ABSTRACT
The possibilities of wireless transmission and the invention of optical fibres have brought up a great revolution in the field of communication. The Optical fibre technology has paved the way for many noticeable developments and spared the necessity to solve the increasing traffic problems.
This paper focuses on "Photonics" which is becoming a powerful tool to solve the drawbacks present in the existing systems. This technology, with the photonic switching technique as its major highlight, satisfies the needs for maximum data transmission with higher speed and this has led the way for miniaturization of existing switching systems. Photonic switching finds applications in both advanced WDM and ATM. The various steps required for the efficient functioning of the high – capacity switches are elucidated and discussed. The trends in optical communication networks such as broadband digital networks and Information Superhighway are also presented in this paper.
The Optical free-space communication systems show significant advantages in comparison to alternate microwave systems. Photonics Technology is becoming a promising factor in space communication systems too, to reduce its growing complexity due to the increase in the transmission requirements.
INTRODUCTION:
The word Communication means the "exchange of information". Ever since the past, ‘Communication’ has become an essential and unavoidable thing in everyone’s part of life. The different basic types of information to be exchanged are voice, video, and data. In the not-too-distant past, there were separate branches of telecommunications. The telephone and the radio provided voice delivery, TV delivered the image, and computers process the data. Today these systems have coalesced. Today, the communication network has widened its base covering even remotest of areas.
The various communication systems use different modus of operandi. Fibre optic communication systems use light – a carrier with the highest frequency among all the practical signals. This is why fibre optic communication systems have the highest information carrying capacity and this is what makes these systems the linchpin of modern telecommunications. The fundamental goal of designing networks is to provide bandwidth at 'low' cost to the end user. The information carrying capacity of a telecommunication system is proportional to its bandwidth.
Since the optical networks came into vogue, the optical fibre communication technologies have been undergoing a great progress toward higher capacity and longer repeated spans. In order to satisfy the requirements for high-speed data transmission with higher capacity and longer repeatability, all-optical networks are used. Photonics is the technology used in all-optical networks which is capable of handling data in the order of Tbit/s. the application of photonics range from trunk networks to LANs and subscribes loops.
PHOTONICS TECHNOLOGY:
Photonics is the technology implied in all-optical networks in order to accommodate the features that are not available in the existing networks. The key techniques used in all-optical networks (all-photonics networks) are,
The main drawback present in the currently existing system is due to the switches, which are exclusively based on electronic designs supporting a maximum bit rate of about 10Gbit/s. The fibre-optics offer the possibility to transmit bit rates upto some Tbit/s which open the door towards ultrafast information superhighways. Hence, the use of electronics switches has become a bottleneck for the advancement in fibre optics through photonics.
In order to overcome the drawback in the currently existing system, the photonics technology may be used by integrating fibre-based transmission, photonic switching and optical broadband access with all-optical networks. Thus the photonics technology, as a first step in an evolution process of its realization, will primarily be employed to support and upgrade existing network and network components. In this step, photonics technology will be used to,
Optical interconnects:
Photonics technology has started to enter the electronic domain of present switching systems, which are exclusively based on electronic processing by using optical interconnects. Because of the greater increase in the speed of switching on using fibre-based interconnections, optical interconnects can be regarded as a pilot-technique bringing photonics technology into the crossconnect offices. (Fig. Ph 1 & Ph 2)
Since the electronic components (due to its high functionality) cannot be scratched off completely the throughput of switching office, which controls both signal traffic maintenance and dropping, can be upgraded by function sharing. In the process of function, sharing routing is done by photonics and switching & dropping is done by electronics.
All-optical networking can pave the way for a simpler, less hierarchical network. It is conceivable that by using the low-loss window of optical fibre (1.2m m-1.6m m length) we could construct multiple access networks carrying a total traffic of perhaps 50Tbit/s roughly four orders of magnitude greater than the traffic flow through a high-capacity electronic switch. Though we are far from realizing this high potential network, the more sophisticated functions (routing and switching) can be certainly achieved by using photonics.
Photonic switching:
A multiplicity of rates and burstiness of traffic sources lead naturally to systems based on the fast packet switching concept since the transmission technology in broadband network is based on optical technology and the enormous bandwidth (257bit/s) offered by the optical medium, the photonics and fast packet switching techniques are required to support a high throughput of data in the range of Tbit/s. Photonics switching devices and systems are required for both advanced WDM and ATM applications as well as these techniques play an important role in high capacity, communication network.
In order to use photonics switching it is required to bring the fibre from point-to-point link into networks. As a first step, photonics will be used to perform the routing function of a crossconnect office which improves its power and throughput with an end-to-end optically routed connection (eg., by means of WDM), electronic switching equipment is by past offering the advantage of bit rate transparency and capacity enhancement. In this step, switching function will still be performed by electronics. In the second step, electronics in the switching part of a cross connect office can be replaced by optics also, which finally results in an all-optical crossconnects (OXC). Photonics crossconnects must be able to spatially route individual wavelength channels on to the desired output fibre. Thus, we have to distinguish space-division switches composed of a wavelength independent cross point with wavelength multiplexers and demultiplexers, wavelength-selective cross points and full non-blocking crossconnects with wavelength-converting elements to avoid potential collisions of two identical wavelengths on the same output fibre. An apt example for OXC is an all-photonics 8X8 crossconnect, shown in 
Fig Ph 3 , which looks after the fibre-to-fibre switching and wavelengths switching.
