Hiroyuki Takahashi

This letter presents an ultra-broadband amplifier architecture that extends the bandwidth of conventional distributed amplifiers beyond their fundamental limitations. The proposed approach combines a baseband (BB) distributed amplifier with a radio frequency (RF) band amplifier, forming a “BB-RF amplifier” topology. To ensure smooth transition between the two bands, band joint adjusters (BJAs) based on transmission lines are introduced to mitigate phase mismatch at the band joint frequency. The amplifier was fabricated in 250 nm InP double heterojunction bipolar transistor (DHBT) technology with a maximum oscillation frequency ( $f_{\max }$ ) of 480GHz. Measurements show a gain of 11 dB at 1 GHz and a bandwidth of 270 GHz, ranging from 1 to 271 GHz. The $f_{\max }$ -normalized bandwidth is 0.56, marking the highest value ever reported.

We experimentally demonstrate long-haul C+L-band transmission using an inline repeater that combines erbium-doped fiber amplifiers (EDFAs) and low-loss variable-spectrum equalizers (LLVSEs). Our LLVSEs offer a potentially low-cost solution for compensating for spectral distortions in a C+L-band wavelength division multiplexing (WDM) signal, primarily caused by EDFA gain tilts and inter-channel stimulated Raman scattering (SRS), even at repeater sites. To evaluate their equalization capability, C+L-band recirculating transmission experiments were conducted with spectral equalization performed solely by the LLVSEs in the recirculating loop. Net bitrates exceeded 1 Tbps per channel for transmission distances up to 1120 km, and total net bitrates of 74.9, 67.0, and 59.2 Tbps were achieved after 640-, 1120-, and 1600-km transmission, respectively. These results demonstrate the effectiveness of LLVSEs in long-haul transmission and their potential to enhance the performance of multiband WDM transmission systems.

We demonstrate ultra-wideband fiber-optic transmission over an ITU-T G.654.E-compliant cutoff-shifted single-mode fiber (CSF), which has low-loss and low-nonlinearity characteristics. By extending the transmission bandwidth to long wavelengths in the U-band and the extremely long wavelength band (X-band), a signal bandwidth reached 18.6 THz within 1530.53–1702.64 nm without using wavelengths below 1530 nm, the maximum cutoff wavelength defined in G.654.x. Using an optical inline lumped repeater combining erbium-doped fiber amplifiers and optical parametric waveband converters based on in-house periodically poled lithium niobate waveguides, we conducted an 80-km-span C + L + U + X-band lumped-amplified long-haul transmission up to 3040 km. We achieved net transmission capacity of 133.9 Tbps at 1040 km (13 × 80 km), 108.0 Tbps at 2000 km (25 × 80 km), and 87.6 Tbps at 3040 km (38 × 80 km). These results demonstrate that ultra-wideband transmission above 18 THz can be achieved using only the single-mode region of CSFs by extending bandwidth to long wavelengths with the support of inter-channel stimulated Raman scattering.

Terahertz waves exhibit highly directional behavior and are easily blocked by obstacles, so for them to be used for wireless communications, they require a technology that can construct arbitrary propagation paths. A Huygens metasurface is a promising technology for this purpose because it can achieve bending of electromagnetic waves by applying a phase distribution to the wavefront using two-layer metal cells that provide a full (2π) phase shift range by adjustment of their geometry. However, the unit cells of Huygens metasurfaces tend to be large, with a quantized phase distribution that less accurately represents the ideal continuous phase distribution for beamforming and thus degrades diffraction efficiency. Here, we propose a glide-symmetric Huygens metasurface that is based on electric LC resonators with a much smaller unit cell than a given wavelength, in which the split parts are offset by a certain distance. We describe the design methodology and experimentally demonstrate large deflection angles up to 50° by using a metasurface having a phase distribution with a quantization number of six. The proposed structure is useful for forming arbitrary propagation paths in terahertz-band communication systems.

