**2.1 Point-to-point and star topology**

86 Selected Topics on Optical Amplifiers in Present Scenario

signal loss (0.3 dB/km for 1550 nm, and 0.5 dB/km for 1310 nm wavelengths) to distribute the RF signals to the RAUs. This way the RAUs are simplified significantly, as they only need to perform optoelectronic conversion and amplification functions. For broadband services the frequencies are in the millimetre wave range (like 60 GHz fibre radio link). Such a concept is based upon an optical link between the central station and the RAU in a picocellular structure [Ng'oma]. Some of the advantages and benefits of the RoF technology compared with electronic signal distribution are the following: low attenuation loss, large bandwidth, immunity to radio frequency interference, easy installation and maintenance, multi-operator and multi-service operation, and dynamic resource allocation. These benefits can translate into major system installation and operational savings, especially in widecoverage broadband wireless communication systems, where a high density of RAUs is

The SOA is a potential candidate for an electro-optic transceiver (transmitter and receiver) in a RoF network. The SOA operates as a modulator to add a new channel, as a detector to drop the needed channel and as an in-line amplifier to amplify the other channels, simultaneously. It realizes a compact, small size and cost-effective radio repeater for signal distribution. Fig.1. shows a simplified system setup representing the SOA based RAU. The output power of the laser source is modulated with the 1st information channel applying a SOA-modulator. The SOA transceiver detects the 1st channel and adds the 2nd channel in the 1st stage. In the 2nd stage the SOA transceiver adds the 3rd channel, but it can detect the 1st or 2nd channel selecting by tuneable filter. Finally the receiver side every information channels can be extracted. The information channels and the bias of the SOA are separated by bias tee circuit. The separation of add and drop channels can be achieved in the electrical regime by an electrical branching filter (1st stage) or an electrical circulator with bandpass filter (2nd stage). In the first case the realisation of a reconfigurable add/drop multiplexer is difficult, in the second case an electrical filter is needed also. In the drop branch a high power RF amplifier provides the suitable power at the antenna output, and a low noise amplifier is

Other advanced devices like electro-absorption transceiver for signal remodulation or polarization rotation remodulator have been designed though only modulation is executed, whilst with semiconductor amplifiers it's possible to perform amplification, modulation and detection with the same optical device. The signal distribution of RoF systems can be

**1st Stage 2nd Stage** 

or 2nd **Branching**

**Low Noise Amplifier**

**SOA**

3rd ch. 1st

**Power Amplifier** **Detector** 

channel Ibias

1 st & 2nd & 3rd

realised by point-to-point, point-to-multipoint, bus, ring and open loop topologies.

Ibias

ch.

**Power Amplifier**

2nd ch. 1st

Fig. 1. Simplified system setup with SOA transceivers

**Low Noise Amplifier**

**filter**

necessary.

necessary in the add branch.

m

**SOA** 

**Source SOA**

1st channel

**Laser** Popt

The Point-to-point topology is the simplest method; it is a permanent link between two endpoints. In a star topology, each RAU is connected to a central unit with a point-to-point connection. The star topology is considered the easiest topology to design and implement. Adding additional nodes is simple, but the central unit represents a single point of failure. Fig. 2. represents the star topology of RoF system. Bi-directional data transmission can be achieved over duplex optical fibre. On the other hand the uplink and downlink can be separated by optical circulator. Anyhow, the RAU consist optical source for electrical-tooptical conversion.

Fig. 2. Star topology, RoF system with traditional Electrical-to-Optical (E/O) and Optical-to-Electrical (O/E) converters

For more simplified RAU structure, the laser source of uplink optical carrier can be moved to the central unit and the continuous wave (CW) optical signal is transmitted over the optical fibre to the RAU. So, there are no injection of light is added at the RAU for the uplink transmission. The optical carrier is attenuated; hence some amplification should be required, in order to arrive to the maximum number of users and larger distances. Bidirectional amplification is possible by EDFAs (erbium doped fiber amplifiers) but the price is high and an additional high pump power is required. In this way, SOAs are suitable to be positioned at the simplified RAU. They not only carry out the modulation and detection but also offer an additional optical gain.

Fig. 3. Point-to-point topology RoF system applying SOA transceiver and centralised laser sources

Multi-Functional SOAs in Microwave Photonic Systems 89

uplink

downlink

downlink

**OADM** 

**SOA Transceiver** 

uplink

**OADM** 

**OADM** 

**SOA Transceiver** 

downlink uplink

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

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

**SOA Transceiver** 

λ1 mod

**Laser Source1**

CW

**Laser Source n**

mm wave oscillator

Data **Detector**

Data input **Modulator**

mm wave oscillator

**LPF** 

output

powerful.

λ2 mod λn-1 mod

λn CW λn+1 CW

> λn mod λn+1 mod

**MUX** 

uplink downlink

**DEMUX**

Fig. 5. WDM - RoF open loop concept with SOA transceivers

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