Introduction
An OTDR (Optical Time Domain Reflectometer) launch cable, also known as a launch fiber or launch lead, is an essential tool used in optical fiber testing to accurately measure the characteristics of an optical fiber link. It helps overcome the limitations and uncertainties associated with the initial “dead zone” of an OTDR, which is the region near the OTDR’s output where the instrument is blind to reflections and events.
Here’s how an OTDR launch cable works:
- Purpose of Launch Cable: When an OTDR sends a pulse of light into an optical fiber, the initial pulse gets partially reflected back due to the connector at the OTDR’s output and the initial dead zone of the OTDR itself. This reflection can mask the true beginning of the fiber under test and any potential events, such as splices, connectors, or breaks, located within the initial dead zone.
- Launch Cable Construction: A launch cable is typically a length of optical fiber with connectors on both ends. It’s designed to be attached to the OTDR’s output port on one end and connected to the fiber under test on the other end. The launch cable can be of the same type as the fiber under test or a similar one. It’s usually longer than the dead zone of the OTDR, allowing the instrument to settle down before detecting any reflections from events within the dead zone.
- OTDR Measurement Process: When performing a measurement with an OTDR using a launch cable: The OTDR sends a pulse of light into the launch cable, which then travels through the cable and enters the fiber under test. As the light travels through the fiber under test, it interacts with various events like splices, connectors, bends, and breaks. These events cause reflections and scattering of light. Some of the light reflects back into the launch cable, travels back to the OTDR, and is detected by the instrument. The OTDR analyzes the time delay between the transmitted pulse and the received reflections to calculate the distance to each event within the fiber under test. It also measures the magnitude of the reflections, which provides information about the loss or gain at those events.
- Benefits of Launch Cables: Launch cables have a few important benefits: Dead Zone Reduction: By using a launch cable longer than the OTDR’s dead zone, the initial reflection from the OTDR’s connectors and dead zone is pushed beyond the range of interest, allowing more accurate measurement of events near the beginning of the fiber. Event Separation: The launch cable helps separate events within the fiber under test from the initial OTDR reflection, improving the accuracy of event detection and measurement.Stabilization: The launch cable allows the OTDR’s output power to stabilize before reaching the fiber under test, ensuring consistent measurements.
