Frequency-noise measurements of optical frequency combs by multiple fringe-side discriminator

Abstract

The frequency noise of an optical frequency comb is routinely measured through the hetherodyne beat of one comb tooth against a stable continuous-wave laser. After frequency-to-voltage conversion, the beatnote is sent to a spectrum analyzer to retrive the power spectral density of the frequency noise. Because narrow-linewidth continuous-wave lasers are available only at certain wavelengths, heterodyning the comb tooth can be challenging. We present a new technique for direct characterization of the frequency noise of an optical frequency comb, requiring no supplementary reference lasers and easily applicable in all spectral regions from the terahertz to the ultraviolet. The technique is based on the combination of a low finesse Fabry-Perot resonator and the so-called "fringe-side locking" method, usually adopted to characterize the spectral purity of single-frequency lasers, here generalized to optical frequency combs. The effectiveness of this technique is demonstrated with an Er-fiber comb source across the wavelength range from 1 to 2 μm.

Figure 1. Principle of the multiple frequency discriminators for FN measurement of OFC.

The comb lines at frequency are pre-filtered with a filtering factor using a low-finesse Fabry-Perot cavity with free spectral range . Cavity resonances are labeled by order , corresponding to the round-trip phase shift of pulses expressed as integer multiple of . A slow feedback loop keeps the overall Fabry-Perot transmission locked to 75% of the maximum. This implies that each filtered comb line of order is weakly locked to the side of the corresponding resonance of order , and is probed with similar slopes. The exact operating point of each single frequency discriminator (i.e. Fabry-Perot resonance) around the nominal 75-level is slightly different mainly because efficient comb-cavity coupling in the presence of non-zero offset frequency requires detuned from .

Figure 2. Schematic of the setup for linewidth measurement of comb tooth.

PZT: piezo-electric transducer; PD: photodiode; Amp: high-bandwidth electrical amplifier; HV Amp: high-voltage amplifier; PID: proportional-integral-derivative controller; ESA: electrical spectrum analyzer.

Figure 3

Figure 4

(a) Voltage applied to the PZT of the scanning FP. (b) Transmission of the scanning FP injected by the comb (red line) and single-frequency laser (blue line) as measured by balanced detection scheme. (c) Expanded view of resonances of the comb (main peak) and the single-frequency laser (translated for comparison) as indicated by the arrows.

Figure 5

(a,b) PSD of FN of the filtered comb spectra and the FP cavity. (c) Linewidth of the filtered comb spectra and the FP cavity as a function of observation time; the inset shows the linewidth across the comb spectrum at 1-ms observation time.

Figure 6. Setup for the coherence transfer from the single-frequency Er:fiber laser to the filtered comb at 1560 nm.

DDS: direct digital synthesizer; SC: supercontinuum.

Figure 7

(a) FN PSD of the free-running comb at 1560 nm (orange), single-frequency Er:fiber laser (green), comb phase-locked to the single-frequency Er:fiber laser (blue), and PSD (red) calculated by summing the frequency noise of the loop error signal and the single-frequency Er:fiber laser. (b) Coherent peak of the beatnote between the comb and the single-frequency Er:fiber laser in locked conditions. (c) Phase noise PSD and integrated phase noise of the loop error signal.