FOA Guide


Fiber Optic Measurement Units: "dB" and "dBm"

Whenever tests are performed on fiber optic networks, the results are displayed on a power meter, OLTS or OTDR readout in units of “dB.” Optical loss is measured in “dB” which is a relative measurement, while absolute optical power is measured in “dBm,” which is dB  relative to 1mw optical power Loss is a negative number (like –3.2 dB) while power measurements can be either positive (greater than the reference) or negative (less than the reference.) Should a meter be set for dB or dBm?

In the early days of fiber optics, source output power was usually measured in milliwatts and loss was measured in dB or deciBels. Over the years, all measurements migrated to dB for convenience. This was when the confusion began.
 
Confused? Many fiber optic techs are too. Let’s see if we can clear up some of the confusion.


Typical Measurement Values in Fiber Optics
Here are some typical measurements in fiber optics of optical power and loss. You may want to come back to this section as you read the explanations of dB and dBm below.


Typical power levels measured by an optical power meter:

Telecom transmitters: 0 to +10 dBm (1 to 10 milliwatts), Receivers: -30 dBm (1 microwatt)
DWDM systems with fiber amplifiers: +10  to +20 dBm (10 to 100 milliwatts), Receivers: -20 to -30 dBm (1-10 microwatt)
Data links and LANs: 0 to -10 dBm (1 to 10 milliwatts, 850 VCSEL), -10 to -16 dBm (25 to 100 microwatts, LED), Receivers: -16 to -30 dBm (1-25 microwatts)

Typical losses of fiber optic components which can be measured by a light source and power meter (LSPM), OLTS or OTDR:

Fiber attenuation: Multimode: 3 dB/km at 850 nm (50% loss of power per km), 1 dB/km at 1300 nm (21% loss/km), Singlemode: 0.4 dB/km at 1310 nm (9% loss/km), 0.25 dB/km at 1550 nm (5.5% loss/km)
Connectors: 0.3 - 0.75 dB (7-16%)
Splices: 0.05-0.3 dB (1-7%)


Measuring Power
When we make fiber optic measurements, we are measuring the power in the light coming out of the end of a fiber. The measurement may be optical power from a test source, a transmitter or the input of receiver, measured in dBm, which is "absolute" power - absolute in that it refers to power calibrated to a national standard, so two people testing the same fiber output with different power meters calibrated to the same standard will measure the same power level within measurement uncertainty of the meters calibration. If we are measuring loss, like of a connection, we measure power before the connection and after the connection and compare the two measurements to get a relative measurement of loss in dB.

This page will try to explain this in more detail.

The primary calibration standards we use for power measurements, maintained by NIST (the US National Institute of Standards and Technology,) are actually determined by the heating effect of the power of the light as it is absorbed in a detector. Every fiber optic power meter sold is calibrated traceable to the NIST standard in the US or similar primary standards worldwide so different meters should measure the same power, within the limits of calibration uncertainty.

Optical power in fiber optics is similar to the heating power of a light bulb, just at much lower power levels. While a light bulb may put out 100 watts, most fiber optic sources are in the milliwatt range (0.001 watts), so you won’t feel the power coming out of a fiber and it’s generally not harmful. (Except for DWDM systems with fiber amplifiers or lasers used for surgery or welding. DWDM systems with fiber amplifiers can carry enough power to pit the fiber end or even ignite cables if a fiber in cracked!) Remember you can't see this light either because it is in the infrared wavelengths beyond the sensitivity of the human eye.

Read more on optical power measurements. 

Understanding dB
In the very early days of fiber optics, source output power was usually measured in milliwatts and loss was calculated in dB or deciBels. Over the years, all measurements migrated to dB for convenience. This was when the confusion began.

Loss measurements were generally measured in dB since dB is a ratio of two power levels, one of which is considered the reference value - that's "0 dB" for loss measurements. dB is a logarithmic scale (remember “logs” from high school math?) where each 10 dB represents a ratio of 10 times.

3 dB is a ratio of 2 times the reference power (gain)
-3 dB is a ratio of 1/2 (loss)
10 dB is a ratio of 10  (gain)
-10 dB is a ratio of 1/10 (loss)
20 dB is a ratio of 100 (gain)
-20 dB is a ratio of 1/100 (loss)
30 dB is a ratio of 1000 (gain)
-30 dB is a ratio of 1/1000, (loss), etc. 
When the two powers are equal, dB = 0, a result of the log scale used in dB but a convenient value that’s easily remembered.

