- Email: [email protected]
Development of a tunable IR lidar system
Optics and Lasers in Engineering 37 (2002) 521–532
Development of a tunable IR lidar system S. Amorusoa,e, A. Amodeob, M. Armenantec, A. Bosellid,e, L. Monaa, M. Pandolfia, G. Pappalardob, R. Velottad,e, N. Spinellid,e,*, X. Wangd,e Dipartimento di Ingegneria e Fisica dell’Ambiente, Universita" della Basilicata, C. da Macchia Romana, I-85100 Potenza, Italy b Istituto di Metodologie Avanzate di Analisi AmbientaleFC.N.R. Area della Ricerca di PotenzaFC. da S. Loja, I-85050 Tito Scalo, Potenza, Italy c INFN - Sezione di Napoli, Dipartimento di Scienze Fisiche, Universita di Napoli, via Cintia 26, I-80126 Napoli, Italy d Dipartimento di Scienze Fisiche, Universita" di Napoli, Complesso Universitario di Monte S. Angelo, via Cintia, I-80126 Napoli, Italy e Istituto Nazionale per la Fisica della Materia, Unita" di Napoli, Complesso Universitario di Monte S. Angelo, via Cintia, I-80126 Napoli, Italy a
Received 19 December 2000; received in revised form 23 May 2001; accepted 28 May 2001
Abstract A differential absorption lidar system (DIAL) based on a continuously tunable optical parametric amplifier (OPA) pumped by a Nd : YAG laser (200 mJ at l ¼ 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 mm), 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 cm1 (FWHM), a spectral purity larger than 99% and a frequency stability better than 1 pm h1, 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. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Lidar; Dial; Tunable laser
*Corresponding author. Dipartimento di Scienze Fisiche, Universit"a di Napoli, Complesso Universitario di Monte S. Angelo, via Cintia, I-80126 Napoli, Italy. Tel.: +39-081-676-261; fax: +39081-676-346. E-mail address: [email protected] (N. Spinelli). 0143-8166/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 3 - 8 1 6 6 ( 0 1 ) 0 0 1 1 5 - 4
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1. Introduction The development of a tunable IR differential absorption lidar (DIAL) system for range-resolved remote sensing of atmospheric pollution is a great challenge, due to the large number of pollutants displaying absorption lines in such a spectral region [1]. Tunable IR laser radiation over a large spectral range (up to several mm) cannot be produced by conventional tunable laser sources (e.g. dye lasers), and IR lidar systems using discretely tunable laser sources have been employed in the past. These systems exploited coincidence between laser lines and pollutants absorption lines, but the absorption line of the gas to be studied was often not ideal in terms of line strength, interference from other atmospheric gases, and high sensitivity on atmospheric temperature and pressure changes. Laser sources continuously tunable on a discretely large spectral IR range would be preferable. Tunable coherent light sources (lasers and their nonlinear counterparts) have played a vital role in spectroscopic sensing and environmental analysis, as well as in basic science. Among others, pulsed optical parametric oscillators (OPO) and amplifiers (OPA) had an impressive resurgence of interest in the last decade [2–6]. The development of such light sources and the availability of a new generation of near IR operating phototubes and photodiodes has encouraged the development of DIAL systems in this spectral region. In particular, injection seeded OPOs and OPAs represent a great potential for lidar applications due to their broad tunability, small bandwidth and high spectral purity. These are fundamental requirements for DIAL applications [7]. In this paper, we report on the development of a tunable near IR lidar system based on Nd : YAG pumped optical parametric amplifier lasers. The laser transmitter emits E3 ns laser pulse at a maximum repetition rate of 100 Hz and is tunable in the spectral region 0.4–2.5 mm. This lidar is primarily devoted to DIAL measurements of atmospheric constituents and pollutants in the first few kilometers of height. Moreover, it can also be used to perform measurements of atmospheric aerosol backscattering profiles in the eye-safe spectral region (l > 1:4 mm). In order to test the DIAL system performance, water vapor has been selected due to its natural presence in the atmosphere in easily detectable concentration. Here, we report on DIAL measurements of water vapor obtained in the near infrared spectral region above 1 mm. For these measurements, couples of wavelengths lon =loff have been carefully selected by requiring low absorption from other molecules, low dependence of differential absorption cross section on temperature and optimal differential absorption optical depth at the maximum range of the measurement [7]. Moreover, from the off-line signal, the performances in term of aerosol backscattering coefficient measurement have been also examined. This paper is organized as follows: Section 2 contains an overview of the DIAL technique, while the description of the experimental system is reported in Section 3. Finally, in Section 4 preliminary results in terms of water vapor content and aerosol backscattering profiles are presented and discussed.
