What Are the Measurement Units for Power

8/3/2019 What Are the Measurement Units for Power

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What are the measurement units for power?Optical power is measured in linear units of milliwatts (mW), microwatts(uW - really the greek letter "mu"W), nanowatts (nW) and decibels (dB).What is the difference between "dBm" and "dB"?dB is a ratio of two powers, for example the loss in a fiber optic cable.

When power is measured in linear units (mW, uW or nW), dB is calculatedon a log scale using this formula:

power (dB) = 10 log (power1/power2)

If we are measuring absolute power levels, the measurement is generallyreferenced to 1 milliwatt (mW), is expressed as "dBm" and the calculationbecomes:

power (dBm) = 10 log (power/1 mW)

Thus 1 mW = 0 dBm, 1 uW = -30 dBm, 1 nW = -60 dBm and two equalpowers compared are 0dB (eg. power being the same, there is no loss.)

What power level should a source have?It depends on the type of source. When coupled into a good test cable, thesource output power will be in these ranges:LED: -10 to -25 dBm into 62.5/125 fiberTelcom/LAN laser: 0 to -13 dBm into singlemode fiber, to +20 dBm withDWDM and fiber amplifier systemsCATV Laser : +16 to 0 dBm into singlemode fiberWhat power level should a receiver see?

It depends on the network and type of source. When measured at the endof the network cable, the source output power will usually be in theseranges:LAN/LED: -20 to -35 dBm into 62.5/125 fiberTelcom/LAN laser: -20 to -45 dBm into singlemode fiberCATV Laser : 0 to -10 dBm into singlemode fiber

How do you calculate a loss budget?The loss budget is a calculation of how much attenuation a link shouldhave. You compare that loss to the dynamic range of the networking

equipment to see if the range and link loss are compatible.How accurate are fiber optic power meters?All optical power meters which are calibrated to NIST (the US standardsbody) or any national standards lab will measure optical power to anuncertainty of about +/- 0.2 dB or 5%. Therefore, since every power meterhas an uncertainty of +/- 0.2 dB, any two meters can differ by 0.4 dB in theworst case (one at +0.2 dB and one at -0.2 dB) even if both are within their


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specification!More information on calibration uncertainty.

Are more complex or higher priced FO power meters more accurate?The high priced meters offer better dynamic range and more features, but

not better absolute measurement uncertainty.Why is the measurement uncertainty so high? That is because there arethree to four calibration transfers from the NIST absolute optical powerstandard before the meter is delivered to the customer. The NIST standardhas an uncertainty of about 1% itself and every transfer adds errors ofabout 1%. More information on calibration uncertainty.

Why do most meters only offer calibrations at a few wavelengths?NIST only offers calibrations at 850, 1300 and 1550 nm, so those metersthat have calibrations at other wavelengths have to extrapolate to thosevalues, increasing the measurement uncertainty at those wavelengths.

If my source is at a slightly different wavelength from the standardcalibration wavelength, doesn't that add to measurement error?Perhaps, but the wavelength of most sources is not known by the personmaking the measurement. If everyone uses meters calibrated at only a fewspecific wavelengths, everyone can be testing to the same standard andwill get more closely correlated mesurements on sources of unknownwavelengths .

Fiber Optic Cable LossStandard: FOTP-171 for cable assemblies

Standard: OFSTP-14 for the installed multimode cable plant, OFSTP-7 forthe installed singlemode cable plant

As of 2011, OFSTP-14 has been replaced by an international standard that as of

mid-2011 is very controversial. First of all, it allows the use of either insertion losstesting with a light source and power meter or OTDR testing. This was predicatedon comparisons of OTDR tests on cable plants for 10GbE of less than 2 dB loss.

For other more typical multimode links of 5-10 dB, the two methods will generally

give divergent results. Since insertion loss is designed to test according to how thelink will be used, it should be the test used for longer links. The new standard also

includes a new metric for measuring mode power distribution in multimode fiber

calledEncircled Flux (EF). It is also controversial, but international documentssay the older source with mandrel wrap meets their requirements. Here is a more

detailed explanation of all the options in cable testing.

