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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.

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.

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

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 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.

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.

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.

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 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.

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.

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.

In this paper, a high-refractive-index (∼2.0) and low-propagation-loss tantalum pentoxide (Ta2O5) waveguide is reported. We fabricated Ta2O5 strip optical waveguides with a cross section of 400 nm × 10 µm by chemical solution deposition (CSD) followed by CF4 reactive ion etching. The optimum fabrication steps make it possible to obtain the Ta2O5 strip optical waveguides with a propagation loss of less than 1 dB/cm at 830 nm, which is significant for optoelectronic integrated circuits in the 0.8 µm wavelength range.

We describe FMCW reflectometry for long optical fibers by using an external-cavity laser diode as a light source. The propagation loss of optical fibers was measured, and the gain of a fiber amplifier was also estimated.

High resolution FMCW reflectometry by the maximum entropy method is described. The spatial resolution is improved by a factor of 4 as compared with that by the Fourier analysis

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.

In order to improve the sensitivity of a fiber-optic polarimetric sensor, a new scheme utilizing a birefringent fiber loop with midloop frequency shifters is proposed. In this scheme, heterodyne signals with an integral multiple-phase sensitivity can be obtained in the frequency domain. The validity of the scheme is confirmed experimentally by a bulk-optic configuration. One-order-of-magnitude improvement in sensitivity without changing the dynamic range is expected from the calculation of the signal-to-noise ratio, even in the case of a 40% excess loss in the optical loop.