Abstract:
This specification discloses a reflective optical circulator, which uses an optical reflective device to reflect an incident light beam from an optical port so that the reflected light beams further pass through all optical devices (i.e., all sorts of optical crystals) on the optical paths. With a proper reciprocal-non-reciprocal optical crystal combination, a particular linear polarization direction is generated to guide the reflected beams to the next optical port. The invention achieves the effect of repeatedly using crystals, lowering the number of crystals and the length of the optical circulator. On the other hand, all optical ports can be installed on the same side of the optical circulator, minimizing the device and making it easy to use.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of Invention  
           [0002]    The invention relates to an optical device and, in particular, to an optical circulator with several optical ports installed on the same side.  
           [0003]    2. Related Art  
           [0004]    Optical circulators are a kind of optical passive device with at least three optical ports. Light entering a first optical port is output from a second optical port and light entering the second optical port is output from a third optical port. When there are more than three ports, light entering the i&#39;th optical port is output from the (i+1)&#39;th optical port. Therefore, the optical path inside the optical circulator is irreversible.  
           [0005]    Different optical ports of most known optical circulators are not situated on the same axis. Polarizing beam splitters (PBSs) have to be used, as proposed in the U.S. Pat. No. 5,878,176. They do not only have higher prices but also larger sizes. To decrease the volume of the products, most people design all optical ports on the same axis. There are several means to implement this. For example, the U.S. Pat. No. 5,921,422 uses a thermally expanded core (TEC) fiber. The U.S. Pat. Nos. 5,973,823 and 6,049,427 can both effectively minimize the product volume by aligning optical ports on the same axis. To lower product prices and to facilitate product assemblies, The U.S. Pat. No. 5,973,823 utilizes the relative angle between a multi-layer Faraday spin crystal and a birefringent crystal optical axis so as to abandon the need for half wave plates. The U.S. Pat. No. 6,002,512 employs a latchable Faraday spin crystal to decrease the number of half wave plates. The U.S. Pat. Nos. 5,921,039 and 6,049,426 do not only have all optical ports on the same axis but also need two-core fiber collimator to among the three optical ports, greatly lowering the cost and volume. The U.S. Pat. Nos. 6,014,244, 6,014,475, and 6,088,491 insert one or several lenses among crystals to change the optical path. Nevertheless, the above-cited references have a general feature: the i&#39;th optical port and the (i+1)&#39;th optical port are on different ends of the optical circulator. Therefore, their optical circulator products have a longer length and require more crystals. The U.S. Pat. Nos. 6,097,869 and 6,111,695 both use one reflective mirror to make all optical ports on the same side. However, the optical ports of the U.S. Pat. No. 6,097,869 are composed of TEC fibers. Each optical port requires an extra convergent lens. The U.S. Pat. No. 6,111,695 totally needs three birefringent crystals to achieve the circulation function, resulting in more length and cost.  
         SUMMARY OF THE INVENTION  
         [0006]    An objective of the invention is to decrease the number of crystals needed in an optical circulator and the length of the optical circulator, thus providing an optical circulator with a small volume.  
           [0007]    Another objective of the invention is to provide an optical circulator with all its optical ports situated on the same side.  
           [0008]    The invention uses an optical reflective device so that a light beam entering through an optical port is reflected and passes through all optical devices (i e., all optical crystals) on its optical path so as to be guided to the next optical port. Through such a design, all crystal can be repeatedly used to reduce the number of crystals needed and the length of the optical circulator.  
           [0009]    The invention uses a miniaturized fiber collimator as the I/O port of the circulator. Aside from reducing the area of crystals and shortening the crystal lengths, it further has feature of an extremely good expandability. The invention uses a non-reciprocal reflector, therefore all optical ports of the optical circulator can be installed on the same side, simultaneously achieving the circulation function and the optical designs of no polarization dependent loss (PDL) and no polarization mode dispersion (PMD).  
