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Abstract: A novel transport system, with directly modulating community antenna television (CATV) signal downstream and phase-remodulating radio-over-fiber (ROF) signal upstream, is proposed and experimentally demonstrated. By modulating multi-carriers CATV signal in amplitude domain and ROF signal in phase domain, a single optical wavelength is able to serve these two applications simultaneous. To be the first system of communicating CATV signal downstream and phase remodulating radio signal upstream, the performances of transmitting CATV and radio signals over 20 km single-mode fiber (SMF) were experimentally verified. Good performances of carrier-to-noise ratio (CNR), composite second-order (CSO), and composite triple beat (CTB) were obtained for CATV applications as well as high extinction ration (ER) and low bit error rate (BER) values were measured for ROF transmission.
©2010 Optical Society of America
1. Introduction
Optical wavelength re-use technique is popular and widely employed in modern optical transmission systems due to its economic and colorless characteristics [1,2]. By replacing a dedicated laser diode with colorless device in each consumer premise, the optical network installation process will become easier, and the service providers will be able to flexibly manage network resources. In literature, numbers of optical carrier remodulation schemes are developed base on Mach-Zehnder modulator (MZM), reflective semiconductor optical amplifier (RSOA) and phase modulator (PM) [3–8]. In these devices, MZM and RSOA are a sort of amplitude modulator (AM), and PM is used to modulate electrical signal in phase domain. The MZM provides a good platform to modulate high frequency radio signal or high data rate signal with one or multiple optical carriers. Nevertheless, additional power supply and continue optical wave are required [9,10]. RSOA on the other hand can purify the received optical carrier and then remodulate upstream signal on it [11]. No additional continue optical wave is required from a central office (CO) demonstrating a resourcefully application in wavelength utilization. However, RSOA with its limited operation speed is not able to support high frequency/data rate applications. To overcome the limitation, PMs are recently applied in optical fiber transmission system. Different with those optical amplitude remodulation schemes, the PM modulation systems utilize optical phase shifting to record signal state, which provides high robustness to against fiber nonlinearities with high gain and low noise figure. All of these advancements and no bias requirement characteristic make it popular in optical fiber systems.
In current literature, the PM remodulation schemes are concentrating on digital communication systems only [12,13]. Multi-carrier analog community antenna television (CATV) transport systems in combination with PM remodulation method is not been reported yet. Optical CATV transport system is the first large scale analog optical commercial structure and is widespread throughout the cable industry [14]. Introducing PM remodulation method into such optical CATV transport systems provides an additional benefit to dig out the potential of optical fiber. We therefore investigate the possibility of utilizing a PM to colorlessly remodulate optical multi-carrier analog CATV signal in a 20-km reached system.
In this paper, a novel transport system with directly modulating CATV downstream and PM-remodulating radio-over-fiber (ROF) signal upstream is proposed and experimentally demonstrated to share the same optical carrier in both directions. To be the first one of employing a PM as wavelength remodulator in CATV transport system, the downstream light source was successfully remodulated with RF signals for upstream. Good performances of CATV RF parameters, carrier-to-noise ratio (CNR), composite second-order (CSO) and composite triple beat (CTB) were obtained for downstream transmission as well as high extinction ratio (ER) and low bit error rate (BER) values were achieved for upstream over a 20-km single-mode fiber (SMF) transmission.
2. Experimental setup
Figure 1 shows the schematic structure of our proposed phase reused bidirectional CATV/ROF transport system. To simulate multi-carrier analog CATV signals, 33 NTSC-channels (CH79-116; 553.25-745.25MHz; 6MHz/CH) were generated by a Matrix SX-16 signal generator. These 33 channels, as show in the Fig. 1 insert (i), were directly fed into a distributed feedback laser diode (DFB LD) with 1562.99nm central wavelength and 40mA threshold current. The modulated signal was then transmitted through a 20-km SMF for downstream transmission. In the optical network unit (ONU), the CATV signal is split by a 1 × 2 optical coupler (OC). 90% of the downstream signal was received by a CATV receiver and analyzed by a CATV analyzer. The rest 10% signal was passed through a polarization controller (PC) to control its polarization state before phase reused by a PM. In the upstream direction, a 100 Mbps nonreturn-to-zero (NRZ) data stream is mixed with a 7.5 GHz RF carrier to produce binary phase-shift keying (BPSK) data signal. Subsequently, the mixed signal is fed into the RF port of the PM and transmitted through another 20-km SMF span to the optical line terminal.
