Fiber Optic technologies in broadcast sphere

By Pavel Platov

Review contents:

Fiber Optic technologies in broadcast sphere Profitt Company equipment for Fiber Optic links of video, audio and data transmission Axon equipment BKtel equipment for Fiber Optic networks Fiber Optics in TV and Radio broadcasting Kramer Electronics equipment for Fiber Optic lines of TV signal transmission Network Electronics equipment Snell & Wilcox Fiber Optic receivers and transmitters Equipment of Telecast Fiber Systems Company Optical node getting closer to the household Fiber Optic equipment from Vistek

Why Fiber Optic cables?

Companies involved into broadcasting are become using fiber optic cables as a base of their systems’ infrastructure. At least there where it was necessary to develop long distance links or where it was presumed the modernization and the move to the Hi Definition (HD) signals in a future. Optic Fiber cables characterize by very low losses and very high influence protection that’s why they allow transmitting HD signals with no gap fillers and amplifiers over the distances several thousands times more than the copper core cables do.

Fiber Optic cables types

Different types of Fiber Optic cables are produced and each of them is dedicated to exact usage. But all Fiber Optic cables can be divided to two main categories: multi wave (multi mode) and single wave (single mode). The last one sometimes called “mono wave” (Mono mode). What is a difference so?

Fiber Optic cables work according to full inner reflection principle and they consist of two parts: core and outside wrapping. In a process of going along inner core the light wave bounces to the boundary in between of a core and a wrapping and reflects from it. Wave’s ongoing itself is a series of “bounces” along the total length of a Fiber Optic cable.

Typical Fiber Optic cable’s design

Single mode (above) and multi mode cables

Multi mode cables in that or other degree suffer from losses caused by diffraction (dispersion). Dispersion appears in those cases when fall angle of a light wave in a fiber exceeds the critical angle necessary for the full inner reflection and it causes then to this wave loss.

Single wave fibers are used for transmission of much more uniform light as a single wave. It doesn’t suffer from dispersion effects than its signal can be transmitted over pretty longer distances and with much more transfer speed.

But single mode fibers have its weaknesses two which are big diameter and wide wave range. That’s why connection via multi mode cable is simpler than via single mode one. Besides, single mode cable usually costs more because it requires much more uniform light beam sources and the manufacturing of thinner cores is labor consuming.

In a multi mode optical cable many light waves go simultaneously and pretty large cable’s core handles them all. In a single mode optical cable a core is quite small and it holds the only wave.

Lasers of LEDs?

Two types of light sources are usually used: light emission diodes (LED) and laser diodes. Deep inspection of relative advantages of each source type is off this article theme. But it is necessary to note that LEDs lack operational speed required for high performance applications based on single mode cables. That’s why they are used with multi mode Fiber Optic cables over which a digital video signal with standard resolution and up to 270 Mbit/s rate transfers. Additionally they emit wider wave range and are non-useful for those applications in which waves combining or multiplexing occur.

Lased diodes are able to emit light waves of different lengths and differ with higher power. Modules of most manufacturers allow selecting output power of a laser diode for any particular system. And the ability to choose light wave lengths for each signal makes it possible to combine or multiplex several different signals transferred via single cable.

Typical LED

Generation of a light wave for transmission over Fiber Optic cable is the only half of a task. It is required also in some way to detect it in the other side of a system. Usually it performs by use of PIN photo diode. Although receiver can detect multi mode signal one have to look for the diode compliance with the type of signal transmitted because the connectors’ and ends’ design on the receiving photo diode must be optimized according to cable type.

To avoid photo diodes overload it is important to provide correct input power. Power fed to photo diode will be always lower than that had been generated by transmitter because even simplest Fiber Optic links have some losses.

Transmitters, receivers and transceivers

In first, Fiber Optic transmitters the electrical and electric-optical elements were separate modules. Modern transmitters have hybrid design. Laser and integral chips modulating light combined into the single compact module which allows reaching higher modulation frequencies and high reliability. Such module is a electrical-to-optical converter in which output light signal intensity is modulated by input digital electric signal.

Typical laser diode

Dichroic mirror operation scheme

At the beginning the solution was extremely simple. To the moment of the practical implementation of Fiber Optic transmission systems semiconductor lasers were created operating in three bands: 850, 1310 and 1550 nm. Since the spectral distance between spectral lines of these lasers is large than summing them and inputting their light into fiber do not require even high quality multiplexers and de-multiplexers. Usual dichroic mirrors were enough.

But such solution didn’t have sufficient practical mean. Fiber’s properties in various areas of a specter are too different. If light with 1550 nm wavelength in modern fibers can distributes up to 100 km distance than the light with 850 nm wavelength mostly total attenuates after 100 km.

Modern integral optics allows creation cheap and handy in use optical transmitting modules which combine in a single crystal the laser, the modulator and semiconductor amplifier. There are optical transmitting modules developed combining multi lasers, simultaneously generating signals with several wavelengths as well as multiplexer and semiconductor optical power amplifier.

Spectral multiplexing caused by the appearance of 1550 nm band lasers which have a gap over operational wavelength less than 1 nm enhanced dramatically. Usage of resonators with distributed feedback based on Bragg’s diffraction grids and external modulators has allowed creating lasers with the emission line width of 0.05 nm.

Sometimes a laser is followed by adjustable attenuator smoothly decreasing laser’s power. Laser’s signal attenuation degree is being chosen according to the first re-generator in a communication line. In a case when several transmitters with different wavelengths are used in parallel it requires appropriate attenuators to use for spectral power distribution flattening.

In DWDM systems most wide used are DFB lasers with Fabry-Perot resonators. In that the diffraction grid is made on a surface of active laser’s crystal part which provides precise selection of laser’s wavelength by means of optical feedback.

A sample of CDWM technology usage in broadcasting

The most important characteristics in optical receiver choosing are spectral sensitivity (current to signal’s optical power ratio A/W depending on wavelength), threshold sensitivity (input signal’s level at which it can not be detected due to photo receiver’s noise), spectral and electrical bandwidth, dynamic range, and noise level. Allowed value of each characteristic depends of its exact implementation. For instance, noise characteristics become more valuable when ahead of photo receiver an optical high power preamplifier is installed.

One more technological direction is linked with the success of DWDM. This is an All-Optical Networks. In such networks all the operations on multiplexing and de-multiplexing, input and output and cross-commutation (routing) of user information are performed with no signal conversion from optical form to electrical and vice versa.

The main advantages of DWDM technology:

• Further increase of a factor of an optical fiber’s frequency potential;
• Excellent scalability — increase of a total network speed by means of adding new spectral channels with no need of all main modules replacement;
• Economical effectiveness due to avoiding electrical re-generating in long distance networks;
• Independence from data transfer protocol — technological “transparency” allowing to transfer via optical link the traffic of any network type (for example it allows adding to SDI/HD-SDI TV signal the RS-422/232 or GPI control signals);
• Independence the spectral channels from each other;
• Compliance with the Ethernet, Gigabit Ethernet and 10 GE families of technologies;
• ITU-T level standardization.

The main disadvantage of DWDM technology is the complexity of equipment quality providing and than not so affordable prices. That’s why when the high density of an optical modulation is not required (less than 10 channels over single cable) the SWDM equipment is usually used as an alternative solution.