Velocity Measurement Of Self-Mixing Speckle In Erbium-doped Fiber Ring Laser

Self-mixing speckle combines the self-mixing interference with traditional speckle, which introduces speckle phenomenon into laser resonator cavity. Compared with the conventional two-beam interference, it is compact, simple, easy to adjust and have only one optical axis. Self-mixing speckle is a new optical measurement technique developed in recent years. Compared with semiconductor laser, there are lots of advantages for fiber laser:smaller in size, wider tuning range, better monochromatic, more laser output spectrum, lower laser threshold, and higher efficiency. Erbium-doped fiber laser whose wavelength is the window for fiber communication is developing rapidly and it is widely used in fiber communication and fiber sensing field.In this paper, a new velocity measurement method of self-mixing speckle based on intensity autocorrelation function in an erbium-doped ring laser is studied. Theoretically an analytical model of optical feedback is established based on the erbium-doped fiber ring laser. The light amplification equation is deduced from the rate equation of the erbium three-level system. The alteration of laser output power and frequency under optical feedback is studied by using the stable solution of the light amplification equation. The autocorrelation analysis of self-mixing speckle signal is made utilizing the optical feedback theory of fiber ring laser, and a relationship between the autocorrelation frequency of speckle signals and the velocities of moving object is theoretically obtained. A velocity measurement system of Self-mixing speckle based on an erbium-doped fiber ring laser is designed。Self-mixing speckle signals is collected with a dynamic target at different velocities. The autocorrelation frequency is calculated through the autocorrelation analysis, and the linear relationship between autocorrelation frequency and the velocity of the target is gained. Experimental result agrees with the theoretical derivation, it indicates that the autocorrelation of self-mixing speckle in a fiber ring laser can be applied to velocity measurement.