No description available.

The motivation of this thesis was to develop efficient and improved optical excitation sources for applications in confocal laser scanning microscopy (CLSM) and multiphoton laser scanning microscopy (MPLSM), with emphasis on extending the wavelength coverage of existing laser sources using nonlinear optical methods. This included quantitative and qualitative structural analysis of periodically poled nonlinear materials using MPLSM. These materials, used in nonlinear optical frequency conversion, rely upon consistent poling lengths for quasi-phase matched operation. The described technique provided a non-destructive assessment of inhomogeneities within the crystal structure, which may impact upon frequency conversion efficiency processes. Following this analysis, innovative pump geometries were investigated for ultra-short pulsed singly resonant synchronously pumped optical parametric oscillators (SPOPOs). Through application of a novel bi-directional pump geometry, an increase in peak power of up to 90 % was observed, with peak powers in excess of 18.8 kW generated. This substantially outperformed any pump geometry previously implemented. This source was then applied to three photon laser scanning microscopy. Next, a visible, wavelength tunable, ultra-short pulsed source based on sum frequency mixing was developed for MPLSM at wavelengths shorter than 700nm. With average output power of ~ 150 mW, the source was applied to MPLSM of biological and non-biological UV excitable samples and results were compared with the longer wavelength Ti:Sappphire system. Finally, a SC source and Ti:Sapphire laser were applied for optical beam induced current (OBIC) microscopy of an InGaN LED to provide information regarding the spectral response of the diode and imaging of the active region. This provided additional information regarding inhomogeneity and hence efficiency.

Nonlinear optics enables the manipulation of the spectral and temporal features of light. We used the tailorable guidance properties of photonic crystal fibres to control and enhance nonlinear processeswith the aim of improving nonlinearity based optical sources. We utilised modern, high power, Ytterbium fibre lasers to pump either single photonic crystal fibres or a cascade of fibres with differing properties. Further extension of our control was realised with specifically tapered photonic crystal fibres which allowed for a continuous change in the fibre characteristics along their length. The majority of our work was concerned with supercontinuum generation. For continuous wave pumping we developed a statistical model of the distribution of soliton energies arising from modulational instability and used it to understand the optimum dispersion for efficient continuum expansion. A two-fold increase in spectral width was demonstrated, along with studies of the noise properties and pump bandwidth dependence of the continuum. For picosecond pumping we found that the supercontinuum bandwidth was limited by the four wave mixing phase-matching available in a single fibre. A technique to overcome this by using a cascade of fibres with different dispersion profiles was developed. Further improvement was achieved by using novel tapered PCFs to continuously extend the phase-matching. Analysis of this case showed that a key role was played by soliton trapping of dispersive waves and that our tapers strongly enhanced this effect. We demonstrated supercontinua spanning 0.34-2.4 ¹mwith an unprecedented spectral power; up to 5 mW/nm. The use of long, dispersion decreasing photonic crystal fibres enabled us to demonstrate adiabatic soliton compression at 1.06 ¹m. From a survey of fibre structures we found that working around the second zero dispersion wavelength was optimal as this allows for decreasing dispersion without decreasing the nonlinearity. We achieved compression ratios of over 15.

No description available.

We investigate the feasibility of simultaneous suppressing of the amplification noise and nonlinearity, representing the most fundamental limiting factors in modern optical communication. To accomplish this task we developed a general design optimisation technique, based on concepts of noise and nonlinearity management. We demonstrate the immense efficiency of the novel approach by applying it to a design optimisation of transmission lines with periodic dispersion compensation using Raman and hybrid Raman-EDFA amplification. Moreover, we showed, using nonlinearity management considerations, that the optimal performance in high bit-rate dispersion managed fibre systems with hybrid amplification is achieved for a certain amplifier spacing – which is different from commonly known optimal noise performance corresponding to fully distributed amplification. Required for an accurate estimation of the bit error rate, the complete knowledge of signal statistics is crucial for modern transmission links with strong inherent nonlinearity. Therefore, we implemented the advanced multicanonical Monte Carlo (MMC) method, acknowledged for its efficiency in estimating distribution tails. We have accurately computed acknowledged for its efficiency in estimating distribution tails. We have accurately computed marginal probability density functions for soliton parameters, by numerical modelling of Fokker-Plank equation applying the MMC simulation technique. Moreover, applying a powerful MMC method we have studied the BER penalty caused by deviations from the optimal decision level in systems employing in-line 2R optical regeneration. We have demonstrated that in such systems the analytical linear approximation that makes a better fit in the central part of the regenerator nonlinear transfer function produces more accurate approximation of the BER and BER penalty. We present a statistical analysis of RZ-DPSK optical signal at direct detection receiver with Mach-Zehnder interferometer demodulation

