Chapter 2 Simulation study of the APSK format using zero-nulling method
2.3 Simulation results of the APSK format and zero-nulling method
2.3.2 Simulation results of the APSK transmission
Figure 2.6 shows the transmission performance of the APSK system. This figure shows the transmission performance of the ASK signal and the PSK signal as a function of the transmission distance and the ER of the ASK signal. Figure 2.7 shows the transmission performance after 1500km of the ASK and the PSK signals as a function of the extinction ratio of the ASK signal. Each point is an averaged value over 16 WDM channels for both figures. As shown in these figures, when the ER increased, the Q-factor of the ASK signal also increased. This means the performance became better when the ER increased. On the other hand, the performance of the PSK signal degraded as the ER increased. Figure 2.7 shows a clear trade off between the ASK signal and the PSK signal. As seen in this figure, the optimized point of the ER was around 5 to 6dB. It means the best ER for the APSK transmission system was 5 to 6dB.
0
0 500 1000 1500 2000 2500 3000 3500 4000
3dB 4dB 5dB 6dB 7dB 10dB 13dB
ASK PSK
Transmission distance (km)
Fig. 2.6 Transmission performance of the APSK signal
0 5 10 15 20
0 5 10 15
Extinction ratio of ASK signal
Q-factor (dB)
ASK PSK
Fig. 2.7 Transmission performance of the APSK signal after 1500km 2.3.3 Zero-nulling APSK format
As seen in the performance of the APSK system shown in the previous section, there is a clear trade off between the ASK signal and the PSK signal due to the ER of the ASK signal.
The PSK signal suffers degradation when the extinction ratio of the ASK signal is large, and the reason of the degradation can be attributed to the PSK information in the space level of the ASK signal. The optical noise causes a significant degradation on this PSK information because the optical power is small. In order to improve the transmission performance, a method is proposed. As the PSK information in the ASK space level suffers significant degradation, if this PSK signal can be ignored, the transmission performance can be improved. Figure 2.8 shows an explanation of this method schematically. This method is named as “zero-nulling” method. The PSK information is carried only when the ASK signal is mark, and the PSK information is nulled while the ASK signal is zero.
(A) Original APSK (B) Zero-nulling APSK Fig. 2.8 A schematic to explain the zero-nulling method
Because the PSK information is ignored on the ASK space level, and the mark ratio is one half in the general case, the capacity of the PSK signal becomes half compared to the original APSK format. As a result, the total effective bit-rate becomes 75% of that of the original APSK system. It is the trade off between this method and the original APSK system.
Another issue of this method is that it is impossible to use the delay demodulation for the detection of the PSK signal. Still, it should be possible to utilize the homodyne/heterodyne detection with the digital signal processing [3] to obtain the phase information imposed only when the ASK signal is the mark.
2.3.4 Simulation results of the zero-nulling APSK transmission
The effectiveness of the “zero-nulling” method was confirmed through the numerical simulations. The performance of the original APSK and the zero-nulling APSK was confirmed. As the total effective bit-rate was 75% of the original APSK system using the zero-nulling method, 7.5Gbits/s APSK system was acting as a reference to compare the performance of the APSK system using the zero-nulling method. The result is shown in figure 2.9. The extinction ratio of the ASK signal was set to 13dB for both the original and the zero-nulling APSK. As shown in this figure, the PSK signal had a significant improvement after using the zero-nulling method.
0 5 10 15 20 25
0 500 1000 1500 2000 2500 3000 3500 4000
Transmission distance (km)
Q-factor (dB)
ASK PSK ASK PSK Zero-nulling
Original (7.5G)
Fig. 2.9 Transmission performance of the APSK signal with and without zero-nulling method.
Figure 2.10 shows the phase eye diagrams after 1500km transmission. As shown in figure (a), the eye diagram was almost closed for the original APSK, but the eye in figure (b) was much clearly opened for the zero-nulling method. This result also shows the effectiveness of the zero-nulling method for the long distance transmission.
(a) (b) Fig. 2.10 Phase eye diagrams after 1500km transmission.
(a) Original APSK, (b) Zero-nulling APSK
2.4 Conclusion
In this chapter, the numerical simulation of the APSK format for the long-haul optical fiber transmission was conducted. It was observed that there was a clear trade off between the ASK signal performance and the PSK signal performance in the APSK system caused by the ER. It was found that the optimized point of the ER was 5 to 6dB after a few thousands kilometers transmission. In order to improve the transmission performance, the zero-nulling method was proposed. As seen in the numerical simulation results, the improvement was clearly observed by this method.
References in this chapter
[1] G. P. Agrawal, Nonlinear Fiber Optics, Academic Press, 2001.
