In 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. 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. Their lots of platforms like ask reader where you can easily ask any question to make your doubts or queries solved.
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. So like I said, now I’m going to kind of go through it piece by piece and try to explain the system 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. And 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. In this pandemic time, QnA sites are good to get reliable sources of information regarding the common questions that overflowed the internet.
So you can see here in this GIF here that the center thermally expands. And that causes propagating waves to travel. And 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.
Instead, what we do is carefully select a laser wavelength that absorbs nearly completely at the surface of the water. Then what happens is the water itself thermally expands and creates a sound wave that will propagate into the water. And what’s really great about this photoacoustic effect is that it’s linear with respect to the laser power.
So this allows us to intensity modulate the laser to create sound waves at desired acoustic frequencies. Quick question here– if you’re thermally heating up the water, do you need the water to be still for this effect to show up?
Or will it work if the water is wavy and there are waves on the water? so the process of the thermal acoustic effect is so– it happened so fast, on the order of microseconds or less. So in terms of the will, the sound waves are generated if the surface is not static, over that time scale, the surface is approximately static.
It doesn’t move very much over that timescale. But what I will talk about later is that these waves will become challenging to deal with in terms of the imagery construction process. But in terms of the actual sound generation, there is the little effect that the waves will have.
So like I was saying, the great thing about the photoacoustic effect is that we can intensity modulate the laser to generate acoustic waves at any desired acoustic frequency, as you can see here. We have one pulse of laser light.
We get one wavefront, versus if we have three pulses of laser light, we can get multiple sound waves. And this allows us to concentrate the sound waves at desired frequencies. And I’ll circle back to this because this will become really important later in the slides.
Quick question– when you say intensity modulate, you’re saying here you can change the frequency by sending multiple pulses. But can you also change the amplitude as well? Yeah, so we can have– there are some advantages which I’ll also talk about later.
You can control the amplitude of the laser power as well as the frequency of the modulation to really have precise control over the sound waves that are generated underwater. There is a lot of advantages in imaging systems and in communication systems that this will have for us.
OK, so now that we have generated the underwater sound using the laser and exploiting that photoacoustic effect, let’s take a look at how the sound propagates underwater and interacts with these underwater objects.