What type of source is used for testing singlemode fibers?

• A. Light Emitting Diode
• B. Vertical-Cavity Surface-Emitting Laser
• C. Laser
• D. Superluminescent Diode

Answer: (C)

Testing singlemode fiber relies on injecting light that travels in a single spatial mode and matches the tiny core of the fiber. A laser is ideal because it emits a narrow-linewidth, highly collimated beam with high spatial coherence, which can be launched into the fiber efficiently. This keeps the light in the fundamental mode and provides stable, repeatable measurements for insertion loss and OTDR results. Light sources like LEDs or SLEDs produce broader spectra and multiple spatial modes, leading to coupling into higher-order modes and more measurement variability. VCSELs are typically used for short-range, multimode links rather than standard singlemode testing. So, a laser best fulfills the needs of singlemode fiber testing.