Feed-forward coherent link from a comb to a diode laser: Application to widely tunable cavity ring-down spectroscopy

Abstract

We apply a feed-forward frequency control scheme to establish a phase-coherent link from an optical frequency comb to a distributed feedback (DFB) diode laser: This allows us to exploit the full laser tuning range (up to 1 THz) with the linewidth and frequency accuracy of the comb modes. The approach relies on the combination of an RF single-sideband modulator (SSM) and of an electro-optical SSM, providing a correction bandwidth in excess of 10 MHz and a comb-referenced RF-driven agile tuning over several GHz. As a demonstration, we obtain a 0.3 THz cavity ring-down scan of the low-pressure methane absorption spectrum. The spectral resolution is 100 kHz, limited by the self-referenced comb, starting from a DFB diode linewidth of 3 MHz. To illustrate the spectral resolution, we obtain saturation dips for the 2ν R(6) methane multiplet at μbar pressure. Repeated measurements of the Lamb-dip positions provide a statistical uncertainty in the kHz range.

Comb coherence-transfer and cavity ring-down saturation spectroscopy around 1.65 μm: kHz-accurate frequencies of transitions in the 2ν3 band of 12CH4

“…The principle of operation of the CCT technique is schematized in Fig. 1 and was previously introduced in Gotti et al 17 Telecom fibered DFB DLs are used as sources of tunable probe radiation. We have used five DLs (butterfly packaged, by NEL) to cover a spectral region limited at long wavelengths by the working range of InGaAs photodiodes and at short wavelengths by the falling strength of methane transitions.…”

“…This sideband is an almost perfect phase clone of the comb mode, shifted by the tunable RF offset. 17,30 The selected side-band at the output of the optical SSB carries from 5 to 10% of the input power, typically 1-2 mW. In order to enhance the signal/noise of ring-down decay signals at the cavity output and to also achieve stronger saturation levels of the molecular transitions, we used a semiconductor fiber boost optical amplifier (BOA, from Thorlabs) with a gain of about 15.…”

“…16 However, only recent advances in laser technology and introduction of high sensitivity detection techniques, such as cavity ring-down spectroscopy (CRDS), make saturation spectroscopy attractive for a wide range of problems in high precision molecular spectroscopy and optical metrology. 6,[17][18][19][20][21][22][23][24][25][26][27] It is important to underline that application of high-finesse optical resonators for saturation spectroscopy demands laser radiation of high spectral purity. A first reason is that Lamb dips are spectrally narrow: Lorentzian widths of less than 500 kHz are typically observed.…”

“…Then, switching of the DFB DL between consecutive OFC teeth allows for gap-free tuning across the full tuning range of the DL. 17,30 Even wider coverage is easily obtained by switching several telecom fibered DFB DLs in the same setup, with minimal adjustments. As a demonstration of the efficiency and robustness of the CCT technique we present a first broadband application by harvesting an extensive set of high quality Lamb dip profiles for the 2n 3 overtone band of 12 CH 4 , across the range spanned by 5 fibered DFB DLs.…”

“…As previously illustrated, 17 by exploiting the standard frequency control of a DFB DL through its operating current and temperature, it is possible to obtain continuous piece-wise tuning of the CCT source limited only by the DL spectral range, typically spanning 1 THz (33 cm À1 ). However, short-range RF tuning is sufficient here to cover each group of lines composing the 12 CH 4 multiplets in the 2n 3 rovibrational band (in particular those with higher rotational excitation).…”