In the multimode option of the Models 15XX and 14XX, how is the light coupled from the fiber to the detector?
In order to eliminate overfilling the 25-µm diameter detector, the multimode versions use a GRIN lens at the end. The GRIN lens has a broadband AR-coating to reduce back-reflection. For the detector models that cover the 400-1650 nm wavelength range, the multimode option decreases the usable wavelength range to 550-1330 nm. Due to chromatic aberrations in the lens, wavelengths shorter than 550 nm will focus in front of the photodiode overfilling the active area. Wavelengths longer than 1330 nm will focus behind the photodiode also overfilling the active area. Overfilling the active area creates longer tails for pulsed applications and reduces the overall frequency response.
