What are the three functional components of an optical power meter?

Practically every measurement in Fibre optics refers to optical power. The power output of a transmitter or the input to receiver are "absolute" optical power measurements, that is, you measure the actual value of the power. Loss is a "relative" power measurement, the difference between the power coupled into a component like a cable or a connector and the power that is transmitted through it. This difference is what we call optical loss and defines the performance of a cable, connector, splice, etc.

Measuring power

Power in a fibre optic system is like voltage in an electrical circuit - it's what makes things happen! It's important to have enough power, but not too much. Too little power and the receiver may not be able to distinguish the signal from noise; too much power overloads the receiver and causes errors too.

Measuring power requires only a power meter (most come with a screw-on adapter that matches the connector being tested) and a little help from the network electronics to turn on the transmitter. Remember when you measure power, the meter must be set to the proper range (usually dBm, sometimes microwatts, but never "dB" - that's a relative power range used only for testing loss!) and the proper wavelengths, matching the source being used. Refer to the instructions that come with the test equipment for setup and measurement instructions (and don't wait until you get to the job site to try the equipment)!

To measure power, attach the meter to the cable that has the output you want to measure. That can be at the receiver to measure receiver power, or to a reference test cable (tested and known to be good) that is attached to the transmitter, acting as the "source", to measure transmitter power. Turn on the transmitter/source and note the power the meter measures. Compare it to the specified power for the system and make sure it's enough power but not too much.

Testing loss

Loss testing is the difference between the power coupled into the cable at the transmitter end and what comes out at the receiver end. Testing for loss requires measuring the optical power lost in a cable (including connectors, splices, etc) with a fibre optic source and power meter by mating the cable being tested to known good reference cable.

In addition to our power meter, we will need a test source. The test source should match the type of source (LED or laser) and wavelength (850, 1300, 1550 nm). Again, read the instructions that come with the unit carefully.

We also need one or two reference cables, depending on the test we wish to perform. The accuracy of the measurement we make will depend on the quality of your reference cables. Always test your reference cables by the single ended method shown below to make sure they're good before you start testing other cables!

Next we need to set our reference power for loss - our "0 dB" value. Correct setting of the launch power is critical to making good loss measurements!

Clean your connectors and set up your equipment like this:

1. Turn on the source and select the wavelength you want for the loss test.
2. Turn on the meter, select the "dBm" or "dB" range and select the wavelength you want for the loss test.
3. Measure the power at the meter. This is your reference power level for all loss measurements. If your meter has a "zero" function, set this as your "0" reference.

Some reference books and manuals show setting the reference power for loss using both a launch and receive cable mated with a mating adapter. This method is acceptable for some tests, but will reduce the loss you measure by the amount of loss between your reference cables when you set your "0dB loss" reference. Also, if either the launch or receive cable is bad, setting the reference with both cables hides the fact. Then you could begin testing with bad launch cables making all your loss measurements wrong. EIA/TIA 568 calls for a single cable reference, while OFSTP-14 allows either method.

Measuring loss

There are two methods that are used to measure loss, which we call "single-ended loss" and "double-ended loss". Single-ended loss uses only the launch cable, while double-ended loss uses a receive cable attached to the meter also.

Single-ended loss is measured by mating the cable you want to test to the reference launch cable and measuring the power out the far end with the meter. When you do this you measure 1. the loss of the connector mated to the launch cable and 2. the loss of any fibre, splices or other connectors in the cable you are testing. This method is described in FOTP-171 and is shown in the drawing. Reverse the cable to test the connector on the other end.

In a double-ended loss test, you attach the cable to test between two reference cables, one attached to the source and one to the meter. This way, you measure two connectors' loses, one on each end, plus the loss of all the cable or cables in between. This is the method specified in OFSTP-14, the test for loss in an installed cable plant.

What loss should you get when testing cables?

While it is difficult to generalise, here are some guidelines:

• For each connector, figure 0.5 dB loss (0.7 max)
• For each splice, figure 0.2 dB
• For multimode fibre, the loss is about 3 dB per km for 850 nm sources, 1 dB per km for 1300 nm. This roughly translates into a loss of 0.1 dB per 100 feet for 850 nm, 0.1 dB per 300 feet for 1300 nm.
• For singlemode fibre, the loss is about 0.5 dB per km for 1300 nm sources, 0.4 dB per km for 1550 nm. This roughly translates into a loss of 0.1 dB per 600 feet for 1300 nm, 0.1 dB per 750 feet for 1300 nm.

So for the loss of a cable plant, calculate the approximate loss as:

(0.5 dB x number of connectors) + (0.2 dB x number of splices) + Fibre loss on the total length of cable

Troubleshooting hints

• If you have high loss in a cable, make sure to reverse it and test in the opposite direction using the single-ended method. Since the single ended test only tests the connector on one end, you can isolate a bad connector - it's the one at the launch cable end (mated to the launch cable) on the test when you measure high loss.
• High loss in the double ended test should be isolated by retesting single-ended and reversing the direction of test to see if the end connector is bad. If the loss is the same, you need to either test each segment separately to isolate the bad segment or, if it is long enough, use an OTDR.
• If you see no light through the cable (very high loss - only darkness when tested with your visual tracer), it's probably one of the connectors, and you have few options. The best one is to isolate the problem cable, cut the connector off one end (flip a coin to choose) and hope it was the bad one (well, you have a 50-50 chance!)

To ensure the signal transmission performance in fiber optic network, optical power should be well controlled. Optical power should not be too high or too low. And it should be within the scope of the device’s requirement. To achieve accurate measurement, optical power meter is usually used to test the optical power. But How to use optical power meter? This post will make an illustration of the power meter components and then state how to use optical power meter.

