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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.

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.

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.

Two huge earthquakes struck Kumamoto, Japan, in April 2016, forcing residents to evacuate. Few studies have reported early-phase disease and injury trends among evacuees following major inland earthquakes. We evaluated the trends among evacuees who visited a medical clinic in a shelter located at the epicenter of the 2016 Kumamoto earthquakes. The clinic opened on April 15, the day after the foreshock, and closed 3 weeks later. We reviewed medical charts related to 929 outpatient visits and conducted descriptive analyses. The evacuees experienced mild injuries and common diseases. The types of diseases changed weekly. Elderly people needed medical support for longer than other age groups. Future earthquakes may be inevitable, but establishing arrangements for medical needs or making precautions for infectious diseases in shelters could reduce the effects of earthquake-related health problems.

Laser frequency stabilization is one of the essential technique in atomic, molecular, and optical physics, quantum optics, optical communications, and fundamental physics. Optical offset locking is the technique to stabilize optical frequency of a single-mode laser (slave laser) with respect to another well-stabilized laser (master laser). Optical phase lock loop (OPLL) [1] is one of the offset locking schemes, with which the slave laser frequency can be stabilized very precisely (less than the uncertainty of 10−18). However, especially in noisy experimental circumstances, the OPLL does not work because its capture range is not so wide that the locking fails for large amount of frequency jitters. In order to keep the locked condition, another offset locking with wider capture range is sometimes required. Examples of such locking schemes are digital processing technique, optical frequency locking to resonance frequency of a high-finesse cavity [2], offset locking with an electrical delay line [3], and offset locking with electric LC resonant circuit [4]. The offset locking with the electric LC circuit (LC locking) has been introduced by W.-Y. Cheng et al, and with employing this scheme with the OPLL the beat-note frequency can be stabilized as precise as one millihertz.

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 4

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 time-domain 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.

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.

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 42.

A scheme to resolve individual comb modes by heterodyne detection with a continuous-wave laser for direct comb spectroscopy is proposed and demonstrated. This is simple and suitable for application to high-resolution spectroscopy.

The nonlinear absorption process and associated phase shift in coupled optical cavities are studied experimentally by observing optical resonance properties. In the coupled cavity configuration, two optical cavities, one for a fundamental beam and the other for a second-harmonic (SH) beam, are coupled by a nonlinear crystal for the second-harmonic generation (SHG). The cavity for the SH beam effectively extends the nonlinear crystal length so that the frequency-conversion efficiency of the SHG, which is proportional to the square of the crystal length, is enhanced. In the observation, power reduction of the fundamental beam at resonance is observed as a small dip in the resonance curve. According to a model calculation, it is found that this power reduction is caused by blue-induced infrared absorption.

We have demonstrated direct-comb molecular spectroscopy in which an individual-comb mode is resolved by heterodyne detection with a continuous-wave (cw) laser. This simple, fast, and sensitive technique was demonstrated for atomic spectroscopy in [ Appl. Phys. Lett.101, 051105 ( 2012)], but is more suitable for molecular spectroscopy. Four rotation-vibration transitions of methane have simultaneously been recorded in a Doppler-limited resolution using a spectrally broadened comb based on an Er-doped fiber laser and a cw 1651-nm distributed-feedback laser diode. Even though the power level of the comb modes and the sample absorption are weaker than those of the previous studies, in this work the cw laser frequency is stabilized to one of the comb modes, and the data are thereby averaged for extended periods of time, resulting an improvement in sensitivity.

Optical bistability in a passive ring cavity with no designed nonlinear materials is experimentally observed in the near-infrared region. The nonlinear material that induces the optical bistability is considered to be water molecules in atmosphere. In the model calculation, it is found that the optical bistability is induced by transversal distribution of optical power-dependent refractive index of water in the Gaussian beam, which causes the power-dependent Gouy phase shift in the cavity. The model calculation agrees qualitatively with the experimental results. This result suggests that the Gouy phase shift has an effect on cavity-enhanced molecular spectroscopy.

The repetition rate of an erbium-doped fiber comb is multiplied by an optical cavity for filtering, and the optical spectral distribution is broadened in a highly nonlinear fiber. For erbium-doped fiber combs, filtering cavities are commonly used to obtain the high-repetition-rate combs by the repetition rate multiplication. After the multiplication, optical spectral broadening by a highly nonlinear fiber is necessary for applications to optical frequency measurements in broad bandwidth and astronomy. In this study, the broadened spectrum covers over the range from 1050 to 2250 nm, which is over an octave so that the determination of the filtered mode numbers is feasible. The optical power of the broadened beam within a narrow optical bandwidth is also measured in the time domain and in the frequency domain to examine the broadening property.