Quadrature Detector using Variable Attenuator Plate and Circular Polarizer Sheets. Optical Layout (Left), Parts (Middle), Typical Assembly (Right)
Quad-Sin-Cos Decoder Parts List
Prt# Qty Description ---------------------------------------------------------------------------- AP1 1 Variable attenuator plate used as adjustable NPBS CP1 1 Pieces of Circular Polarizer (CP) sheet (or cut into two pieces) CS1 4 Microscope cover slips to attach CP pieces (round or square) PD1 1 Photodiode for Sin/A channel PD2 1 Photodiode for Cos/B channel R1 1 Bias protect resistor C1 1 Bias bypass capacitor PCB1 1 Quad Decoder mounting PCB (optional) STB1 1 4 pole screw terminal block
The photo shows the typical optical parts and photodiodes.
The kit will include 1 or 2 ~1x1" pieces of CP that can be cut as needed since the beam is likely to have a small diameter. If mounted directly to the photodiodes, only two ~4x4 mm pieces are required. There is protective film on both sides of the CP sheet which must be carefully peeled off of the pieces actually used since it messes with the polarization. The side of the CP with the QWP has an adhesive on it. This can be used to stick a piece directly to the A Channel PD. Adding a bit of adhesive around the edges is still recommended. The piece for the B Channel can be attached to its PD with a bit of clear adhesive or adhesive only on the edges, and its QWP-side should be protected with a cover slip, piece thereof, or a drop of clear adhesive so that it doesn't collect crud. Confirm that whatever adhesive is used doesn't itself mess with the polarization! This doesn't matter between the CP and PD, but would interfere with the proper behavior for the input to the QWP. Make sure the cut sides are the same orientation as they were originally and parallel to the X and Y axes of the interferometer.
The Attenuator Plate (AP) is used as a Non-Polarizing Beam-Splitter (NPBS). For best results, it should be oriented so that the incident beam is near-normal to minimize the effects of it being close to the Brewster angle, which would result in the X and Y polarized components being very unequal. It could also be rotated to be at 45 degrees with respect to them but that might be tricky to mount. Orienting it approximately as drawn should be acceptable.
Mounting may be tricky in any event unless just stuck down with hot-melt glue. Ideally, the AP should be able to be moved along the screw-axis to adjust the split ratio to be 1:1, though based on experience, the best setting is probably at the location where it is most dense, and trim-pots for the Sin/Cos input circuit can compensate for unequal PD sensitivities. One simple approach is to solder the PDs into a small piece of prototyping and add loops of thick wires to act as a guide for the AP. Then add the hot-melt once the split ratio has been set. For something more sophisticated, a 3-D printer may come in handy. ;-) Note that the glass in the attenuator plate is angled so the reflected beam is directed slightly up or down, which needs to be taken into account in the mounting of the PD for the reflected beam (B Channel). Or, the glass plate can be removed from the plastic frame and mounted however you like.
Once the parts are secured, a polarized laser or one with a separate polarizer set at 45 degrees in front of it can be used to adjust the AP position and the PD sensitivity. If the the ratio cannot be set to 1:1 even at the highest reflectance-end of the AP, fine tune it using the load resistors/trim-pots. Going further will require the actual outputs of the interferometer to confirm quadrature operation.
The right-most photo, above, shows a possible serving suggestion. ;-) It uses a simple custom PCB for mounting the PDs and bias resistor, and has a 4 pole screw terminal block for wire connections. A 4 pin header can also be used (not included but available). The LP is a piece of CP stuck to the PD in the foreground using the adhesive already on its QWP-side. The QWP is a piece of CP attached to the other PD using 5-minute Epoxy on the LP-side. An additional drop of Epoxy can be put on the QWP-side to cover its adhesive and prevent it from collecting crud. Its behavior is virtually identical to that of an NPBSC and pair of Thorlabs DET110s (at least at low freqencies), at less than 1/100th the cost. But a cover would be worthwhile to block ambient light from hitting the PDs.
Quad-Sin-Cos Interface to Atmega 328P Nano 3.0/µMD0
Here is the very long detailed parts list for driving a TTL-compatible device like µMD0. :) The numbering refers to schematic above:
Prt# Description Comments ----------------------------------------------------------------------- U1 LM393P, 8 pin DIP Dual voltage comparator C1 0.1 µF capacitor AC bypass for PDs C2 0.1 µF capacitor Power supply bypass for U1 LED1 LED, red Thresholded Sin, A LED2 LED, HB green Thresholded Sin, B R0 Resistor, 1K, 1/4W Photodiode protection R1 Trip-pot, 100K Sin (Channel A) load resistor* R2 Trim-pot, 10K Sin (Channel A) threshold R3 Resistor, 100K Sin (Channel A) trim-pot isolation R4 Resistor, 470M, 1/4W Sin (Channel A) hysteresis R5 Resistor, 2K, 1/4W Red LED current limiting R6 Resistor, 1K, 1/4W Channel A pullup R7 Trim-pot, 100K Cos (Channel B) load resistor* R8 Trim-pot, 10K Cos (Channel B) threshold R9 Resistor, 100K, 1/4W Cos (Channel B) trim-pot isolation R10 Resistor, 470M, 1/4W Cos (Channel B) hysteresis R11 Resistor, 47K, 1/4W Green LED current limiting R12 Resistor, 1K, 1/4W Channel B pullup
R1 and R6 can serve as load resistors for the photodiodes (as shown) if bandwidth is not critical, or as the feedback resistors for proper trans-impedance amplifiers. R0 is simply to protect the PDs should they be installed backwards, and the power supply should a PD fail shorted.
Most of these parts are also included in the kit for µMD0, which includes a solderless breadboard as well. A similar circuit on a solderless breadboard is shown below.
This version doesn't have the two trim-pots R1 and R7. To make space for them, the other parts can be moved 4 holes to the right.
And here is a version to interface directly to the Teensy used in µMD2:
Note the 3.3 V source from the Teensy for the output pullups. This is critical to the health of the Teensy. Higher voltage (i.e., 5 V) may damage it.
And finally here is a version to generate RS422 for use with SG-µMD2 or other compatible device:
Additional parts:
Prt# Description Comments ----------------------------------------------------------------------- U2 UA9638, 8 pin DIP Dual RS422 line driver C3 0.1 µF capacitor Power supply bypass for U2
The first is/was used by Teletrac for their single frequency laser-based systems.
Or to download the PDF: Teletrac 150 Optical Receiver 1 Schematic.
In addition to the Sin and Cos channels, it also has an Int (intensity) channel which adjust the offset to accomodate varying signal levels.
The second is a circuit for which I am contemplating designing a PCB.
Or to download the PDF: Sam's Quadrature Interface 2.
It's a combination of a dual trans-impedance pre-amp, gain stage, threshold to convert to Sin-A-B, and RS422 line drivers.