# fringe locking

Discussion in 'Electronic Design' started by Níkola Heímpel, Sep 6, 2004.

1. ### Níkola HeímpelGuest

I have a Fabry-Perot interferometer and would like to lock the
interferometer cavity to one point of the transmission fringe. I
searched for basic informations on how to design the circuit for the
piezo voltage, but so far I haven't found something like "fringe locking
for dummies"...
I found a few descriptions (not very detailed though) of methods using
two photodiodes, one on a bright and one on a dark fringe, then
basically amplifying the difference and feeding it into the piezo
actuator. But is there also a way to do this with one diode? Or what
exactly would I have to look at if I wanted to design a circuit for two
derivative control, guessing I would need at least p. and i, but no idea
how to put this in electronics.
If you would know a webpage/book/paper or something to read on this
topic, that would also help me a lot.
Nikola

2. ### Tim WescottGuest

One diode would be subject to drift; two will drift together, and their
difference will be much less drifty. I would be inclined to set the two
diodes so they straddle a peak and lock the thing so the diode outputs
are equal.

The circuit below shows a basic PI compensator using an opamp. The
proportional gain = R2/R1, the integral gain = 1/(R2*C), and you want to
set R3=R1 to minimize your input offset voltage (unless you have a FET
input opamp). If you need to know this because of a brain fart then
proceed. If you need to know this because of a lack of op-amp theory
then get a good book, read it, and proceed.

R3
___
.-|___|-.
| |
=== |
GND | |\
'-|+\
___ | >--------o---o Vout
Vin o-|___|-----o----|-/ |
| |/ |
R1 | |
| |
| ___ || |
'-|___|---||-----'
||

R2 C

created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de