S pecial Topic FSK Modulation Scheme for High-Speed Optical Transmission Nan Chi, Wuliang Fang, Yufeng Shao, Junwen Zhang, and Li Tao
Clock
Data VOA1
PC1
CW1
generation method in a high-speed label switching system. Section 6 concludes the paper.
2 RZ-FSK Transmitter and Receiver
50∶50 MZDI
MZM 1 PC2
CW2
MZM 2
PM FSK Generation RZ-FSK Generation (a)
OSC 30∶70
PD1
PC3
OBPF
+ -
VOA2
LPF
BERT
AWG PD2
RZ-FSK Detection
Transmission (dB)
(b)
Constructive Interference
Destructive Interference
1/Tb
f1
Inverted Data
f2
FTS
Frequency (Hz)
Fig. 1(a) shows the schematic of the RZ-FSK transmitter, and Fig. 1(b) shows the schematic of the RZ-FSK receiver. A low-speed RZ-FSK signal can be produced by directly modulating distributed feedback (DFB) lasers. The lasers are driven by a bias current far above threshold, and a relatively small modulation current is added. To generate an FSK signal at 40 Gb/s and above, two continuous-wave (CW) lasers with carefully selected center frequency are combined by a 3 dB coupler and fed into the Mach-Zehnder modulator (MZM1) or phase modulator (PM). The combined input is modulated by NRZ data and then demodulated to intensity modulation by a Mach-Zehnder delay interferometer (MZDI). The MZDI is imbalanced by the introduction of a one-bit time-delay line. The wavelengths of the two beams are carefully selected so that one beam is at the maximum transmission of the MZDI (constructive interference) and the other is at the minimum transmission of the MZDI (destructive interference). Thus, the (N +1)/2 frequency tone spacing (FTS) is Hz, where Tb N = 1, 2, 3,...,n and Tb is a bit period. The center frequencies of the two beams are f1 and f2. The optical field exiting a phase modulator for f1 is given by
(
Data
)
(
E1(t )=E 0·exp j 2πf1Tb+Φ 1+Φ 2 ·cos 2πf1Tb+φ 2 2 (c)
AWG: array waveguide grating BERT: bit-error ratio tester CW: continues-wave laser. FSK: frequency-shift keying LPF: low pass filter MZDI: Mach-Zehnder delay interferometer MZM: Mach-Zehnder modulator
OBPF: optical band-pass filter OSC: oscilloscope PC: polarization controller PD: photodiode. PM: phase modulator VOA: variable optical attenuator
▲Figure 1. Principle of RZ-FSK generation and detection and the generated FSK signal by experiment. (a) Configuration of our proposed transmitter, (b) configuration of the used receiver, (c) measured FSK waveforms of the two tones. described, and the principles of RZ-FSK generation and detection are given. In section 3, we use simulations to compare FSK, DPSK, and RZ-FSK at 40 Gb/s for 12 spans of 80 km SMF. RZ-FSK is more tolerant to nonlinearity and chromatic dispersion than the other modulation formats. We also analyze an RZ-FSK signal in a 4 × 40 Gb/s WDM transmission system. In section 4, we experiment with a 40 Gb/s RZ-FSK signal over 100 km SMF with full dispersion compensation. We compare transparent wavelength conversion based on four-wave mixing (FWM) in a semiconductor optical amplifier (SOA) with transparent wavelength conversion based on FWM in a highly nonlinear dispersion-shifted fiber (HNDSF). The feasibility of all-optical signal processing of a 40 Gb/s RZ-FSK signal is validated. In section 5, we discuss the use of the RZ-FSK
)
(1)
and the optical field exiting the phase modulator for f2 is given by
(
)
(
E2(t )=E 0·exp j 2πf2Tb+Φ 1+Φ 2 ·cos 2πf2Tb+φ 2 2
)
(2)
where Φ 1 and Φ 2 are the phases of the neighboring bits, and the data information is reflected in the phase difference φ. The f1 center frequency is given by
f1=m /Tb (m =0,1,2,3,...,M )
(3)
and the f2 center frequency is given by
f2=f1+ 2n +1 2Tb
(n = 0, 1, 2, 3,..., N )
(4)
However, in a real transmitter, the rise and fall time of the input signal needs to be taken into consideration. If 1/f1 = Tb /m (m = 0, 1, 2, 3,..., M ), then Tb /M must be greater than the rise time of the input-signal pulse width. If Tb /m is too small, the phase information generated by the MZM1 or PM could not be transferred to the intensity signal in the MZDI. Even if the phase information is accurately transferred to the amplitude information, the difference between the generated amplitude information for binary data 1 and 0 is small. The rise and fall time of the input signal should not exceed Tb /m. With this prerequisite, the maximum value of m can be calculated. The bandwidth of the transmitted signal is a function of the carrier’ s rise and fall times, and it should be determined September 2012 Vol.10 No.3
ZTE COMMUNICATIONS
03