High-precision wavelength calibration of astronomical spectrographs with laser frequency combs

Abstract

We describe a possible new technique for precise wavelength calibration of high‐resolution astronomical spectrographs using femtosecond‐pulsed mode‐locked lasers controlled by stable oscillators such as atomic clocks. Such ‘frequency combs’ provide a series of narrow modes which are uniformly spaced according to the laser's pulse repetition rate and whose absolute frequencies are known a priori with relative precision better than 10−12. Simulations of frequency comb spectra show that the photon‐limited wavelength calibration precision achievable with existing echelle spectrographs should be ∼1 cm s−1 when integrated over a 4000 Å range. Moreover, comb spectra may be used to accurately characterize distortions of the wavelength scale introduced by the spectrograph and detector system. The simulations show that frequency combs with pulse repetition rates of 5–30 GHz are required, given the typical resolving power of existing and possible future echelle spectrographs. Achieving such high repetition rates, together with the desire to produce all comb modes with uniform intensity over the entire optical range, represents the only significant challenges in the design of a practical system. Frequency comb systems may remove wavelength calibration uncertainties from all practical spectroscopic experiments, even those combining data from different telescopes over many decades.

Full Stabilization of a Microresonator-Based Optical Frequency Comb

“…A remaining challenge for selfreferenced frequency comb generators [8] on a chip will be the generation of octave-spanning combs in a microresonator, which could be achieved by improving the optical quality factors and flattening the microcavity's dispersion [26]. The stabilization of a microcavity frequency comb in conjunction with mode spacings in the microwave domain is an important step towards a low cost, small-sized frequency comb generator for spectroscopy applications, astrophysical spectrometer calibration [27], arbitrary optical waveform generation, optical distribution of microwave clock signals [28], and high capacity telecommunication.…”

The photospheric solar oxygen project

“…The set-up is based on the high-resolution vertical Echelle spectrograph of the VTT and employs a laser frequency comb (LFC) wavelength calibration source (see e.g. Murphy et al 2007;Wilken et al 2012;Steinmetz et al 2008) and a single-mode fibre feed (SMFF). The wavelength calibration obtained from the LFC is accurate to 1 m s −1 .…”

Optical frequency comb generation from a monolithic microresonator

“…In contrast to femtosecond mode-locked lasers[16] the present work represents an enabling step towards a monolithic optical frequency comb generator allowing significant reduction in size, cost and power consumption. Moreover, the present approach can operate at previously unattainable repetition rates [17] exceeding 100 GHz which are useful in applications where the access to individual comb modes is required, such as optical waveform synthesis [18], high capacity telecommunications or astrophysical spectrometer calibration [19].…”

“…In contrast to femtosecond mode-locked lasers[16] the present work represents an enabling step towards a monolithic optical frequency comb generator allowing significant reduction in size, cost and power consumption. Moreover, the present approach can operate at previously unattainable repetition rates [17] exceeding 100 GHz which are useful in applications where the access to individual comb modes is required, such as optical waveform synthesis [18], high capacity telecommunications or astrophysical spectrometer calibration [19].Optical microcavities [20] are owing to their long temporal and small spatial light confinement ideally suited for nonlinear frequency conversion, which has led to a dramatic improvement in the threshold of nonlinear optical light conversion [21]. In contrast to stimulated gain, parametric frequency conversion [22] does not involve coupling to a dissipative reservoir, is broadband as it does not rely on atomic or molecular resonances and constitutes a phase sensitive amplification process, making it uniquely suited for tunable frequency conversion.…”