We demonstrate radio-over-fiber (RoF) transmission with a 10-W feed power-over-fiber (PWoF) using a conventional multimode fiber (MMF). In this scheme, the modal dispersion and feed light crosstalk in the MMF are effectively mitigated by the combination of center-launching (CL) and offset-launching (OL) techniques. The CL is used for propagating the feed light into lower-order modes in the MMF, while the OL is used not only for propagating the optical data signals into higher-order modes in the MMF, but also for mitigating the modal dispersion. We successfully achieved significant improvement in the RoF transmission performance with the 10-W feed PWoF and extended the link length up to 4 km, owing to the two techniques.
© 2018 Optical Society of America
Mobile data traffic growth is accelerating the evolution of radio-over-fiber (RoF) networks, which transmit radio-frequency (RF) signals using optical fibers [1,2]. In addition, a reduction in the cell size of remote antenna units (RAUs) is required to support higher data rates of RF signals, and a huge number of RAUs have to be installed in densely populated areas. Thus, in RoF networks, it is important to provide cost-effective installation, operation, and maintenance for the RAUs.
Power-over-fiber (PWoF) is a simple way to simultaneously transmit optical data and power into optical fibers. In RoF networks, the use of PWoF is effectively used to centralize the required power source in a central station (CS) and to deliver the feed light with optical data signals in the same cables. As optical fibers are nonconductive power lines, unlike electrical cables, PWoF is also useful for preventing lightning damage for the CS. In addition, a few research groups have shown that the introduction of sleep mode power control of RAUs offers up to 60% energy saving in the mobile networks [3,4]. Therefore, if we are able to control the delivered power according to the data traffic in the link via PWoF, the overall power in the network will be more efficiently reduced. On the other hand, as the electric power required for driving a conventional femto-cell-type RAU is at least several watt-classes, single-mode fibers (SMFs), which are most popular and widely used for optical fiber communications, are not suitable for PWoF links. This is because simultaneous optical data signals and feed light transmission in its small core area strictly limits the available feed light power.
To solve this problem, a few approaches based on space-division-multiplexing (SDM) have been already reported so far. One is to use different optical fibers for the data signals and the feed light transmission, which is the simplest SDM technology to separate the data signals and the feed light . Another is to use multi-core fibers (MCFs), which enable us to transmit the data signals and feed light into each different core of the MCF . We also have proposed and demonstrated an optically powered RoF system using double-clad fibers (DCFs), which consisted of a single-mode (SM) core and a multimode inner cladding [7–11]. The use of DCFs enables us to simultaneously transmit broadband optical data signal and high-power feed light without large crosstalk between the signal and the feed light, owing to the double-core structure of the DCFs. Indeed, we have achieved good transmission performance with negligible power penalties in the bidirectional RoF transmission  and the optically controlled beam steering system  during 60-W optical feeding.
Multimode fibers (MMFs) are also widely used for optical fiber communications and are practical candidates for PWoF links, because MMFs are pre-existing fibers without requiring a large-scale deployment of new fibers, and have core area, which is much larger than that of the SMFs. However, owing to the differential delay in the MMFs, it is difficult to increase the transmission speed of MMF links. This is called “modal dispersion.” To solve this problem, a number of approaches have been reported so far. In particular, center-launching (CL) [12,13] and offset-launching (OL)  techniques, which adjust the beam launching position of the input optical signal into the MMF core, are widely used to restrict the modal excitation to either higher- or lower-order propagation modes.
In optically powered RoF systems, high-power feed light sources are required to deliver optical power sufficient to drive an RAU. However, in general, the laser quality of the feed light sources in terms of output wavelength, power stabilities, and relative intensity noise is much lower than that of the conventional laser diodes (LDs) used as optical transmitters. In addition, the output power of the feed light is much higher than that of conventional LDs. Therefore, high-power feed light injection into the PWoF links gives rise to the signal quality degradation of optical data signals due to large crosstalk between the signal and the feed light. In particular, the crosstalk will be remarkable in the case of single-core fiber links such as SMFs and MMFs, because the signal and the feed light will have to be transmitted in the same core area together. Thus, it is very important to mitigate the feed light crosstalk in the PWoF links.
