The dominant free space optical (FSO) system adopts an optical fiber as a light transfer unit for transmitting or receiving of communication or beacon signals. The optical fiber is usually placed in a pinhole fiber connector which prevents direct measurement of optical instruments. Traditional test method fails to guide the accurate alignment of an FSO system. Here we propose a new method for point spread function (PSF) modeling by fiber coupling efficiency measurement. First we show the convolution effect of a multimode fiber with a numerical simulation using a focal spot with uniform irradiation. The coupling efficiency map versus the lateral translation has a flattop at the center and drops to zero at the edge, by which the focal spot diameter can be determined. A further simulation shows that the beam profile of a focal spot with rotational symmetry can be derived from coupling efficiency map by deconvolution. We build the mathematical model of the deconvolution method and recover the beam profile with simulated data. With proper modeling and data smoothing, the PSF of a FSO system is recovered with great consistency to the simulation data. The recovered profile can be used for guidance with the alignment of the system. Although the simulated data is rotationally symmetric, the deconvolution method can be improved in the future to be compatible with focal spot with arbitrary beam profile. The method can also be useful in applications such as laser beam profiling, online system testing, phase retrieval and so on.
The recent advances in the optics manufacturing industry to achieve the capability of fabricating rotationally nonsymmetric optical quality surfaces have considerably stimulated the optical designs with freeform components. This opens up new horizons for novel optical systems with larger fields of view and higher performance, or significantly more compact in volume at equal performance compared to conventional systems. A bottleneck to the broad industrial applications of freeform optics remains the lack of a high performance optical metrology tool capable of measuring significant surface departures and slopes of the parts. To address this issue, we have developed a fiber-based swept-source optical coherence tomography (SS-OCT) system for point-cloud freeform metrology, where two-axis galvanometer scanners are leveraged for high-speed lateral scans. We specifically designed a custom all-reflective achromatic pupil relay system to achieve a diffraction-limited scanning configuration. Coupled with a large field-of-view (FOV) telecentric scan lens, the imaging covers 28.9 mm × 28.9 mm FOV with 35 μm lateral resolution and more than 600 μm depth of focus. Freeform metrology is demonstrated for an Alvarez surface of 400 μm surface sag. The high sensitivity of the SS-OCT system allows for capturing the slope variations of the part up to the maximum slope that is 5 degrees in this case. Specific surface reconstruction, rendering and fitting algorithms were developed to evaluate the metrology results and investigate the accuracy and precision of the measurements.
The interferometric method is widely used in the precision measurement, including the surface quality of the large-aperture mirror. The laser interference technology has been developing rapidly as the laser sources become more and more mature and reliable. We adopted the laser diode as the source for the sake of the short coherent wavelength of it for the optical path difference of the system is quite shorter as several wavelengths, and the power of laser diode is sufficient for measurement and safe to human eye. The 673nm linearly laser was selected and we construct a novel form of interferometric system as we called ‘Closed Loop’, comprised of polarizing optical components, such as polarizing prism and quartz wave plate, the light from the source split by which into measuring beam and referencing beam, they’ve both reflected by the measuring mirror, after the two beams transforming into circular polarization and spinning in the opposite directions we induced the polarized light synchronous phase shift interference technology to get the detecting fringes, which transfers the phase shifting in time domain to space, so that we did not need to consider the precise-controlled shift of optical path difference, which will introduce the disturbance of the air current and vibration. We got the interference fringes from four different CCD cameras well-alignment, and the fringes are shifted into four different phases of 0, π/2, π, and 3π/2 in time. After obtaining the images from the CCD cameras, we need to align the interference fringes pixel to pixel from different CCD cameras, and synthesis the rough morphology, after getting rid of systematic error, we could calculate the surface accuracy of the measuring mirror. This novel design detecting method could be applied into measuring the optical system aberration, and it would develop into the setup of the portable structural interferometer and widely used in different measuring circumstances.
The state of conformal optics is described in this article, it explains the concept of the conformal optics and its
application in both military and daily life, the difficulties and solutions to them in the design, also together with the
developing of the tools and techniques which make the use of conformal optics into reality is included. And in the last
part of the article, the comparison of some testing methods of aspheric surface is described.