Laser angle measurements

[mr_byte31]

Hi Folks,

I work on a small project using cheap laser modules and i would like to measure offset of photodiodes from center of laser light source. angle error shall be in range of 0.1 degree.

I prepared 2 figures to show the concept :

I have two photodiodes (red circles) 20cm far from each others and a laser source which has the power curve on the right side.


if the middle point of the two diodes are exactly on the center of the beam this mean they are centered in the beam.


if the two diodes are shifted upwards there exists angle between the center of the beam and middle point.



I have two ways in my mind to solve the problem :

• Measure the difference in received power for each diode
• Measure the difference in phase shift of each diode

Both techniques shall reflect the angle of shift. I don't think that measuring the power would result in good accuracy.
I believe the phase shift shall be a more accurate technique. I have to modulate the light source with Sine wave to be able to implement the measure phase difference.

I don't have good background in analog stuff and I am missing the components names that can help to measure the difference in phase shift between the photodiodes.

any recommendations ?

[PM3295]

Maybe a fast time-interval counter can work. One pulse edge from one PD to start the counter and the delayed edge on the other PD to stop the counter. From the count, the angle can be calculated.

[Catcatcat]

To begin with, pay attention to TDC7200PWR



PIC controllers have such a CTMU module, it can be used for Precision Time Measurement. (AN1375)

[mr_byte31]

measuring the time interval will not work !   I already thought of it. it will give very very low resolution.

Let me clarify with numbers:

min Distance = min Time x velocity of light

velocity of light = C = 3e8


assume TDC with resolution 30ps. This will give min distance of  30ps x 3e8 m/s = 9 cm.

This mean I can only measure multiple time of 9cm.