Patent Publication Number: US-2002009254-A1

Title: High switching speed digital faraday rotator device and optical switches reduced cross talk and state sensing capability

Description:
[0001] This Application claims a priority date of Jul. 24, 2000 benefited from a previously filed Provisional Patent Application No. 60/220,386 filed on Jul. 24, 2000. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] The invention relates generally to method and apparatus for optical signal transmission. More particularly, this invention relates to bi-stable polarization control method and apparatus for configuring high-speed optical switches.  
       [0004] 2. Descriptions of the Prior Art  
       [0005] As more fiber optic network systems are installed for carrying out optical signal transmissions, a technical challenge is still faced by those of ordinary skill to provide optical switches with high switching speed and long term operation reliability. An optical network system typically incorporates one or more switches to direct the optical paths for transmitting the optical signal to the desired destinations. In addition to the general requirements of low insertion loss, small cross talk, high extinction ratio, low polarization-dependent loss, etc., increasingly there is a demand for optical switches that have a high switching speed and good long-term reliability. The switching speed and reliability requirements are particularly important for optical network systems that demand high performance and long-term reliable signal transmissions.  
       [0006] Most optical switches implemented with prior art technology use mechanical switches, which utilize moving parts for controlling the optical signal transmission through different paths. Due to the need to mechanically move the optical element(s), switching speed is very limited, typically in millisecond range. Furthermore, the moving part is susceptible to material fatigue and worn out of linkages, particularly to these components connected to the moving parts. Long-term reliability becomes a major problem for design, operation, and maintenance of the optical network signal transmission systems as that discussed by P. G. Hale and R. Kompfner, in a paper “Mechanical Optical-Fiber Switch,”  Electron. Lett.  12, 388 (1976).  
       [0007] In order to overcome these difficulties, non-mechanical switches are implemented. The non-mechanical optical switches control the optical transmission paths of light by controlling the polarization state of a light by applying either magneto-optical (MO) or electro-optical (EO) control mechanism on the transmission of the light. In the case of using magneto-optic effect for controlling the switching operations, the typical device is composed of a soft Faraday rotator and an electromagnet. The magnetically soft Faraday rotator is located inside a cylindrical electromagnet that has coil windings around a soft magnet. Control of the magnetization state of the Faraday rotator is achieved by controlling the directions of the driving current in the coil. The drawback of this scheme is that it requires a continuous high current source to maintain the magnetization state in the Faraday rotator, resulting in high power consumption.  
       [0008] This problem can be alleviated by the more efficient, but sophisticated, electromagnet designs. Several prior art references discussed about these techniques, specifically in U.S. Pat. No. 5,048,937 entitled “Faraday Rotator Device and Optical Switch Containing same,” issued on Sep. 17, 1991 to Shigeru Takeda and Satoshi Makio. An article entitled “Non-mechanical optical switch for single-mode fibers” was published in  Applied Optics , Vol. 21, No.23,4229-4234, 1982 by M. Shirasaki, H. Nakajima, T. Obokata, and K. Asama. Another article entitled “Magneto-optical 2×2 switch for single-mode fibers,” was published in  Applied Optics , Vol.23, No.19,3272-3276, 1984, by M. Shirasaki, F. Wada, H. Takainatsu, H Nakajima, and K. Asama. Here, a different electromagnet using semi-hard magnetic core material instead of the conventional soft magnets combining with a driving current pulse with finite time duration reduces the need for a continuous power supply. However, the material properties of the semi-hard magnet has to be carefully optimized so that it is not too hard to drive, yet hard enough to sustain the required remnant state. Specific details can be referred to U.S. Pat. No. 5,627,924, entitled “Article Comprising a non-mechanical optical fiber switch,” issued on May 6, 1997 to S. Jin, I. Royer and T Tiefel. These devices however require complicated electromagnet design. Additionally, the devices are more expensive because sophisticated magnet with optimized material property has to be used.  
       [0009] Another difficulty faced by conventional optical switch is the cross talk between channels. All above-mentioned optical switches can be designed based on optical analyses to satisfy specifications providing tolerance ranges of different design parameters. However, when these designs are practically implemented, cross talks are introduced due to imprecise polarization rotation caused by optical misalignment or variations in temperature, crystal thickness and wavelength, etc. For the purpose of maintaining signal integrity during data transmission, particularly for high bandwidth optical transmission systems, a robust mechanism to reduce cross talk is necessary. One method for reducing the cross talk is to remove the unwanted optical signals away from the main optical path. This can be accomplished by using a series of polarization rotating devices (PRDs).  
