Proposal of Arbitrary THz Pulse Train Generation with Tunable Duration and Period
By Che Ming
In theory, any waveform can be expanded into a Fourier series. Conversely, the inverse process can be utilized to synthesize arbitrary waveform signals. In Figure 1, an electro-optic frequency comb is initially employed to generate discrete and equidistant frequency lines. Subsequently, the optical frequency comb (OFC, or optical pulse) is filtered through a wavelength-selective switch (WSS), filting continuous-wave (CW) signals for each frequency. Amplitude and phase modulation for each frequency are then individually adjusted by optical modulators. The modulation signals are calculated inversely through the Fourier series of the desired pulse signal in Figure 2. This can be achieved through optimization algorithms or neural networks to obtain optimal solutions. The CW optical signals at different frequencies are coupled into a THz photomixer (such as a UTC-PD) to achieve the conversion from optical pulses to THz pulses. Due to optical heterodyne down-conversion of the UTC-PD, the spectral width and repetition frequency of the OFC can be effectively translated to the duration and period of the THz pulses. An OFC is that an optical carrier modulated by an electro-optic modulator, where changing the frequency of the injected modulating signal adjusts the period of the generated THz pulses, and the spectral width of the OFC selected by the WSS determines the duration.
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