KEYWORDS: Digital signal processing, Optical fibers, Modulation, Short haul, Signal attenuation, Distortion, Remote sensing, Transceivers, Signal detection, Single sideband modulation
Significant increase of intra and inter data center traffic has been expected by the rapid spread of various network applications like SNS, IoT, mobile and cloud computing, and the needs for ultra-high speed and cost-effective short- to medium-reach optical fiber links beyond 100-Gbit/s is becoming larger and larger. Such high-speed links typically use multilevel modulation to lower signaling speed, which in turn face serious challenges in limited loss budget and waveform distortion tolerance. One of the promising techniques to overcome them is the use of advanced digital signal processing (DSP) and we review various DSP applications for short-to-medium reach applications.
Higher-order optical multilevel signaling is a key technique for future ultra-high speed signal transmission at
100 Gbit/s or beyond. In this paper, we review various multilevel transmitter and receiver designs realizing higher-order
multilevel signaling with precise optical field modulation and demodulation capabilities.
The optical multilevel modulation is one of the attractive candidates to significantly increase the channel bit rate and total capacity of future optical fiber communications. We review various multilevel modulation schemes proposed to date, including our experimental results of the 8-level and 16-level APSK modulation/demodulation schemes at 10 Gsymbol/s. We also discuss the advantages and disadvantages, and future issues of the optical multilevel modulation schemes.
A compact optical displacement sensor is reported which is based on the light undulation caused by the optical feedback on a frequency modulated laser diode. For applying this effect to the metrology, the relationship between the feedback rate and the oscillation behavior of the laser diode was investigated experimentally. As the result, it was found that the higher order of the output undulation could be suppressed by keeping the feedback rate low, and displacement measurement was performed with a resolution of 25 nm for the target distance between a few cm and 30 cm. In addition, an improved current modulation scheme for a laser diode as well as a new signal processing techniques for improving the performance is reported. As a further application, this sensor system was extended for precise measurement of two-dimensional displacement by utilizing a spherical retro-reflector.
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