**2. Power over fiber**

This section presents the technical principles of PoF including a short history, the main elements (high‐power lasers, fibers, and photovoltaic converters (PVs)), and their main limitations (laser power, converter efficiency, and fiber‐fuse effect), circuits, topologies and networks, application on multimode (MM), single mode (SMF) and special fibers, and finally the future perspectives.

#### **2.1. Historical overview**

#### *2.1.1. Telecom*

The concept of PoF was born together with the first optical fiber developments. Optical fiber with low attenuation (less than 20 dB/km) was developed in 1970, and the first fiber optic link was installed in Chicago in 1976, USA. To the best of author's knowledge, DeLoach et al. [1] published the first work about PoF in 1978. They proposed to use PoF to operate a sound alert of a telephone remotely. Continuing the previous experiment, in 1979, Miller and Lawry [2] implemented a two‐way speech communication between an electrically powered station and an optically powered station, and in 1982, Miller et al. [3] demonstrated a bidirectional speech‐television communication over a single optical fiber, with emergency optical powering of the remote station telephone. A great result of those researches was the development of a highly efficient photovoltaic converter based on GaAlAs.

other words, beyond their classical applications, optical fibers can also be used to transport optical energy to powering electric or electronic devices remotely. This is the concept of PoF. Since that time in the 1970s, many applications have emerged for PoF mainly in two different

Why PoF is interesting? The reasons depend on the applications but they are associated to the

For telecom applications, the key factors are (i) PoF eliminates the necessity of batteries, solar panels, and long cupper feeder wires in the remote sites, improving the reliability and the security of the system; (ii) PoF permits the reduction of the space and the installation cost in

For electrical utilities, there are at least four key factors: (i) PoF uses optical fiber, which is made of nonconductive material. This characteristic is important because in most applications in electrical world, the sensors are placed in high voltage. Any conductance in high‐voltage elements can create current leaks; (ii) optical fiber is immune to electromagnetic interferences. The electrical world environment is polluted of electromagnetic interferences; therefore, optical fiber can transmit signals without quality degradation; (iii) the optical fiber eliminates the need to run conductive copper wire into a high‐ground potential rise (GPR) zone. GPR arises when lightning strikes occur in substations and can cause severe interference problems in electronic equipment and systems. Considering that the sensor using PoF has a complete galvanic isolation to the ground potential, it is practically immune to the GPR effects; and (iv) there are many low‐cost/low‐power/high‐efficiency electronic sensors available for transmission lines and substations monitoring that can be supplied by PoF without incurring in the

This chapter describes a revision of PoF, its technical principles, main elements, technologies,

This section presents the technical principles of PoF including a short history, the main elements (high‐power lasers, fibers, and photovoltaic converters (PVs)), and their main limitations (laser power, converter efficiency, and fiber‐fuse effect), circuits, topologies and networks, application on multimode (MM), single mode (SMF) and special fibers, and finally

The concept of PoF was born together with the first optical fiber developments. Optical fiber with low attenuation (less than 20 dB/km) was developed in 1970, and the first fiber optic

and the applications in telecom and in utilities, developed by the author's group.

areas: telecommunications and utilities.

remote sites, which is very important to telecom companies.

optical fiber characteristics.

256 Optical Fiber and Wireless Communications

described problems.

**2. Power over fiber**

the future perspectives.

**2.1. Historical overview**

*2.1.1. Telecom*

In 1993, Banwell et al. investigated the issues of PoF application in fiber‐in‐the‐loop (FITL) [4]. In this work, many technical and cost aspects have been considered in the analysis. Almost a decade after that, Miyakawa et al. [5–7] presented fiber optic power and signal transmission systems considering the application of DC (direct current) powering to information equipment such as personal computers.

Werthen and Cohen [8] in 2006 and 2008 [9] proposed the use of PoF for driving low‐power switches or actuators to assuring path diversity in upstream data delivery in passive optical network (PON) architectures and for providing battery backup power in case of power outages.

In 2007, Wake et al. [10] studied the combination of radio over fiber and PoF, and in [11], they described optically powered radio‐over‐fiber remote units designed and constructed for distributed antenna system applications.

Sato and Matsuura [12] and Matsuura and Sato [13] demonstrated in 2013 the transmission of RF signal and high‐power light using a double‐clad fiber (DCF). A DCF has a single‐mode core and a multimode inner clad. The RF signal and the optical powering signal travel, respectively, through core and in the inner clad of DCF.

In 2014, Penze et al. [14] proposed powering PON extenders using PoF. In this research, SOAs (semiconductor optical amplifier) were powered using PoF. The SOAs were used in a bidirectional amplification of signals of Gigabit PON (GPON) and 10 Gigabit PON (XGPON). In the same year, Ikeda [15] proposed a PoF instead of metal waveguides in order to protect microwave radio equipment from lightning.

Yan et al. [16], in 2015, developed a wireless sensor system based on PoF to realize a flexible distributed sensing over a middle distance, in environments of high voltage, strong magnetic field, flammable, and explosives. In the same year, Matsuura and Sato [17] demonstrated experimentally a bidirectional radio over fiber using a DCF for optically powered remote antenna units. The feasibility of the technique was demonstrated by bidirectional RoF transmission over a 100‐m DCF optically feeding with 4.0 W, and in [18], Sato et al. demonstrated a bidirectional radio‐over‐fiber transmission using a double‐clad fiber with 40‐W optical power feeding. In the same year, Lee et al. [19] described the concept of cloud radio access network (CRAN) based on PON exploiting PoF, andSuto et al. [20] detailed the challenges of QoE‐ guaranteed and power‐efficient network operation for CRAN based on PON exploiting PoF.

