Dear colleagues,
We have observed large fluctuations in laser power in our setup and traced them down to fluctuations in laser beam polarisation. Attached is a schematic of the relevant part of our setup. We have experienced large fluctuations in laser intensity at the sample (after the microscope objective). Attached is a plot of laser power measured at the sample plane. We have checked the laser itself and its output power is stable. We have checked also the laser power at the exit of the optical fibre collimator (before the polarising beamsplitter) at different time points (when the power after the objective was around its maximum and when its was around its minimum) and it remains stable as well. From that we concluded that it is the plane of the polarisation that is changing in time. We have then checked the laser alone and this time placed a polariser between the laser and the power-meter. The power is still stable, showing that the polarisation of the laser output is stable in time. So there's nothing wrong with the laser itself.
Do you have any idea what could be the source of those fluctuations? I know that reflection of laser beam back to the cavity can destabilise the laser, but can this make the plane of polariation rotate? And can it happen on such a slow timescale? Thanks for your answers.
How accurately is the fiber rotation adjusted to the laser polarisation? This has to be aligned in the very-few-degrees range to get a stable polarization (high PER) at the output. The PM-fiber is in fact a very thick retardation plate. One wave retardation between the two polarizations every ~2mm length. Small temperature changes influence the retardation and the polarization state if alignment is not precise or if there is excessive strain in the fiber ferrule from glueing.
As Alexander Dräbenstedt says, for high stability, it is necessary to align the input polarisation state with one of the PM fibre's polarisation eigenmodes. However, you observed the power at the sample plane varied by a factor 28 from 23.26 mW to 0.83 mW. This is not the result of a small angular alignment error at the laser.
The power from the laser is stable. For differential phase drift in the PM fibre to account for this, the fibre would need to be rotated very close to 45° to an incident linear polarisation state for this to be possible. Alternatively a circularly polarised laser output at the input to the PM fibre could explain the observations.
Can you confirm that the output of the laser is linearly polarised and aligned at least approximately with the PM fibre stress axis?
Can you confirm that the output of the fibre collimator is not only stable, but also linearly polarised? Does the power through a polariser fall close to zero when the polariser is rotated through 90°?
If we eliminate polarisation rotation between laser and beamsplitter as cause, I can only think that back-reflection might be responsible. Reflections at or beyond the rear surface of the quarter wave plate will be returned in the orthogonal polarisation state to the incident beam. Most of this should be reflected, rather than transmitted by the polarising beamsplitter, but some of the reflected power could reach the laser.
Are you using the standard Hubner fibre coupling accessory?
https://hubner-photonics.com/products/lasers/options-accessories/
There appears to be a gap of several mm between laser and mounting plate. Can you insert a polarising filter here to further suppress back reflections, and assess the impact on stability?
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