Microwave photonics, which brings together the worlds of radiofrequency engineering and optoelectronics, has attracted great interest from both the research community and the commercial sector over the past 30 years and is set to have a bright future. The technology makes it possible to have functions in microwave systems that are complex or even not directly possible in the radiofrequency domain and also creates new opportunities for telecommunication networks. Here we introduce the technology to the photonics community and summarize recent research and important applications.

Microwave photonics combines two worlds

저작근 근전도에 관한 정상교합자와 ii급 부정교합자의 비교 연구

Recent changes in how people consume multimedia services are causing an explosive increase in mobile traffic. With more and more people using wireless networks, the demand for the ultra-fast wireless communications systems is increasing. To date, this demand has been accommodated with advanced modulation schemes and signal-processing technologies at microwave frequencies. However, without increasing the carrier frequencies for more spectral resources, it may be quite difficult to keep up with the needs of users. Although there are several alternative bands, recent advances in terahertz-wave (THz-wave) technologies have attracted attention due to the huge bandwidth of THz waves and its potential for use in wireless communications. The frequency band of 275 ~ 3000 GHz , which has not been allocated for specific uses yet, is especially of interest for future wireless systems with data rates of 10 Gb/s or higher. Although THz communications is still in a very early stage of development, there have been lots of reports that show its potential. In this review, we will examine the current progress of THz-wave technologies related to communications applications and discuss some issues that need to be considered for the future of THz communications.

Present and Future of Terahertz Communications

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 increasing demand of unoccupied and unregulated bandwidth for wireless communication systems will inevitably lead to the extension of operation frequencies toward the lower THz frequency range. Higher carrier frequencies will allow for fast transmission of huge amounts of data as needed for new emerging applications. Despite the tremendous hurdles that have to be overcome with regard to sources and detectors, circuit and antenna technology and system architecture to realize ultrafast data transmission in a scenario with extensive transmission loss, a new area of research is beginning to form. In this article we give an overview of emerging technologies and system research that might lead to ubiquitous THz communication systems in the future.

A Review on Terahertz Communications Research

A new ultrafast photodiode, uni-traveling-carrier photodiode (UTC-PD) is proposed, and its photoresponse characterization on fabricated devices is presented. The prime feature of this PD is much higher output saturation current than that in a conventional pin-PD. This is achieved by reducing the space charge effect by utilizing electron velocity overshoot in the carrier collecting layer. A 20 μm2-area UTC-PD fabricated with MOVPE-grown InP/InGaAs heterostructure generated an output voltage as high as 2 V for a 25 Ω load while maintaining an f3dB of 80 GHz. Proper device operations at high photocurrent densities up to 400 kA/cm2 were observed.

Uni-traveling-carrier photodiodes

There has been an increasing interest in photonic generation of RF signals in the millimeter-wave (30 GHz ∼ 300 GHz) and/or terahertz-wave (0.1 THz ∼ 10 THz) regions, and photodiodes play a key role in it. This paper reviews re- cent progress in the high-power RF photodiodes such as Uni- Traveling-Carrier-Photodiodes (UTC-PDs), which operate at these frequencies. Several approaches to increasing both the bandwidth and output power of photodiodes are discussed, and promising applications to broadband wireless communications and spectroscopic sensing are described. Photodiode chip is bonded to the substrate with the antenna. This is a symbolic figure of RF photonics in this article. Antenna

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High-power RF photodiodes and their applications

We present a monochromatic sub-terahertz signal generation technique using an optical comb signal, arrayed waveguide gratings (AWGs), and a uni-traveling carrier photodiode (UTC-PD) for spectroscopic applications. This scheme offers random or continuous frequency tuning in the range between 100 GHz and up to 1 THz. In addition, since a RF synthesizer is employed as a reference signal source of the photonic frequency multiplier, frequency locking with external instruments and reliable operation are offered. Highly coherent optical comb signal for the photonic frequency multiplication provides a narrow linewidth and very low phase noise in the generated sub-terahertz signal. For 125 GHz, the phase noise is approximately -92 dBc/Hz at the offset frequency of 10 kHz. This is larger than that of the 25-GHz RF source by about 13 dB and agrees well with the theory regarding phase noise multiplications due to frequency multiplication. For generating monochromatic signals, unwanted spurious signals are suppressed in the optical domain over a wide range with two AWGs, and the suppression ratio is expected to be better than 46 dBc. Utilizing the implemented sub-terahertz signal generator with a J-band UTC-PD module, absorption lines of N2O were measured in the frequency range between 240 and 360 GHz and compared with theoretical calculations.

Broadband-Frequency-Tunable Sub-Terahertz Wave Generation Using an Optical Comb, AWGs, Optical Switches, and a Uni-Traveling Carrier Photodiode for Spectroscopic Applications

A high-output-power photomixer module operating at 200-500 GHz has been developed. The module consists of a uni-traveling-carrier photodiode, a PD-waveguide mode-conversion coupler, and a compact metal package with a rectangular-waveguide that outputs sub-terahertz wave. The fabricated module exhibits a 10- dB down bandwidth of about 300 GHz and an output power as high as -2.7 dBm at 350 GHz. This is the highest value ever reported for a photomixer module operating at 350-GHz band.

High-power and broadband sub-terahertz wave generation using a J-band photomixer module with rectangular-waveguide output port

The uni-traveling-carrier photodiode (UTC-PD) is a kind of pin junction photodiode that selectively uses electrons as active carriers. The diode structure has a relatively thin p-type absorber where electrons are generated as minority carriers, and then they diffuse and/or field-accelerate toward the collector. Since the electrons travel in the depleted collector at a ballistically high velocity, the photoresponse performance of a UTC-PD is superior to that of a conventional pin-PD. In this tutorial, the basics of the current response in a UTC-PD, the electron transport in the p-type absorber, and the performance of a terahertz-wave UTC photomixer, as a representative, are described.

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Hiroshi Ito

Papers

Uni-traveling-carrier photodiodes

High-power RF photodiodes and their applications

Broadband-Frequency-Tunable Sub-Terahertz Wave Generation Using an Optical Comb, AWGs, Optical Switches, and a Uni-Traveling Carrier Photodiode for Spectroscopic Applications

High-power and broadband sub-terahertz wave generation using a J-band photomixer module with rectangular-waveguide output port

Uni-traveling-carrier photodiodes