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Author

Ryo Oe

Bio: Ryo Oe is an academic researcher from University of Tokushima. The author has contributed to research in topics: Optical fiber & Refractive index. The author has an hindex of 7, co-authored 29 publications receiving 126 citations.

Papers
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Journal ArticleDOI
TL;DR: In this article, a dual terahertz (THz) comb spectroscopy system with an intracavity Lyot filter and a single free-running fiber laser is presented.
Abstract: Dual terahertz (THz) comb spectroscopy enables high spectral resolution, high spectral accuracy, and broad spectral coverage; however, the requirement for dual stabilized femtosecond lasers hampers its versatility. We here report the first demonstration of dual THz comb spectroscopy using a single free-running fibre laser. By tuning the cavity-loss-dependent gain profile with an intracavity Lyot filter together with precise management of the cavity length and dispersion, dual-wavelength comb light beams with slightly detuned repetition frequencies are generated in a single laser cavity. Due to sharing of the same cavity, such comb light beams suffer from common-mode fluctuation of the repetition frequency, and hence the corresponding frequency difference between them is passively stable around a few hundred hertz within millihertz fluctuation. While greatly reducing the size, complexity, and cost of the laser source by use of a single free-running fibre laser, the dual THz comb spectroscopy system maintains a spectral bandwidth and dynamic range of spectral power comparable to a system equipped with dual stabilized fibre lasers, and can be effectively applied to high-precision spectroscopy of acetonitrile gas at atmospheric pressure. The demonstrated results indicate that this system is an attractive solution for practical applications of THz spectroscopy and other applications.

33 citations

Journal ArticleDOI
TL;DR: In this paper, a coherent link between the optical and radio frequencies is proposed to enable high-precision refractive index measurement based on frequency measurement in radio-frequency (RF) region.
Abstract: Optical frequency combs (OFCs) have attracted attention as optical frequency rulers due to their tooth-like discrete spectra together with their inherent mode-locking nature and phase-locking control to a frequency standard. Based on this concept, their applications until now have been demonstrated in the fields of optical frequency metrology. However, if the utility of OFCs can be further expanded beyond their application by exploiting new aspects of OFCs, this will lead to new developments in optical metrology and instrumentation. Here, we report a fiber sensing application of OFCs based on a coherent link between the optical and radio frequencies, enabling high-precision refractive index measurement based on frequency measurement in radio-frequency (RF) region. Our technique encodes a refractive index change of a liquid sample into a repetition frequency of OFC by a combination of an intracavity multi-mode-interference fiber sensor and wavelength dispersion of a cavity fiber. Then, the change in refractive index is read out by measuring the repetition frequency in RF region based on a frequency standard. Use of an OFC as a photonic RF converter will lead to the development of new applications in high-precision fiber sensing with the help of functional fiber sensors and precise RF measurement.

24 citations

Journal ArticleDOI
TL;DR: In this paper, a coherent frequency link between the optical and radio-frequency regions, enabling high-precision refractive index measurement of a liquid sample based on frequency measurement in radio frequency region is presented.
Abstract: Optical frequency combs have attracted attention as optical frequency rulers due to their tooth-like discrete spectra together with their inherent mode-locking nature and phase-locking control to a frequency standard. Based on this concept, their applications until now have been demonstrated in the fields of optical frequency metrology and optical distance metrology. However, if the utility of optical combs can be further expanded beyond their optical-frequency-ruler-based application by exploiting new aspects of optical combs, this will lead to new developments in optical metrology and instrumentation. Here, we report a fibre sensing application of optical combs based on a coherent frequency link between the optical and radio-frequency regions, enabling high-precision refractive index measurement of a liquid sample based on frequency measurement in radio-frequency region. Our technique encodes a refractive index change of a liquid sample into a radio-frequency comb by a combination of an intracavity multi-mode-interference fibre sensor and wavelength dispersion of a cavity fibre. Then, the change in refractive index is read out by measuring the repetition frequency of the radio-frequency comb with a frequency counter and a frequency standard. Use of an optical comb as a photonic radio-frequency converter will lead to the development of new applications in high-precision fibre sensing with the help of functional fibre sensors and precise radio-frequency measurement.

