Chapter 3 Experimental setup and results
3.3 Simulation and experiment of FSK
As stated in the previous section, the critical limit with respect to the smallest GVD measurement is the original timing position variation δt0 from the ASM laser source. In this section we try to build up a new measurement light source by CW light modulation instead of a pulse laser source. In this respect, the bi-wavelength sweeping light source is the simplest form to be tried. Thus we will use a FSK modulator and a Mach Zehnder Modulator to carry out the new measurement source for replacing the ASM laser. In Fig. 3.7 we adopt an EO modulation scheme that is similar to the recently developed scheme for generating high frequency Radio-Over-Fiber vector signals [1]. The original timing variation should be zero or very small for this kind of frequency modulated pulse light source.
MZ-a
90o Dual parallel
modulator
Fig. 3.7 FSK pulse light source
The system can be separated into three parts which are the light source, FSK modulator and Mach Zehnder Modulator . The light source is a continuous wave laser (CW laser) at 1550nm which can be considered as a signal carrier.
The FSK modulator can provide double frequency switch and the experimental measurement results are shown in Fig. 3.8, Fig. 3.9, and Fig.3.10.
1555.4 1555.6 1555.8 1556.0 1556.2 -70
-60 -50 -40 -30
Level (dB)
Optical frequency (nm)
Fig. 3.8 Double sideband with FSK modulator
In the double sideband case, the central frequency is at 1555.8 nm, which is determined by the CW laser. The wavelength difference between the two double side peaks is 0.322 nm
1555.4 1555.6 1555.8 1556.0 1556.2 -70
-60 -50 -40 -30 -20
Level (dBm)
Wavelength (nm)
Fig. 3.9 Single sideband at 1555.951 nm
1555.4 1555.6 1555.8 1556.0 1556.2
Optical frequency (nm)
Fig. 3.10 Single sideband at 1555.951 nm
Then, the Mach Zehnder Modulator driven by a pattern generator can produce 5G pulse trains. The optical spectra are in Fig. 3.11, Fig. 3.12 and Fig.
3.13.
1555.4 1555.6 1555.8 1556.0 1556.2 -80
Fig. 3.11 Double sideband with FSK modulator and MZM
Compared with Fig. 3.8 we can see that extra side peaks are generated in Fig. 3.11. and the frequency difference is about 5GHz. Since the MZM will produce phase modulation.
1555.4 1555.6 1555.8 1556.0 1556.2 -70
-60 -50 -40 -30 -20
Levrel (dBm)
Optical frequency (nm)
Fig. 3.12 Single sideband at right side with MZM
1555.4 1555.6 1555.8 1556.0 1556.2 -70
-60 -50 -40 -30 -20
Level (dBm)
Optical frequency (nm)
Fig. 3.13 Single sideband at left side with MZM
We have observed the RF spectra in 5GHz, 10GHz, 15GHz and 20GHz without connecting the test fiber respectively. The RF spectra are in Fig. 3.14, Fig. 3.15, Fig. 3.16 and Fig. 3.17.
4.997 4.998 4.999 5.000 5.001 5.002 5.003 -120
-100 -80 -60 -40 -20
Level (dBm)
Frequency (GHz)
Fig. 3.14 RF spectrum at 5GHz without connecting fiber
9.997 9.998 9.999 10.000 10.001 10.002 10.003 -130
-120 -110 -100 -90 -80 -70 -60 -50
Level (nm)
Frequency (GHz)
b_t_b_10G
Fig. 3.15 RF spectrum at 10GHz without connecting fiber
14.997 14.998 14.999 15.000 15.001 15.002 15.003
Fig. 3.16 RF spectrum at 15GHz without connecting fiber
19.997 19.998 19.999 20.000 20.001 20.002 20.003 -140
Fig. 3.17 RF spectrum at 20GHz without connecting fiber
From Fig. 3.14 to Fig.17, we can see four sidepeaks around the central frequency and the frequency difference between the peaks is 1MHz. There are some undesired sidepeaks in the figures. They may be due to the fact that the interference cancellation of the FSK modulator is not ideal and the fact that the central wavelength of the used tunable laser is drifting in time. One may be able
to select a more stable tunable laser and a more accurate band pass filter to solve the problem.
When the system is connected with a SMF fiber, the measurement figures are illustrated as Fig. 3.18, Fig. 3.19, Fig. 3.20 and Fig. 3.21.