 
Absolute optical power is measured in dBm or dB referenced to 1 milliwatt, about the power of a typical laser, and expressed as dBm. Here is a graph that shows the relationship of dBm to milliwatts and microwatts.

dB to Watts

The actual equation used to calculate dB when the power is measured in watts is:

dB

Using this equation, 10 dB is a ratio of 10 times (either 10 times as much or one-tenth as much), 20 dB is a ratio of 100, 30 dB is a ratio of 1000, etc. When the two optical powers compared are equal, dB = 0, a result of the log scale used in dB but a convenient value that’s easily remembered.

More on dB math below.

In most fiber optic power meters, the readings are in dB, not watts, so the measurement of dB is expressed more simply - no logs, just subtraction of two values in dB:

dB = measured power(dB) - reference power (dB)

The table below shows the ratio of power for dB differences in power:

dB (gain)

Power ratio

dB (loss)

Power ratio

0

1.000

0

1.000

0.1

1.023

-0.1

0.977

0.2

1.047

-0.2

0.955

0.3

1.072

-0.3

0.933

0.4

1.096

-0.4

0.912

0.5

1.122

-0.5

0.891

0.6

1.148

-0.6

0.871

0.7

1.175

-0.7

0.851

0.8

1.202

-0.8

0.832

0.9

1.230

-0.9

0.813

1

1.259

-1

0.794

2

1.585

-2

0.631

3

1.995

-3

0.501

4

2.512

-4

0.398

5

3.162

-5

0.316

6

3.981

-6

0.251

7

5.012

-7

0.200

8

6.310

-8

0.158

9

7.943

-9

0.126

10

10

-10

0.1

20

100

-20

0.01

30

1000

-30

0.001

40

10000

-40

0.0001

50

100000

-50

0.00001

60

1000000

-60

0.000001

Compare the positive and negative dB across the rows. The ratio of the positive dB is the inverse of the negative dB, e.g. +10dB is a ratio of 10 times and -10 dB is a ratio of 1/10 or 0.1. Thus 10 dB is a ratio of 10 times: +10 dB means the power measured is 10 times greater than the reference power and -10 dB is one-tenth as much. Some of the numbers are easy to remember and may be useful. For example, +3 dB is a factor of two in power and -3 dB is a factor of one-half.

When the two optical powers compared are equal, dB = 0, a convenient value that is easily remembered. If the measured power is higher than the reference power, dB will be a positive number, but if it is lower than the reference power, it will be negative. Thus measurements of loss are expressed as negative numbers.


dB

Here is an Excel spreadsheet that calculates dB/power ratio and dBm/milliwatts.


Measuring Power
Measurements of optical power are expressed in units of dBm. The “m” in dBm refers to the reference power which is 1 milliwatt. Thus a source with a power level of 0 dBm has a power of 1 milliwatt. Likewise, -10 dBm is 0.1 milliwatt and +10 dBm is 10 milliwatts. Fiber optic sources may vary from -20dBm to +20dBm and receiver power may go as low as -40dBm.

dBm = 10 log (measured power / 1mw)

When the power measured is 1mw, the equation becomes:

dBm = 10 log (1mw / 1mw) = 10 log (1) = 0 dBm

or

dBm = measured power(dB) - reference power (0dB) = dB = measured power(dB) - 0

If the power is greater than 1mw, say 2mw, the equation becomes:

dBm = 10 log (2mw/ 1mw) = 10 log (2) = +3dBm (rounded off a little)

If the power is less than 1mw, say 0.5mw, the equation becomes:

dBm = 10 log (0.5mw/ 1mw) = 10 log (0.5) = -3dBm (rounded off a little)

That's not hard to remember. Positive dBm means power greater than 1mw and negative means less than 1mw. A good laser source for a singlemode link will have a power output of ~ +3 to +6 dBm - 2-4mw - coupled into the fiber. A VCSEL for multimode links should have a power around 0dBm - 1mw. And a LED, used in older multimode links, has a typical power of -10 dBm - 0.1mw or 100microwatts.

Example:

W to dB

Here is an example of the conversion of watts to dBm. This meter is reading 25microwatts - that's 0.025milliwatts. If we convert to dBm, it becomes -16.0dBm. We can easily figure this out using the table of power ratios above. -10dBm is 1/10 of a milliwatt or 0.100mW. -6dB below that is a factor of 0.25 so 0.1mW X 0.25 = 0.025mW or 25microwatts. The other way to figure it is -10dB is 1/10 and -6dB is 0.25 or 1/4th so -16dBm is 1/40milliwatt or 0.025milliwatts or 25microwatts.

More on dB math below.
 
