Chapter 3 Experimental investigation of APSK format focusing on extinction ratio
3.3 Results and discussions focusing on the effect of the ER
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 fiber C’ can be calculated as
C’ =C/n (4.1)
C’=2.032491241*108(m/s).
As the fiber length L of the setup was 497.189055km, according to those parameters, the unit time period T to pass through the transmission line became
T=L/C’ =2.446205155(ms) (4.2)
Based on this calculation, the unit time period was determined.
The optical switch 1 in figure 4.1 turns on at the first period as shown in figure 4.2. The signal from the transmitter is transmitted to the transmission line and the receiver through 3dB coupler. At this moment, the receiver receives the signal that does not pass through the transmission line. In other words, the receiver receives the signal that performs under back to back situation. The same signal from another output from the 3dB coupler passes through the transmission line to switch 2, which turns on at the second period to let the signal passing through the 3dB coupler again. If the trigger to the error detector (ED) of the receiver turns on in this period like the red-solid line shown in figure 4.2, the ED can measure the transmission performance after the first loop. It means the measured performance is after 500km transmission when the red-solid line is used as the trigger to the ED. This signal not only transmits to the receiver, but also transmits to the transmission line again. Therefore, the switch 2 turns on at the third period to transmits the signal after the second round trip, and so on. As the figure 4.2 shows, the switch 2 turns on four units, it means this recirculating loop setup can transmit the signal over 2000km. The transmission distance can be increased as the periods of switch 2 are increased. The transmission distance is determined by number of recirculation through the loop. The actual distance to be measured is determined by the trigger position of the ED. As figure 4.2 shows, the performance of after 500km transmission can be measured when the red-solid line is used for the trigger of the ED, and the performance of after 1500km transmission can be measured when the blue-dotted line is used for the trigger of the ED. This is the operation principle of the recirculating loop.
Fig. 4.2 Timing trigger of the recirculating loop 4.2.3 Setup to test the zero-nulling APSK system
Figure 4.3 shows a schematic diagram to test the APSK system using the zero-nulling method. The experimental setup was almost the same as the experimental setup shown in chapter 3. The transmission distance was increased to 500km with 50km repeater span. The most important difference was the transmitter. Two individual PPGs were used to generate different signal patterns. The PSK signal was generated by a LiNbO3 phase modulator. The modulation bit-rate and the pattern were 11.43Gbit/s and Pseudo Random Bits Sequence (PRBS) 215-1, respectively. The ASK signal was generated by a LiNbO3 MZ modulator. The modulation bit-rate was 11.43Gbit/s, and the test pattern was 32 bits sequence special pattern.
This pattern had ten consecutive mark bits, ten consecutive space bits, and six consecutive mark and space bits pair like a schematic illustration shown in figure 4.4. The reason of using this kind of special pattern is explained in the next paragraph. The ASK modulator was driven by a RZ signal through a RZ converter, and the PSK modulator was driven by a NRZ signal.
T SW1
SW2
ED
time
time
time
RZ-Conv EDL
Fig. 4.3 Schematic diagram for testing the zero-nulling method
he zero-nulling method transmits the PSK information only when the ASK signal has a mark level. Therefore, if we can measure the BER of the PSK signal bit which corresponds to the ASK mark bit, we can evaluate the performance of the zero-nulling method experimentally.
Therefore, a commercially available BER test set (Anritsu, MT1810A) was utilized to measure the performance of the zero-nulling method in the APSK system
.
The testequipment has 32 individual error counters, and the bit-error count is accumulated for each counter. This means that the BER can be measured by 32 different bit positions. The bit window measurement function enables or disables the BER measurement of a specific bit within these 32 bit positions [1]. The mechanism of the testing method is illustrated in figure 4.5. The squares with broad line in the figure are the chosen positions that will be ignored in the BER measurement. The BER test set can ignore any position in the 32 counters, and the testing position can be measured repeatedly to accumulate the bit errors in the counters. This is the reason why this experiment used the special 32 bits sequence for the ASK modulation.
