Suppression of fiber modal noise by using deformable mirror

The modal noise in optical fiber limits the measurement accuracy of the next generation of high precision radial velocity spectrograph. Especially when using coherent light sources for wavelength calibration, modal noise can induce radial velocity errors that reduce the accuracy of radial velocity measurement. In order to suppress optical fiber modal noise, a technical method based on deformable mirror to dynamically change speckle pattern is proposed. The method smoothes the speckle pattern caused by optical fiber and improves the centroid stability of calibration spectrum by dynamically changing the shape of deformable mirror in one single exposure time, so as to ensure the measurement accuracy of radial velocity. A 632.8 nm He-Ne laser is used as a coherent calibration light source to verify the effectiveness of the device. Experimental results show that the optical fiber modal noise can be effectively suppressed when a single exposure image contains about 10⁵ speckle patterns. For the spectrograph with resolution R=100000, the radial velocity calibration error caused by optical fiber modal noise is about 19.8 cm/s in use of one single calibration line, which is similar to the effect of other suppression methods in the world. And this method can not only improve the energy utilization rate but also not affect the service life of optical fiber.