Power Meter (OPM) Test & Inspection
An Optical Power Meter (OPM) is a device used to measure optical power out from the end of a fiber optic cable or connector. Also known simply as a Power Meter.
When an optical power meter is combined with an Optical Light Source (OLS), it becomes an Optical Loss Test Set (OLTS) and can measure end-to-end attenuation. It can also be used with a Visual Fault Locator (VFL).
Optical power meter measures the amount of optical power in a fiber. It is analogous to the volt-ohm-millimeter (VOM) used in electronics. Most models handle several wavelengths and are adjustable. The power level can be indicated in µw or in dB (absolute) ordBm (relative to one milliwatt or as dBr( relative to a previously noted value). Some of them are available with internal memories to store the day’s work and a thermal printer for hard copies. Although above function can be automatically determined by the meter itself. However, there is usually a range switch which determines the range of signal power expected.
Choose a power meter with the following features
1: Calibrated against National Institute of Standard & Technology:
If the light source and power meter are to be used to check an installation or repair on a commercial basis, the customer will need assurance that the instrument are telling the truth. The proof of this is provided by a calibration certificate for each instrument which must be renewed at intervals, usually annually. This calibration must be carried out by an authorized company whose instruments themselves are calibrated against the appropriate national standards . In this way, we can trace the accuracy back to its source.
2: Power Measurement (dBm):
The range of the meter with different sensor heads, is -80 dBm (10µw) to +33 dBm (2W).
3: Zeroing Function (dB):
Power meter zero itself on the dBr scale.
4: 0.01 dB Resolution:
Resolution is selectable at 0.1 or 0.01dB. Resolution determines whether the reading will be one or two decimal places.
5: Selectable wavelengths:
Different sensors are available for use at different power levels and operating wavelengths from 400 to 1800 nm.
6: Standard batteries:
Standards batteries should be choose for smooth & stable performance of the power meter.
7: Size and weight:
Handheld power meter can normally be connected for testing at different wavelengths 850,1310 and 1550nm
Draw Backs of Optical Power Meter:
If a fiber optic link has become unreliable, then meter will only show the overall loss of the system while unable to indicate which part of the system is responsible for that additional loss.
Another drawback to the use of an optical power meter in many applications( especially in dark fibers measurements) is that both ends of the fiber must be available.
These problems are solved by OTDR.
Power Loss measurement using Power meter:
The meter can be used for a number of measurements. The benefit of using power meter is that they are able to measure the actual power loss of the fiber system. Power meter will describe its use of:
A: Fiber attenuation measurements. ( Fiber Loss Measurement)
- Having dBr mode
- Having no dBr Mode
B: Connector Insertion loss measurement. (Insertion Loss Test)
A: Fiber Loss Measurement
1: Having dBr mode. Connect the meters together using the patch cord . Select the required wavelength on the power meter. Switch to dBm mode. Wait until the readings have stabilized. At this stage the power meter will indicate the incoming power level in dBm. The patch cord loss will be approximately (0.05 dBm) small enough to be ignored. Set the power meter to dBr and it will accept the incoming light level as the reference level. The reading will now change to 0 dBr. It is important to note that both the light source & power meter should remain ON in order to remember the level set as 0 dBr. Disconnect the patch cord. Connect the fiber to be tested at the transmitter and plug in the light source. To the far end, connect the power meter. The power meter will immediately show a new figure such as -8.2 dBr. This is the loss of the overall system.
2: Having no dBr mode:
Step 1: Light Source Output:
Read the light source power out in dBm. Let’s assume this to be -10 dBm
Step 2: Power out from Link:
Leaving the meters switched on and connect them to fiber optic link. The power meter would now indicate -18.2 dBm.
Step 3: Net Result:
The loss in fiber optic link is then the difference between step1 & step2
(-10.0) – (-18.2) = 8.2 dB
B: Insertion Loss Test
First measure the power through a length of fiber. Cut the fiber from the center and then put a connector or splice to the exposed ends. Again measure the power through the cable. The difference between the two readings is the insertion loss contributed by the connector or splice.