Abstract:
A waveguide type liquid-crystal optical switch including: an optical waveguide having a pair of first and second cores close to each other for switching an optical path between the pair of first and second cores; a third core made of nematic liquid crystal enclosed in between a pair of oriented films and oriented in a predetermined direction by the pair of oriented films, the third core being disposed in any one of a space covering the first and second cores in parallel with a plane containing optical axes of the first and second cores, a space sandwiched between the first and second cores and a space covering upper surfaces of the first and second cores so as to be laid over the first and second cores; a first electrode disposed on a surface of the third core opposite to the first and second cores so as to cover a gap portion between the first and second cores; second and third electrodes disposed as a pair, between which electrodes the first electrode is put, the second and third electrodes provided for orienting liquid crystal molecules in a direction perpendicular to the direction of orientation of the oriented films; and a clad for collectively surrounding the first, second and third cores and the first, second and third electrodes.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to an optical switch, for example, used in an optical communication system. More specifically, it relates to a waveguide type liquid-crystal optical switch for switching an optical path between cores by liquid crystal. 
   Optical communication has become popular in ordinary home use in recent years because a great volume of information can be transmitted/received at a high speed. The optical communication can be achieved by a transmission system using optical fibers. Various optical components such as a fiber type optical coupler, a waveguide type optical multiplexer/demultiplexer, a free-space propagation type optical multiplexer/demultiplexer, an optical switch, etc. have been developed to distribute an optical signal to respective terminals. 
   Of these optical components, the optical switch is important as an optical communication exchanger because it has a function of switching an optical path. Various types of optical switches are heretofore known as ones used in optical communication. Of these optical switches, an optical switch of the type using an optical waveguide for switching a propagating path of light by various kinds of physical phenomena has an advantage of high reliability and high speed because this type optical switch has no mechanically movable portion. As this type optical switch, there is known an optical switch using an optical waveguide such as a dielectric crystal waveguide of LiNbO 3  having an electro-optical or acousto-optical effect, a semiconductor waveguide using carrier injection, or a silica waveguide using a thermo-optical effect. 
   An optical switch using liquid crystal is also known as this type optical switch having such an optical waveguide. Liquid crystal has an electro-optical effect in the wide sense in which the refractive index of the liquid crystal varies in accordance with application of electric field. Liquid crystal further has the following properties: it can be actuated by a low voltage; it has high reliability as represented by satisfactory results in use for display; and it can be produced efficiently and inexpensively. For example, such a waveguide type liquid-crystal optical switch has been described in JP-A-5-165068. The waveguide type liquid-crystal optical switch has a structure in which: two single mode optical core patterns having coupling portions parallel and close to each other are formed on a lower clad; a lower electrode is further formed on a part of the lower clad corresponding to the coupling portions; and the coupling portions are filled with oriented liquid crystal and sealed with a glass plate having an upper electrode. 
   In the waveguide type liquid-crystal optical switch, however, the liquid crystal and the lower electrode are formed so that the nearly whole of the lower clad is covered with the liquid crystal and the lower electrode. Accordingly, because the refractive index of the clad varies in a wide region of the clad to disturb the waveguide mode, large crosstalk regarded as being fatal to the switch is generated. Moreover, if there are some liquid crystal molecules not oriented in a predetermined direction, scattering loss due to the non-oriented liquid crystal molecules increases because the coupling portions of the waveguide cores come into contact with the liquid crystal at three surfaces. Further, loss due to the lower electrode becomes very large because the lower electrode is located extremely close to the coupling portions of the wavelength cores. In addition, there is a problem that polarization dependence is high because the optical switch has a structure in which no electrode but a pair of upper and lower electrodes can be disposed. 
   SUMMARY OF THE INVENTION 
   The invention is developed in consideration of such circumstances and an object of the invention is to provide a waveguide type liquid-crystal optical switch which has an advantage of low power consumption, low cost and high reliability and which is improved in crosstalk and insertion loss and free from polarization dependence. 
   To achieve the foregoing object, the invention provides first, second and third waveguide type liquid-crystal optical switches as follows. 
