Optical power meter

Last updated May 23, 2022

An optical power meter (OPM) is a device used to measure the power in an optical signal. The term usually refers to a device for testing average power in fiber optic systems. Other general purpose light power measuring devices are usually called radiometers, photometers, laser power meters (can be photodiode sensors or thermopile laser sensors), light meters or lux meters.

A typical optical power meter consists of a calibrated sensor, measuring amplifier and display. The sensor primarily consists of a photodiode selected for the appropriate range of wavelengths 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 broad spectrum 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 instruments or combined with other test functions such as an Optical Light Source (OLS), Visual Fault Locator (VFL), or as sub-system in a larger or modular instrument. Commonly, a power meter on its own is used to measure absolute optical power, or used with a matched light source to measure loss.

When combined with a light source, the instrument is called an Optical Loss Test Set, or OLTS, typically used to measure optical power and end-to-end optical loss. More advanced OLTS may incorporate two or more power meters, and so can measure Optical Return Loss. GR-198, Generic Requirements for Hand-Held Stabilized Light Sources, Optical Power Meters, Reflectance Meters, and Optical Loss Test Sets, discusses OLTS equipment in depth.

Alternatively, an Optical Time Domain Reflectometer (OTDR) can measure optical link loss if its markers are set at the terminus points for which the fiber loss is desired. However, this is an indirect measurement. A single-direction measurement may quite inaccurate if there are multiple fibers in a link, since the back-scatter coefficient is variable between fibers. Accuracy can be increased if a bidirectional average is made. GR-196, Generic Requirements for Optical Time Domain Reflectometer (OTDR) Type Equipment, discusses OTDR equipment in depth.

Sensors

The major semiconductor sensor types are Silicon (Si), Germanium (Ge) and Indium Gallium Arsenide (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 circuit, however, in addition to their basic wavelength response characteristics, each one has some other particular characteristics:

Si detectors tend to saturate 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.

Power measuring range

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

Calibration and accuracy

Optical Power Meter calibration and accuracy is a contentious issue. The accuracy of most primary reference standards (e.g. Weight, Time, Length, Volt, 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, linearity variations, beam geometry variations, detector saturation.

Therefore, achieving a good level of practical instrument accuracy and linearity is something that requires considerable 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 International Laboratory Accreditation Cooperation (ILAC) accredited calibration, which includes metrological traceability to national standards and external laboratory accreditation to ISO/IEC 17025 to improve confidence in overall accuracy claims.

Extended sensitivity meters

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.

Pulse power measurement

Optical power meters usually display time-averaged power. So for pulse measurements, the signal duty cycle must be known to calculate the peak power value. However, the instantaneous peak power must be less than the maximum meter reading, or the detector may saturate, resulting in wrong average readings. Also, at low pulse repetition rates, some meters with data or tone detection may produce improper or no readings. A class of "high power" meters has some type of optical attenuating element in front of the detector, typically allowing about a 20 dB increase in maximum power reading. Above this level, an entirely different class of "laser power meter" instrument is used, usually based on thermal detection.

Common fiber optic test applications

Measuring the absolute power in a fiber optic signal. For this application, the power meter needs to be properly calibrated at the wavelength being tested, and set to this wavelength.
Measuring the optical loss in a fiber, in combination with a suitable stable light source. Since this is a relative test, accurate calibration is not a particular requirement, unless two or more meters are being used due to distance issues. If a more complex two-way loss test is performed, then power meter calibration can be ignored, even when two meters are used.
Some instruments are equipped for optical test tone detection, to assist in quick cable continuity testing. Standard test tones are usually 270 Hz, 1 kHz, 2 kHz. Some units can also determine one of 12 tones,^[1] for ribbon fiber continuity testing.

Test automation

Typical test automation features usually apply to loss testing applications, and include:

The ability to set the unit to read 0 dB at a reference power level, typically the test source.
The ability to store readings into internal memory, for subsequent recall and download to a computer.
The ability to synchronize the wavelength with a test source, so that the meter sets to the source wavelength. This requires a specifically matched source. The simplest way of achieving this, is by recognizing a test tone, but the best way is by transfer of data. The data method has benefits that the source can send additional useful data such as nominal source power level, serial number etc.

