1. ECEN 4616/5616 Optoelectronic Design
Class website with past lectures, various files, and assignments:
(The first assignment will be posted here on 1/22)
To view video recordings of past lectures, go to:
and select “course login” from the upper right corner of the page.
Lecture #26: 3/14/14

2. Light Field Microscope
The light field capture concept would seem to be of great interest to microscopists, as it would allow post-capture exploration of the depth details of the object. The extreme loss of resolution, however, has prevented the technique from being used as more than just a demonstration.
Scanning Light Field Microscope:
Here is a schematic of a confocal scanning microscope:
Pixelated Detector
The pinhole could also be a lenslet. The main idea is that, at each scan position, the lightfield for that position is detected instead of just the total light. The resolution needed would not be great – perhaps a 10x10 photodiode array could be used which would be readable at thousands of frames/second, so would not slow the scan too much.
Object is scanned in x-y
Instead of an image a one depth, the entire light field is captured over the scan.

4. Scanning Through a Lens:
(The Scan Relay)
An angular scan can be generated by a motor-driven mirrors. A common type is called a “galvanometer mirror”:

5. Scanning Through a Lens:
(The Scan Relay)
The two-mirror system works OK, as long as the scan doesn’t have to go through a remote aperture, such as the entrance pupil of a microscope objective. Just using a free-space mirror setup results in significant beam traversing on the aperture, and consequently does not allow the use of the full aperture or the full resolution of the objective:
Special lenses (with their own terminology) are designed to work with just this kind of free-space scan generation method.
High-quality microscope lenses, however, are not designed to work this way and would suffer significant loss of resolution if a pupil-traversing scan were used.

6. Scanning Through a Lens:
(The Scan Relay)
The solution is to use a scan relay:
This allows the beam to fill the entire entrance pupil. The objective can therefore operate at its full design F/# and resolution.

7. Scanning Through a Lens:
(The Scan Relay)
Scan relays can be chained:
The X-scanning mirror is relayed onto the Y-scanning mirror, and the resulting x-y scan is relayed into the entrance pupil of the objective:
X-scan
Both sides show the same scan direction – we’ll correct this when we put it in Zemax.
Y-scan

8. Scanning Through a Lens:
(The Scan Relay)
A compact X-Y scan relay:
Concave mirrors are used to relay the first scan mirror onto the second.
Lenses are used to relay the resultant x-y scan into the objective lens entrance pupil.

9. Scanning Through a Lens:
(The Scan Relay)
Scanned Source and Image
Un-Scanned source
Objective
Scanner
Light Source
De-Scanned Image
Detector
There is an advantage to using the scanner both ways – the return signal is “De-Scanned” and hence is stationary regardless of where the scanned illumination is on the sample.

11. Designing a Scan Relay in Zemax
We want to scan through a 10X microscope objective. We would like to scan a collimated beam, so we decide to use an “infinity-corrected” objective. This is an objective designed for producing an image at infinity, instead of at the end of the microscope tube like a standard objective.
(Of course you could just re-focus a standard objective to produce an image at infinity, but microscope objectives are so specialized and highly optimized that better results are had with an objective designed for each specific task.)
Observer
Eyepiece:
Standard objective
Object
Exit Pupil of Objective
Camera, focused at infinity:
∞ corrected objective

12. Designing a Scan Relay in Zemax
To get the optical parameters, we look at a high-end, 10X, infinity-corrected objective -- Edmund’s , selling for $795.00:
A Gaussian layout shows how to calculate the exit pupil size and max scan angle from the data sheet:
Type
Plan APO
Magnification
10X
Numerical Aperture NA
0.28
Working Distance (mm)
33.5
Focal Length FL (mm) 20
Resolving Power (μm) 1
Depth of Focus (μm)
3.6
Field of View, 24 Diameter Field Eyepiece (mm)
2.4 f θ h ϕ
FOV
So, we will plan to scan an 11mm dia beam ± 3o .