           [0010]    The invention uses a proper reciprocal-non-reciprocal optical crystal combination to generate a specific linear polarization direction to selectively generate light beam walk-off, satisfying the irreversibility property of the optical path within the optical circulator. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The present invention will become more fully understood from the detailed description given hereinbelow illustration only, and thus are not limitative of the present invention, and wherein:  
         [0012]    [0012]FIG. 1 schematically shows a basic structure of the optical circulator disclosed in the invention;  
         [0013]    [0013]FIG. 2 is a first embodiment structure of the optical reflective device;  
         [0014]    [0014]FIG. 3 is a second embodiment structure of the optical reflective device;  
         [0015]    [0015]FIG. 4 is a third embodiment structure of the optical reflective device;  
         [0016]    [0016]FIG. 5 is a fourth embodiment structure of the optical reflective device;  
         [0017]    [0017]FIG. 6 shows the optical structure according to the first embodiment of the disclosed micro-reflective optical circulator;  
         [0018]    [0018]FIGS. 7A through 7J show detailed crystal orientations and optical polarizations along the paths in the propagation direction of FIG. 6;  
         [0019]    [0019]FIG. 8 shows the optical structure according to the second embodiment of the disclosed micro-reflective optical circulator;  
         [0020]    [0020]FIGS. 9A through 9H show detailed crystal orientations and optical polarizations along the paths in the propagation direction of FIG. 8;  
         [0021]    [0021]FIG. 10 shows the optical structure according to the third embodiment of the disclosed micro-reflective optical circulator;  
         [0022]    [0022]FIGS. 11A through 11J schematically show the beam polarization direction of the first crystal structure in FIG. 10;  
         [0023]    [0023]FIGS. 12A through 12H schematically show the beam polarization direction of the second crystal structure in FIG. 10;  
         [0024]    [0024]FIG. 13 shows the optical structure according to the fourth embodiment of the disclosed micro-reflective optical circulator; and  
         [0025]    [0025]FIGS. 14A through 14J schematically show the beam polarization direction of the crystal structure in FIG. 13. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    With reference to FIG. 1, the disclosed reflective optical circulator  1  includes at least three optical ports  11 ,  12 ,  13  located at the same side of the reflective circulator  1 . The other side of the optical circulator  1  has an optical reflective device  20 . Between the three optical ports  11 ,  12 ,  13  (hereinafter as port  1 , port  2 , port  3 , respectively) and the optical reflective device  20  includes an optical crystal combination composed of crystals with several different optical properties. Due to the design of the optical reflective device  20 , the light beam entering port 1  11  reaches the optical reflective device  20  after passing through several crystals  2 . It is reflected by the optical reflective device  20  back to pass through the crystals in reverse direction and is output from port 2  12 . Similarly, the light beam entering port 2  12  is reflected by the optical reflective device  20  and output from port 3  13 . The behavior that a light beam entering the i&#39;th optical port is output from the (i+1)&#39;th optical port is the irreversibility property of the optical path within the light circulator.  
         [0027]    Known optical circulators use at least one birefringent crystal to produce the walk-off effect. This effect separates an input light beam into linearly polarized light beams with different linear polarization directions and different optical paths. With a proper combination of reciprocal-non-reciprocal optical crystals, the input light beam can proceed according to a predetermined path, thus satisfying the irreversibility requirement for the optical path within the optical circulator. For birefringent crystals made of anisotropic crystals, incident light beams can be classified according to their polarizations into extraordinary rays (E-rays) and ordinary rays (O-rays), whose polarization directions are perpendicular to each other. For a linearly polarized light beam, the polarization directions of the above two rays differ by 90 degrees. The O-ray satisfies the Snell&#39;s Law and the wave propagation direction is parallel to the energy propagation direction. The propagation direction of an E-ray usually is not parallel to that of an O-ray, and its energy propagation direction differs according to the optical axis of the crystal (that is, the walk-off direction). This is called the walk-off phenomenon. Due to the walk-off phenomenon, linearly polarized light beams with different polarization directions have different optical path lengths. If no compensation or process is taken, the incident light beam may have the problem of signal distortion after leaving the optical circulator.  
