**3.2.1 Operation principles**

90 Selected Topics on Optical Amplifiers in Present Scenario

sources are required for both up- and down-link transmission. But, one approach is re-

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

ONU

ONU

RSOA

BPF

Ibias

ONU

upstream baseband

downstream baseband

A W G

frequency

subcarrier *upstream*

LPF PD PD

The investigation of potential multifunctional devices is very important task for the realisation study of the proposed system concepts. Multifunctional facilities of SOAs are

There are several applications of SOAs in today and the future analogue and digital optical networks. The optical losses are compensated by optical amplifiers (OAs), it is well used in loss limited systems as post, in-line or pre-amplifier. The SOA is a semiconductor based, small size, compact, low cost, current driven device, which amplifies the incoming optical signal directly in the optical regime, without any optical/electrical conversion. Moreover, the semiconductor technology offers a wide flexibility in the choice of the gain peak wavelength by just appropriately choosing the material composition of the active layer. Another key advantage is that these devices can be integrated with other active or passive optical components to generate more complex functionalities. Finally, they are potentially

The detection means optical to electrical conversion. The optical detector is characterized by the responsivity, the sensitivity and frequency response. The SOA-detector responsivity can

presented by combining optical amplification with modulation and photo-detection.

baseband *downstream*

Optical power

modulation of down stream signal at ONU for upstream transmission.

TRX

TRX

TRX

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

cheap, thanks to the mature technology basis.

A W G

transmission is possible [Kang].

laser

**3. Multi-functionality** 

**3.1 Amplification** 

**3.2 Detection function** 

upstream baseband

downstream baseband

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

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