1. Introduction
Power over fiber (PoF) technology, as an efficient means of energy transmission, plays an increasingly important role in modern communications [1,2,3,4,5], industrial applications [6,7,8,9], and energy management [10,11]. This technology utilizes optical fiber as the medium for transmitting energy, enabling long-distance energy transmission with low loss. Compared with traditional power transmission methods, it offers several advantages, including long transmission distances, high resistance to electromagnetic interference, and low attenuation, making it the preferred choice for power supply in some special fields [12,13,14,15]. Within PoF systems, different types of optical fibers exhibit distinct advantages based on their performance characteristics [16,17,18,19,20]. Single-mode fiber (SMF), known for its high transmission bandwidth, is critical in long-distance communication and high-speed data transmission. However, as the demand for power transmission increases, the small core diameter of SMF limits its power density, thereby constraining the output performance of high-power PoF systems. Multicore fiber enhances transmission capacity and flexibility by integrating multiple cores within a single fiber. Yet, the crosstalk between cores remains a significant challenge that limits its output performance. Double-clad fiber optimizes the optical signal transmission path and improves the energy transmission efficiency through the specially designed cladding structure. However, the high manufacturing cost and stringent design and processing requirements present substantial barriers to its widespread adoption. Micro-structured fiber, which incorporates micro- and nano-scale features to regulate optical signal propagation, offers promising potential for high-efficiency energy transmission. Nevertheless, the complexity of its manufacturing process and the high technical difficulty involved remain challenges to its practical application. In contrast, multimode fiber (MMF) has a relatively low manufacturing cost and is easy to install and maintain. Moreover, its larger core diameter can support the higher power transmission [14,21,22]. Therefore, the development of MMF optic networks provides a solid platform for further in-depth application of the PoF technology.
With the increasing demand for transmission power and distance, MMF-based PoF systems face a few challenges, especially during high-power laser injection, where the system performance is significantly affected due to the occurrence of Stimulated Raman Scattering (SRS) effects [23]. Several solutions have been proposed to obtain better performance output, including spectral modulation using filters, optimization of the laser pulse morphology using chirp techniques, and tuning of the input signal characteristics by tunable seed sources [24,25,26,27]. To further achieve higher output power in long-distance PoF systems, this paper applies the offset-launching (OL) scheme, which has been previously applied to mode dispersion suppression and signal bandwidth enhancement [28,29,30], to the 49.4 km PoF system using MMF. Moreover, the performance of PoF systems is affected by various factors including the light source and the launch conditions. We analyze the effects of OL distance and coherence on the transmission performance of the long-distance PoF system through continuous-wave light sources. The work in this paper provides a new solution idea for the performance optimization of the high-power and long-distance MMF-based PoF system.
In this study, the effects of coherence and OL distance on the power performance of the 49.4 km PoF systems are comprehensively analyzed. The experimental PoF system is constructed based on the OL scheme, utilizing the 49.4 km conventional graded-index MMF and four distinct light sources. These sources include a low-power 8 nm laser source controlled by an upper computer and high-power continuous-wave lasers (HPCWLs) with linewidths of 2 nm, 12 nm, and 20 nm, respectively. The power transmission characteristics using the HPCWLs are analyzed and the performance in different OL distances is compared with that of a SMF-based PoF system. The effects of light source coherence and OL distance on the maximum output power are discussed and compared. The results show that the 20 nm PoF system with slightly weaker coherence exhibits superior performance at shorter transmission distances (~36 km), while the 12 nm PoF system with slightly stronger coherence performs best as the transmission distance increases (~49.4 km). As the transmission distance further increases, the 2 nm linewidth source is predicted to yield higher output power. For OL distance, the optimal OL distance can be selected according to specific application requirements (light source and transmission distance). Specifically, the 20 nm PoF system achieves the best performance within an OL range of 0–12.5 μm. Beyond 15 μm, the 12 nm linewidth source demonstrates superior performance. Based on the above-detailed analysis of the different PoF systems, we give the optimal light source and OL distance configurations for different transmission distances. The work in this paper provides ideas for the optimization of high-power and long-distance PoF systems and provides an experimental basis for energy supply such as that of submarine optical fiber cable far-end devices and distributed sensor networks.
