Underwater communications in the ocean between vessels such as ships and submarines have traditionally relied on sound waves since light does not travel far in salt water. But sound waves are limited in how much information they can send.
“The emerging consensus is to use very high-power lasers to produce beams powerful enough so that they can get from source to target, even with the high losses associated with salt water,” said Associate Professor Siddharth Ramachandran (ECE). Light-based underwater links of this sort would enable a slew of applications that require high-bandwidth data transfer whether for civilian or military applications, he added.
The communications directorate of the Office of Naval Research (ONR) is interested in Ramachandran’s work and recently gave him $1.22 million to support a two-and-a-half-year project, “High-Power Blue-Green Lasers for Communications,” that will focus on studying and developing novel nonlinear processes in fibers that enable light generation through intermodal nonlinearities. The hope is that this will pave the way toward high-power, all fiber, blue-green lasers.
Making lasers in the blue-green range of the spectrum is already a challenge, with semiconductor technology just beginning to address that problem at low power levels. Traditional fiber lasers can achieve much higher power levels, but currently there are no high-power fiber lasers that can emit blue or green light without additional costly components. That’s where Ramachandran comes in.
Such lasers would not only have to be high-power but also have the capability to tune, on demand, from blue to green. This is because deep seas appear blue in color while more shallow waters have a turquoise or green tinge. These lasers would need to be able adjust to the color changes as marine vessels navigate different parts of the ocean.
Ramachandran’s discovery that higher-order spatial modes in a fiber are, in fact, more stable than the conventional mode, enables a variety of nonlinear mixing processes to be exploited. This additional design capability would allow the use of nonlinear optics inside fibers to generate high-power laser beams in colors not achievable by today’s technology.