A new depolarization channel has beenimplemented in the BarcelonaTech University(UPC) multi-wavelength lidar system. The opticaland mechanical designs are presented. The specialconfiguration of the total power channel is alsodetailed, with the relevant aspects in measurement inversion.  Some  preliminary  measurements  arepresented for Saharan dust intrusion events.Peer ReviewedPostprint (published version

Barragán Cuesta, Rubén

Comerón Tejero, Adolfo

Muñoz Porcar, Constantino

Rocadenbosch Burillo, Francisco

Rodríguez Gómez, Alejandro Antonio

Sicard, Michaël

Vidal Calvo, Eric

EPJ Web of Conferences

English

A new depolarization channel has been implemented in the BarcelonaTech University (UPC) multi-wavelength lidar system. The optical and mechanical designs are presented. The special configuration of the total power channel is also detailed, with the relevant aspects in measurement inversion. Some preliminary measurements are presented for Saharan dust intrusion events

Alejandro Rodríguez-Gómez

Michaël Sicard

Constantino Muñoz-Porcar

Rubén Barragán

Adolfo Comerón

Francesc Rocadenbosch

Eric Vidal

EDP Sciences OAI-PMH repository (1.2.0)

Depolarization channel for barcelona lidar. Implementation and preliminary measurements.

A new depolarization channel has been implemented in the BarcelonaTech University (UPC) multi-wavelength lidar system. The optical and mechanical designs are presented. The special configuration of the total power channel is also detailed, with the relevant aspects in measurement inversion. Some preliminary measurements are presented for Saharan dust intrusion events.</jats:p

