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The two-tone modulation behavior of a semiconductor laser followed by a Mach-Zehnder interferometer is theoretically analyzed under the small-signal assumption. Illustrations are given of the signal at the Mach-Zehnder output, showing the influence of the interferometer free spectral range on the second-order harmonic (2HD) as well as the third-order intermodulation distortion (IMD). The direct intensity modulation (IM) distortions are also reported together with those of frequency modulation (FM) to facilitate direct comparison. The results show that a Mach-Zehnder with low free spectral range is required to achieve low FM distortion levels over those of the direct IM.