CHAPTER 1 Introduction
1.2 Motivation
Recently, the simultaneous modulation and transmission of a RF signal and a BB signal has been demonstrated [2-5]. However, [2-3] the generated hybrid BB and RF signals suffer from performance fading problem caused by fiber dispersion.
Therefore, a dispersion-shifting fiber (DSF) is employed to transmit the hybrid signals.
This negative effect limits implementation to green field application only, rather than the most common application with already installed standard single mode fiber (SSMF). Furthermore, [4-5] only one signal is modulated on the optical subcarrier such that the BB and RF signals are identical after square-law PD detection. Hence, a simple and cost-effective modulation and transmission of the independent BB and RF signals without periodical performance fading due to fiber dispersion are required.
Chapter 2
The new architecture of hybrid optical access network
2.1 Preface
There are three parts in optical communication systems : optical transmitter, communication channel and optical receiver. Optical transmitter converts an electrical input signal into the corresponding optical signal and then launch it into the optical fiber serving as a communication channel. The role of an optical receiver is to convert the optical signal back into electrical form and recover the data transmitted through the lightwave system. In this chapter, we will do an introduction about the external integrated modulator, constructing a model of RoF system and constructing a model of new hybrid optical access network.
2.2 Mach-Zehnder Modulator (MZM)
Direct modulation and external modulation are two modulations of generated optical signal. When the bit rate of direct modulation signal is above 10 Gb/s, the frequency chirp imposed on signal becomes large enough. Hence, it is difficult to apply direct modulation to generate microwave/mm-wave. However, the bandwidth of signal generated by external modulator can exceed 10 Gb/s. Presently, most RoF systems are using external modulation with Mach-Zehnder modulator (MZM) or Electro-Absorption Modulator (EAM). The most commonly used MZM are based on LiNbO3 (lithium niobate) technology. According to the applied electric field, there are two types of LiNbO3 device : x-cut and z-cut. According to number of electrode, there are two types of LiNbO3 device: dual-drive Mach-Zehnder modulator (DD-MZM) and single-drive Mach-Zehnder modulator (SD-MZM) [6].
2.3 External integrated modulator
The SD-MZM has two arms and an electrode. The optical phase in each arm can be controlled by changing the voltage applied on the electrode. When the lightwaves are in phase, the modulator is in “on” state. On the other hand, when the lightwaves are in opposite phase, the modulator is in “off ” state, and the lightwave can not propagate by waveguide for output. When the amplitude imbalance between the arms duo to fabrication errors in MZM structure existed, there is some residual lightwave even in the “off ” state. The residual lightwave will limit the extinction ratio.
However, we can compensate the amplitude imbalance by using the intensity trimmers, as shown in Figure 2-1. The intensity trimmers can be constructed by MZ structures. [7]
Intensity trimmer
Intensity trimmer
Fig 2-1 The Mach-Zehnder modulator has two intensity trimmers on the arms
Figure 2-2 shows the structure of an external integrated modulator. The external integrated modulator using x-cut LiNbO3 [8-9], consists of three SD-MZM.
Two sub-MZMs (MZ-a and MZ-b) are embedded in each arm of the main modulator (MZ-c).
Optical output MZ-a
Optical input
MZ-b MZ-c
Fig. 2-2 The structure of external integrated modulator
2.4 The architecture of RoF system
2.4.1 Optical transmitter
Optical transmitter concludes optical source, modulator, etc.. Presently, most RoF systems are using laser as light source. The advantages of laser are compact size, high efficiency, good reliability small emissive area compatible with fiber core dimensions, and possibility of direct modulation at relatively high frequency [14]. The modulator is used for converting electrical signal into optical form. Because the external integrated modulator was composed of MZMs, we select MZM as modulator to build the architecture of optical transmitter.
There are two schemes of optical transmitter generated optical signal. One scheme is used two MZM. First MZM generates optical carrier which carried the data.
The output optical signal is BB signal. The other MZM generates optical subcarrier which carried the BB signal and then output the RF signal, as shown in Fig. 2-3 (a).
The other scheme is used a mixer to get up-converted electrical signal and then send it into a MZM to generate the optical signal, as shown in Fig. 2-3 (b). Fig. 2-3 (c) shows the duty cycle of subcarrier biased at different points in the transfer function.
