Lecture 08. Fundamentals of Lidar Remote Sensing (6) Basic Lidar Architecture q Basic Lidar Architecture q Configurations vs. Arrangements q Transceiver with HOE q A real example: STAR Na Doppler Lidar q Another example: MRI Fe Doppler Lidar q Summary 1
Physical Picture of Lidar Equation β(λ,λ L,θ,R) ΔR T(λ L,R) T(λ,R) N L (λ L ) η(λ,λ L )G(R) A R 2 2
Basic Architecture of LIDAR Transmitter (Light Source) Receiver (Light Collection & Detection) Data Acquisition & Control System 3
Function of Transmitter q A transmitter is to provide laser pulses that meet certain requirements depending on application needs (e.g., wavelength, frequency accuracy, bandwidth, pulse duration time, pulse energy, repetition rate, divergence angle, polarization, etc). q Usually, transmitter consists of lasers, collimating optics, diagnostic equipment, wavelength control system, and beam steering. q For sophisticated lidars with spectral analysis capabilities, the lidar transmitter is usually the most challenging part of a lidar. The properties of the lidar transmitter determine the performance of the lidar system. q Most modern lidars use nano-second (ns) pulsed lasers, while some uses cw lasers with bistatic configuration or pulse coding, and some uses femto-second (fs) lasers. 4
Function of Receiver q A receiver is to collect and detect returned photon signals while compressing the background noise. Usually, it consists of telescopes, filters, collimating optics, photon detectors, pulse discriminators, polarization discrimination, optical fibers, etc. q The bandwidth of the filters determines whether the receiver can spectrally distinguish the returned photons or is just used as a photon bucket. q Lidar receivers have undergone significant development over the last 20 years, especially for narrowband spectral analyzers, multi-channel spectral detection, and multi-pixel array, etc. q Depending on the optical and mechanical designs, the efficiencies of receivers can vary dramatically. Recent paper by Smith and Chu [2015] in Applied Optics provides a good example of how to achieve high-efficiency receivers -- http://cires1.colorado.edu/science/groups/chu/pubs/documents/ 2015AO_SmithChu_HighEfficiencyLidarArchitecture.pdf 5
Function of Data Acquisition and Control System q Data acquisition and control system are to record returned data and corresponding time-of-flight, provide system control and coordination to transmitter and receiver. q Usually, it consists of multi-channel scalers which have very precise clock so can record time precisely, discriminator, computer and software. q This part has become more and more important to modern lidars. Recording every single pulse return has been done by several groups, enabling various data acquisition modes. q DAQs have also been developed for remote operation of lidars without attendance. 6
LIDAR Configurations: Bistatic vs. Monostatic q Bistatic configuration involves a considerable separation of the transmitter and receiver to achieve spatial resolution in optical probing study. q Monostatic configuration has the transmitter and receiver locating at the same location, so that in effect one has a single-ended system. The precise determination of range is enabled by the nanosecond pulsed lasers via time of flight (TOF). q A monostatic lidar can have either coaxial or biaxial arrangement. 7
Basic Configurations of LIDAR Bistatic and Monostatic Δz R = c Δt 2 z Transmitter Receiver Bistatic Configuration Pulsed Laser A Monostatic Configuration 8
Coaxial vs. Biaxial Arrangements q In a coaxial system, the axis of the laser beam is coincident with the axis of the receiver optics. q In the biaxial arrangement, 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 saturating photo-detector. 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. 9
Biaxial Arrangement 10
Coaxial Arrangement 11
Fancy Architecture of LIDAR Transceiver (Light Source Light Collection Lidar Detection) Data Acquisition & Control System Transceiver with holographic optical element (HOE) Courtesy to Geary Schwemmer 12
STAR Na LIDAR Schematic Receiver PMT Discriminated PMT Signal PC Data Acquisition & Control < Mirror Control (RS - 485) > < Camera Control (USB) > Newtonian > Q- Switch Trigger > TTL B N C > Locking Feedback > Analog/Digital/Counter Channel Connections Transmitter SHG 532 nm 1064 nm Frequency- Doubled Nd:YAG Pulsed, Q- Switched Laser Shutter Driver Oscilloscope Doppler Free Spectroscopy AOM Driver Photodiode Hot Na Vapor Cell ND Filter SM Fiber Wave- meter λ/2 λ/4 Shutter 20 cm AOM 10 cm AOM 20 cm Acousto-Optic Modulation Periscope Pulsed Dye Amplifier λ/2 To Sky CMOS Optical isolator Master Oscillator CW 532 nm Pump Laser Collimator Single- Mode Ring Dye Laser Iris Ring Dye Laser Control Box Actuated Steering Mirror [J. A. Smith, W. Fong, W. Huang, and X. Chu, University of Colorado] 13
Ring Dye Laser ICA 589nm 532nm Brewster Plate From Verdi 1. Four mirror + Dye jet form the laser resonance cavity. 2. Unidirectional lasing prevents spatial hole-burning. 3. Rhomb compensates the astigmatism effect. 4. Optical diode forces the unidirectional lasing. 5. BRF + ICA (etalons) select frequency and narrow bandwidth. 6. Brewster plate + RCA + M3 PZT actively control frequency. 14
Na Doppler-Free Spectroscopy & Laser Frequency Lock e a D 2a D 2b b 3-Level Explanation 15
Acousto-Optical Modulator Hardware Mirror Shutter L 3 AO Shifter + L 2 To PDA AO Shifter _ L 1 Polarized Beam Splitter Ring Laser Light λ/4 Waveplate Shutter Drive CH1 Crystal Oscillator 315MHz AO Driver CH2 λ/2 Waveplate Optical Isolator Explanation: Doppler shift or Photon/Phonon Annihilation Diffracted Beam Ultrasonic Transducer θ Electric Input λs AO Crystal θ Sound wave fronts Incident Beam Diffracted Beam Ultrasonic Transducer θ λs Sound wave fronts Electric Input AO Crystal θ Incident Beam (b) (a) 16
Pulsed Amplification Fast Photo-Diode Linear Phase Amplifier Digitizer Computer λ/2 cw PBS AOM Mirror ND Filter Optical Isolator λ/2 Injection-Seeded Frequency-Doubled Nd:YAG Laser CL1 CL2 PDA DC1 DC2 DC3 1. Amplified Spontaneous Emission (ASE) 2. Injection-seeded Nd:YAG laser 3. PDA chirp caused by pulsed amplification 17
LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 2016 STAR Na Doppler Lidars Na layer bottomside Turbulenc (?) on 8 Dec 2011 @ Boulder 82 81 Altitude [km] 80 79 78 77 76 Δz = 24 m; Δt = 3 s 75 74 52 53 54 55 56 57 58 59 UTC past midnight [min] Na Doppler Lidar Table Mountain Lidar Facility @ Boulder 18
LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 2016 MRI Fe Doppler LIDAR First Light Containerized MRI lidar hard to take photos Come to Table Mountain to see it by your own eyes! 19
Summary q Basic lidar architecture includes transmitter, receiver and data acquisition and control system. Each has special functions. There are bistatic and monostatic configurations, and coaxial and biaxial arrangements. q Two real lidars are used as examples to examine the basic concepts of lidar picture and lidar architecture. q High level lidar systems are sophisticated, mainly on the transmitter (laser) aspect. But receiver and DAQ also strongly affect system performance. q We will offer field tours this semester to the Table Mountain Lidar Facility for students in our class. Please contact John Smith, Cao Chen, Jian Zhao, and Yuli Han for the tours. 20