How to choose an optical detector

Bruce Robertson

Which detector do I need?

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:

InGaAs (Indium Gallium Arsenide)

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

Ge (Germanium)

  • Modest accuracy from 850 - 1550 nm, up to the maximum meter range.
  • Broad single mode & multimode testing.

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

  • Work on WDM systems above 1550 nm. Calibration constants vary by about 2 dB between 1550 -1625 nm, with high temperature sensitivity. Supporting graphs provided below. 
  • 1550 nm testing is also affected in cold temperatures.
  • Precision or laboratory grade accuracy. Ge is inherently non-linear by about 0.04 dB. It has temperature sensitivity, and responsivity is variable across the detector surface.

Si (Silicon)

  • 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.

H series / high power (attenuated InGaAs)

  • 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

XL series / large area

  • 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 power level is the system?

Maximum system power level expectations

  • The large majority of LAN or Telco digital transmission systems have maximum power levels below +5 dBm.
  • Specialist systems, typically RF or 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 tends to be a specialized or R&D system. Typically either no connector, or expanded beam connectors are used, due to the extreme power density.

Minimum system power level expectations

  • 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.

Useful graphs

Power meter responsivity wavelength dependence 

Optical Detector Responsivity Graph

  • Higher is better. More linear is better.

 

Ge vs InGaAs responsivity @ 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 response drift at low temperature

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

 


Applications

This application note assists in the following application areas...


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