[2] J. R. Costa, C. R. Paiva, and A. M. Barbosa, “Modified split-step Fourier method for the numerical simulation of soliton amplification in erbium-doped fibers with forward-propagating noise,” IEEE J. of Quantum Electronics, VOL.37, NO.1, pp.145-152, 2001
[3] O. V. Sinkin, R. Holzlöhner, J. Zweck, and C. R. Menyuk, “Optimization of the split-step Fourier method in modeling optical-fiber communications systems,” IEEE J. of Lightwave Technol., VOL.21, NO.1, pp.61-68, 2003
[4] X. Wei, X. Liu, and C. Xu, “Numerical simulation of the SPM penalty in a 10-Gb/s RZ-DPSK system,” IEEE Photon. Technol. Lett., VOL.15, NO.11, pp.1636-1638, 2003.
[5] K. Kikuchi, “Phase-diversity homodyne detection of multilevel optical modulation with digital carrier phase estimation,” IEEE J. of Selected Topics in Quantum Electron., VOL.12, NO.4, pp.563-570, 2006.
Chapter 3 Experimental investigation of APSK format focusing on extinction ratio
3.1 Introduction
Following the theoretical investigation conducted in chapter 2, experimental investigation was carried out to confirm the results of the numerical simulation. This chapter focuses on the effect of the ER of the ASK signal in the APSK system. As discussed in chapter 2, there is a clear trade off between the ASK signal performance and the PSK signal performance in the APSK system through the ER. Therefore, purpose of this chapter is to confirm this trade off of the performance experimentally. Section 3.2 explains the experimental setup in detailed and shows the parameters of the transmission line. The transmission performance is briefly explained in section 3.3.
3.2 Experimental setup
In this section, the detail of the system setup is explained. Figure 3.1 shows a schematic diagram of the experimental setup. The explanation of the experimental setup is separated into three parts: the transmitter, the transmission line, and the receiver. The goal of this experiment is to clarify the effect of the ER in the APSK system, and to find out the optimized ER to get the best performance of the APSK system.
RZ-Conv
Fig. 3.1 Schematic diagram of the APSK transmission system setup
3.2.1 Transmitter
Figure 3.2 is a schematic diagram of the transmitter. At the transmitter, there was a DFB-LD emitting at 1550.9nm, and an ASK modulator and a PSK modulator were connected in series after the Distributed FeedBack Laser Diode (DFB-LD). The ASK modulator was made from the LiNbO3 material and based on a Mach-Zehnder (MZ)interferometer [1,2]. The Non-Return-to-Zero (NRZ) signal from the pulse pattern generator (PPG) was converted to the RZ format through the RZ converter, and it was fed to the ASK modulator. The PSK modulator was driven by another NRZ signal from the PPG. The reason of using the RZ format for the ASK signal was the limitation of the test equipment, because the ASK receiver only had the clock recovery function for the RZ signal. It was necessary to use the RZ format to retrieve the clock timing after the transmission, otherwise, the measurement of the BER might have some problems. An electrical delay line (EDL) was added between the PPG and the PSK modulator in order to synchronize the timing of the electrical signals to the ASK modulator and the PSK modulator. In order to adjust the ER of the ASK signal, the driving voltage and the bias voltage to the ASK modulator was controlled. The modulation bit-rate and the pattern were 10.66Gbit/s and 215-1, respectively.
Fig. 3.2 Schematic diagram of the transmitter
3.2.2 Transmission line
Figure 3.3 shows a schematic diagram of the transmission line. Seven spans of the optical fibers and seven EDFAs were used for 330km transmission line. The optical fibers used for the experiment were the NZDSF and the SMF. The parameters of these fibers are summarized in Table 3-1. The NZDSF had a negative chromatic dispersion, and the SMF compensated the accumulated negative dispersion by its positive dispersion. The repeater span length of the NZDSF was 50km, while that of the SMF was 30km. The loss of each span was 11.5dB for the NZDSF and 7dB for the SMF. The dispersion map of 330km transmission line is shown in figure 3.4. The first to the third and the fifth to the seventh spans were the NZDSF, and the fourth span was the SMF. This kind of the dispersion map is commonly
the transmission line was 1553.5nm. The EDFAs were used to compensate the loss of the fibers. The output power of the EDFA repeater was set to +3dBm. This was the optimized point that were tested. If the repeater output power was too small, the Signal to Noise Ratio (SNR) decreased. On the other hand, if the repeater output power was too large, it caused large nonlinear effect like a Brillouin scattering or a Self Phase Modulation (SPM). These nonlinear effects degraded the transmission performance. After the transmission line, an Optical Band Pass Filter (OBPF) was inserted to reduce the accumulated ASE noise of the EDFAs.
