The rapid developments in optical and electronic technologies have accelerated developments of solid state laser technology. The diode-pumped solid state laser has the merits of the diode laser, such as compactness, low cost, and the merits of the solid state laser, such as high laser quality, high conversion efficiency, long lifetime, and simple structure. There use in laser applications is very cost-effective in terms of material consumption, which is typically one-thousandth that of bulk material. In addition, heat dissipation in the gain medium can be significantly alleviated because highly heat-conductive material can be applied to the circumference of the crystal fiber. So, it was applicated in electronics, communication and medicine widely. The laser-heated pedestal growth (LHPG) method is now a well-established technique for the growth of single-crystal fibers. It is crucible free and can therefore produce high-purity, low-defect-density single crystals. Interface loss is one of the dominant factors that reduce the efficiency of crystal fiber lasers, although cladding with a dielectric coating or in-diffusion of the gain core has been utilized to suppress this interface loss. Using a gradient-index Nd:YAG crystal fiber with peak Nd concentration up to 1.6-atm.%, we recently demonstrated a laser power of 145 mW and slope efficiency 28.9%. Peak Nd concentration up to 3.6-atm.% Nd:YAG crystal fiber with a 20-um core was grown, which could eliminate the interface loss and enhance the efficiency of crystal fiber lasers to be compatible with bulk solid-state lasers.

Anti-resonant hollow core fibre is a new kind of optical fibre waveguide in which light is trapped in a hollow core surrounded by the capillary formed microstructured cladding. This fibre exhibits high damage threshold, low dispersion and ultra-low nonlinearity with relatively low loss of a few tens of dB/km. Its intrinsic feature of multimode delivery limits the applications with high requirements of single mode transmission. In my thesis, I demonstrate how the design of hollow core fibre can be improved with single mode guidance. S2 imaging measurement was used to analyse the mode content of the solid core fibres. In my research, I established S2 measurement to measure the mode contents in hollow core fibres for the first time. Two hollow core fibres with 8 capillaries and 7 capillaries in their claddings were fabricated in same fashion and showed differences in low attenuations. By comparing the mode contents in both of the fibres via S2 imaging measurement, 7-capillary HCF was demonstrated to give better performance on single mode guidance. Among the applications of the HCF, the property of delivering high power in HCF makes the gas filled HCF laser possible. In my research, a continuous-wave mid-infrared acetylene filled hollow core laser was built with a slope efficiency of 33% and an output power of over 1 watt at the wavelength region of 3.1~3.2 μm. The pump source is an Erbium-doped fibre amplified tunable laser diode which works at C-band wavelength. The fibre without the gain medium has two transmission bands with low attenuation of 0.037 dB/m and 0.063 dB/m at pumping and lasing wavelengths respectively. This laser system works in either cavity-based configuration or single pass ASE configuration. The latter configuration shows a better performance in high output power and high slope efficiency. The optimized laser system was studied experimentally with the proper fibre length and gas pressure. This laser system could be extended to be filled with other molecules to longer wavelengths and has potential for high power output.