Terahertz waves from 0.1 to 0.5 THz are promising candidates for high-speed, high-capacity wireless communication networks and high-resolution sensing applications; however, with the increase in operating frequency, atmospheric attenuation and free-space losses are significant issues to overcome. Commonly used dielectric and metasurface lenses have attracted considerable attention for the enhancement of beam power characteristics, but these solutions still lack the adjustability necessary for dynamic operation, which is necessary for the development of high-efficiency systems. Here, we present a 0.3 THz band adjustable focus lens that allows for dynamic control of the focal length and beam width in response to the operating conditions. The dynamic focus lens is based on a polarization-independent 3-layer ABA-type Jerusalem cross geometry, which allows for thinner, lightweight, and more efficient devices. In the experimental verification, we have confirmed that dynamic tuning of focus can provide over 10 dB (900%) power increase and a focal length tuning ratio of 1:22 (f=0.05−1.1m). We also confirmed that the lens can operate in a wide 40 GHz (0.28–0.32 THz) frequency range, which allowed for transmission data rates of over 123 Gbps, the highest ever achieved for metasurface devices in communication. This work will have a significant impact on the development of stable communication in future high-speed wireless networks operating in terahertz bands to adjust performance in response to environmental conditions. It will also support the further development of high-resolution imaging and sensing applications.

This letter presents a 150-GHz-bandwidth baseband amplifier module with integrated dc-block functions. The baseband amplifier IC was fabricated in 250-nm InP HBT technology with ${f} _{\mathbf {T}}$ / ${f} _{\mathbf {max}}$ of 460/480 GHz. The IC itself has a bandwidth of 195 GHz and is mounted on a quartz glass substrate inside the package by flip-chip bonding. The silicon capacitors are also mounted on the same substrate to form dc-block functions for the input/output of the amplifier IC. The assembled baseband amplifier module is equipped with 0.8-mm coaxial connectors, which cover a frequency range up to 150 GHz, for its input/output interfaces, and has a footprint as small as $10.0\times 15.4$ mm. The module has a gain of 6.5 dB at a frequency of 1 GHz and a 3-dB bandwidth of over 150 GHz, ranging from 312 kHz to above 150 GHz. The measured output of the 1-dB compression point (OP1 dB) was 9.2 and 8.8 dBm at 50 and 100 GHz, respectively. The ultra broadband baseband signal amplification capability of the module was successfully demonstrated on a 160-GBd four-level pulse-amplitude modulation (PAM-4) signal. To the best of the authors’ knowledge, this is the first demonstration of a 150-GHz-bandwidth baseband amplifier module that has dc-block functions.

This article presents a 300-GHz band transmitter (TX) and receiver (RX) wireless front end (FE) with a differential architecture based on our in-house InP-based high electron mobility transistor (InP-HEMT) technology. To resolve the issue of the signal-to-noise-and-distortion ratio (SNDR) deterioration caused by the local oscillator (LO) leakage (LOL) in the integrated FE, we quantitatively investigated the effect of the LOL on SNDR by means of link budget calculation and derived the necessary power/phase imbalances in differential architectures to sufficiently reject the LOL. To achieve a sufficiently high LOL rejection, we propose the use of a 300-GHz band LO power amplifier (PA) with a high common-mode rejection ratio (CMRR). The target value of the LO PA’s CMRR (>15 dB) is also derived by the analysis to clarify the relationship between the power/phase imbalances and the CMRR. The unit amplifier in the LO PA achieves the simulated CMRR of more than 15 dB over the full range of the WR3.4 band (220–330 GHz) thanks to utilizing a common-mode rejection circuit (CMRC) based on passive components. By using the integrated circuit of LO PA and a mixer, we experimentally confirmed that the LOL of the TX is less than −33 dBm with the LO frequency range of 224–330 GHz and LO power of 0 dBm. The fabricated 300-GHz band TX and RX achieve a high conversion gain (CG) of above 5 dB over wide bandwidths of 35 and 56 GHz, respectively. We successfully demonstrated 16QAM 160-Gb/s over-the-air (OTA) data transmission using the fabricated FE with the link distance of 43 m. To the extent of the authors’ knowledge, this is the highest reported data rate achieved with a 300-GHz band FE.