In 2016, Minamoto and Matsuura [21] and Matsuura and Minamoto [22] presented an optically controlled beam steering system using 60‐W PoF in the fiber to feed remote antenna units, and in [23], Yoneyama et al. demonstrated a 1.3‐μm dual‐channel radio‐over‐fiber system using a 300‐m double‐clad fiber feed with 30‐W optical power of a PoF system. In the same year, Umezawa et al. [24] reported a high‐conversion gain in a high‐speed optical receiver based on PoF supply and designed to work with radio over fiber at 100‐GHz region. In [25, 26], the same authors proposed the use of a multicore‐based radio and PoF in a transmission using a 100‐GHz photo receiver.

#### *2.1.2. Utilities and other industries*

In 1980, Caspers and Neumann [27] first described a PoF method used to supply active electronic circuits at high potential. In the experiment, they used four infrared light‐emitting diodes (LEDs) emitting at a wavelength of 940 nm which were connected in parallel and coupled to an uncladded glass rod of 40‐mm diameter and a length of 1 m with an attenuation smaller than 200 dB/km at a wavelength of 850 nm. Four square silica solar cells of 20 mm length were connected in series in this experiment.

Ohte et al., in 1984 [28], demonstrated a transducer with optical‐fiber data link to provide electrical isolation. A pulse‐position–modulated optical signal was used in transmitted signal by optical fiber from a remote converter to mainframe. Wavelength division multiplexing (WDM) was used to transmit the modulated signal and the optical feed power in the same optical fiber.

In 1988, Trisno and Wobschall [29] described a method for improving the efficiency of the conversion of pulsed optical power to electrical power by photovoltaic. The system required a pulsed power source with a storage capacitor to hold energy for a time after the optical power is turned off. In the same year, Bjork et al. [30] demonstrated the ability to provide data communication a variety of off‐the‐shelf electronic transducers in a single fiber with proper attention to the synchronization issues and in [31] Lenz and Bjork presented a comparative study of four concepts for standardizing and multiplexing of fiber optic sensors for aircraft applications and compared them with PoF.

Kirkham and Johnston [32] presented in 1989 an optical frequency–modulated (FM) data link whose remote electronic sensors were optically powered. An application to current measurement is described in a high‐voltage line by means of a linear coupler.

In 1991, Yamagata et al. [33] described a PoF system to measuring gas density of extra high‐ voltage substation gas‐insulated switchgear. This system employed a method of matching the impedances of load and photodiode by optical electrical conversion of pulsed light with a photodiode and boosting the voltage with a transformer. In [34], Sai presented an optimization of this method. In the same year, Nieuwkoop et al. [35] described an alternative system to increase the voltage to the remote sensor using a low‐cost coil.

Spillman and Crowne [36] presented in 1995 a throttle level angle (TLA)‐sensing system applied to aircrafts utilizing a capacitance‐based rotary position transducer which was powered and interrogated via light from a single MM optical fiber. The system used a unique GaAs device that served as both a power converter and optical data transmitter. In the same year, Tardy et al. [37] described a PoF system designed to measure current in high voltage; Pember et al. [38] demonstrated a PoF network concept; and Dubaniewicz and Chilton [39] described a PoF system to gas monitoring in mines.

In 1996, Werthen et al. [40] presented an optically powered current transducer based on shunt technology using PoF and detailed a practical use of this technique. Wang et al. [41] and Zhijing [42] detailed the use of PoF to measure the pressure of liquids in 1998. In 2005, Turán et al. [43] review the basic properties of PoF, their key elements, and its industrial applications.

Bottger et al. [44], in 2008, presented an optically powered video camera link that allows acquiring and communicating a 100‐Mb/s video stream over a distance of hundreds of meters. In [45], M. Roger demonstrated an optically powered sensor network with subscribers consuming less than 1‐μW average power, and an optically powered high‐speed video link transmitting data at a bit rate of 100 Mbit/s.

In 2010, Rosolem et al. [46] described the results of a PoF sensing system for monitoring partial discharges on hydro generators and de Nazaré and Werneck [47] proposed a monitoring system based on PoF to measure the temperature and current of transmission lines.

Audo et al. [48], in 2011, proposed PoF‐based architecture to extend multidisciplinary‐cabled networks or to create a dedicated submarine hydrophone or seismometer network, and in 2012, Lau et al. [49] reported a prototype optically remote‐powered subsea video monitoring system that provides an alternative approach to powering subsea video cameras. In the same year, Tanaka and Kurokawa [50] presented a review of fiber optic sensor network that employs PoF. A hybrid sensor network with wireless sensors is also achievable by introducing wireless/optical interface nodes.

In 2014, Rosolem et al. [51] presented a PoF system to long‐distance applications based on charge/discharge of super‐capacitors. The system was installed in a transmission line tower, and it was connected to the substation using an optical ground wire cable (OPWG), and in [52], Silva et al. proposed a new MM fiber optic cable to transmit optical energy for long reach in PoF systems.

Rosolem et al. [53], in 2015, presented the results of a PoF system to monitor high‐voltage switchgear using video cameras and free space optics (FSO), and in [54], Rosolem et al. proposed the use of PoF and FSO to measure current in high‐voltage transmission lines.

In 2016, Zhang et al. [55] proposed a 15‐kV silicon carbide (SiC) MOSFET gate drive using PoF and replaced the traditional design based on isolation transformer.

