CW DFB RT diode laser-based sensor for trace-gas detection of ethane using a novel compact multipass gas absorption cell

The development of a continuous wave, thermoelectrically cooled (TEC), distributed feedback diode laser-based spectroscopic trace-gas sensor for ultra-sensitive and selective ethane (C₂H₆) concentration measurements is reported. The sensor platform used tunable diode laser absorption spectroscopy (TDLAS) and wavelength modulation spectroscopy as the detection technique. TDLAS was performed using an ultra-compact 57.6 m effective optical path length innovative spherical multipass cell capable of 459 passes between two mirrors separated by 12.5 cm and optimized for the 2.5–4 μm range TEC mercury–cadmium–telluride detector. For an interference-free C₂H₆ absorption line located at 2,976.8 cm^?1, a 1σ minimum detection limit of 740 pptv with a 1 s lock-in amplifier time constant was achieved.     

Single-QCL-based absorption sensor for simultaneous trace-gas detection of CH4 and N2O

A quantum cascade laser (QCL)-based absorption sensor for the simultaneous dual-species monitoring of CH₄ and N₂O was developed using a novel compact multipass gas cell (MGC). This sensor uses a thermoelectrically cooled, continuous wave, distributed feedback QCL operating at ~7.8 µm. The QCL wavelength was scanned over two neighboring CH₄ (1275.04 cm^?1) and N₂O (1274.61 cm^?1) lines at a 1 Hz repetition rate. Wavelength modulation spectroscopy (f = 10 kHz) with second harmonic (2f) detection was performed to enhance the signal-to-noise ratio. An ultra-compact MGC (16.9 cm long and a 225 ml sampling volume) was utilized to achieve an effective optical path length of 57.6 m. With such a sensor configuration, a detection limit of 5.9 ppb for CH₄ and 2.6 ppb for N₂O was achieved, respectively, at 1-s averaging time.     

Crystal growth,optical spectroscopy,and continuous-wave laser operation of Ho:KLu(WO4)2 crystals

We present the crystal growth, optical spectroscopy, and room temperature continuous-wave (CW) laser operation of monoclinic Ho:KLu(WO₄)₂ crystals. Macro defect-free crystals of several dopant concentrations were grown by top-seeded solution growth slow-cooling method. The evolution of unit cell parameters with holmium doping level and temperature was studied using X-ray powder diffraction. The spectroscopic properties were characterized in terms of room- and low-temperature optical absorption and photoluminescence. From low-temperature optical absorption measurements, the energy of the Stark levels was determined. Calculation of the emission and gain cross sections is presented. CW laser action was realized for 3 and 5 at. % Ho-doped KLu(WO₄)₂ by in-band pumping using a Tm:KLu(WO₄)₂ pump laser. A maximum output power of 507 mW with a slope efficiency of ~38 % with respect to the incident power was achieved at 2,080 nm with the Ho:KLu(WO₄)₂ laser.     

A mid-infrared scanned-wavelength laser absorption sensor for carbon monoxide and temperature measurements from 900 to 4000 K

Mid-infrared laser absorption sensors based on quantum cascade laser (QCL) technology offer the potential for high-sensitivity, selective, and high-speed measurements of temperature and concentration for species of interest in high-temperature environments, such as those found in combustion devices. A new mid-infrared QCL absorption sensor for carbon monoxide and temperature measurements has been developed near the intensity peak of the CO fundamental band at 4.6 μm, providing orders-of-magnitude greater sensitivity than the overtone bands accessible with telecommunications lasers. The sensor is capable of probing the R(9), R(10), R(17), and R(18) transitions of the CO fundamental ro-vibrational band which are located at frequencies where H₂O and CO₂ spectral interference is minimal. Temperature measurements are made via scanned-wavelength two-line ratio techniques using either the R(9) and R(17) or the R(10) and R(18) line pairs. The high-speed (1–2 kHz) scanned-wavelength sensor is demonstrated in room-temperature gas cell measurements of CO and, to demonstrate the potential of the sensor for high-temperature thermometry, in shock-heated gases containing CO for a very wide range of temperature (950–3500 K) near 1 atm. To our knowledge, these measurements represent the first use of QCL-based absorption sensor for thermometry at elevated combustion-like temperatures. The high-temperature measurements of CO mole fraction and temperature agree with the post-reflected-shock conditions within ±1.5% and ±1.2% (1σ deviation), respectively.     

