Search

Wireless communications using highly directive terahertz (THz) waves exhibit lower immunity to obstructions than microwaves, limiting their applications. This study demonstrates an obstacle-tolerant THz wireless link established by a self-healing Bessel beam at 300 GHz. The Bessel beam is generated by sending a Gaussian beam through a dielectric axicon lens. Furthermore, experiments were conducted to investigate the short-range transmission (98 mm) reception characteristics with and without the dielectric cubic obstacle (7.5 λ in size) and the metallic obstacle (22 λ in length and 8 λ in width) in the Gaussian beam and self-healing Bessel beam cross sections. The maximum attenuation of the received power due to obstruction was 8.8 and 2.2 dB for the Gaussian and Bessel beams, respectively, when the dielectric obstacle is placed in the middle of the transmission path (49 mm from a transmission lens). This study further investigated the bit error rate (BER) characteristics (1 Gbps, on–off keying) with the dielectric obstacle crossing the beam cross section. When the obstacle crosses the Gaussian beam, the BER degrades as the obstacle approaches the optical axis, breaking the wireless link. In contrast, when the obstacle crosses the Bessel beam cross section, the BER is maintained at <3.8 × 10−3 (the forward error correction limit), and the wireless link is maintained. A self-healing beam, such as the Bessel beam, makes the THz wireless link more tolerant than the Gaussian beam to obstacle and expand applications for THz wireless communications. [ABSTRACT FROM AUTHOR]

To improve the accuracy of material identification in millimeter- and terahertz-wave nondestructive imaging during security and other inspections, it is important to perform both amplitude and phase imaging. Highly coherent sources generated based on the frequency multiplication of stable microwave sources are promising for these applications. However, this method reduces the fidelity of the image due to the interference phenomenon with reflected waves on the sample surface. In this paper, we propose and demonstrate amplitude and phase imaging using an incoherent free-running self-oscillating device. We used a Gunn oscillator operating at 122.5 GHz as a millimeter-wave source and a photoconductive antenna as a detector. The frequency and phase noise of the source are canceled by the low-frequency electronics, which realize phase imaging using an incoherent source. We performed amplitude and phase imaging on low-loss polypropylene objects and obtained ∼22 times higher imaging contrast in the latter than in the former. Furthermore, we were able to obtain high-fidelity imaging results with less interference in both amplitude and phase imaging. The proposed system is reliable and inexpensive because the source operates with a DC power source only, and the frequency and phase noise-cancellation electronics consist of low-frequency circuits. Moreover, because the noise-cancellation electronics are independent of the RF frequency, the system is easily extendible to the higher frequency region to improve its spatial resolution. This paves the way for the realization of a diode-based, sensitive, and low-cost integrated system that allows both amplitude and phase imaging for material identification in security inspections. [ABSTRACT FROM AUTHOR]

A superheterodyne transmission scheme is adopted and analyzed in a 300 GHz wireless point-to-point link. This was realized using two different intermediate frequency (IF) systems. The first uses fast digital synthesis which provides an IF signal centered around a carrier frequency of 10 GHz. The second involves the usage of commercially available mixers, which work as direct up- and down-converters, to generate the IF input and output. The radio frequency components are based on millimeterwave monolithic integrated circuits at a center frequency of 300 GHz. Transmission experiments over distances up to 10 m are carried out. Data rates of up to 60 Gbps using the first IF option and up to 24 Gbps using the second IF option are achieved. Modulation formats up to 32QAM are successfully transmitted. The linearity of this link and of its components is analyzed in detail. Two local oscillators (LOs), a photonics-based source and a commercially available electronic source are employed and compared. This work validates the concept of superheterodyne architecture for integration in a beyond-5G network, supplying important guidelines that have to be taken into account in the design steps of a future wireless system. [ABSTRACT FROM AUTHOR]