We can show the relationship of dBm and milliwatts by the graph at the top of the page or a version of the table shown below it.


dBm

Milliwatts

dBm

Milliwatts

0

1.000

0

1.000

0.1

1.023

-0.1

0.977

0.2

1.047

-0.2

0.955

0.3

1.072

-0.3

0.933

0.4

1.096

-0.4

0.912

0.5

1.122

-0.5

0.891

0.6

1.148

-0.6

0.871

0.7

1.175

-0.7

0.851

0.8

1.202

-0.8

0.832

0.9

1.230

-0.9

0.813

1

1.259

-1

0.794

2

1.585

-2

0.631

3

1.995

-3

0.501

4

2.512

-4

0.398

5

3.162

-5

0.316

6

3.981

-6

0.251

7

5.012

-7

0.200

8

6.310

-8

0.158

9

7.943

-9

0.126

10

10

-10

0.1

20

100

-20

0.01

30

1,000

-30

0.001

40

10,000

-40

0.0001

50

100,000

-50

0.00001

60

1,000,000

-60

0.000001



Measuring Loss

If we have loss in a fiber optic system, the measured power is less than the reference power, so the ratio of measured power to reference power is less than 1 and the log is negative, making dB a negative number. When we set the reference value, the meter reads “0 dB” because the reference value we set and the value the meter is measuring is the same. Then when we measure loss, the power measured is less, so the meter will read “ – 3.0 dB” for example, if the tested power is half the reference value. Although meters measure a negative number for loss, convention has us saying the loss is a positive number, so we say the loss is 3.0 dB when the meter reads  – 3.0 dB.

Here is a short movie of what happens when we induce loss in a cable by stressing it and watch the display of a power meter. We start at -20.0dBm and after stress is added to the cable to cause loss, the power level goes down to -22.3dBm, showing our stress on the cable caused 2.3dB loss.

dB on power meter

Here is the math of calculating this loss:

dB = measured power(dB) - reference power (dB) = -22.3 dBm- (-20dBm) = -22.3 + 20 = 2.3 dB (remember that subtracting a negative number has two minuses with becomes a +.)

More on dB math below.

 
Look at this animated simulation of a laser/singlemode system with 1mw power from the source and watch the meter reading.

attenuation in a link
Watch carefully as the transmitter couples a signal into the fiber. As the signal pulse travels down the fiber, it is attenuated by the fiber, suffers more loss in the connection, then is attenuated more until it reaches the receiver. See how the power in the signal decreases as it travels down the fiber, becoming more negative when measured in dBm.

Note 1: If you are used to making measurements of loss with a light source and power meter, you are used to loss being a negative number. But some manufacturers of optical loss test sets, which include a source and meter, show dB loss as a positive number. They were probably confused by the fact that everybody says "the loss is X dB" not "the loss is -X dB. And they never looked at the math. Or learned math.

Note 2: Sometime in the past the IEC redefined attenuation by flipping the power measured and the power reference to make attenuation a positive number (and therefore gain an negative number.)  This is averse to all other standards that use dB and mathematical convention.
Undoubtedly some instrument manufacturer wanted the definition that way and had no broad knowledge of measurement convention. Nor did they understand fiber optic power meters. We assume it was just to make an optical loss test set read a positive number, but it has certainly confused many people. See below.

FOA has a simulator to help you learn the process of measuring loss with a light source and power meter.


Power-Measuring Instruments
Instruments that measure in dB can be either optical power meters or optical loss test sets (OLTS). The optical power meter usually reads in dBm for power measurements or dB with respect to a user-set reference value for loss. While most power meters have ranges of +3 to –50 dBm, most sources are in the range of 0 to –10 dBm for lasers and –10 to –20 dBm for LEDs. Only lasers used in CATV or long-haul telephone systems with fiber amplifiers have powers high enough to be really dangerous, up to +20 dBm – that’s 100 milliwatts or a tenth of a watt!

The OLTS or the power meter on the dB scale measures relative power or loss with respect to the reference level set by the user. The range they measure will be determined by the output power of the source in the unit and the sensitivity of the detector. For multimode fiber, an OLTS using a LED source will usually measure over a range of 0-30 dB, more than adequate for most multimode cable plants which are under 10 dB loss. Singlemode networks use lasers and may have loss ranges of up to 30-40 dB for long-haul telecom systems, but campus cabling using singlemode may only have 1-3 dB loss. Thus a singlemode OLTS may be different for short and long systems.

Read more on fiber optic instruments.

Conclusion
If you remember that dB is for measuring loss, dBm is for measuring power and the more negative a number is, the higher the loss, it’s hard to go wrong. Set your zero before measuring loss and check it occasionally while making measurements.


Here is an Excel spreadsheet that calculates dB/power ratio and dBm/milliwatts.


More on calibration and metrology (the science of measurements) in fiber optics.


