Quantum cascade (QC) lasers employ intersubband electronic transitions in semiconductor quantum well structures to generate emission at specific engineered wavelengths. QC devices have been particularly successful as mid-infrared emitters in the 4- to 12-μm wavelength range, a spectral regime that is difficult to access with interband diode lasers. As cascade devices, QC lasers can also be designed with many gain stages, which, combined with optimized doping and optical design, has enabled the development of lasers with remarkably high continuous output power (in excess of 1 W). One of the most important applications of mid-infrared QC lasers is quantitative gas detection using absorption spectroscopy, where a single-frequency laser is used to interrogate specific absorption lines of a target compound. While high output power is essential in certain applications, many in situ absorption spectrometers require only milliwatt-level output to effectively measure low levels of compounds of interest with strong absorption lines in the mid-infrared regime.
For portable laser-based instruments, including absorption spectrometers on mobile platforms, the power consumption of the laser source can often be the limiting factor in reducing size and overall input power. In portable spectrometer applications where less than 10 mW is required, it is nontrivial to design a QC laser with proportionally less input power, while maintaining continuous-wave (CW) laser emission near room temperature. The goal of this effort is to produce single-mode QC lasers suitable for in situ spectroscopy instruments with power consumption below 1 W, including power required for temperature stabilization.
An optoelectronic design and fabrication process was developed for reducing QC laser power consumption for devices emitting in the 4- to 5-μm wavelength range. The process yields single-mode lasers with stable, tunable emission and low input power for CW operation at room temperature. The combination of low power dissipation and high temperature functionality enables operation of complete laser modules (including integrated thermoelectric coolers) with watt-level input. Furthermore, these lasers require only one epitaxial growth process to fabricate the active quantum wells and cladding layers. These devices open up new possibilities for instrument applications where power is limited, including portable infrared absorption spectrometers.
Lasers were fabricated from wafers with epitaxial InGaAs/AlInAs quantum wells and InP cladding layers. The QC active region was designed with a two-phonon resonance structure, although the unique aspects of the lasers could be applied to other optimized active region designs at various wavelengths. Epitaxial growth was performed using molecular beam epitaxy; however, the unique aspects of these devices could be adapted to structures grown using alternative processes such as metal-organic chemical vapor deposition. These lasers achieve stable room-temperature CW emission with low power consumption, with narrow waveguide ridges and short cavity lengths, which en ables high electrical current densities in the active region without large ab solute current. As an alternate to buried-heterostructure distributed-feedback architectures, the devices use sidewall gratings formed during the waveguide ridge etching process, with precise pitch and profile control facilitated by a planar lithography process, as well as thin dielectric ridge isolation barriers with a thick electroplated top metal contact for effective heat dissipation.