## **2.2. Technical principles of PoF**

units, and in [23], Yoneyama et al. demonstrated a 1.3‐μm dual‐channel radio‐over‐fiber system using a 300‐m double‐clad fiber feed with 30‐W optical power of a PoF system. In the same year, Umezawa et al. [24] reported a high‐conversion gain in a high‐speed optical receiver based on PoF supply and designed to work with radio over fiber at 100‐GHz region. In [25, 26], the same authors proposed the use of a multicore‐based radio and PoF in a trans-

In 1980, Caspers and Neumann [27] first described a PoF method used to supply active electronic circuits at high potential. In the experiment, they used four infrared light‐emitting diodes (LEDs) emitting at a wavelength of 940 nm which were connected in parallel and coupled to an uncladded glass rod of 40‐mm diameter and a length of 1 m with an attenuation smaller than 200 dB/km at a wavelength of 850 nm. Four square silica solar cells of 20 mm

Ohte et al., in 1984 [28], demonstrated a transducer with optical‐fiber data link to provide electrical isolation. A pulse‐position–modulated optical signal was used in transmitted signal by optical fiber from a remote converter to mainframe. Wavelength division multiplexing (WDM) was used to transmit the modulated signal and the optical feed power in the same optical fiber.

In 1988, Trisno and Wobschall [29] described a method for improving the efficiency of the conversion of pulsed optical power to electrical power by photovoltaic. The system required a pulsed power source with a storage capacitor to hold energy for a time after the optical power is turned off. In the same year, Bjork et al. [30] demonstrated the ability to provide data communication a variety of off‐the‐shelf electronic transducers in a single fiber with proper attention to the synchronization issues and in [31] Lenz and Bjork presented a comparative study of four concepts for standardizing and multiplexing of fiber optic sensors for aircraft

Kirkham and Johnston [32] presented in 1989 an optical frequency–modulated (FM) data link whose remote electronic sensors were optically powered. An application to current measure-

In 1991, Yamagata et al. [33] described a PoF system to measuring gas density of extra high‐ voltage substation gas‐insulated switchgear. This system employed a method of matching the impedances of load and photodiode by optical electrical conversion of pulsed light with a photodiode and boosting the voltage with a transformer. In [34], Sai presented an optimization of this method. In the same year, Nieuwkoop et al. [35] described an alternative system

Spillman and Crowne [36] presented in 1995 a throttle level angle (TLA)‐sensing system applied to aircrafts utilizing a capacitance‐based rotary position transducer which was powered and interrogated via light from a single MM optical fiber. The system used a unique GaAs device that served as both a power converter and optical data transmitter. In the same year, Tardy et al. [37] described a PoF system designed to measure current in high voltage;

ment is described in a high‐voltage line by means of a linear coupler.

to increase the voltage to the remote sensor using a low‐cost coil.

mission using a 100‐GHz photo receiver.

length were connected in series in this experiment.

applications and compared them with PoF.

*2.1.2. Utilities and other industries*

258 Optical Fiber and Wireless Communications

A generic PoF system is shown in **Figure 1**. In the left side, the control unit composed by the high‐power optical source (HPOS) unit together with the optical reception unit (ORU) is shown, which receives signals from the remote sensor unit, shown on the right side of **Figure 1**. Two optical fibers connect the local control unit to the remote unit. The optical fibers may be standard SMF optical fibers or MM fibers. In the remote unit, a photovoltaic converter

**Figure 1.** Generic PoF system diagram showing its main elements HPOS, OF, PV, LD, electronic circuit, and sensors.

detects the power transmitted by the HPOS. The electrical energy produced by the photovoltaic converter is used to power up a low‐threshold laser (LD), electronic circuits, and sensors of the remote unit. The optical fibers and the remote unit can be installed in a hazardous environment, such as high‐voltage substations, oil refineries, mines, oil tanks, water reservoirs, and sea depth.

The biggest challenge for a generic PoF system is to provide to a load the highest possible power, at the greatest possible distance, with the highest reliability. There are limits today to PoF applications and the limits are attributed to technological, physical, and cost aspects of PoF elements.

The amount of power that the PoF system can deliver is determined to a great extent by its components: laser, fiber, and photovoltaic cell. The following parameters can be used to evaluate the delivered electric power (*P*Load) to an electronic load: the maximum transmitted optical power without causing damages in the optical fiber (*P*MaxFiber); the optical power of the HPOS (*P*HPOS); the optical power in the PV input (*P*In); the total loss on the fiber (*α*Fiber); the total loss of the optical connectors (*α*Conn); the link distance (*L*); and the efficiency of the PV (*η*PV). The power delivered to the extender can be expressed by

$$P\_{\rm load} = P\_{\rm ln} \cdot \eta\_{\rm PV} \tag{1}$$

$$P\_{\rm ln} = P\_{\rm H^{\rm COS}} \dots \,\, a\_{\rm Hbar} \dots \,\, a\_{\rm Conn} \tag{2}$$

$$
\alpha\_{\text{Fhor}} = 10^{(-\ell.a\_{\text{two}}/10)} \tag{3}
$$

$$
\alpha\_{\text{Com}} = 10^{\left( -\alpha\_{\text{c.s.o.}}/10 \right)} \tag{4}
$$

where *α*FiberdB is the fiber attenuation in dB/km and *α*ConndB is the total connector loss in dB. Notice that

$$P\_{\text{HTCS}} \le P\_{\text{MaxFlow}} \tag{5}$$

In the following subsections, the main elements of PoF will be described, and **Figure 2** will be used as a reference for further discussion. **Figure 2** shows a qualitative graph of some parameters of the PoF elements, such as the loss of the optical fibers and the responsivity of PV cells. The main spectral bands for HPOS are also shown in **Figure 2**.

**Figure 2.** Qualitative graph of some PoF parameters.

#### **2.3. Main elements of PoF**

detects the power transmitted by the HPOS. The electrical energy produced by the photovoltaic converter is used to power up a low‐threshold laser (LD), electronic circuits, and sensors of the remote unit. The optical fibers and the remote unit can be installed in a hazardous environment, such as high‐voltage substations, oil refineries, mines, oil tanks, water reservoirs,

**Figure 1.** Generic PoF system diagram showing its main elements HPOS, OF, PV, LD, electronic circuit, and sensors.

The biggest challenge for a generic PoF system is to provide to a load the highest possible power, at the greatest possible distance, with the highest reliability. There are limits today to PoF applications and the limits are attributed to technological, physical, and cost aspects of

The amount of power that the PoF system can deliver is determined to a great extent by its components: laser, fiber, and photovoltaic cell. The following parameters can be used to evaluate the delivered electric power (*P*Load) to an electronic load: the maximum transmitted optical power without causing damages in the optical fiber (*P*MaxFiber); the optical power of the HPOS (*P*HPOS); the optical power in the PV input (*P*In); the total loss on the fiber (*α*Fiber); the total loss of the optical connectors (*α*Conn); the link distance (*L*); and the efficiency of the PV (*η*PV).