13. Designing a Scan Relay in Zemax
We’ll try to make the scan relay from achromats. We arbitrarily pick 25 mm focal length, to minimize the space taken by the scan relays.
Edmund’s #45174 looks like a likely candidate:
#45876 was identified as a “NIR” achromat on it’s layout screen, so we bypassed that.

14. Designing a Scan Relay in Zemax
We let Zemax insert the lens for us. We place a stop surface in front of the lens, where the scanning mirror will eventually go, and use quick focus to focus the lens.
We also set the aperture to ‘Entrance Pupil Diameter” = 11mm, the wavelength to the F,d,C lines, and the fields to one on-axis and one at 3.5 degrees:

15. Designing a Scan Relay in Zemax
Looking at just the Chief Rays, we realize that we need to put the stop at the front focal point, so that the Chief Ray of the off-axis beam will be parallel to the axis behind the lens. Thisk will keep the relay symmetric.
How do we decide when it is parallel?
Poking around in the Merit Function help window, we find an operand that measures a ray’s angle with respect to the axis. This should tell us when the ray is parallel.

16. Designing a Scan Relay in Zemax
We’ll use the “Universal Plot” feature to find the right standoff distance for the mirror. This is accessed through “Analysis – Universal Plot – Universal Plot 1D” from the Zemax menu along the top of the main window.
The Universal Plot function allows us to plot almost any construction variable against any Merit Operand (or the entire Merit Function).
We set the window up to plot the standoff distance of the scanning mirror against the angle of the off-axis chief ray.
Explain the ‘Hx, Hy’, and ‘Px, Py’ values: The H’s refer to the normalized field coordinates and the P’s the ray’s coordinates in the entrance pupil. The Chief Ray always goes through the center of the pupil (and hence stop).
We expect the values (in radians) to be fairly small, so putting a ‘0’ for ‘Max plot value’ is a way of letting Zemax decide the plot scale.

17. Designing a Scan Relay in Zemax
The plot clearly indicates a best position:
In this case, the answer is mm.
Zooming in on the plot and hovering the cursor at the minimum lets the position indicator on the top of the window tell you where the thickness value should be:

18. Designing a Scan Relay in Zemax
The Layout looks OK:
But, the MTF and Chromatic Focal Shift plots show that this lens isn’t going to work. We will need to optimize it.

19. Designing a Scan Relay in Zemax
We make the thicknesses and curvatures variable:
And, we add operands for Chromatic Focal Shift (ACXL), parallel Chief ray after the lens (RANG), and focal length (EFFL) to the Merit Function Editor:

20. Designing a Scan Relay in Zemax
We then install the Default Merit Function, with constraints on glass and air thicknesses. After some experimentation with the relative weights on our own operands compared to the DMS, we settle on a DMS weight of 100.
How do you know you have the right weights? Basic balancing of the % of the MF from each component, plus trial and error. ‘F3’ (undo) is a useful shortcut key to know here.

21. Designing a Scan Relay in Zemax
After optimization, the lens looks like this:
But, the MTF is still unacceptable, for use with a microscope.
The Chromatic focal shift looks better:

22. Designing a Scan Relay in Zemax
One last option (before we change our requirements, or give up on doublets) is to let Zemax substitute other glasses.
We make use of a newer Zemax feature called the “Glass Substitution Template”. This is accessed from “Tools – Design – Glass Substitution Template”.
This automates the restriction of the acceptable glasses, and even makes it easy for you to build your own glass catalogs. The options are the ones used in the example (on page 40) of “Getting Started with Zemax”, a very useful reference.
Unfortunately, this produced negligible improvement.

23. Designing a Scan Relay in Zemax
We have to conclude that using a doublet for meeting the design requirements doesn’t look promising. Our options are to add an element to the design and try to find a triplet that works, or to reduce the requirements.
If we reduce the requirements to an entrance pupil of 3mm diameter, and re-optimize, we find a reasonable solution. Further experimentation shows that we can even increase the scan angle to 10 degrees with good results:
This wouldn’t work for the original purpose (scanning through a 10X microscope objective), but would be useful for other scan jobs; Perhaps scanning through a stereo microscope, which has a much longer working distance and operates at smaller NA than the standard microscope objective.

