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A full-duplex lightwave transport system employing phase-modulated radio-over-fiber (RoF) and intensity-remodulated CATV signals in two-way transmission is proposed and experimentally demonstrated. The transmission performances of RoF and CATV signals are investigated in bidirectional way, with the assistance of only one optical sideband and optical single sideband (SSB) schemes at the receiving sites. The experimental results show that the limitation on the optical modulation index (OMI) of the downlink RoF signal can be relaxed due to the constant intensity of phase modulation scheme. Impressive transmission performances of bit error rate (BER), carrier-to-noise ratio (CNR), composite second-order (CSO), and composite triple-beat (CTB) were obtained over two 20-km single-mode fiber (SMF) links. This proposed system reveals an outstanding one with economy and convenience to be installed.
©2011 Optical Society of America
1. Introduction
Wavelength reuse schemes are popular and widely used in full-duplex lightwave transport systems thanks to their economic and convenient installation characteristics [1]. By replacing wavelength-selected laser diodes (LDs) with colorless devices in the subscriber premises, the network service providers can install the optical network flexibly. In the previous studies, some wavelength reuse and optical carrier remodulation schemes were developed based on reflective semiconductor optical amplifier (RSOA) and phase modulator (PM) [2–4] et al. For these schemes, RSOA is employed to modulate electrical signal in intensity domain, whereas PM is used to modulate electrical signal in phase domain. RSOA can erase the downstream signal and then remodulate the optical carrier for uplink transmission [5–7]. No additional optical carrier is required to demonstrate an efficient application for wavelength reuse. However, RSOA with its constraint is not able to support high-speed baseband (BB) signal for transmission. As to the PM, PM with multiple optical carrier output characteristic provides a platform to modulate high-frequency radio frequency (RF) or high-speed BB signal [8,9]. Different with the intensity modulation systems, the phase modulation ones utilize optical phase shifting to record signal state, in which providing high robustness to against fiber nonlinearities with high gain and low noise figure. All of these benefits and no dc bias requirement characteristic make it popular in lightwave transport systems. Nevertheless, with the effect of the intensity modulation-to-phase modulation conversion, the intensity modulation downlink signal with large optical modulation index (OMI) will induce performances degradations on the phase modulation uplink signal. As intensity modulator (IM) is deployed for downlink transmission and PM is deployed for uplink transmission, the OMI of the downlink signal should be sacrificed to allow the minimum required performances of the uplink signal. To overcome the limitation, PM can be applied in full-duplex lightwave transport systems and used for downlink transmission; whereas IM is used for uplink transmission. Thanks to the constant intensity of phase modulation downlink signal, the constraint on OMI of the downlink signal can be considerably relaxed compared to the systems based on the intensity modulation downlink one. Moreover, such a configuration also brings better power budget for the intensity modulation uplink signal, due to lower phase modulation-to-intensity modulation conversion effect.
Full-duplex lightwave transport systems employing radio-over-fiber (RoF) and CATV signals, in which RF passband (PB) and CATV signals are transmitted over fiber links, have attracted much attentions in recent years. The performances of RoF/CATV full-duplex lightwave transport systems are evaluated and analyzed by parameters such as bit error rate (BER), carrier-to-noise ratio (CNR), composite second-order (CSO), and composite triple-beat (CTB) [10,11]. Since these parameters are seriously deteriorated by distortions induced by systems; therefore, it is important to suppress or reduce the distortions as transmitting optical signals in full-duplex RoF/CATV transport systems. In recent studies, full-duplex lightwave transport systems are very attractive for single-mode fiber (SMF) transmission [12]. However, full-duplex lightwave transport systems employing phase-modulated RoF and intensity-remodulated CATV signals over SMF links have not been addressed. In this paper, a full-duplex lightwave transport system using phase-modulated RoF and intensity-remodulated CATV signals over two 20-km SMFs links is proposed and experimentally demonstrated. Introducing PM modulation scheme into the RoF transport systems provides an additional benefit to dig out the potential of PM. The extent of utilizing a PM to colorlessly modulate optical signal in a 20-km reached system is investigated. To be the first one of employing PMs as downlink modulators in full-duplex lightwave transport systems, the transmitting light signals are successfully intensity-remodulated with CATV signals for uplink transmissions. With the assistance of only one optical sideband and optical single sideband (SSB) schemes at the receiving sites, low BER is achieved for RoF application, as well as good performance of CNR, CSO, and CTB are obtained for CATV signal transmission.
