Remote sensing using open-path dual-comb spectroscopy

Kevin C. Cossel , ... Brian R. Washburn , in Advances in Spectroscopic Monitoring of the Atmosphere, 2021

2.1 Introduction

Optical frequency combs are a unique class of lasers that combine the advantages of continuous wave (cw) laser technology—tight spatial coherence and high spectral resolution—with broadband spectral coverage approaching that of white light lamps (Cundiff and Ye, 2003). The spectrum of a frequency comb is composed of individual modes or "comb teeth" that behave very much like an array of many thousands to millions of individual, perfectly evenly spaced cw laser lines spanning the entire comb spectrum—typically tens to hundreds of nanometers of bandwidth—all with a high level of mutual phase coherence. 1 Optical frequency combs were originally developed two decades ago to count the cycles of optical atomic clocks (Telle et al., 1999; Reichert et al., 1999; Holzwarth et al., 2000; Diddams et al., 2000; Udem et al., 2001) but have since found their way into a broad field of other applications (Newbury, 2011; Diddams, 2010; Fortier and Baumann, 2019). Their development resulted in one-half of the Nobel Prize in Physics in 2005 (Hall, 2006; Hansch, 2006).

Frequency combs are well suited for atmospheric sensing, in part because they can probe many atmospheric species simultaneously, covering entire molecular bands at very high spectral resolution. This is illustrated in Fig. 2.1, which shows a broad spectrum before and after gas absorption as well as the underlying closely spaced comb teeth. Additionally, frequency combs are a high-brightness, minimally divergent source that can measure integrated gas concentrations over distances exceeding 10   kilometers in both daytime and nighttime. In order to preserve the high spectral resolution during data acquisition, the spectrometer needs to resolve each comb tooth. This can be accomplished using a technique called dual-comb spectroscopy (DCS) (Coddington et al., 2016), which uses a second optical frequency comb to read out the first comb. There exist other techniques to perform spectroscopy using frequency combs (see Adler et al., 2010; Maslowski et al., 2014; Weichman et al., 2019; Picqué and Hansch, 2019 for reviews), but DCS is the only measurement technique so far to have used frequency combs in field environments for long open-path atmospheric measurements, and thus we focus on DCS for the rest of the chapter. We first motivate the use of DCS for open-path spectroscopy and then describe frequency combs and in particular dual-frequency comb spectroscopy, including a discussion of hardware components and spectral analysis with a focus on the instruments used by the authors of this chapter. The next section discusses several recent applications of open-path measurements using DCS. The final section looks at potential future improvements in the DCS technology.

Figure 2.1. Overview of frequency comb spectroscopy. Light from the comb, made up of tens to hundreds of thousands of individual "comb teeth," passes through a sample resulting in absorption.

2.1.1 Long open-path atmospheric measurements

A significant advantage of long open-path measurements on the hectometer to kilometers scale is that they also allow for characterization of trace gas emissions from potentially heterogeneous source types including area sources and multiple distributed sources (Flesch et al., 2009; Schafer et al., 2012; Lin et al., 2019). For example, methane emissions from multiple wells in a square kilometer area can be monitored with a single open-path spectrometer. By integrating signals in a horizontal "atmospheric column," an aggregate signal can be derived, which makes long open-path measurements less likely to miss the plume from a source compared to a point measurement. In particular, point measurements are subject to local or near-field features in emission source characteristics and atmospheric dispersion caused, for example, by turbulence or buildings. Plumes from point sources can be hard to reliably detect downwind with a point measurement, and semistochastic variability in local micrometeorological conditions can make predictability of plume locations problematic. Path-integrated measurements over large distances relax this dimension of uncertainty due to the plume needing only to intersect a portion of the beam path. This enables open-path systems to make continuous measurements of heterogenous sources, which allows for better determination of temporal variability of the sources. Open-path measurements can also be applied to both point and area sources because of the much larger measurement footprint relative to a point sensor (Section 2.3.1) enabling detection of full plumes from area sources.

