A growing demand for video/data transmitted the Internet and other networks increases need for higher speeds in transmission and delivery systems, especially at what is known as “the last mile”, i.e. the last part of the transmission line reaching a user. Nowadays, telecommunications operators are deploying a large number of fibre links in order to deliver broadband access to the user based on Passive Optical Networks (PONs). With the increasing number of fibres and cables, the probability of fault at some point of the PON also increases, generating a demand for fibre inspection.
The need for optical fibre maintenance functions for prevention and fault maintenance has been extensively discussed, for instance by International Telecommunication Union's Telecommunication Standardization Sector Study Group 6 (ITU-T SG6). Most studies considered by the group were based on the use of traditional Optical Time Domain Reflectometry (OTDR) in order to localize faults with reasonable accuracy without in optical networks without interfering with data transmission.
It is desirable to implement an inspection solution that neither interferes with data transmission, nor requires high additional capital and operational expenses. Furthermore, the implementation of Subcarrier Multiplexed (SCM) signals in PONs enables the use of a subcarrier for fibre monitoring purposes, rather than monitoring faults over a separate optical carrier, which would require a separate transmission and detection scheme.
Some solutions proposed for fault location are based on the coherent Optical Frequency Domain Reflectometry (OFDR) technique (see Oberson et al., “Optical Frequency Domain Reflectometry with a Narrow Linewidth Fiber Laser,” IEEE Photonics Technology Letters, Vol. 12, No. 7, July 2000, 867-869), which provides accuracy for few hundred meters to a few kilometers fiber monitoring. Nonetheless, this approach is associated with the employment of high coherency lasers making it prohibitive in association with SCM PON monitoring. Other solutions based on the incoherent OFDR have been proposed (see Ryu, Dagenais, “Incoherent Optical Frequency Domain Reflectometry for Health Monitoring of Avionics Fiber Optics Networks,” Avionics, Fiber-Optics and Photonics Technology Conference, IEEE, Sep. 30, 2008-Oct. 2, 2008 15-16; Shao et al., U.S. Pat. No. 8,514,381) but both use broadband frequency sweep of the modulation frequency, which is incompatible with subcarrier data transmission and can only detect localized reflections at devices in the network.
Another proposed solution is based on the measurement of the frequency response of the reflected signal from the fiber by suppressing the Rayleigh scattering and detecting reflections along the fiber (see Nakayama, et al., “Optical Fiber Fault Locator by the Step Frequency Method,” Applied Optics, Vol. 26, No. 3, Feb. 1, 1987, 440-443). However this solution is unable to detect non-reflective losses, which are a common fault in optical networks.