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金沢大学理工研究域フロンティア工学系, More than a decade, multipoint sensor systems involving fiber Bragg gratings (FBGs) have been widely used in various implementations, especially the structural health monitoring system (SHMS). In this paper, a three-points FBG sensor was successfully analyzed by combination of incoherent frequency modulated continuous wave (I-FMCW) optical ranging system and a vertical-cavity surface-emitting laser (VCSEL) as a wavelength-tunable laser source. This system is not only capable of reading out the FBG sensors, but also identifying the location of the installed FBGs. The I-FMCW can be conducted by intensity modulation of a low-cost commercial VCSEL. Furthermore, the wavelength of the VCSEL can also be tuned by the injection current change to match the Bragg wavelength of FBGs. The detected signal from the FBGs and the reference signal from the signal generator are electrically mixed to obtain the beat frequency. A thermo-electric controller (TEC) controls temperature of one of the FBGs in the range of 25° C ~ 45° C, and contrary the others are kept at room temperature. As well as measuring temperature, one of the FBGs is strained in the range of 369 με ~2137 με. The measurement result shows that the system can clearly distinguish all FBGs installed in totally 6.6 km-long optical fiber. In addition, the Bragg wavelength shift of the FBGs according to temperature as well as strain changes was successfully measured. The system can be scaled up more than three FBGs and extended scale area, and is a candidate as a low-cost and simple FBG interrogator for a long-range SHMS.

The device durability of inverted organic solar cells (OSCs) is investigated based on Y6, which is an effective nonfullerene acceptor for high-performance OSCs. The durability of Y6-based inverted OSCs is poor and it can be caused by aggregation of Y6 in the bulk-heterojunction layer due to heating by continuous photo-irradiation (≈65 °C, 100 mW cm

金沢大学理工研究域フロンティア工学系, We have developed three-dimensional object profiling system using a highly accurate frequency-modulated continuous-wave optical ranging system. A DFB laser and a vertical-cavity surface-emitting laser (VCSEL) are used as the laser source and the optical frequency is swept by the injection current modulation with a symmetric triangular wave. The influence of nonlinearity in the optical frequency sweep is canceled by utilizing k-sampling technique. Since the optical frequency sweep range of a VCSEL is larger than that of a DFB laser, high spatial resolution and high ranging accuracy are achieved by using a VCSEL. The spatial resolution evaluated by the full-width at half-maximum (FWHM) is 460 μm and the standard deviation of the ranging accuracy is σ = 2.7, μm when a VCSEL is used as the frequency-swept laser source, and the FWHM is 2.3 mm and σ = 14.8 μ m when a DFB laser is used as the frequency-swept laser source. Accordingly, fine and clear object profiling of a coin and a printed circuit board is achieved by using a VCSEL as the frequency-swept laser source. Finally, we speed up the measurement time by increasing the repetition frequency of the injection current modulation and optimizing timing of the data acquisition, the galvano mirror scan and the fast Fourier transform analysis by taking account of the transient response of the galvano mirror, and the fastest measurement time of 22.6 s is achieved for 201 × 201 measurement points, which is four times faster than our previous results. We also discuss the effect of moving average filtering and median filtering to improve profiling quality.

The FMCW reflectometry is a high-resolution ranging system using an optical frequency swept laser source. The beat spectrum consists in the interference signal will analysis by FFT analysis. At the asymmetric method, the timing of the data acquisition called raising period is different with the timing of FFT analysis called the falling period. The exact percentage of both will determine quality profiling and measurement time. In our experiment, we compare the measurement time between 350 and 600 points of the number of the acquired data with the theoretical measurement time. The measurement time of theoretical theory is closer with the measurement time using 350 points of the number of the acquired data. This study proved that the clear profile is generated using the repetition frequency of the injection current modulation 1.8 kHz with the condition is the raising period (Tr = 155 μ) is shorter than the falling period (Tf = 400) at the raising mode in 22.7 sec by using 350 points of the number of the acquired data.

金沢大学理工研究域フロンティア工学系, Quadrant silicon avalanche photodiodes (APDs) were fabricated by standard 0.18µm CMOS process, and were characterized at 405nm wavelength for Blu-ray applications. The size of each APD element is 50×50µm2. The dark current was 10pA at low bias voltage, and low crosstalk of about -80dB between adjacent APD elements was achieved. Although the responsivity is less than 0.1A/W at low bias voltage, the responsivity is enhanced to more than 1A/W at less than 10V bias voltage due to avalanche amplification. The wide bandwidth of 1.5GHz was achieved with the responsivity of more than 1A/W, which is limited by the capacitance of the APD. We believe that the fabricated quadrant APD is a promising photodiode for multi-layer Blu-ray system.