         [0028]    Therefore, the invention utilizes an optical reflective device  20  with an optical path compensation function to solve this problem. When two linearly polarized beams B 1  and B 2  with some optical path difference in between travel along different paths to the optical reflective device  20  and get reflected, the polarization mode dispersion (PMD) function of the optical reflective device  20  compensates for such an optical path difference. The disclosed optical reflective device  20  has basically two embodiments:  
         [0029]    First Embodiment:  
         [0030]    The basic principle is to use two optical devices with different reflection indices to form different optical paths. Because of the speed difference of two linearly polarized beams along paths with different reflection indices due to the walk-off effect, the total optical path become the same, thus achieving the PMD compensation function. The optical reflective device  20  in the first embodiment is basically a non-reciprocal polarization control crystal  21  (FIG. 2), such as a Faraday rotator or a quarter-wave plate. The beam incident surface on the crystal  21  has an anti-reflection (AR) coating, and the other surface is a reflective surface  23 . The reflective surface  23  can be a high-reflection (HR) coating or a high-reflection mirror.  
         [0031]    As shown in FIG. 2, a high-refraction optical crystal  24  is inserted between the crystal  21  and the reflection surface  23 . The optical crystal  24  is in the optical path of one of the two beams linearly polarized in different directions, particularly the one (B 2 ) with a shorter optical path. Due to the insertion of the optical crystal  24 , the linearly polarized beam B 1  with a longer optical path travels through a section of air after passing through the crystal  21 , reaches the reflection surface  23  and gets reflected into the crystal  21 . Since the refraction index of the high-refraction optical crystal  24  is far greater than that of the air, the speed of the beam B 2  with a shorter optical path in the optical crystal  24  is slower than that of the other beam B 1  in the air. Thus, the beam B 1  obtains a proper PMD compensation.  
         [0032]    With further reference to FIG. 3, another structure of the invention is to form on the back surface of the crystal  21  (the surface opposite to the beam incident surface) one half AR  25  and the other half HR  26 . The HR  26  is formed in the optical path of the beam B 1  with a longer optical path and the AR  25  in the optical path of the beam B 2  with a shorter optical path. Afterwards, an optical crystal  24  with a high-refraction index is attached onto the same surface. The other surface of the optical crystal  24  is formed with the above-mentioned reflective surface  23 . In this embodiment, one can use glass as the material for the optical crystal  24 , which becomes a mirror after attaching the reflective surface  23 . This will greatly simplify the structure of FIG. 2. Basically, the high-refraction optical crystal  24  has a relatively higher refraction index to the air. Such an optical crystal can be made of silicon or optical glass.  
         [0033]    Second Embodiment:  
         [0034]    It uses a reflective device that is equivalent to two 45-degree mirrors. As shown in FIG. 4, the optical reflective device  20  is composed of a non-reciprocal polarization control crystal  21  and a right-angle prism  27 . In another example shown in FIG. 5, it is composed of a non-reciprocal polarization control crystal  21  and two 45-degree mirrors  28   a ,  28   b . Therefore, the linearly polarized beam B 1  returns along the optical path that the linearly polarized beam B 2  enters the optical reflective device  20  after being reflected by the prism  27  or the two 45-degree mirrors  28   a ,  28   b . Similarly, the linearly polarized beam B 2  returns along the optical path that the linearly polarized beam B 1  enters the optical reflective device  20  after being reflected by the prism  27  or the two 45-degree mirrors  28   a ,  28   b . Therefore, the two walk-off linearly polarized beams B 1 , B 2  have exactly the same optical path except in opposite directions, achieving the PMD compensation effect. In FIG. 4 and FIG. 5, the optics can be properly designed so that the polarization states of the two linearly polarized beams B 1  and B 2  that enter the optical reflective device  20  are orthogonal to each other. The optical reflective device  20  then does not need to have a non-reciprocal polarization control crystal  21  under this arrangement, thus further reducing the cost and offering the ease of fabrication.  
         [0035]    One should also understand from FIGS. 2 through 5 that due to the action of the non-reciprocal polarization control crystal  21 , the polarization directions of the linearly polarized beams B 1 , B 2  entering the optical reflective device  20  are first rotated by 45 degrees after passing through the crystal  21  and by another 45 degrees after being reflected away from the optical reflective device  20  and passing through the crystal  21 . So their linear polarization directions are indicated by the symbols in the drawings, being rotated by 90 degrees. That is, an E-ray becomes an O-ray.  
