A continuously tunable long-wavelength cw IR source for high-resolution spectroscopy and trace-gas detection

A new widely tunable source in the infrared for use in high-resolution spectroscopy and trace-gas detection is described. This spectroscopic source is based on Difference Frequency Generation (DFG) in gallium selenide (GaSe) and is continuously tunable in the 8.8–15.0 m wavelength region. Such a DFG source operates at room temperature which makes it a useful alternative to a lead-salt diode-laser- based detection system that requires cryogenic temperatures and numerous individual diode lasers.Prof. F. P. Schäfer on the occasion of his 65th birthday.     

Laser excess noise and interferometric effects in frequency-modulated diode-laser spectrometers

Tunable Diode-Laser Absorption Spectroscopy (TDLAS) is increasingly being used to measure trace-gas concentrations down to low part-per-billion levels (1 ppbv = 10^–9 volume mixing ratio). Semiconductor lead-salt diode lasers give access to the mid-infrared spectral region and the application of high-Frequency Modulation (FM) schemes can further improve the sensitivity and detection speed of modern instrumentation. Several factors influence or even limit spectrometer performance. The central elements in such spectrometers are lead-salt diode lasers. Experimental data will be presented, which demonstrate that high-frequency excess-noise contributions above several MHz can be attributed to mode hopping and mode partition noise during multimode laser operation. Additionally it will be discussed how a FM-TDLAS spectrometer can be interpreted as an optimized Michelson interferometer for absolute distance measurements and, therefore, is extremely sensitive towards drift effects. The higher the modulation frequency, the higher is the drift sensitivity of the spectrometer due to interferometric effects. These drift effects are a second factor affecting ultrasensitive measurements. While wideband-laser noise characteristics call for high modulation frequencies, the aforementioned interferometric effects in the spectrometer require low modulation frequencies.     

Detection of methane in air using diode-laser pumped difference-frequency generation near 3.2 μm

Spectroscopic detection of the methane in natural air using an 800 nm diode laser and a diode-pumped 1064 nm Nd:YAG laser to produce tunable light near 3.2 µm is reported. The lasers were pump sources for ring-cavity-enhanced tunable difference-frequency mixing in AgGaS₂. IR frequency tuning between 3076 and 3183 cm^–1 was performed by crystal rotation and tuning of the extended-cavity diode laser. Feedback stabilization of the IR power reduced intensity noise below the detector noise level. Direct absorption and wavelength-modulation (2f) spectroscopy of the methane in natural air at 10.7 kPa (80 torr) were performed in a 1 m single-pass cell with 1 µW probe power. Methane has also been detected using a 3.2 µm confocal build-up cavity in conjunction with an intracavity absorption cell. The best methane detection limit observed was 12 ppb m (Hz.)^–1/2.     

Computer controlled diode laser spectrometer with a helium evaporation cryostat and spectroscopy of thev 3 vibration of NCO

A computer-controlled diode laser spectrometer for the 1200 to 2500 cm^–1 spectral region is described. The spectrometer has been applied to high resolution spectroscopy of the NCO radical at 5.2 m. The lead-salt diode lasers are cooled to their operating temperature with a temperature adjustable helium evaporation cryostat. Computer-controlled tuning procedures for the frequency tuning of the diode lasers have been developed; they are independent of tables describing the tuning characteristics of the diode lasers. 41 lines of the antisymmetric stretching-vibrationv ₃ of the linear NCO radical have been observed. We were able to detect vibration-rotation transitions in both² _1/2 and² _3/2 fine structure sublevels. These measurements led to the precise determination of additional molecular constants.     

High Resolution Linear Laser Absorption Spectroscopy-Review

Throughout the optical spectral region from the visible to the far infrared, lasers have in recent years become increasingly important for high resolution spectroscopy in both laboratory measurements and practical applications. In general, laser sources can be classified into broadly tunable and discretely tunable types with the gas laser belonging to the latter. While gas lasers oscillate in narrow lines scattered throughout the optical spectral region, broadly tunable laser emissions cover a range of wide spectral region depending on the lasing media and operating characteristics. For example, the Pb-salt diode laser covers the spectral region from about 2.6 to 30 μm, the dye laser from 400 to 700 nm, the color center laser from 0.8 to 3.3 μm, the difference frequency spectrometer from 2.2 to 4.2 μm and the spin-flip Raman laser from 5.2 to 6.0 μm. The very limited tunability of the gas laser can sometimes be extended to many times the Doppler width by Zeeman tuning the gain medium or pressure broadening the gain profile or electrooptically modulating the laser output frequency with a tunable microwave source.     

