Supermodes of multiple-stripe quantum-well heterostructure laser diodes operated (cw, 300 K) in an external-grating cavity

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Abstract
The far-field supermode patterns of a phase-locked multiple-stripe quantum-well heterostructure (QWH) laser diode are described as a function of injection current and emission wavelength, the latter controlled by an external grating. The external-grating cavity is used to isolate single or multiple supermodes of the multiple-stripe QWH laser (Pout>170 mW cw, λ∼7400 Å). The progression of supermode patterns consists of a discrete set of mode configurations for each longitudinal mode of the spectrum. The progression is cyclic with a ∼2.8-Å period which corresponds to the longitudinal mode spacing of the diode. Under high gain conditions, i.e., near the center of the recombination-radiation spectrum or at higher current levels, continuous tunability is observed with gradual transitions between supermode eigenstates. As the gain is reduced (low current), the number of supermodes observed decreases until only the in-phase pattern, i.e., each emitter at the same phase, remains above threshold. The far-field patterns range from a double-lobe pattern with a 10° peak separation (5 μm between emitter phase reversals) to a narrow (<2° full angle half power) single-lobe in-phase pattern. The experimental data are compared to the results of coupled-mode analysis.