WHAT IS THE PRINCIPLE OF OTDR?

As we can see in the figure shown above that OTDR Machine contains a light source (mainly a laser) and a receiver along with a coupler or circulator. The coupler is connected with the fiber under test through a front panel connector.
The laser produces a short and intense light beam. These pulses are directed into the fiber link under test through a fiber optic coupler. A coupler splits the transmitted light pulse into two halves. Due to this, not all the transmitted pulse is directed inside fiber.
However, despite using a coupler if we use a circulator then this wastage of transmitted signal can be avoided. As circulators are highly directional devices that direct the overall light signal into the fiber as well as sends the reflected or scattered light signal into the detector.
By inserting circulators in the operational unit of fiber optic tester, the dynamic range of the equipment can be improved. However, it also causes the overall cost of the system to increase considerably as circulator is highly expensive in comparison to couplers.