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2. The DIAL technique The lidar techniques allow to measure atmospheric properties and constituents over large extension areas in real time, with good spatial resolution, and very low interference with the atmospheric target [8]. Such techniques involve the transmission of a laser pulse and the detection of the backscattered radiation by using a telescope. The DIAL technique consists in transmitting two different wavelengths: one (lon ) is resonant with an absorption line of the species under investigation, the other (loff ) is tuned on the line wing. If a laser pulse of duration tL ; wavelength l and peak power P0 is transmitted along an atmospheric path, the received power from a distance z at time tðz ¼ ct=2Þ; Pl ðzÞ; in the single scattering hypothesis, is given by the following expression [9]: Z z ct L 2 al ðrÞ dr ; ð1Þ Pl ðzÞ ¼ P0 bl ðzÞAr ðzÞz exp 2 2 0 where c is the speed of light, Ar ðzÞ is the effective receiver area, bl ðzÞ and al ðrÞ are the volume backscattering and extinction coefficient of the atmosphere at wavelength l; where a l ¼ sl N þ x l :
ð2Þ
In Eq. (2), sl is the absorption cross section and N the gas density, and xl is the coefficient associated with any other extinction processes (atmospheric extinction and absorption from other interfering molecules), thus sl N is the contribution from the absorbing gas under investigationU The ratio of the backscattered signals at lon and loff allows to estimate the concentration of the species under study. In fact, since lon and loff are very close each other, the backscattering and extinction coefficients at the two wavelengths are almost equal (bon Eboff ; xon Exoff ) , then from Eqs. (1) and (2) follows: 1 d Poff ðzÞ ln NðzÞ ¼ ; ð3Þ 2ðson soff Þ dz Pon ðzÞ where the subscript on (off) indicates that the value refers to lon ðloff Þ: Since major pollutant species display absorption lines in the near IR, this spectral region is particularly suitable for the application of the DIAL technique, especially in the lower troposphere [10] and for the monitoring of industrial plants emissions and urban areas. However, the two wavelengths must be carefully chosen in order to (i) avoid interference from other atmospheric molecules absorption lines; (ii) minimize temperature dependence and (iii) optimize optical depth. An analysis of the influence of these factors as well as the definition of a set of criteria to select the best couple of wavelengths (lon and loff ) to be used in near IR DIAL measurements for different molecular species have been recently reported [7]. For DIAL applications, the lidar transmitter has to follow some requirements: (i) the wavelength must be tunable to match appropriate absorption lines of the investigated molecule; (ii) the bandwidth and the spectral stability should be considerably smaller than the linewidth of the absorption line; (iii) a high spectral
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purity (>99%) has to be guaranteed; (iv) a sufficient pulse energy and average power is required and (v) a high repetition rate is desirable. The influence of the bandwidth on DIAL measurement accuracy has been the subject of different studies in the past [11–13]. From results reported in Ref. [11], an error of the order of 1% is obtainable only if the ratio between the laser bandwidth and the molecule absorption linewidth is less than 0.3. Otherwise significant errors can be introduced. In the near IR domain, absorption spectra of simple molecules consist of separate vibro-rotational lines with typical pressure broadened widths of 0.1–0.2 cm1 (FWHM) [1]. This fact imposes the use of a very narrowband laser source for DIAL measurements in the IR region (e.g. DlE0:03 cm1 for measurements up to few km of height).
3. System description 3.1. Transmitter system The transmitter is based on two master oscillator OPA lasers alternatively pumped by a Nd : YAG laser operating at a maximum repetition rate of 100 Hz. The pump laser is a master oscillator-power amplifier (MOPA) Nd : YAG system (Coherent INFINITY). The oscillator is a diode pumped solid state (DPSS) delivering TEM00 and single longitudinal mode output pulse (2 transform limit, 250 MHz). The DPSS output beam is filtered by passing through a pinhole to produce a near top-hat profile at the entrance of the amplifier system. The power amplifier is a two-rod, double pass chain flashlamp pumped Nd : YAG amplifier system exploiting relay imaging and phase-conjugate optics to reduce birefringence losses and phase-front distortions. Second and third harmonic generation is accomplished by nonlinear conversion in BBO crystals. The output beams of the three harmonics (1064, 532 and 355 nm) are separated by dichroic mirrors and show near top-hat profiles in the near field. The tunable source is a Scanmate OPPO laser source (Lambda Physics) modified to work in OPA configuration. In our system, which is schematically shown in Fig. 1, the frequency tripled output of the Nd : YAG laser is splitted in two beams: the first one (E20% of the total laser pulse energy) pumps a narrowband pulsed tunable dye laser with a linewidth p0.03 cm1, while the second one is transported to the OPA system through a relay imaging optical delay line system. Almost 60% of the input energy reaches the two BBO crystals synchronously with the seeder beam from the dye laser, producing the signal (at the same wavelength of the seeder beam) and idler output beams. A dichroic mirror separates the two OPA beams from the remaining part of the third harmonics, which is sent to a beam dump. The OPA provides the generation of two narrow bandwidths, low divergence beams tunable in the range 0.41–0.71 mm (signal) and 0.71–2.5 mm (idler), with a conversion efficiency of E25%. The unique design of the dye laser unit of the Lambda Physik Scanmate OPPO provides a very low ASE background content of the output radiation (o102). The overall background of the OPA output radiation
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Nd:YAG laser BBO Crystals 3ω
Relay Imaging system and delay line
signal + idler
}
Dye laser
{
OPA
} Beam dump
Fig. 1. Schematic of the transmitter system.
is even lower than the master oscillator input radiation, of the order of 104, due to the narrow amplification bandwidth of the OPA system [6], thus, providing the output beam with a spectral purity larger than 99%. Signal and idler wavelengths from the OPA lasers come out simultaneously and are separated by a dichroic mirror. All lasers are controlled by personal computers. The main features of the laser transmitter are summarized in Table 1. The accuracy of the laser tuning is a crucial requirement for DIAL measurements, therefore photoacoustic spectroscopy (PAS) has been employed to obtain a reliable on-line monitoring of the tuning of the laser line [14]. Photoacoustic spectroscopy Table 1 Main characteristics of the transmitter system Pump laser Wavelength and pulse energy Pulse duration Pulse repetition rate Beam divergence Bandwidth OPA laser Tuning range Pulse energya Linewidth Divergence Pulsewidth Wavelength resetability Spectral purity Frequency stability a
l ¼ 355 nm E ¼ 200 mJ p3 ns up to 100 Hz p0.7 mrad 250 MHz 410–710 nm (signal) 710–2500 nm (idler) signalE10–20 mJ idlerE1–10 mJ p0.04 cm1 p3 mrad (typical) o pump laser 1 pm >99% o1 pm h1
Typical values. The exact pulse energy depends on the wavelength and is variable in the tuning range as a function of the dye-seeder efficiency.
