Developing turnkey phase-stabilized optical frequency combs (OFCs) made from commercial off-the-shelf (COTS) components is an important step in making OFC technology widely applicable. This has been demonstrated in a recent paper by researchers at NIST, University of Colorado Boulder, and Menhir Photonics AG, a Swiss firm. OFCs have revolutionized spectroscopy, metrology, and time keeping by providing a coherent link between optical and RF oscillator frequencies. For example, in dual-comb spectroscopy (DCS), two stabilized OFCs with slightly differing pulse repetition frequency (PRF) are mixed, generating an RF comb which enables one to measure a high resolution optical spectrum with a single photodetector. The acquisition speed scales with the square of the PRF, so using a PRF in the GHz range allows for fast measurements, though without the need for the high speed electronics that would be required at higher PRF. The authors use a 1 GHz PRF, passively mode locked laser at 1.56 µm, manufactured by Menhir, as their oscillator. They amplify and compress the pulses to 0.68 nJ and 89 fs, respectively, in a dispersion managed amplifier made entirely of COTS components, and then generate an octave-spanning frequency comb in a length of highly nonlinear fiber. The carrier envelope offset frequency fceo
is measured using an f
interferometer, and phase locked to a low noise RF generator, where fceo
has 438 mrad of integrated phase noise when locked. At the same time, they lock the PRF by beating one of the comb lines with a CW laser at 1550 nm, giving an integrated phase noise of 40.6 mrad, limited by the frequency noise of the CW laser. They demonstrate similar performance using a second turnkey mode locked laser, which highlights the robustness of their design and its suitability for DCS. In future work, the authors propose down-converting the comb to the mid-IR to access the molecular fingerprint and functional group spectral regions.
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