       [0010] Therefore, a need still exists in the art to provide a simple and compact switching device with high switching speed and long term reliability without requiring complicated electromagnets design and expensive materials such that these limitations and difficulties can be resolved. Another patent application filed by the applicant of this invention entitled “High Switching Speed Digital Faraday Rotator Device and Optical Switches Containing the Same” (Ser. No. 60/216,056 filed on Jul. 5, 2000 and Ser. No. 09/784,703 filed on Feb. 14, 2001) is hereby incorporated by reference in the patent application. The key to that patent application is the utilization of a semi-hard or hard magneto-optical crystal in the Faraday rotator instead of the soft magneto-optical crystal used in the prior arts. By using the rotator devices, the need for both a continuous current source and various complicated electromagnets designs is eliminated. However, in the meantime, a person of ordinary skill in the art still has a need to significantly reduce the cross talk in these switching devices for the purpose of maintaining data integrity in signal transmissions.  
       [0011] Furthermore, in practical application, it is often required to detect the state of optical switches in communication systems, especially at the time of power on. Since the above mentioned magneto-optic switches do not provide the capability for detecting state of the switches at power on, there is still a need in the art for a new configuration and method for detecting the state of a switch to satisfy such requirements.  
       SUMMARY OF THE PRESENT INVENTION  
       [0012] It is the object of the present invention to provide a new, compact non-mechanical, non-blocking and high speed optical switch to reduce cross talk and to provide state sensing capability. The first object is achieved by removing the unwanted lights away from the main optical path by using a series of polarization rotating devices (PRDs), such as the digital Faraday rotator device disclosed in the US patent application No. 60/216,056, or liquid crystal, or many EO crystals. The first PRD combining with a Wollaston prism splits the light beam into two: one carries the main optical intensity, while the other contains the leakage signal resulting from the imprecise polarization rotation. Subsequently, a second PRD is used to switch the polarization of the leakage signal into a state so that it is not able to merge into the main optical path in a later stage. More PRDs can be cascaded to further improve the effectiveness of the cross talk reduction in the expense of cost and complexity. Two design examples illustrating the working concept for both 1×2 and 2×2 switches are disclosed in this invention.  
       [0013] The above mentioned 1×2 and 2×2 MO switches, including the ones in U.S. patent No. 60/216,056, are all one directional switches. Bi-directional MO switches can be constructed by replacing the half wave-plate right before or after the switchable digital Faraday rotator with a high coercivity, fixed Faraday rotator. The cross talk reduction for the bi-directional switches is also disclosed in this invention.  
       [0014] The basic concept of sensing the sate of the magneto-optic switch is realized by detecting the magnetization-state of the Faraday rotator. Since the operation of a Faraday rotator generates magnetic flux at the crystal surfaces, a magnetic field sensor is placed near the Faraday rotator to sense the field direction to accurately determine the state of the magneto-optic switch.  
       [0015] These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed descriptions of the preferred embodiment that is illustrated in the various drawing figures. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0016]FIG. 1: Schematic of a polarization independent 1×2 optical switch using Wollaston prism: (a) Side view, (b) Top view, (c) Polarization states in the case of switching from I→O 1 . All 3 polarization rotating devices are in a state of maintaining the incoming light polarization. (d) Polarization states in the case of switching from I→O 2 . All 3 polarization rotating devices are in a state of rotating the incoming light polarization by 90°.  