Improved signal processing in pulsed and cw spin-flip Raman laser spectrometers

Double beam optical techniques which allow direct spectroscopic measurements using both pulsed and cw tunable spin-flip Raman lasers have been developed. The systems provide a high resolution spectrometer (spectral linewidths 0.03 cm^-1 pulsed and 0.003 cm^-1 cw) in the range 5.3–6μm. Line frequency measurements are accurate to 0.01 cm^-1 and intensity measurements to a few percent. Results of molecular spectroscopy provide a comparison of the relative performance of each system.     

An approach of open-path gas sensor based on tunable diode laser absorption spectroscopy

Tunable diode laser absorption spectroscopy (TDLAS) is a new method to detect trace-gas qualitatively or quantificationally based on the scan characteristic of the diode laser to obtain the absorption spectroscopy in the characteristic absorption region. A time-sharing scanning open-path TDLAS system using two near infrared distributed feedback (DFB) tunable diode lasers is designed to detect CH4 and H2S in leakage of natural gas. A low-cost Fresnel lens is used in this system as receiving optics which receives the laser beam reflected by a solid corner cube reflector with a distance of up to about 60 m. High sensitivity is achieved by means of wavelength-modulation spectroscopy with second-harmonic detection. The minimum detection limits of 1.1 ppm·m for CH4 and 15 ppm·m for H2S are demonstrated with a total optical path of 120 m. The simulation monitoring experiment of nature gas leakage was carried out with this system. According to the receiving light efficiency of optical system and detectable minimum light intensity of detection, the detectable optical path of the system can achieve 1 - 2 km. The sensor is suitable for natural gas leakage monitoring application.     

A DFG spectrometer at 3 μm for high resolution molecular spectroscopy and trace gas detection

We report the realization and characterization of a spectrometer based on difference frequency generation using a periodically poled lithium niobate crystal. As signal and pump lasers we used a diode-pumped Nd–YAG laser (λ=1.064 μm) and an extended cavity semiconductor diode laser (λ=0.785 μm), respectively. The mid-infrared coherent radiation was produced at 3 μm with a maximum power of about 160 nW obtained with 340 mW of signal and only 3.4 mW of pump. That corresponds to an efficiency of 0.01%/Wcm, which is in good agreement with other data available in literature. The generated radiation around 3 μm has allowed us to study fundamental absorption bands of molecules of great atmospheric and physical interest such as water vapor and the hydroxyl free radical. In this work we report preliminary spectroscopic results concerning the ν₁ 5₄₁→6₅₂ H₂O line at 2.968 μm. In particular, for this line we provide the first experimental estimation of self-, N₂- and O₂-broadening coefficients.     

Determination of absorption length of CO2 and high power diode laser radiation for ordinary Portland cement and its influence on the depth of melting

The laser beam absorption lengths of CO₂ and a high power diode laser (HPDL) radiation for concrete have been determined. By employing Beer–Lambert’s law the absorption lengths for concrete of CO₂ and a HPDL radiation were 470±22 μm and 177±15 μm, respectively. Indeed, this was borne out somewhat from a cross-sectional analysis of the melt region produced by both lasers which showed melting occurred to a greater depth when the CO₂ laser was used.     

Near-infrared sources in the 1–1.3 μm region by efficient stimulated Raman emission in glass fibers

Low-loss glass fiber waveguides are found to be excellent media for Raman lasers and amplifiers in the near-infrared region of the spectrum. Multiwavelength emission in the 1–1.3 μm range is readily obtained by efficient stimulated Raman scattering in single-mode silica fibers. With a 1.064 μm pulsed pump of 250 W in a 175-m, 6-μm diameter single-mode silica fiber we observed four orders of Stokes radiation at 1.12 μm, 1.18 μm, 1.23 μm and 1.3 μm, respectively. Our results imply that pulsed tunable stimulated Raman emission in this wavelength region is possible using kW tunable infrared dye lasers near 1 μm as pumps. These sources are useful for studying the dispersion of glass fibers as well as for other spectroscopic applications.     

Development of a tunable IR lidar system

A differential absorption lidar system (DIAL) based on a continuously tunable optical parametric amplifier (OPA) pumped by a Nd : YAG laser (200 mJ at λ=355 nm) operating at a maximum pulse repetition rate of 100 Hz has been developed. The system provides continuously tunable coherent radiation in the Visible–near IR range (0.4–2.5 μm), allowing to perform DIAL measurements in a spectral region where most of atmospheric constituents and pollutants display absorption lines. The spectral width of the OPA system is line-narrowed by using a master oscillator dye laser as seeder, achieving a linewidth of 0.04 cm⁻¹ (FWHM), a spectral purity larger than 99% and a frequency stability better than 1 pm h⁻¹, with an output energy in the IR of 1–10 mJ. The OPA system was used to perform DIAL measurements in the lower troposphere. Preliminary results in terms of water vapor content and aerosol backscattering profiles are presented and discussed.     

