Photonic-crystal distributed-feedback lasers for the midwave infrared

Theoretical modelling indicates that photonic-crystal distributed-feedback (PCDFB) lasers should be capable of providing near-diffraction-limited, spectrally pure sources of midwave-infrared radiation. The authors have studied both 2nd-order and 1st-order optically pumped PCDFB lasers with ´W´ active regions. Whereas it was not possible to optimise the 2nd-order grating due to the limitations of optical lithography, the 1st-order grating was fabricated using electron-beam lithography with a nearly optimum set of coupling coefficients. The resonant wavelength of the 1st-order grating remained somewhat longer than the gain peak at all temperatures up to the maximum for laser operation. In spite of the substantial detuning, the beam quality improved by a factor of five over angled-grating (α-DFB) lasers fabricated from similar material, and was slightly better than that for the 2nd-order grating at stripe widths <175 μ. Although the devices were operated in the pulsed mode, a considerable narrowing of the multi-mode spectral linewidth was also realised.