Fig. 1. Expected transfer efficiencies of wireless power charge technologies as a function of transfer distance.

Fig. 2. Structure of OWPT.

Fig. 3. (a) Block diagram and (b) photograph of the experimental setup for underwater OWPT. The transmitter is located at the left side in the block diagram, whereas it is at the right side in the photograph.

Fig. 4. Optical output power and E/O conversion efficiency of the LD.

Fig. 5. Voltage–current graph of the PD by changing the load resistance when the LD is operating at the most efficient condition.

Fig. 6. Voltage–current graph of the solar cell by changing the load resistance when the LD is operating at the most efficient condition.

Fig. 7. E/O conversion efficiency of the LD, O/E conversion efficiency of the PD, and the total back-to-back transfer efficiency as a function of LD optical power.

Fig. 8. E/O conversion efficiency of the LD, O/E conversion efficiency of the solar cell, and the total back-to-back transfer efficiency as a function of LD optical power.

Fig. 9. Transfer efficiencies of the underwater OWPT as a function of transfer distance. The dashed line is the analytic graph of the sea water with the attenuation parameter of 0.69  m−1.

Fig. 10. Expected transfer efficiency of the seawater OWPT as a function of transfer distance by using the measured attenuation parameter.