CWDM System Testing Process

With the explosion of CWDM, it is very necessary to formulate a basic testing procedure to certifying and troubleshooting CWDM networks during installation and maintenance. Today, one of the most commonly available test methods is the use of an OTDR or power source and meter, which is capable of testing the most commonly wavelengths, 1310, 1490, 1550 and 1625nm.

This article here is based on the pre-connectorized plug and play CWDM systems that allow for connecting to test equipment in the field:

In the multiplexing module of a pre-connectorized CWDM system, wavelengths are added to the network through the filters and transmitted through the common port. The transmitted wavelengths enter the COM port in the de-multiplexing module and are dropped. All other wavelengths present at the MUX/DeMux module are went through the express port.

Most of today’s OTDRs have expanded capability for testing wavelengths in addition to 1310 and 1550 nm. The OTDR allows partial testing of such system offered in test equipment source. The OTDR allows partial testing of these systems by using the flexibility of pre-connectorized solutions. This is done by switching connections within the CWDM field terminal to allow for testing portions of the non-1310/1550 nm optical paths.

To test the 1310nm, the first step is to test the downstream portion of a system at 1310 nm by connecting the OTDR to the 1310 nm input on the CWDM MUX located at the headend. Then switch the test leads over the the upstream side and repeat. Test method is the same for both the downstream and upstream paths.

1550 nm testing is performed similarly by switching the test leads to the 1550nm ports. If additional wavelengths are present, you need to follow the procedures below:

Using the 1550 nm test wavelength, switch the OTDR connection to the 1550 nm input port on the headend MUX. Have a technician stationed at the field terminal connect the drop cable leg connectors for the 1570 nm customer to the 1550 nm port on the Mux/demux device. What should be noted is that in a play and plug solution this should not require repositioning where the drop cable passes through the OSP terminal. Test the downstream 1570 nm passive link at 1550 nm, and then repeat for the 1570 nm upstream side. When testing is complete, have the technician switch the connections for the 1570 nm drop back to the 1570 nm ports on the field MUX/DeMUX device as shown in Figure 6. Repeat this process for the 1590 nm, 1610 nm drop cables and other wavelengths present. Finally, test the 1550 nm path normally with the 1550 nm drop cable connected to the 1550nm MUX/DeMUX ports.

Since the OTDRs is able to test at 1490 or 1625 nm, the drop cables under test could be connected to the EXP port of the module and tested at 1490 or 1625 nm respective wavelength, without having to connect each to the 1550 nm port. Otherwise the procedure is the same.

As CWDM network become more and more common the data they carrying has also become critical. The procedure introduced here allows for testing modular pre-connectorized CWDM systems with standard optical test equipments. Relative channel power can be measured with a wide-band fiber optic power meter at the filter outputs or at other points in the network with the aid of a wavelength selective test device or with an optical spectrum analyzer.

Using Fiber Optic Power Meter to Test Optic Power Level

Fiber optic communication equipment is based on the optical power level between the transmitter and the receiver. The difference of the optical power level between them is the loss of the cabling plant. To measure the power loss of them, an optical power meter is needed to conduct a power loss testing.

fiber optic power meter is typically consist of a solid state detector, signal conditioning circuitry and a digital display of power. To interface to the large variety of fiber optic connectors in use, some form of removable connector adapter is usually provided. The power meter is calibrated at the same wavelength at the source output such as multimode 850 or 1300nm, single mode, 1310, 1490 and/or 1550nm, POF. Meters for POF systems are usually calibrated at 650 and 850nm. The wavelengths used in POF systems.

When performing the test, use the optical power meter adapter to mate to the connector type on the cable. The connectorized reference patch cables must be the same fiber type and size as the cable plant and have connectors compatible to those on the source and cables.

Power meters are calibrated to read in dB reference to one milliwatt of optical power. Some meters of a relative dB scale also, useful for loss measurements since the reference value may be set to 0 dB on the output of the test source. Occasionally, lab meters may also measure in linear units like milliwatts, microwatts and nanowatts.

Optical Power Testing Procedure:
Turn on the power meter to allow time to warm-up.
Set meter to wavelength of the source and “dBm” to measure calibrated optical power.
Clean all connectors and mating adapters.
Attach reference cable or fiber patch cord to source if testing source power or disconnect cable from receiver.
Attach power meter to end of cable and read measured power.

To reduce the measurement uncertainty, you must calibrate the optical power meter according the manufacturers specified intervals. Clean all connectors and remove the meter adapter periodically to clean the adapters and power meter detector. To avoid the stress loss, please don’t bend the fiber optic cables during the testing.

Optic power testing is only one the main part of fiber optic testing. Most test procedures for fiber optic component specifications have been standardized by national and international standards which are converted in procedures for measuring absolute optical power, cable and connector loss and the effects of many environment factors such as temperature, pressure, flexing, etc. Basice fiber optic testing instruments are the fiber optic power meter, optical light source, OTDR and fiber inspection microscope.