Self-mixing Interference In Fiber Laser And Its Application For Displacement And Step Height Measurement

Laser self-mixing interference, which is also called laser feedback effect or injected modulation effect, refers to be a portion of light emitted from a laser source is reflected or scattered back by an external object and is coupled into the laser cavity again. The feedback light carrying the motion information of the external object mixes with the light inside the cavity and then modulates the output power and the frequency of the laser. The characterisitics of the output light is similar to that of the conventional two-beam interference, thus the phenomenon is called laser self-mixing interference. When the laser self-mixing interference is used for measurement, it has the same phase sensitivity as that of the conventional interferometer. But it does not need the reference optical path, so its structure is more simple and compact. It has been a well-known measurement technique widely used for the precise measurement of displacement,vibration and speed, and it can be used for measuring the object with low reflectance.The effect of optical fiber laser self-mixing interference and its application in displacement measurement and step height measurement has been reaesrched. Linear-cavity fiber laser and single-wavelength fiber ring laser have been designed. And a new type of two-independent-wavelength fiber ring laser has also been developed. The self-mixing interferences of each kind of laser have been realised. Displacement and step height are also measured respectively with the single-wavelength self-mixing interference and double-wavelength self-mixing interference. When the displacement is measured with the single-wavelength slef-mixing interference, the intensity of the output light of the laser is modified to be fringes that are tilted sawtooth-like waveforms in same direction in which the measured object moves. The measurement range of the system can be as large as6cm, and the measurement precision can be34.3nm when a high-precision piezoelectric scanner drives the measured object to move450μ m in a steady speed during experiments. Step height measurement has been performed with the self-mixing interference of the two-independent-wavelength of fiber ring laser, the measurement range can be as large as the order of minimeter. The factors that could influence the measurement precision have been analysed.