Beam combining of laser arrays with high efficiency and good beam quality for power and radiance (brightness) scaling is a long-standing problem in laser technology. Recently, significant progress has been made using wavelength (spectral) techniques and coherent (phased array) techniques, which has led to the demonstration of beam combining of a large semiconductor diode laser array (100 array elements) with near-diffraction-limited output (M/sup 2//spl sim/1.3) at significant power (35 W). This paper provides an overview of progress in beam combining and highlights some of the tradeoffs among beam-combining techniques.

Laser beam combining for high-power, high-radiance sources

The objective of this paper is to present an overview of topics related to one of the fundamental parameters for semiconductor lasers-the linewidth broadening factor α that describes the coupling between carrier-concentration-induced variations of real and imaginary parts of susceptibility. After introducing the definition of α and discussing its dependence on carrier concentration, photon energy, and temperature, we give a short historical summary on how the concept of α evolved over the past two decades. This is followed by a discussion of α dependence on device structure in gain-guided and subdimensional lasers (quantum wells and quantum wires). The bulk of the paper is devoted to a detailed review of laser properties influenced by α and of associated methods of estimating the value of α. Results of measurements reported to date are collected and the most reliable methods are indicated.

Linewidth broadening factor in semiconductor lasers--An overview

We discuss a number of theoretical and experimental issues in quantum well lasers with emphasis on the basic behavior of the gain, the field spectrum, and the modulation dynamics It is revealed that the use of quantum well structures results in improvement of these properties and brings several new concepts to optical semiconductor devices

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Quantum well lasers--Gain, spectra, dynamics

A new coupled mode formulation for parallel dielectric waveguides is described. The results apply to any guided modes (TE, TM, or hybrid) in waveguides of arbitrary cross-section, dissimilar index, and nonidentical shape. Additional index perturbations not included within the waveguides are encompassed by the theory. Propagation constants and mode patterns for the coupled modes computed according to this theory are shown to agree very well with numerical solutions for the system modes when the latter can be determined. Moreover, the new results are more accurate than those obtained from prior coupled mode formulations. It is shown that even for Iossless guides the coupling coefficients from waveguide "b" to "a" and from "a" to "b," described by k ab and k ba respectively, are not related by their complex conjugates if the guides are not identical.

Coupled mode theory of parallel waveguides

The optical characteristics of phase-locked semiconductor laser arrays are formulated in terms of the array supermodes, which are the eigenmodes of the composite-array waveguide, by using coupled-mode theory. These supermodes are employed to calculate the near fields, the far fields, and the difference in the longitudinal-mode oscillation wavelengths of the array. It is shown that the broadening in the far-field beam divergence, as well as the broadening of each of the longitudinal modes that were observed in phase-locked arrays, may arise from the excitation of an increasing number of supermodes at increasing pumping levels.

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Supermode analysis of phase-locked arrays of semiconductor lasers

We propose nonuniform structures of phase-locked diode lasers, which make it possible to discriminate efficiently against all the higher order array supermodes (lateral modes). In these nonuniform arrays, the effective mode index in each channel varies across the array. Consequently, the envelopes of the various supermodes, including the highest order one, differ significantly from each other. Thus, by proper tailoring of the gain distribution across the array, one can conveniently select the fundamental supermode. Such fundamental supermode oscillation is essential in order to obtain single lobe, diffraction limited beams and minimal spectral spread from phase-locked laser arrays.

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Chirped arrays of diode lasers for supermode control

A new monolithic phase‐locked semiconductor laser array has been fabricated. Employing two‐level metallization, each of the eight elements in the array has a separate contact, thus making it possible to compensate for device nonuniformities and control the near‐field and far‐field patterns. Threshold currents are approximately 60 mA for each 5‐μm‐wide laser in the array. Phase locking has been observed via the narrowing of the far‐field pattern. Experimental results are compared to those obtained from the same arrays operated with all the lasers connected in parallel.

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Phase-locked semiconductor laser array with separate contacts

A formalism is developed which results in simple analytic expressions for the electron energy splittings and the wave functions in coupled quantum well structures of the type employed in new laser and transistor configurations. The limits of validity of the formalism are explored.

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Approximate analytic solution for electronic wave functions and energies in coupled quantum wells

An eight‐element phase‐locked array of index‐guided separate confinement ridge AlGaAs diode lasers is fabricated. In this array the absorption of light in the region between lasers is negligible and the gain profile across the array is nearly uniform. Unlike most other arrays, this array oscillates in its fundamental mode. Stable radiation patterns of near diffraction‐limited single narrow beam with 1.6° width are obtained. The beam width approaches the theoretical limit for the present array structure.

Fundamental mode oscillation of a buried ridge waveguide laser array

We demonstrate a single contact tailored gain‐guided array in which the gain profile across the array is made strongly asymmetric by varying the width of the contact stripes. A proton isolated array of six (GaAl)As lasers with 5‐μm separations and widths varying linearly between 3 and 8 μm had a single lobed far field 2° wide, close to the diffraction limit for a single supermode. Fabrication of this device is simple, and suited to large‐scale processing techniques. We also show that in such an asymmetric gain‐guided array the fundamental mode is favored over higher order modes, and that higher order modes can have single lobed far‐field patterns differing only slightly from that of the fundamental.

/pdf/single-contact-tailored-gain-phased-array-of-semiconductor-32840w313b.pdf

C. Lindsey

Papers

Chirped arrays of diode lasers for supermode control

Phase-locked semiconductor laser array with separate contacts

Approximate analytic solution for electronic wave functions and energies in coupled quantum wells

Fundamental mode oscillation of a buried ridge waveguide laser array

Single contact tailored gain phased array of semiconductor lasers