2.1.

Laser Diode

An LD is a semiconductor laser that is electrically pumped. A portion of the energy produced by the recombination of electrons and holes in the active region is released as photons. An LD behaves like mirrors with variable reflectivity when its end facets are coated or uncoated, producing an effective laser resonator. The feedback and ultimate gain of the radiation’s stimulated emission are advantageous to an LD. Besides that, LDs offer a wide range of wavelengths, ranging from ultraviolet (UV) to infrared (IR). LDs produce a stream of coherent and highly directional photons compared with another light source such as LED, which produces incoherent light that typically has a vast divergence.

In the PoF system, LDs act as an optical power transmitter, which is usually driven by a laser controller and coupled with an optical fiber. LD provides the most effective high-power supply of photons in converting electric power to optical power, generating optical conversion efficiency between 30% and 70%.²⁸ This was demonstrated by Peters et al. in 2007 where high power 940 nm LDs with up to 76% electrical-to-optical power conversion efficiency.²⁹ In addition, LDs provide more wavelength options in the PoF system spectrum, ranging from 780 to 1650 nm for connection with optical fiber compared with any other light sources such as LED. Additionally, fiber lasers are also used as a light source; however, degradation of the cladding caused by high /power or the power stability remains unsolved an issue.³⁰

Other than that, the power of LDs is dependent on the wavelength choices of the laser.³¹ The operating temperatures of the laser are preferably monitored during full power operation of the laser. A TEC is used to keep a constant LD temperature in operation at 25°C. Temperature control is important to maintain the operating lifetime of the LD by protecting it from thermal aging.³² On another note, passive cooling can be engineered into the PoF system. Since thermal energy is transported conductively from a copper heat sink to the system baseplate without the use of a cooling liquid, passive cooling is less expensive and difficult. It is better to use passive cooling to reduce system costs.³³

In terms of the wavelength, the semiconductor used as the active medium determines the operating wavelength of an LD. A suitable wavelength of the LD is required to improve the efficiency of the PoF systems.³⁴ The most used wavelength in past research was 808 to 840 nm. This is because 808 nm is a suitable wavelength for minimal optical fiber loss for a transmission distance that is $<1.3\mathrm{km}$.²⁴ While the optical fiber cables that have the distance of $<1.3\mathrm{km}$ should use 808 to 840 nm, it has been recorded that a wavelength of 1480 nm should be used for optical fiber cables with longer distance to provide better efficiency of the PoF system.²²

2.2.

Optical Fiber

The optical power produced by LDs is transmitted through an optical fiber. The light signals produced by LDs at one end of the cable are transmitted across the fiber optic backbone and reflected to the core as the light reaches the fiber optic cladding, thus preserving the light inside the core. SMF and MMF are two types of optical fiber cables that are commonly used. SMF has a small inner core ($9/10\mu \mathrm{m}$), which has only one direction of light and the MMF contains a larger inner core ($50/62.5\mu \mathrm{m}$).²⁶ MMF has a greater core diameter than SMF. Therefore, it has more than one transmission mode. In comparison, MMF is typically used for short-distance transmission, whereas SMF is used for longer distances. SMF has smaller core regions resulting in smaller optical power transfer, which is not ideal for high power distribution. Although MMF has a larger core area compared with SMF, the disadvantage is that the transmission speed of the MMF is being delayed by the large core area and eventually leading to modal dispersion.³⁵

In addition, the interference between optical power and transmission data arises in the same core.³⁶ Another type of fiber that has potential and is being used in recent research to achieve higher power is DCF.³⁷ To maximize the optical power of SMF, DCF has been made to transfer high power via the cladding of SMF fiber. DCF contains a small single-mode core and large inner cladding that wraps the single-mode core. Therefore the laser light will propagate in a single-mode core surrounded by an inner cladding with a large core effective area over 240 times larger than that of the SMF core.³⁸ Consequently, unlike SMF and MMF, DCF is chosen for high optical power and simultaneously transmits optical data.³⁹ There is also step-index plastic optical fiber (SI-POF) that has been recently used to integrate PoF in home networks with a data transmission speed of up to multiple Gbit/s.⁴⁰ The experiments showed that several mW of optical power could be delivered without significantly degrading the signal quality and only produced $1\times {10}^{-10}$ bit error ratio. While various types of optical fiber cables are available, different types are selected based on the required applications. Table 1 summarizes the specifications of SMF, MMF, DCF, and SI-POF cables. The highest transmission distance was reported by López-Cardona et al.⁵¹ the PoF system was transmitted through SMF optic cable over 14.43 km with an optical loss of 3.89 dB.

