Abstract:
In some embodiments, an optical switch includes multiple individually-retractable wedge switching prisms stacked in a longitudinal channel, corresponding plural transverse-translation rhomboid prisms extending transversely away from the longitudinal channel, and corresponding plural fiber collimators oriented longitudinally and aligned along a transverse line on both sides of the longitudinal channel. To switch light to a selected fiber collimator, its corresponding wedge switching prism is inserted in the longitudinal channel to deflect light to a corresponding transverse-translation prism and on to the selected fiber collimator; the other switching prisms are retracted from the channel. In some embodiments, longitudinally-adjacent switching prisms are oriented in opposite directions. A reverse-deflection wedge prism can be provided between each switching prism and its corresponding transverse-translation prism. The described preferred systems allow improved system stability, as well as ease of manufacturing and alignment.

Description:
BACKGROUND 
   The invention relates to optical system and methods, and in particular to optical switches for use in optical systems such as fiber optic networks. 
   Optical switches are useful for a variety of applications, including fiber optic communications. In one design approach, optomechanical components are used to direct light from a desired optical input to a desired optical output. Conventional optomechanical switches include switches employing moving prisms and switches employing moving fibers, among others. 
   If insertion losses are to be maintained within an acceptable range, the various components of an optical switch ordinarily need to be precisely aligned relative to each other. Precise alignment requirements can significantly increase manufacturing costs, reduce manufacturing yields, and constrain the temperature ranges and vibration intensities to which the switches can be subjected. 
   SUMMARY 
   According to one aspect, an optical switch includes a first optical port, a plurality of second optical ports, and a plurality of switching units each capable of selectively optically coupling the first optical port to a selected second optical port. A switching unit corresponding to a selected second optical port includes a fixed, transverse-translation rhomboid prism, and an individually-movable wedge switching prism movable between a first switching position situated in a generally-longitudinal optical path, and a retracted second switching position situated outside the longitudinal optical path. The wedge switching prism in the first switching position deflects light traveling generally along the longitudinal optical path to optically couple the first optical port to the selected second optical port through the transverse-translation prism. The wedge switching prism in the second switching position does not optically couple the first optical port to the selected second optical port. 
   According to another aspect, an optical switch comprises a first optical port, a second optical port, a third optical port, a first fixed transverse-translation reflector, a second fixed transverse-translation reflector, a first wedge switching prism movable between a first switching position and a second switching position, and a second wedge switching prism movable between a third switching position and a fourth switching position. The first wedge switching prism in the first switching position deflects generally-longitudinal light to optically couple the first optical port to the second optical port through the first transverse-translation reflector. The first wedge switching prism in the second switching position does not optically couple the first optical port to the second optical port. The second wedge switching prism in the third switching position deflects generally-longitudinal light to optically couple the first optical port to the third optical port through the second transverse-translation reflector. The second wedge switching prism in the fourth switching position does not optically couple the first optical port to the third optical port. 
   According to another aspect, an optical switching method includes inserting a first wedge switching prism into a longitudinal light path to establish an optical connection between a first optical port and a second optical port through a first fixed transverse-translation reflector facing the second optical port; and removing the first wedge switching prism from the longitudinal path and inserting a second wedge switching prism into the longitudinal light path to establish an optical connection between the first optical port and a third optical port through a second fixed transverse-translation reflector facing the third optical port. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and advantages of the present invention will become better understood upon reading the following detailed description and upon reference to the drawings where: 
       FIG. 1  shows an optical switch comprising multiple switching units each corresponding to an optical output port, each switching unit including a transverse-translation rhomboid prism and a retractable wedge switching prism, according to some embodiments of the present invention. 
     FIGS.  2 -A–D show optical paths corresponding to four switching states of the optical switch of  FIG. 1 , according to some embodiments of the present invention. 
     FIGS.  3 -A–B illustrate a symmetric and an asymmetric wedge prism, respectively, according to some embodiments of the present invention. 
       FIG. 4  shows computed data on the dependence of the deflection angle on the incident angle for an exemplary wedge prism, according to some embodiments of the present invention. 