Rare-earth doped silica fibers have been used for many years to create continuous-wave lasers, and Er-doped fiber amplifiers are now widely used in telecommunications. In addition, cladding pumped fiber allows the efficient conversion of multimode radiation from high power, low cost, broad-stripe semiconductor laser diodes into the single-mode emission of fiber lasers. With its broad gain bandwidth and high optical conversion efficiency, Yb-doped silica fiber represents an attractive medium for the generation and amplification of high energy ultrashort optical pulses. However, these potential advantages of Yb-doped silica fiber as a gain and nonlinear medium for mode-locked lasers and ultrashort pulse amplifiers have been less well studied, and it was not until 1999 that significant research interest first appeared in Yb-fiber chirp pulse amplifier (CPA) systems. This thesis describes the development of the first practical and stable, femtosecond, Yb-fiber oscillator, and of an Yb-fiber amplifier based CPA system (pulses ~10 μJ, <500 fs). Novel aspects of the system include the use of a high extinction ratio Electro-Optic modulator for pulse selection, and the development of a compact chirped-fiber-Bragg-grating (CFBG) pulse stretcher that provides both 2nd and 3rd order chirp compensation. Recently published theoretical results have demonstrated that the asymptotic solution for ultrashort pulses in a high gain fiber amplifier is a linearly chirped pulse, which can therefore be recompressed with a standard grating compressor. This thesis reports the first experimental comparison of nonlinear pulse evolution towards the asymptotic form using a cascaded amplifier system. The "direct amplification" system was constructed by removing the CPA stretcher grating, which also enabled the use of a less dispersive and more compact compressor. Further system development should lead to the generation of ultrashort pulses at high average power levels and >100 kHz repetition rates. Holey fiber (HF) is a recently developed technology that uses rings of air holes around a solid core to confine the optical field by average-index effects. Fibers are highly suitable for applications using nonlinear optics because of the tightly confined mode and long interaction lengths. The increased mode confinement possible using HF means that small-core, high air-fill fraction HF are an attractive nonlinear medium. Furthermore, the high index contrast in such fibers can create a strong (anomalous) waveguide contribution to the dispersion, and such HFs can have anomalous dispersion at wavelengths <1.3 μm, where conventional fiber has normal dispersion. Therefore HFs can support solitons in new wavelength bands. This thesis reports the first demonstration of linear dispersion compensation, soliton transmission, and visible continuum generation seeded by a 1.06 μm Yb-fiber source. In addition, an experimental study is reported that used HF seeded from a Ti:Sapphire laser to generate continuum in distinct transverse spatial modes of a HF. Numerical simulations suggested that the observed enhancement in UV generation from a higher order mode could be due to differences in the dispersion profiles of the fundamental and higher order transverse modes. Finally, the development of a novel source of <200 fs pulses, continuously tuneable in wavelength from 1.06-1.33 μm, based on the soliton self-frequency-shift principle, is described. The source was constructed from a diode-pumped Yb-doped HF amplifier, and the Yb-fiber oscillator described above. The diode pump power controlled the output wavelength.

This thesis reports on the experimental study of high-power, high-energy, cladding-pumped, rare-earth (Yb3+, Er3+/Yb3+)-doped fibre lasers. Some of the main capabilities of fibre lasers such as: High brightness and thermal properties were exploited for the development of a variety of continuous wave (CW) and Q-switched devices, whose characteristics also includes compactness. Our devices could already be considered an option for several applications. The 25-year long scientific and commercial evolution that fibre lasers have experienced is discussed in the first two chapters. The invention of Erbium-doped fibre amplifiers (EDFA's) and Internet were two major breakthroughs, which launched the need of WDM systems and laser sources. Fibre lasers, are now considered a flexible and powerful device whose technology has finally reached its maturity. Cladding pumping is the technique employed in these experiments in order to pump double clad fibre lasers using high power, broad stripes and bars. In this work, several inner cladding shapes have been used to overcome the normally high mismatch between diode laser beams and inner cladding areas of fibre lasers. Chapter Three consists of a review of cladding-pumped fibre lasers. It describes how inner cladding geometry and pump absorption limits the output power scalability of these devices. Nonlinear effects and amplified spontaneous emission are also studied due to their implication they have over fibre lasers performance. Results on conventional, continuous wave (CW) fibre lasers including fibre characterization and employed launching techniques are described in Chapter Four. A new method to obtain high intensity laser beam output from an Yb3+-doped, cladding-pumped, highly multimode fibre laser has been proposed. In this experiment, we propose the use of fibre tapers to increase intensity and improve beam quality. In CW regime, our results show an intensity increase of ~3.5 times with a low power penalty of ~1 dB. Also, without tapering, a maximum output power of 21-W was reached with a slope efficiency of >80%. Using a simple set of optic elements such as a l/2 waveplate, a polarizing beam-splitter and a bulk grating, we investigated the polarization characteristics of an Yb3+ fibre laser, from which we obtained 6.5 W of single polarization tunable output in the range of 1070 to 1106 nm. As a free running laser, the system produced 18 W at 1090 nm and showed a threshold of 1.8 Watts. The experiment is our first approach for developing a reliable high-power Yb3+-doped fibre source, that could be used in conjunction with optical parametric oscillators (OPO) and amplifiers (OPA) to frequency convert to a broad band of wavelengths. Using a new design of ytterbium-doped fibre made in-house with the conventional modified chemical vapor deposition (MCVD) process, we explored the possibilities of energy storage with such a large mode area (LMA) fibre. The fibre system was capable of delivering energetic pulses of >2 mJ, which could suggest the feasibility of a pulsed fibre laser in the region of tens of milli-Joules. The experiment is described in Chapter Six, on which the experiment that uses the tapered fibre laser in Q-Switched regime is also described and compared to LMA fibre laser. Gaussian-type pulses were obtained which reached pulse energies of 0.6 mJ at 4 kHz using a tapered fibre laser and 1.3 mJ at 500 Hz using conventional laser, corresponding to average powers of 2.1 Watts for the tapered laser and 0.8 watts for the conventional laser. Er3+/Yb3+-doped fibre lasers were part of our experimental work. This co-doping technique allows pumping of Yb3+ ions using broad-stripe high-power pump sources to reach much higher output power levels. Efficient energy transfer from excited Ytterbium ions into Erbium is achieved. From a preliminary study, the fibre laser showed a threshold of 160 mW and a slope efficiency of 49% with respect to absorbed pump power. The maximum output power was 6.2 watts at 1535 nm and a linewidth of 1 nm. One of our co-doped fibre devices produced 16.8 W of continuous wave, multimode laser power at the interesting wavelength of operation of 1550 nm. Finally, conclusions and future work are included in Chapter Eight.