*P*Load = *P*In . *η*PV (1)

*P*In = *P*HPOS . *α*Fiber . *α*Conn (2)

*α*Fiber = 10(−*L*.*α*FiberdB/10) (3)

*α*Conn = 10(−*α*ConndB/10) (4)

where *α*FiberdB is the fiber attenuation in dB/km and *α*ConndB is the total connector loss in dB.

*P*HPOS ≤ *P*MaxFiber (5)

In the following subsections, the main elements of PoF will be described, and **Figure 2** will be used as a reference for further discussion. **Figure 2** shows a qualitative graph of some parameters of the PoF elements, such as the loss of the optical fibers and the responsivity of PV cells.

The power delivered to the extender can be expressed by

The main spectral bands for HPOS are also shown in **Figure 2**.

and sea depth.

260 Optical Fiber and Wireless Communications

PoF elements.

Notice that

#### *2.3.1. High‐power optical source*

**Figure 2** shows the main spectral bands of HPOS. There are four main bands: 800, 950, 1050, and 1480 nm. The HPOSs for these bands have applications in medicine, pumping, thermal printing, and industrial applications. The power of these HPOSs can reach from 2 to 10 W for TO220 packaging depending on the wavelength band and from 6 to 650 W for laser modules. The output fiber core diameter for these HPOSs ranges from 50 to 600 μm depending on the HPOS output power. These fibers are MM types. For SMF fibers, the output power changes from 0.2 to 1.0 W.

Normally, an SMA (Sub‐Miniature A) high‐power type with metal ferrule is used in the HPOS output optical fiber. This metal ferrule is used for thermal dissipation in the HPOS connection to the fiber link. The high‐power SMA connector utilizes air‐gap‐ferrule technology that eliminates the materials near the fiber end face that absorbs energy (e.g., epoxy). This absorption can damage the connector end face [56].

HPOS can also transmit information to the remote unit. The information signal is modulated in the continuous optical power generated by the HPOS. In general, this signal transmits commands to be executed in the remote unit, such as requesting the sensor parameters transmission, switching the sensor, and so on.

The most common modulation formats used to modulate the HPOS are on‐off keying (OOK), pulse‐width modulation (PWM), or frequency modulation. The extinction ratio of this signal can be high (100%) for few milliseconds like burst signals or low (less than 5%) during all the time of optical power sending. The extinction ratio is the ratio of the power used to transmit a logic level "1," to the power used to transmit a logic level "0." **Figure 3** shows an oscilloscope trace (voltage vs. time) of a telecommand signal transmitted over the optical supply power.

The main HPOS parameters required for a PoF project are maximum output power (*P*HPOS), the operation wavelength (*λ*op), the output fiber core diameter (*D*core), the numerical aperture of the fiber (NA), and connector loss *α*ConndB.

#### *2.3.2. Optical fiber*

The next element in the PoF system after the HPOS is the optical fiber, which is responsible to send the optical power from the HPOS to the PV. The fiber has two main parameters in the PoF system design: its attenuation and its limit of transmitted optical power.

The attenuation in modern optical fiber is basically caused by the intrinsic loss due to Rayleigh backscattering. Silica fibers are glasses that have materials with microscopic variations of density and refractive index. This effect gives rise to energy losses due to the scattered light. The loss due to the Rayleigh backscattering has high dependence with the operation wavelength (*1/λop 4* ). The typical loss in the HPOS bands is 2.8 (800 nm), 1.8 (950 nm), 1.4 (1050 nm), and 0.25 dB (1480 nm).

The loss of optical fiber is not a big problem for the major PoF applications that has dozens of meters of fibers. It can be a problem for distances longer than 1 km. The major problem is regarding the maximum transmitted optical power. Next, the physical mechanism that limits the power in optical fiber will be detailed. After this limit, a degradation of the fiber core and the fiber coating occurs.

**Figure 3.** Oscilloscope trace showing a telecommand signal transmitted over the optical supply power.

Optical fibers when subjected to reduced diameter curvatures exhibit further attenuation of the optical signal. This attenuation may be significant when the bending diameter reaches a critical value. The attenuation of the optical signal in the bending zone of the fiber is related to the leakage of the fiber core region signal to the cladding region. The material of the outer‐ coating layer of the optical fiber absorbs the signal lost to the cladding region. When a high‐ power optical signal is propagated in the fiber, the energy absorbed by the coating in the zone of curvature is high, generating a local increase in temperature in the coating [56].

logic level "1," to the power used to transmit a logic level "0." **Figure 3** shows an oscilloscope trace (voltage vs. time) of a telecommand signal transmitted over the optical supply power.

The main HPOS parameters required for a PoF project are maximum output power (*P*HPOS), the operation wavelength (*λ*op), the output fiber core diameter (*D*core), the numerical aperture

The next element in the PoF system after the HPOS is the optical fiber, which is responsible to send the optical power from the HPOS to the PV. The fiber has two main parameters in the

The attenuation in modern optical fiber is basically caused by the intrinsic loss due to Rayleigh backscattering. Silica fibers are glasses that have materials with microscopic variations of density and refractive index. This effect gives rise to energy losses due to the scattered light. The loss due to the Rayleigh backscattering has high dependence with the

The loss of optical fiber is not a big problem for the major PoF applications that has dozens of meters of fibers. It can be a problem for distances longer than 1 km. The major problem is regarding the maximum transmitted optical power. Next, the physical mechanism that limits the power in optical fiber will be detailed. After this limit, a degradation of the fiber core and

). The typical loss in the HPOS bands is 2.8 (800 nm), 1.8 (950

PoF system design: its attenuation and its limit of transmitted optical power.