   (1) A first waveguide type liquid-crystal optical switch including: an optical waveguide including a pair of first and second cores close to each other for switching an optical path between the pair of first and second cores; a third core made of nematic liquid crystal enclosed in between a pair of oriented films and oriented in a predetermined direction by the pair of oriented films, the third core being disposed in any one of a space covering the first and second cores in parallel with a plane containing optical axes of the first and second cores, a space sandwiched between the first and second cores and a space covering upper surfaces of the first and second cores so as to be laid over the first and second cores; a first electrode disposed on a surface of the third core opposite to the first and second cores so as to cover a gap portion between the first and second cores; second and third electrodes disposed as a pair, between which electrodes the first electrode is put, the second and third electrodes being provided for orienting liquid crystal molecules in a direction perpendicular to the direction of orientation of the oriented films; and a clad for collectively surrounding the first, second and third cores and the first, second and third electrodes. 
   (2) A second waveguide type liquid-crystal optical switch including: an optical waveguide including a pair of first and second cores close to each other for switching an optical path between the pair of first and second cores; a third core made of nematic liquid crystal enclosed in between a pair of oriented films and oriented in a predetermined direction by the pair of oriented films, the third core being disposed in any one of a space covering the first and second cores in parallel with a plane containing optical axes of the first and second cores, a space sandwiched between the first and second cores and a space covering upper surfaces of the first and second cores so as to be laid over the first and second cores; a pair of electrodes disposed on a surface of the third core opposite to the first and second cores, the pair of electrodes being provided for orienting liquid crystal molecules in a direction perpendicular to the direction of orientation of the oriented films; and a clad for collectively surrounding the first, second and third cores and the pair of electrodes. 
   (3) A third waveguide type liquid-crystal optical switch including: an optical waveguide including a pair of first and second cores close to each other for switching an optical path between the pair of first and second cores; a third core made of nematic liquid crystal enclosed in between a pair of oriented films and oriented in a predetermined direction by the pair of oriented films, the third core being disposed in a space sandwiched between the first and second cores; two pairs of electrodes disposed on an outer upper surface of the third core close to the first core and an outer lower surface of the third core close to the second core, respectively, along opposite sides of the first and second cores; and a clad for collectively surrounding the first, second and third cores and the two pairs of electrodes. 
   The present disclosure relates to the subject matter contained in Japanese patent application No. P2002-036618 (filed on Feb. 14, 2002), which is expressly incorporated herein by reference in its entirety. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plan view showing a first waveguide type liquid-crystal optical switch according to an embodiment of the invention. 
       FIG. 2  is a sectional view taken along the line A—A in FIG.  1 . 
       FIGS. 3A and 3B  are plan views showing an example of the optical path switching state of the waveguide type liquid-crystal optical switch depicted in FIG.  1 . 
       FIGS. 4A and 4B  are sectional views for explaining the theory of optical path switching of the waveguide type liquid-crystal optical switch depicted in FIG.  1 . 
       FIG. 5  is a sectional view showing a first waveguide type liquid-crystal optical switch according to another embodiment of the invention. 
       FIG. 6  is a sectional view showing a first waveguide type liquid-crystal optical switch according to a further embodiment of the invention. 
       FIG. 7  is a sectional view showing a third waveguide type liquid-crystal optical switch according to an embodiment of the invention. 
       FIG. 8  is a plan view showing a double gate type optical switch system having four waveguide type liquid-crystal optical switches connected to one another according to the invention. 
       FIG. 9  is a plan view showing a double gate type optical switch system having two waveguide type liquid-crystal optical switches connected to each other according to the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The invention will be described below in detail with reference to the drawings. 
