Why Do You Need a PON Power Meter?

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The
requirements for testing fiber optic networks will vary depending on the
specific type of network as well as the network designer’s overall test
requirements. Regardless of type, there are two basic or generic pieces of
Optical Test Equipment that will be used; an Optical Time Domain Reflectomer or
OTDR, and a pair of optical test equipment pieces that are referred to as a
Power Meter & Light Source. These tests are typically measured in
“dB”. The term dB is the expression of attenuation or power loss over
an optical fiber as it travels from a termination end to a point along the
fiber’s path. Once an optical fiber is connected to a piece of active equipment
then all tests are in dBm. The active equipment will be transmitting an actual
or real optical power at a specific wavelength and referenced at 1mW.

 

A
PON
power meter is essential for field technicians installing or
maintaining any type of PON network. PON Power Meters are able to
simultaneously test upstream and downstream through optical fibers, at 1490nm,
1550nm, and 1310nm wavelengths, as well as estimate signals of the voice, data,
and video streams.

 

The
term PON stands for “passive optical network”. A PON is a fiber optic
telecommunications network that delivers broadband to transmit data over fiber
optic cables to the customer premises. Its architecture implements a
point-to-point or a point-to-multipoint arrangement of nodes in a
communications network. A Point-to-Multipoint network uses a single fiber to
serve multiple endpoints by using unpowered or passive fiber optic splitters. A
splitter is used to divide the fiber bandwidth among multiple access points.
Passive optical networks are often referred to as the “last mile”
between an internet service provider (ISP) and its customers.

 

Point-To-Point
or P-T-P type optical network

 

A
P-T-P network is a network that has two termination points and nothing
in-between. As with all fiber optic networks, when it is being constructed the
fibers must be terminated to allow for any tests to be performed. So one end of
the network is terminated and an OTDR test is performed on each fiber to ensure
that the termination and length of fiber beyond does not have any issues. That
test result will be stored for future needs and noted in “dB”. If the
network requires splicing then after the fibers are spliced, the OTDR again is
used to ensure that the splice and added fiber length again meets requirements.
The testing with the OTDR continues and is completed after the end of each
fiber is terminated. At this point, another set of tests is required which is
commonly referred to as an End-To-End test. This test requires the use of a
light source and a power meter and again all test results are stored. The
optical power meter will be set to “dB” and referenced to a light
source which is typically called “Zeroing”. The units are then moved
to opposing ends and the field technicians will send and receive wavelengths
specified by the designer. Again this is a measurement that will be used
against the designer’s overall Link Loss Budget. The P-T-P network will have
its termination ends referred to as “A” and “B” and at
least two unique wavelengths will be sent and received over each fiber. This is
typically required to ensure that any wavelength used by a transmitter can be used
between the two that are specified. The network designer will define these
wavelengths as well as provide a label for these ends. The technician that is
performing the tests will reference these labels in any reporting back to the
designer.

 

Point-To-Multipoint
PON type network

 

Now
when a Point-To-Multipoint network is constructed such as a Passive Optical
Network or PON many of the tests and test equipment remain the same but will
require a few special features. The OTDR testing during construction remains
the same, with tests performed each time a fiber is terminated or spliced.
Again, this continues to the far ends of the fibers after they are terminated.
Once all fibers are terminated, again they will be tested using a power meter
and light source. The need for special equipment is required for the activation
phase of a PON network and that’s where the similarity between a P-T-P and a
P-T-Multipoint ends.

 

The
PON network activation phase begins by connecting a power meter up to an active
piece of equipment called an Optical Line Terminal or OLT and set to the
appropriate wavelength and the unit is set to “dBm” and this becomes
the referenced power.

 

NOTE:
There are several generations of PON network OLTs which use different
wavelengths so the optical power meter must have the capability to be set to
those wavelengths. GPON is 1490nm, XGPON is 1577nm and NGPON has multiple
wavelengths ranging from 1596nm to 1602nm.

 

As
the links are connected out to the far end, the technician repeats the test and
ensures that there are no issues. This testing continues out to the far end,
which in a PON network there is a piece of active equipment called an Optical
Network Terminal or ONT or at times called an Optical Network Unit or ONU.
Regardless, that piece of equipment receives light from the OLT transmitter and
communicates back to the OLT with its own transmitter. The ONT cannot
communicate back to an OLT without first receiving the OLT’s transmitter’s
wavelength. At this time, there is an absolute need to use a specialized
optical power meter which can measure the OLT’s power and allow that power to
pass through and provide the signal to the ONT/ONU so it can send back a
signal.

 

The
PON meter has two test ports; one is named DROP and the other ONT or ONU. The technician
connects the drop, which is connected via a fiber all the way back to the OLT
into the port named DROP and then connects the ONT/ONU connectorized fiber
pigtail into the ONT/ONU port. Now the PON meter is in-line between the OLT and
ONT/ONU and allows for the OLT to communicate with the ONT/ONU. The technician
again will be observing the incoming OLT power level, as well as the outgoing
ONT/ONU power level. If all is well, the drop is connected to the ONT/ONU and
service activation tasks can continue.

 

I
hope you have found this article informative and that you learned a bit about
FTTx networks.