Design and Implementation for optical fiber communication system using frequency shift coding

: In this research, optical communication coding systems are designed and constructed by utilizing Frequency Shift Code (FSC) technique. Calculations of the system quality represented by signal to noise ratio (S/N), Bit Error Rate (BER),and Power budget are done. In FSC system, the data of Nonreturn-to–zero (NRZ ) with bit rate at 190 kb/s was entered into FSC encoder circuit in transmitter unit. This data modulates the laser source HFCT-5205 with wavelength at 1310 nm by Intensity Modulation (IM) method, then this data is transferred through Single Mode (SM) optical fiber. The recovery of the NRZ is achieved using decoder circuit in receiver unit. The calculations of BER and S/N for FSC system at maximum fiber length at 61.2 km equal to 2.30551×10 -12 , 47.88526 dB respectively. The power budget for FSC system was calculated to be 29 dB. Results show that the BER increases when the received optical power decreases the due to increase of the optical fiber length61.2 km. while S/N decreases. The optical power budget increases as the transmitted optical power increases .


Introduction:
Diverse communication and signal processing technologies utilize specially coded signal formats in order to achieve desirable capabilities such as error correction, interference rejection, and secrecy.In 1963, in an effort to standardize data communication codes, the United States adopted the bell system model 33 teletype code as the United States of America Standard Code for Information Interchange (USASII), better known simply as ASCII-63.ASCII is a 7-bit character set which has 128 combinations that means 7 bits in every block [1] Since conventional NRZ code can prove unsuitable, especially for high data rate, for transmitting data in optical fiber, the best pattern for high speed transmission is Manchester coding or frequency shift coding .Manchester transmission is alternate 1S and 0S while the best pattern for high speed frequency shift transmission is all Zeros , as shown in figure 1. FSC waveform shows that the levels change at the leading edge of each bit regardless of the bit value .For ones, encoder give an additional change at each bits center ,zero have no second level change per bit [2] .The main purpose of Frequency Shift encoder is to convert the NRZ data waveform into FSC data waveform with the transition at center bit for each logic 1 bits from NRZ data , and in addition to transition at leading edge, for each 0 from NRZ data .
There are three major multiple access approaches .Each user is allocated a specific frequency (wavelength)

Materials and Methods:
Shift code system is shown in figure 2 .This figure shows the principle components of the link which contains the transmitter(encoder and modulator),the receiver( demodulator and decoder ) , and the optical fiber as transmission channel for the link .A NRZ data rates at 190 kbits/s was entered into FSC encoder .This NRZ data that generated by voice communication represented by time division multiplexing system (TDM).The optical source which provide the electrical-optical conversion is a semiconductor laser diode(LD), The package of this laser is designed to allow repeatable coupling into single mode optical fiber.Frequency shift code signal is used to drive an optical source(1310 nm) using Intensity Modulation (IM).This laser spectral width is 1nm, optical rise time is 2nsec, and the average optical output power is -10 dBm that was measured by using optical power meter OPM-4.

Digital NRZ Encoding Optical
Signal data Fiber

Fig.(2):General Block Diagram of Frequency Shift Coding System
The specification of the fiber that used in this system are core to cladding diameter equal to 10/100 micrometer , the whole all diameter is 3.2 nm, the attenuation is 0.2 dB for wavelength at 1310 nm , the numerical aperture is 0.14 and the acceptance angle is 8.05° .
The receiver consists of an optical detector which drives a future electrical stage and hence provides demodulation of the optical carrier.PIN photodetector was used in the receiver unit .Figure 3 shows the block diagram of the Frequency Shift decoder

Calculation and Results:
The waveforms from the Cathode Ray Oscillscope (CRO)of the input NRZ data and the output data FSC of the transnitter unit are shown in figure 4. The relationship between the BER with the received optical power as a function of fiber length of the SM optical fiber in FSC system is shown in figure 7

B-Optical Power Budget
To ensure that the optical fiber system has sufficient power for correct operation, the power budget must be calculated, the power budget is given by [ Where Ptx is the transmitted optical power from the laser diode.Pmin is the minimum detectable power at the receiver unit.From equation 5 the optical power budget equal 29dB and increases as the transmitted optical power increases.

Conclusion:
The encoded data stream shows successful demonstration of FSC generation techniques with high quality system of different fiber length(1-61.2) km., so FCC code technique is better suited for optical communication because of the large output signal to noise ratio S/N and low bit error rate BER of the received signal

Fig. ( 7
Fig. (7) :The relation ship between the logarithm of BER with the received optical power in single mode optical fiber channel of Frequency shift code system.