Radiofrequency (RF) spectrum is defined as the power spectrum of the electromagnetic field. Light, one of the electromagnetic field, can be chosen as the carrier of information, with the advantages, e.g. large bandwidth, anti-electromagnetic interference. With the rapid development of optical communication system and microwave-photonics, RF spectrum analysis of optical signal has beenwidelyapplied inresearches such as optical performance monitoring, wireless communication, radar system. The conventional scheme is based on the electrical spectrum analyzer (ESA), in which the optical envelop is first obtained by a fast square-law photo-detector, and then followed with an ESA to resolve the RF spectrum. However, the electronic components restrict the measurement bandwidthto bearound 100 GHz, which hinders its applications for optical signals with ultra-large bandwidth (>100 GHz). Recentlya scheme based on cross-phase modulation (XPM) has demonstrated with the bandwidth beyond 100 GHz.In this scheme, the XPM is used to transfer the RF spectrum of the optical signal under test to the probe’s optical spectrum and the transferred RF spectrum is acquired by the optical spectrum analyzer (OSA) based on the spatial grating. Leveragingtheinstantaneous nonlinear effect, the scheme surpasses thebandwidth limitations of electronic componentsand has demonstrated the bandwidth as high as 2.5 THz. However, its frame rate is confined below kHz, mainly limited by the low speed of thespatialgratingbasedOSA. In conclusion, the aforementioned schemes are incapable of observing the broadband RF spectrum of optical signal in real-time.
LeveragingtheXPM and the parametric spectro-temporal analyzer (PASTA), Dr.Chi Zhangand studentLiao Chenfrom theOptoelectronic Device and Integration Lab (OEDI), WNLO, proposed areal-time broadbandRFspectrum analyzer, which realized over800 GHz observation bandwidth and91MHz frame rate, as well as3.8-GHz frequency resolution.In this scheme, XPM in the highly-nonlinear fiber is used to modulate the temporal intensity of the optical signal under test into the probe’s phase, thus the RF spectrumcarried bythe signal is coverted to the optical spectrum of the probe, then the probe’s optical spectrum is mapped into the time domain with the PASTA.Finally, a photo-detectorandanoscilloscopecan directly capture the spectrum of the tested signal in real-time.
In April 17th, 2017, the investigation result entitled “Real-time broadband radio frequency spectrum analyzer based on parametric spectro-temporal analyzer (PASTA),” was published inOptics Express(vol. 25, no. 8, pp. 9416-9425, 2017).
Figure1. Principle ofthe proposedreal-time broadband RF spectrum analyzer
Figure2.(a) Measurement bandwidth. (b)The frequency resolution.(c)Twoframes under500-GHz 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.
