1.55-microm single-mode VCSEL frequency chirp behavior is investigated in under-threshold and above-threshold operating conditions for different VCSEL-to-VCSEL injection locking configurations with respect to free-running case. We experimentally evaluated the capability of adjusting the frequency chirp, reducing its value and inverting the sign. The control over the frequency chirp is obtained changing the wavelength detuning and power injection ratio between the VCSEL master and the VCSEL slave. Advantages of the chirp inversion are demonstrated for 10 Gb/s error-free propagation at 1.55-microm over 40-km standard single mode fiber without any dispersion compensation.