Wide dynamic range and real-time reagent identification and imaging using multi-wavelength terahertz parametric generation and machine learning.

In this study, we propose a technique for identifying and imaging reagents through shielding over a wide dynamic range using a real-time terahertz (THz) spectroscopy system with multi-wavelength THz parametric generation/detection and machine learning. To quickly identify reagents through shielding, the spectral information of the "detection Stokes beam" is used for reagent recognition via machine learning. In general THz wave-based reagent identification, continuous spectra are acquired and analyzed quantitatively by post-processing. In actual applications, however, such as testing for illicit drugs in mail, the technology must be able to quickly identify reagents as opposed to quantifying the amount present. In multi-wavelength THz parametric generation/detection, THz spectral information can be measured instantly using a "multi-wavelength detection Stokes beam" and near-infrared (NIR) camera. Moreover, machine learning enables reagent identification in real-time and over a wide dynamic range. Furthermore, by plotting the identification results as pixel values, the spatial distribution of reagents can be imaged at high speed without the need for post-processing.

Noise-free terahertz-wave parametric generator.

We achieved noise-free terahertz (THz)-wave output from an injection-seeded THz-wave parametric generator (is-TPG) employing high-power injection seeding. A conventional is-TPG uses a weak continuous-wave (CW) seed beam. The position in which broadband noise is generated (via spontaneous parametric down-conversion) and the position of the THz signal overlap. Thus, the output features broadband TPG noise, reducing the signal-to-noise ratio. To solve this problem, we shifted the position in which the THz signal is generated to the front of the crystal; we separated the signal from broadband TPG noise using a high-powered, pulsed seed beam that was 107-fold more powerful than the CW seed beam. Thus, we extracted only the THz signal; we achieved a noise-free is-TPG. This system features a signal-to-noise ratio of 95 dB, approximately 40 dB better than the signal-to-noise ratio of the conventional system.

Real-time wide dynamic range spectrometer using a rapidly wavelength-switchable terahertz parametric source.

In this study, we demonstrate real-time terahertz (THz) spectroscopy using a rapidly wavelength-switchable injection-seeded THz parametric generator. We developed a wavelength-switchable external cavity diode laser using a digital micromirror device as a seed source for the generator. We realized fast acquisition of THz spectra by switching the wavelength of the laser for each pump beam pulse. This system can rapidly switch wavelengths and easily increase the number of measurement wavelengths, and it also has a wide dynamic range, of more than 75 dB, and high stability. Furthermore, by combining this system with THz parametric detection, all wavelengths can be detected in a single frame using a near infrared camera for real-time reagent measurement.

High-power ASE-free fast wavelength-switchable external cavity diode laser.

We developed a high-power amplified spontaneous emission (ASE)-free fast wavelength-switchable external cavity diode laser (ECDL) using a digital micromirror device (DMD) as the wavelength selector. Generally, with a conventional fast wavelength-switchable ECDL with a DMD, the output power is limited by the damage threshold of the DMD. However, with our ECDL, a high-power output was realized by optimizing the beam focus on the DMD. In addition, an ASE-free stable output was realized through the introduction of a ring cavity. As a result, we successfully developed a fast wavelength-switchable ECDL realizing a high-power ASE-free output of over 300 mW.

Highly sensitive multi-stage terahertz parametric detector.

In this Letter, we developed a high-sensitivity multi-stage terahertz (THz)-wave parametric detection system that operates at room temperature. This detection system has high sensitivity over a wide wavelength range through upconversion of a THz wave to near-infrared light. The broadband noise associated with parametric generation limited the detection sensitivity in the previous setup; however, in the multi-stage configuration using multiple LiNbO3 crystals, the THz parametric detection sensitivity was improved by spatially eliminating the broadband noise using an iris between the former and latter stages. With this improvement, the minimum detectable sensitivity at 1.05 THz approached 130 zJ (zJ=10-21J), which is equivalent to 90 photons or less. Furthermore, by combining this detector with an injection-seeded THz-wave parametric generator, which is a high-power, tunable THz-wave source, the THz-wave measurement system achieved a maximum dynamic range of 125 dB.

Terahertz tag identifiable through shielding materials using machine learning.

In recent years, there has been great interest in chipless radio-frequency identification (RFID) devices that work in the terahertz (THz) frequency range. Despite advances in RFID technology, its practical use in the THz range has yet to be realized, due to cost and detection accuracy issues associated with shielding materials. In this study, we propose two types of low-cost THz-tags; one is based on the thickness variation of coated polyethylene and the other on the fingerprint spectra of reagents. In the proposed approach, machine learning, specifically a deep-learning method, is used for high-precision tag identification even with weak signals, or when the spectrum is disturbed by passing through shielding materials. We achieved almost 100% identification accuracy despite using an inexpensive tag placed under thick shielding materials with an attenuation rate of about -50 dB. Furthermore, real-time tag identification was demonstrated by combining a multiwavelength injection-seeded THz parametric generator and a convolutional neural network.

Adaptive spatiotemporal optical pulse front tilt using a digital micromirror device and its terahertz application.

We report a new method to temporally and spatially manipulate the pulse front tilt (PFT) intensity profile of an ultrashort optical pulse using a commercial microelectromechanical system, also known as a digital micromirror device (DMD). For our demonstration, we show terahertz generation in a lithium niobate crystal using the PFT pumping scheme derived from a DMD chip. The adaptive functionality of the DMD could be a convenient alternative to the more conventional grating required to generate a laser beam with a PFT intensity profile that is typically used for efficient optical rectification in noncollinear phase-matching conditions. In contrast to a grating, PFT using DMD does not suffer from wavelength dispersion, and exhibits overlap properties between grating and a stair-step echelon mirror.

Non-destructive drug inspection in covering materials using a terahertz spectral imaging system with injection-seeded terahertz parametric generation and detection.

In 2003, we reported the first-ever development of a spectral imaging system for illicit drugs detection using a terahertz (THz) wave parametric oscillator (TPO) [K. Kawase et al., Opt. Exp. 11(20), 2549 2003]. The system has a dynamic range below four orders of magnitude, which enables it to identify reagents only through thin envelopes using spectral imaging. Recently, we succeeded in developing a high power and high sensitivity THz wave spectral imaging system using injection-seeded THz parametric generation and detection. A dynamic range in excess of 80 dB has been obtained, which is much higher than that of the 2003 system. In this study, the new spectral imaging system successfully identified reagents through thicker material than the thin envelopes used previously.

Terahertz wave three-dimensional computed tomography based on injection-seeded terahertz wave parametric emitter and detector.

We demonstrate a high dynamic range, three-dimensional (3-D) terahertz (THz) wave computed tomography system in which frequency tunable, Fourier transform-limited, high-power THz waves are emitted by an injection-seeded parametric source and ultrasensitive detection of THz waves is accomplished by heterodyne detection. This system covers the frequency range of 0.95 to 2.7 THz and has a maximum dynamic range in excess of nine orders of magnitude, enabling the acquisition of high-resolution 3-D tomographic images of samples with strong THz absorption. As an illustration, we obtained 3-D computed tomographic images of a pencil and a plastic product with an internal defect that demonstrates the potential applications of our imaging system in non-destructive testing and evaluation of industrial products.