CHAPTER 2 The new architecture of hybrid optical access network
2.5 The new model of hybrid optical access network
In section 2.3.2, there is an introduction of three modulation schemes of optical RF signal. In this experiment, SSB scheme can not be applied to generate optical RF signal because it needs a DD-MZM.
Fig. 2-9 (a) and (b) schematically depicts the hybrid optical access network system. A BB signal is modulated on the optical carrier and a RF signal on the optical double sideband. The RF signal using DSBCS modulation scheme is generated at
MZ-a biased at the minimum transmission point and the RF signal using DSB modulation scheme is generated at MZ-a biased at the middle transmission point, as shown in inset (i) of Fig. 2-9 (a) and (b). Moreover, the BB signal can be modulated at the optical carrier, as shown in the inset (ii) of Fig. 2-9 (a) and (b). The optical BB signal is generated at MZ-b. The optical RF signal and BB signal are combined at MZ-c which is hybrid signal, biased at the maximum transmission point, as shown in inset (iii) of Fig. 2-9 (a) and (b). At a remote node, a fiber grating is utilized to separate these two signals, as shown in inset (iv) and (v) of Fig. 2-9 (a) and (b), and each signal is transmitted to the corresponding application.
(a)
(b)
Fig. 2-9The model of hybrid optical access network. (a) use DSBCS scheme to generate RF signal (b) use DSB scheme to generate RF signal.
Chapter 3
Generation of RF Signal Using Only One Single-Electrode Mach-Zehnder Modulator
3.1 Conventional experiment
In the conventional DSBCS modulation scheme, the BB signal is generated using a SD-MZM biased at quadrature and then up-converted using a DD-MZM biased at the minimum transmission point. The power penalty of RF signal after 20km transmission is negligible. After 40km, the power penalty is 2 dB [5]. In this chapter, we will build a new experimental setup from the RoF architecture.
3.2 Experimental components parameter
Name Model Parameter Note
Mixer 1
Double balanced mixer, Up-converted signal
Double balanced mixer, Down-converted signal Driver
amplifier
Picosecond 5865 Max output Vp-p : 7V
RF signal is limited by driver amplifier EDFA Nice, EDFA20A Gain：approximate 20dB Amplify signal Optical BPF JDS Uniphase,
TB4500B
Bandwidth:50GHz Filter out ASE noise
Photo detector u2T, XPRV2022 3dB bandwidth: 38GHz PIN photo diode/TIA module
3.3 Experimental setup and results using a DD-MZM based on DSBCS modulation
3.3.1 Experimental setup
Figure 3-1 shows the experimental setup used for optical microwave generation and transmission based on DSBCS modulation scheme. Due to lack of high frequency components in our laboratory, we use a 622 Mb/s optical BB signal carrying a 10 GHz optical microwave. The continuous-wave (CW) laser is generated by a distributed feedback laser, and the emission wavelength is 1540 nm. The BB signal is 622 Mb/s pseudo random bit sequence (PRBS) signal with a word length of 231-1 and up-converted with the 5 GHz clock as shown in inset (i) of Figure 3-1. The up-converted signal is amplified to maximum peak-to-peak voltage (Vp-p) of 7 volt, limited by the driver amplifier. The CW laser is modulated via external SD-MZM or DD-MZM with half-wave voltage (Vπ) of 5 volt. In order to realize DSBCS modulation, the MZM is biased at the minimum transmission point. The repetition frequency of the generated optical microwave is 10 GHz. The optical microwave and spectrum are shown in insets (ii) and (iii) of Figure 3-1. The generated optical signal is amplified by EDFA and then filtered by a optical tunable filter with a bandwidth of 0.4 nm. The power of RF signal which entered fiber is set to 0 dBm to reduce the effect of both fiber nonlinearity and dispersion changing the duty cycle of optical microwaves.
