Hirochika Nakajima

Quasi-phase-matched adhered ridge waveguide in LiNbO3

Uniform periodically poled domains were demonstrated in an insulator-sandwiched structure with z-cut lithium niobate wafers for an adhered ridge waveguide. Efficient quasi-phase-matched second harmonic generation was achieved in a mechanically defined ridge waveguide. Strong confinement in the waveguide produced second-harmonic-generation normalized conversion efficiency of 370%∕Wcm2 in the 1.5μm wavelength region.

640-Gbit/s Data Transmission and Clock Recovery Using an Ultrafast Periodically Poled Lithium Niobate Device

This paper presents the first demonstration of the use of a periodically poled lithium niobate device for signal processing at 640 Gbit/s. Clock recovery is performed successfully using the lithium niobate device, and the clock signal is used to control a nonlinear fiber-based demultiplexer. The full 640-Gbit/s system gives error-free performance with no pattern dependence and there is less than 1-dB power penalty after 50-km fiber transmission.

High Extinction-Ratio Integrated Mach–Zehnder Modulator With Active Y-Branch for Optical SSB Signal Generation

We propose a high extinction-ratio (ER) Mach-Zehnder modulator (MZM) with an active Y-branch optical intensity trimmer. In addition, we investigate a high ER integrated MZM for optical single-sideband (SSB) signal generation, where the active Y-branch trimmers are embedded in a dual-parallel MZM. Optical SSB generation with high suppression ratio of main carrier (47 dB) and undesired sidebands (>42 dB) is achieved by optimizing the optical amplitude imbalances corresponding to the ER.

High-gain, wide-dynamic-range parametric interaction in Mg-doped LiNbO 3 quasi-phase-matched adhered ridge waveguide

With recent developments and optimizations for quasi-phase-matched adhered ridge waveguide (QPM-ARW), outstanding performances containing efficient amplification were demonstrated by difference frequency generation (DFG) and optical parametric amplification (OPA). A maximum channel conversion efficiency of +7.6 dB (570%) was achieved in a telecommunication band using a 50 mm-long device, when coupling with 160 mW pump. Simultaneously, the input signal was amplified up to +9.5 dB (890%).

Characterization of optical absorption and polarization dependence of single-layer graphene integrated on a silicon wire waveguide

Optical absorption efficiency of graphene integrated onto a silicon photonic platform was characterized at around 1.55-µm optical telecommunications wavelength. Micro-Raman spectroscopy performed after the completion of all fabrication processes confirmed that transferred chemical-vapor-deposited graphene is a single layer (>90% coverage) without any significant damage. Absorption efficiencies of the single-layer graphene (SLG) on a silicon wire waveguide, obtained by measuring different lengths (cutback method) of the SLG from 2.5 to 200 µm, were 0.09 and 0.05 dB/µm for TE- and TM-polarized light. The unusual relationship in the polarization dependency can be explained by strong surface-plasmon-porlariton support in the TM mode.

70-mm-long periodically poled Mg-doped stoichiometric LiNbO3 devices for nanosecond optical parametric generation

A low-threshold, nanosecond, optical parametric generator (OPG) was produced by using a 70-mm-long, periodically poled, Mg-doped, stoichiometric LiNbO3 (PPMgSLN). The periodically poled structure was obtained by applying an electric field of 1.8kV∕mm, which is lower than the coercive field of 3.9kV∕mm. The PPMgSLN OPG pumped with a 6ns, 6.8kHz, Nd:YAG microchip laser had a generation threshold of 12MW∕cm2 and a slope efficiency of 37% of signal at 1.61μm.

Simple Modeling and Characterization of Stress Migration Phenomena in Cu Interconnects

Many observations of stress-induced voids beneath vias in wide Cu lines have been performed to analyze stress migration phenomena. Most of the voids that caused fatal failures of circuits accompanied grain boundaries in the lower lines. Finite element method calculations were performed to obtain the stress distribution around a via sandwiched between wide upper and lower lines. Based on these results, a void growth model for the Cu stress migration phenomena has been proposed by applying the Hull and Rimmer theory. This model takes two diffusion paths, such as a grain boundary and a barrier/Cu interface, into consideration. Compared with experimental results, the proposed model successfully explained the mean time to failure dependence on the temperature and geometrical parameters of Cu interconnects.

High extinction-ratio characteristics over 60 db Mach-Zehnder modulator with asymmetric power splitting y-branches on x-cut Ti:LiNbO 3

We investigated a Mach-Zehnder modulator with asymmetric power splitting Y-branches. Output characteristics of a modulator depend on the relation between the power splitting ratios of the front and back Y-branches. By tuning the power splitting ratio of the front Y-branch, we achieved maximum power output operation and high extinction-ratio operation respectively.

320 Gbps to 10 GHz sub-clock recovery using a PPLN-based opto-electronic phase-locked loop

We present successful extraction of a 10 GHz clock from single-wavelength 160 and 320 Gbps OTDM data streams, using an opto-electronic phase-locked loop based on three-wave mixing in periodically-poled lithium niobate as a phase comparator.

Influence of graphene on quality factor variation in a silicon ring resonator

Selectively patterned graphene is integrated onto a silicon ring resonator to investigate the quality factor (Q factor) variation. The Q factor sharply decreases from 7900 to 1200 as the patterned graphene length increases from 0 to 20 μm. A numerical estimation, which takes into account optical absorption by graphene, shows an exponential damping of the Q factor with increasing graphene length and is consistent with the experimental result. We expect these fundamental characterizations to be helpful in developing graphene-integrated silicon photonics applications.