19 citations

Journal ArticleDOI
TL;DR: In this paper, a dual-wavelength pulsed light beams with slightly detuned repetition frequencies are generated in a single laser cavity using an intracavity Lyot filter.
Abstract: Dual THz comb spectroscopy has the potential to be used as universal THz spectroscopy with high spectral resolution, high spectral accuracy, and broad spectral coverage; however, the requirement for dual stabilized femtosecond lasers hampers its versatility due to the bulky size, high complexity, and high cost. We here report the first demonstration of dual THz comb spectroscopy using a single free-running fibre laser. By tuning the cavity-loss-dependent gain profile with an intracavity Lyot filter together with precise management of the cavity length and dispersion, dual-wavelength pulsed light beams with slightly detuned repetition frequencies are generated in a single laser cavity. Due to sharing of the same cavity, such pulsed light beams suffer from common-mode fluctuation of the repetition frequency, and hence the corresponding frequency difference between them is passively stable around a few hundred hertz within millihertz fluctuation. This considerably stable frequency difference enables dual THz comb spectroscopy with a single free-running fibre laser. While greatly reducing the size, complexity, and cost of the laser source by use of a single free-running fibre laser, the dual THz comb spectroscopy system maintains a spectral bandwidth and dynamic range of spectral power comparable to a system equipped with dual stabilized fibre lasers, and can be effectively applied to high-precision spectroscopy of acetonitrile gas at atmospheric pressure. The demonstrated results indicate that this system is an attractive solution for practical applications of not only THz spectroscopy but also THz-pulse-based measurements.

18 citations

Journal ArticleDOI
TL;DR: In this article, a tunable external cavity laser diode phase-locked to an optical frequency comb was constructed to generate multiple synthetic wavelengths within the range of 32 µm to 1.20 m. The axial dynamic range, defined as the ratio of the axial range ( = 2.0 mm) to the Axial resolution ( = 34 nm), achieved 5.9 × 105, which is larger than that of previous synthetic wavelength DH.
Abstract: Digital holography (DH) is a promising method for non-contact surface topography because the reconstructed phase image can visualize the nanometer unevenness in a sample. However, the axial range of this method is limited to the range of the optical wavelength due to the phase wrapping ambiguity. Although the use of two different wavelengths of light and the resulting synthetic wavelength, i.e., synthetic wavelength DH, can expand the axial range up to several hundreds of millimeters, its axial precision does not reach sub-micrometer. In this article, we constructed a tunable external cavity laser diode phase-locked to an optical frequency comb, namely, an optical-comb-referenced frequency synthesizer, enabling us to generate multiple synthetic wavelengths within the range of 32 µm to 1.20 m. A multiple cascade link of the phase images among an optical wavelength ( = 1.520 µm) and 5 different synthetic wavelengths ( = 32.39 µm, 99.98 µm, 400.0 µm, 1003 µm, and 4021 µm) enables the shape measurement of a reflective millimeter-sized stepped surface with the axial resolution of 34 nm. The axial dynamic range, defined as the ratio of the axial range ( = 2.0 mm) to the axial resolution ( = 34 nm), achieves 5.9 × 105, which is larger than that of previous synthetic wavelength DH. Such a wide axial dynamic range capability will further expand the application field of DH for large objects with meter dimensions.

18 citations


Cited by
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Journal ArticleDOI
TL;DR: The developed high-coherence ultra-broadband dual-comb fiber laser with capability of fCEO detection and frequency measurement using a self-referencing technique is likely to be a highly effective tool in practical, high-sensitivity, ultra-Broadband applications.
Abstract: Dual-comb spectroscopy has emerged as an attractive spectroscopic tool for high-speed, high-resolution, and high-sensitivity broadband spectroscopy. It exhibits certain advantages when compared to the conventional Fourier-transform spectroscopy. However, the high cost of the conventional system, which is based on two mode-locked lasers and a complex servo system with a common single-frequency laser, limits the applicability of the dual-comb spectroscopy system. In this study, we overcame this problem with a bidirectional dual-comb fiber laser that generates two high-coherence ultra-broadband frequency combs with slightly different repetition rates (frep). The two direct outputs from the single-laser cavity displayed broad spectra of > 50 nm; moreover, an excessively small difference in the repetition rate (< 1.5 Hz) was achieved with high relative stability, owing to passive common-mode noise cancellation. With this slight difference in the repetition rate, the applicable optical spectral bandwidth in dual-comb spectroscopy could attain ~479 THz (~3,888 nm). In addition, we successfully generated high-coherence ultra-broadband frequency combs via nonlinear spectral broadening and detected high signal-to-noise-ratio carrier–envelope offset frequency (fCEO) beat signals using the self-referencing technique. We also demonstrated the high relative stability between the two fCEO beat signals and tunability. To our knowledge, this is the first demonstration of fCEO detection and frequency measurement using a self-referencing technique for a dual-comb fiber laser. The developed high-coherence ultra-broadband dual-comb fiber laser with capability of fCEO detection is likely to be a highly effective tool in practical, high-sensitivity, ultra-broadband applications.