   (First Waveguide Type Liquid-Crystal Optical Switch) 
     FIG. 1  is a plan view from the core side (from the bottom in FIG.  2 ), showing a first waveguide type liquid-crystal optical switch  10  according to an embodiment of the invention.  FIG. 2  is a sectional view taken along the line A—A in FIG.  1 . As shown in  FIGS. 1 and 2 , the waveguide type liquid-crystal optical switch  10  is divided into two, that is, an upper substrate  8 A and a lower substrate  8 B. In the lower substrate  8 B, a first core  1 A and a second core  1 B are buried along the axis of the lower substrate  8 B. Incidentally, in the following description, the first and second cores  1 A and  1 B are referred to as “cores  1 A and  1 B” respectively for short. The cores  1 A and  1 B are close and parallel to each other with a predetermined length in the central portion and the distance between the cores  1 A and  1 B is increasing toward opposite edges. The close parallel portions form a so-called directional coupler. 
   The upper and lower substrates  8 A and  8 B are made of a material having a refractive index lower than that of the material forming the cores  1 A and  1 B. For example, (SiO 2 —TiO 2 ) having a refractive index of 1.523 can be used as the material of the upper and lower substrates  8 A and  8 B. The lower substrate  8 B surrounds the cores  1 A and  1 B and serves as a clad  3 . On the other hand, for example, (SiCl 4 —TiCl 4 ) having a refractive index of 1.530 can be used as the material of the cores  1 A and  1 B. 
   A third core  4  is provided so that the lower substrate  8 B is entirely covered with the third core  4 . The third core  4  contains nematic liquid crystal sealed in sealing members  9  and upper and lower oriented films  7 A and  7 B. The sealing members  9  are provided along opposite edges of the lower substrate  8 B. The thickness of liquid crystal of the third core  4  is adjusted by spacers  11 . 
   The kind of nematic liquid crystal constituting the third core  4  is not limited. For example, 4-(4-pentylcyclohexyl)cyanobenzene can be used as the nematic liquid crystal. A known material may be used as the material of each of the upper and lower oriented films  7 A and  7 B. For example, a polyamide film subjected to a rubbing treatment can be used as each of the upper and lower oriented films  7 A and  7 B. 
   A first electrode  6 B is formed on a surface of the upper oriented film  7 A so that the gap between the pair of cores  1 A and  1 B is covered with the first electrode  6 B. Second and third electrodes  6 A and  6 C are formed on opposite sides of the first electrode  6 B and in slightly outward positions compared with the cores  1 A and  1 B respectively. The first, second and third electrodes  6 B,  6 A and  6 C can be controlled respectively individually. The direction of orientation of liquid crystal molecules due to the second and third electrodes  6 A and  6 C is decided so as to cross perpendicularly the direction of orientation of the oriented films  7 A and  7 B. 
   Preferably, each of the cores  1 A and  1 B is formed so as to be far, by a distance of not shorter than a half of the width W of each of the cores  1 A and  1 B, from the respective electrodes. Although a transparent electrode material such as ITO can be used as the material of each of the electrodes, absorption becomes considerably large if infrared rays are used as a medium of transmission. Therefore, loss due to the absorption can be suppressed when the electrode-core distance is defined as described above. 
   In the waveguide type liquid-crystal optical switch  10  configured as described above, the orientation of liquid crystal molecules in the third core  4  can be controlled by adjustment of a voltage applied between the pair of electrodes  6 A and  6 B to thereby switch the optical path between the cores  1 A and  1 B. For example, each of optical signals  5 A and  5 B can be transmitted from an incident side core to the third core  4  and further transferred to the other core as shown in FIG.  3 A. For example, the optical signal transferred thus to the other core can be transmitted to the third core  4  again and further transferred to the incident side core as shown in FIG.  3 B. Incidentally, in the following description, the case where each optical signal is output from the same core as the core on which the optical signal is incident is hereinafter called “ON state” (FIG.  3 B), and the case where each optical signal is output from the other core than the core on which the optical signal is incident is hereinafter called “OFF state” (FIG.  3 A). 