This Letter presents a novel method to simultaneously mitigate modal dispersion and feed light crosstalk of the optical data signals by using a combination of CL and OL techniques in a conventional MMF. In this scheme, the CL technique is used for propagating the feed light into lower-order modes in the MMF, while the OL technique is used for propagating optical data signals into higher-order modes and mitigating the modal dispersion in the MMF. Moreover, the different modal excitations and propagations play an important role in mitigating the crosstalk between the signal and the feed light. Although the optical data signals and the feed light are not completely separated in space, the different modal propagation is effective in mitigating the crosstalk. To show the feasibility, we have successfully demonstrated downlink and uplink RoF transmissions with around 10-W feed PWoF over a conventional 4-km MMF, for the first time, to the best of our knowledge.
Figure 1 shows the experimental setup for optically powered downlink RoF transmission using a 2 km or 4 km MMF. A continuous-wave (CW) light was generated from a distributed-feedback LD (DFB-LD) with a wavelength of 1310 nm. The light was modulated using a intensity modulator (LNM) with an electrical data generated by a signal generator (SG). These data were based on IEEE standard, wireless local area network (WLAN) with orthogonal frequency division multiplexing (OFDM) and 64 level quadrature amplitude modulation (64-QAM) format. The carrier frequency and bit rate of the signal were 2.45 GHz and 54 Mbit/s, respectively. The modulated data (analog RoF) signal was amplified by a 1.3-μm semiconductor optical amplifier (SOA). Isolators (ISOs) at the input and the output ports of the SOA were used to eliminate the laser oscillation, while a bandpass filter (BPF) with a 3-dB bandwidth of 1 nm was used to remove the amplified spontaneous emission noise induced by the SOA. After passing through the BPF, the RoF signal was injected into a commercially available mode-conditioning patch (MCP) cable, which consisted of at around 10-μm offset components between the input SMF and the output MMF with conventional optical connectors, to launch the RoF signal into the MMF link by the OL technique. The outside appearance of the MCP is shown in Fig. 1. In the OL technique, the mitigation effect of the modal dispersion strongly depends on the offset position , i.e., the distance of the center core position between the input SMF and the output MMF, as shown in the inset of Fig. 1. Therefore, we evaluated and compared the bandwidth enhancement performance of the multiple MCPs and carefully selected an MCP for this experiment. The feed light for the PWoF was generated from a commercially available high-power fiber laser (HPFL, 3SP Group Inc., ML-CW-R-TKS-1550) with a wavelength of 1550 nm and maximum output power of 9.7 W. The spectrum linewidth was approximately 3 nm. The SMF output of the HPFL was connected to the MMF output by the CL technique to propagate the feed light into lower-order modes in the MMF link. The RoF signal and the feed light were combined using a 1310/1550 nm wavelength-division-multiplexing, multimode optical coupler (WDMC) with high-power handling and transmitted into a conventional 2 km or 4 km graded-index MMF with a core diameter of 62.5-μm (OM1). After the MMF transmission, the combined RoF signal and feed light were divided using a 1310/1550 nm WDMC. The RoF signal was converted into electrical signal using a photo diode (PD), and the signal quality was evaluated using a signal analyzer (SA) in terms of error-vector magnitude (EVM). A variable electrical attenuator (ATT) was used to adjust the signal power injected into the PD. In addition, we measured the transmitted feed light power to evaluate the delivered feed power and power transmission efficiency using an optical power meter (OPM) at the receiver. In the case of the RoF uplink transmission, the RoF transmitter and receiver were replaced, and the transmission performance was measured using the SA.