Small changes in mirror displacement cause variations in the reflection pattern and corresponding optical path length in a multi-pass absorption cell. Herein, we propose a path length calibration-free laser absorption spectrometer based on a frequency modulated continuous wave (FMCW) ranging system. The absorption coefficient of atmospheric oxygen and the optical path length in a multi-pass absorption cell were measured using direct absorption spectroscopy and an FMCW ranging system, respectively. Further, the measured optical path length in the multi-pass absorption cell was compared with ray-tracing simulations. In addition, we demonstrated that the optical absorption coefficients measured by the direct absorption spectroscopy were not affected by changes in the distance between mirrors.

Avalanche photodiodes fabricated by CMOS process (CMOS-APDs) have features of high avalanche gain below 10V, wide bandwidth over 5GHz, and easy integration with electronic circuits. In CMOS-APDs, guard ring structure is introduced for high-speed operation by canceling photo-generated carriers in the substrate at the sacrifice of the responsivity. We describe here wavelength dependence of the responsivity and the bandwidth of the CMOS-APDs with shorted and opened guard ring structure. © 2017 Elsevier Ltd, Embargo Period 12 months

nMOS-type and pMOS-type silicon avalanche photodiodes (APDs) were fabricated by standard 0.18µm CMOS process, and the current-voltage characteristic and the frequency response of the APDs with and without guard ring structure were measured. The role of the guard ring is cancellation of photo-generated carriers in a deep layer and a substrate. The bandwidth of the APD is enhanced with the guard ring structure at a sacrifice of the responsivity. Based on comparison of nMOS-type and pMOS-type APDs, the nMOS-type APD is more suitable for high-speed operation. The bandwidth is enhanced with decreasing the spacing of interdigital electrodes due to decreased carrier transit time and with decreasing the detection area and the PAD size for RF probing due to decreased device capacitance. The maximum bandwidth was achieved with the avalanche gain of about 10. Finally, we fabricated a nMOS-type APD with the electrode spacing of 0.84µm, the detection area of 10×10µm2, the PAD size for RF probing of 30×30µm2, and with the guard ring structure. The maximum bandwidth of 8.4GHz was achieved along with the gain-bandwidth product of 280GHz.

Silicon avalanche photodiode (APD) was fabricated by standard 0.18 μm CMOS process. The current-voltage characteristic and frequency response was measured for the APD with and without guard ring. With the guard ring around the perimeter of the diode junction, it shows a better performance for the maximum bandwidth but in contrast lower in responsivity. To enhance the bandwidth, the detection area and the PAD size for RF probing are optimized to 10 × 10 μm2 and 30 × 30 μm2, respectively, to decrease the device capacitance, the spacing of interdigital electrode is narrowed to 0.84 μm to decrease carrier transit time, and by cancelling the carriers photo-generated in the deep layer and the substrate because the carriers are slow diffusion carriers. As a result, the maximum bandwidth of 8 GHz was achieved along with a gain-bandwidth product of 280 GHz. © 2015 IEEE.

We have developed high resolution and highly accurate FMCW optical ranging system using a VCSEL, and applied it to three-dimensional object profiling. The spatial resolution of 0.9 mm and the measurement accuracy of 36 μm are achieved, and we can successfully image a coin and a printed circuit board.

For the realization of optoelectronic integrated circuits, it is required to incident light perpendicularly to a planar Si photodiode. We propose a high-efficient vertical optical coupler using an amorphous Si optical waveguide grating coupler with top reflector, which is transparent at 850 nm wavelength range. The optical waveguide (width of 300 nm $$\times $$ height of 100 nm) coupler is analyzed by using finite element method. The coupling efficiency of 80 % is calculated at the grating period of 380 nm, the duty ratio of 0.75 and the depth of 35–65 nm with top metal reflector.

金沢大学理工研究域フロンティア工学系, Optical frequency chirp of a DFB laser is linearized by modifying the modulation waveform using an interference signal of an interferometer, and high spatial resolution FMCW ranging system was realized., Conference on Lasers and Electro-Optics/Pacific Rim 2018Hong Kong China29 July–3 August 2018

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.

The VCSEL using the Coumarin6 as the active layer was fabricated by the wafer bonding method, and measured by the blue LD pumping under pulsed condition. The laser emission was observed at the wavelength of 545 nm. The obtained threshold pump power and estimated threshold current density were about 160 mW and 3.2 kA/cm2.