Wavelength-modulation spectroscopy near 2.5 μm for H2O and temperature in high-pressure and -temperature gases

The design and validation of a tunable diode laser (TDL) sensor for temperature and H₂O in high-pressure and -temperature gases are presented. High-fidelity measurements are enabled through the use of: (1) strong H₂O fundamental-band absorption near 2.5 μm, (2) calibration-free first-harmonic-normalized wavelength-modulation spectroscopy with second-harmonic detection (WMS-2f/1f), (3) an experimentally derived and validated spectroscopic database, and (4) a new approach to selecting the optimal wavelength and modulation depth of each laser. This sensor uses two TDLs near 2,474 and 2,482 nm that were fiber coupled in free space and frequency multiplexed to enable measurements along a single line-of-sight. The lasers were modulated at 35 and 45.5 kHz, respectively, to achieve a sensor bandwidth of 4.5 kHz. This sensor was validated in a shock tube at temperatures and pressures ranging from 1,000 to 2,700 K and 8 to 50 bar. There the sensor resolved transients and recovered the known steady-state temperature and H₂O mole fraction with a precision of 3.2 and 2.6 % RMS, respectively.     

Characterisation of the potential of frequency modulation and optical feedback locking for cavity-enhanced absorption spectroscopy

A combination of optical feedback self-locking of a continuous-wave distributed feedback diode laser to a V-shaped high finesse cavity, laser phase modulation at a frequency equal to the free spectral range of the V-cavity and detection of the transmitted laser beam at this high modulation frequency is described for the possible application in cavity-enhanced absorption spectroscopy. In order to estimate the noise level of an absorbance baseline, the triplet of frequency modulated light, i.e. the central laser frequency and the two sidebands, were transmitted through both the V-cavity in open air and a 1.5-cm long optical cell placed behind the cavity output mirror and filled with acetylene (C₂H₂) at low pressure. The performance of the setup was evaluated from the measured relative intensity noise on the cavity output (normalised by the bandwidth) and the frequency modulation absorption signals induced by C₂H₂ absorption in the 1.5-cm cell. From these data, we estimate that the noise-equivalent absorption sensitivity of 2.1 × 10^?11 cm^?1 Hz^?1/2—by a factor of 11.7 above the shot-noise limit—can be achieved for C₂H₂ absorption spectra extracted from the heterodyne beat signals recorded at the transmission maxima intensity peaks of the successive TEM₀₀ resonances.     

An optical system for measuring nitric oxide using spectral separation techniques

An optical sensor based on differential absorption spectroscopy for real-time monitoring of industrial nitric oxide (NO) gas emission is described. The influence of gas absorption interference from sulfur dioxide (SO₂) in the environment was considered and a spectral separation technique was developed in order to eliminate this interference effect. The absorption spectrum of SO₂ around 226 nm was evaluated by the SO₂ concentration obtained using the experimentally recorded absorption spectrum around 300 nm. The absorption spectrum of NO around 226 nm was obtained by subtracting the absorption of SO₂ from the integral absorption spectrum of SO₂ and NO. The concentration measurements were performed at atmospheric pressure. The technique was found to have a lower detection limit of 0.8 ppm for NO per meter path length (SNR=2) and be immune from the influence from SO₂ on the NO measurement. The sensor based on this technique was successfully employed for in situ measurement of SO₂ and NO concentrations in the flue gas emitted from an industrial coal-fired boiler.     

