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 imaging experiment of Methylidyne (CH) radical generated from methane combustion on a swirl burner was carried out using Planar Laser Induced Fluorescence technology (PLIF), and the C-X (0,0) band was selected as the excitation method. The laser wavelength and laser energy have a significant impact on the signal strength and signal-to-noise ratio of CH radical, and the imaging quality of CH reached the best with the laser wavelength and energy are of 314.415 nm and 0.3 mJ. The fluorescence signal of CH radical from swirling flame mainly distributed on the outside of the image and exists in a narrow area. As the of equivalence ratio of methane combustion increased, the signal of CH radical gradually strengthened, and the flame profile of chemiluminescence gradually evolved from "V" type to "M" type, and the number of vortices formed by CH radical from PLIF imaging gradually increased.
A single-laser-shot N2 Q-branch Coherent Anti-Stokes Raman Scattering (CARS) is used to measure the instantaneous temperature of supersonic combustion in kerosene/air flame with Mach 2.6. The Unstable-resonator spatially enhanced detection (USED) phase matching is used to reduce turbulence effects and to improve the CARS signal intensity. An USED CARS measurement system, which has a high spatial solution of ~100μm in diameter, and a CARS spectrum calculating and fitting program CARSCF are developed. The CARS signal in supersonic combustion is measured and then used to calculate temperature, the results show that, during kerosene/Air ignited in Mach 2.6, the CARS signal first rise rapidly then fall sharply and finally rise slowly, while the temperature increase sharply and then decrease slowly and the average temperature is 1970 ± 144K with 6.5% of repeatability.
A nanosecond pulsed dielectric barrier discharge (ns-DBD) setup is built to preliminarily analyze effect of different discharge repetition (600Hz ~ 1800Hz) and voltage (6.0kV ~ 10.4kV) on CH4-air diffusion flame. Emission spectral is used to understand temperature and relative change of components’ concentrations in diffusion flame, such as O radical (777nm band), OH radical (308nm band). Plasma induced consumption process of repetitive pulsed nanosecond discharges on O radical has been directly observed at 900Hz discharge repetition. Based on time resolved emission spectral, significant changes of OH radical distribution at early discharge stage can be observed, which can be due to air-discharge plasma, and rapidly recover in microsecond scale due to rapidly consumption of generated OH radical. Besides, great differences in reaction time scale of OH radical (half-value period of OH radical consumption ~81.8μs) and O atom (half-value period of O atom consumption ~3.6min) is observed, corresponding to different chemical reaction mechanism of O atom and OH radical. A model based on rate equation is built to describe generation and consumption process of O atom and OH radical, which can also well predict voltage behavior of 777nm band at steady state (6.0kV ~ 10.4kV).
Planar laser induced fluorescence (PLIF) is a powerful tool to visualize the flame structure, especially for the turbulent flame. In this paper, we employ OH-PLIF technique to analyze the structure of a supersonic ethylene jet flame on a turbulent burner. This burner consists of a central jet and hot coflow. The Mach numbers of the jet vary from 1.0 to 1.6, corresponding to Reynolds numbers ranging from 40893 to 65455. The flame structures are imaged by OH-PLIF measurement. The measurement results reveal that the OH concentrations decrease with the increase of jet velocity or decrease of the O2 fraction. And the extinction and re-ignition of flame take place when the jet velocity is high or the O2 fraction is small. These measurement results help to understand the interaction between flame and highly turbulent flow.
Due to non-interruption of laser intensity and dye content, two-colour Laser Induced Fluorescence (LIF) ratio thermometry approach is widely used in the studies of fluid. Ratio of temperature sensitive dye Photo Luminescence (PL) intensity at two wave bands with different temperature sensitivity can efficiently remove interruption of laser intensity and dye content in time and space. To achieve high temperature sensitivity and Signal to Noise Ratio (SNR) in these technique, selection of two wave bands’ peak wavelengths and band widths should be carefully considered. In this work, influences of peak wavelengths, band widths and SNR to temperature sensitivity of this two-colour LIF ratio thermometry approach are discussed. Temperature property of a traditional temperature sensitive dye (rhodamine B) aqueous solution is studied in a wide temperature range from -10°C to 90°C by spectroscopic method. A non-linear fitting method based on Arrhenius equation is present to accurate describe rhodamine B PL intensity decay along with increasing of temperature, achieving significant improved fitting accuracy compared with traditional linear fitting model. Based on this non-linear fitting method, influences of filters’ center wavelengths and band widths to temperature sensitivity are analyzed. These results give very important information of filter’s selection to ensure sufficient temperature sensitivity and SNR in two-colour LIF ratio thermometry approach.
Cavity structure is used to increase the Interferometric Rayleigh scattering signal intensity. By using ZEMAX method, we simulate a special cavity mode comprising two spherical reflectors with different size, including the focal length and the diameter. The simulations suggest that the parallel beam can reflect repeatedly in the resonant cavity and concentrate on the focus. Besides, the reflection times and the ray width can reach about 50 and 2.1 cm after some feasible solutions.
Planar laser induced fluorescence (PLIF) has been a very important species analysis approach in combustion research, but is most often presented qualitatively. Therefore, another supplementary techniques are needed for quantitative PLIF measurement. In this paper, we propose a quantitative OH concentration measurement method using PLIF calibrated by cavity ring-down spectroscopy (CRDS). The CRDS measurement is firstly applied to a methane-air atmospheric pressure flame on a McKenna burner and determine the OH absolute density. Then the PLIF signal is calibrated by the determined OH concentration on the same flame under the same condition. The calibrated PLIF setup is fixed, and another PLIF setup is added to form a two-line OHPLIF thermometry to measure the 2D temperature distribution. Finally, a quantitative OH-PLIF measurement method is provided for the turbulent premixed flame on a Bunsen burner based on this setup.
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