The Difference Between dB and dBm in Fiber Optics

Key Takeaways

• The units dB and dBm stands for decibel and decibel milliwatt, respectively.

• A measurement of 0 dBm using an optical power meter indicates 1 milliWatt of power.

•  The unit dB expresses the difference between two dBm values.

Optical fibers transmit optical power from the transmitter to the receiver. A typical fiber optic system consists of a source emitting optical power, a path formed by fiber optic cables, and a receiver. No fiber optic system is ideal, which implies there are losses in the system. 

Whenever tests are conducted on fiber optics, the quantities of interest are optical power and optical power loss. The optical power in fiber optic cables is measured in dBm, whereas optical power loss is measured in dB. It is possible to express optical power and power loss in the same unit, but the general practice is to use different units. The difference between dB and dBm in fiber optics is a common discussion point. In this article, we will explore why dB and dBm are used in fiber optics and what, exactly, their differences are.

Optical Power and Optical Power Loss in Fiber Optics

In fiber-optic technology, optical power is used to send data from transmitter to receiver. The optical power is measured in fiber optics at the transmitter end and receiver end. The optical power corresponds to the heating power of the light source. With high-power lasers, it is possible to measure the heat that is absorbed by the detector. With light sources of low power levels, the sensitivity of the detector is low and measurements are difficult. 

Optical Power Measurements

For measuring the optical power at the transmitter end, a test cable is inserted via connectors to the fiber optic source and the measurement is taken. For measuring the optical power at the receiver end, the receiver receptacle is disconnected from the fiber optic cable, and the cable optical output power is measured using an optical meter. The optical power measurement is usually expressed in dBm.

Optical Power Loss

When the optical power travels from the transmitter to the receiver, light energy gets lost due to scattering and absorption. The fiber loss, fiber optic connector loss, and losses in splices can reduce the light levels below acceptable limits. That is why the optical power loss calculation is important when you are looking for reliable fiber-optic communication. It also determines how long the fiber optic cable can be extended without disturbing communications. The optical fiber loss is usually expressed in dB. 

The Difference Between dB and dBm in Fiber Optics

The units dB and dBm stand for decibel and decibel milliwatt, respectively. One difference between dB and dBm in fiber optics is that the unit dB represents the variation in the quantities measured, whereas dBm refers to the current value of the quantity. The dB measurements (as given in the equation below) are based on comparisons between two quantities or measurements made with respect to a reference. The dBm measurements are absolute measurements. The unit dB expresses the difference between two dBm values. 

dB = 10 log (measured power/reference power) 

The unit dBm refers to the power level at the transmitter and receiver ends of the cable. Or, it is appropriate to say the power injected or power received in the fiber optic cables is expressed in dBm. 

A measurement of 0 dBm using an optical power meter indicates 1 milliWatt of power. The letter ‘m’ refers to milliwatt in dBm. The difference between the transmitter power (dBm) and receiver power (dBm) in fiber optic cables gives the optical power loss, which is expressed in dB. Even though the loss is negative, we express it as a positive value followed by dB. When the optical power level is halved or doubled, there is a 3 dB decrease or increase in the optical strength.

Both dBm and dB are in logarithmic scales and are non-linear measurement units. The dBm measurement is relative to milliwatts. The dBm values can be converted into watts, whereas this conversion is not possible with dB. 

It is important to understand the difference between dB and dBm in fiber optic measurements when working on optical communication systems. Optical power and optical power loss are common in all optically energized systems. Cadence’s software can help you in designing electronics, photonics, or fiber optic systems. 

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