Near-infrared methane sensor based on a distributed feedback laser

In this article, a near-infrared methane detection system using tunable diode laser absorption spectroscopy technology was designed and implemented. The distributed feedback laser was driven by a self-developed temperature and current controller to allow scanning the selected absorption wavelength at 1654?nm. Laser temperature fluctuation was lower than 0.01%, and the output emission wavelength was linear and stable. The emitted beam passed a reflective gas chamber and was received by the Indium Gallium Arsenide photodiode detector. Through a data acquisition card, a digital lock-in amplifier was developed to extract the second harmonics with real-time monitoring and adjustment. Based on Allan deviation analysis, the limit of detection was about 48?ppm with a path length of 30?cm, at an integration time of 6?s. The experimental results revealed a maximum detection error of less than 3% at a gas concentration higher than 100?ppm. The fluctuations rates in long-term (9?hr) stability measurements for 1?×?10³?ppm and 1?×?10⁴?ppm methane samples were 0.8% and 0.48%, respectively, indicating good stability for the sensor. In the control module design, compared with previous reports on methane detection systems, the current system uses a self-developed temperature controller, a current driver and a signal processor, to allow real-time display and adjustments. The potential for adjustable wavelength scanning is available for multi-gas detection based on a single detection system.     

Gas cells for tunable diode laser absorption spectroscopy employing optical diffusers. Part 1: single and dual pass cells

New designs for gas cells are presented that incorporate transmissive or reflective optical diffusers. These components offer simple alignment and can disrupt the formation of optical etalons. We analyse the performance–limiting effects in these cells of random laser speckle (both objective and subjective speckle), interferometric speckle and self-mixing interference, and show how designs can be optimised. A simple, single pass transmissive gas cell has been studied using wavelength modulation spectroscopy to measure methane at 1651 nm. We have demonstrated a short-term noise equivalent absorbance (NEA, 1σ) of 2×10⁻⁵, but longer term drift of up to 3×10⁻⁴ over 22 hours.     

Cavity-enhanced optical frequency comb spectroscopy in the mid-infrared application to trace detection of hydrogen peroxide

We demonstrate the first cavity-enhanced optical frequency comb spectroscopy in the mid-infrared wavelength region and report the sensitive real-time trace detection of hydrogen peroxide in the presence of a large amount of water. The experimental apparatus is based on a mid-infrared optical parametric oscillator synchronously pumped by a high-power Yb:fiber laser, a high-finesse broadband cavity, and a fast-scanning Fourier transform spectrometer with autobalancing detection. The comb spectrum with a bandwidth of 200 nm centered around 3.76 μm is simultaneously coupled to the cavity and both degrees of freedom of the comb, i.e. the repetition rate and carrier envelope offset frequency, are locked to the cavity to ensure stable transmission. The autobalancing detection scheme reduces the intensity noise by a factor of 300, and a sensitivity of 5.4×10^?9 cm^?1?Hz^?1/2 with a resolution of 800 MHz is achieved (corresponding to 6.9×10^?11 cm^?1?Hz^?1/2 per spectral element for 6000 resolved elements). This yields a noise equivalent detection limit for hydrogen peroxide of 8 parts-per-billion (ppb); in the presence of 2.8 % of water the detection limit is 130 ppb. Spectra of acetylene, methane, and nitrous oxide at atmospheric pressure are also presented, and a line-shape model is developed to simulate the experimental data.     

Near-Infrared Methane Detection Device Using Wavelength-Modulated Distributed Feedback Diode Laser around 1.654 µm

By using a wavelength-modulated distributed feedback laser centered at 1.654 µm, a stand-alone near-infrared methane detection device was experimentally proposed based on the tunable diode laser absorption spectroscopy technique. An intelligent temperature controller, a scan and modulation module, and a cost-effective lock-in amplifier were developed to drive the distributed feedback laser and extract the second harmonic signal. Experimental results show that the relative detection error is less than 7% within the detection range of 0–10⁶ ppm, and the limit of detection is about 11 ppm with an absorption length of 0.2 m. Long-term monitoring on two gas samples (10³ ppm and 2 × 10⁵ ppm) suggests good stability with the maximum detection errors smaller than 7% and 2.5%, respectively. Due to careful design and integration, the developed near-infrared sensor reveals competitive performances compared with our previously reported sensing devices at the mid-infrared region.     