This paper is organized as follows. Section 2 gives the overall experimental design and the related configuration of the PoF system components. Section 3 gives the validation of the effectiveness of the OL scheme. Section 4 analyzes the output performance of the high-power continuous wave PoF system with different coherence. Section 5 comprehensively analyzes the effect of the light source coherence and the OL distance on the output performance of the 49.4 km long-distance PoF system. Section 6 discusses the energy utilization rate performance. Section 7 gives a summary of our main work.
2. Experimental Design and Configuration
Figure 1 presents the block diagram of the PoF system based on the OL scheme, incorporating four distinct light sources, all with central wavelengths of 1550 nm. The laser pigtail is an SMF with a core diameter of 8.2 μm, which is directly fused to the 49.4 km MMF link by controlling the OL distance, and the MMF in the system is the conventional graded-index OM1 fiber with a core diameter of 62.5 μm. Notably, we use the direct fusion solution in the 49.4 km PoF system, where the laser’s pigtail and the MMF are fused directly using the fusion splicer without the need for spatial collimation, lenses and other components. This will greatly expand its application range, allowing it to be flexibly installed in long-distance high-power PoF systems for underground, underwater and other engineering scenarios. Figure 1a illustrates the schematic of the OL node. Figure 1b shows the schematic of the OL scheme, where the OL distance is defined as the separation between the cores of the SMF and MMF. Figure 1c gives the schematic of the traditional center-launching (CL) scheme (i.e., 0 μm OL distance) for comparison. Figure 1d gives a PoF link using a low-power laser source with a linewidth of 8 nm, controlled by an upper computer, where the maximum output reaches 4.2 W. At the end of the optical fiber link, a 1:99 beam splitter is used for power distribution, delivering power with a ratio of 1 into the optical spectrum analyzer (OSA) for spectral observation and power with a ratio of 99 into the optical power meter (OPM) for measurement of the transmitted power, which will prevent the spectrometer from being damaged by the high-power laser and enable simultaneous measurement of both the power and the output spectrum. Figure 1e illustrates the PoF link using three different HPCWLs with 2 nm, 12 nm, and 20 nm linewidth, and the corresponding maximum output powers are 9.89 W, 14.88 W, and 9.29 W, respectively. The laser pigtail is offset and launched into the MMF link, and the high-power laser is transmitted through the MMF and then evaluated by the OPM at the end of the fiber to assess the output power.
3. Validation of the Effectiveness of the OL Scheme
In this section, the effectiveness of the OL scheme for the MMF-based PoF system is firstly verified based on a low-power laser source. We analyze the maximum output power as well as the spectral perspective of the output beam at the end of the fiber. Before proceeding, the definition of the SRS threshold is first given. As the input power increases, the output power reaches a stable value due to the occurrence and accumulation of the SRS effect, and the input power corresponding to this stable value is defined as the SRS threshold power.
The nature of OL is to realize multimode excitation by controlling the OL distance [21,28]. In this paper, by controlling the distance between the laser’s single-mode pigtail and the MMF link, the light from the laser is directly coupled into the MMF. The multiple modes in the MMF link are excited, and the power is loaded in multiple modes for transmission, which improves the output power in the PoF system. The maximum output power of the PoF system using the 8 nm laser source at different OL distances is given in Figure 2. The maximum output power firstly increases with increasing OL distance, and decreases with increasing OL distance beyond a certain OL distance, which is limited by the geometrical properties of the MMF itself. Despite the decreasing trend, the maximum output power at 15 μm is still better than that at 0 μm OL distance. Figure 3 further gives the output spectra of the 0 μm OL distance at the SRS threshold power and the output spectra at 15 μm OL distance at the same input power. Compared with the 0 μm OL distance, the multiple modes excited under 15 μm OL conditions broaden the forward Raman Stokes wave in the 1660–1680 nm band, resulting in a slow tail, and the output power is more concentrated around 1550 nm. From the maximum output power and spectral point of view, the OL scheme is effective in the PoF system and can significantly increase the output power.
