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Abstract
A wireless optical power transfer (WOPT) system using an erbium-doped fiber amplifier as an optical power source is proposed to achieve long range, high power, and hazard-free power delivery in the air. The transmitter generates a wide band of amplified spontaneous emission around the central wavelength of 1550 nm. A wavelength division multiplexing (WDM) filter (λ=1552.25 nm) is deployed to obtain a safe narrowband beam illuminating the receiver units. A ball lens retroreflector reflects a small portion of the incident beam back to the transmitter, establishing a closed ring resonance loop. An improved safety mechanism is proposed to terminate the resonance when an obstacle blocks the transmitter-receiver line of sight. The measured incident power of 1 W decreases to 0.79 mW after the WDM filter is deployed which is well within defined maximum permissible exposure standards. For the demonstration of free-space transmission, transmitter-receiver separation is extended to 30 m. The experimental results show that a single-channel WOPT system provides an optical power of 400 mW with a channel linewidth of 1.027 nm over 30 m and an electrical power of 85 mW is acquired using a gallium antimonide photovoltaic.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The demand for wireless power transfer (WPT) systems is increasing with the growing needs of portable electronic gadgets, internet of things devices, electric vehicle charging, and smart mobile phones. In addition, the advent of 5G mobile communication that consumes significant amounts of power is deriving the demand of, long-range, low-cost, and low-latency wireless power transfer techniques [1–3]. The two major branches of wireless power transfer are near and far field free space power transmission. In recent years, various power transfer techniques have been demonstrated including magnetic induction or capacitive coupling between primary and secondary coils [4–7]. Although, techniques such as resonance-based magnetic and capacitive coupling power transfer methods exhibit high efficiency, and multiple receiver charging, they are limited to short distances of a few mm up to 5 m [8,9].
Long-distance WPT techniques include radiofrequency or microwave, monochromatic laser beam, and ultrasound WPT techniques. Radio frequency wireless power transmission (RFWPT) has also been widely investigated for long-range wireless power transmission [10–14]. Various studies have been conducted using RFWPT at 900 MHz, 2.4 GHz, 5.8 GHz, and 24 GHz (mm-Wave) transmission in free space [15,16]. In a previous study, a RFWPT system at 915 MHz that achieves 10% power transfer efficiency at 15 m was experimentally demonstrated [17]. In contrast, radio frequency WPT at 5.8 GHz provides 0.24% power transfer efficiency at 5 m, which decreases with increasing distance from the transmitter [16]. Another study reported a receiving power of 100mW from a transmitted power of 32 W at 4 m using 5–6 GHz frequencies [18]. Moreover, 2.4 GHz and 24 GHz systems have also been demonstrated recently. However, the 2.4 GHz system has limitations in terms of safe power transmission and the 24 GHz system results in a bulkier and expensive design [19–21]. Over the past decade, industrial startup companies such as Energous, Ossia, Powercast, and GURU have gained interest in wireless charging solutions using RFWPT. Ossia Corp. for instance has introduced a WPT system based on phase conjugation using 2.4 GHz and 5.8 GHz frequencies the latter of which can deliver 2–3 W at 1 m, 1 W at 2 m, and 10–50 mW at 10 m [22,23].
WPT using RF/microwave power transmission is either restricted by safety limits or fails to provide sufficient power levels while maintaining efficiency at long range. These systems face challenges primarily in terms of antenna/rectenna designs, diffraction, interference, and environmental issues [24]. Despite the potential of RFWPT systems, the dispersive nature of EM waves used in these systems tends to increase free space losses, multipath fading, and hazardous radiation. All these factors contribute to the infeasibility of attaining high energy efficiency using RFWPT [25].
In addition, optical power transmission using monochromatic laser light can achieve wireless transmission over large distances at high-power energy levels. However, achieving safe energy levels specified by the International Electrotechnical Commission (IEC) is one of the downfalls in laser-based wireless energy transmission [26].
