Optical frequency combs (OFCs) are characterized by a comb-like structure in the frequency domain, where the frequency spacing between the comb teeth is equal, and the phases are mutually locked. OFCs have widespread applications in precision measurements, spectroscopy, and optical communications. In particular, in the field of microwave photonics, optical frequency combs serve as a bridge linking the optical frequency domain and the radio frequency (RF) domain. Among various OFC generation techniques, cascaded electro-optic modulator (EOM)-based combs have garnered significant attention in academia due to their inherent advantages, such as flexible control of comb tooth spacing (FSR), adjustable comb line number, and insensitivity to optical wavelength.
However, in cascaded EOM-based OFC systems, due to fabrication processes and material limitations, the half-wave voltage (Vπ) of the electro-optic modulators is typically large. To achieve deeper modulation depth and generate more spectral lines, it is often necessary to cascade multiple modulators. A key challenge lies in achieving phase synchronization of the driving signals between the cascaded EOMs.
This research published entitled "Automatic phase- matching technique for cascaded electro-optic frequency combs" in Chinese Optics Letters aims to address the phase synchronization issue of driving signals between multiple modulators in a cascaded EOM-based OFC system. A novel and rapid automatic phase-matching method is proposed, which resolves the inefficiency and real-time issues associated with traditional manual adjustment methods. This approach does not require complex artificial intelligence or deep learning algorithms, yet enables fast and stable phase matching.
The study focuses on analyzing the relationship between the optical power of specific spectral lines in the OFC and the phase of the driving signals applied to the EOMs. By leveraging this relationship, a feedback circuit based on a microcontroller is designed, which allows real-time monitoring of optical power changes. A simple control algorithm is employed to drive an electrically controlled phase shifter, automatically adjusting the phase of the driving signals to achieve phase synchronization between cascaded EOMs. The specific experimental system schematic and the resulting optical frequency comb are shown in Figure 1. The system is capable of rapidly flattening the optical frequency comb within 2 seconds, with real-time feedback, ensuring stable operation of the cascaded EOM-based OFC system over long periods. The flat and stable OFC generated by this method presents new possibilities for the development of microwave photonics and optical communication systems in the future.
Future work will focus on optimizing the microcontroller feedback circuit to achieve more precise phase synchronization while reducing costs. Additionally, efforts will be made to explore the integration of this technology with cascaded EOM-based optical frequency comb systems. These endeavors will further refine the automatic phase-matching mechanism and promote its broader application in various practical scenarios.
Fig. 1. Structural diagram of the automatic phase-matching system for optical frequency combs generated by cascaded electro-optical modulation.
