Extremely compact high-power fiber lasers operating at eye-safe telecom wavelength of 1.5 μm have been achieved by systematic experimental studies. Heavily Er3+-Yb3+-codoped phosphate glasses have been chosen as the host glass for our fibers and 1.5 μm lasers have been realized when pumped with 975 nm laser diodes.The first short-length cladding-pumped fiber laser with watt-level CW output power has been demonstrated by an 11-cm-long doped step-index phosphate fiber. Without active cooling, 1.5 W output power at 1535 nm has been obtained.Thermoelectric cooler has been used for heavily doped phosphate step-index fibers. A dual-end-pumped actively cooled fiber laser has generated more than 11 W CW output power at 1535 nm from an 11.9 cm long active fiber. A fully 3-dimensional thermal analysis has been performed to calculate the internal temperature distribution of the short-length fiber laser and the simulated results have been experimentally verified.Phosphate glass microstructured optical fibers (MOFs) with large active cores have been fabricated. The first demonstrated short phosphate MOF laser has generated > 3 W single-mode CW output power from an 11-cm-long fiber. The impacts of depressed-core-index and annealing upon MOF's modal property have been systematically investigated. Extremely compact high-power fiber laser is demonstrated by a heavily doped MOF laser and > 4.5 W single-mode CW output power is delivered from a 3.5 cm long fiber. Finally, a high-power single-frequency fiber laser is realized by splicing a FBG with a 3.8 cm long MOF, which achieves > 2 W single-frequency output power.

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.

Narrow linewidth tunable fiber lasers have become an important tool in research and in-field applications thanks to their high beam quality, great spectral performance, compact structure and environmental robustness. Many methods have been introduced and developed throughout decades to fulfill the need for rapid wavelength adjustment of these fiber lasers. While maintaining a high-level spectral performance, the ease of manipulation and cost effectiveness of the tuning operation are considered beneficial for the lasers used in real world, and therefore, they are the main focus of the work presented in this thesis.                                        An accurate tuning method with narrow linewidth and compact configuration was achieved in the first work. A polarization independent semiconductor optical amplifier (SOA) was used as the gain medium in a unidirectional fiber ring cavity with a circulator connected to a 6-meter long chirped fiber Bragg grating (CFBG). The laser wavelength was chosen by setting the modulation frequency of the SOA the same as the harmonics of the fundamental repetition rate of the light reflected at a specific point on the CFBG. Careful management of the drive current and pulse width helped to generate laser light of narrow linewidth (less than 0.03 nm) and with low power variation (1.46 dB) over a tuning range of 40 nm.                                                      One example of the application of a tunable fiber source was discussed in the second work. An efficient Erbium Ytterbium fiber amplifier, which is seeded by a distributed feedback laser, was designed for continuous-wave differential absorption light detection and ranging (CW DIAL) of atmospheric CO2-concentration. It had a linewidth of 3 MHz, a tuning range of 2 nm over the CO2 absorption peaks at 1.572 μm and an output power of 1.3 W. Wavelength tuning is achieved by adjusting the drive current to the seed laser. Results from the initial CW DIAL testing demonstrate that this tunable fiber source meets the high demands for range resolved atmospheric CO2 monitoring.                                                                                  To conclude, two narrow linewidth tunable fiber lasers have been demonstrated based on different tuning mechanism. The rapid and accurate tuning operation with low output power variation is achieved in both works. The great spectral properties of these fiber sources make them powerful tools to be used in applications such as optical communication, remote sensing, spectroscopy, optical coherence tomography, and many more. / Smalbandiga avstämbara fiberlasrar har blivit viktiga instrument inom forskning och andra tillämpningar på grund av deras höga strålkvalitet, goda spektrala egenskaper, kompakthet och stora miljötålighet. Under de senaste decennierna har flera metoder introducerats och utvecklats för att kunna göra snabba våglängdsjusteringar för sådana lasrar, med bibehållande av gott spektralt uppförande. Billiga, smalbandiga, enkelt avstämbara lasrar är viktiga för praktiska tillämpningar, och arbetet i denna avhandling har handlat om att utveckla nya sådana och studera deras egenskaper. I det första arbetet togs en precis avstämningsmetod med smal linjebredd tillsammans med en kompakt utformning fram. En polarisationsoberoende optisk förstärkare av halvledartyp (SOA) användes som fotonkälla i en fiberring-kavitet med en fiberoptisk cirkulatorn kopplad till ett 6 meter långt fiber- Braggitter, ett s.k. ¨chirpat¨, eller kvittrat fiber- Braggitter,( CFBG). Laservåglängden valdes genom att sätta modulationsfrekvensen för SOA-förstärkaren till önskad resonansfrekvens i kaviteten. Endast ljus som reflekterades i en bestämd punkt i CFGBgittret förstärks då, vilket svarade mot den önskade våglängden. Med noggrann inställning av drivström fick man då en smal pulsbredd (mindre än 0,03 nm), avstämbart över ett intervall på 40 nm och med bara en liten variation i effekten (1,46 dB). I det andra arbetet utvecklades en annan avstämbar fiberbaserad laserkälla för att användas i en ny gasmätningsmetod av atmosfären. Metoden utnyttjar kontinuerligt ljus och differentiell absorptions-LIDAR (CW DIAL), där man mäter återspritt ljus från den önskade molekylen på, eller vid sidan om en absorptionslinje hos gasen. Från resultaten kan man sen räkna ut den lokala koncentrationen av gasen med hög spatiell noggrannhet. Lasersystemet bestod av en Yb-Er-fiberförstärkare matad från en smalbandig, avstämbar diodlaser (DFBlaser). Systemet hade en linjebredd på 3 MHz, en avstämbarhet på 2 nm avpassat för absorptionstopparna för CO! vid 1,572 µm, och en uteffekt på 1,3 W. Våglängden kunde ändras genom justering av drivströmmen till DFB-lasern. Systemet användes för att mäta CO! med den nya tekniken för första gången och resultaten visade att den avstämbara fiberkällan motsvarar de höga krav som ställs på monitorering av CO! i atmosfären. Sammanfattningsvis har två avstämbara fiberlasrar med smal linjebredd, baserade på olika avstämningsmekanismer, demonstrerats. För båda lasrarna kan en snabb och noggrann avstämning uppnås, med liten variation i uteffekt. Dessa fiberkällors goda spektrala egenskaper gör dem till kraftfulla verktyg för användning i olika tillämpningar som t.ex. optisk kommunikation, fjärranalys, spektroskopi och optisk koherenstomografi (OCT).

