A prototype of large wide field telescope is a Cassegrain telescope which covers 2° field of view with two hyperbolic mirrors, a 0.5 m primary mirror and a 0.2 m secondary mirror with multiple correction lenses. To fulfill the optical and mechanical performance requirements in design and development phase extensive finite element analyses using NX NASTRAN and optical analyses with CODE V and PCFRINGE have been conducted for the structure of optical system. Analyses include static deformation (gravity and thermal), frequency, dynamic response analysis, and optical performance evaluations for minimum optical deformation. Image motion is also calculated based on line of sight sensitivity equations integrated in finite element models. A parametric process was performed for the design optimization to produce highest fundamental frequency for a given weight, as well as to deal with the normal concerns about global performance.
We propose a new method based on direct laser writing to fabricate reference chromium patterns on a silicon wafer. Our
method is able to fabricate a maximum 360-mm-diameter pattern with 651-nm position uncertainty. The minimum
pattern size is about 0.8 μm (line width value) and the maximum available height of the pattern is slightly over 400 nm.
Computer Generated Holograms (CGH) for optical test are commonly consisted of one main pattern for testing aspheric
surface and some alignment patterns for aligning the interferometer, CGH, and the test optics. To align the CGH plate
and the test optics, we designed the alignment CGHs modified from the cat's eye alignment method, which are consisted
of a couple of CGH patterns. The incident beam passed through the one part of the alignment CGH pattern is focused
onto the one radius position of the test aspheric surface, and is reflected to the other part, and vice versa. This method has
several merits compared to the conventional cat's eye alignment method. First, this method can be used in testing optics
with a center hole, and the center part of CGH plate can be assigned to the alignment pattern. Second, the alignment
pattern becomes a concentric circular arc pattern. The whole CGH patterns including the main pattern and alignment
patterns are consisted of only concentric circular fringes. This concentric circular pattern can be easily made by the polar
coordinated writer with circular scanning. The required diffraction angle becomes relatively small, so the 1st order
diffraction beams instead of the 3rd order diffraction beam can be used as alignment beams, and the visibility can be
improved. This alignment method also is more sensitive to the tilt and the lateral shift of the test aspheric surface. Using
this alignment pattern, a 200 mm diameter F/0.5 aspheric mirror and a 600 mm diameter F/0.9 mirror were tested.
The auto-focusing is one of the important parts in the automated vision inspection or measurement using optical
microscopes. Moreover, laser micromachining or laser lithography requires a high speed and precision auto-focusing. In
this paper, we propose and realize an auto-focusing system using two cylindrical lenses, which is the enhanced version of
the previous astigmatism method. It shows very good performances, especially very high speed and the largest
defocusing range in comparison with the previous astigmatic methods. The performance of our auto-focusing system was
evaluated by tracing the linear stage whose position was monitored by a commercial laser interferometer.
KRISS Space Optics Research Center has tested large aspheric surfaces by using interferometry and a series of computer-generated hologram (CGH). In this case it is necessary to fabricate various CGHs in the laboratory level. To address this purpose we are developing and improving a simple and precise laser writing system which uses a cylindrical or circular coordinate. In our system 300 mm diameter CGH can be fabricated with 0.8 μm spatial resolution in radial direction. The writing source Ar+ laser is stabilized by intensity feedback, and gives us approximately 800 mW after the stabilization process at 457.9 nm wavelength. The stabilized beam power is controlled again to make 256 different intensity levels. We also used an auto-focusing technique with astigmatic lenses for the purpose of focusing the writing beam on the material surface.
Total thickness variation (TTV) is one of the important specifications of glass wafer. Glass wafers are thin and transparent parallel plates. In order to measure a flat surface by interferometer, at least one reference flat of same size is required. And the interference between two reflected wavefronts by the front and rear surfaces of the glass wafer also exists. Therefore interferometric measurements of thin glass wafers are not easy. So TTV is mainly measured not by interferometer, but by thickness gauge devices. But these devices measure only the TTV of several positions of glass wafer and don't measure the whole area. To measure the whole area or sufficient points, it requires more time. We developed a relatively simple and inexpensive interferometric TTV measurement method using Haidinger interferometer. This method can be applied to large glass wafers without large reference flat.