After transmitted over standard single mode fiber (SSMF), the transmitted optical microwave signal is converted into an electrical microwave signal by a PIN PD with a 3 dB bandwidth of 38 GHz, and the converted electrical signal is amplified by an electrical amplifier. In the BB path, a LPF with a 3 dB bandwidth of 622 MHz is inserted to reject the undesired RF components. In the other path, the microwave signal is down-converted by a mixer with a 10 GHz clock, and then passes through a
LPF with a 3 dB bandwidth of 622 MHz. The eye diagrams of the down-converted and BB signals are shown in insets (iv) and (v) of Fig.3-1. Both the down-converted and BB signals are tested by a bit-error-ratio (BER) tester. We set the fiber length to be 25, 50 and 75 km.
Fig. 3-1 Experimental setup for optical microwave generation based on DSBCS modulation scheme using one MZM.
3.3.2 Optimal condition for RF signal
Figure 3-2 and Figure 3-3 show the variation of the receiver sensitivities of the down-converted and BB signals with different modulation index (MI, MI=Vp-p/2Vπ).
For the down-converted signal, the receiver sensitivity increases first and then decreases when MI ranges from 1 to 0.13, and the best sensitivity is at MI equal to 0.43. For the BB signal, no receiver sensitivity penalty is observed when MI decrease from 1 to 0.43.
The MZM nonlinearity and optical carrier suppression ratio (OCSR) are closely related to MI. As RF MI for MZM decreases, the MZM nonlinearity and OCSR decrease. The reduction of the MZM nonlinearity makes the duty cycle of optical microwaves closer to 0.5 as shown in Figure 3-4. At the same optical power, smaller
duty cycle of optical microwaves has higher peak power, resulting in better receiver sensitivity of the down-converted signal. However, low OCSR means that the RF component of optical power is relative low and the dc component of optical power at the center wavelength is relative high as shown in Figure 3-5. This incurs worse receiver sensitivity of the down-converted signal. Therefore, there is a trade-off for the receiver sensitivity of the down-converted signal between the MZM nonlinearity and OCSR when we decrease MI. When the optimal MI is 0.43, the receiver sensitivities of the BB and down-converted signals at BER of 10-9 are -22.6 dBm and -22.3 dBm. The receiver sensitivity of the down-converted signal has 1 dB improvement when MI changes from 1 to 0.43.
-1.2
Power Penalty (dB) RF Signal
-25 -24.5 -24 -23.5 -23 -22.5 -22 -21.5 -21 -20.5
Fig. 3-2 BER curves (a) and power penalty (b) of down-converted signal for different MI.
-0.2
Power Penalty (dB) BB Signal
-26 -25 -24 -23 -22 -21
Fig. 3-3 BER curves (a) and power penalty (b) of BB signal for different MI.
MI=1 MI=0.66 MI=0.43 MI=0.28 MI=0.18
Fig. 3-4 Duty cycles of optical microwaves based on DSBCS modulation. The optical microwave power is 0dBm. The optical power scale is 0.8 mW/div and the time scale is 20 ps/div.
MI=1 MI=0.66 MI=0.43 MI=0.28 MI=0.18
Fig. 3-5 The OCSR of optical microwaves based on DSBCS modulation. The resolution is 0.01nm.
3.3.3 Transmission results
After optical microwaves with optical power of 0 dBm, using the optimal MI equal to 0.43, are transmitted over 25 km, 50 km, and 75 km SSMF, no power penalty for the receiver sensitivities of the BB and down-converted signals at BER of 10-9 is observed as shown in Fig.3-6.
-26 -25 -24 -23 -22 -21 -20
BTB 25km 50km 75km
Power Penalty (dB)
RF Signal BB Signal
Fig. 3-6 BER curves using one DD-MZM with MI of 0.43 after transmitted over 25 km, 50 km and 75 km SSMF.
