定义：
利用法拉第效应的光学隔离器。

法拉第隔离器是光学隔离器的一种，在特定方向的光可以通过而相反方向的光则不能通过。法拉第隔离器利用了法拉第旋转器，并且包含了光学隔离器最重要的技术。 
另一种类似的器件是光环形器，具有三个光学端口。光从端口1进入，会从端口2出来，而从端口2进去的光会传到端口3，而端口3进去的光则从端口1出来。 
 

偏振敏感的法拉第隔离器
最简单的偏振敏感的法拉第隔离器是只有特定方向线偏振的入射光束才能通过。这里，准直的线偏振入射光先通过第一个偏振器（图1），损耗非常小，然后经过45° 法拉第旋转器，再经过第二个偏振器，其偏振方向相比于第一个偏振器也旋转了45° ，于是光通过时的损耗仍然很小。 

图1：偏振敏感的法拉第隔离器装置图。双箭头表示前向和后向传播光束的偏振方向。 
 

图2：不同功率下法拉第隔离器的选择。 

如果光经过反射后以相同的偏振态进入隔离器的出射端口，仍然可以无损耗的通过第二个偏振器。然后通过法拉第旋转器其偏振态又旋转了45°，因此光不能通过第一个偏振器，或者说光会进入另一个单独的出射端口。 
出射偏振器在光被反射回去并且偏振状态改变的情况下非常重要。 
如果法拉第旋转器的旋转角度偏离45°（由于制备误差或者工作于非设计的波长处），也可以调节输出偏振器的偏振方向来得到最大的透射，但是隔离度会减小。最好优化偏振器的指向来得到最大的隔离度，前向传播时存在一定的插入损耗是可以接受的。 
 

偏振不敏感的法拉第隔离器
偏振不敏感的法拉第隔离器的入射光束可以处于任意的偏振态。这一器件在光纤光学中会用到，因为大多数光纤不是保偏的。尤其是光纤通信系统中的光偏振态通常不是确定的，因此法拉第隔离器及其它器件需要工作于任意偏振态。 
偏振不敏感的隔离器的工作原理为：首先将入射光中正交的两偏振分量采用某种偏振器在空间上分离，然后进入法拉第隔离器，再在第二个偏振器中合在一起。 
 

图3：偏振不敏感的法拉第隔离器的示意图，前向透射率很高，而后向传播光束则在空间上被分离开。如果是光纤耦合的器件，还需要在两边各放置一个透镜用来准直和聚焦光束。 
图3是光纤光学中常用的一种偏振不敏感隔离器。这里，偏振器为双折射楔，所用的材料为TiO2。后向传播的任意偏振态的光都可以透射出来，但是光束位置发生偏移（图3），因此不能进入到入射光纤中。这一器件可以制作的非常小，如图4：

图4：光纤耦合的法拉第隔离器。还存在具有类似外壳的法拉第镜，只具有一个光纤端口，可以使反射光的偏振态旋转 90° 。 

偏振不敏感隔离器不需要保持偏振态，因为两正交偏振分量间的相对相位是任意变化的。相位变化与温度和波长有关。 
 

插入损耗和隔离度

许多应用中需要很小的插入损耗，即前向波传播损耗。插入损耗由下列因子决定：采用的偏振器类型和质量，抗反射涂层的反射损耗，以及旋转角 45° 的准确性。 
光学隔离器另一个重要的性能指标为隔离度，也就是对后向反射光的衰减程度，也称为回波损耗。法拉第隔离器的典型回波损耗为30-40 dB。高功率装置很难得到很高的隔离度，因为这是光束在法拉第介质中占据的面积更大，因此更易受到场非各向同性效应的影响。采用的偏振器的类型和是否对准也非常重要。低隔离度可能由于工作波长不是出于最优波长，也可能来自于未对准或者入射光的发散角很大，还有可能是由于当隔离器工作于高功率时产生的热效应。 
如果单个隔离器不能得到足够的回波损耗，可以采用两个或者多个隔离器结合在一起。存在将两个隔离器封装到一个外壳中的器件（双级隔离器）。组合器件足够的隔离度可以用来拓宽工作光带宽。 
这一可调谐的隔离器允许调整偏振器的方向，因此可以通过调谐对使器件对不同波长的隔离度最大。这样，单个器件就可以实现很大的工作波长范围，例如，从500 nm到750 nm，范围主要由抗反射涂层的带宽决定。为了优化某一固定装置的带宽（例如，用于可调谐钛蓝宝石激光器中），需要法拉第隔离器的介质的维尔德常数对波长依赖性很小。 
 

