High Power Gain Guided Index Antiguided Fiber Lasers and Amplifiers

Abstract Increasing the core size of high-power fiber lasers and amplifiers is highly desired in order to mitigate the unwanted nonlinear optical effects and raise the optical damage threshold. If the core size of conventional index-guided (IG) optical fibers increases, the fiber will become multimode, because it is very difficult to control and fine-tune the index step between the core and cladding to satisfy the single mode condition. Siegman proposed Gain-guided index-antiguided (GG-IAG) fibers as a possible platform for ultra-large-core single-mode operation for lasers and amplifiers. In this thesis, the beam-quality factor M2 for the fundamental LP01 mode of a step-index fiber with finite and infinite cladding diameter is calculated in the presence of gain as a function of the complex generalized V number. The numerical results agree with analytical work that obtained in our group. It is shown that the M2 value of a single-mode gain-guided fiber laser can be arbitrarily large. The results are important for the interpretation of the beam-quality measurements in recent experiments on single-mode gain-guided fiber lasers. It is also shown that the conventional infinite cladding diameter approximation cannot be used for index-antiguided gain-guided fibers, and the rigorous analysis is required for accurate prediction of the beam quality factor, as reported in recent experimental measurements. We also highlight the key reasons behind the poor power efficiency observed in multiple experiments in gain guided index-antiguided (GG-IAG) fiber amplifiers and lasers. We show that by properly designing the fiber geometrical characteristics, it is possible to considerably improve the power efficiency of GG-IAG fiber amplifiers in end-pumping schemes

The Development Of Scalable Pump Techniques For Gg Iag Fiber Lasers And Passive Athermalization Techniques For Solid State Laser

This dissertation consists of two parts: research pertaining to the development of scalable pump techniques for gain guided index-antiguided fiber lasers and research relating to the development of passive athermalization schemes for solid state lasers. The first section primarily details the development of a side pump scheme that allows for power scaling of gain-guided index anti-guided fibers. While these fibers have been demonstrated in past research, none have used a pump technology capable of pumping with the efficiencies, uniformity, and necessary length to allow for scaling of the fiber lasers to high output powers. The side pumped scheme developed in this section demonstrates a 6 W output power fiber laser with room for improvement in efficiency and beam quality. The second section details work done on the development of technologies for passively athermalizing the output of solid state laser systems. Techniques for passively removing the dependence of laser output power/energy on the operating temperature of the laser system promise to reduce the weight, power consumption, and cost of fielded laser systems. Methods for achieving passive athermalization are discussed, as well as prior research in laser athermalization, background theory, enabling technologies, and experimental results. This work provides the basis for continued research of passive athermalization and the eventual demonstration of this technology

Supersymmetric Laser Arrays

The theoretical framework of supersymmetry (SUSY) aims to relate bosons and fermions -- two profoundly different species of particles -- and their interactions. While this space-time symmetry is seen to provide an elegant solution to many unanswered questions in high-energy physics, its experimental verification has so far remained elusive. Here, we demonstrate that, notions from supersymmetry can be strategically utilized in optics in order to address one of the longstanding challenges in laser science. In this regard, a supersymmetric laser array is realized, capable of emitting exclusively in its fundamental transverse mode. Our results not only pave the way towards devising new schemes for scaling up radiance in integrated lasers, but also on a more fundamental level, they could shed light on the intriguing synergy between non-Hermiticity and supersymmetry

Broad Bandwidth, All-fiber, Thulium-doped Photonic Crystal Fiber Amplifier for Potential Use in Scaling Ultrashort Pulse Peak Powers

Fiber based ultrashort pulse laser sources are desirable for many applications; however generating high peak powers in fiber lasers is primarily limited by the onset of nonlinear effects such as self-phase modulation, stimulated Raman scattering, and self-focusing. Increasing the fiber core diameter mitigates the onset of these nonlinear effects, but also allows unwanted higher-order transverse spatial modes to propagate. Both large core diameters and single-mode propagation can be simultaneously attained using photonic crystal fibers. Thulium-doped fiber lasers are attractive for high peak power ultrashort pulse systems. They offer a broad gain bandwidth, capable of amplifying sub-100 femtosecond pulses. The longer center wavelength at 2 ?m theoretically enables higher peak powers relative to 1 [micro]m systems since nonlinear effects inversely scale with wavelength. Also, the 2 [micro]m emission is desirable to support applications reaching further into the mid-IR. This work evaluates the performance of a novel all-fiber pump combiner that incorporates a thulium-doped photonic crystal fiber. This fully integrated amplifier is characterized and possesses a large gain bandwidth, essentially single-mode propagation, and high degree of polarization. This innovative all-fiber, thulium-doped photonic crystal fiber amplifier has great potential for enabling high peak powers in 2 [micro]m fiber systems; however the current optical-to-optical efficiency is low relative to similar free-space amplifiers. Further development and device optimization will lead to higher efficiencies and improved performance