(a)
(b)
3.4 Experimental setup and results using a SD-MZM based on DSBCS modulation
Based on the result of DSBCS modulation using one DD-MZM, we can generate DSBCS microwaves using only one SD-MZM with MI equal to 0.43 in the same experimental setup. The Vπ of the SD-MZM at 5GHz is 5 volt, and the Vp-p for the MI of 0.43 is 4.3 volt. Fig.3-7 shows the receiver sensitivities of the BB and down-converted signals with optical power of 0 dBm after they are transmitted over 25 km, 50 km and 75 km SSMF. The power receiving penalties for both the BB and down-converted signals at BER of 10-9 are less than 0.3 dB.
-26 -25 -24 -23 -22 -21 -20 -19
BTB 25km 50km 75km
Power Penalty (dB)
RF Signal BB Signal (a)
(b)
Fig. 3-7 BER curves using one SD-MZM with MI of 0.43 after transmitted over 25 km, 50 km and 75 km SSMF.
3.5 Discussions
In this chapter, we experimentally demonstrate a generation of RF signals based on DCBCS modulation scheme using only one SD-MZM. The optimal MI level for driving DD-MZM is 0.43 with 1 dB sensitivity improvement for down-converted signals, and the power penalty after transmitted over 75 km SSMF is less than 0.4dB.
Then this result will be applied to generate RF signal by the sub-MZ of the external integrated modulator.
Chapter 4
Hybrid Optical Access Network Integrating Fiber-to-the-home and Radio-over-fiber Systems
4.1 Preface
In chapter 3, we prove that the generation of RF signal using only one SD-MZM base on DSBCS scheme is workable. Therefore, the result can be tried to apply to the external integrated modulator. If the BB signal could be modulated by the other sub-MZ of the external integrated modulator, that is, simultaneously generate and transmission of a wired-line BB signal and a wireless RF signal on a single wavelength using one external integrated modulator, then FTTH and RoF systems share a single distributed infrastructure. The integrated system is called the hybrid optical access network.
1554.85-20 1554.9 1554.95 1555 1555.05
-10 0
Reflection (dB)
Wavelength (nm)
1554.85 1554.9 1554.95 1555 1555.05-30
-20 -10 0
Transmission (dB)
Reflection Transmission
Fig. 4-1 The reflection spectrum and transmission spectrum of grating
4.2 Experimental components parameter
Table 4-1 shows the experimental components. Figure 4-1 shows the reflection and transmission spectrum of grating.
Name Type number Parameter Note
Mixer 1
Double balanced mixer, in order to generate the
up-converted signal
Double balanced mixer, in order to generate the down-converted signal Driver
amplifier
Picosecond 5865 Max output Vp-p : 7V
RF signal is limited by RF amplifier
Modulation speed : 10Gb/s
Optical bandwidth: Optical BPF JDS Uniphase,
TB4500B
Bandwidth: 0.4nm Filter out ASE noise EDFA Nice, EDFA20A Gain：approximate
20dB
Amplify signal Photo detector u2T, XPRV2022 3dB bandwidth: 38GHz PIN photo diode/TIA module
Grating INDIGO, photonics IP-CS0014-1554.94
Wavelength: 1554.94nm 3dB bandwidth: 4GHz
Separate RF and baseband signals
Table 4-1 Experimental components.
4.3 Experimental setup and results of hybrid signal which RF signal using DSBCS modulation
4.3.1 Experimental setup
Fig.4-2 shows the experimental setup for hybrid signal generation and transmission using one external integrated modulator based on DSBCS modulation.
The CW laser is generated by a tunable laser, and the lasing wavelength is 1554.94nm.
