What is the value of V for step-index fiber?

Value of V for Step-Index Fiber

The V-number or V-parameter of a step-index fiber is a dimensionless value that is crucial in determining the fiber's modal properties, including how many modes it can support. The V-parameter is defined by the equation:

V = (2πa/λ)√(n12 - n22)


  • a is the core radius of the fiber,
  • λ is the operating wavelength,
  • n1 is the refractive index of the core,
  • n2 is the refractive index of the cladding.

The V-parameter is essential for understanding the guiding properties of the fiber. It determines the number of modes a fiber can support. For a step-index fiber:

  • If V < 2.405, the fiber operates in the single-mode regime, supporting only one mode.
  • If V > 2.405, the fiber operates in the multi-mode regime, supporting multiple modes.

The exact number of modes can be calculated for multi-mode operation, but it generally increases with the square of the V-value. The V-parameter not only influences the modal characteristics but also affects the fiber's bandwidth and dispersion properties, making it a fundamental parameter in the design and application of optical fibers.

Back to blog

Leave a comment