Lee W. Casperson

Gain guiding in large-core Bragg fibers

We theoretically analyze gain guiding in large-core Bragg fibers, to be used for large-mode-area laser amplifiers with single-transverse-mode operation. The signal is gain-guided in a low-index core, whereas the pump is guided by the photonic bandgap of the Bragg cladding to achieve good confinement. The high-index layers in the Bragg cladding are half-wave thick at the signal wavelength in order to eliminate Bragg reflection, reducing the Bragg fiber effectively to a step-index fiber for gain guiding.

Gain saturation in gain-guided slab waveguides with large-index antiguiding

We investigate numerically and analytically the effects of gain saturation on the propagation of the fundamental mode in a gain-guided index-antiguided slab waveguide. The propagating mode adapts to gain saturation by becoming less confined, while at the same time its peak intensity increases more slowly. At steady state, both the mode shape and the power remain constant.

Thermal refraction focusing in planar index-antiguided lasers

Thermal refraction focusing in planar index-antiguided lasers is investigated both theoretically and experimentally. An analytical model based on zero-field approximation is presented for treating the combined effects of index antiguiding and thermal focusing. At very low pumping power, the mode is antiguided by the amplifier boundary, whereas at high pumping power it narrows due to thermal focusing. Theoretical results are in reasonable agreement with experimental data.

Gain Saturation and Output Characteristics of Index-Antiguided Planar Waveguide Amplifiers With Homogeneous Broadening

We investigate numerically and analytically the propagation characteristics of gain-guided modes in index-antiguided planar waveguides. We show that the gain-guided mode has modal confinement proportional to the modal gain, and single-transverse mode propagation is very robust in such waveguides. The power evolution of the fundamental mode can be described analytically with very good accuracy. Limiting power and optimum length of the waveguide amplifiers can also be obtained analytically.

Power characteristics of homogeneously broadened index-antiguided waveguide lasers.

A model is reported that describes a bidirectional homogeneously broadened index-antiguided (IAG) slab laser having arbitrary single-pass gain and distributed losses. Maximum extraction efficiency and corresponding optimum output coupling are determined for various values of unsaturated gain and loss per pass. A method is proposed to determine the intrinsic laser parameters from output power measurements.

Stability criteria for spontaneously pulsing gas lasers

Low-threshold spontaneous pulsations are known to occur in the output beams of certain high-gain gas lasers, and good agreement between the experimental observations and numerical models has been achieved. There have also been several analytical studies of threshold criteria for spontaneously pulsing lasers. However, the analytical studies are mostly not applicable to the lasers in which the pulsations have been observed. Stability criteria for high-gain gas lasers are derived in this study, and these criteria are compared with a previously modeled gas laser instability.

Transverse mode competition in index-antiguided waveguide lasers

Transverse mode competition in large-mode-area index-antiguided planar waveguide lasers is investigated. We show that, with very large core width and/or index difference between the core and cladding, high-order modes can oscillate and contribute to output lasing power. We have developed a theoretical model for transverse mode competition that takes into account transverse spatial hole burning. Experiments conducted on index-antiguided waveguide lasers with core width of 220 μm and 400 μm are compared to theory with good agreement.

Thermal refraction defocusing in very-large-core step-index antiguided fiber lasers

Thermal refraction defocusing in very-large-core step-index antiguided fibers is investigated theoretically. Recent analytical methods based on a zero-field approximation for studying the effects of thermal focusing are extended and applied to the defocusing effects that occur in laser media with negative temperature coefficient of refractive index. Intermediate values of thermal defocusing tend to flatten the radial intensity distribution, while strong thermal defocusing is shown to concentrate the intensity near the antiguiding core–cladding boundary. The same analysis is also applicable for very-large-core graded-index fibers with a divergent parabolic index profile.

Transverse mode competition in index-antiguided planar waveguide lasers with large mode area

We study transverse mode competition in index-antiguided planar lasers theoretically and experimentally. Robust fundamental mode operating 10 times above lasing threshold is predicted and confirmed experimentally.

Continuous-wave hybrid index-antiguided and thermal- guided planar waveguide laser with large mode area

We present lasing experiments in index-antiguided (IAG) planar waveguides made of Nd:YAG (refractive index = 1.820) sandwiched by Terbium Gallium Garnet (refractive index = 1.954). For a core width of 220 μm, we observe fundamental mode oscillation with maximal 2.62 W output power and 0.109 slope efficiency. An analytical model is developed taking into account simultaneous index-antiguiding, gain guiding and thermal induced refractive focusing. For a core width of 400 μm, fundamental mode oscillation is only observed near the lasing threshold, suggesting that the maximally core width in IAG waveguides for fundamental mode operation depends on the strength of index antiguiding.