1. Introduction

The rapid development of optical communications has raised the demands for high capacity and high-speed data access not only for single-mode fiber (SMF)-based fiber backbone but also for radio frequency (RF)/optical wireless-based feeder. Through the large bandwidth of optical fiber and the flexibility of RF/optical wireless transmission, fiber-wireless and fiber-invisible laser light communication (IVLLC) integrated systems have proceeded to satisfy the multiple gigabit requirements [1–4]. Fiber-wireless and fiber-IVLLC integrated systems can utilize the advantages of both optical and wireless technologies, they can cover service areas with faster speed and lower cost by virtue of fiber long-haul and RF/optical wireless short-range technologies. A bidirectional fiber-wireless and fiber-IVLLC transmission system based on Mach-Zehnder modulator (MZM)-optoelectronic oscillator (OEO)-based broadband light source (BLS) was demonstrated previously [5]. However, such a bidirectional fiber-wireless and fiber-IVLLC transmission system is not competitive due to sophisticated MZM-OEO-based BLS, complicated wavelength-dependent optical interleavers (OILs), and four expensive intensity modulators are required at the transmitting site. Developing a configuration with potentially simple and cost-effective characteristics to ensure a successful deployment is essential. In this paper, a bidirectional fiber-wireless and fiber-IVLLC integrated system that employs a polarization-orthogonal modulation scheme is proposed and experimentally demonstrated. By means of polarization-orthogonal modulation scheme, the optical carrier and optical sidebands generated by microwave (MW) and millimeter-wave (MMW) signals are polarization-orthogonal and split dramatically [6–8]. It reveals a prominent one with simpler and more economic advantages to separate the optical carrier and the optical sidebands. The downstream light is modulated by a polarization rotator (PR) and a MZM with 50-550 MHz cable television (CATV) signal, 10 Gbps/15 GHz RF data signal, and 10 Gbps/22.5 GHz data signal. The light source, comprising a distributed feedback (DFB) laser diode (LD), a PR and a MZM, is used as a substitute for MZM-OEO-based BLS to overcome the complexity and sophistication of systems. The downstream light is optically promoted from a 10 Gbps/15 GHz RF data signal to 10 Gbps/30 GHz MW and 10 Gbps/60 GHz MMW data signals, and also optically promoted from a 10 Gbps/22.5 GHz RF data signal to 10 Gbps/45 GHz MMW data signal. And further, the optical carrier with downstream 50-550 MHz CATV signal is taken out by a polarization beam splitter (PBS) at the receiving site. The downstream light is reused and remodulated for uplink transmission by a gain-saturated reflective semiconductor optical amplifier (RSOA) with a 5-Gbps baseband (BB) data stream based on fiber-IVLLC convergence. The parameters of carrier-to-noise ratio (CNR), composite second-order (CSO), and composite triple-beat (CTB) are used to evaluate the performances of downstream CATV signal. Meanwhile, the bit error rate (BER) parameter is used to analyze the performances of 10 Gbps/30 GHz MW, 10 Gbps/45 GHz MMW, 10 Gbps/60 GHz MMW downstream data signals, and 5-Gbps BB upstream data stream. Through an in depth observation of such bidirectional fiber-wireless and fiber-IVLLC integrated systems, good performances of CNR, CSO, CTB, and BER are achieved over 40-km SMF and 10-m RF/50-m optical wireless transport.

