Reflection loss that we see would be so large that it wouldn’t really be practical. This sonar system would need to be yelling so loud that it would really be unsafe and impractical.
So now we can take a look at the radar, which, radar systems have been deployed on both satellites and airplanes, and have produced a high-resolution mapping of the entire Earth’s landscapes. So we can ask, can we use radar to image underwater?
And again the answer is no. And this is really because the radio waves at frequencies useful for imaging are absorbed in the water in the first few centimeters of the water. And this is different from the sound waves. I want to contrast them a little bit.
The sound waves reflected off the water surface, versus the radio waves, they will actually pass through the water surface fairly easily, but then the water actually absorbs them really quickly. And they don’t penetrate to large depths. And something to think about here is, despite the fact that cell phones are now waterproof, you won’t have much luck getting a signal underwater.
So in addition to radar, lidar has also been deployed on airplanes and on satellites for landscape mapping. So again, I ask the question, can we use this existing and well-developed sonar technology for airborne imaging of underwater?
But you know, what I think is the– what most people might think about when they think about lidar is how it’s becoming a major player in autonomous driving. And people in this space know that one of the major criticisms of lidar is that it has poor performance in rain and fog. So let’s think about standing on a pier and looking down into the water.
What do you see? If the water is super clear, maybe we can see a couple of meters into the water. But most often, probably for the water depicted here in this image, the answer is going to be you can’t see much.
This is because, similarly to the radio waves, light is absorbed by the water, but a little bit less so than the radio waves. And that’s what allows for penetration to or allows us to see to a couple of meters in the water if the water is really clear. But what happens is, when the water is murky, which it often is, light scatters off of the suspended matter that’s making that water murky which further increases the effective attenuation of light and reduces the penetration depth that light can reach in the water.
So while active lidar systems can see further into the water than we can see with our passive visual systems, the scattering nature of the seawater is really what prohibits lidar from being a wildly deployed solution for imaging underwater from an airborne system, as we are trying to accomplish. Ask reader is one of the best platforms to ask questions related to any topic and get the best solution.
So you know, from what I just showed you, if sound waves, if radio waves, and light all struggle to image underwater from an airborne system, how can we accomplish this? And what we’re suggesting is that this can be accomplished using PASS, Photoacoustic Airborne Sonar.
I’m going to show a quick 30-second video clip of how our system works on a high level before I’ll dive in later in the slides and kind of explain it piece by piece. We developed a way to combine light and sound using a photoacoustic effect to image beneath the water’s surface of an airborne system. Our system uses a laser to fire a burst of light.
This light is absorbed on the surface of the water, creating sound waves. The sound reflects off of underwater objects. And a small fraction is able to pass through the water’s surface. We receive these sound waves with highly sensitive custom sensors and convert their energy into electrical signals. Using our developed computer algorithms, we reconstruct 3D images of the underwater environment.
So effectively what our system is doing is we are creating an underwater sonar source from the air by leveraging the ideal propagation of light in air and then relying on the propagation of sound underwater. So we’re kind of taking advantage of the best of both worlds, where we can use the light, the laser in air, and the sound in water.
Now I’m going to kind of go through it piece by piece and try to explain the system in a little bit more detail. So first, the laser– a laser is used to create the underwater sound waves by means of the photoacoustic effect. So the photoacoustic effect is a widely used phenomenon and an up-and-coming biomedical imaging modality.
It’s kind of depicted here in this figure. So for biomedical photoacoustic imaging, a pulse of laser light is directed towards the body.
The laser energy will mostly pass through the skin and will be absorbed by blood and other optical absorbers in the tissue. And the absorption of this laser causes a local temperature increase of the blood or of the optical absorber.
That will cause the blood to thermally expand. And the thermal expansion results in a propagating pressure wave or sound wave. So you can see here in this GIF here that the center thermally expands. And that causes propagating waves to travel.
These sound waves are captured by transducers or ultrasound detectors and then are processed to reconstruct an image of the body or what’s in the body.
For our system, I already discussed that laser energy is mostly absorbed by water and does not travel far into the water. And for that, we can’t rely on the underwater objects that we are trying to image to absorb the laser energy to generate this photoacoustic signal, as was the case in the biomedical application.