Why do you use a launch cable on the source?You use a launch cable to set the proper test conditions for testing anothercable. The launch cable should match the fiber size and connector type of


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the cable you want to test, and be tested to insure it is a low lossconnector.

Why can't I just attach my cable directly to the source?Sources have a great deal of variety in how they launch light into the cable,

which can cause undesirable variations in loss measurements.Furthermore, the coupled power can vary considerably with each insertion,depending on the alignment of the connector ferrule in the source outputconnector.

What makes a launch or receive cable "good"?A good launch or receive cable will have low loss - less than 0.5 dB losswhen tested in a single-ended FOTP-171 test.

Do I always need a laser source to test singlemode?No, you can use a LED source for short SM patchcords or cables up to

about 5 km long. Longer SM cables will show higher loss with an LED dueto the spectral width of the LED causing higher loss at the upper and lowerends of its spectral output. FOTP-171 actualy calls for LED sources to testSM patchcords to prevent problems with the interference caused by thecoherent light of the laser.

What is a receive cable?The receive cable is used in a double ended test to measure the connectorloss on both ends of the cable.

When doing a double-ended loss test, why don't you set the reference with

both launch and receive cables connected together?There are several reasons why you use the same reference method forsingle-ended and double-ended tests.If you want to measure the loss of connectors on both ends of the cablebeing tested, you need a reference with the launch cable only.A two cable reference removes one connector loss from the measurement,since you include it in the reference. You can also reference with threecables, where you simply replace the middle cable with the cable undertest, but the loss removes two connectors from the test value, as they areincluded in the reference. However all three methods are approved

standard methods and the three cable reference is sometimes the only wayyou can test cable plants with connectors like the MT-RJ that cannot bedirectly connected to test instruments. Here is a more detailed explanation ofall the options in cable testing.

What happens if the launch and/or receive cables have bad connectorswith say 3 dB loss.


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When the zero reference is set, it will include the loss of two badconnectors. When you attach a cable between them to test, you willmeasure erroneously high loss for one or both connectors on the cablebeing tested, invalidating the measurement, so it is very important to keepall test connectors clean.

How do I test cables with different connector types?Use a two or three cable reference as necessary. More.What should my cable loss be? Are my measurements correct?A basic guideline for loss measurements is to estimate the loss using theseapproximate values:Connector loss: 0.5 dB per mated pairSplice loss: 0.2 dB per spliceFiber loss:Multimode: 3 dB/km @ 850 nm, 1 dB/km @ 1300 nmSinglemode: 0.4 dB/km @ 1300 nm, 0.3 dB/km @ 1550 nm For example: a 1.5 km multimode link with 3 connections @ 850 nm:

Loss = (1.5 km X 3 dB/km) + (3 conn X 0.5 dB)Loss = 4.5 dB + 1.5 dB = 6 dB

You should also read this more detailed explanation and this page about"loss budgets".

How Long a Fiber Can My Meter and Source Test?That depends on the output power of the source and the sensitivity of themeter. For example, one of our LED sources will have a maximum outputinto 62.5/125 fiber of about -15dBm. Your meter should be used at power

levels above about 10 dB higher than its minimum spec. A meter can easilyread to -45 dBm (min spec is -55 dBm), giving us a range of 30 dB (-45dBm from -15 dBm gives us 30 dB). At 850 nm and a loss of 3 dB/km,that's 10 km of fiber, less our connector and splice loss, and at 1300 nm,aloss of 1 dB/km, it's 30 km less connector and splice loss, both lots longerthan any networks operating on multimode fiber.For singlemode testing, lasers can give you 0 to -10 dBm output, giving arange of 35 to 55 dB, corresponding to over 100 km of fiber, evenapproacing 200 km at 1550 nm!