In contrast to RF/microwave and ultrasound wireless transmission, resonant beam charging (RBC) is a promising substitute for long-range power transmission with high power levels of up to a few watts. It has two major advantages over RF and ultrasound techniques–: it is unaffected by electromagnetic interference (EMI) and does not require an RF license. The RBC is based on the optical WPT technique that employs retroreflective beamforming schemes to form a closed loop resonating path between the receiver and transmitter. Recently, numerous studies have investigated RBC for energy transfer in the air with large transmitter-receiver separation and high-power conversion efficiency [27,28]. One particular study reported laser diode based power transfer in free space with a receiving power of 10.39 mW over 5.2 m [29]. In another study, a 40 mW power transfer up to 1.6 m was analytically analyzed [30]. A micro-LED light emission source was also demonstrated in a study for low-power underwater applications [31]. Nd: YVO was employed as the gain medium in another study that reported 1064 nm output with intra-cavity second harmonic laser for power and data transmission up to 8 m [32]. Similarly, in a different study, Nd: YVO4 was employed as the gain medium to deliver 4 W of power at an input pump power of 250 W with a transmitter-receiver separation of 3 m [33]. In addition, a 1064 nm fiber-based pumped output was experimentally demonstrated using spherical cat-eye retroreflectors for high amplified spontaneous emission (ASE) and gain using a working resonance cavity. This system delivered a 5.901 W output power in response to 16.1 W incident pumping power [34]. Finally, in a related study, an integrated wireless powered sensor was reported to monitor the heart rate of automobile drivers using a 940 nm power source [35].
Moreover, a novel power transmission technique for multiple receiver charging using spatially distributed laser cavity with built-in safety mechanism was introduced by Jaeyeong Lim, et al., in our laboratory. Semiconductor optical amplifier (SOA) was utilized as the transmitting unit and the RBC technique was employed with a retroreflector and photovoltaic (PV) cell comprising the receiving unit [36,37]. In sum, various RBC schemes using semiconductor-based gain medium that range from 900–1100 nm, for distances up to 12 m, have been reported. Typically, laser diodes are the most suitable power source for RBC systems owing to their high power output, low divergence angle, and improved carrier frequency. Recently, an erbium-doped fiber-based gain medium operating at a wavelength of 1550 nm based on the RBC technique also referred to as distributed laser charging (DLC) via free space was presented [27]. To sum up, the design of an efficient WPT system requires an efficient light source with low divergence and high gain, low beam deviation for longer distances, smaller hazardous wavelength region, and high PV cell optical to electrical energy conversion efficiency.
Based on the aforementioned parameters a long-range wireless optical power transfer (WOPT) system that adheres to the eye safety maximum permissible exposure (MPE) standards defined by the IEC is proposed in this study. An erbium-doped fiber amplifier (EDFA) transmitter is proposed in which a wide band light spectrum is generated by ASE of erbium ions by laser pump excitation, and a retroreflector reflects the incident beam back to the EDFA to complete a resonating cavity. A WDM filter is used for beam illumination into the free space which enables hazard-free propagation. The resonating wavelength of the system is 1550 nm, which ensures risk-free transmission for the eyes compared with other visible or near infrared wavelengths. A Retroreflecting ball lens is utilized to facilitate the self-alignment mechanism between transmitter and receiver with high transmissivity, resulting in maximum power transfer efficiency. Finally, a GaSb PV cell providing 21.5% optical to electrical conversion efficiency is utilized.
The remainder of this article is organized as follows: Section 2, describe the experimental setup for the WOPT system for a transmission distance of up to 30 m in free space, system architecture including transmitter (EDFA), and receiver subunits. Furthermore, we present the advantages of EDFA laser source for the proposed WOPT for high-power delivery over long distances. Section 3 presents an experiment using the proposed system and its results. In addition, the EDFA spectral behavior in power transfer and safety modes, retroreflector transmittance and reflectance, high output power delivery, and efficiencies of the GaSb PV cell, and WOPT are experimentally analyzed by increasing the transmitter-receiver separation. Finally, Section 4, presents the conclusions.