There are several other advantages to long open-path measurements. First, the long path length provides high precision for trace gas retrievals. This is important for gases such as volatile organic compounds (VOCs), which are present in small quantities, as well as for greenhouse gases (GHGs), which are challenging to measure because their signals are often very small changes on a very large background. For example, a typical enhancement for in situ CO2 measurements in the planetary boundary layer (PBL) could be 2   ppm on a 400   ppm background. Another advantage of open-path measurements is the ability to measure the so-called "sticky" molecules. These include species like NH3 and H2O, which can strongly adhere to sampling tubes via electrostatic and dispersion interactions, potentially introducing biases in retrieved concentrations. Open-path measurements such as the ones described in this chapter do not require a sampling line and thus enable more accurate measurements of these trace gases. Finally, measurements from long open-path sensors are inherently better matched to the resolution of meteorological models, which reduces the representation error of the measurements (Ciais et al., 2010). The applications described in Section 2.3 take advantage of one or more of these open-path advantages.

2.1.2 Current techniques

Well-established techniques for long open-path measurements typically fall into two categories: those that use broadband spatially incoherent light sources and those that use lasers in very narrow spectral regions. The first category primarily includes Fourier-transform infrared (FTIR) and differential optical absorption spectroscopy (DOAS) spectrometers, which use active incoherent light sources such as thermal sources or light-emitting diode (LED) or passive scattered solar sunlight as the light source. Spatially incoherent light is difficult to send over long distances, especially at eye-safe levels, so open-path FTIR (Marshall et al., 1994; Russwurm and Childers, 2006; Griffith et al., 2018) and active DOAS (Platt and Perner, 1983; Platt and Stutz, 2008) measurements are typically limited to a few hundred meters of path length or require large telescopes and bulky light sources as done in Stutz et al. (2010). Spectrometers relying on the sun are only operational during daytime and are typically limited to vertical column measurements rather than horizontal column measurements, which make them less well suited to many of the applications discussed later in the chapter. Tunable laser systems with narrow spectral coverage include differential absorption LIDAR (DIAL), where the laser is first tuned online to the center of a gas absorption line and then offline to a nearby baseline wavelength (Johnson et al., 2013; Gibert et al., 2015; Wagner and Plusquellic, 2016), and tunable diode laser absorption spectroscopy (TDLAS) or the related tunable laser dispersion spectroscopy, both of which scan across one or a couple of gas lines (see, e.g., Ku et al., 1975; Thoma et al., 2005; Nikodem and Wysocki, 2012; Bailey et al., 2017; Kuhnreich et al., 2015; Seidel et al., 2015). These conventional laser systems can be launched over long distances but can only analyze a few carefully chosen absorption lines without adding substantial complexity to the systems, for example, an additional laser to measure an additional molecule. Further, to ensure a meaningful gas concentration extraction, wavelength calibration of the instruments is critical, which is unnecessary for DCS instruments (Section 2.2.2).

The DCS technique combines the best of the above techniques: it combines broad spectral coverage that can measure entire absorption bands of multiple trace gases simultaneously with spatially coherent light that can be sent over long paths. Open-path DCS is presently used in a monostatic configuration, with a local transceiver and detector and a retroreflector located at some distance from the transceiver/detector that folds the light path back on itself. This is identical to how open-path TDLAS, open-path FTIR, and active DOAS operate and similar to LIDAR-based gas sensing such as DIAL, where the scattered light from a high-power laser is back reflected and collected.

2.1.3 Advantages of open-path DCS

There are several other advantageous features of DCS that make it optimal for long open-path measurements at an unprecedented high resolution and accuracy. 2 First, as compared to FTIR spectroscopy, the spectral resolution in DCS measurements is not given by the length of a very well-aligned, moving arm, but instead the point spacing for the frequency axis is determined by properties of the frequency comb as discussed more in Section 2.2. This enables DCS to have high spectral resolution (for the measurements discussed here, the resolution was usually 200   MHz or 0.0067   cm−1) in a small package—an individual comb is approximately the size of a tablet and the entire system, including electronics, is about the size of a small desk. Further, frequency combs can be built with off-the-shelf all-polarization-maintaining fiber components making them environmentally robust. In fact, fiber-based comb systems have remained mode locked with sustained 0.5   g acceleration on a shaker table and while being driven around on dirt roads and on a sounding rocket (Sinclair et al., 2014; Lezius et al., 2016).