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.

The lasers operating in the wavelength range of 900 - 1000 nm have caused intense attention because they are in great demands for: 1. Highpower blue and deep UV laser generation 2. High power single-mode pump laser source 3. Light detection and Lidar , etc. And now, there are actually many different types of lasers can generate laser in this wavelength range. For example, Nd and Yb doped fiber laser, Nd and Yb doped glass and crystal lasers, OPO and SHG laser, etc. Among all this options, we decided to study the Nd-doped fiber laser for their outstanding advantages: 1. As fiber laser, it possess all the advantages of any fiber lasers have, such as: high power scalability, excellent beam quality, high spectral and intensity stability, super compactness, robustness and reliability. 2. Comparing to other rare-earth-ion, the Nd^3+ ions have a more broad emission wavelength range from 900-950 nm. My goals for doing this thesis research are:1.Experimentally and theoretically investigate Nd-doped fiber lasers and amplifiers at 9xx nm. 2. Develop 9xx nm single frequency fiber lasers and amplifiers. 3.Obtain directions for developing high power single-frequency Nd-doped fiber laser sources at 9xx nm. To achieve these goals, 1. Nd-doped fiber lasers at 934 nm were investigated. 2. Core-pumped and cladding-pumped Nd-doped fiber amplifiers are also investigated. 3. The simulation of the Nd-doped fiber amplifiers have been done.

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.

A wide band linearly chirped erbium-doped fiber laser has been developed. The erbium-doped fiber laser using a rotating mirror/grating combination as one of the reflectors in a Fabry-Perot laser cavity has been tuned over a 46 nm spectral range. Linearization of the chirp rate has been achieved using feedback from a fiber Fabry-Perot interferometer (FFPI) to adjust the voltage ramp which drives the rotating mirror. In a demonstration of monitoring an array of two fiber Bragg grating (FBG) sensors, a wavelength resolution of 1.7 pm has been achieved. The linearly chirped fiber laser has been used in measuring the optical path difference (OPD) of interferometric fiber optic sensors by performing a Fourier transform of the optical signal. Multiplexing of an array of three FFPI sensors of different lengths has been demonstrated, with an OPD resolution ranging from 3.6 nm to 6.3 nm. Temperature was measured with one of the sensors over the range from 20°C to 610°C with a resolution of 0.02°C. Short FBGs are used to form the two mirrors of a fiber Bragg grating pair interferometer (FBGPI) sensor, so that the mirror reflectances change gradually as a function of temperature. Modulating the drive current of a DFB laser produces chirping of the laser frequency to scan over ~2.5 fringes of the FBGPI reflectance spectrum. Because the fringes are distinguished due to the FBG reflectance change, the ambient temperature can be determined over the range from 24 oC to 367 oC with a resolution of 0.004 oC. Multiplexing of FBGPI sensors of different lengths with a linearly chirped fiber laser has demonstrated improved sensitivity and multiplexing capacity over a conventional FBG WDM system. The FBG spectral peak position and the phase shift of an FBGPI are determined through the convolution of the sensor reflected signal with an appropriately matched reference waveform, even though the reflectance spectra for the FBGs from different sensors overlap over a wide temperature range. A spectral resolution for the FBG reflectance peak of 0.045 GHz (0.36 pm), corresponding to a temperature resolution of 0.035 oC, has been achieved.

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.