The characteristics of a Nd:YLF laser with 1GHz repetition rate at 1053 nm wavelength was investigated. For the generation of 1 GHz mode-locked pulse train we used the combination of a direct method without RF sythesizer and active third-order harmonic mode-locking method with phase modulator. Using the scanning Fabry-Perot spectrometer the laser spectrum was measured. The pulse width was measured using the intensity autocorrelator. We measured the spectra of the CW laser, the FM laser and the FM mode-locked laser by controlling the cavity detunning. From the result it was concluded that the pulse width and spectrum width were sensitive to the etalon effect of the MgO:$LiNbO_3$ in the phase modulator whose length is 20 mm. Due to the etalon effect the laser pulse width was larger than 100 ps and the spectrum width was approximately 3.5 GHz. Without the etalon effect the pulse width became 11-26 ps and the spectrum width was approximately 130 GHz. The spectrum of CW laser contained the etalon effect due to the output mirror whose radius of curvature was 5 m. The maximum of the FM laser width was approximately 250 GHz, which was larger than the mode-locked laser width.
