An optical power meter (OPM) is a device used to measure the power in an signal. The term usually refers to a device for testing average power in systems. Other general purpose light power measuring devices are usually called,, power meters (can be sensors or ), or lux meters. An optical power meter (OPM) is a device used to measure the power in an signal. The term usually refers to a device for testing average power in systems. Other general purpose light power measuring devices are usually called,, power meters (can be sensors or ), or lux meters. A typical optical power meter consists of a , measuring and display. The sensor primarily consists of a selected for the appropriate range of and power levels. On the display unit, the measured optical power and set wavelength is displayed. Power meters are calibrated using a traceable calibration standard. A traditional optical power meter responds to a of light, however, the calibration is wavelength dependent. This is not normally an issue, since the test wavelength is usually known, however, it has a couple of drawbacks. Firstly, the user must set the meter to the correct test wavelength, and secondly, if there are other spurious wavelengths present, then wrong readings will result. Optical power meters are available as stand-alone bench or handheld instrum. The major types are (Si), (Ge) and (InGaAs). Additionally, these may be used with attenuating elements for high optical power testing, or wavelength selective elements so they only respond to particular wavelengths. These all operate in a similar type of, however, in addition to their basic wavelength response characteristics, each one has some other particular characteristics: • Si detectors tend to at relatively low power levels, and they are only useful in the visible and 850 nm bands, where they offer generally good performance.• Ge detectors saturate at the highest power levels, but have poor low power performance, poor general linearity over the entire power range, and are generally temperature sensitive. They are only marginally accurate for "1550 nm" testing, due to a combination of temperature and wavelength affecting responsivity at e.g. 1580 nm, however they provide useful performance over the commonly used 850 / 1300 / 1550 nm wavelength bands, so they are extensively deployed where lower accuracy is acceptable. Other limitations include: non-linearity at low power levels, and poor responsivity uniformity across the detector area.• InGaAs detectors saturate at intermediate levels. They offer generally good performance, but are often very wavelength sensitive around 850 nm. So they are largely used for single-mode fiber testing at 1270 - 1650 nm.An important part of an optical power meter sensor is the fiber optic connector interface. Careful optical design is required to avoid significant accuracy problems when used with the wide variety of fiber types and connectors typically encountered. Another important component is the sensor input amplifier. This needs very careful design to avoid significant performance degradation over a wide range of conditions. A typical OPM is linear from about 0 dBm (1 milli Watt) to about -50 dBm (10 nano Watt), although the display range may be larger. Above 0 dBm is considered "high power", and specially adapted units may measure up to nearly + 30 dBm ( 1 Watt). Below -50 dBm is "low power", and specially adapted units may measure as low as -110 dBm. Irrespective of power meter specifications, testing below about -50 dBm tends to be sensitive to stray ambient light leaking into fibers or connectors. So when testing at "low power", some sort of test range / linearity verification (easily done with attenuators) is advisable. At low power levels, optical signal measurements tend to become noisy, so meters may become very slow due to use of a significant amount of signal averaging. To calculate dBm from power meter output : The linear-to-dBm calculation method is: dB = 10 log ( P1 / P2 ) where P1 = measured power level ( e.g. in mWatts ), P2 = reference power level, which is 1 mW Optical Power Meter and accuracy is a contentious issue. The accuracy of most primary reference standards (e.g.,, Length,, etc.) is known to a high accuracy, typically of the order of 1 part in a billion. However the optical power standards maintained by various National Standards Laboratories, are only defined to about one part in a thousand. By the time this accuracy has been further degraded through successive links, instrument calibration accuracy is usually only a few %. The most accurate field optical power meters claim 1% calibration accuracy. This is orders of magnitude less accurate than a comparable electrical meter. Calibration processes for optical power meters are given in IEC 61315 Ed. 3.0 b:2019 - Calibration of fibre-optic power meters. Further, the in-use accuracy achieved is usually significantly lower than the claimed calibration accuracy, by the time additional factors are taken into account. In typical field applications, factors may include: ambient temperature, optical connector type, wavelength variations, variations, beam variations, detector saturation. Therefore, achieving a good level of practical instrument accuracy and linearity is something that requires design skill, and care in manufacturing. With the increasing global importance in the reliability of data transmission and optical fiber, and also the sharply reducing optical loss margin of these systems in data centres, there is increased emphasis on the accuracy of optical power meters, and also proper traceability compliance via (ILAC) accredited calibration, which includes metrological traceability to national standards and external laboratory accreditation to to improve confidence in overall accuracy claims. A class of laboratory power meters has an extended sensitivity, of the order of -110 dBm. This is achieved by using a very small detector and lens combination, and also a mechanical light chopper at typically 270 Hz, so the meter actually measures AC light. This eliminates unavoidable dc electrical drift effects. If the light chopping is synchronized with an appropriate synchronous (or "lock-in") amplifier, further sensitivity gains are achieved. In practice, such instruments usually achieve lower absolute accuracy due to the small detector diode, and for the same reason, may only be accurate when coupled with single-mode fiber. Occasionally such an instrument may have a cooled detector, though with the modern abandonment of Germanium sensors, and the introduction of InGaAs sensors, this is now increasingly uncommon.
