What is the Dynamic Range of an OTDR?
When certifying or troubleshooting optical fibers in a network using an OTDR, the Dynamic Range is a key parameter of the device that determines the maximum length of the fiber that is observed during a test trace. In more technical terms, it is the distance between the point of the initial backscatter and the noise floor at the end of the fiber under test. The dynamic range value is measured and expressed in decibels (dB) and is essentially an analysis of power levels. OTDRs offering a larger dynamic range value can test longer lengths of fiber compared to those offering a smaller dynamic range value. Therefore, equating a dynamic range value with a fiber distance value is important when evaluating or specifying an OTDR for testing fibers in a network.Calculating Dynamic Range
At the most basic level, the calculation relationship for equating a dynamic range value to a fiber distance includes three components:
- Dynamic range value
- Fiber distance value
- Fiber attenuation value
When seeking to calculate a dynamic range value for a known fiber distance or a fiber distance for a known dynamic range value, the two calculation formulas are shown below:
- Dynamic Range = Fiber Distance * Fiber Attenuation
- Fiber Distance = Dynamic Range / Fiber Attenuation
To put this into a practical example scenario, there is an OTDR with a 27dB useable dynamic range value that will be used to test a standard single mode fiber with a 0.18dB/km attenuation value. What is the approximate maximum fiber distance that can be tested?
Solving for the fiber distance using the equation above, the 27dB dynamic range is divided by 0.18db/km resulting in a fiber distance of 150km. (Alternatively, if the 150km distance was the known variable, using the other equation to solve for dynamic range would be 150km*0.18dB/km = 27dB dynamic range)
Other Important Factors that Influence OTDR Dynamic Range & Signal Distance Values
It is important to note that when considering both the dynamic range and distance values of an OTDR during real-life testing scenarios, there are other factors that will influence these values and must be accounted for when seeking more exact and precise calculations.
Wavelength-Specific Fiber Attenuation
The attenuation specifications of optical fibers change when transmitting at different wavelengths. Because the fiber attenuation value is a key component in the dynamic range to distance formula, a different attenuation value for a given wavelength will result in a change to the dynamic range and distance values.
To demonstrate this, below is a chart generated by Corning® showing the attenuation specifications across the single mode wavelength transmission spectrum for their popular SMF-28® Ultra fiber:
Specification table courtesy of official Corning® SMF-28® Ultra data sheet
As shown, the attenuation value for the 1310nm wavelength is 0.32db/km, but the 1550nm wavelength is 0.18dB/km, a 0.14db/km differential that will change the dynamic range value when calculating using the same distance of fiber.
Wavelength-division multiplexing (WDM) technology is now being utilized globally in single mode systems for increasing data capacity by combining multiple wavelengths onto a single fiber. As a result, a user of a CWDM OTDR that tests multiple wavelengths must recognize and understand that the maximum signal distance will change for a given dynamic at different wavelengths due the attenuation properties of the optical fiber.
Related Technical Note: When using an OTDR for the most accurate measurements, it is also important the device is set to the proper Index of Refraction (IOR) that matches the IOR of the fiber you are testing at that specific wavelength. Since the IOR of a fiber also changes by wavelength, if a user is testing at 1550nm, the device IOR should match the specific fiber IOR at 1550nm, otherwise a fiber length/distance discrepancy will arise.
Additional Fiber Attenuation/Loss Events
In the previous example, the calculation formula is being applied to a single mode optical fiber tested at the 1550nm wavelength in a “perfect” scenario – meaning it is utilizing a single continuous 150km distance of fiber with no other events, network components, or other attenuation-inducing variables aside from the natural attenuation of the fiber itself. In a real network, fibers of any lengths often include numerous attenuation-inducing events like connectors, splices, patch panels, and splitters that increase the total signal loss across the fiber. Accounting for those additional dB attenuation fiber events therefore requires subtracting that value from the usable dB, resulting in a reduced total testing distance. Thus, if there was an estimated 5dB of extra attenuation events in the fiber strand, the usable dynamic range would be reduced to 22dB instead of 27dB, resulting in a maximum fiber distance of 122.22km and more power would be required to reach the end of the 150km fiber.
Example of an OTDR trace with multiple loss-inducing fiber events
OTDR Pulse Width
Another factor that plays a role in the maximum testing distance of a fiber is the pulse width of the OTDR that is being used for the test. Simply put, the longer the distance of fiber, the longer/wider the pulse width must be to reach that target distance. Therefore, even if an OTDR has an appropriate dynamic range level for the test, if the user selects a pulse width that is too narrow, the OTDR signal may fall short of the mark.
Dynamic Range and Fiber Distance Calculator – Download Here!
To help engineering and technical teams quickly equate a dynamic range value with a fiber distance value, M2 Optics has created a helpful Dynamic Range Calculator tool. Simply input a fiber attenuation value, followed by either a known dynamic range or fiber distance, and the tool will calculate the resulting value for you.
If you have questions or would like more information about calculating dynamic range, optical fiber technical specifications, or require optical fiber for accurately simulating fiber links for testing or training purposes, contact M2’s team of fiber experts today.