The RF signal is a 622Mb/s PRBS signal with a word length of 231-1 and up-converted with the 10 GHz clock as shown in inset (i) of Fig.4-2. The up-converted RF signal is amplified to maximum Vp-p of 7V, limited by the RF amplifier. The optical RF signal is generated via MZ-a with half-wave voltage (Vπ) of 5.8V. The MZ-a is biased at the minimum transmission point to realize DSBCS modulation. The repetition frequency of the generated optical microwave is 20GHz, as shown in inset (ii) of Fig.4-2. The BB signal is a 1.25Gb/s PRBS signal with a word length of 231-1; it is sent into MZ-b with Vπ of 5.6V. The eye diagram of the generated optical BB signal is shown in inset (iii) of Fig. 4-2. The optical RF and BB signals are combined in MZ-c with Vπ of 6.9V. The hybrid optical signals are amplified by EDFA to compensate for the loss of the external modulator, yielding a power of 0dBm before transmission over 50km SSMF.
Following transmission over 50km SSMF, the hybrid signals are preamplified by EDFA and then filtered by a tunable optical filter with a bandwidth of 0.4nm. At the remote node, the fiber grating with a 3dB bandwidth of 4GHz is used to separate these two signals, as shown in inset (iv) and (v) of Fig. 4-2, and each signal is sent to the corresponding application. Both optical signals are individually detected by a PIN PD.
For FTTH applications, the electrical BB signal is filtered by an electrical filter with a 3dB bandwidth of 1.25GHz. For wireless applications, the electrical RF signal is down-converted by a mixer with a 20GHz clock before passing through a LPF with a 3dB bandwidth of 622MHz. The eye diagrams of BB and RF signals are shown in inset (vi) and (vii) of Fig. 4-2. Both RF and BB signals are tested by a BER tester and the receiver sensitivities are measured before EDFA pre-amplification. The fiber length is set to 25 and 50km.
Fig.4-2 Experimental setup for RF and BB signal generation and transmission by using one external integrated modulator based on DSBCS modulation.
4.3.2 Optimal condition for RF signal
The RF signal performance is closely related to MZM nonlinearity. To optimize the RF signal performance, the modulation index (MI-RF, MI-RF= Vp-p/2Vπ) for driving MZ-a decreases form 0.6 to 0.1 and no BB signals are sent to MZ-b biased at the minimum transmission point. Figure 4-3 shows the variation of the receiver sensitivity of the RF signal with different MI-RF. The RF receiver sensitivity initially improves and then declines as the MI-RF decreases from 0.6 to 0.1 When MI-RF is 0.48, the RF signals exhibit the best sensitivity and the sensitivity is -39.22dBm.
The optimal MI-RF originates from the trade-off between the MZM nonlinearity and the OCSR for the RF receiver sensitivity when MI-RF is decreased.
As MI-RF decreases, the MZM nonlinearity decreases and the subcarrier varies, as shown in figure 4-4. Hence, the RF sensitivity will improve with MI-RF. However,
the OCSR decreases as MI-RF decreases, as shown in figure 4-5.
Low OCSR means that the optical power of the subcarrier is relative low and that of the optical carrier is relative high. This incurs worse receiver sensitivity of the RF signal. Therefore, there is a trade-off for the receiver sensitivity of the RF signal between the MZM nonlinearity and the OCSR when we decrease MI.
-44 -42 -40 -38 -36 -34 -32 -30
MI-RF=0.6 MI-RF=0.53 MI-RF=0.48 MI-RF=0.4
MI-RF=0.33 MI-RF=0.26 MI-RF=0.17 MI-RF=0.1
Fig 4-4 Subcarrier variation of RF signals for MI-RF from 0.6 to 0.1. The optical microwave power is 0dBm ( Power sale : 600uW/div, Time scale : 10ps/div )
MI-RF=0.6 MI-RF=0.53 MI-RF=0.48 MI-RF=0.4
MI-RF=0.33 MI-RF=0.26 MI-RF=0.17 MI-RF=0.1
Fig 4-5 Optical spectrum of RF signals for MI-RF from 0.6 to 0.1.
(Power scale : 5dB/div, resolution : 0.01nm)
If the optical carrier of RF signal was filtered by the grating, then we can only
consider the impact of MZM nonlinearity. The receiver sensitivity is greatly improved when MI changes from 0.6 to 0.53. As MI-RF changes from 0.53 to 0.1, only small improvement is observed, as shown in figure 4-6. Therefore, the MZM nonlinearity dominates the RF signal performance when MI-RF is between 0.6 and 0.53. As The MI-RF is below 0.5, the OCSR dominates the RF signal performance.
