Table of Contents
- 1 What is negative loss in fiber?
- 2 What does negative dB loss mean?
- 3 How is loss displayed on an OTDR?
- 4 Why is return loss negative?
- 5 When would you use dB reading?
- 6 Why are dB in negative?
- 7 How accurate is an OTDR?
- 8 Is loss a positive or negative?
- 9 What causes a negative loss reading on a patch cord?
- 10 What is the backscatter coefficient of the fiber under test?
What is negative loss in fiber?
Negative loss is caused by the joining of two fibers with different backscatter coeffecients. A higher backscatter coefficient, on the second half of the connection, causes an increase in the back scatter on the other side of the event rather than the normal decrease resulting in what appears to be a negative loss.
What does negative dB loss mean?
So if dB is negative, that means ratio of measured power to reference power is less than 1 – the measured power is less than the reference power or in fiber optic terms, we are measuring a loss.
What is an acceptable dB loss on fiber?
0.5 dB per km @ 1550 nm. These are the minimum requirements. Be aware that fiber specifications typically contain tighter values. For instance, 0.5 dB per mated connector and 3.0 dB per km @ 850 nm.
How is loss displayed on an OTDR?
The power loss is a logarithmic function, so the power is measured and displayed in dB. The amount of light scattered back to the OTDR is proportional to the backscatter of the fiber, peak power of the OTDR test pulse and the length of the pulse sent out.
Why is return loss negative?
Return loss, which measures the amount of light reflected back toward the source, is also expressed in dBs and is always a positive number. Reflectance, which also measures reflection and is expressed in dB, is a negative number. High reflectance is not a good thing.
Is Loss positive or negative?
Gain is always expressed as positive dBs. -Cables, considered passive devices, will always have loss (after all they are lossy). Insertion Loss (attenuation) is always expressed as negative dBs.
When would you use dB reading?
dB is generally used to measure the difference in the power, voltage, or signal strength between two points in a cable or network. dBm is used to measure the strength of a signal at a given point in the cable or network. The abbreviation dB stands for Decibels. The abbreviation dBm stands for Decibel Milliwatts.
Why are dB in negative?
Since the decibel uses a human threshold as a constant, any sound pressure that is lower than the threshold of hearing will register as a negative decibel.
What is OTDR fiber testing?
What is an OTDR? It is a fiber optic instrument used to characterize, troubleshoot and maintain optical telecommunication networks. OTDR testing is performed by transmitting and analyzing pulsed laser light traveling through an optical fiber.
How accurate is an OTDR?
Assuming typical fiber attenuation of 0.20 dB/km at 1550 nm and splices every 2 km (loss of 0.1 dB per splice), a unit such as this one will be able to accurately certify distances of up to 120 km.
Is loss a positive or negative?
Loss also occurs at any connection points along the way such as connectors or splices. Insertion loss is expressed in decibels, or dBs, and should be a positive number as it indicates how much signal was lost by comparing input power to output power. In other words, signals always come out smaller than they go in.
What if there is a negative loss on the fiber AutoTest?
New to DTX 1.3 ( ), at the end of the Fiber Autotest, if there is a negative loss of more than -0.09 dB, a warning will be given. “How can I get a negative loss? Isn’t that a gainer?”
What causes a negative loss reading on a patch cord?
The principle causes of negative loss readings are: Not allowing the source to stabilize The following articles include a step to verify your Test Reference Cords. When you purchase patch cords, you will normally see a loss value enclosed with the patch cord. The generic standard for a mated fiber connection is better than 0.75 dB.
What is the backscatter coefficient of the fiber under test?
The fiber under test may be fiber from XXX vendor which has a backscatter coefficient of -63.3/-70.7 dB, so reflects quite more signal and gives an unexpected trace. By far the best solution is to have a launch lead using the same fiber as the link you are testing or with an equivalent backscatter coefficient.
What is a good mated loss for a fiber connection?
The generic standard for a mated fiber connection is better than 0.75 dB. Most patch cord vendors will achieve a mated loss better than this. What you may not know is that there is a standard for a reference mated connection.