Chirp Evaluation of 850-nm Single Mode VCSEL Exploiting Modal Noise in Standard Single Mode Fiber

The phenomenon of modal noise, which is typically undesired within fiber optic links, is exploited advantageously to characterize vertical cavity surface emitting lasers (VCSELs) operating in the first optical window (wavelength λ ∼ 850 nm), in terms of frequency chirping. The proposed method, which can be applied utilizing ordinary laboratory components and instrumentation, contributes to fill an existing gap. Indeed, the methods which are typically exploited to determine this undesired optical frequency modulation can be straightforwardly applied only to sources operating at wavelengths of 1310 and 1550 nm, while they become unpractical or even inapplicable at wavelengths of 850 nm. Through the proposed technique, the evaluation of chirp coefficient M and phase difference ψ between frequency and intensity modulation (or equivalently linewidth enhancement factor α and chirp scaling factor κ ) of VCSELs can be performed. This allows the correct design of various types of fiber optic links, making possible to take advantage of these low-cost and low-consumption optical sources.