Another key advantage is that DCS adds no instrument lineshape contribution to the measured absorption feature. For fully stabilized frequency combs, the shape of each comb tooth is essentially a delta function with a width much smaller than the spacing between the individual teeth, as shown in Figs. 2.1 and 2.8. For example, the systems discussed here have tooth linewidths on the order of a kilohertz and tooth spacings of ∼ 200   MHz. Thus, the comb tooth width is ∼ 100,000 times narrower than the point spacing. By resolving the absorption at each comb tooth, the DCS instrument lineshape is given just by the comb tooth width and thus is negligible. This can also be seen in Waxman et al. (2017), where the difference between the spectra from two comb systems shows no instrument lineshape residual. In addition, the underlying frequency combs have a frequency axis that is absolutely calibrated against a microwave reference as discussed in Section 2.2.2. Therefore, it is unnecessary to use an external wavelength calibration such as a line lamp or a solar spectrum.

DCS instruments are also immune to potential systematic effects from atmospheric turbulence because they collect a single-shot spectrum at very high temporal resolution—typically a few to 10 milliseconds. Of course, for high signal-to-noise ratio (SNR), many of these individual spectra must be added together, so the time resolution is typically on the order of a half-minute to a few minutes. Because the single-shot sampling rate is faster than any turbulence fluctuations, the measured spectrum is not distorted due to intensity changes during the measurement time. This is in contrast to some other open-path techniques, which can suffer from turbulence-induced intensity fluctuations during the time required to obtain a single spectrum.

Finally, the broad spectral coverage enables measurements of many lines from multiple species simultaneously, which minimizes potential spectral interference effects. For example, open-atmosphere measurements are subject to potentially large and rapid shifts in atmospheric water vapor. The ability to directly measure H2O spectroscopically, with high spectral resolution, means that DCS measurements are less influenced by changes in water vapor concentration. This could happen in other techniques due to cross talk between spectral features or shifts in the absorption baseline due to changing water lines and thus could result in biases in retrieved trace gas concentrations (Webb et al., 1980; Rella et al., 2013; Nara et al., 2012; Burba et al., 2019; Assan et al., 2017; Reum et al., 2019; Warneke et al., 2011; Lin et al., 2020).

In summary, open-path DCS combines broad spectral coverage with high-accuracy spectroscopy in a stable and precise technique for km-scale, path-integrated measurements. These features make open-path DCS an ideal technique for performing atmospheric trace gas measurements.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128150146000087

LIDAR | DIAL

E.V. Browell , ... W.B. Grant , in Encyclopedia of Atmospheric Sciences, 2003

Use of Topographic Targets

When the DIAL measurement does not have sufficient atmospheric backscatter or range resolution for a gas profile measurement, such as for a continuous-wave laser or a low-pulse-energy laser in the infrared, a topographic target may be employed to provide the backscattered laser radiation. This results in a long-path or column measurement of the gas. By using a series of targets at different ranges, it may be possible to obtain some range-resolved information. There are several problems that have to be faced when using topographic targets. One is that unless the target is moving or being scanned, the measurement accuracy will not increase rapidly with the number of pulses averaged. Another concern is that there are sometimes very sharp spectral changes in the reflection features of the materials contained in the topographic targets. Closely spaced DIAL wavelengths will help to reduce any biases due to this effect.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B0122270908002049

Analytical Geochemistry/Inorganic INSTR. Analysis

T. Henkel , J. Gilmour , in Treatise on Geochemistry (Second Edition), 2014

15.22.5.1.2 Sputter-initiated resonance ionization mass spectrometry – Resonant ionization of sputtered neutrals

The first instruments incorporating resonance ionization for ionization of sputtered neutrals were reported in the early 1980s (Parks et al., 1983; Pellin et al., 2010; Winograd et al., 1982 ). Although continuous wave lasers have been used to combine resonance ionization of sputtered neutrals with conventional ion probes ( Perera et al., 1994), the greater ease of achieving saturation of the ionization process with pulsed lasers means that most work has been focused on integrating RIMS with instruments that have pulsed primary beams. Other applications of continuous wave laser-based RIMS are discussed by Wendt and Trautmann (2005). More recently, resonant postionization has been successfully developed and implemented as part of TOF-SIMS instruments by the Argonne group over the last 15 years. Their instruments SARISA and CHARISMA (Pellin et al., 2001; Veryovkin et al., 2004, 2005) are readily available and proven setups exist for several elements (e.g., Mg, Cr, Zr, Mo, Ru, and Ba). The instruments are equipped with Ga+ primary ion guns delivering a submicron lateral resolution, and the specially constructed gridless reflectrons have a wide acceptance volume for high yields, which have been proven to be up to 25% for Mo on the CHARISMA instrument (Veryovkin et al., 2005). A new instrument designed to push instrument performance close to inherent physical limits is under development and is expected to commence analyses in the near future (Stephan et al., 2011). Planned lateral resolution is on the few nanometer scale with the possibility to undertake TOF-SIMS analysis as well as resonant and nonresonant laser postionization measurements. This will allow comprehensive analysis in TOF-SIMS mode in combination with precise isotopic measurements when switching to resonant laser SNMS.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780080959757014315

