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Distributed Laser Charging: A Wireless Power Transfer Approach
Qingqing Zhang, Wen Fang, Qingwen Liu, Jun Wu, Pengfei Xia, Liuqing Yang
TL;DR
Existing wireless charging technologies face limitations in delivering Watt-level power over meter-level distances to IoT and mobile devices. This paper develops a modular distributed laser charging model, validates linear conversion approximations, and derives closed-form maximum transmission efficiency. The resulting efficiency varies with supply power, wavelength, distance, and PV-cell temperature, providing theoretical insight and practical design guidance.
Problem
Traditional WPT technologies face difficulty providing Watt-level power over meter-level distances for IoT and mobile devices.
Method
The paper models DLC as separate electricity-to-laser, laser-transmission, and laser-to-electricity modules, using analytical formulations and validated linear conversion approximations.
Results
The paper derives maximum DLC power transmission efficiency in closed-form and illustrates its dependence on supply power, laser wavelength, transmission distance, and PV-cell temperature.
Takeaways & Limitations
The modular analysis provides theoretical insight into DLC and guidelines for practical system design and deployment.
Takeaways & Limitations
Several important issues remain unaddressed because of space limitations and are left for future work.
Abstract
from arXiv · showhide
Wireless power transfer (WPT) is a promising solution to provide convenient and perpetual energy supplies to electronics. Traditional WPT technologies face the challenge of providing Watt-level power over meter-level distance for Internet of Things (IoT) and mobile devices, such as sensors, controllers, smart-phones, laptops, etc.. Distributed laser charging (DLC), a new WPT alternative, has the potential to solve these problems and enable WPT with the similar experience as WiFi communications. In this paper, we present a multi-module DLC system model, in order to illustrate its physical fundamentals and mathematical formula. This analytical modeling enables the evaluation of power conversion or transmission for each individual module, considering the impacts of laser wavelength, transmission attenuation and photovoltaic-cell (PV-cell) temperature. Based on the linear approximation of electricity-to-laser and laser-to-electricity power conversion validated by measurement and simulation, we derive the maximum power transmission efficiency in closed-form. Thus, we demonstrate the variation of the maximum power transmission efficiency depending on the supply power at the transmitter, laser wavelength, transmission distance, and PV-cell temperature. Similar to the maximization of information transmission capacity in wireless information transfer (WIT), the maximization of the power transmission efficiency is equally important in WPT. Therefore, this work not only provides the insight of DLC in theory, but also offers the guideline of DLC system design in practice.
I. INTRODUCTION
Distributed laser charging addresses the difficulty of delivering sufficient wireless power over long distances to IoT and mobile devices. The paper introduces modular analytical modeling to evaluate conversion and transmission efficiency and guide DLC design.
- Traditional WPT technologies struggle to provide sufficient power over long distances for safely charging IoT and mobile devices.Inductive and magnetic-resonance coupling have short ranges, while electromagnetic radiation has low efficiency and safety constraints.
- DLC can transfer 2-Watt power over a 5-meter distance and supports self-aligning charging without specific device positioning.
- Line-of-sight blocking can stop DLC wireless power transfer immediately, while small receivers and ceiling-mounted transmitters support practical deployment.
- DLC can charge multiple devices simultaneously and provide a WiFi-like wireless charging experience for IoT and mobile devices.
- The paper separates DLC into conceptually independent modules to analyze wavelength, attenuation, and PV-cell temperature effects on power transmission efficiency.It derives maximum power transmission efficiency in closed-form from the modular analysis.
- The proposed model describes module-level power relationships and supplies practical design guidance for DLC implementation.
II. DLC SYSTEM
DLC uses distributed resonating laser components separated between transmitter and receiver to provide self-aligned, intrinsically safe wireless power transfer. Its system model represents electricity-to-laser conversion, laser transmission, and laser-to-electricity conversion as separate modules.
- DLC separates optical components between transmitter and receiver, forming a distributed rather than integrated resonating laser system.
- The transmitter uses a fully reflective mirror and gain medium, while the receiver uses a partially reflective mirror and PV-panel.The mirrors and gain medium form a resonant cavity; photons passing through the receiver mirror form the external-cavity laser.
- DLC self-aligns along the line of sight without requiring specific positioning or tracking.
- Objects blocking the line of sight can stop the intra-cavity laser immediately, providing an intrinsic safety mechanism.
