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     

High-resolution cavity-tuned ringdown spectrometer using a narrow-bandwidth pulsed laser source

A novel method of cavity ringdown spectroscopy is proposed to achieve high spectral resolution with tunable narrow bandwidth pulsed lasers. We demonstrate a cavity-tuned ringdown configuration in which only a single cavity mode is kept excited near the carrier frequency of a narrow bandwidth pulse laser. This is done simply by making a cavity resonance actively track the frequency reference served by the cw injection seed of the pulsed laser source. We present the servo mechanism used in the cavity resonance tracking, reliable procedures for transverse mode matching, and the evidence of single longitudinal mode excitation. The spectrometer performance is tested to record weak molecular overtone features of acetylene around the wavelength of 570 nm, showing cavity tracking stability within 5-MHz uncertainty which overcomes the bandwidth limit of pulsed laser sources itself. PACS 42.62.Fi; 42.60.Da; 33.20.Kf     

Development of a tunable mid-IR difference frequency laser source for highly sensitive airborne trace gas detection

The development of a compact tunable mid-IR laser system at 3.5 μm for quantitative airborne spectroscopic trace gas absorption measurements is reported. The mid-IR laser system is based on difference frequency generation (DFG) in periodically poled LiNbO₃ and utilizes optical fiber amplified near-IR diode and fiber lasers as pump sources operating at 1083 nm and 1562 nm, respectively. This paper describes the optical sensor architecture, performance characteristics of individual pump lasers and DFG, as well as its application to wavelength modulation spectroscopy employing an astigmatic Herriott multi-pass gas absorption cell. This compact system permits detection of formaldehyde with a minimal detectable concentration (1σ replicate precision) of 74 parts-per-trillion by volume (pptv) for 1 min of averaging time and was achieved using calibrated gas standards, zero air background and rapid dual-beam subtraction. This corresponds to a pathlength-normalized replicate fractional absorption sensitivity of 2.5×10^-10 cm^-1. Received: 29 April 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-303/497-1492, E-mail: dr@ucar.edu     

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.     

Real-time detection of 13CH4 in ambient air by use of mid-infrared cavity leak-out spectroscopy

We report on spectroscopic real-time detection of (13)CH(4) in ambient air. Our measurements were carried out by means of cavity leak-out absorption spectroscopy employing a tunable cw laser in the mid-infrared spectral region near lambda = 3 mum. A CO laser in combination with tunable microwave sideband generation was used as the light source. Using a 50-cm-long ringdown cell with R = 99.98% mirrors, we achieved a detection limit of 290 parts in 10(12) (ppt) (13)CH(4) in ambient air (integration time, 100 s). The corresponding noise-equivalent absorption coefficient was 5 x 10(-9)/cm.     

Infrared diode laser spectroscopy

Three types of lasers (double-heterostructure 66 K InAsSb/InAsSbP laser diode, room temperature, multi quantum wells with distributed feedback (MQW with DFB) (GaInAsSb/AlGaAsSb based) diode laser and vertical cavity surface emitting lasers (VCSELs) (GaSb based) have been characterized using Fourier transform emission spectroscopy and compared. The photoacoustic technique was employed to determine the detection limit of formaldehyde (less than 1 ppmV) for the strongest absorption line of the v₃ + v₅ band in the emission region of the GaInAsSb/AlGaAsSb diode laser. The detection limit (less than 10 ppbV) of formaldehyde was achieved in the 2820 cm⁻¹ spectral range in case of InAsSb/InAsSbP laser (fundamental bands of v₁, v₅). Laser sensitive detection (laser absorption together with high resolution Fourier transform infrared technique including direct laser linewidth measurement, infrared photoacoustic detection of neutral molecules (methane, form-aldehyde) is discussed.     

500 nm continuous wave tunable single-frequency mid-IR light source for C-H spectroscopy

A computer controlled tunable mid-IR light source, based on single resonant difference frequency generation (DFG), is experimentally investigated. The DFG process is pumped by an external cavity tapered diode laser, tunable over a spectral range of 30 nm. Grating feedback to the single mode channel of the tapered diode narrows the spectrum and allows for tuning of the emitted spectrum in the range from 780 to 810 nm. The DFG process takes place intra-cavity in a high finesse diode pumped 1064 nm solid state Nd:YVO₄ laser cavity, using periodically poled LiNbO₃ as the nonlinear material. Based on this new approach, a tunable single-frequency output power exceeding 3 mW was obtained in the mid-IR tuning range from 2.9 to 3.4 ??m.     

Near- and mid-infrared laser-optical sensors for gas analysis

Semiconductor diode lasers were first developed in the mid-1960s and found immediate application as much needed tunable sources for high-resolution laser spectroscopy commonly referred to as tunable diode laser absorption spectroscopy (TDLAS). In this paper, currently available semiconductor lasers for spectroscopy in the near- and mid-infrared spectral region based upon gallium arsenide, indium phosphite, antimonides and lead-salt containing compounds will be reviewed together with the main features of TDLAS. Room-temperature measurements of atmospheric carbon dioxide near 2 μm will be discussed and recent results obtained with a fast chemical sensor for methane flux measurements based on lead-salt diode lasers operating near 7.8 μm will be presented.     

