We propose a novel 3-bit/symbol optical data format based on simultaneously modulating dark-return-to-zero (DRZ), differential-phase-shift-keying (DPSK) and polarization-shift-keying (PolSK) on the same optical carrier. Transmission performance is investigated. The proposed data format has a more compact spectrum when compared with other modulation schemes (at the same data rate), such as return-to-zero (RZ), DPSK and PolSK. It is also more tolerant to phase noise when compared with optical-differential-quadrature-phase-shift-keying (ODQPSK).
© 2006 Optical Society of America
Multi-bit per symbol modulation formats, such as optical differential-quadrature phase shift keying (ODQPSK), amplitude-shift-keying/differential-phase-shift-keying (ASK/DPSK), ASK/polarization-shift-keying (ASK/PolSK) or multi-level PolSK schemes have been investigated to enhance spectral efficiency and tolerance to polarization mode dispersion (PMD) in optical communication networks [1–4]. Besides, by modulating the optical signal orthogonally using multiple modulations, the transport capacity and the overall spectral efficiency can be increased. The required bandwidth for the optical electronics at the transmitter and receiver can be reduced. This can lower the cost of the optical systems. However, ODQPSK scheme requires the use of complex encoders and decoders, and ASK/DPSK or ASK/PolSK schemes need precise adjustment of the extinction ratio (ER) of the ASK for superimposing on the DPSK or PolSK modulation signals. Here, we proposed a novel optical data format based on the simultaneous modulations of dark return-to-zero (DRZ) (or inverse-RZ), DPSK and PolSK onto the same optical carrier. It can triple the transmission capacity and achieve a 3-bit/symbol optical transmission without precise ER adjustment and maintenance at each node of the network when compared with the ASK/DPSK  or the ASK/PolSK . As the DRZ signal is half-bit delayed, the crosstalk between the DRZ and the other two modulation formats is small even when high ER DRZ modulation is used.
2. Principle of the proposed scheme
In the transmitter of the proposed 3-bit/symbol modulation scheme [Fig. 1(a)], a continuous wave (CW) signal is launched into a modulator (MOD), which is electrically driven by a DRZ data. The optical DRZ signal is half-bit delayed and launched into a phase modulator (PM) for DPSK signal modulation. The intensity modulated DRZ signal is at a bit-rate R and period T = 1/R. For a pulse width Δt of the DRZ pulse, there is a time interval T - Δt between one bit and the following bit that is constant optical power. The use of this time interval for phase and polarization encoding ensures the integrity for both the DPSK and PolSK signals. Experimental demonstration of a simple DRZ/DPSK transmitter has been reported recently . At the output of the PM, the DRZ/DPSK signal is launched at 45° for PolSK modulation. The polarization modulator (PolM) model used here is the same as . It consists of a polarization beam splitter (PBS) and a polarization beam combiner (PBC). A phase modulator is placed between their horizontal (0°) ports, while their vertical (90°) ports are directly connected. At the output of the PolM, the state of polarization (SOP) of the signal will take the linear values 45° (corresponding to a space on PolSK) or -45° (corresponding to a mark on PolSK).
The receiver of the proposed scheme [Fig. 1(b)] consists of three independent arms to recover the transmitted signals. A splitter at the front-end splits the input power to DRZ receiver (Rx DRZ), which consists of a PIN photodiode for the direct detection of the amplitude modulated signal. After appropriately adjusting the received SOP, a PolSK receiver (Rx PolSK), which consists of a PBS (45° aligned) followed by a balanced detector, demodulates the PolSK. The DPSK receiver (Rx DPSK), which consists of a polarizer (POL) 90° aligned to block the polarization modulated signal, a Mach-Zehnder interferometer (MZI) and a balanced detector, demodulates the DPSK signal. Since the fiber in the transmission link produces an unknown SOP, an automatic polarization controller (APC) at the receiver aligns the input SOP with the polarizing elements of the receiver. The average power detected after the POL is a reference signal to control the APC. As the buried optical fiber networks typically causes only 2° to 10° fluctuations each day in the polarization angles of the propagating signals , a dynamic polarization control can be used to compensate the polarization fluctuations. Bit-error rate (BER) measurement is performed for each signal.
3. Results and discussion
Simulations were performed to assess the feasibility of the proposed system. Each channel carried a 10 Gb/s data and was detected by an erbium-doped fiber amplifier (EDFA)-preamplified receiver (noise figure of 4 dB, Gaussian optical bandpass filter bandwidth for DRZ = 2 nm; DPSK = 1 nm and PolSK = 1 nm) with ideal PIN photodiode. The electrical signal detected after the photodiode was low-pass filtered with a third-order Bessel filter with bandwidth of 7.5 GHz for each modulation format.
Increasing the pulse widths of the DRZ signal may increase the influence of timing jitter on the DPSK and PolSK signals. Figure 2 shows the results of the optical receiver sensitivity at BER of 10-9 under the influence of different pulse widths and ER of the DRZ signal. The BER was estimated by using the build-in function of the OptiSystem, with pseudo-random binary sequence (PRBS) of 27-1. The dotted, solid and dashed lines are the change of receiver sensitivity of the DRZ, DPSK and PolSK signals respectively when different ER of DRZ in the proposed scheme are used. It can be seen that when the pulse width or the ER of the DRZ increases, the receiver sensitivity of the DRZ payload will also increase. The decrease in received optical power requirement for the DRZ signal as the pulse-width is increased, is however at the cost of a decrease in the receiver sensitivities of the DPSK and PolSK signals. As the change in receiver sensitivity of the PolSK label is relatively small (Fig. 2) for different DRZ conditions, we use the intercept of DRZ and DPSK curves to select the specific DRZ pulse width that yields the same performance for the DPSK and DRZ signals. The input optical power to the PolSK arm is higher than that of the DPSK arm (which only uses the vertical linearly polarized component). Hence the sensitivity of the PolSK signal is better. For the 3 × 10 Gb/s data rates considered here, we choose the DRZ pulse width of 35ps and ER of 16 dB (easily achievable in experimental condition) in the simulation.