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has been also employed to measure the idler laser beam linewidth. PAS spectra of H2O molecule have been measured in different spectral regions and compared with computer code generated spectra obtained by using HITRAN [1] data base parameters for water vapor lines. Fig. 2(a)–(b) shows an experimental water vapor PAS spectrum and a simulated one obtained by using a laser linewidth of 0.04 cm1, while in Fig. 3 a narrow spectral region close to the on-line wavelength is reported. The fairly good agreement between the two spectra indicates a E0.04 cm1 linewidth for the idler beam. Similar results have been obtained at different wavelengths, showing that this figure is typical for the idler laser beam linewidth. The signal linewidth has been directly measured by using a wavelength meter (Burleigh WA4500, spectral resolution=0.05 cm1, accuracy 1 pm), obtaining a value comparable with the instrumental resolution, thus showing a linewidth smaller than 0.05 cm1.
Fig. 2. (a) simulated PAS spectrum obtained by using a laser linewidth of 0.04 cm1; (b) experimental water vapor PAS spectrum in the spectral range 1186–1190 nm.
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Fig. 3. Particulars of the experimental PAS spectrum in a narrow region close to the on-line wavelength, and comparison with a simulated PAS spectrum obtained by using a laser linewidth of 0.04 cm1.
For on-line monitoring of the transmitter system, a small portion of the idler output beam is sent to the photoacoustic cell. Moreover, the signal output beam from the OPA is divided with a beam splitter. One beam is sent to a Fabry Perot etalon to control the seeding (by directly watching or recording with a CCD camera the fringe pattern), the other is monitored by the wavelength-meter to check the signal frequency and linewidth stability. Stable operation of the OPA over a period of 1 h has been registered by observing the interference patterns at time intervals of 10 min. The main portion of the idler beam is expanded by a 4 beam expander to reduce the divergence top1 mrad, and transmitted by a series of mirrors and emitted coaxially with the lidar telescope. 3.2. Receiver system The receiver consists of a vertically pointing telescope in Cassegrain configuration with a 0.5 m diameter primary mirror and a combined focal length of 1.8 m. The collected radiation is collimated by a lens and pass through a heat transmitting mirror. Then it is spectrally selected by an IR monochromator (Jobin Ivon H10), finally reaching a near infrared photomultiplier tube (NIR-PMT Hamamatsu R5509-72). The In/InGaAsP photocatode allows an extended spectral response range from 0.9 to 1.7 mm. The photomultiplier is cooled at a temperature of 801C to reduce dark current to E50 nA. The PMT output signal is amplified by a 300 MHz bandwidth preamplifier and sent to the acquisition system. The data acquisition can be accomplished by both analog digitizing and photon counting sampling techniques. Analog signals are sampled by means of transient
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recorder boards or sampling oscilloscopes. The transient recorder (IMTEC T3012) allows the acquisition at a sampling rate of 30 MHz, with a 12 bit resolution. The photon counting chain comprises a 300 MHz discriminator followed by a multichannel scaler board (EG&G Ortec TurboMCS, sampling rate=150 MHz) with a minimum time resolution of 5 ns. All the acquisition system is computer controlled.
4. Results and discussion According to the set of criteria reported in Ref. [7], the two wavelengths reported in Table 2 have been chosen to perform DIAL measurements: lon is tuned to the water vapor absorption line at 1187.869 nm, while loff is 1187.716 nm, where the absorption cross section is almost 60 times smaller. Fig. 4 shows representative range corrected backscattered signals obtained by averaging 1000 laser shots (integration time of 20 s at 50 Hz repetition rate) on each wavelength. Measurements were performed in Potenza (401360 N–151440 E, 820 m a.s.l.) in clear sky condition on 28 June 1999. The value of relative humidity at ground obtained by a sensor located above the lidar station was (8571)% during the measurement. The range corrected backscattered signals z2 Pl ðzÞ (see Eq. (1)) have been obtained from the digitized signals and integrated over a range interval of 15 m, to reduce the signal noise. The backscattered range corrected signals show the expected almost exponential decay following an initial growth due to the geometrical overlap between the area of the laser beam at distance z and the field of view of the receiving system. The differential absorption between the two wavelengths is clearly seen. Moreover, the on-line signal shows appreciable noise beyond about E1 km of height, mainly due to large attenuation of the absorbed wavelength. The corresponding total burden of water vapor as a function of the height above the lidar station has been derived from Eq. (3), and an average water vapor content of (6.170.5) 1022 molecules m3 has been inferred between E300 and E900 m. Fig. 5 shows the water vapor concentration as a function of the height above the lidar station, as obtained from Eq. (3). The vertical resolution has been changed with the altitude (z) to reduce the statistical noise, obtaining a spatial resolution Dz ¼ 15 m for zo250 m, Dz ¼ 22:5 m for z in the range 250–600 m, and Dz ¼ 30 m for z > 600 m. The data show that the water vapor content decreases with the height above the lidar station, going from E1.5 1023 molecules m3 at 200 m to E2.3 1022 molecules m3 at 900 m. The error bars represent the statistical Table 2 Couple of wavelengths used to perform H2O DIAL measurements. lon and loff (nm)are the on- and offline wavelengths, son (m2) and soff (m2)are the corresponding absorption cross sections, while tðzÞ is the on-line optical depth at height z lon (nm) 1187.869
loff (nm) 1187.716
son (m2)
soff (m2) 27
9.05 10
28
1.58 10
t (1 km)
t (2 km)
1.495
2.518
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Fig. 4. Normalized range corrected DIAL backscattered signals at lon ¼ 1187:869 nm and loff ¼ 1187:716 nm. A 15 m range resolution was used. The lon wavelength is more strongly absorbed by the atmospheric water vapor. Potenza, 28 June 1999 (21.30 G.M.T.).