In the experimental setup, the bits position from 10 to 32 were chosen, which were ignored in the BER measurement. After that, the BER test set measure the BER performance of the PSK signal of the bits position from 1 to 9 which corresponds to the consecutive mark bits of the ASK signal.
Fig.4.4 a schematic illustration of special testing pattern
Fig. 4.5 A illustration of bit window
1 - 0 - ….. 1 0 1 - 0 - ….. 1 0 1 2 3 4
1 2 3 4 ….. 31 32 ….. 31 32
….. 31 32 1 ….. 31 32
1 2 3 4 2 3 4
4.3 Results and discussions
4.3.1 OSNR performance of the recirculating loop
The OSNR is confirmed in this section. The reason of testing the OSNR is that, if the OSNR is not correct in the transmission line, some issues cause degradation of the OSNR. To calculate the OSNR associated with a long-haul optical fiber communication system, the total ASE power needs to be calculated. The ASE power can be obtained using equation (4.3) [2].
2 0 ( 1)
A S E sp A o p t
P = n hν N G − Δν (4.3)
where 2nsp = Fn, Fn is the noise figure which is determined by the EDFA. h is Planck’s constant. ν0 is frequency of the signal. NA stands for the number of amplifiers. G is the gain of the amplifier. ΔνOPT is the bandwidth of the optical filter. The OSNR can be calculated using equation (4.3).
in/ ASE
OSNR=P P (4.4)
For the experiment, Fn of EDFA was 5dB, ν0 was 1550.1nm, N was 11, G was A 11.5dB, ΔνOPT was 0.2nm. According to those parameters, the OSNR was 27.8dB after 500km transmission. As shown in figure 4.6, the peak power after 500km transmission was -11.2dBm, and the noise level was about -39.7dBm .Therefore, the OSNR of the experiment was 28.5dB. The difference between the theoretical calculation and the experimental result was 0.7dB. It is small enough to say that the measured OSNR of 500km transmission agreed well to the theoretical calculation. At the third loop (1500km), the OSNR should be decreased by 4.8dB from the theoretical calculation because the number of the amplifiers became triple.
The measured OSNR can be confirmed from figure 4.7. The peak power after 1500km transmission was -14.5dBm, and the noise level was -37.0dBm. Then, the measured OSNR was 22.5dB. The theoretical OSNR was 23.0dB. Therefore, the theoretical calculation and the experimental result were agreed well, and it was confirmed that there was no problem in the transmission line.
(a) The optical spectrum with 10dB/division
(b) The optical spectrum with 0.5dB/division Fig. 4.6 OSNR after 500km transmission
(a) The optical spectrum with 10dB/division
(b) The optical spectrum with 0.5dB/division Fig. 4.7 OSNR after 1500km transmission
4.3.2 Performance of the long distance transmission
At first, the transmission performance of the APSK system as a function of the transmission distance focusing on different ER of the ASK signal was measured. Figure 4.8 shows the results. A clear trade off between the ASK signal and the PSK signal in the APSK system was observed. This experimental result fits qualitatively well with the simulation results in chapter 2.
0 500 1000 1500 2000 2500 3000 3500
Transmission distance (km)
Fig. 4.8 BER as a function of the transmission distance 4.3.3 Performance of the APSK format using the zero-nulling method
The performance of the zero-nulling method was evaluated after 500km transmission.
Figure 4.9 shows the BER performance of the ASK signal in the APSK system as a function of the receiver input power with different ER. As seen in the figure, high ER showed better performance. After the transmission, the performance was degraded due to the ASE noise.
The BER performance of the PSK signal in the APSK system is shown in figure 4.10. As shown in this figure, the BER performance of the PSK signal using the zero-nulling method was clearly improved regardless to the back to back situation and after 500km transmission, even when the ER was equal to 10dB.