4.997 4.998 4.999 5.000 5.001 5.002 5.003 -120
9.997 9.998 9.999 10.000 10.001 10.002 10.003 -120
Fig. 3.19 RF spectrum at 10GHz with 50m SMF
14.997 14.998 14.999 15.000 15.001 15.002 15.003
Fig. 3.20 RF spectrum at 15GHz with 50m SMF
19.997 19.998 19.999 20.000 20.001 20.002 20.003 -120
Fig. 3.21 RF spectrum at 20GHz with 50m SMF
When the light generated from the bi-wavelength sweeping light source goes through the test fiber, the GVD will cause sidepeaks around central frequency components. The relationship between the GVD and the magnitude ratio of sidepeaks has been derived in Eq. (2.29).
The measurement data can be compared with the analytic value and is
shown in Fig. 3.22.
Fig. 3.22 Side-peak ratio △ as a function of the fiber length: experiment (real) and theory (ideal)
From Fig. 3.22 we can understand that ideally the measurement sensitivity is larger for shorter length of test fibers. The reason is because the side-peaks in the RF spectrum don’t exist ideally and will be very sensitive to the dispersion-induced changes. In practice, there are still some magnitudes of side-peaks due to the imperfect of the modulators, which will limit the shortest fiber length (or the smallest dispersion) that can be measured.
In high frequencies, the measurement values seem to lose the accuracy because the high frequency terms are smaller and more sensitive to noises.
Figure 3.23 shows the electrical signal from the pattern generator and Figure 3.24 shows the optical signal from the pattern generator. The pulse width is 100
(ps) and the edge has some distortion. Such pulse shape distortion may also cause some deviation in high frequency signals, although they may be reduced by filtering.
Fig. 3.23 The electrical signal from pattern generator
Fig. 3.24 The optical signal from pattern generator
Although we have not succeeded in experimentally demonstrating the GVD measurement by using FSK signals, the feasibility of the measurement method has been verified by simulation. The simulation results are shown in Fig.
3.25.
1.0 1.5 2.0 2.5 3.0 3.5 4.0
differece between first order and zero order (dB)
Delay (ps)
Fig. 3.25 The simulation of GVD measurement
The main simulation parameters are listed in Table 3.3.
Table 3.3 Definition of simulation value
Numerical unit ps
Wavelength switch frequency 50 MHz
Pulse width 100 ps
Roundtrip time 200 ps
Time window 108 ps
If we consider the experimental conditions where the Δλ is 0.322 nm and the GVD of SMF is 17 ps/km/nm, we can draw the results as in Fig. 3.26.
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 7
89 1011 12 1314 15 1617 1819 2021 2223
differece between first order and zero order (dB)
The length of SMF (km)
5 GHz 15 GHz 25 GHz 35 GHz 45 GHz
Fig. 3.26 Simulation of GVD measurement as a function of the SMF length In Fig. 3.26, the tendency of curves is the same as the analytic results in Fig.
3.22.
Reference
[1] C.-T. Lin, Y.-M Lin, J. Chen, S.-P. Dai, P. T. Shih, P.-C. Peng, and S. Chi,
“Optical direct-detection OFDM signal generation for radio-over-fiber link using frequency doubling scheme with carrier suppression,” Opt. Express, vol 16, pp. 6056-6063, 2008.
Chapter 4 Conclusions
4.1 Summary
In this thesis work, we have proposed and demonstrated a new group velocity measurement method. We use a periodic swept-wavelength pulse light source as the light source for measurement. Theoretically, by using this kind of light source the pulse timing position variation will be increased after the light has passed through a section of dispersive fiber. The pulse timing position variation can then be simply measured by a RF spectrum analyzer. In the experimental side, we have used two kinds of light source to verify the scheme.
One is a sinusoidal periodic swept-wavelength modelocked fiber laser source while the other is a periodic bi-wavelength sweeping pulse modulation light source. For the first case, an asynchronous modelocked Er-fiber soliton laser (ASM laser) has been used as the sinusoidal periodic swept-wavelength pulse light source. The detailed derivation and relation formula have been given in Chapter 2. The RF spectrum of the system is of a Bessel form. Successful experimental demonstration has been given in Chapter 3. For the latter case, we use a system consisted of a frequency shifting keying modulator (FSK modulator) and a Mach Zehnder Modulator (MZM) to be the periodic bi-wavelength sweeping pulse light source. The analytic formula has also been derived in Chapter 2. The RF spectrum of the system is of a sinusoidal form.
Successful simulation demonstration has also been given in Chapter 3. One interesting question then is to compare the measurement sensitivity of the two cases so that one can know how to increase the measurement sensitivity when needed. This will be given in the following section.
We can also compare the commercially available modulation phase-shift technique and the periodic wavelength-swept pulse light method developed in the present thesis. The compared characteristics are listed in Table 3.4.
