Linewidth enhancement factor in electroabsorption modulated lasers

Semiconductor sources with integrated electroabsorption modulators and distributed feedback lasers (EML) are important for high speed (2.5 Gb/s and higher) transmission systems. These devices have very low spectral width under modulation which reduces the effect of fiber chromatic dispersion. The low spectral width is due to low linewidth enhancement factor ((alpha) -factor) of these devices. We report measurement of (alpha) as a function of bias for different EML devices. For EML devices with large isolation resistance between the laser and the modulator, (alpha) varies with the modulator voltage. For some bias level measured (alpha) is close to zero. The value of (alpha) has also been calculated using the band structure parameter of the active region. The measurement and calculation agree. For EML devices with low isolation resistance, the measured (alpha) is found to be nearly independent of modulator voltage. This is explained by considering the effect of leakage current between the laser and the modulator.