Yasuhiro Yamayoshi

Low-switching-energy and high-repetition-frequency all-optical flip-flop operations of a polarization bistable vertical-cavity surface-emitting laser

Subfemtojoule polarization bistable switching in a vertical-cavity surface-emitting laser (VCSEL) is experimentally demonstrated. All-optical flip-flop operation of the VCSEL was performed using two orthogonally polarized injection light pulses. The optimum wavelengths of the two injection pulses for achieving minimum switching power were different and corresponded to the lasing wavelengths of the two polarization states of the laser. The pulse width/switching frequency dependence of the injection pulses showed that a minimum switching energy was obtained at 1ns∕500MHz. A record low switching energy of 0.3fJ has been achieved as well as a record high switching frequency of 10GHz.

Analysis of basic four-wave mixing characteristics in a semiconductor optical amplifier by the finite-difference beam propagation method

We have numerically analyzed nondegenerate four-wave mixing (FWM) among short optical pulses in a semiconductor optical amplifier (SOA) by the finite-difference beam propagation method (FD-BPM). We used the nonlinear propagation equation taking into account gain spectrum dynamic gain saturation which depends on carrier depression, carrier heating, and spectral hole-burning, group velocity dispersion, self-phase modulation, and two-photon absorption. To analyze FWM in an SOA, the evolution in time and spectral domain of two input optical pulses with different frequencies during propagation was calculated. From this simulation, it has become clear that the method me used here is a very useful technique for simulating FWM characteristics in SOAs. We also found that the wavelength dependence of the gain is crucial if the detuning is larger than 1 THz.

Analysis of optical DEMUX characteristics based on four-wave mixing in semiconductor optical amplifiers

We have analyzed the basic characteristics of all-optical demultiplexing (DEMUX) based on four-wave mixing (FWM) in semiconductor optical amplifiers (SOAs) by solving a modified nonlinear Schrodinger equation by the finite-difference beam propagation method. Amplified spontaneous emission noise was not included in our model. The optimum pump pulsewidth for obtaining the high ON-OFF ratio is 1/spl sim/3 ps for 1 ps, 250 Gb/s probe pulses. The shorter limit of the pulsewidth is due to detuning between the pump and probe frequencies, which is determined by the gain bandwidth of the SOA. In order to achieve faster DEMUX operation, an SOA with broader gain bandwidth is required. We also simulated pattern effects in the FWM signal. Power fluctuation in the FWM signal can be reduced by using a strong energy pump pulse and/or weak energy probe pulse. The energy fluctuation of the FWM signal decreases to less than 1% for a 30-bit, 250-Gb/s input probe pulse train with a pulse energy of 0.01 pJ. This small fluctuation should not disturb DEMUX operation. We have also examined DEMUX from time multiplexed signals by repetitive pump pulses. Strong energy pump pulses decrease the FWM signal intensity. However, there is no pattern effect due to gain saturation because the pump pulses are injected continuously.

Analysis of optical phase-conjugate characteristics of picosecond four-wave mixing signals in semiconductor optical amplifiers

We have analyzed for the first time the optical phase-conjugate characteristics of picosecond four-wave mixing (FWM) signals in semiconductor optical amplifiers (SOAs) using the finite-difference beam propagation method (FD-BPM). We show that the optical phase-conjugate characteristics of the FWM signals are strongly dependent on input pump pulsewidths. As a typical example, we have demonstrated that SOAs act as an ideal phase-conjugator, within the confines of reversing the chirp of optical pulses, for a 10-ps input pump pulse and a /spl sim/2.2-ps linearly chirped input probe pulse. When the pulsewidth of pump pulse becomes short, the minimum compressed pulsewidth is obtained by using a fiber shorter in length than the input fiber, but having the same group velocity dispersion as the input fiber. For a much shorter pump pulse such as 1 ps, the short FWM signal can be obtained via the gating characteristics of the FWM. However, only a part of the phase information is copied to the FWM signal due to such gating characteristics. The phase information is also degraded due to the fast nonlinear effect in the SOA. Thus, the pulsewidth is not compressed by propagation through a dispersive medium.

