All-fiber frequency agile triple-frequency comb light source

Abstract

Tricomb spectroscopy unveils a new dimension to standard linear and nonlinear spectroscopic analysis, offering the possibility to reveal the almost real-time evolution of complex systems with unprecedented accuracy. Current triple comb configurations are based on the use of mode-locked lasers, which impose constraints on the comb parameters, and require complex electronic synchronization, thus limiting potential applications. In this paper, we present the experimental demonstration of a new type of all-fiber, self-phase-locked, frequency-agile tri-comb light source. It is based on the nonlinear spectral broadening of three electro-optic modulator-based frequency combs in a three-core fiber. The exploitation of spatial multiplexing of light in optical fibers offers new possibilities to generate broadband-frequency combs that are highly coherent with each other. After characterizing the stability of the source and performing several dual-comb test measurements, we revealed the high mutual coherence between the three combs through the demonstration of a 2-D pump-probe four-wave mixing spectroscopy experiment.

Fig. 1. Experimental set-up.

a Simplified sketch of the experimental setup. b Scanning electron microscope image of the tri-core fiber. c)Measured tri-core input spectrum with a high-resolution optical spectrum analyzer (20 MHz resolution). d Tri-core input temporal shape measured with an optical sampling oscilloscope (700 GHz bandwidth). e–g Output spectra for each core. Insets show a zoom on the spectra. CW continuous wave, MOD intensity modulators, AOM Acousto-optic modulator.

Fig. 2. Frequency stability of Comb2.

a Uncorrelated phase noise. Measured SSB phase noise at 500 MHz carrier frequency wave, at different stages of the setup. At the CW laser output (blue curve), at the tri-core fiber input (orange curve), and at the output (purple curve). The gray dotted lines delimit the different noise regions. The horizontal black dash-line indicates the thermal noise threshold of the photo-detection. b Allan deviation of f_rep at 500 MHz for τ₀ = 1 s for the RF output (blue curve) and the optical output (red curve). c Allan deviation of δf_rep at 50 kHz between Comb1 and Comb 3 for τ₀ = 100 ms and τ_end = 1 h for the RF output (blue curve) and the optical output (red curve). A longer measurement (τ_end = 12 h) is depicted in red dotted line, with the 1-σ error bars at each point and the corresponding τ^−1/2 characteristic white noise fit in gray. SSB single sideband.

Fig. 3. Dual-comb measurements.

a Multi-period interferogram between Combs 2 and 3 with δf_rep = 50 kHz. Inset: Zoom on a single interferogram trace. b Dual-comb spectrum calculated over n = 5 interferograms averaged N = 20,000 times. c Zoom in on the comb structure with n = 50 and N = 2000. d Evolution of the SNR of the RF spectrum as a function of the number of averaged sets (N) at the tri-core fiber input (orange line and dots) and at the output (blue line and dots). e Phase profile at the dual-comb at the output of a standard SMF-28 fiber of 20 cm (green dots) and its quadratic fit (blue line). f Relative coherence between two combs (1 & 2) at the tri-core fiber output (blue curve) and using two separate fiber wounded on the same spools (pink curve).

Fig. 4. FWM tri-comb interferometry.

a, b Scheme of principle. Comb1 (orange) and Comb2 (green) at f_rep1 = 1.25 GHz generate FWM (purple) in a nonlinear fiber, used as the replica of a nonlinear spectroscopic medium. Comb3 (blue) at f_rep3 = f_rep1 + δf_rep down-converts the resulting spectrum in the RF domain. c RF spectrum of a set of n = 5 interferograms averaged over N = 800 times, with zero-delay between the pumps. The inset is a zoom-in of the FWM spectrum around 300 MHz, with n = 10 and N = 80. d Spectrogram of the down-converted FWM. Parameters: f_AOM1 = 100 MHz and f_AOM2 = 200 MHz, δf_rep = 100 kHz, f_rep = 1.25 GHz and F_sampling = 5 GHz. FWM Four-Wave Mixing.