lecture 36. lidar architecture and lidar...

LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 2016

Lecture 36. Lidar Architecture and Lidar Design

q  Introductionq  Lidar Architecture: Configurations & Arrangements

q  Lidar Design: Basic Ideas and Basic Principles

q  Considerations on Various Aspects of Lidar Design

q  Examples of Lidar Design and Development

q  Lidar Calibration

q  Summary

1


Introduction q  Lidar architecture is the art of lidar system instrumentation (including hardware and software).

2


Introduction q  Lidar design is to design a lidar system that meets the measurement goals. Lidar design is based on our understanding of the physical interactions and processes involved and utilizes the lidar simulations to assess the lidar performances, errors, and sensitivities.q  Lidar design includes (1) Choice of what type of lidar to use, based on measurement objectives (subject), measurement requirements (accuracy, precision, and resolution) and operation requirements (reliability, stability, operation difficulty), considering physical interactions and processes involved (how well we know the details), potential signal levels, and available hardware, etc.(2) Choice of what kind of wavelength, bandwidth, and diurnal coverage to use, based on potential return SNR, available hardware, etc.(3) Choice of what kind of laser, frequency control, receiver, filter, detector, DAQ to use, based on measurement requirements, available hardware, etc.(4) Design the lidar system based on above choices, and run simulations or basic tests or building prototypes to predict the lidar performance.

3


LIDAR Architecture

Transmitter(Light Source)

Receiver(Light Collection

& Detection)

Data Acquisition & Control System

4


MRI Fe Doppler Lidar Architecture [Chu et al., ILRC, 2010]

Telescopes (x 3)

ReturnSignals

ReturnSignals

Optical Fibers

LIDAR RECEIVER

FieldStop

CollimatingOptics

DoubleEtalons

InterferenceFilters

Chopper

PMT

FiberSwitcher

OutgoingLaser

744 nmECDL

Dual

Acousto-

Optic

Frequency

Shifters

Pulsed Alexandrite Ring Laser

SHG

Wavelength Meter

Freq. Monitor

372 nm

Fe Doppler-Free Saturation-Absorption

Spectroscopy

Build-upTime

Monitor

LaserBeam

Profiler

Reference

Cavity

Assembly

LIDAR TRANSMITTER

Beam

Expander

& Steerer

372 nmECDL

Electronic Control& Locking Unit

AmplifierPARL Oscillator

Scanning

FPI

AOMPhoto

Diode

Absolute Frequency Reference @ 372 nm (Fe)

Seed Laser @ 744 nmDual Acousto-OpticFrequency Shifters

Optical Heterodyne Detection & Lidar Pulse Control


Beat SignalSampling &Processing

Servo

Loop

372 nm744 nm

PD

744 nm

AOMReal-TimeBeat SignalSampling

Lidar PulseSpectral &

Chirp Analysis

Computer Systems

Signals

DATA ACQUISITION & CONTROL SYSTEM

Multichannel

Scaler Cards

Lidar ControlSoftware and

HardwareSuite

Pulse

DiscriminatorPre-AmplifierData BaseData Retrieval

Subsystem

DAQ

Program

Synchronization, Control, Monitoring, and Recording

5


Novel Architecture of LIDAR

Transceiver(Light Source and

Light Collection/Detection)

Data Acquisition & Control System

q  Transceiver is becoming more and more popular for compact lidars in mobile systems, like ground-based mobile lidars or airborne and spaceborne lidars for the lower atmosphere detection. Many fiber-based Coherent Doppler Lidars are using transceiver designs. 6


Lidar with Holographic Optical Element

Courtesy of Geary Schwemmer

7


LIDAR Configurations: Bistatic and Monostatic

Bistatic Configuration Monostatic Configuration

APulsedLaser

z

Δz

€

R = c ⋅ Δt 2

ReceiverTransmitter

8


LIDAR Arrangements: Biaxial and Coaxial

q  There are considerable amount of lidars using biaxial arrangements although they have monostatic configurations.q  In the biaxial arrangement, the laser beam and the receiver axis are separated, and the laser beam only enters the field of view of the receiver optics beyond some predetermined range.q  Biaxial arrangement helps avoiding near-field backscattered radiation that may saturate photo-detector.q  In the coaxial arrangement, the axis of the laser beam is coincident with the axis of the receiver optics. q  Therefore, the receiver can see the laser beam since the zero range bin. (There are debates on this point – depending on the telescope structure!)q  The near-field backscattering problem in a coaxial system can be overcome by either gating of the photo-detector or use of a fast shutter or chopper to block the near-field scattering.

