**3.4.3 Experimental work**

102 Selected Topics on Optical Amplifiers in Present Scenario

*D fL D fL H ( f ) cos LEF sin*

*f D fL j LEF sin f c*

Where D is the fibre dispersion parameter, L is the fibre length, f is the modulation frequency, c is the speed of light in vacuum, λ is the operating wavelength. For simplicity

The calculated RF responses of 400 km fibre for different chirp parameters of the optical transmitter are depicted in Fig.19. All the results have been calculated for an optical input power of 0 dBm in order to reduce the influence of the nonlinear effects in the fibre. By comparing the results to the reference case of a zero-chirp situation (*LEF*=0), for *LEF*<0, the

**Deep [dB]**


*c*

LEF=-0.5 LEF=-0.2 LEF=-0.1 LEF=0

Fig. 19. Calculated quality of MW/MMW transmission for different SOA and system

In real radio over fibre systems the radio frequency carrier is modulated by digital information. The above presented dispersion compensation technique affects the eye diagram, the BER (Bit Error Rate) and the EVM (Error Vector Magnitude), which are the important quality factors of signal transmission. If the subcarrier frequency is close to one of the frequency notches caused by dispersion the eye diagram closes, the BER and EVM decay, the communication deteriorates or lost. Applying the SOA compensator the eye diagram opens, low BER is obtained and EVM can greatly benefit from the improved link performance. The simulations were executed with different modulation schemes on radioover-fibre systems and VPI software was applied. Fig. 20 presents the quality of 4 QAM (quadrature amplitude modulation) radio signal transmission with different SOA chirp parameter, it shows the Symbol Error rate improvement applying SOA dispersion compensator. The EVM and SER increase versus chirp parameter. If the subcarrier frequency is close to the frequency notch, the communication lost. Applying a SOA dispersion compensator the constellation diagram can be detected, but the diagram rotates

(a) Transfer function with different chirp parameter of SOA, fiber length=100km (b) Rejection frequency and the deep versus chirp parameter of SOA, fiber length=1km

2 2 2 2

 

2 2

*c c*

(13)

**Rejection Frequency [GHz]**

**3.4.2 Calculation and simulation results**  The frequency transfer function of the optical link:

*link*

the linear loss and delay of the fibre are neglected.

012345 **Modulation Frequency [GHz]**

parameters, Optical wavelength=1550nm, D=16.8 ps/(nm·km)

because of the additional phase modulation generating the SOA chirp.

SOA

achievable bandwidth increases.


**Transmission [dB]**

The calculated and simulated results can be verified by experimental work. The SOA under test was driven by different bias (dc) currents. The polarization state of the incoming optical power was optimized by polarization controller. The harmful effect of the optical reflection was eliminated by optical isolators. The required optical power and wavelength were produced by a tuneable laser source. The intensity modulated optical signal was detected by a photodetector. The setup was controlled by a computer program, hence the measurement parameters were carefully set by the program and the measurement results were processed and stored.

The measured RF responses of 50km fibre for different operation points of SOA are depicted in Fig. 21. By comparing the results to the reference case of a zero-chirp situation (*LEF*=0), for *LEF*<0, the achievable bandwidth increases. As the SOA bias current (optical gain) increases the frequency notches of the RF response are reduced and shifted to higher modulation frequencies. Based on the results, we may conclude that the interplay of chirp generated by the saturated SOA and chromatic dispersion enables a significant reduction of the dispersion-induced effect.

Fig. 21. Simulated constellation diagrams (a) without SOA, (b) with SOA compensator, L=500m, fsubcarrier=60GHz, modulation= 2048Mbps, 4QAM

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Fig. 22. Measured RF responses of the optical link for different SOA parameters Length of the optical fiber=50km, Length of the reference optical fiber=4m, Optical wavelength=1550nm, bias current of the SOA=400mA, 200ma, 125mA, 100mA, Optical Gain=15dB, 13.5dB, 4,5dB, -9.5dB
