Abstract:
Methods, systems, and apparatus for optical communications are provided. One of the apparatus includes a first Faraday rotator having an applied magnetic field in a first direction; a second Faraday rotator optically coupled to the first Faraday rotator, the second Faraday rotator having an applied magnetic field in a second direction in opposition to the first direction; and a mirror optically coupled to the second Faraday rotator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit under 35 U.S.C. § 119(e) of the filing date of U.S. Patent Application No. 61/993,424, for “Athermal Faraday Rotator Mirror,” which was filed on Jan. 30, 2014, and which is incorporated here by reference. 
     
    
     BACKGROUND 
       [0002]    This specification relates to optical communications. 
         [0003]    A conventional Faraday rotator mirror can be used in many applications, for example, in a fiber-optic Michelson Interferometer, a laser amplifier, or a sensor device. Conventional Faraday rotator mirrors can be used, e.g., as compensators for induced birefringence in optical fibers. 
       SUMMARY 
       [0004]    In general, one innovative aspect of the subject matter described in this specification can be embodied in apparatuses that include a first Faraday rotator having an applied magnetic field in a first direction; a second Faraday rotator optically coupled to the first Faraday rotator, the second Faraday rotator having an applied magnetic field in a second direction in opposition to the first direction; and a mirror optically coupled to the second Faraday rotator. 
         [0005]    The foregoing and other embodiments can each optionally include one or more of the following features, alone or in combination. The first Faraday rotator includes a first magneto-optic material configured to provide a first polarization rotation to light passing through the first magneto-optic material, and wherein the second Faraday rotator includes a second magneto-optic material configured to provide a second polarization rotation of light passing through the second magneto-optic material. The first polarization direction and the second polarization rotation have opposite signs. The first magnetic-optic material and the second magnetic-optic material are different garnet materials. The first Faraday rotator and the second Faraday rotator are configured to compensate for thermal drift caused by each individual Faraday rotator. The first Faraday rotator and the second Faraday rotator are configured to flatten wavelength dependent polarization rotation caused by individual Faraday rotators of the first and second Faraday rotators. The apparatus further includes an input port configured to input a light beam having one or more signal wavelengths; and an output port configured to output a light beam having one or more signal wavelengths. The polarization direction of the input light beam is rotated by 90 degrees when entering the output port. The input light beam passes through the first Faraday rotator and the second Faraday rotator before being reflected by the mirror, and wherein the light reflected from the mirror passes through the second Faraday rotator and the first Faraday rotator before passing through the output port. 
         [0006]    In general, one innovative aspect of the subject matter described in this specification can be embodied in apparatuses that include a first Faraday rotator having an applied magnetic field in a first direction; and a second Faraday rotator having an applied magnetic field in a second direction in opposition to the first direction, wherein the polarization rotation caused by the second Faraday rotator has an opposite direction than the polarization rotation caused by the first Faraday rotator. 
         [0007]    Particular embodiments of the subject matter described in this specification can be implemented so as to realize one or more of the following advantages. A Faraday rotator mirror is provided that compensates for thermal drift with respect to an applied polarization rotation. The Faraday rotator can also flatten wavelength dependence for applied polarization rotation. 
         [0008]    The details of one or more embodiments of the subject matter of this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is an example of a conventional Faraday rotator. 
           [0010]      FIG. 2  is a plot showing an example of thermal drift of a Faraday rotator mirror of the Faraday rotator mirror of  FIG. 1 . 
           [0011]      FIG. 3  is an example Faraday rotator mirror. 
           [0012]      FIG. 4  is a plot showing an example of wavelength dependence according to the index for different types of Magnetic-Optical materials. 
       
    
    
       [0013]    Like reference numbers and designations in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0014]    A Faraday rotator is intended to provide a specified rotation of a polarization direction of incident light beams. In many applications, there needs to be a very accurate Faraday rotation angle. For example, in a fiber-optic interferometer, Faraday rotator mirrors are used to eliminate interference signal fluctuations due to random polarization direction changes in the optical fibers. The exact rotation provided by a conventional Faraday rotator can vary due to temperature and wavelength changes. 
         [0015]      FIG. 1  is an example of a conventional Faraday rotator mirror  100 . The Faraday rotator mirror  100  includes a magnetic-optical material  102 , a magnetic field generating element  104 , and a mirror  106 . The magnetic-optical material  102  is a material that causes a rotation of polarization of light beams passing through the material in response to an applied magnetic field. For example, the magnetic-optical material  102  can be a garnet material. Garnet materials, for example, thin film garnet materials having particular chemical structures are magneto-optic. Garnet materials can be natural or synthetic including rare-earth doped garnets. 
         [0016]    The magnetic field is caused by the magnetic field generating element  104 . The magnetic field generating element  104  can be, for example, a permanent circular shaped magnet. In some other implementations, the magnetic field generating element  104  can be a wire coil wound onto a circular iron core. An electrical current can be applied to the coil to generate a magnetic field. Under the magnetic field generated by the magnetic field generating element  104 , the polarization of a light beam passing through the magnetic-optical material  102  will be rotated by a specified amount. In particular, the magnetic-optical material  102  can rotate the polarization of an incident light beam by substantially 45 degrees. 
         [0017]    In operation, an input light beam input passes though the magnetic-optical material  102 , undergoing a rotation of the polarization direction of the input light beam by substantially 45 degrees. In some implementations, the input light beam is orthogonally polarized light. In some other implementations, the input light beams have been conditioned to have a single polarization direction upon entering the Faraday rotator mirror  100 . 
         [0018]    The light beam is reflected by the mirror  106  to pass back through the magnetic-optical material  102 , where it undergoes an additional rotation of substantially 45 degrees in the same rotational direction. Thus, after exiting the magnetic-optical material  102  on the reflected path, a total polarization of substantially 90 degrees is realized. 
         [0019]    The rotation angle provided by the magnetic-optical material when the magnetic field is applied typically has some variation due to temperature and the wavelengths of the light beams passing through. 
         [0020]      FIG. 2  is a plot  200  showing an example of thermal drift of a Faraday rotator mirror of the Faraday rotator mirror of  FIG. 1 . In particular, an x-axis corresponds to temperature while a y-axis corresponds to rotation angle in degrees. Each curve illustrates the temperature dependence of a particular wavelength of light, and different curves correspond to different types of magnetic-optical materials. 
         [0021]      FIG. 4  is a plot  400  showing an example of wavelength dependence according to the index for different types of Magnetic-Optical materials. In particular, an x-axis corresponds to wavelength in nanometers while a y-axis corresponds to an index. The Faraday rotate angle of the Faraday rotator mirror of  FIG. 1  will also show the wavelength dependence. 
         [0022]    As noted above, in many applications, there needs to be a very accurate Faraday rotation angle controlling under various conditions. To control for thermal drift, a Faraday rotator mirror can include a composite assembly including a pair of Faraday rotators each having different magnetic-optical materials, as described below with respect to  FIG. 3 . In particular, the different magnetic-optical materials can be different garnet materials. 
         [0023]      FIG. 3  is an example Faraday rotator mirror  300 . The Faraday rotator mirror  300  includes a first rotator  302 , a second rotator  304 , and a mirror  306 . The first rotator  302  includes a first magnetic-optic material  308  and a first magnetic field generating element  310 . The second rotator  304  includes a second magnetic-optic material  312  and a second magnetic field generating element  314 . 
         [0024]    The magnetic field generating elements  310  and  314  can be, for example, a permanent circular shaped magnet or a wire coil wound onto a circular iron core, as described above with respect to  FIG. 1 . The first magnetic-optic material  308  can be a first garnet material while the second magnetic-optic material  312  can be a second garnet material. The garnet materials can have different thermal drift rates. Additionally, the properties of the garnet materials can be different such that when an appropriate magnetic field is applied, a specified amount of polarization rotation occurs to passing light beams. 
         [0025]    The Faraday rotator mirror  300  is arranged such that an input light beam passes through the first magnetic-optic material  308  and the second magnetic-optic material  312  before being reflected by the mirror  306 . The reflected light beam passes back through the second magnetic-optic material  312  and the first magnetic-optic material  308  before exiting the Faraday rotator mirror  300 . 
         [0026]    More specifically, the respective first and second magnetic field generating elements  310  and  314  generate a magnetic field in opposite directions. Consequently, the corresponding first and second magnetic-optic materials  308  and  312  rotate the polarization of incident light in opposite directions. 
         [0027]    An input light beam travelling through the first magnetic-optic material  308  has its polarization rotated by a first rotational angle all. Passing through the second magnetic-optical material  312 , the light beam is rotated by a second rotational angle (−Φ 2 ). Thus, the total rotation after passing through the first and second magnetic-optical materials  308  and  312  is Φ 1 −Φ 2 . After reflection by the mirror  306 , the light beam is again rotated by the second magnetic-optical material  312  by (−Φ 2 ) and by the first magnetic-optical material  308  by Φ 1 . Therefore, the total rotation angle for the exiting light beam is: 2Φ 1 −2Φ 2 . If Φ 1 −Φ 2  is equal to 45 degrees, the device operates as a typical Faraday rotator mirror, e.g., if Φ 1  is 110 degrees and Φ 2  is 65 degrees. The total polarization rotation of the Faraday rotator mirror of  FIG. 3  applied to an existing light beam is therefore 90 degrees. Since the signs are opposite, the thermal drift of the faraday rotator mirror  300  can be compensated, in effect, by cancelling each other out. 
         [0028]    Mathematically, the Faraday rotation angle Φ can be defined as: 
         [0000]      Φ= VBL,    (1)
 
         [0000]    where V is the Verdet Constant, B is the magnetic field, and L is the effective material thickness. The value of the Verdet Constant is material dependent. 
         [0029]    Assuming that two different types of magnetic-optic material are used with different thermal drift rates and applying magnetic fields, it can be shown that: 
         [0000]      Φ 1   −V   1   B   1   L   1  and Φ 2   =V   2   B   2   L   2    (2)
 
         [0000]    If we choose the materials so that: 
         [0000]      Φ 1 −Φ 2 =π/4, and   (3)
 
         [0000]      ∂ T Φ 1 −∂ T Φ 2 =0,   (4)
 
         [0000]    where ∂ T  is the partial derivative with respect to temperature.
 
