Interferometer
Introduction
Here is a presentation by Angie Bu on the state of the interferometer project at CMU as of Spring 2024.
As of May 2024, stepper V2 is aligned by hand or by stepper motor, with human feedback by looking at a microscope, giving microscale precision. In order to create larger circuits using more patterns, smaller feature size, and smaller patterns (in pixels, using the lower cost projector), stepper V3 needs nanoscale precision alignment, which will be supplied using a nanopositioner and displacement measurements from an interferometer. Here we implement a simple and inexpensive interferometer for displacement feedback:
Michelson Interferometer
The Michelson interferometer involves splitting the light source into two arms that travel different lengths, and recombining the two beams which interfere at the detector. We use planar mirrors instead of corner reflectors to ensure that the beam centers (which diverge) match and interfere as intended to produce fringes at the photodiode.
Littrow ECDL
The coherence length of an inexpensive diode laser is around 1mm which is insufficient for the range of motion of the stepper (~2cm). To increase this length for usable interferometry, the source for the interferometer will be an external cavity diode laser (ECDL) instead of just the diode. This is achieved by shining the laser onto a diffraction grating, where the wavelength is chosen by spatially filtering the angle of the reflected light using a pinhole. For simplicity and reducing points of failure, the laser is not tunable, which is one of the main disadvantages of the Littrow configuration.
Optical Isolation
For optical isolation of the interferometer output and the ECDL output, we use polarizers and 90 degree wave rotators (already an inexpensive alternative to half wave plates for rotating the polarization), which is inexpensive compared the setup using a quarter wave plates and polarizing beam splitters by over a thousand dollars. This setup prevents feedback of the mixed light back to the ECDL, and isolates the interferometer output at the detector.
Transimpedance Amplifier & Software Filtering
The small quantity of light is too small to produce a photodiode current, when connected to a resistor, that registers on an Arduino (0-5v), so we boost the dynamic range of the signal using an op amp connected in the simple transimpedance amplifier setup. Because the laser is being operated at low power for short time periods (<100ms) we do not implement precise temperature control. We use Arduino Nano for measurement and control.
Bill of Materials
Part name
Price ($)
Notes
Link
Photodiode
To receiver circuit
Depends on circuit implementation (PCB, soldered parts, breadboard, etc.)
Resistors 1, 10k ohms
To receiver circuit
“
Op amp
To receiver circuit
“
2.5mm screws and washers x 3
<1
Enclosure fastener
Digikey.com
Black plastic
Enclosure 3d printed
Total: $640
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