**3.2.3 Detection of intensity modulated signal**

The detection functionality is studied using the following link (Fig.9.). The optical signal is intensity modulated. The simplified small signal calculation applies sinusoidal modulation part. The optical link attenuates the optical signal and the SOA-detector receives it. The modulation information appears at the electrical connection of the SOA-detector.

Fig. 9. Simplified block diagram for calculation of SOA-detector parameters

The detection functionality was validated by experimental work. During the measurements the SOA-detector under test was driven by the bias (dc) current and a bias tee separated the detected sinusoidal modulation signal. The polarization state of the incoming optical power was optimized by a polarization controller because the measured SOAs were polarization sensitive. The harmful effect of the optical reflection was eliminated by optical isolators. The required optical power and wavelength were produced by a tunable laser source. The intensity modulated optical signal was generated by a Mach-Zehnder modulator (MZM). 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 restored. The detected electrical power was measured with different parameters.

When the bias current increases, i.e. the population inversion and the gain are higher the detected power increases (Fig.10). The diagram follows the shape of the optical gain curve. So the optimum bias current is the same from the viewpoint of the detection and amplification functions. The two curves have reference to two different modulation depths

Optical wavelength=1550nm, Applying isolators to eliminate the optical reflection effect (a) Measured current change versus average optical power, at different temperatures, Bias

The detection functionality is studied using the following link (Fig.9.). The optical signal is intensity modulated. The simplified small signal calculation applies sinusoidal modulation part. The optical link attenuates the optical signal and the SOA-detector receives it. The

PLD **SOA** 

**modulator** <sup>a</sup>PLD amod (1+mcos(t))

The detection functionality was validated by experimental work. During the measurements the SOA-detector under test was driven by the bias (dc) current and a bias tee separated the detected sinusoidal modulation signal. The polarization state of the incoming optical power was optimized by a polarization controller because the measured SOAs were polarization sensitive. The harmful effect of the optical reflection was eliminated by optical isolators. The required optical power and wavelength were produced by a tunable laser source. The intensity modulated optical signal was generated by a Mach-Zehnder modulator (MZM). 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 restored. The

When the bias current increases, i.e. the population inversion and the gain are higher the detected power increases (Fig.10). The diagram follows the shape of the optical gain curve. So the optimum bias current is the same from the viewpoint of the detection and amplification functions. The two curves have reference to two different modulation depths

**detector**

Idet0+Idetcos(t)

(b) Current change versus SOA operation point for different input optical power,

modulation information appears at the electrical connection of the SOA-detector.

Fig. 9. Simplified block diagram for calculation of SOA-detector parameters

detected electrical power was measured with different parameters.

Comparison of measurement and calculation results, Temperature=20C

0 0,5 1 1,5 2 2,5 3 3,5

Current=150mA

CW

T=20C T=25C

Current Change [mA]

0 50 100 150 200 250 300 350 Input Optical Power [mW]

**3.2.3 Detection of intensity modulated signal** 

Imod0+Imodcos(t)

PLDamod(1+mcos(t)) **LD** <sup>a</sup>

DC Information

Fig. 8. Static detection characteristics of SOA-detector

of the incoming intensity modulated signal. The detected current is directly proportional to the modulation depth. Thus, the difference between the two electrical powers is about 14dB.

Fig. 10. Measured detected electrical power versus operation point of SOA-detector with different modulation depths, Optical power at the detector input=5µW, Modulation frequency=3GHz, Optical wavelength=1550nm, Temperature=20C, Applying isolators to eliminate the optical reflection effect

The result of the detection experiments over input optical power and temperature are depicted in Fig.11. In the unsaturated regime the detected electrical signal is square proportional to the optical power, but in the saturation regime the relation goes to linear proportionality. The detection is also temperature sensitive, because the operation of semiconductor devices depends on the temperature. The measurement results show, that the detection efficiency decreases, when the temperature increases.

Fig. 11. Measured detected electrical power versus average input optical power (a) and temperature (b). Optical wavelength=1550nm, Bias current=200mA, Modulation frequency=3GHz, Modulation depth=25%, Isolators