Widely tunable diffraction limited 1000 mW external cavity diode laser in Littman/Metcalf configuration for cavity ring-down spectroscopy

We report on recent progress on external cavity diode lasers (ECDL) using a new concept of a Littman/Metcalf configuration. Within this concept one facet of the diode laser chip is used for coupling to a high quality Littman/Metcalf resonator whereas the other side of the diode laser chip emits the output beam. The alignment of the external resonator is independent from the alignment of the output beam and there is no need for any compromise in the alignment. This results in an improved behavior of the external resonator with the benefit of a drastic increase in power and single mode tuning.We investigated this light source for high resolution spectroscopy in the field of cw-cavity ring-down spectroscopy (CRDS). The monitoring of environmental and medical gases from vehicles or human breath requires a suitable radiation source in the mid-infrared (MIR) between 3 and 5 μm that is frequency stable and can be widely tuned. Since this wavelength cannot be reached via direct emitting room temperature semiconductor lasers, additional techniques like difference frequency generation (DFG) are essential. Tunable difference frequency generation relies on high power, small linewidth, fast tunable, robust laser diode sources with excellent beam quality.With our new compact, alignment-insensitive and robust ECDL concept, we achieved an output power of 1000 mW and an almost Gaussian shaped beam quality (M²<1.2). The coupling efficiency for optical waveguides as well as single mode fibers exceeds 70%. The wavelength is widely tunable within the tuning range of 20 nm via remote control. This laser system operates longitudinally in single mode with a mode-hop free tuning range of more than 150 GHz without current compensation and a side-mode-suppression better than 50 dB. This concept is currently realized within the wavelength regime between 750 and 1080 nm.Our high powered Littman/Metcalf laser system was part of a MIR-light source which utilizes DFG in periodically poled lithium niobate (PPLN) crystals. At the wavelength of 3.3 μm we were able to achieve a high-resolution absorption spectrum of water with four different isotoplogues of H₂O components. This application clearly demonstrates the suitability of this laser for high-precision measurements. PACS 07.57.Ty; 42.55.Px; 42.62.Fi     

Antimonide-based lasers and DFB laser diodes in the 2–2.7 μm wavelength range for absorption spectroscopy

We report on Fabry–Pérot semiconductor lasers and single frequency distributed feedback lasers based on GaInAsSb/AlGaAsSb quantum wells. The laser structures were grown by molecular beam epitaxy on GaSb substrates. The devices were etched either by wet process or by inductively coupled plasma (ICP) process. Electron-beam lithography was used to deposit a metal Bragg grating on each side of the laser ridge to fabricate the DFB lasers. The devices all operate in the continuous wave regime at room temperature with a single frequency emission above 2.6 μm and good tuning properties, making them well adapted to tunable diode laser absorption spectroscopy. PACS 42.55.Px; 42.62.Fi     

InGaAsP/GaAs SCH SQW laser arrays grown by LPE

InGaAsP/InGaP/GaAs separate confinement heterostructure (SCH) single quantum well (SQW) laser structures have been obtained by an improved liquid-phase epitaxy (LPE) process. Wide-contact stripe lasers have been fabricated with threshold current density below 300 A/cm² and cavity length of 800 μm. Finally, with the same grown wafers, 1-cm bar laser diode (LD) arrays are made with 150 μm wide stripes and a maximum fill factor of 30%. Continuous Wave (CW) power output of 20 W has been reached.     

基于可调谐激光吸收光谱技术的脱硝过程中微量逃逸氨气检测实验研究

为了对电厂脱硝过程中逃逸的微量氨气进行在线检测,实验室采用可调谐激光吸收光谱技术对常温常压下以及不同温度下的低浓度氨气进行了测量试验,其中电厂逃逸氨气检测处温度约为650 K。通过分析近红外波段的氨气吸收谱线,并考虑实际测量环境H₂O和CO₂等浓度很大的气体吸收谱线的干扰,实验选取2.25 μm附近的ν₂+ν₃谱线作为浓度检测谱线。为了验证所选谱线对低浓度NH₃的测量能力,实验对H₂O,CO₂和NH₃的吸收谱线进行模拟,发现低浓度NH₃受较大浓度的H₂O和CO₂谱线的干扰较小,尤其是CO₂谱线的干扰可以忽略不计,且2.25 μm处谱线强度远远大于通讯波段1.53 μm处的谱线。基于新型Herriott池以及高温管式炉,结合可调谐激光吸收光谱中的直接吸收技术和波长调制技术,实现了对不同温度下超低浓度NH₃的高分辨率快速检测。常温常压下其线型函数可以利用洛伦兹线型来近似描述,直接吸收测量技术可以使探测极限降低到0.225×10-6。通过采用简单降噪处理技术如多次平均、简单小波分析等,得到不同温度下的谐波信号与浓度具有良好的线性关系,为采用可调谐激光吸收光谱技术进行现场低浓度逃逸氨气检测提供了很好的依据。     

