Ultra-compact VCSEL scanner for high power solid-state beam steering.

We demonstrate the lateral monolithic integration of a tunable first-order surface-grating loaded vertical-cavity surface-emitting laser (VCSEL) and slow-light waveguide with fan-beam steering and amplifier function. Shallow Bragg-grating formed on the surface of a VCSEL section enables the selection of a single slow-light mode, which can be coupled into the integrated long waveguide and amplified through pumping the amplifier above threshold. We obtained over 3W amplified slow-light power with single-mode operation and over 4W amplified quasi-single-mode power under pulsed current injection. To the best of our knowledge, this is the highest output power for single-mode VCSELs. Solid-state beam steering of the device is also demonstrated with 9° fan-beam steering range and 200 resolution points.

Demonstration of real-time structured-light depth sensing based on a solid-state VCSEL beam scanner.

We demonstrated a real-time scanning structured-light depth sensing system based on a solid-state vertical cavity surface-emitting laser (VCSEL) beam scanner integrated with an electro-thermally tunable VCSEL. Through a swept voltage added to the tunable VCSEL, a field of view of 6°×12° could be scanned with a scanning speed of 100 kHz by the beam scanner. Adopting the beam scanner, the real-time depth image with a lateral resolution of 10,000 (20×500) was obtained by measuring a step target placed at 35cm. The frame rate could be >10Hz even if sunlight shot noise is artificially added to the experimental data. By using a higher-speed camera, a potential lateral resolution could be reached at 50,000 (100×500) with a frame rate of > 20Hz. By using flat optics, a compact scanning module offering line pattern with FoV of >40°×20° was also demonstrated. It could help to realize high-resolution and high-accuracy structured-light sensing with a compact module.

Accuracy of wristwatch-type photoplethysmography in detecting atrial fibrillation in daily life.

Aims: Detection of asymptomatic paroxysmal atrial fibrillation is challenging. Smartphone- or smartwatch-based photoplethysmography is efficient at detecting irregular rhythms using pulse waves but is too complex for older patients. We aimed to evaluate the detection accuracy of atrial fibrillation by a wristwatch-type continuous pulse wave monitor (PWM) in daily life. Methods and results: Patients at high risk of atrial fibrillation but with no history of atrial fibrillation (n = 163; mean CHADS2 score, 1.9) and patients with known atrial fibrillation (n = 123, including 34 with persistent atrial fibrillation) underwent PWM and telemetry electrocardiogram recording for 3 days. Risk of atrial fibrillation was judged using the 'Kyorin Atrial Fibrillation Risk Score', a scoring system based on previously reported atrial fibrillation risk scoring systems. The PWM assessed the presence of atrial fibrillation at 30 min intervals, and the results were compared with the telemetry electrocardiogram findings. The PWMs accurately diagnosed two patients with paroxysmal atrial fibrillation in the high-risk group. The PWMs accurately diagnosed 48 of the 55 patients with atrial fibrillation in the known-atrial fibrillation group. The PWM accuracy in detecting patients with atrial fibrillation was as follows: sensitivity, 98.0%; specificity, 90.6%; positive predictive value, 69.4%; negative predictive value, 99.5%. The respective values for intervals with atrial fibrillation were 86.9%, 98.8%, 89.6%, and 98.5%. Conclusion: The wristwatch-type PWM has shown feasibility in detecting atrial fibrillation in daily life and showed the possibility of being used as a screening tool.

Performance of an atrial fibrillation detection algorithm using continuous pulse wave monitoring.