     [0017]FIG. 2: Schematic of a polarization independent 2×2 optical switch using Wollaston prism: (a) Side view, (b) Top view, (c) Polarization states in the case of switching from I 1 →O 2 , I 2 →O 1 .  15 ,  18 ,  22  maintains incoming light polarization.  18 ′ rotates polarization by 90°. (viewing from right hand side of each component), (d) Polarization states in the case of switching from I 1 →O 1 , I 2 →O 2 .  15 ,  18 ,  22  rotate incoming light polarization by 90°.  18 ′ maintains polarization state. (viewing from right hand side of each component)  
     [0018]FIG. 3: A reversible Faraday rotator: (a) architecture (b) polarization state changes for forward traveling light (c) polarization state changes for backward traveling light. FIG. 4: Schematic of a bi-directional MO switch: (a) Side view, (b) Top view, (c) Polarization states in the case of switching from I→O 1 . (looking at the right side of each component; solid line for light traveling forward from I→O 1 , dashed line for light traveling backwards from O 1 →I)  
     [0019]FIG. 5: Architecture of a 1×2 bi-directional MO switch design with cross talk reduction scheme.  
     [0020]FIG. 6: Architecture of a 2×2 bi-directional MO switch design with cross talk reduction scheme.  
     [0021]FIG. 7: Digital Faraday rotator device structure with a magnetic field sensor on the side. 
    
    
     DETAILED DESCRIPTION OF THE METHOD  
     [0022] The basic concept of a polarization independent 1×2 optical switch is shown in FIGS. 1A and 1B as a cross sectional view and a top view respectively. Referring also to FIGS. 1C and 1D for polarization state changes as the beam passes through different components shown in FIGS. 1A and 1B as the Faraday rotator  3 ,  6  and  6 ′ are at two different magnetization states to carry out different polarization rotation operations. First, the light coming out of the fiber end is collimated using a GRIN lens (not shown). Then, the collimated light I passes through a birefringent crystal  1 . The birefringent crystal  1  is used to split the incoming light into extraordinary (e) and ordinary (o) polarization beams as shown in FIGS. 1C and 1D. Next a half-wave plate  2  and a path compensator  2 ′ are used to modify the beams so that both beams have the same polarization parallel to each other as shown in FIGS. 1C and 1D. One of these two beam-components is then passed through a half-wave plate  2  and another beam component passes through a transparent glass plate  2 ′ such that the e-component and o-component have the same polarization. These two beam components are then projected to a digital Faraday rotator device  3 . As shown in FIGS. 1A and 1B, three polarization rotating devices, e.g., three digital Faraday rotator devices  3 ,  6 , and  6 ′ are used to construct the optical switch and, in the mean time, to eliminate the potential cross talk. The applicant of this invention discloses the digital Faraday rotators in the previously filed patent application (No. 60/216,056). To switch the light beam from port I to port O 2  as shown in FIG. 1C, all three PRDs  3 ,  6  and  6 ′ are set to maintain the incoming beam polarization. Ideally, the two beams passing through PRD  3  do not change their polarization states. A Wollaston prism  4  is then used to guide the majority of the light into the upper two quadrants. In the meanwhile any leakage lights, from imprecise polarization rotation due to optical misalignment or variations in temperature, crystal thickness and wavelength, etc., are guided to the bottom two quadrants as shown in FIG. 1( c ). The dashed lines in the figure indicate leakage lights. After passing through the next two PRDs  6 ,  6 ′, a set of half waveplates  7 ,  7 ′ and a birefringent crystal  8 , the upper two beams are merged into the upper right quadrant, which is the output port O 1 . The bottom two beams, in the mean time, are diverged into the lower left quadrant and a location outside of all quadrants, respectively. Both of them will not enter either output port O 1  or output port O 2 . Therefore, the cross talk is removed. To switch the light path from I to O 2 , all PRDs are set to a state, which rotates the incoming beam polarization by 90°. The rest of the working principle is very similar to the previous case, and the polarization state changes after each component are plotted in FIG. 1( d ).  
