19: How to choose an Optical Power Meter Detector

Written by Bruce Roberson and Brian Crook 97-03.Revised Oct 2011 © Kingfisher International

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The correct detector for loss and power measurements on fiber systems will generally be as follows:

Choose an InGaAs (Indium Gallium Arsenide) detector for:

Choose a Ge (Germanium) detector for:

Don’t choose a Ge (Germanium) detector for:

Choose an Si (Silicon) detector for:

Choose an H Series (Attenuated Indium Gallium Arsenide) detector for:

What is the Maximum System Power Level going to be?

The large majority of LAN / Telco digital transmission systems have maximum power levels below +5 dBm.

Specialist systems, typically rf / analogue, without an optical amplifier, have maximum power levels below +15 dBm.

Long distance systems with an optical power amplifier have maximum power levels below +23 dBm.

On installed single mode systems with standard polished-fiber connectors (SC / LC / FC etc), total system power per fiber cannot go above +23 dBm per fiber, owing to connector power density limitations.
In fact for routine operations, +18 dBm is regarded as a maximum, above which special operational precautions are needed to avoid catastrophic connector / system failure.

Anything above +23 dBm is highly specialised, and will be either an r&d system, or will have expanded beam connectors to handle the extreme power density, or no connectors.

What is the Minimum System Power Level going to be?

It’s rare to encounter system power levels below -35 dBm.

Use of a fiber amplifier pre-amp can extend recover sensitivity down to between  -40 to -45 dBm.

Use of a power meter on a cabling system below about -45 dBm is problematic, due to the possibility of stray sunlight leaking into exposed cabling.

Wavelength Dependence

General responsivity graphs for Si, Ge and InGaAs detectors

Detector responsivity graphs @ long wavelengths

Ge and InGaAs detector responsivity from 1500 - 1650 nm

The above graph shows the room temperature response of power meters with Ge & InGaAs detectors as the wavelength is changed beyond 1500 nm. The Ge meter is unsuitable for work on CWDM and DWDM systems above 1550 nm, the InGaAs meter is obviously a much better choice, since it is very stable. This graph uses real measurement data.

Ge detector effect of temperature on responsivity at 1580 nm

The above graph shows how the 1580 nm thermal response of a power meter with a Ge detector changes with temperature. This instability makes Ge power meters basically unsuitable for field work on CWDM and DWDM systems above 1550 nm. The thermal stability below 1550 nm is much better, around 0.2 dB, however it’s never as good as InGaAs. This graph uses real measurement data.