We present experimental results on the output power stabilization of an Ar+ laser for a direct circular laser writing system (CLWS). Instability of the laser output power in the CLWS causes resolution fluctuations of being fabricated diffractive optical elements or computer-generated holograms. For the purpose of reducing the power fluctuations, we have constituted a feedback loop with an acousto-optic modulator, a photodetector, and a servo controller. Here very important things are proper conception of the servo controller and selection of a proper photodetector depending on what kinds of lasers to be controlled. In this system, we have achieved the stability of ± 0.20 % for 12 minutes and the relative intensity noise level of 2.1 x 10-7 Hz-1/2 at 100 Hz. In addition, we applied our system to a 2 mW internal mirror He-Ne laser. As a consequence, we achieved the output power stability of ± 0.12 % for 25 minutes.
The high performance optical system is generally not easy to align. Particularly, if there are off-axis optical components in the system, the alignment is a very serious problem. Many researchers have reported that using sensitivity of some Zernike coefficients is useful in alignment. However, this method has a weak point in that it generates an accurate result only when the misalignments of component are located within the range where the sensitivities of coefficients are linear to the amount of misalignment. We developed the new method which allows larger misalignment at the early stage of alignment. It is to minimize the merit function of Zernike coefficients rather than to use sensitivities of coefficients. In this paper, we analyzed the cons and pros of conventional alignment method and our own method and demonstrated the accuracy and flexibility of our method by aligning the diameter 900 mm Cassegrain type collimator. Even though there was more than 1 mm decenter error, we could align the secondary mirror to the nominal position with just two trials.
MSC (Multi-Spectral Camera) for KOMPSAT2 (Korea Multipurpose Satellite 2) is a Ritchey-Chretien type telescope. Its primary mirror (PM) is a hyperboloidal mirror with 600 mm diameter. To test the PM, we chose a CGH null lens as a null optics. The CGH null lens is much smaller than the Hindle shell, is consist of only one element, and is aligned more easily than the Offner type null lens. The CGH null lens is designed to align and test the aspheric surface at the same time. The CGH lens includes the main part to test the aspheric wavefront from the PM and also includes the alignment part to align the CGH and the PM. The alignment CGH part is consisted of three amplitude type reflection segments to align the CGH and the interferometer, and three phase type transmission segments to align the CGH and the PM. The CGH null lens is made of fused silica by a laser beam writer. We measured the Engineering Model of the PM using this CGH null lens.
Collimator is essential to evaluate and assemble the other telescopes. Its diameter should be larger than that of the target telescope for the correct use. We are currently developing the Cassegrain type collimator of which diameter is 0.9 m. The primary mirror is light-weighted so that its weight is only 70 kg. Due to its structure, the primary mirror can be supported only at the backside of the mirror. This mirror is tested with the combination of null Hartmann test and interferometer. The secondary mirror is tested with a Hindle method. This method requires 600 mm high quality spherical mirror. The distance between the primary and secondary mirror is maintained by the Carbon composite material. The assembly of two mirrors is carried out by the computer aided alignment method. The whole structure is designed to maintain the performance of the collimator under +/-5 degrees of temperature variation.
Most aspheric surfaces have been measured by null lens test or computer generated hologram (CGH) method. This approach, however, often fails when there are many aspherical terms or target surface is very small because it is not easy to design the conventional null lens or CGH. Hartmann test is a good choice for this case because it has a larger dynamic range than the general interferometer. This means that the surface can be measured with a Hartmann test without null correctors or with incomplete null correctors. In this paper, we apply the Hartmann test to the measurement of convex aspheric surface of which diameter is about 16 mm and has 4 additional aspheric terms. In order to measure the surface in real-time, we developed some CGH target that simulates the ideal target surface and the simple optical system. The measurement result can be served as a reference so that the form error of the target surface can be obtained by the subtraction of this reference, to achieve highly accurate measurement in real-time.
In the optical testing of aspheric lenses, the phase shifting lateral shearing interferometer has attracted considerable attention because of its advantage in alignment work. And many phase shifting lateral shearing interferometers were developed. But all of them are time delayed methods which have the time lag between the measurements. When air turbulence and vibration is present, this time lag can be onerous error sources. If we measure all the required interferograms for the phase shifting method at the same time, there should be no time leg. We present a simple instantaneous phase shifting lateral shearing interferometer using a wedge plate and two identical transmission gratings and reconstruct the wavefront error of measured instantaneous phase shifting lateral shearing interferograms.
Most aspheric mirrors have been measured by null lens test or computer generated hologram method. This approach, however, requires that the surface be close to the target shape; otherwise, the testing may not be possible or correct. Hartmann test has an advantage in that it has larger dynamic range than the general interferometer, which means that the surface can be measured with a Hartmann test from the early stage of polishing process. In this paper, authors show the measurement capability of Hartmann sensor, compared to the ZYGO phase-shifting interferometer and implement the Hartmann test to the measurement of 0.9 m aspheric mirror from the beginning of the polishing process. In order to increase the measurement accuracy and reduce the measurement time, authors develop the special null optics. The early stage of aspheric mirror was measured with the inteferometer and Hartmann sensor together, to show the great usefulness of Hartmann test in the measurement of large wavefront error, which can in turn reduce the manufacturing time of large optics.