Fig. 1 Experimental configuration of the proposed phase reused bidirectional CATV/ROF transport system.
As shown in the Fig. 2 , the optical signal at the upstream receiver (Rx) was firstly amplified by an erbium-doped filter amplifier (EDFA) and filtered by an optical band-pass filter (OBPF) to eliminate amplified spontaneous emission noise. The optical signal was consequently passed through a delay interferometer (DI) with a 10 GHz free spectral range (FSR) to transfer the phase modulated signal into intensity modulated one. Generally, a DI consists of a power splitter, a delay line and a combiner. When an optical PM signal is fed into it, the optical signal will be split into two paths. One path is added with a delay line to provide additional one bit delay and another path is designed as a normal pass way. At the end of these two paths, both optical signals are combined to let two adjacent bits interfere with each other at its output port. This interference works as a phase demodulator and presents (absences) of power at a DI output port if two adjacent bits constructively (destructively) interfere with each other [15]. In this experiment, the DI with 100 ps delay in one arm is designed for demodulating a broadband 10 Gbps BPSK signal, so it can be utilized to demodulate the 100Mbps/7.5GHz BPSK signal as well [13]. Following with the DI, the optical signal was attenuated by a variable optical attenuator (VOA), detected by a 10 GHz broadband photodiode (PD) and then filtered by an electrical band-pass filter (BPF) to remove CATV harmonic signal. The purified RF signal, shown in the Fig. 2 insert (iii), was then down-converted by mixing with a 7.5 GHz RF carrier and detected by bit-error-rate (BER) tester.
3. Experimental results and discussions
Figures 3 , 4 , and 5 show the schematic diagrams of the CNR, CSO and CTB definitions as well as the corresponding performances under various CATV channels respectively. In optical fiber CATV transport systems, the higher optical power is received, the better CNR performance can be obtained. The theoretical expression of CNR is [16]:
where each of the CNR terms corresponds to a different element of the transmission system: CNRRIN results from the laser diode relative intensity noise (RIN), CNRth (due to thermal noise) and CNRshot (due to shot noise) are associated with the optical receiver. As shown in Fig. 3(b), the CNR value in each visual carrier, fc, should bigger than 43dB at consumer premises or “snow” phenomenon will present in that channel. It is clear that the measured CNR values (>50dB) in various CATV channels are always higher than the threshold value.Fig. 3 (a) Measured CNR values under various CATV channels, and (b) the schematic diagrams of the CNR definition.
Fig. 4 (a) Measured CSO values under various CATV channels and (b) the schematic diagrams of the CSO definition.
Fig. 5 (a) Measured CTB values under various CATV channels and (b) the schematic diagrams of the CTB definition.
