Frequency chirp under current modulaton in InGaAsP injection lasers

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Abstract
We have measured the frequency chirp in gain-guided, weakly index-guided, and strongly index-guided InGaAsP lasers under direct-current modulation. The measured chirp width is largest for gain-guided and smallest for weakly index-guided ridge waveguide lasers. The chirp width for 1.5-μm InGaAsP lasers is about a factor of 2 larger than that for 1.3-μm InGaAsP lasers of the same structure. The frequency chirping results from a modulaton of the carrier density which modulates the effective refractive index of the guided mode. The frequency chirping can introduce a limitation on the performance of single-frequency injection laser sources for high bit-rate digital transmission in fiber communication systems at 1.55 μm.