Cloud Physics Lidar Instrument
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Primary design considerations for the Cloud Physics Lidar included:
- Make the system as eye-safe as possible.
- Make the system as lightweight as possible, yet rugged.
- Use solid state photon-counting detectors.
In recent years there have been significant advances in the approach to lidar design. A now-proven approach to lidar design is to use a high PRF laser, multiple kHz rather than 10's of Hz, at low pulse energies. Ground based systems of this type, the MicroPulse Lidar, have been in use since the early 90's. The advantages of the high PRF design are the potential of eye safety from low pulse energies, more compact size, greater reliability, and turn-key ease of use. A basic requirement of the high PRF approach is a narrow field of view (along with narrowband filtering) to minimize solar background noise. The FOV is small enough to essentially eliminate multiple scattered signals. The data acquisition is simple photon-counting and the high PRF and pulse averaging results in wide dynamic range.
The overall instrument design is driven by the desire to use photon-counting detection. The system transmits three wavelengths (1064, 532, and 355 nm) simultaneously and collinear. Return signal collected by the telescope is separated by use of dichroics. The 1064 nm return is further separated into polarization components.
The 1064 nm detectors are Perkin-Elmer single photon counting modules (SPCMs). These detectors have approximately 3% quantum efficiency and low thermal noise. The 532 nm channel also utilizes EG&G SPCMs, and the quantum efficiency is approximately 60%. The 355 nm channel uses a photon-counting photomultiplier tube to allow for high quantum efficiency and larger dynamic range for the enhanced Rayleigh signal. Outputs from the detectors are counted by special multi-channel range-gating cards (scalers) made by Sigma Space Corporation.
The optical breadboard is housed in a sealed box to maintain a clean, thermally stable, and dry environment. For vibration isolation the breadboard mounts to the box via a three-point kinematic mounting system. All mechanical and structural design for the CPL was performed by Sigma Space Corporation.
System Parameters
| Parameter |
Value |
| Wavelengths |
1064, 532, and 355 nm |
| Laser type |
solid-state Nd:YVO4 |
| Laser repetition rate |
5 kHz |
| Laser output energy |
50 uJ at 1064 nm
25 uJ at 532 nm
50 uJ at 355 nm
|
| Telescope diameter |
8 inches |
| Telescope type |
off-axis parabola |
| Telescope field of view |
100 microradians |
| Filter bandwidth |
400 pm at 1064 nm
120 pm at 532 nm
150 pm at 355 nm
|
| Detector efficiency |
3% at 1064 nm
60% at 532 nm
10 % at 355 nm
|
| Raw data resolution |
1/10 second
(30 m vertical by 20 m horizontal)
|
| Processed data resolution |
1 second
(30 m vertical by 200 m horizontal)
|