-23 -22 -21 -20 -19 -18
Fig. 4-6 BER curves of RF signal which the optical carrier is filtered by grating and MI-RF changing from 0.6 to 0.1.
-45 -40 -35 -30 -25 -20 -15
Received Power (dBm)
BER
Po w er Pe n alty ( d B )
RF SignalBB Signal (a)
(b)
Fig. 4-7 BER curves (a) and power penalty (b) of both RF and BB signals for MI-BB from 1 to 0.18. MI-RF is fixed at 0.48
4.3.3 Transmission condition for hybrid signal
In order to optimize modulation index for BB signals (MI-BB, MI-BB=
Vp-p/Vπ), the optimal MI-RF for driving MZ-a is fixed at 0.48, and then the BB signal is sent to MZ-b. The MI-BB for driving MZ-b decreases from 1 to 0.18 to yield the same sensitivities of the BB and RF signals. The bias point of MZ-b is adjusted to maximize the extinction ratio of the BB signal as MI-BB varies from 1 to 0.18. Unlike in other works [2-3], the RF and BB signals are generated at the different sub-MZMs.
Various MI-BBs for driving MZ-b cannot influence the RF signal performance when MI-RF for driving MZ-a is maintained at 0.48. As MI-BB decreases, the optical power ratios of the RF and BB signals to the hybrid signals increase and decrease.
Hence, the BB sensitivity increases and the RF sensitivity decreases as MI-BB ranges from 1 to 0.18, as shown in Fig.4-7. When the MI-BB is 0.27, the same sensitivities of the RF and BB signals can be achieved. Under optimal conditions for driving MZ-a and MZ-b, the receiver sensitivities of the RF and BB signals are -37.2dBm and -36.8dBm at BER of 10-9. When the MI-BB is 0.18, the sensitivity of RF is -39.04.
Fig. 4-8 shows the sensitivity measurement of RF signal separated by the grating when the MI-RF is fixed at 0.48 and MI-BB is decreased. It is obviously when the MI-BB is decreased, the sensitivity of RF signal is no variation. It prove that no interference issue exists between RF and BB signal.
The inset (i) ~ (vii) of Figure 4-2 and the inset (i) ~ (v) of Figure 4-9 are eye diagrams and optical spectrums of RF and BB signals when MI-RF and MI-BB were 0.48 and 0.27. At Figure 4-9, the inset (i) and (ii) are optical spectrums of RF and BB signals after modulated, the inset (iii) is optical spectrum of hybrid signal, the inset (iv) and (v) are optical spectrums of RF and BB signals separated from hybrid signal by grating. The inset (v) of Figure 4-9 shows that the grating performance of reflection part is not very good. In order to decrease the influence of grating on BB signal, a LPF was set after O/E converter, as shown in Figure 4-2. We set that MI-RF and MI-BB are 0.48 and 0.27 and transmit the hybrid signal.
-22.5 -22 -21.5 -21 -20.5 -20 -19.5 -19 10-11
10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3
Received Power (dBm)
BER
RF(MI-BB=1) RF(MI-BB=0.8) RF(MI-BB=0.63) RF(MI-BB=0.45) RF(MI-BB=0.27) RF(MI-BB=0.18)
(a)
(b)
Fig. 4-8 Sensitivity measurement (a) and BER curves (b)of RF signal separated by grating as MI-BB decreased
Fig. 4-9 Optical spectrum of BB and RF signals as MI-RF is fixed at 0.48 and MI-BB is fixed at 0.27.
4.3.4 Transmission results of hybrid signal
The optical hybrid signals at an optical power of 0dBm are transmitted over 25km and 50km SSMF. Figure 4-10 plots the BER curves of the RF and BB signals.
