Lesson: Building and Testing A Fiber Optic Link - Part 2 - Passive Optical Network (PON) Link

Objectives: From this lesson you should learn:
How to build a fiber optic link as used in passive optical networks (PON) using media converters and splitters
Testing the fiber optic link

Components
Safety Glasses
Visual Fault Locator (VFL) or the VFL in the power meter if it has one
Fiber optic cable - singlemode with SC connectors, 2 or more
SC connector mating adapters, 4 or more




Fiber Optic Media Converters

Media converters are special fiber optic transceivers used to convert from one type of cable (the media) to another, typically from copper cables to fiber optics, although some media converters will convert from one fiber type to another, e.g. multimode to singlemode. The FOA Guide has a page about media converters you should read before beginning this exercise.

Exercise 2 uses media converters that connect over one fiber while still providing full-duplex (simultaneous) communications bidirectionally over the one fiber. The one fiber bidirectional links are similar to those used in passive optical networks (PONs) used in FTTH (fiber to the home) and optical LANs, so Exercise 2 includes an option to build a PON demonstration.

Media converters are available for purchase online at very reasonable prices. The exercises show how to use media converters that use Ethernet protocols to allow connecting computers to the Internet.



Typical media converters

2 singlemode bidirectional fiber media converters- you will be using singlemode fiber cables from the installation exercises to connect them. If you plan to do both exercises, you will need 2 sets of media converters, one set that operates over two fibers and one set that operates over a single fiber bidirectionally.
You will connect them over 2 or 4 fiber optic cables used in the other exercises

Also you need two Cat 5e patchcords to connect your computer and router to the media converters. Purchase these online if you do not have any.


PON Splitters

You will need one or more PON splitters, also available online. Order a "1 X 2" ( 2port) and a "1 X 4" (4 port) splitter with SC connectors.




Fiber Optic Power Meter




This fiber optic power meter measures in dBm and W and has a dB range for loss measurements.
It has adapters for SC connectors and any connector with 2.5mm ferrules.


Connecting With The Link You Build
To connect with the link, you need an Ethernet connection to the Internet and a device that can connect to the Internet over Ethernet on UTP Cat 5 cable..
Your Internet connection should have a router with Ethernet ports which can be used.
If you have a computer with an Ethernet port, that will be ideal for completing the link. You can also use a WiFi access point connected to your router over UTP Cat 5e cable for the exercise.


Safety:

Always wear safety glasses when doing any of these exercises and dispose of all fiber scraps properly. Safety Rules - Read before beginning any exercises. Do not look directly into the light from the visual fault locator - it's bright!

Exercise

Watch the FOA YouTube Video Of This DIY PON Link Demo Exercise   

Before you start, review the use of fiber optic media converters from the FOA YouTube video on media converters, the FOA Guide page on media converters or the Fiber U MiniCourse on media converters. 

You shouls also review the FOA Guide page on FTTH PON Architectures as you will be building a simulated PON network.


Check Your Cables

1: Attach your cables to the Visual Fault Locator (VFL)

2. Turn the VFL on and ensure the light travels through the fiber. You can see how bright the glow is at the end of the fiber, diffused through the fiber protective cap. This shows how fiber transmits light by total internal reflection as you learned in the lesson on optical fiber.




Repeat for all the cable you are using in this exercise.


Build A Fiber Optic Link

Exercise 1 uses media converters that connect over one fiber like FTTH (fiber to the home) PON (passive optical network) fiber optic links.


 
This link uses two media converters that convert Ethernet on Cat 5 to fiber optics and transmit bidirectionally on one singlemode fiber. The diagram above is the link we will build.

1. Assemble the equipment you need.
2. Power up the media converters and confirm the power light.
3. Connect the two media converters with the singlemode cables, 2 cables connected with mating adapter in the middle as shown in the diagram above. Did your link show the link connection light?
4. Connect your Ethernet devices to the link with UTP Cat 5e cables and confirm the link transmits data. You can check the indicator lights to ensure the bottom light is flashing which shows data is being transmitted.



Confirm your is link transmitting data, connecting our laptop to the Internet over fiber optics, by downloading several websites. This is an FOA YouTube video.



You can open the connection and check the power in both directions from each media converter's transmitter.



 Here are our test results:
We measured -8.34 dBm "downstream" and -12.40 dBm upstream.


Insert a 2-port passive fiber optic splitter:

                      <<<<<<< Downstream--------------------------------------------------Upstream>>>>>>>>>>


(In a FTTH PON network, the subscriber's ONT (optical network terminal simulated by one media converter and the PC) would be on the left and the OLT (optical line terminal) at the head end or central office would be at the right of our page, simulated by the other media converter. That makes signals moving downstream moving to the left and upstream to the right. We will use that orientation throughout this exercise.)

Open the connection in the setup and insert a 2 port (1X2) splitter. Connect one of the cables to the single fiber side and one fiber to the two fiber side. Here is how that link now looks:



The splitter takes one fiber input and splits it into two outputs, and in the opposite direction takes signals from either of the two fibers and transmits it to the single fiber, so it splits the signal in one direction and combines the signals in the opposite direction. Thus our bidirectional link should work through the splitter.

