Toshifumi Hasama

Ultrafast nonlinear effects in hydrogenated amorphous silicon wire waveguide

We, for the first time, present the ultrafast optical nonlinear response of a hydrogenated amorphous silicon (a-Si:H) wire waveguide using femtosecond pulses. We show cross-phase and cross-absorption modulations measured using the heterodyne pump-probe method and estimate the optical Kerr coefficient and two-photon absorption coefficient for the amorphous silicon waveguide. The pumping energy of 0.8 eV is slightly lower than that required to achieve two-photon excitation at the band gap of a-Si:H (approximately 1.7 eV). An ultrafast response of less than 100 fs is observed, which indicates that the free-carrier effect is suppressed by the localized states in the band gap.

All-optical logic based on ultrafast gain and index dynamics in a semiconductor optical amplifier

We investigate nonlinear carrier dynamics in a multiquantum-well semiconductor optical amplifier (SOA) in the context of ultrafast all-optical logic. A rate-equation model is presented that accounts for two-photon absorption, free-carrier absorption, self- and cross phase modulation, carrier heating, spectral, spatial hole burning, and self- and cross polarization modulation. The nonlinear refractive index dynamics is investigated theoretically and experimentally. We find nonlinear phase changes larger than /spl pi/ radians, which recovers on a timescale in the order of 1 ps. We also investigate a nonlinear AND gate that consists of an SOA that is placed in an asymmetric Mach-Zehnder interferometer. We show that the gate can be operated using 800-fJ optical pulses with duration of 200 fs while having a contrast ratio larger than 11 dB.

Ultrahigh-Definition Video Transmission and Extremely Green Optical Networks for Future

The Internet traffic is essentially increasing because the contents are more associated with video, a higher definition video, rather than just text or picture. In fact, next-generation television (TV) standards are being explored toward the so-called 8-k definition, or ultrahigh-definition TV (UHDTV) that requires a bandwidth more than 70 Gb/s for real-time transmission. In the long run, chunks of such video data will eventually prevail over the network, and this trend would ensure a persistent traffic growth for the next decades to come. However, we will point out that the present IP-based technology cannot scale to the increasing traffic for the future mostly due to energy-consumption limits and will become a grave bottleneck for the sustainable growth of the traffic. We will then argue that the only promising solution for this would be the utilization of optical circuit switching, potentially having a few digits better energy efficiency than the present IP-router-based network. As specific implications, we are proposing the concept of the dynamic optical-path network (DOPN). We discuss how to scale the DOPN to the WAN, and show that an extremely green optical network for video-related services is possible at a clean-slate level. Then, we argue that DOPN will be first applied to LANs for broadcasting stations where a technology for scalable network interface cards are essential to accommodate the real-time UHDTV transmissions. Finally, we will briefly introduce our recent demonstration of UHDTV video transmissions using the optical time-division multiplexing based on integratable ultrafast optical devices only.

High‐power discharge‐pumped F2 molecular laser

Output energy of a discharge‐pumped F2 molecular laser has been greatly increased by an extremely intense excitation in a sufficiently high‐pressure laser gas mixture. The maximum vacuum ultraviolet output energy was 112 mJ for an 8 atm laser gas mixture, which is almost an order of magnitude larger than that obtained so far with discharge devices. The peak power deposition into the discharge plasma was estimated to be 23 MW/cm3 for the maximum output energy. Use of a short electrode spacing was found to be effective to increase the output energy.

Pattern-effect-free all-optical wavelength conversion using a hydrogenated amorphous silicon waveguide with ultra-fast carrier decay

Ultra-fast carrier decay, recently discovered in a hydrogenated amorphous silicon waveguide, can be exploited for pattern-effect-free all-optical signal processing based on optical Kerr nonlinearity. In this study, we utilized a 10 Gbit/s RZ-OOK data stream as a pump for degenerate four-wave mixing in a low-loss hydrogenated amorphous silicon waveguide. The propagation loss of the waveguide used was 1.0±0.2 dB/cm at 1550 nm. Unlike crystalline silicon waveguides, no noticeable difference was observed in the BER characteristics between the cases of PRBS 2(7)-1 and 2(31)-1.

Ultrafast refractive-index dynamics in a multiquantum-well semiconductor optical amplifier

We investigate ultrafast refractive index dynamics in a multiquantum-well InGaAsP-InGaAs semiconductor optical amplifier that is operated in the gain regime by using a pump-and-probe approach. The pump-and-probe pulses are cross-linearly polarized. We observe a phase shift of 200/spl deg/ if the amplifier is pumped with 120 mA of current, but find that the phase shift vanishes if the injection current is increased to 160 mA. Our results indicate a contribution of two-photon absorption to the nonlinear phase shift that opposes the phase shift introduced by the gain. Finally, we observe that the phase shift comes up and disappears within a picosecond.

Green-to-Yellow Continuous-Wave Operation of BeZnCdSe Quantum-Well Laser Diodes at Room Temperature

We have grown laser diode structures using highly strained BeZnCdSe quantum wells by molecular beam epitaxy and successfully obtained continuous-wave lasing in the green-to-yellow spectral region (543–570 nm) at room temperature. The green-to-yellow lasing color was tuned by simply varying the Cd content of the quaternary BeZnCdSe quantum well. The threshold current densities of 20-µm-wide lasers were found to be sufficiently low (less than 0.85 kA/cm2). This result demonstrates that BeZnCdSe is a promising material for use as the active layer in high-performance green-to-yellow laser diodes.

Ultrafast All-Optical Refractive Index Modulation in Intersubband Transition Switch Using InGaAs/AlAs/AlAsSb Quantum Well

Large, very fast phase modulation was observed in transverse electric (TE) probe light when an intersubband transition (ISBT) switch module using an InGaAs/AlAs/AlAsSb quantum well was pumped by transverse magnetic (TM) light. The phase shift amounted to 1.88 rad for a pump pulse energy of 4 pJ (fiber input), with very fast response. This phenomenon is explained by the change in plasma dispersion caused by the redistribution of electrons among subbands having different effective masses. This phase modulation will enable us to realize various novel ultrafast all-optical devices for signal processing.

Efficiency of a capacitor‐transfer‐type discharge excimer laser with automatic preionization

Energy transfer aspects in a capacitor‐transfer‐type discharge excimer laser with automatic preionization (API) have been studied to improve the laser efficiency. Detailed measurements on the discharge and output characteristics of the XeCl laser show that efficient energy transfer from the storage capacitor to the discharge capacitor is essential for improvement in the overall efficiency. The circuit analysis based on the measurements suggests that the API discharge excimer laser could be operated with an electrical efficiency of about 4%. The overall efficiencies of 2.9, 2.8, and 1.3% have been obtained with the XeCl, KrF, and ArF lasers, respectively.

50 J discharge-pumped XeCl laser

An X-ray preionized XeCl laser with a wide aperture of 10*10 cm/sup 2/ is described. The density for preionization electrons is estimated from an X-ray energy distribution to be greater than 6*10/sup 9/ cm/sup -3/ under operational gas conditions. In high pressure operation at 4500 torr, sufficient preionization has led to successful discharge-breakdown at a low E/N of 6.4*10/sup -17/ V-cm/sup 2/, resulting in a high electrical efficiency of 3.1% with an output energy of 17.6 J. An output energy of 50 J in an 85-ns (FWHM) optical pulse has been extracted from an active volume of about 10:1 for a pulse-forming-line (PFL) voltage of 400 kV. The effects of impedance transformation of a pulse-transmission-line (PTL) following the PFL have been investigated using two types of PTL with output impedances of 0.26 and 0.48 Omega . >