The Materials Science and Engineering of Advanced YB-Doped Glasses and Fibers for High-Power Lasers

This research studies and yields new understandings into the materials science and engineering of advanced multicomponent glass systems, which is critical for next generation fiber lasers operating at high output powers. This begins with the study and development of Yb-doped glasses in the Al2O3-P2O5-SiO2 (APS) ternary system, fabricated using modified chemical vapor deposition (MCVD), that, despite being highly doped, possess an average refractive index matched to that of silica (SiO2). The highly doped active core material was subsequently processed through a multiple stack-and-draw process to realize a single fiber with high doping, compositionally-tailored index, and scalability for fiber lasers. Based on the knowledge gained in this first focal area, further strategic compositional tailoring to influence the glass’ photoelastic and thermo-optic coefficient, was performed in order to understand and realize significant decreases in Brillouin and thermal-Rayleigh scattering, which instigate parasitic stimulated Brillouin scattering (SBS) and transverse mode instabilities (TMI) in high power fiber lasers. In addition to understanding the composition / structure / properties of these glasses, a double-clad fiber laser will be fabricated, scaled to over 1 kW of output laser power, and studied in order to relate the materials science and engineering of multiple glass systems and fibers designs to laser performance and properties

Laser peak power scaling and beam quality improvement with Ytterbium rod-type fiber amplifiers made by powder sinter technology

Fiber amplifiers with a robust monolithic seed coupling and very high peak power in a near diffraction-limited beam are increasingly demanded by many industrial applications in laser materials processing. A large mode area fiber is used to scale up the peak power and suppress the nonlinear effects. An approach of local adiabatic taper is proposed to provide a monolithic signal path and selectively excite the fundamental mode in highly multimode fiber. The powder-sintering technology was employed to achieve rod-type fibers with excellent refractive index homogeneity. First experiments were performed with 56m core diameter rod fibers. While non-tapered fiber amplifier achieved a peak power of 544kW, tapered amplifier reached 230kW. For comparable average powers of 10W, the taper improves the beam quality from M2 values of about 10 to 3.5, while the monolithic seed coupling significantly improves the beam stability. It was observed that the dopants diffuse during the tapering process because of high temperature, possibly providing further sources for coupling to higher order modes. Second experiments with improved rod-type fiber amplifiers (reduce the Al3+-content of fiber core and use suitable material of outer clad to mitigate the diffusion problem) delivered 2ns pulses with peak powers of 210kW for the non-tapered rod and 140kW for the tapered rod (limited by facet damage). For the tapered fiber, the beam quality was between 1.3 and 1.7, significantly improved compared to the beam quality of the non-tapered fiber (M2 = 3.3 ~ 4.5). An endcap was adopted for the tapered fiber amplifier and the peak power is scaled up to 375kW in the nearly diffraction limited region. For future work, the confined doped fibers and a picosecond seed laser source are envisioned

Near diffraction limited high-power narrow-linewidth Er3+-doped fiber amplifiers : developments towards laser sources at 1.5 [my]m wavelength for gravitational wave astronomy

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Highly Doped Phosphate Glass Fibers for Compact Lasers and Amplifiers: A Review

In recent years, the exploitation of compact laser sources and amplifiers in fiber form has found extensive applications in industrial and scientific fields. The fiber format offers compactness, high beam quality through single-mode regime and excellent heat dissipation, thus leading to high laser reliability and long-term stability. The realization of devices based on this technology requires an active medium with high optical gain over a short length to increase efficiency while mitigating nonlinear optical effects. Multicomponent phosphate glasses meet these requirements thanks to the high solubility of rare-earth ions in their glass matrix, alongside with high emission cross-sections, chemical stability and high optical damage threshold. In this paper, we review recent advances in the field thanks to the combination of highly-doped phosphate glasses and innovative fiber drawing techniques. We also present the main performance achievements and outlook both in continuous wave (CW) and pulsed mode regimes