     [0023] A 2×2 switch is designed using the similar concept. However, four PRDs are required instead of three. As shown in FIG. 2( a ), two incoming beams I 1  and I 2  are spaced further apart by a GRIN lens  11  and a wedge prism  12 . Each beam is then split into e and o beams by a birefringent crystal  13 . A set of half waveplates  14 ,  14 ′ are then used to modify the four beams so that they all have the same polarization state as shown in FIGS.  2 ( c ) and  2 ( d ). Next, these four beams are projected into one of the two different paths by the combination of a PRD  15 , a Wollaston prism  16  and a wedge prism  17 , depending upon the state of the PRD  15 . To switch the light beam from I 1  to O 2  and from I 2  to O 1 , PRDs  15 ,  18 , and  22  are set to maintain the polarization state of the incoming beams while PRD  18 ′ is set to change the incoming beam polarization by 90°. In this case, the majority of the lights are transmitted through the top four quadrants after  17 . Only leakage lights go into the bottom four quadrants. PRD  18 ′ then rotates the polarization of the bottom leakage beams by 90°. The next Wollaston prism  21  then deflects these beams away from the main optical path so that the leakage lights is prevented from merging into either exit ports in a later stage. Therefore, the cross talk is eliminated. In the mean time, PRD  18  maintains the polarization state for the top four beams. However, prism  19  is used to swap the top and the bottom two beams in the top four quadrants. The wedge prism  20  and Wollaston prism  21  bring these four beams into the main optical path. After the following PRD  22 , waveplates  23 ,  23 ′, birefringent crystal  24 , wedge prism  25 , and GRIN lens  26 , the beams in top two quadrants are merged into output port  01  and the beams at the bottom are merged into output port O 2  as shown in FIG. 2( c ). The switching from I 1  to O 2  and from I 2  to O 1  is therefore completed. To switch from I 1  to O 1  and I 2  to O 2 , PRDs  15 , 18 , and  22  are set to rotate the incoming beam polarization by 90° while PDR  18 ′ maintains the incoming beam polarization. The rest of the switching operations are very similar to the previous case, and the switching process is illustrated in FIG. 2( d ).  
     [0024] The above 1×2 and 2×2 switches both use Wollaston prisms to split or combine the beams. The Wollaston prisms can be easily replaced with birefringent crystals and the design concept remains the same. Also, the basic switching element does not have to be digital Faraday rotator. Any polarization control unit including liquid crystal, many EO crystals, etc. or combinations of many different kind of polarization control units can be used to replace the digital Faraday rotators without deviation from the design concept disclosed in the current invention.  
     [0025] All above-mentioned MO switch designs, including the ones in the previously filed patent application No. 60/216,056, are described as with one directional light transmission only. The direction of light propagation is not reversible when the Faraday rotator is used because the Faraday rotator is an irreversible device for light propagation. A bi-directional switch can be achieved by replacing the conventional irreversible Faraday rotator with a reversible digital Faraday rotator configuration as shown in FIG. 3. The reversible digital Faraday rotator is composed of a switchable digital Faraday rotator  28  followed by a high coercivity, fixed Faraday rotator  30  in replacement of the half waveplate used in the conventional irreversible digital Faraday rotator. A shielding soft ferrite core  29  can be inserted between the two Faraday rotators so that when the switchable digital Faraday rotator  28  is switching, the stray magnetic field generated from the switchable Faraday rotator  28  is mostly absorbed by the soft ferrite core  29 . The fixed Faraday rotator  30  will therefore not be affected by the switching and its magnetization state remains the same at all time. To further ensure the stability, the coercivity of the fixed Faraday rotator  30  is preferably increased to be at least twice as much as that of the first Faraday rotator. The operational principles for the reversible Faraday rotator are shown in FIGS. 3B and 3C. When a vertically polarized input light passes through the switchable Faraday rotator  28 , the polarization is rotated by 45° clockwise as shown in FIG. 3B. After passing through the second fixed Faraday rotator  30 , the polarization continues to rotate by another 45° clockwise so that the final output light polarization  31  is in the horizontal direction. The reverse process is achieved for a horizontally polarized light transmitted into the output port  31  as shown in FIG. 3C. After passing through the fixed Faraday rotator  30 , the polarization is adjusted to a direction pointing to 45° count-clockwise, which is different from a polarization state as the light transmitted in the forward direction. However, after passing through the switchable Faraday rotator  28 , the polarization rotates back to the vertical direction. The light propagation through the optical switch is now reversible when this design arrangement is implemented and the optical switch can be utilized as reversible optical switch for bi-directional applications.  
