Types of Optical Time Domain Reflectometers
Different Categories of OTDR's
Looking at OTDR's can be confusing to people because there are a wide variety of different types of OTDR's (Optical Time Domain Reflectometer) to choose from. There are many different applications that will require you to have different functions available on your optical time domain reflectometer depending on the job that is at hand. Below we will discuss a range of OTDR's that will help to explain some of the differences. There are applications that require a higher dynamic range (measurement range) and there are OTDR's that will dig into some deeper, more complex applications. Either way, you need to be able to understand the different categories of OTDR's when you are testing, to better know the type that is needed for a job or even the type of OTDR testing that your company will be doing consistently.
Contractor OTDR's
There will be a higher variance of what is needed and what is required by a contractor. Depending on the day or the service provider or the work that is being done can change the scope of the Optical Time Domain Reflectometer that is needed to get the job done. Often when getting an OTDR, a contractor will look at specific features they might need or what will be the most flexible OTDR for the needs that could potentially arise for them. A modular OTDR would be a good start if you are looking for a certain capability now but down the road, you could get into a couple of other types of applications. A modular OTDR gives you the flexibility and ability to adapt to your testing needs. It has a mainframe and it allows you to mix and match modules. So if you just need single mode wavelengths now but eventually you will get into multimode then you can buy the mainframe with just a single mode OTDR module then later, when it is needed, buy a multimode module and swap them in and out. Another popular testing configuration is the CWDM and DWDM OTDR. There are modules for these as well, and they can be purchased to fit into an existing mainframe. So for contractors, there is the mindset of being flexible or having a couple of different types of OTDR's that will allow them to be able to perform the necessary tests for any project. They will be discussed in more detail later on.
PON OTDR's
PON or Passive Optical Network OTDR's are primarily used for Fiber To The Home (FTTH) applications. Fiber To The Home has several different characteristics that are essential to test for, so you need to make sure you have the correct equipment in the PON OTDR. One key component that is used in PON installations is the fiber optic splitter or coupler. These can be several different size splitters such as 1x4, 1x8, 1x16, 1x32, and even 1x64. Typically we are discussing the two higher count ones when talking about PON applications. When you shoot through these higher count splitters there is a higher level of attenuation or loss of signal that is expected. Typical loss when shooting through a 1x32 fiber optic splitter is between 17dB and 19dB. A PON OTDR has a higher dynamic range, allowing it to pass through, not only the splitter, but down the entire length of fiber optic cable that needs to be tested.
Another important feature on a PON OTDR is the wavelength that is specific to live fiber testing. There are actually two different ones, 1625nm and 1650nm, that are available but they do the same function. These wavelengths are essential when testing not only live fiber but also when verifying that a new fiber optic link will be equipped to handle this wavelength when the system is up and running. There are two different types of ports that you can get with one of those two wavelengths. One is a filtered wavelength port, with 1625nm or 1650nm. This port will be a separate specific wavelength port that will allow you to test fibers that are considered a live line. This means the fiber will have other wavelengths such as 1310nm and 1550nm already running through it, hence the need for the different wavelength of 1625nm. Also when it is filtered it will only be measuring the 1625nm wavelength and you won't need to worry about the other wavelengths that are in the live network. They get filtered out from reaching the OTDR detector, thus not corrupting the test results.
CWDM/DWDM
When talking about complex OTDR's this is where the CWDM (Coarse Wavelength Division Multiplexing) and DWDM (Dense Wavelength Division Multiplexing) OTDR's are no exception. Wavelength Division Multiplexing is the process of combining multiple signals at different wavelengths that are running over a single fiber. Coarse wavelengths are separated by 20 nanometers where dense wavelengths are separated by 0.4 nanometers in the spacing of the wavelengths.
In fiber optics, the low loss wavelengths, ranging from 1260nm to 1625nm are split into five different wavelength bands. These bands are the O, E, S, C, and L bands. The two bands of wavelengths that are effective for DWDM are the C and L bands. The C Band is 1530nm to 1565nm and the L band is 1565nm to 1625nm with the spacing mentioned above (0.4nm). When dealing with these applications you will actually need a dedicated OTDR that has the light source wavelengths that will be used in your system.
For CWDM there are different spectrums that can be used. The range is from 1460nm to 1625nm with a total of 18 different wavelengths that can be used. Along with the C and L bands, the S band is also utilized. The S band ranges from 1460nm to 1530nm. So the OTDR's that you will use to test them will have to be able to cover all of these different wavelengths.
These OTDR's are not cheap and they are wavelength specific. If you are testing these kinds of systems, along with other applications it would be smart to have a modular OTDR mainframe that allows you to switch modules in and out. These systems are typically used when trying to fit more bandwidth into an existing fiber link. By using the different wavelengths you are able to send several signals over a single fiber and they will not interfere with each other. The DWDM versions can be tunable so that they can cover the entire span of wavelengths that will be used. Another thing that is notable, and that you should keep in mind, is that when using DWDM the equipment used needs to be in a temperature controlled environment because fluctuations in temperature can and will affect the test results.
Long Haul
When you have to run fiber that is going a longer than usual distance (60+ km) these are known as long haul applications. An exaggerated example would be running fiber optic cable from one end of a state to the other end of the state - this would be considered a long haul application. In order to be able to test these greater spans of fiber, you will need an OTDR that has a very high dynamic range. They go up to 50dB for dynamic range and that will allow you to test close to 200 kilometers at the 1550nm wavelength.
Last Mile/High Resolution
The OTDR's that are referred to as the "Last Mile" are typically used in Fiber To The Home applications but can also be used when testing shorter runs of fiber and even patch cord connections. These OTDR's typically having a lower dynamic range, usually around 30dB. They tend to have a higher resolution that allows you to test very short runs which gives you about 1 foot of dead zone. The lower the pulse width the higher the sampling resolution and the lowest resolution tends to be around 3 nanoseconds. This allows you to be able to test the shorter runs such as drops to a house or even a fiber that runs up a tower to an antenna. It allows you to be able to run this short pulse width and be able to see all your events that happen in that tight span. When choosing an OTDR, make sure you know what spec you need for this, as not all OTDR's have that low of a pulse width to provide the higher resolution that is needed. When looking at a spec sheet you will be able to see the range of pulse widths that an OTDR will cover. If 3 nanoseconds are needed and you purchase an OTDR that only goes down to 10 nanoseconds then you will not be able to turn in the correct reports.
Low priced
There are some low price OTDR's out there that will assist in getting the job that is needed done. These lower cost hand held OTDR's typically have a smaller dynamic range and do not have the bells and whistles that the more in depth, complex machines boast. So if you are just looking for a machine that will measure distance on shorter run applications, and you are not looking to have an elaborate print out of your OTDR test, you would look at this type of OTDR. We see these used in applications such as maintenance of a network. You may need to identify the distance where a fiber break has occurred so you can fix it quickly and get the optical network back up and running. This type of OTDR typically does not offer pass/fail certified reporting software like some of the other higher end, more expensive units. These are simple to use and get the job done for the basic needs.