High-rate reactive magnetron sputtering of zirconia films for laser optics applications

ZrO₂ exhibits low optical absorption in the near-UV range and is one of the highest laser-induced damage threshold (LIDT) materials; it is, therefore, very attractive for laser optics applications. This paper reports explorations of reactive sputtering technology for deposition of ZrO₂ films with low extinction coefficient k values in the UV spectrum region at low substrate temperature. A high deposition rate (64 % of the pure metal rate) process is obtained by employing active feedback reactive gas control which creates a stable and repeatable deposition processes in the transition region. Substrate heating at 200 °C was found to have no significant effect on the optical ZrO₂ film properties. The addition of nitrogen to a closed-loop controlled process was found to have mostly negative effects in terms of deposition rate and optical properties. Open-loop O₂ gas-regulated ZrO₂ film deposition is slow and requires elevated (200 °C) substrate temperature or post-deposition annealing to reduce absorption losses. Refractive indices of the films were distributed in the range n = 2.05–2.20 at 1,000 nm and extinction coefficients were in the range k = 0.6 × 10^?4 and 4.8 × 10^?3 at 350 nm. X-ray diffraction analysis showed crystalline ZrO₂ films consisted of monoclinic + tetragonal phases when produced in Ar/O₂ atmosphere and monoclinic + rhombohedral or a single rhombohedral phase when produced in Ar/O₂ + N₂. Optical and physical properties of the ZrO₂ layers produced in this study are suitable for high-power laser applications in the near-UV range.     

Sensitive detection of CO2 concentration and temperature for hot gases using quantum-cascade laser absorption spectroscopy near 4.2 μm

Mid-infrared quantum-cascade laser (QCL) absorption spectroscopy of CO₂ near 4.2 μm has been developed for measurement of temperature and concentration in hot gases. With stronger absorption line-strengths than transitions near 1.5, 2.0, and 2.7 μm used previously, the fundamental band (00⁰1–00⁰0) of CO₂ near 4.2 μm provides greatly enhanced sensitivity and accuracy to sense CO₂ in high-temperature gases. Line R(74) and line R(96) are chosen as optimum pair for sensitive temperature measurements due to their high-temperature sensitivity, equal signal-to-noise ratio (SNR), weak interference of H₂O transitions, as well as relatively strong line-strengths in high temperature and weak absorption in room temperature. The high-resolution absorption spectrum of the far wings of the R-branch (R56–R100) in the fundamental vibrational band of CO₂ is measured in a heated cell over the range 2,384–2,396 cm^?1 at different temperatures from 700 to 1,200 K. Taking three factors into consideration, including SNR, concentration detectability, and uncertainty sensitivity, the absorption line R(74) is selected to calculate CO₂ concentration. The tunable QCL absorption sensor is validated in mixtures of CO₂ and N₂ in a static cell for temperature range of 700–1,200 K, achieving an accuracy of ±6 K for temperature and ±5 % for concentration measurements.     

A quantum cascade laser-based optical feedback cavity-enhanced absorption spectrometer for the simultaneous measurement of CH4 and N2O in air

Optical feedback cavity-enhanced absorption spectroscopy (OF CEAS) has been demonstrated with a thermoelectrically cooled continuous wave distributed feedback quantum cascade laser (QCL) operating at wavelengths around 7.84 μm. The QCL is coupled to an optical cavity which creates an absorption pathlength greater than 1000 m. The experimental design allows optical feedback of infra-red light, resonant within the cavity, to the QCL, which initiates self-locking at each TEM₀₀ cavity mode frequency excited. The QCL linewidth is narrowed to below the mode linewidth, greatly increasing the efficiency of injection of light into the cavity. At the frequency of each longitudinal cavity mode, the absorption coefficient of an intracavity sample is obtained from the transmission at the mode maximum, measured with a thermoelectrically cooled detector: spectral line profiles of CH₄ and N₂O in ambient air were recorded simultaneously and with a resolution of 0.01386 cm^?1. A minimum detectable absorption coefficient of 5.5×10^?8 cm^?1 was demonstrated after an averaging time of 1 s for this completely thermoelectrically cooled system. The bandwidth-normalised limit for a single cavity mode is 5.6×10^?9 cm^?1?Hz^?1/2 (1σ).     

Silver nanoparticles produced by PLD in vacuum: role of the laser wavelength used

In this work we report the results of investigation of silver (Ag) nanoparticles prepared on a silica substrate by laser ablation. Our attention was focused on the mean diameter, size distribution and optical absorption properties of nanoparticles prepared in vacuum by using different laser wavelengths. The fundamental wavelength and the second, third, and fourth harmonics of a nanosecond Nd:YAG laser were used for nanoparticles fabrication. The corresponding values of the laser fluence for each wavelength were: 0.6 J/cm² at 266 nm, 0.8 J/cm² at 355 nm, 2.8 J/cm² at 532 nm, and 2 J/cm² at 1064 nm. The Ag nanoparticles produced have mean diameters in the range from 2 nm to 12 nm as the nanoparticles’ size decreases with the decrease of the wavelength used. The presence of the Ag nanoparticles was also evidenced by the appearance of a strong optical absorption band in the measured UV-VIS spectra associated with surface plasmon resonance (SPR). A redshift and widening of the absorption peak were observed as the laser wavelength was increased. Some additional investigations were performed in order to clarify the structure of the Ag nanoparticles.     