In a standard frequency allocation CATV transport system, when multiple even-spaced channels are transmitted via nonlinear devices, some discrete distortion products are presented through various combinations of CATV carriers. From the statistics record of those compose products, the worse CSO distortion are found at ±0.75 and ±1.25 MHz from the visual carrier and the worse CTB distortion are found at the same frequency of a CATV carrier [16]. For example, when the carrier frequencies for CH6, CH7, CH13 and CH15 are 83.25, 175.25, 211.25 and 127.25 MHz respectively, the frequency summations of CH7 and CH13 as well as CH6 and CH15 are 386.5 and 210.5 MHz respectively. Since the carrier frequency for CH51 is 385.25 MHz, we can see that this CSO distortion induced by CH7 and CH13 is located at CH51 (386.5 MHz - 385.25 MHz = 1.25 MHz). In addition, another CSO distortion induced by CH6 and CH15 is located at CH13 (210.5 MHz – 211.25 MHz = 0.75 MHz). Similarly, when the carrier frequencies for CH80 and CH81 are 559.25 and 565.25 MHz, the composite triple best product of double CH81 frequency minus CH80 frequency is 571.25 MHz (2 × 565.25MHz - 559.25MHz = 571.25 MHz). Because the carrier frequency for CH82 is 571.25 MHz, we can also see that this CTB distortion inducted by CH80 and CH81 is located at the same carrier frequency of CH82. These two examples illustrate a small portion of CSO and CTB distortions presenting in CATV transport systems. The detail CSO/CTB distortions can be stated as [17]:
where m is the optical modulation index, D is the chromatic dispersion coefficient, λc is the optical carrier wavelength, L is the fiber length, f is the RF frequency, is the chromatic fiber dispersion, is the optical linewidth, NCSO and NCTB are the product counts of CSO and CTB distortions respectively. According to these two equations, we can find that the NCSO and NCTB are not the only parameters to affect the final CSO and CTB performances. Some other factors such as the RF frequency will also influence the transmission performance. When the RF frequency is increased, the CSO/CTB distortions will be increased as well. In other words, the CSO/CTB distortions in high CATV channel range are serious than in low CATV channel range, so the overall transmission performances will gradually reduce when the channel number is increased. Since those compose products are close to CATV signals, the required CSO/CTB performances are generally more stringent than the CNR demand. Poor CSO/CTB performance will seriously destroy video quality and some unwanted “diagonal stripes” and “thin white horizontal stripes” will present in TV screen. In the calculation of CSO performance, the mathematical formula of CSO is:From (5), we can observed that the lower power different between the carrier and second-order products, the worse CATV parameter we can get. To guarantee acceptable quality of service (QoS), the CSO values should higher than 53dB at the consumer premises. Similar with the CSO measurement, the definition of the CTB measurement is:
Since the triple order distortion products in a CATV system is located at the same frequency of a CATV carrier, the relative RF carrier needs to be turn on and off when measuring the CTB values. The recorded values can then be used to calculate the CTB performance which must higher than 43 dB at client premise. It is clear from the Fig. 4 and Fig. 5 that the measured CNR, CSO or CTB values are higher than 50/67/66 dB and the power penalties are small than 2dB.
In parallel with verifying CATV performance, the measured BER values and eye diagrams of upstream 100Mbps/7.5GHz signal are presented in Fig. 6 . The received optical power levels at the BER of 10−9 are −15.7dBm and −13.3 dBm for CATV off and CATV on scenarios. A power penalty of about 2.4 dB is presented in our system. Nevertheless an error free transmission is achieved to demonstrate the possibility of employing a PM modulator to remodulate multi-carrier CATV signal with high frequency RF signal.
4. Conclusions
A novel phase remodulation scheme employing CATV in downstream direction and ROF in upstream transmission is proposed and demonstrated. To be the first one of reusing the phase of a lightwave in a multi-carrier analog CATV transport system, the transmission performances of CATV and RF signals are experimentally demonstrated. Different with employing MZM to remodulate the amplitude of a lightwave, no additional laserdiode and power supply are required in our system. In addition, by modulating upstream signals in phase domain, the upstream data rate is expected to extend the limitation in RSOA remodulation systems. Through a serious investigation, the downstream light source is successfully remodulated with RF signals for upstream transmission. Impressive CNR, CSO and CTB performance were obtained for downstream transmission and acceptable upstream ER and BER values are accompanied in upstream. Since no additional laser diode and no dc-bias are required in the upstream circuit, the proposed colorless transport system not only presents its advancement in high frequency application but also reveals its economic and convenient in installation process.
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