60 citations

Journal ArticleDOI
22 May 2020-iScience
TL;DR: This paper explicitly shows that the nonlinear interaction between ultrafast pulses and optical fibers plays the essential role and the toolbox of ultrafast fiber lasers will continue to expand and provide solutions to scientific and industrial problems.

58 citations

Journal ArticleDOI
TL;DR: In this paper, two free-running quantum cascade lasers (QCLs) were used to generate approximately 120 GHz wide combs centered at 4.2 THz, with slightly different repetition frequencies.
Abstract: Due to its fast and high-resolution characteristics, dual-comb spectroscopy has attracted an increasing amount of interest since its first demonstration. In the terahertz frequency range where abundant absorption lines (fingerprints) of molecules are located, multiheterodyne spectroscopy that employs the dual-comb technique shows an advantage in real-time spectral detection over the traditional Fourier transform infrared or time domain spectroscopies. Here, we demonstrate compact terahertz dual-comb spectroscopy based on quantum cascade lasers (QCLs). In our experiment, two free-running QCLs generate approximately 120 GHz wide combs centered at 4.2 THz, with slightly different repetition frequencies. We observe that ∼490 nW terahertz power coupling of one laser into the other suffices for laser-self-detecting the dual-comb spectrum that is registered by a microwave spectrum analyzer. Furthermore, we demonstrate practical terahertz transmission dual-comb spectroscopy with our device, by implementing a shor...

47 citations

Journal ArticleDOI
TL;DR: In this article, a hybrid dual frequency comb spectrometer for line-center determination of methanol molecular transitions is presented, combining the advantages of a THz QCL-FC with the accuracy and absolute frequency referencing provided by a free-standing, optically-rectified THz frequency comb.
Abstract: Four-wave-mixing-based quantum cascade laser frequency combs (QCL-FC) are a powerful photonic tool, driving a recent revolution in major molecular fingerprint regions, i.e. mid- and far-infrared domains. Their compact and frequency-agile design, together with their high optical power and spectral purity, promise to deliver an all-in-one source for the most challenging spectroscopic applications. Here, we demonstrate a metrological-grade hybrid dual comb spectrometer, combining the advantages of a THz QCL-FC with the accuracy and absolute frequency referencing provided by a free-standing, optically-rectified THz frequency comb. A proof-of-principle application to methanol molecular transitions is presented. The multi-heterodyne molecular spectra retrieved provide state-of-the-art results in line-center determination, achieving the same precision as currently available molecular databases. The devised setup provides a solid platform for a new generation of THz spectrometers, paving the way to more refined and sophisticated systems exploiting full phase control of QCL-FCs, or Doppler-free spectroscopic schemes. THz dual comb spectroscopy may be performed using down-converted or quantum cascade laser systems, forcing a choice between absolute frequency referencing and high sensitivity. Here, these strengths are combined in a hybrid, dual frequency comb spectrometer, capable of high-accuracy measurements on molecular transitions.

43 citations

Journal ArticleDOI
TL;DR: In this paper, the authors present the research status and prospects of several common continuous-wave (CW) terahertz medical imaging systems and applications of THz medical imaging in biological tissues.
Abstract: In the past few decades, the applications of terahertz (THz) spectroscopy and imaging technology have seen significant developments in the fields of biology, medical diagnosis, food safety, and nondestructive testing. Label-free diagnosis of malignant tumours has been obtained and also achieved significant development in THz biomedical imaging. This review mainly presents the research status and prospects of several common continuous-wave (CW) THz medical imaging systems and applications of THz medical imaging in biological tissues. Here, we first introduce the properties of THz waves and how these properties play a role in biomedical imaging. Then, we analyse both the advantages and disadvantages of the CW THz imaging methods and the progress of these methods in THz biomedical imaging in recent ten years. Finally, we summarise the obstacles in the way of the application of THz bio-imaging application technology in clinical detection, which need to be investigated and overcome in the future.

38 citations