19: How to choose an Optical Detector

Written by Bruce Roberson and Brian Crook 97-03.Revised June 2007 © Kingfisher International

Introduction

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

Choose an InGaAs (Indium Gallium Arsenide) detector for:

General & Precision measurements over 850 - 1650 nm (new Kingfisher B series InGaAs detectors) up to +5 dBm.
General & Precision measurements over 1000 - 1650 nm (other InGaAs detectors) up to +5 dBm.
CWDM or DWDM bands up to +5 dBm.

Choose a Ge (Germanium) detector for:

Modest accuracy over 850 -1550 nm up to +10 dBm.

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

Work on WDM systems above 1550 nm, despite attempts by some competitors to say otherwise. If in any doubt, just look at the graph below. Calibration constants vary by about 2 dB between 1550 and 1625 nm, and are also very temperature sensitive.
1550 nm systems if cold temperatures are expected.
Precision or laboratory grade accuracy. Ge is inherently non-linear by about 0.04 dB and has some temperature sensitivity.

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

High power Measurements. These detectors are more expensive and have slightly lower accuracy than normal InGaAs detectors, so only specify them if actually required. H3B detectors are for very high power operation up to +27 dBm, and H5 detectors are for up to +15 dBm.

Choose an Si (Silicon) detector for:

Precision measurement at 600 - 1000 nm up to 0 dBm

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.

More Information

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