Now let’s further suppose that the focusing beam enters your optical bench at some angle, and you are required to take this incoming beam and output a collimated beam that is parallel to the surface of the optical bench. This is the kind of thing that often occurs in real life, and it can be easily entered into ZEMAX if you follow a methodical approach.

The attached files (OAP_starter.ZMX and OAP_ starter.SES) give a starting point for the problem we are to solve.  Surfaces 1 through 9 represent the legacy optical system, which produces a beam that comes to a focus at f/8 at some angle relative to the optical bench. For the purposes of the current example, there’s no need to pay much attention to what is going on in surfaces 1—9, just realize that these surfaces produce a perfect focus at an angle to the optical bench.  For this example, we’ll assume that the optical bench is parallel to the X-Z plane, as shown in the layout picture below.

First, let’s add a parabola at Surface 11, located exactly one focal length (635mm) after the focal point.  Type in the values shown in the highlighted cells below:

It’s a good idea to keep an eye on the spot diagram as you make changes to a parabola, because it will give you an early warning if you make a mistake.  Remember: we’re producing a collimated beam from this parabola, so we’ll need to make a slight adjustment before we check the spot diagram.  Open up the System | General dialog box, and then click on the Aperture tab.  We need to set the system to make calculations based on “Afocal Image Space.”

A quick check of the spot diagram now shows that the parabola is working perfectly, with zero spot size. Now we want to move the parabola off-axis.  When we hit a parabola with a collimated beam (as is done in another Knowledge Base article) it’s a straightforward matter to simply decenter the optical axis before the parabola and then use a chief-ray solve after the parabola to tilt the optical axis around to follow the focused beam.  The idea is that when you “Decenter Y” before the parabola (to move the parabola off-axis), you will have to “Tilt About X” after the parabola (to get the optical axis to line up with the direction the beam is exiting).

Unfortunately, since our system starts with a focused point, we have to do things a little bit backwards.  But we do have a plan: we’ll put Coordinate Breaks around our parabola, and then we’ll make it so that there is a “Tilt About X” before the parabola (to get the parabola optical axis to line up with the direction the beam is entering) and then a “Decenter Y” after the parabola (to move the optical axis over to where the exiting beam is). 

Add a Coordinate Break before the parabola surface (to tilt the parabola) and another one after (to decenter the axis):

Now let’s put a Chief Ray Solve on the Decenter Y of Surface 13, and then we’ll add a little bit of Tilt About X to Surface 11:

First thing we notice is that the Chief Ray Solve on Surface 13 has no effect.  We expect the optical axis to have to move over after hitting the tilted parabola.  That’s a very subtle clue that something is not quite right.  Because we were watching the Spot Diagram, we got a much less-subtle clue that something was very wrong:

The problem is that we need to make our Tilt About X before moving forward 635mm from the focus spot.  That way, the optical axis will change direction, and then the beam will travel 635mm.  Now when the parabola gets drawn, it will be centered someplace away from where the beam ends up.  This will have the effect of decentering the parabola.

So we need to go back and change the Thickness of Surface 10 to 0.00000 and change the Thickness of Surface 11 to 635mm:

Now we’re back on track.  The spot diagram looks perfect again.  However, our guess of 5 degrees for Tilt About X was not quite right, as the Decenter Y is not 190.5mm.  We could guess at different values for Tilt About X until the Decenter Y is exactly 190.5mm.  But this is Zemax: we don’t have to guess.  We can put the power of optimization to work for us.  Open the Merit Function Editor.  Add one operand: PMVA, Parameter Value (I’ve also added a comment line, but you don’t have to).  We want a target value of 190.5 for Parameter#2 on Surface#13, with a weight of 1:

Now we set the “Tilt About X” on Surface 11 to be a Variable:

And then we optimize (Tools | Optimization | Optimization | Automatic).  The Merit Function value should go all the way down to 0.0000000 in a matter of seconds.

The parabola is now decentered and tilted just right. 

We’ll add some thickness after Surface 13, so we can look at the collimated beam coming off the OAP:

And here’s our layout:

We have just one more thing to do: we need to make the output beam parallel to the optical bench (the X-Z plane).  To do this, we’re going to clock the parabola (Tilt About Z) until the beam has the correct Y-angle.