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Optical Power Measurement Solutions for Telecom Applications

Over 55 years ago, optical fiber was already contemplated as a more reliable alternative to copper wire in telecommunications. Optical cable can carry much more data than copper wire, is free of electromagnetic interference, and ensures less loss of a signal due to attenuation or noise.

Just to get an idea of the difference, research has shown that copper cable can lose 94% of the signal over a distance of only 100 meters, while optical cable loses only 3% over the same distance!

Lasers and LEDs for optical telecom will usually be in the near-infrared (=NIR) range of 1260 to 1625 nm, instead of visible light. This is because there are fewer losses due to attenuation and dispersion of a signal as the wavelength increases–so it’s preferable to use NIR wavelengths. However, using optical signals for telecom has its own unique problems:

  • For testing the equipment, you need specialized sensors for the spectral range and power range used in telecommunications.
  • There are losses as a beam exits a fiber, going either into another fiber, or into free space. For this, we may require a fiber optic adapter and connectors.
  • Since the beam diverges upon exiting a fiber, we may require an integrating sphere to capture the entire beam. 

Sensors for Telecom Lasers

Telecom applications can employ lasers over a very wide range of powers–which requires sensors suitable for anywhere from 10 picowatts to 300 milliwatts. Photodiode detectors, such as Ophir’s PD300-IRG (Fig. 1), are specially designed for monitoring Telecom wavelength signals, both from an optical fiber or through free space. The PD300-IRG has a spectral range of 800 nm to 1700 nm, and a power range of 10 picowatts to 200 milliwatts.

Analyzing the sensor output

There are a number of options for analyzing the sensor output. The output can be read out by a suitable stand-alone meter, such as the StarBright or Vega meter (Fig. 2). Ophir’s meters are “plug-and-play”, so the sensors and meters can be interchanged and each sensor’s calibration and related data moves with the sensor. Also, Ophir meters can be hooked up to a PC by means of an RS232 cable, a USB cable, or an Ethernet cable, depending on the model of the meter.

There are also PC interfaces, such as the Juno+ (Fig. 3), which allows data to be read out directly from the sensor to a PC. Also, you have the option of using Ophir’s LabVIEW software, in order to analyze the data, or using any other data analysis tool on your PC.

Figure 1 : The PD300-IRG Sensor
Figure 2: The StarBright and Vega Stand-Alone Meters
Juno+
Figure 3: The Juno+ PC Interface, that hooks up the sensor to your PC

Fiber Optic Adapters and Connectors

When connecting a fiber to a detector, there are special connectors and adapters for different fiber types. Different fiber types you may encounter are an FC (Ferrule Connector—Fig. 4), or an ST (Straight-Tip—Fig. 5), and the like. These may require a bracket to hold the fiber in place on the sensor.

Figure 4: FC Fiber Adapter
Figure 5 :ST Fiber Adapter

Integrating Sphere Sensors

If a beam diverges significantly upon leaving an optical fiber, an integrating sphere should be added to the sensor to capture the beam. (If we’re dealing with an optical fiber with a low numerical aperture, we can use a photodiode sensor without an integrating sphere). For example, the 3A-IS-IRG (Fig. 6) is a photodiode sensor with an integrating sphere that can capture beams with divergence angles up to ±40 degrees. There are also larger integrating spheres that can be fitted and calibrated with a variety of detectors, such as the IS6-D-VIS (Integrating Sphere 6”-Divergent-for visible-light detectors–Fig. 7).

Figure 6: The 3A-IS-IRG Photodiode Sensor, with Built-In Integrating Sphere
Figure 7: The IS6-D-VIS 6″ Integrating Sphere

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