Hagyong Kihm

Fiber-diffraction interferometer for vibration desensitization

We describe a new interferometer designed to obtain high immunity to vibration by generating the reference wave by means of fiber diffraction so as to share a common path with the measurement wave. A spatial phase shifter is added to secure vibration desensitization by capturing four phase-shifted interferograms simultaneously without time delay. Experimental results prove that the proposed interferometer is capable of providing stable measurements with a level of fringe stabilization of less than 1 nm in a typical workshop environment equipped with no excessive ground isolation for antivibration.

Nonparaxial free-space diffraction from oblique end faces of single-mode optical fibers

We investigate free-space diffraction of light that emanates from obliquely cleaved end faces of single-mode fibers. Emphasis is placed on precise prediction of the wave-front sphericity of fiber-generating waves in the nonparaxial Fresnel propagation region spanning an entire hemispherical observation surface. Rayleigh-Sommerfeld scalar diffraction theory is used to produce an analytic closed-form solution with a nonparaxial approximation. The result allows the wave fronts sphericity and the amplitude distribution of fiber-generating waves to be evaluated precisely with less computation than for existing numerical or infinite-series solutions.

Oblique fiber optic diffraction interferometer for testing spherical mirrors

We present a new type of point diffraction interferometer that employs single-mode optical fibers instead of traditional pinholes. Two optical fibers with different exit endfaces are used; one is normally cut and the other obliquely cleaved. The normally cut fiber is used to gener- ate a near-perfect spherical reference wave. The test wave is produced from the oblique fiber that performs two functions: generating a spherical wave to illuminate the test optics, and reflecting the test wave for common-path combination with the reference wave. The interferometer proposed in this investigation is found suitable for testing large size mir- rors with a relatively simple overall hardware configuration.

A point-diffraction interferometer with vibration-desensitizing capability

We present a new type of point-diffraction interferometer specially designed for industrial use with high immunity to external vibration encountered in the course of measurement process. The proposed interferometer uses thermally-expanded fibers instead of conventional pinholes as the point-diffraction source to obtain a high quality reference wave with an additional advantage of relatively easy alignment of optical components. Vibration desensitization is realized through a common-path configuration that allows the influence of vibration to affect both the reference and measurement waves identically so that it is subsequently cancelled out during the interference of the two waves. A spatial phase shifter is added to capture four phase-shifted interferograms simultaneously without time delay using a single camera to avoid vibration effects. Experimental results demonstrate that the proposed interferometer is capable of providing stable measurements with a level of fringe stabilization of less than 1 nanometer in a typical workshop environment equipped with no excessive ground isolation for anti-vibration.

Fiber optic diffraction interferometer for testing spherical mirrors

Light emanating from the polished end of a single-mode fiber forms into a near-perfect spherical wave within a finite solid angle, which can readily be explained by diffraction theory. Utilizing the useful phenomenon, we in this paper present a fiber optic diffraction interferometer that has specially been devised for testing spherical mirrors. The interferometer adopts three optical fibers; one is for generating spherical reference wave, another is for illuminating the mirror under test and the other is for calibrating the interferometer. A special assembly of sliding prism driven by a PZT actuator provides necessary phase shifts with a high immunity to environmental disturbances. In conclusion, the proposed fiber optic diffraction interferometer enables us to achieve a measurement accuracy of an order of magnitude better than conventional testing schemes using the Fizeau interferometers of which measurement accuracy is ultimately limited by the reference surface to (lambda) /50, where λ is the wavelength of the source.

Point-diffraction fiber interferometer for vibration desensitization

We present a new type of point-diffraction interferometer specially designed for industrial use to obtain high immunity to external vibration encountered in the course of measurement process. The proposed interferometer uses thermally- expanded fibers instead of conventional pinholes as the point-diffraction source to obtain a high quality reference wave with an additional advantage of relatively easy alignment of interferometric optical setup. Vibration desensitization is realized through a common-path configuration that allows the influence of vibration to identically affect both the reference wave and the measurement wave and be subsequently cancelled out during the interference of the two waves. A new spatial phase shifter is also added to capture four phase-shifted interferograms simultaneously without time delay using a single camera to avoid vibration effect. Experimental results for a spherical concave mirror prove that the proposed interferometer is capable of providing stable measurements with a level of fringe stabilization of less than 1 nanometer in a typical workshop environment equipped with no excessive ground isolation for anti-vibration. Also, we verify that the proposed interferometer using a short coherence source is applicable to the surface metrology for defect inspection of transparent substrates such as liquid crystal display panels.

Nonparaxial Fresnel diffraction from oblique end facets of optical fibers

When optical fibers are connected with other optical or opto-mechanical components, free space propagation phenomena of the light emitted from cleaved end facets of fiber need to be precisely known to maximize coupling efficiency. Besides, optical fibers are widely used in interferometers design as point-diffraction sources to replace conventional pinholes, in which case the far-field wave front of the light emitted from fibers is of major concern. End facets may be cleaved normal to the optical axis of fibers or with oblique angles to suit specific purposes such as anti-reflection or propagation direction alteration. In this investigation, diffraction from oblique end facets of single-mode fibers is studied with emphasis on Fresnel propagation in nonparaxial zones based on the Rayleigh-Sommerfeld scalar diffraction theory. The result is a closed-form explicit solution expressed in terms of spherical coordinates, which enables to determine the propagation field generated over an entire hemispherical observation surface. In comparison to exiting solutions of numerical or infinite series forms, the explicit solution obtained in this investigation saves a considerable amount of computation time and provides better estimation accuracy. Finally, as an example, the wave front sphericity emitted from an oblique single-mode fiber used in a new design of the Fizeau interferometer is determined and discussed in detail.

Oblique point-diffraction source for interferometer design

We describe an oblique point-diffraction source, which is made from a single-mode optical fiber whose end is specially cut with an oblique face angle of 28.8° to the optical axis of the fiber. Analytical and empirical investigations reveal that, like ordinary fibers cut with zero face angle, the oblique fiber generates a high-quality spherical wave into free space by means of point diffraction at its end, but the diffracted wave has a propagation angle of 45° to the surface normal. This inclined nature of point diffraction is useful in many optical designs because it allows the returning wave of the original diffracted wave to be reflect at a right angle at the end face of the oblique fiber without additional optics. An exemplary use of the oblique fiber is demonstrated in an enhanced optical design of Fizeau interferometer, in which the troublesome combination of a pinhole and a beam splitter is effectively replaced only with a single oblique fiber. Another example is shown in a phase-shifting diffraction interferometer that has been specially designed for testing concave mirrors of low f-numbers. The use of oblique fibers enables to use the entire full numerical aperture of the diffracted wave from a single-mode fiber for optical testing without any overlap between the reference and the test waves.