Rapid measurement of cavity ringdown absorption spectra with a swept-frequency laser

A novel cavity ringdown spectrometer, incorporating a miniature continuous-wave swept-frequency laser that is widely tunable, requires less than 1 s to record wide-ranging absorption spectra with high sensitivity in a single rapid sweep of the laser frequency. The free spectral range of the ringdown cavity defines a sampling grid to measure absorbance-dependent ringdown times at successive cavity-resonance frequencies. The spectrometer has a single-ended transmitter-receiver configuration based on retro-reflected optical-heterodyne detection and exploiting fibre-optical telecommunications components. This swept-frequency approach to cavity ringdown spectroscopy yields a simple, low-cost, compact, rugged, versatile instrument for efficient sensing of gases. The performance of the spectrometer is demonstrated by measuring weak near-infrared rovibrational spectra of carbon dioxide gas within the 1.5–1.6 m wavelength range. PACS 42.62.Fi; 07.57.Ty; 07.07.Df     

Absorption line profile recovery based on residual amplitude modulation and first harmonic integration methods in photoacoustic gas sensing

Two methods for recovery of gas absorption line profiles are presented in this paper using photoacoustic spectroscopy and tunable diode laser spectroscopy (TDLS) with wavelength modulation (WM). A theoretical analysis based on Fourier coefficients is given in order to describe the various components that arise under simultaneous intensity and frequency modulation. The first method makes use of the residual amplitude modulation (RAM) signal which is always present in current modulation of distributed feedback (DFB) tunable diode lasers. The second method involves integration of a near-pure first harmonic derivative signal, separated from other distorting components by appropriate choice of the lock-in detection phase in the case of low modulation index. Good agreement is obtained with both methods between the experimental results and the theoretical simulation for the P17 absorption line of acetylene at 1535.39 nm but the second method gives a much improved accuracy and signal-to-noise ratio in lineshape recovery with photoacoustic spectroscopy.     

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.     

用于铷蒸气激光泵浦的窄线宽阵列半导体激光器

在激光二极管LD泵浦铷蒸气激光器中,窄线宽半导体激光是实现铷蒸气激光高效率输出的关键技术之一。基于体布拉格光栅（VBG）外腔技术,实现了40 W功率0.14 nm线宽的780 nm激光输出。采用半导体制冷片（TEC）控制VBG温度,使得该激光器空气中波长可从779.35 nm调谐至780.10 nm,可用于铷蒸气激光的高效泵浦。     

Application of a characterized difference-frequency laser source to carbon monoxide trace detection

A tunable continuous wave(cw) mid-infrared(MIR) laser based on difference-frequency generation(DFG) in a 1.5-cm long AgGaS2 nonlinear crystal for trace gas detection is reported.Two visible and near-infrared diode lasers were used as pump and signal sources.The MIR-DFG laser was tunable in a wavelength range of 4.75 μm-4.88 μm.The phase-matching(PM) condition was non-critically achieved by adjusting the temperature of the crystal for fixed pairs of input pump and signal wavelengths.The required PM temperatures of the generated MIR-DFG wavelengths have been calculated by using three sets of recent Sellmeier equations and the temperature-dispersion equations of AgGaS2 given by Willer U,et al.(Willer U,Blanke T and Schade W 2001 Appl.Opt.40 5439).Then the calculated PM temperatures are compared with the experimental values.The performance of the MIR-DFG laser is shown by the trace detection of the P(16) carbon monoxide(12C16O) absorption line in a laboratory-fabricated absorption cell.The enhanced sensitivity of about 0.6×10 4 was obtained through the long path absorption provided by consecutive reflections between coated cylindrical mirrors of a constructed cell.     

可调谐二极管激光吸收光谱法监测大气痕量气体中的浓度标定方法研究

可调谐二极管激光吸收光谱(TDLAS)技术是利用二极管激光器波长调谐特性,获得被测气体的特征吸收光谱范围内的吸收光谱,从而对污染气体进行定性或者定量分析。在空气痕量气体检测中,一般需要和长光程吸收池相结合使用。可调谐二极管激光吸收光谱法就是在可调谐二极管激光器与长光程吸收池技术相结合的基础上发展起来的一种新的痕量气体检测方法。这种方法不仅精度较高,选择性强而且响应速度快。已经广泛用于大气中多种痕量气体的检测以及地面的痕量气体和气体泄漏的检测。在大部分痕量气体检测仪器中需要精确地对检测气体进行在线的浓度标定,文章介绍了一个简单而精确的浓度标定方法,从理论上进行了分析,然后通过实验证明了这种方法的可行性。     

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.     