         [0036]    The following description uses an optical circulator with four optical ports as an example to explain the complete structure of several embodiment reflective optical circulators disclosed herein. The structure of a first reflective optical circulator  1   a  is shown in FIG. 6.  
         [0037]    Starting from the optical ports  31 ˜ 34  along the optical axis are a birefringent crystal  30 , a non-reciprocal polarization crystal  40 , a first pair of birefringent crystals  50   a ,  50   b , a second pair of birefringent crystals  60   a ,  60   b , and a non-reciprocal optical reflective device  20 . The birefringent crystal  30  has its walk-off direction in the +x direction for light propagating along the z-axis. It functions as a polarization splitter/combiner. One end of the optical path connects to optical ports  31 ,  32 ,  33 ,  34 . The non-reciprocal polarization crystal  40  rotates a linearly polarized beam clockwise by 45 degrees. The first pair of birefringent crystals  50   a ,  50   b  has orthogonal walk-off directions and functions as a forward (the direction of the linearly polarized beam entering the reflective optical circulator  1   a ) displacer. The second pair of birefringent crystals  60   a ,  60   b  has orthogonal walk-off directions and functions as a backward (the direction of the linearly polarized beam leaving the reflective optical circulator  1   a ) displacer.  
         [0038]    The reciprocal crystal refers to a crystal that the polarization direction of a beam does not change traveling back and forth once in the z-direction. However, for non-reciprocal crystals, the change in the polarization direction is additive. One usually uses a half-wave plate as the reciprocal crystal, and the non-reciprocal crystal can be a Faraday rotator or a quarter-wave plate.  
         [0039]    [0039]FIGS. 7A through 7J indicate the polarization directions of a linearly polarized light beam passing through various crystals in the reflective optical circulator  1   a  in FIG. 6. In the drawings, we use circles and their diameters to indicate the polarization directions of the light beam. First, the linearly polarized light beams  711 ,  721 ,  731  enter the optical ports  31 ,  32 ,  33  along their forward directions (FIG. 7A). After passing through the birefringent crystal  30 , they are separated into E-rays  712   a ,  722   a ,  732   a  and O-rays  712   b ,  722   b ,  732   b  due to the walk-off effect (the walk-off direction is the +x direction), as shown in FIG. 7B. After passing through the non-reciprocal polarization crystal  40 , the E-rays and O-rays are rotated by 45 degrees in the same direction (clockwise) into linearly polarized beams  713   a ,  723   a ,  733   a  (45 degrees with respect to the +x axis on the x-y plane) and  713   b ,  723   b ,  733   b  (−45 degrees with respect to the +y axis on the x-y plane), respectively, as shown in FIG. 7C. The beams further pass through the first pair of birefringent crystals  50   a ,  50   b . Due to the walk-off effect (the walk-off directions are −45 degrees with respect to the +x axis and −45 degrees with respect to the +y axis, respectively, on the x-y plane), the beams are displaced toward the second optical port  32  along the optical axes of the birefringent crystals  50   a ,  50   b , becoming the linearly polarized beams  714   a ,  724   a ,  734   a  and  714   b ,  724   b ,  734   b , respectively (FIG. 7D). The walk-off directions of the second pair of birefringent crystals  60   a ,  60   b  are 135 degrees with respect to the +y axis and −135 degrees with respect to the +y axis, respectively, on the x-y plane. Therefore, the linearly polarized beams  714   a ,  724   a ,  734   a  and  714   b ,  724   b ,  734   b  directly pass through the second pair of birefringent crystals  60   a ,  60   b  and become beams  715   a ,  725   a ,  735   a  and  715   b ,  725   b ,  735   b , respectively. Therefore, the beams enter the optical reflective device  20  without any polarization direction changed (FIG. 7E).  