-37 -32 -27 -22 -17 -12 -7
Fig. 4.9 Performance of the ASK signal in the APSK system
-37 -32 -27 -22 -17 -12 -7
(b) after 500 km transmission
Fig. 4.10 Performance of the PSK signal in the APSK system
As shown in figure 4.10, the PSK signal using the zero-nulling method exhibited a clear improvement compared to the conventional APSK format. As mentioned in chapter 2, it is impossible to use the delay demodulation in this scheme, but a measurement technique using the delay demodulation was developed and the effectiveness of the zero-nulling method was confirmed experimentally. For the actual implementation of this method, it is needed to find some method to obtain the phase information imposed only when the ASK signal is equal to
“one”.
4.4 Conclusion
The performance of the long-haul APSK system as a function of the ER was confirmed through the experiment in this chapter. A clear trade off between the ASK performance and the PSK performance was observed. The qualitative measurement of the zero-nulling method was conducted in this chapter, and the performance of the APSK system was clearly improved by using the zero-nulling method.
References in this chapter
[1] Anritsu MU181040A Operation Manual
[2] G. P. Agrawal, Fiber-optic communication systems, Wiley Interscience, Third Edition.
Chapter 5 Discussion of simulation and experimental results
5.1 Introduction
This chapter discusses the results obtained so far. At first, the results of the numerical simulations are discussed in section 5.2. Section 5.3 discusses the results of the experimental investigations. Some phenomenon shown in the experimental results are pointed out and discussed what kind of mechanism causes this kind of result. Section 5.4 compares the results of numerical simulations and experimental investigations. Some results of the experiments agree the numerical simulations, but some are not. The reasons of this kind of discrepancies are discussed in this section.
5.2 Discussion of the simulation results
Figure 5.1 shows the simulation result of the transmission performance of the APSK signal as a function of the transmission distance with different ER of the ASK signal, which was already explained in chapter 2. As seen in this figure, the transmission performance degrades as the transmission distance extends. Because the chromatic dispersion was compensated by the dispersion map and the receiving terminal, the degradation was mainly due to the accumulated ASE noise. The accumulated ASE noise degraded the SNR, and the SNR degraded the transmission performance. Figure 5.2 summarizes the performance of the APSK signal after 1500km transmission. The ER was set from 3dB to 13dB. The transmission performance of the ASK signal became better when the ER increased, while the transmission performance of the PSK signal became better when the ER of the ASK signal decreased.
These results show a clear trade-off between the ASK performance and the PSK performance. The reason of the PSK performance degradation could be attributed to the degradation of the phase information in the space signal of the ASK format. In the definition, the optical power of the mark level was fixed. Therefore, high ER meant the optical power of the space level was small. As the space signal of the ASK format suffered the effect of the optical amplifier noise more severely, the information of the PSK format on this part was damaged, and the overall performance of the PSK signal was degraded. As shown in figure 5.2, the performances of the ASK signal and the PSK signal could be compromised. The optimum extinction ratio for 1500km transmission should be around 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. 5.1 Simulated transmission performance of the APSK signal
0
Extinction ratio of ASK signal
Q-factor (dB)
ASK PSK
Fig. 5.2 Simulated transmission performance of the APSK signal after 1500km transmission
In figures 5.1 and 5.2, the performances of the APSK signal as a function of the ER and the transmission distance are described. Then, next step is the discussion of the zero-nulling method. Figure 5.3 shows the transmission performance of the APSK signal with and without the zero-nulling method. For the ASK signal, the performance of the original ASK signal (7.5G) was slightly better than the zero-nulling APSK because of the noise reduction
corresponding to the bit-rate reduction. The performance of 7.5G ASK signal was slightly better than 10G ASK signal. On the other hand, the PSK performance of the zero-nulling APSK signal was greatly improved compared to the original APSK signal. This simulation result showed that the zero-nulling method was proved to be quite effective to improve the long-distance transmission performance of the APSK format. On the other hand, because there was no PSK information in the ASK space level, the transmission capacity was reduced compared to the original APSK system.