uncertainty on the concentration measurements. During each single measurement both the average lidar echo and its standard deviation were acquired. Then, the standard deviation is used to propagate statistical signal fluctuations on the water vapor concentration by means of a Monte Carlo procedure. From the off-line wavelength, the range resolved backscattering coefficient from tropospheric aerosols can be derived, as shown in Fig. 6. This has been done also to demonstrate the utility of the tunable IR lidar system to perform aerosol backscattering profile measurement in the near IR. The data reported in Fig. 6 have been obtained by smoothing the digitized signals over a range interval of 15 m, to reduce the signal noise. Then the backscattering coefficient was calculated by using the iterative procedure method described in Ref. [15], and normalizing the lidar signal to the molecular contribution to backscattered radiation in an aerosol-free region above the PBL layer. The results show a backscattering coefficient going from 2 107 m1 sr1 at E300 m to E1 108 m1 sr1 above E1200 m, with some structure showing the presence of thin aerosol layers around E800 and E1100 m.
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Fig. 5. Measured values of the water vapor concentration as a function of the height above the lidar station (K). A variable range resolution is used to reduce the noise. Potenza, 28 June 1999 (21.30 G.M.T.).
The main outcome of such result is the possibility to obtain range resolved aerosol backscattering measurements in the lower troposphere in the near IR region, which gives rise to a good prospect to perform such kind of measurements in the eye-safe region above 1.4 mm. Moreover, due to the high repetition rate a short acquisition time is needed to obtain backscattering profiles showing the possibility to follow atmospheric dynamics with high temporal resolution.
5. Summary A DIAL system based on a continuously tunable optical parametric amplifier pumped by a Nd : YAG laser 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 mm), allowing to perform DIAL measurements in
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Fig. 6. Aerosol backscattering coefficient at 1187.716 nm as a function of the height above the lidar station. Potenza, 28 June 1999 (21.30 G.M.T.).
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 cm1 (FWHM), a spectral purity larger than 99% and a frequency stability better than 1 pm h1, with an output energy in the IR of several mJ. The OPA system was used to perform water vapor DIAL measurements in the lower troposphere (up to E1 km of height). Preliminary range resolved water vapor measurements have been demonstrated. Moreover, the possibility to obtain simultaneous range resolved aerosol backscattering measurements in the lower troposphere in the near IR region have been demonstrated by using the off-line elastic echo.
Acknowledgements This research was partially supported by the European Community and Istituto Nazionale per la Fisica della Materia (INFM) in the framework of the Project
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‘‘Progetto SUDFTecniche ottiche innovative per il monitoraggio ambientale e piani di tutela e risanamento’’.
References [1] Rothman LS, Rinsland CP, Goldman A, Massie ST, Edwards DP, Flaud JM, Perrin A, Dana V, Mandin JY, Schroeder J, McCann A, Gamache RR, Wattson RB, Yoshino K, Chance K, Jucks K, Brown LR, Nemtchinov V, Varanasi P. The HITRAN molecular spectroscopic database and HAWKS: 1996 edition. J Quant Spectrosc Radiat Transfer 1998;60:665–710. [2] Orr BJ, Johnson MJ, Haub JG. Spectroscopic applications of pulsed tunable optical parametric oscillators. In: Duarte FJ, editor. Tunable laser application. New York: Marcel Dekker, 1995. p. 11–82. [3] Barnes NP. Optical parametric oscillator. In: Duarte FJ, editor. Tunable lasers handbook. New York: Academic, 1995. p. 293–348. [4] Fix A, Ehret G. Injection seeded optical parametric oscillator system for water vapor DIAL measurements. In: Ansman A, Neuber R, Rairoux P, Wandinger U, editors. Advances in atmospheric remote sensing with lidar. New York: Springer, 1997. p. 313–6. [5] Gardiner TD, Milton MJT, Molero F, Woods PT. Infrared DIAL measurements with an injectionseeded OPO. In: Ansman A, Neuber R, Rairoux P, Wandinger U, editors. Advances in atmospheric remote sensing with lidar. New York: Springer, 1997. p. 451–4. [6] Ehret G, Fix A, Wei V, Poberaj G, Baumert T. Diode-laser-seeded optical parametric oscillator for airborne water vapor DIAL application in the upper troposphere and lower stratosphere. Appl Phys B 1998;67:427–31. [7] Ambrico PF, Amodeo A, Di Girolamo P, Spinelli N. Sensitivity analysis of differential absortpion lidar measurements in the mid-infrared region. Appl Opt 2000;39:6847–65. [8] Measures RM. Laser remote sensing: fundamentals and applications. New York: Wiley, 1984. [9] Collis RTH, Russel PB. Lidar measurements of particles and gases by elastic backscattering and differential absorption. In: Hinkley ED, editor. Laser monitoring of the atmosphere. Berlin: Springer, 1976. [10] Megie G, Menzies RT. Complementarity of UV and IR differential absorption lidar for global measurements of atmospheric species. Appl Opt 1980;19:1173–83. [11] Cahen C, M!egie GJ. Quant Spectrosc Radiat Transfer 1981;25:151. [12] Ansmann A, Boesenberg J. Correction scheme for spectral broadening by Raleigh scattering in differential absorbtion lidar measurements of water vapor in the troposphere. Appl Opt 1980;26:3026. . [13] Bosenberg J. A differential absorption lidar system for high resolution water vapor measurements in the troposphere. Max–Planck Institut fur . Meteorologie Report No. 71, 1991. [14] Ambrico PF, Amodeo A, Amoruso S, Armenante M, Berardi V, Boselli A, Bruzzese R, Capobianco R, Di Girolamo P, Fiorani L, Pappalardo G, Spinelli N, Velotta R. Multiparametric lidar from UV to mid-IR. Laser Optoelektronik 1997;29:62–9. [15] Di Girolamo P, Ambrico PF, Amodeo A, Boselli A, Pappalardo G, Spinelli N. Aerosol observation by lidar in the nocturnal boundary layer. Appl Opt 1999;38:4585.