- The system model decomposes wireless power transfer into electricity-to-laser conversion, laser transmission, and laser-to-electricity conversion.
- A. Electricity-to-Laser Conversion: Laser generation requires the stimulating current to exceed a threshold, and electricity-to-laser efficiency is then evaluated from the supply-to-laser relationship.
B. Laser Transmission
The laser-transmission module models power attenuation through air as a function of distance and atmospheric conditions. Its efficiency approaches 100% at negligible distance and depends on wavelength through the attenuation coefficient.
- Laser power decreases during transmission through air, with attenuation depending on transmission distance and air quality.The model assumes a constant laser diameter, achievable by controlling transmitter and receiver aperture diameters.
- Laser transmission efficiency is modeled using received power, attenuation coefficient, and transmission distance.
- When transmission distance approaches zero, laser transmission efficiency approaches 100% and received laser power approximates transmitted laser power.
- The attenuation coefficient depends on wavelength, visibility, and scattering-particle size distribution.
C. Laser-to-Electricity Conversion
The DLC receiver models laser-to-electricity conversion with a single-diode PV-panel model, relating received laser power to voltage, current, output power, and efficiency.
- PV-panel model: A single-diode equivalent circuit models the PV-panel's laser-to-electricity conversion.The panel converts received laser power Pr into electrical output power Po with efficiency ηle.
- PV-panel model: The PV-panel output voltage Vo and current Io are characterized through the diode model.The model uses the PV-panel short-circuit current, saturation current, and thermal voltage.
- PV-panel model: The thermal voltage Vm depends on the PV-panel ideality factor, Boltzmann constant, and absolute PV-cell temperature.These parameters determine the thermal-voltage term in the diode equation.
- Conversion efficiency: The output power Po is obtained from Io and Vo, and ηle depends on Po and received laser power Pr.Thus, the PV-panel conversion stage maps received laser power to electrical output and its conversion efficiency.
D. DLC Power Transmission Efficiency
The paper combines the modeled DLC modules into an overall power-transmission efficiency from transmitter supply power to receiver output power.
- System efficiency: The overall DLC power-transmission efficiency is derived from the analytical models of the individual modules.The efficiency spans the transmitter, laser transmission, and receiver conversion stages.
- System efficiency: Table I lists the transmission or conversion efficiency of each DLC module and the complete system.The table organizes the component efficiencies used to evaluate end-to-end power transfer.
- Evaluation: The numerical evaluation follows the analytical derivation of the system efficiency.The paper next evaluates the modeled DLC system quantitatively.
IV. NUMERICAL EVALUATION
The numerical evaluation examines how laser wavelength, transmission attenuation, and PV-cell temperature affect individual modules and overall DLC efficiency.
- Evaluation scope: The DLC system efficiency varies with laser wavelength, transmission attenuation, and PV-cell temperature.These factors are evaluated for both individual modules and the overall system.
- Evaluation scope: The evaluation studies the impacts of these factors on each DLC module's performance.The analysis separates module-level effects before considering overall system behavior.
- Evaluation method: The numerical evaluation is implemented in MATLAB and Simulink.These tools are used to evaluate the analytical DLC system model.
A. Electricity-to-Laser Conversion
The electricity-to-laser stage is evaluated using measured 810nm and 1550nm laser systems and a linear supply-power approximation. The approximation fits the measured operating range, while conversion efficiency rises after threshold and reaches wavelength-dependent plateaus.
- Measurements: Measured supply power, laser power, stimulating current, and stimulating voltage are evaluated for 810nm and 1550nm laser systems.The measurements cover laser systems operating around 800–820nm and 1540–1560nm.
- Linear approximation: The relationship between laser power Pl and supply power Ps is approximated with a linear formula.The approximation is used to model electricity-to-laser power conversion.
- Linear approximation: The fitted curves match measurements well within the given supply-power and laser-power ranges.This validates the linear approximation over the evaluated operating ranges.
- Conversion efficiency: Electricity-to-laser conversion efficiency ηel begins increasing at the supply-power threshold associated with the laser current threshold.The efficiency then approaches a plateau as supply power increases.
- Conversion efficiency: The ηel plateau is around 43% for 810nm and 31% for 1550nm.The 810nm plateau is higher than the 1550nm plateau.
B. Laser Transmission
Laser transmission efficiency decays exponentially with distance and depends on visibility and wavelength. Shorter-wavelength 810nm transmission attenuates more than 1550nm in clear air and haze, while fog removes the wavelength dependence described here.
