Spectral Tailoring of Electro-Optic Comb for Tuneable THz wave Generation and MWP RF Filter
Loading...
Author
Chin, Sanghoon
Obrzud, Ewelina
DOI
Abstract
In this paper, we present both experimental and simulation results on the use of electro-optic (EO) frequency comb light sources for various applications, particularly in the generation of frequency-tuneable terahertz (THz) waves and dynamically reconfigurable microwave photonic (MWP) RF filters. These applications are effectively realized by a simple and robust technique that involves spectral tailoring of the optical spectrum of the EO comb that is generated through sinusoidal hybrid phase and amplitude modulation of a seed laser operating in the C-band. The process begins by modulating the optical phase of the seed laser using a phase electro-optic modulator (EOM). This results in the generation of a broadband optical frequency comb with a line spacing corresponding to the applied RF modulation frequency. The amplitude of each spectral line is essentially revealed to be Bessel functions of the first kind, shaping the frequency comb's spectrum. To further refine the comb's output, the generated multicarrier signal is passed through an amplitude-EOM, which reduces the optical power of the comb lines near the central seed laser frequency. The output spectrum at the modulator is in principle determined by the superposition of the modulation patterns from both the phase and amplitude modulators. By adjusting the relative phase of the two modulators and the DC bias of the amplitude-EOM, the optical spectrum can be dynamically controlled. In most use cases, the amplitude-EOM is used to generate a flat optical frequency comb. However, in this work, we first optimize the parameters of the amplitude-EOM to suppress a large number of sidebands around the central frequency, creating a two-tone-like light source. The frequency separation of the two tones can be easily tuned by adjusting the RF power applied to the phase-EOM, enabling the control of the carrier frequency for the generated THz waves. As a proof-of-concept, we demonstrate the generation of a dual-frequency-like light source with a spectral spacing of 86.4 GHz, achieved by modulating the concatenated EOMs at 10.8 GHz.
In the second application, we use the spectrally engineered EO comb light source as a Brillouin pump in optical fibers to realize MWP RF filters with dynamic reconfiguration capabilities. By tuning the amplitude and/or frequency of the RF signal applied to the modulators, we can adjust the characteristics of the filter. This technique offers a bandwidth-tuneable single-resonance MWP filter when the flat-top comb is used, or a double-resonance filter with dynamically adjustable spectral distance when the two-tone-like comb source is employed. For proof-of-concept, we experimentally demonstrate MWP band-pass filters with tuneable bandwidths of over 100 MHz and extinction ratios greater than 30 dB. Notice that the maximum achievable bandwidth was practically limited by the available RF source in our laboratory, but it’s not a fundamental limit. In addition, we successfully demonstrate a double-resonance MWP band-pass filter with spectral distance tunability up to approximately 100 MHz, which is also restricted by the employed RF power.
To the best of our knowledge, this is the first experimental demonstration of implementing the spectral engineering of EO comb sources to generate tuneable THz waves and create dynamically reconfigurable MWP RF filters. We believe that such a simple but powerful technique has the large potential to provide a competitive solution for a wide range of RF, millimetre-wave and THz wave applications, offering a flexible and scalable platform for future advancements in photonics.
Publication Reference
Space Microwave Week ESA, Noordwijk (NL)
Year
2025-05-12