Radio-frequency line-by-line Fourier synthesis based on optical soliton microcombs

Radio-frequency (RF) waveform synthesis enables wide applications in wireless communications, radar systems, and electronic frequency-domain and time-domain testing.

 

Conventionally, electrical arbitrary waveforms are acquired by digital-to-analog converter (DAC) technologies. However, because of the constraints of conversion speed, complexity, and costs, DAC has difficulty to keep up with the ever-increasing demand of ultrawide bandwidth, which is critical to the developments of ultrahigh-speed wireless communications.

 

In the past decade, photonic-based RF arbitrary waveform synthesis approaches have demonstrated key advantages in analog bandwidth when compared to conventional electronic solutions. The optical approaches benefit from the ultrabroad optical bandwidth, low power dissipation and remoting through radio-over-fiber technology. Various techniques have been proposed and demonstrated, including frequency-to-time mapping, direct time-domain synthesis, and spectral Fourier synthesis.

 

However, the existing photonic methods either rely on long optical delay lines or require low repetition rate mode-locked lasers, which are difficult to achieve on chip scale. This limits the potential of the mass-scale integration of RF waveform synthesis on a photonic chip.

 

To address this issue, the research group led by Prof. Xu Yi from University of Virginia, demonstrated radio-frequency arbitrary waveform generation (AWG) using integrated microresonator soliton frequency combs, which provides a viable path to fully integrated photonic-based ultrawide-bandwidth RF AWG on a chip. The research results are published in Photonics Research, Volume 10, No. 4, 2022 (Beichen Wang, Zijiao Yang, Shuman Sun, Xu Yi. Radio-frequency line-by-line Fourier synthesis based on optical soliton microcombs[J]. Photonics Research, 2022, 10(4): 04000932).

 

In this study, the RF arbitrary waveforms are generated with optical dual-microresonator soliton frequency combs through Fourier synthesis, which creates one-to-one mapping between the frequency and temporal profiles of a waveform. The high repetition rate of microresonator soliton frequency combs facilitates line-by-line amplitude and phase control of individual optical frequency comb lines.

 

By generating the optical dual-comb with the same pump laser and photomixing them on a photodiode, an RF frequency comb with zero envelope offset frequency is created, and its comb lines derive their amplitudes and phases from the optical dual-comb. Therefore, a complete discrete Fourier series in the frequency domain can be constructed for RF arbitrary waveform synthesis in the time domain, as shown in Fig.1.

 

Fig.1 Concept of RF line-by-line Fourier synthesis with dual-microresonator solitons.

 

As a result, a series of temporal waveforms, including tunable Gaussian, triangle, square, and "UVA"-like logo, is demonstrated to illustrate arbitrary waveform synthesis. All critical components in this method, including soliton microcombs, wavelength multiplexers/demultiplexers, intensity and phase modulators, optical amplifiers, and ultrafast photodiodes, are compatible with photonic integration.

 

'With the rapid developments of soliton microcomb technology, we are excited to see its potentials to miniaturize the photonic-based RF Fourier synthesis on a chip.' Wang said.

 

Compared to the conventional electronic AWG solution, the ultrahigh analog bandwidth has been the key advantage of photonic AWG systems. The analog bandwidth of the dual-comb Fourier synthesis method is ultimately limited by the Nyquist frequency of optical coherent sampling, i.e., half of the optical frequency comb repetition rate, and the bandwidth of the photodiode. The Nyquist frequency of dual-microcomb can range from a few gigahertz to a few hundred gigahertz. And the photodiodes with bandwidth exceeding hundreds of GHz has been demonstrated previously.

 

'Achieving a higher analog bandwidth is our next goal. In our experiment, the analog bandwidth of the waveforms is 3 GHz, which is limited by our oscilloscope bandwidth.' Yang said. 'However, we expect to push it to 50 GHz to 100 GHz in the future.'

 

This article reports the generation of radio-frequency arbitrary waveform generations using optical dual-microresonator soliton frequency combs, which has broad applications in ultrawide-bandwidth wireless communications, radar systems, and electronic testing. This approach provides not only the possibility of precise Fourier synthesis at microwave and millimeter-wave frequencies, but also a viable path to fully integrated chip-scale photonic-based RF AWG.