A recess-gate-type n-channel depletion mode metal–semiconductor field-effect transistor (MESFET), a metal–oxide–semiconductor field-effect transistor (MOSFET) prepared by UV and ozone oxidation, and metal–insulator–semiconductor field-effect transistors (MISFETs) prepared by oxidation followed by nitrogen plasma treatment for different time durations with an electron cyclotron resonance system (oxinitridation) were fabricated using an ex-situ process and equally current controlled recessed wafers. The MISFETs, especially the longest-nitrided one, showed the highest pinch-off gate voltage (-1.5 V) and the highest peak transconductance (170 mS/mm) at a 0.9 V gate voltage. The MOSFET showed the largest drain current, whereas the longest-nitrided MISFET showed the smallest drain current, owing to the difference in applicable forward gate voltage. These were clearly confirmed with statistical data. The transconductance obtained reproduces that of our previous experiment, and its increase is due to the improvement of crystallographic order in the vicinity of the insulator/semiconductor interface. The different applicable gate voltage implies that the subsequent nitridation weakens the barrier effect of the oxidized layer. An AlGaAs layer, as the mother material for oxinitridation, improves it.

We have reported that the oxi-nitridation of GaAs forms an insulator–semiconductor interface without deteriorating the crystallographic order of GaAs, and is applicable to the fabrication of compound semiconductor devices with metal–insulator–semiconductor (MIS) gates. A problem with oxi-nitridation is that nitrogen plasma ashes and thins the photoresist and hence restricts the possible nitrogen processing times even if a long nitridation is desirable for a better interface quality. To counteract this restriction, we developed a new processing technique utilizing an 0.3-µm-thick Al layer as a mask for selective etching, oxi-nitridation and lift-off. A high transconductance (185 mS/mm) and sharp pinch-off were obtained by a long (8 h) nitridation.

A lasing charactrization of a Brillouin/erbium optical fiber laser (BEFL) is experimentally discussed. In the BEFL, an erbium-doped fiber amplifier (EDFA) is incorporated into the Brillouin laser resonator to enhance small Brillouin gain, which makes the configuration of the Brillouin laser resonator easy and flexible. The experimental results show that the output power of the BEFL has a threshold against the Brillouin pump power, and above the Brillouin threshold, the output power increases linearly with the EDFA pump'power. The BEFL threshold decreases with increasing the length of the optical fiber in the laser resonator used as a Brillouin gain medium. The BEFL oscillates in a stable single longitudinal mode because the bandwidth of the Brillouin gain profile is very narrow (∼ 30 MHz). The relative intensity noise (RIN) and the spectral lineshape were measured. The noise floor level decreases with increasing the EDFA pump power, and the full-width at half maximum of the BEFL was measured to be about 8 kHz.

The electronic states of Ga-terminated and As-terminated GaAs(100)-(1×1) surfaces and those surfaces with adsorbed O and S are studied by molecular orbital calculation using small cluster models and the discrete variational Xα method. Without adsorption of any foreign atoms, the Ga-terminated surface has 4p-related surface states in the middle of the forbidden band. While, the As-terminated surface has 4p-related surface states in the lower half of the forbidden band. The adsorption sites of chalcogen atoms on Ga- and As-terminated surfaces are determined ab initio by using density functional theory (Gaussian 98). With adsorbed chalcogen atoms, the density of the surface states in the forbidden band of the surface Ga layer markedly decreases, and that of the surface As layer increases. The adsorbates have states in the lower half of the band gap (Ga-terminated surface) or in the valence band (As-terminated surface). The results suggest that a chalcogen adsorbed Ga-terminated and bare As-terminated surface have low surface densities in the upper half of the band gap.

Oxidation by the UV & ozone process, nitridation by the nitrogen helicon-wave-excited plasma process, and the combination of these processes are applied to (100) GaAs wafers. An atomic force microscope, X-ray photoelectron spectroscopy, a transmission electron microscope, photoluminescence and electrical characteristics (current–voltage and capacitance–voltage) were used to analyze the influences of these processes on the structure and composition of the surfaces and the interfaces. Metal–insulator–semiconductor (MIS) diodes and Schottky diodes were fabricated in order to investigate the electrical influences of these processes. The oxidation slightly disorders GaAs surfaces. Nitridation of a bare surface creates about a 2-nm-thick strongly disordered layer, which strongly deteriorates the electrical and photoluminescence characteristics. Nitridation of oxidated wafers (oxi-nitridation) forms firm amorphous GaON layers, which contain GaN, with very flat and sharp GaON/GaAs interfaces, where crystal disorder is hardly observed. It improves the current–voltage (I–V) and capacitance–voltage (C–V) characteristics and the photoluminescence intensity. Results of the structural and the electrical characterizations qualitatively coincide well with each other.