高功率工作

当隔离器工作于很高的功率时，需要考虑下列效应： 

• 在平均功率很高时，法拉第旋转器的寄生吸收效应会引起显著的热透镜效应，会使光束形状发生畸变，因此影响隔离度。 
• 高功率装置都需要有不能通过光的出射端口，而不是采用内部减震器，是为了避免装置内部的热效应。 
• 高峰值功率可能会引起光损伤。为了避免损伤，需要足够大的入射孔径。 
• 高峰值功率还会引起自聚焦效应。功率极限由入射光束半径以及装置长度共同决定。 

不仅法拉第隔离器，偏振器也需要满足高功率工作的要求。 
商用高功率法拉第隔离器可以在功率高达100 W时工作，并且光束失真也不严重。有些应用中，尤其是结合高功率光纤激光器和放大器的应用需要更高的功率，目前正在研发更先进的装置可以在高于1 kW的功率情况下工作。对于光纤耦合的装置，对功率有更大的限制，通常需要在100 W以下。 
 

光纤法拉第隔离器

玻璃光纤的维尔德常数可以很高。采用这种光纤可以制作光纤法拉第隔离器。尽管采用目前的技术得到的器件性能还不能与体装置比拟，但是其对未来的光纤激光器光源非常有用。 
 

应用

法拉第隔离器和环形器在激光器技术中有许多应用： 

• 通常，它们用于保护激光器和放大器，阻止背向反射光。在放大器链的各级放大器之间放置隔离器，是为了阻止背向反射光和放大的自发辐射产生的对放大器的不利效应。在光纤通信系统中，偏振不敏感的光纤耦合隔离器通常用在光纤放大器之前或者之后。 
• 各种干涉仪和其它体器件、光纤光学器件会采用环形器将反射光与前向传播光束分离开。这可以实现双通放大器。 
• 激光器谐振腔中（例如光纤激光器）会利用偏振敏感隔离器的偏振特性来得到线偏振光或者采用非线性偏振旋转实现模式锁定。 

单纯的法拉第旋转器除了用做隔离器，还有很多其它的用途。

Definition: optical isolators based on the Faraday effect

More general term: optical isolators

A Faraday isolator is a kind of optical isolator, i.e., a device which transmits light in a certain direction while blocking light in the opposite direction. Faraday isolators are based on Faraday rotators and constitute the technologically most important type of optical isolators.

A variant is an optical circulator, having at least three optical ports. Light injected into port 1 will exit at port 2, while input at port 2 will be sent to port 3, and input at port 3 to port 1.

Polarization-sensitive Faraday Isolators

The simplest type of Faraday isolator is polarization-sensitive in the sense that it works only when the input beam has a prescribed direction of linear polarization. Here, a properly polarized and collimated input beam passes a first polarizer (pol 1 in Figure 1) with minimum loss, then a 45° Faraday rotator, and finally another polarizer (pol 2) with its transmitting axis being rotated by 45°, such that the transmission losses are small.

Figure 1: Setup of a polarization-sensitive Faraday isolator, containing a Faraday rotator between two polarizers. The double arrows indicate the polarization directions of a forward and backward-propagating beam.

Figure 2: A selection of Faraday isolators for different power levels. The photograph was kindly provided by GMP Switzerland.

When light is reflected back to the output port of the isolator with an unchanged polarization state, it can fully transmit the output polarizer (pol 2). Then, however, its polarization direction is rotated by another 45° in the Faraday rotator, so that this light will be blocked at the input polarizer, or can be sent to the separate output port.

Note that the output polarizer (pol 2) is important if light may be reflected back with a modified polarization state.

If the rotation angle of the Faraday rotator somewhat deviates from 45° (either due to fabrication errors or due to operation away from the design wavelength), the orientation of the output polarizer may still be adjusted for maximum transmission, but in that case the degree of isolation is reduced. It may be better to optimize that polarizer's orientation for maximum isolation, while accepting a somewhat higher insertion loss in forward direction.

Polarization-insensitive Faraday Isolators

A polarization-insensitive Faraday isolator is a device which works for arbitrary polarization of the input beam. Such devices are often required in the context of fiber optics, because many fibers are not polarization-maintaining. In particular, optical fiber communication systems are usually operated with undefined polarization state, and Faraday isolators as well as other components are then required to work with arbitrary polarization states.

The basic principle of a polarization-insensitive isolator is to first spatially separate the orthogonal polarization components of the input beam with some kind of polarizer, then send both through a Faraday rotator, and combine them again in the second polarizer.

Figure 3: Setup of a polarization-insensitive Faraday isolator, with high transmittance in forward direction (top) and a spatial offset for backward propagation (bottom). (The beam angles are exaggerated for better clarity.) In a fiber-coupled device, there is an additional lens on each side for collimating and re-focusing the beam.

A type of device as frequently used in fiber optics is shown in Figure 3. Here, the polarizers are birefringent wedges, made e.g. of rutile (TiO₂). Backward-propagating light of any polarization direction can be transmitted, but will appear with an offset beam position (Figure 3, bottom), and will thus not be launched back into the input fiber. Such devices can be fitted into rather compact packages, as shown in Figure 4.

