Hi Yusof, Thanks for the question. The OSA uses a grating-based monochromator which has a fairly long open-beam path. So there is definitely transmission through air in the instrument. This can be noticed molecular absorption lines near 1400nm wavelength, especially under humid ambient conditions.
Thanks. Now it make sense as I noticed there were some absorption near 1400 nm band.
Could you please let me know how long those open-beam path? This information is very crucial to me as I'm developing humidity sensor currently. I try to look in the datasheet but could not find that information.