A bidirectional lightwave transport system employing direct modulation CATV and phase remodulation radio-over-fiber (ROF) signals in two-way transmission is proposed and experimentally demonstrated. To be the first system of reusing the phase of the transmitting lightwave with multi-carrier analog CATV signal, the transmission performances of CATV and ROF signals are investigated in bidirectional way, with the help of optical band-pass filters (OBPFs) at the receiving sites. Through a serious investigation, the transmitting light sources are successfully remodulated with RF signals for transmission. Brilliant transmission performances of composite second-order (CSO), composite triple-beat (CTB), and bit error rate (BER) were obtained; accompanied with acceptable carrier-to-noise ratio (CNR) value. This proposed system reveals a prominent one with economy and convenience to be installed.
©2010 Optical Society of America
Optical wavelength reuse scheme is popular and widely employed in bidirectional lightwave transport systems due to its economic and colorless characteristics . By replacing a dedicated laser diode (LD) with colorless device in each subscriber premise, the optical network installation process will become easier, and the service providers can manage the network resources flexibly. In the previous literature, some optical carrier remodulation schemes were developed base on Mach-Zehnder modulator (MZM), reflective semiconductor optical amplifier (RSOA), and phase modulator (PM) [2–4] et al. For these schemes, MZM and RSOA are used to modulate electrical signal in intensity domain, whereas PM is utilized to modulate electrical signal in phase domain. The MZM provides a good platform to modulate radio frequency (RF) signal or high-speed baseband signal with one or multiple optical carriers. However, additional dc bias and continue optical wave are required . RSOA can purify the received optical carrier and then remodulate upstream signal on it . No additional continue optical wave is required to demonstrate an efficiently application in wavelength utilization. Nevertheless, RSOA with its limitation is not able to support high frequency RF or high-speed baseband signal applications. To overcome the limitation, PM is recently applied in lightwave transport systems . 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 benefits and no dc bias requirement characteristic make it popular in lightwave transport systems.
CATV integrating with radio-over-fiber (ROF) transport systems, in which a wide area are connected by fibers as well as CATV and RF signals are transmitted over fiber links, have attracted much attentions. The performances of CATV/ROF transport systems are evaluated by parameters such as carrier-to-noise ratio (CNR), composite second-order (CSO), composite triple-beat (CTB), and bit error rate (BER) [8–10]. These parameters are seriously deteriorated by distortions induced by systems. Thereby, it is important to eliminate or suppress the distortions induced by systems when transmitting optical signals in CATV/ROF transport systems. In recent researches, bidirectional transport in lightwave systems is a very attractive option for single-mode fiber (SMF) transmission. However, bidirectional transport of intensity modulation CATV signal and phase remodulation ROF signal over SMFs transmission has not yet been studied. In this paper, a bidirectional transport system with directly modulating CATV and PM-remodulating ROF signals is proposed and experimentally demonstrated. Introducing PM remodulation scheme into the optical CATV transport systems provides an additional benefit to dig out the potential of PM. The extent of utilizing a PM to colorlessly remodulate optical signal in a 20-km reached system is investigated. To be the first one of employing PMs as wavelength remodulators in bidirectional lightwave transport systems, the transmitting light sources are successfully remodulated with RF signals for transmission. With the help of optical band-pass filter (OBPF) at the receiving sites; the CATV distortion signal, RF power degradation and sensitivity to dispersion are reduced. In contrast to a similar lightwave transport system without employing OBPF, impressive performances of CSO, CTB, and BER were obtained accompanied with acceptable CNR value over a 20-km SMF transmission.
