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.

The objective of the proposed research is to develop multiwavelength lasers as cost-efficient sources for broadband optical access networks. Todays telecommunications networks have widely adopted optical fiber as the backbone transmission medium. Optical fiber systems are promising candidates for the broadband access networks to offer high-speed and future-proof services. To harness the available bandwidth in fiber and to meet the ever-growing bandwidth demand, wavelength division multiplexing (WDM) techniques have been investigated. There have been intense research activities for the creation of new low-cost laser sources for such emerging applications. In this context, multiwavelength fiber ring lasers have been significantly investigated as they present many advantages, including simple structure, low-cost, and selectable multiwavelength operation. We propose a new laser system architecture that emits alternate multiwavelength picosecond pulses operating at room temperature. Optical signal generation is based on a single active component, an unbalanced Mach-Zehnder interferometer, inserted in an actively mode-locked erbium-doped fiber ring laser to provide both intensity modulation and wavelength-selective filtering. Time and frequency controls of the light emission are reached by inserting an additional modulator and a periodic filter in the cavity. This approach focuses on the application of multiwavelength lasers as sources for WDM passive optical networks.

Recently, with the escalating demands for optical communication, the need to use broadband laser light sources in optical communication network system has increased. Henceforward, the broadband characteristes of Cr4+:YAG crystal fiber possess signifies its indispensability. Furthermore, Yb3+:YAG-silica also has its advantages in high power laser domain. In this thesis, the crystal fiber grown by the laser heated pedestal growth method is used as the laser gain medium with fused silica packaging technique. Cr4+:YAG double-clad crystal fiber with a core diameter as small as 11 £gm was achieved. Moreover, a Yb3+:YAG-silica layer was formed due to the strong inter-diffusion between silica capillary and Yb3+:YAG crystal. When the silica all diffused into the Yb3+:YAG, a Yb3+:YAG-silica fiber with 125-£gm core was obtained with waveguide structure. By directly coating the optical thin films onto the end faces of the two types of fibers, the laser configuration is compact and cost effective. Besides, heat dissipation is also improved. By Cu-Al alloy packaging, a record-low Cr4+:YAG double-clad crystal fiber laser was achieved with threshold of 0.75 mW and a record-high slope efficiency of 6.9% at room temperature. And we also successfully fabricate the Yb3+:YAG-silica fiber laser with low threshold (100 mW) and high efficiency (67.2%) at room temperature. In fiber laser development, we have successfully fabricated the coating of high-reflective thin films which match the faces of fiber heterostructure (single cladding and double cladding structures). It forms a cavity with anti-reflectivity for pumping wavelength and high-reflectivity for lasing wavelength. For these reasons, low threshold, high slope efficiency, and stable laser output have been achieved. Finally, through different thin-film designs, the strain effect between thin film and heterosubstrate is significantly reduced, which facilitates the realization of high performance fiber lasers.

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.

Thulium (Tm) based high repetition rate compact optical frequency comb sources operating in the 2 µm regime with femtosecond pulse durations enable a wide range of applications such as precise micro-machining, spectroscopy and metrology. Applications such as metrology and spectroscopy rely on the stability of mode-locked lasers (MLLs) which provide extreme precision, yet, the complex dynamics of such highly nonlinear systems result in unstable events which could hinder the normal operation of a MLL. MLL as a nonlinear system inherently exists a wide variety of complex attractors, which are sets of states that the system tends to evolve toward, exhibiting unique behaviors. Complex phenomena including pulsating solitons, chaotic solitons, period-doubling, soliton explosion, etc., have been predicted theoretically and observed experimentally in the past decade. However, most experimental observations rely on conventional characterization methods, which are limited to the scanning speed of the spectrometer and the electronic speed of photodetector and digitizer, so that the details of the non-repetitive events can be buried. In recent years, a technique called dispersive Fourier transform (DFT) has been developed and allows consecutive recordings of the pulse-to-pulse spectral evolution of a femtosecond pulse train, opening a whole new world of nonlinear dynamics in MLL. In this dissertation, we first demonstrate the ability of scaling the repetition rate of a Tm MLL to repetition rate as high as 1.25 GHz through miniaturizing the cavity. Our approach of maintaining comparable pulse energies while scaling the repetition rates allows a high-quality femtosecond mode-locking performance with low noise performance in Tm soliton lasers. Then we experimentally study the transition dynamics between consecutive multi-pulsing states through adjusting pump power with a constant rate in an erbium-doped fiber laser, specifically the build-up and annihilation of soliton pulses between a double pulsing and a three-pulse state utilizing DFT. To investigate real-time laser dynamics in Tm based laser systems, we propose and develop a DFT system that up-converts the signal to the 1 µm regime via second harmonics generation (SHG) and stretches the signal in a long spool of single-mode fiber to realize DFT. This approach overcomes the limitation of bandwidth of 2 µm photodetector and high intrinsic absorption of 2 µm light in fused silica fibers. The SHG-DFT system is used to study dynamics of both explosions in a chaotic state between stable single-pulsing and double-pulsing state, and explosions induced by soliton collision in a dual-wavelength vector soliton state. We also study dynamics of transient regimes in a Tm-doped fiber ring laser that can be switched between conventional soliton and dissipative soliton, revealing how spectral filtering plays a role in obtaining stable stationary states. / 2022-11-23T00:00:00Z

