It is demonstrated for the first time that spatial filtering of partially reflected irradiance of light from a directly modulated semiconductor laser diode produces a weak but appreciable amount of harmonic distortions. The laser diode discussed here is assembled to be misaligned in such a way that there is significant reflection of light from the submount attached to the heat sink. This geometry of laser and submount gives rise to interfered light intensities of simultaneous AM- and FM-modulated outputs in the far-field region. Numerical calculations are presented for a variety of laser structures and FM-modulation parameters. The second-order harmonic distortion is found to disclose multiple peaks with respect to the aperture radius of a receiving optical system.
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