**2.3 Hybrid WDM/SCM passive optical network**

Fiber-to-the-home (FTTH) technology is one of the main research objectives in the "broadband for all" concept that encourages the development of optical access infrastructure. In order to fulfil this concept, cost effective solutions must be developed to be able to offer future-proof broad-band connections to end users at a reasonable cost [Arellano]. A key element in access networks is the optical network unit (ONU) of the customer premises equipment, having a direct impact on the cost per customer, whereas, the access part represents the main segment of the total capital cost; thus, simple ONUs need to be designed. Other key desirable characteristics of an access network are the use of one single fiber for both upstream and downstream transmission in order to reduce network size and connection complexity of the outside plant [Prat], the elimination of the laser source at the ONU, thus avoiding its stabilization and provisioning, and, if possible, all ONUs being wavelength independent, to fit in a transparent wavelength-division-multiplexing scenario of a future FTTH (fiber-to-the-home) network. The SOA-modulator-detector may be used in a basic bidirectional single-fiber single-wavelength scenario for a FTTH network. However it works as a point-to-point connection, hence a reflective structure (RSOA) is more powerful.

The present passive optical network (PON) is standardised. The future, upgraded systems are waiting for standardisation. Wavelength division multiplexed-passive optical network (WDM-PON) is a promising solution for the future high-speed access networks such as FTTH or fiber to the office (FTTO) by reasons of large capacity, network security, protocol transparency and upgradability [Kang]. However, because of relatively expensive WDM components, the WDM-PON has been considered as a next-generation. Recently, to overcome this problem, there have been several proposals. Normally, two wavelength

Multi-Functional SOAs in Microwave Photonic Systems 91

be determined from the variation in the injection current. The magnitude and purity of the detected signal depend on the modulation signal, the bias current, the input power and the

Two different mechanisms induce detection in the SOA. Operated at an injection current corresponding to an electron density below transparency, the device works as a photodetector and the detection signal arises due to absorption of the injected light and the creation of electron-hole pairs. However at injection current above transparency, that is the amplifying regime, the injected optical signal will cause stimulated transitions, which will reduce the carrier density in the gain medium. Due to these two different mechanisms of

> 0 50 100 150 200 Bias Current [mA]

Fig. 7. Detection behaviour of SOA. Measured detected power and detection responsivity versus operation point of the SOA, optical wavelength=1550nm, Optical power at the input

The static photo-detection current is not given data in data sheets of traditionally used SOAs. In unsaturated regime this curve can be approximate calculated from the optical gain (proportional with G-1) and the input optical power (proportional with Pin). However in

Fig.8a depicts the electrical current as a function of the input optical power of SOA-detector. The relationship between the input optical power and the output electrical current is given by the detector responsivity, it can be determined from this curve. However this static photo-detection current is operation point and temperature sensitive. Fig.8b represents the shape of the calculated curve (based on optical gain-bias current curve), that follows the

of the SOA-detector=40µW, Modulation depth=20%, Temperature=20C

several cases it can only be determined by measurements.

Transparency





Detected Power [dBm]



interaction, the detected electrical signal will change polarity at transparency (Fig.7.).

operation parameters of the SOA [Udvary1].

0

0,01

0,02

0,03

Responsivity [A/W]

**3.2.2 Static characteristics** 

measured data.

0,04

0,05

**3.2.1 Operation principles** 

sources are required for both up- and down-link transmission. But, one approach is remodulation of down stream signal at ONU for upstream transmission.

The downstream signal modulates directly a laser diode, in upstream transmission the downstream signal re-modulates the optical carrier using a SOA with the SubCarrier Multiplexed (SCM) technique. No additional high cost devices are required such as external modulator and optical amplifier. A WDM-PON employing a SOA as a modulator has some advantages. The SOA gives additional gain for incident optical power to overcome device and transmission losses. Hence the SOA may be used as a modulator which accomplishes both modulation and amplification (Fig.6.). The SOA which operated in gain saturation region can reduce the intensity noise of optical signal. Due to the mixture of WDM and SCM techniques, a simple ONU which shares the same wavelength both up- and down-link transmission is possible [Kang].

Fig. 6. Hybrid WDM/SCM PON with multifunctional SOA
