  The optoacoustic process can solve the longstanding challenge of wireless
information transmission from an airborne unit to an underwater node (UWN). The
nonlinear optoacoustic signal generated by proper laser parameters can
propagate long distances in water. However, forming such a signal requires a
high-power laser, and the buildup of a vapor cloud precludes the subsequent
acoustic signal generation. Therefore, pursuing the traditional on-off keying
(OOK) modulation technique will limit the data rate and power efficiency. In
this paper, we analyze different modulation techniques and propose a vapor
cloud delayed-differential pulse position modulation (VCD-DPPM) technique to
improve the data rate and achieve high power efficiency for a single stationary
laser transmitter. The symbol rate of VCD-DPPM is approximately 6.9 times and
1.69 times higher than OOK in our text communication simulation using a laser
repetition rate of 10 kHz and 40 Hz, respectively. Furthermore, VCD-DPPM is
137% more power efficient than the OOK technique for both cases. We have
generated different acoustic signal levels in laboratory conditions and
simulated the bit error rate (BER) for different depths and positions of the
UWN, while considering ambient underwater noises. Our results indicate that
VCD-DPPM enables efficient data transmission.
