Optical Sinc-shaped Nyquist pulse source based on a single Mach-Zehnder modulator

Abstract

We present a precise and flexible Sinc-shaped Nyquist pulse source based on a single Mach-Zehnder modulator. The use of multi-harmonic RF signals provides direct control on the Nyquist pulse train generation. The variable comb source is demonstrated by selecting the number of RF components used to drive the optical modulator, leading from 3 to 9 comb lines. Furthermore, different operating points are reported to show the multi-harmonic comb generator potential to provide near-perfect Nyquist pulse source with pure rectangular optical spectrum or with minimized insertion loss. Keywords: Sinc-shaped Nyquist pulse, optical frequency comb, electro-optic modulation. INTRODUCTIONGeneration of Sinc-shaped optical Nyquist pulses is essential for a wide range of applications in optical telecommunications, RF photonics, and optical signal processing. Indeed, due to its rectangular spectrum and zero inter-symbol interference (ISI) in time domain, the newly developed optical orthogonal time division multiplexing (O-OTDM) and Nyquist WDM technique [1] can achieve comparable spectral efficiency as optical orthogonal frequency division multiplexing (O-OFDM), while the receiver complexity is reduced and it is less sensitive to the nonlinearities of the link. Besides, since the Nyquist pulse train corresponds to a number of optical frequency combs in spectrum with equal amplitude and linear spectral phase, it can be utilized to implement optical or microwave filters with rectangular spectrum [2]. In addition, all-optical sampling using Nyquist pulses has recently been demonstrated based on external modulators with tunable sampling speed [3], thereby avoiding the need for nonlinear mixing with short optical pulse as the gating function.Therefore, considerable efforts have been placed to generate Sinc-shaped Nyquist pulses with a variety of methods [4][5][6][7][8]. By cascading two Mach-Zehnder modulators (MZM) with synchronized RF frequencies (f, 3f) to each of the modulator, near-ideal Sinc-shaped Nyquist pulses were obtained with the corresponding 9-line comb spectrum [4]. Sinc-shaped Nyquist pulses were also demonstrated using a single dual-parallel Mach-Zehnder modulator (DP-MZM) with 5 spectral comb lines [5]. In [6], by putting a segment of dispersion compensating fiber (DCF) in between a phase modulator (PM) and a MZM, serving as a modified lens, Nyquist pulses were generated with a 11-line comb spectrum after a selective bandpass filter. In order to expand the spectrum, and therefore narrowing the Nyquist pulse width, four-wave mixing technique were utilized with the help of a waveshaper to tailor the spectrum [7]. Indeed, liquid crystal spatial light modulator can be used to implement line-by-line shaping of mode locked laser spectrum or super-continuum comb source into Nyquist pulses, with the full control of the amplitudes and the relative phases [8]. In order to obtain flatter and wider optical combs, more than one RF clock were used in [9] to smooth the central region of the output spectrum from a single asym...