LASER-BASED TECHNIQUES

A. Quentmeier , in Encyclopedia of Analytical Science (Second Edition), 2005

Applications

The great potential of LIFS for diagnostics on spectrochemical excitation sources and atomizers, preferentially when using pulsed dye lasers, enable the measurement of various important spectroscopic quantities, such as transition lifetimes, collisional coefficients, and quenching rates. Tunable narrowband CW lasers, however, allow the determination of the profile and shift of spectral lines, thus yielding the excitation temperature, electron number density, and information about the different processes responsible for the observed line broadening. The analytical applications make use of the unsurpassed sensitivity of LIFS, especially in combination with electrothermal atomizers, such as the graphite tube furnace or the tungsten filament. The detection limits obtained with the analysis of metal traces in environmental samples or in biological liquids (e.g., Pb, Pt, Se in blood or serum) are in the nanogram per gram to picogram per gram range or even better. LIFS has also been applied to the determination of lithium and uranium isotopes in solid samples using a Nd:YAG laser for LA, and a pulsed tunable dye laser or a CW DL for excitation.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B0123693977003083

Measurements of aerosol optical properties using spectroscopic techniques

Tomoki Nakayama , ... Weijun Zhang , in Advances in Spectroscopic Monitoring of the Atmosphere, 2021

7.4.2 Cavity ring-down/enhanced albedometer

7.4.2.1 Tube cell configuration

A schematic diagram of the combination of CRDS with a tube cell–based reciprocal nephelometer developed by Strawa et al. (2003) for simultaneous measurement of aerosol extinction and scattering coefficients is shown as Fig. 7.25. The probe light source was a cw-laser diode operating at 690   nm. Extinction was measured by the difference between the ring-down time of particle-free air ( τ 0 ) and particulate-laden sample ( τ ) using Eq. (7.34). One wall of the cavity cell was made of BK-7 glass for scattering measurements with a Lambertian diffuser. The scattering coefficient is proportional to the ratio of the scattering intensity (I scat ) and the transmitted intensity from the cavity (I trans ) (Zhao et al., 2014):

Figure 7.25. Optical layout of an albedometer combining CRDS with a reciprocal nephelometer.

 Reproduced from Strawa, A.W., Castaneda, R., Owano, T., Baer, D.S., Paldus, B. A., 2003. The measurement of aerosol optical properties using continuous wave cavity ring-down techniques, J. Atmos. Oceanic Technol. 20, 454–465, https://doi.org/10.1175/1520-0426(2003)20<454:TMOAOP>2.0.CO;2.

(7.39) b scat = I scat I trans ( 1 − R ) ( 1 + R ) d K = I scat I trans K ′

where K and K′ are the experimentally determined calibration constants that consider the differences in collection efficiency and response of different type detectors. The reported scattering truncation angle was about 5 degrees.

To get single particle absorption and scattering information, a modified three-mirror CRDS albedometer operating at 672   nm was developed by Sanford et al. (2008). Total scattering (integrated over 8.5 to 171.5 degrees) was collected with a spherical–ellipsoidal mirror pair in the middle of the cavity cell. The forward and backward scattered lights at angles between 4.6 and 8.5 degrees from the beam axis were separately collected with fiber optics cables at each side of the scattering cell for the optical sizing measurement.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128150146000038

THERMAL LENSING SPECTROMETRY

T. Imasaka , in Encyclopedia of Analytical Science (Second Edition), 2005

Exciting Laser

The enhancement factor calculated theoretically is obtained experimentally only when an ideal laser source is used. The enhancement factor is strongly affected by the beam quality of the laser. Thus, a laser with a single transverse mode must be used to obtain an enhancement factor close to the theoretical one. A CW laser, such as an argon ion laser, is usually operated in the single mode and is preferred in thermal lensing spectrometry. A pulsed laser, such as a dye laser pumped by an excimer laser, provides rather poor beam quality, unless the beam shape is specially controlled by using a pinhole followed by amplification. The enhancement factor obtained experimentally in pulsed thermal lens spectrometry is usually small in comparison with the theoretical value. Aberration of the focusing lens may also affect the enhancement factor and should be minimized by careful selection of the lens.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B012369397700618X

