Fiber Optic Series: Understanding dB and dBm

When conducting tests on fiber optic networks, the results are typically presented on a meter readout in dB. In this context, optical loss is quantified in dB, while optical power is measured in dBm. It's common for both loss and power measurements to yield negative values, causing confusion for many fiber optic technicians.

In fiber optic measurements, we evaluate the power carried by the light. The standards for power measurements, set by the United States National Institute of Standards and Technology (NIST), are determined based on the heating effect of light absorbed in a detector. Every fiber optic power meter is calibrated traceable to the NIST standard, ensuring consistency among different meters within calibration uncertainty limits.

Optical power in fiber optics is akin to the heating power of a light bulb but at significantly lower power levels. Unlike a 100W light bulb, most fiber optic sources operate in the milliwatt to microwatt range (0.001 to 0.000001W), making the power emitted from a fiber negligible and generally non-harmful.

In the early days of fiber optics, source output power was measured in milliwatts, and loss was measured in decibels (dB). Over time, all measurements shifted to dB for convenience.

Loss measurements were conventionally expressed in dB, as it represents a ratio of two power levels with one serving as the reference. The logarithmic scale of dB, where each 10 dB signifies a ratio of 10, provides a convenient and easily memorable value. When there's loss in a fiber optic system, the measured power is less than the reference power, resulting in a negative logarithmic value and a negative dB reading on the meter. Despite the meter displaying a negative number, convention dictates referring to the loss as a positive value. For example, a meter reading of "-3.0 dB" signifies a loss of 3.0 dB.

Optical power measurements use the unit dBm, with the "m" denoting the reference power, set at 1mW. Thus, a source with a power level of 0 dBm corresponds to 1mW. Instruments measuring in dB can be optical power meters or optical loss test sets (OLTS), with optical power meters usually reading in dBm for power measurements or dB concerning a user-set reference value for loss.

Loss (dB) = -10 log(Po/Pi) or 10 log(Pi/Po)

Common measurement values in fiber optics

Below are typical measurements in fiber optics for optical power and loss:

Typical Power Levels Measured by an Optical Power Meter:

Telecom Transmitters:

Range: 0 to +10 dBm (1 to 10 milliwatts)

Receivers: -30 dBm (1 microwatt)

DWDM Systems with Fiber Amplifiers:

Range: +10 to +20 dBm (10 to 100 milliwatts)

Receivers: -20 to -30 dBm (1-10 microwatts)

Data Links and LANs:

Range: 0 to -10 dBm (1 to 10 milliwatts, 850 VCSEL)

Range: -10 to -16 dBm (25 to 100 microwatts, LED)

Receivers: -16 to -30 dBm (1-25 microwatts)

Typical Losses of Fiber Optic Components 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)


Range: 0.3 - 0.75 dB (7-16%)


Range: 0.05-0.3 dB (1-7%)

Power-measuring instruments

Instruments utilizing dB measurements can be optical power meters or optical loss test sets (OLTS). The optical power meter typically indicates readings in dBm for power measurements or dB concerning a user-set reference value for loss. While the majority of power meters have ranges spanning from +3 to –50 dBm, most sources fall within the range of 0 to –10 dBm for lasers and –10 to –20 dBm for LEDs. It's worth noting that only lasers employed in CATV or long-haul telephone systems, equipped with fiber amplifiers, possess powers high enough to pose genuine danger, reaching up to +20 dBm – equivalent to 100 milliwatts or a tenth of a watt!

The OLTS or the power meter on the dB scale gauges relative power or loss relative to the reference level set by the user. The measurement range is influenced by the output power of the source in the unit and the sensitivity of the detector. For multimode fiber, an OLTS utilizing an LED source typically covers a range of 0-30 dB, which proves more than sufficient for the majority of multimode cable plants with losses under 10 dB. On the other hand, single-mode networks employ lasers and might exhibit loss ranges of up to 30-40 dB for long-haul telecom systems. However, campus cabling with single-mode may only incur 1-3 dB loss. Consequently, a single-mode OLTS may vary between short and long systems.


In summary, dB measures loss, dBm measures power, and the more negative the dB value, the higher the loss. It's crucial to set the zero before measuring loss and periodically check it during measurements to ensure accurate results.

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