The accuracy of fiber positioning is crucial for the observation of multi-target fiber spectral telescopes such as LAMOST (Large Sky Area Multi-Object Fiber Spectroscopy Telescope). Most of the methods used for fiber positioning are openloop control or semi-closed-loop control, the efficiency cannot meet the requirements of next-generation spectral telescopes. Considering that, this paper proposes a fiber positioning method that can achieve a completely closed loop without blind spots based on SMART (Special-shaped Micro-lens Aimer for Real-time Targeting) and a dual-rotary positioning mechanism. The entire correction process can be described as follows: first, the light intensity obtained by the 6-channel photodetector is stored in a buffer. Then the pulses required for correction are calculated based on the data in the buffer and the correction algorithm by the computer. The running command is then sent to the stepper motor controller using serial communication to drive the stepper motor. After the movement, the program will run again to verify if the correction is done. After selecting a position in the first quadrant, 8 directions were selected to conduct automatic correction experiments after the optical fiber position offset. The experimental results show that the average farthest distance that the method could correct is 600μm, and 75.9% positioning accuracy under our closed control method can reach 10μm, 94.8% positioning accuracy can reach 20μm, 100% positioning accuracy can reach 30μm. For corrections within the 500μm offset distance, 97.5 % of the correction time is within one minute.
Fiber spectroscopy technology is important in many areas of astronomical surveys. The fiber is used to transfer light from the telescope to the spectrograph. On the detector of the spectrograph, the image of fiber ends after dispersing can be obtained. In multi-mode fiber, multiple modes propagating in the fiber form a granular speckle pattern on the fiber end. In high-resolution spectral measurement, the speckle disturbs the energy distribution of the spot and reduces spectral resolution. The influence of fibers with different parameters on the centroid shift, signal-noise ratio, and radial power spectrum under artificial and mechanical disturbance is explored in this paper. The experimental results show that when the number of modes propagating in the fiber is higher, the precision of the centroid offset of the speckle is higher. Under the same disturbance condition, the speckle suppression effect is better with more mode numbers. This will be a reference for the parameter selection of optical fiber in the new instrument.
Optical fiber spectroscopy technology is widely used in astronomical surveys. Due to the flexibility and long-distance transmission characteristics of the fiber, astronomical observation can gain larger scale and higher precision spectral data. Nowadays, a lot of representative technologies have been presented to enhance spectral resolution, including fiber integral field spectroscopy, fiber positioning technology in the sky survey, adaptive optics, and photonic lantern technology. Fiber spectroscopy technology plays a crucial role in astronomy. The long-distance transmission characteristics of fibers separate the telescope from the spectrograph. The intrinsic flexibility of fibers lends itself readily to reconfigurable sampling of the field. The method to improve the spectral resolution has been gradually proposed. Fiber integral field spectroscopy is one of the most typical techniques to enhance the spectrum resolution. The flexible combination of fiber bundle and microlens is used to improve the sampling rate of target stars and fitting factor. In the observation of target stars by a single fiber, the alignment accuracy between the fiber and the star image determines the spectrum resolution. In the multi-object telescope, the position of a large number of multi-mode fibers needs to be detected. As a kind of optical waveguide device with multi-mode and single-mode conversion, photonic lantern can convert the energy collected by multi-mode fiber into the output of single-mode fiber. This review introduces optical fiber technology on astronomical observation.
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.
The small-field radiotherapy is a developing technique because it can reduce the damage to normal tissues. However measurement of the dose distributions of small radiation fields in radiotherapy is a challenge. In this work, we designed and produced an optic fiber X-ray sensor array with high spatial resolution. The sensing array includes 7 sensing probes connecting to an 8-channel optical switch and a photon counting detector (PCD). To verify the practicality of the system, these sensors were measured under a 10×10 cm2 field of a medical linear accelerator. For the small-field application, the dose distribution of radiotherapy fields 1×1 cm2 and 0.8×0.8 cm2 were measured. The distribution of these small radiotherapy fields were given based on the experimental results.
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.
It is well-known that the surface roughness of materials plays an important role in the operation and performance of technological systems. The roughness influences key parameters, such as friction and wear, and is directly connected to the functionality and durability of the respective system. Tactile methods are widely used for the measurement of surface roughness, but a destructive measurement procedure and the lack of feasibility of online monitoring are crucial drawbacks. In the last decades, several non-contact, usually optical systems for surface roughness measurements have been developed, e.g., white light interferometry, light scatter analysis, or speckle correlation. These techniques are in turn often unable to assign the roughness to a certain surface area or involve inappropriate adjustment procedures. One promising and straightforward optical measurement method is the surface roughness measurement by analyzing the fringe visibility of an interferometric fringe pattern. In our work, we employed a spatial light modulator in the interferometric setup to vary the fringe visibility and provide a stable and reliable measurement system. In previous research, either the averaged fringe visibility or the fringe visibility along a defined observation profile were analyzed. In this article, the analysis of the fringe visibility is extended to generate a complete roughness map of the measurement target. Thus, surface defects or areas of different roughness can be easily located.
The inspection of technical surfaces is often performed by two-wavelength electronic speckle-pattern interferometry (ESPI) combined with a phase-shifting procedure. As in conventional specular interferometry, the characteristic fringe spacing in the generated interferogram is defined by the applied wavelengths and the sensitivity is therefore constant in one fringe pattern. Subsequently, this technique is limited to surface structures with similar phase gradients and low structural density. To measure more complex structures, a high-resolution generated reference wavefront (HRGW) is adapted to the measurement object for local sensitivity adaption. The feasibility of this principle is directly linked to the functionality of the used spatial light modulator (SLM). A key factor of a proper phase-control is the structural setup of the SLM. In this article, the general influence of the microstructure of SLMs in adaptive ESPI is evaluated.
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