Single-Mode Distributed-Feedback ``W'' Diode and Interband Cascade Lasers in the Mid-Infrared

To obtain spectrally pure output, we have fabricated narrow index--guided ridges with lateral distributed-feedback (DFB) line gratings on both ``W'' diode and interband cascade lasers. The ``W'' diode structure containing a GaSb $p$-SCH etch-stop layer was chemically etched into a 5 $\mu $m ridge and a first-order DFB grating constructed on both side walls of the ridge. For the interband cascade laser, a first-order top DFB grating was patterned on top of a chemically-etched ridge that was 15 $\mu $m wide. The low-loss DFB mode was roughly resonant with the gain peak at $T$ = 165 K for the ``W'' diode and at $T$ = 110 K for the interband cascade laser. The sidemode suppression ratios were definitely $>$ 20 dB for both devices, and all of the features above 30 dB appeared to result from instrument noise rather than actual parasitic modes. Just beyond the stop band on the long-wavelength side of the ``W'' diode DFB, a series of longitudinal modes became apparent at $>$ 30 dB suppression. For the narrow-ridge waveguide DFB devices, the temperature ranges over which single-mode lasing were successfully obtained were 140-162 K for the ``W'' diode, for which $\lambda $ = 3.195 $\sim $ 3.202 $\mu $m (0.29 nm/K), and 110-125 K for the interband cascade laser, for which $\lambda $ = 3.452 $\sim $ 3.456 $\mu $m (0.27 nm/K).