Abstract:
This invention discloses to a multi-stage optical isolator having a large centrally disposed birefringent crystal and two smaller crystals sized to separate in a polarization dependent manner into sub-beams and combine light that is separated into sub-beams at an output end thereof. By providing a single large crystal centrally disposed, less alignment is required due to fewer alignment critical components being required. Furthermore, the isolator in accordance with this invention is well suited to high-power applications.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to a multi-stage optical isolator and more particularly to an at least two stage optical isolator that requires only a single birefringent element between two non-reciprocal rotating elements. 
     BACKGROUND OF THE INVENTION 
     Optical isolators are used in a variety of applications in optical communication systems. Generally, optical isolators are used to prevent reflective portions of transmitted signals from re-entering the transmitting device. Many older prior art designs prevent reflections from re-entering a transmitting device in a polarization-selective manner. However, in certain circumstances where a transmission system causes uncontrollable changes in polarization, the polarization state of a signal may be unknown, and thus, these earlier polarization dependent designs are not considered to be practical. Thus, as of late, a large effort has been undertaken to develop an isolator that is polarization independent. It is also desired to have an optical isolator that is capable of isolating high power optical signals without compromising the longevity of the isolator. 
     One prior art polarization independent optical isolator is described in U.S. Pat. No. 5,033,830 issued Jul. 23, 1991 in the name of Jameson and entitled Polarization Independent Optical Isolator. Jameson describes an isolator having a single birefringent plate, a pair of stacked reciprocal rotators, a Faraday rotator, and a reflector positioned in tandem adjacent to the birefringent plate. In a forward (transmitting) direction, a lightwave signal exiting an optical fiber is split into a pair of orthogonal rays by the birefringent plate. The orthogonal rays then pass through a first reciprocal rotator and the Faraday rotator which provides 22.5° of rotation. The rotated rays are then redirected by the reflector back though the Faraday rotator. After passing through the second reciprocal rotator, the orthogonal rays re-enter the same birefringent plate where they are recombined and launched in an output fiber. Since a Faraday rotator is a non-reciprocal device, any signal traveling through the isolator in the reverse (isolation) direction will be split on both passes through the birefringent plate such that neither will intercept the input fiber. In practice, Jameson&#39;s single stage isolator described above, may provide adequate isolation; however, in some instances, increased isolation may be required. Such additional isolation has been known to be provided by using a multi-stage optical isolating device; generally these multi-stage devices are costly to manufacture often requiring nearly double the number of optical components that a single stage device requires; more importantly, aligning nearly twice as many components with one another can be difficult, time-consuming, costly, and generally increase the overall alignment error. 
     For example, U.S. Pat. No. 5,581,640 in the name Pan et al. Assigned to E-tek Dynamics, Inc. describes a multi-stage optical isolator wherein two polarizers in the form of a birefringent crystal wedges of lithium niobate are used as the birefringent material of the polarizers. The polarizers in prior art FIG. 1 (shown as FIG. 6A of the &#39;640 patent) are shown as spaced-apart crystal wedges having complementary slanted faces. However, it is not clear from the specification whether the space between the optical elements, shown between all of the elements in the device, is merely for the purpose of illustration. For example, if the gapped end faces of elements  64   a  and  64   b  in FIG. 1 are air gapped, then the end faces would likely require anti-reflection (AR) coating. 
     The instant invention obviates both the requirement of AR coating two crystals disposed between two non-reciprocal polarization rotating elements, and, obviates using two such crystals, thereby obviating the requirement for adhesive between such crystals. 
     Since the instant invention obviates the requirement of two crystals disposed between two non-reciprocal polarization rotating elements, it thereby obviates the complex and difficult component alignment that is required when using two such crystals are in tandem in a multi-stage isolator. 
