Abstract:
An optical switch including a first polarization converter and a second polarization converter which is provided in series with the first polarization converter. 
     The first and second converters are able to select as emitted light one of two perpendicular polarized light components of incident light having a desired wavelength (λ 0 ) in a polarized state. The first polarization converter converts the incident light having wavelength λ 0  into a polarized light component corresponding to one of two points positioned on opposite sides of a Poincar/e/  sphere. The two points on the Poincar/e/  sphere are obtained by rotating the incident light +90° or -90°. The incident light is rotated around an axis perpendicular to an axis passing through a point corresponding to the incident light on the Poincar/e/  sphere and the center of the Poincar/e/ sphere. The second polarization converter converts the polarized light obtained by the first polarization converter into polarized light corresponding to a point obtained by rotating the two polarized light components at +90° or -90° around an axis passing through the above-mentioned two points on the Poincar/e/  sphere.

Description:
BACKGROUD OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical switch, more particularly to an optical switch provided with an element for converting an optical polarization plane, such as a Faraday rotator or an element using an electric optical effect (hereinafter, &#34;electro-optic element&#34;). 
     2. Description of the Prior Art 
     Optical switches, integral components of optical communication systems, generally make use of Faraday rotators or electro-optic elements so as to convert a plane of polarization of light into two perpendicular polarized light components. 
     Such optical switches, however, are effective only for light having predetermined wavelengths. If the light has a wavelength other than the predetermined wavelength, the converted polarized light components will not be perpendicular, resulting in cross talk and obstructing correct optical communications. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an optical switch whose effectiveness is independent of the wavelength of light. 
     It is another object to provide an optical switch which has a high extinction ratio. 
     It is still another object to provide an optical switch enabling correct optical communications. 
     According to the present invention, there is provided an optical switch comprising a first polarization converter and a second polarization converter provided in series with the first polarization converter. The first and second converters are able to select as emitted light one of two perpendicular polarized light components of incident light having a desired wavelength (λ 0 ) in a polarized state. The first polarization converter converts the incident light having the wavelength λ 0  into a polarized light component corresponding to one of two points positioned on opposite sides of a Poincar/e/  sphere and obtained by rotating the incident light +90° or -90° around an axis perpendicular to an axis passing through a point corresponding to the incident light on the Poincar/e/  sphere and the center of the Poincar/e/  sphere. The second polarization converter converts the polarized light obtained by the first polarization converter into polarized light corresponding to a point obtained by rotating the two polarized light components +90° or -90° around an axis passing through the above-mentioned two points on the Poincar/e/  sphere. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further objects and advantages of the present invention will be apparent from the following description made in reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic diagram of a conventional optical switch; 
     FIG. 2 is a diagram for explaining a polarization state of the optical switch of FIG. 1; 
     FIG. 3 is a graph of the relationship between wavelength and an extinction ratio in an optical switch; 
     FIGS. 4 and 5 are schematic diagrams of embodiments of the present invention, constituting a polarization converting portion of an optical switch with a Faraday rotator and an electro-optic device; 
     FIG. 6 is a diagram for explaining the polarization conversion in the embodiment of FIG. 4 with reference to a Poincar/e/  sphere; 
     FIG. 7 is a diagram for explaining polarization conversion in the embodiment of FIG. 5 with reference to a Poincar/e/  sphere; 
     FIG. 8 is a diagram for explaining FIG. 6 in detail, and 
     FIGS. 9 and 10 are schematic diagrams showing other embodiments of the present invention, constituting a polarization converting portion of an optical switch with electro-optic devices or Faraday rotators. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before describing the preferred embodiments of the present invention, a more detailed explanation of the prior art will be given for reference purposes. 
     FIG. 1 is a schematic view of a conventional optical switch. As shown in FIG. 1, a conventional optical switch comprises a Faraday rotator 1, a polarizer 2, and a polarized light separation device 3. A 45° Faraday rotator made of, for example, yttrium-iron-garnet (YIG) is preferably used. A magnet (not shown) is placed over the Faraday rotator 1. 
     In such an optical switch, when light X linearly polarized by the polarizer 2 is passed through the 45° Faraday rotator 1, polarized light P 1  and P 2  rotated ±45° or -45° around the passing direction with respect to the incident light P 0  according to the direction of the magnetic field in the Faraday rotator 1 are obtained, as shown in FIG. 2. The polarized light P 1  and P 2  may, if desired, be passed through an analyzer (not shown). 
