Zong-Ru Yu

Development of the fast astigmatic auto-focus microscope system

In this paper, a fast auto-focus microscope system was developed based on the astigmatic method. A collimated infrared laser beam was employed in the infinite-corrected microscope optical axis by the beam splitter and reflected by the sample surface. By embedding an astigmatic lens in the system, the reflected laser beam has different focal lengths in the sagittal and tangential planes. As the microscopes relative distance varies, the reflected laser beam shape also varies and can be detected by an embedded four-quadrant photodiode, i.e., the focus error signal (FES) can be found. Then, a fast auto-focus system can be realized by converting the FES to the microscopes defocus distance. We designed an astigmatic auto-focus system for a 20× objective lens with a ±50 µm working range, and this system could also be used for 10× and 5× objectives with ±200 µm and ±800 µm working ranges, respectively.

Study of air-driving fluid jet polishing

In this study, we proposed an air-driving fluid jet polishing (FJP) system which draws slurry utilizing an air/water mixer. The air-driving FJP system is mainly comprised by an air/water mixer, slurry tank with stirrer, compressed air, pressure and flow rate regulators, and a nozzle. The high speed air flow in the air/water mixer draws out the slurry from the slurry tank, and the slurry is guided to mix with air flow inside the nozzle cavity. Then, the combined fluid slurry is emitted from the nozzle. The air-driving FJP system was preliminarily tested on N-BK7 and ZERODUR® plates with different air pressure and processing time. The test results show that the air-driving system could get a Gaussian-like removal shape with 3 kg/cm2 compressed air source and the depth of removal is about 100 nm within 5 seconds processing time. The compressed air improves the kinetic energy of each abrasive, and makes it more efficient in material removal. Furthermore, the Gaussian-like removal shape is more convenient for tool path planning and surface waviness control of corrective polishing.

Design, tolerance analysis, fabrication, and testing of a 6-in. dual-wavelength transmission sphere for a Fizeau interferometer

Abstract. This paper presents the validation of the design of a 6-in. f/2.2 dual-wavelength transmission sphere (TS) based on the Fizeau interferometer. The TS was verified at a wavelength of 632.8 nm and is compatible for measuring the transmitted wavefront error of an i-line lithography lens. The achromatic design is imperative for a dual wavelength TS and requires more lenses for correcting the chromatic focal shift. Because the overall weight of the TS should be controlled within the load range of the piezoelectric transducers of the interferometer, the fabrication and mounting of lenses with high aspect ratios are challengeable. The mounting of the reference surface based on three-point mounting was successful for the residual reference wavefront error under peak-to-valley (PV) λ/10. Furthermore, the reference wavefront is typically restricted within PV 5λ to avoid distorted interference fringes. Therefore, we built a double-pass interferometer model for tolerance analysis, and the error budget facilitated decision-making regarding the suitable specifications of lens manufacturing and assembly for cost efficiency. The test results demonstrated that the deformation of the reference wavefront and the residual reference wavefront error met the critical specification at 632.8 nm and that the achromatic TS is compatible for measuring i-line lithography lenses.

Freeform surface machining error compensation method for ultra-precision slow tool servo diamond turning

In this research, ultra-precision slow tool servo (STS) diamond turning technique has been adopted to generate a freeform surface. In the previous studied, we have developed a model of three-dimensional (3-D) tool shape compensation for generating 3-D tool path in STS diamond turning of asymmetrically freeform surface. However, the form error is not acceptable when the surface sagitta or tangential slope variation too large. Therefore, the surface form error compensation method has been developed in this studied. The surface form error has been compensated from 3μm to less than 1μm by the compensation method.

Study on measurement of 160 mm convex hyperbolic mirror for Cassegrain reflecting system

In this study, the measurement of a 160 mm convex hyperbolic mirror by using the 6 inches phase shifting interferometer and the CGH (computer-generated hologram) in a vertical setup is presented. The wavefront errors of the metrology system including the reference flat and CGH flat due to gravity effect are measured and calibrated by using random testing and null cavity testing with and without CGH. Then, the real form error of each single sub-aperture could be acquired by subtracting the system wavefront errors. In this study, we measured form errors of 10 off-axis sub-apertures with equal angle space and then stitched them to establish the whole irregularity in the shape of the mirror in 160 mm diameter. Finally, we imported the irregularity in the shape of the mirror to the CNC aspheric polishing machine for correction polishing. Combining the aspheric metrology technique and the CNC aspheric polishing technique, a 160 mm secondary mirror for the Cassegrain reflecting system was finished within PV 0.15μm and RMS 17.9 nm.

