The Original Fibre Optic Communication Spectrum Band: The O Band

The O band was the original or first official spectrum range in optical fibre optic communications. It includes the wavelengths ranging from 1260 nm to 1360 nm. The first fibre optic cable trials and deployments in the mid-70s were very short range ranging from a few metres to several kilometres. In 1977 the first phone voice traffic traversed a local fibre optic link in Long Beach, California. The Dorset, UK police deployed a fibre optic link in 1975, but unfortunately, I have been unable to ascertain the specific application. NORAD used fibre optic cables to connect computers at its underground Cheyenne Mountain headquarters in 1975. Note that this 1970 experiments used 850 nanometers as the semiconductor lasers were not capable of longer wavelengths. The O band became the de facto standard in the early 80s when the industry migrated from the early multimode fibres to single mode fibres and gallium arsenic enables lasers to to achieve the longer wavelengths of the O band. By 1988 commercial networking technology was able to use single mode fibre to generate a wavelength at 1300 nanometers that transmitted 1.7 Gbps with 50 kilometer spacings. 

The O band beat out competing spectrum ranges to emerge as the initial standard frequency range because it enjoyed the lowest optical attenuation at that time given manufacturing practices. Attenuation is how quickly a laser light pulse loses intensity as it traverses a glass fibre. As the signal weakens, so does digital legibility. The message becomes unreadable. Intensity loss reflects glass absorbing the laser emitted photons or  scattering them. O band also has another advantage, namely zero chromatic dispersion. This is a huge edge as chromatic dispersion means that different light frequencies travel at different speeds through solid materials. At transmission rates exceeding 10 gigabits per second a pulse of light consisting of a narrow band of frequencies smears. Instead of arriving at the intended time slot, some of the light arrives before and after the intended slot. So imagine three consecutive time slots whose correct digital representation is 0,1,0. The zeros are represented by an absence of light and the one by a pulse of light. In this example chromatic dispersion can fool optical receivers into thinking it has received three consecutive light pulses when in fact there was only one intended for the second slot. So the receiver sees 1,1,1 when the correct digital sequence is 0,1,0. The resulting errors garble information thereby raising the bit error rate and rendering the optical channel unusable. Chromatic dispersion results in a single pulse occupying several consecutive time slots. This is the crux of the problem. 

Later as manufacturing improved, the C band emerged as enjoying the lowest attenuation or power loss. A position it still holds today. However, chromatic dispersion in the C band requires the use of digital signal processing (DSP) in order to achieve acceptable bit error rates above 10 Gbps. DSP interprets the received light using correction factors designed to recover the original signal. Essentially the computer calculates the expected chromatic dispersion and corrects for it. 

In contrast, the O band suffers from no chromatic dispersion. DSP chips are expensive so O band transmission is cheaper and easier to deploy than its C band counterpart. Consequently, most data centre customers use the O band for both intra-building and campus connectivity. A campus is a collection of data centres in close proximity so that no optical amplification is necessary for fibre links between them. Typically campus buildings are connected using dark fibre cross connects. 

A new emerging O band use is the Last Mile where profitable mass deployment demands low cost electronics. The absence of any need for DSP chips has triggered growing use of the O band in passive optical networks that split light into frequency ranges using passive optical splitters in order to deliver service to residential or commercial buildings. Passive last mile optical networks are all about inexpensive network connectivity for mass markets so avoiding digital signal processors as well the associated power requirement is a huge advantage. 

The chart below outlines the potential optical transmission bands and their various attributes. O band and C band are today's optical work horses. Because the L band's attenuation is only slightly above the C band's and is compatible with existing Layer 1 network technology, carriers are tentatively starting to use it as well. Arelion is using the L band on US East Coast routes. The L band is also used on the PLCN subsea cable that links Taiwan and the Philippines to Los Angeles. Hong Kong was part of the original deployment, but that segment was decommissioned due to US government pressure.