Measuring Power
| Measuring Power |
| Measuring power of transmission |
| One of the most important testing procedures because it states whether transmitters and transmitter links are maintaining their minimum specifications Optical Power is stated in A linear value in the unit of W (mW, �W, or nW) A logarithmic value in the unit of dBm Formula for dBM absolute power level relative to 1 mW x dBm=10∙logâ¡ã€–(opt.power P [mW])/(1 mW)〗 Formula to convert dBm to mW P [mW]=〖10〗^((x [dBm])/10) Positive value for dBm shows that the power is greater than the reference value of 1 mW Negative value for dBm shows that the power is smaller than the reference value of 1 mW (ex: 3 db in reduction is a 50% loss) On average power levels for POF transmissions are between -2 dBm (0.63 mW) and -26 dBm (0.0025 mW) There are various test devices to measure the power loss in a POF transmission with accuracy up to .01 dB Devices usually consist of a laser or LED transmitter and a photodiode receiver Types of power testing methods FOPT-171 A FOTP-171 test uses only a single launch reference cable to test the cable. This method allows testing a single cable from either end to find out if one connector is bad. It�s main use is testing patchcords to insure both connectors are good, but it can also be used to troubleshoot installed cables where one connector is suspected of being bad. The 0 dB loss reference is made by connecting the power meter to the output of the launch cable and measuring the power output. The cable under test is connected to the launch cable and the meter. The loss measured is only the loss of the mated connectors and any loss of the fiber in the cable, usually very small when testing patchcords this way. The fact that the connector on the launch cable and the cable under test are mated directly to the meter, with it�s large detector, means that the connection loss to the meter is calibrated out of the loss test, allowing testing of only the connector mated to the launch cable. Reference document: FOTM, Chapter 6,7,9,17, DVVC, Chapter 12,13, 14 OFSTP-14/OFSTP-7 OFSTP-14/OFSTP-7 are used for testing installed and terminated cable plants, where we want to test the connectors on each end and everything in between. So we use a meter and source with two reference cables – one on each end. The big issue with this test method is how one sets the 0 dB reference. Reference document: FOTM, Chapter 6,7,9,17, DVVC, Chapter 12,13, 14 OFSTP-14/OFSTP-7 offers three options on how one sets the 0 dB reference. Method B: with one reference cable (the launch cable) This method sets the �0 dB reference� with the power meter measuring the output of the launch cable directly, so that no connector loss is included when setting the reference. Then when testing a cable with both launch and receive cables, the loss includes the loss of both connectors on the cable under test and the loss of all the components in between. Method A: with two cables (launch and receive cables) This method sets the �0 dB reference� with the launch cable mated to the receive cable, so that one mated connector loss is included when setting the reference. Then when testing a cable with both launch and receive cables, the loss includes the loss of connectors on the cable under test and the loss of all the components in between, less the loss of the mated connectors included in the reference. Method C: with three cables (launch, receive and a �golden� reference cables) This method sets the �0 dB reference� with the launch cable and the receive cable, plus a �golden� reference cable mated to them, so that two mated connector losses and any fiber loss in the third cable are included when setting the reference. Then when testing a cable with both launch and receive cables, the loss includes the loss of connectors on the cable under test and the loss of all the components in between, less the loss of the mated connectors included in the reference. Reference document: FOTM, Chapter 6,7,9,17, DVVC, Chapter 12,13, 14 |