Heterodyne interferometry using acousto-optic frequency combs

Summary form only given. Optical frequency combs (OFCs) have led to an impressive number of achievements, including multi-heterodyne spectroscopy [1]. Mode-locked OFCs have been widely used due to their exceptional spectral bandwidth, but they usually require sophisticated locking methods to control the free spectral range (FSR) and the absolute frequency of the comb. A promising alternative is provided by electro-optic OFCs, where multiple mutually coherent sidebands are generated by intensity and/or phase modulation of a single stable CW laser. The comb FSR is easily reconfigurable by changing the frequency of the modulation signals. However the number of lines is typically limited to ~10s. This quantity can be increased to 100s but at the expense of conducting spectral broadening by light propagation through a nonlinear fiber [2, 3]. Alternatively, pseudo-random EO phase modulation leads to the generation of hundreds of mutually coherent lines, but then the comb bandwidth is limited to a few GHz [4].Here we demonstrate a simple solution to generate OFCs from a single CW laser that produces at least hundreds of mutually coherent lines. The FSR of the comb can be controlled over orders of magnitude, between the kHz range to tens of MHz, and the comb bandwidth varies accordingly from the MHz range to tens of GHz [5]. Our source is based on a frequency-shifting loop (FSL) [6]. A fiber loop containing an acousto-optics frequency shifter (AOFS), a fiber amplifier (EDFA) and a tunable bandpass filter (BPF) is seeded with a CW narrow linewidth (subkHz) laser (Fig 1.a). At each roundtrip in the loop, the frequency of the light is shifted a quantity given by the frequency of the signal driving the AOFS, generating a comb with a FSR equal to Δ. The FSR typically takes values of a few tens of MHz, but it can also be reduced to the MHz or even kHz level, by using two AOFS (AOFS1 and AOFS2) providing frequency shifts with opposite signs. A coupler enables to extract a fraction of the intracavity field. To demonstrate the phase coherence of the comb and to prove the capabilities of the FSL for ultrahigh resolution spectroscopy, we measured the transmission function (intensity and phase) of a Fabry-Perot cavity (FPC) (FSR = 1.6 GHz, finesse = 250). This measurement was performed using the so-called self-heterodyne technique [4], which basically consists of detecting the beat notes between the OFC and the seed laser, whose frequency is shifted by another acousto-optic modulator (AOFS3).