arXiv subjects

Kenju Otsuka

Publications and source records attributed to Kenju Otsuka.

11 recordsLinked to original sources

arXiv2025

Harmonics-assisted optical phase amplification with a self-mixing thin-slice Nd:GdVO4 laser operating in the self-induced skew cosh Gaussian mode

Harmonic-assisted phase amplification was achieved in a 300-micrometer-thick Nd:GdVO4 laser in the self-mixing interference scheme. The key event is the self-induced skew cosh Gaussian (e.g., skew-chG) mode oscillation in a thin-slice solid-state laser with wide-aperture laser-diode pumping. The skew-chG mode was proved to be formed by the phase-locking of nearly frequency-degenerate TEM00 and annular fields. The resultant modal-interference-induced gain modulation at the beat frequency between the two modal fields, which is far above the relaxation oscillation frequency, increased experimental self-mixing modulation bandwidth accordingly. Fifty-fold phase amplification was achieved in a strong optical feedback regime.

physics.optics↗

arXiv2023

Spiking ruby revisited: self-induced periodic spiking oscillations leading to chaotic state in a Cr:Al2O3 laser with cw 532-nm pumping

This paper reexamines a 60 year old mystery of spiking behavior in ruby lasers with a cw 532 nm pump, paying special attention to mode matching between the pump and lasing beam within a ruby crystal placed in a semi-confocal laser cavity. Periodic spiking oscillations were observed in a limited pump power regime, where spikes obeying the generic asymmetric hyperbolic function appeared at a repetition rate around 50 kHz and with a 130-150 ns width and 0.1-0.6 microjoule energy depending on the pump power. The physics of the spiking behavior based on Kleinman's mechanical approach and a plausible interpretation for the periodic spiking oscillation in terms of self-induced mode matching between the pump and laser beams through the self-induced Kerr-lens effect are addressed. The statistical nature inherent to spiking and the associated self-organized critical behavior in quasi-periodic spiking oscillations as well as chaotic states occurring outside the periodic spiking regime are also explored from a nonlinear dynamics point of view.

physics.optics↗

arXiv2022

Self-mixing interference in a thin-slice solid-state laser with few feedback photons per observation period

Formation of a peculiar lasing pattern leading to a TEM00-like mode accompanied by a nonzero bright outer-ring emission (namely, a modified TEM00 mode) was observed in a 300-μm-thick LiNdP4O12 (LNP) laser with reflective flat end mirrors that was pumped with a laser diode whose focused spot size was larger than the lasing beam spot size determined by the thermal lens effect. The photon lifetimes of the laser oscillations were found to be greatly shortened to τ_{p} = 6.74 ps as compared with those in pure TEM00 mode operation, while the fluorescence lifetime was τ= 130 μs. The formation of the peculiar transverse mode was theoretically treated in terms of the pump-intensity-dependent thermal lens effect, which yields the lasing beam spot sizes in the resultant optical cavity. In addition, transverse spatial hole burning of population inversions due to the preceding transverse eigenmode intensity and the associated anti-guidance of the pure TEM00 field leading to formation of the TEM00 with an emergent outer ring emission was shown to appear with increasing pump power through the electric lens (i.e., population lens) effect, in which almost all of the population inversion contributes to lasing with a pronounced effective modal gain. Self-mixing laser Doppler velocimetry (LDV) experiments and numerical simulations revealed that spontaneous emission factors in the peculiar mode oscillations were greatly decreased in comparison with those in pure TEM00 mode operation. The present thin-slice solid-state laser possessing a large fluorescence-to-photon lifetime and decreased spontaneous emission factor enabled us to detect LDV signals in the regime of few feedback photons per observation period.

physics.optics↗

arXiv2021

Nonlinear dynamics of a self-mixing thin-slice solid-state laser subjected to Doppler shifted optical feedback

Chaotic oscillations of a linearly polarized single longitudinal-mode thin-slice Nd:GdVO4 laser placed in a self-mixing laser Doppler velocity scheme were dynamic characterized in terms the intensity probability distribution, joint time-frequency analysis and short-term Fourier transformation of temporal evolutions, and the degree of disorder in the amplitude and phase of the long-term temporal evolutions. The transition from chaotic relaxation oscillations (RO) to chaotic spiking oscillations (SO) was explored via the chaotic itinerancy (CI) regime by increasing the feedback ratio toward the laser from a rotating scattering object. The intensity probability distribution was found to change from an exponential decay in the RO regime to an inverse power law in the SO regime, which manifests itself in self-organized critical behavior, while stochastic subharmonic frequency locking among the two periodicities of RO and SO takes place in the CI regime featuring quantum-noise (spontaneous emission)-induced order in the amplitude and phase of the spiking oscillations. All of the experimental results were reproduced by numerical simulations of a model equation of a single-mode self-mixing solid-state laser subjected to Doppler-shifted optical feedback from a rotating scattering object.

physics.optics↗

arXiv2020

High degree of chaos synchronization in pairs of transverse modes with parity-symmetric polarizations in a thin-slice solid-state vector laser and application to polarimetric secure self-mixing metrology