9


Biaxial Arrangement

10


Coaxial Arrangement

11


ALOMAR RMR Lidar Transmitter q  The ALOMAR RMR lidar is optimized for maximum receiving signal to measure atmospheric temperatures, winds and aerosols simultaneously. Therefore, it is a complex twin-lidar system consisting of two power lasers, two receiving telescopes, and one optical bench for spectral separation and filtering of the light received from the atmosphere. q  The power lasers are pulsed Nd:YAG lasers emitting the fundamental (1064 nm), second (532 nm), and third (355 nm) harmonic wavelengths simultaneously. Both power lasers are seeded by a single external cw laser, which is frequency stabilized to iodine absorption spectroscopy, to generate laser pulses of high spectral stability. Using beam direction stabilization systems the laser beams are guided into beam widening telescopes (BWT) for reduction of the beam divergence by a factor of 20. After that the beams with 20-cm diameters are guided by a second set of beam direction stabilization systems into the atmosphere.q  For collection of the backscattered light two quasi-Cassegrain telescopes with 1.8-m primary mirrors are used which can be tilted up to 30◦ off-zenith while covering an azimuth range of 90◦ each. They are installed in such a way that one telescope is able to access the north-to-west quadrant (NWT), the other one the south-to-east quadrant (SET). The light received from the atmosphere is guided by optical fibers to the input of the optical bench. For investigations of the polarization characteristics of the light in the visible and ultraviolet spectral range, polarizers are integrated in the focal optics of the receiving telescopes. 12

[Fiedler et al, ILRC, 2008]


ALOMAR RMR Lidar Receivers q  At the input of the optical bench a rotating segmented mirror (fiber switch) is used to feed the light of both telescopes synchronized to the laser pulses into the single set of receiving optics. q  In the following, the light is separated and filtered by spectral range and intensity to produce 15 different channels: q  1064 nm (two channels, Rayleigh-/Mie-scattering on air molecules & aerosols), q  532 nm (three channels, Rayleigh-/Mie-scattering on air molecules & aerosols), q  355 nm (three channels, Rayleigh-/Mie-scattering on air molecules & aerosols), q  608 nm (two channels, N2 vibrational Raman-scattering excited by 532 nm), q  387 nm (one channel, N2 vibrational Raman scattering excited by 355 nm), q  530.4 nm and 529.1 nm (two channels, N2 + O2 rotational Raman-scattering excited by 532 nm). q  Two additional channels at 532 nm are placed behind an iodine absorption cell for analyzing the Doppler shift. q  Using photomultipliers (PMT) and avalanche photodiodes (APD) the light is converted into electrical signals which are altitude resolved by counters and processed and stored on a computer. [Fiedler et al, ILRC, 2008] 13


ALOMAR RMR Lidar @ Andoya

[Fiedler et al, ILRC, 2008] 14


Basic Ideas of Lidar Design q  The key of lidar design is the understanding of physical interactions and processes involved, the lidar simulations, and the choices of lidar type, configuration, arrangement, hardware and software to meet the measurement goals (subject, accuracy, precision, resolution, coverage).q  The basic procedure of lidar design includes (1) Study of physical interactions, processes, and spectroscopy for their applications in the lidar field. Study of lidar principles and technologies.(2) Choice of what type of lidar to use, based on measurement objectives and requirements (subject, accuracy, precision, resolution, reliability, stability, operation difficulty, etc).(3) Choice of what kind of wavelength, bandwidth, and diurnal coverage to use, based on potential return SNR, available hardware, etc.(4) Choice of what kind of laser, frequency control, receiver, detector, filter, and DAQ to use, based on measurement requirements, available hardware, etc.(5) Design the lidar system based on above choices, and run simulations or basic tests or prototypes to predict the lidar performance. 15


