Analyzing and measuring the diode laser’s linewidth affected by the driving current’s white noise

Abstract

It is important to study the effect of injection current’s noise on the diode laser’s linewidth. In this paper, the effect of the white noise applied to the injection current on the linewidth of diode laser is investigated theoretically and experimentally. Theoretically, by adding the non-Markov noise term into the Langevin equation, the output laser’s linewidth of the diode laser with the white noise current is obtained. In experiment, an optical Fabry–Perot (F–P) cavity with a linewidth of ∼ 1.83 M H z and a finesse of ∼ 136 is employed to measure a 1560.5 nm distributed-feedback (DFB)-type diode laser’s linewidth. With an increase of the white noise intensity, the laser linewidth is broadened from ∼ 2 to ∼ 60 M H z gradually, and the line shape is characterized evolving from Lorentzian function to Gaussian function. At the same level of white noise intensity, the laser linewidth decreases with increase of the injection current. The accuracy of our measurements is verified based on the fiber-delayed and acousto-optic modulator (AOM)-shifted self-heterodyne scheme and the optical F–P cavity linewidth measurement method. The 1560.5 nm laser is boosted by using an erbium-doped fiber amplifier (EDFA) and is frequency doubled to 780.25 nm by a fiber-pigtailed, single-pass PPMgO:LN waveguide module, and the laser linewidth is visually observed and verified by broadening of the saturated absorption spectra of rubidium atoms. Clearly this linewidth manipulation method of diode lasers based on broadband white noise current coupling can be extended to other wavelength diode lasers and can be applied to laser measurement, laser communication, and other fields for different requirements.