Thus the wavelengths (time-slots) are routed and switched separately in an all-optical crossconnect (OXC) which is logically same as traditional crossconnects. Thus on employing fibre-based transmission and all-photonics crossconnect, the network becomes all-optical except the subscriber loop where optics normally stops at the last mile to the home.
Photonic switching in Wavelength Division Multiplexing (WDM):
Wavelength Division Multiplexing (WDM) will be employed to upgrade the transport network by realizing an optical overlay network. The use of WDM into networks offers advantages in terms of flexibility, scalability, modularity and transparency. Transparency is in particular the enabling factor towards all-optical networks. An all-optical network employing WDM consist of two important network elements, the OXC and the optical add-drop multiplexer (OADM). The basic function of an OXC is to crossconnect wavelength channels separately between a number of incoming and outgoing fibre lines. An OADM has more specialized application and can be much reduced in size compared to an OXC. An OADM will preferably be used in optical WDM rings or buses.
TRENDS IN OPTICAL COMMUNICATION NETWORKS
Broadband digital networks:
The current telephony networks with a bit rates of 64kBit/s are globally switchable. Broadband is a service or system requiring transmission channels capable of supporting rates greater than the primary rate, which is at present 2.048Mbit/s. In order to meet the requirements to solve the future traffic problem, the broadband system must be capable of supporting rates equal to the STM – 1 rate (155.52 Mbit/s). Therefore, if we increase the line rate to 155.52Mbit/s and keep everything else the same, then we need an aggregate throughput capability of 1.5 Tbit/s! Thus, the broadening of the bandwidth of the telecommunication network brings with it the need for faster, higher capacity switching system. Thus, an important ingredient of the future broadband networks will be a very high-capacity switch for the public network. Currently, switches are exclusively based on electronic designs, but the advent of multi – Gbit/s optic transmission systems may cause this to become network bottlenecks. Declining transmission costs and powerful database technologies will continue to push the trend toward 
fewer numbers of larger switches and increased utilization of network reconfiguration at the path and facilities levels. A typical broadband configuration is shown in Fig Ph 4.
The techniques required for a broadband network in order to support a line bit rates of 155.52 Mbit/s or more are
· Optical overlay network
· Wavelength division multiplexing (WDM)
· Fibre-in-the-loop (FITL)
· Photonics asynchronous transfer mode and
· Passive optical network (PON)
Information superhighway:
The information superhighway is a network, which supports high bit rate and all types of services. A network based on information superhighways should the following important network feature characterize atleast:
As a high-speed backbone network, an information super highway connects other networks through junctions, which are known as gateways. The preferred topology of the information superhighway is a ring. As a broadband pipe, the information superhighway is an ideal medium to transport interactive multimedia signals. These require low-loss transmission links, which are able to carry ultrahigh bit rates. Since optical fibers are able to operate up to some Tbit/s, fibre optic is a key technology of information superhighways.
OPTICAL SPACE COMMUNICATION:
Optical free-space communication systems for inter-satellite links and deep space missions show significant advantages in comparison to alternate microwave systems. The complexity of space communication system is growing rapidly with the increase in the transmission requirements. Most of these systems are exclusively based on microwave links. The optical communication systems become increasingly attractive as the interest in high capacity and long-distance space links grows. Advances in laser communication system architecture and optical component technology make such high capacity links feasible.
The potential use of optical free-space communication systems can be divided into three main applications as shown in Fig Ph 6
The promise of an optical communication system is mainly based on the strongly improved gain and reduction in the beam width of the antenna. Most of the differences
between optical and microwave space links are due to the difference in wavelength. The much shorter optical wavelength results in a very small beam width and a very small spot size.
As shown in Fig Ph 7, spot size of an optical beam transmitted from planet Mars is only about 10% of the earth’s diameter, whereas in case of microwave beam it is about 100 times. The small spot size of an optical beam results in substantial increase in the received optical power and therefore improvement in the receiver performance. The use of optical technology in space communication offers a tremendous reduction in antenna diameter and weight as compared to microwave technology. This is quite promising for satellite communication particularly when the data rate is high.
LINEAMENTS OF PHOTONICS:
L More data handling capacity in the order of Tbit/s
L High – speed optical pulse generation and modulation
L Soliton transmission
L Ultrafast Information Superhighway
L Miniaturization of Switching systems
L Multiple access networks with more traffic handling capacity
L Optical Space Communication systems
CONCLUSION:
Although, Photonics is under research, its features have sown seeds for its realization. The more sophisticated functions can be achieved by upgrading the existing networks and network components using photonics. Thus, the uses of photonic technology in the present day communication network throw a promising light in improving efficiency and capability of the systems.