During testing & commissioning of fiber optics cable you might encounter unexplained or unexpected higher fiber loss. You have done your best in getting the lowest possible fiber jointing loss, you took proper and extra care during fiber installation by avoiding micro & macro bending and pulling cable with minimum force and yet you still encounter unexplained fiber optic cable loss.
The first thing you should do is to make sure that your fiber optics test equipment is properly set up. The second thing that you should take note is to have a clean fiber optics ferrule end surface. During installation your fiber optics connectors might exposed to dusty environment or you forget to use cap to cover your connector ferrule.
You could use either cotton with 99.99% Isoprophy alcohol to clean the end surface of fiber optics ferrule or following easy to use fiber cleaning method.
Saturday, May 9, 2009
dB versus Optical Power Loss
dB vs. Optical Power Loss
In designing an optical link, the optical link loss budget must be calculated. This reflects the difference of power generated by the transmitter compared to the amount of light the receiver is set to receive. In calculating link loss budgets, first consider the output power of the transmitter. For example, most multimode transceivers have a minimum transmission value of -10dB; maximum receiver sensitivity is set at -17dB. The result is that in a multimode application, with a standard transceiver at 850nm, the dB budget is -7dB. This means that up to 7dB of power can be lost in the cable plant loss and the receiver will still detect an optical signal.
The following table shows the percentage of power lost and its corresponding dB value. At -7dB, only 20% of power is received and yet the optical signal is received successfully. A theoretically perfect optical device would have no internal losses and would transmit 100% of the power, thus having 0dB.
Using the dB budget determined by the transceiver, determine the attenuation value to be used. In the example given above, there is a dB budget of -7db. Assuming a tolerance of +/-1db, an attention value of between -6dB and -8dB is optimal. But there is the need to eliminate false failures, so an attenuation value of -6dB comes close to maximum stress for the device under test without the fear of incurring a failure rate of 50%.
In designing an optical link, the optical link loss budget must be calculated. This reflects the difference of power generated by the transmitter compared to the amount of light the receiver is set to receive. In calculating link loss budgets, first consider the output power of the transmitter. For example, most multimode transceivers have a minimum transmission value of -10dB; maximum receiver sensitivity is set at -17dB. The result is that in a multimode application, with a standard transceiver at 850nm, the dB budget is -7dB. This means that up to 7dB of power can be lost in the cable plant loss and the receiver will still detect an optical signal.
The following table shows the percentage of power lost and its corresponding dB value. At -7dB, only 20% of power is received and yet the optical signal is received successfully. A theoretically perfect optical device would have no internal losses and would transmit 100% of the power, thus having 0dB.
Using the dB budget determined by the transceiver, determine the attenuation value to be used. In the example given above, there is a dB budget of -7db. Assuming a tolerance of +/-1db, an attention value of between -6dB and -8dB is optimal. But there is the need to eliminate false failures, so an attenuation value of -6dB comes close to maximum stress for the device under test without the fear of incurring a failure rate of 50%.
Fiber Optics Test Equipment - Power Meter & Light Source
Power Meter & Light Source are used as final fiber optics cable test requirement. The test would indicate either fiber optics loss (after installation and jointings) is within specified fiber budget loss. If final fiber optics loss measurement is less than optimal fiber budget loss then the fiber optics would be accepted and finally handed over to client.
How to conduct Fiber Optics Loss ?
Power in a fiber 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.
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!
Turn on the source and select the wavelength you want for the loss test. Turn on the meter, select the "dBm" or "dB" range and select the wavelength you want for the loss test. 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.
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 fiber, 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.
While it is difficult to generalize, here are some guidelines:
- For each connector, figure 0.5 dB loss (0.7 max)
- For each splice, figure 0.2 dB
- For multimode fiber, 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 fiber, 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:
Fiber Budget Loss = (0.5 dB X # connectors) + (0.2 dB x # splices) + fiber loss on the total length of cable
How to conduct Fiber Optics Loss ?
Power in a fiber 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 fiber 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:
Turn on the source and select the wavelength you want for the loss test. Turn on the meter, select the "dBm" or "dB" range and select the wavelength you want for the loss test. 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.
Testing 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 fiber, 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 generalize, here are some guidelines:- For each connector, figure 0.5 dB loss (0.7 max)
- For each splice, figure 0.2 dB
- For multimode fiber, 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 fiber, 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:
Fiber Budget Loss = (0.5 dB X # connectors) + (0.2 dB x # splices) + fiber loss on the total length of cable
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