         [0040]    The linearly polarized light beams  715   a ,  725   a ,  735   a  and  715   b ,  725   b ,  735   b  are reflected by the optical reflective device  20  and become the linearly polarized beams  715   a ′,  725   a ′,  735   a ′ and  715   b ′,  725   b ′,  735   b ′, respectively (FIG. 7F). The polarization directions of the beams  715   a ′,  725   a ′,  735   a ′ and  715   b ′,  725   b ′,  735   b ′ are orthogonal to those of the beams  715   a ,  725   a ,  735   a  and  715   b ,  725   b ,  735   b  before being reflected off the optical reflective device  20 , respectively. The reflected beams further pass through the second pair of birefringent crystals  60   a ,  60   b . Due to the walk-off effect, the beams are displaced toward the second optical port  32 , becoming the linearly polarized light beams  714   a ′,  724   a ′,  734   a ′and  714   b ′,  724   b ′,  734   b ′, respectively (FIG. 7G). The beams pass through the first pair of birefringent crystals  50   a ,  50   b  with none of their polarization directions changed, becoming the linearly polarized light beams  713   a ′,  723   a ′,  733   a ′ and  713   b ′,  723   b ′,  733   b ′, respectively (FIG. 7H). Further passing through the non-reciprocal polarization crystal  40 , the beams are rotated clockwise by 45 degrees and become the beams  712   a ′,  722   a ′,  732   a ′ and  712   b ′,  722   b ′,  732   b ′, respectively (FIG. 71). Finally, the beams pass through the birefringent crystal  30  and combine together to form the linearly polarized beams  711 ′,  721 ′,  731 ′, respectively, which then leave the optical circulator from the second, third and fourth optical ports  32 ,  33 ,  34  (FIG. 7J).  
         [0041]    [0041]FIG. 8 discloses the structure of a second micro-reflective optical circulator lb, which includes a first birefringent crystal  30 , a pair of non-reciprocal polarization crystals  40   a ,  40   b , a second birefringent crystal  30   a , and a non-reciprocal optical reflective device  20 . The first birefringent crystal  30  has a c-axis orientation θ=φ=45° and functions as a polarization splitter/combiner in the 45° direction viewed on the x-y plane. One end of its optical path connects to the optical ports  31 ,  32 ,  33 ,  34 . The pair of non-reciprocal polarization crystals  40   a ,  40   b  rotate the linearly polarized light beam by 45 degrees, but in opposite directions. The second birefringent crystal  30   a  has a walk-off direction θ=45°, φ=0° and functions as a displacer in the y direction.  
         [0042]    [0042]FIGS. 9A through 9H indicate the polarization directions of a linearly polarized beam passing through various crystals in the reflective optical circulator lb in FIG. 8. First, the linearly polarized light beams  811 ,  821 ,  831  enter the optical ports  31 ,  32 ,  33  along their forward directions (FIG. 9A). Taking the beam  811  entering the first optical port  31  as an example, after passing through the first birefringent crystal  30 , it is separated into an E-ray  812   a  and an O-ray  812   b  due to the walk-off effect (FIG. 9B). The beams then pass through pair of non-reciprocal polarization crystals  40   a ,  40   b . The crystal  40   a  rotates the beam counterclockwise by 45 degrees and the crystal  40   b  rotates the beam clockwise by 45 degrees. At the moment, the two polarized beams become the linearly polarized beams  813   a  and  813   b  with the same polarization direction (FIG. 9C).  
         [0043]    The beams further pass through the second birefringent crystals  30   a , both beams are O-rays relative to the second birefringent crystal  30   a . Therefore, they do not experience the walk-off effect when passing through the second birefringent crystal  30   a  and become the linearly polarized beams  814   a ,  814   b  (FIG. 9D). After being reflected by the optical reflective device  20 , the beams become the linearly polarized beams  814   a ′,  814   b ′ with their polarization directions rotated by 90 degrees (FIG. 9E). When passing the second birefringent crystal  30   a  again, they experience the walk-off effect in the +y direction, becoming the linearly polarized beams  813   a ′,  813   b ′ (FIG. 9F). The beams  813   a ′,  813   b ′ further pass through the pair of non-reciprocal polarization crystals  40   a ,  40   b  and are rotated into the linearly polarized beams  812   a ′,  812   b ′ with orthogonal polarization directions (FIG. 9G). Finally, they pass through the first birefringent crystal  30  and are combined into the beams  811 ′,  821 ′,  831 ′, which are then leave the reflective optical circulator lb from the optical ports  32 ,  33 ,  34  (FIG. 9H).  
