Recent years, many OTDR manufacturers and suppliers declare their OTDRs support online testing, but some times, it is a trap if you do not understand what is OTDR online testing clearly. This artical will explain what is OTDR online testing.

1. OTDR working wavlength
OTDR emits a specific wavelength of light pulse into the tested fiber, while at the same time, OTDR receives backscattered and reflected light from the tested fiber. OTDR analyzes the received optical light and forms a curve, which is displayed on the screen. Please note that the wavelength of the backscattered and reflected light received by OTDR is consistent with the wavelength of the light emitted by OTDR. That is to say, the specific wavelength of optical light used in OTDR testing are emitted by OTDR itself. This wavelength is called the OTDR working wavelength.

2. Fiber network service wavelength
Fiber optic networks transmit information through specific wavelength optical signals. Due to the characteristics of optical fibers, at wavelengths of 1310nm, 1490nm, and 1550nm, the loss is relatively small, so these three wavelengths are usually used for signal transmission. The following table briefly lists the wavelengths used in common GPON, EPON, and CWDM technologies.

3. when no service wavelength in fiber network
Throughout the OTDR testing process, OTDR only needs to use the wavelength of light emitted by itself. It does not need or want to have optical signals of other wavelengths in the tested fiber. If there is, it needs to be filtered out.
In the daily OTDR test scenario, it can be roughly divided into the acceptance testing before the network is opened and the maintenance test after the network is opened. During the acceptance testing, the optical cable is usually just laid and the network service has not been opened, so there is no optical signal in the optical fiber. At this time, it is very friendly to OTDR. Considering that the service wavelength for future operation is 1310/1490/1550nm, it is recommended to select the corresponding wavelength for testing on OTDR, which can better simulate the fiber optic link condition during operation. Of course, other wavelengths such as 1610nm or 1525nm can also be chosen, because although the fiber exhibits slight differences in loss at different wavelengths, these differences are not significant due to the close proximity of wavelengths, and they only affect the loss and do not affect the testing of length and event points. Therefore, for the tested fiber without optical signals, any wavelength can be selected for testing. Due to the lower cost of 1310nm/1550nm lasers, it is customary to use 1310nm/1550nm for testing.

4. when there is an optical signal in the tested fiber
When conducting fiber optic maintenance or troubleshooting, maintenance personnel generally do not disconnect the fiber optic link from the OLT, as a PON port on an OLT typically manages dozens of ONUs. Once not online, the impact range is large. Therefore, during troubleshooting, technical personnel usually conduct online testing. At this point, we have learned that the so-called OTDR online testing refers to testing when there is an optical signal present in the fiber optic link.

5. Testing of GPON/EPON optical networks
5.1. There is a 1490nm downlink signal in the link
In traditional GPON/EPON networks, upstream signals are transmitted at a wavelength of 1310nm, while downstream signals are transmitted at a wavelength of 1490nm. When testing with 1550nm wavelength OTDR from the ONU to the OLT side, the 1490nm wavelength light in the link will also enter the OTDR, which affects the testing. Therefore, it is necessary to filter out the 1490nm wavelength optical signal present in the link. A specialized passive component filter is a good choice. Usually, it is customary to refer to OTDR with integrated filters as online testing

5.2 There are 1490nm and 1550nm downlink signals in the link
In GPON/EPON optical networks, 1550nm cable TV video signals are generally added, so the downstream wavelengths of 1490nm and 1550nm will coexist in the fiber optic link, making it impossible to use 1550nm OTDR for online testing. It is necessary to find another wavelength as the testing wavelength for OTDR. Considering cost factors, OTDR at 1610nm is a good choice. This way, a filter can be built into the 1610nm OTDR to filter out all wavelengths except for 1610nm.

6. CWDM fiber network testing
Due to the use of a transmission wavelength of 1610nm in the CWDM fiber optic communication system, OTDR with a working wavelength of 1610nm cannot be used. At this point, the 1625nm OTDR came in handy. So, 1310nm/1550nm/1625nm OTDR is the most widely used. The 1310nm/1550nm wavelengths are mainly used for non online testing. 1625nm is mainly used for online testing. However, due to the proximity of 1625nm to 1610nm, its isolation is relatively low. Sometimes, there may be problems that cannot be measured online. Therefore, in recent years, 1650nm online OTDR has been developed. Due to the relatively high cost of 1650nm lasers, in general, if users purchase 1650nm OTDR, its main purpose is to conduct online testing. In this way, most of them only purchase OTDR with a single wavelength of 1650nm.