Bandwidth and repetition rate programmable Nyquist sinc-shaped pulse train source based on intensity modulators and four-wave mixing

Abstract

We propose and experimentally demonstrate an all-optical Nyquist sinc-shaped pulse train source based on intensity modulation and four-wave mixing. The proposed scheme allows for the tunability of the bandwidth and the full flexibility of the repetition rate in the limit of the electronic bandwidth of the modulators used through the flexible synthesis of rectangular frequency combs. With the ever-growing demand for telecommunication bandwidth, highly efficient spectral techniques are currently thoroughly investigated in order to optimize the capacity of fiber optics networks. Two digital techniques are commonly used: orthogonal-frequency division multiplexing (OFDM), where a superchannel composed of sincshaped subcarriers is generated, and Nyquist-WDM where the data symbols are carried by Nyquist-shaped pulses in the time domain [1]. Recently, the concept of orthogonal time-division multiplexing (O-TDM) of optically generated Nyquist pulses was demonstrated [2]. In this scheme, the ultra-short Nyquist pulses are generated all-optically and time multiplexed with no intersymbol interference (ISI), taking advantage of the orthogonality of Nyquist pulses. Various techniques to generate optical Nyquist pulses have been demonstrated including spectral reshaping of mode-locked laser [2] or fiber optical parametric amplification using one degenerated pump [3] or two dissimilar frequency pumps in order to achieve uniform pulse generation over a wide bandwidth [4]. These techniques allow for the generation of Nyquist pulses of a few picoseconds duration down to sub-picosecond. However, reducing the complexity of these schemes to a viable solution is not straightforward. Moreover, the generated Nyquist pulses exhibit a non-rectangular spectrum, which leads to the necessity of guard band between WDM channels. To overcome these limitations, another technique based on phase-locked flat-comb generation was demonstrated to obtain sinc-shaped pulse trains [5]. The proofof-principle setup was based on intensity modulators driven by radio-frequency (RF) tones. This simple setup is cost-effective, but the generated bandwidth remains limited to three times the electronic bandwidth of the modulators. In order to overcome the bandwidth limitation, we show that a nonlinear stage based on Kerr effect in highly nonlinear fiber (HNLF) can be used to expand the bandwidth of the generated comb while maintaining the high flexibility and reconfigurability advantages of the original technique. The proposed principle for the Nyquist sinc-shaped pulse train generation is sketched in Fig. 1. Three stages of rectangular-shaped in-phase optical frequency comb generators (RI-OFCG's) are used to achieve a Nyquist-sinc pulse train from an initial optical continuous wave (CW) [ Fig. 1(a)]. The first and third stages are based on intensity modulation, while the second stage is based on four-wave mixing (FWM) [6,7] combined with a wavelength selective switch (WSS) [8].At the first RI-OFCG stage, the CW input is modulated via an intensity modulator (IM 1 ...

Reconfigurable Filter-free Sinc-shaped RF Photonic Filters Based on Rectangular Optical Frequency Comb

“…We show that the filter bandwidth is easily reconfigured by tuning the DC bias and RF power to the MZMs. Narrower filters can be reached by nonlinear expansion of the comb [9] while tuning of the FSR can be obtained by adjusting f 1 and f 2 . …”

Versatile High Repetition Rate 2-μm Pulsed Source Based on Wideband Parametric Conversion

“…This translates into reducing the pump repetition rate while keeping the same pulse duration and should impact the source output in the same way. On the other hand it is possible to keep the same spectral line spacing and broaden it with a third modulator or via FWM in a HNLF [23] before boosting it. This means that the pump pulse duration would be decreased at constant repetition rate.…”

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

“…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].…”

“…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].…”

“…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].…”