Mid-IR Photonic-Crystal Interband Cascade Lasers

Photonic-crystal distributed-feedback (PCDFB) semiconductor lasers have the potential to maintain optical coherence over very large areas. We report an electrically pumped PCDFB laser operating in a true single mode in the mid-infrared. A two-dimensional grating was formed on top of an interband cascade laser emitting at 3.3 $\mu $m by patterning a high-index Ge layer. The grating and the 400-$\mu $m-wide gain stripe were tilted by 20$^{o}$ with respect to the facet. Current spreading was prevented by ion bombarding the region outside the gain stripe rather than etching of a ridge. The gain region at the back of the cavity was also terminated by ion bombardment, since feedback from the back facet is undesirable. A single mode was emitted with maximum cw output power $>$ 60 mW, resolution-limited spectral linewidth (side-mode suppression ratio 27 dB), and single-lobe spatial far-field with angular full width at half maximum of 0.5$^{o}$. Comparison of the near and far field patterns indicated effective $M^{2} \quad \approx $ 3. The observation of low efficiency is thought to be due primarily to inadequate grating coupling, which can be remedied by thickening the Ge layer.