**Figure 3.** Oscilloscope trace showing a telecommand signal transmitted over the optical supply power.

of the fiber (NA), and connector loss *α*ConndB.

*4*

nm), 1.4 (1050 nm), and 0.25 dB (1480 nm).

*2.3.2. Optical fiber*

262 Optical Fiber and Wireless Communications

operation wavelength (*1/λop*

the fiber coating occurs.

The heating of the coating of the optical fiber leads to its degradation, implying a consequent reduction of the useful life of the fiber. This degradation of the optical fiber is related to the reduction of the volume of the coating in the zone of curvature.

The second problem resulting from the propagation of high‐power signals is the occurrence of the "fiber‐fuse" effect on the optical fibers. This phenomenon was first observed in 1987 in a standard SMF fiber with optical intensity exceeding 5 MW/cm2 at 1064 nm [57]. The fiber‐fuse effect was initiated at a point of the fiber with a high‐temperature value, then propagating toward the optical source with a velocity of about 1 m/s and emitting an optical signal in the spectral region of the visible. The propagation of this high‐temperature zone is similar to the burning of a wick, which gave rise to the name of the "fiber‐fuse" effect. After propagation of the fiber‐fuse effect, the optical‐fiber core presents a periodic bubble chain (see **Figure 4**) and is permanently destroyed, being unable to guide an optical signal.

Currently, the most accepted general explanation for the fiber‐fuse effect relates to the ignition of this effect with the increase of fiber optic absorption at a point with a high‐temperature value [56]. In turn, the increase in optical signal absorption is responsible for the catastrophic temperature increase in the fiber core, reaching values higher than the vaporization temperature of the silica. Through the thermal diffusion mechanism, this high‐temperature zone is transmitted to neighboring regions and the process evolves toward the optical source. Thus, the "fiber‐fuse" effect is related to the increase in fiber optic absorption and the thermal diffusion process.

**Figure 4.** Schematic of a fiber‐fuse effect and a photo of an optical fiber core presenting a periodic bubble chain due to fiber‐fuse effect.

It should be noted that ignition and propagation of the fiber‐fuse effect occurs only if the power of the optical signal in the fiber is maintained above a threshold value. According to [56], this threshold is proportional to the diameter of the fiber modal field (MDF) and dependent on the wavelength of the optical signal. A typical SMF fiber (MDF = 7.8 μm) presents a threshold of 1.5 W at 1467 nm. In other study [58], it was observed that an SMF fiber type 28F (MDF = 7.8 μm) presents a threshold of 1.0 W, and an MM fiber of 62.5‐μm core (MDF = 12.0 μm) presents a threshold of 4.0 W at 1060 nm.

In order to increase the optical power in SMF fibers, many efforts have been made to transmit the high power by the cladding of the SMF fiber. In [21, 22], an optically controlled beam steering system using 60 W transmitted in the cladding of the fiber to feed remote antenna units is presented.

The propagation of signals with optical power exceeding the threshold required for ignition and propagation of the fiber‐fuse effect is not sufficient to trigger it, and an ignition point characterized by having a high‐temperature value is also required. This point of ignition occurs in contaminated and/or degraded connectors or in optical fibers subject to tight bending. As described previously, in optical fibers subjected to curvatures of reduced diameters, an additional attenuation of the optical signal occurs, which in combination with high‐power signals generates a considerable localized heating.

Optical connectors can be easily contaminated with dust or organic particles that are common in outdoor environments. This additional attenuation in the connectors, contaminated and/or degraded, may be considerable and when associated with high‐power signals also generate localized heating.

PoF is generally used in hazardous environment requiring rugged optical cables for this kind of environment. It is desirable that such cables containing fiber from 100‐ to 1000‐μm core diameter be available on the market, since PoF requires fibers with large core diameter. The development of a rugged PoF optical cable using 100‐μm fiber core for long‐distance applications is described in [52].

The main OF parameters required for a PoF project are maximum transmitted optical power without causing damages (*P*MaxFiber), fiber attenuation (*α*FiberdB) in dB/km at the operation wavelength (*λ*op), the fiber core diameter (*D*core), and the numerical aperture of the fiber (NA).

#### *2.3.3. Photovoltaic converter*

A photovoltaic power converter is the element where the light will be finally transformed in electricity to supply the control unit and the sensors. PV is one (or more) photodiode that operates in photovoltaic mode. When the supply optical power reaches the PV, electron‐hole pairs (carriers) are generated creating a photocurrent. The total current is the photocurrent produced by the supply optical power minus the dark current, which is related to the reverse saturation current. This photocurrent (*I* ph) is proportional to the incident supply optical power according to Eq. (6)

$$I\_{ph} = P\_{\rm ln} \cdot R(\lambda\_{\rm op}) \tag{6}$$

*R*(*λ*op) is the spectral responsivity of the cell, which is dependent on the HPOS operation wavelength (**Figure 2**).

