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Author

Shuko Yokoyama

Bio: Shuko Yokoyama is an academic researcher from Osaka University. The author has contributed to research in topics: Terahertz radiation & Frequency comb. The author has an hindex of 15, co-authored 51 publications receiving 1018 citations.


Papers
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Journal ArticleDOI
TL;DR: In this article, a stable terahertz frequency comb from a photoconductive tera-hertz emitter driven by a stabilized femtosecond laser was observed with frequency accuracy of 2.5×10−7 and resolution of 81.8MHz.
Abstract: We report a terahertz spectroscopy technique based on a stable terahertz frequency comb from a photoconductive terahertz emitter driven by a stabilized femtosecond laser. To this end, a photocurrent frequency comb is induced in a photoconductive terahertz detector by instantaneous photogating with another detuned femtosecond laser and is applied to read out the terahertz frequency comb. The detailed structure of the terahertz frequency comb was clearly observed with frequency accuracy of 2.5×10−7 and resolution of 81.8MHz using multifrequency-heterodyning photoconductive detection, which in turn is caused by the slightly mismatched frequency spacing between terahertz and photocurrent frequency combs.

235 citations

Journal ArticleDOI
TL;DR: A THz spectrum analyzer is proposed to measure the absolute frequency and spectral shape of continuous-wave terahertz waves and has the potential to become a powerful tool for THz frequency metrology.
Abstract: Precision frequency measurements of terahertz (THz) waves are required to establish metrology in the THz spectral region. However, frequency measurement techniques in this region are immature. We propose a THz spectrum analyzer to measure the absolute frequency and spectral shape of continuous-wave THz waves. Based on a stable frequency comb generated in a photoconductive antenna, the absolute frequency of a sub- THz test source was determined at a precision of 2.8 x 10(-11). Furthermore, the spectral bandwidth of the THz spectrum analyzer can be extended over 1 THz, as demonstrated by measurement of a THz test source. This spectrum analyzer has the potential to become a powerful tool for THz frequency metrology.

105 citations

Journal ArticleDOI
TL;DR: This work uses a combination of a spectral interleaving and dual-comb spectroscopy in the terahertz (THz) region to achieve a spectral sampling interval equal to the mode linewidth rather than the mode spacing.
Abstract: Optical frequency combs are innovative tools for broadband spectroscopy because a series of comb modes can serve as frequency markers that are traceable to a microwave frequency standard. However, a mode distribution that is too discrete limits the spectral sampling interval to the mode frequency spacing even though individual mode linewidth is sufficiently narrow. Here, using a combination of a spectral interleaving and dual-comb spectroscopy in the terahertz (THz) region, we achieved a spectral sampling interval equal to the mode linewidth rather than the mode spacing. The spectrally interleaved THz comb was realized by sweeping the laser repetition frequency and interleaving additional frequency marks. In low-pressure gas spectroscopy, we achieved an improved spectral sampling density of 2.5 MHz and enhanced spectral accuracy of 8.39 × 10−7 in the THz region. The proposed method is a powerful tool for simultaneously achieving high resolution, high accuracy, and broad spectral coverage in THz spectroscopy.

87 citations

Journal ArticleDOI
TL;DR: In this article, two techniques for terahertz frequency metrology based on frequency comb, namely, a THz-comb-referenced spectrum analyzer and a continuously tunable, single-frequency continuous-wave (CW)-THz generator, are reviewed.
Abstract: Two techniques for terahertz (THz) frequency metrology based on frequency comb, namely, a THz-comb-referenced spectrum analyzer and a continuously tunable, single-frequency continuous-wave (CW)-THz generator, are reviewed. Since the frequency comb enables to coherently link the frequency among microwave, optical, and THz regions, it is possible to establish the THz frequency metrology traceable to time of the SI base units. Using a THz-comb-referenced spectrum analyzer based on a stable THz comb generated in a photoconductive antenna for THz detection, the absolute frequency of CW test sources in the sub-THz and THz regions was determined at a precision of 10-11. Furthermore, a continuously tunable, single-frequency CW-THz generator was demonstrated around 120 GHz by photomixing of an accurately tunable CW laser and a tightly fixed CW laser in the optical frequency region, phase locked to two independent optical combs. The combination of the CW-THz generator with the THz-comb-referenced spectrum analyzer will open the door for establishment of frequency metrology in the THz region.

81 citations

Journal ArticleDOI
TL;DR: A novel method of phase measurement of terahertz intermode beat in optical frequency comb, to enhance the dynamic-range of distance meter was proposed and distance measurement with 130 GHz equivalent frequency was realized.
Abstract: We propose a distance meter that utilizes an intermode beat of terahertz frequency in an optical frequency comb to perform high resolution and high dynamic range absolute distance measurements. The proposed system is based on a novel method, called multiheterodyne cross-correlation detection, in which intermode beat frequencies are scaled down to radio frequencies by optical mixing of two detuned optical frequency combs with a nonlinear optical crystal. Using this method, we obtained a 1.056 THz intermode beat and achieved a distance resolution of 0.820 μm from its phase measurement. Absolute distance measurement using 1.056 THz and 8.187 GHz intermode beats was also demonstrated in the range of 10 mm, resulting in a precision of 0.688 μm.

