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3-Bit/Symbol Optical Modulation-Demodulation Simultaneously Using DRZ, DPSK and PolSK 1
C. W. Chow1, C. H. Kwok2, Y. Liu2, H. K. Tsang2 and Chinlon Lin2 Photonic System Group, Tyndall National Institute and Dept. of Physics, University College Cork, Ireland 2 Dept. of Electronic Engineering, The Chinese University of Hong Kong, Shatin, Hong Kong. Email:
[email protected]
Abstract: 3-bit/symbol optical modulation scheme is proposed using dark-return-to-zero (DRZ), differential-phase-shift-keying (DPSK) and polarization-shift-keying (PolSK). Transmission performance of the 3x10Gb/s signal is investigated.
©2006 Optical Society of America
OCIS codes: (060.4080) Modulation, (060.5060) Phase modulation
1. Introduction Multi-bit/symbol modulation, such as optical differential quadrature phase shift keying (ODQPSK), amplitude shift keying (ASK)/DPSK and other orthogonal modulation schemes can enhance spectral efficiency, tolerate to PMD and reduce bandwidth and costs of electronics at transmitter and receiver [1-2]. But ODQPSK requires complex encoders and decoders, and ASK/DPSK needs precise extinction ratio (ER) of ASK. Here, we proposed a novel optical modulation, achieving 3-bit/symbol using DRZ, DPSK and PolSK modulations, without reducing the ER of the three signals. 2. Principle of the Proposed Scheme (a)
DRZ Encoder
DPSK Encoder
(b)
PolSK
From Link
45o CW
MOD
PM
APC
PolM
To Link
Rx PolSK
BERT
Rx DPSK
BERT
Rx DRZ
BERT
POL 90o
Fig. 1. Schematic of the (a) transmitter and (b) receiver of the proposed 3-bit/symbol modulation scheme. MOD: modulator, PM: phase modulator, APC: automatic polarization controller, POL: polarizer, and BERT: bit-error-rate tester.
In the transmitter [Fig. 1(a)], a CW laser is launched into a modulator (MOD) for DRZ encoding. It is half-bit delayed and launched into a phase modulator (PM) for DPSK modulation. For a pulse width ∆t of the DRZ pulse, there is a time interval T - ∆t that is constant power, where T is the bit period. The signal is then launched at 45o for PolSK modulation. The polarization modulator (PolM) used is the same as [3]. The state of polarization (SOP) of the signal will take the linear values 45o (corresponding to a space on PolSK) or -45o (corresponding to a mark on PolSK), irrespective of the DPSK signal that is initially present. The receiver [Fig. 1(b)] consists of three independent arms. A splitter splits the input power to a PIN photodiode for DRZ detection [inside Rx DRZ (DRZ receiver)]. After appropriately adjusting the received SOP, a PolSK receiver (Rx PolSK), which consists of a PBS (45o aligned) followed by a balanced detector, demodulates the PolSK. The DPSK receiver (Rx DPSK) has a polarizer (POL) 90o aligned to block the polarization modulated signal, a Mach-Zehnder interferometer (MZI) and a balanced detector to demodulate the DPSK. An automatic polarization controller (APC) aligns the input SOP with the polarizing elements of the receiver. Bit-error rate (BER) measurements are performed. 3. Results and Discussion Fig. 2 shows the simulation results of the receiver sensitivity at 10 Gb/s BER of 10-9 under different pulse width and ER of the DRZ signal. The dotted, solid and dashed lines are the receiver sensitivity of the DRZ, DPSK and PolSK respectively. As the change in receiver sensitivity of the PolSK label is relatively small (Fig. 2) at different DRZ conditions, we consider the most appropriate choice for the DRZ pulse width is 35 ps and ER ≥ 16 dB (the interception of DRZ and DPSK curves). Pulse width of 35 ps and ER of 16 dB were used in simulation. Fiber birefringence was included to generate differential group delay (DGD) and evaluate the system tolerance to PMD. The fiber principal state of polarization (PSP) and the DGD were changed to show the eye opening factor of the signals (Fig. 3). By rotating the PSP of the transmission fiber at horizontal (0o/180o) and vertical axes (90o), the DPSK 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 eye closure at these angles, as shown in Fig. 3(c). The received eye diagrams (with optical amplifier noise) are shown in Fig. 4. As the DRZ is half-bit delayed, the influence of the DRZ on the other two signals is negligible. Fig. 5(a) shows the eye opening factor of the three signals when different amounts of polarization dependent loss (PDL) is introduced into the system. The eye opening factors of DRZ and DPSK are reduced to 66% and 83% respectively at PDL of 3 dB. We also calculated the effect of
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polarization misalignment on the DPSK and PolSK detections, as shown in Fig. 5(b) and Fig. 5(c) respectively. The DPSK is more sensitive to the polarization misalignment than the PolSK. Fig. 5(d) shows the power penalties of DPSK and PolSK by timing misalignment with the DRZ. DRZ DPSK
-26
PolSK
20ps DRZ to 45ps DRZ
(a) DRZ
-28
receiver sensitivity (dBm)
(a) DRZ 20ps DRZ to 45ps DRZ
-30
-32
Simulation condition
-34
(b) DPSK
(b) DPSK
-36
20ps DRZ to 45ps DRZ -38 5
7
9 11 13 15 extinction ration of DRZ (dB)
17
(c) PolSK
19
Fig. 2 Receiver sensitivity at BER of 10-9 under different pulse width and extinction ratio of the DRZ. The pulse width of the DRZ is changed from 20 ps to 45 ps with step size of 5 ps.