Considerations on Lidar Design q  What type of lidar: Mie, Rayleigh, Raman, resonance fluorescence, DIAL, coherent, direction-detection Doppler, fluorescence, rangefinder, altimeter, HSRL, white light lidar, etc. ?q  Bistatic or monostatic?q  Biaxial or coaxial?q  Geometrical overlapq  Uplooking or downlooking?q  Care about only scattering or only timing or both?q  Wavelength for transmitter and receiverq  Tunable or not?q  Bandwidth for transmitter and receiverq  Frequency stability for transmitter and receiverq  Power/energy consideration (especially in space-, balloon- or UAV-borne)q  Nighttime or full diurnal capability?q  Mobile or not?q  Volume, mass, cost, reliability, robustness, operation, etc.? 16


Further Considerations q  Multiple wavelengths or not?q  Doppler shift and how much?q  Polarization detection or not?q  Pulse repetition rateq  Beam divergenceq  Layer saturation (to Fe, Na, K, etc. layers or other atoms/molecules)q  Photo detector dynamic rangeq  Bin width and resolutionq  Record every pulse or not?q  Record system parameters or not?q  Timing control (how accurate and how precise it must be?)q  Need precise beam point control or not?q  Need real time data reduction or not?q  Eye safe or not? … …

17


Choice of Lidar Types q  To choose the right type, we have to know how many types of lidars are available and the capabilities and limitations/issues of each type of lidar. This is why we have gone through all types of lidars in our class to give you a comprehensive overview.q  The choice of lidar type is mainly driven by the measurement goals, available expertise, and available hardware.Ø  Mie and Rayleigh lidars (polarization discrimination or not)Ø  Pure Rotational Raman lidarØ  Vibrational-Rotational Raman lidarØ  Differential absorption lidarØ  Broadband resonance fluorescence lidarØ  Narrowband resonance-fluorescence Doppler lidarØ  Coherent Doppler lidar (heterodyne and homodyne)Ø  Direct-detection Doppler lidar (interferometer and absorption based)Ø  High-spectral-resolution lidar (interferometer and absorption based)Ø  Fluorescence lidar (range resolved or not)Ø  Laser range-finder and laser altimeterØ  White light lidar or optical comb lidar or lidar+spectrometer … ... 18


Capabilities and Limitations q  Mie/Rayleigh lidar: aerosol/cloud occurrence, geometry, size, shape (with polarization and multi-wavelength detection), density; atmospheric density and temperature (with Rayleigh integration technique) in aerosol-free region, …q  Pure-Rotational Raman lidar: temperature in lower atmosphere, aerosols, speciesq  Vibrational-Rotational Raman lidar: temperature in lower atmosphere when aerosols present; species; help derive aerosol extinction and backscatter coefficientsq  Differential absorption lidar: various species in lower atmosphere, temperatureq  Broadband resonance fluorescence lidar: various species and/or temperature in MLT (Boltzmann), Rayleigh temperature above 30 km, aerosols/cloudsq  Narrowband resonance-fluorescence Doppler lidar: various species, temperature and wind in MLT, Rayleigh temp (& wind) above 30 km, aerosols/cloudsq  Coherent Doppler lidar: high-resolution winds, aerosols in the lower atmosphereq  Direct-detection Doppler lidar: wind and/or temperature in lower & middle atmosq  High-spectral-resolution lidar: aerosol optical properties, wind, or tempq  Fluorescence lidar: species in liquid or solid statesq  Laser range-finder and laser altimeter: range and altitude determinationq  White light lidar: simultaneous detection of multiple species

19


Configuration, Arrangement, Direction q  Most modern lidars (consisting of pulsed lasers) use monostatic configuration with either biaxial or coaxial arrangement. Lidars consisting of cw laser transmitters usually take bistatic configuration, in order to distinguish range, if coding technique is not applied.q  The choice of biaxial or coaxial arrangement is usually determined by the detection range. If near-field range is desired, coaxial arrangement is preferred as it provides full overlap of receiver field-of-view with laser beam. If near-field range is not desired, biaxial arrangement may help prevent the saturation of photo-detector by strong near-field scattering. Scanning capability can also come into play for the selection of biaxial or coaxial.q  Groundbased lidars are usually uplooking, while spaceborne lidars are usually downlooking. Airborne lidars can be either uplooking or downlooking, depending on application needs.q  The reason to care about up- or down-looking is the fact that atmospheric density decreases with altitude nearly exponentially. So the signal strength for up- or down-looking lidars will be quite different.

