OCT with a Visible Broadband Light Source Applied to High-Resolution Nondestructive Inspection for Semiconductor Optical Devices

Optical coherence tomography with a visible broadband light source (vis-OCT) was developed for high-resolution and nondestructive measurements of semiconductor optical devices. Although a near-infrared (NIR) light source should be used for medical OCT to obtain deep penetration of biological samples, a visible broadband light source is available as a low-coherence light source for industrial products. Vis-OCT provides higher axial resolution than NIR-OCT, because the axial resolution of an OCT image is proportional to the square of the center wavelength of the light source. We developed vis-OCT with an axial resolution of less than 1 μm in air and obtained cross-sectional profiles and images of ridge-type waveguides having heights and widths of several μm. Additionally, we performed cross-sectional measurements and imaging of a stacked semiconductor thin layer. The measured values were similar to those measured by scanning electron microscopy, and the effectiveness of vis-OCT for nondestructive inspection of semiconductor optical devices was demonstrated

Development of a broadband superluminescent diode based on self-assembled InAs quantum dots and demonstration of high-axial-resolution optical coherence tomography imaging

We developed a near-infrared (NIR) superluminescent diode (SLD) based on self-assembled InAs quantum dots (QDs) and demonstrated high-axial-resolution optical coherence tomography (OCT) imaging using this QD-based SLD (QD-SLD). The QD-SLD utilized InAs QDs with controlled emission wavelengths as a NIR broadband light emitter, and a tilted waveguide with segmented electrodes was prepared for edge-emitting broadband electroluminescence (EL) spanning approximately 1–1.3 μm. The bandwidth of the EL spectrum was increased up to 144 nm at a temperature of 25 °C controlled using a thermoelectric cooler. The inverse Fourier transform of the EL spectrum predicted a minimum resolution of 3.6 μm in air. The QD-SLD was subsequently introduced into a spectral-domain (SD)-OCT setup, and SD-OCT imaging was performed for industrial and biological test samples. The OCT images obtained using the QD-SLD showed an axial resolution of ~4 μm, which was almost the same as that predicted from the spectrum. This axial resolution is less than the typical size of a single biological cell (~5 μm), and the practical demonstration of high-axial-resolution OCT imaging shows the application of QD-SLDs as a compact OCT light source, which enables the development of a portable OCT system

Integration of Emission-wavelength-controlled InAs Quantum Dots for Ultrabroadband Near-infrared Light Source

Near-infrared (NIR) light sources are widely utilized in biological and medical imaging systems owing to their long penetration depth in living tissues. In a recently developed biomedical non-invasive cross-sectional imaging system, called optical coherence tomography (OCT), a broadband spectrum is also required, because OCT is based on low coherence interferometry. To meet these operational requirements, we have developed a NIR broadband light source by integrating self-assembled InAs quantum dots (QDs) grown on a GaAs substrate (InAs/GaAs QDs) with different emission wavelengths. In this review, we introduce the developed light sources and QD growth techniques that are used to control the emission wavelength for broadband emission spectra with center wavelengths of 1.05 and 1.3 μm. Although the strain-induced Stranski-Krastanov (S-K) mode-grown InAs/GaAs QDs normally emit light at a wavelength of around 1.2 μm, the central emission wavelength can be controlled to be between 0.9–1.4 μm by the use of an In-flush technique, the insertion of a strain-reducing layer (SRL) and bi-layer QD growth techniques. These techniques are useful for applying InAs/GaAs QDs as NIR broadband light sources and are especially suitable for our proposed spectral-shape-controllable broadband NIR light source. The potential of this light source for improving the performance of OCT systems is discussed

1.1 μm waveband tunable laser using emission-wavelength-controlled InAs quantum dots for swept-source optical coherence tomography applications

In this study, an optical gain chip using emission-wavelength-controlled self-assembled InAs quantum dots (QDs) was developed for swept-source optical coherence tomography (SS-OCT) applications. The optical characterizations indicated that the QDs emission wavelength and optical gain spectra were controlled in the 1.1μm waveband by optimizing the QDs growth conditions. This waveband is useful for obtaining a large imaging depth of OCT because of an optimal balance between absorption and scattering in biological samples. In addition, continuous tunable lasing in the waveband was achieved by introducing the QD-based gain chip into a grating-coupled external cavity. This tunable laser was introduced into an SS-OCT setup, and the point spread function (PSF) was evaluated. The PSF position was observed to vary according to the optical path length differences. These results demonstrate the feasibility of the application of emission-wavelength-controlled QDs for SS-OCT

Volume crystallization and microwave dielectric properties of indialite/cordierite glass by TiO2 addition

Abstract Indialite/cordierite (Mg₂2Al₄Si₅O₁₈) glass-ceramics with a low dielectric constant of 4.7 and a high Qf of >200 × 10³ GHz are predicted for use as micro/millimeter-wave materials in the fifth generation (5G) mobile communication systems. The glass-ceramics have a serious cracking problem caused by the anisotropic crystal growth during the surface crystallization. In this paper, the cracking was prevented by adding TiO₂ which acts as a seed. The glass-ceramics produced without cracking were composed of spherical crystals of approximately 10 μm diameter, formed by volume crystallization. Precipitated phases of the glass-ceramics crystallized at 1200–1350 °C/10 h and 20 h were indialite, cordierite, Al₂TiO₅ and rutile. The glass-ceramics crystallized for 10 h were analyzed by the Rietveld method. Indialite precipitated as an intermediate metastable compound at the lower temperature of 1200 °C and transformed to cordierite at the crystallization temperature. The reaction between cordierite and TiO₂ produced the new Al₂TiO₅ phase. The amounts of Al₂TiO₅ and rutile affected the microwave dielectric properties. In particular, the amount of rutile affected the TCf. In the cases of 10 wt % added TiO₂, and the crystallized at 1250 °C for 10/20 h the TCf values were improved to -2/-8 ppm/°C