Fig. 3.3 Schematic diagram of the transmission line
Table 3-1 Parameters of the transmission fiber
Parameters NZDSF SMF
Chromatic dispersion (ps/km/nm) -2 20
Dispersion slope (ps/km/nm2) 0.128 0.59 Transmission loss (dB/km) 0.23 0.23
-400 -200 0 200 400
0 100 200 300
distance(km)
cumulative dispersion(ps/nm/km)
330km
Fig. 3.4 Dispersion map of the transmission line
3.2.3 Receiver
A schematic of the optical receiver is shown in figure 3.5. The EDFA acted as a preamplifier to improve the receiver sensitivity. The OBPF was used to reduce the ASE noise of the preamplifier. The coupler was used to split the optical power to the PDs (PD) of the ASK receiver and the PSK receiver. The ASK detector provided a recovered clock to measure the BER performance. The DPSK demodulator was used to demodulate the PSK signal. The Differential Phase Shifting Keying (DPSK) demodulator was using 1-bit delay scheme to switch the PSK information to amplitude signal. This amplitude signal was fed to the balanced PD. After that, the optical signal was converted to the electrical signal through this balanced PD. The PSK signal used the same clock to measure the BER performance, because the ASK signal and the PSK signal was synchronized. The performance of the ASK signal and the PSK signal was measured separately using this optical receiver.
Fig. 3.5 Schematic diagram of the receiver
3.3 Results and discussions focusing on the effect of the ER 3.3.1 Optical spectrum of the transmission line
The optical spectrum before transmission is shown in figure 3.6. The OSNR is 55dB at 0.01nm resolution. After 330km transmission, the ASE noise was accumulated. It is easy to observe this in figure 3.7. The ASE noise was generated by the EDFAs. As the signal power decreases after transmission through the optical fibers, the EDFAs are used to compensate the optical power to the next transmission stage, but the EDFAs not only increase the optical power of the signals but also add the optical noise. Therefore, the ASE noise accumulates in each repeater. After 330km transmission, the accumulated ASE noise power became significant, and if the PD received all of the noise power, the received signal was severely degraded. In order to overcome the degradation, the OBPF was used. As the OBPF did not remove the optical noise at the signal wavelength, the OSNR at the signal wavelength could not be improved by the OBPF. Without an OBPF, ASE noise of far apart from the signal wavelength could act as noise at the receiver, this ASE noise could be removed by the OBPF.
Therefore, if we compare the performance without the OBPF, the OBPF improved the transmission performance.
As shown in figure 3.8, the ASE noise far apart the signal wavelength was removed by the OBPF. It showed a clear optical spectrum compared to figure 3.7. Because another EDFA was added after the OBPF, the ASE noise in left side of the signal wavelength was increased as shown in figure 3.8.
Fig. 3.6 Optical spectrum before transmission
Fig. 3.7 Optical spectrum after 330km transmission
Fig. 3.8 Optical spectrum after using OBPF
3.3.2 Eye diagram of the ASK and the PSK signals
Figure 3.9 shows the electrical eye diagram of the ASK signal with different ERs for back-to-back and after 330km transmission. Because the electrical amplifier was included in the receiver, the electrical eye could not show the amplitude improvement clearly. As the ER increased, the eye opening became clearer in back to back situation. In the definition of ER, high ER means the difference between the mark level and the space level is large. Therefore, it is reasonable to observe a good eye opening in high ER case. After 330kmtransmission, the eye was degraded due to the accumulated ASE noise. It was expected that when the ER was small, the performance degradation due to the accumulated ASE noise was more significant compared to the high ER case. Even though the electrical amplifier affected the eye diagram shown in figure 3.9, the eye opening became clearer as the ER increased. For the PSK signal, the eye diagram is shown in figure 3.10. The eye became closed as the ER increased. The reason of this phenomenon can be attributed to the degradation of the PSK information in the ASK space level. The detailed explanation had mentioned in Chapter 2.2. After 330km transmission, the PSK signal suffered degradation due to the accumulated ASE noise, and it was clearly observed in the eye diagram after transmission.