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In recent years laser technology has increasingly developed with the use of fiber lasers and this has provided the possibility to implement different techniques in the defense industry. LADAR is at the forefront of these techniques. Fiber lasers constitute a perfect source for LADAR systems due to their excellent robustness, compact size and high-power generation capability. In this study we will explore the development of a pulsed fiber laser source for a LADAR system that can obtain high resolution 3D images in eye-safe region. A high power, all fiber integrated erbium system with strictly single mode operation in eye-safe region based on MOPA (master oscillator power amplifier) configuration with seed source and amplifier part was developed. Both the use of an actively mode locked laser with erbium doped fiber and fiber coupled modulated distributed feedback diode laser were investigated as seed sources for the amplifier part. Both erbium doped single clad fiber and erbium-ytterbium doped double clad gain fiber were used in this amplifier system. After amplification of the actively mode locked laser, 12 W of average optical power was obtained through single mode fiber with 1ns pulse duration at 10 MHz which corresponds to 1.2 kW peak power. For the fiber coupled DFB diode laser, 9.5W average power was obtained with around 8 ns duration pulses at 100 kHz and about 9.2 W average power was also obtained with around 700 ps duration pulses at 1 MHz through strictly single mode fiber at the output of the same amplifier system as was used in the actively mode locked seed source. In both cases calculated peak power was around 10 kW v which is estimated as the highest peak power for an all fiber integrated system with single mode operation. The development of such a fiber system with high power capability, compact size and free of misalignment is expected to be useful for LADAR application as well as other areas such as eye surgery, 3D silicon processing or any other material processing applications.

Fiber lasers represent the state-of-the-art in laser technology and hold great promise for a wide range of applications because they have a minimum of exposed optical interfaces, very high efficiency, and are capable of exceptional beam quality. In the near future, the most important markets such as micromachining, automotive, biomedical and military applications will begin to use this technology. The scope of this thesis is to design and develop a short picosecond pulsed fiber laser using rare-earth doped fiber as a gain medium. The proposed master oscillator power amplifier (MOPA) will be used to generate pulses with high repetition rates. In this study, first we explain the basic theoretical background of nonlinear optics and fiber laser. Then, the numerical simulation will be introduced to explain how the laser system design and optimization. The simulation is based on nonlinear Schr&ouml / dinger equation with the method of split-step evaluation. The brief theoretical background and simulation results of the laser system will be shown. Finally, the experimental study of the developmental fiber laser system that comprises an oscillator, preamplifier and power amplifier will be discussed.

Compact high power fiber lasers and the vertical-external-cavity surface-emitting lasers (VECSELs) are promising candidates for high power laser sources with diffraction-limited beam quality and are currently the subject of intensive research and development. Here three large mode area fiber lasers, namely, the photonic crystal fiber (PCF) laser, the multicore fiber (MCF) laser, and the multimode interference (MMI) fiber laser, as well as the VECSEL are modeled and designed.For the PCF laser, the effective refractive index and the effective core radius of the PCF are investigated using vectorial approaches and reformulated. Then, the classical step-index fiber theory is extended to PCFs, resulting in a highly efficient vectorial effective-index method for the design and analysis of PCFs. The new approach is employed to analyze the modal properties of the PCF lasers with depressed-index cores and to effectively estimate the number of guided modes for PCFs.The MCF laser, consisting of an active MCF and a passive coreless fiber, is modeled using the vectorial mode expansion method developed in this work. The results illustrate that the mode selection in the MCF laser by the coreless fiber section is determined by the MMI effect, not the Talbot effect. Based on the MMI and self-imaging in multimode fibers, the vectorial mode expansion approach is employed to design the first MMI fiber laser demonstrated experimentally.For the design and modeling of VECSELs, the optical, thermal, and structural properties of common material systems are investigated and the most reliable material models are summarized. The nanoscale heat transport theory is applied for the first time, to the best of my knowledge, to design and model VECSELs. In addition, the most accurate strain compensation approach is selected for VECSELs incorporating strained quantum wells to maintain structural stability. The design principles for the VECSEL subcavity are elaborated and applied to design a 1040nm VECSEL subcavity that has been demonstrated for high power operation of VECSELs where near diffraction-limited output over 20 W is obtained. Physical modeling of the VECSEL is also discussed and used to compare VECSEL subcavity designs on the laser level.