10-Gb/s slow-light performance based on SBS effect in optical fiber using NRZ and PSBT modulation formats

For the first time, we have demonstrated error-free operation of slow-light via SBS in optical fiber for 10-Gb/s NRZ and PSBT formats. The PSBT signal sensivity is 5.2dB better than the NRZ case for a 35-ps delay. Introduction Recently, slow-light has attracted much interest due to potential applications in future optical networks, such as optical buffering and tunable time delay. Among all proposed techniques, slow-light mechanism based on Stimulated Brillouin Scattering (SBS) in optical fibre has attracted extensive attention [1-4]. The problem of narrow-band SBS gain has been solved by direct modulation of a pump laser diode using a Gaussian noise source [2]. Until now, the system performance for 10-Gb/s signal transmitted in SBS based slow-light have been investigated only in case of differential-phase-shiftedkeying (DPSK) modulation format [3]. However, the sensitivity was degraded by ~10dB for a 42-ps delaytime due to SBS filtering effect and chromatic dispersion accompanied with slow-light. In this paper, we investigate the system performance of 10-Gb/s non-return-to-zero (NRZ) and phaseshaped binary transmission (PSBT) modulation formats in SBS based slow-light delay line. It is well-known that PSBT format allows high spectral efficiency and strong dispersion-tolerance [5]. Here, we propose to utilize the PSBT format to reduce the signal distortions in slow-light. All the results in term of sensitivity and power penalty have been measured at a bit-error-rate (BER) of 10. For the 10-Gb/s PSBT signal, a negative power penalty, i.e. -2 dB for a 25 ps delay-time, has been obtained. When the delay-time is increased to 35 ps, the sensitivity of the PSBT signal is 5.2 dB better than the NRZ signal case. Maximum of ~50 ps delay with error-free operation can be obtained only by using PSBT format. Experimental setup Fig.1 depicts the experimental set-up. The signal transmitter consists of a laser diode (LD1) operating at 1548.26nm, and a Mach-Zehnder modulator (MZM) driven by a 10-Gb/s pseudo-random bit sequence (PRBS). A 10-Gb/s PBST modulation format can be achieved by filtering the electrical NRZ signal using a 5 order Bessel filter with a ~2.7GHz cut-off frequency [6]. A 2-1 word length sequence is used for BER measurements due to non-optimized 3-GHz PSBT electrical filter and the driver [7]. The inset of Fig.1 corresponds to the eye diagram of the PSBT signal. The signal is launched into a 20-km long Truewave (TW) fibre with a ~10.75GHz Brillouin frequency shift. The SBS pump source is a directly modulated laser diode (LD2), whose central wavelength can be precisely controlled by temperature. The pump LD is modulated by a Gaussian noise source (Tektronix AFG3252), and subsequently boosted by a high power erbium-doped fibre amplifier (EDFA). We have obtained ~12GHz SBS pump bandwidth, resulting in ~7GHz gain bandwidth for a 22-dBm pump power.