Graded-Index Technologies for Faster Transmission

After the studies on transparency, a complete high-speed optical network based on POFs became realistic. However, despite these advances, the SI POF had a limitation in bandwidth, requiring the excitation of several tens of thousands of modes for transmission. The large core increases modal dispersion and drastically degrades the bandwidth to approximately several hundred megahertz over 100 m. The concept of fiber bandwidth originates from the general theory of time-invariant linear systems [42]. If the optical fiber can be treated as a linear system, its input and output powers in the time domain are described simply as follows  

In other words, the output pulse response Poatit) of the fiber can be calculated through the convolution of the input pulse and the impulse response function h t) of the fiber. The period T between input pulses should be wider than the expected time spread of the output pulses. In the frequency domain. Equation AI7 can be expressed as follows  

which is the power transfer function of the fiber at the baseband frequency /, is the Fourier transform of h t) 

The optical bandwidth of the fiber is defined by the Fourier-transformed H(f). This is normally done in terms of the -3-dB bandwidth, which is the modulation frequency at which the optical power of H(f) falls to one-half the value of the zero frequency modulation. The larger the -3-dB bandwidth/ 3 g, the narrower the output pulse and the higher the possible transmission capacity. 

Typically, modal dispersion is the dominant factor degrading the bandwidth for multimode optical fibers such as SI POFs. However, as mentioned above, it can be minimized by forming a quadratic refractive index profile in the core region and controlling the propagation speed of each mode. This method was first developed by Nishizawa et al [13, 16] for glass fibers, and subsequently it was adapted to POFs. The first GI POF was reported in 1982 [43]. Initially, the GI profile was formed by copolymerizing two or three kinds of monomers with 

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