Visibility oscillation in a multimode laser interferometer signal and its use in optimizing path lengths.

Area of Science:

• Optical Physics
• Interferometry
• Laser Metrology

Background:

• Interference signal visibility (V) in multimode laser interferometers is affected by optical path length differences (Z(S)) and spectral characteristics (k(S)).
• Traditional visibility (V) is unreliable for quantifying path length differences due to laser variations and system inefficiencies.
• Accurate optical path length measurement is crucial for systems using long optical fibers, where spatial measurements are insufficient.

Purpose of the Study:

• To derive and validate a new parameter (W) for reliably quantifying optical path length differences in multimode laser interferometers.
• To demonstrate the utility of parameter W for path length balancing, particularly in systems with long single-mode optical fibers.
• To compare theoretical calculations with empirical measurements of W using different HeNe lasers.

Main Methods:

• Derivation of interference visibility (V) as a function of optical path length difference (Z(S)) and spectral separation (k(S)).
• Definition of parameter W as the ratio of maximum to minimum V variations with k(S) for a given Z(S).
• Experimental validation using two HeNe lasers, one with known and one with unknown gain profiles, in a heterodyne interferometer.

Main Results:

• Parameter W is shown to be an intrinsic laser property, insensitive to configurational details, making it reliable for path length balancing.
• Empirical determination of W from high-bandwidth waveform records in a heterodyne setup, even with spontaneously varying k(S).
• Qualitative agreement between theoretical and experimental W values validates the technique for diverse multimode laser systems.

Conclusions:

• Parameter W offers a robust solution for accurate optical path length difference measurement and balancing in interferometers.
• The developed method is particularly advantageous for applications involving long optical fibers where precise path length control is critical.
• The technique is validated across different laser types, demonstrating its broad applicability in optical metrology.