Final answer:
Intensity fluctuations in a Mach-Zehnder interferometer's reference arm might be caused by thermal fluctuations, environmental vibrations, changes in the multimode laser's coherence, misalignment of optics, or electronic noise in the detection system. All these factors need to be investigated and mitigated to ensure stable operation.
Step-by-step explanation:
Fluctuations in the intensity of the reference arm of a Mach-Zehnder interferometer, when all other variables are controlled and ambient light is accounted for, could stem from various sources. One possibility to consider is thermal fluctuations that affect the path length on a microscopic scale, leading to variations in interference. Another possibility could be vibrations or acoustic noise affecting the stability of the interferometer's components, especially if the system is not adequately isolated from environmental disturbances.
Moreover, even though the laser is multimode, the coherence length should be considered. Changes in the mode structure or power distribution between modes might cause fluctuation in the reference arm, even if the sample arm is blocked. It is also important to examine the alignment of the optics and the stability of all optical mounts and components, as slight misalignments can cause significant intensity variations due to the sensitivity of the interferometric setup.
Lastly, electronic noise or instability in the detection system, such as the photodetector or DAQ apparatus, may lead to apparent intensity fluctuations that are not actually present in the light path. Ensuring that all electrical connections are secure and noise sources are minimized is thus crucial for accurate measurements.