In developing terahertz (THz) technologies that are more suitable for industrial applications, we have focused on research on continuous-wave (CW) THz technologies to develop small, low-cost, and multifunctional THz devices and systems. In the course of this research, we have developed several key devices such as widely tunable compact beating sources in the form of dual mode lasers, THz emitters, including nano-electrode-photomixers and uni-traveling carrier photodiode photomixers, and highly sensitive THz detectors, such as Schottky barrier diodes (SBDs). In this study, along with our recently obtained results that demonstrate the enhanced performance of these devices, we also present an example of a practical industrial application of our CW THz system: a nondestructive evaluation (NDE) system. The system described can be applied in the car manufacturing factory as an NDE technique to find process errors. Although further improvements to photonics-based THz technologies are necessary, we believe that efforts in this field will begin an era of THz technologies as a widely-used industrial technique.
Recently, a wide interest has been gathered in using terahertz (THz) waves as the carrier waves for the next generation of broadband wireless communications. Upon this objective, the photonics technologies are very attractive for their usefulness in signal generations, modulations and detections with enhanced bandwidth and data rates, and the readiness in combining to the existing fiber-optic or wireless networks. In this paper, as a preliminary step toward the THz wireless communications, a THz wireless interconnection system with a broadband antenna-integrated uni-traveling-carrier photodiode (UTC-PD) and a Shottky-barrier diode (SBD) module will be presented. In our system, optical beating signals are generated and digitally modulated by the optical intensity modulator driven by a pulse pattern generator (PPG). As the receiver a SBD and an IF filter followed by a low-noise preamplifier and a limiting amplifier was used. With a 6-mA photocurrent of the UTC-PD which corresponds to the transmitter output power of about 30 μW at 280 GHz, an error-free (BER<10-9) transmission has been achieved at 2.5 Gbit/s which is limited by a limiting amplifier. With this system, a 1.485-Gbit/s video signal with a high-definition serial digital interface format was successfully transmitted over a wireless link.
Terahertz (THz) waves have been actively studied for the applications of astronomy, communications, analytical science and bio-technologies due to their low energy and high frequency. For example, THz systems can carry more information with faster rates than GHz systems. Besides, THz waves can be applied to imaging, sensing, and spectroscopy. Furthermore, THz waves can be used for non-destructive and non-harmful tomography of living objects. In this reasons, Schottky barrier diodes (SBD) have been widely used as a THz detector for their ultrafast carrier transport, high responsivity, high sensitivity, and excellent noise equivalent power. Furthermore, SBD detectors envisage developing THz applications at low cost, excellent capability, and high yield. Since the major concerns in the THz detectors for THz imaging systems are the realizations of the real-time image acquisitions via a reduced acquisition time, rather than the conventional raster scans that obtains an image by pixel-by-pixel acquisitions, a line-scan based systems utilizes an array detector with an 1 × n SBD array is preferable.
In this study, we fabricated the InGaAs based SBD array detectors with broadband antennas of log-spiral and square-spiral patterns. To optimize leakage current and ideality factor, the dependence to the doping levels of ohmic and Schottky layers have been investigated. In addition, the dependence to the capacitance and resistance to anode size are also examined as well. As a consequence, the real-time THz imaging with our InGaAs SBD array detector have been successfully obtained.
Based on either a SOI wafer or a bulk-silicon wafer, we discuss silicon photonic devices and integrations for chip-level
optical interconnects. We present the low-voltage silicon PICs on a SOI wafer, where Si modulators and Ge-on-Si
photodetectors are monolithically-integrated for intra-chip or inter-chip interconnects over 40 Gb/s. For future chip-level
integration, the 50 Gb/s small-sized depletion-type MZ modulator with the vertically-dipped PN-depletion-junction
(VDJ) is also presented. We report vertical-illumination-type Ge photodetectors on bulk-silicon wafers, with high
performances up to 50 Gb/s. We present the bulk-silicon platform for practical implementation of chip-level
interconnects, and the performance of the photonic transceiver silicon chip.
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.
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.
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