A high degree of chaos synchronization among transverse mode pairs with parity-symmetric polarizations was demonstrated in a laser-diode pumped thin-slice c-cut Nd: GdVO4 laser. The amplitude correlation coefficient greater than R > 0.994 resulted from a non-reversing mirror symmetric-polarization-dependent structural change in the modal patterns, where chaotic dynamics of the whole system consisting of qualitatively different dynamics, which depend on the polarization crossing angle, resembled that of a chaotic single mode laser. A self-organized sender-receiver type of chaos synchronization of a single pair of modes among an infinite number of parity-symmetric polarizations and the associated chaotic camouflaging as well as extracting experiment of a self-mixing solid-state laser Doppler velocimetry signal were demonstrated.

physics.optics↗

arXiv2019

High degree of chaos synchronization of a single pair of transverse modes with different polarizations in vector lasers subjected to self-mixing modulation

Synchronization of chaos among a single pair of transverse modes featuring intermode dynamical non-independence, resulting from cross-saturation of modal population inversions and coherent modal field coupling, is explored in a thin-slice solid-state laser with coupled orthogonally polarized transverse modes operating in quasi-locked states. The chaotic signals that are created in one of the pairing modes (sender) by self-mixing modulations are shown to be transferred solely to the partner mode (receiver) among all the other modal fields with different polarizations. The experimental results are reproduced in a simulation of a model equation of coupled laser modes subjected to self-mixing modulations.

physics.optics↗

arXiv2019

Dynamic characterization of beating sinusoidal wave oscillations in a thin-slice solid-state laser with coupled orthogonally polarized transverse modes

Dynamic characterization of self-induced beating sinusoidal wave (BSW) oscillations and related phenomena, which were previously explored in a thin-slice dual-polarization solid-state laser operating in the regime of incomplete mode-locking of orthogonally polarized transverse eigenmodes, namely, a quasi-locked state [Laser Physics Letters, 15 (2018) 075001], is performed in terms of the amplitude correlation coefficient and intensity circulation among various orthogonally polarized mode pairs. Polarization-dependent symmetry-breaking featuring a nonreciprocal intensity flow for different orthogonally polarized mode pairs operating in the BSW state are found. The singular state is hidden in a nonlinear system where a perfect mode-locked BSW state with a large amplitude correlation coefficient featuring reciprocal intensity flow is established for a transverse mode pair possessing particular polarization directions that are related to the intensity ratio of orthogonally polarized transverse eigenmodes. Self-organized chaos synchronization is shown to take place for such a singular transverse mode pair subjected to self-mixing modulations.

physics.optics↗

arXiv2008

Spontaneous Generation of Vortex Array Beams from a Thin-Slice Solid-State Laser with Wide-Aperture Laser-Diode Pumping

We studied complex lasing pattern formations in a thin-slice solid-state laser with wide-aperture laser-diode end-pumping. Radial and rectangular vortex arrays were found to be formed in a controlled fashion with symmetric and asymmetric pump beam profiles, respectively. Most of these vortices exhibited single-frequency oscillations arising from a spontaneous process of transverse mode locking of nearly degenerate modes assisted by the laser nonlinearity. Single-frequency rectangular array beams consisting of a large number of vortices, e.g., closely packed 36 or 46 vortex pixels, were generated, originating from Ince-Gaussian modes excited by the asymmetric pumping.

physics.optics↗

arXiv2008

Polarization properties of laser-diode-pumped micro-grained Nd:YAG ceramic lasers

Detailed polarization properties have been examined in laser-diode-pumped (LD-pumped) micro-grained ceramic Nd:YAG lasers in different microchip cavity configurations. Stable linearly-polarized single-frequency oscillations, whose polarization direction coincide with that of an LD pump light, were observed in an external cavity scheme. While, in the case of a thin-slice laser scheme with coated reflective ends, elliptically-polarized single-frequency operations took place in the low pump-power regime and dynamic instabilities appeared, featuring self-induced antiphase modulations among counter-rotating circularly-polarized components having slightly different lasing frequencies, with increasing the pump power.

physics.optics↗

arXiv2008

Brownian Motion Captured with a Self-Mixing Laser

We measured the overall motion of Brownian particles suspended in water by a self-mixing thin-slice solid-state laser with extreme optical sensitivity. From the demodulated signal of laser intensity fluctuations through self-mixing modulations by the interference between the lasing field and Doppler-shifted scattered fields from Brownian particles, it was found that the changes over time in random walks of small particles suspended in water result in different overall dynamics when the field of vision for particles seen by the laser beam (scale of the observation) is changed. At a small focal volume of the laser beam, in which the relevant diffusion broadening is observed, the overall motion which can be represented by the motion of a ''virtual'' single particle whose velocity obeys a double-peaked non-Gaussian distribution function. The fast motion of a virtual particle, featuring fractional Brownian motion, which leads to a pure random walk with increasing the observation time, has been characterized in terms of mean-square displacements of a virtual particle.

physics.optics↗

arXiv1999

Non-stationary Characteristics of the instability in a Single-mode Laser with Fiber Feedback

Chaotic bursts are observed in a single-mode microchip Nd:YVO$_4$ laser with fiber feedback. The physical characteristic of the instability is a random switching between two different dynamical states, i.e., the noise-driven relaxation oscillation and the chaotic spiking oscillation. As the feedback strength varied, transition which features the strong interplay between two states exhibits and the dynamical switching is found to be non-stationary at the transition.

physics.optics↗