What is an Optical Power Meter?

An Optical Power Meter is a special instrument used to measure the power of light emitted from the end of a fiber optic cable. This device is capable of accurately measuring the light within the exact wavelength and power range. They are also suitable for measuring power levels ranging from -15 to -35 dBm for multimode links and 0 to -40 dBm for single-mode links. They are typically adaptable to various connectors, including SC, ST, FC, SMA, LC, MU, and more. 

The standard unit for measuring optical power is dBm, which stands for decibels referenced to one milliwatt. The reason for using a logarithmic scale is due to the vast dynamic range of fiber optic links, which can exceed 1000 dB. Although some power levels may be expressed in microwatts, many meters are capable of directly measuring them. Most power meters are designed to operate at 850 nm and 1300 nm because these wavelengths are commonly used in fiber optic communications.

Optical power meters are designed to measure the average optical power, rather than the peak power, which makes them susceptible to the duty cycle of the transmitted data. Therefore, it is crucial to specify the test conditions when measuring the optical power of a transmitter or receiver in relation to the data being transmitted. In most networks, a diagnostic test signal is available for this purpose.

Block diagram of Optical Power Meter

The optical power meter block diagram consists of a photodiode, logarithmic current to voltage converter IC, microcontroller and an LCD display.

The photodiode is the primary light-sensing element in the optical power meter. It converts incoming light energy into electrical current. The amount of current generated is proportional to the intensity of the incident light. The output from photodiode is fed to logarithmic current to voltage converter IC. This IC is responsible for converting the logarithmic current generated by the photodiode into a corresponding voltage. It allows for a wide dynamic range of light measurements, as the optical power can span several orders of magnitude. The output from IC is given to the microcontroller for analog-to-digital conversion and current integration. 

The microcontroller serves multiple functions in the optical power meter. It includes an Analog-to-Digital Converter (ADC) that converts the analog voltage from the logarithmic converter into a digital signal. The microcontroller also performs current integration, which involves measuring and accumulating the photodiode current over a specific time period. This integration is essential for obtaining accurate power measurements. After this, the signal is sent to the LCD (Liquid Crystal Display) for optical power display. LCD is used to present the measured optical power to the user. The microcontroller processes the integrated current and converts it into a corresponding power value. This power value is then displayed on the LCD screen, allowing the user to read the optical power level.

Types of Optical Power Meter 

There are different types of optical power meters available. The measurement uncertainty of almost all fiber optic power meters is constrained by the physical limitations of transferring standards with optical connectors. Typically, most meters have an uncertainty of around +/-5 % or roughly 0.2 dB, irrespective of the display resolution.

Some of the different types of optical power meters are:

• Field Optical Power Meters
• Laboratory Optical Power Meters
• Specialized Fiber Optic Power Meters

Field optical power meters usually exhibit a resolution of 0.1 dB, whereas laboratory meters typically exhibit a higher resolution of 0.01 dB. Some specialized fiber optic power meters are available with a resolution as high as 0.001 dB.

When selecting the appropriate resolution for a measurement, it should be based on the specific test requirements. For laboratory measurements of low-loss patch cables, connectors, and fiber splices, a resolution of 0.01 dB is suitable, provided that the test conditions are carefully controlled to achieve an uncertainty of 0.05 dB or less.

When performing field measurements of absolute power, the precision is limited by the absolute calibration uncertainty, which may affect the accuracy of the measurement. However, relative power measurements can be made with a precision of 0.1 dB. 

Test Source for Optical Power Testing

The test source is a portable alternative to the source utilized in the fiber-optic communication network. It stimulates the signal in the fiber to test for loss using a power meter. Therefore, the test source must match the wavelength and source type of the system source.

Typically, LEDs at both 850 nm and 1300 nm are utilized to test multimode fiber, while lasers at 1310 nm and 1550 nm are used to test single-mode fiber. The test source for multimode fiber is usually an LED, except for high-speed networks such as Gigabit Ethernet which require laser sources. LEDs can be used to test single-mode fibers that are shorter than 5000 meters, but for long single-mode fibers, a laser source should be used instead.

It is crucial to match the wavelength of the test light source to the network requirements. While frequently referring to 850 nm, 1300 nm, or 1550 nm, the actual source wavelength may differ. This difference becomes significant for lengthy fiber connections, as commonly found in WANs or long-distance networks.

The attenuation coefficient of fiber is dependent on the wavelength of the source, which makes the spectral characteristics of the source significant. Therefore, when testing extended fiber lengths, it may be necessary to adjust for losses that deviate from the nominal source wavelength.

Applications of Optical Power Meter

Optical power meters are frequently used in telecommunications and networking to test and measure the power of optical signals transmitted through fiber optic cables. They help to ensure that the signal power is within the specified range and that the signal is not too weak or too strong, which can cause signal degradation or damage to the equipment.

They are also used in the manufacturing and testing of fiber optic components, such as cables, connectors, and transceivers. These optical power meters help verify the performance and quality of these components by measuring the output power of the devices and identifying any issues.

Optical power meters are used in medical and scientific research to measure the output power of lasers and other light sources. They are useful for calibrating equipment, ensuring consistent output, and verifying that the device is operating safely.

They are essential tools for the maintenance and troubleshooting of fiber optic networks. To identify and locate issues such as breaks, bends, and poor connections, as well as diagnose problems related to signal quality and strength, these optical power meters are used.

Optical power meters can be used in educational settings to teach students about fiber optic technology and measurement techniques. They help students understand the principles of light transmission and measurement and provide hands-on experience with industry-standard equipment.