The feasibility of establishing a full-duplex radio-over-fiber (RoF) link based on a dual-polarization MZM was demonstrated previously [6]. In addition, the feasibility of establishing multi-service RoF links based on a phase-coherent orthogonal lightwave generator was illustrated formerly [7,8]. However, a bidirectional fiber-wireless and fiber-IVLLC integrated system with hybrid CATV/MW/MMW/BB signal based on polarization-orthogonal modulation scheme has not been reported. In this work, an architecture of a bidirectional fiber-wireless and fiber-IVLLC integrated system adopting a polarization-orthogonal modulation scheme is presented. To the best of our knowledge, it is the first time to successfully establish a bidirectional fiber-wireless and fiber-IVLLC integrated system with hybrid CATV/MW/MMW/BB signal based on polarization-orthogonal modulation scheme. As the architecture is changed from RoF links to fiber-wireless and fiber-IVLLC integrations, the configuration complexity will increase greatly. And further, as the transmission signal is changed from MW signal to hybrid CATV/MW/MMW/BB signal, the technique challenge will increase greatly. To guarantee successful design of a bidirectional fiber-wireless and fiber-IVLLC integrated system, system designers will have to overcome the configuration complexity and technique challenge. Such bidirectional fiber-wireless and fiber-IVLLC integrated systems for hybrid CATV/MW/MMW/BB signal transmission will be an attractive approach for providing heterogeneous services, including CATV, Internet, and telecommunication services. This approach can prominently present advancements for the integration fiber backbone and RF/optical wireless-based feeder.

2. Experimental setup

The configuration of the proposed bidirectional fiber-wireless and fiber-IVLLC integrated systems based on polarization-orthogonal modulation scheme is shown in Fig. 1. A light source, comprising a DFB LD with a central wavelength of 1540.16 nm, a PR and a MZM, is employed at the transmission site to generate optical carrier and multiple optical sidebands. When a lightwave is modulated by an MZM driven by RF signals, optical sidebands will be generated. The number of optical sidebands that can be generated relies on the amplitude of the driven RF signals on the MZM [9]. To apply this characteristic, we adjust proper RF signals to drive the MZM, by which resulting in the appropriate number of optical sidebands. The MZM is modulated with hybrid 50-550 MHz CATV, 10 Gbps/15 GHz, and 10 Gbps/22.5 GHz RF signals. A total of 77 channels (CH2-78; 50-550 MHz) generated from a multiple signal generator are used to simulate the CATV signal. The function of the PR is used to rotate the polarization axis of a polarized light beam by an angle of θ [inset (a) of Fig. 1]. Due to the electro-optical property of the LiNbO3 crystal, the half-wave voltage V_π in the x-axis is about 3.58 times of that in the y-axis [10]. In result, the maximum modulation efficiency is obtained as the polarization direction of the input lightwave is parallel to the principal axis of MZM, and the modulation for the x-axis component of the light is negligible. Since the MZM is operated at the minimum transmission point (null-bias point), yet the y-axis component of the light is modulated in the format of optical carrier suppression and optical sidebands enhancement, and the x-axis component of the light is modulated in the format of only optical carrier. Thereby, the y-axis component of the light is modulated in the format of optical carrier suppression with 30-GHz/45-GHz/60-GHz MW/MMW signals, and the x-axis component of the light is modulated in the format of only optical carrier with CATV signal [inset (b) of Fig. 1]. The optical signal is then amplified using an erbium-doped fiber amplifier (EDFA). The output power and noise figure of EDFA are 17 dBm and 4.5 dB, respectively, at an input power of 0 dBm. A variable optical attenuator (VOA) is positioned after the EDFA to reduce distortion as the optical power launched into the fiber decreases. Optical circulator1 (OC1) and an OC2 are placed after the VOA to bridge downstream and upstream lightwaves. Over a 40-km SMF transport, the lights are separated by a polarization controller (PC) and a PBS. The y-axis component of the light [inset (c) of Fig. 1] is split by a 1 × 2 optical splitter and then fed into OIL1 and OIL2 to separate the optical signal into odd and even sidebands. OIL1 has an input channel spacing of 15-GHz and an output channel spacing of 30-GHz; whereas OIL2 has an input channel spacing of 22.5-GHz and an output channel spacing of 45-GHz. For OIL1, the −1 and + 1 optical sidebands are utilized for the 30-GHz MW downlink transmission, and the −2 and + 2 optical sidebands are utilized for the 60-GHz MMW downlink transmission. For OIL2, the −1 and + 1 optical sidebands are utilized for the 45-GHz MMW downlink transmission. The x-axis component of the light [inset (d) of Fig. 1] is also split by a 1 × 2 optical splitter. One optical signal is passed through an EDFA, attenuated by a VOA, and launched into a 50-m free-space optical (FSO) link with doublet lens scheme (doublet lens1 and doublet lens2). The output power and noise figure of EDFA are 13 dBm and 4.2 dB, respectively, at an input power of 0 dBm. Over a 50-m FSO link, the 50-550 MHz optical signal is received by a CATV receiver, fed into a push-pull scheme for nonlinear distortions suppression [11], and supplied to a spectrum analyzer for CNR, CSO, and CTB performance analysis. The other optical signal is circulated using an OC (OC3), reused and remodulated using a gain-saturated RSOA for uplink transmission.

