Lab 1 Paraxial Optics Lab in 106A

Look at paraxial optics rules Use a bi-convex singlet at 1:1 conjugates Do it double pass so can see image Lateral magnification Axial magnification See aberrations due to misalignment Measure optical thickness Placement between pupil and image

Test setup Use an autostigmatic microscope Produces “perfect” point source of light (stigmatic) Views reflected image on CCD camera Is calibrated in real units Autostigmatic mode Autocollimator mode

Hardware used

Details of set up Object will be point source projected by an autostigmatic microscope Lens will be a biconvex singlet used at roughly f/8 cone at 1:1 conjugates The image will be located at the center of a ball, or convex mirror, so the light will be reflected back into the microscope The video camera allows precise centering and focus of the return image The ball reflector can be tricky since a Cat’s eye reflection can be gotten off the surface. How can you know you are focused at the center of the ball? For those interested there is a Zemax file called Lab 1 1to1 relay with CB’s to permit tilts and decenters

Initial alignment Set the laser source to MAX so the light from the object is visible See that the light is going roughly through the center of the lens Find the focused spot and move the ball until the spot is centered Find the reflected light and move the ball until it goes through the lens Find the focused spot near the objective and move the ball until the spot returning overlaps the spot coming from the objective The return image should be visible on the video screen Move the ball axially to find best focus. The screen will be saturated Turn the intensity to MIN and continue the focus and centering Move the ball back until the spot is focused on the surface (Cat’s eye) Center the reference cross using the SET REF button

Initial alignment (con’t) Move the ball toward the lens until the spot is again focused Move the ball laterally to bring the spot on the reference cross Does this suggest how to measure a radius of curvature? Does the image look symmetrical? We’ll learn how to align things better later How sensitive is the adjustment of the ball? Touch the reflecting surface of the ball (lightly) What happens to the image? Clean the ball surface with alcohol and a swab (gently) Now we are ready for some measurements of image motion

Nominal optical setup Point source at object position Biconvex lens at roughly 1:1 conjugates (Thorlabs LB4941) BK7 Ball centered on image to act as a convex mirror to return light to detector

Image and wavefront metrics -1 -.5 0 .5 1 Defocus (mm)

Effect of lens decenter Decenter lens 0.05 mm in +Y direction Look at spot, now aberrated with astigmatism and coma Let Zemax move ball sideways so image at center of ball Look at spot again Moved lens .07 mm, then moved return mirror .135 mm to restore image .07/.135 ~ 200/380 or s/(s+s’)

Effect of lens tilt Tilt lens 5 degree in +alpha direction Look at spot, now aberrated with astigmatism and coma Let Zemax move ball sideways so image at center of ball Look at spot again Tilted lens 5 degrees, then moved ball -.06 mm to re-optimize image Tilt has less effect but alignment doesn’t work well to restore image

Effect of adding plane parallel window Total track increases by 1.87 mm, while first order (n – 1)t/n = 2.16 mm However, window adds spherical aberration that affects focus Spot before window after Wavefront before after

Effect of tilting window 15 degrees Wavefront no longer symmetric Image shows astigmatism Ball moved 1.063 mm near to theory but not much bigger

Effect of thin poor quality window Insert a microscope slide into the beam in place of the window. The slide in a poor quality window Move the window by hand laterally. Position the window close to the lens and close to the ball. What is the difference in effect on the image?

Conclusion Record your results in your lab book Photograph some of the test setups for later viewing When finished, return the setup to its original condition so the next group is set to go right from the beginning If there is not time to finish all the suggested experiments it is better to do and record what you can in a professional manner rather than rush through the experiments The goal is to learn, have fun while doing so and be proud of your work