定义：
由于未校准引起的激光器、光学谐振腔或者其它器件的灵敏度。

当激光器中的一个部件未对准的情况下，其性能通常就会被破坏的很严重。通常引起的效应包括输出功率的减小，光束质量变差，激光器噪声增大，以及输出光束位置和方向发生变化，从而影响其应用。
不仅接触光学元件会造成未对准的情况，另外对激光器外壳的压力或者热效应也会引起未对准。热效应主要来自于实验室中温度的变化或者激光器某些部分自身产生的热效应。
在高功率激光器中，只需要一小部分的光通过高反射率的激光反射镜射到另一个反射镜上就足够了，这一过程产生的热会通过热膨胀而引起一些弯曲。它可以在几分钟的工作时间内破坏掉激光器的性能。
在固态体激光器中，未对准通常是指激光器谐振腔中光束位置的平移。光束位置可能受任一光学元件的位置影响，尤其是反射镜的倾斜。另外，泵浦光束位置的改变也会引起未对准的情况，因为这会引起热透镜的不对称。
这种不对称性会引起腔内激光光束的倾斜。当增益很大的情况下，增益导引也会影响光束。

图1：改变热透镜聚焦功率（与泵浦功率成正比）后激光器谐振腔的稳定区域。实现代表不同位置处的光束半径，虚线代表将输出耦合器偏斜1mrad时激光器晶体上光束的水平偏移。未校准灵敏度往稳定边界的左边分开，在稳定区域I处数值很小。
最直接的减小未对准效应的方法是采用很精准的机械装置对准，激光器装置也非常稳定，包括稳定的基座等。但是，还需要知道激光器谐振腔的灵敏度与未对准之间的关系与谐振腔设计有多大的关系。尤其是，对于一个稳定的谐振腔来说，它与选取的稳定区域有关系，根据Magni命名法则，区域I比区域II要高一个数量级。未校准灵敏度在区域II的一个边界甚至是发散的。因此，激光器不建议工作在该点附近，但是由于激光器谐振腔设计的时候很少考虑这一问题所以经常会发生这种情况。最后，如果激光器工作在TEM00模式，且并未严格设计，那么稍微的不对准就会激发高阶模式。
除了设计谐振腔，仔细检查泵浦强度也是非常关键的。
具有很大基模面积的谐振腔尤其对未对准非常敏感。在这种情况下，采用非稳定谐振腔会更适合。
未对准也可以从动力学角度去看：谐振腔反射镜的机械振动转移到输出光束的波束指向涨落，谐振腔的设计业决定了该影响的大小。因此，优化设计使未对准灵敏度降低对波束指向稳定有好处。
在谐振腔设计方面需要权衡一些量，例如需要在大模式尺寸和低未对准灵敏度之间权衡，以及在区域I工作和其它性质之间权衡。因此，好的谐振腔设计涉及到非常复杂的各种要求，从而能够找到最优的谐振腔性质的组合。
尽管这项工作非常复杂，但是非常值得花费精力，因为这会引起激光器性能很大的改变。谐振腔从类型II到类型I的改变会使对准更加简单，也可以得到更稳定更大的输出功率，即使在不改变增益介质模式尺寸的情况下。优化谐振腔的设计也可以缩短需要的预热时间。

Definition: the sensitivity of a laser, an optical resonator or some other device with respect to misalignment

The performance of a laser often deteriorates seriously when one of its components is misaligned. Typical effects are a drop of output power, a deteriorated beam quality, and increased laser noise, apart from a modified output beam position and/or direction which can affect applications.

Misalignment may be caused not only by touching an optical element, but also by mechanical stress applied to the laser housing or by thermal effects. The latter can arise from temperature changes in the laboratory or from heating caused by parts of the laser itself. In high-power lasers, it may be sufficient that some small fraction of the light as transmitted by a highly reflective laser mirror hits another mirror mount, and the resulting heating causes some bending through thermal expansion. This can degrade the laser performance within a few minutes of operation.

In solid-state bulk lasers, the effect of misalignment is often strongly related to a shift of the beam position in the laser resonator. This beam position can be affected by the angular position of any optical element, in particular by tilting of mirrors. Further, an effective misalignment can also result from a change in the pump beam position because this can cause some asymmetry of the thermal lens. Such an asymmetry can cause the intracavity laser beam to be deflected. The beam can also be affected by gain guiding if the gain is high.

Figure 1: Stability zones of a laser resonator for variation of the thermal lens focusing power (which is proportional to the pump power). The solid lines show the beam radius at various locations, and the dashed curve shows the horizontal offset of the beam at the laser crystal when the output coupler is tilted by 1 mrad. The alignment sensitivity diverges at the left stability edge, and is much smaller in stability zone I (which is the right one in that case).

The straightforward (but not necessarily the most effective) way to reduce misalignment effects is to use very precise mechanics for alignment and a very stable mechanical laser setup, including a rigid base plate which is ideally somewhat decoupled from the laser housing. It is also important, however, to know that the sensitivity of a laser resonator against misalignment can depend strongly on the detailed resonator design. In particular, for a stable resonator it depends on which stability zone is chosen: zone I according to the nomenclature of Magni [2] can be an order of magnitude better than zone II (see Figure 1). The alignment sensitivity does even diverge at one of the edges of zone II. Obviously, it is not advisable to operate a laser close to such a point, but this does happen since many laser resonators are designed without being aware of such issues. Finally, a painfully sensitive alignment can result from operating a poorly designed TEM₀₀ laser in a situation where slight misalignment can cause the excitation of higher-order modes. Apart from good resonator design, carefully checking the pump intensity profile at an early stage can be a crucial measure.

Resonators with a large fundamental mode area are often particularly sensitive to misalignment. In that regime, a solution with a so-called unstable resonator can be substantially more robust.

Misalignment also has a dynamic aspect: mechanical vibrations of resonator mirrors can translate into beam pointing fluctuations of the output beam, and the resonator design determines how large this influence is. Therefore, design optimization for low alignment sensitivity can also contribute good beam pointing stability.

There are a number of trade-offs in resonator design, e.g. between large mode sizes and low alignment sensitivity, or between zone-I operation and certain other properties. Therefore, a good resonator design requires a comprehensive treatment of all requirements in order to find a good combination of resonator properties. Although this can be a complicated task, it can be very worthwhile to engage in it because it can make a huge difference in performance. The change from a type II to a type I resonator can make the alignment much easier and allow more output power to be obtained with better stability, even if the mode size in the gain medium is not modified. It is also possible to shorten the required warm-up time of a laser by optimizing the design.

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