Development of a tunable IR lidar system S. Amorusoa,e, A. Amodeob, M. Armenantec, A. Bosellid,e, L. Monaa, M. Pandolfia, G. Pappalardob, R. Velottad,e, N. Spinellid,e,*, X. Wangd,e Dipartimento di Ingegneria e Fisica dell’Ambiente, Universita" della Basilicata, C. da Macchia Romana, I-85100 Potenza, Italy b Istituto di Metodologie Avanzate di Analisi AmbientaleFC.N.R. Area della Ricerca di PotenzaFC. da S. Loja, I-85050 Tito Scalo, Potenza, Italy c INFN - Sezione di Napoli, Dipartimento di Scienze Fisiche, Universita di Napoli, via Cintia 26, I-80126 Napoli, Italy d Dipartimento di Scienze Fisiche, Universita" di Napoli, Complesso Universitario di Monte S. Angelo, via Cintia, I-80126 Napoli, Italy e Istituto Nazionale per la Fisica della Materia, Unita" di Napoli, Complesso Universitario di Monte S. Angelo, via Cintia, I-80126 Napoli, Italy a
Received 19 December 2000; received in revised form 23 May 2001; accepted 28 May 2001
Abstract A differential absorption lidar system (DIAL) based on a continuously tunable optical parametric amplifier (OPA) pumped by a Nd : YAG laser (200 mJ at l ¼ 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 mm), 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 cm1 (FWHM), a spectral purity larger than 99% and a frequency stability better than 1 pm h1, 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. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Lidar; Dial; Tunable laser
*Corresponding author. Dipartimento di Scienze Fisiche, Universit"a di Napoli, Complesso Universitario di Monte S. Angelo, via Cintia, I-80126 Napoli, Italy. Tel.: +39-081-676-261; fax: +39081-676-346. E-mail address: [email protected] (N. Spinelli). 0143-8166/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 3 - 8 1 6 6 ( 0 1 ) 0 0 1 1 5 - 4
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1. Introduction The development of a tunable IR differential absorption lidar (DIAL) system for range-resolved remote sensing of atmospheric pollution is a great challenge, due to the large number of pollutants displaying absorption lines in such a spectral region [1]. Tunable IR laser radiation over a large spectral range (up to several mm) cannot be produced by conventional tunable laser sources (e.g. dye lasers), and IR lidar systems using discretely tunable laser sources have been employed in the past. These systems exploited coincidence between laser lines and pollutants absorption lines, but the absorption line of the gas to be studied was often not ideal in terms of line strength, interference from other atmospheric gases, and high sensitivity on atmospheric temperature and pressure changes. Laser sources continuously tunable on a discretely large spectral IR range would be preferable. Tunable coherent light sources (lasers and their nonlinear counterparts) have played a vital role in spectroscopic sensing and environmental analysis, as well as in basic science. Among others, pulsed optical parametric oscillators (OPO) and amplifiers (OPA) had an impressive resurgence of interest in the last decade [2–6]. The development of such light sources and the availability of a new generation of near IR operating phototubes and photodiodes has encouraged the development of DIAL systems in this spectral region. In particular, injection seeded OPOs and OPAs represent a great potential for lidar applications due to their broad tunability, small bandwidth and high spectral purity. These are fundamental requirements for DIAL applications [7]. In this paper, we report on the development of a tunable near IR lidar system based on Nd : YAG pumped optical parametric amplifier lasers. The laser transmitter emits E3 ns laser pulse at a maximum repetition rate of 100 Hz and is tunable in the spectral region 0.4–2.5 mm. This lidar is primarily devoted to DIAL measurements of atmospheric constituents and pollutants in the first few kilometers of height. Moreover, it can also be used to perform measurements of atmospheric aerosol backscattering profiles in the eye-safe spectral region (l > 1:4 mm). In order to test the DIAL system performance, water vapor has been selected due to its natural presence in the atmosphere in easily detectable concentration. Here, we report on DIAL measurements of water vapor obtained in the near infrared spectral region above 1 mm. For these measurements, couples of wavelengths lon =loff have been carefully selected by requiring low absorption from other molecules, low dependence of differential absorption cross section on temperature and optimal differential absorption optical depth at the maximum range of the measurement [7]. Moreover, from the off-line signal, the performances in term of aerosol backscattering coefficient measurement have been also examined. This paper is organized as follows: Section 2 contains an overview of the DIAL technique, while the description of the experimental system is reported in Section 3. Finally, in Section 4 preliminary results in terms of water vapor content and aerosol backscattering profiles are presented and discussed.