Polymer Characterization

E. Braeken , J. Hofkens , in Polymer Science: A Comprehensive Reference, 2012

2.18.2.1.1 Pulsed versus continuous excitation

Due to their collimated emission, high output power, and monochromaticity, lasers are the excitation source of choice in single-molecule experiments. Moreover, because the electronic transitions of most dyes are in the visible region of the electromagnetic spectrum, visible lasers are normally used. Broadly speaking, we can divide the available lasers into two categories: continuous wave (CW) and pulsed lasers. As the names imply, CW lasers emit continuous, constant intensity radiation, whereas pulsed lasers emit regularly spaced and very short light pulses. These pulses usually have repetition rates in the MHz range and pulse widths in the range of 100 femtoseconds to nanoseconds. Using pulsed lasers has the advantage of allowing for the determination of excited-state lifetimes and of easier analysis of photon coincidence (Section 2.18.2.4). This can be important as excited-state lifetimes are generally very sensitive to the molecular structure and environment. Unfortunately, because the total laser power is packed into very short laser pulses the resulting high peak intensities in each pulse can induce nonlinear or high-energy processes, including formation of higher-energy excited states (e.g., singlet states above the first excited singlet state) that can react in various ways 1 (e.g., triplet formation or photobleaching) or may affect the molecular environment itself. In contrast, CW excitation normally possesses none of these side effects, but generally does not allow for the determination of the excited-state lifetime. Traditional laser sources usually included CW gas lasers, which are routinely available yet suffer from a low energy efficiency, or complex titanium:sapphire pulsed laser systems. However, in recent years, a large number of pulsed and continuous diode lasers have become commercially available. These diode lasers tend to be comparatively inexpensive and convenient to use, though the quality of the output beam is often rather poor. Fortunately, this is not a problem in many experiments and if required this can be solved by coupling the laser to a single-mode optical fiber.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780444533494000388

LIDAR | Differential Absorption Lidar

S. Ismail , E.V. Browell , in Encyclopedia of Atmospheric Sciences (Second Edition), 2015

Use of Topographic Targets

When the DIAL measurement does not have sufficient atmospheric backscatter or does not have range resolution for a gas profile measurement, or there is a requirement for a very highprecision measurement, such as for CO2 and O2, a topographic target may be employed to provide the backscattered laser radiation. This DIAL measurement results in a long-path or column measurement of the gas, which is known as an IPDA measurement. Low-pulse-energy high-repetition-rate laser systems and intensity-modulated (IM) continuous-wave (CW) laser systems often need to use the IPDA method. There are several issues that need to be addressed when using topographic targets, including (1) the need for accurate range information that can be achieved by proper design of the pulsed or pulse-encoded laser systems and during their associated data retrieval methods; (2) rapid variations in the height of topographic targets that can be minimized either by simultaneous measurements of on and off signals from these targets or by randomizing these effects by collecting a large number of samples; (3) the introduction of bias in IPDA measurements due to very sharp spectral changes in the reflection features of the materials contained in the topographic targets, which can be reduced by selecting closely spaced on and off wavelengths; and (4) interferences in IPDA measurement by intervening clouds and aerosols that need to be compensated for, particularly when using IM–CW measurement techniques. Another factor to consider is that unless the target is moving, such as from an aircraft or satellite, or is being scanned, the measurement accuracy will not necessarily increase rapidly with the number of pulses averaged.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780123822253002048

Trace gas measurements using cavity ring-down spectroscopy

Shui-Ming Hu , in Advances in Spectroscopic Monitoring of the Atmosphere, 2021

8.4 Summary

CRDS is one of the cavity-enhanced absorption spectroscopy methods using a high-finesse optical cavity to increase the effective absorption path length. In CRDS, the decay curve is measured instead of the transmittance of the cavity, which also reduces the influence due to laser power noise. The detectable minimum absorption coefficient can easily reach the level less than 10−7  cm−1. By using cw lasers for single mode excitation, the sensitivity of cw-CRDS devices can be further improved, and the best reported sensitivity reaches 10−13  cm−1. As an absorption spectroscopy method, CRDS directly measures the absorption coefficient of the sample gas, which is a great advantage for quantitative trace detection. Since the sample cell used in CRDS has a simple structure and does not require a complicated multipass optical scheme, it is possible to miniaturize the sample cell. For some applications where the sample volume is limited, the CRDS method is very competitive.

