Phase portraits' for characterizing advanced lasers

摘要

Lasers play a crucial role in dense WDM systems, and so the characterization of their performance is critical [1]. Commercially, lasers are characterized by parameters such as: linewidth, typical static spectra, side-mode-suppression ratio, modulation response, slope efficiency and wavelength stability. However, the dynamic performance is rarely specified in detail, such as the time-resolved optical frequency waveform of the modulated laser, and the interplay between instantaneous power and frequency. The time relationship between the power and optical frequency crucial if the pulse evolution in fibers is to be calculated, for example. Furthermore, in tunable devices, the time and frequency while the laser is being re-tuned is critical, as the tunable laser could pollute live channels during tuning [2]. In this paper, it is shown that the 'phase portrait' of instantaneous time versus frequency gives valuable information on the transient and stability characteristics of fixed and tunable semiconductor lasers. From the recognizable shapes in the portrait, the deficiencies in the laser are easily identified. Furthermore, the impact of frequency and power deviations can be assessed. This is topical because commercial instrumentation to measure instantaneous time and frequency is now available [3], 50 that phase portraits are easily plotted from the two measurements.