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Acceptance Angle in Fiber Optics

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Definition: the maximum incidence angle of a light ray which can be used for injecting light into a fiber core or waveguide

German: Akzeptanzwinkel in der Faseroptik

Category: fiber optics and waveguides

How to cite the article

The acceptance angle of an optical fiber is defined based on a purely geometrical consideration (ray optics): it is the maximum angle of a ray (against the fiber axis) hitting the fiber core which allows the incident light to be guided by the core. The sine of that acceptable angle is called the numerical aperture, and it is essentially determined by the refractive index contrast between core and cladding of the fiber, assuming that the incident beam comes from air or vacuum.

acceptance angle of a fiber

Figure 1: An incident light ray is first refracted and then undergoes total internal reflection at the core–cladding interface. However, that works only if the incidence angle is not too large.

Of course, the concept of ray optics is not fully appropriate for describing the operation details of optical fibers, because wave aspects are important – particularly for fibers with small core such as single-mode fibers. A real light beam (for example, a laser beam) is not well resembled by a ray, since it inevitably has both a finite beam radius and a finite beam divergence. Therefore, there is in reality not a well-defined transition between guidance and non-guidance, when a beam angle is varied. However, the acceptance angle gives at least some estimate concerning how large an incidence angle may be for efficiently launching light.

Note that the term acceptance angle also plays a role in nonlinear optics – see the article on critical phase matching.

See also: numerical aperture, fibers, waveguides, total internal reflection, fiber optics

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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.


Watch our quick video tour!

Single-mode and Multi­mode 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,

  • analyze dispersive and nonlinear signal distortions
  • investigate the impact of amplifier noise
  • optimize nonlinear management and the placement of amplifiers

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

Fiber Lasers

fiber laser

For example, analyze and optimize the

  • power conversion efficiency
  • wavelength tuning range
  • Q switching dynamics
  • femtosecond pulse generation with mode locking

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

Ultrafast Fiber Lasers and Amplifiers

fiber laser

For example, study

  • pulse formation mechanisms
  • impact of nonlinearities and chromatic dispersion
  • parabolic pulse amplification
  • feedback sensitivity
  • supercontinuum generation

Apply any sequence of elements to your pulses!

… and even Bulk Devices

regenerative amplifier

For example, study

  • Q switching dynamics
  • mode-locking behavior
  • impact of nonlinearities and chromatic dispersion
  • influence of a saturable absorber
  • chirped-pulse amplification
  • regenerative amplification

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
  • propagation constants
  • 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.

Boost your competence, efficiency and creativity!

  • Stop fishing in the dark! Develop a clear quantitative understanding of your devices.
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  • That way, get most efficient in the lab.
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  • Get new ideas by playing with your models.

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