0
0 500 1000 1500 2000 2500 3000 3500 4000
Transmission distance (km)
Fig. 5.3 Transmission performance of the APSK signal with and without the zero-nulling method
5.3 Discussion of the experimental results
This section focuses on the results investigated experimentally. The phenomenon of the transmission performance is discussed in this section. Figure 5.4 shows the BER performance as a function of the transmission distance and the ER. As seen in this figure, the performance of the ASK signal was getting better as the ER was increased. On the other hand, the performance of the PSK signal was degraded as the ER was increased. This experimental result showed a clear trade off between the ASK signal and the PSK signal.
Next, the performance of the zero-nulling method was evaluated after 500km transmission. Because the bit window function was not compatible with the burst mode measurement required for the recirculating loop experiment, only the straight line transmission (500km) result was measured. Figure 5.5 shows the BER performance of the PSK signal in back to back situation and after 500km transmission. The performance was
evaluated as a function of the ER of the ASK signal. As seen in figure 5.5(a), the performance of the PSK signal degraded as the extinction ratio was increased. It should be noted that the power penalty of 6dB ER case was the largest among five cases. When the ER was high, the PSK information in the ASK space level suffered significant degradation due to the small optical power.
0 500 1000 1500 2000 2500 3000 3500
Transmission distance (km)
Fig. 5.4 BER performance as a function of the transmission distance
The performance after 500km transmission is shown in figure 5.5(b). The performance was degraded significantly compared to the back to back situation. Less than 10-9 BER could be achieved only when 3 dB ER case and with the zero-nulling method case. The error floor was clearly observed when the ER was 4, 5, and 6 dB. It could be explained that the PSK signal in the ASK space level suffered signal to noise ratio degradation due to the accumulated ASE noise, and it caused significant transmission penalty to exhibit an error floor after 500km transmission.
As a matter of fact, we could only achieve synchronous loss both in back to back situation and after 500 km transmission when the extinction ratio was 10dB. On the other hand, we could achieve less than 10-9 BER even when the extinction ratio was 10dB with the zero-nulling method. From this result, it can be said that zero-nulling method was quite effective to improve the transmission performance of the APSK system.
Figure 5.6 shows the eye diagrams of the PSK signal in different ER case. It was easy to observe the phenomenon of eye closure in high ER case. Those figures provided an evidence of the PSK information degradation in the ASK space level due to the small optical power.
After the transmission, the degradation became more significant due to the accumulated ASE noise in the transmission line. When the ER was 10dB, the eye diagram was almost fully closed. This was the reason why we could only achieve synchronous loss both in back to back situation and after 500 km transmission when the extinction ratio was 10dB.
Figure 5.7 shows the eye diagrams of the PSK signal with the zero-nulling method.
These eye diagrams correspond to the special pattern of 32 bits sequence. As the pattern has ten consecutive mark bits, ten consecutive space bits, and six consecutive mark and space bits pair in the ASK signal, a clear eye opening of nine consecutive bits was observed in figure 5.7(a). In addition, an intermediate eye opening area was observed before the large eye opening area. As 1-bit delay demodulation scheme were used in this experiment, consecutive mark bits of the ASK signal showed the large eye opening area, and ten consecutive mark bits of the ASK signal corresponded to nine consecutive bits of the PSK signal after the demodulation. The intermediate eye opening area was generated by six consecutive mark and space bits pair in the ASK signal. Consecutive space bits of the ASK signal showed the worst eye opening area. Figure 5.7(b) shows the eye diagram after 500km transmission. Because the ASE noise caused significant signal to noise ratio degradation, only large eye opening area and no eye opening area were observed. Those figures also show the degradation of the PSK information in the ASK space level clearly.