There exists a beautiful analogy between the paraxial diffraction ofmonochromaticbeams 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 astronomyand electronic warfare, where real-time monitoring of a RF signal is required. However, conventionalspectrum analyzerbased on electronic technologyhas 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 asanimprovement, a single tone condition orpost processing based on digital signal processoris still required, whichhinders ultrafast measurement ofanarbitrarysignal.
Leveragingthe space-time duality, Dr.Chi Zhangand student Yuhua Duanfrom theOptoelectronic Device and Integration Lab (OEDI), WNLO, proposed a ultrafastRFspectrum analysis system based onall-optical Fourier transform and temporalmagnification, 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 laseris implemented a Fourier transform temporally by passing a long fiber, thenthe RF source under test is multiplexed withanelectro-optical modulator, followed by a dispersiveinverseFourier transform. According to the convolution theorem ofFourier transform, the RFfrequency will bemappedto the timeposition of the ultrashort pulses.After temporally stretched by a time-lens based magnification system, a photo-detectorandanoscilloscopecan 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 inOptics Express(vol. 25, no.7, pp.7520-7529, 2017).
Figure1. Principle ofthe proposed ultrafastRFanalyzer
Figure2.(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 ofChina (Grants No. 61631166003, 61675081, 61505060, 61320106016, and 61125501), theNatural Science Foundation of Hubei Province (Grant No. 2015CFB173), and the DirectorFund of WNLO.
