**3.4.1 Basic operation**

When the incoming optical power of the laser amplifier is intensity modulated, the optical gain is affected in both magnitude and phase via the modulation of the complex refractive index caused by the electron density. Consequently, in SOA the optical signal becomes amplitude modulated (AM) and phase modulated (PM). It can be modelled using the Linewidth Enhancement Factor (LEF=Henry factor=α factor) approximation. Measurements of LEF can are found in the literature and have shown that LEF is not a mere constant factor, but it is for instance a function of bias current, wavelength and input optical power. In the unsaturated region the LEF value ranges from 2 to 7 for GaAs and GaInAsP conventional lasers and from 1.5 to 2 for quantum well lasers [Occhi]. However, the chirping parameter which is positive for light sources and unsaturated optical amplifiers is negative for saturated amplifiers [Watanabe]. It cancels the positive chirp-parameter of modulator, causing asymmetrical optical power between the sidebands [Lee] and the optical amplification causes RF signal gain [Marti]. However the SOA adds noticeable noise to the system.

Multi-Functional SOAs in Microwave Photonic Systems 103

SER without SOA SER with SOA


Fig. 20. Simulated Symbol Error Rate at frequency notch versus chirp parameter of SOA, L=500m, fsubcarrier=60GHz, modulation= 2048Mbps, 4QAM, without and with SOA

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

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

Fig. 21. Simulated constellation diagrams (a) without SOA, (b) with SOA compensator,

L=500m, fsubcarrier=60GHz, modulation= 2048Mbps, 4QAM

0,00001

**3.4.3 Experimental work** 

the dispersion-induced effect.

and stored.

0,0001

0,001

**Simbol Error Rate**

0,01

0,1

1
