by Din » Thu Jan 06, 2022 10:33 am
Rotating the detector would detect no difference if the beam was polarised, since detector measure power, irrespective of polarisation.
I'm assuming the measurements were identical - the meter position (relative to mirrors), the input beam profile and mirror orientation were the same in both cases. I'm also assuming that the input beam is collimated. In this case, the only solution I can think of is that there is an aberration on the surface of the dielectric, causing the beam profile to change. Either the beam radius is decreasing, so that the aperture of the meter was not filled, or the beam radius is increasing, and there are stray reflections hitting the detector. Considering the increase in measured power, I would suspect the former. To test this, I'd put a card right up against the dielectric, and trace the beam profile as you move away from the mirror. if the beam radius is decreasing, it would explain your observation.
Joy thinks that the detector could be picking up something in the room. I would assume that if both measurements were taken under identical conditions, then this stray light would affect both mirrors the same way.
Rotating the detector would detect no difference if the beam was polarised, since detector measure power, irrespective of polarisation.
I'm assuming the measurements were identical - the meter position (relative to mirrors), the input beam profile and mirror orientation were the same in both cases. I'm also assuming that the input beam is collimated. In this case, the only solution I can think of is that there is an aberration on the surface of the dielectric, causing the beam profile to change. Either the beam radius is decreasing, so that the aperture of the meter was not filled, or the beam radius is increasing, and there are stray reflections hitting the detector. Considering the increase in measured power, I would suspect the former. To test this, I'd put a card right up against the dielectric, and trace the beam profile as you move away from the mirror. if the beam radius is decreasing, it would explain your observation.
Joy thinks that the detector could be picking up something in the room. I would assume that if both measurements were taken under identical conditions, then this stray light would affect both mirrors the same way.