2. Experimental setup
A long-range high power WOPT system based on the RBC technique is presented in this study. The experimental setup used to evaluate this system is illustrated in Fig. 1. The transmitting unit incorporates an EDFA, a circulator, and a WDM passband filter. The receiver subsystem comprises a retroreflecting spherical lens and PV cell. The operating central wavelength of the EDFA is 1550 nm, which is considered as the safest wavelength compared with other short wavelength counterparts [17]. An EDFA module (DWDM-EDFA-MP31, Shanghai B&A Technology) with an LD pump power operating at 976 nm induces an output with a 1550 nm central wavelength. The ASE spectrum of EDFA is analyzed using optical spectrum analyzer (OSA; AQ6370C, Yokogawa) that range from 1525 nm to 1570 nm. The bias current is set at 5 A to obtain an optical power of 1 W at the output port of the EDFA. The illuminating beam from the EDFA output is injected into a circulator, to ensure that the incoming reflected signal does not affect the output light signal. A 99:1 optical tap coupler is applied to observe the OSA spectrum and optical power of the transmitting optical light beam (explained in Section 3).
The light signal emanating from the output port of the circulator enters a WDM filter (DWDM 100G-1550, ETERN Optoelectronics). A narrow spectral bandwidth of 1.5 nm at a peak of 1552.25 nm is separated from the ASE spectrum generated by the EDFA. Therefore, a WDM filter was employed to achieve an optical power within the limits of MPE standards for free space propagation. After it passes through the WDM filter, the laser beam is directed into a collimator (TC25APC-1550, Thorlabs) that emits the parallel light beam at its output into the wireless medium. The receiver section comprises a retroreflector to reverse a portion of the incident light back to the transmitter, thereby enabling an easier line of sight positioning of the receiver. A spherical ball lens is employed as the retroreflector owing to its adequate transmittivity and reflectivity split ratio. In context of this study, reflectivity is defined as the amount of signal that bounces back to the transmitter. The reflected signal from the ball lens follows the resonating path and enters the transmitter’s gain medium for amplification, thereby completing the resonance cavity and inducing a power transfer mode. If the resonance cavity is uninterrupted the power continues to transfer from the transmitter to the receiver. In the event that a foreign object interrupts the resonance path, the resonance immediately stops, and the system automatically shifts to a safe power transfer mode. In this mode, the transmitter produces an incredibly low intensity light that does not pose any risk to humans.
3. Results and discussion
3.1 EDFA spectra without resonance
In the first experiment, the ASE output light spectrum of EDFA generated by laser pumping at 976 nm was analyzed using an OSA with a range from 1525 nm to 1570 nm. The generated ASE spectrum of light acts as a power source in the proposed system. A narrow band was filtered out from the generated ASE spectrum using a WDM filter having ITU’s CH31. The output spectrum of the WDM filter from the passband port was analyzed using the OSA shown in Fig. 2(b). The filtered narrow spectrum comprises a low-intensity light beam as compared with the complete ASE optical beam which ensures that a hazard-proof power transfer mode is accomplished. The rest of the blocked spectrum can be analyzed from the reflection port of the WDM filter illustrated in Fig. 2(c). The optical power measured from the output of the WDM filter’s passband port was 0.79 mW which is within the hazard-free limits according to MPE standards.
Fig. 2. (a) Broadband ASE spectra of optical power source in non-resonance mode (b) EDFA’s narrowband filtered spectra around 1552 nm wavelength. (c) Wavelength spectra of EDFA when resonance is established (d) EDFA reflection band spectra when there is no resonance.