Confirm the link still works.



To prove the splitter sends the same signals out of each of the outputs, change your connection on the 2 fiber side to the other fiber output of the splitter and confirm the link still works.

Since the splitter divides the signal into 2 parts, the optical power of the signal is cut in half (a loss of 3 dB) plus some additional loss for the inefficiency of the splitter. Measure the downstream loss of the splitter as follows:



Disconnect the incoming fiber on the input (one fiber) side of the splitter and measure the output of the cable (not the input of the splitter) and record that value. Meter set for dBm, 1310 nm, reading _____dBm.

Disconnect the fiber on the output of the splitter and measure the output of both fibers, Meter set for dBm, 1310 nm, reading _____dBm on one fiber and _____dBm on the other fiber.

How much loss do you see on the splitter? How much difference is there between the two fibers?

Here are our test results:
We measured -11.92 dBm from output 1 and -11.70 dBm from output 2. Since the input was measured at -8.34 dBm, the loss was -3.58 dB for output 1 and -3.36 dB for output 2, about what we expect from a 2 port splitter.





Insert a 4-port (1X4) passive fiber optic splitter:

Remove the 2-port (1X2) splitter and insert a 4 port splitter. Connect one of the cables to the single fiber side and one fiber to the four fiber side.

                      <<<<<<< Downstream--------------------------------------------------Upstream>>>>>>>>>>

 
Here is how that link now looks:




Confirm the link still works.



To prove the splitter sends the same signals out of each of the outputs, change your connection on the 4 fiber side to the other fiber outputs of the splitter and confirm the link still works.

Since the splitter divides the signal into 4 parts, the optical power of the signal is cut in one-fourth (a loss of 6 dB) plus some additional loss for the inefficiency of the splitter. Measure the downstream loss of the splitter as follows:



Disconnect the incoming fiber on the input (one fiber) side of the splitter and measure the output of the cable (not the input of the splitter) and record that value. Meter set for dBm, 1310 nm, reading _____dBm.

Disconnect the fiber on the output of the splitter and measure the output of all 4 fibers, Meter set for dBm, 1310 nm,
Fiber 1 reading _____dBm .
Fiber 2 reading _____dBm .
Fiber 3 reading _____dBm .
Fiber 4 reading _____dBm .

How much loss do you see on the splitter? How much difference is there between the 4 fibers?

Here are our test results:
Fiber 1 reading -15.78 dBm, 7.44 dB loss,
Fiber 2 reading -15.55 dBm, 7.21 dB loss .
Fiber 3 reading -15.60 dBm, 7.26 dB loss .
Fiber 4 reading -15.59 dBm, 7.25 dB loss


Cascade Splitters Like A FTTH PON Network

FTTH PON networks often cascade splitters to optimize the use of fiber in the network. Let's simulate a FTTH PON network with 2 levels of splitting using the 2-port and 4-port splitters we used above. Connect you setup like this:



                      <<<<<<< Downstream--------------------------------------------------Upstream>>>>>>>>>>

 
And the equipment setup looks like this:




Confirm the link still works.



To prove the splitter sends the same signals out of each of the outputs, change your connection on the 4 fiber side to the other fiber outputs of the splitter and confirm the link still works. You may also want to change the 4-port splitter connection to the other output of the 2-port splitter.

Since the splitters divides the signal into 8 parts (1X2 plus 1X4 equals 1X8), the optical power of the signal is cut in one-eighth (a loss of 9 dB) plus some additional loss for the inefficiency of the splitter. Measure the downstream loss of the splitter as follows:



Disconnect the incoming fiber on the input of the 2 port splitter and measure the output of the cable (not the input of the splitter) and record that value. Meter set for dBm, 1310 nm, reading _____dBm.

Then measure the power on the output of the 4 port splitter on any of the 3 unused fibers and measure the power there:
Meter set for dBm, 1310 nm, reading _____dBm.

How much loss do you measure across the two cascaded splitters? ______ dB.

Here are our test results:
Fiber 1 reading -18.83 dBm, 10.49 dB loss,
Fiber 2 reading -18.56 dBm, 10.22 dB loss .
Fiber 3 reading -18.61 dBm, 10.27 dB loss .
Fiber 4 reading -18.56 dBm, 10.22 dB loss


 
You have successfully completed this exercise when you have built a fiber optic link using the media converters and used it to transmit data in a simulated FTTH PON network. You also should have tested the power in the link and the loss of the cable plant.

Note: Actual FTTH PONs use special protocols to allow multiple ONTs on the output of splitters. This involves addressing each uniquely and encrypting data to ensure the privacy of each user. The media converters here do not have FTTH PON type protocols to allow using multiple media converters on this simulated network. See the FOA Guide for more information on FTTH PON Networks.

Complete the exercise and fill in your Scorecard.


Return to Lesson Plan

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