     [0026] A bi-directional one-by-two optical switch architecture is shown in FIG. 4. The reversible Faraday rotator is used instead of the conventional irreversible Faraday rotator. FIG. 4C shows the polarization state of each component when this bi-directional 1×2 switch is set to transmitting light from input port I to output port O 1 . At this state, if the light travels backwards from O 1  to I, polarization state remains the same as that of the light traveling forward for most of the components, except the fixed Faraday rotator  36 . The polarization after the fixed Faraday rotator  36  rotates the polarization by 45° counter-clockwise instead of 45° clockwise at the same location when the light travels in the forward direction. However, after passing through the switchable Faraday rotator  34 , the polarization state reverses back to the same state as for the case with forward traveling light. Therefore, with the two Faraday rotators placed adjacent to each other, the switch becomes bi-directional.  
     [0027] The similar concept can be applied to the bi-directional MO switch family in general, which includes any switch dimensions such as 1×1, 1×2, 2×2 and even higher dimensions. The key is the use of reversible Faraday rotator instead of the conventional irreversible Faraday rotator.  
     [0028] The cross talk reduction scheme for the bi-directional switch is similar to that of the one directional switch as shown in FIGS. 1 and 2. As long as the reversible Faraday rotator replaces the irreversible Faraday rotator, the above mentioned cross talk reduction technique can be directly used for the bi-directional switches. However, as described below, the implementation for the bi-directional switch can be further simplified.  
     [0029] Since the reversible digital Faraday rotator contains two separate 45° MO crystals, it rotates the incoming light polarization by either 0° or 90° depending upon the magnetization states of the two crystals. For the 0 20  case, if the first MO crystal rotates the polarization by +45°, the second crystal must rotate the polarization by the same amount, but in the opposite direction at −45°. Since the two crystals have very similar optical properties, any imprecise polarization rotation caused by temperature or wavelength changes will be compensated by itself, which means the 0° rotation is very precise. Therefore, when the reversible digital Faraday rotator is in the state of rotating polarization by 0°, no leakage light will be generated after the following Wollaston prism. As a consequence, no PRD is needed in the branch which otherwise includes the cross-talking light signal. The implementation of the cross talk reduction scheme is therefore simplified. A 1×2 bi-directional switch with simplified cross talk reduction is shown in FIG. 5. Comparing to FIG. 1 a , the PRD  6 ′ is removed because the design here assumes that the PRD  6  (or PRD  45  in FIG. 5) contains the light path which has the same polarization state as the one right before the first PRD  3  (or PRD  42  in FIG. 5). Similarly, a 2×2 design example is shown in FIG. 6. Again, comparing to FIG. 2 a , either PRD  18  or  18 ′ can be removed depending upon which path contains the cross-talking signal when the first PRD  3  is in the 0° state.  
     [0030] In order to detect the magnetization state of the Faraday rotator, a magnetic field sensor such as Hall sensor, magneto-resistive (MR) sensor, or giant magneto-resistive (GMR) sensor, etc., can be used to detect the magnetic fringing field generated from the crystal of the Faraday rotator. As shown in FIG. 7A and FIG. 7B, the sensor  115  is positioned very close to the crystal surface to achieve adequate signal output, yet, the sensor  115  is placed away from the main optical axis in order not to block the transmission of the light beams.  
     [0031] When the pulsed current passes through the coil winding  110  around the magneto-optic crystal  105 , it generates a pulsed magnetic field and the direction of the field is towards left inside the coil. If the peak magnitude of the pulsed magnetic field exceeds the remanence coercivity of the magneto-optic crystal, the direction of the magnetization of the crystal is shown in FIG. 7A. The polarization  120  of the light passing through the said crystal is rotated by an angle θ with respect to that of incoming light. The direction of the fringing field generated by the said crystal is shown in FIG. 7A when the current in the coil goes to zero. When the opposite polarity pulsed current passing through the coil  110  is used, the polarization  120  of the light passing through the said crystal is rotated by an angle −θ with respect to that of incoming light. The direction of the remnant field generated by the said crystal is shown in FIG. 7B.  
     [0032] Therefore, the sensor is employed to generate two discernible output levels when the magneto-optic crystal is at two different states. The magnetization direction in the Faraday rotator is then determined based on the output levels provided by the sensor. The same sensor structure can be used in any magneto-optic switches using digital Faraday rotator. Detection of the state of a switch is therefore achieved through the sensing and determination of the magnetization direction of the Faraday rotator.  
     [0033] Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is not to be interpreted as the limit. Various alterations and modifications will no doubt become apparent to those skilled in the art after reading the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the true spirit and scope of the invention.