Table 3.4 Comparison between the modulation phase-shift technique and the periodic wavelength-swept pulse light method
Measurement (ASM Er-fiber laser )
Light source Tunable laser Periodic
wavelength-swept pulse light source Observe signal Phase (RF signal) Pulse timing position
variation (light signal) Observe instrument RF network analyzer RF spectral analyzer Wavelength measurement GVD measurement range
unit length
0.1 ps/nm ~ 1 μs/nm 0.875 ps/nm ~ 25.5 ps/nm
Measurement accuracy (ps) ±0.16 ±0.125 Measurement accuracy
(ps/nm)
× ±0.175 High order dispersion measurable immeasurable
Measurement speed slower faster
Cost higher lower
From the table one can notice some important difference. The MPS can measure the higher order dispersion value owing to the higher wavelength measurement resolution. Thus, the periodic wavelength-swept pulse light method is more suitable to measure the group velocity dispersion (the second
order dispersion) for broadband test components. However, the periodic wavelength-swept pulse light method has better time delay measurement resolution. Also the measurement speed of periodic wavelength-swept pulse light method is faster than MPS because the MPS must scan the RF signal from 40 MHz to 3 GHz per wavelength. Finally, the cost for the MPS method is more expensive then that for the periodic wavelength-swept pulse light method because the MPS method uses a RF network analyzer in the system and the RF network analyzer is more expensive then the RF spectral analyzer.
4.2 Analysis
From Eq. (2.24) and Eq. (2.29) we can get Fig. 4.1 and Fig. 4.2, respectively.
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
Fig. 4.1 Side-peak ratio in the ideal sinusoidal swept-wavelength pulse light source case
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0
5 10 15 20 25 30 35
L (km)
delta (dB)
ESA 5G ESA 10G ESA 15G ESA 20G
Fig. 4.2 Side-peak ratio in the periodic bi-wavelength sweeping pulse light source case
Comparing Fig. 4.1 and Fig. 4.2, we can know the sensitivity of measurement for shorter fiber lengths is higher in Fig. 4.1 than in Fig. 4.2. But, if we use the ASM laser as the sinusoidal periodic swept-wavelength pulse light source, then the original timing variation δ must be considered. The results t0 are shown in Fig. 4.3 and Fig. 4.4.
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
Fig. 4.3 Side-peak ratio in the ASM laser case
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Fig. 4.4 The relation between timing variation and delay time with non-zero δ t0 in the ASM laser case
In Fig. 4.3 and Fig. 4.4, the wavelength variation (Δ ) is equal to 0.677 λ (nm), the GVD of SMF is 17 ps/km/nm and the original timing variation δ is t0
0.3 (ps). From Fig. 4.3 and Fig. 4.4, we can see that δ will reduce the t0 measurement sensitivity for short length test fibers. Therefore the original timing variation should be reduced or avoided in the proposed measurement system if possible.
If we use the binary FSK modulation light source to be our measurement source, then in principle the original timing variation from the laser itself can be avoided and thus the measurement sensitivity can be improved. However, the unwanted sidepeaks shown in Fig 3.14 -Fig. 3-17 will limit the achievable measurement sensitivity. These unwanted sidepeaks should be suppressed in the proposed measurement system if possible.
In the FSK experiment, the frequency difference between two double frequency peaks is 0.322 (nm), or equivalently the 20GHz. Fig. 4.5, Fig. 4.6 and Fig. 4.7 show the side-peak ratio relation for different Δλ.
Fig. 4.5 The relation of SMF length and Δ with Δλ=0.1nm
Fig. 4.6 The relation of SMF length and Δ with Δλ=0.677nm
Fig. 4.7 The relation of SMF length and Δ with Δλ=1 nm
From Fig. 4.5, Fig. 4.6 and Fig. 4.7, we can know that the bigger Δ will λ give rise to higher measurement sensitivity. In the ASM fiber laser case, the central frequency variation Δ is 0.835 nm, which is achieved by using λ nonlinear effects and a 10G modulator. In the FSK case, one will need to use a 50GHz modulator for achieving the same Δ . This shows one important λ
advantage of the ASM laser for the proposed GVD measurement method.
4.3 Future work
To further exploit the advantages of the ASM laser, we can try to reduce its timing variation δt0 and to increase its central frequency variation Δ as λ much as possible. In this way the measurement sensitivity of the proposed method can become the biggest advantage when compared to other approaches.
Another possible future work is to develop wavelength-swept light sources based on fast wide-range tunable optical filters. In this way the timing variation
t0
δ can be reduced and the central frequency difference can also be made very large.