Wireless laser spectroscopic sensor node for atmospheric CO2 monitoring—laboratory and field test

We developed a low-power, portable, wireless laser spectroscopic sensor for atmospheric CO₂ monitoring. The sensor is based on tunable diode laser absorption spectroscopy with a 2-μm wavelength VCSEL as a source and wavelength modulation technique for spectroscopic signal detection. The sensor allows measurement of CO₂ concentration changes with a 1σ sensitivity of 0.14 ppmv?Hz^?1/2. This sensor was both laboratory and field tested under varying environmental conditions. It was used to measure a soil respiration rate of topsoil in the lab and of forest floors in the field. Measurement results are compared with those of commercial non-dispersive infrared sensors and very good agreement is found.     

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.     

Laser radiation effects on optical and structural properties of nanostructure CdSe thin film

Cadmium selenide (CdSe) thin films were deposited on a glass substrate using the thermal evaporation method at room temperature. The changes in the optical properties (optical band gap and absorption coefficient) after irradiation by TEA N₂ laser at different energies were measured in the wavelength range 190–800 nm using a spectrophotometer. It was found that the optical band gap is decreased after irradiating the thin films. The samples were characterized using X-ray diffraction (XRD), and the grain size of the CdSe thin film was calculated from XRD data, which was found to be 41.47 nm as-deposited. It was also found that grain size increases with laser exposure. The samples were characterized using a scanning electron microscope and it was found that big clusters were formed after irradiation by TEA N₂ laser.     

Application of wavelength-scanned wavelength-modulation spectroscopy H2O absorption measurements in an engineering-scale high-pressure coal gasifier

A real-time, in situ water vapor (H₂O) sensor using a tunable diode laser near 1,352 nm was developed to continuously monitor water vapor in the synthesis gas of an engineering-scale high-pressure coal gasifier. Wavelength-scanned wavelength-modulation spectroscopy with second harmonic detection (WMS-2f) was used to determine the absorption magnitude. The 1f-normalized, WMS-2f signal (WMS-2f/1f) was insensitive to non-absorption transmission losses including beam steering and light scattering by the particulate in the synthesis gas. A fitting strategy was used to simultaneously determine the water vapor mole fraction and the collisional-broadening width of the transition from the scanned 1f-normalized WMS-2f waveform at pressures up to 15 atm, which can be used for large absorbance values. This strategy is analogous to the fitting strategy for wavelength-scanned direct absorption measurements. In a test campaign at the US National Carbon Capture Center, the sensor demonstrated a water vapor detection limit of ~800 ppm (25 Hz bandwidth) at conditions with more than 99.99 % non-absorption transmission losses. Successful unattended monitoring was demonstrated over a 435 h period. Strong correlations between the sensor measurements and transient gasifier operation conditions were observed, demonstrating the capability of laser absorption to monitor the gasification process.     

Bulk crystal growth and efficient diode-pumped laser performance of Yb3+:Sc2SiO5

A bulk crystal of Yb:Sc₂SiO₅ (Yb:SSO) with favorable thermal properties was successfully obtained by the Czochralski method. The energy level diagrams for Yb:SSO crystal were determined by optical spectroscopic analysis and semi-empirical crystal-field calculations using the simple overlap model. The full width at half maximum of the absorption band centering at 976 nm was calculated to be 24 nm with a peak absorption cross-section of 9.2×10^-21 cm². The largest ground-state splitting of Yb³⁺ ions is up to 1027 cm^-1 in a SSO crystal host. Efficient diode-pumped laser performance of Yb:SSO was primarily demonstrated with a slope efficiency of 45% and output power of 3.55 W.     