   When an equiphase electric potential is applied to the second and third electrodes  6 A and  6 C with respect to the first electrode  6 B as shown in  FIG. 4A  to switch the optical path, an electric field going from the first electrode  6 B to the second and third electrodes  6 A and  6 C is generated in the third core  4  as represented by the arrows in  FIG. 4A  to thereby make an electric field going from the first electrode  6 B to the cores  1 A and  1 B dominant. As a result, variation in coefficient of coupling to polarized light in TE mode (hereafter, polarized light in TE mode being referred to simply as TE polarized light) becomes dominant. On the other hand, when a reversed-phase potential with the potential of the first electrode  6 B as a reference potential is applied to the second and third electrodes  6 A and  6 C as shown in  FIG. 4B , an electric field going from the second electrode  6 A to the third electrode  6 C becomes dominant in the third core  4  as represented by the arrows in  FIG. 4B  to thereby make variation in coefficient of coupling to polarized light in TM mode (hereafter, polarized light in TM mode being referred to simply as TM polarized light) dominant. When the voltage and phase of the second and third electrodes  6 A and  6 C with respect to the first electrode  6 B are adjusted in the aforementioned manner, the ON/OFF state both for TE polarized light and for TM polarized light can be adjusted to eliminate polarization dependence. 
   Generally, variation in refractive index of liquid crystal is larger by the order of tens of times than the refractive index difference between the material forming the cores  1 A and  1 B and the material forming the clad  3 . Hence, the switching can be surely performed by a low voltage. 
   Incidentally, the switching mode in the case where nematic liquid crystal used in the third core  4  exhibits positive variation in refractive index is reversed to the switching mode in the case where the nematic liquid crystal exhibits negative variation in refractive index. 
   Further, the structure itself can be changed. For example, the third core  4  may be formed in a gap between parallel portions of the cores  1 A and  1 B as shown in  FIG. 5  which is a sectional view similar to FIG.  2 . That is, a lower oriented film  7 B is formed in a lower portion of a groove which is formed along the gap between the cores  1 A and  1 B so as to extend from an upper surface of a lower substrate  8 B. The groove is filled with nematic liquid crystal. The whole surface of the lower substrate  8 B is covered with an upper oriented film  7 A to thereby form the third core  4 . Further, first, second and third electrodes  6 B,  6 A and  6 C are provided on an upper surface of the upper oriented film  7 A in the same manner as described above. In the waveguide type liquid-crystal optical switch  10  configured thus, only two, left and right side surfaces of the third core  4  in  FIG. 5  come into contact with the cores  1 A and  1 B respectively. Hence, crosstalk is reduced greatly and loss is reduced compared with the related-art waveguide type liquid-crystal optical switch in which the third core  4  is formed so that the cores  1 A and  1 B are substantially entirely covered with the third core  4 . 
   Incidentally, in the waveguide type liquid-crystal optical switch  10  configured as described above, the lower oriented film  7 B may be omitted so that only the upper oriented film  7 A is provided because it is difficult to form the lower oriented film  7 B. Or the lower oriented film  7 B of the third core  4  (or the lower portion of the groove) may be formed so as to extent to a position deeper than the cores  1 A and  1 B. 
   Further, the third core  4  may be formed so that only the upper surfaces of the cores  1 A and  1 B are covered with the third core  4  as shown in  FIG. 6  which is a sectional view similar to FIG.  5 . Although the lower oriented film  7 B of the third core  4  may be brought into contact with the upper surfaces of the cores  1 A and  1 B, a very slight gap may be provided between each of the upper surfaces of the cores  1 A and  1 B and the lower oriented film  7 B if there is fear of optical influence of the lower oriented film  7 B. 
   (Second Waveguide Type Liquid-Crystal Optical Switch) 
   Though not shown, a second waveguide type liquid-crystal optical switch according to another embodiment of the invention has a structure in which any one of the three electrodes  6 A,  6 B and  6 C is removed from each example of the first waveguide type liquid-crystal optical switch  10 . Incidentally, in this case, switching of the optical path by the third core  4  is limited to either polarized light. When, for example, the first electrode  6 B is removed, an optical path for TE polarized light can be switched because a horizontal electric field between the second and third electrodes  6 A and  6 C becomes dominant (see FIG.  4 B). When, for example, the second or third electrode  6 A or  6 C is removed, an optical path for TM polarized light can be switched because an electric field going toward the core side becomes dominant (see FIG.  4 A). 
