Koichi Iiyama

Extended-range FMCW reflectometry using an optical loop with a frequency shifter

We propose a novel method to extend the measurement range of FMCW reflectometry. In this method, an optical loop with a frequency shifter (working frequency f/sub FS/Hz) is incorporated in the reference arm of the reflectometry. As a result, the interference signal corresponding to the reference beam that circulates N found in the loop appears around (N/spl times/f/sub FS/) Hz, which means that the signals from different measurement ranges can be detected in different frequency domains. Therefore, it is possible to extend the measurement range. In the experiment, the measurement range is extended from 15 m to 48 m.

Linearizing optical frequency-sweep of a laser diode for FMCW reflectometry

We propose and demonstrate a novel linearizing method of optical frequency-sweep of a laser diode for frequency-modulated continuous-wave (FMCW) reflectometry. In order to linearly sweep the optical frequency, we adopt a reference interferometer and an electric phase comparator. The interference beat signal of the reference interferometer is phase-compared with an external reference rectangular signal having a fixed frequency near the interference beat signal frequency by a lock-in amplifier. The error signal from the lock-in amplifier is fed back to the modulating signal of the injection current of the laser. Thus, a phase-locked loop composed of optical and electric circuits can be established, and the beat signal frequency is locked to the frequency of the reference signal. The optical frequency of the laser diode is, therefore, excellently linearly swept in time. In order to experimentally confirm the linearity of the proposed method, we apply this frequency-swept laser diode to the FMCW reflectometry. Resultingly, the improvement of the linearity is estimated to be about 10 dB. And the theoretically limited spatial resolution of the FMCW reflectometry is achieved. The backscattered light in optical waveguide devices is measured by the FMCW reflectometry using the proposed light source, and the propagation loss of a single-mode glass waveguide is successfully evaluated.

Loss measurement in optical waveguide devices by coherent frequency-modulated continuous-wave reflectometry

We describe experimental results of loss measurement in optical waveguide devices by coherent frequency-modulated continuous-wave reflectometry composed of a frequency-swept semiconductor laser and a two-beam interferometer. The backscattered light in a potassium-ion-exchanged single-mode optical waveguide is successfully detected, and the loss along the optical waveguide is evaluated to be 1.0 dB/cm.

Hole-Injection-Type and Electron-Injection-Type Silicon Avalanche Photodiodes Fabricated by Standard 0.18-

A hole-injection-type and an electron-injection-type Si avalanche photodiode (APD) were fabricated by a standard 0.18-μm complementary metal-oxide-semiconductor process. The avalanche amplifications are observed below 10 V of the bias voltage, and the maximum avalanche gains were 493 and 417 for the hole-injection-type and the electron-injection-type APDs, respectively. The maximum bandwidth is 3.4 GHz, and the gain-bandwidth products were 90 and 180 GHz for the hole-injection-type and the electron-injection-type APDs, respectively.

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A simple method for measuring the linewidth enhancement factor alpha of a semiconductor laser by optical injection locking is described. Without knowledge of the absolute values of detuning and the optical injection level, the value of alpha is evaluated from only changes in optical power in the stable optical injection-locking state. The value of alpha of a 0.83-microm channeled substrate planar laser is evaluated to be 2.65 +/- 0.2. The measurement error of this method is also discussed.

m CMOS Process

A gate insulating layer with single nm-order thickness for suppressing gate leakage current is one of the key factors in extending downsizing limits, based upon the scaling rule, of field-effect-type transistors. We describe the fabrication and characterization of GaAs MISFETs with a nm-thin oxidized layer as the gate insulating layer, which is formed by an ultraviolet (UV) and ozone process. The UV and ozone process forms oxidized GaAs layers near the surface, which effectively suppress the reverse leakage current by several orders of magnitude. The fabricated GaAs MISFET can operate not only in the depletion mode, but also in the accumulation mode up to 3 V gate voltage for 8-nm-thick oxidized layers due to the current blocking effect of the oxidized layer. A current cutoff frequency of 6 GHz and a maximum oscillation frequency of 8 GHz are obtained for a GaAs MISFET with 1-/spl mu/m gate length and 8-nm-thick oxidized layers.

Simple method for measuring the linewidth enhancement factor of semiconductor lasers by optical injection locking.

Directional couplers are widely used as one of the key components of optical integrated circuits. However, the coupling efficiency of the conventional directional coupler is highly sensitive to wavelength. This sensitivity degrades the characteristics of devices that contain directional couplers for wavelength division multiplexing transmission. A curved directional coupler has been proposed using silica optical waveguide as one of the coupler which realize wavelength insensitive, small footprint and tolerant to fabrication. In this paper, we theoretically investigated this curved coupler using Si wire waveguide and got results that the curved coupler whose bending radius of 21 μm and coupling length of 7.40 μm can reduce the wavelength dependence and achieve about a sevenfold enhancement of operational bandwidth in the transmittance variation range of -3 ± 0.1 dB compared with conventional directional coupler.

Fabrication of GaAs MISFET with nm-thin oxidized layer formed by UV and ozone process

High-resolution frequency-modulated continuous-wave (FMCW) reflectometry is realized by using a single-mode vertical-cavity surface-emitting laser (VCSEL) as a frequency-swept light source for the first time. The optical frequency of the VCSEL is swept by the injection current modulation. The experimental spatial resolution of 250 μm was achieved, which is the best value when an injection current tuned laser diode is used as a frequency-swept light source.

Reduction of Wavelength Dependence of Coupling Characteristics Using Si Optical Waveguide Curved Directional Coupler

A silicon lateral photodiode is fabricated by standard 0.18µm CMOS process, and the optical detection property is characterized. The photodiode has interdigital electrode structure with the electrode width of 0.22µm and the electrode spacing of 0.6µm. At 830nm wavelength, the responsivity is 0.12A/W at low bias voltage, and is increased to 0.6A/W due to avalanche amplification. The bandwidth is also enhanced from 12MHz at low bias voltage to 100MHz at the bias voltage close to the breakdown voltage.

High-Resolution FMCW Reflectometry Using a Single-Mode Vertical-Cavity Surface-Emitting Laser

InAlAs was oxidized by UV and ozone process. Nanometer-order thin-oxide layers were generated in proportion to the square root of the process period, although their correlation with the period is poor compared with that of InGaAs. Oxidation initially decreased photoluminescence intensity from the InAlAs surface and then gradually increased it when the process period exceeded 15 min until it reached 4 h (crystallographic-order degradation followed by recovery). Plasma nitridation of InAlAs exhibited a gradual and monotonic decrease in photoluminescence intensity. Lattice-matched InAlAs/InGaAs metal–oxide–semiconductor high electron mobility transistors were fabricated utilizing gate oxide layers which were formed by 4 h oxidation. A transconductance of 200 mS/mm was obtained using a 1.5-µm-gate-length device, in a high-forward-gate-bias region although hysteresis was observed in the current–voltage curve.