Crossref

UPCommons. Portal del coneixement obert de la UPC

DEPOLARIZATION CHANNEL FOR BARCELONA LIDAR. IMPLEMENTATION AND PRELIMINARY MEASUREMENTS. Alejandro Rodríguez-Gómez1*, Michaël Sicard1,2, Constantino Muñoz-Porcar1, Rubén Barragán1,2, Adolfo Comerón1, Francesc Rocadenbosch1,2, Eric Vidal3 1RSLab, BarcelonaTech University (UPC), Barcelona, Spain *alejandro@tsc.upc.edu 2Ciències i Tecnologies de l'Espai - Centre de Recerca de l'Aeronàutica i de l'Espai / Institut d'Estudis Espacials de Catalunya (CTE-CRAE / IEEC), UPC, Barcelona, Spain 3UTC Building & Industrial Systems, Spain ABSTRACT A new depolarization channel has been implemented in the BarcelonaTech University (UPC) multi-wavelength lidar system. The optical and mechanical designs are presented. The special configuration of the total power channel is also detailed, with the relevant aspects in measurement inversion. Some preliminary measurements are presented for Saharan dust intrusion events. 1 INTRODUCTION The capability of measuring depolarization of the backscattered light, together with multi-wavelength detection, improves the ability of a lidar system to identify the aerosols present in the atmosphere, as it has been pointed out by different authors [1]–[4]. The BarcelonaTech University (UPC) Remote Sensing Lab (RSLab) has implemented a new depolarization channel for its lidar system. The depolarization channel has been tested with measurements during Saharan dust intrusion events taking place on May and June 2016. 2 THE UPC LIDAR MAIN RECEIVER CONFIGURATION The lidar system at UPC [5] presents a non-common receiver configuration that is sketched in Figure 1.  A 14-inch telescope collects the backscattered light, which is delivered to the poly-chromator unit by means of a 3-m long multi-mode fiber bundle of 3 mm diameter. The poly-chromator delivers light to the 6 photo-receivers tuned at 1064, 532 and 355 nm (elastic backscattering) and 607, 407 and 387 nm (Raman channels). According to fiber optics theory [6], a multi-mode fiber will not preserve the polarization of the guided light.   Figure 1 UPC lidar receiver configuration, including a depolarizing multi-mode fiber bundle. This fact makes that the overall behavior of the receiver is non-sensitive to the polarization of the incoming light. So the main receiver becomes total-power collecting. To check this feature, we measured the depolarization introduced by the fiber bundle, with a simple test set, presented in Figure 2. The laser beam passes through a linear polarizer before being fed to the fiber bundle.   Figure 2 Test set for checking the polarization behavior of the fiber bundle The polarization state of the output light was measured in two different situations: with the fiber bundle cable placed straight and rolled up. The results are plotted in Figure 3. The measured circularities are 0.96 for the cable straight and 0.93 for the cable rolled up.  This depolarization capability is also useful to make our system insensitive to the di-attenuation of the different beam-splitters used in the poly-chromator unit. Poly-chromatorFiber bundledetectordetectordetectordetectordetectorMain telescopePolarizer Polarization analyzer Fiber bundleEPJ Web of Conferences 176, 01029 (2018) https://doi.org/10.1051/epjconf/201817601029ILRC 28© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). Figure 3 Polarization state measurement at the output of the fiber bundle cable for two situations: straight (blue line) and rolled up (red line) cable.[7] 3 NEW DEPOLARIZATION CHANNEL The depolarization channel has been built by means of an auxiliary telescope attached to the lidar mechanical setup. The auxiliary telescope is, in fact, a 300-mm focal distance, 70-mm aperture diameter, TAIR-3S telephoto lens. The optical setup, sketched in Figure 4, includes an eye-piece lens that produces an image of the telescope aperture onto the photo-receiver (a Licel R9880U PMT module) sensitive area. This arrangement allows distributing the collected light over the PMT sensitive surface, while a 1-mm field-stop iris limits the field of view to approx. 3.3 mrad.  Figure 4 Optical setup of the depolarization channel A polarization analyzer located at the focal plane of the telephoto lens selects the desired polarization of the collected light. When performing measurements, the position of the analyzer is oriented to collect the cross-polar component; when performing the needed calibration of the channel [4] the analyzer is oriented alternatively at +45 and -45 degrees from the nominal position. Finally, an interference filter at 532 nm is located near the photo-receiver surface. According to this arrangement, the collected light rays are not parallel when passing through the filter. This fact implies that the filter effective spectral width is greater than the specified value (0.5 nm). This has implications especially in the molecular contribution to the depolarized backscattered light [8] and must be taken into account when processing the measurement results. 