定义：
一种利用非线性材料实现激光频率为输入光频率四倍的效应

四倍频是一种非线性频率变换，得到的光的频率是输入激光的频率的四倍，也就是说输出光的波长是输入光的四分之一。这一过程可以通过使用两个连续的倍频器实现（图1）。另一种可能的方法是使用一个倍频器和两个连续的利用剩余泵浦光的和频产生过程，但这种方法并不常见。 

图 1一个典型四倍频装置：一个波长为1064nm的红外输入光束通过一个倍频过程后产生了一个波长为532nm的绿光，该绿光再次经过倍频后产生了波长为266nm的光。 
常见的四倍频过程如下：一个波长为1064nm的由Nd：YAG激光器产生的连续光或者脉冲光利用一个倍频器（基于LBO，三硼酸锂）产生了波长为532nm的绿光，然后该绿光利于第二个倍频器（基于CLBO，硼酸锂铯）产生波长为266nm的光。这种的紫外光在很多领域都有着广泛的应用，如：泵浦染料激光器或光学参量振荡器、用于拉曼光谱、用于材料加工（例如光纤布拉格光栅的制备）。

 由晶体退化导致的有限工作寿命 如在三倍频一文中提到的，由于强烈的紫外光的作用，会导致非线性晶体的退化。对于四倍频过程，这种短波长的紫外光同样会造成晶体和其他光学器件寿命变短的问题，有时还更为严重。更多内容可以参阅三倍频。

Definition: the phenomenon that an input laser beam generates a beam with four times the optical frequency

Alternative term: fourth-harmonic generation

More general terms: nonlinear frequency conversion

Frequency quadrupling is a process of nonlinear frequency conversion where the resulting optical frequency is four times that of the input laser beam, which means that the wavelength is reduced by a factor of 4. That can be accomplished with two sequential frequency doublers (Figure 1). Another possibility would be to use a single frequency doubler and two sum frequency generation stages for mixing with residual pump light, but that approach is not common.

Figure 1: A typical configuration for frequency quadrupling: an infrared input beam at 1064 nm generates a green 532-nm wave in a first frequency doubler, and a second frequency doubler converts this to light at 266 nm.

A common type of frequency quadrupling configuration begins with a continuous-wave or pulsed Nd:YAG laser at 1064 nm for generating 532-nm light in a first frequency doubler stage (based e.g. on LBO = lithium triborate) and then 266 nm in a second stage (based e.g. on CLBO = cesium lithium borate). Such ultraviolet light is useful e.g. for pumping a dye laser or an optical parametric oscillator, for Raman spectroscopy in flames, or for laser material processing, e.g. the writing of fiber Bragg gratings.

Limited Lifetime due to Crystal Degradation

As explained in the article on frequency tripling, nonlinear crystals can be degraded during operation by the intense ultraviolet light. For frequency quadrupling, the correspondingly shorter UV wavelength tentatively cause stronger problems of that type, compared with frequency tripling, and lead to short lifetimes of crystals and other optics. Otherwise, similar aspects apply as discussed in the article on frequency tripling.

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