Single-modulator, direct frequency comb spectroscopy via serrodyne modulation

Abstract

Traditional electro-optic frequency comb spectrometers rely upon the use of an acousto-optic modulator (AOM) to provide a differential frequency shift between probe and local oscillator (LO) legs of the interferometer. Here we show that these modulators can be replaced by an electro-optic phase modulator (EOM) which is driven by a sawtooth waveform to induce serrodyne modulation. This approach enables direct frequency comb spectroscopy to be performed with a single dual-drive Mach-Zehnder modulator (DD-MZM), allowing for lower differential phase noise. Further, this method allows for simpler production of integrated photonic comb spectrometers on the chip scale.

Figure 1.

(a) Schematic of a typical self-heterodyne spectrometer. (b) Schematic of the single-modulator, direct frequency comb spectrometer presented herein. Abbreviations are direct digital synthesizer (DDS), external-cavity diode laser (ECDL), arbitrary waveform generator (AWG), electro-optic phase modulator (EOM), acousto-optic modulator (AOM), device under test (DUT), photodiode detector (DET), dual-drive Mach Zehnder modulator (DD-MZM).

Fig. 2.

Computed power spectra for sawtooth modulation of the laser phase corresponding to amplitudes of $\pi /2$ (upper panels), $\pi$ (lower panels) and indicated sawtooth symmetry factors, $\beta$. Here the x-axes represent normalized detuning about $f_0$ expressed as integer orders, $m=({f-f_0)/f}_r$.

Fig. 3.

Typical self-heterodyne comb power spectrum shown as an average of 500 individual power spectra with a total acquisition time of 0.5 s. As can be seen in the inset, the large positive- and negative-order comb teeth are separated by 120 kHz (i.e., twice the serrodyne frequency). The serrodyne harmonics are also visible as the lower magnitude comb teeth with a spacing of 60 kHz.

Fig. 4.

Typical spectrum of a given optical cavity mode of the optomechanical accelerometer as well as the corresponding Fano lineshape [15] fit. This spectrum was acquired in 16.7 μs with no averaging performed. Elevated noise is seen near zero detuning as this corresponds to DC in the self-heterodyne spectrum.

Fig. 5.

Observed cavity mode detuning recorded with the DD-MZM spectrometer when the electromechanical shaker table was driven with an amplitude-modulated 200 Hz sine wave.