In recent years, wideband near-infrared light sources have been required in the field of optical coherence tomography (OCT), optical communication, and biometric measurement systems. Broadening of nearinfrared light source is significant in various purpose such as improvement of spatial resolution in the depth direction of OCT observation, increase of communication traffic, high sensitive biological tissue observation by molecular vibrational absorption spectroscopy, and so on. Development of super continuum light source and super luminescent diode (SLD) attracts attention to satisfy such a demand. In this special issue, we focused on the research activities related on the development of near-infrared light source and its applications.

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In this paper, we reviewed the scientific investigations of ultra-wideband supercontinuum (SC) generation with optical fibers. Thanks to the invention of photonic crystal fibers and highly nonlinear fibers, octave spanning supercontinuum can be generated with sub-nJ level ultrashort pulse pumping. Nowadays, we can generate coherent, low noise SC, which is useful for optical frequency comb, highresolution optical coherence tomography, etc. Historical background, generation mechanism, their applications, recent progress, and future prospects were briefly discussed.

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We report a MEMS (Micro-Electro-Mechanically-Systems) based high speed wavelength swept NIR (Near Infrared) VCSEL (Vertical-Cavity Surface-Emitting-Laser) source operating at 1050 nm wavelength range for the SS-OCT (swept-source optical-coherence-tomography) application. An electrically pumped half-VCSEL with a semiconductor bottom DBR (Distributed Brag Reflector) is used for its superior reliability and simple optical configuration as a light source. The developed tunable VCSEL is composed of silicon on insulator; SOI-MEMS based diaphragm mirror with a high reflective dielectric coating, and is mounted by the thermo-compression bonding of gold onto a half-VCSEL chip with strained InGaAs multi-quantum wells gain medium. Wavelength tuning range over 88 nm is achieved by mechanical means. This tuning range corresponds to 8.3% tuning ratio which is currently a record for electrically pumped tunable VCSELs. We also explained its application in SS-OCT sensing.

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This paper reviews a near-infrared (NIR) superluminescent diode (SLD) based on self-assembled InAs quantum dots (QDs). Self-assembled InAs QDs exhibiting a broadband NIR spectrum due to their size and In composite distributions are suitable for optical coherence tomography (OCT) applications. We developed a QD-based SLD (QD-SLD) and demonstrated high-axial-resolution OCT imaging using the QD-SLD. InAs QD layers with controlled emission wavelengths were grown on GaAs, and an edgeemitting SLD with a tilted waveguide and segmented electrodes was fabricated. An electroluminescence spectrum spanning approximately 1 - 1.3 μm with a bandwidth of 144 nm was obtained, and the inverse Fourier transform of the spectrum predicted an axial-resolution of 3.6 μm in air. OCT images of test samples obtained with the QD-SLD actually indicated an axial resolution of ~4 μm, which is less than that of a conventional OCT (10 - 15 μm). This practical imaging demonstrates the potential of the QDSLD as a high-axial-resolution OCT light source.

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Quantum dot (QD) have many attractive characteristics for development advanced optical gain devices, such as broadband and high-robustness QD laser, high-speed QD optical amplifier, and so on. In this paper, we briefly described characteristics of ultra-broadband wavelength tunable QD lasers, heterogeneous QD laser integrated with silicon photonic circuit, a QD comb laser for multiple frequency carrier generation, and a two-tone laser for convergence photonic and wireless communications. We also discussed a utilization of novel wavelength bands of Thousand-band and O-band for optical communications, since an operating waveband of the InAs/GaAs QD optical gain is suitable for the T+O-bands. A construction of over 1000-ch huge optical switch fabric was successfully realized using the broadband QD optical gain devices. It is expected that the QD optical gain devices will become breakthrough technology for creating novel broadband photonic devices and constructing of further advanced photonic network system for short and middle range communications.

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Polychromatic reconstruction (PCR) is a novel holographic reconstruction method that utilizes a spectrally broad light source for a readout probe beam. It enables us to reconstruct stored images at any desired wavelengths even though the probe wavelength is very different from the recorded one. On the other hand, the broadband spectrum of the probe beam considerably degrades the Bragg selectivity and increases the angular separation between multiplexed holograms, which lowers the storage density in holographic data storage systems. However, such a drawback can be overcome if an additional suitable optical component is used in the imaging system. The readout tolerance in the PCR method is also much larger than conventional monochromatic readout. Such unique and attractive features of the PCR method were numerically and experimentally demonstrated.

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Tissue oximeters using near-infrared spectroscopy (NIRS) have been applied for evaluating the viability of cerebral tissue in surgery and for monitoring muscle oxygenation in sport medicine and rehabilitation. We developed NIRS devices for muscles and the brain to measure deep and superficial tissues as well as a finger-mounted oximeter for the fetus and a neonate by miniaturizing the optical probe and analyzing light propagation in the layered tissue model. Various applications are also progressing to evaluate the flaps oxygenation and observe cerebral cortex activity. We outlined the principle of oximetry, its medical device development, and its applications.

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