Osamu Tadanaga

Multiple quasi-phase-matched LiNbO3 wavelength converter with a continuously phase-modulated domain structure.

We have devised a novel device structure for a multiple quasi-phase-matched wavelength converter. Optimized continuous phase modulation of a periodic domain structure makes possible multichannel pumping with minimum loss of efficiency. Using the device, we demonstrate variable and simultaneous wavelength conversion of wavelength-division multiplexed signals.

Efficient 3-μm difference frequency generation using direct-bonded quasi-phase-matched LiNbO3 ridge waveguides

We fabricate 50-mm-long direct-bonded quasi-phase-matched LiNbO3 ridge waveguides for difference frequency generation in the 3-μm wavelength range. Conversion efficiency of 40%/W is achieved using a 1-μm-band pump and a 1.55-μm-band signal, and a 0.26-mW output is obtained. We also use the device to demonstrate methane gas detection at around 3.3μm.

Efficient entanglement distribution over 200 kilometers.

Here we report the first demonstration of entanglement distribution over a record distance of 200 km which is of sufficient fidelity to realize secure communication. In contrast to previous entanglement distribution schemes, we use detection elements based on practical avalanche photodiodes (APDs) operating in a self-differencing mode. These APDs are low-cost, compact and easy to operate requiring only electrical cooling to achieve high single photon detection efficiency. The self-differencing APDs in combination with a reliable parametric down-conversion source demonstrate that entanglement distribution over ultra-long distances has become both possible and practical. Consequently the outlook is extremely promising for real world entanglement-based communication between distantly separated parties.

Generation of pulsed polarization-entangled photon pairs in a 1.55-µm band with a periodically poled lithium niobate waveguide and an orthogonal polarization delay circuit

We report a scheme for generating pulsed polarization-entangled photon pairs based on conversion from time-bin entanglement to polarization entanglement by use of an orthogonal polarization delay circuit and post-selection. We have experimentally demonstrated the scheme, using a periodically poled lithium niobate waveguide, and successfully obtained polarization entanglement in the 1.55-microm telecom wavelength band.

Highly Efficient Wavelength Converter Using Direct-Bonded PPZnLN Ridge Waveguide

We fabricated a periodically poled and ZnO-doped LiNbO3 ridge waveguide by employing direct bonding and dry etching techniques. We obtained a second-harmonic generation (SHG) conversion efficiency of 2400%/W, and converted 92% of the pump light into SH light at a pump power of 160 mW. We developed a fiber-coupled module using the fabricated ridge waveguide. The high conversion efficiency and high damage resistance of the ridge waveguide result in the parametric amplification of the signal and converted signal lights. The low insertion loss of the module (-4 dB) and sufficient parametric conversion gain (+8 dB) enable us to achieve a wavelength converter with + 4 dB fiber-to-fiber gain, which means the wavelength converter operates without loss.

Multiple quasi-phase-matched device using continuous phase modulation of /spl chi//sup (2)/ grating and its application to variable wavelength conversion

We propose a new multiple quasi-phase-matched wavelength converter based on the continuous phase modulation of a /spl chi//sup (2)/ grating for use in variable wavelength conversion. A numerical study shows that the proposed device exhibits a high conversion efficiency, flexible design, and robust fabrication tolerance. A waveguide device fabricated by annealed proton exchange agrees well with the numerical design. Fine-tuning the device enabled us to demonstrate variable wavelength conversion between signals on the standard optical frequency grid. Using the device, we also demonstrated fast (<100 ps) wavelength switching of 4-channel 40-Gb/s signals. The obtained results clearly show that the proposed multiple quasi-phase-matched devices will be useful when constructing future flexible photonic networks.

Reducing photorefractive effect in periodically poled ZnO- and MgO-doped LiNbO3 wavelength converters

The photorefractive effect in annealed proton-exchanged waveguides in periodically poled nondoped, MgO- and ZnO-doped LiNbO3 was evaluated by monitoring the quasiphase matching (QPM) wavelength shift induced by a 0.784-μm-irradiating light. The QPM wavelength shift was reduced at room temperature by a factor of 3–6 in ZnO- and MgO-doped samples compared with the nondoped samples within a 104–105-W/cm2-irradiation intensity range. The doped samples exhibited no significant wavelength shifts when the temperature was raised to slightly above room temperature (50–60 °C).

Long-distance distribution of time-bin entangled photon pairs over 100 km using frequency up-conversion detectors.

We report an experimental demonstration of the distribution of time-bin entangled photon pairs over 100 km of optical fiber. In our experiment, 1.5-mum non-degenerated time-bin entangled photon pairs were generated with a periodically poled lithium niobate (PPLN) waveguide by using the parametric down conversion process. Combining this approach with ultra-low-loss filters to eliminate the pump light and separate signal and idler photons, we obtained an efficient entangled photon pair source. To detect the photons, we used single-photon detectors based on frequency up-conversion. These detectors operated in a non-gated mode so that we could use a pulse stream of time correlated entangled photon pairs at a high repetition frequency (1 GHz). Using these elements, we distributed time-bin entangled photon pairs over 100 km of dispersion shifted fiber and performed a two-photon interference experiment. We obtained a coincidence fringe of 81.6% visibility without subtracting any background noise, such as accidental coincidence or dark count, which was good enough to violate Bells inequality. Thus, we successfully distributed time-bin entangled photon pairs over 100 km.

Long-distance entanglement-based quantum key distribution over optical fiber.

We report the first entanglement-based quantum key distribution (QKD) experiment over a 100-km optical fiber. We used superconducting single photon detectors based on NbN nanowires that provide high-speed single photon detection for the 1.5-mum telecom band, an efficient entangled photon pair source that consists of a fiber coupled periodically poled lithium niobate waveguide and ultra low loss filters, and planar lightwave circuit Mach-Zehnder interferometers (MZIs) with ultra stable operation. These characteristics enabled us to perform an entanglement-based QKD experiment over a 100-km optical fiber. In the experiment, which lasted approximately 8 hours, we successfully generated a 16 kbit sifted key with a quantum bit error rate of 6.9 % at a rate of 0.59 bits per second, from which we were able to distill a 3.9 kbit secure key.

High-power, tunable difference frequency generation source for absorption spectroscopy based on a ridge waveguide periodically poled lithium niobate crystal.

A novel waveguide for difference frequency generation in the mid-IR spectral region at 3.52 mum is characterized. High mid-IR power of 15 mW and an external conversion efficiency of up to 19 %W( -1) have been obtained. An optical beam propagation factor M(2) =1.18 was determined using the second moment method. A simple 2-f absorption spectra demonstrates the potential of this mid-IR source for high precision trace gas sensing applications.