The Potassium lidar
The potassium lidar is designed to probe the terrestrial potassium atom layer using the K(D1) resonance wavelength. Over the distance of 80 - 110 km it performs a remote spectroscopy of the potassium layer. Thus, scanning the Doppler broadened hyperfine structure it measures a height resolved picture of the thermal movement of the atoms. From this picture the temperature profile can be calculated.
The heart of the transmitting unit as shown in the right column is a specially developed solid state alexandrite laser, emitting light pulses with the necessary properties. The wavelength is controlled by the seederlaser and measured for every single laser pulse with the spectrumanalyser. Since 2000 the receiving unit is equipped with a daylight filter which allows measurements under full daylight conditions.
Transmitter
| Laser active material | 2 alexandrite crystals, Cr3+ :BeAl2O4 |
| Geometry | ring resonator, unidirectional |
| Wavelength | 770 nm, tunable |
Mode structure | TEM00 |
| Bandwidth | << 20MHZ (FWHM) |
| Polarisation | linear |
| Repetition rate | 35 pulses per second |
| Pulseenergy | 150 mJ |
| Energy jitter | 5% (1σ) |
| Pulse length | 250 ns (FWHM) |
| Beam width | 2.5 mm, behind AWT 6.7 mm |
| Beam divergence (full angle) | 0.45 mrad, behind AWT 0.3 mrad |
Receiver
| Telescope | d = 80 cm, f = 199.5 cm |
| Field of view | 192 µrad |
| Broad band filter | IF filter (1nm, Τ=70%) |
| Narrow band filter | 2 potassium FADOF (1800 G), 124° C, T = 75% |
| Transmission | 50 % |
| Contrast (FADOF) | 1:10000 |
| Band width (FWHM) | 6.7 pm |
| Temperature shift | ±0.5 K |
| Detector | APD |
| Photon efficiency | 68% @ 770 nm |
Publications
- J. Lautenbach, J. Höffner, F.-J. Lübken, M. Kopp und M. Gerding, Ten-year climatology of potassium number density at 54°N, 12°E, J. Atmos. Solar-Terr. Phys., 162, 172-177, doi:10.1016/j.jastp.2017.06.010, 2017.
- C. Fricke-Begemann, M. Alpers und J. Höffner, Daylight rejection with a new receiver for potassium resonance temperature lidars, Opt. Lett., 27(21), 1932-1934, 2002.
- J. Höffner und C. Fricke-Begemann, Accurate lidar temperatures with narrowband filters, Opt. Lett., 30(8), 890-892, 2005.
