Fiber characterization

When it comes to high-speed data transmission, anywhere from 10G to 800G, thorough and complete assessment and characterization of fiber quality becomes imperative. Beyond ensuring fundamental parameters like insertion loss (IL) and optical return loss (ORL) are within acceptable limits, it is also vital to qualify the fiber link to ensure that it can effectively carry customer traffic. At these speeds, the optical testing and measurement of polarization mode dispersion (PMD) and chromatic dispersion (CD) become critical considerations.

Upholding quality and reliability

The foremost objective of any communication network is to preserve the quality and reliability of optical signal transmission. Whether building new networks or upgrading existing ones, thorough characterization is necessary to prevent induced bit-error-rate (BER) levels from exceeding system requirements. In that view, EXFO provides a comprehensive array of fiber characterization solutions for today’s networks, as per recommendations from ITU-T G.650.3.

Fiber characterization gauges the network's capability to accommodate various applications. Characterization offers an overview of the entire link, encompassing interconnection points, splices, and fiber segments. By validating IL, ORL, CD and PMD, the fiber's suitability for traffic transmission can be confirmed, serving as a benchmark for subsequent commissioning and troubleshooting activities.

Essential tests to perform based on ITU-T G.650.3 recommendations

Inspection of connectors endfaces

Field technicians inspect connector endfaces to identify any contamination, defects and scratches against established standards like IEC/IPC or user-defined criteria.

Automated and accurate pass/fail diagnostics for connectors can greatly accelerate field operations while ensuring that the no. 1 cause of network failures (i.e., dirty or damaged connectors) has been dealt with. Specialized tools such as fiber- inspection probes, and cleaning kits are indispensable for this purpose.

Monitoring insertion loss (IL)

Irrespective of fiber types and wavelengths, monitoring IL remains crucial across varied bending angles. While the angles may vary, the underlying behavior remains consistent. Excessive loss is generated through fixed-angle bending, potentially leading to identification inaccuracies.

IL can be tested with an optical power meter, an optical loss test set (OLTS) or an optical time-domain reflectometer (OTDR).

Assessing optical return loss (ORL)

Arising from the reflective properties of network components like fiber, mechanical connections or substandard splices, ORL assumes significance due to its potential impact on transmitter performance and service quality.

ORL can be quantified using a backreflection tester or an OLTS equipped with a backreflection option.

Characterizing the optical link and elements

OTDRs are pivotal in the comprehensive characterization of fiber links, facilitating the measurement of loss, reflectance, and attenuation across all network connections, splices, and splitters. Additionally, OTDRs offer precise distance measurements.

Automated OTDR threshold setting, multipulse acquisitions and analysis (as performed by iOLM) can provide precise icon-based mapping of all elements on the link at the push of a button, avoiding the risk of human errors.

Analyzing dispersion

Chromatic dispersion (CD) and polarization mode dispersion (PMD) can both lead to an increase in bit error rates. CD is pulse broadening due to different wavelengths inside a pulse of light traveling at different speeds. PMD is pulse broadening caused by the differences in propagation velocities for different polarization states. CD increases linearly with distance (e.g., doubling the distance doubles the CD), and PMD increases with the square root of the distance (e.g., multiplying the distance by four doubles the PMD).

There are two types of test solutions available in the industry:

Dual-ended testers, which require a source at one end and a detector at the other.
Single-ended testers, which combine the source and the detector within a single instrument.

According to the ITU-T G.650.3 guidelines, it is not necessary for this measurement to be conducted from both ends of the link. The single-ended approach therefore provides reliable measurements with half of the resources dispatched to the field, significantly reducing operational expenses.

Evaluating spectral attenuation

Attenuation denotes power loss attributed to various scattering and absorption phenomena. The objective is to gauge the transmitted intensity of a spectral pulse of light across multiple wavelengths to calculate attenuation rate accurately.

Spectral attenuation can be tested with an optical spectrum analyzer (OSA).