Measurements of CO2 in a multipass cell and in a hollow-core photonic bandgap fiber at 2 μm

Recently, hollow-core photonic bandgap fibers (HC-PBFs) for use in the 2 μm wavelength region have become available. We have employed tunable diode laser absorption spectroscopy (TDLAS) to quantify CO₂ in nitrogen, injected into a HC-PBF. Our spectrometer contains both an HC-PBF-based absorption cell and an astigmatic Herriott multipass gas cell. The Herriott cell was used for comparison with the HC-PBF cell. The HC-PBF cell’s sensitivity and limit of detection were calculated to be 3.5×10^?4 cm^?1?Hz^?1/2 and 59 ppm?m, respectively. To substantiate the spectrometer performance, a measurement was done in the Herriott cell probing a reference gas mixture with nominal 400 μmol/mol CO₂ in N₂. The spectrometric results were in good agreement with the reference value. The relative standard uncertainty of the spectrometric result was found to be at the ±2 % level.     

Simultaneous detection of trace gases using multiplexed tunable diode lasers and a photoacoustic cell containing a cantilever microphone

We report what we believe to be a novel demonstration of simultaneous detection of multiple trace gases by near-IR tunable diode laser photoacoustic spectroscopy using a cell containing a cantilever microphone. Simultaneous detection of carbon monoxide (CO), ethyne (C₂H₂), methane (CH₄) and combined carbon monoxide/carbon dioxide (CO+CO₂) in nitrogen-based gas mixtures was achieved by modulation frequency division multiplexing the outputs of four near-IR tunable diode lasers. Normalized noise-equivalent absorption coefficients of 3.4×10^?9, 3.6×10^?9 and 1.4×10^?9 cm^?1?W?Hz^?1/2 were obtained for the simultaneous detection of CO, C₂H₂ and CH₄ at atmospheric pressure. These corresponded to noise-equivalent detection limits of 249.6 ppmv (CO), 1.5 ppmv (C₂H₂) and 293.7 ppmv (CH₄) respectively over a measurement period of 2.6 s at the relevant laser power. The performance of the system was not influenced by the number of lasers deployed, the main source of noise arising from ambient acoustic effects. The results confirm that small-volume photoacoustic cells can be used with low optical power tunable diode lasers for rapid simultaneous detection of trace gases with high sensitivity and specificity.     

Measurement of the spectral distribution of a multi-mode dye laser light by using speckle patterns

A simple method of measuring the wavelength γ₁ and the spectral line spacing Δγ of a multi-mode dye laser light by using the correlated speckle patterns produced at the far-field of a diffuser is proposed. Examples of the obtained values of γ₁ and Δγ are 6.0 × 10³ Å and 3.0 × 10¹ Å, respectively.     

Acetylene detection based on diode laser QEPAS: combined wavelength and residual amplitude modulation

Quartz-enhanced photoacoustic spectroscopy (QEPAS) is demonstrated for acetylene detection at atmospheric pressure and room temperature with a fiber-coupled distributed feedback (DFB) diode laser operating at ~1.53 μm. An efficient approach for gas concentration calibration is demonstrated. The effect of residual amplitude modulation on the performance of wavelength modulated QEPAS is investigated theoretically and experimentally. With optimized spectrophone parameters and modulation depth, a minimum detectable limit (1σ) of ~2 part-per-million volume (ppmv) was achieved with an 8.44-mW diode laser, which corresponds to a normalized noise equivalent coefficient (1σ) of 6.16 × 10^?8 cm^?1 W/Hz^1/2.     

