Fibre Optics is often perceived as a futuristic technology within the Indian Cable TV industry. As is the case with most futuristic technologies, factual technical details are hard to come by. The CATV operator then usually assumes either, that Fibre Optics is a panacea for all his problems or that Fibre Optics is too expensive for him to implement. Often the CATV operator assumes both the above. As is frequently the case, the truth is somewhere in-between. To fully understand and appreciate the capabilities and limitations of a fibre optic system, lets take a close look at the basics. A copper (coaxial conductor ) based CATV system utilises aRF carrier which is Amplitude Modulated ( AM) by the program signal. This modulated carrier is then carried on a cable for distribution. The signal is fed to different points using Tap-offsand Splitters which divide the signal as required. The signal is then fed into a TV set whichprovides the final picture.
The basic Fibre Optic system is quite similar to a copper based system, but for a few importantdifferences. Before we get down to details, it is relevant to note that not all Fibre Optic (F.O. systems use amplitude modulation. Fibre Optic systems used in Communications such as Telephony, utilise digital transmission and are quite different from the CATV Fibre Optic transmission systems. Contrary to the popular perception, it is not possible to use the digital F.O. systems employed for telephony, for CATV signals. The range of equipment and signal processing are totally different. These series of articles will be restricted only to A.M. Fibre Optic systems used for CATV signal distribution.
In a CATV F.O. system, the carrier not a R.F. signal, but a light beam. Just like an RF signal, light is also part of the electromagnetic spectrum, only of a much higher frequency. Since the frequency is so much higher, (Typically 200,000 GHz.) a single carrier can be used to modulate 80 or more television signals simultaneously ! This is one of the main advantages of a Fibre Optic System.... a very large bandwidth. The light beam, needs to be of a single frequency (or colour), i.e. “ Monochromatic “ and all the light should be in-phase, i.e. “Coherent ". These requirements are easily met by a LASER. Hence, just as an Oscillator is used as a source of a carrier in a copper system, a laser is used as the source for the light beam.
The brightness (amplitude or intensity) of the Laser beam is varied by the incoming signal simply by changing the current fed to the laser. This produces an Amplitude Modulated light beam, which is Coherent and monochromatic. This single carrier usually carries 70 to 80channels of TV programs.

In the early days of Fibre Optics, a LASER used to typically be a large unwieldy piece of equipment, that required an entire room to house it. This image of the laser, was further reinforced by science fiction stories of mad scientists in the labs, toiling over giant Lasers capable of vaporising air planes in mid-air ! While there do exist today, Lasers that are capable of burning a hole through a thick slab of steel, most commercial lasers are mundane pieces of equipment, called Laser Diodes. Every CD player (Video, Audio or computer ) uses a laser diode to read the information from the disc. This laser diode is the size of a regular LED and is also fabricated using similar production facilities. These laser diodes cost approximately US $100 (Rs. 4,200) each. Of course, a range of products are available, with different intensitiesand wavelengths. CATV F.O. systems utilise lasers emitting light at 1310 or 1550 nanometers.
This wavelength is in the near infra red region, not visible to the human eye. The light from the laser is quite powerful, particularly since it is focused into a narrow beam. This beam, though invisible, can injure the retina if the light is directed, even accidentally, to the eye. For safety reasons, most Laser equipment carry statutory warning near active lasers. Several CATV transmitters even have a provision to automatically reduce the laser intensity if the output port is left open, or a fault develops in the distribution system. A Fibre Optic Transmitter typically costs approximately Rs. 4 to 5 Lakhs each. One of the most important factors that influences price of the transmitter, is the output power of the laser. A more powerful laser permits longer runs of cable from the F.O. Transmitter to the Receiver. The lasers initially developed had a life of about 10,000 hours. The more powerful lasers had an even shorter life. The technology has now advanced significantly, and now Scientific Atlanta claim a M.T.B.F. (Mean Time Before Failure) of 100,000 hours, fortheir Fibre Optic transmitters. This is equivalent to over 10 years of continuous use.