Design and development of bi-directional viewer

Bi-directional viewer just likes a lens of camera. Unlike conventional camera lens, the bi-directional viewer captures not only front view but also side view around the bi-directional viewer. The barrel of bi-directional viewer was designed to have cuts in the side to allow side light to come through, and we use two aspherical mirrors to receive and change the side light to converge on the main optical path. The front view and side view can be imaged simultaneously on an image sensor. Commercial aspherized achromatic lens and micro video lens were also used to keep the image quality and miniaturize the size of the bi-directional viewer respectively. We report on the development of bi-directional viewer, including optical design, optical simulation, optomechanics design, fabrication techniques, assembly procedure and performance testing. The MTF of the bi-directional viewer is also measured and discussed in this paper. We successfully design and realize a bi-directional viewer, which can perform 100 lp/mm spatial frequency image in detail above 20% contrast in the 7.6 mm front view area. And we keep the dimensions of this device within 35 mm in total length and Φ20 mm in diameter.

Development of the micro displacement measurement system based on astigmatic method

A non-contact micro displacement measurement system with a long working distance is presented based on the astigmatic technology. The astigmatic method principle is to detect the shape change of the reflected laser beam on a four-quadrant photodiode. The laser beam shape changes with respect to the positions of the reflect mirror, because of the focal length difference of the astigmatic lens in sagittal and tangential planes. The test results of the implemented micro displacement measurement system show good performances of accuracy and reliability.

Freeform mirror polishing for compensation on non-symmetry system aberrations of remote sensing instrument

Cassegrain optical systems are widely used in remote sensing instrument. Cassegrain telescope is composed of a primary mirror (M1), a secondary mirror (M2), and a set of correction lenses. The system aberrations of telescope could be corrected and balanced by M1 and M2. In the event of deformation of telescope assembly, the non-symmetry aberrations will be induced to the optical system and reduce the optical performance. The non-symmetry aberrations can be measured after completing M1 and M2 assembly and alignment processes. Compensating this identified error to M1 or M2 can improve the optical performance of the telescope system. The error compensation on M2 is more efficient due to its smaller aperture and quickly assembly and de-assembly processes. In this study, we map the system wavefront error caused by deformation of mirror supporting and gravity onto the designed aspheric surface of M2. The surface of M2 becomes a freeform from aspheric. The polishing process combines the techniques of conventional lapping and CNC polishing. We apply the conventional spherical lapping process to quick remove the sub-surface damage (SSD) layer and to get the accurate radius of best fit sphere of the designed aspheric surface with fine surface texture simultaneously. The polishing and metrology processes were performed by using Zeeko IRP1000 polisher and QED ASI. A Φ 150 mm mirror with freeform surface was completed.

Parameter investigation of air-driving fluid jet polishing

Air-driving fluid jet polishing (FJP) technique was first presented in 2011. Slurry was drop out due to Venturi effect inside the atomizer which is the main component of air-driving FJP system, and was guided to mix with air flow by the nozzle. The Venturi effect and the added high speed air flow provide slurry more kinetic energy to impact the optical surface. Therefore, the air-driving FJP system has a rotational symmetrical Gaussian-like removal profile with lower air pressure and normal incidence configuration. In this paper, we investigate oblique incidence polishing to find the optimal material removal performance of the technique, including removal shape and depth and surface roughness. Different oblique angles ranged from 80 to 20 degree were tested. The air-driving FJP system was adapted upon a CNC machine, so not only single point polishing but also straight line polishing with constant feed rate can be carried out. We report on the performance of oblique air-driving FJP in different air pressure and processing time, and also the material removal of dynamic polishing for N-BK7, Fused Silica and ZERODUR®. The results indicate oblique incidence can get a Gaussian-like removal shape, and improve the surface roughness. The air-driving FJP not only has the advantages of conventional fluid jet polishing, such as no tool wears, cutter interference and debris deposition problems, but also has excellent material removal rate with lower energy.

A portable non-contact displacement sensor and its application of lens centration error measurement

We present a portable non-contact displacement sensor (NCDS) based on astigmatic method for micron displacement measurement. The NCDS are composed of a collimated laser, a polarized beam splitter, a 1/4 wave plate, an aspheric objective lens, an astigmatic lens and a four-quadrant photodiode. A visible laser source is adopted for easier alignment and usage. The dimension of the sensor is limited to 115 mm x 36 mm x 56 mm, and a control box is used for dealing with signal and power control between the sensor and computer. The NCDS performs micron-accuracy with ±30 μm working range and the working distance is constrained in few millimeters. We also demonstrate the application of the NCDS for lens centration error measurement, which is similar to the total indicator runout (TIR) or edge thickness difference (ETD) of a lens measurement using contact dial indicator. This application has advantage for measuring lens made in soft materials that would be starched by using contact dial indicator.