High precision alignment between the fiber core in the focal plane and the image of the target star is of great significance for the observation of multi-target telescopes. In this work, we propose and demonstrate a Special-shaped Micro-lens Aimer for Real-time Targeting, namely SMART, combining a special-shaped microlens and a fiber bundle to realize online alignment and improve the coupling efficiency of fibers. The platform in the center of the microlens transmits the starlight to the science fiber of the fiber bundle without changes in focal ratio. Six side micro-lenses couple leakage light to six feedback fibers and return misalignment signals. The structural parameters of SMART are well designed. Fresnel diffraction theory is applied to build a model for simulating the performance of SMART. In the SMART measurement, a pinhole with a diameter of 200 μm is used to imitate the effect of atmospheric turbulence during astronomical observations. Experimental results indicate that when the image spot is offset relative to the science fiber, the misaligned direction and displacement distance are identified by the signal of feedback fibers in SMART with a resolution of 0.02 mm and a detection range of 0.08 mm to 0.26 mm.
Fiber scrambling is important in high-precision calibration systems for radial velocity measurement for searching for exoplanets. As for laser frequency combs, the modal noise of significant laser speckles can occur due to the strong coherence of the light source, which can be effectively suppressed by vibrating the fiber. However, the fibers used for scientific target detection are coupled with polychromatic light from the celestial body. This study focuses on the fiber mode noise and length dependency under white light conditions, and proposes a new fiber scrambling method of combining different types of fiber to achieve high scrambling gain. The results show that the fiber mode noise increases with decreasing length, and that there is also significant mode noise when the fiber is less than 2m, resulting in a speckle-like pattern as the modal pattern in the near field. The combination of non-circular fibers and graded index fibers can effectively reduce mode noise and improve the scrambling gain.
A fiber IFU with 8064 fibers is designed and manufactured for the Fiber Arrayed Solar Optical Telescope. 8064 fibers are divided to two 2D arrays for different polarization states and 12 pseudo fiber slits for 12 spectrometers. There are many relative techniques have been developed during this process. The hexagon microlens array fits the 100% filling factor. The quartz micropores plate guarantee the positioning accuracy among different temperatures. The 18m fiber cables with special designs transfer the signal with low focal ratio degradation. The quartz V-grooves are used to control the positions of the fibers to form those pseudo slits. Besides, a six-dimensional alignment system and a fast alignment and detection system are built to align the microlens array with micropores and measure the focal ratio, transmission efficiency and alignment accuracy of the IFU, respectively.
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.
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