Second-harmonic detection with tunable diode lasers — Comparison of experiment and theory

A series of experiments are carried out by current modulating a tunable diode laser, and slowly ramping the wavelength to scan weak absorption lines in gases at pressures ranging from 2 to 60 Torr. A lock-in amplifier detects the second harmonic (2f) of the modulation frequency, and the experimental 2f signals are compared with theory. Detailed measurements are made on Lorentzian, Voigt, and Gaussian line profiles, over a wide range of modulation amplitudes. Excellent agreement between experiment and calculation is obtained in all cases. This quantitative understanding enables one to derive true lineshapes and linewidths of very weak absorption lines from measurements of 2f lineshapes only. Results are applicable to trace gas detection using tunable diode lasers, and to other areas of spectroscopy and magnetic resonance where harmonic detection techniques are routinely employed to monitor weak signals.This work was supported in part by the National Science and Engineering Research Council, Canada     

Spectroscopic diagnostics of chemical processes: applications of tunable optical parametric oscillators

Optical parametric oscillator (OPO) and amplifier (OPA) devices are useful for spectroscopic sensing of chemical processes in laboratory, industrial, and environmental settings. This is particularly true of nanosecond-pulsed, continuously tunable OPO/OPA systems, for which we survey a variety of instrumental strategies, together with actual spectroscopic measurements. The relative merits of OPO wavelength control by intracavity gratings and by injection seeding are considered. A major innovation comprises an OPO with a ring cavity based on periodically poled lithium niobate (PPLN) and injection-seeded by a single-mode tunable diode laser (TDL). Active cavity control by an ‘intensity dip’ method yields an optical bandwidth ≤0.005 cm^-1 (150 MHz), which compares favourably with the performance of advanced grating-tuned OPO/OPA systems. A novel adaptation of this TDL-seeded PPLN OPO employs a compact, inexpensive multimode pump laser, with which it is still possible to obtain continuously tunable single-mode signal output. Cavity ringdown (CRD) spectroscopy also figures prominently, with infrared (IR) CRD spectra from both grating-scanned and TDL-seeded OPOs reported. Finally, a tunable ultraviolet (UV) source, combining a TDL-seeded passive-cavity OPO and a sum-frequency generation stage, is developed for measurements of time-resolved IR-UV double resonance spectra of acetylene and UV laser-induced fluorescence spectra of nitric oxide. Received: 28 March 2000 / Published online: 13 September 2000     

SG-DBR半导体激光器的光谱技术研究

可调谐半导体激光吸收光谱作为一种高灵敏度、高选择性、非侵入的痕量气体实时检测技术,已在大气监测、工业控制等方面得到广泛应用。采用一种新型宽带可调谐的SG-DBR半导体激光器(可调谐范围1 520~1 570 nm)作光源,并通过自编程序对该激光器设定了18个通道,输出波长分别对应CO,CO₂以及H₂O的吸收谱线中心位置,设计和构建了一个基于近红外可调谐半导体激光吸收光谱的多组分气体光谱测量系统,描述了相关的光学系统设置,结合波长调制(wm)的二次谐波技术测量其中14个通道(分别对应CO和CO₂的吸收谱线)的吸收光谱,系统获得的CO和CO₂峰值吸收探测极限能够达到10-5。实验结果验证了SG-DBR激光器在波长调制吸收光谱多组分气体检测领域的可行性。在实际应用过程中使用单个SG-DBR激光器可以实现多组分气体的同时测量,有效降低设备成本和系统复杂性。     

Mid-IR difference frequency laser-based sensors for ambient CH4, CO, and N2O monitoring

A new mid-infrared sensor platform is described, which combines difference frequency generation (DFG)-based tunable laser sources with simple direct absorption spectroscopy. DFG lasers operating in the 3–5 micron window are tuned to access a variety of species in the C–H, N–O, and C–O stretch regions. The sensors are capable of sub-ppb detection of key greenhouse gas species as well as common pollutants and tracer species. Specific examples of sensor data obtained for methane, nitrous oxide, and carbon monoxide are presented, including relevant time series data and associated Allan Variances. The platform provides a cost-effective alternative to other laser-based approaches in some cases, performing at similar or superior levels. Emphasis on achieving key performance metrics driven by World Meteorological Organization guidelines for Global Air Watch program and other applications is highlighted.     

Two schemes for trace detection using cavity ringdown spectroscopy

We describe and compare two schemes of high-sensitivity cavity ringdown spectroscopy (CRDS), both functioning with telecom diode lasers. The first (cw-CRDS) gives high spectral resolution, which is useful for low-pressure trace detection or for laboratory spectroscopy applications. We present a compact prototype partly based on fiber technology. The second scheme exploits optical feedback (of-CRDS) and results in a much simpler setup, more appropriate for realizing low-cost trace-detection devices. PACS 42.62.Fi; 87.64.Ni; 42.68.Ca; 82.80.Gk