Impact of nonlinear spectral broadening on the phase noise properties of electro-optic frequency comb

TL;DR

The paper examines how nonlinear spectral broadening impacts the phase noise of electro-optic frequency combs. Using coherent detection, DSP, and subspace tracking, it shows that nonlinear broadening preserves the standard phase-noise model driven by the seed laser and RF source, effectively behaving as if the EO comb were driven with higher RF power. The key findings are that no extra NL phase-noise terms appear (φ^{(i)}_{NL}(m,t) = 0), the repetition-rate noise remains unchanged (N_1 = 1), and the central comb line remains at m^{*} = 0 with N_0 = 1. Practically, this implies broadbanded EO combs from nonlinear stages can maintain low phase noise, with broadening acting as power amplification rather than a noise source; however, generality across different broadening schemes requires further investigation.

Abstract

Electro-optic modulation is an attractive approach for generating flat, stable, and low-noise optical frequency combs with relatively high power per comb line. However, a key limitation of electro-optic combs is the restricted number of comb lines imposed by the available RF source power. To overcome this limitation, a nonlinear spectral broadening stage is typically employed. The phase noise characteristics of an electro-optic comb are well described by the standard phase noise model, which depends on two parameters: the seed laser and the RF source phase noise. A fundamental question that arises is how nonlinear broadening processes affect the phase noise properties of the expanded comb. To address this, we employ coherent detection, digital signal processing, and subspace tracking. Our experimental results show that the nonlinearly broadened comb preserves the standard phase noise model of the input electro-optic comb. In other words, the nonlinear processes neither introduce additional phase noise terms nor amplify the existing contributions from the seed laser and RF source. Hence, nonlinear broadening can be viewed as equivalent to driving the electro-optic comb with a much higher RF modulation power.

Figures (11)

• Figure 1: Experimental setup for generation and characterization of parametric optical frequency comb. DSO - Digital Signal Oscilloscope, BP - Bandpass Filter, UW - Unwrapping, LD - Linear Detrending.
• Figure 2: Dispersion profile of the PM-HNLF2.
• Figure 3: Optical frequency comb spectrum after the nonlinear broadening stage. Dashed boxes indicate the measured comb-lines.
• Figure 4: Illustration of the heterodyne measurement using groups of 5 comb-lines.
• Figure 5: Evolution of eigenvalues and eigenvectors for 5 measured comb-lines after each parametric stage: (a–b) EO comb; (c–d) NALM; (e–f) expanded comb at 1520 nm; (g–h) expanded comb at 1580 nm.