A physical model of the Littman configuration external cavity diode laser is established based on the angular spectrum theory, and the external cavity transfer function is derived. The wavelength-dependent coupling efficiency is calculated, obtaining the external cavity filtering envelope. Based on this model, we numerically simulate the filtering envelope with different parameters. The simulation shows that the filtering bandwidth of the external cavity narrows with the decrease of the collimator focal length. The wave aberrations of the collimator lens seriously degrade the peak reflectivity and filtering bandwidth of the external cavity. The impact of the external cavity length and the beam misalignment on the filtering function is studied, providing a theoretical basis for the error analysis of the optical path design.
In this paper, based on the amplified spontaneous emission (ASE)-induced limited-band white noise modulation, a short-coherence laser source is developed for the wafer dynamic profilometry. All-fiber Mach-Zehnder optical path is constructed, obtaining the orthogonal-polarized light source with optical path matching for short-coherence dynamic interferometry. The 220-μm coherence length and 28 % side lobe suppression ratio (SLSR) of the modulated laser are realized in Fizeau interferometer. The dynamic profilometry of a 100-mm high-Al-doped glass wafer with 1.5-mm thickness is realized in the common-path interferometer, overcoming the interference fringe crosstalk induced by the front and rear transparent surfaces.
The traditional underwater detection methods using sonar have some limitations, for example, in close range detection, the low frequency detection ability of sonar system is weak and the detection accuracy is low. In this paper, a new type of fiber Bragg grating hydrophone which can detect low frequency signals is developed. the hydrogel and fiber grating are combined in structure, and a combined packaging structure is designed, which can effectively improve the low frequency sensitivity of the hydrophone. The new fiber Bragg grating hydrophone works in the frequency range of 50-1000Hz and has good response characteristics for underwater low frequency signals. Underwater optical detection technology has high detection accuracy in close range detection, and will become a powerful supplement to underwater acoustic detection technology. However, in the water environment, the contrast of the image obtained by the optical imaging system is very low, so it is difficult to process and analyze the image information effectively. In this paper, the underwater polarization imaging system and algorithm are studied, and a target enhancement technology of underwater polarization imaging based on generating countermeasure network is proposed.Firstly, the circularly polarized light is used for active illumination, and then the underwater polarization imaging based on Stokes vector is carried out, and the images of different polarization states are collected respectively. finally, these polarization images are input into the generation countermeasure network trained by "combined image restoration of circular polarization and linear polarization", and the polarization images are fused to generate multi-polarization high-resolution fusion images.
Recently, much attention has been paid to optical components with complex surfaces because they can significantly optimize the aberrations of optical systems. However, a complex surface introduces several difficulties for precise measurement. To address this challenge, we propose tilted-wave point diffraction interferometer (TPDI) for measurements of complex surfaces. TPDI combines the advantages of tilted-wave interferometry and point diffraction interferometry, thus enabling high-precision measurements of complex surfaces; moreover, this method has demonstrated good efficiency and versatility. We adopt a fiber array (FA) to generate diffractive tilted waves to compensate for the local gradient of the test specimen. Unlike the classical PDI, which focuses more on the beam quality of the on-axis diffractive wave, the proposed system requires better quality for both the on-axis and off-axis diffractive waves. Therefore, the main parameters of the FA that may introduce measurement errors are analyzed, including the end face roughness of the array, the quality of diffractive wavefronts, and the initial optical path differences. Finally, the measurement result of the high-order hyperboloid verifies that our TPDI is effective and its precision of testing a complex surface is better than λ / 37 (rms, λ = 632.8 nm).
In high-power fiber lasers, stimulated Raman scattering (SRS) is a major factor limiting its power increase. In order to analyze the influence of the SRS effect on high-power fiber lasers, based on the traditional SRS model, we propose a theoretical model of multimode fiber lasers and fiber amplifiers containing the SRS effect. Then, the theoretical model is used to analyze the influence of various structural parameters of the fiber laser on the SRS effect. According to the simulation analysis results, we constructed a fiber laser system based on a master oscillator power amplifier configuration to carry out verification experiments for various structural parameters. The experimental results show that shortening the length of the gain fiber and the transmitting fiber, choosing a signal light of longer wavelength, and increasing the fiber core diameter can suppress the influence of the SRS effect on the fiber laser. And we apply the analysis results to 3 kW fiber laser system, and successfully achieve the suppression of SRS effect in the highpower fiber laser.
Free-form optics has been attracted huge interest since it can significantly improve the performance of an optical system with a simpler structure. However, optical testing for free-form surfaces is usually more difficult compared with traditional ones. Although a variety of interferometers can achieve measurements with nanometer-scale precision, it suffers from the problems of a complex system configuration, a limited measurement range and relatively high requirements of testing conditions, etc. In contrast, phase measuring deflectometry (PMD) which has the benefits of a simple system structure, a large dynamic range and a high measurement accuracy is gradually becoming a powerful tool for free-form surface testing. Nevertheless, multiple groups of fringe patterns are required to sequentially display in two orthogonal directions to obtain the corresponding surface gradients in the classical PMD measurement. Therefore, a speedy detection of free-form surfaces is generally blocked. To overcome the above shortcoming, a spatial color-encoded phase-shifting strategy is put forward for PMD to acquire absolute phases with only four color images in this paper. Experimental results demonstrate the effectiveness of the proposed method as well.
