# Mode Matching

Definition: the precise spatial matching of the electric field distributions of laser beams and resonator modes or waveguide modes

German: Modenanpassung

Categories: general optics, optical resonators

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Author: Dr. RĂ¼diger Paschotta

In many situations, it is necessary to spatially match a laser beam precisely to another beam or a mode in order to obtain some kind of efficient coupling. Examples are:

- A beam from a laser has to be coupled into an optical fiber.
- A laser beam must be matched to a passive optical resonator which should act, e.g., as a spatial and/or spectral filter (→
*mode cleaner cavities*). - For injection locking, the mode of a master laser has to be matched to that of the slave laser.

The necessary matching of modes means not only creating a good spatial overlap of the intensity profiles, but also matching the optical phase profiles. In other words, the wavefronts of the beams will then be matched. If the complex amplitude profiles of two beams (having the same optical frequency) are well matched in a certain plane, they will remain well matched during further propagation.

Mode matching can be achieved by using suitable relay optics (typically some combination of curved mirrors or lenses), provided that the beam quality of the initial beam is close to diffraction-limited.

Mathematically, the quality of mode matching can be quantified with an overlap integral. The following formula, involving the square of such an overlap integral, calculates the coupling efficiency concerning optical powers:

where *E*_{1} and *E*_{2} are the complex electric fields in a plane, referring e.g. to a laser beam and the field of a resonator or waveguide mode, and the integration spans the whole beam cross-section.
That quantity is preserved during propagation in free space.

A similar overlap integral can be used for calculating complex mode *amplitudes*.

If the beam from a frequency-tunable single-frequency laser hits a symmetric Fabry–Pérot interferometer and the laser frequency is tuned over the whole free spectral range of the resonator, the transmitted light can be used to analyze the degree of mode matching. For perfect matching to a cavity mode (typically the fundamental Gaussian mode), complete transmission of the resonator can be observed when the resonance condition is met, whereas other resonances (corresponding to other resonator modes) can not be excited.

## Questions and Comments from Users

2021-09-17

How to couple light into a waveguide which does not have a precise mode description but only a rough mode width; how to use the formula?

Answer from the author:

You can at least normally assume that the wavefronts of the modes are flat. As a first guess, you may then assume a Gaussian amplitude profile.

2021-10-28

Do the two fields E1 and E2 have to be normalized to the maximum?

Answer from the author:

No – the formula contains the normalization.
You can easily see, for example, that the result won't change if you double *E*_{1}.

2021-11-23

What if we do not have the phase information, but measured the intensity profiles with a camera? I guess one could work with the square root of the intensity, but that would only obtain an upper limit, wouldn't it?

Answer from the author:

You need the full phase information to do any calculations on mode matching.By taking the square root of the intensity profile, you are effectively assuming a flat phase profile. That might serve as an upper limit for the possible mode matching efficiency, if the ideal output field has flat phase fronts.

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See also: modes, cavities, laser beams, diffraction-limited beams

and other articles in the categories general optics, optical resonators

2021-02-15

How does one account for polarization in this equation?

Answer from the author:

Not at all. The calculation is based on scalar fields, i.e., neglecting polarization. If you want to calculate the overlap often arbitrary polarized field with a polarized mode, you first have to decompose the field into its polarization components.