Yahei Koyamada

Development of a distributed sensing technique using Brillouin scattering

This paper reviews the developments of a distributed strain and temperature sensing technique that uses Brillouin scattering in single-mode optical fibers. This technique is based on strain- and temperature-induced changes in the Brillouin frequency shift. Several approaches for measuring the weak Brillouin line are compared. >

Simulating and designing Brillouin gain spectrum in single-mode fibers

For many fiber applications, the Brillouin gain spectrum (BGS) contains important information including the Brillouin frequency shift, the Brillouin spontaneous linewidth, and the Brillouin gain coefficient. This paper is the first, to the best of our knowledge, to present an accurate numerical simulation of the BGS in single-mode fibers. The simulated and measured BGS were in good agreement. Through repeated numerical simulations, we revealed a tendency of the peak Brillouin gain coefficient that determines the stimulated Brillouin scattering threshold.

Fiber-Optic Distributed Strain and Temperature Sensing With Very High Measurand Resolution Over Long Range Using Coherent OTDR

We describe a novel fiber-optic technique for measuring distributed strain and temperature that uses a coherent optical time-domain reflectometer (OTDR) with a precisely frequency-controlled light source. Using this technique, we achieved temperature measurements in an 8-km-long fiber with a resolution of 0.01degC and a spatial resolution of one meter. This temperature resolution is two orders of magnitude better than that provided by the Brillouin based sensing technique.

Coherent self-heterodyne Brillouin OTDR for measurement of Brillouin frequency shift distribution in optical fibers

Time domain reflectometry of spontaneously Brillouin scattered lightwaves in a single-mode optical fiber is demonstrated with a coherent self-heterodyne detection system employing a recently proposed frequency translator, a DFB laser diode, and erbium-doped fiber amplifiers. Since the probe pulse frequency is up-converted by the translator by an amount approximately equal to the Brillouin frequency shift, the self-heterodyne beat frequency can be reduced to a sufficiently low frequency in the IF band. The system enables one-end measurement of the Brillouin frequency shift distribution in optical fibers with a single way dynamic range (SWDR) of 16 dB and a frequency resolution of 5 MHz for a spatial resolution of 100 m. >

Characteristics and reduction of coherent fading noise in Rayleigh backscattering measurement for optical fibers and components

The characteristics of fading noise in Rayleigh backscattering measurements made with coherent lightwaves such as in coherent-OTDR (optical time-domain reflectometry) and coherent-OFDR (optical frequency-domain reflectometry) are studied. The effects of frequency shift averaging on fading noise reduction are clarified theoretically, and the relationships between measurement accuracy and other parameters, such as spatial resolution and frequency variation range are derived. The calculated results of loss measurement accuracy are in good agreement with experimental data. The formula can also be applied to low-coherence interferometric OTDR. >

Fading noise reduction in coherent OTDR

The authors describe a reduction in fading noise which causes amplitude fluctuation on a backscattered trace in coherent OTDR (optical time-domain reflectometry). In order to reduce the fading noise, the optical frequency of the DFB-LD (distributed feedback laser diode) is changed by changing the LD temperature during the integrations of the backscattered signals. At the same time, the state of polarization of the launched signal pulses is changed. The amplitude fluctuation has been reduced to as low as 0.06 dB. Measurements were performed on a 10 km length of conventional single-mode fiber.<<ETX>>

Numerical analysis of core-mode to radiation-mode coupling in long-period fiber gratings

This letter presents, to the best of our knowledge, the first analysis of core-mode to radiation-mode coupling in long-period fiber gratings (LPGs) that are surrounded with dielectric material whose refractive index is higher than that of the cladding. We calculated core-mode transmission spectra through LPGs by integrating numerically the coupled-mode equations between the core mode and a continuum of radiation modes. We show calculated core-mode transmission spectra that exhibit such loss band continuums as have already been observed experimentally but not yet theoretically estimated.

Novel Technique to Improve Spatial Resolution in Brillouin Optical Time-Domain Reflectometry

A novel Brillouin optical time-domain reflectometry (BOTDR) system, called a double-pulse BOTDR (DP-BOTDR) system, is proposed for measuring distributed strain and temperature in a fiber with a sub-meter spatial resolution. Our experiment confirmed that the DP-BOTDR system enables us to measure the distributed Brillouin frequency shift, i.e., the distributed strain and temperature, with a spatial resolution of 20 cm. This spatial resolution is five times better than that provided by the conventional single-pulse BOTDR system.

Carrier-envelope-phase stabilized chirped-pulse amplification system scalable to higher pulse energies

We have demonstrated a carrier-envelope phase (CEP) stabilized chirped-pulse amplification (CPA) system employing a grating-based pulse stretcher and compressor and a regenerative amplifier for the first time. In addition to stabilizing the carrier-envelope offset phase of a laser oscillator, a new pulse selection method referenced to the carrier-envelope offset beat signal was introduced. The pulse-selection method is more robust against the carrier-envelope offset phase fluctuations than a simple pulse-clock dividing method. We observed a stable fringe in a self-referencing spectrum interferometry of the amplified pulse, which implies that the CEP of amplified pulse is stabilized. We also measured the effect of the beam angle change on the CEP of amplified pulses. The result demonstrates that the CEP stabilized CPA is scalable to higher-pulse energies.

Coherent self-heterodyne detection of spontaneously Brillouin-scattered light waves in a single-mode fiber.

Time-domain reflectometry of spontaneous Brillouin scattering in a single-mode optical fiber is performed with a coherent self-heterodyne detection system containing a recently proposed external frequency translator and a single light-wave source. The light wave is divided into probe and reference light waves. The frequency of the probe light wave is upconverted by the translator by an amount approximately equal to the Brillouin frequency shift. The frequency-converted probe is launched into the fiber and spontaneously Brillouin scattered. As the frequency of the scattered probe is downconverted to near that of the reference light wave, coherent self-heterodyne detection of spontaneous Brillouin scattering becomes possible without having to use a fast-speed detector.