For the light to be injected into the fibre efficiently, it must enter the fibre at an angle greater than the "Critical Angle ". While it is not necessary to dwell deep into the Physics of the process, it is important to bear in mind that a Fibre Optic system need to confirm to some very tightly controlled, critical parameters, both during installation and operation. The connectors in a F.O. system therefore play a very critical role, and need to be fitted with the utmost care.
The Fibre Optic Cable Basically consists of an optically transparent core. This core can be made of either Glass or plastic. The plastic core has a much higher loss and is not used in large, high bandwidth systems, such as those required for CATV applications. CATV Fibre Optic cable consists of a Silica (Glass) core of a very fine diameter - about 125 microns (1000 microns = 1 millimetre). Physically this means that each core is about the thickness of a human hair. Each glass core is then covered with a protective sheath or “Cladding ". Several such cladded cores can be packed into a small sized cable. Manufactures, such as Commscope usually provide a minimum of 4 cores and 100 or even more cores per cable. These cores are the enclosed in a mechanically robust jacket. Fibre Optic cables are also classified as Single Mode & Multi Mode. CATV F.O. systems utilise only Single Mode, glass fibre.

Over the years, extensive research has been done for the glass used to manufacture the glass fibre. During research, it was observed, that though there was a reasonably high transmission loss for light traveling in glass, if the glass was doped with certain elements, the glass transmitted particular wavelengths of light with almost no loss at all. Today, practical F.O. systems offer transmission loss of only 0.5dB per Km ! Of this, the glass fibre contributes about 0.2 dB and other losses such as joints and connectors, and other stay losses account for the balance.
In coaxial CATV systems, the carriers are referred to by their frequencies. However in Optical systems the frequencies are very large, and it is more convenient to refer to these by their wavelengths,usually in nano meters. ( 1 nano meter is 1 millionth of a millimeter.) As an example, the frequencies used for F.O.transmissions are approx. 200,000 GHz .It is simpler to refer to it as 1300nm. The particular wavelengths at which there is a minimal loss, are referred to as “Transmission Windows ". The loss/ attenuation falls to a verylow value, around 2 wavelengths: 1310 nm and 1550nm. CATV fibre optic systems all operate only at these wavelengths to minimise transmission losses.
Most optical transmitters available today, operate at 1310 nm and provide an optical (light) output ranging from 6 dBm (4 mW) to 12 dBm (16 mW). The cost of the optical transmitter sharply increases as its light output capability increases. As an example, a 6 dBm optical transmitter operating at1310 nm would cost approximately Rs.3 lakhs (all prices indicated in this article are approximate and include prevailing customs duty and freight). In comparison a 12 dBm optical transmitter operating at the same wave length would cost approximately Rs.7 lakhs.

Optical Transmitters are also available that operate at 1550 nm. This is a relatively new technology that aims to operate with the absolute minimum loss possible on the cable. However, like all new technologies it is very expensive. A 1550 nm Optical Transmitter with an output of 6 dBm would cost almost Rs.30 to 40 lakhs. From the above it is apparent that Optical output is very very expensive and the Fiber Optic System designer must buy a transmitter that exactly meet his need without too much of extra optical power output (technically referred to as Optical Margin), or else it would almost double his cost of equipment ! All along, in the design of Fiber Optics systems, extreme care and precautions are taken to minimise the optical loss. Current state-of-the-art systems utilise 1550 nm signals. These signals have a slightly lower loss on the cable. Contrary to what was stated in Part I of this article (which was actually written and published almost 3 years ago in this magazine), Direct Optical Amplifiers are available for 1550 nm systems. These Amplifiers accept an optical input and provide an optical output without going through an intermediate RF conversion. These Optical Amplifiers utilize Erbium technology and are popularly referred to as ED-FA Amplifiers.
ED-FA Amplifiers accept an input of approximately 6 dBm and provide a total optical output of approximately 22 dBm. This very high optical output of 25 dBm is often split and distributed over 4 outputs each of approximately 16 dBm. Due to the much higher optical output power from EDFA Amplifiers, a 1550 nm Fiber Optic System typically utilises a STAR based distribution topology or layout. Compared to this, the 1310 nm Fiber Optic System typically utilise a tree and branch distribution structure.
One must keep in mind that all RF inputs for distribution over a CATV network are electrical signals. Hence the Headend always puts out an electrical signal. Further the customers TV set also requires an electrical signal. This clearly indicates that both the input and output must ultimately be electrical signals. The electrical signals need copper coaxial cable for their distribution. Therefore all Fiber Optic CATV Networks must also deploy copper coax for distribution to the customers TV set, that is they must be a mix of both Fiber and Copper. These are termed as Hybrid (Mixed) Systems. Since the Fiber Distribution System utilises electronics only at the sending and receiving end, distortion can only be generated either by the Transmitter or the Receiver. Fiber Optic.
Distribution Does Substantially Lower The Distortion Between the Headend and any customer, there would be only 1 Fiber Optic Transmitter and Receiver, the distortion would essentially be from these 2 devices only for the Fiber System. The final coaxial distribution would of course add its own distortion but that too would be low since typically not more than 2 to 3 Amplifiers would be in the path between the Fiber Optic Receiver and the customer. Thus, Fiber Optic distribution does substantially lower the distortion.