4 PRINCIPLE OF OPERATION [4] The primary depolarization measurement is done by comparing the signals detected by the depolarization and total power channels            90º , ··Dep DepTot Tot TotS R V R P RS R V R P R   (1) where  P R and  TotP R  are the cross-polar and total powers collected while  DepV R and  TotV R are the depolarization and total power channels responsivities, as a function of height over the lidar R. The primary observable is      *90º ,90º , DepTOTS RRS R    (2) In parallel to the measurement process, a channel calibration (with respect to the total power channel) must be performed to obtain the system channel ratio      *DepTotV RV RV R   (3) The calibration is done by performing two short measurements with the polarization analyzer rotated +45 and -45 degrees from the nominal position,  90º 45º ,DepS R  and  90º 45º ,DepS R . The system calibration profile is then computed by comparing with the signal obtained simultaneously from the total power channel,  TotS R : Telephoto lensPolarization analyzer1fEye‐piece lensFilterPMT moduleLicel R9880UField‐stop iris2EPJ Web of Conferences 176, 01029 (2018) https://doi.org/10.1051/epjconf/201817601029ILRC 28       1 2*1 290º 45º , 90º 45º ,2 ·Dep DepTot TotS R S RV RS R S R    (4) Figure 5 shows the result of a calibration performed on December 2nd 2016. As it can be noted, a spatial average is performed  Figure 5 System channel-ratio calibration performed on December 2nd 2016. The volume depolarization ratio is then computed        ** *90º ,90º ,V RRV R R     (5) Finally, the particle depolarization ratio is computed by using concurrently the results of the molecular  m R  and particle  p R   backscattering coefficients of the atmosphere under measure, via an elastic or Raman inversion of the total power 532-nm channel:                 1 · · 11 · 1m V V mpm VR R R R RRR R R                                         (6) where:        m pmR RRR    (7) and  m R , the molecular volume depolarization ratio is usually considered a constant, which depends on the temperature and the spectral width of the photo-receiver [8]. 5 RESULTS  Two Saharan dust intrusion events took place around Barcelona area in late May and early June 2016. Measurements were performed with the UPC lidar and simultaneously with a co-located Micro Pulse Lidar, associated to MPLnet [9]. This Micro Pulse Lidar provides the range-corrected total power and depolarization signals and calculates the volume depolarization in real time. Figure 6 and Figure 7 show two consecutive measurements during May 26th. They show a Saharan dust layer around 3.7 km altitude, with particle depolarization ratio exceeding 20% (more than 15% is characteristic of this kind of aerosol). Both measurements show good agreement between the UPC lidar and the MPL.  Figure 6 Measurement performed on May 26th 2016, 14:24 UTC  Figure 7 Measurement performed on May 26th 2016, 15:40 UTC. Figure 8 and Figure 9 show measurements taken, respectively, in the beginning and in the maximum of an intrusion event in June 2016. Figure 8 shows a small value of volume depolarization up to 5 km, while Figure 9 shows a great mass of aerosol between 3 and 4 km height, 0 1 2 3 4 5 6 7 8 9012345Height AGL (km)System ratioBA161202a  From +45ºFrom -45ºAverageFiltered0 0.1 0.2 0.3 0.4 0.5012345Depolarization ratiosAltitude (km agl)  VpV(MPL)0 0.1 0.2 0.3 0.4 0.5012345Depolarization ratiosAltitude (km agl)  VpV(MPL)3EPJ Web of Conferences 176, 01029 (2018) https://doi.org/10.1051/epjconf/201817601029ILRC 28also with particle depolarization over 20%. In this latest measurement, there is a noticeable difference with the MPL measurement, very likely due to some mechanical tolerance in the telescope pointing not considered because of the absence of a calibration on the day of the measurement.  Figure 8 Measurement performed on June 7th 2016, 8:37 UTC.  Figure 9 Measurement performed on June 10th 2016, 13:17 UTC 6 CONCLUSIONS The new depolarization channel of the UPC lidar has been introduced with details of architecture and principle of operation. Measurements performed during two Saharan dust intrusion events are used to test the new channel. Particle depolarization ratios varying between 10 and 40 % are found in the dust layers. Comparisons of the volume depolarization with an MPL system show a good agreement between both systems and demonstrate the reliability of the new depolarization channel of the UPC multi-wavelength lidar.  ACKNOWLEDGEMENTS This work has been financed by the European Union (H2020, grant 654169, ACTRIS-2), the European Fund for Regional Development, the Spanish Government (grant TEC2015-63832-P) and the Catalan Government (grant 2014 SGR 583). The authors also acknowledge the priceless help of Joaquim Giner and Josep Pastor, both members of UPC technical staff.  References [1] K. Sassen, “Polarization in Lidar,” in Lidar, C. Weitkamp, Ed. New York: Springer-Verlag, 2005, pp. 19–42. [2] K. Sassen, J. Zhu, P. Webley, K. Dean, and P. Cobb, “Volcanic ash plume identification using polarization lidar: Augustine eruption, Alaska,” Geophys. Res. Lett., vol. 34, no. 8, Apr. 2007. [3] G. Wandinger, Ulla; Ansmann, Albert; Mattis, Ina; Müller, Detlef; Pappalardo, “Calipso and beyond: long-term ground-based support of space-borne aerosols and cloud lidar missions” in 24th International Laser Radar Conference, 2008, pp. 715–718. [4] V. Freudenthaler et al., “Depolarization ratio profiling at several wavelengths in pure Saharan dust during SAMUM 2006,” Tellus, Ser. B Chem. Phys. Meteorol., vol. 61, no. 1, pp. 165–179, 2009. [5] D. Kumar et al., “Six-channel polychromator design and implementation for the UPC elastic/Raman lidar,” SPIE Int. Symp. - Remote Sens. Eur., vol. 8182, p. 81820W–1–81820W–10, 2011. [6] G. P. Agrawal, Fiber-Optic Communications Systems, Third Edition., vol. 6. 2002. [7] E. Vidal, “Disseny d’un canal de despolarització a 532 nm per al lidar d’EARLINET de la UPC,” BarcelonaTech, 2013. [8] A. Behrendt and T. Nakamura, “Calculation of the calibration constant of polarization lidar and its dependency on atmospheric temperature,” Opt. Express, vol. 10, no. 16, p. 805, Aug. 2002. [9] NASA, “MPLNet,” 2017. [Online]. Available: https://mplnet.gsfc.nasa.gov/. [Accessed: 11-Jan-2017].  0 0.1 0.2 0.3 0.4 0.5012345Depolarization ratiosAltitude (km agl)  VpV(MPL)0 0.1 0.2 0.3 0.4 0.5012345Depolarization ratiosAltitude (km agl)  VpV(MPL)4EPJ Web of Conferences 176, 01029 (2018) https://doi.org/10.1051/epjconf/201817601029ILRC 28