Figure 4: A fiber-coupled Faraday isolator. The photograph was kindly provided by Microwave Photonic Systems, Inc. There are also Faraday mirrors in similar housings, having only one fiber port and exhibiting a 90° polarization rotation for reflected light.

Note that a polarization-insensitive isolator will in general not preserve the polarization state (or simply rotate it), since there is an arbitrary relative phase change between the two polarization components. That phase change depends on temperature and wavelength.

Insertion Loss and Degree of Isolation

For many applications, a low insertion loss, i.e., a low loss for forward-propagation light, is essential. The insertion loss is determined by several factors: the type and quality of the polarizers used, residual reflection losses on anti-reflection coatings, and the precision and uniformity with which a 45° rotation angle is obtained in the rotator.

Another important performance figure of an optical isolator is the degree of isolation, i.e., the attenuation achieved for back-reflected light, also called return loss. Typical Faraday isolators achieve a return loss of the order of 30 to 40 dB. A high degree of isolation is generally more difficult to achieve for high-power devices, where the beam in the Faraday medium covers are larger area and is thus more sensitive to field inhomogeneities. The quality and alignment of the polarizers used is also important. A lower degree of isolation may result from operation at a non-optimized wavelength, from improper alignment or large divergence of the input, or from thermal effects when the isolator is operated with a high optical average power.

If the return loss achievable with a single isolator is insufficient, a combination of two (or even more) isolators may be used. There are devices available where two isolators are packaged into one housing (double-stage isolators). Such a combination may also be used to widen the optical bandwidth with sufficient isolation.

Some “tunable” isolators allow the user to adjust the angular orientation of the polarizers, and thus to optimize the isolation for different wavelengths. In that way, a single device may be used to cover a broad wavelength region, e.g. from 500 nm to 750 nm, limited only by the bandwidth of the anti-reflection coatings. For optimizing the bandwidth of a fixed setting (as required e.g. for use with a tunable titanium–sapphire laser), one requires a Faraday rotator based on a medium with a low relative wavelength dependence of the Verdet constant.

Dual-stage Isolators

In some cases, a sufficiently high degree of isolation (e.g. 60 dB) can be achieved only with a dual-stage isolator. Essentially, this means that one uses two Faraday isolators in sequence; of course, only a single polarizer is needed between the two rotators.

The two isolators may either rotate the polarization in the same direction, resulting in a total rotation angle of 90°, or they rotate in opposite directions, generating no net rotation.

The insertion loss will typically be somewhat higher with a dual-stage device.

High-power Operation

For operation of an isolator at very high optical power levels, various effects have to be considered:

• For high average powers, there is thermally induced depolarization, which can affect the degree of isolation. In addition, the parasitic absorption in the Faraday rotator can cause substantial thermal lensing, which distorts the beam profile.
• High-power devices usually have exit ports for the rejected light, rather than internal absorbers, in order to avoid the associated heating inside the device.
• For high peak powers, optical damage may occur. To prevent this, a sufficiently large input aperture is required.
• High peak powers may also lead to self-focusing. The power limit depends not only on the input beam radius but also on the length of the device.

Of course, not only the Faraday rotator, but also the polarizers have to be suitable for high-power operation.

Commercially available high-power Faraday isolators can handle optical average powers up to the order of 100 W with not too strong beam distortions. Particularly some applications in combination with high-power fiber lasers and amplifiers demand higher powers, and more advanced devices for powers of the order of 1 kW appear are being developed. For fiber-coupled devices, the power levels are more limited, usually to well below 100 W.

Fiber-based Faraday Isolators

It is possible to fabricate optical fibers from glasses with a high Verdet constant [1, 4, 10]. With such fibers, one can make entirely fiber-based (not only fiber-coupled) Faraday isolators. Although the performance achieved so far with that method does not reach that of bulk devices, this development may be useful for future fiber laser sources.

Applications

Faraday isolators and circulators find many applications in laser technology:

• In many cases, they are used to protect some laser or amplifier against back-reflected light. Amplifier chains sometimes contain several isolators between the different amplifier stages, preventing not only back-reflected light but also amplified spontaneous emission from having detrimental effects. In optical fiber communication systems, polarization-insensitive fiber-coupled isolators are frequently used e.g. before and after any fiber amplifiers.
• Various kinds of interferometers and other devices (e.g. amplifiers) in bulk or fiber-optic form use circulators for separating reflected light from the forward-propagating beam. This allows e.g. to realize a double pass through an amplifier.
• The polarizing property of a polarization-dependent isolator within a laser resonator (e.g. of a fiber laser) may be exploited for enforcing a linear polarization state or for mode locking with nonlinear polarization rotation.

There are also various applications of pure Faraday rotators, not functioning as isolators.

Bibliography