Procedure 1 : Setting the interferometer
1) Setting with visible light
Reference (called also first) path (or beam) will be the path with the mirror which has the piezo control (see fig. 1).
First (mirrors,apertures,etc), second and ordinal numbers are defined in the light's propagation direction.
-Set the removable mirror (which is on a magnetic base so if you have to put it again it comes back to its initial position), the first PBS and waveplates roughly (cf. fig 1).
-Choose the height wanted for the beam with respect to camera height for example. You can use a post with a horizontal metallic rod to have the reference height. You can write a thin line on a white cardboard across the width to have another height reference (Write also a line longitudinally on the center of the card). The post with the rod shall be used to set height of the optical elements added after, meanwhile the card will be useful to see the red laser and control its height.
- Put the laser red source and block its position totally. It must not move after.
- Before PBS, put an aperture to work with a small beam. Alternatively, you can choose an iris aperture which does not totally close (so if you open the iris aperture you can come back to the reference settings defined by the aperture closed as much as possible).
- Lock aperture's position.
(1.3) Then, proceed optical element by optical element, in the order of light propagation :
- Set the PBS so it sends the reference beam alongside the optical table. To have a good alignment, screw a post on the same line (line drawn by the holes in the table) as far as possible and place the white card in front of it to see the beam and set alignment.
- To fix the eventual tilt, use two movable apertures placed on the line (or look manually with the white card in two different points on the line). Generally, red beam may be hard to observe due to optics not designed for visible wavelengths. In this case try to work in the dark room and after opening the aperture until it is visible.
- Add the mirror with its piezo and set alignment with the same procedure at the other side.
- Set mirrors of the other path with same procedure.
- Add the second BS, and set it in a way beams from both paths are superposed.
This step is critical, this BS has to have adjusters so that it can be rotated and translated (3 degrees of freedom) because we need to assure two beams intercepting at the same location on the BS and an orientation to obtain rigorous colinearity. Try to be very precise.
2) Setting with infrared light
- First, add an aperture on the path you want, far of the first one, centered on the red light beam. Now, both installed apertures will be used to align infrared beam.
- Set the infrared source and block its position. Put a lens with a short focal length to collimate the beam (with the goal to minimize energy losses). An infrared card is needed to see if the beam after the lens is not divergent and to set the system. Working in the dark is preferable to perform the adjustments with good conditions.
- Put the two mirrors which will serve to align infrared beam with the path of red light, as in fig 2.
- Now repeat the following procedure until it converges : (step 1) Aim the first aperture center with the first mirror
(step 2) Aim the second aperture center with the second mirror. It converges since distance between first mirror and first aperture is smaller than distance between second mirror and second aperture.
- Put the camera after the second PBS and get interference translating the piezo mirror with the manual actuator.
(2.2) Now the goal being is to have only one disc on the screen, meaning we have two colinears and superposed beams, the following step aims to do that.
- Rotate a little bit (it is very sensitive) the second BS searching for the optimum (bigger fringes that can be seen).
- Next, adjust carrefully the last mirror before the BS (on the second path), trying to have bigger fringes. You should not forget that fringes orientation indicates tilt orientation. You can try to set vertical direction and horizontal direction independently (firstly get totally vertical fringes and after that fix horizontal tilt for example)
If on the image, the spot of the second beam is getting away from the reference beam, it means beams are becoming collinear but less superposed. With the PBS only, superpose the second beam with the reference beam.
During this manipulation reference path should not be changed (if you do that it becomes very hard to go back to red light settings if you need it after).
Procedure 2 : making the microscope
- In our setup (see fig. 4) camera is mounted on a cage system with the tube lens on it and the lens for magnification of the reference beam.
- Block the second beam (placing an object on the path) and align the cage system with the reference beam. Then, mount an aperture on the extremity of the cage system and repeat the following procedure until it converges: (step 1) center the reference beam on the camera moving the post which carries the camera side (step 2) center the aperture with the beam moving the other post (other extremity). Direction where it has to be moved is visible closing the aperture as much as possible.
- Lock position of the cage system. Normally tube lens and the lens of the reference path are aligned and centered on the reference beam.
- Ensure taht the second lens will be approximatively at the same distance of the tube lens than the first one, and use a base mount with a micrometer screw.
- Take the mount of the second lens (the 'objective' ) and install an aperture on it to center the beam on it.
- Put the lens (or a longer focal length lens if it is needed for the first settings because it is easier), and with the Vernier micrometer find interferences in the image.
- Using the adjusters on the lens mount, try to have bigger fringes and finally rings.
- Due to the fact that the beam is a little bit diverging and it has not exactly the same profile everywhere on the path crossed (curvature can change a little bit, something we don't see normally but we can see with interferences), we see rings and not only one spot evidencing longitudinal interferences between two strictly identical beams (same wavefront). With the Vernier micrometer, adjust the distance objective-tube lens to get finally the largest possible interference rings. In practice one ring observed on the camera corresponds to acceptable conditions for measurements without complex procedure for phase reconstruction. At this stage, the microscope is ready for measurements (see fig.3 for schematic of the final setup)
Procedure 3 (optional) : calibration measurment for performance validation
A thin gold sample with modifications or defaults on it (typically local removal of the film) is good to verify the performance of the assembled system. Modifications can be measured with an AFM microscope, and knowing its refractive index, we can evaluate if consistent results are obtain by both methods.