Depolarization channel for barcelona lidar. Implementation and preliminary measurements

UPCommons (Universitat Politècnica de Catalunya)

DEPOLARIZATION CHANNEL FOR BARCELONA LIDAR. IMPLEMENTATION AND PRELIMINARY MEASUREMENTS. Alejandro Rodríguez-Gómez1*, Michaël Sicard1,2, Constantino Muñoz-Porcar1, Rubén Barragán1,2, Adolfo Comerón1, Francesc Rocadenbosch1,2, Eric Vidal3 1RSLab, BarcelonaTech University (UPC), Barcelona, Spain *alejandro@tsc.upc.edu 2Ciències i Tecnologies de l'Espai - Centre de Recerca de l'Aeronàutica i de l'Espai / Institut d'Estudis Espacials de Catalunya (CTE-CRAE / IEEC), UPC, Barcelona, Spain 3UTC Building & Industrial Systems, Spain ABSTRACT A new depolarization channel has been implemented in the BarcelonaTech University (UPC) multi-wavelength lidar system. The optical and mechanical designs are presented. The special configuration of the total power channel is also detailed, with the relevant aspects in measurement inversion. Some preliminary measurements are presented for Saharan dust intrusion events. 1 INTRODUCTION The capability of measuring depolarization of the backscattered light, together with multi-wavelength detection, improves the ability of a lidar system to identify the aerosols present in the atmosphere, as it has been pointed out by different authors [1]–[4]. The BarcelonaTech University (UPC) Remote Sensing Lab (RSLab) has implemented a new depolarization channel for its lidar system. The depolarization channel has been tested with measurements during Saharan dust intrusion events taking place on May and June 2016. 2 THE UPC LIDAR MAIN RECEIVER CONFIGURATION The lidar system at UPC [5] presents a non-common receiver configuration that is sketched in Figure 1.  A 14-inch telescope collects the backscattered light, which is delivered to the poly-chromator unit by means of a 3-m long multi-mode fiber bundle of 3 mm diameter. The poly-chromator delivers light to the 6 photo-receivers tuned at 1064, 532 and 355 nm (elastic backscattering) and 607, 407 and 387 nm (Raman channels). According to fiber optics theory [6], a multi-mode fiber will not preserve the polarization of the guided light.   Figure 1 UPC lidar receiver configuration, including a depolarizing multi-mode fiber bundle. This fact makes that the overall behavior of the receiver is non-sensitive to the polarization of the incoming light. So the main receiver becomes total-power collecting. To check this feature, we measured the depolarization introduced by the fiber bundle, with a simple test set, presented in Figure 2. The laser beam passes through a linear polarizer before being fed to the fiber bundle.   Figure 2 Test set for checking the polarization behavior of the fiber bundle The polarization state of the output light was measured in two different situations: with the fiber bundle cable placed straight and rolled up. The results are plotted in Figure 3. The measured circularities are 0.96 for the cable straight and 0.93 for the cable rolled up.  This depolarization capability is also useful to make our system insensitive to the di-attenuation of the different beam-splitters used in the poly-chromator unit. Poly-chromatorFiber bundledetectordetectordetectordetectordetectorMain telescopePolarizer Polarization analyzer Fiber bundleEPJ Web of Conferences 176, 01029 (2018) https://doi.org/10.1051/epjconf/201817601029ILRC 28© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). Figure 3 Polarization state measurement at the output of the fiber bundle cable for two situations: straight (blue line) and rolled up (red line) cable.[7] 3 NEW DEPOLARIZATION CHANNEL The depolarization channel has been built by means of an auxiliary telescope attached to the lidar mechanical setup. The auxiliary telescope is, in fact, a 300-mm focal distance, 70-mm aperture diameter, TAIR-3S telephoto lens. The optical setup, sketched in Figure 4, includes an eye-piece lens that produces an image of the telescope aperture onto the photo-receiver (a Licel R9880U PMT module) sensitive area. This arrangement allows distributing the collected light over the PMT sensitive surface, while a 1-mm field-stop iris limits the field of view to approx. 