Table 1

Characteristics of MMF, SMF, DCF, and other types of fiber cables that are commonly used during the optical transmission.

2.3.

Photovoltaic Power Converter

The receiver unit hosts PV cells where the optical power is converted to electrical power to supply the load. The most common semiconductor materials used are Gallium Arsenide (GaAs), Indium Gallium Arsenide (InGaAs),⁵² Gallium Antimonide (GaSb), and Silicon (Si).⁵³ The conversion efficiency and the maximum electrical power output are the most important indicators of PV cell performance. There are also significant differences in energy gap and open-circuit voltage ($V_{oc}$) between these materials, which affect the minimum electrical energy required for the semiconductors to conduct electricity. The energy gap of GaAs material is 1.4 eV and produces 1.1 V of output voltage,⁸ and Si material has 1.12 eV with 0.7 V of output voltage level, which are lower than those of GaAs. InGaAs and GaSb have the same energy gap and the output voltage level of 0.7 eV and 0.5 V, respectively.⁷ The efficiency of these materials for PoF mechanism in different experiments were reported; Si has a maximum efficiency of 42.9% at 980 nm,⁵⁴ whereas InGaAs and GaSb have 45% and 49% peak efficiency at 1550 nm³⁰ and 1680 nm⁵⁵ respectively. GaAs is a good material for PV since it has a direct and large bandgap material and has high tolerance over a wide range of temperatures. GaAs PPC was reported to possess a maximum efficiency of more than 60% at 800-850 nm range⁵⁶ and another paper by Helmers et al.⁵⁷ reported a conversion efficiency of 68.9% for GaAs-based power converter under 858 nm monochromatic light. In another study by Fafard et al.,⁵⁸ it was demonstrated that GaAs-based laser power converters at 808 nm at 150 K were found to have a conversion efficiency of 74.7% and 68.9% at a laser illumination of 858 nm in another research.⁵⁹ The main limitation of GaAs is that it has a minimal spectral response. However, the transmission wavelength at 850 nm is still in the first minima attenuation window.⁵⁴ To increase the efficiency of a GaAs PPC, it was reported that a heterostructure architecture can increase conversion efficiency up to 65%⁶⁰ and allows for applications with larger output powers and new wavelengths.⁶¹ For InGaAs PPC, the highest performance is shown by heterojunction solar cell, which can be implemented in the application of PPC.⁶² The significance of high PPC efficiency was demonstrated in applications such as photonic boost converter and optically-isolated DC supply.⁶³

Table 2 shows the measured performance of common PPC such as Si, GaAs, and InGaAs for high-intensity laser application in terms of open-circuit voltage, energy gap, bandgap type, sensitivity, operating wavelength, quantum efficiency, and conversion efficiency. The conversion efficiency is defined as how much electrical power can be produced for every 1 W of optical power. Based on the table below, Si’s indirect band gap makes it a less than perfect optical material. However, Si can still be a promising material for PPC applications, with a conversion efficiency of up to 40% at 850 nm wavelength, and apparently has low manufacturing cost. Besides that, different efficiency levels for GaAs cells have been reported mostly due to GaAs-based power converters’ output voltage can be increased by interconnecting converter segments or multijunction PV cells.³⁰ While InGaAs and GaSb have output voltage levels lower than GaAs, 0.5 V, which is too low to supply power to any devices. These materials are not an option for the high performance of PoF systems. However, GaSb converters may benefit wireless applications since it has been used as a material to develop high-intensity laser power beaming⁵⁵ that can be used for wireless power transmission.⁷⁰ InGaAs is reported effective at 1550 nm while GaSb at 1315 nm.³⁸ Another study reported that PPCs made of InGaAs materials have the capacity to transmit electrical energy over several kilometers.⁷¹ Fafard and Mason also demonstrated in their research that InGaAs/InP PPCs for 1470 nm may generate electrical output voltages of more than 4 to 5 V indicating the capability of long-wavelength converter.⁷² At 1520 nm InGaAs PPC, it was found that the maximum room temperature conversion efficiency was 36.9%.⁷³ In most cases, PPC contributes most of the power loss in the PoF system since they do not generally have high conversion efficiency. A study on PPC material found that high bandgap material such as ZnS and 6H–SiC with bandgaps of 3.54 and 3 eV, respectively, contributed to a higher efficiency.⁷⁴ However, the doping limitation remains a major challenge that would have to be addressed. Hence, each PPC has disadvantages and limitations that rely more on wavelength and materials. For example, GaAs PPCs have high efficiency, voltage per cell ($>1\mathrm{V}$), whereas Si PPCs are advantageous in terms of having a mature planar technology. InGaAs and GaSb, on the other hand, have a very low fiber attenuation compared with GaAs.