2. Experimental setup
Figure 1 shows the experimental configuration of our proposed full-duplex lightwave transport systems with phase-modulated RoF and intensity-remodulated CATV signals. Two distributed feedback (DFB) LDs, with wavelengths of 1549.15 (λ1) and 1553.95 (λ2) nm, are fed into two PMs for phase-modulated RoF signals and two IMs for intensity-remodulated CATV ones. A 1.25-Gbps data stream is mixed with a 10-GHz RF carrier to generate the 1.25Gbps/10GHz RF PB signal, and the resulting RF PB signal is applied to the PM. When the lightwave is modulated by a PM driven by a RF PB signal, some sidebands will be generated. How many sidebands can be generated depends on the amplitude of the driven RF PB signal on the PM. Here, we use an appropriate RF PB signal to drive the PM, in which resulting in small second-order sidebands after phase modulation. Only the first-order sidebands are generated, and the peak of the first-order sidebands is 10 GHz away from the optical carrier of the lightwave. λ1 is used for RoF signal downstream and CATV signal upstream transmissions; while λ2 is used for RoF signal upstream and CATV signal downstream transmissions. Downstream transmission is defined as transmitting signal from left side to right one; whereas upstream transmission is defined as transmitting signal from right side to left one. For λ1 transmission, the phase-modulated RoF signal is transmitted through a 20-km SMF via two optical circulators (OCs; OC1 and OC2) for downlink transmission. At the receiving site, the RoF signal is split by a 1×2 optical splitter. One of the downstream signals is consequently passed through a delay interferometer (DI) with a 10 GHz free spectral range (FSR) to transfer the phase modulated signal into the intensity modulated one. Following with the DI, the optical signal is passed through an optical band-pass filter (OBPF), directly detected by a 10 GHz broadband photodiode (PD), and fed into a BER tester (BERT) for BER analysis. The OBPF exhibits a 3-dB bandwidth of 0.2 nm and a 40-dB bandwidth of 0.3 nm. Here, the function of the OBPF is to pick up the upper sideband of RoF signal; i.e., to convert the optical double sideband (DSB) format into the only one optical sideband format. The OBPF is worth deploying due to excellent optical characteristics including sharp cutoff in the transmission spectrum and environment stability. The optical spectrum before the DI (Fig. 1 insert (i)) is given in Fig. 2(a) . And further, the optical spectra before (Fig. 1 insert (ii)) and after (Fig. 1 insert (iii)) the OBPF are shown in Fig. 2(b) and (c), respectively.
Fig. 1 The experimental configuration of our proposed full-duplex lightwave transport systems with phase-modulated RoF and intensity-remodulated CATV signals.
Fig. 2 (a) The optical spectrum before (Fig. 1 insert (i)) the DI. (b) The optical spectrum before (Fig. 1 insert (ii)) the OBPF. (c) The optical spectrum after (Fig. 1 insert (iii)) the OBPF. (d) The optical spectrum before (Fig. 1 insert (iv)) the OBPF. (e) The optical spectrum after (Fig. 1 insert (v)) the OBPF.
The other downstream signal is passed through a polarization controller (PC) to control its polarization state before intensity remodulation by an IM. For the up-link transmission, channels 79-116 (553.25-745.25 MHz; 6MHz/CH) generated from a multiple signal generator (MATRIX SX-16) were directly fed into an IM, with an optical modulation index (OMI) of ~3.5% per channel. The optical signal was firstly amplified by an erbium-doped filter amplifier (EDFA), transmitted by the other 20-km SMF link via two OCs (OC3 and OC4), went through an OBPF, and received by a CATV receiver. All CATV parameters (CNR, CSO, and CTB) were analyzed by using an HP-8591C CATV analyzer. Here, the function of the OBPF is to convert the optical DSB format into the optical SSB one. The optical spectra before (Fig. 1 insert (iv)) and after (Fig. 1 insert (v)) the OBPF are shown in Fig. 2(d) and (e), respectively. The upstream signal can be sent back either by the same fiber or by the other fiber to avoid the crosstalk of the downstream signal. In order to minimize the crosstalk, we use two fibers for downlink and uplink transmissions. Moreover, for better performance of the CATV receiver, the received optical power level needs to be kept at 0 ~ +3 dBm. For hybrid fiber/coax (HFC) access application, the CATV signal is broadcast to all subscribers after received by the CATV receiver. To meet the CNR/CSO/CTB demands at the optical node (≥50/60/60 dB) and the subscriber (≥43/53/53 dB), the maximum subscriber numbers for each CATV receiver are 200.