Optical Time Domain Reflectometers (OTDRs)

Reference: Understanding OTDRs

Can't I use an OTDRto test cable loss?Well, Yes and No. The OTDR will measure the loss in the cable plant, butusing a technique based on backscatter signals that indirectly measureloss, unlike a source and power meter that measure loss directly. TheOTDR measurement technique doesn't correllate well with the source andmeter. Since the source and meter tests loss just like the transmission link,


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all standard cable plant tests specify using a source and meter to measureloss. You must always use them to test the cable plant loss. Even theoutside plant singlemode test standard, OFSTP-7, says you should acceptonly the source/meter results.

Why do I use an OTDR

?Use the OTDR for troubleshooting. If you have a cable break, especially inthe outside plant, the OTDR is the best way to find it. You can also use it toverify splice loss (but test both ways and average to get a reliablemeasurement) or find problems with back reflection (optical return loss).

How do I see close features with an OTDR?The blind spot of an OTDR caused by crosstalk from the test pulse can beovercome by using a "pulse suppressor", a long (1 km is normal) length ofcable to allow the OTDR to settle down after the initial pulse.

Bandwidth Measurements

Do I need to test bandwidth?Generally no. Most systems are specified for use with a minimumbandwidth fiber and most fiber is much better than minimum specification.Besides test equipment is not cost effective or readily available for fielduse. Manufacturers of fiber and cables have the expensive lab equipmentto reliably test bandwidth (or actually dispersion), but there are no goodfield testers. If you need bandwidth data for an unusual application, ask themanufacturer of the fiber or cable to assist you or use a simulationprogram, which are available from some manufacturers of fiber. One exception is long distance networks which need testing for chromatic

dispersion (CD) and polarization mode dispersion (PMD). Here is more on CD and

PMD.

Reflectance and Optical Return LossStandard: FOTP-107More on Reflectance Testing

What is Reflectance and Optical Return Loss?Reflectance is the light reflected back from a connector or splice. OpticalReturn Loss (ORL) is generally used to combine the reflectance fromconnectors or splices with the backscatter from the fiber, so the term isprimarily used for longer cable runs. Reflectance was once called backreflection, but that term, which is really redundant, has lost favor.


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When do I need to test optical return loss?Reflectance or optical return loss mostly affects very high bitrate digital oranalog singlemode systems. None of today's multimode systems are verysensitive to reflectance or ORL although high amounts can createbackground noise in short links adversely affecting BER or data transfer.

For affecting laser sources, ORL is only important in the first fewconnectors in the cable nearest the laser transmitter. But some short SMcable plants used in premises systems are too short to attenuate thereflected power from connectors so undergoes multiple reflections until itbuilds up background noise that cab affect receivers.

How do I test optical return loss?Use an OTDR on cable plants, OCWR on patchcords. ORL testing withwhat people call a ORL tester ( or what Telcordia/Bellcore calls an OCWRor optical continuous wave reflectomenter) is only applicable to shortpatchcords. If you try using them on an installed cable plant, the integrated

backscatter from the length of the fiber will overwhelm the connector backreflection. Twenty km of fiber gives about the same backscatter as aconnector with 20 dB ORL and you cannot tell where the backscattercomes from! It's better to use an OTDR to find ORL problems on installedcable plant.

How accurate are ORL measurements?The measurement uncertainty of ORL is very high, about +/-1 dB forsinglemode and +/-5dB for multimode, according to round robin resultsfrom standards committees. This is a function of the difficulty in creating a

reference for the measurement, the reflection is very small compared to thetest signal causing noise problems and the dependence of themeasurement on the connector on the test apparatus is very high. Tominimize uncertainty, keep the connections extremely clean and inspectthe connectors continuously with a microscope and repolish when needed.Making measurements to a 0.01 dB resolution is ridiculous; remember thisis a +/- 1 dB measurement.

Do I need special instruments to test ORL?No, the special ORL tester is unnecessary. A good laser source and powermeter, along with a coupler that costs a few hundred dollars will make a

very good tester.

What Are the Measurement Units for Power

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