It should be noted that ignition and propagation of the fiber‐fuse effect occurs only if the power of the optical signal in the fiber is maintained above a threshold value. According to [56], this threshold is proportional to the diameter of the fiber modal field (MDF) and dependent on the wavelength of the optical signal. A typical SMF fiber (MDF = 7.8 μm) presents a threshold of 1.5 W at 1467 nm. In other study [58], it was observed that an SMF fiber type 28F (MDF = 7.8 μm) presents a threshold of 1.0 W, and an MM fiber of 62.5‐μm core (MDF = 12.0

In order to increase the optical power in SMF fibers, many efforts have been made to transmit the high power by the cladding of the SMF fiber. In [21, 22], an optically controlled beam steering system using 60 W transmitted in the cladding of the fiber to feed remote antenna

The propagation of signals with optical power exceeding the threshold required for ignition and propagation of the fiber‐fuse effect is not sufficient to trigger it, and an ignition point characterized by having a high‐temperature value is also required. This point of ignition occurs in contaminated and/or degraded connectors or in optical fibers subject to tight bending. As described previously, in optical fibers subjected to curvatures of reduced diameters, an additional attenuation of the optical signal occurs, which in combination with high‐power

Optical connectors can be easily contaminated with dust or organic particles that are common in outdoor environments. This additional attenuation in the connectors, contaminated and/or degraded, may be considerable and when associated with high‐power signals also generate

PoF is generally used in hazardous environment requiring rugged optical cables for this kind of environment. It is desirable that such cables containing fiber from 100‐ to 1000‐μm core diameter be available on the market, since PoF requires fibers with large core diameter. The development of a rugged PoF optical cable using 100‐μm fiber core for long‐distance applica-

The main OF parameters required for a PoF project are maximum transmitted optical power without causing damages (*P*MaxFiber), fiber attenuation (*α*FiberdB) in dB/km at the operation wavelength (*λ*op), the fiber core diameter (*D*core), and the numerical aperture of the fiber (NA).

A photovoltaic power converter is the element where the light will be finally transformed in electricity to supply the control unit and the sensors. PV is one (or more) photodiode that operates in photovoltaic mode. When the supply optical power reaches the PV, electron‐hole pairs (carriers) are generated creating a photocurrent. The total current is the photocurrent produced by the supply optical power minus the dark current, which is related to the reverse

ph) is proportional to the incident supply optical power

ph = *P*In . *R*(*λ*op ) (6)

μm) presents a threshold of 4.0 W at 1060 nm.

signals generates a considerable localized heating.

units is presented.

264 Optical Fiber and Wireless Communications

localized heating.

tions is described in [52].

*2.3.3. Photovoltaic converter*

according to Eq. (6)

saturation current. This photocurrent (*I*

*I*

The most important photovoltaic parameter is the conversion efficiency (*η*). It is defined as the maximum electric power produced by the PV (*P*elecmax) divided by the incident light power under standard light conditions or as defined by Eq. (7):

$$\eta = P\_{\text{obcmax}} / P\_{\text{Pn}} = (\text{V}\_{\text{OC}} \dots \text{I}\_{\text{SC}} \text{ FF}) / (P\_{\text{Pn}}) \tag{7}$$

where *V*oc is the open‐circuit voltage, *I*SC is the short circuit current, and FF is the fill factor of PV.

Many PV types are actually made of micro‐PV arrays [59, 60]. In this way, connecting the micro‐PVs in parallel to the output voltage can reach an adequate level. **Figure 5** shows a schematic arrangement of a commercial PV array with four elements compared with 100‐μm‐core optical‐fiber diameter. This type of fiber is normally used in fiber‐packaged PVs.

The fill factor is the parameter that evaluates the junction quality and series resistance of the PV array. The fill factor is the ratio of the maximum electric output power to the product of VOC and ISC.

A comparative study of PV efficiency for the materials Si, GaAs, and Ge is founded in [61]. PV devices made of Si or GaAs proper for PoF uses can be founded in market. Unfortunately, these devices operate in the spectrum region where the fiber does not have the minimum attenuation (**Figure 2**). For operation in 1310 or 1550 nm, the PV arrays can be obtained easily connecting many InGaAs or Ge photodiodes in series or in parallel using an optical splitter. This is a topic for the next section.

The main PV parameters required for a PoF project are the maximum output electric power (*P*elecmax), the maximum optical input power without damaging the PV (*P*Inmax), the operation spectral band (*λ*op), the efficiency (*η*), open‐circuit voltage (*V*oc), and the short‐circuit current (*I* SC).

**Figure 5.** Schematic arrangement of a commercial PV array with four elements compared with 100‐μm‐core optical fiber diameter.

#### *2.3.4. Basic remote unit circuits*

The electronic circuits of unit control including LD driver and sensors need to work in a specific voltage (*V*cc), and they consume an electrical current (*I* load). To provide the required voltage and current, some basic circuits can be designed. The most basic circuits use just one PV and a DC‐DC converter to stabilize the voltage, as shown in **Figure 6(a)**. In this case, the PV should be an array type to provide voltage higher than 3 V, although there are many integrated circuits that work below 3 V. If the PV chosen has low‐output voltage (lower than 2V), the circuit shown in **Figure 6(b)** can be used. In this circuit, the DC‐DC converter was substituted by a step‐up converter, which elevates the PV output voltage to the voltage required by the circuit. Other possibility to elevate the voltage is using an optical splitter (with *n* output ports) connected to the optical fiber and the PVs (or photodetectors) connected in series (**Figure 6(c)**). Typically, splitters with *n* = 4–8 have been used, to provide *V*cc ranging from 2.4 to 4.8 V. Each splitter output port is connected to one PV. This is an easy solution to use in spectral bands where there is no commercial PV array available, such as in 1310 and 1550 nm. **Figure 6(d)** shows a different version of **Figure 6(c)** circuit. In this case, the association of the PVs is made in parallel in order to increase the current available to the circuit. Certainly, the association of PVs can be made simultaneously in parallel and in series.

**Figure 6.** Some types of circuits used in the design of PoF remote units, (a) direct conversion, (b) conversion using a step‐up converter, (c) PVs connected in series, (d) PVs connected in parallel, and (e) charging circuit using a capacitor.

