Infra-red heterodyne laser radar for remote sensing of air pollutants by range-resolved differential absorption

An infra-red heterodyne laser radar scheme incorporating range-resolved differential absorption using elastically backscattered, pulsed signal from distributed particles in the atmosphere is proposed and analysed. The basic requirements of the system arrangement and operating parameters involved in this scheme are discussed in detail. This method is shown to offer a remote and spatially-resolved measurement of various molecuar pollutants dispersed in the air for ranges up to several km and detection sensitivities to less than 0.1 ppm with laser pulses of 100 W average power. This power is 10³ to 10⁶ times smaller than that required with the conventional direct detection technique in the infra-red. The measuring accuracy and its limiting processes inherent to the infra-red range-resolved differential absorption method are also analysed.     

Cavity ring down spectroscopy: detection of trace amounts of substance

We describe several applications of cavity ring-down spectroscopy (CRDS) for trace matter detection. NO₂ sensor was constructed in our team using this technique and blue-violet lasers (395–440 nm). Its sensitivity is better than single ppb. CRDS at 627 nm was used for detection of NO₃. Successful monitoring of N₂O in air requires high precision mid-infrared spectroscopy. These sensors might be used for atmospheric purity monitoring as well as for explosives detection. Here, the spectroscopy on sharp vibronic molecular resonances is performed. Therefore the single mode lasers which can be tuned to selected molecular lines are used. Similarly, the spectroscopy at 936 nm was used for sensitive water vapour detection. The opportunity of construction of H₂O sensor reaching the sensitivity about 10 ppb is also discussed.     

A PbSe diode laser spectrometer to be used in air pollution monitoring and mineral prospecting

A high resolution infra-red PbSe diode laser spectrometer has been developed for application in air pollution monitoring and mineral prospecting. This instrument possesses three cells for measurement and calibration. Correlation techniques have been applied to increase the sensitivity. SO₂ concentrations of around 5 ppm have been monitored at 7.3m with the diode operating at 77 K.     

Photoacoustic spectroscopy for trace gas detection with cryogenic and room-temperature continuous-wave quantum cascade lasers

The main characteristics that a sensor must possess for trace gas detection and pollution monitoring are high sensitivity, high selectivity and the capability to perform in situ measurements. The photacoustic Helmholtz sensor developed in Reims, used in conjunction with powerful Quantum Cascade Lasers (QCLs), fulfils all these requirements. The best cell response is # 1200 V W⁻¹ cm and the corresponding ultimate sensitivity is j 3.3 × 10⁻¹⁰ W cm⁻¹¹ Hz^−11/2. This efficient sensor is used with mid-infrared QCLs from Alpes Lasers to reach the strong fundamental absorption bands of some atmospheric gases. A first cryogenic QCL emitting at 7.9 μm demonstrates the detection of methane in air with a detection limit of 3 ppb. A detection limit of 20 ppb of NO in air is demonstrated using another cryogenic QCL emitting in the 5.4 μm region. Real in-situ measurements can be achieved only with room-temperature QCLs. A room-temperature QCL emitting in the 7.9 μm region demonstrates the simultaneous detection of methane and nitrous oxide in air (17 and 7 ppb detection limit, respectively). All these reliable measurements allow the estimated detection limit for various atmospheric gases using quantum cascade lasers to be obtained. Each gas absorbing in the infrared may be detected at a detection limit in the ppb or low-ppb range.     

Theoretical investigation of chirped mirrors in semiconductor lasers

This paper reports a theoretical design of chirped mirrors in 1.3-μm double-section semiconductor lasers to achieve high reflectivity and dispersion compensation over a broad bandwidth. Analytic expressions for reflectivity, group delay and group delay dispersion are derived. We use for the first time chirped air/semiconductor layer pairs as mirrors for higher-order dispersion compensation in semiconductor lasers. Our optimised calculations demonstrate that the broad-band mirrors designed consist of a total of only 12 air/semiconductor layers and achieve a reflectivity higher than 99.8%, a smooth group delay and almost stable dispersion in the laser cavity over a 100-nm bandwidth. Due to a high index contrast of both types of the layers, n _l = 1, n _h~ 3.5, a high-reflectivity bandwidth of > 700 nm is obtained in 1.3-μm semiconductor lasers. We also compare our results with that of a commercial simulation program and show a good agreement between them. As a conclusion, we assume from the theoretical results that air/semiconductor layer pairs with varying thicknesses used at one end of double-section semiconductor lasers can lead to femtosecond optical pulse generation using mode-locking techniques. An erratum to this article can be found at .     

