METR Advanced Atmospheric Radiation Dave Turner Lecture 11

What is happening here? MPL and CAPABL laser beams above Summit, Greenland Photo by Ed Stockard

Active Optical Remote Sensing Before advent of the laser, searchlights were used as the optical source – Aerosol scattering measurements performed in the 1930s

Hulburt ’ s Searchlight Observations Journal of the Optical Society of America, 1937

Active Optical Remote Sensing Before advent of the laser, searchlights were used as the optical source – Aerosol scattering measurements performed in the 1930s Laser invented in 1960

Creation of the “ Death Ray ” LASER (light amplification by stimulated emission of radiation) was a very hot research topic in late 1950s due to a theoretical paper First working laser built by Theodore Maiman, a junior researcher at Hughes Research in May 1960 Maiman ’ s budget was very miniscule ($50 K) Pursued the use of ruby as the lasing medium, even after the “ experts ” had long ruled out its candidacy Maiman ’ s first paper on a working laser was rejected! Battle over patent rights to the laser was waged for over 20 years

Theodore Maiman

Laser 101 Medium is “ pumped ” with external energy, exciting the photons in the medium to higher energy levels A few atoms in the medium relax and emit photons These photons cause the emission of more photons from adjacent atoms Photons which run parallel to medium bounce back and forth off of the mirrors and energy builds Monochromatic single- phase collimated light exits partially silvered mirror Q-switch increases power by acting as a door

Active Optical Remote Sensing Before advent of the laser, searchlights were used as the optical source – Aerosol scattering measurements performed in the 1930s Laser invented in 1960 First laser remote sensing measurements were in May echoes recorded from the moon The real advance in laser remote sensing came with the invention of the Q-switch

Photon Detection Energy scattered back to lidar from atmosphere is (typically) weak Need way to amplify the signal to make it detectable Photomultiplier tubes provide one way (other methods are similar and different) Works together with a “discriminator” to actually count the number of photons impinging upon the photocathode

Type of Eye Damage Depends on Wavelength

Maximum Permissible Exposure (MPE) MPE values are much smaller for visible wavelengths! Gases/particles in atmosphere often have relatively weak scattering signals, so temptation is to increase laser energy to increase signal

Ways to Make Lidars Eyesafe Transmit smaller laser pulse energies Expand the outgoing beam size to reduce energy density Turn off beam when something gets within FOV of the lidar (using radar)

Single Channel Lidar Equation Two-way transmission Backscatter “Detected” signal Laser output power Telescope area Range squared loss of signal System efficiency Overlap function Background “Afterpulse”

Corrections / Calibrations Dead-time (pulse pileup) Afterpulse Background subtraction Overlap Calibration

Observed Signal in Photons Counted [MHz] Range [km] ARM Raman Lidar Signals (10-s, 7.5-m averaging) H 2 O Raman Channel N 2 Raman Channel Combined Channel Background in H 2 O channel Background in N 2 channel Background in Combined channel Overlap

Observed Signal in Photons Counted [MHz] Range [km] H 2 O Raman Channel N 2 Raman Channel Combined Channel ARM Raman Lidar Signals (10-s, 7.5-m averaging) Afterpulsing in combined channel (due to reflection off protective window above telescope)

Overlap Geometrical near-field effect Depends on field of view (FOV) of receiver, laser beam divergence, focus range of the receiver, distance between the transmitter and receiver (if bistatic), other geometrical issues…

Many Types of Lidar Single channel elastic lidar – E.g., micropulse lidar, ceilometer Raman lidar High spectral resolution lidar (HSRL) Doppler lidar Polarization lidar

Raman Scattering Cross-Sections Energy shift for H2O Raman scattering: 3652 cm -1 Energy shift for N2 Raman scattering: 2331 cm -1

Principle behind Rotational Raman Lidar

ARM Raman Lidar Aft-Optics

Principle Behind HSRL Systems

Lidar Single Channel Lidar

The HSRL and Raman Technique Molecules 2 Equations 2 Unknowns Molecular channel “Combined” channel

Example Signals in Cirrus Combined Channel Molecular Channel Computed Molecular Backscatter (arb)

Overlap Correction When Taking Ratio of Two Signals Many lidar-derived products are actually ratios of two different signals – E.g., water vapor mixing ratio from Raman lidar is ratio of S h2o (z) / S n2 (z) The “overlap” of this ratio is the ratio of the two overlap functions If the two channels are well-aligned to each other, then the overlap functions would reduce to unity – Not often true in reality…

HSRL Data Combined Channel Only (E.g., like a single-channel lidar)

HSRL Data Backscatter Coefficient (derived from ratio of combined to molecular channel)

HSRL Data Depolarization Ratio (derived by ratio of cross-polarized to co-polarized combined return)

Example Raman Lidar water vapor mixing ratio data