The Optical Power Meters are the standard testers in a typical fiber optic technician’s tool kit. It is an invaluable tool during installation and restoration.
The main function of optical power meter is to display the incident power on the photodiode.Optical power meters measure transmitted and received optical power. For transmitted power, the power meter is connect directly to the optical transmitter output. For received power, the optical transmitter is connect to the fiber system. the power level is read using the power meter at the point on the fiber cable where the optical receiver would be.
Currently, power meter photodiodes use Silicon (for multimode applications), Germanium (for single-mode and multimode applications), and Indium Gallium Arsenide (InGaAs) (for single-mode and multimode applications) technologies. As shown in the following figure, InGaAs photodiodes are more adapted to the 1625 nm wavelength than Germanium (Ge) photodiodes, because Ge photodiodes are quite sensitive and drop off rapidly at the 1600 nm window.
Features found on more sophisticated optical power meters may include temperature stabilization. The ability to calibrate to different wavelengths. Also the ability to display the power relative to “reference” input. The ability to introduce attenuation, and a high power option.
The requirements for a power meter vary depending on the application. Power meters must have enough power to measure the output of the transmitter (to verify operation). They must also be sensitive enough, though, to measure the received power at the far (receive) end of the link. Long-haul telephony systems and cable TV systems use transmitters with outputs as high as +16 dBm and amplifiers with outputs as high as +30 dBm. Receiver power levels can be as low as –36 dBm in systems that use an optical pre- amplifier. In local area networks (LANs), though, both receiver and transmitter power levels are much lower.
The difference between the maximum input and the minimum sensitivity of the power meter is termed the dynamic range. While the dynamic range for a given meter has limits. Moreover the useful power range can be extended beyond the dynamic range by placing an attenuator in front of the power meter input. However, this limits the low-end sensitivity of the power meter.
For high power mode, use an internal or external attenuator. If we use an internal attenuator, it can be either fix or switch.
The typical dynamic ranges
Typical dynamic range requirements for power meters are as follows:
- +20 to –70 dBm for standard power applications
- +26 to –55 dBm for high power applications such as Analog RF transmission in cable TV (CATV) or video overlay in passive optical network (PON) systems. –20 to –60 dBm for LAN applications
Insertion Loss and Cut Back Measurements
The most accurate way to measure overall attenuation in a fiber is to inject a known level of light in one end and measure the level of light that exits at the other end. To measure insertion loss light sources and power meters are the main instruments recommend by ITU-T G650.1. And International Electrotechnical Commission (IEC) 61350 This measurement requires access to both ends of the fiber.
Cut Back Method
The cut back method is the most accurate measurement, but it is also destructive and cannot be applied in the field. For this reason,we don’t use it during installation and maintenance. Testing using the cut back method requires first measuring the attenuation of the length of fiber under test. Then, From the source a part of the length of the fiber is cut back. The attenuation is measure as a reference. Subtracting the two values provides the attenuation of the cut fiber.
Insertion Loss Method
The insertion loss method is a non-destructive method and can measure the attenuation across a fiber, a passive component, or an optical link. With this substitution method, measure the output from a source fiber and a reference fiber directly. Then, obtain a measurement with the fiber under test added to the system. The difference between the two results provides the attenuation of the fiber.The purpose of the first or reference measurement is to cancel out, as much as possible, the losses caused by the various patch cables.Contamination of the optical connector end face can cause significant change in optical power.
It is important to inspect the end face of both connectors to verify. That they are clean and un damage before mating.
The insertion loss method uses two steps to measure the attenuation along a fiber link . It is important to note that significant variations can occur in attenuation measurements. if precautions are not taken with the injection conditions.