(a) back to back (ER 3dB) (b) after 330km transmission (ER 3dB)
(c) back to back (ER 4dB) (d) after 330km transmission (ER 4dB)
(e) back to back (ER 5dB) (f) after 330km transmission (ER 5dB)
Fig. 3.9 Eye diagram of the ASK signal
(a) back to back (ER 3dB) (b) after 330km transmission (ER 3dB)
(c) back to back (ER 4dB) (d) after 330km transmission (ER 4dB)
(e) back to back (ER 5dB) (f) after 330km transmission (ER 5dB)
Fig. 3.10 Eye diagram of the PSK signal
3.3.3 Performance of the ASK signal
Figure 3.11 shows the BER performance of the ASK signal. Figure 3.11 (a) shows the back-to-back performance and (b) shows the performance after 330km transmission. As seen in figure 3.11 (a), the performance of the ASK signal was improved when the ER was increased. The performance of the ASK signal after the transmission was degraded as shown in figure 3.11 (b). The reason of the degradation can be attributed to the ASE noise accumulation. Fig. 3.11 Performance of the ASK signal
3.3.4 Performance of the PSK signal
Figure 3.12 shows the BER performance of the PSK signal. Figure 3.12 (a) shows the back-to-back performance and (b) shows the performance after 330km transmission. As seen in the figure, the performance was degraded as the ER increased. As seen in figure 3.12 (b), the error floor was clearly observed when the ER was 5dB. According to the results of the ASK signal and the PSK signal shown in figure 3.11 and 3.12, high ER improved the transmission performance of the ASK signal, and degraded the performance of the PSK signal. On the other hand, the PSK signal had better performance in low ER case, but the performance of the ASK signal was degraded. Therefore, a tendency of the trade off between the ASK signal and the PSK signal was observed clearly.
-37 -32 -27 -22 Fig. 3.12 Performance of the PSK signal
The power penalties at 10-9 BER as a function of the ER are shown in figure 3.13 The PSK signal could not achieve 10-9 BER when the ER was 5dB, so it was not plotted in the figure. As shown in this figure, the power penalties of the PSK signal increased as the ER increased. On the other hand, the power penalties of the ASK signal decreased as the ER increased. These results showed a clear trade-off between the ASK signal performance and the PSK signal performance of the APSK transmission.
0 1 2 3 4
2 2.5 3 3.5 4 4.5 5 5.5
Extinction Ratio(dB)
Power Penalty(dB) ASK
PSK
Fig. 3.12 Power penalty of ASK and PSK at BER=10-9
3.4 Conclusion
This chapter discussed the experimental investigation of the APSK system as a function of the ER. The transmission performance showed in this chapter demonstrated that high ER improved the transmission performance of the ASK signal and degraded that of the PSK signal. The PSK signal performance was improved when the ER was decreased. After the transmission, this tendency was clearer. These results show the clear trade off between the ASK signal performance and the PSK signal performance in the APSK system as expected from the theoretical study in chapter 2.
References in this chapter
[1] G.. P. Agrawal, Fiber-optic communication systems, Wiley Interscience, Third Edition.
[2] L. Thylen, “Integrated optics in LiNbO3: recent developments in devices for telecommunications,” IEEE J. of Lightwave Technol,. VOL 6, NO.6, pp.847-861, 1988.
Chapter 4 Experimental investigation of the APSK format using zero-nulling method
4.1 Introduction
This chapter focuses on the experimental investigation of the APSK system using the zero-nulling method. At first, the experimental setup of the recirculating loop is explained.
This experimental setup is used to measure the performance of the long-haul optical fiber communication systems. After that, the experimental investigation focusing on the long-haul optical fiber communication system using the APSK modulation format as a function of the ER is discussed. Finally, the experiment of the APSK system using the zero-nulling method is conducted and the result is shown in section 4.3.3.
4.2 Experimental setup
4.2.1 Experimental setup of the APSK system with the recirculating loop
Figure 4.1 shows a schematic diagram of the recirculating loop setup. The experimental setup was almost the same with the previous experimental setup shown in chapter 3 except the transmission line and the wavelength of the DFB-LD. The wavelength of the DFB-LD was set to 1550.2 nm in order to reduce the Chromatic Dispersion (CD) in the transmission line. The length of the transmission line was increased to 500km in order to investigate the APSK system performance after long distance transmission. In addition, optical switch 1 and switch 2 were used to control the signal transmission through the optical fiber loop. Detail about the control of the recirculating loop is described in the next section
EDL
EDFA OBPF
Optical switch 1
SMF
NZ DSF
x4 NZ DSF x5
Optical switch 2OBPF
RZ-Conv PPG
DFB-LD ASK
detector
Fig. 4.1 Schematic diagram of the recirculating loop setup 4.2.2 Recirculating loop
The recirculating loop experiment is very useful for evaluating the long-haul optical fiber communication systems. The length of the transmission line needed for the long-haul optical fiber communication system is a few thousand kilometers. It is very difficult to demonstrate this kind of the long transmission distance using straight line experimental setup.
Therefore, the experimental setup of the recirculating loop is utilized to simulate the system performance of the long-haul optical fiber communication system.
Figure 4.2 shows the timing trigger of the recirculating loop. The unit period of the time is determined by the length of the transmission line. As the refractive index of the fiber is 1.475 and the speed of the light in vacuum is C=2.99792458*108(m/s), the light speed in the
Figure 4.2 shows the timing trigger of the recirculating loop. The unit period of the time is determined by the length of the transmission line. As the refractive index of the fiber is 1.475 and the speed of the light in vacuum is C=2.99792458*108(m/s), the light speed in the