Wavelength-selective meters

An increasingly common special-purpose OPM, commonly called a "PON Power Meter" is designed to hook into a live PON (Passive Optical Network) circuit, and simultaneously test the optical power in different directions and wavelengths. This unit is essentially a triple power meter, with a collection of wavelength filters and optical couplers. Proper calibration is complicated by the varying duty cycle of the measured optical signals. It may have a simple pass/ fail display, to facilitate easy use by operators with little expertise.

Wavelength sensitivity of fiber optic power meter is a problem when using a photodiode for voltage current measurement. If the temperature-sensitive measurement replaces voltage-current measurement by photodiode the wavelength sensitivity of an OPM can be reduced. Thus if the photodiode is reverse biased by a constant voltage source and supplied with constant current, when triggered by light the junction dissipates power. The temperature of the junction rises and the temperature rise measured by thermistor is directly proportional to the Optical power. Due to constant current supply, the reflection of power to photodiode is nearly zero and the transition to and fro of electrons between valence band and conduction band is stable.

Related Research Articles

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A photodiode is a semiconductor p–n junction device that converts light into an electrical current. The current is generated when photons are absorbed in the photodiode. Photodiodes may contain optical filters, built-in lenses, and may have large or small surface areas. Photodiodes usually have a slower response time as their surface area increases. The common, traditional solar cell used to generate electric solar power is a large area photodiode.

UV spectroscopy or UV–visible spectrophotometry refers to absorption spectroscopy or reflectance spectroscopy in part of the ultraviolet and the full, adjacent visible regions of the electromagnetic spectrum. Being relatively inexpensive and easily implemented, this methodology is widely used in diverse applied and fundamental applications. The only requirement is that the sample absorb in the UV-vis region, i.e. be a chromophore. Absorption spectroscopy is complementary to fluorescence spectroscopy. Parameters of interest, besides the wavelength of measurement, are absorbance (A) or Transmittance (%T) or Reflectance (%R), and its change with time.

Spectrophotometry Deals with visible light, near-ultraviolet, and near-infrared, but does not cover time-resolved spectroscopic techniques

Spectrophotometry is a branch of electromagnetic spectroscopy concerned with the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength. Spectrophotometry uses photometers, known as spectrophotometers, that can measure the intensity of a light beam at different wavelengths. Although spectrophotometry is most commonly applied to ultraviolet, visible, and infrared radiation, modern spectrophotometers can interrogate wide swaths of the electromagnetic spectrum, including x-ray, ultraviolet, visible, infrared, and/or microwave wavelengths.

A photometer is an instrument that measures the strength of electromagnetic radiation in the range from ultraviolet to infrared and including the visible spectrum. Most photometers convert light into an electric current using a photoresistor, photodiode, or photomultiplier.

Near-infrared spectroscopy (NIRS) is a spectroscopic method that uses the near-infrared region of the electromagnetic spectrum. Typical applications include medical and physiological diagnostics and research including blood sugar, pulse oximetry, functional neuroimaging, sports medicine, elite sports training, ergonomics, rehabilitation, neonatal research, brain computer interface, urology, and neurology. There are also applications in other areas as well such as pharmaceutical, food and agrochemical quality control, atmospheric chemistry, combustion research and astronomy.

A pyranometer is a type of actinometer used for measuring solar irradiance on a planar surface and it is designed to measure the solar radiation flux density (W/m²) from the hemisphere above within a wavelength range 0.3 μm to 3 μm. The name pyranometer stems from the Greek words πῦρ (pyr), meaning "fire", and ἄνω (ano), meaning "above, sky".