3.2. EDFA spectra with resonance
A narrow band light beam from the passband-port of the WDM filter travels through the free space and strikes the retroreflector surface. A narrow portion of this incident light ray is reflected back to the receiver, which becomes the input signal of the EDFA and is amplified within the gain medium of the optically pumped EDFA at 976 nm. Therefore, a resonant cavity is established between the transmitter and receiver via a wireless channel. Figure 2(c) shows the ASE gain spectra of the resonating ring cavity measured using the 1:99 coupler and OSA. The peak power around 1552 nm indicates that the transferred energy at a particular narrow width wavelength depends on the center wavelength of the WDM filter. Moreover, the optical signal to noise ratio (OSNR) of the observed spectra was observed to be 40 dB. In the resonating mode, the EDFA generates a broadband light beam by the phenomenon of ASE in free space from the passband of the WDM filter, as explained in Section 2. Subsequently, the system enters the power transfer mode until the line of sight is obstructed by an external object, and power transfer mode is achieved until the line of sight is interrupted by an external object. The resonating channel analyzed by using OSA at an interval of 1000 sweeps by setting resolution at 0.02 nm is plotted in Fig. 2. (c). Its linewidth defined as the full at width half maximum (FWHM) was measured to be 1.027 nm. When an object obstructs the line of sight, the input signal is lost, and the resonance is terminated. Consequently, the WOPT switches to the safety mode.
To evaluate the performance of the ball lens as a retroreflector, the power variation for increasing beam size is measured and plotted as a reflectivity and transmittivity percentage. In general, the beam diameter can be presented in different ways including knife-edge, FWHM, 1/e2, and D86 [38]. In this experiment, we examined the beam width using a 1/e2 Gaussian beam through a beam profiler (BP209-VIS, Thorlabs). The 1/e2 is the width of beam excluding 1/e2 times the maximum intensity i.e. 13.5% of the maximum intensity portion of beam on the marginal axis [39]. First, the percentage of the transmitted and reflected power of the retroreflector were measured by changing the refractive index of the retroreflector material. The optical power in free space was recorded using an optical power meter (PM20A, Thorlabs) and a photodiode (S121C, Thorlabs). Two different ball lens retroreflectors of a fixed diameter of 10 mm, i.e., the glass type S-LAH79, and N-BK57 with refractive indices of n = 2.003 and n = 1.51, respectively were analyzed. The experimental results of reflectivity and transmittivity are plotted in Fig. 3. In addition, the transmitted power and reflected power for coupling back to the transmitter was recorded when the beam was aligned with retroreflector. The distance between the transmitting unit and receiving retroreflector was maintained at 2 m and the projected beam spot size was constant at 4.5 mm. Furthermore, the results were calculated in resonating mode. Figure 3(a) shows that a maximum reflectivity of 9.45% of the total input power projected to the retroreflector was achieved using the S-LAH79 ball lens.
Fig. 3. (a) Plot of reflectivity and transmittivity of (S-LAH-79, n = 2.003), and (N-BK57, n = 1.51) retroreflectors as a function of beam size, respectively (b) Effect of increasing the retroreflector diameter on reflectivity and transmittivity.
In Fig. 3(a), the acquired data of S- LAH-79 is plotted, and a maximum reflectivity of 9.29% is observed. Moreover, the reflectivity decreases with increasing beam size diameter. To be precise, at a beam spot diameter of 11.5 mm. It is observed that the reflectivity of S-LAH79 (n = 2.003) and N-BK57 (n = 1.51) reduces to 6.8%, and 1.1%, respectively. Importantly, the peak transmitted power percentage of the ball lens with n = 1.51 is 96.25%, which is better compared with 90.67% of the ball lens with a refractive index of n = 2.Nevertheless, to maximize the transmitted power, the ratio between reflectivity and transmittivity must be optimized. As shown in Fig. 3(a), the transmittivity of retroreflectors S-LAH79 (n = 2.003) and N-BK57 (n = 1.51) also decreases at larger beam diameters up to 81%, and 82.4%, respectively. Figure 3 shows that the power of the received beam after it passes through the ball lens at an effective focal length decreases considerably with an increase in the beam spot size diameter for both the retroreflectors. Similarly, the reflective optical power that serves as the input signal of the EDFA also decreases with an increase in the beam spot size diameter, resulting in low power transfer efficiency. The measured reflectivity of incident light beam shows that a retroreflector with a refractive index 2.003 (S-LAH-79, Edmund optics) has the highest efficiency among other retroreflective ball lenses. On the other hand, despite exhibiting reliable transmittivity, the N-BK57 results in a lower overall efficiency. This reduction in overall efficiency can be attributed to the fact that the system depends upon the EDFA gain, which is dependent on the reflectivity of the retroreflector. Hence, a retroreflector with a larger value of reflectivity i.e., the S-LAH79 (n = 2.003) lens with a reflectivity of 9.29% and transmittivity of 90.67% provides higher system efficiency. In conclusion, based on these results, the beam size must be optimized according to the diameter of the retroreflecting ball lens to enhance the effectiveness of power transmission.