     It is an object of this invention to provide a relatively low-cost optical isolator that is particularly well suited to carrying high-power optical signals. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, there is provided, a multi-stage optical isolator having at least two stages, comprising: 
     an input birefringent crystal for splitting an input beam into two orthogonally polarized sub-beams; 
     an output birefringent crystal for combining two orthogonally polarized beams into a single output beam at an output end thereof; 
     first rotating means for rotating the polarization of the beams received from an output end of the input birefringent crystal; 
     second rotating means for rotating the polarization of the beams directed to the input end of the output birefringent crystal; and 
     a centrally disposed birefringent crystal substantially equal to the sum of the thicknesses of the input and the output crystals, disposed between the first and second rotating means for merging two rotated beams received from the first rotating means, and for diverging said beams after they combine therein to provide two separated beams to the second rotating means. 
     In accordance with the invention, a two stage optical isolator is provided having two birefringent crystals, one at each end thereof, each having a thickness of “t”, two at least non-reciprocal rotators disposed between the two birefringent crystals; and a single birefringent crystal having a thickness of substantially at disposed between the two non-reciprocal rotators. 
     In accordance with the invention, a two stage optical isolator, having less than four birefringent crystals, comprise a centrally disposed birefringent crystal; two non-reciprocal rotating means, one on each side of said centrally disposed birefringent crystal; and two other birefringent crystals, one on each end of the two stage optical isolator, for respectively splitting and combining two orthogonally polarized beams; wherein the centrally disposed birefringent crystal is thicker that the combined thickness of the two other birefringent crystals; and wherein the thickness of the centrally disposed crystal is such that when two separated beams orthogonally polarized by a distance “d” received from one of the other birefringent crystals are launched into the centrally disposed crystal, the two beams converge and subsequently diverge within the centrally disposed crystal to exit the centrally disposed crystal having a separation “d”. 
     Advantageously, the isolator in accordance with this invention is found to provide a high degree of isolation of an incoming optical signal with less overall cost per device. 
     Advantageously the isolator in accordance with this invention is well suited to isolating high-power optical signals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention will now be described in conjunction with the accompanying drawings in which: 
     FIG. 1 is a side view of a prior art dual-stage optical isolator; 
     FIG. 2 a  is a side view of a prior art four-stage optical isolator; 
     FIG. 2 b  is a side view of another prior art two-stage isolator; 
     FIG. 2 c  is a top view of the isolator shown in FIG. 2 b;    
     FIG. 3 is a side view of a two-stage optical isolator in accordance with a preferred embodiment of the invention; 
     FIGS. 3 a  through  3   i  illustrate the state of an input beam as it becomes divided in a polarization dependent manner and as its polarization is rotated; 
     FIGS. 4 a  through  4   i  illustrate the state of a beam launched backward into the device of FIG. 3 i  and illustrates the isolation achieved; 
     FIG. 5 is a side view of a double stage isolator consisting of two non-reciprocal rotating elements and three crystals, wherein a centrally disposed crystal is substantially thicker than input and output crystals; 
     FIGS. 6 a  through  6   f  illustrate the state of an input beam as it becomes divided in a polarization dependent manner and as its polarization is rotated as it travels through the device shown in FIG. 5; 
     FIGS. 7 a  through  7   f  illustrate the state of a beam launched backward into the device of FIG.  5  and illustrates the isolation achieved; 
    
    
     DETAILED DESCRIPTION 
     Referring now to prior art FIG. 1 a two-stage polarization independent optical isolator is shown having oppositely oriented lithium niobate crystals  62 A and  64 B at opposite ends, and having Faraday rotators  63 A and  63 B disposed inside of crystals  62 A and  64 B. Sandwiched between the two Faraday rotators  63 A and  63 B are two crystals  64 A and  62 B having complementary slanted inwardly facing end faces. This arrangement of components is quite typical; in order to provide isolation independent of the state of incoming light, the incoming beam must be separated into two sub-beams having two orthogonal polarizations and, the two beams must pass through a non-reciprocal rotator after which the beams must be recombined. This occurs within components  62 A,  63 A, and  64 A. The second stage of isolation is afforded by replicating this via components  62 B,  63 B and  64 B, which are aligned with the components  62 A,  63 A and  64 A in a housing  69 . 