20


Wavelength Considerations q  Many factors determine the wavelength selection: Ø  First, the detection subject - whether a specific wavelength is required, e.g., Na or Fe atomic transition wavelength, or H2O differential absorption wavelength, or multiple species of interests. Ø  Second, signal-to-noise ratio considerations: Rayleigh (λ-4), Mie (λ-2 to λ): e.g., Coherent lidar (Mie vs Rayleigh); VR Raman lidar (N2 vs. O2)Ø  Third, transmission of laser light through the medium (e.g., atmosphere or water). Ø  Fourth, the solar background intensity - low solar radiation is desirable to benefit signal-to-noise ratio (SNR) in daytime. Usually UV solar radiation is lower than visible and IR. Fraunhofer lines may be utilized.Ø  Fifth, available hardware (wavelength vs. power/energy) is often to be a major limitation. Lasers, photo detectors, optics, opto-electronics, etc.Ø  Another important factor in determining wavelength is eye-safety. UV and far IR are safer for people because our eyes cannot focus the light with wavelengths in these regions. Our eyes have much higher damage threshold in these wavelengths than visible light or near IR. 21


Fraunhofer Lines q  Fraunhofer lines are named after the German physicist Joseph von Fraunhofer (1787-1826). Fraunhofer lines in solar radiation are a set of several hundred dark lines appearing against the bright background of the continuous solar spectrum. They are produced by absorption of light by the cooler gases in the Sun's outer atmosphere at frequencies corresponding to the atomic transition frequencies of these gases, such as atomic H, Fe, Na, K, Ca, Mg, Li, etc, or by oxygen of the Earth’s atmosphere. q  Lidar operating at the wavelengths in deep Fraunhofer lines benefits from the lower solar background for daytime operations.

0

2000

4000

6000

8000

10000

371.4 371.6 371.8 372.0 372.2 372.4 372.6

Inte

nsity

Wavelength (nm)

Fe Fraunhofer LineAbsorption

Feature

Solar Spectrum at Ground Level

http://bass2000.obspm.fr/solar_spect.php?PHPSESSID=b1fb4b5e30286b1698ae5b2f330b3318 22


Bandwidth Considerations q  Possible combinations of transmitter and receiver

BroadbandTransmitter

BroadbandReceiver+

NarrowbandTransmitter

BroadbandReceiver+

NarrowbandTransmitter

NarrowbandReceiver+

BroadbandTransmitter

NarrowbandReceiver+

Conventional Mie, Rayleigh, Raman Scatter lidar, Broadband Resonance Fluorescence lidar, Differential Absorption lidar

Narrowband Resonance Fluorescence Doppler lidar,Differential Absorption lidar

Coherent Doppler lidar, Direct-Detection Wind lidar, Rayleigh Doppler lidar.High-Spectral-Resolution lidar

Potential broadband resonance fluorescence temperature lidar,Potential Rayleigh and Raman temperature lidar 23


Nighttime-Only & Full Diurnal q  This is mainly a consideration on background suppression to ensure sufficient signal-to-noise ratio (SNR).q  Even for nighttime-only operation, interference filters are necessary to suppress background (like moon or star or city light) and ensure safe operation of photo detectors.q  Daytime operation needs extra suppression on much stronger solar background. Usually extra spectral filters with very narrow bandwidth are needed. Two major narrowband spectral filters: F-P etalons and atomic/molecular spectral filters (like Faraday filter or iodine filter).q  Spatial filter or minimized field-of-view (FOV) is also very necessary to largely suppress the solar background. Of course, this may be limited by layer saturation, geometrical overlap and alignment issues.q  FOV usually should be larger than the laser beam divergence to ensure that the receive sees the full lidar beam. When a tight FOV is used, active alignment/stabilization (beam steering) system may be necessary to ensure the FOV contains the full beam at all times.

24


Transmitter & Receiver q  Depending on application needs and lidar types, there may be several possible combinations of transmitter and receiver to satisfy the same goal. Choose the best one depending on science need, technical feasibility, cost, performance, reliability, etc. q  To choose tunable lasers or not depends on the application needs, e.g., resonance fluorescence and DIAL lidars usually need to be tunable, while conventional Mie, Rayleigh, and Raman scattering lidars can use fixed wavelengths.q  Selection of pulse energy, repetition rate, and duration time, mainly concerns the SNR, measurement resolution, as well as cost, volume, mass, etc. to the entire system, along with eye safety.q  Selection of telescope area, type, configuration; detector type, size, quantum efficiency, maximum count rate; filter type, size, bandwidth, transmission, mainly concerns the SNR, measurement resolution, as well as cost, volume, mass, etc. to the entire system.

