Iwao Hosako

At the Dawn of a New Era in Terahertz Technology

The National Institute of Information and Communications Technology (NICT, Japan) started the Terahertz Project in April 2006. Its fundamental purpose in the next five years is to enable a nationwide technical infrastructure to be created for diverse applications of terahertz technology. The technical infrastructure includes the development of semiconductor devices such as terahertz quantum cascade lasers, terahertz-range quantum well photodetectors, and high-precision tunable continuous wave sources. It also includes pulsed terahertz measurement systems, modeling and measurement of atmospheric propagation, and the establishment of a framework to construct a materials database in the terahertz range including standardization of the measurement protocol. These are common technical infrastructure even in any terahertz systems. In this article, we report the current status of developments in these fields such as terahertz quantum cascade lasers (THz-QCLs) (with peak power of 30 mW, 3.1 THz), terahertz-range quantum well photodetectors (THz-QWPs) (tuned at 3 THz) an ultrawideband terahertz time domain spectroscopy (THz-TDS) system (with measurement range of from 0.1 to 15 THz), an example of a database for materials of fine art, and results obtained from measuring atmospheric propagation.

Terahertz spectroscopy for art conservation

Terahertz spectroscopy has rapidly progressed, and is being applied to various research fields as a new non-invasive examination method. Scientific analysis is important for the conservation of art, as it can help to reveal the history of a work and to determine the proper materials for its restoration. Mid-infrared spectroscopy and X-ray analysis are currently used to identify organic and inorganic materials, respectively. Terahertz spectroscopy is expected to identify composites themselves, and to give clear and direct information for art conservation. We have collected terahertz spectra of various historical and modern materials, and found that terahertz spectroscopy can distinguish the pigments, binders and their mixtures. The experimental results prove that terahertz spectroscopy as a new non-invasive examination method can contribute to art history and to restorations. The material database can also contribute to other fields such as earth science, the printing industry, and the food and pharmaceutical research fields.

Application of terahertz spectroscopy for character recognition in a medieval manuscript

Terahertz (THz) technology is a focus of attention in research on applied optics. We have applied THz spectroscopy and THz-imaging method to text recognition of a medieval manuscript made from sheepskin. Based on the database which contains more than 200 spectra of art materials, the red ink on the manuscript was estimated as Cinnabar. The red ink text and stains on the text were successfully distinguished by the THz-Time Domain Spectroscopy (TDS) imaging with a component spatial pattern analysis. THz spectroscopy can be used as a non-invasive analysis method for conservation science of cultural properties.

Optical and millimeter-wave radio seamless MIMO transmission based on a radio over fiber technology

Multi-input multi-output (MIMO) transmission of two millimeter-wave radio signals seamlessly converted from polarization-division-multiplexed quadrature-phase-shift-keying optical signals is successfully demonstrated, where a radio access unit basically consisting of only optical-to-electrical converters and a radio receiver performs total signal equalization of both the optical and the radio paths and demodulation with digital signal processing (DSP). Orthogonally polarized optical components that are directly converted to two-channel radio components can be demultiplexed and demodulated with high-speed DSP as in optical digital coherent detection. 20-Gbaud optical and radio seamless MIMO transmission provides a total capacity of 74.4 Gb/s with a forward error correction overhead of 7%.

Detection of terahertz radiation from quantum cascade laser, using vanadium oxide microbolometer focal plane arrays

The authors tried real-time imaging of THz radiation from Quantum Cascade Laser (QCL), using vanadium oxide (VOx) microbolometer focal plane arrays (FPAs) of 320x240 with pitches of 37 μm and 23.5 μm as well as 640x480 with 23.5μm pitch. The QCL has such parameters as 3.1 THz emission frequency (97μm in wavelength), 300-400 nsec pulse width, 1.07 msec repetition period, 30 mW peak intensity, 15K operation temperature. The THz radiation from QCL is collimated by off-axis parabola (OAP) and focused on FPA by another OAP. The 10 μm range infrared radiation from scene is blocked by sapphire disk or metal mesh filter. Noise Equivalent Power (NEP) at 3.1 THz is estimated to be 200~400 pW.

Nonequilibrium Green’s function calculation for four-level scheme terahertz quantum cascade lasers

We have calculated the performance of a recently proposed four-level scheme terahertz quantum cascade laser (4L terahertz-QCL) with the nonequilibrium Green’s function method. The calculation result for 40 K showed that the 4L QCL has a larger terahertz gain than the conventional resonant phonon QCL. This is because a large number of electrons accumulate in the upper lasing level and contribute to lasing in the new scheme. When the temperature is increased, the advantage of gain decreases due to thermally activated phonon scattering.

Coherent Radio-Over-Fiber and Millimeter-Wave Radio Seamless Transmission System for Resilient Access Networks

We propose a millimeter-wave (MMW) coherent radio-over-fiber (RoF) transmission system for application to an access network with a direct broadband last-mile wireless connection to an optical fiber network. The coherent RoF system comprises an optical two-tone RoF signal generator with an advanced modulation format for high-throughput transmission and employs a digital-signal-processing-aided coherent detection technique for MMW radio, which is a technique similar to that of optical digital coherent detection. As proof of concept, a 20-GBd quadrature phase-shift keying RoF transmission over 20 km of optical fiber before a radio transmission over 20 m of air is demonstrated. The results of the radio transmissions using Cassegrain-type antenna pairs with a gain of 50 dBi are consistent with theoretical estimations; this suggests the possibility of developing a midrange transmission system using a high-power MMW amplifier.

20-Gb/s QPSK W-band (75-110GHz) wireless link in free space using radio-over-fiber technique

We demonstrate 20-Gb/s W-band wireless transmission in free space with a distance of 30mm using optical signal generation. Optically synthesized QPSK signal and direct optical upconversion technique ease generation of W-band RoF signals for dual purpose of wireline and wireless transmission link. A W-band radio receiver with W-band frequency downconversion and digital signal processing will be applicable for any W-band radio detection.

Broadband wavelength-tunable ultrashort pulse source using a Mach-Zehnder modulator and dispersion-flattened dispersion-decreasing fiber

The broadband wavelength tunability of femtosecond pulse generation using a Mach-Zehnder-modulator-based flat-comb generator (MZ-FCG) and a dispersion-flattened dispersion-decreasing fiber (DF-DDF) was demonstrated. Near-Fourier-transform-limit picosecond pulses generated from the MZ-FCG were compressed into femtosecond pulses by adiabatic soliton compression. By tuning the wavelength of the input cw light, 200 fs, 10 GHz pulses were generated in the wavelength range of 1,535 to 1,570 nm. Such wide-range wavelength tunability was realized by both the independence of a comb-flattening condition from the inputted wavelength and the dispersion flatness of the DF-DDF.

Widely repetition-tunable 200 fs pulse source using a Mach–Zehnder-modulator-based flat comb generator and dispersion-flattened dispersion-decreasing fiber

By combining a Mach-Zehnder-modulator-based flat comb generator (MZ-FCG) with a dispersion-flattened dispersion-decreasing fiber, femtosecond pulses have been generated from a cw light. Near-Fourier-transform-limit picosecond pulses generated from the MZ-FCG were compressed into femtosecond order by pulse compression. Our system enables flexible tuning of the repetition rate and pulse width, because those depend on the driving signal of the MZ-FCG. Pulse trains of 200 fs width were continuously and stably generated without mode hopping, with a repetition rate range from 5 to 17 GHz. Our system consists of a modulator and compression fiber; thus, the configuration is simpler and more stable.