A typical optical power meter uses a diode sensor to measure output power from the source. The sensor responsivity is dependent on the wavelength of the laser.
Calibrate the meter with the transfer standard.
Calibration is a common process that engineers use to check the accuracy and performance of measuring equipment. Unless you work in a laboratory that specializes in calibration, you probably don't think of calibration in the context of fiber-optic (FO) test equipment, but FO optical power meter and other instruments require periodic calibrations to meet their claimed accuracy.
To calibrate an optical power meter, you need to use a transfer standard, which is a thermal detector that is spectrally insensitive over the wavelength regions of interest for FO power measurements. The standard is usually connected to a measurement system that includes laser diodes, fibers, connectors, fiber splitters, monitor detectors, and lenses.
During the calibration process, the technician first measures one laser source and then substitutes the signal from another laser source. The second source is then adjusted until the meter's output matches the first source's power output, as shown in Figure 3.
This process has been used to calibrate optical power meters for years, but it still has inherent uncertainties. These include inconsistencies in optical coupling, about 1% at every transfer, and slight variations in wavelength calibration.
However, despite the uncertainty, NIST has been working to reduce this uncertainty through continued collaboration with instrument manufacturers and private calibration labs. The best result so far has been a worst-case uncertainty of less than 5%, which is within the combined standard uncertainty for NIST primary standards.
The main reason that the calibration of optical power meters has been difficult is the semiconductor detectors in these instruments. These semiconductors exhibit strong wavelength dependence. This makes them difficult to use in a wide range of FO wavelength bands.
For this reason, most manufacturers have characterized the performance of their devices at selected wavelengths. This allows them to apply correction factors that adjust the readings of the power meters at non-calibrated wavelengths.
Using a transfer standard to calibrate an optical power meter can be a simple and effective way to correct for this wavelength dependence in the detectors. The resulting measured value can be compared with the corresponding reference standard to verify the accuracy of the meter's reading.
Calibrate the detector.
Optical power meters are often used to measure the amount of light entering or leaving an optical fiber. To be accurate, the detector needs to be calibrated at the correct wavelength and power level.
A power meter's accuracy is affected by many factors, including the detector, beam geometry and electronics design. These effects are often difficult to accurately assess, but they must be taken into account when choosing an instrument for a specific application, or comparing one with a similar instrument in another manufacturer's catalog.
Most optical pon power meter use silicon (Si), Germanium (Ge) or Indium-Gallium-Arsenide (InGaAs) semiconductor detectors, which are sensitive to light in the wavelength ranges and power levels common to fiber optic testing. They can saturate at higher power levels, and the level at which this occurs depends partly on the beam profile or geometry, and partly on the instrument electronics design.
However, it is important to select a detector with good responsivity to the wavelengths you want to test, and one that has a high level of detector uniformity. Photodiode-based instruments tend to suffer from this, and there are better options available with integrating spheres or other specialised sensor heads that provide the best detector uniformity.
Alternatively, some manufacturers offer high power meters that include a "high power" attenuating filter in front of the detector, or specialised meter electronics. These have significantly lower accuracy than an integrating sphere, so should only be specified where this is truly necessary.
Attenuating elements can have varying levels of wavelength sensitivity, coherence sensitivity, polarization sensitivity and reflection levels. These factors can have a huge effect on accuracy, so always check the attenuating element specification carefully.
The most common type of detector for power meters is a photodiode, which is a silicon-based device that can detect incoming and outgoing optical signals at a particular wavelength and power level. The resulting signal can be amplified and displayed on the display screen.
An alternative to an optical power meter is a calorimeter that uses a laser diode to generate a pulse of light, and measures the heat from the laser radiation. These can be useful in measuring the output of some pulsed fiber optic transmitters, but they are usually less accurate than an optical power meter because the laser diode instability affects both the reading and the time-averaged readings of the detector.
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Calibrate the splitter.
The optical power meter is a great tool for testing fiber loss, continuity and transmission quality. However, it does not function as well if it is not properly calibrated. In particular, it is crucial to calibrate the splitter before you use it in your tests.
A typical optical power meter consists of a sensor, measuring amplifier and display. The sensors are typically photodiodes selected for the appropriate range of wavelengths and power levels. The meter displays the measured optical power and set wavelength on the display unit.
Optical power meters are typically wavelength dependent, and a properly calibrated instrument will respond to only the correct test wavelength. If there are other spurious wavelengths present, then wrong readings will result. This is a problem in many applications, especially in the field where different wavelengths are often used simultaneously.
One solution to this issue is the use of a fiber splitter. In particular, a fixed makeup-flow splitter can reduce errors by diverting some power to a monitor while passing the rest of the light directly to the meter.
The meter can also be calibrated using the tunable laser system. This is an excellent and affordable solution for wavelength calibration in a lab or on-site.
To calibrate the meter using this technique, you simply select the appropriate tunable laser diode (typically 1310 nm or 1550 nm) for your application. The tunable diode is connected to the meter and the beam from the diode is then collimated with a special fixture before it is delivered to the ECPR chopper wheel. The resulting averaged ECPR power reading is then transferred to the monitor. This is the most efficient and accurate way to achieve this feat.
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Calibrate the power meter.
Every fiber optic meter, whether used by a test lab or by a customer on an OTDR, must be calibrated to ensure accuracy. Calibration is also important for maintaining compliance with customer contracts and regulations.
To perform a calibration of an optical power meter, the meter must be paired with a transfer standard or reference instrument that has a wavelength range of the same order as the meter. This enables the meter to be tested against a reference measurement, which typically has a worst-case uncertainty of less than 5% (or 0.2 dB).
The transfer standard used in the calibration is usually a laser source plate or a collimated-beam or connectorized-fiber configuration, which is a set-up of a light source that produces an output at 1310 or 1550 nm. ECPR or a similar transfer standard from esz AG is usually used for this type of calibration, although some meters may have their own transfer standards.
Once the meter has been paired with the transfer standard, the meter must be stabilized so that its readings are in agreement with the reference measurements after the power is applied. Stabilization should be done at a rate of +-0.003 dB over 15 minutes and better than +-0.001 dB over a few minutes to minimize fluctuations in the readings.
After stabilization, the meter must be compared to the reference readings using a sequential method. The sequential method combines the power readings on the two instruments at different times and measures the average difference. This difference is called the calibration factor and is a measure of the meter's accuracy.
Optical power meters that have been operating for some time are often found to have a slope, or error, that can affect the accuracy of its readings. A slight change in the slope can make a big difference, so it's best to have it checked periodically.
Performing a spindown calibration before every ride is essential for maintaining accuracy on your cycling computer. This can be done by pressing the "Set Zero" button within your AXS app or on a compatible cycling computer. This will zero out the power output to your bike, ensuring that it's measuring accurately before you start pedaling.
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