OPTICAL FIBER COMMUNICATION AND ITS NON-LINEAR CHARACTERISTICS
In 1950, it was realized that the Bit-rate distance product would have been increased if the carrier were Optical Waves. After development of Coherent Light source in 1960, the idea of using glass material as transmission medium was introduced. Kao and Hockham made Optical Fiber in 1966 but the loss was 1000dB/km [1]. An optical fiber consisting less than 20db/km attenuation was introduced by Kapron. Then the era of Fiber Optic System was to begin.
The figuration of Optical Fiber is similar to thread except that the thread constructed from glass or silica or plastic material with large refractive index and diameter of some microns. The four segment jacket, buffer, cladding and core are confined in strata. The refractive index of the segment dwindles after each layer and the core material has greater refractive index. The data signal propagates through the fiber like the water flowing in pipe in which one end is coupled with the transmitter and the other is the receiver. Optical Fiber communication can contain the large bandwidth compared to other transmission system.
To utilize the large bandwidth and suffice the proliferating demand of the consumers, multiplexing system is developed. Depending on the transmission rate, interference, signal to noise ratio, bit error rate etc., the data signal is multiplexed and passed through the fiber at approximately low power loss and distortion. Different multiplexing system is introduced keeping in mind their working principle such as Wavelength Division Multiplexing requires strict management of wavelength and Time Division Multiplexing necessitates the management of time. Another method of multiplexing is acknowledged on account of their inherent security is Code Division Multiple Access (CDMA). CDMA has the feature of accessing network asynchronously and simultaneously. In this method, the data and pseudo code are X-ORed. The codes such as Pseudorandom Noise (PN) Short or Long code, Gold Sequence, Khasami Sequence, Walsh code, Hadamard code etc. is the choice of selection of individual users. Multi-wavelength Optical CDMA can be encoded using two dimensional (2-D) codes in time domain and also in frequency domain.
The electromagnetic and radio frequency interference do not affect the light containing information in optical fiber and the bandwidth is much greater than the metal wires (coaxial, copper cables). Metal wires are ponderous, corrosive, more power consuming, can be deteriorated easily. The linear characteristics of Optical Fiber comprise attenuation, noise, interferences, dispersion etc. Optical fiber has the capability of transoceanic transmission. Though optical fiber communication is far better than metal wire communication, some noise and interferences are originated from the Avalanche Photo Diode (APD) for instance Receiver noise, Dark current noise, Thermal noise, shot noise, Multiple-access interference (MAI), Optical beating interference etc. Dispersion is the phenomenon of the wrong way of passing light carrying information in which the wavelength and refractive index varies with each other. Group velocity Dispersion occurs when a pulse consisting multiple frequencies passes through fiber, according to that material of the fiber the velocity differs and as a result the frequencies progress to the end at different time causing broadening of the pulse.
Another dispersion that distorts the data is Chromatic dispersion. As light possesses seven wavelengths of different colors, with each color of light refractive index changes and thus the passing of data is delayed.
The non-linear characteristics originate from the third order susceptibility of the optical fiber material. The Non-linear response of frequency, polarization, phase or path of incident light, intensity variation arises some optical characteristics named Self Phase Modulation, Cross Phase Modulation, Four Wave Mixing, Stimulated Raman Scattering, Stimulated Brillouin Scattering and the Kerr-nonlinearity. These phenomenons are called Non-linear characteristics of optical fiber. The Four wave Mixing phenomena create many frequencies which are actually not sent by the transmitters. At least three frequencies are needed to generate other unwanted frequencies. In multi-channel system, many frequencies with mis-matching phase are propagated at a time through the optical fiber resulting overlap among them. These overlapping generate new frequencies and mix with the data frequency. Thus at the receiving end, the distorted data is detected and the real information is lost or include errors. It can be easily understood that if there are multi channels, the number of frequency combinations will be large in number and it will probably increase the error in each channel. FWM depends on the launched power. If the launched power is high, the unwanted signal power will be also high while increasing the error or leading to crosstalk. FWM efficiency increases with less chromatic dispersion and with the rise of signal frequency separation, efficiency decreases. In case of Cross Phase Modulation (XPM) two channels are required. In a multi-channel system, when two-channels having different group velocity propagate in the same fiber, while passing each other overlapping occur and this induce phase of one channel to the other. This is XPM and the phase of one channel is also changed by its own intensity which indicates Self Phase Modulation (SPM). If the walk over occurs with high power channel, the more power intensity causes more induce of phase spreading the pulse. The phase induced by XPM is more severe than SPM (approximately twice of SPM). In multi-wavelength system, the phase variation will be huge and the transmitted pulse will overlap with each other and thus the receiver will not be able to differentiate and detect the real information.
Reference: 1. Fiber Optic Communication System- Govind P. Agrawal.