24. Designing a Scan Relay in Zemax
The other option, adding an element, does produce a lens with reasonable properties for our initial requirements:
Symmetry is somewhat surprising, given that the image and object distances are anything but symmetric, but should work well in the overall relay, which is symmetric.
The symmetry of the design comes only from the optimizer – it was not constrained. Some variants even have the same first and last glass types.

25. Designing a Scan Relay in Zemax
I’ll use the first design (with doublets and a reduced Entrance Pupil) for the rest of the demonstration, as the fields don’t overlap so much and are more easily seen. The proceedure would be exactly the same with the triplet.
First, we add a second lens to make a complete relay stage. Notice that the second lens is reversed, which can be done manually, or by using the ‘Tools – Modify – Reverse Elements’ tool. Regardless, inspect the LDE to make sure everything came out correctly:
Also, we are only displaying the central and edge scan fields, for clarity.

26. Designing a Scan Relay in Zemax
The stop position (surface #1) is going to be where our first scan mirror goes, so ask Zemax to add a fold mirror at that position:
(We get this dialog box from ‘Tools – Modify – Add Fold Mirror’.)
You have to switch to ‘3D layout’, which looks like this:
For some reason, Zemax reverses L and R in the 3D layout. You can fix this by rotating 180 degrees about the Y-axis in the ‘Settings’ dialog of the window.
And the LDE looks like this:

27. Designing a Scan Relay in Zemax
Add another surface to the front of the system, with some arbitrary distance. Then the layout looks like this (after rotating and re-setting the ‘first surface’ to 1 in the settings tab of the window):
However, we would like to have the scan initiated at the mirror location, rather than magically appearing out of space. How do we do that?

28. Designing a Scan Relay in Zemax
First, we remove all but the on-axis field:
Then we change the surface type of the mirror to ‘Tilted’. Notice that this type has parameters for X and Y tilts, in terms of tangents.
I’ve given the mirror some Semi-Diameter larger than the beam, so it can be seen.
The system now looks like this:

29. Designing a Scan Relay in Zemax
We open the Multi-Configuration Editor (MCE), add a second configuration (from the edit tab). We add an operand which is the 2nd parameter of surface 3 (the scan mirror surface), and add the tangents for ±5 degrees:
How do we know that the Y-tangent column is Parameter #2? Just click in that column (in the LDE) in a row where it is unused:
Why 5 degrees, not 10?

30. Designing a Scan Relay in Zemax
If we set the 3D Layout window to show ‘All Configurations’, and select ‘Color rays by Configuration’ in the ‘Settings’ menu, we see this:
The scan is now being generated by the motion of the mirror.
And being relayed to the location of the next mirror.

31. Designing a Scan Relay in Zemax
To add the next relay, we start by copying and pasting these surfaces onto the end of the LDE:

32. Designing a Scan Relay in Zemax
At first, we get some weird results:
(It’s a good idea to leave the ‘Delete Vignetted’ box in ‘Settings’ unchecked, so you can see where things are going wrong.)
After we change the copied ‘Coordinate Break’ surface to a ‘Standard’ surface, things look better:
Again, we’ve made the surface (#12) at the first relay larger so it can be easily seen, in anticipation of making it the next scan mirror:

33. Designing a Scan Relay in Zemax
We add another fold mirror, this time a 90 degree Y-tilt (the first was in the X-direction):

34. Designing a Scan Relay in Zemax
The results are a little harder to visualize, but you can by playing around with the rotation settings (or use the arrow and page up and page down keys).

35. Designing a Scan Relay in Zemax
We change the second mirror surface to type ‘Tilted’:
And add another operand and two more configurations to the MCE, so we can excerise both the X and Y scans independently:

36. Designing a Scan Relay in Zemax
The result is complicated to display, but we now have a Y-scan mirror relayed to an X-scan mirror, relayed to an Exit Pupil, which can be the Entrance Pupil of the optical system we want to scan through.