Reduction of modal noise due to fiber/photodetector misalignment in SSMF links based on 850 nm VCSELs through a low-cost polymer coupling structure

A low-cost polymer-based structure is proposed to improve the coupling between a fiber end section and photodetector active surface in optical links based on standard single-mode fiber (SSMF), which employs vertical cavity surface emitting lasers operating at 850 nm, i.e., below the SSMF cutoff wavelength. Considering receivers as small-area detectors, which are generally necessary to guarantee high-speed operation but at the same time are particularly subject to power fluctuations due to modal noise (whose impact is in turn enhanced in the presence of fiber-to-photodetector misalignment), significant achievements are demonstrated by employing the presented structure. Indeed, in the presence of a misalignment of ±4 to ±6 µm, which is nowadays typically achievable, the relative optical power fluctuations due to modal noise reduce in the presented case more than four times (2.5% from more than 10%) with respect to the case of butt-coupling, which implies an increase of the same factor in the output signal-to-noise ratio at the receiver end.