Laser Guide Stars Developments
Within the Telescope Systems Division the Laser Guide Stars Group is responsible for the development and deployment of Laser Guide Stars projectors on ESO facilities, to study novel LGS-AO sensing schemes and to pursue related technologies. The, by now classical approach, is to use a narrow-line laser emitting at a sodium resonance line wavelength to create a yellow artificial “star” in the ~ 95 km altitude sodium cloud around the Earth. When working with an Adaptive Optics system, this beacon provides a bright reference source to correct atmospheric turbulence in real time in fields devoid of bright enough natural stars; note however that a moderately bright natural star is still needed to correct global image motion in the field (see a short tutorial here)
The first such facility has been installed in Paranal on the VLT Unit Telescope #4 (Yepun) in 2005. Until 2012, it used the PARSEC dye laser developed by MPE-Garching and MPIA-Heidelberg. Two adaptive optics assisted instruments, also installed at Yepun, will use that facility, viz. the NACO IR imager and the SINFONI near-IR 3-dimensional spectrometer. In 2012, PARSEC was replaced by PARLA, a Raman fiber laser.
ESO is using its development 20 watt CW laser at La Palma, Canary Islands, to perform field tests of technologies related to laser guide star systems (LGS). These tests are strategic to guide the technology developments related to LGS-AO for large and extremely large telescopes.
The activities are done in collaboration with other member states institutes, at the 4.2m William Herschel Telescope (WHT), using the Canary adaptive optics test bench, and the ESO Wendelstein laser guide star unit, (Ref SPIE 2012). The Wendelstein laser, developed at ESO labs, uses the same technologies as the Toptica engineered lasers, which are installed on the VLT UT4 telescope and have been ordered for the ELT laser guide star units.
One of the activities of the R&D field tests is to optimize the laser emission format for maximum brightness of the laser guide star. We have run experiments in the past for different parameters of the laser emission, and are now planning to evaluate experimentally the effect of frequency chirping, i.e. changing repeatedly and periodically the laser emission frequency.
From our models of the sodium atom illumination at the mesosphere, the emission frequency chirping should improve the LGS brightness by a factor at least 1.5. Other experts claim even higher gains in LGS brightness. Frequency chirping means to periodically ramp the laser emission frequency over 100--200 MHz to optimize the number of atoms which are able to interact with the laser photons.
Why would laser chirping raise the LGS brightness?
The mesospheric sodium atoms that absorb and re-emit photons towards the ground are affected by recoil and hence get Doppler shifted by 50 kHz per photon emission on average. After a number of absorptions and re-emissions, the laser photons cannot interact with the atom because of its accumulated Doppler shift, the atom has moved to a different velocity class.
By changing laser frequency, we follow the atoms into the next velocity class where also other atoms are present that had not been interacting with the laser before. So the total number of atoms interacting with the laser increases while following their Doppler shift, a sort of snow-plowing through the velocity classes of the mesospheric atoms.
The frequency shifting of the laser frequency is effective for periods corresponding to the mean collision time of the sodium atoms in the mesosphere, which we can assume for now to be 150--200 microseconds. It will have to be optimized experimentally. So the laser frequency should be varied in a sawtooth manner, at a repetition rate of 7 kHz, and for frequency ranges of about 200 MHz.
How can we obtain chirping with Wendelstein?
To implement frequency chirping, a novel modification of the current laser controls has to be done. Following the scheme of our laser, we will modulate the seed emission wavelength by changing the applied current while synchronously varying the SHG cavity optical length. This length change is accomplished using the internal piezo actuator of the cavity to keep the optical path length an integral multiple of the emitted wavelength while it is shifting. We will apply a sawtooth signal via the commercial Toptica seed laser current controller. We intend to test this scheme first in the ESO laser laboratory, then perform field tests at the Observatorio del Roque de los Muchachos.
After demonstrating that the sawtooth effectively makes the desired, periodic wavelength shifts for the chirping, we will apply the same sawtooth signal, possibly smoothed on the steep side, to the commercial Toptica Digilock unit, or else to the voltage amplifier of the piezo controller.
We may need to apply amplitude and phase offsets between the seed and the piezo amplifier sawtooth ramps to compensate for the sensitivities and delays in the electronics, making sure the piezo sawtooth is synchronized with the seed sawtooth signal.
To check the performance in the lab we will initially do the chirping slowly and verify the wavelength shifting using an Optical Spectrum Analyzer based on a confocal cavity. Then we will raise the speed and verify via a fast response diode that the output power remains constant, proving that the seed laser frequency and the SHG cavity resonance remain tuned and in phase.
We will in parallel investigate, via simulations and modeling, if the repumper D1 line needs a different sawtooth ramp slope or amplitude, to optimize the LGS brightness. In that case we will make further field experiments to demonstrate the modeling results.
The field experiments will be done using the ESO Wendelstein Laser Guide Star Unit. During the experiment, we will initially verify that the frequency changes of the seed laser emission and the cavity resonance are the same, first slowly chirping and checking with the Optical Spectrum Analyzer, then fast by monitoring the stability of the laser emission with a fast photodiode.
Finally, we will do sky tests and monitor the return flux using the photometrically calibrated receiver telescope, alternating between chirping and no chirping, while exploring optimal settings for the chirping (speed of the frequency shift and range).
In case the modeling indicates that with a different chirping of the D1 repumper line we obtain even better LGS brightnesses, we will introduce in the RF generator of the WLGSU the possibility to remote controlling the D1 emission frequency.
We note that this chirping approach can also be combined with sum-frequency generation (SFG). Alternatively, it can be combined with doubly-resonant SHG schemes that double the main laser line at 589.159 nanometers, but also a rempumper line at 1.7 GHz shifted to the blue. Yet another possibility is the combine the chirping with singly-resonant SHG, after which the repumper line is added using a phase modulator external to the SHG cavity. We foresee that chirping can be rather easily added to laser systems as those employed in the 4LGSF (4-Laser Guide Star Facility) on UT4 of the VLT.