In the second step, the performance of the retroreflector was practically measured by increasing the diameter of the retroreflector and keeping the spot size of the light beam constant. The refractive index of the retroreflectors was kept constant at 2.003 (S-LAH-79, Edmund optics). Figure 3(b) shows the transmittivity and reflectivity percentage readings of the incident beam acquired by changing the retroreflectors diameter. The results plotted in Fig. 3(b) clearly demonstrate that the reflective signal from the surface of the retroreflector is large in the case of a 10 mm ball lens and decreases as the size of retroreflector decreases. Owing to the sphericity of a retroreflectors of a larger diameter the incident rays bounce back more precisely to the transmitter unit, whereas for smaller diameters the projected beam reflects at different angles from the incident angle. Hence, power transmission efficiency can be enhanced by selecting a retroreflector with the appropriate diameter. Measured results revealed that retroreflectors with small diameters of 4 mm, 6 mm, and 8mm performed worse when compared with a retroreflector with a diameter of 10 mm. A comparison of the results plotted in Figs. 3(a) and 3(b) indicates that the power transfer effectiveness of the system depends upon the refractive index of the retroreflector, diameter of the retroreflector (ball lens), diameter of the incident laser beam, and divergence of the beam spot size in free space. In addition, an extra highly reflective (HR) coating on half of the retroreflector’s surface improves reflective signal percentage. Hence, by the adaptation of HR coating the performance of retroreflector can be fine-tuned. The beam spot size should be less than the diameter of retroreflector for better performance. In sum, careful optimization of each of the above-mentioned parameters plays a vital role in enhancing the retroreflector’s efficiency.
In the third experiment, the performance of the GaSb PV cell (JX Crystals) was monitored. The GaSb PV cell dimensions are 14.5 mm × 10.4 mm. It was manufactured by JX Crystals with anode and cathode power lines is deployed in the receiver unit. GaSb is a material that can effectively absorb photons at longer wavelengths up to 1.8 micrometers [40]. The conversion efficiency of photogenerated power induced by the optical beam projection on the PV cell is illustrated in Fig. 4. Firstly, the capability of the GaSb PV cell to convert photons into electrons was measured by increasing the transmitter-receiver separation distance. In the receiver unit, the PV cell was placed directly behind the ball lens in the resonance mode of the proposed RBC system. The DC electrical output power was recorded across the anode and cathode of the PV cell to calculate the PV cell efficiency percentage. Figure 4(a) shows the result of PV cell efficiency and optical power plotted as a function of increasing transmitter-receiver distance.
Fig. 4. (a). Plot of PV cell efficiency and illuminated light power as a function of distance between transmitter and receiver. Figure 4. (b). Variation in PV cell optical to electrical conversion efficiency as function of optical power.
The acquired data indicates that the conversion efficiency of the GaSb PV cell decays at a higher optical power. The efficiency of the PV cell at an optical power of 400 mW at 30 m was observed to be 20.38%. The results reveal that the PV cell efficiency decreases with increasing power density per unit area. This variation in PV cell quantum efficiency can be primarily attributed to the formation of a p-n junction and the unavailability of surface passivation [41]
Secondly, the maximum power point for the GaSb PV cell was detected by tuning the photogenerated power. The transmitter-receiver separation was fixed at 1 m and the cell efficiency was recorded in power transfer mode. The acquired results are shown in Fig. 4(b). The ratio of input optical power to the output electrical power was examined as a function of increasing optical power on the PV cell surface. Previous studies have reported that the conversion efficiency of the GaSb PV cell in a wavelength range of 1500 nm is 34.8% [42].