Voltages
Fig 2-3 (a) and (b) are two schemes of transmitter and (c) is duty cycle of subcarrier LO
(c) : minimum point
: middle point : maximum point LO: local oscillator
biased at different points in the transfer function.
2.4.2 Optical microwave/mm-wave signal generations based on LiNbO3 MZM
The microwave and mm-wave generations are key techniques in RoF systems.The optical mm-waves using external MZM based on double-sideband (DSB), single-sideband (SSB), and double-sideband with optical carrier suppression (DSBCS) modulation schemes have been demonstrated [10-12], as shown in Fig. 2-4. Generated optical signal by setting the bias voltage of MZM at quadrature point, the DSB modulation experiences performance fading problems due to fiber dispersion, resulting in degradation of the receiver sensitivity. What is RF fading? When an optical signal is modulated by an electrical RF signal, fiber chromatic dispersion causes the detected RF signal power to have a periodic fading characteristic [12].
The SSB signal is generated when a phase difference of π/2 is applied between the two RF electrodes of the DD-MZM biased at quadrature point. Although the SSB modulation can reduce the impairment of fiber dispersion, it suffers worse receiver sensitivity than DSB modulation [11]. In [5], DSBCS modulation is demonstrated at the mm-wave range with the best receiver sensitivity, lowest spectral occupancy, lowest bandwidth requirement for RF signal, electrical amplifier, and optical modulator, and smallest power penalty of receiver sensitivity after long transmitted distance [5][13].
2.4.3 Communication channel
Communication channel concludes fiber, optical amplifier, etc.. Presently, most RoF systems are using single-mode fiber (SMF) or dispersion compensated fiber (DCF) as the transmission medium. When the optical signal transmits in optical fiber, dispersion will be happened. DCF is use to compensate dispersion. The transmission distance of any fiber-optic communication system is eventually limited by fiber losses.
For long-haul systems, the loss limitation has traditionally been overcome using
regenerator witch the optical signal is first converted into an electric current and then regenerated using a transmitter. Such regenerators become quite complex and expensive for WDM lightwave systems. An alternative approach to loss management makes use of optical amplifiers, which amplify the optical signal directly without requiring its conversion to the electric domain [14]. Presently, most RoF systems are using erbium-doped fiber amplifier (EDFA). An optical band-pass filter (OBPF) is necessary to filter out the ASE noise. The model of communication channel is shown in Fig. 2-5.
Fig 2-4 Optical microwave/mm-wave modulation scheme by using MZM
Data Clock
EDFA OBPF fiber Modulated
optical signal
Fig 2-5 The model of communication channel in a RoF system
2.4.4 Demodulation of optical microwave/millimeter-wave signal
Optical receiver concludes photo-detector (PD), demodulator, etc.. PD usually consists of the photo diode and the trans-impedance amplifier (TIA). In the microwave or the mm-wave system, the PIN diode is usually used because it has lower transit time. The function of TIA is to convert photo-current to output voltage.
The BB and RF signals are identical after square-law PD detection. We can get RF signal by using a mixer to drop down RF signal to baseband then filtered by low-pass filter (LPF).
After getting down-converted signal, it will be sent into a signal tester to test the quality, just like bit-error-rate (BER) tester or oscilloscope, as shown in Fig. 2-6.
PD LPF
mixer
LO after transmitting BERT
optical signal
Fig 2-6 The model of receiver in a RoF system
Combining the transmitter with communication channel and receiver, that is the
model of RoF system, as shown in Fig. 2-7. We select the scheme of Fig. 2-3 (b) to become the transmitter in the model of RoF system.Laser MZM data
mixer
LO
EDFA OBPF fiber
LPF PD
mixer
LO BERT
Fig 2-7 The model of RoF system
Fig. 2-8 shows the model of BB signal generation. Not the same as the model of RoF system, the mixer which set to raise or drop down frequency is not necessary in the model of BB signal generation. But the modulation speed of the MZM needs to cover the data rate of the BB signal.
Laser MZM
data
EDFA OBPF fiber
LPF PD BERT
Fig 2-8 The model of BB signal generation
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
For FTTH applications, the electrical BB signal is filtered by an electrical filter with a