定义：
测量激光线宽的技术，基于光束与其自身经过时间延时后的光之间的拍音得到。

图1：测量激光线宽的自外差测量装置

自外差方法是一种用于测量激光器线宽（光谱宽度）的外差技术，尤其是单频激光器。基本装置如图1所示。一部分激光光束进入长光纤中从而得到时间延迟。另一部分光束则进入由恒定频率（通常为几十 MHz）驱动的声光调制器（AOM），使光束产生等于该恒定频率的频移。两束光后来通过分束器叠加在一起，光探测器（通常为光二极管）记录得到的拍音（中心位于AOM频率）。通过这个信号可以计算出激光器的线宽。 

如果时间延时足够长，叠加的光束基本上是不相干的，这时输出光谱仅仅为激光输出光谱的自卷积，这时激光线宽很容易算出来。但是，这里足够长是指在一种特殊情形中，是比较难以给出明确答案的问题。常用的标准是延迟长度要大于相干长度，而相干长度则来源于要测量的线宽。这在白噪声情况下表达是准确的，但是如果存在更高的低频噪声时则是不正确的，例如窄线宽光纤激光器中会存在后者的噪声。如果不完全理解这些问题，得到的测量线宽值可能会是错误的（通常偏低）。 

自外差装置中的延迟时间可看做是有限测量时间，表明小于测量时间倒数的噪声频率影响很小。这样如果瞬时频率涨落具有很大的1/f噪声，那么当时间延迟比较短时，测量得到的线宽值也比较小，这是大多数激光二极管的情形。 

在测量非常窄线宽激光器（长相干长度）时，获得非相干光束需要很长的延迟时间，这是比较不实际的。在这种情况下，也可以采用较短的时间延迟，但是数据处理会非常复杂。另一个方法是采用这种技术的延伸，即采用循环光纤环。 
 

Definition: a technique for laser linewidth measurements, based on a beat note between the beam and a delayed version of itself

The self-heterodyne method [1] is a heterodyne technique which can be used to measure the linewidth (width of the optical spectrum) of a laser, particularly a single-frequency laser. The basic setup is shown in Figure 1. One portion of the laser beam is sent through a long optical fiber which provides some time delay. Another portion is sent through an acousto-optic modulator (AOM), which is driven with a constant frequency (typically some tens of megahertz) and shifts all the optical frequency components by that frequency. Both beams are finally superimposed on a beam splitter, and the resulting beat note (centered at the AOM frequency) is recorded with a photodetector (typically a photodiode). From this signal, the laser linewidth can then be calculated.

Figure 1: Setup for self-heterodyne measurement of a laser linewidth.

For sufficiently long delays, the superimposed beams are essentially uncorrelated, and the output spectrum becomes a simple self-convolution of the laser output spectrum [1], from which the laser linewidth is easily retrieved. However, what “sufficiently long” means in a specific case, can be a difficult question. The criterion which is usually used is that the delay length must be larger than the coherence length, and the latter is derived from the (measured) linewidth itself. This is correct for white frequency noise, but not in situations with higher (e.g. 1 / f) low-frequency noise [5], as can occur e.g. for narrow-linewidth fiber lasers. If such issues are not completely understood, the measured linewidth values may be wrong (often too low).

The delay time in the self-heterodyne setup can be interpreted as a finite measurement time, which implies that the impact of noise frequencies below that inverse measurement time is low [2]. A consequence of that is that measurements with shorter delay time yield smaller linewidth values if the instantaneous frequency fluctuations exhibit substantial 1 / f noise, as is the case for many laser diodes.

For lasers with a very narrow linewidth (long coherence length), it may not be practical to have a long enough time delay to obtain uncorrelated beams. In that case, shorter delays can be used, but the data processing then has to be substantially more sophisticated. Another possibility is to use an extension of this method, based on recirculating fiber loops [3].

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