Polarization Bistable Characteristics of Mesa Structure 980 nm Vertical-Cavity Surface-Emitting Lasers

The polarization bistable characteristics of 980 nm vertical-cavity surface-emitting lasers (VCSELs) with a mesa structure were experimentally studied. To achieve this, a distributed Bragg reflector mirror was processed into a rectangular mesa structure waveguide using inductively coupled plasma reactive ion etching and was buried with a polyimide layer. The VCSELs oscillated with single frequency and the lowest order transverse mode under CW operation at room temperature. Additionally, the VCSELs with a nearly-square mesa structure oscillated in either of the two orthogonal linear polarization states. Bidirectional polarization switching was demonstrated by two injection lights for the orthogonal linear polarization states. These results, together with the former results of polarization bistablity, which were observed in another type of VCSEL [Kawaguchi et al.: Electron. Lett. 31 (1995) 109], suggest that polarization bistability is a basic characteristic of square-shaped waveguide VCSELs.

An Analysis on the Driving Force and Optimum Frequency of a Noncontact-Type Ultrasonic Motor

A Noncontact-type ultrasonic motor driven by the radiation pressure of the sound waves can be rotated well and miniaturized. In the previous study, the maximum revolution speed reached 4,000 rpm in the case of a small ultrasonic motor of 5 mm diameter and 0.5 mm thickness. In this paper, a consideration of the driving force of such a noncontact ultrasonic motor is described.

Sound Field Characteristics in Air Gaps of Noncontact Ultrasonic Motor Driven by Two Flexural Standing Wave Vibration Disks

The sound field characteristics in the air gaps of the noncontact ultrasonic motor driven by two flexural standing wave vibration disks were analyzed by finite element method (FEM). Standing wave sound fields with an opposite phase were generated in the two air gaps by a single driving stator. Traveling wave sound fields in the two air gaps are formed as the superposition of the standing wave sound fields generated by two stators whose temporal phases and spatial positions are different from each other. The traveling direction of the calculated sound fields coincided with the rotating direction of the rotor observed in the experiments. The intensity of the sound field generated in the air gap by the single driving stator was measured by detecting the voltage induced in another stator. A gap distance wider than 0.5 mm is required for a high revolution speed because the sound field generated in the gap on the opposite side of the rotor is weak in an air gap narrower than 0.3 mm.

Improvement of Characteristics of Noncontact Ultrasonic Motor Using Acoustically Coupled Two Air Gaps

The improvement of the rotational characteristics of a noncontact ultrasonic motor using a flexural traveling wave vibration disk was experimentally tried using an acoustic reflector and a double-disk rotor. By attaching the acoustic reflector over the single-disk rotor, a revolution speed of about 2300 rpm was obtained, which was about 1.4 times the maximum value without the reflector. Using a double-disk rotor instead of a single-disk rotor, a revolution speed of about 2800 rpm was achieved. From the observation of the air flow in the air gap, it was confirmed that the increase in revolution speed is attributed to the acoustic streaming induced in the air gap newly formed by the reflector and the double-disk rotor. The acoustic streaming in the new air gap is caused by the acoustic coupling between the double air gaps through the open edges of the air gaps.

A Method of Measuring the Vibration Level Dependence of Impedance-Type Equivalent Circuit Constants

In piezoelectric transducers with high electromechanical coupling and finite dielectric loss angle, the mechanical quality factor of the transducer resonating under the short-circuit condition of the electric terminal is usually lower than that under the open-circuit condition. This fact is well explained by the impedance-type improved equivalent circuit including the effect of dielectric loss. This circuit is available for design of high power ultrasonic devices. A method of measuring the vibration level dependence of the impedance-type circuit constants is described and some experimental results are shown.

Optimum Design for Noncontact Ultrasonic Motor with Flexurally Vibrating Disk Using an Equivalent Circuit Considering Viscosity of Air

The optimum design for a noncontact ultrasonic motor with a flexurally vibrating disk has been investigated by analyzing the sound pressure in the air gap using an equivalent circuit considering the viscosity of air. The calculated results indicate that the sound pressure is affected by the viscosity and mass effect in the air gap, which are changed by the gap distance. The experimental results of revolution speed measured at various gap distances agree qualitatively with the sound pressure calculated using the equivalent circuit. In the case of wider gaps, a design satisfying the resonant condition of the air gap is necessary for optimizing the motor because the air gap resonates by the mass effect. However, in motors with an air gap narrower than about 50 µm, a higher speed rotation can be obtained for a wide range of rotor diameters and for a wide frequency range without consideration of the air gap resonance because the air viscosity effect becomes dominant.