25


Major Research Instrumentation (MRI) Fe/Rayleigh/Mie Doppler Lidar

q  Why choosing Fe Doppler lidar? Science needs are the main motivations.-- High product of abundance, cross-section, power and aperture for the best measurements of temperature and wind (high SNR)-- Large measurement range (30 to 155 km) considering strong Rayleigh scattering-- Interesting science questions on Fe chemistry, dynamics and altitude coverageq  Pulsed Alexandrite Ring Laser (PARL) and frequency control-- Tunable and solid-state laser for mobile-- Bias-free and single pulse frequency controlq  Wavelength selection (372 nm)-- Larger (>2) effective cross-sectionq  Narrowband laser + broadband receiver leading to the best accuracy and precisionq  3-frequency tech. for the highest resolutionq  Choice of receiver, filter, DAQ: Newtonian telescopes for low costq  Platform: Lidar containers for mobile deployment and modular designsq  Potential expansion in the future: Candidate for Whole Atmosphere Lidar 26

z5Fo

a5D

3d64s4p

3d6 4s2 0.000 cm−1

415.932

26874.549

27166.819

cm−1

cm−1

cm−1

372nm 374nm

ΔE ≅ 416cm−1

D5 4

D5 3

F5 5oF5 4o

D5 4o

z 5Do3d64s4p

D5 3o

25900 cm−126140 cm−1

386nm 389nm


MRI Fe Doppler Lidar Design [Chu et al., ILRC, 2010]

Telescopes (x 3)

ReturnSignals

ReturnSignals

Optical Fibers

LIDAR RECEIVER

FieldStop

CollimatingOptics

DoubleEtalons

InterferenceFilters

Chopper

PMT

FiberSwitcher

OutgoingLaser

744 nmECDL

Dual

Acousto-

Optic

Frequency

Shifters

Pulsed Alexandrite Ring Laser

SHG

Wavelength Meter

Freq. Monitor

372 nm

Fe Doppler-Free Saturation-Absorption

Spectroscopy

Build-upTime

Monitor

LaserBeam

Profiler

Reference

Cavity

Assembly

LIDAR TRANSMITTER

Beam

Expander

& Steerer

372 nmECDL


AmplifierPARL Oscillator

Scanning

FPI

AOMPhoto

Diode

Absolute Frequency Reference @ 372 nm (Fe)

Seed Laser @ 744 nmDual Acousto-OpticFrequency Shifters

Optical Heterodyne Detection & Lidar Pulse Control


Beat SignalSampling &Processing

Servo

Loop

372 nm744 nm

PD

744 nm

AOMReal-TimeBeat SignalSampling

Lidar PulseSpectral &

Chirp Analysis

Computer Systems

Signals

DATA ACQUISITION & CONTROL SYSTEM

Multichannel

Scaler Cards

Lidar ControlSoftware and

HardwareSuite

Pulse

DiscriminatorPre-AmplifierData BaseData Retrieval

Subsystem

DAQ

Program

Synchronization, Control, Monitoring, and Recording

27


Optical Heterodyne Detection of Laser Pulse

−200 0 200 400 600−0.02

0

0.02

0.04

0.06

0.08

Time (ns)

Inte

nsity

(a.u

.)

150 200 250 300−0.02

0

0.02

0.04

0.06

0.08

Time (ns)

Inte

nsity

(a.u

.)

200 220 240 260 280−0.02

0

0.02

0.04

0.06

0.08

Time (ns)

Inte

nsity

(a.u

.)

220 240 260−0.02

0

0.02

0.04

0.06

0.08

Time (ns)

Inte

nsity

(a.u

.)−700−500−300−100 100 300 500 700 900 1100

−0.04

−0.02

0

0.02

0.04

0.06

0.08

Time (ns)

Inte

nsity

(a.u

.)