4. Output Performance of 49.4 km PoF System
The HPCWL with better coherence and higher power are more often used as light sources in the PoF systems. Therefore, in the following analysis, we focus on the output performance of 49.4 km PoF systems with different linewidths based on the OL scheme. The maximum output powers of 2 nm, 12 nm, and 20 nm PoF systems at different OL distances are analyzed sequentially, and the performance is compared with that of SMF power delivery systems.
The variation curves of the maximum output power with OL distance for the 49.4 km PoF system with a 2 nm narrow-linewidth HPCWL as the light source are given in Figure 4a. The maximum output power also shows a trend of increasing and then decreasing with the OL distance, and the maximum output power at 30 μm OL distance is still higher than that at 0 μm. The performance comparison curves of the MMF system using different OL distances with the SMF-based PoF system are given in Figure 4b, where the 12.5 μm, 15 μm, and 17.5 μm OL distances with superior performance are selected. We can see that the maximum output power of the OL scheme outperforms the SMF power transmission link at all transmission distances. The maximum output power at 17.5 μm OL distance is even about 4.5 times that of the single-mode system. The above fully demonstrates that the OL scheme in 49.4 km long-distance PoF systems with the 2 nm HPCWL can achieve a significant increase in output power, even far exceeding that of single-mode systems.
The linewidth of the light source is further increased, and the variation curves of the maximum output power with OL distance for the 49.4 km PoF system with the 12 nm HPCWL as the light source are given in Figure 5a. In the fiber length range of 10.8 km–49.4 km, the maximum output powers are all higher than the conventional scheme. This indicates that the OL scheme plays a crucial role in power enhancement in the 49.4 km MMF-based PoF systems. The performance comparison curves with SMF are further given in Figure 5b. It is evident that the maximum output power of the 12 nm PoF system based on the OL scheme still performs better than that of the SMF-based PoF link. Notably, the OL nodes selected here for relatively superior performance are 15 μm, 17.5 μm, and 20 μm, differing slightly from the OL nodes in the 2 nm light source condition, and this will be analyzed in detail later.
In addition, the maximum output power of a HPCWL with a wider linewidth of 20 nm as the light source is also measured and compared with the performance of a SMF power delivery system, as shown in Figure 6. It is observed in Figure 6a that the OL scheme still has a significant increase in output power over 49.4 km MMF link. In detail, the maximum output power can reach 3.79 W, 2.35 W, 1.40 W, 0.65 W, 0.35 W, and 0.20 W at all transmission distances, respectively. The conventional scheme (i.e., 0 μm) is only 1.31 W, 0.90 W, 0.44 W, 0.17 W, 0.13 W, 0.07 W. Moreover, the performance comparison curves with the single-mode power transmission system are given in Figure 6b, and the OL nodes are selected in 12.5 μm, 15 μm, 17.5 μm, respectively. When increasing the linewidth of the light source to 20 nm, the transmission performance of the OL scheme is much better than that using the SMF in the range of 10–36 km, and even better than that of the G.654E fiber with an ultra-low loss coefficient. However, it is slightly different in the 41–49.4 km transmission range compared to the 2 nm and 12 nm PoF system. The maximum output power of the 20 nm PoF system based on the OL scheme is on par with the output performance of the G.654E fiber. Therefore, we consider that while the OL scheme improves the output power of the 49.4 km long-distance PoF system, its enhancement performance is also affected by the linewidth of the light source, i.e., coherence.
5. Effect of Light Source Coherence and OL Distance on PoF System Performance
To more explicitly give the effects of light source coherence and OL distance on the performance of the 49.4 km long-distance PoF system, this section presents a detailed analysis of the output power variation as a function of input power, considering light sources with varying linewidths (i.e., differing coherence properties). Additionally, the variation of the maximum output power for transmission distance (i.e., fiber length) is explored for different OL distances. The optimal OL distance and the corresponding maximum output power for each light source under varying transmission distances are determined. The work in this part will provide a detailed configuration selection for the practical application of long-distance PoF systems.