BACKGROUND: Detecting asymptomatic and undiagnosed atrial fibrillation (AF) is increasingly important. Recently, we developed a wristwatch-based pulse wave monitor (PWM; Seiko Epson, Japan) capable of long-term recording, with an automatic diagnosis algorithm that uses frequency-based pulse wave analysis. The aim of this study was to evaluate the validity of continuous pulse wave monitoring for detection of AF. METHODS: During the electrophysiological study (EPS) in patients with AF, simultaneous pulse wave monitoring and Holter electrocardiograms (ECG) were recorded (n = 136, mean age 62.7 ± 10.9 years). The diagnostic accuracy of the PWM for AF was compared to the Holter ECG diagnosis. Standard performance metrics (sensitivity [Se], specificity [Sp], positive predictive value [PPV], and negative predictive value [NPV]) were calculated. The duration-based measurements were based on the diagnosis concordance ratios for the duration of time between diagnosis detected by the PWM and true diagnosis by the Holter ECG (AF or not AF). The episode-based performance metrics were based on the proportion of episodes appropriately detected with the PWM relative to episodes determined by the Holter ECG. RESULTS: The total recording time was 1,542,770 s (AF: 270,945 s). A high diagnostic Sp (patient average: 96.4%, cumulative: 97.7%) and NPV (patient average: 95.1%, cumulative: 96.8%) were obtained in the duration-based results. In the episode-based metrics, all indices significantly improved with longer AF episode durations. CONCLUSIONS: Continuous pulse wave monitoring can provide accurate and dependable information to aid in AF diagnosis. A high validity in confirming freedom from AF was shown by a high NPV.

Fan-beam steering device using a photonic crystal slow-light waveguide with surface diffraction grating.

Compact non-mechanical beam steering devices are desired not only for current common applications, but also for advanced applications such as light detection and ranging. We use a Si photonic crystal slow-light waveguide with a diffraction grating, which radiates the guided mode to free space and steers a fan beam by sweeping the wavelength. Due to its large angular dispersion, slow light enhances the steering range without degrading the beam quality, resulting in more resolution points. We fabricated 600 µm devices and observed a 23° steering range and a beam divergence of 0.23°, which resulted in 100 resolution points.

Enhancing the modulation bandwidth of VCSELs to the millimeter-waveband using strong transverse slow-light feedback.

We present modeling on the millimeter (mm)-wave modulation of vertical-cavity surface-emitting laser (VCSEL) with a transverse coupled cavity (TCC). We show that strong slow-light feedback can induce 300% boosting of the modulation bandwidth of the TCC-VCSEL. Also, the strong lateral feedback can induce resonance modulation over passbands centered on frequencies as high as 3.8 times the VCSEL bandwidth. The slow-light feedback is modeled by a time-delay rate equation model that takes into account the multiple round trips as well as the optical loss and phase delay in each round trip in the feedback cavity.

Low-voltage, high-speed and compact electro-absorption modulator laterally integrated with 980-nm VCSEL.

We present a compact electro-absorption slow-light modulator laterally-integrated with a 980-nm VCSEL. We figured out the small signal modulation response for different modulator lengths. While the 3-dB small-signal modulation bandwidth of conventional directly modulated VCSELs on the same epi-wafer structure was limited below 10 GHz, we obtained a modulation bandwidth over 21 GHz for a 30 µm long modulator. We also demonstrated large signal modulation up to 25 Gbps with a low driving voltage below 600 mV(pp) and an extinction ratio of 4 dB for the modulator length of 50 µm. Prospects of much higher speed (> 40 Gbps) were examined with reducing the size of the modulator. Also, the tapered waveguide coupling structure enables "quasi-unidirectional coupling", which reduces the optical feedback in a VCSEL from the integrated modulator.

Athermal and widely tunable VCSEL with bimorph micromachined mirror.

We demonstrate an athermal and electrostatically-tunable 850 nm-band MEMS VCSEL for the first time. The thermal wavelength drift is compensated by the thermal actuation of a cantilever-suspended mirror with a bimorph effect. At the same time, the resonant wavelength can be continuously tuned by electro-static force as a voltage is applied in the cantilever structure. A continuous wavelength tuning of 10 nm is obtained with a low thermal wavelength drift, which is 10 times smaller than that of conventional VCSELs. Our athermal and tunable VCSELs enable us to reduce the channel spacing in course wavelength division multiplexing optical interconnects even under uncooled operations.

