ABSTRACT
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 $ \pm 4 $±4 to $ \pm 6\;{\unicode{x00B5}{\rm m}} $±6µm, 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.
ABSTRACT
Direct modulation of a laser source is often utilized in realizing optical fiber connections where the cost of the entire system must be kept at a low level. An undesired consequence of this choice is the onset of the laser frequency chirp effect, which is detrimental in the case of either digital or analog links, and must be evaluated with precision in order to perform an accurate design of the whole system. Various methods of evaluation of the chirp parameters have been proposed, and the choice among them is typically made on the basis of the laboratory equipment available at the moment. This paper adds a further element to the set of possible choices, since it presents a method for the evaluation of the adiabatic chirp factor in distributed feedback (DFB) laser sources, which exploits a simple interferometric scheme, guarantees low cost, and shows, at the same time, good accuracy of the results.