**Figure 6(e)** shows a charging circuit type. This type of circuit is used to supply loads in long‐distance PoF links where the available input optical power level in the PV is not enough to provide a necessary electric power. In this circuit, when the capacitor C connected in the PV output is charged in the maximum PV voltage output, an analog switch connects the capacitor to a DC‐ DC converter that stabilizes its output voltage for the load for a specific time period. This type of circuit should be used only if the sensors do not require measuring their parameters all the time.

#### **2.4. PoF topologies**

*2.3.4. Basic remote unit circuits*

266 Optical Fiber and Wireless Communications

The electronic circuits of unit control including LD driver and sensors need to work in a

voltage and current, some basic circuits can be designed. The most basic circuits use just one PV and a DC‐DC converter to stabilize the voltage, as shown in **Figure 6(a)**. In this case, the PV should be an array type to provide voltage higher than 3 V, although there are many integrated circuits that work below 3 V. If the PV chosen has low‐output voltage (lower than 2V), the circuit shown in **Figure 6(b)** can be used. In this circuit, the DC‐DC converter was substituted by a step‐up converter, which elevates the PV output voltage to the voltage required by the circuit. Other possibility to elevate the voltage is using an optical splitter (with *n* output ports) connected to the optical fiber and the PVs (or photodetectors) connected in series (**Figure 6(c)**). Typically, splitters with *n* = 4–8 have been used, to provide *V*cc ranging from 2.4 to 4.8 V. Each splitter output port is connected to one PV. This is an easy solution to use in spectral bands where there is no commercial PV array available, such as in 1310 and 1550 nm. **Figure 6(d)** shows a different version of **Figure 6(c)** circuit. In this case, the association of the PVs is made in parallel in order to increase the current available to the circuit. Certainly, the

**Figure 6.** Some types of circuits used in the design of PoF remote units, (a) direct conversion, (b) conversion using a step‐up converter, (c) PVs connected in series, (d) PVs connected in parallel, and (e) charging circuit using a capacitor.

load). To provide the required

specific voltage (*V*cc), and they consume an electrical current (*I*

association of PVs can be made simultaneously in parallel and in series.

The connection between the control units and remote units in PoF systems is usually made in the same way of telecommunications links. **Figure 7(a)** shows a typical PoF link using two fibers. However, in some cases the quantity of fibers is a limiting factor in specific application. WDM can be used to multiply the supply optical power transmitted by the HPOS and the sensor signal transmitted by the LD in the remote unit. The WDM device must support the high power of the HPOS. WDM devices for high‐power applications can support around 2 W of handling power. The wavelength pairs used in a WDM PoF system as the one shown in **Figure 7(b)** are as follows:


**Figure 7.** Illustrations of some topologies for PoF systems, (a) two fibers topology, (b) bidirectional WDM topology, and (c) tree topology.

PoF networks have also been reported. A typical tree topology (**Figure 7(c)**) permits the connection of a single control unit to many remote units. This type of network works in a similar way as a passive optical network used in telecommunications.

### **2.5. Example of PoF link calculations**

Using Eqs. (1)–(5), it is possible to calculate the maximum PoF link for two cases, using SMF and MM fibers. **Table 1** shows the parameters used in this calculation.

**Figure 8** shows a plot of the delivered power (*P*Load) to an electronic load as a function of link distance. It can be observed that MM fiber option is better until 2.2 km. After this distance, the SMF option is better.


**Table 1.** Parameters used to calculate the maximum PoF link distance.

In utilities, the major applications of PoF occur in the substations, which the maximum internal distances are less than 0.2 km. For telecommunications, the distances in the major application are longer than 2 km.

**Figure 8.** Plot of the delivered power to an electronic load as a function of link distance.

### **2.6. PoF applications developed at CPqD**

PoF networks have also been reported. A typical tree topology (**Figure 7(c)**) permits the connection of a single control unit to many remote units. This type of network works in a similar

Using Eqs. (1)–(5), it is possible to calculate the maximum PoF link for two cases, using SMF

**Figure 8** shows a plot of the delivered power (*P*Load) to an electronic load as a function of link distance. It can be observed that MM fiber option is better until 2.2 km. After this distance, the

In utilities, the major applications of PoF occur in the substations, which the maximum internal distances are less than 0.2 km. For telecommunications, the distances in the major applica-

way as a passive optical network used in telecommunications.

**Parameter/fiber SMF MM** HPOS optical power 1.0 W 2.0 W Wavelength 1480 nm 830 nm Total connectors attenuation 1.0 dB 1.0 dB Fiber attenuation 0.25 dB/km 3.0 dB/km

PV efficiency 0.2 0.4 PV material InGaAs GaAs

**Table 1.** Parameters used to calculate the maximum PoF link distance.

**Figure 8.** Plot of the delivered power to an electronic load as a function of link distance.

and MM fibers. **Table 1** shows the parameters used in this calculation.

**2.5. Example of PoF link calculations**

268 Optical Fiber and Wireless Communications

SMF option is better.

tion are longer than 2 km.

This section presents some PoF systems and sensors developed by the author's group for telecommunications and for utilities applications. Most PoF systems for utilities have been tested in field trials.

#### *2.6.1. PoF applications in telecommunications*

In telecommunication, we developed a new approach to powering PON extenders using PoF [14]. PON extenders are elements that recover the downstream and upstream signals. These signals are highly attenuated by the splitters and by link length in any PON. However, the use of extenders in PON modifies its passive characteristic since extenders require electrical supply to work. Using PoF to supply, the extenders turns them virtually passives.

Using PoF to eliminate the batteries located at telecom remote sites, the reliability and the security of the system are improved. Furthermore, PoF can reduce the copper cables theft, used to power the extenders in many cases. We demonstrated the performance of a fiber‐powered extender using semiconductor optical amplifiers in a 10‐gigabit passive optical network system and a gigabit passive optical network system (G‐PON) setup using a 1:32 splitter and 50‐km reach. The extender was powered from a remote site placed in the access area using a 62.5‐**μ**m MM fiber at 830 nm. **Figure 9** shows a schematic of the PON using the fiber‐powered extender and the photo of the extender.

