According to Edholm’s law, the demand for point-to-point bandwidth in wireless short-range communications has doubled every 18 months over the last 25 years It can be predicted that data rates of around 5–10 Gb/s will be required in ten years In order to achieve 10 Gb/s data rates, the carrier frequencies need to be increased beyond 100 GHz Over the past ten years, several groups have considered the prospects of using sub-terahertz (THz) and THz waves (100–2000 GHz) as a means to transmit data wirelessly Some of the reported advantages of THz communications links are inherently higher bandwidth compared to millimeter wave links, less susceptibility to scintillation effects than infrared wireless links, and the ability to use THz links for secure communications Our goal of this paper is to provide a comprehensive review of wireless sub-THz and THz communications

Review of terahertz and subterahertz wireless communications

The unitraveling-carrier photodiode (UTC-PD) is a novel photodiode that utilizes only electrons as the active carriers. This unique feature is the key for its ability to achieve excellent high-speed and high-output characteristics simultaneously. To date, a record 3-dB bandwidth of 310 GHz and a millimeter-wave output power of over 20 mW at 100 GHz have been achieved. The superior capability of the UTC-PD for generating very large high-bit-rate electrical signals as well as a very high RF output power in millimeter/submillimeter ranges can lead to innovations in various systems, such as broadband optical communications systems, wireless communications systems, and high-frequency measurement systems. Accomplishments include photoreceivers of up to 160 Gb/s, error-free DEMUX operations using an integrated UTC-PD driven optical gate of up to 320 Gb/s, a 10-Gb/s millimeter-wave wireless link at 120 GHz, submillimeter-wave generation at frequencies of up to 1.5 THz, and photonic frequency conversion with an efficiency of -8 dB at 60 GHz. For the practical use, various types of modules, such as a 1-mm coaxial connector module, a rectangular-waveguide output module, and a quasi-optic module, have been developed. The superior reliability and stability are also confirmed demonstrating usefulness of the UTC-PD for the system applications.

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High-speed and high-output InP-InGaAs unitraveling-carrier photodiodes

A 1 k-pixel camera chip for active terahertz video recording at room-temperature has been fully integrated in a 65-nm CMOS bulk process technology. The 32 × 32 pixel array consists of 1024 differential on-chip ring antennas coupled to NMOS direct detectors operated well-beyond their cutoff frequency based on the principle of distributed resistive self-mixing. It includes row and column select and integrate-and-dump circuitry capable of capturing terahertz videos up to 500 fps. The camera chip has been packaged together with a 41.7-dBi silicon lens (measured at 856 GHz) in a 5 × 5 × 3 cm3 camera module. It is designed for continuous-wave illumination (no lock-in technique required). In this video-mode the camera operates up to 500 fps. At 856 GHz it achieves a responsivity Rv of about 115 kV/W (incl. a 5-dB VGA gain) and a total noise equivalent power (NEPtotal) of about 12 nW integrated over its 500-kHz video bandwidth. At a 5-kHz chopping frequency (non-video mode) a single pixel can provide a maximum responsivity Rv of 140 kV/W (incl. a 5-dB VGA gain) and a minimum noise equivalent power ( NEP) of 100 pW/√Hz at 856 GHz. The wide-band antenna and pixel design achieves a 3-dB bandwidth of at least 790-960 GHz.

A 1 k-Pixel Video Camera for 0.7–1.1 Terahertz Imaging Applications in 65-nm CMOS

We demonstrate two modified uni-traveling carrier photodiode (MUTC) structures that incorporate a charge or “cliff” layer to attain high-saturation-current. MUTC1 achieved responsivity of 0.82 A/W and 134 mA saturation current at -6-V and 20 GHz. The MUTC2 structure, which has higher doping density in the cliff layer and thinner absorption region, exhibited a higher saturation current of 144 mA (at -5-V) and an improved 3 dB bandwidth of 24 GHz; however, the responsivity was reduced to 0.69 A/W. For MUTC2, a high-saturation-current bandwidth product of 3456 GHz mA has been achieved. An intermodulation distortion figure of merit, IP3, > dBm at 20 GHz was observed for both MUTC structures.

