How to choose an optical detector


When selecting a fiber optical light meter, it is very important to correctly match the power meter detector type to the application. The correct detector for loss and power measurements on fiber systems will generally be as follows:

Choose an InGaAs (Indium Gallium Arsenide) detector for:

  • General & Precision measurements over 1000 - 1650 nm (other InGaAs detectors) up to up to the maximum meter range.
  • CWDM or DWDM bands up to the maximum meter range.
  • Single mode-only testing.

Choose a Ge (Germanium) detector for:

  • Modest accuracy over 850 -1550 nm up to the maximum meter range.
  • General single mode & multimode testing

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, has some temperature sensitivity, and responsivity is more variable across the detector surface.

Choose an Si (Silicon) detector for:

  • Precision measurement at 600 - 1000 nm up to the maximum meter range.
  • POF, HCS and 850nm-only testing. It won't work for eg 1300 nm and longer

Choose a High Power 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. Kingfisher power meters can be used up to the maximum display range.
  • The peak signal power must be within the meter range.
  • High power meters have less Autotest sensitivity, which is a consideration for loss testing.
  • The KI2600-H5 offers the best balance for most high power users, with up to +24 dBm range & reasonable Autotest senstivity

Choose a Large Area Series (XL) detector for:

  • POF, MPO and other fibers above 200u active light diameter.
  • Select from Si, Ge or InGaAs using the above guidelines. 
  • HCS is 200u core diameter, so a standard meter is just OK for HCS installers, but a large area detector gives laboratory grade results.
  • MPO connectors with 16 and 32 fibers require the larger Ge7 detector, which also works with other MPO fiber counts.
  • The KI2600XL-H3B offers the highest power level of +33 dBm

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

Detector responsivity graphs @ long wavelengths

Ge and InGaAs detector responsivity from 1500 - 1650 nm

Ge detector effect of temperature on responsivity at 1580 nm

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