Frequency tuning of long-wavelength VCSELs

Tuning properties of long-wavelength VCSELs have been studied experimentally, for the first time to our knowledge. Injection current and temperature tuning rates of two VCSELs operating near 1512 and 1577 nm have been measured using a Fabry-Perot etalon with free spectral range 0.056 cm−1. A 100-Hz saw-tooth modulation with depths of modulation of ∼10% or less was superimposed on a direct injection current (dc bias) to tune lasers in narrow spectral intervals (0.3-1.2 cm−1) around a central frequency set by the dc bias. The lasers have been found to be capable of being tuned faster at higher levels of dc bias. The enhancement factors were up to ∼2 and ∼3 for the 1512- and 1577-nm lasers, respectively, as compared with their tuning rates measured at the levels of the dc bias close to the threshold of lasing. A linear dependence between injection current tuning rates and the levels of dc bias has been observed. Temperature tuning coefficients have been proved to be independent of the laser heat sink temperature and of the dc bias. Frequency tuning curves were approximated with a second-order polynomial. The frequencies of more than 40 absorption lines of CO, CO2, H2O and NH3 known from spectral databases were compared with the calculated frequencies. The accuracy of the approximation was found to be within 0.2 cm−1 for spectral intervals up to 38 cm−1. The dependence of current tuning rates of the VCSELs on dc bias was shown to be taken into account for accurate analysis of absorption line profiles. The results obtained can be used for precise spectroscopic measurements with long-wavelength VCSELs.