2. Experimental setup
Figure 1 shows the schematic architecture of our proposed bidirectional transport systems with directly modulating CATV and PM-remodulating ROF signals. Channels 79-116 (553.25-745.25 MHz; 6MHz/CH) generated from a multiple signal generator (MATRIX SX-16) were directly fed into two distributed feedback (DFB) LDs, with optical modulation index (OMI) of ~3.5% per channel. The two wavelengths of λ1 and λ2 for directly modulated CATV and phase remodulated ROF signals are 1549.15 and 1553.95 nm, respectively. λ1 is used for CATV downstream and ROF upstream transmissions; while λ2 is used for CATV upstream and ROF 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 directly modulated CATV signal is transmitted through a 20-km SMF via two optical circulators (OCs; OC1 and OC2) for down-link transmission. At the receiving site, the CATV signal is split by a 1 × 2 optical splitter. One of the downstream signal is passed through an OBPF, received by a CATV receiver, and CNR/CSO/CTB parameters are analyzed by an HP-8591C CATV analyzer. Here the function of the OBPF is to convert the optical double sideband (DSB) format (as shown in the Fig. 1 insert (i)) into the optical single sideband (SSB) one (as shown in the Fig. 1 insert (ii)). The OBPF (in front of CATV receiver) exhibits a central wavelength of 1549.23 nm, a 3-dB bandwidth of 0.2 nm, and a 40-dB bandwidth of 0.29 nm. The OBPF is worth deploying due to excellent optical characteristics including sharp cutoff in the transmission spectrum and environment stability. For better performance of the CATV receiver, the received optical power level needs to be kept at −3 ~ + 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 subscriber (≥43/53/53 dB), the maximum subscriber numbers for each CATV receiver are 200. The other downstream signal is passed through a polarization controller (PC) to control its polarization state before phase reused by a PM. For the up-link transmission, a 100-Mbps data stream is mixed with a 7.5-GHz RF carrier (7.5 GHz oscillator is available at our laboratory) to generate the 100Mbps/7.5GHz RF data signal for the compatible worldwide interoperability for microwave access (WiMAX) application. The resulting RF data signal is supplied to the PM, transmitted through the other 20-km SMF link via two OCs (OC3 and OC4), then received and analyzed by an upstream receiver (Rx). When the lightwave is modulated by a PM driven by a RF data signal, some sidebands will be generated. How many sidebands can be generated depends on the amplitude of the driven RF data signal on the PM. Here, we use a small RF data signal to drive the PM, the OMI of PM is 2.8%. Because the RF data signal is small, the second-order sidebands after modulation are small. Only the first-order sidebands are generated, and the peak of the first-order sidebands is 7.5 GHz away from the optical carrier of the lightwave, as shown in the Fig. 1 insert (iii). 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. Here, we use two fibers for down/up-link transmissions to minimize the crosstalk.
As shown in the Fig. 2 , the optical signal at the upstream Rx was firstly amplified by an erbium-doped filter amplifier (EDFA), and 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. Following with the DI, the optical signal was attenuated by a variable optical attenuator (VOA), passed through an OBPF, directly detected by a 10 GHz broadband photodiode (PD), and fed into a BER tester for BER analysis. The OBPF (in front of PD) exhibits a central wavelength of 1549.28 nm, a 3-dB bandwidth of 0.2 nm, and a 40-dB bandwidth of 0.3 nm. The OBPF at the Rx plays one role: to pick up the upper sideband of ROF signal; i.e., to convert the DSB format into only one optical sideband format (as shown in the Fig. 2 insert (i) and (ii)).
3. Experimental results and discussions
Each of the system ends has a local light source (DFB LD), the light source for PM remodulation can be come from the one locally generated; i.e., not from the other end. For λ1 transmission, since the downstream modulation scheme is intensity modulation (IM), the optical power level of downstream signal and the effect of IM to PM conversion should be considered on the performance of PM-remodulating upstream signal. To allow for the minimum required performance of the PM-remodulating upstream signal, the light source for PM remodulation should come from the other end, not from the one locally generated.