金沢大学理工研究域電子情報学系, 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. © 2011 IEEE.

金沢大学理工研究域電子情報学系, Amorphous polyethylene terephthalate (PET) optical waveguides are fabricated by the spin coating method and the optical properties are characterized. The refractive index measured by a spectroscopic ellipsometer is 1.5656, 1.5560, 1.5489, and 1.5477 at 633-, 830-, 1310-, and 1550-nm wavelength, respectively. Multimode optical waveguides with 12-μm thickness and 46-μm width are fabricated by mechanical grinding using a dicing saw to form the core ridge, and the propagation loss is measured by the cut-back method to be 0.30, 0.12, 0.35, and 0.70 dB/cm for 660-, 830-, 1310-, and 1550-nm wavelength, respectively. © 2011 IEEE.

金沢大学理工研究域電子情報学系, A hole-injection-type and an electron-injection-type Si avalanche photodiode (APD) were fabricated by a standard 0.18-μm complementary metaloxidesemiconductor 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. © 2006 IEEE.

We have proposed and demonstrated a method of linearizing the optical frequency sweep of a laser diode by superimposing a rectangular signal on the modulating triangular signal for high-resolution FMCW reflectometry. First, we describe the principle of the proposed method. The waveform of the optical frequency sweep is simulated by using a simplified model, in which only the frequency modulation response is taken into account. The optimum amplitude of the rectangular signal and the linearity of the optical frequency sweep are discussed. Next, the proposed method is applied to FMCW reflectometry to confirm the usefulness of the method. The spatial resolution is minimized when the amplitude of the rectangular signal normalized to the amplitude of the triangular signal is between 0.5 and 0.7. It is difficult to linearize the optical frequency sweep perfectly by using the proposed method. However, the method is very effective because the linearity of the optical frequency sweep can be significantly improved despite the simple configuration. © 1998 Scripta Technica, Electron Comm Jpn Pt 2, 81(10): 30–36, 1998

金沢大学工学部, We describe a frequency domain detection scheme of coherence multiplexed sensor signals by using an optical loop with a frequency shifter. The optical path differences of the sensors are adjusted to integer-multiple of the loop length of the optical loop, and then the the sensor signals are multiplexed in the frequency domain, appearing at the integer-multiple of the working frequency of the frequency shifter. In the experiment, four Michelson-type sensors are successfully multiplexed with , the sensitivities of about 2 mrad/√Hz. The sensitivities are also theoretically calculated which are in good agreement with the experimental results. The optimum design of the optical loop is also discussed.

Silicon avalanche photodiodes (APD) fabricated by 0.18 μm CMOS process with different interdigital electrode spacing were characterized at 850 nm wavelength. The largest bandwidth of 7 GHz was achieved for the APD with 1 μm electrode spacing, and the gain-bandwidth product was 270 GHz.

We fabricated an InGaAs metal-semiconductor-metal (MSM) photodetector bonded on Si substrate by Benzocyclobutene bonding method. The responsivity of 0.035 A/W at 1.55 μm-wavelenght and the dark current of 29 nA at the bias voltage of 6V were obtained. The bandwidth of 3 GHz was obtained at the bias voltage of 10 V.

We theoretically investigate the approach to enhance the bandwidth of couplers. We present a Si/SiO2 bent 2 × 2 coupler. Its splitting ratio has a low sensitivity to wavelength and fabrication changes.

In order to realize the Si based optical waveguide, which can propagate the absorption wavelength in Si, we theoretically and experimentally investigate the material propagation loss of silicon-on-insulator slot waveguide at 1050 nm.

We fabricated Si photodiode from 0.35 µm BiCMOS process and measured the sensitivity and the frequency response at the wavelength of 650nm. The responsivity was 0.11 A/W and the bandwidth was about 1 GHz.

We propose a high-efficient vertical optical coupler using an amorphous Si waveguide grating coupler with top reflector. The coupling efficiency of 80% is calculated at the grating period of 380 nm and the duty ratio of 0.75 with top metal reflector.

PZT and PLZT films on Si substrates were deposited to realize electro-optic modulators on LSI chip. The coercive field and remnant polarization of PZT and PLZT film were 148kV/cm and 50µC/cm2, 170kV/cm and 32µC/cm2, respectively.

