NOTICE: This webpage and associated files is provided for reference only. This is not a kit site! It
is a collection of my work here at the
University of Toronto in the
Physics department. If you are considering using any schematics, designs, or anything else from here then be warned
that you had better know something of what you are about to do. No design is guaranteed in any way, including
workable schematic, board layout, HDL code, embedded software, user software, component selection, documentation, webpages,
All that said, if it says here it works then for me it worked. To make the project work may have involved undocumented
additions, changes, deletions, tweaks, tunings, alterations, modifications, adjustments, waving of a wand while wearing
a pointy black hat, appeals to electron deities and just plain doing whatever it takes to make the project work.
For Xingxing Xing ("Triple X") in Aephraim's lab. This project generates pairs of entangled 780nm photons, the rest is details.
There are five sections:
ECL Laser: A 780nm laser with temperature and current control.
Injection Laser: A780nm laser seeded by the ECL laser, with temperature and current control.
A rubidium reference to measure laser frequency and control a piezo that changes the ECL laser's external cavity resonance.
A 780nm piezo controlled up converter cavity with a BBO crystal, the power density is increased so that the crystal is
non-linear and two 780nm photons up convert to a single 390nm photon.
A 390nm piezo controlled cavity with a KD*P crystal used to down convert the 390nm photon to a pair of 780nm photons.
1. ECL Laser
The 780nm <50mW laser has both temperature and current controls. It is run in constant current mode. The
temperature is adjusted with a TEC (thermo-electric cooler, a "Peltier device"). It uses an external cavity,
see the next section, 3.
2. Injection Laser
The 780nm 80mW laser has both temperature and current controls. It is run in constant current mode. The
temperature is adjusted with a TEC (thermo-electric cooler, a "Peltier device").
3. Rb Reference and Piezo
A polarization adjustment configuration detects the peak of a Rb absorption line. Two optical outputs are detected.
The first beam goes directly through the cell and is detected. The second beam counter-propagates on
itself through the cell and with a polarizer, gives a differentiated output (maximum just before the peak wavelength,
minimum just after and middle amplitude either side). Using the first beam as a reference, the second is
subtracted and the resultant error signal passes through the virtual zero when exactly on frequency.
The error signal is split into two paths. A very low frequency filter sends a correction signal to the laser
temperature. A higher bandwidth filter passes the error signal to the piezo. Both the slow temperature correction
and the faster piezo correction add to keep the laser on frequency.
4. 780nm Up Converter Cavity
A BBO crystal is in the cavity to up convert pairs of 780nm photons into single 390nm photons. A photodiode
monitors leakage of an end mirror. A piezo adjusts cavity size. The cavity power is controlled
to a preset level by monitoring the photodiode and adjusting the piezo. The preset level is near the
maximum obtainable level in the cavity but slightly offset from the peak so as to provide a sloped transfer
curve between piezo voltage and photodiode level.
5. 390nm Down Converter Cavity
A KD*P crystal in the cavity down converts 390nm photons to pairs of 780nm photons. The cavity operates
in two modes: operate and calibrate. When operating, the piezo is frozen at the calibrated voltage
so that the cavity resonates at 390nm. Occasionally the cavity has to be calibrated. For calibration,
a 780nm beam is switched into the cavity, a photodiode monitors the cavity power while the piezo is adjusted.
When the photodiode has reached the peak, the piezo voltage is frozen, the shutter blocks off the
780nm calibration beam and normal operation resumes.