190 200 210 220 230 240 250 260 270−1

0

1

2

3

4 x 10−3

Beat Frequency (MHz)

Pow

er D

ensi

ty (a

.u.)

−50 −40 −30 −20 −10 0 10 20 30−1

0

1

2

3

4 x 10−3

Frequency Chirp Relative to Seed Laser (MHz)

Pow

er D

ensi

ty (a

.u.)

−200 0 200 400 600−0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

Inte

nsity

(a.u

.)

−200 0 200 400 600

−20

−10

0

10

20

Time (ns)

Freq

uenc

y C

hirp

(MH

z)

[Chu and Huang, ILRC, 2010]

28


Lidar Simulations

Simulated MRI lidar photon count returns for both day and night measurements and simulated MRI Doppler lidar measurements of temperature and wind for 1-km resolution and 1-min integration at an off-zenith angle of 35° in nighttime configuration. The errors are less than 1 K and 1 m/s at the Fe layer peak. Comparable features can be achieved with 10-min integration in daytime conditions.

29


MRI Fe Doppler LIDAR – First Light

30

Containerized MRI lidar – hard to take photosCome to Table Mountain to see it by your own eyes!


ST

AR

Lid

ar D

esig

nBNC

Oscilloscope

Acousto-Optic Modulation

Master Oscillatorλ/2

λ/2

λ/4

CW532nmPumpLaser

10cm AOMAOM20cm 20cmShutter IrisCollimator

Periscope

Opticalisolator

Frequency-DoubledNd:YAGPulsed,Q-SwitchedLaser

Single-ModeRingDyeLaser

PulsedDyeAmplifier

Photodiode

Doppler Free SpectroscopyHotNaVaporCell

Wave-meterSMFiber

NDFilter

ToSky

AOMDriver

RingDyeLaserControlBox

ActuatedSteeringMirror

>LockingFeedback>

ShutterDriver

CMOS

532 nm1064 nm

<CameraControl(USB)>

Receiver Data Acquisition & Control

SHG

Transmitter

TTL

PC

PMT

Newtonian

<MirrorControl(RS-485)>

DiscriminatedPMTSignal

>Q-SwitchTrigger>Analog/Digital/CounterChannel Connections

Based on a presentation by Smith, Fong, et al. at CEDAR 2012


STAR Lidar Timeline

20122011201017 28 39 4101112 56 17 28 39 4101112 566 7

TransmitterPulse energy Pulse rep. rate Pulse divergence angle AOM shifting frequency Transmission of lens surface

15-18 mJ 30 Hz

840 µrad 480 Hz

3 @ 95 %

June2010,STARlidarlaunchedatCUBoulder,TableMountainLidarObservatory,witha16''trueCassegraintelescopeasreceiver.


FromAugusttoSeptember2010,wehavesimultaneousobservationofNaandFe.TheNalayerlevelisabout15-20countpershot.

f0f+f-

20122011201017 28 39 4101112 56 17 28 39 4101112 566 7

16''CassegrainTelescope


20122011201017 28 39 4101112 56 17 28 39 4101112 566 7

16''CassegrainTelescope

InMarch2011,photoncountwasimprovedbyadjustingthepositionofthefiber.Thephotoncountreached50-60countpershot.


Test32''Newtoniantelescope,modifieditforSTARlidarreceiver.Secondarymirrorwasremoved.TheinitialtestofNalevelwasabout140-200countpershot.

20122011201017 28 39 4101112 56 17 28 39 4101112 566 7

f0f+f-


f0f+f-

Meanphotoncount:507Peakphotoncount:807

20122011201017 28 39 4101112 56 17 28 39 4101112 566 7

Primefocusreceiver0 10 20 30 40 50 60 70 80100

101

102

103

Altitude [km]

Cou

nts

Simulation, Aug. 7, 2011, 10:01 UTC

Simulation (Ta=83%)

Averaged Data

ThesimulationshowsgoodagreementwithobservationsatRayleighregionfrom50-70km,assuming83%fortheloweratmospheretransmission.Suggestingthatpeakperformanceisattained.