Fiber birefringence was included in order to generate differential group delay (DGD) to evaluate the system tolerance to PMD. The fiber principal state of polarization (PSP) and the DGD were changed to show the eye opening factor (EOF) of the DRZ, DPSK and PolSK signals (Fig. 3). By rotating the PSP of the transmission fiber at horizontal (0°/180°) and vertical axes (90°), the DPSK signal is nearly unaffected (since DPSK signal is present at the vertical component of the beam) as shown in Fig. 3(b), while the PolSK signal has suffered slightly eye closure at these angles, as shown in Fig. 3(c). The input optical power to the PolSK arm is higher than that of the DPSK arm (which only uses the vertical linearly polarized component). Hence the receiver sensitivity of the PolSK signal is better. By changing the power splitting ratio at the PBS in the transmitter, the same base-line receiver sensitivity can be achieved for the signals. We simulated the PMD-induced eye opening penalty (EOP) of the proposed scheme with conventional 30 Gb/s RZ, DPSK and PolSK. The normalized EOPs (at worse case) at mean DGD 20 ps for the DPSK and PolSK in the 3-bit/symbol modulation are 0.13 dB and 0.17 dB respectively, while that in the conventional DPSK and PolSK are 0.45 dB and 0.73 dB respectively. The normalized EOPs (at worse case) of the DRZ and the RZ are 1.5 dB and 0.01 dB respectively, showing that RZ signal is more tolerance to PMD. Chromatic dispersion tolerance of the 3-bit/symbol modulation format was studied. The chromatic dispersion tolerance of the DRZ, DPSK and PolSK signals are ±370 ps/nm, ±2100 ps/nm and ±1680 ps/nm respectively when EOP < 3 dB.
The received eye diagrams of the 3-bit/symbol modulation scheme are shown in Fig. 4. As the DRZ signal is half-bit delayed, the influence of the DRZ modulation on the DPSK and PolSK modulations is small. Some of the inter-symbol interference (ISI) contributed by the DRZ on DPSK and PolSK was filtered out by the electrical low pass filter at the receiver. Figure 5(a) shows the eye opening factors of each modulation signal in the 3-bit/symbol modulation scheme when polarization dependent loss (PDL) is introduced into the system during transmission. The eye opening factors of DRZ and DPSK are reduced to 66% and 83% when PDL of 3 dB is introduced during transmission. We simulated the effect of PDL on conventional 10 Gb/s RZ and DPSK, showing that both signals have EOP of < 0.1 dB at PDL of 3 dB. We also calculated the effect of polarization misalignment in the receiver during the PolSK and DPSK detections of the proposed scheme (DRZ detection is polarization insensitive), as shown in Fig. 5(b) and Fig. 5(c) respectively. The DPSK is more sensitive to the polarization misalignment than the PolSK. Figure 5(d) shows the power penalty induced to the DPSK and PolSK of the proposed scheme by timing misalignment with the DRZ. The tolerance for 1-dB penalty is about 40 ps, which is larger than 20 ps reported in another 3-bit/symbol optical modulation scheme using inv-RZ and DQPSK .
Figure 6 shows the optical spectra of the proposed scheme, together with RZ (Gaussian pulse, bit-rate = 30 Gb/s, full-width half-maximum pulse-width = 35 % of bit-rate, ER = 20 dB), DPSK (bit-rate = 30 Gb/s, rise and fall time = 8 ps) and PolSK (bit-rate = 30 Gb/s, rise and fall time = 8 ps) modulations, with resolution bandwidth of 0.01 nm. In order to provide more accurate spectra, PRBS of 223-1 was used. The 20-dB spectral widths of the proposed scheme is 0.32 nm, which is the narrowest when compared with RZ (20-dB width: 0.49 nm), DPSK (20-dB width: 1.45 nm) and PolSK (20-dB width: 0.48 nm). The spectral shape of the proposed scheme is due to the superposition of the DRZ and the phase modulated signals . By deploying polarization multiplexing to the DPSK/Inv-RZ  may enhance the spectral efficiency, but this needs polarization demultiplexing at the receiver. We compared the proposed 3-bit/symbol modulation scheme with the polarization multiplexing of the DPSK/Inv-RZ, showing that both modulation formats have similar shape of optical spectra; and the same 10- and 20-dB spectral widths. Performance comparison with other advanced modulation (e.g. ODQPSK) was also performed. The proposed scheme is much more tolerant to phase noise than ODQPSK. Figure 7 shows the influence of proposed modulation and ODQPSK (at 30 Gb/s) at different laser linewidths. The greater tolerance of the proposed scheme toward phase noise is due to the fact that in ODQPSK, the phase distance between symbols is only 90°.
We propose a novel 3-bit/symbol optical data format using simultaneously DRZ, DPSK and PolSK modulations. Transmission performance (e.g. PMD, PDL, polarization and timing misalignment) was analyzed. The proposed scheme has a more compact spectrum when compared with other modulation schemes, such as RZ, DPSK and PolSK. It is also more tolerant to phase noise when compared with ODQPSK system.
This work was supported by the Research Grants Council of Hong Kong under Earmarked Grant CUHK4198/03E. C. W. Chow would like to thank A. D. Ellis and P. D. Townsend of the Photonic Systems Group, Tyndall National Institute for their support and discussion.
References and links
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