差分吸收光谱法测量大气污染的浓度反演方法研究

介绍了差分光学吸收光谱法(DOAS)测量大气污染气体浓度的基本原理,描述了对测量光谱所作的一些必要处理,对最小二乘法作了简单介绍,并将它用在DOAS方法中的浓度反演中,通过与当地监测站的数据进行对比,证明了最小二乘法非常适用于DOAS方法中的浓度反演. 关键词： 差分光学吸收光谱法 光谱处理 最小二乘法 环境监测     

High power, long pulse, CH3I far infra-red laser

A high power, far infra-red laser, pumped by a long pulse high energy CO₂ laser, has been developed to produce 8 μs pulses at 447 μm. The influence of the molecular weight of the far infra-red gas on the absorption of the pump energy is discussed. The results show that the heavy molecules are most convenient for long pulse far infra-red lasers.     

Remote air pollution measurement

The availability of high energy tunable laser sources has extended the possibilities for remote air pollution measurement. This paper presents a discussion and comparison of the Raman method, the resonance and fluorescence backscatter method, long path absorption methods and the differential absorption method. A comparison of the above remote detection methods shows that the absorption methods offer the most sensitivity at the least required transmitted energy. Topographical absorption provides the advantage of a single ended measurement, and differential absorption offers the additional advantage of a fully depth resolved absorption measurement. Recent experimental results confirming the range and sensitivity of the methods are presented. Future widespread use of remote pollution monitoring will probably be limited to the infra-red to interact with molecular spectral bands and to meet eye safety requirements.     

Infra-red lasers and their applications

Properties and applications of infra-red lasers are reviewed, with particular emphasis on those likely to be fundametnally important in energy production, materials and data processing.     

Infrared laser radar technique using heterodyne detection for range-resolved sensing of air pollutants

An infrared, pulsed heterodyne laser radar using differential absorption technique with tunable lasers is proposed as a practical means of measuring dispersed air pollutants with range resolution. A system analysis shows that this scheme offers an adequate sensitivity for ranges up to several km.     

A laser fluorimeter for direct cardiac metabolism investigation

We have developed a device which includes two laser sources (nitrogen and dye lasers), an optical system, an optical fibre able to transmit both ultra-violet and infra-red light, photoreceivers, and an analogue circuit feeding a microcomputer in order to perform an in situ and on line study of NADH/NAD ratios by fluorimetry.The paper includes the scientific basis of the method, the characteristics of each part of the apparatus and possible applications to the study of cardiac metabolism or any organ that can be reached through catheterism, fibrescopy or simple transcutaneous puncture. Examples of experiments carried out to appreciate the effects of anoxia or ischemia on the tissular NADH level are presented.     

Assessment of free electron lasers

Free electron lasers represent a radical alternative to conventional lasers, being potentially the most flexible, high power and efficient generators of coherent radiation from the ultra-violet to the far infra-red. In this review the properties of free electron lasers are discussed from both a theoretical and an experimental viewpoint, and some likely areas of application for these devices are outlined.     

Laser spectroscopic instrumentation and techniques: long-path monitoring by resonance absorption

Tunable laser spectroscopy has necessitated the introduction of new measurement techniques, particularly with regard to the wavelength calibration. This paper highlights some of these requirements and surveys laser spectroscopic measurements which have already been performed in the infrared. The rather unique properties of tunable infrared lasers also make them useful for air pollution monitoring. In particular, several tunable laser techniques have been used for long-path monitoring of ambient gases in the atmosphere by resonance absorption. The results of these measurements and prospects for the future will be discussed.     

Whispering gallery microcavity lasers

Whispering gallery mode (WGM) optical microresonators have attracted intense interests in the past decades. The combination of high quality factors (Q) and small mode volumes of modes in WGM resonators significantly enhances the light‐matter interactions, making them excellent cavities for achieving low threshold and narrow linewidth lasers. In this Review, the progress in WGM microcavity lasers is summarized, and the laser performance considering resonator geometries and materials as well as lasing mechanisms is discussed. Label‐free detection using WGM resonators has emerged as highly sensitive detection schemes. However, the resolution is mainly limited by the cavity Q factor which determines the mode linewidth. Microcavity lasers, due to their narrow laser spectral width, could greatly improve the detection resolution. Some recent developments in sensing using microcavity lasers are discussed.     

A simple carbon dioxide laser amplifier suitable for optically pumped infra-red laser applications

A simple amplifier tube has been designed which, when used with already well-developed, stable single-frequency CO₂ laser oscillators, produces a cw power output of up to 40 W suitable for pumping far infra-red lasers.     