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2. The DIAL technique The lidar techniques allow to measure atmospheric properties and constituents over large extension areas in real time, with good spatial resolution, and very low interference with the atmospheric target [8]. Such techniques involve the transmission of a laser pulse and the detection of the backscattered radiation by using a telescope. The DIAL technique consists in transmitting two different wavelengths: one (lon ) is resonant with an absorption line of the species under investigation, the other (loff ) is tuned on the line wing. If a laser pulse of duration tL ; wavelength l and peak power P0 is transmitted along an atmospheric path, the received power from a distance z at time tðz ¼ ct=2Þ; Pl ðzÞ; in the single scattering hypothesis, is given by the following expression [9]: Z z ct L 2 al ðrÞ dr ; ð1Þ Pl ðzÞ ¼ P0 bl ðzÞAr ðzÞz exp 2 2 0 where c is the speed of light, Ar ðzÞ is the effective receiver area, bl ðzÞ and al ðrÞ are the volume backscattering and extinction coefficient of the atmosphere at wavelength l; where a l ¼ sl N þ x l :
ð2Þ
In Eq. (2), sl is the absorption cross section and N the gas density, and xl is the coefficient associated with any other extinction processes (atmospheric extinction and absorption from other interfering molecules), thus sl N is the contribution from the absorbing gas under investigationU The ratio of the backscattered signals at lon and loff allows to estimate the concentration of the species under study. In fact, since lon and loff are very close each other, the backscattering and extinction coefficients at the two wavelengths are almost equal (bon Eboff ; xon Exoff ) , then from Eqs. (1) and (2) follows: 1 d Poff ðzÞ ln NðzÞ ¼ ; ð3Þ 2ðson soff Þ dz Pon ðzÞ where the subscript on (off) indicates that the value refers to lon ðloff Þ: Since major pollutant species display absorption lines in the near IR, this spectral region is particularly suitable for the application of the DIAL technique, especially in the lower troposphere [10] and for the monitoring of industrial plants emissions and urban areas. However, the two wavelengths must be carefully chosen in order to (i) avoid interference from other atmospheric molecules absorption lines; (ii) minimize temperature dependence and (iii) optimize optical depth. An analysis of the influence of these factors as well as the definition of a set of criteria to select the best couple of wavelengths (lon and loff ) to be used in near IR DIAL measurements for different molecular species have been recently reported [7]. For DIAL applications, the lidar transmitter has to follow some requirements: (i) the wavelength must be tunable to match appropriate absorption lines of the investigated molecule; (ii) the bandwidth and the spectral stability should be considerably smaller than the linewidth of the absorption line; (iii) a high spectral
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purity (>99%) has to be guaranteed; (iv) a sufficient pulse energy and average power is required and (v) a high repetition rate is desirable. The influence of the bandwidth on DIAL measurement accuracy has been the subject of different studies in the past [11–13]. From results reported in Ref. [11], an error of the order of 1% is obtainable only if the ratio between the laser bandwidth and the molecule absorption linewidth is less than 0.3. Otherwise significant errors can be introduced. In the near IR domain, absorption spectra of simple molecules consist of separate vibro-rotational lines with typical pressure broadened widths of 0.1–0.2 cm1 (FWHM) [1]. This fact imposes the use of a very narrowband laser source for DIAL measurements in the IR region (e.g. DlE0:03 cm1 for measurements up to few km of height).
3. System description 3.1. Transmitter system The transmitter is based on two master oscillator OPA lasers alternatively pumped by a Nd : YAG laser operating at a maximum repetition rate of 100 Hz. The pump laser is a master oscillator-power amplifier (MOPA) Nd : YAG system (Coherent INFINITY). The oscillator is a diode pumped solid state (DPSS) delivering TEM00 and single longitudinal mode output pulse (2 transform limit, 250 MHz). The DPSS output beam is filtered by passing through a pinhole to produce a near top-hat profile at the entrance of the amplifier system. The power amplifier is a two-rod, double pass chain flashlamp pumped Nd : YAG amplifier system exploiting relay imaging and phase-conjugate optics to reduce birefringence losses and phase-front distortions. Second and third harmonic generation is accomplished by nonlinear conversion in BBO crystals. The output beams of the three harmonics (1064, 532 and 355 nm) are separated by dichroic mirrors and show near top-hat profiles in the near field. The tunable source is a Scanmate OPPO laser source (Lambda Physics) modified to work in OPA configuration. In our system, which is schematically shown in Fig. 1, the frequency tripled output of the Nd : YAG laser is splitted in two beams: the first one (E20% of the total laser pulse energy) pumps a narrowband pulsed tunable dye laser with a linewidth p0.03 cm1, while the second one is transported to the OPA system through a relay imaging optical delay line system. Almost 60% of the input energy reaches the two BBO crystals synchronously with the seeder beam from the dye laser, producing the signal (at the same wavelength of the seeder beam) and idler output beams. A dichroic mirror separates the two OPA beams from the remaining part of the third harmonics, which is sent to a beam dump. The OPA provides the generation of two narrow bandwidths, low divergence beams tunable in the range 0.41–0.71 mm (signal) and 0.71–2.5 mm (idler), with a conversion efficiency of E25%. The unique design of the dye laser unit of the Lambda Physik Scanmate OPPO provides a very low ASE background content of the output radiation (o102). The overall background of the OPA output radiation
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Nd:YAG laser BBO Crystals 3ω
Relay Imaging system and delay line
signal + idler
}
Dye laser
{
OPA
} Beam dump
Fig. 1. Schematic of the transmitter system.
