定义：
不同的谐振器模式在相同增益介质中发生激光放大的现象，并导致交叉饱和效应。

常用的（很有价值）概念是描述光以谐振模式在光学谐振器中循环流通。每个模式的光就可以具有一些参数来描述，除了光学频率和极化，还有如电场强度（或总功率）和光相位等。当只有少数模式参与激光过程，则总体参数的量很小，
在这种简单的基础上可以描述各种物理现象。 
所有激光谐振器中的模式都经历了在同一增益介质中的光学放大，如激光晶体中，很大程度上会在空间重叠。这会导致模式竞争或增益竞争的现象。然而，这种概念并没有精确的定义，在不同文献中则以多种不同的含义出现： 

1. 不同模式在同一增益介质中放大，这会导致交叉饱和效应，一个模式的受激发射导致的增益饱和，不仅为是它自身（自饱和），而且其它模式也是这样 
2. 多个模式中的功率分配现象是不稳定的 

当然，前者作为条件可以推断出后者的现象。 
在前一词的含义下，模式竞争原则上是可以量化的：在模式的强度分配相同的条件下是可以的，这样，自饱和与交叉饱和强度是一样的（假定发射截面相同）。如果强度分配重叠是不完整的，那么模式竞争就会降低。两个简单的例子： 

1. 在线性谐振器中，每个谐振器模式在增益介质中形成驻波模式，与拥有不同频率的模式有是有区别的。因此，如果一个模式获得很大功率及强的饱和增益，另一个模式可能会看到饱和增益的减少，其驻波模式在不同位置存在波腹（→空间烧孔）。 
2. 如果激光谐振器包含色散棱镜，不同的光频率的模式在增益介质中可能会获得一个相对的横向偏移。 

强模式竞争情况下（有大的模式强度分布重叠），则可能出现如：单向环形激光器，在稳态下激发出一个单一的模式（→单模运作，单频激光器）：拥有很高的净增益的模式会是增益饱和，使它平衡了损耗，和其它的模式将会有一个负的增益，使它的功率渐渐消失消失。
一些减少模式竞争的情况可能更加复杂：一个强大的激光模式不能饱和其它模式的增益水平到能阻止发射激光的程度。然后会有复杂非线性动力学现象，常常由交叉饱和效应所控制，但也可能通过微小的模式耦合效应来影响。根据不同的细节，可能出现几个模式中稳定
的功率分布，或者（更频繁）由额外的激光噪声导致的不稳定的功率分布。 讨论表明，强的模式重叠会导致一个稳定的情形。根据定义，这可能被看作是一种强的模式竞争情况（因为强的重叠），或相反的弱竞争情况（状态是稳定的，没有表现出明显的竞争）。

Definition: the phenomenon that different resonator modes experience laser amplification in the same gain medium, leading to cross-saturation effects

A frequently used (and very valuable) concept is to describe the light circulating in an optical resonator in terms of resonator modes. The light in each mode can then be characterized with few parameters, e.g. electric field amplitude (or total power) and optical phase, apart from optical frequency and polarization. When only a few modes participate e.g. in the lasing process, the overall number of parameters is small, and various physical phenomena can be described on that simple basis.

The modes of a laser resonator all experience optical amplification in the same gain medium, e.g. a laser crystal, in which they spatially overlap to a significant extent. This leads to the phenomenon of mode competition or gain competition. Unfortunately, the terms are not precisely defined, and different meanings appear to be used in the literature:

• the fact that different modes experience amplification in the same gain medium, and that this leads to cross-saturation effects, where stimulated emission by one mode causes gain saturation not only for itself (self-saturation), but also for the other modes
• the phenomenon that the power distribution over several modes is unstable

Of course, the last-mentioned phenomenon can be a consequence of the former condition.

In the case of the former meaning of the term, mode competition can in principle be quantified: it is complete if the intensity distributions of the modes are identical, so that self- and cross-saturation are equally strong (assuming that the emission cross sections are also identical). Mode competition is then reduced if the overlap of the intensity distributions is incomplete. Two simple examples are:

• In a linear resonator, each resonator mode forms a standing-wave pattern in the gain medium, which differs for modes with different frequencies. Therefore, if one mode acquires a large power and strongly saturates its gain, another mode may see a reduced amount of gain saturation, as its standing-wave pattern has its anti-nodes at different positions (→ spatial hole burning).
• If a laser resonator contains a dispersive prism, modes with different optical frequencies may obtain a relative transverse offset in the gain medium.

In a situation with strong mode competition (in the sense of strong overlap of mode intensity distributions), which may occur, e.g., in a unidirectional ring laser, a single mode may be excited in the steady state (→ single-mode operation, single-frequency lasers): the mode with the highest net gain will saturate the gain so that it exactly balances its losses, and any other mode will then experience a negative net gain, which causes its power to fade away.

Situations with reduced mode competition can be more complicated: a strong lasing mode can not saturate the gain for the others to a level which prevents them from lasing. One then has a situation with complicated nonlinear dynamics, often dominated by cross-saturation effects, but possibly also influenced even by tiny mode coupling effects. Depending on various details, a stable distribution of optical power over several modes may occur, or (more frequently) an unstable power distribution, which causes additional laser noise.

The discussion has shown that strong mode overlap can lead to a stable situation. Depending on the definition of terms, this may be considered as a situation with strong mode competition (because of the strong overlap), or on the contrary as a situation with weak competition (as the situation is stable, not exhibiting explicit signs of competition).

Bibliography