Fresnel zone plate with apodized aperture for hard X-ray Gaussian beam optics.

Fresnel zone plates with apodized apertures [apodization FZPs (A-FZPs)] have been developed to realise Gaussian beam optics in the hard X-ray region. The designed zone depth of A-FZPs gradually decreases from the center to peripheral regions. Such a zone structure forms a Gaussian-like smooth-shouldered aperture function which optically behaves as an apodization filter and produces a Gaussian-like focusing spot profile. Optical properties of two types of A-FZP, i.e. a circular type and a one-dimensional type, have been evaluated by using a microbeam knife-edge scan test, and have been carefully compared with those of normal FZP optics. Advantages of using A-FZPs are introduced.

First demonstration of high density optical interconnects integrated with lasers, optical modulators, and photodetectors on single silicon substrate.

Engineers are currently facing some technical issues in support of the exponential performance growths in information industries. One of the most serious issues is a bottleneck of inter-chip interconnects. We propose a new "Photonics-Electronics Convergence System" concept. High density optical interconnects integrated with a 13-channel arrayed laser diode, silicon optical modulators, germanium photodetectors, and silicon optical waveguides on single silicon substrate were demonstrated for the first time using this system. A 5-Gbps error free data transmission and a 3.5-Tbps/cm(2) transmission density were achieved. We believe that this technology will solve the bandwidth bottleneck problem among LSI chips in the future.

All-silicon and in-line integration of variable optical attenuators and photodetectors based on submicrometer rib waveguides.

We demonstrate a monolithic integration of variable optical attenuators (VOAs) and photodetectors (PDs) based on submicrometer silicon (Si) rib waveguide with p-i-n diode structure for near infrared (NIR) light. To make the Si PD absorptive for NIR, we introduced lattice defects at the rib core by means of argon ion implantation. At reverse bias of 5 V, the PD exhibits dark current of approximately 1 nA, responsivity of approximately 65 mA/W at 1560-nm wavelength, and a 3-dB cut-off frequency of approximately 350 MHz, while the VOA shows approximately 100 MHz. The PD has an absorption coefficient as low as approximately 0.5 cm(-1), which is favorable for an in in-line PD configuration, where the PD absorbs a small portion of the optical power. For DC light, the PD precisely detects the optical power attenuated by the VOA with a detection range of over 40 dB. The 3-dB cut-off frequency of synchronous operation between the VOA and PD is approximately 50 MHz, which is limited by RF noise originating from the VOA drive current. Putting an isolation groove between the VOA and PD is effective for avoiding performance degradation in DC and RF operation.

Long-distance entanglement-based quantum key distribution experiment using practical detectors.

We report an entanglement-based quantum key distribution experiment that we performed over 100 km of optical fiber using a practical source and detectors. We used a silicon-based photon-pair source that generated high-purity time-bin entangled photons, and high-speed single photon detectors based on InGaAs/InP avalanche photodiodes with the sinusoidal gating technique. To calculate the secure key rate, we employed a security proof that validated the use of practical detectors. As a result, we confirmed the successful generation of sifted keys over 100 km of optical fiber with a key rate of 4.8 bit/s and an error rate of 9.1%, with which we can distill secure keys with a key rate of 0.15 bit/s.

Monolithic integration and synchronous operation of germanium photodetectors and silicon variable optical attenuators.

We demonstrate the monolithic integration of germanium (Ge) p-i-n photodetector (PDs) with silicon (Si) variable optical attenuator (VOAs) based on submicrometer Si rib waveguide. A PD is connected to a VOA along the waveguide via a tap coupler. The PDs exhibit low dark current of ~60 nA and large responsivity of ~0.8 A/W at the reverse bias of 1 V at room temperature. These characteristics are uniform over the chip scale. The PDs generate photocurrents precisely with respect to DC optical power attenuated by the VOAs. Two devices work synchronously for modulated optical signals as well. 3-dB cut-off frequency of the VOA is ~100 MHz, while that of the PD is ~1 GHz. The synchronous response speed is limited by the VOA response speed. This is the first demonstration, to the best of our knowledge, of monolithic integration of Ge PDs with high-carrier-injection-based optical modulation devices based on Si.

Influence of carrier lifetime on performance of silicon p-i-n variable optical attenuators fabricated on submicrometer rib waveguides.

We investigated influence of carrier lifetime on performance of silicon (Si) p-i-n variable optical attenuators (VOAs) on submicrometer Si rib waveguides. VOAs were fabricated with and without intentional implantation of lattice defects into their intrinsic region. Carrier lifetime was measured by pulse responses for normal incidence of picosecond laser pulse of 775 nm to the VOA, as approximately 1 ns and approximately 7 ns for the VOAs with and without defects, respectively. Carrier lifetime is determined by the sum of surface recombination and Auger recombination for VOAs without defects, while Schockley-Read-Hall recombination is dominant for the VOA with defects. As a result, attenuation efficiency (dB/mA) is 0.2-0.7 and 0.04-0.1, while 3-dB bandwidth is 40-100 MHz and over 200 MHz for the VOAs with and without defects, respectively. There is a trade-off relation between attenuation and response speed of the VOAs with respect to carrier lifetime i.e., attenuation efficiency is linearly proportional to the carrier lifetime, whereas response speed is inversely proportional to it.