Absolute line intensity measurements in the ν2 bands of HDO and D2O using a tunable diode laser spectrometer

Absolute line intensities for several individual vibration-rotation transitions in the ν₂ bands of HDO and D₂O have been determined at room temperature from laboratory spectra recorded with a tunable diode laser spectrometer. Two tunable semiconductor diode lasers operating in the 1250- to 1350- and 1060- to 1140-cm⁻¹ spectral regions have been used in the recording of the data. Comparisons are made with previously published results where appropriate.     

Trace gas monitoring with infrared laser-based detection schemes

The success of laser-based trace gas sensing techniques crucially depends on the availability and performance of tunable laser sources combined with appropriate detection schemes. Besides near-infrared diode lasers, continuously tunable midinfrared quantum cascade lasers and nonlinear optical laser sources are preferentially employed today. Detection schemes are based on sensitive absorption measurements and comprise direct absorption in multi-pass cells as well as photoacoustic and cavity ringdown techniques in various configurations. We illustrate the performance of several systems implemented in our laboratory. These include time-resolved multicomponent traffic emission measurements with a mobile CO₂-laser photoacoustic system, a diode-laser based cavity ringdown device for measurements of impurities in industrial process control, isotope ratio measurements with a difference frequency (DFG) laser source combined with balanced path length detection, detection of methylamines for breath analysis with both a near-IR diode laser and a DFG source, and finally, acetone measurements with a heatable multipass cell intended for vapor phase studies on doping agents in urine samples. PACS 33.20.Ea; 42.62.Fi; 42.72.Ai; 87.64.km; 92.60.Sz     

Infrared laser stark shift spectroscopy in ammonia

The Stark effect in ammonia has been theoretically and experimentally analyzed using lead salt tunable diode laser absorption spectroscopy and CO₂ laser absorption spectroscopy of several absorption lines around 1050 cm^–1 applied to an all-optical sensor for measuring of electric field strength. Measurements of the Stark splitting effect of theaR(5,K) ammonia lines forK=1–5 as well as for the sR(3,K) lines forK=0–3 have been made at Doppler broadening pressures and for several different electric field strengths. Theoretical electric field dependent energy levels have been evaluated by diagonalization of a 6×6 energy matrix constructed using both electric field independent and dependent terms. From the theoretical analysis the resolution can be predicted and optimized both in the Doppler broadened and in the pressure broadened regimes. The predicted resolution is 0.5% at an electric field strength of 20 kV/cm. The theoretical calculations and the experimental data recorded with the tunable diode laser system were compared with independent measurements made with a CO₂ laser system. The agreement between experimentally recorded and theoretically calculated spectra is good which indicates that the theoretical model is satisfactory for our purposes. The contribution from the normally forbidden ssR(5, 3) ammonia line to the absorption at theP(12) CO₂ laser line in the 9 m band is briefly discussed.     

Photoacoustic monitoring of gases using a novel laser source tunable around 2.5 μm

We present the first spectroscopic measurements using a tunable solid state Cr²+:ZnSe laser emitting at wavelengths between 2.2 μm and 2.8 μm. Photoacoustic measurements on various gases such as methane, carbon monoxide, carbon dioxide, water vapour, nitrous oxide, and ambient air were carried out. In this paper, we present measurements on methane, nitrous oxide, and ambient air. The deduced detection limits are in the low ppm or sub-ppm range, e.g., 0.2 ppm for carbon dioxide, 0.8 ppm for methane and 2.7 ppm for carbon monoxide.     

Compact eye-safe laser sources based on OPOs with KTP or PPKTP crystals

We report on compact eye-safe nanosecond laser sources emitting in the 1.5 μm wavelength range based on non-critically phase-matched parametric interaction in optical parametric oscillators (OPOs) with KTP and periodically poled KTP (PPKTP) crystals, pumped by the fundamental frequency of Nd:YAG lasers. As much as 250 μJ signal pulse energy at 1.5 μm wavelength, 6.5 ns FWHM pulse-width, has been obtained in a PPKTP-OPO, extracavity pumped by a Nd:YAG microlaser oscillator–amplifier at 650 μJ pump pulse energy, 8 ns pulse-width. A single signal pulse of 2.7-mJ output energy at 1.57 μm wavelength, less than 5 ns pulse-width, was generated in a KTP-OPO, intracavity pumped by a passively Q-switched Nd:YAG laser.