OTDR Block Diagram
The OTDR consists of a laser light source, an optical sensor, a coupler/splitter, a display section, and a controller section.
Laser Light Source
The laser diode sends out pulses of light on command from the controller. You can select the duration of the pulse (the Pulse Width) for different measuring conditions. The light goes through the coupler-splitter and into the fiber under test (FUT). Some OTDRs have two lasers to allow for testing fibers at two different wavelengths. Only one laser is used at a time.You can easily switch between the two with the press of a button.
The coupler/splitter has three ports — one each for the source, the fiber under test, and the sensor. It is a device that allows light to travel only in specific directions: FROM the laser source TO the fiber under test, and FROM the fiber under test TO the sensor. Light is NOT allowed to go directly from the source to the sensor. Thus, pulses from the source go out into the fiber under test, and the returning backscatter and Fresnel reflections are routed to the sensor.
Optical Sensor Section
The sensor is a photo detector that measures the power level of the light coming in from the fiber under test. It converts the optical power in the light to a corresponding electrical level — the higher the optical power, the higher the electrical level put out. OTDR sensors are specially designed to measure the extremely low levels of backscattered light. The sensor section includes an electrical amplifier to further boost the electrical signal level. The power of a Fresnel reflection can be up to 40,000 times higher than that of backscatter, and may be more than the sensor can measure — thus overloading the sensor, driving it into saturation. The electrical output level is subsequently “clipped” at the sensor’s maximum output level. Therefore, whenever a test pulse encounters an end of a fiber — whether at a mechanical splice or at the end of the fiber — it causes the sensor to be “blinded” for as long as the pulse occurs. This blind period is known as the “dead zone.”
The controller is the brains of the OTDR. It tells the laser when to pulse; it gets the power levels from the sensor; it calculates the distance to scattering and reflecting points in the fiber; it stores the individual data points; and it sends the information to the display section. A major component of the controller section is a very accurate clock circuit which is used to precisely measure the time difference between when the laser pulses and when the sensor detects returning light. By multiplying this round-trip pulse travel time by the speed of light in fiber (which is the speed of light in free space corrected by the Index of Refraction), the round-trip distance is calculated. The distance from the OTDR to the point (one-way distance) is simply half of the round-trip distance. Since backscattering occurs all along a fiber, there is a continuous flow of light back into the OTDR. The controller samples the level measured by the sensor at regular time intervals to get its data points. Each data point is described by its sequence time (which relates to distance from the OTDR) and power level. Because the original pulse gets weaker as it travels down the fiber (due to Rayleigh scattering induced loss), the corresponding returned backscatter level gets weaker further down the fiber. Therefore, the data points generally have decreasing power levels from start to end. But when a Fresnel reflection occurs, the power level of the corresponding data point for that location suddenly goes up to its maximum level — way above the level of the backscatter just prior to it. When the controller has gathered all its data points it plots the information on the display screen. The first data point is displayed at the left edge of the graph as the starting point of the fiber. Its vertical position is based on its returned signal power level: a higher power is plotted higher up on the graph. Subsequent data points are placed to the right, one data point every resolution setting. The resultant trace is a sloping line that runs from the upper left towards the lower right. The slope of the line indicates its loss per-unit-distance (dB/km) value. Steep slopes mean larger dB/km values. Data points corresponding to backscatter level make up the line. Fresnel reflections look like spikes coming up from the backscatter level. A sudden shift of the backscatter level indicates a “point loss,” that may indicate either a fusion splice or a stress point in the fiber where light is escaping.
The display section is a CRT or LCD screen that shows the data points that make up the fiber trace, and displays the OTDR set-up conditions and measurements. Most OTDR displays connect the data points with a line to provide a clearer look at the overall trace. You can manipulate cursors on the screen to select any point on the fiber trace. The distance to the cursor is displayed on the screen. An OTDR with two cursors will display the distances to each cursor and the difference in backscatter levels between them. You can choose the type of measurement being made with the cursors, such as 2-Point Loss, dB/Km, Splice Loss, and Reflectance. The measurement results are shown on the display.