#### *2.6.2. PoF applications in utilities*

In utilities, we developed applications for monitoring hydrogenerators, power transformers, switchgears, and for transmission lines.

**Figure 9.** Schematic of a PON using extender powered by a PoF system and the extender photo.

**Figure 10** shows the applications for partial discharges monitoring in hydrogenerators [46] and for 500‐kV power transformers [62]. Partial discharges on high‐voltage equipment insulation are a symptom of fragility of the dielectric capacity. The growing of partial discharges can cause serious consequences for the equipment and the electrical system. Partial discharges generate some physical effects, such as conducted and radiated electromagnetic pulses, light, acoustic noise, localized temperature variation, and chemical reactions. In the developed PoF‐monitoring systems, we used a powered antenna to detect the radiated electromagnetic pulses of discharges.

For hydrogenerator monitoring (**Figure 10(a)**), the sensor is composed by one dipole meander antenna, one photovoltaic converter, and one semiconductor laser. Some passive components such as resistors, capacitors, and inductors are omitted in the schematic of sensor circuit to simplification. This system was installed in the Eletrobrás' Coaracy Nunes power plant, which is located in Amapá state, in the north of Brazil.

The sensor for monitoring the high‐voltage power transformers (**Figure 10(b)**) is composed by one monopole antenna, one photovoltaic converter, one field‐effect transistor (FET) amplifier, and one semiconductor laser. The FET was used in this sensor to increase the sensitivity of the sensor in this particular application. This system was installed in the Cemig's Neves substation, which is located in Minas Gerais state, in the southeast of Brazil.

**Figure 10** shows the schematic circuit and sensor photo of antenna powered by fiber for partial discharges monitoring in (a) hydrogenerators and (b) high‐voltage power transformers.

**Figure 10.** Schematic circuit and sensor photo of antenna powered by fiber for partial discharges monitoring in (a) hydrogenerators and (b) high‐voltage power transformers.

**Figure 11** shows some applications using more complex electronic circuits with PoF. **Figure 11(a)** shows a 138‐kV switchgear‐monitoring application using fiber‐powered video cameras to inspect the quality contact of the switchgear [53]. In this application, the three phases of 138‐kV switchgear were monitored using three sensors. The connection of the sensors to the control unit used the tree topology shown in **Figure 7(c)**. This system was installed in the Cemig's Bonsucesso substation, which is located in Minas Gerais state, in the southeast of Brazil.

**Figure 11(b)** shows a fiber‐powered camera installed in a 138‐kV transmission line tower [51]. The camera was used to monitor possible invasions in the security area of transmission line. The circuit of the camera works in a noncontinuous regime (circuit of **Figure 6(e)**) since the power transmission was done using an SMF fiber embedded in an optical ground wire cable. This system was installed in the Cemig's Bonsucesso/Gutierrez transmission line, which is located in Minas Gerais state, in the southeast of Brazil.

Remembering that for electrical utilities, there are key factors to use PoF. Optical fiber is made of nonconductive material. In high‐voltage environment, any conductance can

**Figure 11.** PoF applications with fiber‐powered video cameras, (a) 138‐kV switchgear sensor shown in the circle (above) and a sensor image (below) and (b) 138‐kV transmission line sensor shown in the circle (above) and a sensor image (below).

**Figure 10.** Schematic circuit and sensor photo of antenna powered by fiber for partial discharges monitoring in (a)

**Figure 10** shows the applications for partial discharges monitoring in hydrogenerators [46] and for 500‐kV power transformers [62]. Partial discharges on high‐voltage equipment insulation are a symptom of fragility of the dielectric capacity. The growing of partial discharges can cause serious consequences for the equipment and the electrical system. Partial discharges generate some physical effects, such as conducted and radiated electromagnetic pulses, light, acoustic noise, localized temperature variation, and chemical reactions. In the developed PoF‐monitoring systems, we used a powered antenna to detect the radiated electromagnetic

For hydrogenerator monitoring (**Figure 10(a)**), the sensor is composed by one dipole meander antenna, one photovoltaic converter, and one semiconductor laser. Some passive components such as resistors, capacitors, and inductors are omitted in the schematic of sensor circuit to simplification. This system was installed in the Eletrobrás' Coaracy Nunes power plant,

The sensor for monitoring the high‐voltage power transformers (**Figure 10(b)**) is composed by one monopole antenna, one photovoltaic converter, one field‐effect transistor (FET) amplifier, and one semiconductor laser. The FET was used in this sensor to increase the sensitivity of the sensor in this particular application. This system was installed in the Cemig's Neves

**Figure 10** shows the schematic circuit and sensor photo of antenna powered by fiber for partial discharges monitoring in (a) hydrogenerators and (b) high‐voltage power transformers.

substation, which is located in Minas Gerais state, in the southeast of Brazil.

hydrogenerators and (b) high‐voltage power transformers.

pulses of discharges.

270 Optical Fiber and Wireless Communications

which is located in Amapá state, in the north of Brazil.

create current leaks. Optical fiber is immune to electromagnetic interferences. The electrical world environment is polluted of electromagnetic interferences; therefore, optical fiber can transmit signals without quality degradation. The optical fiber eliminates the need to run conductive copper wire into a GPR zone. GPR arises when lightning strikes occur in substations and can cause severe interference problems in electronic equipment and systems. PoF sensors have a complete galvanic isolation to the ground potential, then they are practically immune to the GPR effects and finally, as we show, there are many low‐cost/ low‐power/high‐efficiency electronic sensors available for transmission lines and substations monitoring that can be supplied by PoF increasing the monitoring capacity for utilities companies.
