# Simple photodiode connected to lock-in amplifier

• baptiste
The phase shift between the A.O. modulator control signal and the reflected microwave amplitude is something that you would want to measure.f

#### baptiste

Hello.

I am currently modifying a semiconductor experiment. A laser beam is modulated with an acousto-optical modulator and illuminate the semiconductor sample. The sample is probed by microwaves. The reflected microwave signal go through a demodulator, and then in a lock-in amplifier. Then I am studying the phase shift between the A.O. modulator control signal and the reflected microwave amplitude.

My problem is presently to calibrate the system by measuring the phase shift introduced by the acousto-optical moculator.

My idea is to deviate a part of the modulated laser beam, to send it to a fast photodiode, and to inject the diode photocurrent into the current imput of the lock-in amplifier.

Do you think that this idea is good?
I am fearing some problems with the imput impedance of the lock-in amplifier or something like that...

Sincerely

Baptiste

As from a third person perspective, you approach is good. This falls into control engineering problem, if am right. For this, a negative feedback is needed.
Input impedance of lock in-amplifier may be considered. But, since you are experimenting, find the transfer function of open system(without acousto-optical modulator feedback signal). Figure in both input and required output of the system...Usually for control engineering problems, the neceassy condition is difference between output and input,..ie...error is zero.
I would like you to look at something like phase locked loop: https://en.wikipedia.org/wiki/Phase-locked_loop:

Dear Alpharup

I had a look to the wikipedia article.
Presently, I have no feedback from the acousto optical modulator, but I would like to build one (with a photodiode), which should mesure the phase shift introduced by the modulator (compared to the signal going out of the frequency générator, which control the modulator)
What is according to you the open system that I should measure?

I am in contact with a fast photodiode supplier, maybe they will give me tips about the way to use it.

Sincerely

Dear Alpharup

What is according to you the open system that I should measure?

Sincerely
A laser beam is modulated with an acousto-optical modulator and illuminate the semiconductor sample. The sample is probed by microwaves. The reflected microwave signal go through a demodulator, and then in a lock-in amplifier.
The input here is the electrical signal fed as input to the acoust-optical modulator. I do presume that electrical signals(going to acuoto-optical modulator) control the manner in which incoming laser is modulated.
The output here is the electrical output from the lock in amplifier.
You have both input and output for the system. This, I would assume as open system.