Superluminescent (SL) light sources, which offer the beneficial combination of a broad emission bandwidth with low temporal and high spatial coherence, have many potential industrial and medical applications. In particular, mid-infrared (MIR) SL light sources that are suitable for use in an optical coherence tomography system would be attractive for the biomedical imaging of cancerous tissues and collagen, for example. However, an appropriate MIR SL light source that offers the milliwatt power level needed for such applications has so far been lacking.

Mei C. Zheng and co-workers at Princeton University and AdTech Optics Inc. in the USA have now developed such a light source based on a quantum cascade (QC) device implemented in a spiral-shaped cavity. The 12-mm-long device emits an output power of 57 mW at a temperature of 250 K (Opt. Express 23, 2713–2719; 2015).

Credit: OSA

The QC active core was based on InGaAs/AlInAs multi-quantum wells and was designed to have an emission wavelength around 5 μm at 80 K. The team adopted a spiral waveguide design to achieve a long waveguide and thus a higher SL power while still retaining a compact footprint.

The device was composed of a waveguide (25 μm wide and 6 μm deep) arranged in a spiral shape (with a minimum spiral radius of 380 μm), which, after several turns, transitions to a straight waveguide that is 1,325 μm long. The straight waveguide was tilted from the cleavage plane by 17° to suppress the residual reflection from the front facets. The spiral design allowed a total waveguide length of 12 mm, with a footprint of 3 × 3 mm2.

Light, current, and voltage characterization of the device was performed in pulsed mode with a current pulse width of 100 ns and a repetition rate of 5 kHz. Optical emission from the device was collimated and focused onto a room temperature HgCdTe detector by a pair of ZnSe lenses. An SL power of 57 mW was obtained just below the lasing threshold of 2.5 kA cm−2. The emission spectra showed a broad Gaussian shape with a full width at half maximum of 56 cm−1 wavenumbers. A coherence length of 107 μm was determined from the interferograms.

“The next challenge of this work is to increase the SL power even more by further suppressing the lasing threshold in addition to reducing the input power required so that this device can be more efficient,” Zheng told Nature Photonics.