Today we will be using an optical raytracing program, Zeemax, in conjunction with optical breadboards, a laser, and lenses to learn more about optics.

When working on the breadboards, DO NOT LOOK INTO THE LASER BEAM. These HeNe lasers have low power, but you should be careful. There will likely be stray reflections bouncing around, but the power in stray beams will be very low. The lasers will be aligned with the rail on the breadboard, so you do not have to touch the laser.

When using Zemax, DO NOT SAVE ANY CHANGES.

Introduction to Zemax: Zemax is a ray tracing program. That means that it takes individual rays fo light and propagates them through an optical system (lens, mirrors, apertures [holes], and obscurations [solid blocks]) according to geometrical optics. Two things to remember from geometrical optics: ø_i=ø_r for reflection and n₁ sin ø₁ =n₂ sin ø₂ for refraction. We will be dealing with refraction at lenses.

To start learning Zemax, we're going to work through a short setup together. Your computers should already show a Zemax setup with three windows. The top one is where you enter the optics description. This window is much like a spreadsheet. Remember that the first column is the surface number. If you look at surface 1, you will see the Thickness (column 5) is 20 mm (All units are in mm). This means that there are 20 mm between surface 1 and surface 2. Surface 1 is actually a 25 mm diameter circular hole, but don't worry about how to set that right now. The lower left window shows a 2D layout of your 3D optical system. You can see a vertical black line with a horizontal blue line coming out of it. The black line is your 25 mm aperture seen edge on and the blue line is a single light ray traveling 20 mm to the end of your system. The lower right diagram, the spot diagram, shows the location of all the rays Zemax is following at a particular place in the system. Once we get started, these two windows will be more interesting. At the top of the screen are some pull-down menus and some buttons that access Zemax features.

Click on the Gen button at the top left of the screen. In the window that pops up, enter 12.5 for the Aperture Value. Click on the layout window and notice that you now see three lines traveling horizontally across the window. You have set the aperture to 12.5 mm and this is reflected in the size of bundle of rays Zemax is now tracing.

Since the breadboarding will be done with HeNe lasers, we want to the wavelength of light in Zemax to be the same wavelength as the HeNe's, 0.6328 µm. Click on the Wav button near the top left and enter 0.6328 as wavelength number 1.

We're now going to insert a lens. Click on the Len button near the top right. Find lens number 45098 and click on it so it's highlighted. The information in the window tells you the lens has a focal length of 25mm and a diameter of 25 mm. Click on Append near the bottom of the lens pop-up window. In the dialog box that appears, you want to have the lens append after surface 2, click OK, and Exit from the lens window. You should now have more entries in your description window and you should see a that the parallel lines in the layout window now enter into a lens (but don't leave).

Look at the description window. Since Zemax just deals with surfaces, the lens takes two rows of the spreadsheet: one for each surface. The fourth column is Radius, which tells you the radius of curvature of that surface. Only one surface of the system is curved; most of the others are flat (Radius = INFINITY). The sixth column is Glass. Most of the time our system is in air, but the lens is made out of SF5 glass. Zemax has a catalog of glasses that includes the index of refraction for each glass. The seventh column is Semi-diameter, wich is a way of saying radius that won't be confused with radius of curvature.

Here are some last general tips: Highlight surface 4 (it should say Sto instead of having a number) where it says Standard . Now delete this surface by hitting the delete key which is on the right side of the keyboard. Note that the insert key is just above it if you ever want to insert a surface. By clicking the left mouse button in one of the lower windows, you activate it. If the window is already activated, you can draw a box using the left mouse button to zoom in on a region. On the bar for the window is Unzoom. If you click with the right button on the lower windows, you bring up a dialog box for that window that let's you change the wavelength shown, the field shown, and other parameters for that window.

EXERCISES:

We now want to bring the rays to a focus. Inside the lens, you can see the rays starting to converge. With a longer distance after the lens, they should come to a focus. Click in Thickness for surface 3 so that it is highlighted. What should we enter?

Bring up the Image analysis window by hitting Cntrl-J. This should show a grid pattern. Change the focus a bit and see what effect it has on the grid. Can you get the entire grid in good focus? Note the spot size as you change focus. You will have to read the units on the window, which can be hard. You might want to make it full screen to read it and then shrink it back again.

Make the lens go out of focus by doubling the distance after its back surface. Make note of the current lens properties. Now, change the properties of the lens such that you bring the light back into focus. Make the image grid as sharp as you can. Go back to the original lens properties and change something else.

Add some extra wavelengths. Add in 0.45, 0.55, and 0.75 µm by hitting the Wav button and typing in the numbers in wavelengths 2, 3, and 4. Go back to the image, spot, and layout diagrams and show these other wavelengths (use the right button in the windows). What do you notice? Why does this happen?

Go to the File menu and open setup 1 (without saving any changes!). This reloads the

original lens #45098 (which is over on the breadboards). Add an additional lens to the end of the system that will give you collimated light (parallel rays) with the same size beam.

Reload setup 1 again and this time make a beam expander. Note the relationship between the focal lengths of the two lenses, their distance apart, and the size of the input and output beams.

If there is still time, play with the surface marked Coord Break. In particular click on its column 9 (should be called de-center Y). Make this value 2 mm. Can you describe what this did? You might want to reduce the size of your aperture (Gen button).

If there is still time, get someone to load in a sample program for you like the Hubble space

telescope or a Cooke lens. Remember never to save your changes!

BREADBOARD OPTICS: The laser beams should be aligned with the rails on the breadboards and they should be producing pretty good collimated beams. Try not to touch the lasers. When you have to touch them, for example turning them off, be careful. Don't worry too much, though. If the laser gets bumped, you get to re-align it.

Put a mask in the collimated beam and note what the pattern looks like on the card. Does it matter where the mask is inserted? Why is this?

From the lens kit, put the 45098 lens in a holder. You should wear a cloth glove when handling the lens to avoid putting smudges on it. With the curved side of the lens facing the laser, use the lens to bring the light to a focus. What is distance from the lens to the focus? There is a nice way of telling when you are at a sharp focus called the knife-edge test. You slowly insert a card into the beam near focus and watch the spot. Watch what happens when the card is upstream of the focus vs when the card is downstream of the focus. What do you notice? Now insert the card right at the focus. The spot should blink out suddenly. Can you draw a sketch of the light rays to explain what's going on?

Insert the mask at various places and notice what happens. What does this tell you about magnification and distance from the focus? What happens when the mask is in the collimated beam? Do you notice a change in the focused light?

Now choose a second lens from the lens kit such that you can produce a collimated beam from the two lenses. What is the focal lengths of the two lenses? What is the distance between the two lenses? What is the ratio of the beam sizes? What happens if you move the second lens closer to or further from the first one? What happens when you insert the mask in various places?