Line strength and collisional broadening coefficients of H2O at 2.7 μm for natural gas quality assurance applications

We employed tunable diode laser absorption spectroscopy to measure the line strength, the methane (CH₄), ethane (C₂H₆) and the propane (C₃H₈) broadening coefficients for the 523–422 H₂O transition at 3619.61 cm^?1. Water amount fractions generated by a stable and accurate humidity transfer standard, traceable to the SI units via the German national humidity standard, were used to calibrate the spectroscopic line strength measurements. We focus on the traceability of the measured line data to the SI and on uncertainty assessments following the guidelines of the Guide to the Expression of Uncertainty in Measurement. We determined the line strength to be (8.42 ± 0.07)×10^?20 cm^?1/(cm^?2 molecule) corresponding to a relative uncertainty of ±0.8%. To the best of our knowledge, we report the first methane, ethane and propane broadening coefficients of (8.037 ± 0.056)×10^?5 cm^?1/hPa, (9.077 ± 0.064)×10^?5 cm^?1/hPa and (10.469 ± 0.073)×10^?5 cm^?1/hPa for the 523–422 H₂O transition at 3619.61 cm^?1, respectively. The relative combined uncertainties of the stated CH₄, C₂H₆ and C₃H₈ broadening coefficients are in the ±0.7% range.     

A trace methane gas sensor using mid-infrared quantum cascaded laser at 7.5 μm

Presented is a compact instrument developed for in situ high-stable and sensitive continuous measurement of trace gases in air, with results shown for ambient methane (CH₄) concentration. This instrument takes advantage of recent technology in thermoelectrically cooled pulsed Fabry–Perot (FP) quantum cascaded (QC) laser driving in a pulse mode operating at 7.5 μm to monitor a well-isolated spectral line near the ν4 fundamental band of CH₄. A high-quality liquid nitrogen cooled mercury cadmium telluride mid-infrared detector with time discriminating electronics is used along with a total reflection coated gold ellipsoid mirror offering 20 cm single pass optical absorption in an open-path cell to achieve stability of 5.2 × 10^?3 under experimental condition of 200 ppm measured ambient CH₄. The instrument operates continuously, and integrated software for laser control using direct absorption provides quantitative trace gas measurements without calibration. One may substitute a QC laser operating at a different wavelength to measure other gases. The instrument can be applied to field measurements of gases of environmental concern.     

Photo-emission studies from Zn cathodes under plasma phase

In this paper, we report investigations of the electron emission from pure Zn cathodes irradiated by UV laser pulses of 23 ns (full-width at half-maximum) at a wavelength of 248 nm (5 eV). The metal cathodes were tested in a vacuum photodiode chamber at 10^?5 Pa. They were irradiated at normal incidence and the anode–cathode distance was set at 3 mm. The maximum applied accelerating voltage was 18 kV, limited by the electrical breakdown of the photodiode gap. Under the above experimental conditions, a maximum applied electric field of 6 MV/m resulted. In the saturation regime, the measured quantum efficiency value increased with the accelerating voltage due to the plasma formation. The highest output current was achieved with 14 mJ laser energy, 18 kV accelerating voltage and its value was 12 A, corresponding to a global quantum efficiency (GQE) approximately of 1×10^?4. The temporal quantum efficiency was 1.0×10^?4 at the laser pulse onset time and 1.4×10^?4 at the pulse tail. We calculated the target temperature at the maximum laser energy. Its value allowed us to obtain output pulses of the same laser temporal profile. Tests performed with a lower laser photon energy (4.02 eV) demonstrated a GQE of two orders of magnitude lower.     

Artifact reduction in the CSPAD detectors used for LCLS experiments

The existence of noise and column‐wise artifacts in the CSPAD‐140K detector and in a module of the CSPAD‐2.3M large camera, respectively, is reported for the L730 and L867 experiments performed at the CXI Instrument at the Linac Coherent Light Source (LCLS), in low‐flux and low signal‐to‐noise ratio regime. Possible remedies are discussed and an additional step in the preprocessing of data is introduced, which consists of performing a median subtraction along the columns of the detector modules. Thus, we reduce the overall variation in the photon count distribution, lowering the mean false‐positive photon detection rate by about 4% (from 5.57 × 10^?5 to 5.32 × 10^?5 photon counts pixel^?1 frame^?1 in L867, cxi86715) and 7% (from 1.70 × 10^‐3 to 1.58 × 10^?3 photon counts pixel^?1 frame^?1 in L730, cxi73013), and the standard deviation in false‐positive photon count per shot by 15% and 35%, while not making our average photon detection threshold more stringent. Such improvements in detector noise reduction and artifact removal constitute a step forward in the development of flash X‐ray imaging techniques for high‐resolution, low‐signal and in serial nano‐crystallography experiments at X‐ray free‐electron laser facilities.     

