With the rapid development of optical light systems，such as 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. Recentlyan all-optical RF spectrumanalyzer, named a light intensity spectrum analyzer (LISA) has demonstrated with a breakthrough observationbandwidth of 800 GHz.In this scheme, the XPM is used to transfer the RF spectrum of the optical signal under test to a continuous wave (CW) probe’s optical spectrum and the transferred RF spectrum is acquired by the optical spectrum analyzer (OSA).
Leveragingtheinstantaneous nonlinear effect, the scheme surpasses thebandwidth limitations of electronic componentsand has demonstrated the bandwidth as high as 2.5 THz based on the nonlinear waveguide. However, its frame rate is confined below kHz, mainly limited by the low speed of thespatialgratingbasedOSA. LeveragingtheXPM and temporal convolution, Dr.Chi Zhangand studentLiao Chenfrom theOptoelectronic Device and Integration Lab (OEDI), WNLO, proposedthe Frequency-domain light intensity spectrumanalyzer (f-LISA),which can be regarded as the frequency-domain counterpart of a conventional light intensity spectrum analyzer (LISA).In this scheme, the intensity of the signal under test is modulated onto the Fourier domain of an ultrashort pulse rather than the time domain of a CW probe via the XPM,while the temporal convolution helps to enable the RF spectrum to be temporally resolved with a high frame rate.Finally, a photo-detectorandanoscilloscopecan directly capture the spectrum of the tested signal in real-time.Thef-LISA hasrealized over800 GHz observation bandwidth and94MHz frame rate, as well as1.25-GHz frequency resolutionand has successfully characterized the dynamic RF spectrum of an ultrafast wavelength-switching signal with a 10-ns switching interval.
In July 15th, 2017, the investigation result entitled “Frequency-domain light intensity spectrum
analyzer based on temporal convolution,” was published inOptics Letters (Vol. 42 (14), pp. 2726-2729, 2017).
Figure1. Schematic diagrams of the two schemes. (a) Time-domain
LISA. (b) Frequency-domain LISA.
Figure2.(a) Measurement bandwidth. (b)The frequency resolution.(c)Real-time monitoring of fast wavelength-switching processes
The work was partially supported by grants from the National Natural Science Foundation of China (Grants No. 61125501, 61320106016, 61505060,61631166003, 61675081), the Natural Science Foundation of Hubei Province (Grant No. 2015CFB173), and the Director Fund of WNLO.
