Optical fiber crack detection instrument (OLI) is based on the combination of optical coherent detection technology and optical heterodyne detection technology. The basic principle of OLI is shown in the figure below.

The broadband continuous light emitted by the light source is divided into two paths by the coupler, one of which is used as the reference light, and the other is sent into the optical fiber to be tested as the detection signal light. When the probe light propagates forward in the optical fiber, echo signals will be continuously generated. These echo signal lights and the reference light are reflected back to the coupler after passing through the reflector for beat frequency interference, and are detected by the photodetector. The motor controls the movement of the mirror Z to change the optical path of the reference light.

The photocurrent detected by the photodetector can be expressed as:

Among them, β is the photoelectric conversion coefficient. The first three items in the above expression are all filtered out (two are DC items and one is high-frequency items) , leaving only the last beat frequency item. W _L -WS is the _beat frequency f _b , and the beat frequency signal is detected by designing a band-pass photoelectric conversion circuit.

According to the theory of optical interference, for interference to occur, the optical path difference must be within the coherence length range, and the coherence length of broadband light is very short. When the mirror moves, the echo signal returned from the DUT is at an equal distance from the mirror. The reflected signal beats. By processing the final beat frequency signal, the strength of the reflected signal at each point on the DUT link can be mapped to the reflectivity of the point (ie, the vertical coordinate of the curve) , and the actual interference position of the DUT corresponds to the corresponding distance of the mirror Z movement (ie, the curve Abscissa) , thus forming the OLI distance-reflectance curve.