Figure 3(a) , 3(b) and 3(c) show the measured CNR, CSO and CTB values under NTSC channel number, respectively. For a repeaterless fiber optical CATV system (i.e., without optical amplifier), the approximated expression for CNR is stated as :Eq. (1), large carrier level or smaller noise level will result in better CNR performance. Chromatic dispersion-induced distortion is not considered a relevant factor to affect the CNR performance, since it is one of the distortions (not noise). Due to 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 format. 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 back-to-back (BTB) case. This CNR degradation can be attributed to the fiber losses reducing the signal received power.
In addition to the CNR performance, the CSO and CTB requirements are generally more stringent than the CNR demand in CATV systems. To guarantee acceptable quality of service (QoS), the CSO and CTB values should higher than 65 dB. While measuring the CSO and CTB performances, we send the CATV signals in both directions simultaneously. For the CSO performance, there exists a power penalty of ~4 dB between the BTB case and optical DSB format because of distortions generated from frequency chirp in combination with fiber dispersion. Nevertheless, CSO performance improvement of about 3 dB is achieved as SSB format is utilized. Improved result is owing to the use of OBPF to reduce the degradation factor of fiber dispersion. The use of OBPF filter converts the optical DSB format into optical SSB one. Optical SSB format, which removes a half of the optical spectrum, is deserved to obtain a dispersion benefit since that the optical signal spectrum has been reduced by a factor of two. The CSO distortion is given by :
As to the CTB performance, there exists a power penalty of ~4 dB between the BTB case and optical DSB format because of fiber dispersion-induced distortion and crosstalk arises from the adjacent channel. However, CTB performance improvement of about 3 dB is obtained as SSB format is employed. Improved result is due to the use of OBPF to reduce the fiber dispersion-induced distortion and to suppress the linear crosstalk from the adjacent channel. Total crosstalk includes linear, cross phase modulation-transmission slope (XPM-TS), optical Kerr effect followed by polarization-dependent loss (OKE-PDL), stimulated Raman scattering (SRS) and XPM crosstalk. Linear crosstalk is larger than any other crosstalk, and the linear crosstalk that arises from the incomplete isolation of the channel output can be expressed as :
In parallel with verifying CATV performance, the measured BER curves of 100Mbps/7.5GHz data channel are presented in Fig. 4 . At the Rx, the DI with 100 ps delay in one arm is designed for demodulating a broadband 10 Gbps data signal, thus it can be utilized to demodulate the 100Mbps/7.5GHz data signal. For CATV on, the received optical power levels at the BER of 10−9 are −15 (with OBPF) and −12.8 (without OBPF) dBm, respectively. For CATV off, the received optical power levels at the BER of 10−9 are −17.5 (with OBPF) and −15.2 (without OBPF) dBm, respectively. Power penalties of 2.2 and 2.3 dB are presented in systems due to the cancellation of RF power degradation. The power penalty of our proposed systems is improved of 2.2 and 1.7 dB compared with [2,14], it meets the high-quality ROF demand. 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 can be avoided for transmission. An error free transmission is achieved to demonstrate the possibility of employing a PM to remodulate the optical signal with CATV multiple carriers.
A novel bidirectional system employing direct modulation CATV and phase remodulation ROF signals is proposed and experimentally demonstrated. To be the first system of reusing the phase of the transmission lightwave with multi-carrier analog CATV signal, the transmission performances of CATV and ROF signals are investigated in bidirectional way, with the help of OBPFs. For CATV signal with SSB format, since one of the sidebands is deleted before receiving, the RF power degradation induced by fiber dispersion can be suppressed. In this way, the optical spectral efficiency is improved and the fiber dispersion-induced distortion is reduced. For ROF signal with only one optical sideband format, since optical carrier and one of the sidebands are eliminated before detecting, the RF power degradation induced by fiber dispersion can be avoided. In this way, the baseband data signal is obtained directly from the optical sideband. It is shown to be a promising solution since expensive and sophisticated RF devices are not involved in systems. Through a serious investigation, the transmitting light sources are successfully remodulated with RF signals for transmission. Good transmission performances of CSO, CTB, and BER were obtained in the proposed system; accompanied with acceptable CNR value (≥50 dB). CSO, CTB, and power penalty (receiver sensitivity) performances improvements of about 3, 3, and 2.2 dB are achieved, respectively. This proposed system is shown to be an outstanding one not only presents its advancement in high frequency application but also reveals its economy and convenience to be installed.
