So now I am very confused because as I mentioned the output current of my photodiode is too large when compared to the laser input and I cannot figure out why that is the case.
When I turn on the LASER and connect a multimeter across the PD the output is about 2mA and as I try and move the PD around it reaches 7mA. Which doesn't make any sense.

 

This is the photodiode output across a 1K Standard Resistor. The light were turned off and the photodiode was covered. I am not sure what this current could be.

 

Hi sorry to bother you all but this large output current has me confused. I don't understand why is it 6mA when LASER power is 1.5mW and responsivity of PD is 1.03A/W for my wavelength.
Can someone explain what am I missing here?

 

That does seem an awful lot of current. What do you get at the output of the circuit with no light on the photodiode?

The circuit you're using is very simple, and the only reasons I can think of for anomalous results would be either a bad connection somewhere, a wrong connection, or a defective component.

 

That does seem an awful lot of current. What do you get at the output of the circuit with no light on the photodiode?

The circuit you're using is very simple, and the only reasons I can think of for anomalous results would be either a bad connection somewhere, a wrong connection, or a defective component.

I am not sure(as I do not have the data on me now) but this 6mA current is with the laser measured with a multimeter across the photodiode output.
The laser is coupled to a collimator.

 

I am not sure(as Ido not have the data on me now) but this 6mA current is with the laser measured with a multimeter across the photodiode output.

That doesn't tell me much. Is that with the photodiode alone, not connected in the circuit?

The laser is coupled to a collimator.

I don't see how that's relevant here.

 

Yes it is just photodiode alone across the multimeter to measure the current.

 

Also moving the when I move the photodiode even a little bit the current changes which is again strange.
I thought maybe the beam is not collimated and the total light on active area keeps changing.

 

Sounds to me like your laser is more powerful than advertised, or the photodiode is more sensitive.

 

Why does the pic in post #22 look like 50 Hz to me?

All sorts of wierd things can happen with I-V converters. I never did high speed, but i did a 4 terminal one with bias and current supression over 4 ranges (+-100 mA max).

 

Sounds to me like your laser is more powerful than advertised, or the photodiode is more sensitive.

I am not sure what my next step should be also the output current changes as I move the PD does it mean that the laser diameter is bigger than the active area?

I am stuck and not sure what my next step should be...

 

I don't know either; while I've worked with photodiodes before, I've had very little experience with lasers.

 

Can you try putting a series resistor in the + and - leads of the OP-AMP. It can be up to about 100 ohms to 10K, but the higher the resistance the higher the noise. They should at least be equal.

Also reading the datasheet, your feedback cap is lower than the suggested feedback cap.

I didn't quite understand what the behavior of the circuit was with a resistor.

What about lead dress, connections and power supply? i.e. the construction technique. My application was for solar cells and chopped monochomatic light and scanned laser light. One design oscillated like crazy with large area cells.

 

Right now I am stuck with the photodiode and laser as their performance seems very weird.
The laser is Thorlabs LPS-1550-FC where output optical power is 1.5mW and as I mentioned the photodiode is FGA21 the current output of photodiode is about which is way too large and does not make any sense to me.
I am not sure what I am missing here.

 

UPDATE:

I now am able to observe fringes from the photodiode. The problem seem to have been with alignment and collimation(I am not sure what the problem was). As I increased the distance from laser source I could get this output on exciting one mirror.

The feedback capacitance for TIA is 20pf and gain is 4.7KOhm.

I am still not sure why the output current of the photodiode is so large.
If I am correct the output current is : Photodiode Sensitivity* Photodiode Active area*laser intensity

 

If I do not have any feedback capacitance there seem to be these oscillations.


I connected a 200pF feedback capacitor but I still observe these oscillations. Could this be some type of noise?