Table 2

Measured PV cell performances for high-intensity laser application according to material type, open circuit voltage, energy gap, bandgap type, sensitive wavelength, and experimental results of conversion efficiency.

2.4.

Performance Improvements of PoF System

The efficiency of PoF systems can be improved by considering the specifications of the transmitter, optical fiber, and receiver in the PoF system. The main component of the transmitter unit is the LD, which is used to convert the electrical power to optical power and transmit to optical fiber. The limitation of optical fiber is a maximum power limit that can be used for a PoF system. This is due to the fiber-fuse effect, resulting in heat increases in the optical fiber.⁷⁵ It is stated that the limitation power at 1467 nm for SMF is 1.5 W, and SMF with type 28 F is 1.0 W. On the other hand, it is recorded that the maximum power of MMF at 1060 nm is 4.0 W.²²

The main component of the receiver unit is the PPC, which transforms the optical power to electrical power and delivers it to electronic devices. The maximum electrical power output and PV conversion efficiency are two of the most important limitations at the receiver unit. Among the PV cell listed in Table 2, the GaAs PV cell is reported to be the most commonly used PV cell with conversion efficiencies of 51% to 54%. Most experiments choose GaAs, due to their better responsivity to shorter wavelengths and high-temperature resistance.⁷⁶ It can be concluded that the overall efficiency of PoF systems can be optimized if the suitable LD, optical fiber, and receiver are selected in an experiment. However, the efficiency of the overall system may need to be compromised if there are limited options for materials for a specific application.

3.1.

Experimental Setup and Applications

Figure 2 shows the summary of several applications that implement the usage of PoF technology. The PoF system in the laboratory and commercial applications are further discussed in terms of system efficiencies, types of LDs, optic fibers and PV converters, power loss, and limitations as well as improvements of the applications.

Fig. 2

The various applications of the PoF system in different industries and the power output range for each of the applications.

3.1.7.

Others

PoF technology has also been implemented in various other applications. A combination of radio over fiber and remote optical powering over optical fiber was investigated by Wake et al.³⁵ The system used a high-power LD with a maximum output of 2 W into $62.5\mu \mathrm{m}$ core diameter of MMF and 830 nm of wavelength. The optical power obtained from the laser is distributed over 300 m MMF, and the output power from PVC is regulated at 3.3 V. The system is designed for distributed antenna system application.⁴² According to Perhirin et al.,⁶ an optical prototype dedicated to low-power submarine sensors that use PoF technology was demonstrated. The LD used has a central wavelength 1500 to 1600 nm and the power is transmitted along 10 km of optical fiber. The optoelectric conversion that occurs at the PPC contributes to the overall system efficiency of 22.5%. In 2014, PoF using 100 m of DCF for radio over fiber systems was introduced by Matsuura and Sato.³⁸ The optical power obtained is up to 4.0 W, and the electrical power output to powering the base stations for wireless local area network (WLAN) is more than 400 mW.³⁸ In 2015, Sato et al.³⁹ demonstrated PoF using DCF optically powered ROF systems with 40 W of optical feeding obtained. In the same year, Matsuura et al.⁸⁹ presented 60 W optical power obtained over a 300 m DCF with improved system efficiency. In 2017, PoF was used for a fishing camera over MMF as the fishing line. The electrical power output obtained is 413 mW with 1.27 W of optical power required from high-power LDs.⁸ In 2018, Kamiyama et al.³⁷ used PoF for multichannel data signals and power distribution using a DCF. The maximum optical power feed is 60 W, and the optical power delivered to a remote unit is then converted into electrical power by several PPC.