3. Experimental results and discussions
A phase-modulated optical carrier is generated by modulating the phase of a continuous wave (CW) light source, and its complex amplitude can be expressed as [13]:
where and are the amplitude and angular frequency of the CW, β is the OMI, is the Bessel function of the first kind of order n, and is the angular frequency of the driving signal. The phase-modulated optical carrier features multiple optical sidebands centered at . It is clear that, from the Eq. (1), the amount of sideband is mainly affected by the β (OMI) value. The amount of sideband is proportionally increased with the increasing OMI value. A large OMI allows a PM to obtain many sidebands output; however; a small OMI allows a PM to obtain only the first-order sidebands output.In parallel with verifying CATV performance, the measured BER curves of 1.25Gbps/10GHz data channel are presented in Fig. 3 . For CATV on, the received optical power levels at the BER of 10−9 are −15.5 (with OBPF) and −13.2 (without OBPF) dBm, respectively. For CATV off, the received optical power levels at the BER of 10−9 are −18.3 (with OBPF) and −15.9 (without OBPF) dBm, respectively. Power penalties of 2.3 and 2.4 dB are obtained in systems due to the cancellation of RF power degradation induced by fiber dispersion. In DSB system, fiber dispersion leads to RF power degradation, in which causing fading problem and resulting in system performance degradation. In only one optical sideband system, since optical carrier and one of the sidebands are eliminated before detecting, the RF power degradation induced by fiber dispersion can be avoided. An error free transmission is achieved to demonstrate the feasibility of employing a PM to modulate the RF PB signal and an IM to remodulate the CATV one. In addition, the eye diagrams of different cases in Fig. 3 are shown in Fig. 4(a) (20km and CATV off (with OBPF)), (b) (20km and CATV off (without OBPF)), (c) (20km and CATV on (with OBPF)), and (d) (20km and CATV on (without OBPF)), respectively. More undesired jitter and amplitude fluctuations are induced in Fig. 4(d) case; however, clear and open eye diagrams are obtained in Fig. 4(a), (b), and (c) cases.
Fig. 4 (a) Eye diagram for case of 20km and CATV off (with OBPF).(b) Eye diagram for case of 20km and CATV off (without OBPF).(c) Eye diagram for case of 20km and CATV on (with OBPF).(d) Eye diagram for case of 20km and CATV on (without OBPF).
Figure 5(a), (b) and (c) show the measured CNR, CSO and CTB values under NTSC channel number, respectively. For fiber optical CATV transport systems, the theoretical expression for CNR is [14]:
where CNRRIN results from the relative intensity noise (RIN) of LD; CNRth (due to thermal noise) and CNRshot (due to shot noise) are associated with the optical receiver; CNRsig-sp (due to signal-spontaneous beat noise) and CNRsp-sp (due to spontaneous-spontaneous beat noise) are associated with the EDFA. Owing to the insertion loss of OBPF (0.5 dB, in front of CATV receiver), the CNR value of systems with SSB format is degraded about 0.5 dB compared to the systems with DSB one. However, systems with SSB format still meet the CNR performance demand (≥50 dB). And further, it can be seen that the CNR value of systems with DSB format is deteriorated about 2 dB compared to the back-to-back (BTB) case. This CNR degradation can be attributed to the fiber loss, in which reducing the received optical power. The CNR value in AM-VSB channels would increase 1 dB as the optical power launched into the EDFA is increased 3 dBm.Fig. 5 (a) The measured CNR values under NTSC channel number. (b) The measured CSO values under NTSC channel number. (c) The measured CTB values under NTSC channel number.
For the CSO and CTB performances, there exist power penalties of ~6 dB between the BTB cases and optical DSB formats because of fiber dispersion-induced distortions. Nevertheless, CSO and CTB performances improvements of about 3 dB are achieved as SSB formats are used. These improvement results are due to the conversion of optical DSB format into optical SSB format to decrease the linewidth of the optical signal, in which leading to the reduction of the fiber dispersion. In an intensity modulation, the CSO and CTB distortions can be expressed as [15]:
where NCSO and NCTB are the product counts, is the gain of the EDFA, is the linewidth of the optical signal. It is effective to introduce optical SSB format to reduce the optical linewidth so that total fiber dispersion is reduced. There would be significant reductions in the CSO and CTB distortions since the CSO and CTB distortions are due to fiber dispersion.4. Conclusions
A full-duplex lightwave transport system employing phase-modulated RoF and intensity-remodulated CATV signals is proposed and experimentally demonstrated. Through a serious investigation, the transmitting light sources are successfully remodulated with CATV signals for uplink transmission. Impressive transmission performances of BER, CNR, CSO, and CTB are obtained over two 20-km SMF links. This proposed system reveals a prominent one not only presents its advancement in broadband application but also reveals its economy and convenience to be installed.
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