Yoh Imai

Direct velocity sensing of flow distribution based on low-coherence interferometry

A new sensing method for measuring flow velocity distribution directly by using low-coherence interference techniques is proposed and demonstrated. In this method a temporally fluctuating signal, not the Doppler frequency shift, is detected. Theoretical analysis shows that a spectrum of light backscattered from a particle takes a Gaussian form whose width is simply proportional to the flow velocity. The measured velocity is in good agreement with the actual flow velocity derived from the flow rate. The dynamic range of this sensing method is governed by the frequency range of the fast-Fourier-transform processor used and is estimated to be 1.4×10-4–14xa0m/s. The depth position can be adjusted with an accuracy of approximately 30 μm, which is determined by the coherence length of the light source.

Dependence of stimulated Brillouin scattering on temperature distribution in polarization-maintaining fibers

The dependence of stimulated Brillouin scattering (SBS) on a graded temperature distribution applied periodically to a polarization-maintaining fiber is investigated. The SBS intensity decreases with an increase in the temperature difference. SBS disappears at a 37 degrees C temperature difference, where a 10-mW pumping light of 1.319- mu m wavelength is launched into a fiber of 1.6-km length. The SBS threshold increases as the temperature difference increases.<<ETX>>

Coherence effect on nonlinear dynamics in fiber-optic ring resonator

Dependence of nonlinear dynamics generated in an optical fiber ring resonator on source coherence is investigated numerically. Chaotic and periodic behaviors are found to have reduced moduli with maintained patterns which become time independent as source spectrum width increases. Output bifurcation characteristics are independent of source coherence but are dependent on loss and other resonator parameters. Coherence length which is longer than resonator length is required for chaotic output. The critical spectrum width at which output becomes time independent is inversely proportional to fiber ring resonator length. In addition, critical spectrum width decreases with increased fiber ring resonator loss.

Fluorescence-Based Temperature Sensing Using Erbium-Doped Optical Fibers with 1.48 μm Pumping

Temperature sensing using 1.54 μm fluorescence at the transition between the energy levels4I15/2 (ground state) and4I13/2 generated in an erbium-doped fiber with 1.48 μm pumping is proposed. The fluorescence has a peculiar spectral profile that possesses two peaks around 1.530 μm and 1.552 μm wavelengths. The temperature-dependent fluorescence is investigated in the temperature range between -50°C and 90°C. The power ratio between the two peaks increases with an increase in temperature. The sensitivity of the ratio is 0.007/°C on average in the measured temperature range. The total fluorescence power and the absorption loss at λ = 1480 nm in the fiber decrease as the temperature increases. Optical fiber temperature sensing immune from the fluctuation in pumping power can be performed using the peak power ratio and/or the absorption loss.

Measurements of thermal effects on four-photon mixing conversion efficiency in an optical fiber.

Thermal effects on the conversion efficiency of four-photon mixing (FPM) power in a silica optical fiber are measured. When the temperature of the fiber changes from 9 to 350 degrees C, the first Stokes power decreases with a temperature coefficient of -0.33%/ degrees C as the temperature increases to 200 degrees C and then becomes saturated with a further increase in temperature. This temperature characteristic of the first Stokes power reflects the temperature characteristics of the nonlinear refractive index of the fiber, which are similar to its FPM temperature dependence.

High-particle-density flowmetry by use of a SLD

An interferometric velocilnetry using a SLD for measuring a high particle density liquid flow is examined. The proposed scheme enables us to specify the velocity at a chosen depth in a multiple scattering medium which has been impossible in a conventional speckle velocilnetry. 1.

Polarization characteristics of amplified Rayleigh backscattered light in erbium-doped optical fibers

Polarization characteristics of Rayleigh backscattered light in erbium-doped optical fiber amplifiers are investigated by using 1.54-?m signal light pumped by 1.48-?m light. The Rayleigh backscattered equivalent reflectance for the signal light was ? 49.9 dB in a 30-m-length erbium-doped fiber and was increased and was amplified with the pump light. Linear polarization in the input signal light was almost preserved in the Rayleigh backscattered light in the absence of pumping. The degree of polarization was degraded from 88% to 52% with an increase in the pump power.

Velocity vector sensing by using fiber optic low-coherence interferometer

Velocity vector sensing by using fiber optic low coherent interferometer is proposed and demonstrated. The direction and absolute value of the flow velocity can be measured from a peak position of a correlation between two backscattering lights from probing points on the x-, y-, and z-axis in flow.

Velocity vector sensing with correlation detection by using fiber-optic low coherent interferometer

Velocity vector sensing by using fiber-optic low coherent interferometer is proposed and demonstrated. The velocity vector component is measured directly from a peak position of correlation signal between two backscattering lights from flow.

Measurements of velocity distribution along depth using low coherence interferometry

A new optical measurement method of a flow velocity distribution along the depth using low coherence interference techniques is proposed and demonstrated. In the proposed method, fluctuations of the interference signal formed with the reference and backscattering lights are detected by the optical heterodyne scheme. It is shown experimentally that the width of the spectrum of the interference signal is proportional to the flow velocity. The dynamic range is determined by the time constants of the fast Fourier transform (FFT) processor and lock-in amplifier used in the experiment.