A. E. Siegman's research while affiliated with Stanford University and other places

Single mode laser action in a diode-pumped gain-guided, index- antiguided Nd3+-doped phosphate glass fiber having a 200-μm-diameter core is demonstrated. Near-Gaussian beam quality was maintained, even when pumped up to four times the threshold pump power, indicating robust lowest order mode oscillation. Subtle differences associated with the effectiveness of diode pumping gain-guided, index-antiguided fibers are discussed.

Single mode laser oscillation is achieved in a 200 μum diameter core end pumped gain guided index anti-guided fiber. The near-Gaussian beam quality of the laser is maintained at up to four times laser threshold.

Optical fibers with large diameter cores having a negative index step from cladding to core combined with an adequately large gain coefficient in the core provide near-optimal mode properties for fiber lasers delivering robust single transverse mode operation with very large mode areas. Basic properties and simple design formulas for such fibers are presented.

The authors report laser oscillation in what appears to be a single transverse mode with very large mode area in optical fibers having heavily Nd-doped 100 μ m diameter cores with refractive index significantly lower in the core than in the surrounding cladding. Since fibers of this type cannot support conventional index-guided modes, their results appear to confirm a recent analysis which predicts gain-guided single-mode propagation in index antiguided fibers, provided the gain coefficient in the core exceeds a threshold value. Fibers of this type may be of significant interest for amplifiers and oscillators having large power outputs and/or small nonlinear pulse distortion.

This memo discusses the resonant modes of coupled fiber structures consisting of N optical fibers linearly coupled together by a lossless fiber coupling assembly of some sort. Results include expressions for the statistical distributions of reflectivities and internal mode losses in N -to-1 coupling systems as a function of the number of fibers N in the system; expressions for the power output variation as increasing numbers of fibers are lit up or activated in such a system; and discussions of the far-field beam properties likely to be exhibited by elementary coupled fiber optic arrays. c 2004 Optical Society of America Note: A few of the figures in this memo have been copied from earlier publications as a temporary measure. I hope the original authors will not object to this, but these figures should not be reproduced further until independent versions have been prepared or proper permissions have been obtained.

We propose a generalized radiation-field quantization formalism, where quantization does not have to be referenced to a set of power-orthogonal eigenmodes as conventionally required. This formalism can be used to directly quantize the true system eigenmodes, which can be non-power-orthogonal due to the open nature of the system or the gain/loss medium involved in the system. We apply this generalized field quantization to the laser linewidth problem, in particular, lasers with non-power-orthogonal oscillation modes, and derive the excess-noise factor in a fully quantum-mechanical framework. We also show that, despite the excess-noise factor for oscillating modes, the total spatially averaged decay rate for the laser atoms remains unchanged.

Optical fibers in which gain-guiding effects are significant or even dominant compared with conventional index guiding may become of practical interest for future high-power single-mode fiber lasers. I derive the propagation characteristics of symmetrical slab waveguides and cylindrical optical fibers having arbitrary amounts of mixed gain and index guiding, assuming a single uniform transverse profile for both the gain and the refractive-index steps. Optical fibers of this type are best characterized by using a complex-valued v-squared parameter in place of the real-valued v parameter commonly used to describe conventional index-guided optical fibers.

The conventional Fourier transform has a well-known uncertainty relation that is defined in terms of the first and second moments of both a function and its Fourier transform. It is also well known that Gaussian functions, when translated to an arbitrary centre and supplemented by a linear phase factor, provide a complete set of minimum uncertainty states (MUSs) that exactly satisfies this uncertainty relation. A similarly general set of MUSs and uncertainty relations are derived here for discrete and/or periodic generalizations of the Fourier transform, namely for the discrete Fourier transform and the Fourier series. These extensions require a modified definition for the width of a periodic distribution, and they lead to more complex uncertainty relations that turn out to depend on the centroid location and mean frequency of the distribution. The derivations lead to novel generalizations of Hermite–Gaussian functions and, like Gaussians, the MUSs can play a special role in a range of Fourier applications.

Work has continued on laser beam analysis, laser beam characterization, and laser beam quality. Un expected analytical and numerical result have been developed, and experimental work on the development of quasi-monlithic diode-pumped unstable resonator lasers continues. A number of papers have been published or are in course of preparation, as detailed in the report.