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A hybrid wireless-over-fiber (WoF) transmission system based on multiple injection-locked Fabry–Perot laser diodes (FP LDs) is proposed and experimentally demonstrated. Unlike the traditional hybrid WoF transmission systems that require multiple distributed feedback (DFB) LDs to support different kinds of services, the proposed system employs multiple injection-locked FP LDs to provide different kinds of applications. Such a hybrid WoF transmission system delivers downstream intensity-modulated 20-GHz microwave (MW)/60-GHz millimeter-wave (MMW)/550-MHz cable television (CATV) signals and upstream phase-remodulated 20-GHz MW signal. Excellent bit error rate (BER), carrier-to-noise ratio (CNR), composite second-order (CSO), and composite triple-beat (CTB) are observed over a 40-km single-mode fiber (SMF) and a 4-m radio frequency (RF) wireless transport. Such a hybrid WoF transmission system has practical applications for fiber-wireless convergence to provide broadband integrated services, including telecommunication, data communication, and CATV services.
© 2015 Optical Society of America
1. Introduction
By combining the capacity of the optical fiber network with the ubiquity and mobility of the wireless network, wireless-over-fiber (WoF) transmission systems form a powerful platform for supporting and creating future unforeseen applications and services. A network that can simultaneously provide both wired and wireless communications must be established because of the increased requirements in bandwidth and access data rate for various applications [1–4]. Transmitting hybrid microwave (MW), millimeter-wave (MMW), and cable television (CATV) signals over a WoF transmission system has several advantages, including high-speed access rate, large bandwidth, and wide service area. Conventionally, a hybrid MW/MMW/CATV WoF transmission system requires multiple distributed feedback laser diodes (DFB LDs) to support different kinds of services. However, multiple DFB LDs increases the complexity and cost of systems. Therefore, a configuration with potentially simple and cost-effective characteristics must be established to ensure the practical implementation of a hybrid WoF transmission system. Recently, the Fabry–Perot (FP) LD transmitter has become an attractive option for lightwave transmission systems because of its lower cost compared with the DFB LD transmitter. Nevertheless, an FP LD with multiple longitudinal modes exhibits a wide spectrum spread that causes a higher intensity noise and a higher fiber dispersion-induced distortion. Previous studies employ the injection-locked FP LD in MMW, radio-over-fiber, and hybrid wavelength-division-multiplexing (WDM) lightwave transmission systems [5–7]. However, its application in hybrid WoF transmission systems has not been reported. Injection locking technique, which can amplify the injection-locked sideband and suppress other sidebands [8], is expected to demonstrate favorable transmission performances in hybrid MW/MMW/CATV WoF transmission systems. It is attractive because it avoids the need of multiple DFB LDs with selected wavelengths. In this paper, a hybrid MW/MMW/CATV WoF transmission system based on multiple injection-locked FP LDs is proposed and experimentally demonstrated. By applying the injection locking technique at the transmitting site, such a hybrid WoF transmission system transports downstream intensity-modulated 20-GHz MW, 60-GHz MMW, and 550-MHz CATV signals and upstream phase-remodulated 20-GHz MW signal. The downstream light is intensity modulated with 5 Gbps/20 GHz MW and 50-550 MHz CATV signals, and optically promoted from 5 Gbps/20 GHz MW to 5 Gbps/60 GHz MMW data signal. The downstream light is successfully phase-remodulated with 5 Gbps/20 GHz MW data signal for the upstream light. For up-link transmission, an optical phase modulation (PM)-to-intensity modulation (IM) conversion is employed to convert the optical PM signal into the optical IM one. The bit error rate (BER) is used to analyze the performances of the downstream 5 Gbps/20 GHz MW and 5 Gbps/60 GHz MMW data signals, as well as the upstream 5 Gbps/20 GHz MW data signal. Carrier-to-noise ratio (CNR), composite second-order (CSO), and composite triple-beat (CTB) are used to analyze the performances of the downstream CATV signal. The performances of downstream CATV signal are improved by the introduction of half-split-band and injection locking techniques [9,10]. Over a 40-km single-mode fiber (SMF) and a 4-m RF wireless transport, an in-depth investigation of hybrid WoF transmission systems reveals that BER, CNR, CSO, and CTB perform brilliantly. Such a hybrid WoF transmission system has several advantages in broadband applications and fiber-wireless convergence.