     Similarly, the four-stage device shown in FIG. 2 a  illustrates the same type of device wherein a second two-stage device  80  is provided after the first two-stage isolator  70 . 
     As was mentioned above, when an air gap is provided between component end faces, an AR coating is typically required. 
     FIGS. 2 b  and  2   c  illustrate another isolator wherein two single stage isolators, including rutile crystals  20   a  and  20   b,  half-wave plates  22   a  and  22   b,  and non-reciprocal rotators  24   a  and  24   b,  have their output and input crystals  26   a  and  26   b  respectively joined. 
     Turning now to FIG. 3, a preferred embodiment of a two-stage optical isolator in accordance with the invention is shown having two rutile crystals  20   a  and  20   b  for separating an incoming beam into two orthogonally polarized sub-beams and, for combining two orthogonally polarized sub-beams into a single output beam, respectively. Between the crystals  20   a  and  20   b  are two half-wave plates  22   a  and  22   b,  and two non-reciprocal rotators  24   a  and  24   b  in the form of Faraday rotators. A rutile crystal  26  having a thickness of substantially about equal to the combined thickness of the rutile crystals  20   a  and  20   b  is located centrally between the Faraday rotators  24   a  and half-waveplate  22   b.  In operation when the incoming beam incident upon the rutile crystal  20   a  is separated into the two orthogonally polarized sub-beams, the beams first pass through the reciprocal rotating half-wave plate  24   a  and are rotated by 45°. Subsequently as the beams propagate toward the output end of the device after traversing the plate  24   a,  they are simultaneously rotated a second time in a same direction by another 45°. The beams then enter the thick crystal  26  and converge/combine and focus at a location substantially about halfway along the fast axis of the crystal. In contrast to conventional two-stage isolators which at this stage launch the combined beams into a second isolator chip, the beams in this instance, after combining, splitting from one another by crossing over each other and continuing along their original paths. The two beams are each subsequently, simultaneously rotated 45° by the second half-wave plate  22   b  and are then rotated by the Faraday rotator  24   b,  afterwhich they are combined by the last rutile crystal  20   b  at an output port. 
     The advantages of utilizing the single crystal  26  as opposed to the two crystals  64 A and  62 B as shown in prior art FIG. 1, are numerous. When two crystals are used, and lateral or longitudinal alignment is not perfect isolation may be lessened. 
     Conveniently, by using a single crystal these drawbacks are obviated, and when a high power optical signal is focused substantially about the centre of the single crystal  26  along its fast axis, the difficulty of focusing the beam at the interface of two crystals is avoided. 
     Referring now to FIGS. 3 a  through  3   i,  a transmission path is defined from the input port through elements  20   a,    22   a,    24   a,    26 ,  24   b,    22   b,  and  20   b  to the output port (from left to right). In an alternative embodiment (not shown), the order of the reciprocal and non-reciprocal rotators may be reversed. 
     Calcite or rutile crystals are preferably used for the above refraction crystal plates  20   a    20   b  and  26 . The non-reciprocal rotators are preferably a Faraday rotating elements  24   a,    24   b  that uses a Y.I.G crystal or Bi-added thin film crystal. The composition of the Bi-added thin film crystals include a combination of, for example, (YbTbBi) 3 Fe 5 O 12  and (GdBi) 3 (GeAlGa) 5 O 12 , or of Y.I.G. and Y 3x Bi x Fe 5 O 12 . 
