It has been recognized for more than a decade that laser frequency combs (LFC) offer significant advantages for astronomic spectrograph calibration versus traditional methods such as halogen discharge lamps in terms of spectral marker density, stability, accuracy and traceability. Only LFCs support radial velocity measurements with a precision of a few centimeters per second as required for the detection of earthlike exoplanets and their atmospheres or for measurements of variations of fundamental constants (Huke et al., 2017). To match the ideal calibration marker density of about 1 line per 2.4 spectrograph resolution elements (resulting from a trade-off between avoidance of line blending and high density sampling), it is required to thin out mode-locked comb sources to a mode spacing of several GHz using Fabry-Perot (FP) cavities. As an example, the desired spacing for the High Resolution Spectrograph (HIRES) for the European Extremely Large Telescope (E-ELT) is 7 GHz between 1155 nm and 2000 nm and 20 GHz between 375 nm and 435 nm with intermediate values at integer multiples of 1 GHz for other channels between the above mentioned ones (Huke et al., 2017). As such, a comb laser with a repetition rate of 1 GHz is ideal because it permits the generation of all channels from a single source with different FP filter cavities applied to the different channel. While comb lasers with lower repetition rates (e.g. fiber sources with ~250 MHz) would also fulfill this requirement, their repetition rate is too low and has significant disadvantages compared to 1 GHz: 1) they need to be thinned out significantly more leading to corresponding increased power losses that need to be compensated for by amplification steps which have their own additional problems, 2) FP filtering with sufficient side mode suppression becomes more challenging the closer the modes are to begin with and potentially leading to significant systematic calibration errors due to line asymmetries (Huke et al., 2017), (Charsley et al., 2017).
Laser Quantum has worked with two groups to provide calibration sources for astronomic spectrographs. The group around Derryck Reid and Richard McCracken at Heriot-Watt University have built a 15 GHz spaced comb to calibrate the red channel of the spectrograph at the South African Large Telescope (SALT). For simplicity, they have employed a comb with locked repetition rate but free-running offset frequency which is tracked and achieved a calibration improvement by a factor of two (McCracken et al., 2017). Work at SALT is ongoing towards the use of a fully stabilized comb and potentially a 10 GHz repetition rate Ti:sapphire laser (Laser Quantum’s model taccor x10). The below image shows the resolved 15 GHz comb on the echelle spectrograph together with light from a discharge lamp.
More recently we have collaborated with a group from the Harvard-Smithsonian Center for Astrophysics to calibrate the HARPS-N spectrograph at the Telescopio Nazionale Galileo (TNG) on the island of La Palma. In this case the turn-key LFC based on a taccor comb from Laser Quantum based on a taccor power 10 laser and offset frequency stabilization from our partner Menlo Systems was fully stabilized and used to calibrate an 80 nm band about 570 nm with a stability of one centimeter per second within half an hour of averaging time (Ravi et al., 2017).
1. Huke, P. et al., 2017. Optical Measurement Systems for Industrial Inspection X, Proc. SPIE. Vol. 10329
2. Charsley J. et al., 2017. Presented at: SPIE Optical Metrology
3. McCracken R. et al., 2017. Wavelength calibration of a high resolution spectrograph with a partially stabilized 15-GHz astrocomb from 550 to 890 nm. Optics Express Vol. 6450
4. Ravi, A. et al., 2017. Astro-comb calibrator using a turn-key laser frequency comb. Astro-Ph.IM https://arxiv.org/abs/1705.07192 [Accessed 2 October 2017]