Write the scientific term for the parallel laser beam that interferes with the information-bearing rays reflected from an object in holography.
As we get started, let’s remind ourselves of just what holography is. When we take a picture of some object, we’re able to do that because light from the object is incident on our imaging surface. Our imager, our camera, consists of an array of pixels which measure the intensity of the light incident on each one. Now, according to this sketch, the way we’ve drawn the light coming off of our object, the tree, towards our imager, it looks as though the light as it travels from the object to our imager is moving in a straight line, array. And indeed, that is a good way to model the light as it moves.
But we know something else about light as well. Light shows the properties of a wave. This means it has a sinusoidally varying amplitude. And this amplitude repeats back on itself every wavelength of the wave. Now, it’s not that our camera-imaging device ignores the amplitude of the wave, indeed it’s that amplitude — which we can show here on our plot, the difference between the axis and either the maximum or the minimum value of the wave — that gets converted to an image intensity. In other words, it shows us what we see on this grid, the pixels of our image. But the fact that light can be modelled as a wave means that it has other properties in addition to its amplitude.
One of those properties is called its phase. This has to do with where along the wavelength we encounter this wave. Whether at the beginning, that would correspond to a phase angle of zero, or at the peak of phase angle of 90 degrees or back at the middle, 180 degrees, and so on. Imagine that this wave of light is moving left to right on to some vertical surface. When the wave reaches that surface, not only will it have a particular amplitude, it will have a particular phase. For example, our wave could meet the surface at this point at its phase angle of 360 degrees. Or it could meet the surface at this phase or at this phase or this one or really anyone in between the phases that we’ve drawn. Here’s why all this matters.
We know that when we look at a picture of something, it’s a two-dimensional representation of a three-dimensional object. But why is that? Why do we lose one dimension of information? Well, it has to do with the fact that our imager ignores the phase information in the light coming from the object it’s making an image of. If we could keep that information along with the intensity of the light, then we can make a three dimensional representation of a three dimensional object. And that is what the process of holography includes. We create an image, yes, but not just a two-dimensional image. Instead, we deliberately set up our imager so that we can keep the phase information of the light coming off of our object.
Here’s how that works. First, we start with the object we want to take a three-dimensional image of, in this case a stuffed teddy bear. Now, remember that if we just shine light on our object and then collect that light on an imaging plane, we will get a picture or an image of our object. But it will only be two-dimensional. It won’t have the phase information of the light rays added in. And when we say phase information, here’s what we mean. Let’s say that all the light that we use to illuminate our object comes from a coherent light source; that is, a laser. That means that all of the light that comes out of the laser is in phase. The various waves have the same exact phase relationship to one another. That is, the peaks and the troughs of every wave line up with one another.
When all that 3D light reaches our object, it interacts with the object depending on how far it has to travel. Maybe the belly of our teddy bear is closer to the source of light than any other point on the bear. So the light that gets reflected off that point of our object has a different phase relationship compared to the light that gets reflected off another part, say one of the paws of the bear. This means that after interacting with our object, these rays of light don’t have the same phase relationship with one another like they did before. They’re out of phase. But, we wonder how much has their phase changed? Well, with our current imaging set-up, we can’t tell. All we can tell is that when the rays reach our imaging plane, we’re able to measure their intensity.
But there’s a way to modify our set-up so that this phase information of the light reflected off of our object can be found out. What it will take to keep that phase information, not to lose it, is a reference. Say we do this. Say that into the path of our laser beam, we put what’s called a beam splitter. What a beam splitter does is it lets half of the light through. And then it sends the other half of the light at a 90-degree angle to that. By putting the beam splitter in the path of this laser beam then, what we’ve created is two separate paths of light. But they had the same source, the laser. All that’s happened is that we’ve put a bit of optic in the original path to create these two rays.
Now, one of these two rays, as we see, interacts with the object we want to make an image of. The other one, though, goes on, bounces off a mirror, and then is sent towards our imaging surface. Now, here’s the key to this whole process. At the imaging surface, the two separate beams that we’ve created by splitting the original beam come back together. And what’s more, one of those two beams which has bounced off of our object now has a different phase relationship than the other beam which has it. So then, at this imaging surface where the two beams meet, they interfere with one another. That means that the phase information of the beams combines and is recorded on this photographic plate.
So then, it’s not just the intensity of the light coming off of our object that we’re able to record, we’re also able to record the phase relationship of that light to a reference beam. And it’s through that interference that we’re able to develop a three-dimensional representation of our object, a hologram. We could say that the rays of light coming off of our object are the information-bearing rays. And the ones we compare them to are the reference rays or the reference beam. That is the scientific term for the parallel laser beam, located here and here, that interferes with our information-bearing rays, here, coming off of our object that help to create a hologram. We call that beam the reference beam.