Then the tot thermal drift of the Faraday rotate angle can be cancelled out.
 
         [0030]    Combining Equations (1), (2), (3) and (4), can provide: 
         [0000]      ∂ T  Φ 1 −∂ T  Φ 2 =∂ T  ( B   1   V   1   L   1 )− 7   T ( B   2   V   2   L   2 )=0,   (5)
 
         [0000]    and 
         [0000]        B   1   V   1   L   1   −B   2   V   2   L   2 =π/4.   (6)
 
         [0031]    Under Saturated region: 
         [0000]    The magnetic field strength B 1  and B 2  can be treated as constants Therefore:
 
Equation (5) can be rewritten as:
 
         [0000]      ∂ T  Φ 1 −∂ T Φ 2   =B[∂   T ( V   1   L   1 )−∂ T ( V   2   L   2 )]= B [L   1∂T ( V   1 )+ V   1   L   1α1   −L   2∂T ( V   2 )+ V   2   L   2α2 ]=0(6-2)
 
         [0032]    Additionally Equation (6) can be written as, 
         [0000]        B   1   V   1   L   1   −B   2   V   2   L   2   =B ( V   1   L   1   −V   2   L   2 )=π/4   (7)
 
         [0033]    The change in the Faraday rotation angle with respect to temperature ΔΦ)/ΔT, namely the thermal drift, can therefore be written for the first magnetic-optic material as: 
         [0000]      ΔΦ 1   =ΔT×∂   T ( B   1   V   1   L   1 )=Δ T×BL   1 (∂ T ( V   1 )+ V   1α1 ).   (8)
 
         [0000]    And for the second magnetic-optic material as: 
         [0000]      ΔΦ 2   =ΔT×∂   T ( B   2   V   2   L   2 )=Δ T×BL   2 (∂ T ( V   2 )+ V   2α2 );   (9)
 
         [0000]    Where, α 1  and α 2  are the thermal expansion coefficients of the magnetic-optical materials. 
         [0034]    For magnetic-optical material 1 GTD(for example), let 
         [0000]      (∂ T    V   1 )/ V   1 +α 1 =γ 1    (10)
 
         [0000]    And meanwhile, for magnetic-optical material 2 GLB(for example), let 
         [0000]      (∂ T    V   2 ) V   2 +α 2 =γ 2    (11)
 
         [0000]      Let Φ 1 =( r   1 ) (π/4); Φ 2 =( r   2 ) (π/4);   (12)
 
         [0000]    Equation (3) becomes 
         [0000]        r   1   −r   2 =1;   (13)
 
         [0000]    and Equation (4) becomes 
         [0000]      γ 1    r   1 −γ 2    r   2 =0;   (14)
 
         [0000]    γ 1  and γ 2  can be obtained from known data about the particular materials, e.g., from material data sheets. 
         [0000]        r   2 =γ 1 /(γ 2 −γ1); (15)
 
         [0000]        r   1 =γ 2 /(γ 2 −γ 1 );   (16)
 
         [0035]    Consequently, the materials and their Faraday rotation angles Φ 1  and Φ 2  can be specified to obtain a temperature insensitive 90 degree Faraday rotator mirror. 
         [0036]    A wavelength insensitive 90 degree Faraday rotator mirror can be designed in a similar way. 
         [0037]    The wavelength dependence of the Faraday rotation provided by the Faraday rotator mirror of  FIG. 3  can be described as follows: 
         [0000]      (Φ)(λ)= V (λ)× B×L.    (17)
 
         [0038]    The Verdet constant has a wavelength dependence that can be described as: 
         [0000]        V (λ)=(π/λ)×[ n (λ)−1 /n (λ)]×[ A+B /(λ 2 −λ 2 )];   (18)
 
         [0000]    where A and B are dispersion constants.
 
The conditions to get wavelength insensitive 90 degree Faraday rotator mirror are:
 
         [0000]      Φ 1 (λ c )−Φ 2 (λ c )=π/4;   (19)
 
         [0000]      ∂ λ Φ 1 −∂ λ Φ 2 =∂ -80  ( B   1   V   1   L   1 )−∂ λ ( B   2   V   2   L   2 )=0   (20)
 
         [0000]    Under saturated region; 
         [0000]      ( L   1 )(∂ λ )( V   1 )−( L   2 )(∂ λ )( V   2 )=0   (21)
 
         [0039]    In a similar way like in designing temperature insensitive device, the materials and their Faraday rotation angles Φ 1  and Φ 2  can be carefully specified to get a wavelength insensitive 90 degree Faraday rotator mirror. 
         [0040]    While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular inventions. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
         [0041]    Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system modules and components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. 
         [0042]    Particular embodiments of the subject matter have been described. Other embodiments are within the scope of the following claims. For example, the actions recited in the claims can be performed in a different order and still achieve desirable results. As one example, the processes depicted in the accompanying figures do not necessarily require the particular order shown, or sequential order, to achieve desirable results. In certain implementations, multitasking and parallel processing may be advantageous.