     This polarization of light is efficiently effected only for light having a certain wavelength. Namely, the Faraday rotator 1 is designed for polarization of only light having a wavelength λ 0 . To rotate by 45° linearly polarized light having, for example, a wavelength of 1.3 μm, a 45° Faraday rotator having a length of 2.1 mm is necessary. 
     When, the wavelength is shorter than that required, the Faraday rotation is deviated to P 1  &#39; and P 2  &#39; shown in FIG. 2, resulting in crosstalk in the optical switch. Thus, when the wavelength deviates from the required value, i.e., λ 0 , the extinction ratio of the optical switch is severely lowered, as shown in FIG. 3. 
     FIGS. 4 and 5 are schematic diagrams of first and second embodiments of an optical switch according to the present invention, respectively polarization. 
     As shown in FIG. 4, a polarization converting portion of an optical switch comprises a 45° Faraday rotator 1 and an electro-optic element 4. 
     In this optical switch, the direction of the principal axis of the electro-optic element 4 corresponds with the direction of polarized light incident thereto. Further, the electro-optic element 4 is operated at voltages corresponding to a phase difference of +/-90° with respect to the wavelength λ 0 . 
     In the optical switch of FIG. 4, the 45° Faraday rotator 1 has a wavelength dependency. Thus, when a deviation Δλ in the wavelength of light passing through the Faraday rotator 1 occurs, a deviation corresponding to kΔλ (k: a constant determined by the Faraday rotator) occurs in the angle of optical rotation. 
     FIG. 6 is a diagram for explaining polarization conversion in the embodiment of FIG. 4 with reference to a Poincar/e/  sphere. Linearly polarized light incident to the Faraday rotator 1 in FIG. 4 is shown as a point A on the equator of the Poincar/e/  sphere of FIG. 6. When the wavelength of the light is λ 0 , the point A is rotated by +90° or -90° around the axis passing through the Arctic and the Antarctic points of the Poincar/e/  sphere by the Faraday rotator, whereby the point A is moved to a point P 1  or a point P 2 . 
     When the wavelength of the light has deviated, the point P 1  or the point P 2  moves along the equator of the sphere. As shown in FIG. 6, the points P 1  and P 2  are moved to points P 1  &#39; and P 2  &#39; respectively when the wavelength of the light is shorter than λ 0 . 
     Then, the points P 1  &#39; and P 2  &#39; are rotated by +90° and -90° with respect to an axis passing through the points P 1  and P 2  by using the electro-optic element 4, which operates as the second polarization converter, and the points P 1  &#39; and P 2  &#39; aremoved to points P 3  &#39; and P 4  &#39;, respectively. As a result, the points P 3  &#39; and P 4  &#39; are arranged so that they are substantially opposed to each other with respect to the center O of the Poincar/e/ sphere. This mechanism will be explained with reference to FIG. 8, which is a diagram for explaining FIG. 6 in detail. 
     In FIG. 8, the deviated point P 2  &#39; of the point P 2  is shown. The point P 2  &#39; of a shorter wavelength is moved not to the point P 4  rotated by 90° from the point P 2 , but to the point P 4  &#39; rotated more than the point P 4 . In this case, the deviation angle α 2  formed by lines P 4  O0 and P 4  &#39;O is remarkably small and negligible in comparison with the deviation angle α 1  due to only the first polarization. 
     Thus, the points P 3  &#39; and P 4  &#39; are oppositely arranged on the Poincar/e/  sphere with respect to the center thereof, with the result that polarized light corresponding to the point P 3  &#39; is perpendicular with polarized light corresponding to the point P 4  &#39;. 
     FIG. 7 is a diagram for explaining the polarization conversion in the embodiment of FIG. 5. In FIG. 5, the electro-optic element 4 is arranged so that the direction of the principal axis is slanted by 45° to the direction of polarized light incident on the electro-optic element 4. Linearly polarized light incident on the electro-optic element 4 is shown as a point B positioned on the equator of the Poincar/e/  sphere. The linearly polarized light, having a wavelength of λ 0 , is rotated by +90° or -90° around an axis CO0 perpendicular to an axis BO0 passing through the point B and the center O of the Poincar/e/  sphere, by using the electro-optic element 4. As a result, the point B is moved to the point Q 2  or Q 2 , i.e., the Arctic point or the Antarctic point. 
     When the wavelength of light is deviated from the required wavelength, the point Q 1  or Q 2  is moved along the curved line passing through the points Q 1 , B, and Q 2 . Namely, as shown in FIG. 7, the points Q 1  and Q 2  are moved to points Q 1  &#39; and Q 2  &#39; when the wavelength of the light is shorter than λ 0 . 