Table 3 shows a summary of the different PoF experiments conducted and their respective applications from 2008 to 2022. Most of the experiments that are listed in the table involved sending power and data simultaneously to low-power sensors and communication devices such as remote antenna units (RAU) and remote radio heads (RRH). The overall system efficiency stated in the experiments listed ranges from 9% to as high as 28.6%, which was achieved by Matsuura et al.³ The experiment used an erbium-doped fiber amplifier (EDFA) and bandpass filters to increase the output power and reduce noise, respectively. The tapered fiber bundle combiner (TFBC) used in the research was also fabricated by tapering and fusing the bundles of fiber with the DCF they used to reduce connection loss. Apart from the overall system efficiency, it is also observed that the most frequently used wavelength operation is 808 nm due to its compatibility with most PV cells and power converter also its availability in the commercial market. Moreover, that 808 nm frequency can improve system energy efficiency for silica fibers and applications with a link length of $<1.3\mathrm{km}$.¹³

Table 3

A summary of the different PoF applications available and its values of power, types of fiber, efficiency, PV material, and loss.

3.2.

Commercialized PoF Systems

There are a range of commercial systems that can be observed at in-depth. There are several available commercialized PoF systems from companies that are able to deliver power to remote and inaccessible areas. One of the PoF systems that have been commercialized operates at 830 nm wavelength, which provides electrical distribution to remote locations.⁹⁶ One of their novelties is that they have supercapacitors to ensure a smooth and constant voltage is supplied to the remotely powered devices. Since the PoF system uses MMF with a large core size, the electrical output power declines over a certain distance because of the modal dispersion inside the cable.⁴⁹ Therefore, the PoF system is more suitable for short-distance applications. Another commercialized PoF system used a wavelength of 976 nm for smart monitoring system for electrical assets such as switchgears and busbars. The optical power of the transmitted light is converted into electrical power using the sensor modules.⁹⁷ It provides power for sensors that continuously monitor temperature, humidity, and partial discharge. Other than that, at the wavelength of 750 to 850 nm and 900 to 1000 nm, a different PoF system allows different power inputs with a maximum power output of 1 W to be compatible with various laser sources. Fiber optic cable with core sizes of $62.5/100\mu \mathrm{m}$ is used to transmit power.⁹⁸ The designed system provides electrical sources in remote, inaccessible, hazardous, electrically noisy, or exposed to extreme weather locations. The typical application of the system is to power up low wattage electronic devices such as sensors, actuators, data communication transceivers, GPS receivers, and gauges, with the limitation of 1 W output power. Overall, each commercialized PoF system has been designed to produce the necessary output power and voltage for their respective applications. However, all of them have their own set of limitations. The optimization of these systems can be explored more thoroughly by using numerical methods or techniques involving artificial intelligence, such as a genetic algorithm.

3.3.

Patented PoF Systems

Table 4 lists the commercial patents of PoF that focuses on the various methods of transmitting power and data simultaneously to sensors and communication devices. These patents initially started from designing the optical network that can transmit power and data. Since then, more patents have been created that contain methods for providing digital data services and power in an optical fiber-based distributed communication system. The keywords, power over fiber, was used for patent search on Espacenet¹¹⁸ and Patentscope.¹¹⁹ The keywords were entered into all fields/any field section of the databases in September 2022. The patents in Table 4 discuss industrial applications of PoF systems, such as controlling remote antenna units and acoustic sensing for a barrier.

Table 4

The summary of PoF systems that have been patented from the year 2008 to 2022.

There are two patents in 2010 that highlighted the methods of transmitting power through optical fiber. The first patent focused more on the method of transmitting power and data simultaneously to network equipment such as wireless access points using an optical network. The method uses the electrical signal that includes low- and high-frequency data signal to transmit power and data. A filter is used to separate the data and power signal before the data signal is processed. The inventor of the patent highlighted that the conversion efficiency and the amount of current produced depend on the equipment’s power rating that needs to be powered so that the method can have different performance based on the applications. The second patent also proposed a method of distributing power through an optical fiber that includes a receiver unit sending a feedback signal to the transmit unit to control and regulate laser sources. The patent improved the safety of the PoF system by first operating the transmit unit at low power levels until a feedback signal indicates that all safety and operational checks have been performed.

As the use of the PoF system increases in industrial applications, two patents were published later in 2012 that proposed providing power in radio frequency applications using converter pairs connected using optical fibers. The converter pair comprises of electrical-to-optical converters that include lasers and optical-to-electrical converters that include photodiodes. The same inventor published both patents, and the inventions helped provide alternatives to creating more picocells that can increase radio frequency network coverage. The widespread use of PoF systems became apparent in 2017 when two patents about providing digital data services in optical-fiber-based communication systems were published. The two patents proposed a more efficient method of supplying power to data service clients instead of providing a standalone power source for each data client.