       FIG. 5  illustrates the sensitivity of the deflection angle to changes in prism positioning for a wedge prism according to some embodiments of the present invention. 
       FIG. 6  shows an optical switch according to some embodiments of the present invention. 
       FIG. 7  shows an optical switch having all optical fibers exiting along one side of the switch, and all ports facing the same direction within the switch, according to some embodiments of the present invention. 
       FIG. 8  shows a 1×8 optical switch according to some embodiments of the present invention. 
       FIG. 9  shows an optical switch having all output collimators positioned on one side of a central longitudinal channel, according to some embodiments of the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   In the following description, it is understood that any recitation of an element refers to at least one element. A set of elements includes one or more elements. A plurality of elements includes two or more elements. A rhomboid prism is a prism having a cross-section shaped as an oblique-angled parallelogram with equal opposite sides, and equal or non-equal adjacent sides. The statement that a first deflection is opposite in sign to a second deflection means that the first deflection is clockwise and the second deflection is counterclockwise with respect to a pre-deflection direction of light travel, or that the first deflection is counterclockwise and the second deflection is clockwise. For simplicity, the description below focuses primarily on 1×N switches having 1 input and N outputs, but the described configurations are optically reversible to yield switches with 1 output and N inputs, as well as extendible to N×M configurations. Using a wedge prism to deflect generally longitudinal light encompasses both deflecting light incident precisely along a longitudinal central axis (e.g. in a configuration with 1 input and N outputs), as well as deflecting incident oblique, generally-longitudinal light onto the central longitudinal central axis (e.g. in a configuration with N inputs and 1 output). Retracting a switching prism encompasses withdrawing the switching prism by any trajectory, including without limitation a linear trajectory or an arcuate in-plane or out-of-plane trajectory. The term transverse encompasses directions that are transverse to a longitudinal direction, including a direction perpendicular to the longitudinal direction. A transverse-translation reflector is a reflector that receives generally-longitudinal incoming light and outputs generally-longitudinal light that is transversely offset relative to the incoming light. 
     FIG. 1  is a schematic diagram of an exemplary 1×N optical switch  10  according to some embodiments of the present invention. Optical switch  10  has a plurality of input and output optical ports, including a first input/output fiber collimator  20  and a plurality of output/input collimators  30 ,  40 ,  50 ,  60 , respectively. Each collimator  20 ,  30 ,  40 ,  50 ,  60  is mechanically and optically coupled to a corresponding optical fiber  21 ,  31 ,  41 ,  51 ,  61 , respectively. The optical axes of collimators  20 – 60  are generally parallel and longitudinal. Collimator  20  faces a direction opposite that of collimators  30 ,  40 ,  50 ,  60 . Collimators  30 ,  40 ,  50 ,  60  are aligned and face the same direction, with their optical axes offset by a set transverse distance. The inter-collimator separation distance is in general larger than the diameter of the collimators. Collimators  20  and  50  are optically aligned facing each other along a common optical axis, and fixed on a switch base plate  12 . 
   In a first, default switching state of switch  10 , optical fiber  21  is optically coupled to optical fiber  51 . Input light incident through fiber  21  becomes a collimated beam traveling along an optical path  22 . Optical path  22  coincides with the optical axes of collimators  20 ,  50 , and forms a central longitudinal optical axis of switch  10 . The collimated light beam traveling along optical path  22  enters collimator  50  and outputs through fiber  51  without deflection by the switching components of switch  10 . The optical path is reversible: light entering through fiber  51  exits through fiber  21 . 
   FIGS.  2 -A–D show the optical paths corresponding to four switching states of optical switch  10 , according to some embodiments of the present invention. In the switching states illustrated in FIGS.  2 -A–D, optical fiber  21  is selectively connected to only one of fibers  51 ,  41 ,  61  and  31 , respectively. 
   As shown in  FIG. 1 , switch  10  includes three switching units capable of selectively optically coupling input collimator  20  to one of the collimators  30 ,  40 ,  50 ,  60 . Each switching unit includes a wedge switching prism, a reverse-deflection wedge prism, and a transverse-translation rhomboid prism, as described below. Three individually-movable wedge switching prisms  70 ,  80 ,  90  are stacked in a longitudinal column space (or pathway)  26  centered along optical path  22 . Each wedge prism  70 ,  80 ,  90  is attached to a mechanical switching device that moves the prism in and out the region between collimator  20  and collimator  50 . The switching device may be an electromagnetic device such as an electric relay. 
   When optical switch  10  is in a first switching state, all wedge prisms  70 ,  80 ,  90  are situated out of the optical path  22 . In other switching states, one of the wedge prisms  70 ,  80 ,  90  is situated in the optical path  22 , with light passing through its transmissive side surfaces, while the other switching prisms are retracted. Wedge prisms  70 ,  80 ,  90  are sequentially arranged along optical path  22 , with their top and bottom surfaces facing a generally transverse direction, alternatively in generally opposite orientations. In  FIG. 1 , a top surface  76  of wedge prism  70  faces the right side, a top surface  86  of wedge prism  80  faces the left side, and a top surface  96  of wedge prism  90  faces the right side. 
