We experimentally demonstrate phase regeneration of a 40-Gb/s differential phase shift keying (DPSK) signal in a 1.7-m long highly nonlinear lead silicate W-type fiber using a degenerate two-pump phase-sensitive amplifier (PSA). Results show an improvement in the Error Vector Magnitude (EVM) and a reduction of almost a factor of 2 in the phase noise of the signal after regeneration for various noise levels at the input.
©2012 Optical Society of America
The increasing demand in traffic capacity driven by ever expanding broadband applications has sparked efforts to enhance transmission efficiency and to reduce networking operating costs. Modulation formats deploying the phase of the signal have begun to replace the previous industry standard of intensity-modulated on-off keyed (OOK) signals, in order to either achieve a better receiver sensitivity as in the case of differential phase shift keying (DPSK), or increase the spectral efficiency as in the case of quaternary-phase shift keying (QPSK). However, phase noise (either linear or nonlinear) poses a limiting factor to the performance of phase encoded signals. Thus, the deployment of more complicated and spectrally efficient modulation formats will require the development of technologies enabling the removal of the phase- and the amplitude- noise from such signals. This has led to an increased research interest in PSAs, which possess a quasi step-like phase transfer characteristic [1, 2] and that have already been demonstrated to efficiently regenerate DPSK [3, 4] and QPSK  signals.
One way to realize PSAs is to utilize four wave mixing (FWM) in fiber optical parametric amplifiers (FOPAs). FOPAs offer various advantages, such as the potential for wide gain bandwidths extending up to several hundred nanometers [6, 7], the flexibility to center their gain profile about any arbitrary wavelength given by the zero-dispersion wavelength (ZDW) of the fiber, the ability to simultaneously parametrically amplify a weak signal and generate a frequency converted idler signal [8, 9], and transparency to the signal repetition rate due to the fast response time of the χ(3) susceptibility of optical fibers . In addition to the attributes outlined above, phase sensitive (PS) FOPAs can offer further advantages such as nearly noiseless amplification , reduction of phase and amplitude noise fluctuations, dispersion compensation and the elimination of modulation instability [12–15].
Key elements in the development of PS-FOPA technology are the realization of a high nonlinear coefficient, a precisely engineered dispersion profile and a high stimulated Brillouin scattering (SBS) threshold in the nonlinear fiber. Such characteristics are generally attractive for the implementation of several all-optical processing applications and subsequently significant research efforts have been devoted to the development of fiber technologies that satisfy (typically, a certain subset of) these characteristics. In recent years, highly nonlinear fibers (HNLFs) based on soft glasses of the likes of bismuth-oxide , lead-silicate  and chalcogenide  have emerged as an alternative technological competitor to standard germanium-doped silica HNLFs. This is due to a number of key characteristics they possess, most importantly their high nonlinear refractive indices and small effective areas enabled by modern microstructure fabrication techniques. Thus, the increase by 2-3 orders of magnitude in the effective nonlinearities which is possible in such glass systems leads to realizing the same net nonlinearity as in silica-based HNLFs but in a much shorter length of fiber, thereby enabling meter or sub-meter long devices with a larger parametric gain bandwidth and better device latency and stability . Furthermore, improvements in the fabrication techniques have reduced the fiber losses and the practical challenges associated with handling and splicing [19,20].
We have previously reported the fabrication of a highly nonlinear lead silicate step-index (W-type) fiber and highlighted its novel optical characteristics, such as the high nonlinearity and low and flat dispersion profile by demonstrating a uniform FWM conversion efficiency of 0 dB across a 40 nm bandwidth . We have also demonstrated the potential of this fiber for realizing broadband phase sensitive amplification without requiring any active SBS suppression schemes . In this paper, we demonstrate the regeneration of a 40-Gb/s DPSK data signal in a 1.7-m sample of this fiber, based on a degenerate two-pump phase sensitive amplifier (2P-PSA) configuration.
