定义：
利用共振效应增强在其中往返运动的光功率的光学腔。

增强腔是一种光学腔（谐振腔），是用于共振增强光功率或者光强的；如果入射光是与腔共振且模式匹配的，那么腔内的光功率远大于入射功率，尤其是腔具有很高的精细度时。 

图1：在两端曲断面处涂覆电介质涂层的单片倍频器。
增强腔也可以加入一些光学元件。例如，加入非线性晶体可以进行有效的非线性频率转换，如倍频或者和频产生。如图一所示，图中为一个单片倍频器。它包含了一个端面具有电介质涂层的非线性晶体，泵浦光可以在其中发生共振。在右侧可以得到倍频光。

尽管非线性过程只能将一小部分的光功率转化到倍频光中，但是谐振腔还可以在一定程度上循环利用未被转化的光。如果能够实现阻抗匹配，即如果入射反射镜的透射率等于谐振腔损耗，那么转化效率会非常高。 如果采用双共振机制，即泵浦光和二次谐波都是共振的，即使功率非常低（几个毫瓦），倍频过程效率也会非常高。但是，双共振条件很难保持。 

需要注意的是，不要将共振倍频与腔内倍频，后者的非线性晶体放置在激光腔内，因此不需要单独的共振腔。 

高效共振增强的条件 
为了使共振腔工作效率很高，需要考虑以下几个因素： 

• 共振条件要求谐振腔长度的误差在一个波长范围内。为了使共振保持更长的时间，通常采用一个电子反馈回路。
• 这一反馈回路或者通过改变激光器的频率使其匹配腔频率，或者通过压电驱动器改变腔长。
• 需要注意的是，对于能够实现很大程度的功率增强的高精细度腔来说，腔和激光器的稳定性必须非常高。 
• 入射光必须与腔是模式匹配的，需要采用合适的几何光学来聚焦和对准。另外，辐射光在传输过程中需要保持衍射极限的光束质量。 
• 通过泵浦光功率的背向反射可使损耗最小，这时增强腔需要满足阻抗匹配。即入射端反射镜的透射系数与共振腔中其它的损耗数值相等。 

锁模激光器中的增强腔 
增强腔通常与单频激光器结合使用，还可以与锁模激光器一起使用。在后者情况下，腔长需要满足一定的条件，即腔中光往返时间等于脉冲间隔的整数倍。
也就是说，腔的自由光谱区是激光器脉冲重复频率的整数倍，因此激光输出的梳齿（频率梳）都满足共振。并且，腔内色散和非线性效应不能太强。

 近年来，有人利用增强腔与很强的超短脉冲结合可以得到很高脉冲重复频率的高次谐波产生。

其中遇到的一个问题是需要很精确的腔内色散补偿，来自于很高的光强在谐振腔反射镜和其它光学元件，以及由于用于高次谐波产生的气体中等离子产生引起的光束畸变。

Definition: optical cavities for exploiting the resonant enhancement of the power of circulating light

Alternative term: resonant enhancement cavities

More general term: optical resonators

An enhancement cavity is an optical cavity (resonator) which is used for resonant enhancement of an optical power or intensity: if the incident light is resonant with the cavity and is mode-matched to it, the intracavity power can be far above the incident power, particularly for a cavity with high finesse, and that aspect is typically exploited for some purpose – typically, for efficient nonlinear frequency conversion. The topography of the cavity can be that of a linear cavity or a ring resonator.

An enhancement resonator can contain other optical components. For example, a nonlinear crystal can be used for efficient nonlinear frequency conversion, such as frequency doubling [1] or sum frequency generation. As an example, Figure 1 shows a monolithic frequency doubler, as described more in detail in Ref. [5]. It consists of a nonlinear crystal with dielectric coatings on the end faces which make the pump wave (red) resonant. Frequency-doubled light is extracted on the right-hand side. Even if the nonlinear process converts only a small fraction of the circulating optical power, the resonator allows for a kind of recycling of the unused light. The conversion can be very efficient if impedance matching is achieved, i.e. if the input mirror transmission equals all other resonator losses.

Figure 1: Monolithic resonant frequency doubler with dielectric coatings on the curved end faces.

Frequency doubling can be efficient at even significantly lower powers (a few milliwatts) by using a doubly resonant scheme, where both pump wave and second-harmonic wave are resonant. However, the double resonance is usually delicate to maintain.

Resonant doubling should not be confused with intracavity frequency doubling, where the nonlinear crystal is placed within the laser cavity, so that there is no need for a separate resonant cavity.

Conditions for Efficient Resonant Enhancement

For efficient operation of an enhancement cavity, several factors have to be considered:

• The resonance condition can usually be fulfilled only for single-frequency light, or for a frequency comb (see below). The resonance condition leads to the requirement that the resonator length must be correct within a fraction of an optical wavelength. An electronic feedback loop is often used for maintaining the resonance over longer times. Such a feedback loop may either adjust the optical frequency of the laser to match the cavity frequency, or adjust the cavity length e.g. via a piezo actuator below a resonator mirror. Note that for high-finesse cavities, which make possible a large power enhancement, the stability requirements on both the cavity and the laser can be very high.
• The incoming radiation must be spatially mode-matched to the cavity, using e.g. appropriate optics for focusing and alignment. The incoming light must usually be delivered with essentially diffraction-limited beam quality.
• To minimize losses via back reflection of pump power, the enhancement cavity should be impedance matched. This means that the transmission coefficient of the input mirror for the pump radiation matches the coefficient quantifying all other losses.

Enhancement Cavities for Mode-locked Lasers

Enhancement cavities are often applied in conjunction with single-frequency lasers, but can also be used with mode-locked lasers. In the latter case, the cavity length has to be chosen such that the cavity round-trip time is an integer multiple of the pulse spacing. In other words, the free spectral range of the cavity must be an integer multiple of the laser's pulse repetition rate, so that all lines of the laser output (→ frequency combs) can be simultaneously resonant. Also, the intracavity chromatic dispersion and nonlinearity should not be too strong [8].

Recently, enhancement cavities have been used with very intense ultrashort pulses in order to obtain high harmonic generation at very high pulse repetition rates [6, 7]. Challenges arise from the need for precise intracavity dispersion compensation, from the very high optical intensities on resonator mirrors and other optics, and from beam distortions due to plasma generation in the gas used for the high harmonic generation.

Bibliography