This application note shows how high-repetition-rate CMOS LineScan cameras can support these techniques by capturing low-intensity return signals, preserving temporal resolution, and enabling synchronized measurements in demanding optical setups.
Laser-based remote sensing is an umbrella term for amyriad of established and emerging techniques with a wide variety ofapplications and use cases. From basic range-finding like time-of-flight LIDARto profilometry and triangulation, lasers are routinely deployed for distanceor position measurements with previously unimaginable accuracy.Laser-scattering techniques have also enabled the measurement of objects anddistances on the order of the wavelength, invisible to the naked eye. Moreadvanced techniques supplement spatial resolution with spectral resolution totake advantage of the spectral properties of laser light. Often termedHyperspectral LIDAR, a variety of spectroscopic techniques have been deployedin this context. Remote lasers can also be used to excite samples triggeringprocesses by which internal mechanisms can be studied or characterized by theirmaterial composition, from a distance. By combining all these technologicaladvancements, measurement instruments can be imagined that are not only remote,but also highly spatially-, spectrally and temporally resolved.
Laser-based remote sensing is often enabled by a pulsedlaser system, the pulse train simultaneously acting as the clock andrange-finder in applications from time-of-flight LIDAR to remote spectroscopictechniques that require triggering. High optical peak power is sometimesrequired, not only for excitation of samples but also so that back-scatteredlight remains detectable even over large distances. This specification oftennecessitates the deployment of a Chirped-Pulse Amplification (CPA) lasersystem, with typical repetition rates on the order of kilohertz. A detectionsystem must be able to detect even low returning light intensities and preservetemporal resolution as best possible. Once again, the need arises forhigh-repetition rate detection that can match the repetition rate of the lasersystem, with low noise and high dynamic range in the optical and NIR frequencyrange.
High-repetition rate CMOS-based LineScan cameras offerunique capabilities that can significantly enhance laser-based remote sensing,particularly in terms of throughput, sensitivity, data quality, and long-termstability. In the following sections we will describe selected laser-basedremote sensing techniques and specifically, the role that a highrepetition-rate LineScan camera can play in advancing the capabilities of thesetup.
LIDAR is typically enabledby pulsed systems that rely on time-of-flight detection. However, continuouslasers can also be deployed by effectively mapping distance onto the pixels ofthe LineScan camera. In this “Scheimpflug” configuration, the probe beamdefines the object plane and is nonparallel to the lens plane. Sharp focusalong the entire beam is then achieved by tilting the image sensor so that theimage plane intersects both the object- and lens-planes. The distance at whichback-scattering of laser light occurs, then determines the pixel whereback-scattered light is detected. This method has been deployed for long-rangemonitoring with use cases in vertical profiling of atmospheric aerosols, remotedetection of gas plumes or pollution layers and even detection of flyinginsects over great ranges for environmental studies.
Whether pulsed or continuouslaser systems are used, a similar configuration can be used in more controlledenvironments. In laser triangulation (profilometry), a laser beam is reflectedoff a probe surface and imaged by the LineScan camera. By monitoring fordeviations the reflecting surface is thereby subjected to interrogation for avariety of use cases like 3D surface scanning, crack detection and industrialquality control. When tunable or multi-wavelength laser sources are deployed,estimates of the size of material imperfections can also be deduced.Analogously, differential absorption can be inferred from changes inbackscattering intensity as the laser wavelength is tuned. This method can beused for remote detection of atmospheric composition.
As with other applications,effective application of LIDAR and similar applications depends on an opticaldetection system that offers
- Fast frame rates foraveraging and capturing low-signal events and matching laser repetition rateswhen pulsed laser systems are deployed
- High signal-to-noise ratio(SNR) to detect subtle changes and low-intensity back-scattered light
- High dynamic range toaccommodate both weak and strong signals, the intensity of returning light isgreatly enhanced when a reflecting surface crosses the probe beam.
The capability of a remote sensing setup consisting of alaser source and LineScan camera, can be expanded by including a grating todisperse backscattered light, thus effectively combining spatial and spectralresolution. This is an emerging technology in environmental monitoring andprecision agriculture, where several methods can be deployed depending on theapplication.
LIF is an analytical technique that examines light emittedby atoms or molecules (fluorescence) after they absorb energy and are excitedto higher electronic states. When these excited species return to lower energystates, they emit photons at characteristic wavelengths. By measuring theintensity and spectrum of this emitted light, you can infer information aboutthe molecular or chemical structure, concentration, and environment of thefluorescent species. By combining this technique with LIDAR, a pulsed laserexcites a distant target (such as vegetation, water, soil, aerosols, orpollutants), and a telescope collects the resulting fluorescence emitted by thetarget with a time-resolution determined, in part, by the speed of the LineScancamera. Due to the spectrometer, the fluorescence is spectrally resolved andcan be analyzed to retrieve chemical or biological information about the targetfrom afar. This method has been deployed for water quality monitoring,vegetation health assessment (chlorophyll emission lines), soil and rockcharacterization, detection and identification of atmospheric pollutants andmany other use cases.
Raman spectroscopy can also be deployed, but the signalsare much weaker and therefore necessitate much greater sensitivity. Ramanscattering is, however, attractive because it does not require a fluorescencesignal (indeed, where fluorescence occurs, it tends to contaminate the desiredsignal but even here, Raman bands can be identified with a well-designedinstrument). Instead, inelastic scattering via vibrational/rotational modes isdetected as a wavelength shift and allows for the detection of highly specificmolecular fingerprints. This also enables identification, quantification andeven temperature measurement of remote samples in gas, liquid or solid phase.Because Raman signals are weak and interference from molecular fluorescence isa major issue, lasers, time-gating, and spectral filtering are essential. Dueto its selectivity, remote Raman sensing is often deployed for detection ofhazardous materials such as explosives.
Where elemental composition needs to be determined, higherpower lasers can be used to create a micro-plasma by laser ablation. The atomicemission lines of the resulting plasma are analysed to identify the atomspresent. The method requires simpler optics but higher peak powers are neededand the method is destructive in nature. The emission lines are typicallyrelatively strong signals and this method is therefore effective over longerranges than is typically achieved using Raman scattering.
High-repetition rate CMOS-based LineScan cameras offer apowerful, flexible, and cost-effective solution for sensitive spectral andspatial detection and monitoring for remote sensing applications. Particularlywhere lasers are deployed, LineScan cameras are easily synchronized even athigh repetition rates to enable high temporal resolution. When a spectrometeris included the camera’s quick integration capabilities allow for time-resolvedspectroscopies to be deployed. More broadly, a CMOS LineScan camera can be aneffective sensor in laser-based remote sensing systems in general, by enablinghigh-speed scanning and precise measurement of laser spot position afterreflection or back-scattering that forms the central operating principle ofLIDAR applications.
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