Distributed Feedback Tunable Diode Lasers Research Articles

Pressure-dependent sensitivity of a single-pass methane detection system using a continuous-wave distributed feedback laser at 3270 nm.

A single-pass, compact, and low-power-consumption methane sensing system is presented, and its sensitivity is examined for a set of reference cells with pressures of 7.4, 74, and 740Torr and the same methane concentration at laboratory temperature of 23°C. A GaSb-based continuous-wave distributed feedback tunable diode laser at 3270nm and wavelength modulation spectroscopy were employed to collect 2f spectra in the ν3, R3 band of CH412. The collected 2f spectra were fitted to a modulated Voigt line profile model. An Allan-Werle variance analysis shows that the best detectivity, 4ppbm, is obtained for the highest pressure cell. At pressures of 74 and 7.4Torr the detectivities are 16 and 28ppbm, respectively. For these low-pressure cells, further sensitivity improvements were achieved using a large modulation depth (m>2.2) at the expense of spectral resolution.

Hydrogen sulphide detection using near-infrared diode laser and compact dense-pattern multipass cell**Project supported by the National Key Research and Development Program of China (Grant Nos. 2016YFC0303900 and 2017YFC0209700).

Sub-ppmv level detection of hydrogen sulphide (H2S) using a 1.578- distributed feedback tunable diode laser combining with wavelength modulation spectroscopy and second harmonic detection scheme is reported. A home-developed novel compact dense-pattern multipass gas cell with an effective optical path length of 29.37 m is used to improve sensitivity and reduce sample volume. Detection parameters are optimized, including modulation frequency and amplitude. The analysis of Allan variance shows that a minimum detectable concentration 60 ppbv is obtained with a lock-in time constant of 10 ms, and a detection limit of 13 ppbv can be achieved by average in 300 s. The demonstrated H2S sensor has a strong penitential application in natural gas process for regulating and controlling H2S concentration.

Compact, fiber-coupled, single-ended laser-absorption-spectroscopy sensors for high-temperature environments.

The design and demonstration of a compact single-ended laser-absorption-spectroscopy sensor for measuring temperature and H2O in high-temperature combustion gases is presented. The primary novelty of this work lies in the design, demonstration, and evaluation of a sensor architecture that uses a single lens to provide single-ended, alignment-free (after initial assembly) measurements of gas properties in a combustor without windows. We demonstrate that the sensor is capable of sustaining operation at temperatures up to at least 625K and is capable of withstanding direct exposure to high-temperature (≈1000 K) flame gases for long durations (at least 30min) without compromising measurement quality. The sensor employs a fiber bundle and a 6mm diameter antireflection-coated lens mounted in a 1/8'' NPT-threaded stainless-steel body to collect laser light that is backscattered off native surfaces. Distributed-feedback tunable diode lasers (TDLs) with a wavelength near 1392nm and 1343nm were used to interrogate well-characterized H2O absorption transitions using wavelength-modulation-spectroscopy techniques. The sensor was demonstrated with measurements of gas temperature and H2O mole fraction in a propane-air burner with a measurement bandwidth up to 25kHz. In addition, this work presents an improved wavelength-modulation spectroscopy spectral-fitting technique that reduces computational time by a factor of 100 compared to previously developed techniques.

Demonstration of an ethane spectrometer for methane source identification.

Methane is an important greenhouse gas and tropospheric ozone precursor. Simultaneous observation of ethane with methane can help identify specific methane source types. Aerodyne Ethane-Mini spectrometers, employing recently available mid-infrared distributed feedback tunable diode lasers (DFB-TDL), provide 1 s ethane measurements with sub-ppb precision. In this work, an Ethane-Mini spectrometer has been integrated into two mobile sampling platforms, a ground vehicle and a small airplane, and used to measure ethane/methane enhancement ratios downwind of methane sources. Methane emissions with precisely known sources are shown to have ethane/methane enhancement ratios that differ greatly depending on the source type. Large differences between biogenic and thermogenic sources are observed. Variation within thermogenic sources are detected and tabulated. Methane emitters are classified by their expected ethane content. Categories include the following: biogenic (<0.2%), dry gas (1-6%), wet gas (>6%), pipeline grade natural gas (<15%), and processed natural gas liquids (>30%). Regional scale observations in the Dallas/Fort Worth area of Texas show two distinct ethane/methane enhancement ratios bridged by a transitional region. These results demonstrate the usefulness of continuous and fast ethane measurements in experimental studies of methane emissions, particularly in the oil and natural gas sector.

Folded dipole antenna for power enhancement of a terahertz photomixer

A folded dipole antenna is proposed to increase the input impedance of an antenna for power enhancement of a terahertz integrated photomixer/antenna because the impedance of a teraherz photomixer is very high compared to the impedance of an antenna. The antenna was optimized using an electromagnetic simulator, and its resistance was increased to 2750 Ω, which is about 40 times as high as the impedance of a self-complementary broadband antenna. The optimized antenna was fabricated on low-temperature-grown GaAs (LTG-GaAs) and measured using a liquidhelium-cooled silicon bolometer. Two distributed-feedback tunable diode lasers were scanned for photomixing at frequencies from 100 to 1300 GHz in steps of 1 GHz. For stable frequency scans, the two lasers were controlled by using a feedback loop system. The measured THz output power of the proposed folded dipole antenna integrated with a THz photomixer showed resonant characteristics and was larger at the resonant frequency bands.

Mars laser hygrometer.

We have designed and built a miniature near-IR tunable diode laser (TDL) spectrometer for measuring in situ the water vapor mixing ratio either in the Martian atmosphere or thermally evolved from Martian soil or ice samples. The laser hygrometer uses a thermoelectrically cooled single-mode distributed-feedback TDL at 1.87 microm to scan over a selected vibration-rotation line of both H2O and CO2 near 5327.3 cm(-1). A working prototype that weighs only 230 g has been built and used to generate spectra whose analysis demonstrates precision sensitivities as fine as 1 part in 10(6) by volume in 1 s or 0.1 part in 10(6) in 10 s at Martian pressures and temperatures. Absolute uncertainties of approximately 5% are calculated.