A mesoscopic dielectric cuboid antenna (DCA), which can be connected to a standard waveguide, is proposed to achieve high directivity with a simple-structure and low-profile antenna compared with that of a horn antenna of same dimensions. We optimized the antenna dimensions based on simulation to maximize the antenna gain of 14.22 dBi, which is 1.9 dB higher than that of the horn antenna with the same dimensions. Simulation was performed both at 300 and 24 GHz. As a proof of concept, we designed and fabricated a scaled DCA with dimensions of 1.2 $\lambda \,\times \,1.2\lambda \,\times \,1.36\lambda$ and experimentally evaluated the radiation pattern at 24 GHz band. The full-width at half-maximum (FWHM) values were approximately 21% and 34% narrower than those of the horn antenna in the E-plane and H-plane, respectively. The frequency characteristic of sensitivity enhancement using the DCA as the reception antenna shows that the DCA is a nonresonant antenna with a wide bandwidth. Narrower FWHMs of the DCA have been discussed with respect to a two-dimensional near-field phase distribution measured using the electrooptic sensing technique. [ABSTRACT FROM AUTHOR]

This paper describes continuous millimeter-wave and terahertz (THz)-wave applications, where telecom-based photonics technologies are efficiently employed to enhance their performance. First, 300-GHz-band wireless communications are described toward real-time error-free transmissions at 50 Gbit/s and beyond. Next, a novel approach to increase a phase measurement sensitivity in THz frequency-domain spectroscopy systems is explained, and a similar technique is successfully applied to the visualization of electric-field radiation and propagation. Finally, as a futuristic study, the manipulation of THz waves with a concept of photonic crystals and its possible applications to platforms in THz integrated systems are presented. [ABSTRACT FROM AUTHOR]

We demonstrate that the simultaneous injection locking of two monolithically integrated distributed feedback (DFB) laser diodes (LDs) with an optical frequency comb is a feasible technique for photonic-based carrier signal generation in the real-time coherent wireless link. The phase of the carrier signal generated by heterodyning the two injection-locked DFB LDs is sufficiently stable to achieve 10-Gb/s error-free (bit-error rate <10^-11 ) coherent wireless transmission in the $W$ -band (75–110 GHz) without active phase stabilization on the transmitter side and digital signal processing on the receiver side. Compared with a direct detection scheme, we show a 17-dB sensitivity improvement using coherent detection. These results open the path toward the development of compact and cost-effective coherent photonic wireless transmitters based on state-of-the-art InP photonic integrated circuit technology, which have wider bandwidth compared with electronics-based transmitters. [ABSTRACT FROM PUBLISHER]

We demonstrate a self-heterodyne terahertz (THz) frequency-domain spectrometer in which commercially available uni-traveling-carrier photodiodes (UTC-PDs) are used as opticalto-electrical (O/E) converter and optoelectronic mixer (OEM). The optical intensity beat generated by mixing two frequency-detuned free-running 1.55 μm laser diodes (LDs) is used as a source for the photomixing at the O/E converter, and as an optical local oscillator (LO) signal for the OEM. The frequency of the optical LO signal is coherently shifted with an electrooptic frequency shifter so that the heterodyne detection is realized using free-running LDs for the beat source. We investigate the sensitivity characteristics of the spectrometer as functions of diode bias voltage and optical LO level. The maximum signal-to-noise ratio of 47 dB for the RF power of -16 dBm is achieved at 300 GHz with the lock-in time constant of 23.4 ms. The standard deviation of the phase measurement is 0.73°, where the amplitude noise limited standard deviation is 0.25°. We measure the system response ranging from 200 to 1450 GHz and find that it fits well the response which is calculated using the frequency characteristics of the output power of the UTC-PDs. Our results show that UTC-PDs are promising candidates not only for the high-power terahertz wave generator, but also for the wideband THz-OEM. Integration of UTC-PDs with free-running LDs on the same substrate with the same process steps would significantly reduce the cost and size of the present spectrometer. [ABSTRACT FROM PUBLISHER]

