Carrier conversion from terahertz wave to dual-wavelength near-infrared light for photonic terahertz detection in wireless communication.

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.

Design and characterization of mesoscopic dielectric cuboid antenna for operation in WR-3.4 waveguide bandwidth (220-330 GHz).

We designed a mesoscopic dielectric cuboid antenna connected to a flangeless WR-3.4 open-ended waveguide, and the antenna characteristics at 300 GHz were examined through simulations and experiments. Simulations confirmed that the flangeless design eliminated the flange-induced ripples in the radiation pattern, whose shape varied with frequency, and that the antenna operated in the full bandwidth of the WR-3.4 waveguide (220-330 GHz). Prototypes were then fabricated based on the simulation findings. A prototype with an antenna aperture area of 1.5 mm [Formula: see text] 1.5 mm and an antenna length of 2.35 mm exhibited an antenna gain of 17.2 dBi at 300 GHz and a voltage standing wave ratio of less than 1.5 throughout the WR-3.4 waveguide bandwidth. The level of the side lobes at about [Formula: see text] degrees in the E-plane pattern was approximately [Formula: see text] dB that of the main lobe. Therefore, the proposed antenna, connected to a flangeless waveguide, is a promising antenna for use in future short-range high-speed terahertz wireless applications such as kiosk downloads and board-to-board communication.

Terahertz scanning microscopy with 2λ depth of field based on photonic nanojet generated by a dielectric cuboid probe.

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.

Asynchronous electric field visualization using an integrated multichannel electro-optic probe.

The higher the frequency, the more complex the scattering, diffraction, multiple reflection, and interference that occur in practical applications such as radar-installed vehicles and transmitter-installed mobile modules, etc. Near-field measurement in "real situations" is important for not only investigating the origin of unpredictable field distortions but also maximizing the system performance by optimal placement of antennas, modules, etc. Here, as an alternative to the previous vector-network-analyzer-based measurement, we propose a new asynchronous approach that visualizes the amplitude and phase distributions of electric near-fields three-dimensionally without placing a reference probe at a fixed point or plugging a cable to the RF source to be measured. We demonstrate the visualization of a frequency-modulated continuous wave (FMCW) signal (24 GHz ± 40 MHz, modulation cycle: 2.5 ms), and show that the measured radiation patterns of a standard horn antenna agree well with the simulation results. We also demonstrate a proof-of-concept experiment that imitates a realistic situation of a bumper installed vehicle to show how the bumper alters the radiation patterns of the FMCW radar signal. The technique is based on photonics and enables measuring in the microwave to millimeter-wave range.

Wireless Data Transmission at Terahertz Carrier Waves Generated from a Hybrid InP-Polymer Dual Tunable DBR Laser Photonic Integrated Circuit.

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.

Mapping of electromagnetic waves generated by free-running self-oscillating devices.

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.

Terahertz balanced self-heterodyne spectrometer with SNR-limited phase-measurement sensitivity.

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.

Terahertz wireless communications based on photonics technologies.

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.

Continuous-wave terahertz field imaging based on photonics-based self-heterodyne electro-optic detection.

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.

Multiplication of optical frequency shift by cascaded electro-optic traveling phase gratings operating above 10 GHz.

We propose and demonstrate an electro-optic (EO) multiplication of a frequency shift using 10 GHz order electrical signal. The principle is based on a successive Bragg diffraction from cascaded EO traveling phase gratings in a traveling wave EO phase modulator. We fabricate a shift frequency doubler using simple domain engineering processes in a LiTaO3 crystal. Frequency shifting of ±32.5 GHz with an efficiency of 60% is demonstrated using a 16.25 GHz modulation signal.

Generation of an optical frequency comb with a Gaussian spectrum using a linear time-to-space mapping system.

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.

Linear time-to-space mapping system using double electrooptic beam deflectors.

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.

Transparent volumetric three-dimensional image display based on the luminescence of a spinning sheet with dissolved Lanthanide(III) complexes.

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.

Electro-optic transparent frequency conversion of a continuous light wave based on multistage phase modulation.

Frequency conversion of a continuous light wave based on multistage phase modulation has been investigated both analytically and numerically. The proposed frequency-conversion process consists of three stages: (i) phase modulation and chirp compression to generate a pulse train, (ii) Doppler shift of the pulse center frequency in a second phase modulation, and (iii) demodulation of the pulse train. By controlling the modulation power we can select the destination frequency from an equally spaced grid separated by the modulation frequency. A conversion efficiency of approximately 40% has been numerically confirmed with respect to a destination frequency of +/- 50 channels. Carrier frequency conversion of an analog data stream is numerically demonstrated.

Generation of flat power-envelope terahertz-wide modulation sidebands from a continuous-wave laser based on an external electro-optic phase modulator.

Flat power-envelope terahertz-wide modulation sidebands are generated by only electro-optic phase modulation of continuous-wave laser light. Generation and power equalization of widespread sidebands are realized simultaneously by spatial distribution of the modulation index within a laser beam cross section by use of simple domain-engineering processes in LiTaO3 electro-optic crystal. Generation of 46 sidebands spaced by 16.25 GHz within a -3-dB bandwidth (over a 1-THz span for a +/- 3-dB bandwidth) is demonstrated.