Kaoru Shimizu

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. >

Field test of quantum key distribution in the Tokyo QKD Network

A secure communication network with quantum key distribution in a metropolitan area is reported. Six different QKD systems are integrated into a mesh-type network. GHz-clocked QKD links enable us to demonstrate the world-first secure TV conferencing over a distance of 45km. The network includes a commercial QKD product for long-term stable operation, and application interface to secure mobile phones. Detection of an eavesdropper, rerouting into a secure path, and key relay via trusted nodes are demonstrated in this network.

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. >

A monolithically integrated polarization entangled photon pair source on a silicon chip

Integrated photonic circuits are one of the most promising platforms for large-scale photonic quantum information systems due to their small physical size and stable interferometers with near-perfect lateral-mode overlaps. Since many quantum information protocols are based on qubits defined by the polarization of photons, we must develop integrated building blocks to generate, manipulate, and measure the polarization-encoded quantum state on a chip. The generation unit is particularly important. Here we show the first integrated polarization-entangled photon pair source on a chip. We have implemented the source as a simple and stable silicon-on-insulator photonic circuit that generates an entangled state with 91 ± 2% fidelity. The source is equipped with versatile interfaces for silica-on-silicon or other types of waveguide platforms that accommodate the polarization manipulation and projection devices as well as pump light sources. Therefore, we are ready for the full-scale implementation of photonic quantum information systems on a chip.

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. >

Measurement of Rayleigh backscattering in single-mode fibers based on coherent OFDR employing a DFB laser diode

Rayleigh backscattering in a single-mode optical fiber was measured by coherent optical frequency-domain reflectometry (C-OFDR) employing a frequency-variable distributed-feedback laser diode (DFB-LD). Backscattered power from the neighborhood of the far end of a 0.5-km-long fiber was measured with a spatial resolution of 50 cm. The insertion of a delay line fiber in the reference optical path and an acoustooptic frequency shifter in the signal optical path made it possible to measure backscattering in a long optical fiber in spite of the short coherence length of the DFB-LD.<<ETX>>

Technique for translating light-wave frequency by using an optical ring circuit containing a frequency shifter

A new technique for the external frequency translation of light waves is proposed. The technique enables the stepwise sweeping of an optical frequency in time over a wide range. The frequency translator is composed of an optical pulse modulator and an optical ring circuit that contains an acousto-optic frequency shifter and an optical amplifier. The pulse launched into the ring circuit undergoes a frequency shift for each complete trip around the ring circuit, and the frequency is translated considerably from the original input pulse. We confirm a frequency translation of as much as 8 GHz for a 1.5-microm light wave, in which the sweeping is strictly linear with respect to time.

Slow light enhanced correlated photon pair generation in photonic-crystal coupled-resonator optical waveguides

We demonstrate the generation of quantum-correlated photon pairs from a Si photonic-crystal coupled-resonator optical waveguide. A slow-light supermode realized by the collective resonance of high-Q and small-mode-volume photonic-crystal cavities successfully enhanced the efficiency of the spontaneous four-wave mixing process. The generation rate of photon pairs was improved by two orders of magnitude compared with that of a photonic-crystal line defect waveguide without a slow-light effect.

Frequency translation of light waves by propagation around an optical ring circuit containing a frequency shifter : I. Experiment

A technique for the external frequency translation of light waves is reported. The technique permits the stepwise sweeping of an optical frequency over a wide range with high linearity with respect to time. The frequency translator is composed of an optical pulse modulator and an optical ring circuit containing an acousto-optic frequency shifter and an optical amplifier. The pulse launched into the ring circuit undergoes a constant frequency shift for each circulation around the circuit and the frequency can be translated to a considerable degree from that of the original input pulse. We report a stepwise frequency translation over approximately 68 GHz for a 1.5-µm light wave with a strictly constant frequency-sweep rate and an approximately constant intensity.

Superconducting Nanowire Single-Photon Detector with Ultralow Dark Count Rate Using Cold Optical Filters

We report the fabrication of a superconducting nanowire single-photon detector (SSPD or SNSPD) with an ultralow dark count rate. By introducing optical band-pass filters at the input of the SSPD and cooling the filters at 3 K, the dark count rate is reduced to less than 1/100 at low bias. An SSPD with 0.1 cps dark count rate and 5.6% system detection efficiency at 1550 nm wavelength is obtained. We show that a quantum key distribution (QKD) over 300 km of fiber is possible based on a numerical calculation assuming a differential phase shift QKD protocol implemented with our SSPDs.