Quantum Cascade Superluminescent Spiral Cavity Devices at 8 $\mu $m with Various Doping Characteristics

Optical Coherence Tomography (OCT) is an imaging technique with biomedical applications; the system requires a superluminescent (SL) light source that has a low temporal coherence for high resolution and high power for imaging through a thicker sample. We focus on the mid-IR region because many fundamental ro-vibrational transitions happen in this `fingerprint' region. Quantum cascade superluminescent (QCSL) emitters are excellent sources for this wavelength. Prior work dealt with QCSL devices with an emission wavelength of 5 $\mu $m, and in this work we extended our work to 8 $\mu $m because there are more interesting biomolecular absorption peaks from proteins and lipids. We designed and fabricated 8 $\mu $m spiral cavity SL emitters with various doping characteristics. Using FT-IR spectrometry, the devices were characterized at various temperatures. Compared to injector region doped devices, active region doped devices showed a slower gain narrowing in the electroluminescent region. Doping in the active region contributes to suppressed lasing, but it does not affect the coherence length that was 320 $\mu $m for both type of devices at lasing threshold at 120 K. The active region doped device had a 7.5 mW power output at 120 K, which makes it a good candidate for an OCT system.