Introducing speckle noise maps for Laser Vibrometry

When coherent light scatters from a surface, which is rough on the scale of the wavelength of the light, a speckle pattern is produced. The Laser Vibrometer measures target vibration velocity in the direction of the incident laser beam and typically samples a region of a speckle pattern on its photodetector. Target motions can cause the speckle pattern to change on the photodetector surface, particularly when target motions are non-normal to the direction of the laser beam. This speckle motion modulates the Doppler signal and adds noise to the demodulated output signal. Periodic target motions can cause the speckle noise to become pseudo-random and produce harmonic peaks, with the same fundamental frequency as the genuine target vibrations, which can be indistinguishable from the genuine target vibrations. Typical speckle noise levels are generally considered to be low-level, but they have not so far been adequately quantified. This paper reports preliminary results quantifying speckle noise levels using controlled experimental configurations incorporating periodic in-plane and tilt target motions. Working with commercial Laser Vibrometers, various target surface finishes and treatments are considered and speckle noise maps are produced for each configuration. For a tilting surface, speckle noise has been quantified at approximately 1 μm s^?1/deg s^?1 while, for surfaces with in-plane motion, the sensitivity to speckle noise has been estimated pessimistically at 0.1% of the in-plane velocity. Ultimately, these speckle noise maps will form a valuable practical resource for the Laser Vibrometer user.     

Cavity-enhanced optical feedback-assisted photo-acoustic spectroscopy with a 10.4 μm external cavity quantum cascade laser

An ultra-sensitive photo-acoustic spectrometer using a 10.4 μm broadly tunable mid-IR external cavity quantum cascade laser (EC-QCL) coupled with optical feedback to an optical power buildup cavity with high reflectivity mirrors was developed and tested. A laser optical power buildup factor of 181 was achieved, which corresponds to an intra-cavity power of 9.6 W at a wavelength of 10.4 μm. With a photo-acoustic resonance cell placed inside the cavity this resulted in the noise-equivalent absorption coefficient of 1.9 × 10^?10 cm^?1 Hz^?1/2, and a normalized noise-equivalent absorption of 1.1 × 10^?11 cm^?1 W Hz^?1/2. A novel photo-acoustic signal normalization technique makes the photo-acoustic spectrometer’s response immune to changes and drifts in the EC-QCL excitation power, EC-QCL to cavity coupling efficiency and cavity mirrors aging and contamination. An automatic lock of the EC-QCL to the cavity and optical feedback phase optimization permitted long wavelength scans within the entire EC-QCL spectral tuning range.     

Detection of C2H2 and HCl using mid-infrared degenerate four-wave mixing with stable beam alignment: towards practical in situ sensing of trace molecular species

A stable and convenient optical system to realize the forward phase-matching geometry for degenerate four-wave mixing (DFWM) is demonstrated in the mid-infrared spectral region by measuring DFWM signals generated in acetylene (C₂H₂) and hydrogen chloride (HCl) molecules by probing the fundamental ro-vibrational transitions. IR laser pulses tunable from 2900 cm^?1 to 3350 cm^?1 with a 0.025 cm^?1 linewidth were obtained using a laser system composed of an injection seeded Nd:YAG laser, a dye laser, and a frequency mixing unit. At room temperature and atmospheric pressure, a detection limit of 35 ppm (～ 9.5×10¹⁴ molecules/cm³) for C₂H₂ was achieved in a gas flow of a C₂H₂/N₂ mixture by scanning the P(11) line of the (010(11)⁰)–(0000⁰0⁰) band. The detection limit of the HCl molecule was measured to be 25 ppm (～6.8×10¹⁴ molecules/cm³) in the same environment by probing the R(4) line. The dependences of signal intensities on molecular concentrations and laser pulse energies were demonstrated using C₂H₂ as the target species. The variations of the signal line shapes with changes in the buffer gas pressures and laser intensities were recorded and analyzed. The experimental setup demonstrated in this work facilitates the practical implementation of in situ, sensitive molecular species sensing with species-specific, spatial and temporal resolution in the spectral region of 2.7–3.3 μm (3000–3700 in cm^?1), where various molecular species important in combustion have absorption bands.     