   In a second switching state, switch  10  establishes an optical connection between fiber  21  and fiber  41 . In the second switching state, wedge prism  70  is inserted in the optical path  22 , while wedge prisms  80 ,  90  are out of the optical path  22 . An input longitudinal light beam traveling along optical path  22  enters prism  70  through a transmissive side surface  72  and leaves prism  70  through an opposite transmissive side surface  74 . A generally-longitudinal deflected light beam  32  forms a deflection angle δ with respect to the input light beam along optical path  22 . Light beam  32  is deflected clockwise by prism  70 . In some embodiments, the deflection angle δ is less than 15°, in particular less than 10°, for example about 5–6° or less. Light beam  32  then enters a fixed-position, reverse-deflection wedge prism  120  located adjacent to optical path  22 . Wedge prism  120  has a wedge angle α generally identical to the wedge angle of movable prism  70 . Wedge prism  120  is located opposite top surface  76  relative to optical path  22 , and is in an orientation opposite to that of prism  70 . Wedge prism  120  is separated from wedge prism  70  by a minimum distance d chosen so that beam  32  is sufficiently separated from a beam traveling along optical path  22  at the location of prism  120 . The inter-beam separation d sin δ is preferably larger than the light beam&#39;s diameter plus a safety margin, such that light beam  32  can fully pass through wedge prism  120  without clipping, while light passing along optical path  22  is not blocked by prism  120 . 
   Wedge prism  120  deflects light beam  32  by a deflection angle δ opposite in sign to the deflection angle imparted by switching prism  70  (counterclockwise with respect to optical path  22 ) so that light beam  32  is parallel to optical path  22  after passage through wedge prism  120 . Beam  32  is then offset transversely by a fixed-position rhomboid prism  122 . Prism  122  has a 45°-parallelogram shape, with two parallel, generally-transverse transmissive surfaces perpendicular to light beam  32 , and two reflective side surfaces forming a 45° angle with the transmissive surfaces. The transverse extent of the transmissive surfaces is determined according to the transverse position of collimator  40 . A distal transverse end of prism  122  faces collimator  49 , while a proximal transverse end is generally adjacent to optical path  22 . Within prism  122 , light beam  32  passes through a first transmissive surface, is reflected by a first side surface, travels across the prism, is further reflected by the other side surface and leaves prism  122  through the other transmissive surface. The section of beam  32  exiting prism  122  is parallel to the beam section entering prism  122 , and is offset by a transverse distance equal to the length of prism  122 . Light beam  32  then enters the aligned collimator  40  and is output through fiber  41 . The optical path described above is reversible: light can be input through fiber  41  and output through fiber  21 . 
   In a third switching state of optical switch  10 , wedge prism  80  is inserted in optical path  22 , while wedge prisms  70 ,  90  are out of optical path  22  and do not establish optical connections between collimator  20  collimators  30 ,  40 , respectively. A light beam  34  initially traveling along optical path  22  is deflected by prism  80  by a deflection angle δ, counterclockwise with respect to optical path  22 , and then deflected by a fixed, reverse-deflection wedge prism  130  by an identical deflection angle of opposite sign, clockwise with respect to optical path  22 . A distance d′ between wedge prism  130  and wedge prism  80  and the corresponding inter-beam separation d′ sin δ are chosen such that light can pass through wedge prism  130  without clipping, while light traveling along optical path  22  is not obstructed by prism  130 . After passage through wedge prism  130 , an incident light beam is transversely offset by a fixed rhomboid prism  132 , enters collimator  60 , and outputs through fiber  61 . 
   In a fourth switching state of optical switch  10 , wedge prism  90  is inserted in optical path  22 , while wedge prisms  70 ,  80  are out of optical path  22 . A light beam  36  initially traveling along optical path  22  is deflected by prism  90  by a deflection angle δ, clockwise with respect to optical path  22 , and then deflected by a fixed, reverse-deflection wedge prism  140  by an identical deflection angle of opposite sign, counterclockwise with respect to optical path  22 . A distance d″ between wedge prism  140  and wedge prism  90  and the corresponding inter-beam separation d″ sin δ are chosen such that light can pass through wedge prism  140  without clipping, while light traveling along optical path  22  is not obstructed by prism  140 . After passage through wedge prism  140 , an incident light beam is transversely offset by a fixed rhomboid prism  142 , enters collimator  30 , and outputs through fiber  31 . 
   Preferably, the two transmissive surfaces of each rhomboid prism  122 ,  132 ,  142  are anti-reflection (AR) coated. The reflective surfaces have cleanness and flatness characteristics of optical quality. The prism material is chosen according to its refractive index n. For total internal reflection inside a material of refractive index n, the incident angle θ with respect to the normal obeys the relation:
 
