PID Controller for Laser Power Stabilization

[@wadhesh]

Hi,

Can somebody help me to know how to control output power of the laser using PID. My setup has HWP attached to KCube motor and Linear polarizer. 

[jjohn1]

Are you trying to stabilize the output from a laser or just set the final output power?  If it is the latter, you can just calibrate it and rotate the waveplate without using PID.

[@wadhesh]

I did the calibration and setup my Set point voltage to the middle of the linear regime of the Sinosoidal curve then I removed PID from labview program. Now the error is simply difference between SP and PV. But I can still see the final output is not stable.

[@wadhesh]

Hi jjohn1,

I did what you suggested and ran the labview program overnight to see how it affects. But the problem is as it is.

I think I did not elaborate what actually I am doing. Basically I am trying to model simple version of Labview program to monitor the final output of the laser after passing through waveplate and linear polarizer. The figure is attached. 

Here is the explanation "Using a waveplate and linear polarizer, I'm attempting to stabilize the laser power output. After hitting the beamsplitter, the beam is sent into detectors in two different directions. let's say 1 and 2. Theoretically, a stabilizing beam in direction 1 must fix the beam in direction 2. However, I have observed that when the output is low or high in one direction, say 1, it acts as feedback to the waveplate, causing it to rotate in that direction. Instead of oscillating continuously to operate as a stabilizer, it follows the direction of the waveplate". How can I solve this?

[jjohn1]

I am not sure there is enough information to answer the question.

What kind of laser are you using?

Is the laser output polarization stable and linear going into the wave plate?

Are you sure the retarder is half wave and not some other value?

A stray back reflection from the wave plate could affect the laser stability, and would change if it is tilted in the rotation mount.

Are you possibly rotating the wave plate in the wrong direction?

From the rise time specification, you probably have about 20MHz detector bandwidth.

I am not sure how fast your control loop is, but you might want to low pass filter the detector.

[@wadhesh]

My bad I forgot to mentioned it.

The laser I'm currently using is 5KHz femtosecond pulsed laser and shining it on the linear polarizer first to get stable polarization output which then pass through the Half wave plate (1100-2000 nm range as I am using beam of 1800 nm). Since it is a pulse laser so I don't think back reflection cause anything.

[@wadhesh]

The full description is here.

[Kyle97330]

I'm not sure you want to be using a PID loop here.  PID loops are usually meant for situations where there is a significant time delay between a change in the input variable and the reaction from the output variable.  For instance, trying to achieve a stable temperature using a TEC.  Photodiodes have a near-instant response so unless your HWP motion is very imprecise it seems unlikely that you would really need this.

It sounds more to me like you would want to create a loop that does the following:

1. Move the motor to a set point.

2. Measure the laser output.

3. Move again to a different point.

4. Measure again.

5. Use the two measurements to establish a slope.

6. Calculate where your target is on the slope, move there.

7. Measure again.

8. If close enough stop.  If not, add this point to the slope calculations, and go back to step 6.

If your measurements are nonlinear you might use a curve fitting rather than a slope after you get 3+ points.

[@wadhesh]

Hi,

Yes, you are correct. I am doing what you have mentioned.

My data follows y=a*sin^2(2x) equation and from this I took several points which are in the linear regime of this curve (MATLAB curve fitting).

After calculating the slope, I set my desired output at π/8, which is the center of the linear regime (0° is min and π/4 is max).

SP=m*π/8+c, (m is slope here);

PV=m*θ+c;

From this, I calculated the angle θ=(PV-SP)/m+π/8.

Now I'm telling my motor stage tied to HWP to rotate to that angle so I can have a stable output.

But the issue is that Arms 1 and 2 do not appear to communicate. Stabilizing Arm 1 does not steady the output of Arm 2.

There is no PID in my current setup. It's a Servo motor. Apologize for not correcting that in my previous drawing.

[jjohn1]

Transmission through a polarizer follows Malus' law T=T0 cos²(theta) (where is your case the half wave plate doubles the rotation rate).  Maybe the sin² should be a cos²?

Off topic:

You do not say what kind of laser you are using.  A fiber laser? A chip laser?  Some pulsed chip lasers can be destroyed by seemingly minor back reflections.  I usually just move the optics further from the laser and tilt them slightly.