- Transmission model: Transmission attenuation is modeled for clear air, haze, and fog using visibility-dependent scattering parameters.The visibility ranges and scattering-particle distribution are specified for the three scenarios.
- Distance and visibility effects: Laser transmission efficiency decays exponentially toward zero as distance increases.For a fixed wavelength, attenuation increases as visibility decreases.
- Wavelength effects: In fog, attenuation is independent of wavelength because the coefficient α has the same value for 810nm and 1550nm.The fog case uses ρ = 0 for both wavelengths.
C. Laser-to-Electricity Conversion
The PV receiver converts received laser power into electrical output whose current, voltage, and power depend on wavelength, input power, and cell temperature. Maximum power occurs at a unique operating point and is reduced by higher temperature, with stronger temperature impact at 1550nm.
- PV conversion model: PV-panel output current, voltage, and power are evaluated for 810nm and 1550nm laser inputs using a standard solar-cell model.The analysis considers laser power and PV-cell temperature as operating variables.
- Maximum power point: For a given received laser power, the maximum output power occurs at a unique maximum power point (MPP).For example, at Pr = 10W, the MPP is 4.64W for 1550nm.
- Electrical characteristics: Output current remains nearly constant below the MPP voltage, then drops rapidly above it; increasing received laser power raises current at a given voltage.Output power rises with voltage up to the MPP and then falls dramatically above the corresponding MPP voltage.
- Temperature effects: Higher PV-cell temperature lowers output power and shifts the MPP upward as temperature declines.The temperature comparison uses 0°C, 25°C, and 50°C at Pr = 10W for both wavelengths.
- Analytical approximation: Linear curve fitting approximates maximum output power as a function of received laser power, and the fitted lines match the MPP points closely.The approximation is used to relate received laser power to the PV-panel’s maximum output power.
- Conversion efficiency: Maximum laser-to-electricity efficiency is lower at higher cell temperature, with temperature having a larger impact at 1550nm than at 810nm.The efficiency trends are shown against received laser power for both wavelengths.
D. DLC Power Transmission Efficiency
The paper combines electricity-to-laser conversion, atmospheric transmission, and laser-to-electricity conversion into a closed-form DLC efficiency model. Efficiency rises with supply power before reaching a plateau, decreases with distance, and varies with wavelength, temperature, and transmission efficiency.
- System model: The model derives the relationship between transmitter supply power Ps and receiver maximum output power Pm by combining the three DLC conversion and transmission stages.The relationship is obtained from the electricity-to-laser, laser-transmission, and laser-to-electricity models.
- Supply-power dependence: Maximum power transmission efficiency ηom initially increases with supply power Ps and then reaches a plateau.This behavior is shown for both 810nm and 1550nm under multiple cell temperatures and transmission efficiencies.
- Distance and environment: At Ps = 40W, ηom decreases as transmission distance increases under clear air, haze, and fog conditions.The distance comparisons also evaluate wavelength and PV-cell temperature.
- System design: Maximum power transmission efficiency ηom increases linearly with laser transmission efficiency ηlt.The relationship is presented as a design guideline for selecting wavelength and determining DLC coverage.
- System design: The numerical evaluation validates the analytical model and supports guidelines for laser-wavelength selection and DLC coverage determination.The paper frames coverage planning as minimizing the number of transmitters needed to cover a given area.
V. CONCLUSIONS
The paper develops distributed laser charging as a theoretically modeled and practically guided wireless power-transfer approach. It identifies unresolved issues involving photovoltaic efficiency, wavelength coverage, voltage conversion, multi-device charging, WPT networking, and future SWIPT applications.
- The paper provides theoretical insight and practical guidance for distributed laser charging system design and deployment.
- PV-panel efficiency is about 50%, motivating further study of panel types, efficiency analysis, and total DLC efficiency.
- Only 810nm and 1550nm laser wavelengths are considered, leaving wider wavelength ranges for future work.
- Future work should examine circuits that convert PV-panel output voltage and current to application-specific charging levels.
- Point-to-multiple-point charging requires further study of access protocols, scheduling algorithms, transmission factors, and adaptive battery-charging optimization.
- Because distributed laser charging naturally supports point-to-multiple-point transfer, WPT network architecture and operating protocols remain open research topics.
- Simultaneous wireless information and power transfer could exploit laser bandwidth for demanding IoT, mobile reality, and ultra-high-definition video applications.