There are also three patents in 2017 and 2020 that have made contributions to PoF system’s innovation. The patent in 2017 is about designing a perimeter monitoring system using fiber-optic distributed acoustic sensing. The system used PoF because any acoustic disturbance will trigger an interrogator module consisting of a laser and photodetector to produce a series of optical pulses. One of the design advantages is that the burying of sensing fiber isolates them from environmental effects and produce better signal-to-noise ratio. The first patent in 2020 is an invention of a sensor system that can be enabled by PoF. One of the claims of the invention is that it can power up multiple sensors using one laser source. However, the patent inventor stated that the PPC within the sensor module might not provide enough continuous power to operate all sensors continuously. The patent of the invention that can solve the problem was published in the same year that proposed a power management system of remote units. Inventions that were patented in 2021 were found to revolve around communication and measurement devices while patents reported in 2022 were on the inventions of temperature sensors and an optical connector of power over fiber system. A patent published on the 28^th of July 2022 claims that their overall power over fiber system capable of efficiently transporting ultrahigh power across great distances with significantly low loss was due to the unreliability of conventional power over fiber systems using silica-based fiber, which constrained transmission and distribution losses, conversion efficiencies at both the transmit and receive ends, and significant attenuation in the transport medium.¹¹⁶ The PoF patents published over the decade have shown how far PoF has progressed in research and the potential of PoF systems in the future.

4.1.

System Efficiency

Several potential ways to increase the efficiency of the overall PoF system involve increasing the efficiency of the transmitter, optical fiber, and receiver. It has been discussed in previous work¹²⁰ that variables such as absorption efficiency, quantum efficiency, storage efficiency, and extraction efficiency depend directly on the choice of laser material. The work concluded that Neodymium (Nd) type lasers were widely used for most applications such as laser delivery and high energy storage. One of the challenges that must be tackled by researchers is to choose what type of laser materials can give the best results for their projects where the performance requirements are different. It is recommended for researchers to consider the four variables mentioned when selecting a laser material for their transmitter.

For the optical fiber, one of the main aspects of selecting the suitable optical fiber cable is the amount of attenuation or loss of the transmitted power. There are low-cost options available such as plastic optical fiber, but one must consider their higher attenuation than conventional glass optical fiber cables.¹²¹ The attenuation loss can happen due to Rayleigh scattering, absorption of light by the optical fiber cable, geometrical interferences at cladding core interface, and attenuation within the optical cladding.⁷⁶ In a study by Matsui et al.,¹²² it has been mentioned that SMF and MMF optic fiber cables have a fixed value of attenuation in dB/km for a given wavelength, which makes them most frequently used for PoF applications. Another important aspect in selecting the suitable optical fiber cable is their capacity on how much power they can withstand. For example, SMF might be unreliable in projects that require high power transmission due to limited power density of the small core effective area.³ Therefore, fiber cables that can withstand higher power, such as DCF and MMF, might be more compatible. After selecting an optical fiber cable based on these two important aspects, there are other challenges in working with optical fiber cables for PoF system, such as splicing losses and mismatching effects.⁵ Using the same type of cable with the same core size throughout the PoF system can increase the efficiency of the transmission medium.

The receiver in the PoF system is a power conversion module that converts optical power from optic fiber cable back to electrical power that can be transmitted to the load. The module consists of PPC devices that are made up of semiconductor materials. The most common semiconductor materials used are Si, GaA, InGaAs, and GaSb. The main differences between these materials can be found in an open-circuit voltage for a single-cell converter and their energy gap.⁷⁶ The type of semiconductor materials needs to be selected by considering five aspects, which are the amount of dark current, sensitivity, wavelength, quantum efficiency, and conversion efficiency.²⁶ The selection of the type of materials that needs to be used can also be different depending on the cost and performance requirement of the projects.

4.2.

Others

PoF systems have also shown some limitations in terms of only being able to supply continuous power to multiple devices from a single laser source.¹⁰⁸ However, there have been inventions from the published patents that can solve the problem by using power management systems.¹²³ In high-power laser application, there may also be a laser safety issue associated with several watts of optical power, leaving the fiber when it is broken. However, systems are already available that include engineering controls as a control measure to avoid the potential hazards.¹²⁴ With more developing research and technology toward the PoF system, the cost of optical components can be reduced. There will be higher potential in terms of available power and conversion efficiency. Eventually, PoF technology can be implemented as a backup supply as an alternative to electrical conductors.