Applications of the CRDS in trace detection have covered almost all the regions from infrared to ultraviolet, and various laser sources have been used. If the coupling of the laser to the optical cavity is well controlled, a cw milliwatt laser is sufficient for high-sensitivity CRDS detection. Near-infrared semiconductor lasers, particularly the DFB diode lasers, having the advantages of low power consumption, convenient tuning, and low cost, are ideal for CRDS applications. Using QCLs or tunable optical parametric oscillators, the spectral range can be easily extended to the midinfrared region where molecules have stronger absorption lines. There is an increasing interest to develop CRDS in the midinfrared, although the laser sources and detectors are not yet as convenient as those used in the near-infrared. In the ultraviolet region, it is expected to achieve higher detection sensitivity because extremely strong electronic transitions can be used for detection. With the development of ultraviolet lasers, including those upconversion sources, combined with evolving coating technology in producing HR ultraviolet mirrors, we can expect more exciting applications in the near future.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128150146000026

Missions and Sensors

X. Sun , in Comprehensive Remote Sensing, 2018

1.15.4.1 Principle of Operation

Lidar sensors can be used to remotely probe a specific spectral absorption line of a certain atmosphere constituent or surface element, either sampling the line at discrete laser wavelengths or scanning across the line continuously. The lidar sensor becomes a laser absorption spectrometer with its own light source instead of sunlight. Compared to conventional spectrometers, laser spectral absorption lidars measure the surface element and/or atmosphere day and night under a uniform lighting condition. The data are also easier to interpret because there is no need for photometric correction due to sun angle, topography, etc. The spectral resolution of a laser spectral absorption lidar is determined by the laser wavelength stability and the line width, which can be much more precise and accurate than the resolution of passive spectrometers. The most commonly used laser spectral absorption lidars up to now are integrated path differential absorption (IPDA) lidars, in which the receiver measures the difference in the surface reflection at a certain atmospheric absorption line (online) and a nearby wavelength (offline), as depicted in Fig. 11 , just like the DIAL atmosphere backscattering lidar but measuring the differential reflectance of the ground surface. Similarly, with the proper choice of the laser wavelengths, a laser absorption spectrometer lidar can also be used to measure the abundance certain elements on the surface but not the atmosphere column.

Fig. 11. Measurement approach of a DIAL IPDA lidar.

IPDA lidars are similar to surface elevation lidars but primarily measure the surface reflectance at multiple laser wavelengths. The received signal of an IPDA lidar is much stronger compared to atmosphere backscattering lidars, but it only measures the total column absorption instead of the vertical profile of the atmosphere constituent. The required SNR is usually high in order to resolve the changes in the abundance of the atmosphere species. For example, the SNR for a CO2 IPDA lidar has to be >   400 in order to detect 1 part per million (ppm) changes out of 400   s CO2 in the atmosphere. Most IPDA lidars use DIAL technique with the online and offline laser wavelength at the spectral absorption line being targeted. The laser transmitters can be continuous wave (CW) lasers with and without modulation. For direct detection IPDA lidars, a CW laser can be intensity modulated (chopped) and the receiver cross-correlates the received signal with the modulation signal, a technique commonly known as lock-in amplifier detection. The lock-in amplifier can detect very weak signal from a relatively strong background noise. A lock-in type IPDA lidar can have several lasers emitting simultaneously at different wavelengths. Each laser is modulated by a different frequency, detected with a common detector, but cross-correlated with different modulation signal (Pruitt et al., 2003). The laser can also be pulsed like in a surface elevation lidar with the laser output alternating between the online and offline wavelengths (Abshire et al., 2010). Coherent detection can also be used for IPDA lidar with either CW or pulsed lasers (Spiers et al., 2002; Koch et al., 2004; Pearson and Collier, 1999; Gibert et al., 2006). Fig. 12 shows a block diagram of laser transmitters for IPDA lidars. Fig. 13 shows block diagrams of the receivers for coherent detection, lock-in detection, and pulsed modulation and detection IPDA lidars (Sun and Abshire, 2012).

Fig. 12. Block diagram of an IPDA lidar transmitter. For a coherent IPDA lidar, the laser is either CW or pulsed. For a direct detection lock-in type IPDA lidar, the lasers are intensity modulated with sine-waves of known frequencies. For a direction detection-pulsed IPDA lidar, the lasers are intensity modulated with a pulse train.

