differential mode delay (original) (raw)

Author: the photonics expert (RP)

Acronyms: DMD, DMGD = differential mode group delay

Definition: the range of time delay values for signals in a telecom fiber

Alternative term: mode group delay

Categories: fiber optics and waveguides, lightwave communications, light pulses

• fiber properties
  • acceptance angle
  • bend losses
  • cut-off wavelength
  • differential mode delay
  • effective mode area
  • effective refractive index
  • group velocity dispersion
  • intermodal dispersion
  • modal bandwidth
  • mode radius
  • polarization beat length
  • propagation constant
  • propagation losses
  • V-number
  • waveguide dispersion
  • zero dispersion wavelength
  • (more topics)

Related: Telecom Fiber With Parabolic Index Profileintermodal dispersiongroup velocitygroup delaymultimode fibersfew-mode fiberstelecom fibersgraded-index fibers

Units: ps/km

Formula symbol: DMD

Page views in 12 months: 625

DOI: 10.61835/v5n Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What is a Differential Mode Delay?

The group velocities of different modes in a multimode fiber are generally different, resulting in mode-dependent group delays for a given length of fiber. This phenomenon of intermodal dispersion is generally a limiting factor for the achievable transmission bandwidth (data rate) in optical fiber communications as far as multimode fibers are used.

Quantifying Differential Mode Delay

For quantifying intermodal dispersion in telecom fibers, one usually specifies the differential mode delay (sometimes also called differential modal delay or differential group delay). This is often essentially understood as the difference between the maximum and minimum time delay (group delay) of a short signal pulse within a certain length of the fiber under test. It must be measured under carefully standardized conditions, e.g. using bandwidth-limited ultrashort pulses with a certain pulse duration well below the DMD result. The pulses should be in a diffraction-limited beam at a certain optical center wavelength, and the time delay should be measured for a range of radial positions of the input beam across the fiber core. Special DMD analyzer tools have been developed for such measurements.

In some cases, the difference in mode delays between two particular modes is considered. In such cases, positive and negative results are possible; indeed, one can design fibers where e.g. the LP11 mode is a higher or lower group velocity than the LP01 mode, or that difference may even change sign due to uncontrolled variations of fiber parameters in the fiber fabrication.

For constant fiber properties along the whole length and without mode coupling, the total difference in group delays is proportional to the fiber length. Therefore, the differential mode delay is often specified in picoseconds per kilometer (ps/km), for example. Its value can substantially depend on the optical wavelength.

Graded-index fibers can be optimized for a small differential mode delay, which however is usually achieved only within a quite limited wavelength range. This limits the application of wavelength division multiplexing. There are special wideband multimode fibers, however, where a low differential mode delay is achieved over a larger wavelength range with the width of e.g. 100 nm.

Typical differential group delays of few-mode fibers with step-index profile are of the order of several picoseconds per meter (ps/m), while graded-index fibers can be made with differential group delays far below 1 ps/m, in some cases even well below 0.1 ps/m = 100 ps/km.

A minimized differential mode delay is not ideal for all applications. For example, intermodal nonlinear effects such as cross-phase modulation can be more strongly disturbing in fiber-optic telecom systems based on fibers with low differential mode delay. In some cases, one uses sequences of fibers having opposite signs of differential mode delay, so that the local DMD is relatively large, and nevertheless the total difference in group delay over long transmission lines is relatively small.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is differential mode delay (DMD)?

Differential mode delay quantifies intermodal dispersion in a multimode fiber. It is defined as the difference between the maximum and minimum group delay for a short light pulse traveling through a certain length of the fiber.

What causes differential mode delay?

It is caused by the different group velocities of the various modes propagating in a multimode fiber. This phenomenon, known as intermodal dispersion, results in different modes arriving at different times over a given distance.

How can differential mode delay be minimized in optical fibers?

Graded-index fibers are specifically designed to minimize differential mode delay. Their refractive index profile is optimized to make the group velocities of different modes very similar, thus reducing the spread in their arrival times.

What are wideband multimode fibers?

Wideband multimode fibers are special graded-index fibers where a low differential mode delay is maintained over a broad wavelength range, for example, 100 nm wide. This makes them suitable for wavelength division multiplexing.

Is a minimal differential mode delay always the best solution?

No — in some telecom systems, a very low differential mode delay can enhance detrimental intermodal nonlinear effects. Sometimes, sequences of fibers with opposite signs of DMD are used for overall dispersion management.

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