High-Saturation-Current Modified Uni-Traveling-Carrier Photodiode With Cliff Layer

This paper reviews the operation, design, and performance of the uni-traveling-carrier-photodiode (UTC-PD). The UTC-PD is a new type of photodiode that uses only electrons as its active carriers and its prime feature is high current operation. A small signal analysis predicts that a UTC-PD can respond to an optical signal as fast as or faster than a pin-PD. A comparison of measured pulse photoresponse data reveals how the saturation mechanisms of the UTC-PD and pin-PD differ. Applications of InP/InGaAs UTC-PDs as optoelectronic drivers are also presented. key words: photodiode, photoreceiver, InP, InGaAs

InP/InGaAs Uni-Traveling-Carrier Photodiodes

We have studied the ultrafast response of uni-traveling-carrier photodiodes with photo-absorption layer doping levels from 2.5 ×1017 to 2.5 ×1018 cm-3. It is found that 3-dB band width increases with the output voltage in the low output region. This enhancement is more prominent for a lower doping level in the photo-absorption layer. From the analysis of the carrier transport in the photo-absorption layer, we attribute the observed enhanced bandwidth as a result of the self-induced electric field associated with carrier injection.

https://iopscience.iop.org/article/10.1143/JJAP.37.1424/pdf

Improved Response of Uni-Traveling-Carrier Photodiodes by Carrier Injection

A pin photodiode having a structure capable improving the frequency response and the saturation output while maintaining the effective internal quantum efficiency and CR time constant. A pin photodiode is formed by: a first semiconductor layer in a first conduction type; a second semiconductor layer in a second conduction type; a third semiconductor layer sandwiched between the first and second semiconductor layers, having a doping concentration lower than those of the first and second semiconductor layers; a fourth semiconductor layer in the first conduction type, provided at one side of the first semiconductor layer opposite to a side at which the third semiconductor layer is provided; and a cathode electrode and an anode electrode connected directly or indirectly to the second semiconductor layer and the fourth semiconductor layer, respectively. The first semiconductor layer has a bandgap energy by which a charge neutrality condition is maintained in at least a part of the first semiconductor layer and the first semiconductor layer is made to function as a light absorption layer, while the second and third semiconductor layers have bandgap energies by which the second and third semiconductor layers are made not to function as a light absorption layer, and the fourth semiconductor layer has a bandgap energy greater than that of the first semiconductor layer.

Pin photodiode with improved frequency response and saturation output

A remote gas sensor based on a continuous-wave sub-terahertz spectrometer with an operation bandwidth between 200 and 500 GHz has been developed. The performance of the developed system was investigated in a full-scale fire. The measured transmission spectra of smoke show the existence of hydrogen cyanide (HCN) and water molecules, which were generated due to the combustion of a foam urethane block. The HCN content estimated from the absorption intensity agrees quite well with that obtained from the chemical analysis of gases sampled from the fire site. These results demonstrate the potential of the developed sensing system as a remote toxic gas sensor at disaster sites.

Remote gas sensing in full-scale fire with sub-terahertz waves

For continuous millimeter and terahertz-wave applications, a two-mode optical signal generation technique that uses two arrayed waveguide gratings and two optical switch units is presented. In addition to easy and fast operation, this scheme offers broadband frequency tunability and high signal purity with a low spurious mode level. Mode spacing, which corresponds to the frequency of the generated MM/THz-wave signal after photomixing, was successfully swept in the range of 200 ~ 550 GHz and the optical spurious mode suppression ratio higher than 25 dBc was achieved. In addition, spurious modes characteristics were investigated by using second harmonic generation (SHG) autocorrelation methods for several frequencies.

Naofumi Shimizu

Papers

Improved Response of Uni-Traveling-Carrier Photodiodes by Carrier Injection

Pin photodiode with improved frequency response and saturation output

Improved Response of Uni-Traveling-Carrier Photodiodes by Carrier Injection

Remote gas sensing in full-scale fire with sub-terahertz waves

Generation of two-mode optical signals with broadband frequency tunability and low spurious signal level