Differential phase-shift keying (DPSK) modulation has received much attention in fiber communications over the past decade. DPSK signals are more resilient to impairments originating from fiber nonlinearities than conventional amplitude-shift keying output, less vulnerable to degradation caused by pattern-dependent effects, and ready for balanced detection. As a result, they enable a 3dB improvement in receiver sensitivity. However, the DPSK receiver structure is more complicated. For example, conversion of the phase information in DPSK signals to the corresponding intensity distribution requires demodulation in which adjacent bits undergo optical interference. To allow processing of different signals transmitted at various bit rates, it is of great interest to demonstrate a functional bit-rate-tunable DPSK demodulator. Here, we use four-wavelength-mixing (FWM) wavelength conversion in a highly nonlinear photonic-crystal fiber — and group velocity dispersion (GVD) in a standard single-mode fiber — to produce a continuously tunable optical delay. By placing the FWM and GVD elements in a fiber-loop mirror, we achieve continuously bit-rate-tunable DPSK demodulation.
The operational principle of the loop-mirror demodulator is illustrated in Figure 1. The device functions as a delay-asymmetric nonlinear-loop mirror (DANLM).1 Its structure resembles that of the optical parametric-loop mirror that was originally proposed to separate input and output signals in a wavelength-conversion experiment.2 If the GVD is sufficiently large, one can obtain asymmetric optical delays — in addition to asymmetric phases — of the converted signals in the interfering branches. With the highly nonlinear photonic-crystal fiber providing a 3dB FWM-conversion bandwidth of 20nm and standard single-mode fibers offering a ~10ps/nm dispersion, a tunable delay of up to 200ps can be obtained. Hence, a DANLM can (in principle) be used to demodulate DPSK signals at any bit rate above 5Gb/s. To operate at different bit rates, one simply needs to tune the wavelength of the continuous-wave (CW) pump for FWM with the DPSK signal.
Figure 1. Schematic illustration of the delay-asymmetric nonlinear-loop mirror (DANLM) for differential phase-shift keying (DPSK) demodulation. GVD: Group velocity dispersion. Δλ, Δτ: Shift in wavelength, phase.
Figure 2 shows the output spectra after the DPSK input and the CW pump have undergone FWM in the photonic-crystal fiber. Figures 2(a) and 2(b) illustrate relative pump tuning of 2.5nm, resulting in a 5nm shift in the converted signal. The spectral spacing between the input and the output is 10 and 5 nm in Figures 2(a) and 2(b), respectively. Hence, with a dispersion of 10ps/nm in the single-mode fiber, DPSK demodulation at 10 and 20Gb/s can be obtained. Note that for a continuously wavelength-tunable CW pump, the delay is also continuously tunable, thus supporting continuous bit-rate tuning for DPSK demodulation. Figure 3 shows the eye diagrams of demodulated DPSK signals. Figures 3(a) and 3(b) correspond to 10 and 20Gb/s measured eyes, respectively. Widely opened eye diagrams are obtained in both cases. Error-free detection was also obtained with a DANLM.
Figure 2. Optical spectra showing four-wave mixing between the DPSK input and the tunable continuous-wave pump: (a) 10nm separation of the input and converted output for demodulating 10Gb/s signals, (b) 5nm separation for demodulating 20Gb/s signals.
Figure 3. Eye diagrams of demodulated DPSK signals (in arbitrary units, a.u.): (a) 10Gb/s, (b) 20Gb/s.
In conclusion, we have demonstrated a continuously bit-rate-tunable DPSK demodulator based on a DANLM. The device relies on FWM wavelength conversion in a highly nonlinear photonic-crystal fiber and GVD in a standard single-mode fiber to provide a precisely tunable asymmetric delay between the counter-propagating interfering branches. Our next steps will focus on addressing the DANLM polarization sensitivity. Further applications in the general area of all-optical processing of variable bit-rate communication signals will also be investigated.
Chester Shu, Mable P. Fok
Department of Electronic Engineering
The Chinese University of Hong Kong
Hong Kong, China