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Ask RP Photonics for advice concerning fiber and waveguide modes. RP Photonics has the powerful software RP Fiber Power with a mode solver for optical fibers – see a demo case. For simpler purposes, there is also the free fiber optics software RP Fiber Calculator.

Definition: self-consistent electric field distributions in waveguides, optical resonators or in free space

German: Moden

Categories: fiber optics and waveguides, general optics, optical resonators

How to cite the article; suggest additional literature

This articles discusses propagation modes of light in free space, in a transparent homogeneous medium, in a waveguide structure, or in an optical resonator. Alternatively, the term “mode” can also mean a mode of laser operation, e.g. continuous-wave mode locking, Q switching, or single-frequency operation.

When some light beam propagates in free space or in a transparent medium, its transverse intensity profile in general changes during propagation (see Figure 1). There are, however, certain electric field distributions which are self-consistent during propagation. What “self-consistent” means, depends on the situation. Different situations are discussed in the following sections.

evolution of a beam intensity profile

Figure 1: Simulated evolution of the intensity profile of a laser beam. The shape of the profile changes during propagation. This would not be the case for a mode.

Free-space Modes

The mathematically simplest kinds of modes in free space (or in an optically homogeneous medium) are plane waves. A plane wave satisfies the wave equation, provided only that the wavelength times the optical frequency matches the phase velocity of light in the medium. During propagation in a direction, a plane wave only changes its oscillation phase, and possibly its amplitude if there is optical loss or gain in a medium.

Although plane waves are mathematically very simple, they cannot resemble any wave occurring in reality, since they have an infinite transverse extent. Therefore, other kinds of modes, which are limited in the transverse spatial dimension, are often of higher interest. The simplest kind of such modes are Gaussian modes. A Gaussian beam expands or contracts during propagation, but is self-consistent in the sense that the amplitude profile is only scaled in the transverse dimension, but has a constant (in that case Gaussian) shape.

Each Gaussian mode is only the simplest member of a whole family of modes, which contains an infinite number of modes. The most frequently used mode families (mode systems) are those of Hermite–Gaussian modes and Laguerre–Gaussian modes. Within such a mode family, the Gaussian mode is the fundamental mode, while all other modes are called higher-order modes and have more complicated intensity profiles (see Figure 2). During propagation, the transverse extent of each higher-order mode changes in proportion to that of the fundamental mode.

intensity distributions of TEM modes

Figure 2: Intensity profiles of the lowest-order Hermite–Gaussian modes, starting with TEM00 (lower left-hand side) and going up to TEM33 (upper right-hand side). The two mode indices indicate the number of zero crossings of the intensity distributions in horizontal and vertical direction, respectively.

Note that for each combination of an optical frequency, a beam axis, a focus position, and some beam radius of the Gaussian mode in the focus, a whole family e.g. of Hermite–Gaussian modes arises.

Waveguide Modes

Waveguide structures are spatially inhomogeneous structures which can guide waves. For light propagating in a waveguide, the self-consistency condition for a mode is more strict than for free-space modes: the shape of the complex amplitude profile in the transverse dimensions must remain exactly constant. Any re-scaling is not allowed, only an overall phase change and a loss or gain of total optical power, which are both described by the propagation constant.

A waveguide has only a finite number of guided propagation modes, the intensity distributions of which have a finite extent around the waveguide core. The number of guided modes, their transverse amplitude profiles and their propagation constants depend on the details of the waveguide structure and on the optical frequency. A single-mode waveguide (e.g. a single-mode fiber) has only a single guided mode per polarization direction. As an example of a multimode waveguide, Figure 3 shows the transverse profiles of all the modes of a multimode fiber.

modes of a fiber

Figure 3: Electric field amplitude profiles for all the guided modes of a fiber (disregarding the distinction between versions with cos lφ and sin lφ) with a top-hat refractive index profile (→ step index fiber) and V number of 11.4. The two colors indicate different signs of electric field values. The diagram has been generated with the RP Fiber Power software.

A waveguide also has cladding modes, the intensity distributions of which essentially fill the whole cladding (and core) region. Optical fibers (even single-mode fibers) have a large number of cladding modes, which often exhibit substantial propagation losses at the outer interface of the cladding.

Modes of fiber and other waveguides can be calculated numerically with so-called mode solvers, which can be part of a fiber simulation software. Depending on whether the waveguides have radially symmetric profiles and are weakly guiding, mode solver algorithms with a different level of complexity and quite different computation times are required.

Resonator Modes

For light in optical resonators (made of bulk-optical elements, not with waveguides), the self-consistency condition for a mode is again different: a mode must reproduce its exact transverse amplitude profile (without any re-scaling) only after a full resonator round trip; during the round trip, the mode profile may change in size and even in shape. On the other hand, the optical phase must also be reproduced after one round trip, i.e., the total experienced phase change must be an integer multiple of 2π. The overall optical power may decrease or increase if there are optical losses or gain in the resonator.

Due to the phase condition, resonator modes can exist only for certain optical frequencies (the resonance frequencies). In general, the round-trip phase shift depends on the intensity pattern of a mode. Therefore, different higher-order modes can have different sets of mode frequencies. In the simpler case of a geometrically stable resonator, there are fundamental (axial) modes with Gaussian shape and higher-order transverse modes e.g. of Hermite–Gaussian shape. Unstable resonators also have modes, but with much more complicated mode properties.

The article on resonator modes gives more details.