Figure 4(b) depicts that the practically examined maximum efficiency of the GaSb solar cell is 29% at an input power of 150 mW. Owing to the long wavelength range performance characteristics and impressive response to infrared light [43], the GaSb PV cell is highly suitable for boosting the power conversion efficiency in a 1500 nm wavelength range. Furthermore, the efficiency of the PV cell can be enhanced by careful construction of the p-n junction depth. In a multi-layer PV cell, the p-n junction width reflects the sheet resistance of the PV cell, which is directly related to ohmic resistance [42]. Therefore, better quantum efficiency can be achieved by reducing ohmic resistance. However, the PV cell efficiency is also dependent on the projected beam size and the size of the PV cell. Therefore, for high output power the PV cell size must be comparable to the incident beam spot size. In short, the beam spot diameter of the projected laser beam and PV cell size should be carefully optimized to enhance electrical conversion efficiency.
The results plotted in Fig. 4(b) indicate a stable optical to electrical conversion efficiency curve for the incident beam. Orderly projection of the incident beam on the active area of the PV cell boosts the received electrical power on the terminals of PV cell. Furthermore, the adjustment of the laser beam shape compared with the PV cell shape, proportionally affects the current density, thereby resulting in maximum power flow.
In the final experiment, the power of the beam after it passes through the retroreflector, i.e., the received output power was recorded by increasing the separation of the receiver from the transmitting unit. Considering the tradeoff between the ball lens size and beam diameter discussed in previous section, the ball lens (S-LAH79, Edmund optics) with the following specifications: refractive index of 2.003, diameter of 10 mm, and numerical aperture of 0.10 was selected as the retroreflector at the receiver side for the experimental setup of the long-range wireless power transfer system. The collimator used for the final experiment based on the beam size is (Thorlabs, TC25APC-1550). The laser beam spot diameter from the output of the collimator was measured to be 4.5 mm using a scanning slit beam profiler (BP209-VIS, Thorlabs) in free space.
The EDFA output laser beam was aligned with the retroreflector and the power in the resonating mode (i.e., the power transfer mode) was measured. Similarly, for the same aligned beam, the resonating mode was terminated using an external object that cutoff the resonance and the power in the non-resonating mode (i.e., the safety power mode) before the retroreflector was measured. The recorded values are plotted in Fig. 5. To demonstrate the power variation and path loss, the power loss budget was observed as follows: The ASE emitted power from the EDFA travels through the circulator port 1-2, 1:99 coupler, and WDM filter facing losses of 2.6 dB, 0.6 dB, and 1.2 dB respectively. In addition, the reflected beam from the retroreflector that passes through the passband port of the WDM filter in the opposite direction and the circulator port 2-3, limit the power by inducing losses of 0.3 dB and 2.7 dB respectively. In conclusion, the system components induce a total loss of 7.4 dB.
Fig. 5. Effect of increasing the transmitter-receiver distance on maximum power transfer in resonating mode, and power in broken resonance mode (i.e., safety power mode). Here, dotted line represents the MPE safe power limit.
