Phase-disturbing speckle-suppressing method in fiber metrology under coherent illumination

摘要

When we use laser to measure the focal ratio degradation of astronomical fibers, we have to reduce the speckle contrast to fit the output spot to a 2-D Gaussian-like function. The origin speckle contrast is near to 100%. The simple average method doesn't work because the speckle patterns are stable. We tried several ways to disturb randomly the transmission phase of the light modes inside the fiber to be tested. Both non-contact fiber-disturb-mode device (NCFDMD) and contact fiber-disturb-mode device (CFDMD) were established and tested. The NCFDMD is to set a vibrating phase plate against the output end of the tested fiber. The CFDMD is to set the vibrating device in the middle of the fiber. Under different vibration frequency we compared the contrast of speckle patterns. We set different exposure time of the CCD camera to check the effects. For NCFDMD, the exposure time should be long enough, for example 30 ms, to guarantee enough different patterns could be collected to suppress the contrast of the speckle and get good Gauss-like pattern. For CFDMD, we compared three kinds of fibers with different core-diameters. We found that 65-70 Hz is the optimized vibration frequency for all fibers and 30 ms is the best exposure time. The introduction of the phase modulation could dramatically suppress the speckle under coherent illumination. The measurement accuracy could be enhanced according to the speckle suppression.