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Construction of Fiber-Optic Networks


Long-haul fiber networks can cover great distances and are usually composed of many fiber sections and various optical components. Key parameters, such as loss and optical return loss (ORL), as well as basic factors like connector cleanliness, need to be closely controlled during network construction to ensure future transmission quality.

Optical testing requirements of metro networks are similar to those of long-haul networks, but the different strategic and economic realities that surround both architectures lead to distinct threshold settings and testing procedures.

 

Test Equipment

Test Type
Power Meter
Light Source
Return Loss Meter
Bidirectional Loss Test Set
 OTDR 
Fiber Inspection Probe
Visual Fault Locator
Total optical loss
     
Back reflection (ORL)
           
Link characterization
         
Visual inspection
         

 

Total Link Loss Measurement (or Optical Power Budget)

While the laser transmitter light is traveling though the fiber link, absorption and scattering effects will slowly attenuate the original signal, and so will elements like connections, splices and other components. Insertion loss is measured to ensure that the link complies with the engineering loss specification (also called loss budget).

The loss budget takes into account the system's tolerances, so it is crucial to meet such specifications to ensure error-free transmission. Testing fiber links in both directions is the norm, as data traffic can be bidirectional and significant loss differences can be observed between directions.

Test Method

Description

Proposed Equipment

Loss testing
The basic tools to measure optical loss are a power meter and a light source positioned at separate ends of the fiber link under test. Taking into account the source power as a reference, link loss is obtained from the remaining power reaching the power meter at the other end.
Bidirectional loss testing
Automated bidirectional optical loss test sets (OLTSs) are used in pairs and contain both a power meter and light source. They provide fully automated bidirectional loss and ORL test results in 10 seconds for up to three wavelengths, as well as a fiber-length measurement. This greatly reduces testing time and risk of human error.
OTDR testing
OTDRs are used to perform complete link characterization, including loss measurements. The OTDR has the ability to measure and locate connections, splices and splitter points within a network.

 

Optical Return Loss Measurement

Optical return loss (ORL, or backreflection) comes from the amount of energy lost within components and fiber due to backreflections. Main causes include the reflective nature of some components including the fiber itself, mechanical connections and poor-quality splices. ORL is a key parameter, as backreflection can affect transmitter performance and, in turn, the service quality delivered to customers. Bad ORL levels can result in:

  • Increased bit error rate
  • Distortion on IPTV and analog video signals
  • Permanent damage to laser transmitters

Test Method

Description

Proposed Equipment

ORL testing
ORL measurement can be performed by using a backreflection tester, or with an OLTS including a backreflection measurement option.
ODTR testing
An OTDR will provide a measurement of Fresnel reflections as well as the fiber's Rayleigh backscattering. The OTDR will then calculate the ORL value for the entire link or a specific section.

 

Fiber Link Characterization

Total link characterization is a key step that provides a snapshot of the entire link including all interconnection points, fusion splices and fiber sections. Link characterization also serve as a future reference when performing commissioning and troubleshooting on the same link.

Test Method

Description

Proposed Equipment

ORL testing
OTDRs are used to perform full link characterization. It has the ability to measure and locate connections, splices and splitter points within a network. The OTDR will measure the loss, reflectance and attenuation for each component of the network while also measuring its location (distance). This instrument will also display a total loss, attenuation and ORL value.

FTB-500 Platform
or FTB-200 
Compact Platform

with FTB-7000 OTDR module

 

Visual Inspection

Instruments such as visual fault locators (VFL) and fiber inspection probes (FIP) can be used to pinpoint problems in a fiber-optic network. These problems include:

  • Fiber reversal
  • Macrobends
  • Dirty or damaged connectors

Test Method

Description

Proposed Equipment

Visual fault location
A visual fault locator uses a laser typically working at 655 nm, which is a visible red laser. Main applications include identifying fiber and locating macrobends in fusion-splice trays.
Video inspection
Fiber inspection probes use a camera with lenses providing 200x or 400x magnification. Special types of adapters allow for the inspection of male connectors such as patchcords and female connectors such as those found in patch panels. An image is displayed on the instrument showing connector endfaces.

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