The power penalties of both signals at a BER of 10-9 are less than 0.4dB.
4.3.5 Discussion
Although the RF signal using DSBCS modulation overcomes the RF fading and has the best receiver sensitivity [5], there is a limitation for the RF signals in our proposed system. The RF signals are restricted to amplitude-shifted keying signals due to elimination of the optical carrier. Besides, there is the other limitation among the RF frequency, the RF signal bandwidth, and the BB signal bandwidth. Therefore, to avoid the interference between RF and BB signals after fiber grating, the total data rate of the hybrid signals should be less than the RF frequency.
-42 -41 -40 -39 -38 -37 -36
Received Power (dBm)
BER
for driving MZ-a and MZ-b are 0.48 and 0.27.
-1
Fig. 4-10 BER curves (a) and power penalty (b) of both RF and BB signals following transmission over 25km and 50 km SSMF. The optimal MI-RF and MI-BB
4.4 Experimental setup and results of hybrid signal which RF signal using DSB modulation
4.4.1 Experimental setup
Figure 4-11 shows the experimental setup for hybrid signal generation and transmission using one external integrated modulator based on DSB modulation. The CW laser is generated by a tunable laser, and the lasing wavelength is 1554.94nm.
The RF signal is a 622Mb/s PRBS signal with a word length of 231-1 and up-converted with the 10 GHz clock as shown in inset (i) of Fig.4-11. The up-converted RF signal is amplified to maximum Vp-p of 7V, limited by the RF amplifier. The optical RF signal is generated via MZ-a with Vπ of 5.8V. The MZ-a is biased at the middle transmission point to realize DSB modulation. The repetition frequency of the generated optical microwave is 10GHz, as shown in inset (ii) of Fig.4-11. The BB signal is a 1.25Gb/s PRBS signal with a word length of 231-1; it is sent into MZ-b with Vπ of 5.6V. The eye diagram of the generated optical BB signal is shown in inset (iii) of Fig. 4-11. The optical RF and BB signals are combined in MZ-c with Vπ of 6.9V. The hybrid optical signals are amplified by an EDFA to compensate for the loss of the external modulator, yielding a power of 0dBm before transmission over 50km SSMF. Following transmission over 50km SSMF, the hybrid signals are preamplified by EDFA and then filtered by a tunable optical filter with a bandwidth of 0.4nm. At the remote node, the fiber grating with a 3dB bandwidth of 4GHz is used to separate these two signals, as shown in inset (iv) and (v) of Fig. 4-11, and each signal is sent to the corresponding application. Both optical signals are individually detected
The RF signal is a 622Mb/s PRBS signal with a word length of 231-1 and up-converted with the 10 GHz clock as shown in inset (i) of Fig.4-11. The up-converted RF signal is amplified to maximum Vp-p of 7V, limited by the RF amplifier. The optical RF signal is generated via MZ-a with Vπ of 5.8V. The MZ-a is biased at the middle transmission point to realize DSB modulation. The repetition frequency of the generated optical microwave is 10GHz, as shown in inset (ii) of Fig.4-11. The BB signal is a 1.25Gb/s PRBS signal with a word length of 231-1; it is sent into MZ-b with Vπ of 5.6V. The eye diagram of the generated optical BB signal is shown in inset (iii) of Fig. 4-11. The optical RF and BB signals are combined in MZ-c with Vπ of 6.9V. The hybrid optical signals are amplified by an EDFA to compensate for the loss of the external modulator, yielding a power of 0dBm before transmission over 50km SSMF. Following transmission over 50km SSMF, the hybrid signals are preamplified by EDFA and then filtered by a tunable optical filter with a bandwidth of 0.4nm. At the remote node, the fiber grating with a 3dB bandwidth of 4GHz is used to separate these two signals, as shown in inset (iv) and (v) of Fig. 4-11, and each signal is sent to the corresponding application. Both optical signals are individually detected