(c) PolSK Fig. 4. Received eye diagrams with optical amplifier noise.
Fig. 3 Eye opening factor versus DGD and PSP orientation.
DRZ
0.4
DPSK
0.2
PolSK
0 0
0.5
1 1.5 2 PDL (dB)
(a) 2.5
3
PolSK 1
(b) 0 20
30 40 50 60 70 rotation angle (degree)
8 6
DPSK
4 2
(c)
0 80
85 90 95 100 rotation angle (degree)
power penalty (dB)
0.6
2
power penalty (dB)
1 0.8
power penalty (dB)
eye opening factor (a.u.)
Fig. 6 shows the optical spectra of the proposed 3x10 Gb/s scheme, RZ, DPSK and PolSK signals each at 30 Gb/s. Although the 3-dB width of the proposed scheme is wider than DRZ and PolSK, the 10-dB and 20-dB width of the proposal (0.1 nm and 0.3 nm respectively) is the narrowest when compared with RZ (10-dB: 0.5 nm; 20-dB: 0.6nm), DPSK (10-dB: 0.3 nm; 20-db: 1.0 nm) and PolSK (10-dB: 0.1nm; 20-dB: 0.5 nm). Comparing performance with other advanced modulation (ODQPSK) was also performed. Fig. 7 shows the influence of proposed modulation and ODQPSK at different laser linewidth. The greater tolerance of the proposed scheme toward phase noise is due to the fact that in ODQPSK, the phase distance between symbols is 90o. 2.5 2
DPSK
1.5
PolSK
1
(d)
0.5 0
10 20 30 40 50 60 70 80 90 relative delay (ps)
3-bit/symbol RZ DPSK PolSK
intensity (10dB/div)
0 -10 -20 -30 -40 1552.8
1553.3 1553.8 w avelength (nm)
1554.3
Fig. 6. Optical spectra of the proposed scheme, RZ, DPSK and PolSK modulation
power penality (dB)
Fig. 5. (a) Eye opening factor of the three modulation signals versus polarization dependent loss (PDL). Power penalties of (b) PolSK and (c) DPSK versus polarization misalignment. (d) Power penalty of DPSK and PolSK versus timing misalignment of the DRZ signal. 8
DRZ DPSK PolSK ODQPSK
6 4
in the proposed 3bit/symbol modulation
2 0 10
15 20 laser linew idth (MHz)
25
Fig. 7. Power penalty versus laser linewidth of the proposed and ODQPSK system.
4. Conclusion We propose a novel 3-bit/symbol optical modulation using DRZ, DPSK and PolSK. Transmission performance (DGD, PDL, polarization and timing misalignment) was analyzed. The proposal has a compact spectrum and is more tolerance to phase noise. Acknowledgment The authors would like to thank Dr. A. D. Ellis and Prof. P. D. Townsend for their useful discussion.
References [1] R. A. Griffin et al “Optical differential quadrature phase-shift keying (oDQPSK) for high capacity optical transmission,” OFC’02 367 (2002). [2] C. W. Chow, et al “All-Optical ASK/DPSK label-swapping and buffering using Fabry-Perot laser diodes,” IEEE JSTQE, 10, 363 (2004). [3] J. Comellas, et al “Quaternary optical transmission system combining phase and polarization-shift keying,” IEEE PTL, 16, 1766 (2004).
