Center frequency shift and reduction of feedback in directly modulated external cavity lasers

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Abstract
It is shown experimentally and theoretically that a center frequency shift occurs when an external cavity laser is directly modulated. The shift can be observed even when the frequency deviation is small compared to the roundtrip frequency of the external cavity and can qualitatively be explained by a reduction in the effective feedback level due to modulation. The frequency shift was measured as a function of modulation frequency and current, and frequency shifts up to 350 MHz were observed.