Considering the beam divergence, the beam diameter and ball lens size are in relatively good agreement. However, the measured data indicates an average transmission loss of 1.76 dB within the resonance at a range of 30 m, which is mutually contributed by air and beam divergence. To calculate the expected gain of the system, the optical power of the beam reflected back as an input to the EDFA was estimated as the power at the receiver after the beam passes through the ball lens in the non-resonating mode as −0.5 dBm. The power measured at the input and output ports of the tap coupler in resonance mode was estimated to be −29 dBm and 10 dBm, respectively. The insertion losses of 1% ports of the coupler were measured to be 20.2 dB and 19.8 dB in the forward and reverse directions, respectively. Accordingly, by eliminating the insertion losses of the coupler ports we estimated a −8.8 dBm reflected input power and 29.8 dBm lasing output power at the coupler ports. Hence, the total power loss in the reverse direction due to the ball lens, free space, and collimator is 8.3 dB. Therefore, the estimated input power to the EDFA by subtracting the circular’s port 2-3 loss of 2.7 dB was −11.5 dBm. In contrast, the power received at the output port of the tap coupler when the beam passed through the coupler in the forward direction was assumed to be 29.8 dBm. Hence, the output power of the EDFA considering the coupler’s port 1-2 loss of 2.6 dB was estimated to be 32.4 dBm in the resonance mode. Therefore, the gain of the EDFA was calculated by considering the input and output port power as −11.5 dBm and 32.4 dBm, respectively. Consequently, the EDFA gain was estimated to be 43.9 dB. It should be noted that the EDFA used in this experiment has a two-stage amplification module to provide high gain and output power. Following this, the EDFA gain and power loss in the proposed setup was compared. Moreover, the EDFA gain observed using OSA in Fig. 2(c) validates the expected gain calculation. The lasing power of the WOPT system is limited by the above mentioned insertion losses. The circulator limits the power by a loss of 2.6 dB and 2.7 dB in the forward and reverse directions, respectively. Furthermore, the EDFA gain calculated by the power loss estimation and the measured OSNR value using OSA have very comparable results. Hence a clear comparison of the EDFA gain and the power loss is investigated for the proposed setup.
In the resonating mode, when the beam is aligned with the retroreflector, the portion of the beam that was reflected back establishes a ring loop cavity between the receiver and the transmitter, and the recorded power after retroreflector at 1 m was 600 mW. The output power of the EDFA was measured to be 1 W using a photodetector (S121C, Thorlabs) at a bias current of 5 A. As illustrated in Fig. 5, the separation between the transmitting and receiver units was increased and transferred power was measured at intervals of 5 m. At the maximum transmitter-receiver separation reported in this study, a power of 400 mW was acquired for a distance of 30 m when the EDFA ASE output power was set at 1 W. The power loss budget describes the constraints in the power transmission at longer distances. Furthermore, the decay in power transmission can be attributed to free space and beam divergence loss.
Similarly, the safety power mode was attained by intentionally obstructing the line of sight. The measured power when the system automatically shifted to the safe mode was 0.65 mW at a transmitter-receiver separation of 1 m. Subsequently, the safety and received power were measured at intervals of 5 m as the separation was increased. The data illustrated in Fig. 5. shows that the safety power range from 0.79 to 0.45 mW corresponding to a separation range of 1–30 m. The plotted values prove that, in the safety mode, the power drops down to a low intensity value that ensures that human organs such as the eye are free from harm. However, the measured power transfer has a decremental curve in contrast to the arc plotted using theoretically derived values. This discrepancy can be attributed to the transmission losses caused by air and beam divergence at a separation distance of 30 m. Therefore, optimizing the beam spot to make it comparable to the retroreflector diameter can effectively reduce the transmission loss in free space.
To investigate and compare the safety mode power, calculations were performed based on the radiant exposure of the beam, saturated intensity of the medium, and cross-sectional area of the illuminating beam. Similar safety parameters were theoretically modeled in the recent research [44]. Based on the resonant cavity beam power of the current setup and the received optical power, that are 1 W and 400 mW, respectively, the MPE for 1550 nm wavelength is calculated to be 10000 J/m2. Moreover, the beam radiant exposure is calculated to be 0.0000314 J/m2. As per the IEC 60825-1 standard for an operating wavelength of 1550 nm, calculated and measured values of our proposed setup are considerably within the defined limits.