We propose and demonstrate a technique for phase change measurement of a continuous terahertz (THz) wave, based on 1.55-\mum photonic techniques. The generation of the THz wave is based on photomixing using a uni-traveling-carrier photodiode. The detection of the THz wave is based on heterodyne mixing of the THz wave, using a low-temperature-grown GaAs photoconductor with frequency-shifted photonic local oscillator signal. The measurement of a phase shift induced by a single sheet of paper with 85-\mum thickness has been demonstrated at 300 GHz. [ABSTRACT FROM PUBLISHER]

We demonstrate for the first time the continuous operation of a Bragg diffraction type electrooptic (EO) frequency shifter using a 16GHz modulation signal. Because frequency shifting is based on the Bragg diffraction from an EO traveling phase grating (ETPG), this device can operate even in the millimeter-wave (>30 GHz) range or higher frequency range. The ETPG is generated based on the interaction between a modulation microwave guided by a microstrip line and a copropagating lightwave guided by a planner waveguide in a domain-engineered LiTaO3 EO crystal. In this work, the modulation power efficiency was improved by a factor of 11 compared with that of bulk devices by thinning the substrate so that the modulation electric field in the optical waveguide was enhanced. A shifting efficiency of 65% was achieved at the modulation power of 3W. [ABSTRACT FROM AUTHOR]

We estimate the phase velocity of a modulation microwave in a quasi-velocity-matched (QVM) electro-optic (EO) phase modulator (QVM-EOM) using EO sampling which is accurate and the most reliable technique for measuring voltage waveforms at an electrode. The substrate of the measured QVM-EOM is a stoichiometric periodically domain-inverted LiTaO3 crystal. The electric field of a standing wave in a resonant microstrip line (width: 0.5 mm, height: 0.5mm) is measured by employing a CdTe crystal as an EO sensor. The wavelength of the traveling microwave at 16.0801 GHz is determined as 3.33mm by fitting the theoretical curve to the measured electric field distribution. The phase velocity is estimated as vm = 5.35 × 107 m/s, though there exists about 5% systematic error due to the perturbation by the EO sensor. Relative dielectric constant of ϵr = 41.5 is led as the maximum likelihood value that derives the estimated phase velocity. [ABSTRACT FROM AUTHOR]

A simple millimeter wave and terahertz (THz) receiver scheme that uses subwavelength focusing of electromagnetic beam on the point-contact detector area with waveguide dimensions is studied. A detection system with such an optical coupling scheme is implemented, where the signal to be detected is coupled to a detector through a mesoscale dielectric particle lens based on terajet effect. We have experimentally demonstrated an enhancement of the point-contact detector sensitivity higher than 6 dB and with 1.5 times decreasing of the noise equivalent power value. The results show that the proposed method could be applied to reduce the size and increase the sensitivity of various THz systems, including imaging, which would enable significant progress in different fields such as physics, medicine, biology, astronomy, etc. [ABSTRACT FROM AUTHOR]

We report on terahertz wireless communication experiments at 0.3 THz using 250-nm gate-length GaAs/AlGaAs field-effect transistor (FET) as a detector and unitraveling-carrier photodiode as a source. The physical mechanism of the detection process is terahertz wave rectification on nonlinearities related to overdamped plasma oscillations in the transistor channel. We present an experimental study of rectification bandwidth and show for the first time that room-temperature direct detection with modulation bandwidth of up to 8 GHz can be achieved, thus showing that nanometer-sized FETs can be used as valuable detectors in all-solid-state terahertz wireless communication systems. [ABSTRACT FROM AUTHOR]

We report a traveling-wave electro-optic sinusoidal deflector which operates in a picosecond time scale. The deflector is fabricated by use of simple domain-engineering processes in a LiTaO3 electro-optic crystal. The shape of the domain inverted region in a traveling-wave electro-optic phase modulator is theoretically designed in consideration of velocity mismatch between an optical group velocity and a microwave phase velocity in a crystal for spatial linear distribution of the modulation index within an optical beam cross section. At a deflection period of 61.5 ps, the maximum resolvable spot number of 13.0±0.6 is demonstrated. Our EOD can be applicable to time-space optical information processing systems with the minimum temporal resolution of 1.5 ps. [ABSTRACT FROM AUTHOR]

