how to decide fiber length in Delayed self-heterodyne method of laser linewidth measurement.Some authors suggest 6X coherence length and some 10X coherence length and some even more... How to choose the optimal fiber length for getting best results?
Note that self-heterodyne linewidth measurements are not recommended for measuring linewidths of order 100kHz at many wavelengths 'typical' diode wavelengths. See the paper by Bennetts et al (External cavity diode lasers with 5kHz linewidth and 200nm tuning range at 1.55μm and methods for linewidth measurement) for a discussion of the issues (cost, length of fibre required).
We circumvented some of these issues (e.g. $10k worth of SM fibre) in my paper on laser linewidths: we use the technique described in reference [41] but a 2km length of cheap telecom single mode fibre (multimode at 780nm) which gives a bandwidth limit of about 25kHz (bandwidth = c / ( pi * L)). The polarisation wanders around a bit so use a polarisation paddle or similar to control the wander.
A better technique is to use a high-finesse Fabry-Perot cavity, which you can find lots of papers on, and can build for a few thousand dollars.
The technique you choose will depending on your wavelength and expected linewidth.
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