Laser Doppler velocimetry, also known as laser Doppler anemometry, is the technique of using the Doppler shift in a laser beam to measure the velocity in transparent or semi-transparent fluid flows or the linear or vibratory motion of opaque, reflecting surfaces. The measurement with laser Doppler anemometry is absolute and linear with velocity and requires no pre-calibration.

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An optical fiber is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means to transmit light between the two ends of the fiber and find wide usage in fiber-optic communications, where they permit transmission over longer distances and at higher bandwidths than electrical cables. Fibers are used instead of metal wires because signals travel along them with less loss; in addition, fibers are immune to electromagnetic interference, a problem from which metal wires suffer. Fibers are also used for illumination and imaging, and are often wrapped in bundles so they may be used to carry light into, or images out of confined spaces, as in the case of a fiberscope. Specially designed fibers are also used for a variety of other applications, some of them being fiber optic sensors and fiber lasers.

Distributed temperature sensing systems (DTS) are optoelectronic devices which measure temperatures by means of optical fibres functioning as linear sensors. Temperatures are recorded along the optical sensor cable, thus not at points, but as a continuous profile. A high accuracy of temperature determination is achieved over great distances. Typically the DTS systems can locate the temperature to a spatial resolution of 1 m with accuracy to within ±1°C at a resolution of 0.01°C. Measurement distances of greater than 30 km can be monitored and some specialised systems can provide even tighter spatial resolutions.

Fiber-optic communication Method of transmitting information

Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of infrared light through an optical fiber. The light is a form of carrier wave that is modulated to carry information. Fiber is preferred over electrical cabling when high bandwidth, long distance, or immunity to electromagnetic interference is required. This type of communication can transmit voice, video, and telemetry through local area networks or across long distances.

Transmissometer Meteorological instrumentation

A transmissometer or transmissiometer is an instrument for measuring the extinction coefficient of the atmosphere and sea water, and for the determination of visual range. It operates by sending a narrow, collimated beam of energy through the propagation medium. A narrow field of view receiver at the designated measurement distance determines how much energy is arriving at the detector, and determines the path transmission and/or extinction coefficient. In a transmissometer the extinction coefficient is determined by measuring direct light transmissivity, and the extinction coefficient is then used to calculate visibility range.

Electro-optical sensors are electronic detectors that convert light, or a change in light, into an electronic signal. These sensors are able to detect electromagnetic radiation from the infrared up to the ultraviolet wavelengths. They are used in many industrial and consumer applications, for example:

A laser beam profiler captures, displays, and records the spatial intensity profile of a laser beam at a particular plane transverse to the beam propagation path. Since there are many types of lasers — ultraviolet, visible, infrared, continuous wave, pulsed, high-power, low-power — there is an assortment of instrumentation for measuring laser beam profiles. No single laser beam profiler can handle every power level, pulse duration, repetition rate, wavelength, and beam size.

A fiber-optic sensor is a sensor that uses optical fiber either as the sensing element, or as a means of relaying signals from a remote sensor to the electronics that process the signals. Fibers have many uses in remote sensing. Depending on the application, fiber may be used because of its small size, or because no electrical power is needed at the remote location, or because many sensors can be multiplexed along the length of a fiber by using light wavelength shift for each sensor, or by sensing the time delay as light passes along the fiber through each sensor. Time delay can be determined using a device such as an optical time-domain reflectometer and wavelength shift can be calculated using an instrument implementing optical frequency domain reflectometry.

EMF measurements are measurements of ambient (surrounding) electromagnetic fields that are performed using particular sensors or probes, such as EMF meters. These probes can be generally considered as antennas although with different characteristics. In fact, probes should not perturb the electromagnetic field and must prevent coupling and reflection as much as possible in order to obtain precise results. There are two main types of EMF measurements:

A colorimeter is a device used in colorimetry that measures the absorbance of particular wavelengths of light by a specific solution. It is commonly used to determine the concentration of a known solute in a given solution by the application of the Beer–Lambert law, which states that the concentration of a solute is proportional to the absorbance.

References

↑ Ribbon Fiber Tone Tester

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[1] Ribbon Fiber Tone Tester

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