In order to test aspheric optical elements, wedge plate lateral shearing interferometer is used. In a wedge plate the thickness varies along the wedge direction. Because of this thickness varying characteristic, the optical path difference between the original wavefront and the sheared wavefront in wedge plate lateral shearing interferometer is changed according to the incident position of the ray. Simply moving the wedge plate in-parallel to the wedge direction gives the phase shift quantity required for phase shifting interferometry. In typical phase shifting methods, piezoelectric transducer(PZT) is mainly used to give phase shifting quantity. But this method requires only one additional linear translator to move the wedge plate. The required moving distance for the phase shift of wavefront in this method is an order of millimeter, whereas the typical moving distance in the other method using a piezoelectric transducer is an order of wavelength. This method has an advantage in the moving distance precision compared with the other methods. Using this wedge plate lateral shearing interferometer, optical wavefront is measured and reconstructed.
A simple phase shifting (PS) technique without moving element in lateral shearing (LS) interferometer is presented. This method uses Murty’s simple LS interferometer with a wedge plate and adds only two identical transmission gratings. The two gratings can make three (or odd number of) equally separated identical test wavefronts. The wedge plate changes the optical path difference between the original wavefront and the sheared wavefront along the wedge direction. So three (or odd number of) interferograms having different phase shift quantity can be obtained simultaneously and 3-step PS interferometry algorithm can be applied.
A holographic polymer-dispersed liquid crystal (HPDLC) thin film is composed of multifunctional acrylate monomer blended with the nematic liquid crystal mixture, and then investigated the real-time diffraction efficiencies for various physical parameters of amount of liquid crystals and applied AC electric fields. It is experimentally shown that the holographic gratings recorded in HPDLC film can be reversibly erased and reconstructed by switching on and off of appropriate applied AC electric field. By using these electro-optic properties we have developed bifocal holographic lenses having two different focal lengths of 300 mm and 400 mm.
In order to use phase shifting interferometry(PSI) in lateral shearing interferometer, piezoelectric transducer(PZT) is mainly used. But because PZT has many error sources and the moving distance for required phase shifting is very short, the phase shifting quantity has always considerable errors. We present a simple phase shifting technique without PZT. Moving the wedge plate parallel to the first wedge surface changes the phase difference between the original wavefront and the sheared wavefront. This method makes the shearing interferometer very simple in spite of using PSI technique. We derive the phase shifting relations originated from the moving wedge and discuss some error factors.
Bessel beams are a special class of diffraction-free solutions of the wave equation which are of practical interest, for example, in precision alignment, laser machining and laser surgery. Therefore efficient methods for the transformation of a Gaussian laser beam into a Bessel beam are needed. We design and fabricate computer generated holograms(CGH) to generate this Bessel beam. Designed CGH is recorded onto the photopolymer which is used as a volume hologram material making CGHOE(computer generated holographic optical element). The principle of the CGH and the propagation characteristics of the Bessel beam are explained. The binary mask designed for generating Bessel beam is copied on photopolymer by the contact copy method. The axial intensity is measured for the beam propagation distances. The propagation property and the intensity profile of the Bessel beam are analyzed.
A method for measuring the wave front errors and the modulation transfer function (MTF) of the large aperture optics is presented. The large aperture Fizeau interferometer with long optical path difference by using a (phi) 400 mm off-axis parabolic mirror is made. The MTF is measured at the wavelength of the interferometer by changing the laser into the partially incoherent light. A bidirectional shearing interferometer is used for collimation testing. The test results of a (phi) 300 mm Cassegrain type satellite telescope made in Korea are presented.
Far-ultraviolet IMaging Spectrograph (FIMS) is a far ultraviolet diffuse imaging spectrometer which will be launched in 2002 as the main payload of KAISTSAT-4. We have designed the optics for observing diffuse emission sources by employing an off-axis parabolic cylinder mirror in front of a slit which guides lights to a diffraction grating. The reflective diffraction grating is an ellipse of rotation providing angular resolution. We describe our plan to measure the off-axis parabolic mirror and our initial experiments to establish the measurement technique. To assist manufacture of the off-axis parabolic cylinder, a cylindrical wavefront generated using computer generated hologram (CGH) will be used during the polishing to check errors in surface profile using the Fizeau interferometer.