We demonstrate a nonpolarimetric technique for the coherent detection of a continuous wave (CW) electromagnetic (EM) radiation using electrooptic (EO) detection. A dual-frequency optical source is used for both the generation and detection of the CW radiation. The detection process is based on an optical homodyning of the probe beam and sideband generated in the EO crystal by the EM field to be detected. The operation is analyzed theoretically and confirmed experimentally in the microwave region. [ABSTRACT FROM AUTHOR]

THz waves are promising wireless carriers for next-generation wireless communications, where a seamless connection from wireless to optical communication is required. In this study, we demonstrate carrier conversion from THz waves to dual-wavelength NIR light injection-locking to an optical frequency comb using asynchronous nonpolarimetric electro-optic downconversion with an electro-optic polymer modulator. THz wave in the W band was detected as a stable photonic RF beat signal of 1 GHz with a signal-to-noise ratio of 20 dB via the proposed THz-to-NIR carrier conversion. In addition, the results imply the potential of the photonic detection of THz waves for wireless-to-optical seamless communication.

We demonstrate terahertz scanning microscopy using a dielectric cuboid probe (DCP). The protruding part of the DCP is inserted into a waveguide, which is commonly used in the millimeter- and terahertz-wave bands, to generate a photonic jet. The DCP does not require free-space optics, making the system very compact. The DCP generates a 300 GHz beam with full width at half maximum (FWHM) of less than wavelength (λ) in the region from the surface to 2λ ahead. This relatively longer depth of field (DOF) is a great advantage when the imaging target is covered with dielectric material and the probe head cannot be brought close to the imaging target. Also, this eliminates the need for precise feedback control of the distance between the uneven sample and probe, thus simplifying the microscopy system. Taking with this advantage, we demonstrate depth imaging with longitudinal and lateral spatial resolutions of about 10 µm (λ/100) and less than 1 mm (λ), respectively, by using the phase data in a reflective imaging configuration. This technology is expected to aid the realization of an inexpensive and compact high-resolution microscopy system with large DOF in the millimeter- and terahertz-wave regions.

We report for the first time the successful wavelength stabilization of two hybrid integrated InP/Polymer DBR lasers through optical injection. The two InP/Polymer DBR lasers are integrated into a photonic integrated circuit, providing an ideal source for millimeter and Terahertz wave generation by optical heterodyne technique. These lasers offer the widest tuning range of the carrier wave demonstrated to date up into the Terahertz range, about 20 nm (2.5 THz) on a single photonic integrated circuit. We demonstrate the application of this source to generate a carrier wave at 330 GHz to establish a wireless data transmission link at a data rate up to 18 Gbit/s. Using a coherent detection scheme we increase the sensitivity by more than 10 dB over direct detection.

Near-field mapping has proven to be a powerful technique for characterizing and diagnosing antennas in the microwave frequency range. However, conventional measurement methods based on a network analyzer cannot be applied to on-chip antenna devices extensively studied for future wireless communication in the millimeter wave (mm-wave) (30-300 GHz) and terahertz (THz) wave (0.1-10 THz) frequency regions. Here, we present a new asynchronous mapping technique to investigate the spatial distribution of not only the amplitude but also the phase of the electric field generated by free-running, self-oscillating generators including CMOS oscillators, Gunn oscillators, resonant tunneling diodes, and quantum cascaded lasers. Using a photonic-electronic hybrid measurement system, a wide frequency coverage, minimal invasiveness of the field to be measured, and phase distribution measurements with a theoretically-limited sensitivity are simultaneously achieved. As a proof-of-concept experiment, we demonstrate the mapping of a mm-wave (77 GHz) generated by a free-running Gunn oscillator and antenna characterization based on near-to-far field transformation.

