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Light Detection And Ranging
Light Detection And Ranging (LIDAR) is a remote sensing technology that measures distance by illuminating a target with a laser and analyzing the backscattered light. In recent years lidar became popular as a technology to make high-resolution maps for geology, seismology, and forestry, among other areas. This innovative technology you can even find it in cell phone applications.
One of the first major applications of lidar was the mapping of the moon during the Apollo 15 mission in 1971. Soon after, lidar for meteorological applications was constructed to measure various components and parameters of the Earth’s atmosphere. Lidar technology has since expanded vastly in capability and systems that are used to perform a range of measurements, including profiling clouds, measuring winds, studying aerosols, and quantifying various atmospheric constituents and parameter from the troposphere to the lower thermosphere.
Principal of Operation
Lidar is an active remote sensing instrument that sends out a pulsed laser beam and receives the light scattered back from the atmosphere. At which distance the scattering process has occurred is calculated from the time elapsed between emission and detection of the light. A variety of information about the scattering medium can be derived from the intensity, spectral composition, and polarization of the scattered light, depending on the emitted light's properties. The scattering and absorption of the laser beam are proportional to the scatterers' density and absorbers in the atmosphere. The various types of lidar instruments are classified according to the atmospheric parameter measured or the scattering process or atmospheric constituent used.
Arecibo Lidar Facility
The work at the Arecibo Lidar Facility is mainly focused on the so-called mesosphere and lower thermosphere (MLT) region at around 85 – 115 km of altitude but also includes the stratosphere down to 30 km and ion observations as high as 160 km altitude. The MLT region is experimentally challenging to access as it is too high for aircrafts or atmospheric balloons and too low to fly satellites because of the atmospheric friction. This region of our atmosphere is where meteors ablate (burn up) because of their speed entering the earth atmosphere and the increasing density of the atmosphere. The meteor trail is leaving behind a permanent layer of metal atoms, molecules, and dust. Resonance lidar can detect the quantity of a given atomic species from the remains of meteors. Atmospheric constituents at these altitudes can provide useful information about temperature, composition, and chemistry. The atomic metal layers can also be used as a tracer to study the dynamics of the upper atmosphere and to understand processes from short-term variabilities to long-term seasonal variations for climatological purposes.
At AO, we have different kinds of resonance lidars, which are based on two different type of laser transmitters: (a) Nd:YAG lasers that pump dye lasers that are tuned to sodium (Na), calcium ion (Ca+) resonance wavelengths and can be configured in Rayleigh mode also, and (b) Alexandrite laser tuned to potassium (K) resonance wavelength, which gives both densities and temperatures in the MLT region.
Potassium Resonance Lidar
The transmitter is a pulsed Alexandrite Ring Laser injection seeded by an external cavity Diode Laser. The diode laser is locked to the central cross-over feature of the Doppler-free spectrum from the K D1 spectral line.
- The diode laser is frequency modulated by a dual-pass acousto- optic modulation setup developed by the lidar group at Colorado State University in order to get the spectral information necessary to measure potassium temperature in the mesopause.
- The receiver consists of an 800 mm diameter Cassegrain telescope that is fiber-optically coupled to receiver optics made up of an optical chopper to block low-altitude return, a narrow-band interference filter to reduce background, and an electronically gated GaAs photomultiplier detector.
Multi-metal Resonance and Rayleigh LidarNa/Ca+ Lidar
This lidar uses a pulsed dye laser pumped by a Nd:YAG laser for its transmitter. The fundamental output can operate in the yellow to near-IR in order to measure Na or K densities. This output can be doubled or mixed with residual IR from the Nd:YAG laser at 1064 nm in order to reach wavelengths for Fe, Ca, Ca+, or for DIAL studies of O3 near 310 nm.Rayleigh Lidar
Stratospheric and Mesospheric Climatology, Waves, and Tides: The Rayleigh lidar can profile atmospheric temperatures in the altitude range from 30 to 80 km. It can also profile winds to above 50 km. This makes it a powerful instrument in the study of middle atmospheric dynamics.