Editing Lidar (section)

===Atmosphere===
[[File:Lidar-IGF-UW-2024 AB-02.jpg|thumb|Near range lidar at Institute of Geophysics, Warsaw, Poland]]
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{{main|Atmospheric lidar}}

Initially, based on [[ruby laser]]s, lidar for meteorological applications was constructed shortly after the invention of the laser and represents one of the first applications of laser technology. Lidar technology has since expanded vastly in capability and lidar systems are used to perform a range of measurements that include profiling clouds, measuring winds, studying [[aerosol]]s, and quantifying various atmospheric components. Atmospheric components can in turn provide useful information including [[Atmospheric pressure|surface pressure]] (by measuring the absorption of [[oxygen]] or [[nitrogen]]), [[greenhouse gas]] emissions ([[carbon dioxide]] and [[methane]]), [[photosynthesis]] (carbon dioxide), [[wildfire|fires]] ([[carbon monoxide]]), and [[humidity]] ([[water vapor]]). Atmospheric lidars can be either ground-based, airborne or satellite-based depending on the type of measurement.

Atmospheric lidar [[remote sensing]] works in two ways:

# by measuring [[backscatter]] from the atmosphere, and 
# by measuring the scattered reflection off the ground (when the lidar is airborne) or other hard surface.

Backscatter from the atmosphere directly gives a measure of clouds and aerosols. Other derived measurements from backscatter such as winds or cirrus ice crystals require careful selecting of the wavelength and/or polarization detected. ''Doppler lidar'' and ''Rayleigh Doppler lidar'' are used to measure temperature and wind speed along the beam by measuring the frequency of the backscattered light. The [[Doppler broadening]] of gases in motion allows the determination of properties via the resulting frequency shift.<ref>{{cite journal | last1 = Li | year = 2011 | first1 = T. |title = Middle atmosphere temperature trend and solar cycle revealed by long-term Rayleigh lidar observations |journal =  Journal of Geophysical Research| volume = 116 | pages = D00P05 |doi=10.1029/2010jd015275 | bibcode=2011JGRD..116.0P05L| url = https://hal.archives-ouvertes.fr/hal-00594272/file/2010JD015275.pdf | doi-access = free }}</ref> Scanning lidars, such as [[NASA]]'s conical-scanning HARLIE, have been used to measure atmospheric wind velocity.<ref>[http://harlie.gsfc.nasa.gov/IHOP2002/Pub&Pats/AMOS%202002%20final.pdf Thomas D. Wilkerson, Geary K. Schwemmer, and Bruce M. Gentry. ''LIDAR Profiling of Aerosols, Clouds, and Winds by Doppler and Non-Doppler Methods'', NASA International H2O Project (2002)] {{webarchive|url=https://web.archive.org/web/20070822232155/http://harlie.gsfc.nasa.gov/IHOP2002/Pub%26Pats/AMOS%202002%20final.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://harlie.gsfc.nasa.gov/IHOP2002/Pub%26Pats/AMOS%202002%20final.pdf |archive-date=2022-10-09 |url-status=live |date=2007-08-22 }}.</ref> The [[ESA]] wind mission ''ADM-Aeolus'' will be equipped with a Doppler lidar system in order to provide global measurements of vertical wind profiles.<ref>[http://www.esa.int/esaLP/ESAES62VMOC_LPadmaeolus_0.html 'Earth Explorers: ADM-Aeolus', ''ESA.org'' (European Space Agency, 6 June 2007)]. Retrieved 8 August 2007.</ref> A doppler lidar system was used in the [[2008 Summer Olympics]] to measure wind fields during the yacht competition.<ref>[http://optics.org/cws/article/research/34878 'Doppler lidar gives Olympic sailors the edge', ''Optics.org'' (3 July, 2008)]. Retrieved 8 July 2008.</ref>

Doppler lidar systems are also now beginning to be successfully applied in the renewable energy sector to acquire wind speed, turbulence, wind veer, and wind shear data. Both pulsed and continuous wave systems are being used. Pulsed systems use signal timing to obtain vertical distance resolution, whereas continuous wave systems rely on detector focusing.

The term ''eolics'' has been proposed to describe the collaborative and interdisciplinary study of wind using computational fluid mechanics simulations and Doppler lidar measurements.<ref>Clive, P. J. M., [https://web.archive.org/web/20140512215311/http://www.sgurrenergy.com/wp-content/uploads/2014/04/TEDxUniversityOfStrathclyde_TheEmergenceOfEolics.pdf The emergence of eolics], TEDx University of Strathclyde (2014). Retrieved 9 May 2014.</ref>