         [0044]    [0044]FIG. 10 discloses the structure of a third micro-reflective optical circulator  1   c , which includes a first birefringent crystal  30 , a polarization rotation control crystal set  4 , a pair of second birefringent crystals  90   a ,  90   b , and a non-reciprocal optical reflective device  20 . The first birefringent crystal  30  functions as a polarization splitter/combiner. One end of its optical path connects to the optical ports  31 ,  32 ,  33 ,  34 . The pair of second birefringent crystals  90   a ,  90   b  has opposite walk-off directions to function as a displacer. The birefringent crystal  90   a  is a forward displacer to generate displacement for forward-traveling light beams. Its walk-off direction is the +y axis. The other birefringent crystal  90   b  functions as a backward displacer to generate displacement for backward-traveling light beams. Its walk-off direction is the −y axis in respective to the forward propagating direction.  
         [0045]    An embodiment of the polarization rotation control crystal set  4  is mainly composed of a first polarization rotation control crystal  41  and a second polarization rotation control crystal  42  (FIGS. 11A through 11J). The first polarization rotation control crystal  41  is a reciprocal crystal that rotates the polarization of a forward-traveling light beam (the direction the light beam enters the optical circulator) clockwise by 45 degrees. The second polarization rotation control crystal  42  is a non-reciprocal crystal that produces a clockwise polarization rotation by 45 degrees. The first birefringent crystal  30  has a walk-off direction in the +x direction. Its internal light beam polarization directions are shown in FIGS. 11A through 11J. The notation meanings are identical to the ones used before.  
         [0046]    Another embodiment of the polarization rotation control crystal set  4  is a non-reciprocal crystal that produces a clockwise polarization rotation by 45 degrees. The first birefringent crystal  30  in FIG. 10 has a walk-off direction that is 45 degrees away from the +x axis on the x-y plane. Its internal light beam polarization directions are shown in FIGS. 12A through 12H. The notation meanings are identical to the ones used before.  
         [0047]    [0047]FIG. 13 discloses the structure of a fourth micro-reflective optical circulator  1   d , which includes a first birefringent crystal  30 , a polarization rotation control crystal set  4 , a second birefringent crystal  90 , and a non-reciprocal optical reflective device  20 . The first birefringent crystal  30  functions as a polarization splitter/combiner. One end of its optical path connects to the optical ports  31 ,  32 ,  33 ,  34 . The second birefringent crystal  90  has a walk-off direction orthogonal to that of the first birefringent crystal  30 . It functions as a displacer.  
         [0048]    An embodiment of the polarization rotation control crystal set  4  is mainly composed of a first polarization rotation control crystal  41  and a set of second polarization rotation control crystals  42  (FIGS. 14A through 14J). The first polarization rotation control crystal  41  is a reciprocal crystal that rotates the polarization of a forward-traveling light beam clockwise by 45 degrees. The set of second polarization rotation control crystals  42  is a non-reciprocal crystal pair that produces clockwise and counterclockwise polarization rotations by 45 degrees, respectively. The first birefringent crystal  30  has a walk-off direction in the +x direction. Its internal light beam polarization directions are shown in FIGS. 14A through 14J. The notation meanings are identical to the ones used before.  
         [0049]    Effects of the Invention  
         [0050]    The invention uses a non-reciprocal optical reflective device so as to repeatedly use the crystals in an optical circulator. It can decrease the number, length, volume and cost of crystals in an optical circulator.  
         [0051]    All optical ports are installed on the same said of the optical circulator, rendering great convenience in uses and extremely good expandability when more ports are needed.  
         [0052]    The invention uses a proper design of birefringent crystal axes to use the same Faraday crystals, flipped by 180 degrees. This can produce the opposite polarization rotation direction to form a latched pair. The invention can use a single Faraday crystal to complete the polarization mode control. Therefore, expensive half-wave plates can be totally abandoned and the device angle errors are lowered to the minimum.  
         [0053]    Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments, will be apparent to persons skilled in the art. It is, therefore, contemplated that the appended claims will cover all modifications that fall within the true scope of the invention.