3.3 mrad.  Figure 4 Optical setup of the depolarization channel A polarization analyzer located at the focal plane of the telephoto lens selects the desired polarization of the collected light. When performing measurements, the position of the analyzer is oriented to collect the cross-polar component; when performing the needed calibration of the channel [4] the analyzer is oriented alternatively at +45 and -45 degrees from the nominal position. Finally, an interference filter at 532 nm is located near the photo-receiver surface. According to this arrangement, the collected light rays are not parallel when passing through the filter. This fact implies that the filter effective spectral width is greater than the specified value (0.5 nm). This has implications especially in the molecular contribution to the depolarized backscattered light [8] and must be taken into account when processing the measurement results. 4 PRINCIPLE OF OPERATION [4] The primary depolarization measurement is done by comparing the signals detected by the depolarization and total power channels            90º , ··Dep DepTot Tot TotS R V R P RS R V R P R  (1) where  P R and  TotP R  are the cross-polar and total powers collected while  DepV R and  TotV R are the depolarization and total power channels responsivities, as a function of height over the lidar R. The primary observable is      * 90º ,90º , DepTOTS RRS R    (2) In parallel to the measurement process, a channel calibration (with respect to the total power channel) must be performed to obtain the system channel ratio      * DepTotV RV RV R   (3) The calibration is done by performing two short measurements with the polarization analyzer rotated +45 and -45 degrees from the nominal position,  90º 45º ,DepS R  and  90º 45º ,DepS R . The system calibration profile is then computed by comparing with the signal obtained simultaneously from the total power channel,  TotS R : Telephoto lensPolarization analyzer1fEye‐piece lensFilterPMT moduleLicel R9880UField‐stop iris2EPJ Web of Conferences 176, 01029 (2018) https://doi.org/10.1051/epjconf/201817601029ILRC 28       1 2*1 290º 45º , 90º 45º ,2 ·Dep DepTot TotS R S RV RS R S R    (4) Figure 5 shows the result of a calibration performed on December 2nd 2016. As it can be noted, a spatial average is performed  Figure 5 System channel-ratio calibration performed on December 2nd 2016. The volume depolarization ratio is then computed        ** *90º ,90º ,V RRV R R  (5) Finally, the particle depolarization ratio is computed by using concurrently the results of the molecular  m R  and particle  p R   backscattering coefficients of the atmosphere under measure, via an elastic or Raman inversion of the total power 532-nm channel:                1 · · 11 · 1m V V mpm VR R R R RRR R R                                       (6) where:       m pmR RRR   (7) and  m R , the molecular volume depolarization ratio is usually considered a constant, which depends on the temperature and the spectral width of the photo-receiver [8]. 5 RESULTS  Two Saharan dust intrusion events took place around Barcelona area in late May and early June 2016. Measurements were performed with the UPC lidar and simultaneously with a co-located Micro Pulse Lidar, associated to MPLnet [9]. This Micro Pulse Lidar provides the range-corrected total power and depolarization signals and calculates the volume depolarization in real time. Figure 6 and Figure 7 show two consecutive measurements during May 26th. They show a Saharan dust layer around 3.7 km altitude, with particle depolarization ratio exceeding 20% (more than 15% is characteristic of this kind of aerosol). Both measurements show good agreement between the UPC lidar and the MPL.  