Tamper detection of fiber optic links via modulation transfer function characterization

Fiber optic communication links have long been touted as providing a secure communications channel for sensitive information. This reputation is largely based on the absence of electromagnetic emanations from the fiber. However, unauthorized access to data in optical fibers is a realistic possibility. Unfortunately it is not always possible to encrypt data transmitted via fiber optic links due to the lofty cost of high data rate encrypters. Therefore, on non-encrypted fiber communications channels, some form of tamper detection scheme is desirable to assure security. The limitations of conventional tamper detection techniques suggests that more sophisticated methods be used. Fiber optic links, like all communications channels, have a band-limited frequency response which produces a characteristic distortion of the signals that propagate down the fiber. A technique has been developed to perform a sensitive characterization of the fiber's frequency response function. This characterization can be exploited for detecting tamper in fiber optic communications links.<>