Papers - Hasegawa, Taro

Improvement in the Sagnac-loop laser frequency stabilization

Taro Hasegawa

Journal of Optical Society of America B (Optica)  42 ( 2 ) 285 - 291 2025.01

Research paper (scientific journal), Single Work, Lead author, Last author, Corresponding author, Accepted

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Evaluation of the Sagnac-loop laser frequency stabilization to sub-Doppler spectral lines: erratum

Taro Hasegawa

Journal of Optical Society of America B (Optica)  42 ( 1 ) 18 - 18 2024.12

Research paper (scientific journal), Single Work, Lead author, Last author, Corresponding author, Accepted

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Phase structure of harmonically mode-locked optical frequency combs

Taro Hasegawa

Optics Express (Optica)  32 ( 5 ) 8092 - 8100 2024.02

Research paper (scientific journal), Single Work, Lead author, Last author, Corresponding author, Accepted

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Carrier phase of a harmonically mode-locked optical frequency comb (OFC) is investigated in detail. While harmonically mode-locked OFCs show promise for high-repetition-rate applications, their mode spacing is not the same as the pulse repetition rate, unlike fundamentally mode-locked OFCs. Consequently, harmonically mode-locked OFCs are unsuitable for applications requiring OFCs with wide mode spacing. This study examines the pulse-to-pulse carrier phase evolution of 4th- and 5th-order harmonically mode-locked OFCs, revealing uneven carrier phase evolution responsible for the narrow mode spacing. The possibility of achieving harmonically mode-locked OFCs with wide mode spacing is suggested by implementing pulse-to-pulse phase modulation to ensure even phase evolution.

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Evaluation of the Sagnac-loop laser frequency stabilization to sub-Doppler spectral lines

Taro Hasegawa

Journal of Optical Society of America B (Optica)  40 ( 3 ) 667 - 672 2023.03

Single Work, Lead author, Last author, Corresponding author, Accepted,  ISSN  07403224

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Laser frequency stability locked to D2 transition of 85Rb at 780 nm by the Sagnac-loop stabilization scheme was evaluated using the heterodyne measurement with an optical frequency comb. This scheme does not require modulation and magnetic field to generate the feedback error signal. Therefore, the setup for the laser frequency stabilization is simple. Because of the mechanical stability of the Sagnac interferometer, the dispersion of atom transition, which is used as an error signal for feedback loop, can be observed with negligibly small effects of mechanical fluctuations. In the experiment, the Allan variance for relative frequency uncertainty was 2.6×10-12 over 64 s averaging.

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Mode spacing of harmonic mode-locked optical frequency combs

Hasegawa T.

Hyperspectral/Multispectral Imaging and Sounding of the Environment in Proceedings Optica Sensing Congress 2023, AIS, FTS, HISE, Sensors, ES - Part of Optical Sensors and Sensing Congress 2023  2023

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Mode spacing of an optical frequency comb of a harmonic mode-locked laser is studied by using an optical cavity. It turns out that mode spacing disagrees with pulse repetition rate for the harmonic mode-locked comb.

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Mode spacing of harmonic mode-locked optical frequency combs

Hasegawa T.

Applied Industrial Spectroscopy in Proceedings Optica Sensing Congress 2023, AIS, FTS, HISE, Sensors, ES 2023  2023

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Mode spacing of an optical frequency comb of a harmonic mode-locked laser is studied by using an optical cavity. It turns out that mode spacing disagrees with pulse repetition rate for the harmonic mode-locked comb.

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Mode spacing of harmonic mode-locked optical frequency combs

Hasegawa T.

Optical Sensors: Proceedings Optica Sensing Congress 2023, AIS, FTS, HISE, Sensors, ES 2023  2023

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Mode spacing of an optical frequency comb of a harmonic mode-locked laser is studied by using an optical cavity. It turns out that mode spacing disagrees with pulse repetition rate for the harmonic mode-locked comb.

Mode spacing multiplication of optical frequency combs without power loss

Taiki Kageyama, Taro Hasegawa

Optics Express (Optica)  30 ( 11 ) 19090 - 19099 2022.05

Research paper (scientific journal), Joint Work, Last author, Corresponding author, Accepted

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We demonstrate the mode spacing multiplication of optical frequency combs (OFCs) using interleaving technique. The pulse train of an OFC after interleaving is phase modulated, and phase demodulation is necessary for mode spacing multiplication. This scheme enables mode spacing multiplication without power loss in principle, which is in contrast to the conventional mode filtering technique. During demonstration, the mode spacing of the OFC of a mode-locked Er-doped fiber laser (repetition rate of 97 MHz) is quadrupled by the interleaving process and successive phase demodulation.

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Fast polarization control for optical frequency combs

Taiki Kageyama, Taro Hasegawa

Optics Express (Optica)  29 ( 23 ) 38477 - 38487 2021.11

Research paper (scientific journal), Joint Work, Accepted

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Polarization of an optical frequency comb is electrically controlled using a waveguide electro-optic phase modulator (WG-EOM). Owing to the low operation voltage and wide electric bandwidth of the WG-EOM, fast polarization control is possible. It is found that birefringence of the WG-EOM and polarization-maintaining optical fibers causes polarization-dependent pulse separation, which makes polarization control of the optical frequency comb impossible. Therefore, compensation of the birefringence is required for polarization control. In the experiment, a delay line in free space is used for birefringence compensation, and pulse-to-pulse polarization control of an optical frequency comb (with a repetition rate of 100 MHz) is demonstrated.