In resonating mode, when the light beam is in line with the retroreflector the back reflected partial portion of the beam establishes a ring loop cavity between the receiver and the transmitter, and the recorded power at 1 m is 600 mW. The laser diode pump power of the EDFA is 1 W which is the output power of the EDFA measured with a photodetector (S121C, Thorlabs) at a bias current of 5 A. As illustrated in the Fig. 5, the separation between the transmitting and receiver unit is increased and transferred power is measured at an interval of 5 m each. At the maximum transmitter-receiver separation reported in this research, 400 mW power was acquired over a distance of 30 m when the EDFA ASE output power is 1 W. The power loss budget describes the constraints in the power transmission at longer distances. Furthermore, the decay in power transmission can be attributed to free space and beam divergence loss.
Similarly, the safety power mode is attained by intentionally blocking the line of sight. The measured power when the system automatically shifts to a safe mode is 0.65 mW at 1 m. As the distance increases, the safety power along with received power is measured at every 5 m interval. The data illustrated in Fig. 5. shows that a secure power ranging from a 1 m to 30 m distance is 0.79 mW to 0.45 mW, respectively. Therefore, it can be inferred from the plotted values that, in safety mode, the power drops down to a secure value which has a low intensity so that human organs including the eyes are not at risk. The measured power transfer has a decremental curve in contrast to the theoretically derived and plotted arc, this is as a result of the transmission losses due to air and beam divergence at 30 m. Optimizing the beam spot size comparably with the retroreflector diameter can reduce the transmission loss in free space.
Finally, the performance efficiency of the proposed system is investigated. The overall efficiency of a WPT system can be further classified into categories: efficiency of the electrical excitation power to laser diode output power ${\eta _1}$, efficiency of laser transmission ${\eta _{2\; }}$, and efficiency of received optical power to electricity ${\eta _{3\; }}$. The total electrical power applied to EDFA pump lasers is 3.925 W generating an output power of 1 W and contributing an efficiency of 25.4%. Subsequently, this generated power of 1 W is reduced to 400 mW at the receiver end, hence the transmission efficiency is 40%. Similarly, the conversion efficiency of the optical power to electricity that is the PV cell efficiency is 21.5%. The performance of a DLC system has been analyzed and calculated in a recent research study [45] according to which the overall efficiency of a DLC system can be expressed as follows:
It can be inferred from Eq. (1) that the overall efficiency is the product of efficiencies of each subsection. Hence the system efficiency is 2.1%.
In conclusion, the power transfer efficiency of the proposed system depends on the high output power, gain, and bandwidth of the EDFAs. In addition, the retroreflector transmittance and reflectance ratio, and optimization of the 1/e2 beam spot, and ball lens diameter can affect the power flowing into and the power transmitted out of the resonance cavity. Furthermore, optimizing the design of the PV cell for the proposed 1550 nm central wavelength power source can significantly enhance the overall efficiency of the system. Therefore, precise adjustments in the aforementioned system parameters can maximize the efficiency of the proposed WOPT.
4. Conclusions
In this study, a long range and high power WOPT system with an EDFA optical source using the RBC technique was demonstrated. A spherical retroreflector was employed to obtain flexibility in the transmitter-receiver alignment. The 1550 nm central wavelength operation ensures a risk-free environment comparable to that provided by short wavelengths, which eliminates human health risks. The proposed optical power delivery system shows that the power transferred through air can be maximized by carefully adjusting the EDFA gain, and PV cell conversion efficiency, loss, and retroreflector size, reflectivity, and beam spot size. To the best of author’s knowledge, this proof-of-concept system is the first of its kind. It is based on the RBC technique and exhibits optical and electrical receiving powers of 400 mW (26.02 dBm) and 85 mW, respectively, up to a transmission distance of 30 m. Moreover, through design optimization and careful PV cell adaptation, the current scheme has the potential to achieve high efficiency over even longer distances.
Funding
Ministry of Science and ICT, South Korea (No. 2022-0-00208).
Acknowledgments
This work was supported by the Institute of Information & Communications Technology Planning & Evaluation (IITP) grant funded by the Korea government (MSIT) (No. 2022-0-00208, Infrared long-range multi-devices wireless charging technology).
Disclosures
Jinyong Ha is the inventor of the patent-pending technologies described in this paper.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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