Narrow linewidth single mode lasers are used in domains such as spectroscopy or metrology. Those high-coherent sources offer spectral full width at half maximum linewidths (FWHM) under 100 kHz, even under 1 kHz. Usual optical spectrum analyzers are not able to solve such coherent spectra: linewidths are measured using a beating method based on self-heterodyne detection, or deduced from the power spectral density of the laser phase noise. We proposed a new method based on optical injection to measure spectral linewidth of highly-coherent laser [1]. The linewidth shape is kown to be a Voigt profile with two contributions: a Lorentzian part linked to the white noise and a gaussian part linked to the 1/f noise. We give in this communication the influence of optical injection on the gaussian and lorentzian contribution, following the injected power and detuning.