定义：
激光光束与谐振腔或者波导模式的电场分布是完全的空间匹配。

在很多情况下，需要将激光光束与另一光束或模式严格匹配从而可以得到有效的耦合。例如： 

• 激光器产生的光束需要耦合进光纤中。 
• 激光光束需要与无源光学谐振腔匹配，作为空间或者光谱滤波器（参阅模清洁腔）。 
• 注入锁定时，主激光器的模式需要与从属激光器的模式匹配。 

模式的匹配不仅要求强度截面在空间上是交叠的，还需要相位截面也是匹配的。如果两光束的复振幅在某一平面上完全匹配，那么在传播过程中会始终保持匹配的。
可以采用合适的替续光学系统实现模式匹配（通常是一些曲面镜或透镜的组合），但是前提是初始光束的光束质量接近于衍射极限。 
数学上，模式匹配量可以由以下交叠积分定量表示： 

其中 E1和E2 是平面上的复电场，代表一束激光光束和谐振腔或者波导模式的场，积分区域为整个光束截面。该积分比值在自由空间中传播时保持不变。 
以上的交叠积分在计算模式振幅时非常重要。在许多情况下，例如将光功率耦合进某一模式时，需要计算以上交叠积分的平方。 
如果光束来自于频率可调谐的单频激光器，射向对称的法布里-珀罗干涉仪，如果把激光频率在谐振腔的整个自由光谱范围内调谐，那么可以利用得到的透射光分析模式匹配的程度。
当与腔模式完全匹配时（通常是基模，即高斯模式），并且满足共振条件时，就能得到谐振腔完全的透射，此时不会激发其它共振模式。

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

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 E₁ and E₂ 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.