The noise contribution from a cable Amplifier increases by just 3 dB for doubling the number of Amplifiers. Hence only 3 dB more noise is generated by a cascade of 64 Amplifiers compared to a cascade of 32 Amplifiers. In contrast, the noise at the Fiber Optic Receiver increases rapidly if the optical signal falls appreciably below O dBm. Every effort should be made to ensure that the light received at the optical receiver is approximately O dBm. If this does not happen due to various factors such as poor quality connector, poor quality of repair splice or degradation of the laser over a period of time, the noise from the Fiber Optic based system may actually exceed that of a copper system. It has been the intention of this article to provide the readers a quick overview of key considerations in Fiber Optic System design as well as to provide a simple, basic working knowledge of how a Fiber Optic CATV Distribution Network is designed. Both Fiber Optic and Coax systems have similar loss calculations. Only, in the case a Fiber Optic System, there is a total " Loss Budget" or " Optical Budget" that needs to be calculated. This " Excess " optical power can be distributed as a loss over the cable, splicing, Optical Splitters, etc. Further, it is not possible to regenerate or amplify the signal in a Fiber Optic Amplifier System (except with Esoteric Optical Amplifiers which cost Rs.30 to 40 lakhs !). It is hoped that this article would help remove readers fears that comprehending a Fiber Optic system is beyond their reach. We welcome any queries that readers may have on the subject.
New technologies often provide unique and even sometimes elegant solutions. However, more often than not, these solutions come with a hefty price tag attached to them. Five years ago, fibre optics was a typical case. It offered distortion free CATV distribution but at a hefty price tag that could not be justified in the Indian context.
However, over the past 2 years, the situation has changed radically. 4 core fibre optic cable today cost less than half the price of quality RG 6 drop cable! Also as seen in the fibre optic survey in ourJune 2004 issue, an 860 MHz fibre optic node can be obtained at a lower price than an 860 MHz amplifier !
Clearly, fibre optic solutions are now no more expensive, or may be even cheaper than RF coaxial CATV distribution. Several Indian CATV networks, particularly those of LMOs have grown on a piece meal basis over the past several years. Often, sections of the LMO's network have utilised poor quality coaxial cables. The quality of workmanship also sometimes leaves a lot to be desired. In some cases it is possible to "clean up" the signal path by changing connectors as well as tapoffs and splitters and using good workmanship.

However, where poor quality coaxial cable has been deployed, particularly if the cable provides inadequate shielding, the only solution may be to change the entire coaxial cable. This solution is extremely expensive. Infact, a 4 core fibre optic cable cost less than Rs 10 per meter compared to over Rs 50 per meter for quality trunk cable.
Fibre optic cable also provides extremely low attenuation. As a result, a fibre optic CATV distribution system does not utilise any amplifiers at regular intervals. This eliminates additional distortion and also makes the system more reliable and immune to power surges and power failures. Clearly, where the LMO plans a distribution system rebuild, fibre optics provides not only a technically superior but possibly a cheaper solution as well.