The ground reflection of an airborne lidar gives a measure of surface reflectivity (assuming the atmospheric transmittance is well known) at the lidar wavelength, however, the ground reflection is typically used for making absorption measurements of the atmosphere. "Differential absorption lidar" (DIAL) measurements utilize two or more closely spaced (less than 1&nbsp;nm) wavelengths to factor out surface reflectivity as well as other transmission losses, since these factors are relatively insensitive to wavelength. When tuned to the appropriate absorption lines of a particular gas, DIAL measurements can be used to determine the concentration (mixing ratio) of that particular gas in the atmosphere. This is referred to as an ''Integrated Path Differential Absorption'' (IPDA) approach, since it is a measure of the integrated absorption along the entire lidar path. IPDA lidars can be either pulsed<ref name=":1">{{cite journal | last1 = Koch | first1 = Grady J. | last2 = Barnes | first2 = Bruce W | last3 = Petros | first3 = Mulugeta | last4 = Beyon | first4 = Jeffrey Y | last5 = Amzajerdian | first5 = Farzin | last6 = Yu | first6 = Jirong | last7 = Davis | first7 = Richard E | last8 = Ismail | first8 = Syed | last9 = Vay | first9 = Stephanie | last10 = Kavaya | first10 = Michael J | last11 = Singh | first11 = Upendra N | year = 2004| title = Coherent Differential Absorption Lidar Measurements of CO2 | journal = Applied Optics | volume = 43 | issue = 26| pages = 5092–5099 | doi = 10.1364/AO.43.005092 | pmid = 15468711 |bibcode = 2004ApOpt..43.5092K }}</ref><ref name=":2">{{Cite journal|last1=Abshire|first1=James B.|last2=Ramanathan|first2=Anand|last3=Riris|first3=Haris|last4=Mao|first4=Jianping|last5=Allan|first5=Graham R.|last6=Hasselbrack|first6=William E.|last7=Weaver|first7=Clark J.|last8=Browell|first8=Edward V.|date=2013-12-30|title=Airborne Measurements of CO2 Column Concentration and Range Using a Pulsed Direct-Detection IPDA Lidar|journal=Remote Sensing|volume=6|issue=1|pages=443–469|doi=10.3390/rs6010443|bibcode=2013RemS....6..443A|doi-access=free|hdl=2060/20150008257|hdl-access=free}}</ref> or CW<ref name=":3">{{cite journal | last1 = Campbell | first1 = Joel F. | year = 2013| title = Nonlinear swept frequency technique for CO2 measurements using a CW laser system | journal = Applied Optics | volume = 52 | issue = 13| pages = 3100–3107 | doi = 10.1364/AO.52.003100 | pmid = 23669780 |arxiv = 1303.4933 |bibcode = 2013ApOpt..52.3100C | s2cid = 45261286 }}</ref> and typically use two or more wavelengths.<ref>{{cite journal | last1 = Dobler | first1 = Jeremy T. | last2 = Harrison | first2 = F. Wallace | last3 = Browell | first3 = Edward V. | last4 = Lin | first4 = Bing | last5 = McGregor | first5 = Doug | last6 = Kooi | first6 = Susan | last7 = Choi | first7 = Yonghoon | last8 = Ismail | first8 = Syed | year = 2013| title = Atmospheric CO2 column measurements with an airborne intensity-modulated continuous wave 1.57 μm fiber laser lidar | journal = Applied Optics | volume = 52 | issue = 12| pages = 2874–2892 | doi = 10.1364/AO.52.002874 | pmid = 23669700 |bibcode = 2013ApOpt..52.2874D | s2cid = 13713360 }}</ref> IPDA lidars have been used for remote sensing of carbon dioxide<ref name=":1"/><ref name=":2" /><ref name=":3"/> and methane.<ref>{{Cite journal|last1=Riris|first1=Haris|last2=Numata|first2=Kenji|last3=Li|first3=Steve|last4=Wu|first4=Stewart|last5=Ramanathan|first5=Anand|last6=Dawsey|first6=Martha|last7=Mao|first7=Jianping|last8=Kawa|first8=Randolph|last9=Abshire|first9=James B.|date=2012-12-01|title=Airborne measurements of atmospheric methane column abundance using a pulsed integrated-path differential absorption lidar|journal=Applied Optics|volume=51|issue=34|doi=10.1364/AO.51.008296|pmid=23207402|issn=1539-4522|bibcode = 2012ApOpt..51.8296R|pages=8296–305|s2cid=207299203}}</ref>

''[[Synthetic array heterodyne detection|Synthetic array]] lidar'' allows imaging lidar without the need for an array detector. It can be used for imaging Doppler velocimetry, ultra-fast frame rate imaging (millions of frames per second), as well as for [[speckle pattern|speckle]] reduction in coherent lidar.<ref name="Strauss">{{cite journal | last1 = Strauss | first1 = C. E. M. | year = 1994 | title = Synthetic-array heterodyne detection: a single-element detector acts as an array | journal =  Optics Letters| volume = 19 | issue = 20| pages = 1609–1611 | doi=10.1364/ol.19.001609|bibcode = 1994OptL...19.1609S | pmid=19855597| url = https://zenodo.org/record/1235660 }}</ref> An extensive lidar bibliography for atmospheric and hydrospheric applications is given by Grant.<ref>Grant, W. B., Lidar for atmospheric and hydrospheric studies, in ''Tunable Laser Applications'', 1st Edition, [[F. J. Duarte|Duarte, F. J.]] Ed. (Marcel Dekker, New York, 1995) Chapter 7.</ref>