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260 kHz mode-spacing optical frequency combs for scan-free high-resolution direct-comb spectroscopy

Taro Hasegawa

Optics Express (The Optical Society)  28 ( 22 ) 32837 - 32846 2020.10

Research paper (scientific journal), Single Work, Accepted

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For scan-free high-resolution direct-comb spectroscopy, mode spacing of an optical
frequency comb is reduced down to 260 kHz by phase modulation. It turns out that timedomain signal is degraded by averaging because of slow optical path length fluctuations and
fast optical pulse timing jitter. In this study, compensation of these effects is introduced, and
signal degradation by averaging is avoided. With demonstrations of direct-comb spectroscopy
with the narrow-mode-spacing optical frequency comb, Doppler-limited absorption spectrum of
methane and reflection spectrum from an optical ring cavity are observed. As a result, detailed
resonance spectral line profile of 8 MHz linewidth for the optical ring cavity is obtained in 50 ms
measurement time.

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Robust offset locking of laser frequency with electronically tunable LC circuits for sub-millihertz uncertainty

Yuhei Seishu, Taro Hasegawa

Applied Physics B (Springer)  125 ( 8 ) 142(1) - 142(8) 2019.07

Research paper (scientific journal), Joint Work, Accepted

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Mode Density Multiplication of an Optical Frequency Comb by N^{2} with Phase Modulation

Hasegawa T., Sasada H.

2018 Conference on Lasers and Electro-Optics Pacific Rim, CLEO-PR 2018 (2018 Conference on Lasers and Electro-Optics Pacific Rim, CLEO-PR 2018)   2019.04

ISSN  9781943580453

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© 2018 OSA. We introduce a simple scheme for mode density multiplication of an optical frequency comb by square of an arbitrary integer with phase modulation. The mode density of an Er fibercomb is multiplied by 4 2 .

Mode density multiplication of an optical frequency comb by N² with phase modulation

Hasegawa T.

Optics Express (Optics Express)  26 ( 19 ) 24551 - 24563 2018.09

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© 2018 Optical Society of America. We introduce a simple scheme for mode-density multiplication of an optical frequency comb (OFC) by a factor of square of an arbitrary integer N using phase modulation. This scheme is employed to multiply the mode density of an erbium-doped fiber laser OFC (repetition rate of 66.87 MHz) by factors of 42, 82, and 3 · 42 using an electro-optic phase modulator. The OFC multiplied by 42 is applied to direct-comb spectroscopy of methane with a spectral resolution of 4.18 MHz.

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A radio-frequency ion trap with string electrodes

KANAI DAISUKE, HASEGAWA TARO

Review of Scientific Instruments (American Institute of Physics)  89 ( 2 ) 023106 2018.02

Research paper (scientific journal), Joint Work, Accepted,  ISSN  00346748

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© 2018 Author(s). A radio-frequency (rf) ion trap with string electrodes is introduced. In this trap configuration, the rf electrodes are made of narrow metal strings, by which a negligibly small portion of light-induced fluorescence (LIF) is blocked. Then the LIF collection solid angle can be maximized. In the demonstration, barium ions are trapped and laser-cooled in the rf trap with string electrodes successfully, and the crystallization is confirmed by the LIF spectrum.

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Mode density multiplication of an optical frequency comb by N ² with phase modulation

Hasegawa T., Sasada H.

Optics InfoBase Conference Papers (Optics InfoBase Conference Papers)  Part F113-CLEOPR 2018 2018

Research paper (scientific journal), Lead author, Last author, Corresponding author, Accepted

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© OSA 2018 We introduce a simple scheme for mode density multiplication of an optical frequency comb by square of an arbitrary integer with phase modulation. The mode density of an Er fibercomb is multiplied by 4 2

Optical bistability induced by Gouy phase shift

HASEGAWA TARO

Journal of Optical Society of America B (The Optical Society)  34 ( 7 ) 1319 - 1326 2017.07

Research paper (scientific journal), Single Work, Accepted

Direct-comb molecular spectroscopy by heterodyne detection with continuous-wave laser for high sensitivity

HASEGAWA TARO, HIROYUKI SASADA

Optics Express (The Optical Society)  25 ( 16 ) A680 - A688 2017.06

Research paper (scientific journal), Joint Work, Accepted

Broadened optical spectrum of repetitionrate-multiplied erbium-doped fiber comb

HASEGAWA TARO

Journal of Optical Society of America B (The Optical Society)  33 ( 10 ) 2057 - 2061 2016.10

Research paper (scientific journal), Single Work, Accepted

Nonlinear absorption and phase shift in coupled optical cavities

HASEGAWA TARO

Physical Review A 92 ( 4 ) 043815 2015.10

Research paper (scientific journal), Single Work, Accepted

Theoretical analysis of mid-infrared molecular spectral line profiles with transversal relaxation in an intense optical field

HASEGAWA TARO

Journal of Optical Society of America B (The Optical Society)  31 ( 8 ) 1879 - 1884 2014.08

Research paper (scientific journal), Single Work, Accepted