In RoF system, optical transmitter includes optical source, optical modulator, RF signal, electrical mixer, electrical amplifier, 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. 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
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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.
Maximum transmission point Minimum transmission point Quadrature point
Figure 2-3 (a) and (b) are two schemes of transmitter and (c) is duty cycle of subcarrier biased at different points in the transfer function. (LO: local
oscillator)
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2.4.2 Optical 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, 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. 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. The DSB signals can be transmitted over several kilo-meters. Therefore, the SSB modulation scheme is proposed to overcome fiber dispersion effect. 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 due to limited optical modulation index (OMI). The DSBCS modulation is demonstrated optical mm-wave generation using DSBCS modulation. It has no performance fading problem and it also provides the best receiver sensitivity because the OMI is always equal to one. The other advantage is that the bandwidth requirement of the transmitter components is less than DSB and SSB modulation. However, the drawback of the DSBCS modulation is that it can’t support vector signals, such as phase shift keying (PSK), quadrature amplitude modulation (QAM), or OFDM signals, which are of utmost importance in wireless applications.
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Figure 2-4 Optical microwave/mm-wave modulation scheme by using MZM
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
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management makes use of optical amplifiers, which amplify the optical signal directly without requiring its conversion to the electric domain [16]. 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.
Figure 2-5 The model of communication channel in a RoF system.
2.4.4 Demodulation of optical 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 photo 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.
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. 2-5 (b) to become the transmitter in the model of ROF system.
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Figure 2-6 The model of receiver in a ROF system.
Figure 2-7 The model of ROF system.
Laser MZM
data
mixer
LO
EDFA OBPF
fiber
LPF PD
mixer
LO BERT
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2.5 The new proposed model of optical modulation system