Optical power meters are devices with a sensor head for receiving a light input beam and an amplifier for measuring the optical power. The display shows the set wavelength, the measured power and possibly other data.
Normally, these instruments come with one photodiode for a particular power range and a sensor head for receiving the input beam. Some special sensor heads have an integrating sphere, which allows measuring divergent input beams with a quite high precision.
How to use?
An optical power meter is a test instrument used to measure the absolute optical power and the loss in fiber optic signals. It consists of a calibrated sensor, measuring amplifier and display unit. The sensor primarily consists of a photodiode that has been selected for the appropriate range of wavelengths and power levels. The measured optical power and the set wavelength are displayed on the device’s display screen.
Optical power meters are available as stand-alone bench or handheld instruments or as part of a larger or modular instrument that includes other test functions such as an Optical Light Source (OLS), Visual Fault Locator (VFL), and/or OTDR. They are commonly used to measure absolute optical power, but they can also be used with a matched light source to determine optical loss.
When testing fiber cables, it is important to use an optical power meter in combination with a light source. During the test, an optical light source will send a specific wavelength of light down the fiber, and the power meter will read that light and determine the amount of loss.
This method is typically more accurate than using a thermal or photodiode-based meter, because the responsivity of optical components is rather wavelength-dependent. This is especially true for lasers, where the responsivity changes significantly with the wavelength. However, it is still possible to achieve a reliable and accurate measurement with a photodiode-based power meter if the device is specifically matched to a laser source.
The wavelength of the light source should be a close match to the wavelength of the equipment being tested, so that the meter will accurately measure the total power and loss in the fiber. This is not as easy to do as it sounds, but there are some options that can help you achieve this.
A matched source will also ensure that the light being sent down the fiber is at a consistent wavelength for a given distance, so that it is easier to determine the loss in the fiber. This method can also be useful when testing long-haul or high-bandwidth fiber networks, since it can reduce ambient light rejection problems.
Power Measurement
Optical power meters are used in fiber optic networks to determine the average optical power levels of transmitters and receivers for a given transmission link. This allows for the proper level of output power, which is important for bandwidth and performance.
Typical power meters have a digital display that displays linear units such as milliwatts or decibels referenced to one watt, and logarithmic units such as dBm. Some meters also have the option to measure microwatts, which are less common than milliwatts.
They may be designed for specific narrow wavelength ranges, offer different telecoms, or possibly even be able to store many power readings so that you can use them in various situations. Some of these meters have automatic wavelength recognition, which is useful when you want to perform a loss test on a fiber.
To perform an optical fiber identifier measurement, first plug the light source into the power meter and select the wavelength that you will be testing. For multimode cables, it is often necessary to attach a mandrel wrap to strip out higher modes of light that can interfere with the test results.
Once you have selected the appropriate wavelength, connect the light source to the power meter and then connect the fiber cable that is as close as possible to the fiber that you are testing. The power meter will get the power value from the light source and this will be displayed on the meter's screen.
Since the optical power meter is not always calibrated to the exact wavelength being tested, you must be careful when choosing the appropriate power measurement range for the fiber that you are testing. Using an incorrect power measurement range can cause erroneous results and can affect your ability to accurately assess fiber loss.
Another thing to consider when measuring optical power is the duty cycle of the data transmitted. You should not use a power meter when there is a high duty cycle, as this can cause the display to become distorted or appear to be stuck.
Optical power measurement is essential for ensuring a high quality fiber network and identifying potential issues before they cause damage to the cable or the end-user. With a wide range of fiber power meters and accessories for installation, maintenance, and troubleshooting, VIAVI offers the right device for every project and budget.
Loss Measurement
Optical loss testing using an optical power meter (OPM) is one of the most accurate methods to measure end-to-end signal loss. It is also the most commonly used technique by telecoms and cable installers to ensure fiber networks are installed to specifications.
During the test, a wavelength of light is sent down the fiber and an optical power meter measures the amount of signal loss. This measurement is displayed in dB, a dimensionless unit which is a ratio of the measured value to a reference value.
The accuracy of an Optical Laser Source Meter is dependent upon the wavelength setting and the sensitivity of the measurement instrument. Low cost field units usually have a resolution of 0.1dB, laboratory meters display 0.01dB, and a few specialized instruments can provide a resolution as high as 0.001dB.
Most handheld optical power meters are very intuitive, presenting simple controls for wavelength selection, memory functions, unit display, and zero/reference buttons to establish baseline values. Many are available with an autorange feature which automatically sets the range to the lowest or highest power level if no user input is provided. However, some manufacturers of metering instruments have reported problems with this feature, particularly in cases where a user makes a few measurements near the autorange point.
Some optical power meters can be configured with integrated visual fault locators to assist in troubleshooting near-end fiber optic issues during installation and testing. This feature allows the technician to quickly and easily identify near-end fibers that may be in need of cleaning or replacement, or may have a problem.
The ability to clean and inspect the connector ends of a fiber optic network is one of the most critical factors in ensuring that an optical power meter will perform as designed and report accurately. Fortunately, some next-generation devices combine fiber cleaning and inspection capabilities with the proven reliability of a handheld power meter for complete end-to-end loss testing. These devices include direct attachment connector designs and fully adjustable test arms to help reduce the risk of debris or contamination compromising the performance of an optical power meter.
Calibration
An optical power meter is one of the most common pieces of test equipment used in fiber optic networks, and it requires regular calibration to maintain accurate and consistent measurements. In some cases, contracts require proof that all instruments used are calibrated annually.
Optical power is a measurement that is fundamental to almost all other measurements made in fiber optics and cable plant. It is used to measure transmitter output and receiver input when testing fiber links, as well as measuring loss of a cable plant or any of its components, such as an OTDR.
Calibration of an optical power meter is performed by using a traceable calibration standard. It is a very important part of the process, and it ensures that the measurement will be correct and repeatable. It also allows the meter to be tested for nonlinearity and absolute power, as these factors will impact the final calibration results.
The calibration method is typically conducted in a laboratory, and it involves the use of an optical power meter, a light source, and a reference power meter. It is essential to consider the spectral bandwidth of the light source, as it may influence the accuracy of the meter and affect the overall measurement. The spectral width of the laser must be narrow enough to minimize the effect of wavelength uncertainty and ensure that the source center wavelength is within +- 10 nm of the meter’s reading.
During calibration, the power meter should be connected to the light source and the optical signal measured as a function of time, so that the source power can be recorded. The power meter must be calibrated for the specific connector and adapter that will be used for the measurement, as well as for environmental operating conditions.
Another key factor in a successful calibration is ensuring that the instrument is in good condition. It must not be exposed to dust or moisture and should be cooled properly during the process. This will reduce the risk of contamination and improve the accuracy of the measurement.
The accuracy of an optical power meter can be affected by the insertion loss and return loss of the light source and the attenuator. These losses are very significant and can cause the meter to be out of specification. Therefore, minimizing these effects during the calibration process can greatly increase the accuracy of the power meter.
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