Random fiber lasers, a new type of fiber laser that uses disordered medium to provide distributed feedback, have drawn considerable interest in the photonics community over the past ten years. Stimulated Brillouin scattering (SBS), with a typical narrow spectral width of ~100 MHz, provides an important gain mechanism for random fiber lasers. Brillouin random fiber laser (BRFL) has shown excellent advantages in generating highly coherent photons and in ultrasound sensing. However, the accompanied large intensity noise in BRFLs hinders its further performance improvement and practical applications. In order to design a low noise BRFL, it is important to explore the fundamental physics behind BRFL and study its output characteristics. This thesis focuses on the study of random lasing mechanism in BRFL, which lays the foundation for the demonstration of a low noise BRFL. The main research results and contributions are as follows: (1) In order to understand the dynamic noise properties of BRFLs, the properties of the acoustic wave generated by BRFL, including its intrinsic spectral width, intensity dynamics, distributed spectrum and distributed intensity statistics are characterized for the first time. The characterization method is based on the SBS enhanced polarization decoupled four wave mixing process, where the pump wave, Stokes wave, probe wave and reflected probe wave are coupled through the fiber density variation induced by the acoustic wave. It is demonstrated that the intrinsic spectral width of the acoustic wave in the Brillouin gain fiber depends on the spectral convolution of pump light and Stokes light. Stochastic behaviour is introduced to the intensity dynamics of the acoustic wave when the linewidth of the pump light (or the Stokes light) is larger than several MHz. The distributed spectra of the dynamic grating are determined by the birefringence of the Brillouin gain fiber, which have maximum change on the order of 10-7 to 10-6 when the BRFL is on operation. Different proportion of optical rogue waves are detected at high gain position and low gain position near the lasing threshold, proving the nonlinear amplification of the SBS process. (2) In order to study the mode selection mechanism of the distributed random feedback and explore new physics phenomenon in BRFLs, the conventional Rayleigh scattering fiber in BRFL is replaced by the artificially controlled random scattering medium. First, weak FBG array with random spacing offers distributed feedback with varied length, which demonstrate the longitudinal mode filter function of the distributed random feedback. Single longitudinal mode operation of BRFL is realized by using appropriate length of the FBG array. Then, scattering from random fiber grating (RFG) with varied grating period is used to provide feedback for BRFL. The enhanced backscattering strength from RFG improves the slope efficiency of BRFL to 29.3% and reduces the lasing threshold to 10.2 mW. By calculating the correlation of the intensity fluctuation spectra from trace to trace, the correlation of two traces is found to be dependent on the specific two chosen traces, demonstrating the replica symmetry breaking phenomenon in photonics. (3) RFG with relatively large refractive index modulation shows potentials in improving the performance of the BRFL. In order to investigate the working mechanism of the RFG, optical frequency domain reflectometry (OFDR) with spatial resolution of 8 μm is employed to characterize the property of RFG. The backscattering strength and spectral response of RFG is highly related to the degree of randomness of RFG. Theoretically, entropy is introduced to build a quantitative relationship between the degree of randomness and backscattering strength of the RFG based on the transfer matrix method. A linear relationship between the average reflectivity of the RFG in dB scale and sub-grating’s entropy is found. Further, based on a polarization maintaining RFG, a low noise BRFL is proposed and demonstrated. Compared to Rayleigh scattering, the polarization maintaining RFG can tolerate environmental perturbation, leading to a 20 dB intensity noise suppression of the BRFL in the low frequency domain from 10 Hz to 1 kHz. (4) The dynamic properties of the slowly varying frequency drift of a dual-wavelength BRFL in polarization maintaining fiber are characterized. Two principal lasing peaks in each polarization are enabled by the combined distributed Rayleigh scattering and the Brillouin gain provided by the polarization maintaining fiber with large birefringence. Polarization dependent and polarization independent spectral variations are studied in the dual-wavelength BRFL due to the environmental perturbation and gain competition. The probability distribution of the lasing frequency exhibits a dip near the mean frequency that is caused by the spectral hole burning. By calculating the matrix of the Pearson correlation coefficient, the internal correlations between different part of random fiber laser spectra are found, which enhances the understanding of the fundamental physics of random lasing process.

