Alan Stummer
Research Lab Technologist

ALPS2 - AOM Laser Power Stabilization revisited

Warning:  This project will probably never happen.  The original ALPS is working well.

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I am curious who uses what.  Are these webpages a waste of time, or are they any help to others?  Are the circuits, software and utilities appearing in other labs?  Please send your comments or suggestions or what you have used (or not) or schematics of your version or pictures or anything!   Email me, or be creative and send a postcard! I want to hear from the vacuum! Links

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, or anything.

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.

Overview

Started July 2009 for Karl Pirch in Joseph's lab.  This is yet another an update of the original ALPS with the latest requirements.  This project controls the laser power delivered to the experiment.  Contrary to its lumpy namesake, ALPS also removes peaks and valleys from the beam.  A laser, or part of a beam, is sent to an AOM.  The AOM's "walking wave" diffracts part of the beam out at an angle - the 1 st order beam.  The amount of light diffracted is roughly proportional to the RF power, creating our working output.  A photodiode taps a bit of this light.  The amplitude of the RF is adjusted by the ALPS such that the desired power is obtained.  To reduce effects of upstream polarization changes, a polarizer may be inserted after the AOM but before the photodiode and experiment.  Other than the original specs, the new requirements are:
  • Differential inputs from both the photodiode (PD) and control input to reduce ground loops.  No common mode range specified.
  • Output range 0V to +5V into 50 Ohms (50 Ohm source impedance).  The user must be aware of the RF amplifier's damage threshold.
  • Output voltage limiting, set by trimpot.
  • TTL input (opto-isolated) to select an output hold mode.  This will use analog switches to disconnect the inputs (input impedance changes are ignored) and freeze the integrator (drift <20%/minunte).
  • Internal P and I gain controls, front panel overall gain control.
  • Front panel switch so user can leave off the saturation alarm.
  • Front panel switch for external/local reference and reference pot.
  • Supply range is 12-35V with power LED.

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