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The magnitude and phase of the small-signal injection current-to-frequency modulation transfer function in GaAlAs semiconductor diode lasers has been measured over the frequency range 100 Hz-1300 MHz using network analyzers. Channeled substrate planar (CSP), buried heterostructure (BH), and crank transverse junction stripe (TJS) laser structures were investigated and will be compared. Approximately 180, 180, and 90° phase differences between the low frequency, thermal frequency modulation (FM), and the high-frequency carrier-density FM was observed for BH, TJS, and CSP laser structures, respectively. The origin of this phase difference and its implications for FM optical communications will be discussed, A rate equation analysis for small-signal injection current modulation (using Mason's flowgraph representation) indicates the presence of a real-axis left half-plane zero in the carrier-density small-signal frequency modulation response. Experimental evidence for this feature has been observed in the measured injection current-to-FM transfer functions.