 

Fig. 1 The configuration of the proposed bidirectional fiber-wireless and fiber-IVLLC integrated systems based on polarization-orthogonal modulation scheme.

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Following the OIL1 output with odd sidebands, the optical signal is passed through an optical band-pass filter (OBPF) [inset (e) of Fig. 1], detected by a photodiode (PD) with 30-GHz, amplified by a power amplifier (PA) with 30-GHz, and wirelessly transmitted by a horn antenna (HA) with 30-GHz. Over a 10-m RF wireless transport, the 10 Gbps/30 GHz MW data signal is received by a HA with 30-GHz, boosted by a low noise amplifier (LNA) with 30-GHz, and down-converted by an envelope detector (ED) with a frequency range of 0.5-10 GHz. After ED detection, the 10-Gbps data stream is captured by a 10-GHz low-pass filter (LPF) and clock/data recovered via 10 Gbps clock/data recovery (CDR). Finally, the 10-Gbps data stream is supplied to a BER tester (BERT) for BER performance evaluation. Meanwhile, following the OIL1 output with even sidebands [inset (f) of Fig. 1], the optical signal is passed through an OBPF to form only one optical sideband [inset (g) of Fig. 1] and inputted into a 50-m FSO link with doublet lens scheme (doublet lens3 and doublet lens4). Over a 50-m FSO link, the optical signal is directly detected by a PD with 10-GHz, boosted by a LNA with 10-GHz, filtered by a LPF with 10-GHz, clock/data recovered via 10 Gbps CDR, and fed into a BERT to analyze the BER performance. Given that 60-GHz MMW has a high atmospheric attenuation, fiber-IVLLC integration is a promising substitute for fiber-wireless integration in a 60-GHz MMW transmission. Here, the optical wireless subsystem is deployed to substitute for the traditional MMW wireless subsystem. Thereby, the costly and sophisticated 60-GHz high-bandwidth RF devices are not involved in such downlink transmission subsystem. Following the OIL2 output with odd sidebands, the optical signal is passed through an OBPF [inset (h) of Fig. 1], detected by a 45-GHz PD, amplified by a 45-GHz PA, and wirelessly transmitted by a 45-GHz HA. Over a 10-m RF wireless transport, the 10 Gbps/45 GHz MMW data signal is received by a 45-GHz HA, boosted by a 45-GHz LNA, down-converted by an ED, filtered by a 10-GHz LPF, clock/data recovered via 10 Gbps CDR, and supplied to a BERT for BER performance analysis.

For uplink transmission, the optical carrier (central carrier) circulated by the OC3 is reused and remodulated by a gain-saturated RSOA. A 5-Gbps data stream, with a pseudorandom binary sequence (PRBS) length of 2¹⁵-1, is directly fed into the RSOA. The remodulated upstream lightwave is circulated by an OC, amplified by an EDFA, attenuated by a VOA, and launched into the same 40 km SMF link. Over a 40-km SMF transport, the optical signal is inputted into a 50-m FSO link with doublet lens scheme (doublet lens5 and doublet lens6). After a 50-m FSO link, the optical signal is detected by a PD with 5-GHz. After PD detection, the detected 5 Gbps data stream is boosted by a LNA with 5-GHz, captured by a LPF with 5-GHz, clock/data recovered via 5 Gbps CDR, and inputted into a BERT to evaluate the BER performance.