2. Experimental setup
The configuration of the proposed hybrid MW/MMW/CATV WoF transmission systems based on multiple injection-locked FP LDs is shown in Fig. 1. The transmitting site includes four FP LDs, four optical isolators, four optical couplers, and four external light injection sources. The FP LD consists of a reliable strained multi-quantum well (SMQW) InGaAsP laser and a fiber pigtail. Such pigtailed FP LD has a low operating current over a wide temperature range (0 °C to + 70 °C). It provides high optical performances for SMF and is used in telecommunication and data communication applications. The output power level of FP LD is 0 dBm at a bias current of 14 mA. To prevent the wavelength drift of the FP LD induced by the temperature variation, a temperature controller is used in the FP LD transmitter. The wavelength variation of the FP LD with temperature controller is 0.003 nm/°C. It is important to delicately control the temperature of FP LD. A maximum central wavelength shift of 0.003 nm/°C is needed to avoid the wavelength drift and performance degradation problems. For the external light injection part, the light is injected through an optical isolator and an optical coupler. The injection power level for each optical channel is 3 dBm. A 5-Gbps data stream is mixed with a 20-GHz RF carrier to generate the 5 Gbps/20 GHz MW data signal. The resulting 5 Gbps/20 GHz MW data signal is split by a 1 × 2 RF splitter and supplied to FP LD1 and FP LD2. Channels 2 to 40 (55.25 MHz to 319.25 MHz) are directly fed into FP LD3 for the intensity-modulated CATV signal, whereas channels 41 to 78 (325.25 MHz to 547.25 MHz) are directly fed into FP LD4 for the intensity-modulated CATV signal. The output of the injection-locked FP LD1 and FP LD2 are fed into the variable optical attenuators (VOAs) to attenuate the optical power levels. The wavelengths of the injected light are 1546.18 (λ1), 1549.68 (λ2), 1554.63 (λ3), and 1556.83 (λ4) nm, respectively. The wavelengths of the injected light must be carefully selected to ensure optimal system performances. All optical signals are combined by using a 4 × 1 optical combiner and are amplified by using an erbium-doped fiber amplifier (EDFA). The output power and noise figure of EDFA are 17 dBm and 4.5 dB at an input power of 0 dBm, respectively. The VOA is introduced after the EDFA, which can result in less distortion considering the less amount of optical power launched into the fiber. Over a 40-km SMF transport, the optical signals are split by a 1 × 3 optical splitter and are transmitted through three separate optical paths. One optical signal goes through the optical band-pass filter1 (OBPF1). Two optical sidebands spaced by 20 GHz are picked up by using the OBPF1 with a 3-dB bandwidth of 0.36 nm. The 5 Gbps/20 GHz MW data signal is then detected by a 20-GHz photodiode (PD), boosted by a 20-GHz power amplifier (PA), and wirelessly transmitted by a 20-GHz horn antenna (HA). Over a 4-m RF wireless transport, the 5 Gbps/20 GHz MW data signal is received by a 20-GHz HA, amplified by a 20-GHz low-noise amplifier (LNA), and down-converted by a 20-GHz local oscillator (LO) and mixer. Data are recovered by using a data recovery scheme and are fed into a BER tester (BERT) for BER performance analysis.
Fig. 1 The configuration of the proposed hybrid MW/MMW/CATV WoF transmission systems based on multiple injection-locked FP LDs.
Another optical signal passes through the OBPF2. Two optical sidebands spaced by 60 GHz are picked up by using the OBPF2 with a 3-dB bandwidth of 0.52 nm. Thus, the 5 Gbps/20 GHz MW data signal is optically promoted to the 5 Gbps/60 GHz MMW one. The 5 Gbps/60 GHz MMW data signal is then detected by a 60-GHz PD, boosted by a 60-GHz PA, and wirelessly transmitted by a 60-GHz HA. Over a 4-m RF wireless transmission, the 5 Gbps/60 GHz MMW data signal is received by a 60-GHz HA, amplified by a 60-GHz LNA, and down-converted by a 60-GHz LO and mixer. Data are recovered using a data recovery scheme. The data signal is eventually supplied to a BERT for BER performance evaluation.
