State-of-the-art laser technology
Alexandrite is a very special laser crystal: It can be tuned over a broad spectrum from 730 to 800 nm. Unlike titanium sapphire (a much better known laser crystal), it can also store abundant energy in the process. Thanks to these properties, it is particularly suitable for high pulse energies.
However, it must be operated with an extremely narrow bandwidth for the LIDAR measurements. This means that only one spatially narrow mode may be pumped in the crystal; otherwise, higher modes would oscillate and the laser would become spectrally more broadband. This was one of the main problems when flash lamps were used – they usually excited multiple modes. The thermo-optical properties are not without their problems either; alexandrite was often operated at well over 100 degrees. "I've never had such a challenging laser crystal," Michael Strotkamp summarizes. "With the diodes, we could directly pump the mode volume, which solved many problems."
Alexandrite absorbs light down to the red range, which is why pump diodes are used at a wavelength of 638 nm. In the beginning, these diodes were bars with 80 W pulse peak power and a beam profile that had to be elaborately homogenized for the first prototypes. In the meantime, the Aachen researchers are using fiber-coupled pump diodes, which considerably simplify the shaping of the pump radiation.
The laser (Image 5) is designed as a ring resonator, i.e. there is no standing, but a circulating laser wave. This is excited with a frequency-stabilized cw laser (seed). For this purpose, a resonator mirror is made to oscillate with a piezo element. Whenever the resonator length corresponds exactly to a multiple of the wavelength of the seed laser, it is coupled into the resonator. The principle is also called "ramp-and-fire." The laser is triggered via a Q-switch, which releases the circulating pulse when its energy is high enough.
The first prototype of the alexandrite laser delivered 0.15 W, at that time with two crystals but without homogenization of the pump radiation. With homogenized pump radiation, the laser delivered more power from only one crystal. Since then, the system has been significantly improved over several iterations. To save space, the beam path (2 m resonator length) is folded several times. Meanwhile, the pump radiation is delivered only via fibers, allowing the pump source to be changed without much effort. The latest pump source delivers 400 watts in pump pulses, and the laser is then operated with an output power of 2.3 W or pulses of 4.6 mJ at 500 Hz. At a 750 Hz repetition rate, it even runs with 2.7 W and 3.6 mJ pulses.
The secrets of the "stardust"
When meteorites burn up in the mesosphere, individual atoms are left behind and float up there for many years. "For atmospheric physics, this is an absolute stroke of luck, because apart from this 'stardust,' there's not much else up there," explains Josef Höffner.
For LIDAR measurements, various atoms or aerosols of the atmosphere are illuminated by the ground-based laser and scatter back individual photons. Measurements with the alexandrite LIDAR at Leibniz IAP use three main effects: Rayleigh, Mie and resonance scattering.
Rayleigh scattering is the reason why our sky is blue. It describes how light is scattered by particles smaller than its wavelength. The scattering is strongly frequency dependent, which is why blue light is scattered more than red light. That is why sunsets are red and the sky is blue. Above about 60 km altitude, the density of oxygen and nitrogen atoms becomes so small that Rayleigh scattering is difficult to measure.
Mie scattering describes the effects on light when it is scattered by particles that have a size similar to the wavelength. In the atmosphere, these are usually aerosols, i.e. dust or volcanic ash, for example. Such particles occur up to an altitude of about 30 km.
Depending on the flight speed of the particles, the Rayleigh and Mie spectra are shifted by the Doppler effect. By comparing such backscattered spectra with the light from the LIDAR source, scientists can calculate the velocity of the particles. More precisely, the spectral shift provides a vector component of the wind speed in the direction of the laser beam. That is why the new LIDAR system measures with beams in five directions. In this way, the wind can be determined well even at high altitudes.
The actual height results from the travel time of the laser beams; they need 0.3 milliseconds for 90 km. To ensure that each pulse can be completely evaluated before the next one follows, the system works with a laser repetition rate of up to 1,000 Hz.