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
Laser is a usual light source in many measurement applications, because it has good coherence. To measure the light transmission of a large-core fiber, we designed and setup a testing system using a He-Ne laser. The light from the laser could be focused to an ideal point to incident the fiber. On the other hand, the output spot from the fiber suffers serious coherent noise, which is called as speckle. Even using multi-image average or removing mask filter, we can’t reduce the influence of the speckle remarkably. So we made a mode-disturbing device to vibrate the fiber at a certain frequency and amplitude. To check the effectiveness of the fiber vibrating method and to find the best parameters for our fiber mode-disturbing device, we set different working frequencies from 0 to 80 Hz. And we set the CCD exposure time at 100ms, 200ms, 500ms and 800ms. According to the experimental results, the speckle is much more sensitive to the working frequency of vibrating the fiber than the exposure time. After comparing different frequencies, 70Hz is chosen as the optimized frequency to effectively suppress the speckle, decrease the speckle contrast. The output of the large-core fiber could fit to a 2-D Gaussian function. So we can measure the diameter of the spot based on the fitting result. Using this mode-disturbing system, we measured the focal ratio degradation of a large-core fiber and studied on the bend effect of the fiber.