The other optical signal is split by a 1 × 2 optical splitter. One optical signal is reflected by FBG1 (λc = 1554.63 nm) and FBG2 (λc = 1556.83 nm) that are combined with optical circulator1 (OC1) before being received by a CATV receiver. All CATV parameters (CNR, CSO, and CTB) are measured and analyzed using an HP-8591C CATV analyzer. The other optical signal is reflected by FBG3 (λc = 1546.18 nm) combined with OC2 before being launched into a phase modulator. The FBG filter (FBG3 + OC2) is used as a narrow OBPF to pick up only one optical sideband. To ensure only one optical sideband is picked up, the FBG filter exhibits a sharp cutoff in the transmission spectrum, with a 3-dB bandwidth of 0.2 nm and a 35-dB bandwidth of 0.34 nm. For up-link transmission, the 5 Gbps/20 GHz MW data signal is fed into a phase modulator for phase remodulation, amplified by an EDFA, attenuated by a VOA, and delivered by another 40-km SMF link. With the effect of the IM-to-IM conversion, the IM downstream signal with large optical modulation index (OMI) will induce performances degradations on the IM upstream signal. As IM are employed for down-link and up-link transmissions, the OMI of the downstream signal should be sacrificed to allow the minimum required performances of the upstream signal. To conquer the limitation, PM is applied in a bidirectional lightwave transmission system and employed for up-link transmission. Due to the constant intensity of PM upstream signal, the constraint on OMI of the IM downstream signal can be considerably relaxed compared to a bidirectional lightwave transmission system based on the IM upstream signal. The output power and noise figure of EDFA are 13 dBm and 3.6 dB at an input power of 0 dBm, respectively. The VOA is introduced after the EDFA, which will result in less distortion as the optical power launched into the fiber is less. The fiber transmission loss is about 0.25 dB/km, and the link budget for up-link transmission is about 13 dB (fiber transmission loss + insertion loss). Thereby, an EDFA is needed to boost the optica signal and a VOA is also needed to adjust the optica power level. Over a 40-km SMF link, the optical signal passes through the OBPF3-based PM-to-IM converter, detected by a 20-GHz PD, boosted by a 20-GHz PA, and wirelessly transmitted by a 20-GHz HA. Over a 4-m RF wireless transport, the 5 Gbps/20 GHz MW data signal is received by a 20-GHz HA, amplified by a 20-GHz LNA, and down-converted by a 20-GHz LO and mixer. Data are recovered using a data recovery scheme. The data signal is eventually supplied to a BERT for BER performance evaluation.
3. Experimental results and discussions
Figure 2(a) shows the optical spectrum of the free-running FP LD1 without 5 Gbps/20 GHz MW data signal modulation. It is obvious that the free-running FP LD1 exhibits a wide spectrum spread. Figure 2(b) shows the optical spectrum of the injection-locked FP LD1 locked at λ1 (1546.18 nm) with 5 Gbps/20 GHz MW data signal modulation. Figure 3 presents the optical spectrum of the injection-locked FP LD2 locked at λ2 (1549.68 nm) with 5 Gbps/20 GHz MW data signal modulation. An injection locking technique enhances the intensity of the injection-locked sideband and suppresses other sidebands. When the lightwave is modulated by a LD driven by the data signal, some optical sidebands will be generated. How many sidebands can be generated depends on the amplitude of the driven data signal on the LD. Here, we use an appropriate data signal to drive the FP LD, by which resulting in small fourth-order sideband after modulation. Only the first-order, second-order, and third-order sidebands are generated; and the channel spacing between each sideband is equal. In Fig. 2(b) and Fig. 3, the bandwidth of center wavelength component looks wider than the left λ1 and λ2 component. This is because λ1 and λ2 component is unmodulated sideband and the center wavelength component is modulated sideband. Figure 4 shows the optical spectrum of the injection-locked FP LD3 locked at λ3 (1554.63 nm) with low-band CATV signal (channels 2-40) modulation. For λ3 to FP LD3 injection locking, the intensity of the injection-locked mode is amplified and the linewidth of the injection-locked mode is reduced.
Fig. 2 (a) The optical spectrum of the free-running FP LD1 without 5 Gbps/20 GHz MW data signal modulation. (b) The optical spectrum of the injection-locked FP LD1 locked at λ1 (1546.18 nm) with 5 Gbps/20 GHz MW data signal modulation.
