Our recent studies in regards of developing portable THz scanner for imaging and spectroscopy systems are presented. In the course, high power tunable continuous wave (CW) THz emitter and high sensitivity THz receiver platforms are presented. Those platforms can be realized with tunable optical beating source, broadband photomixer, arrayed photomixer and Schottky barrier diode, evanescently-coupled photodiodes with high saturation current, and semiconductor optical amplifier (SOA) integrated optical beating source. On the system level, our recent THz thickness measurement systems and the THz line scanner imaging system are presented.
A novel type of semiconductor beating source, a monolithically integrated dual-mode laser, and continuous-wave
terahertz (THz) system adopting it will be investigated. The combined system of the beating source with broadbandantenna-
integrated low-temperature-grown semiconductor photomixers shows the possibility of the realization of the
cost-effective and compact continuous-wave THz systems. Such a system is highly-demanded to examine the THz finger
prints of specimens without limitations. Since the optimized performance depends not only on the characteristics of
functional devices but also module configurations, various approaches such as traveling-wave photomixers, Schottky
barrier diodes, and nano-structure contained photomixers have been investigated to implement high-performance THz
platforms as the main building blocks of a THz system. Semiconductor-based compact and cost-effective photonics
technologies will envisage the bright future of THz systems.
We demonstrate the tunable continuous-wave (CW) terahertz generator based on the λ/4 phase-shifted 1.3 μm dual-mode laser diode (DML) and travelling-wave photodiode (TWPD). The DML and TWPD operate as an optical beat source and terahertz photomixer, respectively. The laser diodes (LDs) operating at the 1.3 μm have more suitable characteristics as optical beat sources than the LDs operating at 1.55 μm because of their high efficiency and better thermal stability. The micro-heaters are integrated on top of each DFB LD for mode beat frequency tuning. The fabricated DML was continuously tuned from 230 GHz to 1485 GHz by increasing the temperature of each DFB section independently via integrated micro-heaters. The high-speed TWPD with an InGaAs absorber was designed and fabricated to efficiently generate the photomixing terahertz CW. A complementary log-periodic antenna was integrated with the TWPD to radiate the generated terahertz wave with minimum reflection in the wide frequency range. The terahertz characteristics of the tunable CW terahertz generator based on the DML and TWPD were measured in a fiber-coupled, homodyne terahertz photomixing system. Our results of the tunable CW terahertz generator show the feasibility of a compact and highly efficient CW terahertz spectrometer and imager.
It represents a viable solution for the realization of a portable biosensor platform that could screen/diagnose acute
myocardial infarction by measuring cardiac marker concentrations such as cardiac troponin I (cTnI), creatine kinase MB
(CK-MB), and myoglobin (MYO) for application to u-health monitoring system. The portable biosensor platform
introduced in this presentation has a more compact structure and a much higher measuring resolution than a conventional
spectrometer system. Portable guided-mode resonance (GMR) biosensor platform was composed of a biosensor chip
stage, an optical pick-up module, and a data display panel. Disposable plastic GMR biosensor chips with nano-grating
patterns were fabricated by injection–molding. Whole blood filtration and label-free immunoassay were performed on
these single chips, automatically. Optical pick-up module was fabricated by using the miniaturized bulk optics and the
interconnecting optical fibers and a tunable VCSEL (vertical cavity surface emitting laser). The reflectance spectrum
from the GMR biosensor was measured by the optical pick-up module. Cardiac markers in human serum with
concentrations less than 0.1ng/mL were analyzed using a GMR biosensor. Analysis time was 30min, which is short
enough to meet clinical requirements. Our results show that the GMR biosensor will be very useful in developing lowcost
portable biosensors that can screen for cardiac diseases.
We demonstrate several optical beating sources based on 1.55 μm photonic devices. Broadband antenna-integrated,
low-temperature-grown (LTG) InGaAs photomixers for widely tunable continuous-wave THz generation and detection
are also verified. The novel optical beat sources show a beat frequency tuning range from 0.3THz to over 1.34 THz. The
dual-mode laser diode (DML) consists of one phase and two active sections. Micro-heaters are used to independently
tune the wavelengths of the two DML laser modes. Broadband antenna-integrated, LTG InGaAs photomixers are used as
THz wave generators and detectors. This use of 1.55 μm photonic devices could connect current THz and InP-based
communication technologies because the well-developed InP-based optoelectronic technologies are already expected to
enable the integration of tunable LD sources with other optical components such as semiconductor optical amplifiers
(SOAs), electro-absorption modulators, and waveguide-type THz photomixers. As well as realizing an optical fibercoupled
THz time-domain spectroscopy (TDS) system, we also successfully achieved continuous frequency tuning of the
CW THz emissions. Our results show that photomixing using the photonic devices is a promising approach to realize
compact, cost-effective, and portable THz spectrometer.
The butt-coupled (BT) sampled grating distributed Bragg reflector laser diode (SGDBR-LD) was designed and fabricated using planar buried heterostructure (PBH), enabling a low threshold current and a stable fundamental transverse mode. The but-coupled SGDBR-LD's with target tuning ranges of 44.4nm was fabricated, and the tuning ranges were experimentally measured to be 44.4nm. The measured peak periods of the fabricated SGDBR-LD's were well matched with theoretical values and output power is close to calculated values. The side mode suppression ratio of more than 35dB was obtained in the whole tuning range. The output power variation was less than 5dB, which is 4dB smaller than that of RWG structure.
Coherent laser light was tuned by cumulative refraction of light waves in a semiconductor laser cavity, while the wavelength of electroluminescence was not tuned by any noticeable amount. This effect is explained by the fact that the laser light has perfect spatial coherence, while the electroluminescence has a spatial coherence of 0.7 µm, smaller than the width of the wavefront. The refraction angle of 2.4 deg means that more than 16 triangular electrodes are involved in the refraction. This observation of coherent tuning may lead to a new optoelectronic device, a coherent filter, which filters out only coherent light from a coherent-incoherent mixture.
As a lightwave having a Gaussian intensity profile passes through a tilted Fabry-Perot filter, the profile is distorted severely depending on the conditions of the parameters. This distortion has not been studied and reported in depth, though the propagation of a Gaussian wave itself has been discussed well in the literature. We show our results of quantitative calculation and discuss it, which we believe will help significantly design waveguides containing gratings or filters.