Research on generation of optical frequency comb using cascaded phase modulator driven by multiple RF signals

Optical frequency combs (OFCs) have important application value in fields such as 5G/6G communication, spectroscopy, and precision measurement. However, existing literature shows that schemes based on mode-locked lasers, microring resonators, or cascaded electro-optic modulators have complex system architectures and high implementation costs, making it difficult to simultaneously achieve a large number of comb lines and good flatness performance. Based on this, a structure for OFC generation using a cascaded phase modulator driven by three radio frequency (RF) signals is designed. The first two RF signals are multiplied to drive the first-stage phase modulator (PM 1), and the third RF signal directly drives the second-stage phase modulator (PM 2) connected in parallel with an optical attenuator. This design achieves effective generation of high-performance OFCs by simplifying the system architecture and optimizing the RF parameters.

The system architecture was illustrated in Fig.1. Firstly, the RF signal S₁(t) at 5 GHz and S₂(t) at 10 GHz were multiplied using a multiplier and then used to drive the PM 1. The modulated light was then divided into upper and lower branches by a 50:50 optical splitter. The upper branch was injected into the PM 2, which was driven by a 45 GHz RF signal S₃(t). The lower branch was attenuated by an optical attenuator with a coefficient of α. Finally, both branches were combined and output through an optical combiner. The frequencies of the three RF signals satisfied the relationship f₁=f₂/2=f₃/9. The three RF signal amplitudes (A₁, A₂, A₃) and the attenuation coefficient α of the attenuator were optimized through simulation. In the experimental verification, an NKT-E15 laser was used as the light source, an MPZ-LN-20 modulator was used for PM 1, and an EOSPACE modulator was used for PM 2. The output was ultimately measured using an optical spectrum analyzer (APEX-AP2081A).

By optimizing the amplitudes of the RF signals and the attenuation coefficient, a high-performance OFC was simulated, generating 45 comb lines with a spacing of 5 GHz, a side-mode suppression ratio of 5.79 dB, and a flatness of 0.87 dB (Fig.2d). Studies of RF perturbations revealed that S₁(t) and S₂(t) played a decisive role in determining the flatness, and deviation of their amplitudes from the optimal values led to significant degradation in flatness (Fig.4b, Fig.5b). S₃(t) could optimize the flatness to below 1 dB (Fig.6b). The experiment successfully generated an OFC with 45 comb lines (Fig.8), although the achieved flatness was relatively poor. The anomalously high-power comb line (marked by the ellipse in Fig.8) was induced by S₃(t). Its root cause was destructive interference caused by a mismatch in the RF source parameters, which confirmed the conclusion regarding RF perturbation sensitivity drawn from the simulation.

A scheme for generating an OFC using a cascaded phase modulator driven by multiple RF signals is designed. Analysis shows that the RF signals S₁(t) and S₂(t) play a decisive role in determining the flatness of the OFC, while the RF signal S₃(t) can achieve an OFC flatness better than 1 dB. The stability of the RF signals must be maintained during OFC operation. Finally, the feasibility of the proposed scheme is verified through experiments. This scheme provides a new approach for OFC generation with a simple structure and controllable performance. Further research will focus on issues such as improving the precision of RF signals and extending broadband flatness.