Ultra-high-precision mid-IR spectrometer II: system description and spectroscopic performance

The development and spectroscopic performance evaluation of an ultra-sensitive, mid-IR spectrometer is reported. The laser system is based upon difference-frequency generation (DFG) at ～3.5 μm by mixing a DFB diode laser at 1562 nm and a DFB fiber laser at 1083 nm using a periodically poled LiNbO₃ crystal. DFG radiation was coupled to a 100?m optical path length astigmatic Herriott cell. Sensitive and selective spectroscopic detection of formaldehyde was performed with second-harmonic detection using Peltier-cooled HgCdTe detectors. By applying computer lock-ins, dual-beam optical noise subtraction, focus matching, thermal stabilization, active wavelength control, and advanced signal processing a sensitivity corresponding to an absorbance ～1.6×10^-7 is achieved for 260 s of averaging.     

Laser ablation of silver in different liquids: Optical and nonlinear optical properties of silver nanoparticles

The optical, structural, and nonlinear optical properties of silver nanoparticles prepared using the method of laser ablation in various liquids at wavelengths of 397, 532, and 795 nm with laser pulses of different duration are studied. An analysis of the dimensional and spectral characteristics of the silver nanoparticles revealed a time dynamics of the nanoparticle size distribution in solutions. It is shown that thermal self-defocusing is observed for the case of nanosecond or shorter pulses generated with a high repetition rate. For picosecond and femtosecond pulses with a low repetition rate, the effects of self-focusing (γ = 3 × 10^?13 cm² W^?1) and saturated absorption (β = ?1.5 × 10^?9 cm W^?1) were observed in the solutions under study. The third-order nonlinear susceptibility of the silver nanoparticles was found to be 5 × 10^?8 esu at a wavelength of 397 nm.     

Optical-feedback cavity-enhanced absorption spectroscopy with a quantum-cascade laser yields the lowest formaldehyde detection limit

We report on the first application of Optical Feedback-Cavity Enhanced Absorption Spectroscopy to formaldehyde trace gas analysis at mid-infrared wavelengths. A continuous-wave room-temperature, distributed-feedback quantum cascade laser emitting around 1,769 cm^?1 has been successfully coupled to an optical cavity with finesse 10,000 in an OF-CEAS spectrometer operating on the ν₂ fundamental absorption band of formaldehyde. This compact setup (easily transportable) is able to monitor H₂CO at ambient concentrations within few seconds, presently limited by the sample exchange rate. The minimum detectable absorption is 1.6 × 10^?9 cm^?1 for a single laser scan (100 ms, 100 data points), with a detectable H₂CO mixing ratio of 60 pptv at 10 Hz. The corresponding detection limit at 1 Hz is 5 × 10^?10 cm^?1, with a normalized figure of merit of 5 × 10^?11cm $^{-1}/\sqrt{\rm Hz}$ (100 data points recorded in each spectrum taken at 10 Hz rate). A preliminary Allan variance analysis shows white noise averaging down to a minimum detection limit of 5 pptv at an optimal integration time of 10 s, which is significantly better than previous results based on multi-pass or cavity-enhanced tunable QCL absorption spectroscopy.     

An IH-QCL based gas sensor for simultaneous detection of methane and acetylene

Extended wavelength tuning of an IH-QCL (integrated heater quantum cascade laser) is exploited for simultaneous detection of methane and acetylene using direct absorption spectroscopy. The integrated heater, placed within few microns of the laser active region, enables wider wavelength tuning than would be possible with a conventional DFB (distributed feedback) QCL. In this work, the laser current and heater resistor current are modulated simultaneously at 25?kHz to tune the laser over 1279.6–1280.1 cm^?1, covering absorption transitions of methane and acetylene. The laser is characterized extensively to understand the dependence of wavelength tuning on modulation frequency, modulation amplitude and phase difference between laser/heater modulation. Thereafter, the designed sensor is validated in both room-temperature static cell experiments and non-reactive high-temperature-measurements in methane-acetylene-argon gas mixtures in the shock tube. Finally, the sensor is applied for simultaneous detection of methane and acetylene during the high-temperature pyrolysis of iso-octane behind reflected shock waves.