Herriott cell spot imaging increases the performance of tunable laser spectrometers.

With the availability of high-power (milliwatts) single-mode tunable laser sources that operate at room temperature across the infrared (IR) region, tunable laser spectrometers have seen an explosion of growth in applications that include commercial, Earth and planetary science, and medical and industrial sensing. While the laser sources themselves have shown steady improvement, the detection architecture of using a single-element detector at one end of a multipass cell has remained unchanged over the last few decades. We present here an innovative new approach using a detector array coupled to an IR-transmissive mirror to image all or part of the multipass spot pattern of the far mirror and record spectra for each pixel. This novel approach offers improved sensitivity, increased dynamic range, laser power normalization, contaminant subtraction, resilience to misalignment, and reduces the instrument power requirement by avoiding the need for "fringe-wash" heaters. With many tens of pixels representing each spot during the laser spectral scan, intensity and optical fringe amplitude and phase information are recorded. This allows selection and manipulation (e.g., co-addition, subtraction) of the pixel output spectra to minimize optical interference fringes thereby increasing sensitivity. We demonstrate a factor of ∼20 sensitivity improvement over traditional single-element detection. Dynamic range increase of a factor of ∼100 is also demonstrated through spot selection representing different pathlengths. Additionally, subtracting the spectrum of the first spot from that of the higher pass normalizes the laser power and removes the contribution of contaminant gas and fringes in the fore-optics region. These initial results show that this imaging method is particularly advantageous for multi-channel laser spectrometers, and, once the image field is analyzed, pixel selection can be used to minimize data rate and volume collection requirements. This technique could be beneficial to enhanced-cavity detection schemes.

Temperature Dependence Study of the Kinetics and Product Yields of the HO2 + CH3C(O)O2 Reaction by Direct Detection of OH and HO2 Radicals Using 2f-IR Wavelength Modulation Spectroscopy.

The HO2 + CH3C(O)O2 reaction consists of three product channels: CH3C(O)OOH + O2 (R1a), CH3C(O)OH + O3 (R1b), and OH + CH3C(O)O + O2 (R1c). The overall rate constant ( k1) and product yields (α1a, α1b, and α1c) were determined over the atmospherically relevant temperature range of 230-294 K at 100 Torr in N2. Time-resolved kinetics measurements were performed in a pulsed laser photolysis experiment in a slow flow cell by employing simultaneous infrared (IR) and ultraviolet (UV) absorption spectroscopy. HO2 and CH3C(O)O2 were formed by Cl-atom reactions with CH3OH and CH3CHO, respectively. Heterodyne near- and mid-infrared (NIR and MIR) wavelength modulation spectroscopy (WMS) was employed to selectively detect HO2 and OH radicals. Ultraviolet absorption at 225 and 250 nm was used to detect various peroxy radicals as well as ozone (O3). These experimental techniques enabled direct measurements of α1c and α1b via time-resolved spectroscopic detection in the MIR and the UV, respectively. At each temperature, experiments were performed at various ratios of initial HO2 and CH3C(O)O2 concentrations to quantify the secondary chemistry. The Arrhenius expression was found to be k1( T) = 1.38-0.63+1.17 × 10-12 exp[(730 ± 170)/ T] cm3 molecule-1 s-1. α1a was temperature-independent while α1b and α1c decreased and increased, respectively, with increasing temperatures. These trends are consistent with the current recommendation by the IUPAC data evaluation. Hydrogen-bonded adducts of HO2 with the precursors, HO2·CH3OH and HO2·CH3CHO, played a role at lower temperatures; as part of this work, rate enhancements of the HO2 self-reaction due to reactions of the adducts with HO2 were also measured.

Passively mode-locked interband cascade optical frequency combs.

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.

Low-dissipation 7.4-µm single-mode quantum cascade lasers without epitaxial regrowth.

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.

Lovastatin corrects ERK pathway hyperactivation in fragile X syndrome: potential of platelet's signaling cascades as new outcome measures in clinical trials.

AIM: To establish whether platelets from fragile X syndrome (FXS) individuals recapitulate FXS mouse neurons' defects in ERK and Akt pathways, and to evaluate the effect of lovastatin on these pathways. METHODS: ERK and Akt phosphorylation (pERK, pAkt) statuses were assessed with quantitative Western blotting before and after a 12-week lovastatin trial. RESULTS: Levels of pERK and pAkt were increased in FXS platelets, and lovastatin specifically normalized ERK activity. Changes in ERK phosphorylation were correlated with clinical response to lovastatin. CONCLUSIONS: Platelets' signaling pathways provide biomarkers that can be used as treatment outcome measures in FXS clinical trials.

Single-mode high-power interband cascade lasers for mid-infrared absorption spectroscopy.

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.