3. Experimental results and discussions

The optical field at the output port of MZM [inset (b) of Fig. 1] can be expressed as [6–8]:

The FSO link is to realize the free-space transmission using a pair of doublet lenses with optical fibers [12]. Transmitting a laser beam through the free-space between the doublet lenses enables the FSO link to work as if the fibers were connected seamlessly. A pair of doublet lenses, as illustrated in Fig. 2, is employed to send out light from an optical fiber to the free-space and to couple light from the free-space into an optical fiber. The function of the doublet lenses is to significantly extend the free-space transmission distance. The doublet lenses connected to optical fibers play important roles to transmit laser beam through the free-space between the two sides. Given that the diameter of the laser beam is smaller than the diameter of the doublet lens1 and doublet lens2; thereby, a pair of doublet lenses is feasible for a 50-m FSO link. For an FSO link, a reduction setup should be implemented at the receiving site to reduce laser beam size for coupling into the ferrule of SMF. The function of the doublet lens2 is to reduce laser beam size for coupling into the ferrule of SMF.

 

Fig. 2 A 50-m FSO link with a pair of doublet lenses (doublet lens1 and doublet lens2).

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FSO link utilizes a narrow laser beam to obtain long free-space transmission distance and high free-space transmission rate. However, a very limited operating coverage is provided in an FSO link even though it can provide long free-space transmission distance and high free-space transmission rate. It means that as laser beam misalignment problem occurs, a rapid performance degradation happens in an FSO link. Nevertheless, with the rapid progress of FSO link, the increasing requirements raise the needs for long free-space transmission distance and high free-space transmission rate. As to the limited operating coverage problem, spatial light modulator (SLM) can be employed in an FSO link to solve the problem [13]. To employ SLM in an FSO link with doublet lens scheme, it cannot only extend the free-space transmission distance but also solve the limited operating coverage problem of an FSO link.

The measured CNR/CSO/CTB values under NTSC channel number (CH2-78), over 40-km SMF transport as well as over 40-km SMF and 50-m free-space transport scenarios, are presented in Fig. 3. Over a 40-km SMF transport, the measured CNR/CSO/CTB values (≥50.5/61/61.6 dB) satisfy the fiber optical CATV CNR/CSO/CTB requirements at the optical node (≥50/60/60 dB). Large CNR/CSO/CTB degradations of 4.2/4/4.1 dB are existed between 40-km SMF transport as well as 40-km SMF and 50-m free-space transport scenarios, due to further transmission over a 50-m free-space link. Further transmission over a 50-m free-space link causes lower received optical power, leads to lower RF carrier level output, and finally results in lower CNR/CSO/CTB values. Over 40-km SMF and 50-m free-space transport, however, the measured CNR/CSO/CTB values (≥46.3/57/57.5 dB) still meet the fiber optical CATV CNR/CSO/CTB demands at the subscriber (≥43/53/53 dB).

 

Fig. 3 The measured CNR/CSO/CTB values under NTSC channel number (CH2-78).

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The measured BER curves of the 10 Gbps/30 GHz MW data signal for back-to-back (BTB) and over 40-km SMF as well as 10-m RF wireless transport scenarios are shown in Fig. 4. At a BER of 10⁻⁹, a power penalty of 4.4 dB is observed between BTB and 40-km SMF as well as 10-m RF wireless transport scenarios. Such a 4.4-dB power penalty is resulted from the fiber dispersion over a 40-km SMF transport and fading effect over a 10-m RF wireless transport. Over a 40-km SMF transport, RF power degradation induced by fiber dispersion degrades BER performance due to the natural characteristic of the two optical sidebands. Over a 10-m RF wireless transport, the fading effect causes the amplitude and phase of the received signal to fluctuate, and thus degrades the BER performance. In addition, the measured BER curves of the 10 Gbps/45 GHz MMW data signal for BTB and over 40-km SMF as well as 10-m free-space transport scenarios are shown in Fig. 5. At a BER of 10⁻⁹, a power penalty of 4.7 dB is observed between BTB and 40-km SMF as well as 10-m RF wireless transport scenarios. Such BER performance degradation is also resulted from the fiber dispersion-induced and RF fading-induced penalties.