A silicon avalanche photodiode fabricated by CMOS process was characterized at 405 nm wavelength for Blu-ray applications. The avalanche gain of 36, the maximum responsivity of 2.61 A/W, and the bandwidth of 300 MHz were achieved.

Oscillations in the higher frequency range are closely related to regional brain hemodynamic changes. Here we investigated this neurovascular coupling in humans in response to electrical stimulation of the right median nerve. In a single-trial study, we simultaneously recorded hemodynamic fluctuations in the somatosensory cortex by near infrared spectroscopy and brain neuronal oscillations by whole-head magnetoencephalography (MEG). The results from six volunteers showed that neural fluctuations at β or γ-band power were correlated with hemodynamic fluctuation during stimulus conditions. These correlations were prominent with a time delay of 5-7 s. This study provides new direct evidence that hemodynamic onset lags specific neural oscillations in the order of seconds in human awake conditions using noninvasive methods.

We fabricated optical waveguides using amorphous silicon deposited by catalytic chemical vapor deposition method. The waveguides were fabricated by photolithography and wet chemical etching. The propagation loss of 15 dB/cm at 830 nm was measured.

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.

金沢大学理工研究域電子情報学系, We propose a novel type Mach-Zehnder interferometric optical switching device using micro-electro mechanical systems (MEMS) as a phase shifter. As the analytical result, the amount of stretch length is 70nm at the applied voltage of 60 V per one phase shifter. The π-shift is achieved using cascade connections of 4-7 shifters. ©2009 IEEE., 9th International Conference on Numerical Simulation of Optoelectronic Devices, NUSOD 2009; Gwangju; 14 September 2009 through 17 September 2009; Category number CFP09817; Code 78843

金沢大学工学部, The reflection-type delayed self-homodyne (RDSH) method presently described is based on the same principle as the delayed self-homodyne/heterodyne (DSH) method but no beam splitter or fiber coupler is required. The delayed beam is produced by utilizing a round-trip propagation passing through a solitary long optical fiber delay line. Therefore, the optical configuration is very simple, and the beat signal has a resolution twice that of the DSH method. However, the RDSH method's drawback is that a large intensity difference between the two beams reduces the level of the beat signal. This can be improved by increasing the reflectivities of the fiber ends by dielectric coating. In this case, multiple reflection at the fiber ends should be taken into account. An improvement of more than 12 dB is analytically predicted and is experimentally confirmed. A heterodyning scheme of the RDSH method is also presented.

Locking bandwidth and FM noise properties of mutually optical injection-locked semiconductor lasers are described, taking into account an optical delay time between lasers. By introducing a figure of merit of optical injection specifying unilaterality of the optical injection, optical injection-locking systems with an arbitrary inverse optical injection from a slave laser to a master laser can be described adequately. It is found from the analysis that for a short delay time, the locking bandwidth and the FM noise depend on both the round-trip phase angle and the figure of merit of optical injection. In the case of a long delay time, the FM noise and the spectral linewidth are reduced significantly with increasing locking bandwidth. Finally, an undulation similar to that in the external cavity semiconductor laser appears in the FM noise spectrum. The experimental results obtained by using semiconductor lasers emitting at 0.83 μm agree well with the analytical results.

Generating mechanisms and measuring methods of the intensity noise and the frequency noise of semiconductor lasers are reviewed. Both types of noise originates from the field flucturation inherent in the spontaneous emission which works as a source of a lasing field. The noise level and its property are determined with lasing mechanism and related physics. Several topics on the noise problem are also shown presented.

We report a design and implementation of lateral silicon photodetectors fabricated on a silicon-on-insulator (SOI) substrate in a complementary CMOS-compatible process. In addition, we disscuss the structure dependences on the frequency and optimum design for a maximum bandwidth. A standard device fabricated with a 210 nm absorbing layer, a finger width of 1.00 µm, a finger spacing of 1.63 µm, a square detector area of 20 × 20 µm2, and a pad size of 60 × 60 µm2 achieved a bandwidth of 12.6 GHz at a bias voltage of 10 V, with a responsivity of 7.5 mA/W at 850 nm wavelength. A photodetector with the same geometry, which was fabricated with a smaller pad size of 30 × 30 µm2, exhibited a bandwidth of 13.6 GHz.

金沢大学工学部, 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 fFS Hz) is incorporated in the reference arm of the reflectometry. As a result, the interference signal corresponding to the reference beam that circulates N rounds in the loop appears around (N × fFS) 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.