sin θ≧1/n.  [3]
 
For BK7, a borosilicate optical glass, the refractive index is n=1.5, and the minimum incident angle θ given by Eq. [3] is 41.8°.
 
   As shown in  FIG. 3-A , wedge prism  70  has two symmetrically-disposed transmissive side surfaces  102 ,  104 , a top surface  106 , and a bottom surface  108 . Transmissive surfaces  102 ,  104  are preferably anti-reflection (AR) coated. The side surfaces  102 ,  104  cross at a point  105 , and form a wedge angle α. An incident light beam  110  enters wedge prism  70  through side surface  102  and is deflected by prism  70 . A deflected light beam  112  forms a deflection angle δ with respect to incident beam  110 . Light beam  112  is deflected toward bottom surface  108 . 
   The deflection angle δ formed between incident beam  110  and deflected beam  112  is given by
 
δ=arc sin [sin α(n 2 −sin 2  θ) 1/2 −cos α sin θ]+θ−α,  [1]
 
where α is the wedge angle, θ is the incident angle, and n is the refractive index of the wedge prism material. When α is a small angle, the deflection angle δ is relatively stable, and is relatively insensitive to variations in the incident angle θ.
 
   A similar relationship between the deflection, incident and wedge angles can be observed for an asymmetric wedge prism  70 ′ shown in  FIG. 3-B . The wedge angle α is typically the most important parameter for a wedge prism; the geometric configuration of the wedge prism is less important. Prism  70 ′ has a wedge angle α defined at a side surface crosspoint  105 ′. The incident angle formed between an incident beam  110 ′ and the normal to the input side surface is denoted by θ′, while the deflection angle formed between incident beam  110 ′ and a deflected beam  112 ′ is denoted by δ′. 
     FIG. 4  shows data on the dependence of the deflection angle δ on the incident angle θ, for a wedge prism with α=11.3° and an index of refraction n of about 1.5. The data of  FIG. 4  is computed according to Eq. [1]. When the incident angle θ varies between 0° and 18°, the deflection angle δ changes only between 5.7° and 5.8°, and the relative change rate dδ/dθ is within ±0.03. The relative insensitivity of the deflection angle to changes in the incident angle ensures a good stability of the output optical beam as well as a good repeatability of the switch, making a wedge prism such as the one described above an ideal choice for movable switching parts. The output beam stability is particularly improved for lower wedge angles α. 
     FIG. 5  illustrates the sensitivity of the deflection angle δ to changes in wedge prism positioning. In  FIG. 5 , an incident beam  110  enters wedge prism  70  along the z-axis. An output beam  112  is located in the z-x plane, and forms a deflection angle δ relative to the z-axis, inclined toward the positive x-direction. If wedge prism  70  is rotated about the z-axis by a small angle β, a resulting output beam  112 ′ is no longer in the z-x plane. The plane containing output beam  112 ′ and the z-axis is rotated by angle β about the z-axis. Such a rotation of prism  70  can be caused by a mechanical disturbance or some other reason. Output beam  112 ′ forms a deviation angle γ relative to output beam  112 . The deviation angle γ is
 γ=β sin δ.  [2] 
Eq. [2] shows that reducing the deflection angle δ leads to a reduction in the sensitivity of the deviation γ to the rotation disturbance β. As an example, for γ=5.7°, Eq. [2] yields γ=0.1 β.
 
