A proactive approach to fibre characterisation can cure a network of common ailments while raising profitability and Quality of Service.
By Steve Wolszczak, Fibre-optic Program Manager, Acterna
Currently, dense wave division multiplexing (DWDM) techniques and transmission rates above 2.5Gbit/s allow service providers to achieve geometrically increased bandwidth over existing fibre networks, without needing to install new cable runs. As DWDM systems use more of the available spectrum, the ability of the system to perform is becoming increasingly dependent on the current health of the fibre plant itself. Thorough evaluation of the fibre is therefore necessary.
The evaluation of installed fibres against a specified standard, such as those established by the ITU, is known as fibre characterisation. This allows service providers to ensure operability of their spans at higher transmission rates and across more channels by providing detailed information about the fibre. This information allows the provider to maximise total network capacity and target capital investments appropriately.
Fibre characterisation is not new for operators of long-haul networks. However, with the increased deployment of high-bit-rate single- and multi-wavelength systems, operators of short- and medium-haul spans are finding that they, too, must quickly get up to speed on fibre characterisation.
Four characteristics that should be considered include chromatic dispersion (CD), polarisation mode dispersion (PMD), optical return loss (ORL) in the C and L band, and spectral attenuation.
Chromatic dispersion describes one of the basic characteristics of fibre and is caused by variation of the fibre index with wavelength. CD generates a delay between wavelengths and broadens the transmission pulse as it travels along the fibre. This, in turn, provokes distortion and increases the bit-error-rate (BER) ratio of the optical system.
As a result, the distance for higher bit rates can become extremely limited. For example, when the transmission rate is increased from 2.5 to 10Gbit/s, the CD-imposed distance limit might be reduced from 640km to less than 100km. And operating at 40Gbit/s, a fibre with similar CD characteristics could then operate only to a maximum of 5km.
CD testing is recommended for new network installations and upgrades operating at or above 10Gbit/s. CD is measured by looking at the fibre link group-delay values as a function of wavelength. From this information, CD can be calculated and the appropriate dispersion compensation modules can be designed into the network.
Polarisation Mode Dispersion is caused by birefringence in a fibre that is not perfectly circular or has suffered external stresses. PMD is caused by a difference between the indexes of the two orthogonal transmission modes in the fibre. This creates a time delay that causes the transmission pulse to broaden when travelling along the fibre. Unlike CD, currently no PMD compensators are commercially available. The results of PMD testing allow the provider to determine the maximum bit rate and channel loading possible for a given span.
Optical Return Loss occurs as a signal loses power after transmission. The extent of signal loss can be determined by the strength of the returned signal compared to the original transmission. Current transmitter designs require low reflections to perform optimally. For example, a 565Mbit/s system might tolerate an ORL of -21dB and discrete reflections of -27 dB; higher-bit-rate systems demand ORLs below -28dB or less and discrete reflections no higher than -45dB in some cases.
To minimise ORL many providers are replacing existing hardware with low reflection connectors such as so-called "ultra" and/or angled PC connectors. Providers are also replacing existing mid-span splices with fusion splices to eliminate that source of reflection.
Spectral attenuation is the measurement of the loss of the optical signal, with respect to wavelength. Variations in attenuation at different wavelengths of fibre are caused by both the fibre design and fibre installation, primarily bending.
In a DWDM network, it is important to have the same received power level for all channels. As the attenuation of the fibre is not flat over the wavelength spectrum, it is important to measure the spectral attenuation profile to equalise the received power levels of the channels.
Until now, many operators have chosen to repair problems when they arise, rather than perform full characterisation of these links. But simply installing the fibre and turning up services is no longer adequate for today's technologies.
Fortunately, service providers can now take steps to limit or even prevent these problems from occurring by performing a thorough evaluation of fibre spans prior to lighting up DWDM or high-speed services. Fibre characterisation will increase Quality of Service, which leads to satisfied customers, improved network capacity, and the comfort of knowing that the network is capable of delivering on its promise.