Doctor of Philosophy / Department of Physics / Brian R. Washburn / Mode-locked lasers that produce short optical pulses in the mid-infrared wavelength region have been sought out for a wide range of applications such as free space communication, molecular spectroscopy, medical diagnostics, and remote sensing. Here, a thulium and holmium (Tm/Ho) co-doped fiber laser that mode-locks in both the solitonic and stretched-pulse regimes is used to produce ultra-short pulses in the 2 [mu]m region. Nonlinear polarization rotation technique is used where fiber nonlinearity is responsible to mode-lock the laser. The anomalous group velocity dispersion of both the single mode and gain fibers used limit the laser operation in the solitonic regime where spectral bandwidth is 10 nm and hence the pulse duration is limited to 996 fs. In order to increase the spectral bandwidth and hence get the shorter pulses the anomalous dispersion of these fibers has to compensate using normal group velocity dispersion fiber in the laser cavity. High numerical aperture fibers, which have normal group velocity dispersion around 2 [mu]m due to its large and positive waveguide dispersion, can be used to compensate the anomalous dispersion of the gain and single mode fibers. We used a high numerical aperture fiber called UHNA4 in the laser cavity in order to compensate the anomalous dispersion of other fibers and mode-locked the laser in stretched pulse regime. The spectral bandwidth of the laser increased to 31 nm with corresponding pulse duration of 450 fs measured from the interferometric autocorrelation. The laser dynamics of the Tm/Ho co-doped fiber laser is also studied while going from the stretched-pulse to solitonic regime by fiber cut-back measurements of normal dispersion fiber. It was clearly observed that both the spectral bandwidth and the pulse duration changed significantly going from one region to the other.

The rapid developments in optical and electronic technologies have accelerated developments of solid state laser technology. The diode-pumped solid state laser has the merits of the diode laser, such as compactness, low cost, and the merits of the solid state laser, such as high laser quality, high conversion efficiency, long lifetime, and simple structure. In addition, the diode-pumped solid state lasers have made it a feature star among lasers. One of the problems in solid state laser is the heat removal. The crystal fiber is used as the laser gain medium in this work to be able to reduce largely the volume of solid-state laser, and improve the heat disappearance. There are many different methods to grow crystal fibers, LHPG method one of the best because single crystal fibers can be grown with small diameters at very fast rate, and accurate control. We have gown high quality Nd:YAG crystal fiber with diameter of 23~285 mm. After cladding, grinding, polishing, and coating, we could ready to fabricate the Nd:YAG crystal fiber laser. We have successfully implemented diode-laser pumped Nd:YAG crystal fiber laser. The lasing threshold power is 143 mW, and the maximum output power is 38 mW. In the feature, we shall improve the cooling system, the cladding, and coating to further increase the conversation efficiency and output power.

High power thulium fiber lasers are interesting for a myriad of applications due to their potential for high average output power, excellent beam quality, compactness, portability, high operating efficiency and broad, eye-safe spectral range from 1.8-2.1 microns. Currently, the majority of thulium laser research effort is being invested into scaling average output powers; however, such output powers are being scaled with no degree of control on laser system output spectrum or temporal behavior. Thulium fiber laser technology is not useful for many of its most important applications without implementation of techniques enabling tunable, narrow spectral widths with appropriate pulse durations for particular applications. This work outlines several techniques for spectral control of thulium fiber lasers and investigates scaling of average laser powers while using these techniques to maintain a desired spectral output. In addition, an examination of operation in both nanosecond and picosecond pulsed regimes and scaling of average powers and pulse energies in these regimes to useful power levels is conducted. The demonstration of thulium fiber laser systems for applications in frequency conversion and spectral beam combination is also discussed. In addition to the experimental results, theoretical modeling of thulium fiber amplifier operation, simple thermal management analysis, as well as practical fiber and system design considerations for future power scaling are presented. Experimental and theoretical results of this work will enable the successful design of future extremely high power spectrally and temporally controlled thulium fiber laser systems.

No description available.

In the first chapter, a Q-switched fiber laser that is capable of generating transform-limited pulses based on single-frequency fiber laser seeded ring cavity is demonstrated. The output pulse width can be tuned from hundreds of nanoseconds to several microseconds. This Q-switched ring cavity fiber laser can operate over the whole C-band. In addition, a theoretical model is developed to numerically study the pulse characteristics, and the numerical results are in good agreements with the experimental results. In the next chapter, a Raman fiber laser is developed for generating signal at 1480 nm. Initial experimental results has demonstrated generating of Raman laser at 1175 nm, 1240 nm, 1315 nm, and 1395 nm wavelength. Finally, a free space fiber amplifier is studied both theoretically and experimentally. The experimental work has demonstrated signal coupling efficiency up to 90% in the NP highly Er/Yb co-doped phosphate fiber.