     FIG. 6  shows a 1×4 optical switch  210  according to some embodiments of the present invention. Switch  210  differs from the switch  10  shown in  FIG. 1  in that switch  210  does not include reverse rotation prisms ( 120 ,  130 ,  140  in  FIG. 1 ), and the assemblies formed by collimators  30 ,  40 ,  60  and rhomboid prisms  142 ,  122 ,  132 , respectively, are appropriately rotated with respect to the longitudinal direction defined by optical path  22 . Rhomboid prisms  122 ,  132 ,  142  are arranged so that their corresponding incident light beams are generally normal to their respective prism transmissive input surfaces. The output beams translated by prisms  122 ,  132 ,  142  form a tilt angle δ with respect to the longitudinal direction defined by optical path  22 . Accordingly, collimators  30 ,  40 ,  60  are oriented at an angle δ with respect to the longitudinal direction defined by optical path  22 . The optical switch shown in  FIG. 6  employs fewer optical parts than the one shown in  FIG. 1 . At the same time, the tilted output collimator positions may require appropriately designed and positioned collimator mounts to achieve desired levels of optical performance. 
     FIG. 7  shows a 1×4 optical switch  310  according to some embodiments of the present invention. An input collimator  20  faces the same direction as collimators  30 ,  40 ,  50 ,  60  within switch  310 . A 45° dovetail prism  340  having a trapezoidal in-plane cross-section faces collimator  20 . A collimated light beam output by collimator  20  enters prism  340  through a front transmissive surface  342 , is totally reflected by a reflective surface  344 , travels transversely within prism  340 , is again reflected by a reflective surface  346 , and leaves prism  340  along a general longitudinal direction opposite to the direction of the input beam. Transmissive surface  342  is anti-reflection (AR) coated, while reflective surfaces  344 ,  346  have cleanness and flatness characteristics of optical quality. The output beam is received by one of collimators  30 ,  40 ,  50 ,  60 , according to the switching state of switch  310 . Three mechanical switching devices  315 ,  325 ,  335  coupled to transverse linearly movable arms  312 ,  322 ,  332  are used to linearly insert and retract movable prisms  70 ,  80 ,  90 , respectively, in and out of the light beam path, along a linear transverse trajectory. Each prism  70 ,  80 ,  90  is mounted at the distal tips of a corresponding arm  312 ,  322 ,  332 . The configuration of  FIG. 7  allows placing all the switch optical fibers on one side of the switch. 
     FIG. 8  shows a 1×8 optical switch  410  according to some embodiments of the present invention. Switch  410  includes an input collimator  420 , eight output collimators  30 ,  30   a–g , a dovetail prism  440 , seven switching wedge prisms  430   a–g , and seven transverse translation assemblies  460   a–g  each including a wedge prism and a rhomboid prism. Switching prisms  430   a–g  are arranged sequentially along a longitudinal direction, and are oriented in alternating opposite directions. When all switching prisms  430   a–g  are retracted, collimators  420  and  30  are optically coupled. When switching prisms  430   a–g  are sequentially inserted, one at a time, into the optical path of the light beam output by collimator  420 , the light beam is deflected by the inserted switching prism  430   a–g  and a corresponding translation assembly  460   a–g  into a corresponding output collimator  30   a–g . Similar 1×N switches with N lower or higher than 8 can be constructed, with N−1 movable switching prisms and N−1 transverse translation assemblies arranged as illustrated in  FIG. 8 . 
   If the switching wedge prisms are arranged alternately in opposite orientations, as shown in  FIG. 8 , beams deflected by adjacent prisms are diverted to opposite sides of the switch longitudinal axis. Such an alternating-orientation design allows the use of wedge prisms having a relatively small deflection angle δ (within a few degrees), which allows improved switching stability, while maintaining sufficient separation between parallel adjacent deflected beams to avoid cross talk. A separation s between parallel adjacent deflection beams obeys the relation:
 
