Since their inception, optical frequency combs have transformed a broad range of technical and scientific disciplines, spanning time keeping to navigation. Recently, dual comb spectroscopy has emerged as an attractive alternative to traditional Fourier transform spectroscopy, since it offers higher measurement sensitivity in a fraction of the time. Midwave infrared (mid-IR) frequency combs are especially promising as an effective means for probing the strong fundamental absorption lines of numerous chemical and biological agents. Mid-IR combs have been realized via frequency down-conversion of a near-IR comb, by optical pumping of a micro-resonator, and beyond 7 μm by four-wave mixing in a quantum cascade laser. In this work, we demonstrate an electrically-driven frequency comb source that spans more than 1 THz of bandwidth centered near 3.6 μm. This is achieved by passively mode-locking an interband cascade laser (ICL) with gain and saturable absorber sections monolithically integrated on the same chip. The new source will significantly enhance the capabilities of mid-IR multi-heterodyne frequency comb spectroscopy systems.

The HO 2 + CH 3 C(O)O 2 reaction consists of three product channels: CH 3 C(O)OOH + O 2 (R1a), CH 3 C(O)OH + O 3 (R1b), and OH + CH 3 C(O)O + O 2 (R1c). The overall rate constant (k 1 ) and product yields (α 1a , α 1b , and α 1c ) were determined over the atmospherically-relevant temperature range of 230 -294 K at 100 Torr in N 2 . Time resolved kinetics measurements were performed in a pulsed laser photolysis experiment in a slow flow cell employing simultaneous infrared (IR) and ultraviolet (UV) absorption spectroscopy. HO 2 and CH 3 C(O)O 2 were formed by Cl-atom reactions with CH 3 OH and CH 3 CHO, respectively. Heterodyne near-and mid-infrared (NIR and MIR) wavelength modulation spectroscopy (WMS) was employed to selectively detect HO 2 and OH radicals. Ultraviolet absorption at 225 nm and 250 nm was used to detect various peroxy radicals as well as ozone (O 3 ). These experimental techniques enabled direct measurements of α 1c and α 1b via time-resolved spectroscopic detection in the MIR and the UV, respectively. At each temperature, experiments were performed at various ratios of initial HO 2 and CH 3 C(O)O 2 concentrations to quantify the secondary chemistry. The Arrhenius expression was found to be k 1 (T) = 1.38 +1.17 −0.63 × 10 −12 exp[(730 ± 170)/T] cm 3 molecule −1 s −1 . α 1a wastemperature-independent while α 1b and α 1c increased and decreased, respectively, with increasing temperatures. These trends are consistent with the current recommendation by the IUPAC data evaluation. 1 Hydrogen-bonded adducts of HO 2 with the precursors, HO 2 · CH 3 OH and HO 2 · CH 3 CHO, played a role at lower temperatures; as part of this work, rate enhancement of the HO 2 self reaction due to reactions of the adducts with HO 2 were also measured.

For high-sensitivity absorption spectroscopy, single-mode light sources capable of emitting high optical output power in the 3 to 5 µm wavelength range are vital. Here, we report on interband cascade lasers that emit 20 mW of optical power in a single spectral mode at room temperature and up to 40 mW at 0 °C using second-order laterally coupled Bragg gratings for distributed feedback. The lasers employ a double-ridge design with a narrow 3-µm-wide top ridge to confine the optical mode and a 9-µm-wide ridge for current confinement. The lasers were developed for an integrated cavity output spectroscopy instrument for stratospheric detection of hydrogen chloride at a wavelength of 3.3746 µm and emit at the target wavelength with more than 34 mW of single-mode power.

Mid‐wave infrared (MIR, 3–5 µm) optical frequency combs (OFC) are of critical importance for spectroscopy of fundamental molecular transitions in space and terrestrial applications. Although in this band OFCs can be obtained via supercontinuum or difference frequency generation using optical pumping schemes, unprecedented source miniaturization, and monolithic design are unique to electrically‐pumped semiconductor laser structures. To date, high‐brightness OFC generation in this region has been demonstrated in quantum‐ and interband cascade lasers (QCL/ICL), yet with sub‐optimal spectral properties. Here, an MIR quantum well diode laser (QWDL) OFC is shown, whose excellent spectral uniformity, narrow optical linewidths, and milliwatt optical power are obtained at a fraction of a watt of power consumption. The continuously tunable source offers ≈1 THz of optical span centered at 3.04 µm, and a repetition rate of 10 GHz. As a proof‐of‐principle, a directly‐battery‐operated MIR dual‐comb source is shown with almost 0.5 THz of optical coverage accessible in the electrical domain in microseconds. These results indicate the high suitability of QWDL OFCs for future chip‐based real‐time sensing systems in the mid‐infrared.

We report continuous-wave operation of single-mode quantum cascade (QC) lasers emitting near 7.4 µm with threshold power consumption below 1 W at temperatures up to 40 °C. The lasers were fabricated with narrow, plasma-etched waveguides and distributed-feedback sidewall gratings clad with sputtered aluminum nitride. In contrast to conventional buried-heterostructure (BH) devices with epitaxial sidewall cladding and in-plane gratings, the devices described here were fabricated without any epitaxial regrowth processes, yet they exhibit power consumption comparable to the lowest-dissipation BH QC lasers reported to date. These low-dissipation devices are designed primarily as light sources for infrared spectroscopy instruments with limited volume, mass, and power budgets.