Fig. 3 The optical spectrum of the injection-locked FP LD2 locked at λ2 (1549.68 nm) with 5 Gbps/20 GHz MW data signal modulation.
Fig. 4 The optical spectrum of the injection-locked FP LD3 locked at λ3 (1554.63 nm) with low-band CATV signal (channels 2-40) modulation.
Figure 5 shows the optical spectrum measured at point (A) of Fig. 1. Given that the FP LD1 and FP LD2 are directly modulated by a 5 Gbps/20 GHz MW data signal, the generated optical sidebands for FP LD1 and FP LD2 are coherent with each other and the channel spacing between the adjacent sidebands is 20 GHz (0.16 nm). Given that the −1 and zero sidebands of FP LD1 are picked up by the OBPF1, the 5 Gbps/20 GHz MW data signal can be obtained over a 40-km SMF link. Similarly, as the −3 and zero sidebands of FP LD2 are picked up by the OBPF2, the 5 Gbps/20 GHz MW data signal can be optically promoted to 5 Gbps/60 GHz MMW. As the zero sidebands (central carriers) of FP LD3 and FP LD4 are picked up by the FBG1 and FBG2 that are combined with OC1, the full-channel (channels 2-78) CATV signal can be obtained.
For up-link transmission, the optical spectra before and after the OBPF3 are shown in Figs. 6(a) and 6(b), respectively. With the assistance of OBPF3, the optical carrier and two optical sidebands of the phase-modulated optical signal are suppressed with different ratios. By using such a tilt filter, the upper sideband with a different phase can be deleted [11]. Therefore, an optical PM-to-IM conversion is achieved successfully and the optical signal can be directly detected by a PD.
Figure 7 shows the measured BER curves of the 5 Gbps/20 GHz MW data signal for back-to-back (BTB) and over a 40-km SMF as well as a 4-m RF wireless transmission scenarios. At a BER of 10−9, a power penalty of 4.1 dB is observed between BTB and 40-km SMF as well as 4-m RF wireless transmission scenarios. Figure 8 shows the measured BER curves of the 5 Gbps/60 GHz MMW data signal for BTB and over a 40-km SMF as well as a 4-m RF wireless transmission scenarios. At a BER of 10−9, a larger power penalty of 4.6 dB is observed between BTB and 40-km SMF as well as 4-m RF wireless transmission scenarios. The 5 Gbps/20 GHz MW data signal is optically promoted to 5 Gbps/60 GHz MMW one. However, RF power degradation is induced by fiber dispersion. Over a 40-km SMF link, the fiber dispersion degrades the transmission performance because of the natural characteristics of the two optical sidebands. In two optical sidebands intensity modulation, if 60 GHz signal fading due to 40 km fiber dispersion occurs, power penalty should be changed much more. However, given that the SMF length is only 40 km, the BER performance degradation due to fiber dispersion is limited. Thus, a fiber dispersion compensation device is not required in a short-haul lightwave transmission system. Over a 4-m RF wireless transmission, the fading effect fluctuates the amplitude and phase for the received signal, which degrades the BER performance. Figures 7 and 8 also show the eye diagrams of the 5 Gbps data channel, obtaining a 5 Gbps data stream from the 5 Gbps/20 GHz MW and 5 Gbps/60 GHz MMW data signals, over a 40-km SMF and a 4-m RF wireless transmission at a BER of 10−9. A clear eye diagram is obtained by using LNA to amplify the 5 Gbps data stream while adding as little noise and distortion as possible and by using a data recovery scheme to suppress the amplitude and phase fluctuations (Fig. 7). However, the amplitude and phase fluctuations in the signal are somewhat observed wherein obtaining 5 Gbps data stream from the 5 Gbps/60 GHz MMW data signal (Fig. 8). This eye diagram degradation is attributed to the fiber dispersion and fading effects.
Fig. 7 The measured BER curves of the 5 Gbps/20 GHz MW data signal for BTB and over a 40-km SMF and a 4-m RF wireless transmission scenarios.
Fig. 8 The measured BER curves of the 5 Gbps/60 GHz MMW data signal for BTB and over a 40-km SMF and a 4-m RF wireless transmission scenarios.
