTI structures. A subsequent theoretical analysis enabled a more suitable strategy to be devised. It involved optimising the cavity optics and using a prism system with variable prism spacing, alone or in tandem with specially tailored GTI structures. Implementation of these findings resulted in pulse durations of around 30-40fs and the elimination of pulse asymmetry, which was attributed to a residual positive cubic phase. However, the appearance of a distinctive modulation in the wings of the pulse provided strong evidence that the pulse durations from the CPM dye laser had become limited by the next higher-order dispersion term; quartic phase. To demonstrate the direct relevance of this work to the more recently developed solid- state laser systems, an alternative all-solid-state femtosecond laser has been described. Based around a Ti:sapphire gain medium, the design of this laser incorporates the essential optimising principles and techniques developed for the CPM dye laser. The proposed system utilises a low-loss, broadband semiconductor saturable absorber mirror to initiate self-modelocking and a hybrid prism-chirped-mirror scheme for broadband intracavity and extracavity quintic-phase-limited dispersion compensation. When fully optimised, it is predicted that this laser should yield pulse durations as short as 5fs