光纤耦合器(fiber couplers)

定义：一种光纤装置，用来将光从一根或几根输入光纤中耦合到一根或多根光纤中，或者从自由空间耦合进光纤中。

光纤耦合器是一种基本的光纤光学器件。需要注意的是，光纤耦合器有两种不同的含义： 

这里指的光纤耦合器是第一种类型，将光在光纤之间耦合。可以采用不同的方法制作耦合器： 

 图1:2*2光纤耦合器。 

图2给出了以上描述第一种类型光纤耦合器中光束传播的数值模拟。其中，光在两光纤纤芯中振荡，最终大部分光强保留在初始光纤中。但是，对于另一个波长的光来说，耦合过程可能差别很大。因此，这种耦合器只能工作在有限的光学带宽中。它可以用作二色性耦合器或合束器，用来将两个波长组分分离或合在一起（例如光纤放大器中的泵浦光和信号光）。 

图2：光纤耦合器的振幅分布，由数值模拟光束传播过程得到的。 光纤耦合器通常都是定向耦合器，即进入入射端口的光不会再返回输入端口中。还有一个特征量是回波损耗，表征反射光相对于入射光的大小。 

光纤合束器的限制因素 耦合损耗如果采用的光纤都是单模的（即给定波长情况下，单位偏振方向上只传播一个模式），耦合器的性能会受到一些物理因素的限制。尤其是，将两个或更多具有相同频率的入射光耦合进单偏振的输出光纤时不可避免的存在很大的附加损耗，当然如果精准的调节和稳定入射光束的相位是可以消除损耗的。即两入射光束合束时需要完全相干。 但是，这一限制在不同入射波长时就不存在了，有的耦合器可以将两个不同波长的光合在一起耦合到一个输出光纤中，而不会产生很大损耗。这种二色性耦合器通常用在光纤放大器中将信号光和泵浦光合在一起。两个输入光的插入损耗都很小（例如，远小于1 dB）。还有对波长敏感的耦合器，可以用在波分复用通信系统中用作复用器，将几个具有不同波长的入射信号结合在一起，或者分离开。 多模光纤合束器可以将两个不相干的光束合在一起，也不会产生耦合损耗。但是，这会使光亮度减小。 带宽大多数的耦合器都工作在有限波长范围内（带宽有限），因为耦合强度与波长有关（通常还与偏振有关）。这是耦合长度一定的耦合器的典型性质。熔融耦合器的典型带宽为几十纳米。上面已经提到了，它可以用作二色性耦合器或者合束器。有时也被称为WDM耦合器（参阅波分复用）。 

典型应用 光纤耦合器的一些典型应用包括： 

Definition: fiber devices for coupling light from one or several input fibers to one or several output fibers, or from free space into a fiber

Fiber couplers belong to the basic components of many fiber-optic setups. Note that the term fiber coupler is used with two different meanings:

This article treats fiber couplers of the first type, coupling light from fibers to fibers. Such couplers can be fabricated in different ways:

Figure 1: A 2-by-2 fiber coupler.

One may omit one of the input ports of a 2-by-2 fiber coupler, obtaining a Y coupler, also called T coupler. It may also be called a tap coupler, particularly if only a small fraction of power is obtained at one output and used e.g. for power monitoring. Couplers with many inputs or outputs are called star couplers; they may be used, e.g., as fiber-optic splitters, e.g. for distributing cable-TV signals.

Figure 2 shows a numerical beam propagation simulation for a fiber coupler based of the first type as explained above. Here, the light distribution oscillates between the two fiber cores, and finally the larger part of the power remains in the original (upper) fiber. For light with other wavelengths, however, the coupling can be very different. Therefore, such couplers work only in a limited optical bandwidth. They can be used as dichroic couplers or beam combiners, for example for separating or combining two wavelength components (such as pump and signal light in a fiber amplifier).

Figure 2: Amplitude distribution in a fiber coupler, obtained with a numerical simulation of beam propagation, done with the software RP Fiber Power.

Fiber couplers are usually directional couplers, which means that essentially no optical power sent into some input port can go back into one of the input ports. There is often a specification of return loss, which indicates how much weaker the back-reflected light is, compared with the input, and is usually quite large (many tens of decibels).

In many cases, all fibers involved are single-mode, i.e., they support only a single mode per polarization direction for a given wavelength. There are then certain physical restrictions on the performance of the coupler. In particular, it is not possible to combine two or more inputs of the same optical frequency into a single-polarization output without significant excess losses, except if the optical phases of the input beams are precisely adjusted and stabilized. That means that the two inputs to be combined would have to be mutually coherent.

However, such a restriction does not occur for different input wavelengths: there are couplers which can combine two inputs at different wavelengths into one output without exhibiting significant losses. Such dichroic couplers are used in fiber amplifiers to combine the signal input and the pump wave. Their insertion loss may be very small (e.g. far below 1 dB) for both inputs. Other wavelength-sensitive couplers are used as multiplexers (WDM couplers) in wavelength division multiplexing (WDM) telecom systems to combine several input channels with different wavelengths, or to separate channels.

Multimode fiber combiners allow the powers of two mutually incoherent beams to be combined without a power loss. However, this will cause some loss of brightness.

Most types of couplers work only in a limited range of wavelength (a limited bandwidth), since the coupling strength is wavelength-dependent (and often also polarization-dependent). This is a typical property of those couplers where the coupling occurs over a certain length. Typical bandwidths of fused couplers are a few tens of nanometers. As mentioned above, they can be used as dichroic couplers or beam combiners. They are sometimes also called WDM couplers (→ wavelength division multiplexing).

Some typical applications of fiber couplers are: