Manabu Kagami

Light-induced self-written three-dimensional optical waveguide

Three-dimensional (3D) optical waveguides were fabricated in a photopolymerizing resin mixture solution by using a multimode optical fiber, without any moving parts. The core portion has formed by the selective photopolymerization of a higher refractive index monomer by Ar+ laser irradiation through the optical fiber. A continuous, straight waveguide was grown by the self-trapping of a guided laser beam. We demonstrated automatic 3D optical circuit formation that enables regrowth after passing through thick transparent glass plates. This growth mechanism also enables automation of the optical interconnection and packaging process, and could potentially contribute to future expansion of optical fiber communications networks.

A 100-m Range 10-Frame/s 340

This paper introduces a single-photon detection technique for time-of-flight distance ranging based on the temporal and spatial correlation of photons. A proof-of-concept prototype achieving depth imaging up to 100 meters with a resolution of 340 × 96 pixels at 10 frames/s was implemented. At the core of the system, a sensor chip comprising 32 macro-pixels based on an array of single-photon avalanche diodes featuring an optical fill factor of 70% was fabricated in a 0.18-μm CMOS. The chip also comprises an array of 32 circuits capable of generating precise triggers upon correlation events as well as of sampling the number of photons involved in each correlation event, and an array of 32 12-b time-to-digital converters. Characterization of the TDC array led to -0.52 LSB and 0.73 LSB of differential and integral nonlinearities, respectively. Quantitative evaluation of the TOF sensor under strong solar background light, i.e., 80 klux, revealed a repeatability error better than 10 cm throughout the distance range of 100 m, thus leading to a relative precision of 0.1%. In the same condition, the relative nonlinearity error was 0.37%. In order to show the suitability of our approach in a real-world situation, experimental results in which the depth sensor was operated in a typical traffic scenario are also reported.

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We introduce an optical time-of-flight image sensor taking advantage of a MEMS-based laser scanning device. Unlike previous approaches, our concept benefits from the high timing resolution and the digital signal flexibility of single-photon pixels in CMOS to allow for a nearly ideal cooperation between the image sensor and the scanning device. This technique enables a high signal-to-background light ratio to be obtained, while simultaneously relaxing the constraint on size of the MEMS mirror. These conditions are critical for devising practical and low-cost depth sensors intended to operate in uncontrolled environments, such as outdoors. A proof-of-concept prototype capable of operating in real-time was implemented. This paper focuses on the design and characterization of a 256 x 64-pixel image sensor, which also comprises an event-driven readout circuit, an array of 64 row-level high-throughput time-to-digital converters, and a 16 Gbit/s global readout circuit. Quantitative evaluation of the sensor under 2 klux of background light revealed a repeatability error of 13.5 cm throughout the distance range of 20 meters.

96-Pixel Time-of-Flight Depth Sensor in 0.18-

We show the feasibility of automatic waveguide formation by means of the self-trapping effect of multimode optical fiber irradiation into a photopolymerizing resin. By using a graded-index multimode optical fiber, we experimentally obtained a straight waveguide of over 20 mm in length. It is shown that its growth properties, such as waveguide shape and diameter, depend on the propagation modal distribution along the optical fiber used for the waveguide formation. Moreover, an all-solid polymer optical waveguide that relies on the selective photopolymerization proceeding into the photopolymerizing resin mixture is also demonstrated. We call this type of waveguide a light-induced self-written optical waveguide. The measured propagation loss in the waveguide is 1.0 dB/cm or less for the wavelength range 0.8/spl sim/1.0 /spl mu/m. The proposed technology is capable of eliminating both costly lenses and the need for an alignment system from optical waveguide devices.

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With the emerging need for high-resolution light detection and ranging (LIDAR) technologies in advanced driver assistance systems (ADAS), we introduce a system-on-a-chip (SoC) that performs time-correlated single-photon counting and complete digital signal processing for a time-of-flight (TOF) sensor. At the core of the 0.18-μm CMOS SoC, we utilize linear arrays of 16 TOF and 32 intensity-only macro-pixels based on single-photon avalanche diodes in an original look-ahead concept, thus acquiring active TOF and passive intensity images simultaneously. The SoC also comprises an array of circuits capable of generating precise triggers upon spatiotemporal correlation events, an array of 64 12-b time-to-digital converters, and 768 kb of SRAM memory. The SoC provides the system-level electronics with a serial and low-bit-rate digital interface for: 1) multi-echo distance; 2) distance reliability; 3) intensity; and 4) passive-only intensity, thus mitigating system-level complexity and cost. A proof-of-concept prototype that achieves depth imaging up to 100 m with a resolution of 202 × 96 pixels at 10 frames/s has been implemented. Quantitative evaluation of the TOF sensor under strong solar background illuminance, i.e., 70 klux, revealed a repeatability error of 14.2 cm throughout the distance range of 100 m, thus leading to a relative precision of 0.14%. Under the same conditions, the relative nonlinearity error was 0.11%. In order to show the suitability of our approach for ADAS-related applications, experimental results in which the depth sensor was operated in typical traffic situations have also been reported.

CMOS

Light-induced self-written (LISW) polymeric optical waveguides were fabricated in a photopolymerizing resin mixture solution using a single-mode optical fiber. In order to realize a straight waveguide core, a pre-UV treatment was performed before core formation. Single-mode propagation of laser light at 1310 nm was realized by appropriately controlling the ratio of the resins in the mixture for the first time. Moreover, optical interconnection between two single-mode fibers was demonstrated based on this technology.

Design and characterization of a 256x64-pixel single-photon imager in CMOS for a MEMS-based laser scanning time-of-flight sensor

We have developed a simultaneous fabrication method using temperature control reactive ion etching (RIE) for channel optical waveguides incorporating plural out-of-plane branching mirrors made from polymer film. By using this method, the etching rate can be adjusted locally by controlling the temperature. This technology also enables the formation of trenches of various depths on the same polymer optical waveguide. We noted from scanning electron microscope (SEM) observations that simultaneous control of the mirror tilt angle and a smooth core surface could be achieved. To be specific, a heat treatment temperature of 130-135/spl deg/C appears to be the optimum to maintain a rectangular cross section and to achieve a sufficiently smooth core surface for a polymethyl methacrylate (PMMA) waveguide. The measured propagation loss is small, in spite of the presence of a high-/spl Delta/ waveguide (/spl Delta/=5.4%). For example, losses of 0.1, 0.3, and 0.7 dB/cm are measured at wavelengths of 650 nm, 850 nm, and 1.3 /spl mu/m, respectively. From far-field pattern (FFP) measurements, we found that the mirror plane was almost rectilinear, and that the reflected light can be captured efficiently by a photodiode. In operational temperature tests, we showed that intensity fluctuations of the coupling light can be reduced to less than 1.5 dB for the temperature range between -25/spl deg/C and +85/spl deg/C by adopting a sandwich structure with glass plates.

Waveguide shape control and loss properties of light-induced self-written (LISW) optical waveguides

Doppler laser radar can improve the precision of speed measurement by about two orders of magnitude compared with time-of-flight range finders, which obtain target speeds by range differentiation. However, in a car environment, the usage of traditional Doppler laser radar schemes is limited, because they do not satisfy the requirement of simultaneously measuring the target range together with speed with high precision. First, in this paper, we describe a new in-car laser radar system and show a new modulation scheme that enables the in-car laser radar to simultaneously measure the target range and speed with high precision. Then, we perform simulations and experiments to verify the accuracy of the proposed method. In the Appendix, a brief review of current widely used laser radar schemes is given. The limitations of these schemes to their employment in car applications are also discussed.

A 0.18-

The realization of polymer optical waveguides that have a large core size and high refractive-index difference (LCHD) Δ transmission characteristics is presented. A fabrication procedure for the waveguide based on vertical dip coating and reactive ion etching has been studied. To achieve the lower propagation loss, this procedure includes two original techniques, i.e., the lamination of thick polymer films and sidewall flattening. With these techniques, Δ of 5.4% and a 80 µm × 83 µm core polymer waveguide with 1.4-dB/cm propagation loss were achieved at 680 nm. The LCHD polymer waveguides are useful for practical power-transmission devices.

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We report a polymer waveguide module that provides bidirectional communication over a single plastic optical fiber (POF) with dual visible wavelength LEDs. The module is constructed using light-induced self-written waveguides, which enables a three-dimensional optical circuit for visible wavelength division multiplexing to be fabricated by an extremely simple process. We demonstrated 250 Mbits/s communication using a pair of these modules that each contained one green (lambda = 495 nm) and one red (lambda = 650 nm) LEDs by measuring the bit error rates. The results indicate that the system could transmit over more than 20 m of POF in full duplex mode.