[Smithetal.,2012]

32''NewtonianTelescope


1.5 mm

Fiber Coupled Configuration

Micrometer

Crossbar

Z-Translation Stage

XY Translation Stage

Fiber

Thorlabs lens tube thread

Pre-fibersetup

PMT Module

Adapter Lens Condenser IF

Pho

toca

thod

e

Lens Tube

Fiber Window

Post-fibersetup

[Smithetal.,2012]


20122011201017 28 39 4101112 56 17 28 39 4101112 566 7

f0

f+f-

Meanphotoncount:1146Peakphotoncount:1504

SwitchbetweenPMTandfibercouplingconfigurations.Withthecarefulalignment,thetwoconfigurationsgavesimilarphotoncountpershot.



20122011201017 28 39 4101112 56 17 28 39 4101112 566 7

Reachover2000countpershotinDec.2011.


Pulseenergy=17mJPrimarydia.=810mmPeakNaphotoncount:2126cnt/shot

f0f+f-

ZEMAXopticaldesignwasakeyinimprovinglidarreceiverefficiencysignificantly.PaperwillbesubmittedtoAppliedOptics[Smithetal.,2012].


Lidar Calibration q  Lidar calibration is a difficult issue for cases when we try to push technology/measurement envelope, because existing instruments have not been able to achieve what you design to achieve. For these cases, you have to fully understand your own system and the entire lidar sensing procedure, including every possible interaction or process involved, and then do a thorough analysis on all possible measurement errors (accuracy, precision, resolution, and stability). Primary vs. secondary atomic clocksq  Understanding your own system and entire procedure is also the key for all cases of lidar calibration. A self-calibration must be made before cross-calibrations with others.q  Make sure your lidar system and data processing are human-error-free!q  In all cases, try to find any possible existing measurements (even not as accurate or resolution as yours) and theoretical/model predictions, and then compare your measurements with them to figure out the similarity and differences. Then analyze the reasons why so.q  Try to operate your lidars with an existing lidar or lidars or other instruments simultaneously and in common-volume, and then compare the measurement results. Be aware of the limitation of each instrument. 40


Lidar Calibration q  Design your measurements so that you can have some internal calibration or at least do some reality check. For example, temperature profile is usually stable but wind is highly variable. Simultaneous temperature and wind measurements can help determine whether the measurements make sense.q  Before the full system calibration, you may want to calibrate each individual pieces, e.g., PMT, filter, laser, etc. Is your PMT or APD saturated? How is your filter function like and is it stable? How is your laser lineshape like and is it stable? Is there any component in your lidar having day-to-day variability? q  For spaceborne or airborne lidars, it may be necessary to set up some ground-based calibration points. Flight over-passes some ground-based lidar stations for simultaneous and common-volume measurements or over-pass some known objects for altimeter calibration.q  If possible, compare with some in-situ measurements.

41


Summary q  Lidar architecture is the art of lidar instrumentation, concerning the lidar hardware and software, lidar configuration and arrangement, etc.q  Lidar architecture consists of lidar transmitter, receiver, and data acquisition and control system. Some have merged transceiver. Basic lidar configurations are bistatic and monostatic configurations. Basic lidar arrangements are biaxial and coaxial arrangements.q  Learning existing lidar systems is a good approach to understand the lidar architecture in depth, especially experiences and issues. It will help the design of a new lidar system.q  Lidar design is based on the understanding of physical interactions and processes involved, the lidar simulations, and the choices of lidar type, configuration, arrangement, hardware and software to design a lidar that meets the measurement goals (subject, accuracy, precision, resolution, reliability, coverage, etc).

42


Summary q  Besides basic architecture, configuration, and arrangement, more considerations should be given to the selection of wavelengths (specific request and solar spectrum intensity), bandwidth of transmitter and receiver (application needs - spectral resolved or not, nighttime-only or full diurnal cycle), laser power/energy, repetition rate, pulse duration time, receiver area, detector efficiency and capability, data acquisition software, and system timing and coordination control. Cost, volume, mass, reliability, etc will also be important when come to reality.q  Careful lidar simulations, laser design, optical design, frequency control, receiver design, DAQ design, etc. are the key to achieve the best performance of a lidar. ZEMAX optical modeling is shown to be a powerful tool to improve lidar optical efficiency.q  Lidar calibration is an important but challenging issue. Thorough understanding of your own lidar system and the entire lidar sensing procedure is the key step to calibrate your lidar. Then comparison with other lidars or other instruments is usually necessary for cross-calibration and at least reality check.

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lecture 36. lidar architecture and lidar...

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