62 citations


Cited by
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Journal Article
TL;DR: In this article, a fast Fourier transform method of topography and interferometry is proposed to discriminate between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour generation techniques.
Abstract: A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

3,742 citations

Journal Article
TL;DR: In this article, a self-scanned 1024 element photodiode array and a minicomputer are used to measure the phase (wavefront) in the interference pattern of an interferometer to lambda/100.
Abstract: A self-scanned 1024 element photodiode array and minicomputer are used to measure the phase (wavefront) in the interference pattern of an interferometer to lambda/100. The photodiode array samples intensities over a 32 x 32 matrix in the interference pattern as the length of the reference arm is varied piezoelectrically. Using these data the minicomputer synchronously detects the phase at each of the 1024 points by a Fourier series method and displays the wavefront in contour and perspective plot on a storage oscilloscope in less than 1 min (Bruning et al. Paper WE16, OSA Annual Meeting, Oct. 1972). The array of intensities is sampled and averaged many times in a random fashion so that the effects of air turbulence, vibrations, and thermal drifts are minimized. Very significant is the fact that wavefront errors in the interferometer are easily determined and may be automatically subtracted from current or subsequent wavefrots. Various programs supporting the measurement system include software for determining the aperture boundary, sum and difference of wavefronts, removal or insertion of tilt and focus errors, and routines for spatial manipulation of wavefronts. FFT programs transform wavefront data into point spread function and modulus and phase of the optical transfer function of lenses. Display programs plot these functions in contour and perspective. The system has been designed to optimize the collection of data to give higher than usual accuracy in measuring the individual elements and final performance of assembled diffraction limited optical systems, and furthermore, the short loop time of a few minutes makes the system an attractive alternative to constraints imposed by test glasses in the optical shop.

1,300 citations

Journal ArticleDOI
20 Apr 2016
TL;DR: This review describes dual-comb spectroscopy and summarizes the current state of the art and suggests that frequency comb technology will continue to mature and could surpass conventional broadbandSpectroscopy for a wide range of laboratory and field applications.
Abstract: Dual-comb spectroscopy is an emerging new spectroscopic tool that exploits the frequency resolution, frequency accuracy, broad bandwidth, and brightness of frequency combs for ultrahigh-resolution, high-sensitivity broadband spectroscopy. By using two coherent frequency combs, dual-comb spectroscopy allows a sample’s spectral response to be measured on a comb tooth-by-tooth basis rapidly and without the size constraints or instrument response limitations of conventional spectrometers. This review describes dual-comb spectroscopy and summarizes the current state of the art. As frequency comb technology progresses, dual-comb spectroscopy will continue to mature and could surpass conventional broadband spectroscopy for a wide range of laboratory and field applications.

1,113 citations

Journal ArticleDOI
TL;DR: The goal of this paper is to provide a comprehensive review of wireless sub-THz and THz communications and report on the reported advantages and challenges of using sub-terahertz andTHz waves as a means to transmit data wirelessly.
Abstract: According to Edholm’s law, the demand for point-to-point bandwidth in wireless short-range communications has doubled every 18 months over the last 25 years It can be predicted that data rates of around 5–10 Gb/s will be required in ten years In order to achieve 10 Gb/s data rates, the carrier frequencies need to be increased beyond 100 GHz Over the past ten years, several groups have considered the prospects of using sub-terahertz (THz) and THz waves (100–2000 GHz) as a means to transmit data wirelessly Some of the reported advantages of THz communications links are inherently higher bandwidth compared to millimeter wave links, less susceptibility to scintillation effects than infrared wireless links, and the ability to use THz links for secure communications Our goal of this paper is to provide a comprehensive review of wireless sub-THz and THz communications

991 citations

Journal ArticleDOI
TL;DR: In this paper, the authors demonstrate a coherent laser ranging system that combines the advantages of time-of-flight and interferometric approaches to provide absolute distance measurements, simultaneously from multiple reflectors, and at low power.
Abstract: The ability to determine absolute distance to an object is one of the most basic measurements of remote sensing. High-precision ranging has important applications in both large-scale manufacturing and in future tight formation-flying satellite missions, where rapid and precise measurements of absolute distance are critical for maintaining the relative pointing and position of the individual satellites. Using two coherent broadband fibre-laser frequency comb sources, we demonstrate a coherent laser ranging system that combines the advantages of time-of-flight and interferometric approaches to provide absolute distance measurements, simultaneously from multiple reflectors, and at low power. The pulse time-of-flight yields a precision of 3 µm with an ambiguity range of 1.5 m in 200 µs. Through the optical carrier phase, the precision is improved to better than 5 nm at 60 ms, and through the radio-frequency phase the ambiguity range is extended to 30 km, potentially providing 2 parts in 1013 ranging at long distances. Using two coherent broadband fibre-laser frequency comb sources, a coherent laser ranging system for absolute distance measurements is demonstrated. Its combination of precision, speed and long range may prove particularly useful for space-based sciences.

789 citations