A coherent optical THz (1.5 THz, 200 µm) source was developed based on pulsed, near IR, fiber lasers, and frequency mixing in nonlinear crystals. The generated THz frequency is determined by the difference frequency of two high peak power pulsed fiber lasers at 1550 nm and 1538 nm. When incident to the crystal, the near IR lasers induce a polarization at their beat frequency which generates the THz radiation. The pulsed fiber lasers are single transverse mode, have high pulse energy and peak powers of 0.38 mJ and 128 kW respectively. They are transform limited at a few ns in duration with very good beam quality of M² ≈ 1.2. The pulse seed was created by modulating a constant laser beam with an electro-optic modulator. An arbitrary waveform generator was used to pre-shape these pulses to compensate for pulse distortion caused by pump gain depletion in the subsequent fiber amplifiers. Pre-amplifiers were constructed using commercial erbium doped silica fiber. Special, highly doped, large core, phosphate fiber was developed in-house to further amplify the pulses, while avoiding nonlinear scattering processes such as stimulated Brillouin scattering and stimulated Raman scattering. THz generation was achieved in both ZnGeP₂ and GaP which were chosen based on their low pump and THz absorption, as well as high nonlinear coefficient. Angle tuning was used to phase match all three optical frequencies in ZnGeP₂ thanks to its birefringence. Layers of GaP ~500 µm thick were pressed together alternately rotated 180° around the normal to quasi-phase match the pump and THz frequencies. To increase the efficiency of the THz generation an external optical cavity was used to enhance and recycle the IR pump pulses. The nonlinear crystal was placed inside the cavity and 151 times enhancement of THz power was observed.

Fiber tapers have attracted much attention and have been successfully employed in various applications, ranging from resonators, filters, interferometers to sensors. This thesis studies the properties of fiber tapers for the purpose of making tapered-based devices and sensors in aerospace related application where small size and light weight are critical. This thesis includes theoretical derivation and experimental verifications of distributed mode coupling in tapered single-mode fibers (SMFs) with high-resolution optical frequency-domain reflectometry (OFDR) technique. The studies are realized with OFDR through phase detection of a Mach-Zehnder interferometer (MZI), which measures local refractive index change relative to the reference arm. The wavelength shifts converted by the phase change give the group index differences between the fundamental mode and higher-order modes of fiber tapers. The energy re-distribution is observed in Rayleigh backscatter amplitude as a function of fiber length with a ~13µm resolution over the entire fiber taper, and group index difference between core and cladding modes is measured with a spatial resolution of ~2cm by using autocorrelation data processing. The thermal and mechanical properties of fiber tapers have also been characterized with OFDR. The cross-correlation wavelength shift is related to the refractive index change of the modes. It is shown that residual stress induced by the tapering process results in the inhomogeneous thermal property, which can be significantly reduced by an annealing treatment. A fiber taper with a waist diameter of ~6µm has a force sensitivity of ~620.83nm/N, ~500 times higher than that of SMF. Furthermore, polarization-preserving character of tapered polarization-maintaining fibers (PMFs) is evaluated by OFDR-based distributed birefringence along tapered PMFs. Three tapered-based micro-fiber devices have been used as effective mode selecting components to build narrow-linewidth tunable Erbium-doped fiber ring lasers. The fabrication is easy and at a low cost. 1) a tapered fiber tip forms multimode interference mechanism; 2) a two-taper MZI has been demonstrated by splitting/combining the fundamental mode and higher-order modes through fiber tapers and is tuned by bending one taper waist; 3) a novel tunable fiber Fabry-Perot filter, consisting of a hollow-core photonic bandgap fiber and a micro-fiber, is employed in the reflection mode.