High-speed data transfer and high-performance imaging can be realized by
using radio-waves in high-frequency bands, such as millimeter-waves and
THz-waves, where wide frequency bands are available. However, the cell
size would be smaller than a few hundred meters, due to large free space
propagation loss and large atmospheric attenuation. Thus, many base
stations, which are connected by networks, are required to offer
nation-wide or global network services by such high-frequency
radio-bands. The networks would be constructed by various transmission
media including optical fibers and fixed wireless links, where many
media converters are required. This paper reviews various technologies
for seamless bridges between radio and optical links. For the time
being, congestion of radio spectrum in THz bands is not significant.
However, if we look at the history of radio-wave technologies, spectral
congestion has been high even in newly developed high frequency bands.
Even in active radio services in millimeter-wave or THz-wave bands,
interference mitigation with passive services such as radio astronomy
and Earth observation satellites is an important issue, as of now. This
paper describes research trends of THz-wave technologies from the point
of view of a figure of merit defined by a product of the carrier
frequency and spectral efficiency, to discuss the significance of
spectral efficiency enhancement in the high-frequency region. Analysis
of power consumption of short distance radio systems is also shown to
discuss expected performance of THz-wave links.