2. Experimental setup and results
The experimental setup is shown in Fig. 1 . The data signal was a 40 Gb/s non-return-to-zero DPSK 231-1 pseudo-random bit sequence centered at a wavelength of 1555.64 nm. To emulate the effects of broadband phase-only noise, the signal was combined with narrowband (3-dB bandwidth of 1 nm) amplified spontaneous emission (ASE) with a Gaussian spectral profile centered at 1562 nm and launched into an HNLF. The ASE induced cross phase modulation (XPM) on the signal, thereby introducing broadband incoherent phase noise to it (see  for more details on the noise emulation system). The magnitude of the phase perturbations was controlled by varying the corresponding power levels of the ASE noise. The distorted signal was then launched into the PSA regenerator, which was set up following a black-box configuration (for a detailed presentation, please refer to ). At the input of the regenerator,the signal was coupled with a continuous wave (CW) signal (Pump 1) operating at a wavelength of 1557.36 nm through an add/drop multiplexer. The data signal (acting as a pump here) and the CW signal were launched into a HNLF to generate a new phase-locked and modulation-free idler wave at 1553.9 nm. The HNLF used had a length of 300 m, a dispersion of - 0.01 ps/nm/km at 1550 nm, a nonlinear coefficient of 10.5 /W/km, and an insertion loss of 0.9 dB. The weak idler was then used to injection-lock a slave semiconductor laser (Pump 2) . The three phase-locked optical fields were gated in time through a Mach-Zehnder modulator driven by 400 ns rectangular pulses with a duty cycle of 1:10. Time gating provided a means to increase the peak power of the pumps and signal by a factor of ten whilst retaining the same average power, so as to overcome the splice losses to the lead-silicate fiber that followed, without the need for excessive EDFA average power. This signal arrangement now formed the two pumps and signal required in a degenerate PS-FOPA (see inset to Fig. 1). The three waves were amplified through a high power EDFA to a total power of approximately 31 dBm, before being fed through a polarization controller and an isolator to the lead silicate W-type fiber where PS amplification was realized. The lead silicate HNLF had a nonlinear coefficient of 820 /W/km, a propagation loss of 2.1 dB/m and a dispersion of −0.8 ps/nm/km at 1550 nm . The sample was spliced through a commercial high-NA silica fiber to a standard single-mode fiber patchcord . A total splice loss of approximately 6.25 dB per splice was estimated (note that this figure is higher than what was reported in  and could be improved with further optimization efforts). At the output of the PSA the signal was filtered to reject the two pumps. Part of the data signal was tapped-off to be used as the feedback in the phase locking portion of the setup and to compensate for any slow (sub-kilohertz) relative phase drifts between the three interacting waves. It consisted of an optical detector, a proportional integral (PI) controller and a piezo-electric transducer fiber stretcher located in the path of one of the pumps. The short length of the soft glass fiber contributed to a reduced acoustic pick-up relative to systems based on silica-based fibers (e.g [2,3].) and resulted in more stable locking of the feedback circuit. At the output of the regenerator, the data signal was characterized using an optical modulation analyzer (OMA), consisting of an optical coherent detector and a real-time data acquisition system.
The relative power of the signal with respect to the pumps was adjusted to ensure optimal phase noise suppression with minimal amplitude degradation, which was achieved when the PSA operated in saturation. In this operating regime, a total phase-sensitive gain variation of approximately 4.5 dB was measured (this is to be contrasted to 14 dB which was measured when the PSA was not saturated – see ). Higher order FWM products were observed at the output of the PSA owing to the low and flat dispersion characteristics of the W-type fiber (Fig. 2 ). Figure 3 presents constellation diagrams of the data signal before and after the regenerator for different noise levels. The signal was first assessed with no added noise,showing similar performance both before and after the regenerator (Fig. 3(a)). With mainly phase noise added to the signal (Figs. 3(b)-3(d)), the constellation diagrams display a clear level of noise squeezing with negligible additional amplitude noise. To better quantify the level of improvement achieved in the signal quality, Fig. 4 shows the root mean square (rms) error vector magnitude (EVM), the phase error and the rms magnitude error of the data signal before and after regeneration. Again the no-added-noise case (the leftmost data point in the graphs) shows almost no penalty due to the presence of the regenerator, and a universal improvement in the EVM of the data signal is observed for all the different noise levels examined. As shown in Fig. 4(b), the level of phase error is reduced by about a factor of 2 for all of the cases that we examined, confirming the regenerator capability for reducing phase noise. A slight (effectively negligible) deterioration in the magnitude error of the regenerated signals is observed in Fig. 4(c), mainly due to the phase-to-amplitude noise transformation occurring in the PSA.
We experimentally demonstrated the regeneration of 40 Gb/s data signals using a saturated phase-sensitive amplifier based on a short length of a highly nonlinear lead-silicate W-type fiber. The short fiber length drastically reduced the latency of the processing system relative to corresponding configurations based on germanium-doped HNLFs, and improved the system stability at the same time. The measured data showed a clear reduction in the phase noise of the signal. Furthermore, the low and flat dispersion profile of the fiber across the C-band highlights its potential for future use in applications requiring wavelength multicasting which will be the subject of future experiments.
We thank OFS Fitel Denmark for providing the silica HNLF used in the experiment. This work has received funding from the European Communities Seventh Framework Programme FP/2007-2013 under grant agreement 224547 (PHASORS) and the EPSRC under grant EP/I01196X: Transforming the Future Internet: The Photonics Hyperhighway. Dr. F. Parmigiani gratefully acknowledges the support from the Royal Academy of Engineering/EPSRC through a University Research Fellowship.
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