Buttons on Optical Power Meter

The functions and operation of optical power meters provided by the market are similar. Generally there are four buttons on the optical power meter: power button, dBm/w button, REF button and λ button. The functions of these buttons are listed in the following:

• Power button: turn the power meter on or off;
• dBm/w button: shift between linear (mW) mode and logarithmic (dBm) mode;
• REF button: press this button to set the current measured power as the referent point;
• λ button: select the calibrated wavelength. The most commonly used wavelengths are 850nm, 980nm, 1310nm, and 1550nm.

Here takes an example of a typical handheld optical power meter (FOPM-104) which is designed by FS.COM as shown in the following picture.

Adapter Type of Optical Power Meter

To use the optical power meter, a length of fiber optic patch cable is usually required to connect the optical power meter interface and the interface of devices requiring test. For instance, if the interface on the fiber optic power meter is FC, the device for testing has a LC interface. Then a length of FC-LC fiber patch cable is needed. Some of the optical power meters have only one fixed optical interface. Some can provide replaceable optical adapter to fit different patch cables. The above mentioned FOPM-104 handheld optical power meter provides three type adapters: SC, FC and ST (as shown in the following picture).

For testing of fiber optic interface like LC, SC, ST and FC, this above power meter is enough. Some optical power meter might have two optical interfaces for common connectors. However, interface like MTP/MPO, optical power meter with special interface should be used. The following picture shows a MTP optical power meter provided by FS.COM, which can be used to test devices or components with MTP interfaces like 40G SR4 QSFP+ transceiver.

Optical Power Measurement Using Optical Power Meter

How to use optical power meter? It can be easy. The following video will take the example of 10G-LR SFP+ Cisco compatible module to illustrate how to use optical power meter for testing. This cisco compatible transceiver will be inserted in Cisco Nexus 9396PX switch. A length of single-mode LC-FC fiber patch cable is required. This is because 10G-LR SFP+ transceiver is a single-mode transceiver working on wavelength of 1310nm. After the optical power meter is connected to the module. Turn on the power button and press λ button to select 1310nm wavelength. At first the power value will change rapidly, then it slows down until still. The final power value will be shown on the screen.

This post introduces the buttons and adapter types of optical power meters, and illustrates how to use optical power meter with the aid of both text and video. Kindly visit Optical Power Meter page or contact for more details.

Related Article: DWDM MUX/DEMUX Insertion Loss Test

                             Optical Power Meter (OPM): A Must for Fiber Cable Testing

With the rapidly development of science technology, the fiber optic cables becomes more and more popular. When you install and terminate fiber optic cables, you need to test them. A test should be conducted for each fiber optic cable plant for three main areas: continuity, loss, and power. In order to do this, you’ll need a fiber optic power meter. There are two types of Optical Power Meters: Optical Power Meter and PON Optical Power Meter. This text mainly states two typical Fiberstore Optical Power Meters: FS6070B and PON Power Meter FS607P.

Optical Power Meter FS6070B
Optical Power Meter FS6070B stands for Ordinary Optical Power Meter. Power Meter FS6070 series with a chip microcomputer control and the large-screen graphics LCD, it can provide precision measurement from 850 to 1625nm wavelength ranges. The FS6070B power meter can test the PM power to the light, light stability, insertion loss, optical transmittance, reflectance of light and so on. It widely used in optical communications, Telecom, Fiber Optical Sensors etc. It is indispensable for scientific research and engineering construction.

Features

• Self-calibration function
• High performance import laser detector
• High sensitivity and large dynamic range
• 0.01dBm precision
• Automatically converted Range
• Small size, light weight, high reliability
• Battery-powered, convenient field work
• Line display or relative loss displayed

Applications

• Fiber optic production and research
• Test and maintain fiber-optic network
• Teach and test optical communication

PON Power Meter FS607P
PON optical power meter is used to design, operate and maintain FTTX networks. It is able to measure the optical power values of voice, data and video signals concurrently. It is an ideal choice for the construction and maintenance of FTTX networks. The FS607P is with high quality HD TFT color LCD, and the dual-port through the design, guarantee the full communication testing from the OLT to the ONT.

Features

• Provide simultaneous measurement at all three wavelengths on the fiber (1490nm, 1550nm, 1310nm)
• Use in Burst mode measurement of 1310nm upstream
• Dual-port through the design, guarantee the full communication testing from the OLT to the ONT
• Three status LEDs represents different optical signal conditions of Pass, Warn and Fail respectively
• Easy to use, simply connect the fiber to read the results
• HD TFT color LCD
• USB communication port enables data transfer, easy and quick
• 200 measurement items can be saved in PON power meter or computer for data review
• Through the background software setting threshold, upload data, calibration wavelength
• Auto power off function, and time settings on the device, the threshold setting

Applications

• FTTx / PON opening: In the opening stage of all PON signals are measured to verify and ensure that meet network standards
• FTTx / PON maintenance: routine maintenance of various transport issues, such as: connector end surface contamination, fiber macro bending, breaking, Optical fault, etc.; that will have a loss of signal or transmission performance simultaneously measure and display 1310 upstream, 1490 downstream and 1550 downstream the power of three wavelength values.

Tips for Selection

• Choose the best probe type and interface type.
• Evaluation of calibration accuracy and manufacturing calibration procedures, and your fiber and connectors to match the required range.
• Make sure the type and the range of your measurement and display resolution is consistent.
• With immediate effect db insertion loss measurements.

Fiberstore offers a full range of optical power meters to meet all your needs. If you have any requirements, please contact us over  or live chat with us.