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 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

We report injection‐locking studies on a gain‐guided diode‐laser array using a single‐frequency cw tunable dye laser master oscillator. By injecting a single end element, both the spatial and spectral emission characteristics of the entire ten‐element array could be controlled. The injection‐locked array produced over 100 mW output in a single‐lobed far‐field beam ( 60 GHz wide. Furthermore, by varying the injection frequency, the far‐field emission angle of the array could be continuously scanned over several degrees at a rate of 2.3×10−2 deg/GHz.

Single‐channel injection locking of a diode‐laser array with a cw dye laser

The lateral modes of broad area lasers are investigated theoretically. The nonlinear interaction between optical field and effective refractive index leads to a saturable nonlinearity in the governing field equation, so that self-modulated solutions are found to be stable with increased current injection above saturation intensity. We derive approximate analytical solutions for traveling wave fields within the broad area laser. The field amplitude consists of a small ripple superimposed on a large dc value. Matching fields at the boundary determines the modulation depth and imparts an overall phase curvature to the traveling wave mode. There are multiple lateral modes for a given set of operating conditions, and modes with successively more lobes in the ripple have greater overall phase curvature. In contrast to the linear problem, several lateral modes can achieve the same modal gain, for a given injected current density, by saturating the gain to different extent. Thus, these modes would exhibit slightly different optical powers.

Self-stabilized nonlinear lateral modes of broad area lasers

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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U. Shreter

Papers

Phase-locked semiconductor laser array with separate contacts