Lateral integration of vertical-cavity surface-emitting laser and slow light Bragg reflector waveguide devices.

We present the modeling and the experiment on the lateral integration of a vertical-cavity surface-emitting laser (VCSEL) and slow light Bragg reflector waveguide devices. The modeling shows an efficient direct-lateral coupling from a VCSEL to an integrated slow light waveguide. The calculated result shows a possibility of 13 dB chip gain and an extinction ratio over 5 dB for a compact slow light semiconductor optical amplifier (SOA) and electroabsorption modulator integrated with a VCSEL, respectively. We demonstrate an SOA-integrated VCSEL, exhibiting the maximum output power over 6 mW. Also, we fabricate a sub-50-µm long electroabsorption modulator laterally integrated with a VCSEL. An extinction ratio of over 15 dB for a voltage swing of 2.0 V is obtained without noticeable change of threshold. In addition, we demonstrate an on-chip electrothermal beam deflector integrated with a VCSEL.

Ultra-high channel-count wavelength demultiplexer based on a Bragg reflector waveguide with large angular dispersion.

We present a new type of wavelength demultiplexers based on a Bragg reflector waveguide, which provides a large angular dispersion of 1~2°/nm. Benefiting from its large steering bandwidth and sharp divergence angles, we record a number of resolution-points (possible channel-count in demultiplexing) over 200 and 1,000 for active-type and passive-type devices, respectively. It is the highest number in various multiplexing elements ever reported. The device size is as small as a few millimeters.

Giant and high-resolution beam steering using slow-light waveguide amplifier.

We propose a novel beam-steering device based on a slow-light waveguide amplifier. In this paper, we present the idea of this steering technique and show its modeling characteristics. Giant steering of the radiation beam is obtained by tuning the wavelength of input light, which is coupled into the Bragg reflector waveguide. A tunable deflection-angle range can be over 40 degrees. High beam coherency and flat intensity distribution enable us to obtain an ultra-large number of resolution-points over 1,000 for few-millimeter long devices.

Slow-light total-internal-reflection switch with bending angle of 30 deg.

Slowing light in a Bragg reflector waveguide is used to miniaturize optical waveguide switches. We can realize a giant equivalent refractive index change induced by carrier injection near a cutoff wavelength due to its large waveguide dispersion. We fabricate and characterize a reflection-type slow-light switch. Input light is reflected at the off state due to an equivalent index difference between an oxide aperture and an oxide region, while it passes through at the on state, since the equivalent index difference is compensated using carrier injection. We obtained a large bending angle of 30° with total internal reflection of slow light.

Monolithically integrated multi-wavelength VCSEL arrays using high-contrast gratings.

We propose a novel design for multi-wavelength arrays of vertical cavity surface-emitting lasers (VCSELs) using high-contrast gratings (HCGs) as top mirrors. A range of VCSEL cavity wavelengths in excess of 100 nm is predicted by modifying only the period and duty-cycle of the high-contrast gratings, while leaving the epitaxial layer thickness unchanged. VCSEL arrays fabricated with this novel design can easily accommodate the entire Er-doped fiber amplifier bandwidth with emission wavelengths defined solely by lithography with no restrictions in physical layout. Further, the entire process is identical to that of solitary VCSELs, facilitating cost-effective manufacturing.

Greatly increased fiber transmission distance with an optically injection-locked vertical-cavity surface-emitting laser.

We demonstrate single-mode fiber transmission distance enhancement up to 120 km of a directly-modulated injection-locked VCSEL modulated by a 10Gb/s NRZ signal. Injection locking induced data pattern inversion of the VCSEL causes adjustable chirp, which greatly extends reach. Both experiments and simulations are shown to explain this phenomenon.