References and links
1. H. H. Lu, H. L. Ma, Y. W. Chuang, Y. C. Chi, C. W. Liao, and H. C. Peng, “Employing injection-locked Fabry-Perot laser diodes to improve bidirectional WDM-PON performances,” Opt. Commun. 270(2), 211–216 (2007). [CrossRef]
2. J. Yu, Z. Jia, T. Wang, and G.-K. Chang, “A novel radio-over-fiber configuration using optical phase modulator to generate an optical mm-wave and centralized lightwave for uplink connection,” IEEE Photon. Technol. Lett. 19(3), 140–142 (2007). [CrossRef]
3. M. Omella, I. Papagiannakis, B. Schrenk, D. Klonidis, J. A. Lázaro, A. N. Birbas, J. Kikidis, J. Prat, and I. Tomkos, “10 Gb/s full-duplex bidirectional transmission with RSOA-based ONU using detuned optical filtering and decision feedback equalization,” Opt. Express 17(7), 5008–5013 (2009). [CrossRef] [PubMed]
4. C. W. Chow, “Wavelength remodulation using DPSK down-and-upstream with high extinction ratio for 10-Gb/s DWDM-passive optical networks,” IEEE Photon. Technol. Lett. 20(1), 12–14 (2008). [CrossRef]
5. X. Yu, J. B. Jensen, D. Zibar, C. Peucheret, and I. T. Monroy, “Converged wireless and wireline access system based on optical phase modulation for both radio-over-fiber and baseband signals,” IEEE Photon. Technol. Lett. 20(21), 1814–1816 (2008). [CrossRef]
6. H. S. Kim, T. T. Pham, Y. Y. Won, and S. K. Han, “Simultaneous wired and wireless 1.25-Gb/s bidirectional WDM-RoF transmission using multiple optical carrier suppression in FP LD,” J. Lightwave Technol. 27(14), 2744–2750 (2009). [CrossRef]
7. H. C. Ji, H. Kim, and Y. C. Chung, “Full-duplex radio-over-fiber system using phase-modulated downlink and intensity-modulated uplink,” IEEE Photon. Technol. Lett. 21(1), 9–11 (2009). [CrossRef]
8. H. H. Lu, A. S. Patra, S. J. Tzeng, H. C. Peng, and W. I. Lin, “Improvement of fiber optical CATV transport systems performance based on lower-frequency side mode injection-locked technique,” IEEE Photon. Technol. Lett. 20(5), 351–353 (2008). [CrossRef]
11. W. Ciciora, J. Farmer, D. Large, and M. Adams, Modern Cable Television Technology, (Elsevier, 2004).
12. H. H. Lu, W. S. Tsai, C. Y. Chen, and H. C. Peng, “CATV/radio-over-fiber transport systems based on EAM and optical SSB modulation technique,” IEEE Photon. Technol. Lett. 16(11), 2565–2567 (2004). [CrossRef]
13. M. R. Phillips and D. M. Ott, “Crosstalk caused by nonideal output filters in WDM lightwave systems,” IEEE Photon. Technol. Lett. 12(8), 1094–1096 (2000). [CrossRef]
14. W. J. Jiang, C. T. Lin, P. T. Shih, J. J. Chen, P. C. Peng, and S. Chi, “A full duplex radio-over-fiber link with multi-level OFDM signal via a single-electrode MZM and wavelength reuse with a RSOA,” Opt. Express 18(3), 2710–2718 (2010). [CrossRef] [PubMed]