Samuel Choi

Frequency-Comb-Based Interference Microscope with a Line-Type Image Sensor

We present a frequency-comb-based interferometric microscope using a broadband comb light source and a line-type image sensor for profilometry and optical tomography. A waveguide-type frequency comb generator and wavelength equalizer were introduced to broaden the comb spectrum. In this paper, we demonstrate the surface profilometry of a coin and the observation of a cross-sectional tomography image of transparent glass. The three-dimensional images were captured using a line-type image sensor without any mechanical moving parts with a frequency interval variation of 25 GHz. The dynamic range in the depth direction was about 1.4 mm and the measurement resolution was 35 µm. The standard deviation of seven thickness measurements was 5 µm.

Multi-gigahertz frequency comb-based interferometry using frequency-variable supercontinuum generated by optical pulse synthesizer

A multi-gigahertz frequency comb (MGFC)-based interferometer was developed for profilometry and tomography using a frequency variable supercontinuum (SC). The comparatively flattened and broadened SC light source with variable multi-gigahertz interval frequency was developed using an optical pulse synthesizer and highly nonlinear dispersion flattened fiber. The generated SC provided a stable interference output with a full width half maximum of 19 μm during interval frequency sweeping of over 400 MHz. We experimentally confirmed that the interference signal exhibited an envelope-only waveform without fringes, which enabled the drastic reduction of the sampling points resulting in high speed measurement. A full-field 3-D image with 320 × 256 × 300 pixels was acquired with a measurement time of only 10 seconds. It was demonstrated that the MGFC-based interferometer with the novel SC light source has the potential for application in a high speed full-field 3-D metrology.

Supercontinuum Comb Generation Using Optical Pulse Synthesizer and Highly Nonlinear Dispersion-Shifted Fiber

We propose a novel supercontinuum (SC) generation technology using an optical pulse synthesizer and highly nonlinear dispersion-shifted fiber. The optical pulse synthesizer enables us to reshape seed pulses optimized for broadband SC generation. 25.0 and 12.5 GHz SC combs were successfully generated by propagating synthesized seed pulses in the highly nonlinear fiber. We investigated how seed pulse conditions including shape, peak power, repetition rate, and center wavelength affect SC spectral bandwidth. We also evaluated interferometric signal waveforms from a Michelson interferometer using the generated SC comb as the light source. Interferometric peak width improved to 35.5 µm when peak power reached 7.9 W for the Gaussian pulse-based SC comb. Numerical simulation results almost agreed with experimental results. The difference between SC combs generated from Gaussian and sech2 pulses was confirmed by the simulations.

Phase-shifting laser diode interferometer using pulse modulation.

A phase-shifting laser diode interferometer that uses direct pulse modulation is proposed and demonstrated. We found that a laser beam with a wide range of wavelength variation at constant optical power could be generated when a pulsed current was injected into the laser diode. We constructed a highly accurate interferometer by using a pair of interferometers. Several experiments, such as observations of temporal interference signals and spatial interferograms, measurement of a concave mirror, and duplicate measurements, confirmed the characteristics of pulse modulation and demonstrated the effectiveness of our technique.

OCT based on multi-frequency sweeping Fizeau interferometer with phase modulating method

The Multi-frequency sweeping Fizeau-type interferometer (MFS-FI) for optical coherence tomography (OCT) is demonstrated. The multi-frequency sweeping by a variable Fabry-Perot filter permits detection of high-order low-coherence interferometric signals in the Fizeau interferometer. The sinusoidal phase modulation technique was utilized to detect accurate interference amplitude and phase distributions of back scattered light from surfaces of a sample. OCT measurements by the MFS-FI were conducted for vibrating glass plates with a frequency of 1 kHz, and cellular tissues fixed with formalin and embedded in paraffin. The tomographic 3-dimensional volume and cross-sectional surface displacements were detected with an accuracy of nano-meters.

Profile measurement of glass sheet using multiple wavelength backpropagation interferometry.

Multiple-wavelength backpropagation interferometry based on a spectral interferometer is proposed for measuring thin glass sheets with nanometer accuracy. The multiwavelength backpropagation method introduced to the spectral interferometer eliminates time-encoded wavelength sweeping and mechanical scanning, which enables high-speed profile measurements. The applicability of the proposed method is experimentally demonstrated through cross-sectional profile and vibrating surface displacement measurements of a glass sheet.

Utilization of frequency information in a linear wavenumber-scanning interferometer for profile measurement of a thin film

The positions of the front and rear surfaces of a silicon dioxide film with 4 μm thickness is measured with a novel and simple method in which both amplitude and phase of a sinusoidal wave signal corresponding to one optical path difference of a reflecting surface is utilized in a linear wavenumber-scanning interferometer. For this utilization, the scanning width and the position of the reference mirror are adjusted exactly to distinguish the two sinusoidal waves corresponding to the two surfaces of the film. The scanning width of the wavenumber and wavelength of the light source are 0.326×10(-3) nm(-1) and 140 nm, respectively.

Highly precise optical pulse synthesis for flat spectrum supercontinuum generation with wide mode spacing

We demonstrate highly precise pulse synthesis technique using an optical pulse synthesizer for spectrally flat supercontinuum generation. The technique achieved further 4-dB suppression of background noise of synthesized pulse and 3-dB suppression of supercontinuum spectrum peak at pump pulse wavelength.

Profile measurement based on spectral interferometer with multi-wavelength back-propagation methods

The multi-wavelength back-propagation (MWB) method enables to determine precisely the optical path different (OPD) longer than the optical wavelength from detecting the amplitude and phase of the interference signal for the multiple wavelengths. In this study, we demonstrate a 1-dimensional thickness profile measurement by the MWB and sinusoidal phase modulation (SPM) technique with a spectral interferometer. The OPD for the front and rear reflecting surfaces of a glass film with the thickness of 100 μm are measured. The thickness profile is successfully measured with repeatability of 2 nm estimated from a standard error between 9 repeatedly-measured profiles.

Profilometry using Fizeau-interferometer based on optical comb interferometry and sinusoidal phase modulation method

A profilometry based on Fizeau interferometer using the optical comb and the sinusoidal phase modulation technique is demonstrated. The optical comb generated with the Fabry-Perot etalon and a SLD is introduced in the Fizeau interferometer. The interval wave number of comb is swept by controlling the resonance length of the Fabry-Perot etalon. The sweeping range is about 500 μm. The displacement measurement is performed by determination of zerophase or π-phase positions. The accuracy estimated from 5 repeated experiments is about 0.2 rad.