For a more specific analysis of the effect of light source coherence on the 49.4 km PoF system performance, the output power characteristic curves of the three continuous-wave sources at different OL distances are next compared. Among them, the three light sources reach the SRS threshold at all transmission distances in the 0–12.5 μm OL range. Figure 7 illustrates the curves of output power with input power with the 10 μm OL distance as an example. The PoF system under all three continuous wave light source conditions experiences SRS nonlinear effects, leading to stabilization of output power at respective levels. This choice ensures the PoF system is in the nonlinear region, avoiding analytical errors associated with systems that remain in the linear transmission region. From Figure 7, the output power initially increases with input power, which corresponds to the linear region typical of optical fiber transmission. As the input power increases, the nonlinear SRS effect due to the high-power laser causes the output power to level off and eventually reach a stable value, and the input power corresponding to this value is also known as the SRS threshold power.
Furthermore, as shown in Figure 7a–d, the PoF system employing the HPCWL with a 20 nm linewidth achieves the highest output power within the transmission range of 10.8 km–36 km, followed by the PoF system using the 12 nm HPCWL, and the system with a 2 nm linewidth yields the lowest output. Beyond 36 km [see Figure 7e,f], the system with a 12 nm linewidth exhibits the maximum output power within the 41 km to 49.4 km range, surpassing the 20 nm system. At longer transmission distances, it is considered that the narrower the linewidth of a light source, the fewer frequency components it contains and the less phase or frequency noise it has. Meanwhile the corresponding coherence is stronger, which is manifested in extremely long coherence lengths [31,32,33]. Its high monochromaticity, high coherence, and low noise properties mean the light wave will maintain phase consistency over longer distances, stabilize the transmission and better maintain the shape of the signal over long-distance links, which can reduce the degradation of the signal. Therefore, under the same transmission distance and OL distance configuration conditions, it is expected that the 2 nm light source with the narrowest linewidth and higher coherence will have the best performance over longer distances.
Correspondingly, the SRS threshold is in one-to-one correspondence with the output power of the system, and a better output performance corresponds to a larger SRS threshold. As can be seen in Figure 7, the 20 nm system with the largest linewidth has the largest SRS threshold, followed by the 12 nm system, and the 2 nm system has the lowest SRS threshold over the transmission range of 10.8 km–36 km. Similarly, beyond 36 km, following the same trend as the best output performance, the 12 nm system with slightly higher coherence has the largest SRS threshold when the transmission distance is larger (41 km–49.4 km). Moreover, the narrower linewidth 2 nm system with the highest coherence is expected to have the largest SRS threshold at longer transmission distances.
From the light source coherence, the wider linewidth source at a single OL distance (any one of 0–12.5 μm) is selected to obtain the highest possible output power at a shorter transmission distance, and the narrower linewidth source is selected to obtain the highest possible output power at a longer transmission distance. The power transmission characteristics at a single OL distance are analyzed above, and some PoF systems have greater output power when the OL distance exceeds 12.5 μm because the SRS threshold has not yet been reached at the rated power of the laser. The power characteristics of PoF systems at different OL distances will be analyzed in detail later.
Figure 8 gives the maximum output power of the 49.4 km PoF system versus fiber length at 0 μm–20 μm OL distance under different light source conditions. Note that due to the limitation of the geometrical characteristics of the MMF itself, when the OL distance exceeds 20 μm, the output performance of the PoF systems with three different linewidths exhibits a slight degradation even though the SRS threshold power is not reached, and thus the system performance after 20 μm is not analyzed here. From Figure 8a–f, it is observed that the 20 nm PoF system has the optimal performance in the transmission range of 10–36 km at the OL distance of 0–12.5 μm, and it has maximum output power at different OL distances. From Figure 8g, at the OL distance of 15 μm, it is no longer the 20 nm PoF system that has the optimal performance in the transmission range of 10.8 km–36 km. Specifically, the 12 nm PoF system performs optimally in the range of 15.8 km, and it is the 20 nm PoF system that performs optimally in the range of 25.2 km–36 km. When the OL distance exceeds 15 μm [see Figure 8h,i], the 12 nm PoF system has optimal performance for all transmission distances in the 17.5 μm–20 μm OL range. Therefore, the OL distance also affects the output performance of the PoF systems using light sources with different linewidths, and the PoFs with different linewidths have their own optimal OL distances.
