Research On Coherent Optical Reflectometer Based On Chirp Pulse Modulation

Optical reflectometer is a measuring instrument to characterize the optical fiber transmission, which has already been widely used for the diagnosis of optical fiber communication networks. Techniques for the reflectometry are developing rapidly in recent years, such as Optical Time Domain Reflectometry(OTDR)?Optical Coherent Domain Reflectometry(OCDR) ?Optical Frequency Domain Reflectometry(OFDR) and so on. Coherent OTDR(COTDR) is based on OTDR, and it greatly enhances the dynamic range and detection distance of the probe signal by using coherent reception in the backscattered signal in the fiber. OCDR can realize high spatial resolution without high speed photoelectric detection and acquisition device, but its measurement distance is restricted to several kilometers because it is restricted by the coherence length of light source and it often needs long delayed optical fiber. The measurement distance for COTDR can be from tens of kilometers to hundreds of kilometers, but its spatial resolution is difficult to be better than 1m. Moreover, the measured distance will be short with the improvement of spatial resolution due to the tradeoff between them.In order to break the contradiction between spatial resolution and measured distance of COTDR, this paper presents a novel COTDR technique based on external dual chirp pulse modulation, in which the probe beam is linear frequency modulated by acousto-optic modulators while the local is continuous light without modulation. The spatial resolution depends on the range of frequency chirp other than pulse width. The new COTDR technique overcomes the contradiction between spatial resolution and the measured distance with the realization of 1.2m in spatial resolution of 58.6 km fiber. Its dynamic range is more than 20 d B.On the basis of the above scheme, the technique of external dual chirp pulse modulation is applied to OCDR. However, the probe and local oscillator beams are linear frequency modulated independently. Moreover, the time delay between the probe and local beams can be adjusted digitally precisely. This technique not only doesn't need high-speed data acquisition device but also overcomes the contradiction between frequency modulation rate and the measured distance with the realization of 3.9 m in spatial resolution of 24.6 km fiber.