Photonics-based frequency-domain terahertz (THz) wave measurement systems have received significant attention in both scientific and industrial fields due to their high-frequency resolution. Highly sensitive phase-measurement systems have been desired in the chemical, material, and biomedical sciences to facilitate microanalysis of materials. Here, we demonstrate a balanced self-heterodyne technique that, for the first time, simultaneously offers wide frequency tunability of more than 2.5 THz and high phase sensitivity, which is limited only by the signal-to-noise ratio (SNR) of the amplitude measurement. Using free-running lasers for THz wave generation and detection, the experimentally achieved minimum detectable optical path length change was 400±50 nm at 2 THz for a SNR of 37.7 ± 0.7 dB, even though the theoretically expected SNR-limited value was 310 ± 20 nm. The phase measurement sensitivity of our system is almost one order of magnitude better than that of the conventional systems in which limitations arise from phase instabilities in the optical components and/or laser linewidth.

There has been an increasing interest in the application of terahertz (THz) waves to broadband wireless communications. In particular, use of frequencies above 275 GHz is one of the strong concerns among radio scientists and engineers, because these frequency bands have not yet been allocated at specific active services, and there is a possibility to employ extremely large bandwidths for ultra-broadband wireless communications. Introduction of photonics technologies for signal generation, modulation and detection is effective not only to enhance the bandwidth and/or the data rate, but also to combine fiber-optic (wired) and wireless networks. This paper reviews recent progress in THz wireless communications using telecom-based photonics technologies towards 100 Gbit/s.

We demonstrate a photonics-based self-heterodyne electro-optic field imaging technique at terahertz (THz) frequency. An optical intensity beat generated by mixing two frequency-detuned free-running lasers is used for both the generation and the detection. The frequency of the beat for detection is shifted by an optical frequency shifter to realize coherent heterodyne measurement with free-running lasers. Neither mechanical delay lines nor phase-locked synthesizers are required for the amplitude and the phase imaging of the THz field, and the system simplicity is thus improved. The amplitude and phase of the THz field (125 GHz) radiated from a horn antenna are simultaneously imaged, and the standard deviation of the phase measurement is found to be 0.18 rad.

We demonstrate the generation of an optical frequency comb (OFC) with a Gaussian spectrum using a continuous-wave (CW) laser, based on spatial convolution of a slit and a periodically moving optical beam spot in a linear time-to-space mapping system. A CW optical beam is linearly mapped to a spatial signal using two sinusoidal electro-optic (EO) deflections and an OFC is extracted by inserting a narrow spatial slit in the Fourier-transform plane of a second EO deflector (EOD). The spectral shape of the OFC corresponds to the spatial beam profile in the near-field region of the second EOD, which can be manipulated by a spatial filter without spectral dispersers. In a proof-of-concept experiment, a 16.25-GHz-spaced, 240-GHz-wide Gaussian-envelope OFC (corresponding to 1.8 ps Gaussian pulse generation) was demonstrated.

We propose and demonstrate a linear time-to-space mapping system, which is based on two times electrooptic sinusoidal beam deflection. The direction of each deflection is set to be mutually orthogonal with the relative deflection phase of pi/2 rad so that the circular optical beam trajectory can be achieved. The beam spot at the observation plane moves with an uniform velocity and as a result linear time-to-space mapping (an uniform temporal resolution through the mapping) can be realized. The proof-of-concept experiment are carried out and the temporal resolution of 5 ps has been demonstrated using traveling-wave type quasi-velosity-matched electrooptic beam deflectors. The developed system is expected to be applied to characterization of ultrafast optical signal or optical arbitrary waveform shaping for modulated microwave/millimeter-wave generation.

We have developed a new type of transparent volumetric three-dimensional (3D) image display in which a thin photopolymer sheet containing Lanthanide(III) complexes is used as a rotational screen. The Lanthanide(III) complexes used in our system are Eu(TTA)(3) Phen, designed for achieving red luminescence (615nm) for an excitation light of 395 nm. An arbitrary luminous point (voxel) is identified by controlling the excitation laser beam direction in synchronization with the photopolymer sheet rotation. The full colorization of the proposed volumetric 3D image display can be realized by using, for example, Eu(TTA)(3) Phen, Tb(ACAC)(3) Phen, and Coumarin 337, simultaneously.