Technical Bulletin N°5

Experts in Next Generation Lightwave Test Equipment

Ultra high resolution optical spectrum analyzer (0.16pm) for relaxation oscillation measurements

AP2040A application note

Optical spectrum of a DFB laser with decreasing injection current Click on the picture for a better resolution graph

The 3 spectrums above are showing the optical spectrum of a DFB laser with decreasing injection current (from left to right) We can see two symetric lobes arising and getting closer to the main peak. This is the signature of the relaxation oscillation resonance in energy exchange between electrons and photons inside the cavity of a semiconductor laser. When injection current I decreases, the resonance amplitude increases while its frequency decrease. This concludes that: - The resonance amplitude is inversely proportional to SQRT(I-Io) (Where Io is the treshold current) - The resonance frequency is proportional to SQRT(I-Io) (Where Io is the treshold current) The maximum direct-modulation rate of a DFB laser is directly related to the location of this resonance. This phenomenon can not be seen with a standard optical spectrum analyzer. Only an ultra high resolution OSA like the AP2040A can show and analyze such fine details.

Comparison between a standard OSA and the AP2040A :

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