s=D sin δ,  [4]
 
where D is the longitudinal separation between the movable wedge prisms corresponding to the two beams. A minimum inter-beam separation s can be determined by the beam&#39;s diameter plus a safety margin. In a switch configured with alternating wedge prism orientations, the minimum longitudinal separation between switching prisms corresponding to parallel deflected beams is twice the minimum separation between adjacent switching prisms. The minimum separation between adjacent switching prisms can be determined by the size of the mechanical switching device, for example. To maintain a given inter-beam separation s, a doubling in the size of the inter-prism longitudinal separation D allows reducing sin δ in half, which allows achieving improved stability. The alternating-orientation configuration of  FIG. 8  allows increasing the longitudinal separation D between prisms corresponding to parallel beams, relative to a configuration using prisms in a single orientation, shown in  FIG. 9 .
 
     FIG. 9  shows a 1×4 optical switch  510  having a set of wedge prisms  450   a, c, e  oriented in the same direction, according to some embodiments of the present invention. Wedge prisms  450   a, c, e  deflect an input light beam on the same side of the switch longitudinal axis, toward translation assemblies  460   a, c, e , respectively, which in turn direct the light beam to output collimators  30   a, c, e , respectively. To achieve a desired level of separation between adjacent parallel beams, a switch in configuration of  FIG. 9  may use larger longitudinal separations between adjacent switching prisms than a switch in the alternating-orientation configuration of  FIG. 8 . 
   The preferred optical switch designs described above allow achieving high switching stabilities in environmental conditions subject to vibrations and/or relatively wide temperature variations. A change in the position or orientation of a small-angle wedge prism has a relatively small effect on the translational position and angular orientation of the deflected light beam. In a switch built as shown in  FIG. 7 , an output power switching repeatability of 0.02 dB was achieved. A maximum insertion loss of 0.6 dB was readily achieved for all ports in switches built as described above. Each input/output optical coupling can be adjusted independently, which allows achieving small insertion losses for all ports. The functioning of the wedge switching prisms is relatively insensitive to geometric parameters of the switching prisms other than the wedge angle, which may allow the use of switching prisms with relaxed tolerances for parameters other than the wedge angle, and thus lower part costs. 
   It will be clear to one skilled in the art that the above embodiments may be altered in many ways without departing from the scope of the invention. For example, in some embodiments, the pair of reflectors provided by a rhomboid prism can be provided by two physically-separate, fixed reflectors formed by prisms or mirrors. The direction of light travel in the configurations described above can be reversed. A N×M switch can be constructed by concatenating all or the internal parts of a 1×N and a M×1 switch as described above. The switching prisms can be retracted out of the optical path in a linear trajectory, or by a rotary arm movable in-plane or out-of-plane, among others. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents.