Faraday rotators (original) (raw)
Definition: devices which can rotate the polarization state of light, exploiting the Faraday effect
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general optics, 
photonic devices
Related: Faraday isolatorsFaraday mirrorsoptical activity
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DOI: 10.61835/s6z Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn
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Contents
What is a Faraday Rotator?
A Faraday rotator is a magneto-optic device, where the linear polarization direction of light propagating through a transparent optical medium is continuously rotated. That rotation is caused by a magnetic field to which the medium is exposed. The magnetic field lines have approximately the same direction as the beam direction, or the opposite direction.
Polarization Rotation Angle; Verdet Constant
The total rotation angle ($\beta$) can be calculated as \beta = V\;B\;L$$
where ($V$) is the Verdet constant of the material, ($B$) is the magnetic flux density (in the direction of propagation), and ($L$) is the length of the rotator medium. Note that the Verdet “constant” usually exhibits a substantial wavelength dependence: It is smaller for longer wavelengths.
Non-reciprocal Behavior
The change in polarization direction is defined only by the magnetic field direction and the sign of the Verdet constant. If some linearly polarized beam is sent through a Faraday rotator and back again after reflection at a mirror, the polarization changes in the two passes add up, rather than canceling each other. This non-reciprocal behavior distinguishes Faraday rotators from various other optical devices with reciprocal behavior. For example, a waveplate may also rotate a polarization direction, but on the backward path (after reflection on a mirror), the polarization change would be reversed.
Effect on Circular Polarization
Concerning the physical origin of the polarization rotation, one may consider a linearly polarized beam as a superposition of two circularly polarized beams. The magnetic field causes a difference in phase velocity between these circularly polarized components, i.e., an induced circular birefringence. The resulting relative phase shift corresponds to a change in the linear polarization direction.
Construction Details of Faraday Rotators
Requirements on Magnets
The magnetic field is usually generated with an assembly of permanent magnets and ferromagnetic materials, which is optimized such that the following goals are achieved to some extent:
- The field strength should be as high as possible, such that a certain rotation angle (e.g. 45°) can be achieved with a short rotator medium. This reduces detrimental effects related to parasitic absorption (and the resulting thermal effects) and to nonlinearities of the medium.
- The magnetic flux density should be as uniform as possible over the region where light is sent through the medium. In that way, a spatially uniform rotation angle is achieved, as required e.g. to achieve high isolation of Faraday isolators.
These goals involve certain design trade-offs. In particular, a larger geometric cross-section with good field homogeneity tends to require stronger and bigger magnets or to reduce the achievable field strength. For such reasons, devices are optimized for different purposes. There are heavy and expensive high-power devices with a large aperture as well as much cheaper miniature devices for low power levels.
Magneto-optic Materials
Apart from a high Verdet constant, the Faraday medium should exhibit a high transparency in the spectral region of interest, a high optical quality, and sometimes also a high optical damage threshold. Besides, birefringence is unwanted. Different materials can be used:
- Often used Faraday media for the near-infrared spectral region are terbium–gallium garnet crystals (TGG), having high Verdet constants. For 1064-nm YAG lasers and vanadate lasers, for example, this is a natural choice.
- Terbium-doped borosilicate glass can be fabricated more flexibly and at lower cost (particularly for large sizes complicated shapes), but exhibit lower Verdet constants and lower thermal conductivity.
- Bismuth-substituted iron garnets (BIG) are principal materials for Faraday rotators in telecommunications, especially around the 1.3–1.5-μm telecom spectral region. For example, one may start with YIG = yttrium iron garnet, Y3Fe5O12) and substitute some of the Y3+ ions with Bi3+ ions to obtain BixY3-xFe5O12 (e.g. with ($x$) = 1). BIG thick films, grown with liquid phase epitaxy, offer high specific Faraday rotation, good transparency for low insertion loss. They are available in several specialized compositions optimized for different operational requirements, e.g. with a focus on low losses or low temperature dependence. As BIG materials are strong ferrimagnets, exhibiting spontaneous magnetization, there are even “latching” devices which do not require an external bias magnet.
- It is also possible to replace some of the iron in BIG with gallium, leading to bismuth iron gallium garnet (BIGG). This can be used to extend the transparency range, but is not common.
- Gadolinium gallium garnet (Gd3Ga5O12) or substituted variants (e.g., SGGG) can be used at cryogenic temperatures.
High Power Operation
For operation with high optical average powers, parasitic absorption in a Faraday rotator can lead to substantial internal heating and consequently to thermal beam distortions. In particular, thermal lensing can occur. Both the power-dependence and the significant optical aberrations of the thermal lens can be very disturbing. A high thermal conductivity helps to reduce temperature gradients.
Additional aspects of high-power operation are discussed in the article on Faraday isolators.
Anti-reflection Coatings
In most cases, reflection losses on the input and output surface of a Faraday rotator are minimized with anti-reflection coatings, designed for the intended range of operation wavelengths. The often high refractive index of these materials makes this particularly important.
The operation bandwidth may be limited by the coatings, but the wavelength dependence of the Verdet constant is another factor.
Tunable Faraday Rotators
In principle, one could make a tunable Faraday rotator by using an electromagnet instead of permanent magnets. This approach is not common, however. Instead, one can introduce a variable relative position of magnets and the Faraday medium, which also effectively leads to a variable magnet field strength. This can be useful, for example, to achieve the optimum degree of rotation for different optical wavelengths.
Applications of Faraday Rotators
Faraday rotators find many applications in laser technology:
- A particularly important application is in Faraday isolators, as needed e.g. to protect lasers and amplifiers against back-reflected light. For that application, the rotation angle should be close to 45° in the spectral region of interest. A highly uniform polarization rotation is desirable for obtaining a large attenuation for back-reflected light.
- A Faraday rotator in a ring laser resonator can be used to introduce round-trip losses which depend on the direction and thus enforce unidirectional operation. As only a very small loss difference is often sufficient, a Faraday rotator providing only a very small rotation angle may be sufficient. An additional half-waveplate may be used to compensate the polarization rotation for one beam direction.
Figure 1: Setup of a double-pass laser amplifier. The Faraday mirror on the right side ensures that the polarization state of light is not distorted after a double pass through the amplifier medium.
- A 45° rotator combined with an end mirror forms a Faraday mirror. If a laser beam is sent through some amplifier (see Figure 1), then reflected at such a Faraday mirror and sent back through the amplifier, the returning beam has a polarization direction which is orthogonal to that of the input beam — even if the polarization state is not preserved within the amplifier. Therefore, a polarizer can reliably separate the counterpropagating beams.
- The latter technique can also be utilized in similar form within laser resonators of certain high-power lasers for minimizing the polarization distortions and thus depolarization loss.
- Fiber-coupled Faraday mirrors are useful e.g. in fiber-optic interferometers and certain fiber lasers.
A variant of isolators are Faraday circulators, having three optical ports.
Frequently Asked Questions
What is a Faraday rotator?
A Faraday rotator is a magneto-optic device that uses a magnetic field to rotate the linear polarization direction of light passing through a transparent optical medium.
What determines the amount of polarization rotation?
The rotation angle is proportional to the magnetic flux density, the length of the medium, and the material's Verdet constant. This constant is specific to the material and depends on the wavelength of the light.
What does it mean that a Faraday rotator is non-reciprocal?
Non-reciprocity means the polarization rotation's direction is determined by the magnetic field, not the light's propagation direction. If light goes forth and back through the rotator, the rotations add up instead of canceling.
What materials are commonly used for Faraday rotators?
Common materials are terbium–gallium garnet (TGG) crystals for the near-infrared, terbium-doped glasses for large apertures, and bismuth-substituted iron garnets (BIG) for telecommunication wavelengths.
What are the main applications of Faraday rotators?
The most important application is in Faraday isolators, which protect lasers from back-reflections. They are also used in ring lasers to ensure unidirectional operation and in Faraday mirrors to compensate for polarization distortions.
Suppliers
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CSRayzer offers free-space Faraday rotators with different clear aperture sizes, including diameters of 3, 5, 6, 7, 8 and 10 mm. The working wavelength can be customized, and typical operation wavelengths include 800/920/1030/1064/1550 nm, with wide adaptability. Using a high-power resistant TGG crystal, the optical pulse damage threshold can reach 10 J/cm2. The clear aperture of the Faraday rotator can be up to 30 mm.
⚙ hardware
Our in-line Faraday rotator is characterized by low insertion loss, high return loss, high extinction ratio and excellent environmental stability and reliability. It is available with fiber input and output. It is ideal for polarization-maintaining fiber amplifiers, fiber lasers, and high-speed communication systems and instrumentation applications.
Bibliography
| [1] | R. H. Stolen and E. H. Turner, “Faraday rotation in highly birefringent optical fibers”, Appl. Opt. 19 (6), 842 (1980), [DOI:10.1364/AO.19.000842] |
|---|---|
| [2] | D. S. Zheleznov et al., “Faraday rotators with short magneto-optical elements for 50-kW laser power”, IEEE J. Quantum Electron. 43 (6), 451 (2007), [DOI:10.1109/JQE.2007.895668] |
| [3] | L. Sun et al., “Compact all-fiber optical Faraday components using 65-wt%-terbium-doped fiber with a record Verdet constant of −32 rad/(Tm)”, Opt. Express 18 (12), 12191 (2010); doi:10.1364/OE.18.012191 |
| [4] | E. A. Mironov, O. V. Palshov and S. S. Balabanov, “High-purity CVD-ZnSe polycrystal as a magneto-active medium for a multikilowatt Faraday isolator”, Opt. Lett. 46 (9), 2119 (2021); doi:10.1364/OL.423632 |
| [5] | J. G. Meyer et al., “Multipass Faraday rotators and isolators”, Opt. Express 32 (17), 29227 (2024); doi:10.1364/OE.520396 |
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