The simplest test we can make on a fiber optical system is to see whether light is getting through and how much is lost. The instruments used for this are called an optical source and a receiver. Together, they form a fiber optic test set.
Most fiber optic test sets work with several different wavelengths. The usual choices are 850, 1310, and 1550nm (nanometer). The receivers can be set for any one of these. The sources you will use are dual wavelength sources. They can provide signals at 1310nm or 1550nm . You might wonder why we need several wavelengths. The basic reason for this is that light of different wavelengths shows different losses in the same cable. In addition, you must “tell” the receiver the wavelength you are using because the detector reacts differently to different wavelengths. To use a test set, you connect the system being tested between the source and the receiver. To do this is not very complicated . You first make sure that the correct adapter is connected to each instrument. Next, connect a commercial cable to the chosen wavelength output of the source. Connect the other end to the input port on the receiver. Turn the source and receiver on (by pushing the appropriately marked button). Press either the WAVELENGTH or the λ button (depending on your meter) until the desired wavelength appears on the display . If your receiver has a zero button , you may zero the meter. To do this, just press the ZERO button . You next connect the cable (or other system) that you are testing between the commercial cable and the receiver. To do this, you will need to use a bulkhead connector . This is a metal piece that has two receptacles, one for the original cable and the other for the system you are testing. Check to make sure that the wavelength at which you are operating is the wavelength you want. Choose the units that you want to use. On some of the meters, your choice will be between dB, dBμ, and dBm. On others, your choices will be dBm or dB. Choose dBμ if it is available. If you have zeroed your meter, the readings will be in dB unless you change to dBm. Finally, you read the result. That’s all there is to the operation.
There is, however, the matter of interpreting the reading. If you choose to get results in dBμ or in dBm, you will need to subtract the reading with the cable you are testing from the reading with just the commercial cable. This difference will tell you the dB loss in the cable being tested . If you have zeroed the meter with the commercial cable, you can skip this step.
It will be helpful if you understand the use of dBm, dBm, and dB in the study of light. The section that follows should be useful.
Decibels, dBm, and dBm
Optical power is often expressed in units called dBm. These units tell us the optical power in a convenient way.
A microwatt is 1 millionth of a watt. Microwatt is abbreviated as mW. An ordinary 100 watt light bulb produces about 2 million microwatts of light power.
A table of some equivalents may be handy .
0 dBm = 1 microwatt (1mW)
-3 dBm = 0.5 microwatts (0.5 mW)
3 dBm = 2 mW
-6 dBm= 0.25mW
6 dBm = 4 mW
-9 dBm = 0.125 mW
9 dBm = 8 mW
-10 dBm= 0.1 mW
10 dBm = 10 mW
If you add 10 to the number of dBms, you multiply the power by 10. So, for example, since 6 dBm is the same as 4 microwatts , 16 dBm is the same as 40 microwatts. In a similar way, if you subtract 10 from a number of dBms , you divide the power by 10. (The actual definition of dBm is given by the following equation:
Power (dBm) = 10log10 (Power in microwatts ).)
A similar unit is called dBm. The equivalents shown above for dBm are correct for dBm except that wherever there is a “m” you substitute an “m.” “m” is the symbol for milli. For example, 3 dBm = 2 milliwatts instead of 2 microwatts. The prefix micro means 1 millionth, while t e prefix milli means 1 thousandth. You may also sometimes see plain dB with out a m or an m after i t. This stands f or decibel s. When i t refers to sound, it has a special meaning that we will not discuss here. When it refers to light, however, it is related to a ratio of powers. If, for example, we say that the difference between two light signals is 6 dB, it means that the power of the stronger signal is 4 times as strong as that of the weaker signal. Similarly, if the difference is 9 dB, the ratio is 8:1. As you can see, these ratios are the same as those for dBm and dBm, except that dBm and dBm express the ratio between the measured signal and a standard, either 1 microwatt or 1 milliwatt.
Discussion Questions
1. What is the power of a 23dBm optical source?
2. What is the power of a –16dBm optical source?
3. A certain source produces 18 microwatts of power. Another source produces 72 microwatts. What is the dB difference between these two sources?
