Fiber Optic Communications Systems
- Friday, 01 February 2008
Communications and, more recently telecommunications, are needs deeply engrained in human history. These needs have significantly evolved over time enabling today’s content-rich (text, music, images and video, etc), real-time and multi-location exchanges through electrical, optical or, more broadly, electromagnetic signals conveyed by different media. Among the more versatile is optical fiber.
Why is that hair-thin silica pipe at the core of an ongoing telecommunication and media revolution? Why is it now so widely deployed, from inside houses to super-capacity backbone pipes, covering an impressive array of applications ranging from biomedical, telemedicine, and industrial (automation, sensors, etc), to space, automotive and, of course, telecommunications fields (voice, data, television, music, video, etc)? The answer is simple — because it has versatility, immunity to external parasitical radiation, and the capacity to transmit tremendous flows of data.
An optical telecommunication system basically consists of a coding element that transforms content into an electrical signal, which is then converted into an optical signal through a modulator that hashes the light coming from a continuous laser source. This light then follows its optical path down the fibers in optical cables that are deployed in a variety of environments (buried, ducts, sewers, aerial, etc). After traveling through the network to reach its destination, the optical signal is detected, converted back to an electrical signal, processed and, in the end, the content is recovered and can be displayed. All of this happens in a fraction of a second.
There are different types of fiber depending on the part of the network where it is deployed, whether in the core or in the access network, or in the campus or local area network (single mode fibers, bend insensitive fibers, multi-mode fibers of different class depending on the capacity-reach to be targeted).
There are also different optical cable constructions and deployment methods for different environments. Optical fibers are long cylindrical strands (125μm diameter) of silica glass. They are obtained by melting large silica rods, or preforms, made of several concentric lathes providing light guiding capabilities, in huge furnaces on top of tall drawing towers. They essentially consist of a central core surrounded by a cladding that has a smaller refractive index, thus allowing internal reflection throughout the fiber length. Typical values for core-cladding index differences range from ≈5 to 30×10-3 with core diameters from ≈5 to 10μm. Key characteristics of fibers depend upon the refractive index profile.
Single-mode light guiding is a prerequisite for long-distance transmissions (from a few kilometers to thousands of kilometers) as it is not possible to compensate for distortions caused by the presence of different light modes inside the fiber. Cut-off wavelength, λc, is a common characteristic of single-mode fibers; it defines the range of wavelength values for which fiber can operate in the single-mode regime (above λc). Typical transmission fibers guarantee a single-mode regime above 1300nm or 1450nm, enabling transmission inside the suited standardized ITU transmission windows (Figure 1).
Loss, which will dictate the pace for regular re-amplification of the optical signals, is another key parameter for line fibers. It is mainly controlled by the characteristics of the silica. At 1550nm, where most systems operate, typical values are ≈ 0.19dB/km (i.e. the signal power is divided by 80 every 100km), close to the fundamental limit for silica. Loss values ≈ 0.17dB/km can be achieved using pure-silica-core structures.
Bends can constitute another source of loss for optical signals guided inside a fiber. The smaller the bend radius, the higher the loss (light leakage at the bend). Fiber can also exhibit micro-bend loss once the surrounding cable construction is done. Recently fibers exhibiting reduced bending sensitivity compared to standard Single-Mode Fibers (SMFs, described in the ITU-T G.652 recommendation), have been introduced to better account for the specific needs of access networks. They are now covered in a newly-released ITU-T G.657 recommendation. To meet both recommendations (G.652.D & G.657.B), an interesting solution consists of including a depressed layer (or “trench”) in the cladding to improve mode confinement without sacrificing other characteristics. This type of structure also offers the advantage of being fully compatible with mature manufacturing technologies. Introducing holes in the cladding, which have a similar impact to that of a trench, is an interesting alternative. Issues involving splice-ability, connectorization, or strength, however, need further investigation.