Entanglement generation using silicon photonic wire waveguide.

This paper reviews recent progress on telecom-band entangled photon-pair sources based on spontaneous four-wave mixing (SFWM) in a silicon photonic wire waveguide. Thanks to the large third order nonlinearity of nano-scale silicon waveguides, we can generate photon pairs efficiently. Moreover, the use of silicon waveguides enable us to avoid the noise photons caused by spontaneous Raman scattering, which has been a serious problem with entanglement sources based on SFWM in dispersion shifted fiber. We successfully demonstrated high-purity time-bin and polarization entanglement generation using 1-cm long silicon waveguides.

Generation of high-purity entangled photon pairs using silicon wire waveguide.

We observed high-purity correlated and entangled photon pairs generated through spontaneous four-wave mixing (SFWM) in a silicon wire waveguide (SWW). Employing a nano-scale silicon waveguide with a low loss mode size converter, we obtained a high coincidence to accidental coincidence ratio (CAR) of around 200 that was larger than that of cooled dispersion shifted fiber (DSF) by a factor 3.2, and observed the two-photon interference fringe of time-bin entangled photons with > 95% visibility without subtracting the accidental coincidences.

Polarization rotator based on silicon wire waveguides.

We describe a polarization rotator based on an off-axis double-core structure consisting of a silicon wire waveguide and a silicon-oxinitride waveguide. The rotator can be made by planar fabrication technology and do not require complex processes, such as three-dimensional structure formation. A rotator with 35-microm long provides the polarization rotation angle of 72 degrees and the polarization extinction ratio of 11 dB with the excess loss of about 1 dB. Our polarization rotator represents a significant step towards accomplishing an optical circuit with polarization diversity based on silicon wire waveguides.

Silicon photonic circuit with polarization diversity.

We devised a silicon photonic circuit with polarization diversity that consists of polarization splitters and polarization rotators. The splitter is based on a simple directional coupler and the rotator has an off-axis double-core structure. Both devices can be made by using planar fabrication technology and require no complex proceses for the fabrication of three-dimensional structures. We fabricated a polarization-independent wavelength filter based on Si wire waveguides as an application of the polarization diversity. The filter consists of the polarization splitters, the rotators, and a ring resonator. The polarization-dependent loss of the filter is about 1 dB. A 10-Gbps data transmission with scrambled polarization is demonstrated.

Generation of polarization entangled photon pairs using silicon wire waveguide.

We report the experimental generation of polarization entangled photon pairs based on spontaneous four-wave mixing in a silicon waveguide. Using a nano-scale silicon wire waveguide placed in a fiber loop, we obtained 1.5-microm band polarization entanglement with two-photon interference visibilities of >83%.

Ultrasmall polarization splitter based on silicon wire waveguides.

We describe an ultrasmall polarization splitter based on a simple directional coupler consisting of silicon wire waveguides. The size is only 7 x 16 microm(2), and the polarization extinction ratio is about 15 dB for a single coupler. A double-coupler structure improves the extinction ratio to over 20 dB. The excess loss is smaller than 0.5 dB for both types of device. In the device, the shape of the high-speed waveform is retained at any angle of polarization. Our polarization splitter represents a first step towards accomplishing an ultrasmall optical circuit with polarization diversity based on silicon wire waveguides.

Four-wave mixing in silicon wire waveguides.

We report the observation of four-wave mixing phenomenon in a simple silicon wire waveguide at the optical powers normally employed in communications systems. The maximum conversion efficiency is about -35 dB in the case of a 1.58-cm-long silicon wire waveguide. The nonlinear refractive index coefficient is found to be 9x10-18 m2/W. This value is not negligible for dense wavelength division multiplexing components, because it predicts the possibility of large crosstalk. On the other hand, with longer waveguide lengths with smaller propagation loss, it would be possible to utilize just a simple silicon wire for practical wavelength conversion. We demonstrate the wavelength conversion for data rate of 10-Gbps using a 5.8-cm-long silicon wire. These characteristics are attributed to the extremely small core of silicon wire waveguides.

Silicon-wire-based ultrasmall lattice filters with wide free spectral ranges.

Using silicon-on-insulator-based silicon-wire waveguides with submicrometer cross sections, we constructed ultrasmall channel-dropping lattice filters for 1.5-microm infrared systems. The waveguide's low-loss bends with 2.5-microm radius reduce the total length of the filter to less than 100 microm and enlarge the free spectral range to more than 80 nm. The measured spectra show fine channel-dropping characteristics, and the results agree well with numerical predictions. Moreover, we have succeeded in tuning the dropping wavelength by adjusting the lengths of the delay lines.