is even lower than the master oscillator input radiation, of the order of 104, due to the narrow amplification bandwidth of the OPA system [6], thus, providing the output beam with a spectral purity larger than 99%. Signal and idler wavelengths from the OPA lasers come out simultaneously and are separated by a dichroic mirror. All lasers are controlled by personal computers. The main features of the laser transmitter are summarized in Table 1. The accuracy of the laser tuning is a crucial requirement for DIAL measurements, therefore photoacoustic spectroscopy (PAS) has been employed to obtain a reliable on-line monitoring of the tuning of the laser line [14]. Photoacoustic spectroscopy Table 1 Main characteristics of the transmitter system Pump laser Wavelength and pulse energy Pulse duration Pulse repetition rate Beam divergence Bandwidth OPA laser Tuning range Pulse energya Linewidth Divergence Pulsewidth Wavelength resetability Spectral purity Frequency stability a
l ¼ 355 nm E ¼ 200 mJ p3 ns up to 100 Hz p0.7 mrad 250 MHz 410–710 nm (signal) 710–2500 nm (idler) signalE10–20 mJ idlerE1–10 mJ p0.04 cm1 p3 mrad (typical) o pump laser 1 pm >99% o1 pm h1
Typical values. The exact pulse energy depends on the wavelength and is variable in the tuning range as a function of the dye-seeder efficiency.
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has been also employed to measure the idler laser beam linewidth. PAS spectra of H2O molecule have been measured in different spectral regions and compared with computer code generated spectra obtained by using HITRAN [1] data base parameters for water vapor lines. Fig. 2(a)–(b) shows an experimental water vapor PAS spectrum and a simulated one obtained by using a laser linewidth of 0.04 cm1, while in Fig. 3 a narrow spectral region close to the on-line wavelength is reported. The fairly good agreement between the two spectra indicates a E0.04 cm1 linewidth for the idler beam. Similar results have been obtained at different wavelengths, showing that this figure is typical for the idler laser beam linewidth. The signal linewidth has been directly measured by using a wavelength meter (Burleigh WA4500, spectral resolution=0.05 cm1, accuracy 1 pm), obtaining a value comparable with the instrumental resolution, thus showing a linewidth smaller than 0.05 cm1.
Fig. 2. (a) simulated PAS spectrum obtained by using a laser linewidth of 0.04 cm1; (b) experimental water vapor PAS spectrum in the spectral range 1186–1190 nm.
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Fig. 3. Particulars of the experimental PAS spectrum in a narrow region close to the on-line wavelength, and comparison with a simulated PAS spectrum obtained by using a laser linewidth of 0.04 cm1.
For on-line monitoring of the transmitter system, a small portion of the idler output beam is sent to the photoacoustic cell. Moreover, the signal output beam from the OPA is divided with a beam splitter. One beam is sent to a Fabry Perot etalon to control the seeding (by directly watching or recording with a CCD camera the fringe pattern), the other is monitored by the wavelength-meter to check the signal frequency and linewidth stability. Stable operation of the OPA over a period of 1 h has been registered by observing the interference patterns at time intervals of 10 min. The main portion of the idler beam is expanded by a 4 beam expander to reduce the divergence top1 mrad, and transmitted by a series of mirrors and emitted coaxially with the lidar telescope. 3.2. Receiver system The receiver consists of a vertically pointing telescope in Cassegrain configuration with a 0.5 m diameter primary mirror and a combined focal length of 1.8 m. The collected radiation is collimated by a lens and pass through a heat transmitting mirror. Then it is spectrally selected by an IR monochromator (Jobin Ivon H10), finally reaching a near infrared photomultiplier tube (NIR-PMT Hamamatsu R5509-72). The In/InGaAsP photocatode allows an extended spectral response range from 0.9 to 1.7 mm. The photomultiplier is cooled at a temperature of 801C to reduce dark current to E50 nA. The PMT output signal is amplified by a 300 MHz bandwidth preamplifier and sent to the acquisition system. The data acquisition can be accomplished by both analog digitizing and photon counting sampling techniques. Analog signals are sampled by means of transient
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recorder boards or sampling oscilloscopes. The transient recorder (IMTEC T3012) allows the acquisition at a sampling rate of 30 MHz, with a 12 bit resolution. The photon counting chain comprises a 300 MHz discriminator followed by a multichannel scaler board (EG&G Ortec TurboMCS, sampling rate=150 MHz) with a minimum time resolution of 5 ns. All the acquisition system is computer controlled.
