定义：
拥有环形谐振腔的激光器。
 
环形激光器是一种有环形谐振腔的激光器。驻波激光谐振腔相比，这种环谐振器允许两种不同传播方向的光在腔内。在许多情况下，单向运作（光传播只在两个可能的方向之一）是谐振腔内引入一个元件强制实现的，这将导致不同的传播方向有不同的损耗（图1）；
这样可与偏振器件相结合形成法拉第旋转器（如激光晶体的布鲁斯特表面表面）。如果实现单向操作，那么在增益介质中没有驻波干涉图样（反射点附近除外），因此没有空间烧孔。因此，单频运作很容易实现。尤其是固态体激光器、单向环形激光器的设计可以被认为是一种获得稳定的单频发射的标准方法。 

图1: 环形激光谐振腔，利用一个光学隔离器实现单向运作。 

图2: VERDI绿光激光器的连接照片。使用一个单向环形激光器谐振腔，可实现低噪声单频运行。这张照片由COHERENT公司提供。 
一种普遍的固态环形激光器叫做非平面环形振荡器，也叫做NPRO或者MISER。这是一个单片的激光设计（图3），整个激光谐振腔只包含涂层的晶体。尽管这种晶体的制造比普通的激光晶体更复杂的，但是校准是相当容易的，而且这样的激光器非常稳定和有很好的鲁棒性。 
也有一些具有环谐振器配置的光纤激光器。光纤环激光器作为锁模激光器通常比单频激光器更常见。常用的配置是8字形的激光器，包含一个非线性环形镜作为有效的可饱和吸收体。

环几何结构不是为了避免空间烧孔效应，而是按照可饱和吸收体原则（非线性环形镜），也是脉冲整形所需。 
还有一些环形激光器，如用于光学陀螺仪中的，双向运作是必需的。激光谐振腔外，与不同传播方向一致的光速的一个拍音可以检测到，拍频可以显示出激光器旋转的角频率（萨格纳克效应），应格外注意的是，要避免反向散射波的相干锁定。
特别的，避免即使是很微弱的寄生反射（不完美的激光反射镜中）也是必要，其可以与反向散射模式相耦合。 

图3: 单片环形激光器的结构（NPRO或MISER）。更多详细细节参见文献非平面环形振荡器。

Definition: lasers with a ring resonator

More general terms: lasers

Opposite term: lasers with linear resonator

A ring laser is a laser with a ring resonator as its laser resonator. In contrast to a standing-wave (or linear) laser resonator, such a ring resonator does not have any end mirrors, hit with normal incidence, and it allows for two different propagation directions of the intracavity light.

Figure 1: A ring laser resonator, where unidirectional operation is enforced with an optical isolator.

Unidirectional Laser Operation

In many cases, unidirectional operation (where light propagates only in one of the two possible directions) is enforced by introducing an element into the resonator which leads to different losses for the propagation directions (Figure 1); this can be, e.g., a Faraday rotator combined with a polarizing element (e.g. a Brewster surface of the laser crystal).

Figure 2: Photograph of the VERDI green laser from Coherent. A unidirectional ring laser resonator is used, enabling low-noise single-frequency operation. The image was kindly provided by Coherent.

Note that one normally requires only a tiny difference in round-trip propagation losses to obtain stable unidirectional operation. This is because in continuous-wave operation lasing leads to gain saturation such that the round-trip gain becomes zero; due to the loss difference, light propagating in the opposite direction then has a slightly negative round-trip gain, which means that it can never acquire any substantial optical power.

If unidirectional operation is achieved, there is no standing-wave interference pattern in the laser gain medium (except near reflection points), and consequently no spatial hole burning. Therefore, single-frequency operation is easily achieved. Particularly for solid-state bulk lasers, unidirectional ring laser designs can be considered as a standard approach to obtain stable single-frequency emission.

Typical Types of Ring Lasers

Nonplanar Ring Oscillators

Figure 3: Setup of a monolithic ring laser (NPRO or MISER). See the article on nonplanar ring oscillators for more details.

A popular kind of solid-state ring laser is the nonplanar ring oscillator, also called NPRO or MISER. This is a monolithic laser design (Figure 3), where the whole laser resonator consists only of a coated laser crystal. While the fabrication of such crystals is somewhat more complicated than for ordinary laser crystals, alignment is fairly easy, and such lasers are very stable and robust.

Fiber Ring Lasers

There are also fiber lasers with ring resonator configurations. Fiber ring lasers are more often mode-locked lasers than single-frequency lasers. A frequently used configuration is that of the figure-eight laser [4], containing a nonlinear loop mirror as effective saturable absorber. The ring geometry then does not serve to avoid spatial hole burning, but rather follows from the principle of the artificial saturable absorber (a nonlinear loop mirror) as required for pulse shaping.

There are other fiber ring lasers, used e.g. in optical gyroscopes, where bidirectional operation is required. Outside the laser resonator, a beat note of the beams corresponding to the different propagation directions can be detected, and the beat frequency reveals the angular frequency of rotation of the laser (Sagnac effect). Special care is often required in order to avoid coherent locking of the counterpropagating waves. In particular, it is necessary to avoid even very weak parasitic reflections (e.g. on imperfect laser mirrors) which can couple the counterpropagating modes.

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