Wide tunability and ultralarge birefringence with 3D hollow waveguide Bragg reflector.

A tunable Bragg reflector based on a 3D hollow waveguide (HWG) has been proposed. Ultrawide tuning ranges of 152 nm and 164 nm, respectively, in Bragg wavelengths of TE and TM modes of the Bragg reflector have been presented experimentally. With a 3D hat-shaped HWG, a giant birefringence of 0.012 has been demonstrated that can be varied with a variable air core to realize tunable polarization-manipulating devices. The ultrawide tuning in Bragg wavelength can be utilized to make a number of widely tunable photonic devices based on the proposed 3D HWG Bragg reflector.

Fabrication and characterization of hollow waveguide optical switch with variable air core.

We demonstrate a novel hollow waveguide optical switch composed of an multi-mode interference (MMI) coupler with a variable air core. The numerical simulation and experiment of the proposed optical switch is carried out for investigating the operation of the switch. Switching operation can be obtained by the mechanical displacement of the air core of an MMI hollow waveguide. A hollow waveguide consists of Au mirrors deposited on two GaAs substrates for optical confinement. The measured result shows a possibility of switching of about 85% optical power fraction with a switch length of 1.1 mm and small displacement (DeltaDcore=3 mum) of an air core thickness. The measured insertion losses of a 1.1 mm long hollow waveguide with 12 and 9 mum air core are 5.4 dB and 5.7 dB, respectively.

Transverse mode control by etch-depth tuning in 1120-nm GaInAs/GaAs photonic crystal vertical-cavity surface-emitting lasers.

Robust and tolerant single-transverse-mode photonic crystal GaInAs vertical-cavity surface-emitting lasers are fabricated and investigated. Triangular lattice patterns of rectangular air holes of various etch-depths are introduced in the top mirror. The stable single-transversemode operation is observed with a large margin of allowance in the etch depth (t = 2.5 +/- 0.6 microm). This stable mode selection mechanism is explained by the mode competition between the two lowest photonic crystal guided modes that are influenced by both the index guiding effect and the etchdepth dependent modal losses.

Tunable hollow waveguide distributed Bragg reflectors with variable air core.

We demonstrate a tunable hollow waveguide distributed Bragg reflector consisting of a grating loaded slab hollow waveguide with a variable air-core. The modeling shows that a change in an air-core thickness enables a large shift of several tens of nanometers in Bragg wavelength due to a change of several percents in a propagation constant. We fabricated a slab hollow waveguide Bragg reflector with 620 mum long and, 190 nm deep 1st-order circular grating composed of SiO2, exhibiting strong Bragg reflection at 1558 nm with an air-core thickness of 10 mum for TM mode. The peak reflectivity is 65% including fiber coupling losses, the 3-dB bandwidth is 2.8 nm and the grating-induced loss is less than 0.5 dB. We demonstrate a 3 nm wavelength tuning of the fabricated hollow waveguide Bragg reflector by changing an air-core thickness from 10 mum to 7.9 mum.

Plasmon Enhanced Optical Near-field Probing of Metal Nanoaperture Surface Emitting Laser.

We demonstrate a metal nano-aperture GaAs vertical cavity surface emitting laser (VCSEL) for sub-wavelength optical near-filed probing, which exhibits the strong plasmon enhancement of both optical near-fields and voltage signals with forming a metal nano-particle in the nano-aperture. The threshold current is as low as 300microA, which shows a potential of nano-probing with low power consumption. We achieved the first demonstration of a plasmon enhanced VCSEL near-field probe. The spatial resolutions of the VCSEL probe with 400 nm and 200 nm apertures are 240nm and 130 nm, respectively. The enhancement factors of the optical near-field and voltage signal with a Au particle are 1.8 and 2, respectively. Our FDTD simulation shows that localized plasmon with a Au particle is very helpful for increasing optical near-field intensity and signal voltage in the VCSEL nano-probing.