 

Fig. 4 The measured BER curves of the 10 Gbps/30 GHz MW data signals for BTB and over 40-km SMF as well as 10-m RF wireless transport scenarios.

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Fig. 5 The measured BER curves of the 10 Gbps/45 GHz MMW data signals for BTB and over 40-km SMF as well as 10-m RF wireless transport scenarios.

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Moreover, the measured BER curves of the 10 Gbps/60 GHz MMW data signal for BTB and over 40-km SMF as well as 50-m free-space transport scenarios are shown in Fig. 6. Under a low BER operation of 10⁻⁹, a large power penalty of 6.2 dB is existed between BTB and 40 km SMF as well as 50-m free-space transport scenarios. Such a 6.2-dB large power penalty can be attributed to fiber dispersion over 40-km SMF transport and optical power attenuation resulting from the 50-m free-space transmission. Longer free-space transmission distance leads to lower received optical power, by which conducing to lower photocurrent induced from PD. As the free-space transmission distance increases, the photocurrent decreases and brings on the decrement of optical signal-to-noise ratio (OSNR) value. Such OSNR decrement results in the degradation of BER performance.

 

Fig. 6 The measured BER curves of the 10 Gbps/60 GHz MMW data signals for BTB and over 40-km SMF as well as 50-m free-space transport scenarios.

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For uplink transmission, the measured BER curves of the 5-Gbps data stream for BTB and over 40-km SMF as well as 50-m free-space transport scenarios are shown in Fig. 7. At a BER of 10⁻⁹, a power penalty of 4.4 dB is existed between BTB and 40-km SMF as well as 50-m free-space transport scenarios. This 4.4 dB power penalty is the result of the fiber dispersion after 40-km SMF transport and further transmission over a 50-m free-space link. Further transmission over a 50-m free-space link leads to lower received optical power and OSNR, and eventually causes worse BER performance. Since both downstream and upstream signals are transmitted by the same SMF and wavelengths, yet Rayleigh backscattering noise will limit the performance systems seriously. The Rayleigh backscattering noise is generated due to both the back-reflection of downstream signal and that of remodulated upstream signal in a RSOA. To reduce the noise caused by the remodulation, the RSOA is operated in the saturation region [14]. As the RSOA injection power is increased, the reflection tolerances of both the upstream and downstream signals are improved to some extent. This is mainly due to a fact that the RSOA gain is reduced as the injection power increased, and consequently the power ratio of the reflected light to the signal light is reduced. The higher the injection power into the RSOA, the more suppressed the downstream CATV signal in the upstream transmission become, and finally improving the BER performance of upstream data stream.

 

Fig. 7 The measured BER curves of the 5-Gbps data stream for BTB and over 40-km SMF as well as 50-m free-space transport scenarios.

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4. Conclusions

A bidirectional fiber-wireless and fiber-IVLLC integrated system based on polarization-orthogonal modulation scheme to deliver downstream 50-550 MHz CATV, 10 Gbps/30 GHz MW, 10 Gbps/45 GHz MMW, and 10 Gbps/60 GHz MMW signals, as well as upstream 5 Gbps BB data stream, is proposed and demonstrated. To our knowledge, it is the first one that employs a polarization-orthogonal modulation scheme in a bidirectional fiber-wireless and fiber-IVLLC integrated system. By virtue of polarization-orthogonal modulation scheme, the optical carrier and the optical sidebands are polarization-orthogonal and split automatically. It reveals a prominent alternative to separate the optical carrier and the optical sidebands. The results show that CNR, CSO, CTB, and BER perform well over 40-km SMF and 10-m RF/50-m optical wireless transport. Such a proposed bidirectional fiber-wireless and fiber-IVLLC integrated system will be a very attractive approach for the integration of fiber backbone and RF/optical wireless feeder.