Similarly, to avoid analysis errors induced in the linear region, we focus on the OL distances that reach the SRS threshold power in each linewidth PoF system. In more detail, Table 1 gives the optimal OL distances for different light sources at different transmission distances. Correspondingly, Table 2 gives the optimal output power corresponding to the SRS threshold power of different light sources at optimal OL distances for different transmission distances. Moreover, the optimal output power can reach 0.36 W over the 49.4 km MMF link. The optimal configurations in Table 1 and Table 2 will provide experimental support for the optimization of long-distance and high-power PoF systems and the energy supply of remote devices. According to the specific practical requirements, the optimal OL can be selected based on the transmission distance and the existing light source to realize the optimal performance output.
6. Performance Discussion of 49.4 km PoF System
Based on the above optimal configurations in Table 1, we obtain the optimal output performance of the 49.4 km PoF system as shown in Table 2. This section discusses the energy utilization rate (the ratio of output power to input power) performance at a transmission distance of 49.4 km, as well as an application example in long-distance energy supply. The energy utilization rate of the PoF system at 0 μm OL distance and the optimal configurations is given in Figure 9. At 10.8 km transmission distance, all systems with different linewidths achieve an energy utilization of about 45%. At all transmission distances, the energy utilization rate of PoF systems with different linewidths fluctuates no more than 3% for 0 μm and optimal OL configuration conditions. With the increase in transmission distance and the accumulation of the fiber’s own loss under long distance transmission, the energy utilization of PoF system shows a decreasing trend. At 49.4 km transmission distance, the output power is 0.08 W at 0 μm OL distance, and the energy utilization rate is 3.2%. Comparatively, this paper achieves an output of 0.36 W in the optimal configuration, at which point the energy utilization rate is 2.7%. Notably, despite a 0.5% difference in energy utilization rate, a relatively high-power output is achieved, which is important for long-distance remote energy supply. As an example, in the distributed sensing network, the 0 μm PoF system has only 0.08 W power output at 49.4 km, and combined with the photoelectric conversion module with 25% photoelectric conversion efficiency, it can realize 0.02 W electric power output, which can support two sensors (temperature, humidity, etc.) with 0.01 W power consumption, whereas an electrical power output of 0.09 W can be realized with optimal configurations, which can support nine sensors. Therefore, the work in this paper can solve the power limitation in long-distance application scenarios. It can expand the coverage range of the distributed sensing network, especially in the scenarios such as underwater and underground corridor tunnels, where a relatively high energy supply of about 50 km can be achieved by a single optical fiber. On the other hand, more sensors of the same type or more types of sensors can be supported at the 50 km node. Therefore, this work will provide an experimental basis for the optimization design and configuration selection of PoF systems for energy supply to remote devices.
7. Conclusions
This paper experimentally and systematically analyzes the effects of light source coherence and OL distance on the power performance in the 49.4 km long-distance PoF system using the conventional graded-index MMF. The PoF experimental system with four different light sources is constructed using 49.4 km MMF and 30 μm OL distance. These include the low-power 8 nm laser source, and the HPCWLs source with different linewidths (2 nm, 12 nm, and 20 nm). The results show that the OL scheme significantly improves power performance across various light source configurations. For coherence, the 20 nm light source performs best in the 10.8 km–36 km PoF link, while the advantage of the 12 nm light source is gradually reflected in the 41 km–49.4 km, and the 2 nm light source is expected to realize the optimal output power in longer-distance. For OL distances, the 20 nm source yields the best performance between 0 and 12.5 μm, while the 12 nm source is more advantageous beyond 15 μm. Additionally, this paper finally gives the optimal light source and OL distance configurations for the different transmission distances, and the maximum output power can up to 0.36 W over 49.4 km. This work provides an experimental basis for the optimal design and configuration selection of long-range and high-power PoF systems.