4. Results and discussion According to the set of criteria reported in Ref. [7], the two wavelengths reported in Table 2 have been chosen to perform DIAL measurements: lon is tuned to the water vapor absorption line at 1187.869 nm, while loff is 1187.716 nm, where the absorption cross section is almost 60 times smaller. Fig. 4 shows representative range corrected backscattered signals obtained by averaging 1000 laser shots (integration time of 20 s at 50 Hz repetition rate) on each wavelength. Measurements were performed in Potenza (401360 N–151440 E, 820 m a.s.l.) in clear sky condition on 28 June 1999. The value of relative humidity at ground obtained by a sensor located above the lidar station was (8571)% during the measurement. The range corrected backscattered signals z2 Pl ðzÞ (see Eq. (1)) have been obtained from the digitized signals and integrated over a range interval of 15 m, to reduce the signal noise. The backscattered range corrected signals show the expected almost exponential decay following an initial growth due to the geometrical overlap between the area of the laser beam at distance z and the field of view of the receiving system. The differential absorption between the two wavelengths is clearly seen. Moreover, the on-line signal shows appreciable noise beyond about E1 km of height, mainly due to large attenuation of the absorbed wavelength. The corresponding total burden of water vapor as a function of the height above the lidar station has been derived from Eq. (3), and an average water vapor content of (6.170.5) 1022 molecules m3 has been inferred between E300 and E900 m. Fig. 5 shows the water vapor concentration as a function of the height above the lidar station, as obtained from Eq. (3). The vertical resolution has been changed with the altitude (z) to reduce the statistical noise, obtaining a spatial resolution Dz ¼ 15 m for zo250 m, Dz ¼ 22:5 m for z in the range 250–600 m, and Dz ¼ 30 m for z > 600 m. The data show that the water vapor content decreases with the height above the lidar station, going from E1.5 1023 molecules m3 at 200 m to E2.3 1022 molecules m3 at 900 m. The error bars represent the statistical Table 2 Couple of wavelengths used to perform H2O DIAL measurements. lon and loff (nm)are the on- and offline wavelengths, son (m2) and soff (m2)are the corresponding absorption cross sections, while tðzÞ is the on-line optical depth at height z lon (nm) 1187.869
loff (nm) 1187.716
son (m2)
soff (m2) 27
9.05 10
28
1.58 10
t (1 km)
t (2 km)
1.495
2.518
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Fig. 4. Normalized range corrected DIAL backscattered signals at lon ¼ 1187:869 nm and loff ¼ 1187:716 nm. A 15 m range resolution was used. The lon wavelength is more strongly absorbed by the atmospheric water vapor. Potenza, 28 June 1999 (21.30 G.M.T.).
uncertainty on the concentration measurements. During each single measurement both the average lidar echo and its standard deviation were acquired. Then, the standard deviation is used to propagate statistical signal fluctuations on the water vapor concentration by means of a Monte Carlo procedure. From the off-line wavelength, the range resolved backscattering coefficient from tropospheric aerosols can be derived, as shown in Fig. 6. This has been done also to demonstrate the utility of the tunable IR lidar system to perform aerosol backscattering profile measurement in the near IR. The data reported in Fig. 6 have been obtained by smoothing the digitized signals over a range interval of 15 m, to reduce the signal noise. Then the backscattering coefficient was calculated by using the iterative procedure method described in Ref. [15], and normalizing the lidar signal to the molecular contribution to backscattered radiation in an aerosol-free region above the PBL layer. The results show a backscattering coefficient going from 2 107 m1 sr1 at E300 m to E1 108 m1 sr1 above E1200 m, with some structure showing the presence of thin aerosol layers around E800 and E1100 m.
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Fig. 5. Measured values of the water vapor concentration as a function of the height above the lidar station (K). A variable range resolution is used to reduce the noise. Potenza, 28 June 1999 (21.30 G.M.T.).
The main outcome of such result is the possibility to obtain range resolved aerosol backscattering measurements in the lower troposphere in the near IR region, which gives rise to a good prospect to perform such kind of measurements in the eye-safe region above 1.4 mm. Moreover, due to the high repetition rate a short acquisition time is needed to obtain backscattering profiles showing the possibility to follow atmospheric dynamics with high temporal resolution.
5. Summary A DIAL system based on a continuously tunable optical parametric amplifier pumped by a Nd : YAG laser 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 mm), allowing to perform DIAL measurements in
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Fig. 6. Aerosol backscattering coefficient at 1187.716 nm as a function of the height above the lidar station. Potenza, 28 June 1999 (21.30 G.M.T.).
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 cm1 (FWHM), a spectral purity larger than 99% and a frequency stability better than 1 pm h1, with an output energy in the IR of several mJ. The OPA system was used to perform water vapor DIAL measurements in the lower troposphere (up to E1 km of height). Preliminary range resolved water vapor measurements have been demonstrated. Moreover, the possibility to obtain simultaneous range resolved aerosol backscattering measurements in the lower troposphere in the near IR region have been demonstrated by using the off-line elastic echo.
Acknowledgements This research was partially supported by the European Community and Istituto Nazionale per la Fisica della Materia (INFM) in the framework of the Project
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‘‘Progetto SUDFTecniche ottiche innovative per il monitoraggio ambientale e piani di tutela e risanamento’’.
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