Author: Optoelectronic Device and Integration Lab (OEDI)

There exists a beautiful analogy between the paraxial diffraction of monochromatic beams confined in space and the dispersion of narrow-band pulses in the time domain, which is known as the space-time duality. Based on this concept, some novel methods and approaches for temporal optical signal processing can be inspired by and extended from the elements or systems used in space optics. Especially, due to the wide spread application in ultrahigh-speed optical signal processing recently, the space-time duality based technologies attracted a wide range of attention. On the other hand, ultrafast analysis of an RF spectrum is of great significance, particularly in applications such as radio astronomy and electronic warfare, where real-time monitoring of a RF signal is required. However, conventional spectrum analyzer based on electronic technology has low measurement speed and limited observation bandwidth, which are difficult to be further improved owing to the electronic bottleneck. Although some photonics-assisted RF analysis approaches have been proposed as an improvement, a single tone condition or post processing based on digital signal processor is still required, which hinders ultrafast measurement of an arbitrary signal.

Leveraging the space-time duality, Dr. Chi Zhang and student Yuhua Duan from the Optoelectronic Device and Integration Lab (OEDI), WNLO, proposed a ultrafast RF spectrum analysis system based on all-optical Fourier transform and temporal magnification, which realized over 20 GHz observation bandwidth and 100 MHz frame rate, as well as sub-GHz frequency resolution. Ultrashort pulses from a mode-locked fiber laser is implemented a Fourier transform  temporally by passing a long fiber, then the RF source under test is multiplexed with an electro-optical modulator, followed by a dispersive inverse Fourier transform. According to the convolution theorem of Fourier transform, the RF frequency will be mapped to the time position of the ultrashort pulses. After temporally stretched by a time-lens based magnification system, a photo-detector and an oscilloscope can directly capture the spectrum of the tested signal in real-time.

In March, 2017, the investigation result entitled “Ultrafast electrical spectrum analyzer based on all-optical Fourier transform and temporal magnification,” was published in Optics Express (vol. 25, no. 7, pp. 7520-7529, 2017).

Figure 1. Principle of the proposed ultrafast RF analyzer

Figure 2. (a) Measurement results of different frequencies. (b) Ten frames under 10GHz frequency

The work was partially supported by grants from the National Natural Science Foundation of China (Grants No. 61631166003, 61675081, 61505060, 61320106016, and 61125501), the Natural Science Foundation of Hubei Province (Grant No. 2015CFB173), and the Director Fund of WNLO.