Conceptualization, Y.L. and Y.G.; methodology, Y.L. and Y.G.; software, Z.Z. (Zhaoyang Zhang); formal analysis, Y.G.; investigation, H.G.; resources, Y.L. and Z.Z. (Zhiguo Zhang); data curation, Y.G.; writing—original draft preparation, Y.G.; writing—review and editing, Y.L.; visualization, Z.Z. (Zhaoyang Zhang) and H.G.; supervision, Z.Z. (Zhiguo Zhang); project administration, Y.L. and Z.Z. (Zhiguo Zhang); funding acquisition, Y.L. and Z.Z. (Zhiguo Zhang). All authors have read and agreed to the published version of the manuscript.
Not applicable. The study did not require ethical approval.
Not applicable. The study did not involve humans.
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.
The authors declare no conflicts of interest.
Footnotes
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Figure 1 (a) Schematic diagram of OL node. (b) Schematic of OL distances. (c) Schematic diagram of 0 μm OL distance. (d) Block diagram of the PoF system based on an 8 nm laser source controlled by the upper computer. (e) Block diagram of PoF system based on HPCWLs.
Figure 2 Variation curves of maximum output power with OL distance.
Figure 3 Comparison of the spectra of the 0 μm OL distance at the SRS threshold power with the OL-based scheme at the same input power, (a) 10.8 km, (b) 15.8 km, (c) 25.2 km, (d) 36 km.
Figure 4 (a) Variation curves of maximum output with OL distance for 49.4 km PoF system with 2 nm narrow linewidth light source, (b) performance comparison curves of MMF power transmission link based on OL scheme with SMF power transmission link.
Figure 5 (a) Variation curves of maximum output with OL distance for 49.4 km PoF system with 12 nm narrow linewidth light source, (b) performance comparison curves of MMF power transmission link based on OL scheme with SMF power transmission link.
Figure 6 (a) Variation curves of maximum output with OL distance for 49.4 km PoF system with 20 nm wider linewidth light source, (b) performance comparison curves of MMF power transmission link based on OL scheme with SMF power transmission link.
Figure 7 Variation curves of output power with input power for PoF system at 49.4 km transmission distance with 10 μm OL distance. (a) 10.8 km, (b) 15.8 km, (c) 25.2 km, (d) 36 km, (e) 41 km, (f) 49.4 km.
Figure 8 Maximum output power versus fiber length for different light sources at 0 μm–20 μm OL distance, and the red dot represents the optimal performance. (a) 0 μm, (b) 2.5 μm, (c) 5 μm, (d) 7.5 μm, (e) 10 μm, (f) 12.5 μm, (g) 15 μm, (h) 17.5 μm, (i) 20 μm.
Figure 9 Energy utilization rate with 0 μm OL distance and optimal configuration.
Optimal OL distance.
| Linewidth (nm) | Optimal OL Distance (μm) | |||||
|---|---|---|---|---|---|---|
| 10.8 km | 15.8 km | 25.2 km | 36 km | 41 km | 49.4 km | |
| 2 | 12.5 | 12.5 | 15 | 15 | 12.5 | 12.5 |
| 12 | 15 | 17.5 | 17.5 | 17.5 | 17.5 | 17.5 |
| 20 | 12.5 | 12.5 | 15 | 15 | 12.5 | 12.5 |
Optimal output power corresponding to
| Linewidth (nm) | Optimal Output Power (W) | |||||
|---|---|---|---|---|---|---|
| 10.8 km | 15.8 km | 25.2 km | 36 km | 41 km | 49.4 km | |
| 2 | 3.44 | 2.16 | 1.30 | 0.59 | 0.42 | 0.24 |
| 12 | 4.74 | 4.09 | 1.78 | 0.78 | 0.65 | 0.36 |
| 20 | 3.79 | 2.35 | 1.40 | 0.65 | 0.35 | 0.20 |
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