Optimization of a compact photoacoustic quantum cascade laser spectrometer for atmospheric flux measurements: application to the detection of methane and nitrous oxide

Room temperature (RT) quantum cascade lasers (QCL) are now available even in continuous wave (cw) mode, which is very promising for in situ gas detectors. Ambient air monitoring requires high sensitivity with robust and simple apparatus. For that purpose, a compact photoacoustic setup was combined with two cw QCLs to measure ambient methane and nitrous oxide in the 8 μm range. The first laser had already been used to calibrate the sensitivity of the photoacoustic cell and a detection limit of 3 ppb of CH₄ with a 1s integration time per point was demonstrated. In situ monitoring with this laser was difficult because of liquid nitrogen cooling. The second laser is a new RT cw QCL with lower power, which enabled one to reach a detection limit of 34 ppb of methane in flow. The loss in sensitivity is mainly due to the weaker power as photoacoustic signal is proportional to light power. The calibration for methane detection leads to an estimated detection limit of 14 ppb for N₂O flux measurements. Various ways of modulation have been tested. The possibility to monitor ambient air CH₄ and N₂O at ground level with this PA spectrometer was demonstrated in flux with these QCLs. PACS 07.88; 92.60.Sz     

Remote sensing with infra-red heterodyne radiometers

The narrow spectral bandwidth capability of heterodyne radiometers can be utilized to construct highly selective pollutant sensing instruments. The infra-red spectral region is favourable due to the availability of pollutant absorption lines and the wavelength dependence of heterodyne sensitivity. We review the types of sensing systems which can make use of heterodyne techniques, including comments about the type of lasers which can serve as local oscillators. Sensitivity calculations of passive heterodyne radiometers to NO, CO₂, and SO₂ are then presented. Our concluding discussion involves the use of a heterodyne receiver in an active NO sensing system which is based on infra-red fluorescence.     

Environmental trace gas detection using laser spectroscopy

This special issue of Applied Physics B – Lasers and Optics attempts to document the current status and trends of environmental trace gas detection through a collection of 32 invited papers motivated in part by the need for and importance of a detailed understanding of our environment. Although numerous traditional optical methods, gas chromatography, and mass spectrometry have served us extremely well in atmospheric and environmental trace gas detection, promising new sensing and precise measurement techniques based on laser spectroscopy have emerged and been successfully used in numerous applications. The concept and timing of this special issue has been stimulated to some extent by recent exciting developments of several novel technologies, such as diode and fiber lasers for the optical communications industry, diode-pumped solid-state lasers, and novel bulk and waveguide infrared nonlinear materials. These can be applied to the ultra-sensitive, highly selective detection and real-time analysis of a large number of trace gas species by means of absorption spectroscopy in the mid-infrared fingerprint region, which contains virtually all the fundamental vibrational modes of molecules. Reduction of cost and complexity makes such spectroscopic sources more universally available and user friendly to both established and new fields that include air quality, atmospheric chemistry, industrial, traffic, and rural emissions, chemical analysis and process control, and medical applications.This issue, consisting of two parts, chronicles some of the most significant and representative current research trends. It is hoped that this issue will inspire new directions to both specialists and newcomers in which to drive this exciting field and envision future applications of environmental sensing.Part I: Spectroscopic air monitoring techniques and instrumentation

Incoherent broad-band cavity-enhanced absorption spectroscopy for simultaneous trace measurements of NO2 and NO3 with a LED source

In the past decade, due to a growing awareness of the importance of air quality and air pollution control, many diagnostic tools and techniques have been developed to detect and quantify the concentration of pollutants such as NO _x , SO _x , CO, and CO₂. We present here an Incoherent Broad-Band Cavity-Enhanced Spectroscopy (IBB-CEAS) set-up which uses a LED emitting around 625 nm for the simultaneous detection of NO₂ and NO₃. The LED light transmitted through a high-finesse optical cavity filled with a gas sample is detected by a low resolution spectrometer. After calibration of the spectrometer with a NO₂ reference sample, a linear multicomponent fit analysis of the absorption spectra allows for simultaneous measurements of NO₂ and NO₃ concentrations in a flow of ambient air. The optimal averaging time is found to be on the order of 400 s and appears to be limited by the drift of the spectrometer. At this averaging time the smallest detectable absorption is 2×10⁻¹⁰ cm⁻¹, which corresponds to detection limits of 600 pptv for NO₂ and 2 pptv for NO₃. This compact and low cost instrument is a promising diagnostic tool for air quality control in urban environments.     

Far infra-red applications of pyroelectric retina tv tubes

A pyroelectric video tube which transforms a thermal image into a CRO display is useful in a number of experiments. A few applications in usually difficult areas have been selected as examples. Text-book demonstrations in either optics or thermodynamics are often easier to do in the infra-red than in the visible. Applications include recording mode patterns of lasers with cw powers as low as 1 mW. The tubes are also useful in controlling the thickness and parallelism of infra-red flats which are apaque in the visible but transparent in parts of the infra-red spectrum.