     Then, points Q 1  &#39; and Q 2  &#39; are rotated by +90° and -90° with respect to an axis passing through the points Q 1  and Q 2  by using the Faraday rotator 1, which is operated as the second polarization converter, and the points Q 1  &#39; and Q 2  &#39; are moved to points Q 3  &#39; and Q 4  &#39;. As a result, the points Q 3  &#39; and Q 4  &#39; are arranged so that they are substantially opposed to each other with respect to the center of the Poincar/e/  sphere, and polarized light corresponding to the point Q 3  &#39; is perpendicular with polarized light corresponding to the point Q 4  &#39;. 
     FIGS. 9 and 10 are schematic diagrams showing other embodiments of the present invention, constituting a polarization converting portion consisting of an optical switch with electro-optic devices and Faraday rotators. 
     As shown in FIG. 9, a polarization converting portion of an optical switch consists of an electro-optic element 4, a quarter-wave plate 5, an electro-optic element 6, and a quarter-wave plate 5&#39;. The plate 5&#39; is used only for rotation of light. 
     The polarization converting portion of an optical switch, shown in FIG. 9, corresponds to that shown in FIG. 4, except the Faraday rotator 1 is replaced by the electro-optic element 4 and the quarter-wave plate 5. 
     The quarter-wave plates 5 and 5&#39; give a phase difference of 90° to incident polarized light and have no wavelength dependency. The principal axis of the quarter-wave plate 5 is parallel or perpendicular to a plane of polarization in linearly polarized light incident on the first polarization converter, i.e., the electro-optic element 4. 
     Since the electro-optic device 4 in FIG. 9 is the same as the electro-optic element 4 in FIG. 5, the conversion of the linearly polarized light on the electro-optic device 4 in FIG. 9 is the same as that explained with reference to FIG. 7, in which the point B is converted to the points Q 1  and Q 2 . After the conversion of light by the electro-optic element 4, the converted light shown as the points Q 1  and Q 2  in FIG. 7 is rotated by 90° around the axis BO by passing through the quarter-wave plate 5, with the result that the points Q 1  and Q 2  in FIG. 7 are converted to points corresponding to points P 1  and P 2  on the Poincar/e/  sphere shown in FIG. 6. Thus, the functions of the electro-optic element 4 and the quarter-wave plate 5 in FIG. 9 are the same as that of the Faraday rotator 1 in FIG. 4. 
     In FIG. 10, the polarization converting portion of an optical switch consists of a Faraday rotator 1, a quarter-wave plate 5, a Faraday rotator 7, and a quarterwave plate 5&#39;. The plate 5&#39; is used only for rotation of light. 
     The polarization converting portion of an optical switch, shown in FIG. 10 corresponds to that shown in FIG. 5, except the electro-optic element 4 is replaced by the Faraday rotator 1 and the quarter-wave plate 5. 
     Since the Faraday rotator 1 in FIG. 10 is the same as the Faraday rotator 1 in FIG. 4, the conversion of the linearly polarized light by the Faraday rotator 1 in FIG. 10 is the same as that explained with reference to FIG. 6, in which the point A is converted to the points P 1  and P 2 . 
     After the conversion of light by the Faraday rotator 1, the converted light shown as the points P 1  and P 2  in FIG. 6 is rotated by 90° around the axis AO by passing through the quarter-wave plate 5, with the result that the points P 1  and P 2  in FIG. 6 are converted to points corresponding to points Q 1  and Q 2  on the Poincar/e/ sphere shown in FIG. 7. 
     Thus, the functions of the Faraday rotator 1 and the quarter-wave plate 5 in FIG. 10 are the same as that of the electro-optic device 4 in FIG. 5. 
     In an optical switch according to the present invention, each of the first and the second polarization converters simultaneously convert light at the rotational angles of +90° and -90° on the Poincar/e/ sphere. 
     Further, to make a plurality of final points on the Poincar/e/  sphere, for example, a plurality of points P 4  &#39; obtained by slight dependency of wavelength, fit one point on the Poincar/e/  sphere, a different half wave plate (not shown) may be used in the optical switch. 
     According to the present invention, even though light emitted from a first polarization converter is converted into one of two non-perpendicular light components, the two light components can be made substantially perpendicular by providing the second polarization converter in series with the first polarization converter, thereby reducing cross talk due to deviation of the wavelength. Namely, the extinction ratio is maintained high over a wide range of wavelengths as shown in FIG. 3. 
     According to the present invention, a YIG 45° Faraday rotator is preferably used as the Faraday rotator, and lithium tantalate (LiTaO 3 ) or lithium niobate (LiNbO 3 ) is preferably used for the electro-optic element.