The ALOMAR Rayleigh/Mie/Raman lidar

The Rayleigh/Mie/Raman (RMR-)Lidar has been installed on the island of Andøya in Northern Norway at (69,28°N, 16.01°E) as part of the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR). It has been a cooperation of the Leibniz-Institute of Atmospheric Physics, the University of Bonn, the Service d'aéronomi of the Centre National de la Recherche Scientifique in Verrières-le-Buisson (France) and the University College London (Great Britain). Since then, it has been extended and improved in many ways until reaching the status described on this page. Starting in 1994, the RMR lidar is used to study temperatures in the middle atmosphre, aerosol layers in der stratosphere, polar stratospheric clouds in the lower stratosphere in winter and noctilucent clouds in the mesopause region in sommer. 

ALOMAR observatory with the two green laser beams of the RMR lidar

Technical design

The ALOMAR RMR lidar is a twin-lidar system consisting of two power lasers, two receiving telescopes, and one optical bench for spectral separation and filtering of the light received from the atmosphere A schematic overview of the lidar system is given in Figure 1.

The power lasers are pulsed Nd:YAG lasers emitting the fundamental (1064 nm), second (532 nm), and third (355 nm) harmonic wavelengths simultaneously. Each laser works with a repetition rate of 30 Hz, in case of both lasers working simultaneously, they are triggered alternately on a pulse-by-pulse basis. Both power lasers are seeded by a single external cw-laser, which is frequency stabilized by iodine absorption spectroscopy to keep a relative wavelength stability of Δλ/λ = 10-9. Due to the seeding, the lasers produce pulses with a high spectral stability and short pulse length (approx. 10 ns). Using beam direction stabilization systems on the laser tables the laser beams are guided into beam widening telescopes (BWT) for reduction of the beam divergence by a factor of 20. Figure 2 shows the design of the laser system with power laser, seeder laser and beam widening telescope. After that the beams, now having a diameter of 20 cm each, are guided by a second set of beam direction stabilization systems using three beam guiding mirrors to emit the beam coaxially to the telescopes into the atmosphere (see Figure 3).

For collection of the backscattered light from the atmosphere two quasi-cassegrain telescopes with 1.8 m primary mirrors are used which can be tilted up to 30° off-zenith while covering an azimuth range of 90° each. They are installed in such a way that one telescope is able to access the north-to-west quadrant (NWT), the other one the south-to-east quadrant (SET). A schematical drawing of the telescopes and the laser beam paths in the telescope hall is shown in Figure 3. The picture in Figure 4 shows the view from above into the telescope hall with the two telescopes. The South-East telescope (SET, left) is tilted 20° to the East while the North-West telescope (NWT, right) is titled 20° to the North. The protective covers of both telescopes are open and one can see the green laser beams. Using the two telescopes, different combinations of viewing directions are possible for observations requiring tilted laser beams, like simultaneous and common-volume measurements with rocketborn in situ experiments in the mesopause region and studies of the horizontal structure of NLC.

The light received from the atmosphere is guided by optical fibers to the input optics of the optical bench. For investigations of the polarization characteristics of the light in the visible (532 nm) and ultraviolet (355 nm) spectral range, turnable polarizers are integrated in the focal optics of the receiving telescopes which can be used when needed.

At the input of the optical bench (see drawing in Figure 5) a rotating segmented mirror (fiber selector) is used to feed the light of both telescopes synchronized to the laser pulses into the single set of receiving optics. A rotating chopper blade blocks the strong signal from the troposphere to prevent detector overload. In the following, the light is separated and filtered by spectral range and intensity to produce 14 different channels: 1064 nm (two detectors, Rayleigh-/Mie-scattering on air molecules and aerosols), 532 nm (three detectors, Rayleigh-/Mie-scattering on air molecules and aerosols), 355 nm (three detectors, Rayleigh-/Mie-scattering on air molecules and aerosols), 608 nm (two detectors, N2 vibrational Raman-scattering excited by 532 nm), 387 nm (one detector, N2 vibrational Raman-scattering excited by 355 nm), 530.4 nm and 529.1 nm (two detector, N2 + O2 rotational Raman-scattering excited by 532 nm). Two more detectors are used to measure winds in the middle atmosphere by the induced Doppler shift. Using photomultipliers (PMT) and avalanche photodiodes (APD) the light scattered in the atmosphere and collected with the telescopes is converted into electrical signals which are altitude resolved by counters and processed and stored on a computer.

The channels for the three laser wavelengths (1064 nm, 532 nm and 355 nm) can be operated all year round and under all daylight conditions due to strong spatial and spectral filtering by a small telescope field-of-view (180 μrad), interference filters and actively stabilized single and double Fabry-Perot etalons having a bandwidth of 10 and 4 pm, respectively. The picture in Figure 6 shows the Fabry-Perot etalons together with other parts of the optical bench.

The whole lidar is computer controlled and automated to a large extent, allowing to operate it by a single trained operator. Our norwegian colleagues operate the RMR lidar whenever permitted by the weather. This is essential for the acquisition of extensive data sets for climatological investigations of the polar middle atmosphere. A more detailed description of the system can be found in the article by von Zahn et al. [2000].

Online analysis

During the lidar soundings a online data visualisation is provided. Outside the sounding periods you will find an overview over the last measument.

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Figure 1: Schematical drawing of the ALOMAR RMR lidar instrument with emitting powerlasers (left), receiving telescopes (middle) and optical bench (right).

Laser system with seeder laser

Figure 2: Schematical drawing of the laser system with seeder laser (upper left), power laser (top) and beam widening telescope (bottom). The beam stabilization system on the laser table is schon in the middle. gezeigt.

Telescope hall

Figure 3: Technical drawing of the telescope hall with laser beam guiding (green) to achieve coaxial emission into the atmosphere and the two tiltable receiving telescopes.

Telescope hall

Figure 4: View from above into the telescope hall with the two telescopes. The South-East telescope (SET, left) is tilted 20° to the East while the North-West telescope (NWT, right) is titled 20° to the North. The protective covers of both telescopes are open and one can see the green laser beams.

Optical bench

Figure 5: Technical drawing of the optical bench. The light from the telescopes is guided through glas fibres to the optical bench. It is fed into the optical bench using a rotating segmented mirror (fiber selector).

Optical bench

Figure 6: Picture of the optical bench without covers. The light is fed into the optical bench from the upper right. The black cylinders are the single and double Fabry-Perot etalons.