To investigate the mitigation effect of the modal dispersion by the OL technique, we measured the EVM characteristics of the 2-km transmitted signals as a function of the received electrical signal power for various launching conditions. The result is shown in Fig. 2. In this experiment, we employed four MCPs at different offset positions and compared the transmission performances. In addition, we also employed a conventional connector instead of the MCP. In this case, the beam of the RoF signal at the SMF output was directly launched into the MMF at around center position of the MMF core. As shown in Fig. 2, the best and the worst EVM characteristics could be obtained by using the MCPs. The difference performance was due to the different offset positions of the MCPs. As the mitigation effect of modal dispersion in the OL technique strongly depends on the offset position , the EVM characteristics reflected the offset position of the MCPs. In the best case, the assumed offset position was approximately 10-μm, while in the worst case, the offset position was approximately 15-μm. In Fig. 2, the EVM characteristics of the transmitted signal determined by the best MCP were better than those of the signal determined by the conventional connector. This indicates that the OL technique is useful in mitigating the modal dispersion in the MMFs. Although the improvement is not very significant, the OL technique is also useful for mitigating the feed light crosstalk with the CL technique. Thus, the use of the OL technique by the MCPs is very effective to simultaneously transmit data and power in the same MMF core. In the following experiments, we used the best MCP for the OL technique.
We measured the delivered feed light power and the power transmission efficiency over 2-km and 4-km MMFs. The result is shown in Fig. 3. As the feed light power increased, the transmitted power increased almost linearly. When the feed light power was set to 9.7 W, the maximum delivered powers over the 2-km and 4-km MMF transmissions were 7.3 W and 6 W, respectively. The calculated power transmission efficiencies, which were defined as the power ratios of the HPFL output and the OPM input, were 73% and 60%, respectively. The reason for achieving such a high-power transmission efficiency was due to the feed light wavelength in the lowest loss transmission band (1.55-μm) and low insertion losses of the optical components inserted into the MMF link, such as the WDMCs. Evidently, the power transmission efficiency was much higher than that of our previously reported PWoF transmission using double-clad fibers [7,9]. If we use gallium (Ga)-based photovoltaic power converters, an ideal optical-to-electrical conversion efficiency of approximately 36.4% can be obtained at around a wavelength of 1550 nm . Thus, the assumed total power transmission loss of the presented PWoF system in the 2 km and 4 km MMF links were 5.76 dB and 6.61 dB, respectively.
To evaluate the RoF transmission performance with the PWoF in the MMFs, we compared the EVM values of the transmitted RoF signals. Figure 4(a) shows the EVM characteristics as a function of the feed light power in the 2-km MMF downlink transmissions. When the feed light power was 0 W, the EVM values of the transmitted signals were close to that of the back-to-back signal (dashed line), and the differences were less than 0.37%. This indicates that the OL technique using the MCP is effective to mitigate the modal dispersion of the RoF signal. As the feed light power increased, the EVM values were gradually increased. In particular, the EVM value of the transmitted signal without the OL technique increased more drastically. When the feed light power was set to 9.7 W, the EVM penalty was approximately 1.0%. The insets show the constellations of the back-to-back and the transmitted signals with and without the OL technique. It was clearly seen that the feed light crosstalk gave rise to the distortion of the constellation. Figure 4(b) shows the EVM characteristics in the 4-km MMF downlink transmission. In this case, in the 4-km MMF transmission without the OL for the RoF signal, the EVM value could not be measured when the feed light power was over 6 W, as it was outside of the measurable EVM range of the SA. However, when we used the OL technique, the EVM values were dramatically reduced, and the available transmission length with the PWoF was extended up to the 4-km MMF link. This was due to the strong mitigation effect on the feed light crosstalk. It was thus found that the combination of the OL technique for the RoF signal and the CL technique for the feed light was very useful for simultaneously mitigating the modal dispersion and the feed light crosstalk.
Figure 5 shows the EVM values of the uplink transmitted signals in the 2 km and 4 km transmissions. The EVM values of each transmission were close to those of the downlink transmitted signals without the feed light power, and were maintained to be almost constant even when the feed light power was increased. This was because the feed light was propagating in a direction opposite to that of the RoF signal, and the counter-propagation condition did not give rise to the significant feed light crosstalk.
In conclusion, we demonstrated the RoF transmission with around 10-W feed PWoF using a conventional MMF. By using CL and OL techniques, the modal dispersion and the feed light crosstalk were effectively mitigated, and good transmission performance was successfully achieved. The presented PWoF link has a simple scheme, which consists of conventional optical fiber and components, and will be useful for cost-effective, optically powered RoF networks.
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