2-μm high-power multiple-frequency single-mode Q-switched Ho:YLF laser for DIAL application

We report on the development and the demonstration of a two-wavelength single-frequency laser oscillator based on Ho:YLF crystal. This laser is especially suitable for application as a transmitter in differential absorption lidar (DIAL)/integrated path differential absorption (IPDA) measurements of atmospheric carbon dioxide (CO₂) using the R30 CO₂ absorption line at 2,050.967 nm. The oscillator consists in a fiber-coupled and free-space solid-state hybrid system and can be used in high-energy middle-rate or moderate-energy high-rate configurations. The latter produced On and Off sequentially single-frequency laser pulses with 13 mJ of energy at a repetition rate of 2 kHz and pulse duration of 42 ns. The pulse energy and frequency stabilities are specially documented in free-running, single-frequency and two-frequency seeding single-mode operations. Standard deviation is 7.7 % for pulse energy and 2 MHz for frequency stability for the two-wavelength seeding operation. Allan variance plot shows that frequency fluctuations are reduced below 70 kHz for 10 s of averaging which is suitable for accurate CO₂ DIAL or IPDA measurements.     

Graphene-based passively Q-switched Nd:KLu(WO4)2 eye-safe laser operating at 1,425 nm

With graphene as saturable absorber, an Nd:KLu(WO₄)₂ eye-safe laser operating at 1,425 nm is demonstrated. To the best of our knowledge, this is the first demonstration that an Nd:KLu(WO₄)₂ laser operates at the eye-safe 1.4-μm region. A maximum total average output power of 170 mW is obtained under the pump power of 9.6 W, corresponding to an optical–optical efficiency of 1.77 %. The minimum pulse width and the highest pulse repetition rate are 153 ns and 97 kHz, respectively. Also the characteristics of the graphene used as saturable absorber for a 1.4-μm laser were studied for the first time.     

Inline multi-harmonic calibration method for open-path atmospheric ammonia measurements

We demonstrate a real-time inline calibration method for an open-path ammonia sensor using a quantum cascade laser (QCL) at 9.06 μm. Ethylene (C₂H₄) has an absorption feature partially offset from the ammonia absorption feature, and the ethylene signal serves as a reference signal for ammonia concentration in real time. Spectroscopic parameters of ammonia and ethylene are measured and compared with the HITRAN database to ensure the accuracy of the calibration. Multiple harmonic wavelength modulation spectroscopy (WMS) signals are used to separate the ambient ammonia and reference ethylene absorption signals. The ammonia signal is detected with the second harmonic (2f), while it is calibrated simultaneously with a high-harmonic (6–12f) signal of ethylene. The interference of ambient ammonia absorption on the ethylene reference signal is shown to be negligible when using ultra high-harmonics (≥6f). This in situ calibration method yields a field precision of 3 % and accuracy of 20 % for open-path atmospheric ammonia measurements.     

Spectroscopic characterisation of Er-doped LuVO4 single crystals

The LuVO₄:Er single crystals were grown by the Czochralski technique. The crystal-field split energy levels of Er³⁺ ion were derived experimentally employing absorption and emission spectra measured at T=10 K. The Judd–Ofelt phenomenological method was used to estimate intensity parameters, radiative lifetimes and branching ratios of luminescence. The excited state dynamics of the LuVO₄:Er systems was investigated and experimental lifetimes of emitting levels were measured. The emission cross section of the ⁴I_13/2→⁴I_15/2 transition in the infrared was calculated by the Füchtbauer–Ladenburg method. The gain cross section, estimated for several inverse-population parameters, allowed us to evaluate a potential laser activity of the LuVO₄:Er system at 1.6 μm. Also, the potential range of the optical pumping was assessed based on absorption spectra achieved at the room temperature. The optical losses related to the green up-converted emission, encountered under the 978 nm excitation between 300 and 670 K were indicated and discussed. Spectroscopic peculiarities of the Er³⁺-doped LuVO₄ crystal were discussed in relation to optical properties of the YVO₄:Er and GdVO₄:Er crystals. Taking into account the high quantum efficiency of the ⁴I_13/2 level, and satisfactory absorption and emission features, the LuVO₄:Er crystal can be considered as a promising active material for laser operation near 1.6 μm.