High-resolution Ultrafast Temporal Optical Systems By Overcoming The Detection Bandwidth Limitation

Leveraging all-optical signal transformation approaches,ultrafast temporal optical measurement systems convert physical quantities under test onto the time axis for rapid and continuous acquisition through a high-speed single-pixel photo-detector and subsequent signal acquisition circuit.However,the requirement for large-bandwidth detection system composed of a photo-detector and a real-time oscilloscope is bringing challenge in terms of measurement resolution to the temporal optical measurement systems.Moreover,high-speed detection devices greatly increase the system cost,thereby reducing the practicality of the temporal optical measurement systems.In addition,large bandwidth is accompanied by high sampling rate,so that high-throughput data will be produced when the systems operate in continuous real-time acquisition mode,which will bring great pressure to the subsequent data processing and storage.For these considerations,this thesis explored some solutions from perspectives of signal bandwidth compression,singnal spectrum synthesis and optical source of the systems,to enhance the measurement resolution under limited detection bandwidth.In brief,the research achievements and contributions of this thesis include:(1)The basic theory of temporal optics based on the space-time duality is studied.The resolution limiting factors of several typical ultrafast temporal optical measurement systems are systematically analyzed,and mainly focusing on that induced by the detection bandwidth.(2)From the idea of signal bandwidth compression,temporal magnification is firstly applied to the time-lens RF spectrum analyzer to reduce the detection bandwidth,without sacrificing the measurement speed.Accordingly,the frequency resolution is enhanced from 31.7 GHz to 1 GHz.This work realized an ultrafast and high-resolution RF spectrum measurement approach.In addition,when the time-lens RF spectrum analyzer is equipped with the dual-comb asynchronous optical sampling,high resolution of 100 MHz can be achieved with detection bandwidth as low as 20 MHz.(3)A novel signal detection technology named as temporally structured illumination is proposed and demonstrated,which is inspired from the spatial structured illumination microscopy.An electro-optic modulator is used to impart a sinusoidal pattern onto the broadband optical signal before detection,which shifts different spectral components into the limited bandwidth of the detection system.Theoretically,this scheme can equivalently double the existing detection bandwidth through a subsequent spectrum synthesis algorithm.In simulation section,the method demonstrated the ability to extend the detection bandwidth from 40 GHz to 80 GHz.In the experimental stage,this temporally structure illumination technology is applied to the time-stretch microscopy,and enhanced the resolution from 6 ?m to 4 ?m under limited detection bandwidth.(4)A novel signal detection technology named as temporal Fourier ptychography is proposed,which is inspired from the Fourier ptychographic microscopy.The broadband optical signal is spectrally divided and then measured by the bandwidth-limited detection system.Based on the intensity waveforms of different spectrum slices,the broadband complex spectrum of the signal can be reconstructed through iterative algorithm.This method extends the detection bandwidth from 20 GHz to 175 GHz in simulation,and provides a new solution to the problem of detection bandwidth limitation in the temporal optical measurement systems.(5)A novel high-speed frequency swept source based on dual comb assisted timefrequency mapping is proposed and demonstrated.Leveraging the time-frequency mapping function of time lens,the stepped time delay created by the dual comb system is converted to stepped wavelength sweeping.Therefore,an extra-cavity frequency sweeping scheme without any mechanical components is realized.Compared with the time-stretch source,this frequency swept source equivalently achieved much larger dispersion amount so that high spectral resolution at pm magnitude can be achieved under ultra-low detection bandwidth of 90 MHz.When applied to the time stretch microscopy,the detection bandwidth requirement is decreased from 3.8 GHz to 90 MHz while maintaining high imaging resolution.