Nyquist pulse, which is characterized by rectangular spectra and sinc-shaped time-domain waveforms, is crucial in Nyquist wavelength-division multiplexing (WDM) and Nyquist optical time-division multiplexing (OTDM), which facilitate substantial enhancements in spectral efficiency and promote super-terabit transmission. Various methodologies have been proposed for generating Nyquist pulse, including nonlinear fiber effects, cascaded modulators, and regenerative mode locking. Among these methods, modulator-based techniques are notable for their capacity to yield high-quality Nyquist pulse while remaining relatively uncomplicated and facilitating miniaturized integration. Duty cycle is one of the most important metrics of Nyquist pulse. In an OTDM system, the duty cycle determines the number of signals that can be multiplexed, whereas in an optical sampling system, it determines the sampling accuracy of the signal. However, Nyquist pulses generated by cascaded modulators have limited duty cycle. First, in most of the aforementioned methods, a high-performance rectangular tunable optical filter (TOF) must be utilized to obtain a Nyquist pulse with adjustable duty cycles, which increases the cost and complexity of the system. Second, the duty cycle of Nyquist pulse generated by cascaded modulators is limited to 0.0372. Hence, a straightforward and effective approach must be devised to generate Nyquist pulse that obviates the necessity for optical filters and offers an adjustable duty cycle.

A Mach?Zehnder modulator (MZM) and an arbitrary waveform generator (AWG) were used in the current experiment. The AWG can be programmed to generate electric-frequency combs (EFCs) with different numbers of comb teeth (N), which are then used to drive the MZM to obtain a Nyquist pulse with an adjustable duty cycle. A real-time oscilloscope (Keysight, MSOV334A) with a photodiode (PD) and an optical spectrum analyzer (Yokogawa, AQ6370D) were used to observe the time-domain waveforms and spectrograms of the Nyquist pulse, respectively. However, the limited bandwidth of the oscilloscope restricts the measurement of narrower pulses. Consequently, the electrical signal frequency is reduced to 1 MHz to determine the minimum available duty cycle. Nevertheless, lower-frequency intervals cannot be measured using the optical spectrum analyzer; therefore, the homodyne method was employed to measure the spectrograms of the optical signal. This approach facilitates the detection of optical frequency combs (OFCs) with a frequency interval of only 1 MHz using an electrical spectrum analyzer (ESA), whereas the low-bandwidth real-time oscilloscope accurately measures the waveform of the Nyquist pulse.

Results show that under a fixed V_PP (peak-to-peak voltage of electrical signals), the side-mode suppression ratio (SMSR) of the OFC decreases as the N of the OFC increases, and the OFC flatness deteriorates as N increases(see Fig. 4). Under a fixed N, an optimal V_PP exists that optimizes the SMSR and flatness. Therefore, considering the SMSR and flatness simultaneously, an OFC with up to 121 comb teeth was generated, whose corresponding time-domain waveform was a Nyquist pulse with a duty cycle of 0.00907 (see Fig. 5). Additionally, the effect of the electrical-signal quality on the Nyquist pulse was investigated. As the maximum frequency of the EFC increases, the quality of the EFC produced by the AWG deteriorates, which primarily manifests in the deterioration of the EFC flatness (see Fig. 6). Therefore, the maximum frequency of the EFC is limited by the sampling rate of the AWG. Furthermore, the deterioration in the EFC flatness worsens the OFC flatness, with an almost identical trend exhibited (see Fig. 7). Consequently, for an AWG bandwidth of 240 MHz, the maximum EFC frequency should be limited to 70 MHz to safeguard the quality of the generated Nyquist pulses. Finally, the phase noises for the EFC and Nyquist pulse register at offsets of -97.17 dBc/Hz@10 kHz and -96.89 dBc@10 kHz, respectively, and show almost identical curves (see Fig. 8). The jitter and phase-noise measurements confirme the relative stability of the Nyquist pulse.

Herein, we present a programmable approach for generating Nyquist pulse. Our method requires only one MZM and one AWG to produce Nyquist pulse with customizable duty cycles. Our experimental results indicate that the SMSR and OFC flatness are sensitive to both N and the electrical-signal power. Specifically, under a constant electrical-signal power, the SMSR and flatness deteriorate as N increases. Meanwhile, under a fixed N, the SMSR and flatness initially improve and then deteriorate as the electrical-signal power increases. Consequently, for each fixed N, an optimal V_PPexists that optimizes the SMSR and flatness. By balancing between the SMSR and flatness, the proposed approach facilitates the generation of an OFC with up to 121 comb teeth. This corresponds to a Nyquist pulse with an exceptionally low duty cycle of 0.00907.