Multi-branch erbium fiber-based femtosecond optical frequency comb for measurement of cavity ring-down spectroscopy

Abstract

<sec>In this paper, we demonstrate an optical frequency comb (OFC) based on an erbium-doped-fiber femtosecond laser, for the measurement of cavity ring-down spectroscopy (CRDS) with wavelengths of 1064, 1083, 1240, 1380, 1500, 1600, 1750 and 2100 nm. We adopt a multi-branch structure to produce high power at the specific wavelengths to meet the requirement for application in the spectral measurement. The OFC is developed by using a mode-locked fiber ring laser based on the nonlinear amplifying loop mirror mechanism. The laser is self-starting by introducing a nonreciprocal phase bias in the cavity and insensitive to the environmental perturbation. Using the chirped pulse amplification and highly nonlinear fibers, the broad spectra at the specific wavelengths are obtained. By optimizing the parameters of the pulses, the power of per mode at each target wavelength is greater than 300 nW.</sec><sec>The<i> </i><i>f</i>_rep is obtained by detecting the output of the femtosecond laser directly, while the<i> </i><i>f</i>_ceo is detected by <i>f</i>-2<i>f</i> interference. The signal-to-noise ratio of the<i> </i><i>f</i>_ceo is about 35 dB with a 300-kHz resolution bandwidth. By controlling the intra-cavity electro-optic modulator and piezoactuator , the <i>f</i>_rep is stabilized with high bandwidth and large range (about megahertz bandwidth and 3 kHz range). The <i>f</i>_ceo is stabilized by using feedback to the pump current of the femtosecond laser dynamically. The in-loop frequency instability degree of the <i>f</i>_ceo, evaluated by the Allan deviation, is approximately 4.95 × 10^–18/<i>τ</i>^1/2 at 1 s and integrates down to 10^–20 level after 2000 s, while that of the <i>f</i>_rep is well below 5.85 × 10^–13/<i>τ</i>. The all polarization-maintaining erbium fiber-based femtosecond optical frequency comb with multi-application branches we demonstrate in this paper is efficient and reliable for many other applications including optical frequency metrology and optical atomic clocks.</sec>