For up-link transmission, the measured BER curves of the 5 Gbps/20 GHz MW data signal for BTB and over 40-km SMF as well as 4-m RF wireless transmission scenarios are shown in Fig. 9. A power penalty of 4.2 dB is obtained between BTB and 40-km SMF as well as 4-m RF wireless transmission scenarios at a BER of 10−9. This finding can be attributed to the use of the PM scheme to reduce the distortions induced by the systems. The PM scheme utilizes the optical phase shift to record signal state, thereby providing high robustness against fiber nonlinearity. Constant power operation on PM reduces the distortions induced by systems, thereby improving the BER performance of systems. Figure 9 also presents the eye diagram of the 5 Gbps data channel, in which a 5 Gbps data stream is obtained from the 5 Gbps/20 GHz MW data signal, over 40-km SMF and 4-m RF wireless transmission at a BER of 10−9. The amplitude and phase fluctuations in the signal are not obvious when the PM scheme is employed. Error-free transmission is achieved to demonstrate the possibility of setting up a hybrid WoF transmission system.
Fig. 9 The measured BER curves of the 5 Gbps/20 GHz MW data signal for BTB and over a 40-km SMF and a 4-m RF wireless transmission scenarios (up-link transmission).
Half-split-band technique divides the CATV signal into low- and high-band signals and deletes the CSO and CTB distortions dramatically, which greatly improves the CSO and CTB performances. The CSO and CTB distortions can be given by [12]:
where m is the OMI, D is the dispersion coefficient, is the optical carrier wavelength, L is the fiber length, is the RF frequency, is the fiber dispersion ( is the spectral width), and NCSO/NCTB are the product counts of CSO/CTB. By using the half-split-band technique, a smaller NCSO/NCTB can be obtained from a smaller channel number. Therefore, part of the CSO and CTB distortions will be deleted automatically in each split-band region.The measured CNR, CSO and CTB values under NTSC channel number (CH2-78) with half-split-band scheme, with and without 3 dBm light injection, are shown in Fig. 10. The CNR value (≥ 50 dB) is increased as the 3 dBm optical power is injected. The CNR value depends on the received optical power level as shown in the following equation:
CNRRIN results from the RIN of LD. CNRth (resulting from thermal noise) and CNRshot (resulting from shot noise) are associated with the optical receiver. The (CNRth + CNRshot) of the systems with a 3 dBm light injection is higher than that of systems without light injection because a higher optical power level is received by the optical receiver. The systems obtain a better CNR performance when the optical receiver receives a higher optical power level. As to the CSO/CTB performances, the CSO/CTB values (≥ 63/62 dB) of systems with 3 dBm light injection are significantly improved. Large CSO and CTB improvements of around 5 dB are achieved, which have resulted from the use of the injection locking technique. Injection locking technique increases the frequency response of LD and decreases the frequency chirp and relative intensity noise of LD, which significantly improves the CSO and CTB performances.Fig. 10 The measured CNR, CSO, and CTB values under NTSC channel number (CH2-78) with half-split-band scheme, with and without 3 dBm light injection.
4. Conclusions
A hybrid WoF transmission system based on multiple injection-locked FP LDs is proposed and experimentally demonstrated. By using the injection locking and half-split-band techniques, this hybrid WoF transmission system successfully delivers downstream intensity-modulated 20-GHz MW/60-GHz MMW/550-MHz CATV signals and upstream phase-remodulated 20-GHz MW signal. Fiber dispersion and polarization mode dispersion (PMD) are the limiting factors for SMF-based lightwave transmission systems. Since the SMF length used in the proposed hybrid WoF transmission systems is only 40 km, yet the performance degradations owing to fiber dispersion and PMD are restricted. It indicates that such proposed hybrid WoF transmission systems are not suitable for systems designed to operate over long distances at high data rates. Whereby, our proposed systems are suitably applicable to the hybrid WoF transmission systems over a short-haul transport. Such a hybrid MW/MMW/CATV WoF transmission system not only has advantages in the integration of fiber backbone and wireless feeder networks, but also provides a communication channel for higher data rate and bandwidth. This transmission system can be used to provide broadband integrated services, including telecommunication, data communication, and CATV services.
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