     Next, in conjunction with FIGS. 3 a  through  3   i,  the detailed operation of the device will now be described. Light entering the rutile  20   a  is followed and explained as it enters and exits each optical element on its forward path through the device (shown as left-to-right). Looking at the end face of the rutile (labeled rutile  1 ) in FIG. 3 a,  the light is shown in a lower central position as a single beam having an unknown polarization state. After traversing the rutile crystal  20   a,  the beam is divided into two sub-beams (o-ray and e-ray) and are incident upon the first half-wave plate (HWP 1 )  22   a  where they are rotated by 45° shown in FIG. 3 c.  The two beams subsequently traverse the first Faraday rotator FR 1   24   a  where they are rotated again by 45° shown in FIG. 3 d  as the beams enter the thick rutile  26 . After propagating halfway through the thick rutile  26 , the beams converge and focus about the centre of  26  as shown in FIG. 3 e.  The beams then diverge in a polarization dependent manner to become two separated orthogonal beams at the shown entering the second half-wave plate HWP 2   22   b  in FIG. 3 f.  The beams are then rotated  22   b  as shown entering FR 2   24   b  in FIG. 3 g.  The beams rotated a second time by  24   b  are shown entering the rutile crystal  20   b.  FIG. 3 i  illustrates the output end of the rutile  20   b  where the beams have recombined to be output in a conventional manner via a lens such as a rod graded index (GRIN) lens to an optical fibre. 
     Turning now to FIGS. 4 a  through  4   i,  isolation is illustrated by use of the diagrams. For example, when light is launched into the second rutile and back reflected light might be, the beam is split by the rutile crystal  20   b  into two sub-beams shown in FIG. 4 b  as entering FR 2   24   b.  FIG. 4 c  illustrates the beam rotated clockwise by 45° at the input end face of HWP 2   22   b.  After passing through HWP 2   22   b,  the beam is rotated counter clockwise by 45° as is shown at the input face of the thick rutile  26  in FIG. 4 d  completing a first stage of isolation. 
     The second stage of isolation continues from the mid-section of the rutile  26  wherein the two separated beams further separate as shown in FIG. 4 f  at the input face of FRI  24   a.  After rotation of 45° by FR 1   24   a  and a 45° rotation by the rutile  22   a,  the beams are shown separated by a large distance affording high isolation. 
     FIG. 5 illustrates another embodiment of the invention wherein the isolating portion of a double stage isolator consists of an input rutile crystal  50   a  (rutile 1 ), a first Faraday rotator  53   a  FR 1  a thicker centrally disposed rutile crystal  55 , followed by a second Faraday rotator  53   b  FR 2  followed by an output rutile crystal  50   b  (rutile 2 ). For simplicity and clarity collimating/focusing input/output lenses are not shown. 
     Referring now to FIGS. 6 a  through  6   f,  states of polarization are shown for an input beam as it is launched into an input end of the isolator and propagates through to the output end. After the beam exits the rutile 1   50   a  in FIG. 6 a,  it is split into two orthogonally polarized sub-beams shown at the input end of the Faraday rotator FR 1   53   a  in FIG. 6 b.  After being rotated the beam is shown in FIG. 6 c  at the input end of the thick rutile  55 . The beam is then shifted by the rutile  55  as shown at the input face of the Faraday rotator FR 2  in FIG. 6 d.  The beam is then rotated by the rotator  53   b  and is shown at the input face of the rutile 2  in FIG. 6 e.  The output rutile crystal  50   b  joins the two beams into a single beam as shown at the output end in FIG. 6 f.    
     FIGS. 7 a  through  7   f  shows a sequence of polarization states for beam of light launched into the isolator backwards toward the input end from the rutile crystal  50   b  toward the rutile crystal  50   a.  FIG. 7 f  shows two separated beams that are disposed a distance away from the “real” input port, denoted “R”. 
     Of course, the two-stage optical isolator shown can be used as a part of an n-stage optical isolator wherein n&gt;2. 
     In summary, the optical isolator in accordance with this invention provides high power handling capacity since the light traversing the device is focused inside the crystal; polarization dependent loss is lessened due to high tolerance; and, insertion loss is lessened due to any mismatch that may otherwise occur between rutile crystals in conventional devices having two centrally disposed crystals; and a high extinction ratio is obtained. Furthermore, all of the above advantages are achieved using a device which is less costly to manufacture than a conventional two-stage isolator. 
     Numerous other embodiments may be envisaged, without departing from the spirit and scope of the invention.