During the installation, commissioning or troubleshooting of an FTTH connection, power metres and OTDR s are used for different tasks. An optical power metre (OPM) measures how much optical power arrives at a specific measurement point. An OTDR (Optical Time Domain Reflectometer), on the other hand, analyses the fibre-optic link and can identify where a fault or abnormal attenuation is located. Put simply, the difference can therefore be summarised in two questions: Power metre: How much optical power is arriving? OFL/OTDR: Where along the fibre-optic link is the problem? Both measurement methods complement each other: if the power measurement indicates an abnormal value, a reflectometric measurement using an OFL or OTDR can then be carried out to pinpoint the cause.
When do I need a power meter for FTTH?
An optical power meter is used to measure the optical signal level at a specific point along the fibre-optic connection. This allows you, for example, to check whether sufficient optical power is reaching the subscriber connection.
For an active GPON or XGS-PON connection, a selective or filtered PON power meter is particularly useful. GPON uses 1490 nm for the downstream and 1310 nm for the upstream, whilst XGS-PON uses 1577 nm and 1270 nm respectively. If several signals are transmitted simultaneously over the same fibre, the individual wavelengths must be measured separately.
A simple overall level is then insufficient for an unambiguous assessment. A selective power metre can measure the individual wavelengths separately and thus determine the respective optical levels.
When do I need an OTDR for FTTH?
If, for example, the level measurement indicates excessive attenuation, this does not reveal where along the fibre-optic link the cause lies. This is where an OTDR comes into play.
An OTDR sends a defined optical pulse into the fibre and analyses the backscattered and reflected signal components. This enables events along the fibre-optic link to be detected and localised.
Depending on the measurement, the following events and characteristics of the fibre-optic link, amongst others, can be identified and analysed:
- Link length and fibre end
- Connectors
- Splices
- Bends
- Attenuation events
- Reflections
The OTDR therefore not only answers the question of whether a problem exists, but also provides information on where on the fibre it is located and what might be causing it.
How does fault-finding work for an FTTH connection?
This results in a straightforward troubleshooting procedure:
Power metres and OTDR s are not competing measurement methods. They answer different questions and can build on each other during fault-finding.
What is the difference between an OFL and an OTDR?
The full scope of measurement and analysis offered by an OTDR is not required for every service call. An Optical Fault Locator (OFL) can be used for rapid fault localisation.
OFLs and OTDR s are based on reflectometric measurements, but serve different purposes. An OFL narrows the measurement down to the information relevant to the service call and guides the technician, as automatically as possible, to any anomalous events. Detected events and their distances are displayed in an easy-to-understand format.
A full-featured OTDR, moreover, provides detailed information about the entire fibre run and enables a more in-depth analysis of attenuation, reflections, splices and connectors.
OFL or OTDR – which method is suitable for which application?
| Question / Application | OFL / simplified fault localisation | Full-featured OTDR |
|---|---|---|
| What do I want to know? | Where is an anomalous event located? | What is happening in detail along the entire fibre-optic route? |
| Typical user | Installation and service engineers | Fibre-optic technician / measurement specialist |
| Operation | Guided, largely automatic evaluation | Automatic or detailed manual analysis |
| Presentation | Events and distance presented in an easy-to-understand format | Event table and complete OTDR curve |
| Locating faults | Yes | Yes |
| Detailed characterisation of splices and connectors | Limited | Yes |
| Determine event attenuation | Limited | Yes |
| Analyse path attenuation | Limited | Yes |
| ORL / Reflection analysis | Limited | Yes |
| Manual detailed analysis | Not a primary objective | Yes |
| Main area of application | Rapid fault-finding on the last mile | Comprehensive acceptance testing and detailed fibre diagnosis |
| Key benefit | Rapid, clear service assessment | Maximum measurement and analysis depth |
Which ARGUS testers are suitable for power meter, OFL and OTDR measurements?
Depending on the application, various ARGUS testers are available for measurement and fault location. The ARGUS F200 and ARGUS F240 can combine PON measurement functions with the Optical Fault Locator (OFL) to simplify fault location. The ARGUS F300 and ARGUS 300 combine PON measurement functions with a fully-fledged OTDR.
This allows fault localisation or detailed fibre analysis to be carried out immediately following an abnormal signal level reading, without having to switch to a separate measuring instrument for the next measurement.
Technical specifications of the ARGUS OTDR
| Specification | ARGUS OTDR |
|---|---|
| Measurement method | Optical Time Domain Reflectometer (OTDR) |
| Wavelengths | 1310 + 1650 nm or 1310 + 1550 nm |
| Dynamic range | up to 37 dB |
| Event dead zone | 0.9 m |
| Attenuation dead zone | 3.5 m |
| PON dead zone | ≤ 25 m, typically 20 m |
| Display range | 250 m to 240 km* |
| Measurement modes | Smart Auto, Expert, Real Time up to 4 Hz |
| Detected events | including connectors, splices and bends |
| Insertion loss (IL) | Yes |
| Optical return loss (ORL) | Yes |
| Macrobend localisation | Yes, by comparing two wavelengths |
| Feed-through fibre can be used | Yes |
