Fibre Characterisation for Next Generation Optical Networks
The ongoing race to develop higher speed optical transmission systems and to exploit the latent bandwidth within an optical network is extending the boundaries of fibre optic networks. As a consequence, optical fibre plant is now often expected to support both Coarse Wavelength Division Multiplexing (CWDM) and Dense Wavelength Division Multiplexing (DWDM) transmission solutions.This is taking place not only in long haul transmission networks but also in metro applications. These new wavelength division transmission technologies exploit the latent capabilities of optical fibre as a communications medium however, they also place very demanding requirements on the optical properties and performance of singlemode fibre.
Telecommunication network operators and service providers are becoming aware of the fact that in order to take advantage of these Next Generation Optical transmission technologies, there is a need for a detailed analysis of their fibre cable plant to determine its ability to allow network migration to 10Gbps, 40Gbps or even 100 Gbps systems. In response FOSS has now developed a full suite of field based fibre characterisation services to allow a client to fully understand the capabilities of their fibre network plant to support these new network solutions.
Our Fibre Characterisation service secures the integrity of your underlying fibre network. With thorough capture and evaluation of its physical properties, such as optical attenuation, reflectance/scatter, and dispersion, you ensure every capital and operational decision for network growth is accurate and properly defined for future technologies.
FOSS Engineers will test your fibre and provide a comprehensive final report with detailed test results and recommendations, documenting; ILM, ORL, OTDR traces, CD, PMD and AP, for each fibre tested. In addition to providing valuable wavelength, equipment choice, and capacity threshold information, fibre characterisation will help you identify life expectancy and potential issues that could lead to service interruptions and loss of revenue.
Value Added Services
While performing fibre characterisation services, our Field Engineering staff are fully certified to perform detailed site surveys and/or site audits for CO equipment identification and placement. Capturing this critical data early in the project process provides added value by minimising redeployments and identifying potential network constraints. This combined effort contributes to both financial and time savings keeping your projects under budget and on schedule.
Detailed Fibre Characterisation Reports:
- Site Survey Data Capture
- Early Identification/Verification
- Professional Recommendations
- Full Certification
Fibre Characterisation Test Descriptions
Insertion Loss Measurement
Insertion Loss is the decrease in transmitted signal power, resulting from the insertion of any number of devices, along the fibre path. Contributors to Insertion Loss include, but are not limited to; connectors, splices, splitters, fibre type, bends, and the optical equipment itself. Insertion Loss is measured at the receiver and is relative to the signal power delivered from the insertion point. Insertion Loss should be as small as possible for maximum power transfer.
Optical Return Loss (ORL) Measurement
ORL is a measured ratio of optical power arriving at an interface, relative to the optical power which is being reflected back from that same interface. The higher the ORL value, the better. Back reflections can cause interference to the transmitted source resulting in higher bit error rates (BER), fluctuations in power and permanent damage to the light source.
Optical Time Domain Reflectometer (OTDR) Measurement
OTDR measures the loss and reflectance of optical signals created by splices, bends, fibre mismatches, and various connection points. . An OTDR test set injects a short, intense laser pulse into the optical fibre and measures the backscatter and reflection of light as a function of time. The reflected light is analysed to determine the location of any fibre optic anomalies. Figure 1 below shows a simplistic example of what an OTDR graph might look like.
Figure 1: Graph Showing loss / splice and reflectance.
Chromatic Dispersion Measurement (CD) in the C and L bands
Chromatic Dispersion is the temporal spreading of light pulses travelling at various speeds down a fibre optic cable. This variation in speed causes the wavelengths to broaden as they travel down the fibre until the pulses overlap and BER increases. This overlap of pulses can be compensated for by adding specifically designed dispersion compensating modules along the optical path. Fibre Characterisation test results are critical to identifying the proper location and amount of CD compensation to insert within the optical path.
Figure 2: Effects of Chromatic Dispersion.
Polarisation Mode Dispersion (PMD) Measurement
PMD is a form of dispersion where the different polarisations of light in a fibre arrive at different times. Normally, the different polarisations of light travel at the same speed but as the speed of the transmitted light increases over distance, random imperfections and asymmetries in the fibre increase the PMD effect. This in turn causes an increase of errors at the optical receiver. Temperature or physical changes in the fibre also contribute to the PMD effect. Compensating for PMD is difficult and expensive therefore, a ‘dispersion penalty’ is usually added to the link budget, in order to compensate for PMD.
Figure 3: Polarization Mode Dispersion Graph of Single Mode Fibre.
Effects of Polarisation Mode Dispersion (PMD)
In single mode fibre, light is guided through the whole core and in a part of the cladding (referring to mode field diameter), so that there is only a single propagation mode. However, due to bi-renfringence, the propagation mode is degenerated into two orthogonal modes, defining the two Principal States of Polarisation (PSPs). These two PSPs travel at different speeds. The arrival time difference at the output of the media (fibre) is called the Differential Group Delay (DGD).
Figure 4 : Pulse Broadening effect due to PMD in High Ditrate Digital Systems.
Attenuation Profile (AP)
The attenuation profile provides the total loss of the fibre cable as a function of wavelength over the wavelength range. Wavelength range can cover specific bands such as C&L bands for DWDM systems (1530-1625nm) as well as S band (1460-1530nm). For CWDM systems the full wavelength range of 1271 to 1611 can be specified for 8 and 16 channels systems. Data provided by the attenuation profile can identify fibre types and provide information on the maximum channel count.
Figure 5: Attenuation Profile of a fibre link.
FOSS certified Engineers will assess the health of your fibre optic network, and provide you with invaluable data to allow you to determine your immediate and future network upgrade path.