Jae-Bong Song

Laser output power stabilization for direct laser writing system by using an acousto-optic modulator

We present experimental results on the output power stabilization of an Ar(+) laser for a direct laser writing system (LWS). Instability of the laser output power in the LWS cause 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. In this system, we have achieved the stability of +/-0.20% for 12 min and the relative intensity noise level of 2.1 x 10(-7) Hz(-12) 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 min.

Testing of steep convex aspheric surface with a Hartmann sensor by using a CGH

Most aspheric surfaces have been tested by interferometer with some null correctors. This approach, however, often fails when there are many aspherical terms or test surface is very steep because it is not easy to design the conventional null lens or CGH (Computer Generated Hologram). On the other hand, 3-D profilometer can measure aspheric surfaces without any null correctors; however, it takes some time to measure, which makes it unsuitable for the production line in the factory. In this paper, we apply the Hartmann test to the measurement of steep convex aspheric surfaces of which diameter is about 16 mm. In order to increase the measurement accuracy, we calibrated the test setup using a CGH that simulates the ideal test surface. We demonstrated that the significant amount of error in the test setup could be removed by this calibration process. The test results showed only 2 nm rms WFE (wave front error) difference even though the WFE of test setup was worsened by more than 0.13 mum rms. Since this method makes it possible to measure highly aspheric surface quickly and accurately, it can be used in the production line.

Null Hartmann test for the fabrication of large aspheric surfaces

Most aspheric mirrors have been tested by the null lens or computer-generated hologram method. This approach, however, requires that the shape of the surface be similar to the target shape; otherwise testing may not be possible or correct. The Hartmann test has an advantage in that it has a larger dynamic range than a general interferometer, which means that the surface can be tested beginning at an early stage of the polishing process. We suggest use of the null Hartmann test in conjunction with a phase-shifting interferometer for the measurement of a 0.9-m aspheric concave mirror. This setup was able to measure the surface with a large surface form error as well as with a small error without sacrificing any measurement accuracy. Using this setup, we have successfully polished a surface to remove approximately 1 microm of peak-to-valley wave-front error of a total of 39 microm of error during 1 month of polishing.

Modeling Alignment Experiment Errors for Improved Computer-Aided Alignment

Contrary to the academic interests of other existing studies elsewhere, this study deals with how the alignment algorithms such as sensitivity or Differential Wavefront Sampling (DWS) can be better used under effects from field, compensator positioning and environmental errors unavoidable from the shop-floor alignment work. First, the influences of aforementioned errors to the alignment state estimation was investigated with the algorithms. The environmental error was then found to be the dominant factor influencing the alignment state prediction accuracy. Having understood such relationship between the distorted system wavefront caused by the error sources and the alignment state prediction, we used it for simulated and experimental alignment runs for Infrared Optical System (IROS). The difference between trial alignment runs and experiment was quite close, independent of alignment methods; 6 nm rms for sensitivity method and 13 nm rms for DWS. This demonstrates the practical usefulness and importance of the prior error analysis using the alignment algorithms before the actual alignment runs begin. The error analysis methodology, its application to the actual alignment of IROS and their results are described together with their implications.

Computer aided alignment using Zernike coefficients

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.

High-speed and precision auto-focusing system for direct laser lithography

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.

Development of Cassegrain type 0.9-m collimator

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.

Optical test of aspheric elements using instantaneous wedge plate phase shifting lateral shearing interferometer

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.

Fabrication of 300-mm silicon reference wafer by using direct laser writer

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.

Modified alignment CGHs for aspheric surface test

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 cats 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 cats 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.