Application of the Mode Concept

In many photonic devices, light propagates only in a single mode. For example, single-mode operation of a laser means that only a single mode of its laser resonator is excited (i.e., carries a significant optical power). If the lasing mode is a Gaussian mode, the output is close to diffraction-limited, i.e. it has an ideal beam quality. As another example, a single-mode fiber guarantees a fixed intensity profile at its output, assuming that all light launched into cladding modes (unguided modes) is lost before the fiber end is reached. The mode of a single-mode fiber normally has a shape which is similar to that of a Gaussian.

In other cases, it is often convenient to decompose all the propagating light into different modes. That decomposition means that for each mode some mode amplitude (a complex number) is calculated for the given light field, usually using some overlap integral. The basic advantage of such a procedure is that it is known how all the modes propagate: for each mode, there is only a phase change which can be calculated from the propagation constant, and possibly some change in optical power. The total intensity and phase profile can then be calculated for any position simply by adding up the contributions of different modes. This procedure can greatly simplify numerical simulations: a large number of amplitudes, resembling e.g. a two-dimensional optical field distribution with many samples on a fine grid, can be replaced with a relatively small number of mode amplitudes (excitation coefficients), and thus often strongly reduce the demands both in terms of required computer memory and computation time.

The mode concept is useful even if the propagation conditions somewhat deviate from those for which the modes have been calculated. In such cases, mode coupling can occur: light from some mode can be coupled to one or several other modes. This is usually described with coupled differential equations for the mode amplitudes. Such mode coupling can be caused, e.g., by nonlinear interactions at high optical intensities or by external disturbances which act on a waveguide.

Bibliography

[1]E. Snitzer, “Cylindrical dielectric waveguide modes”, JOSA 51, 491 (1961)
[2]D. Gloge, “Weakly Guiding Fibers”, Appl. Opt. 10 (10), 2252 (1971)
[3]R. Paschotta, tutorial on "Passive Fiber Optics", Part 2: Fiber Modes
[4]R. Paschotta, case study on fiber modes

(Suggest additional literature!)

See also: mode radius, effective mode area, effective refractive index, mode matching, optical resonators, resonator modes, fibers, fiber simulation software, LP modes, higher-order modes, waveguides, mode coupling, Spotlight article 2006-12-03, Spotlight article 2007-10-11, Spotlight article 2008-04-15

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The Encyclopedia of Laser Physics and Technology is also available in the form of a two-volume book. Maybe you would enjoy reading it also in that form! The print version has a carefully designed layout and can be considered a must-have for any institute library, laser research group, or laser company. You may order the print version via Wiley-VCH.

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RP Fiber Power – the versatile Fiber Optics Software

An Amazing Tool

RP Fiber Power software

This amazing tool is extremely helpful for the development of passive and active fiber devices.

ASE

Watch our quick video tour!

Single-mode and Multi­mode Fibers

fibers

Calculate mode properties such as

  • amplitude distributions (near field and far field)
  • effective mode area
  • effective index
  • group delay and chromatic dispersion

Also calculate fiber coupling efficiencies; simulate effects of bending, nonlinear self-focusing or gain guiding on beam propagation, higher-order soliton propagation, etc.

Arbitrary Index Profiles

A fiber's index profile may be more complicated than just a circle:

special fibers

Here, we "printed" some letters, translated this into an index profile and initial optical field, propagated the light over some distance and plotted the output field – all automated with a little script code.

Fiber Couplers, Double-clad Fibers, Multicore Fibers, …

fiber devices

Simulate pump absorption in double-clad fibers, study beam propagation in fiber couplers, light propagation in tapered fibers, analyze the impact of bending, cross-saturation effects in amplifiers, leaky modes, etc.

Fiber Amplifiers

fiber amplifier

For example, calculate

  • gain and saturation characteristics (for continuous or pulsed operation)
  • energy transfers in erbium-ytterbium-doped amplifier fibers
  • influence of quenching effects, amplified spontaneous emission etc.

in single amplifier stages or in multi-stage amplifier systems, with double-clad fibers, etc.

Fiber-optic Telecom Systems

eye diagram

For example,

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Find out in detail what is going on in such a system!

Fiber Lasers

fiber laser

For example, analyze and optimize the

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for lasers based on double-clad fiber, with linear or ring resonator, etc.

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fiber laser

For example, study

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Apply any sequence of elements to your pulses!

… and even Bulk Devices

regenerative amplifier

For example, study

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RP Fiber Power is an extremely versatile tool!

Mode Solver

fiber modes

For example, calculate

  • amplitude and intensity profiles
  • effective mode areas
  • cut-off wavelengths
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  • group velocities
  • chromatic dispersion

All this is calculated with high efficiency!

Beam Propagation

beam propagation

Propagate optical field with arbitrary wavefronts through fibers. These may be asymmetric, bent, tapered, exhibit random disturbances, etc.

See our demo video for numerical beam propagation.

Laser-active Ions

level scheme

Work with the standard gain model, or define your own level scheme!

Can include different ions, energy transfers, upconversion and quenching effects, complicated pumping schemes, etc.

Multiple Pump and Signal Waves, ASE

optical channels

Define multiple pump and signal waves and many ASE channels – each one with its own transverse intensity profile, loss coefficient etc.

The power calculations are highly efficient and reliable.

Simple Use and High Flexibility Combined

For simpler tasks, use convenient forms:

signal parameters

Script code is automatically generated and can then be modified by the user. A powerful script language gives you an unparalleled flexibility!

High-quality Documentation and Competent Support

The carefully prepared comprehensive documentation includes a PDF manual and an interactive online help system.

Competent technical support is provided: the developer himself will help you and make sure that any problem is solved!

Our support is like included technical consulting.

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