Hi ,
I want to monitor supercontinuum on OSA and for that purpose first I couple supercontinuum into SMF (PM-980) and other end to OSA. I am wondering since the loss is wavelength dependent and at shorter wavelengths we have high attenuation constant (~6dB/km), I could see red light in the generated supercontinuum but OSA does not show any IR wavelength and SC starts from 800nm to 1600nm. Is it because of high attenuation on those wavelengths or something else is going on. What type of fiber one should use in such situation.
Thanks
I think the PM-980 works up to 1550 nm and if the OSA can measure light at those wavelengths you should be good. So I expect the way you couple light into the fiber, can you tell us more about it? Is it coupled using a fiber or a lens?
The numerical aperture (NA) of the PM-980 is about 0.11 (depending of the supplier). You must fit that value with your lens to optimize the coupling, using the formula: NA = R/f, where R is the radius of your laser beam (diameter/2) and f is the focal length of your lens. Also the beam should be centered and perpendicular to the lens. If one side of your lens is flat you can use the reflection of the beam to align (the refection should overlap the incident beam). The fiber should be parallel to the laser beam. It is easier to couple shorter wavelength. So using the OSA or the red light at the end of the fiber using a white paper, oyou can optimize the output using your 3-axes stage. Once the shorter wavelength is optimized, you can optimize the longest wavelength coupling by slightly moving the lens away from the fiber (the focal length is longer for longer wavelength).
By the way, here is a trick to optimize coupling using the OSA. You can scan the wavelengths for only a single wavelength, for example from 900 nm to 900 nm. This allow to see in real time the variation of the power at this wavelength (while moving the 3-axes stage) instead of waiting the full scan and it is easier to visualize precisely an increase or decrease. I hope you understand what I mean!
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