What is the difference between this "spectral width" and the laser linewidth (FWHM)? I am currently looking at the 1550nm fibre-coupled DFB laser diodes on Alibaba for use in interferometry experiments. These are commonly used in optical communications applications, but it seems to me that they would still be suitable for interferometry experiments. The problem is that they don't list a a linewidth (FWHM), which I want to know for my application. However, many of them do list "spectral width"/"spectrum width", $\Delta \lambda$, at -3db or -20db, in units of nanometers. This "spectral width" value seems to be close to what I would expect for linewidth (in terms of FWHM), but a bit higher. See this example:

What is the difference between this "spectral width" and the laser linewidth (FWHM)?
What information can I gain from this "spectral width" value, in terms of the linewidth of the laser? For instance, if "spectral width" is actually always greater than linewidth (FWHM), then, despite not knowing the linewidth (FWHM), can I at least assume that the linewidth (FWHM) of the laser is significantly lower than the "spectral width" (in the above case, significantly lower than 0.5 nm)?

This question is related, but I don't think it answers my question.
 A: 
many of them do list "spectral width"/"spectrum width", $\Delta \lambda$, at -3db or -20db, in units of nanometers.

A spectral width at -3 dB (half intensity) is exactly what a FWHM is.
Depending on the vendor and product, there might be a typical spectrum plot included in the datasheet that makes this more clear.

This "spectral width" value seems to be close to what I would expect for linewidth (in terms of FWHM), but a bit higher.

Remember that the maximum specification is a commitment from the vendor not to sell you any device with a wider spectral width. If their typical width is 0.5 nm but most of their customers don't require a spectral width narrower than 1 nm, it's better for them to specify 1 nm and not have to reject the occasional laser that produces 0.6 or 0.7 nm.
For customers who require a narrower spectral width, they may be able to produce a custom part number that is tested to tighter limits.
Also keep in mind that based on the notes after the table, these lasers are designed for cable TV distribution applications, not interferometry. And, given you're buying these on Alibaba, and the center wavelength uncertainty is extremely wide, these lasers may be rejected parts from a more tightly spec'ed product.