More pages of information on fiber optic measurements


Return to the FOA Guide Table of Contents 

Return to the FOA home page 



Understanding dB Math

Let’s start with the equation that defines dB that should be familiar to most of you, the equation for attenuation in fiber optics:
dB math

Let’s do some simplification. First manipulate the equation to get the “10” over to the left side of the equation by dividing both sides by 10:
 dB math

Now we need to deal with what is a “log” or logarithm function. A logarithm is the exponent or “power” to which a base must be raised to yield a given number, for example:
dB math


Based on that, we can further manipulate the equation above to get the equation expressed as 10 to the power of dB/10:

dB math
So if we convert 20dB this way, showing it step by step,
 

dB math
 
Thus 20 dB means the ratio of measured power to reference power is 100:1. Likewise 10dB is a factor of 10 and 30dB is a factor of 1000.
 

Now there is one more thing to learn about logarithms, they can be positive or negative numbers. Consider this where dB is negative:

dB math


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.



 

Note: Sometime in the past the IEC redefined attenuation thusly:


IEC definition of attenuation in dB

where (quoting from the standard)
  • A is the attenuation, in dB
  • P1 is the optical power traversing cross-section 1 (e.g. before the attenuation you are measuring - what we would call the "0dB" reference in testing cables)
  • P2 is the optical power traversing cross-section 2. (e.g. after the attenuation you are measuring - what we would call the measurement of loss in testing cables)
Note 1 to entry: Attenuation is a measure of the decreasing optical power in a fibre at a given wavelength. It depends on the nature and length of the fibre and is also affected by measurement conditions.

What Happened?
As we traced this definition in other IEC standards, we find they are variations of this definition, and one specifically states that it expresses attenuation as a positive term. 

So there you have it - why attenuation is positive - and therefore gain - like a gainer on an OTDR - is a negative number. The IEC standards just turned the measurement upside down - reversing "Measured Power" and "Reference Power" to get the term to become a positive number in dB when it's attenuation.

IEC is unique. See
References below. Undoubtedly some instrument manufacturer wanted the definition that way and had no broad knowledge of measurement convention. Nor did they understand how fiber optic power meters work.

Three issues with the IEC definition:

First:
There are several reasons to object to this from a mathematical and measurement standpoint. When you measure something against a reference, it's common to divide the measured value by the reference. Thus if something is getting smaller, like attenuation, and the change is the measured value decreases by 50% or half, you expect the ratio of powers to be a number less than 1 because the value has decreased, in this case the ration would be 1/2 or 0.5 0r 50%.

Consider what happens when using the equation above. If P1 is the reference and P2 the value after it decreases, the ratio for the example above would be 2. Wouldn't anybody assume that the measured value had increased instead of decreased it the ratio was 2? 


Second: There are several reasons to object to this from a mathematical and measurement standpoint. When you measure something against a reference, it's common to divide the measured value by the reference - like we do defining dBm where the reference is 1mw.

dBm definition

We checked and the TIA and IEC standards for measuring power, FOTP-95, still defines dBm this way. That's good, because we're used to negative dBm being power smaller than 1mW and positive dBm being power larger than 1mW.

However if one makes an attenuation measurement using a fiber optic power meter calibrated in dB and you used the "Zero" control to set the reference
, the resulting measurement of loss will be a negative number. Likewise if you measure the two powers in dBm, the resulting measurement of loss will be a negative number, if you understand negative numbers.

Note: dBm is defined as Power(measured)/Power(1mw) (see FOTP-95, Sec. 6.2) and if dBm were defined in this upside down manner, power levels below 1mW would be positive numbers, not negative as they are now, and power levels above 1mW would be negative! How's that for confusing.


Third: The definition assumes you are making measurements in linear units - Watts, milliwatts or microwatts, then calculating dB. Does anyone do that anymore? We don't think so. Instruments measure in dB and dBm. Recognizing that, some standards actually tell you how to calculate using simple subtraction of dB or dBm measurements but reverse the values so loss is positive and gain negative.

Maybe it's time to drop the definition from the standards or at least provide descriptions of how one makes measurements in dB.


References: The method for calculation of attenuation in dB IEC uses in these fiber optic standards is definitely not how measurements are normally defined. In fact we looked at several dozen websites and the result was 100% - attenuation is a negative value.
Rapid tables  
Wikipedia- If P is greater than P0 then LP is positive; if P is less than P0 then LP is negative.  
Wikipedia - definitions of the International Systems of Quantities - If P is greater than P0 then LP is positive; if P is less than P0 then LP is negative
TonTechnik-Rechner - see Electric Power (telephone)  
UC San Diego Neurophysics - they get it! - (-3dB = half power)  
UC Santa Cruz - with the measured value less than the reference, we get a negative dB value 
Henry Ott Consultants -  The unit can be used to express power gain (P2>P1), or power loss (P2<P1) -- in the latter case the result will be a negative number.
Electronics Notes - Where there is a loss, the deciBel equation will return a negative value  


 


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