Figure 6 Measurement performed on May 26th 2016, 14:24 UTC  Figure 7 Measurement performed on May 26th 2016, 15:40 UTC. Figure 8 and Figure 9 show measurements taken, respectively, in the beginning and in the maximum of an intrusion event in June 2016. Figure 8 shows a small value of volume depolarization up to 5 km, while Figure 9 shows a great mass of aerosol between 3 and 4 km height, 0 1 2 3 4 5 6 7 8 9012345Height AGL (km)System ratioBA161202a  From +45ºFrom -45ºAverageFiltered0 0.1 0.2 0.3 0.4 0.5012345Depolarization ratiosAltitude (km agl)  VpV(MPL)0 0.1 0.2 0.3 0.4 0.5012345Depolarization ratiosAltitude (km agl)  VpV(MPL)3EPJ Web of Conferences 176, 01029 (2018) https://doi.org/10.1051/epjconf/201817601029ILRC 28also with particle depolarization over 20%. In this latest measurement, there is a noticeable difference with the MPL measurement, very likely due to some mechanical tolerance in the telescope pointing not considered because of the absence of a calibration on the day of the measurement.  Figure 8 Measurement performed on June 7th 2016, 8:37 UTC.  Figure 9 Measurement performed on June 10th 2016, 13:17 UTC 6 CONCLUSIONS The new depolarization channel of the UPC lidar has been introduced with details of architecture and principle of operation. Measurements performed during two Saharan dust intrusion events are used to test the new channel. Particle depolarization ratios varying between 10 and 40 % are found in the dust layers. Comparisons of the volume depolarization with an MPL system show a good agreement between both systems and demonstrate the reliability of the new depolarization channel of the UPC multi-wavelength lidar.  ACKNOWLEDGEMENTS This work has been financed by the European Union (H2020, grant 654169, ACTRIS-2), the European Fund for Regional Development, the Spanish Government (grant TEC2015-63832-P) and the Catalan Government (grant 2014 SGR 583). The authors also acknowledge the priceless help of Joaquim Giner and Josep Pastor, both members of UPC technical staff.  References [1] K. Sassen, “Polarization in Lidar,” in Lidar, C. Weitkamp, Ed. New York: Springer-Verlag, 2005, pp. 19–42. [2] K. Sassen, J. Zhu, P. Webley, K. Dean, and P. Cobb, “Volcanic ash plume identification using polarization lidar: Augustine eruption, Alaska,” Geophys. Res. Lett., vol. 34, no. 8, Apr. 2007. [3] G. Wandinger, Ulla; Ansmann, Albert; Mattis, Ina; Müller, Detlef; Pappalardo, “Calipso and beyond: long-term ground-based support of space-borne aerosols and cloud lidar missions” in 24th International Laser Radar Conference, 2008, pp. 715–718. [4] V. Freudenthaler et al., “Depolarization ratio profiling at several wavelengths in pure Saharan dust during SAMUM 2006,” Tellus, Ser. B Chem. Phys. Meteorol., vol. 61, no. 1, pp. 165–179, 2009. [5] D. Kumar et al., “Six-channel polychromator design and implementation for the UPC elastic/Raman lidar,” SPIE Int. Symp. - Remote Sens. Eur., vol. 8182, p. 81820W–1–81820W–10, 2011. [6] G. P. Agrawal, Fiber-Optic Communications Systems, Third Edition., vol. 6. 2002. [7] E. Vidal, “Disseny d’un canal de despolarització a 532 nm per al lidar d’EARLINET de la UPC,” BarcelonaTech, 2013. [8] A. Behrendt and T. Nakamura, “Calculation of the calibration constant of polarization lidar and its dependency on atmospheric temperature,” Opt. Express, vol. 10, no. 16, p. 805, Aug. 2002. [9] NASA, “MPLNet,” 2017. [Online]. Available: https://mplnet.gsfc.nasa.gov/. [Accessed: 11-Jan-2017].  0 0.1 0.2 0.3 0.4 0.5012345Depolarization ratiosAltitude (km agl)  VpV(MPL)0 0.1 0.2 0.3 0.4 0.5012345Depolarization ratiosAltitude (km agl)  VpV(MPL)4EPJ Web of Conferences 176, 01029 (2018) https://doi.org/10.1051/epjconf/201817601029ILRC 28

A new depolarization channel has beenimplemented in the BarcelonaTech University(UPC) multi-wavelength lidar system. The opticaland mechanical designs are presented. The specialconfiguration of the total power channel is alsodetailed, with the relevant aspects in measurement inversion.  Some  preliminary  measurements  arepresented for Saharan dust intrusion events.Peer Reviewe

Vidal, Eric

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Depolarization channel for barcelona lidar. Implementation and preliminary measurements

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