Fig. 13. Block diagrams of IPDA lidar receiver for: (A) coherent detection, (B) direct detection with sine-wave laser intensity modulation and lock-in type detection, and (C) direct detection with pulsed modulation and detection (Sun and Abshire, 2012).

The major advantages of the coherent IPDA lidars are the high signal gain at the heterodyne or homodyne detection and narrow bandwidth. Coherent lidars can use ordinary photodiodes without internal photoelectron multiplication gain. The combined received signal multiplied by the local oscillator laser is usually sufficient to overcome the detector electronics noise. The signal output from the detector is proportional to the electromagnetic field of the received optical signal, but frequency downshifted to a RF frequency. One can use mature RF techniques to process the signal, such as Fast Fourier Transform (FFT), frequency and phase locks, and narrow band filtering. The major challenges for cohere IPDA lidars are spatial mode matching of the received and the local oscillator lasers and speckle noise. The receiver telescope size cannot exceed the coherent size of the speckles. The speckle size at the receiver is inversely proportional to the laser footprint size on the surface, which is limited by the size of the laser collimator.

The major advantages of lock-in type direct detection IPDA lidars are simultaneous online–offline laser illumination on the surface and low peak laser power requirement. The major disadvantages are the continued receiver integration time, susceptibility to background noise, and lower SNR compared to a pulsed direct detection IPDA lidar under the same average laser power (Sun and Abshire, 2012). A pulsed IPDA lidar provides not only the surface return but also the atmosphere backscattering profile. It integrates signal only about the laser pulse arrival time and gates out solar background noise and other spurious signals. It can readily identify clouds and aerosol and provide simultaneous range measurement, just like a surface elevation lidar. The major challenge for a pulse IPDA lidar is the requirement for high peak power of the laser transmitter.

The choice of the laser wavelengths directly affects the performance of the IPDA lidars. The offline laser wavelength should be close to the online laser wavelength so that the effects from other atmosphere constituents and instrument are nearly identical and can be canceled out when taking the ratio of the two in post signal processing. The offline wavelength has also been away from spectral absorption lines of any other constituents. On the other hand, the offline laser wavelengths should not be too far from the online laser wavelength so that they can pass a common narrow band optical filter to reduce the solar background light during daytime measurements. The signal levels at the offline laser wavelengths are stronger and have higher SNR. The signal at online wavelength has a lower SNR due to the absorption. The laser wavelength has to be precisely controlled via an automatically control loop in reference to the gas absorption line. Any offset and jitter in the laser wavelengths directly affect the lidar measurement precision and biases.

Another consideration in the laser wavelength selection for a DIAL IPDA lidar is the weighting function that the total column atmosphere gas molecule density is integrated from that of each atmosphere layer. The measurement at the peak absorption has more weight on the atmosphere molecule density at high altitude while the primary interest is often in the changes of molecule density at lower altitude. The online laser wavelength is sometimes placed on the side of the absorption line where the measurement is more sensitive to the line broadening by the pressure and water vapor and has more weight on the atmosphere at lower altitude.

More than two laser wavelengths can be used in DIAL IPDA lidars to provide additional information about the atmosphere species being measured and help to reduce the measurement biases. For example, the Multifunctional Fiber Laser Lidar (MFLL) has two offline laser wavelengths, one on each side of the CO2 absorption line at 1571.061   nm, to calibrate out potential variation in the receiver optical transmission and gain versus wavelength and to improve the offline measurement SNR (Dobler, 2013). The 2-μm triple-pulsed IPDA lidar uses three laser wavelengths to simultaneously measure CO2 and H2O (Refaat et al., 2015).

Multiwavelength sampling of an absorption line can also be used in IPDA lidar, which gives the line shape besides the total column absorption (Ben-David et al., 1992). The data from a multiwavelength sampling IPDA lidar can be used to estimate the Doppler shift and line broadening via curve fitting (Ben-David et al., 1992; Chen et al., 2014). It significantly reduces the effects of instrument artifacts, such as laser wavelength jitter and uneven receiver optics transmission. Multiwavelength sampling is not necessarily the more efficient methods in terms of measurement precision versus average laser power because the wavelength samples are placed across the entire absorption line instead of where it gives the highest receiver SNR. Nevertheless, it provides means to control measurement biases and additional information about absorption line shape.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780124095489103276