As a core part of phase retrieval, phase unwrapping is widely used in the field of phase-based optical testing. In the past few decades, a large number of phase unwrapping methods have been proposed which can be divided into the pathdependent and the path-independent ones. In general speaking, the former is faster but more susceptible to noises, while the latter is time-consuming but more robust. In consideration of the aforementioned problems, a phase unwrapping algorithm based on iterative zonal reconstruction technique is put forward in this investigation. For a wrapped phase map with a rectangle pupil, the proposed method can directly and speedy achieve the phase unwrapping with a simply zonal reconstruction. For the case of an arbitrary pupil, the phase unwrapping is realized by an iteratively compensated zonal reconstruction with a slightly increased amount of calculation. Simulations and experiments have been carried out to demonstrate the effectiveness of the proposed method as well.
In a high-power laser system, thermally induced aberrations, which can influence the laser beam quality, are caused by thermal deformations of the optical elements when they are irradiated by the laser. We evaluate the mechanism responsible for the influence of thermally induced aberrations on the laser beam quality factor (M2) using the finite element method and angular spectrum theory. The thermally induced aberrations are decomposed into their constituent Zernike terms, showing that the influence of the thermally induced aberrations on M2 comprises a mixed contribution of different aberrations. The M2 of a fiber laser is simulated and measured when the laser transmits through organic glass. The simulation results have a good consistency with the experimental results, indicating that the method used in this study is effective when analyzing the influence of thermally induced aberrations on M2.
Smaller and lighter optical systems with better performance can be built by the use of freeform optics.
However, most optical systems were constrained to traditional surfaces for the accurate metrology of
freeform surface is a challenge so far unsolved. One high-precision approach to measure freeform
surface with less time and expense is using tilted wave interferometer. A lens array is placed in the test
path of the interferometer, which can generate light source array that locally compensate the gradient of
test surface. But each source generated by lens array is not ideal spherical wave which contains
aberrations. In addition, the sources cannot be activated individually during the measurement, so that it
is impossible to perform an irregular source array according to the gradient variation of each test
surface. Thus, a novel technique based on fiber array is proposed for generating irregular source array.
Whereas, the position deviation of each fiber and phase difference produced by the length of each fiber
affect the measurement result. In this paper, the consequences of above errors are analyzed. A
calibration method can obtain the exact spatial coordinates of each fiber is suggested to calculate the
position deviation of each fiber. Meanwhile, a method based on Mach-Zehnder interference system is
presented, which can get phase difference produced by the length of each fiber accurately. Afterwards,
the data obtained by the two calibration methods are introduced into the mathematical model of system
error for eliminating the measurement error introduced by the use of fiber array. An elliptical mirror is
measured by our tilted wave interferometer based on fiber array showing the feasibility of the proposed
methods.
Compared with conventional optical elements, free-form surface optical elements, as a kind of nonrotationally symmetrical shaped component, can provide more freedom in optical design, optimize the structure of the optical system, and improve its performance. However, the difficulties involved in the measurement of free-form elements restrict their manufacture and application. A tilted-wave-interferometer (TWI) can achieve high precision in free-form surface measurement, but it requires higher space attitude error control. We analyze the relation between the alignment error and the measurement error introduced by the misalignment in free-form surface metrology with TWI. The attitude control method in the rotation direction is proposed based on the moire fringe technique. Then, combining it with the five-dimensional space attitude control method of aspherical elements, we put forward an alignment error control process in measuring the free-form surface. An experiment of measuring a free-form surface using TWI shows the effectiveness of our method.
As is well-known, optical testing has begun to emerge as a limiting factor in the application of freeform surfaces. In all kinds of published freeform optical metrology, the tilted-wave-interferometer (TWI) is the precise and flexible method for testing a freeform surface as it can compensate the local surface’s deviation from its best fit sphere by using a set of tilted waves. In the process of measurement with TWI, accurate assessment of the test surface error from the fringes plays a key role. We present a method for evaluation and characterization of surface aberrations in TWI by combining computer-generated wave technology and a retrace errors elimination algorithm. The feasibility of the method is proved by the simulation and experimental results.
Optical freeform surfaces are complex surfaces with non-rotational symmetry that break through the limitations of conventional optical element, and are widely used in advanced optics application for system configuration simplifying and performance enhancing. Due to the geometrical complexity and optical particularity of optical freeform surfaces, there is, as yet, a lack of precision freeform surfaces testing. Computer generated hologram (CGH) null testing method are discussed in this paper to test the optical freeform surfaces such as off-axis aspheric surfaces. CGH design based on ray tracing and NURBS interpolation are included. Simuation in Zemax is given to verify the result of calculation. The alignment and fiducial sections are added to the CGH to lead the alignment of the freeform surface and CGH with sixdimensional adjustment. The CGH was designed and fabricated to test an off-axis aspheric with Fizeau configuration.
A general analytical transient temperature field expression of KDP crystal irradiated by sinusoidal modulated laser is obtained by the integral transform method, based on the heat conduction equation of the orthotropic material. The influence of radius, power and frequency of the laser on the transient temperature field of KDP crystal is simulated by Matlab. The results show that the temperature of material has stepwise distribution with time, which shows periodical stable distribution after some time, and it increases with the decrease of the laser radius and the increase of the laser power. The above results provide a theoretical basis for the photothermal displacement technology used in the measurement of the opto-thermal parameters of the KDP crystal.
The main method of the attitude measurement is to get series of graphs for analyzing the changing attitude of the
projectile by utilizing multiple shadowgraph-taking camera stations.However,the premise of accurate results is that
feature points on graphs must match its spatial coordinate in a certain spatial coordinate.Addressing to this
problem,combining the optical lever principle and the PSD,we propose a method of calibrating the spatial coordinate
coherence of the long distance and multiple shadowgraph-taking camera stations,and analyze the measurement accuracy
and repeatability of this method.According to the results,the accuracy of this calibration method is superior to 0.1mm .
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