Akiko Hirai

Broad-spectrum frequency comb generation and carrier-envelope offset frequency measurement by second-harmonic generation of a mode-locked fiber laser

Frequency comb spanning more than one octave has been achieved by injecting the second harmonic generation (780 nm) of a mode-locked fiber laser (1.56 /spl mu/m) into a photonic crystal fiber. We propose and realize a novel interferometric scheme for observing the carrier-envelope offset frequency.

Low-coherence tandem interferometer for measurement of group refractive index without knowledge of the thickness of the test sample

A new low-coherence interferometric technique is proposed for measuring the group refractive indices of dispersive samples with high accuracy. A tandem configuration of interferometers is used to compensate for the asymmetrical distortion of interferograms that results from the broad spectrum of the light source. The group refractive index can be measured without knowledge of the geometrical thickness of the sample under test. The proposed technique can successfully measure even a thick sample. Computer calculations have shown the effectiveness of the proposed technique, which was verified by preliminary experiments; the difference between the experimental result and the catalog data was 7 x 10(-4).

High-sensitivity surface-profile measurements by heterodyne white-light interferometer

A tandem white-light interferometer has been developed, based on the heterodyne technique, for highly accurate and sensitive surface profiling of a three-dimensional (3-D) object. Two acousto-optic modulators (AOMs) and two spherical reflecting mirrors are used in a Michelson interferometer to provide the optical frequency shift for white light. By using this technique in the tandem interferometer, the profiles of mirrorlike and diffusing surfaces of 3-D objects are measured with heterodyne signals of 200 kHz. The optical path difference of the interferometer can be stabilized by simultaneously using coherent and incoherent light. The experimental results show a 27-fold improvement in the stability of the interferometer and the possibility of achieving high accuracy of several tens of nanometers.

A white-light interferometer using a lamp source and heterodyne detection with acousto-optic modulators

Abstract A heterodyne technique for white-light interferometer with a lamp source, which uses two acousto-optic modulators, is developed for high-sensitivity detection of weak light. By using converging input beams input into a Michelson interferometer with spherical mirrors, the spectral dependence of the modulators is canceled, and the white-light heterodyne interference fringes are generated at 200 kHz. Using a tandem interferometer, the object surface which has a low surface reflectivity of less than 10 −4 was detected with a good signal-to-noise ratio.

Fully interferometric three-dimensional imaging spectrometry using hyperbolic-type volume interferogram

A signal-processing method is proposed in the fully interferometric three-dimensional (3D) imaging spectrometry. This processing computes a 3D interferogram, in which recorded fringe patterns do not directly reflect wavefront forms propagated from a polychromatic light source under measurement. This paper presents a procedure for signal processing including a synthesis of the 3D interferogram and retrieval of a set of spectral components of 3D images. We demonstrate retrieving 3D images for spectral components of two planar light sources by means of the proposed method. The procedure to synthesize the 3D interferogram in this method suggests the possibility of direct measurement of the 3D interferogram.

High-accuracy interferometer with a prism pair for measurement of the absolute refractive index of glass

Interferometer for precise refractive-index measurement of glass-prism without prior knowledge is developed using a direct comparison of optical-path-changes in glass and air. Preliminary result shows uncertainty of 4.4 x 10-5 agreeing with the manufacturerpsilas values.

Measurement of group refractive index wavelength dependence using a low-coherence tandem interferometer

A technique for the measurement of the group refractive index wavelength dependence of optical materials using a low-coherence tandem interferometer and a spectrometer is proposed. Four channeled spectra resulting from interferences of light beams from different pairs of optical paths are used for the calculation of optical path differences. The group refractive index wavelength dependence is calculated from these optical path differences generated from the sample under measurement. No a priori information of the geometric thickness of a sample is required. The wavelength dependence of the group refractive index of the samples BK7 parallel plate of 5.200 and 10.025 mm from 675 to 850 nm is experimentally measured with an accuracy of the order of 10(-5) and a repeatability of the order of 10(-9).

A two-color heterodyne interferometer for measuring the refractive index of air using an optical diffraction grating

A new two-color heterodyne interferometer using an optical grating was proposed for measuring the refractive index of air. The heterodyne modulation is realized by moving the diffraction grating. The method allows us to measure the refractive index with high accuracy by using a 1:06 lm YAG laser and its second-harmonic wave because two beams (the fundamental and the second-harmonic wave) are in common path completely. Moreover, good signal-to-noise ratio (SNR) can be obtained by the heterodyne interference. Using the refractive index calculated by Edl e equation as a reference, that measured by present method was experimentally evaluated having an accuracy of about 1 � 10 � 6 within a distance of 100 mm. 2002 Elsevier Science B.V. All rights reserved.

Gauge block interferometer using three frequency-stabilized lasers

We have developed a gauge block measurement system that uses three frequency-stabilized lasers. The stabilized lasers are as follows: an I2 stabilized offset locked He-Ne laser, an I2-stabilized Nd:YAG laser, and a Rb-stabilized diode laser. The I2-stabilized offset locked He-Ne laser is commercially available and its relative wavelength uncertainty is 2.5 X 10-11. An I2-stabilized Nd:YAG laser and a Rb-stabilized diode laser was developed in our institute and their relative wavelength uncertainties are 5 X 10-12 and 1 X 10-9, respectively. In the measurement system, laser beams were introduced to the interferometer using an optical multimode fiber. An interferometer fringe pattern was taken using a CCD camera and the excess fraction parts were calculated from the fringe pattern using the Fourier transform method. The excess fraction part obtained from the Rb-stabilized semiconductor laser was used only to determine the integer part of the fringe order, because the accuracy and stability of the wavelength were not sufficient for the long gauge block measurements. This interferometer can measure gauge blocks of up to 1000 nm long and the standard uncertainty of the interferometer is about 75 nm for a 1000 mm long gauge block.

White-light interferometry using pseudo random-modulation for high-sensitivity and high-selectivity measurements

We developed a white-light interferometer that uses pseudo random-modulation for high-sensitivity, high-selectivity measurements of the surface profile of three-dimensional objects. The pseudo random-modulation is achieved by using a multi-segment mirror, where each segment has a different height in a maximum shift-register sequence (M-sequence). Preliminary experiments, where the white-light source was a halogen lamp and the test object was a plane mirror, show that our M-sequence modulated white-light interferometer (MMWI) had a four-time improvement in the signal-to-noise (SNR) ratio compared with that of a conventional interferometer.