   Accordingly, in the second waveguide type liquid-crystal optical switch  10 , when optical signals  5 A and  5 B are polarized at random, a polarizing unit such as a wave plate for polarizing the optical signals  5 A and  5 B into either TE polarized light or TM polarized light is separately provided on the input side of the waveguide type liquid-crystal optical switch  10 . 
   (Third Waveguide Type Liquid-Crystal Optical Switch) 
   A third waveguide type liquid-crystal optical switch  10  according to a further embodiment of the invention is shown in  FIG. 7  which is a sectional view similar to FIG.  6 . In third the waveguide type liquid-crystal optical switch  10 , the third core  4  is disposed between the upper substrate  8 A and the lower substrate  8 B. A core  1 A and a pair of upper electrodes  6 A and  6 C are provided on a surface of the upper oriented film  7 A so that the core  1 A is disposed between the pair of upper electrodes  6 A and  6 C. The other core  1 B and a pair of lower electrodes  12 A and  12 C are provided on a surface of the lower oriented film  7 B so that the core  1 B and the pair of lower electrodes  12 A and  12 C are located opposite to the core  1 A and the pair of upper electrodes  6 A and  6 C respectively. That is, the pair of cores  1 A and  1 B, the pair of upper electrodes  6 A and  6 C and the pair of lower electrodes  12 A and  12 C are provided so that the third core  4  is clamped. The thickness of liquid crystal is adjusted by spacers not shown. The third core  4  is sealed with sealing members  9 . 
   In the third waveguide type liquid-crystal optical switch  10 , when the upper electrodes  6 A and  6 C are kept equal in electric potential while an electric potential is applied to the lower electrodes  12 A and  12 C, variation in coefficient of coupling for TE polarized light becomes dominant. When a pair of opposite electrodes (e.g.,  6 A and  12 A) are kept equal in electric potential while an electric potential is applied to the other pair of opposite electrodes (e.g.,  6 C and  12 C), variation in coefficient of coupling for TM polarized light becomes dominant. Accordingly, polarization dependence can be eliminated in the same manner as in the first waveguide type liquid-crystal optical switch. Incidentally, pairs of electrodes between which a voltage is applied may be combined variously. For example, a pair of electrodes  6 A and  12 C and a pair of electrodes  6 C and  12 A may be combined. 
   (Double Gate Type Optical Switch System) 
   An even number of waveguide type liquid-crystal optical switches  10  as defined above can be connected to one another to form a single optical switch-like structure having a pair of input ends and a pair of output ends as a whole. In the invention, the configuration having such an even number of waveguide type liquid-crystal optical switches  10  connected to one another and functioning as a single optical switch as a whole is defined as “double gate type optical switch system”. 
     FIG. 8  is a typical view showing a double gate type optical switch system having four waveguide type liquid-crystal optical switches as an example of the double gate type optical switch system. The double gate type optical switch system shown in  FIG. 8  is configured as follows. One output end c of a first waveguide type liquid-crystal optical switch  10 A is connected to one input end e of a second waveguide type liquid-crystal optical switch  10 B by an optical waveguide  20 . The other output end d of the first waveguide type liquid-crystal optical switch  10 A is connected to one input end m of a fourth waveguide type liquid-crystal optical switch  10 D by an optical waveguide  20 . One output end k of a third waveguide type liquid-crystal optical switch  10 C is connected to the other input end f of the second waveguide type liquid-crystal optical switch  10 B by an optical waveguide  20 . The other output end l of the third waveguide type liquid-crystal optical switch  10 C is connected to the other input end n of the fourth waveguide type liquid-crystal optical switch  10 D by an optical waveguide  20 . One input end a of the first waveguide type liquid-crystal optical switch  10 A is used as a first input port (IN 1 ). One input end j of the third waveguide type liquid-crystal optical switch  10 C is used as a second input port (IN 2 ). One output end g of the second waveguide type liquid-crystal optical switch  10 B is used as a first output port (OUT 1 ). One output end p of the fourth waveguide type liquid-crystal optical switch  10 D is used as a second output port (OUT 2 ). Accordingly, the other output end h of the second waveguide type liquid-crystal optical switch  10 B is terminated as a dummy port  1 , and the other output end o of the fourth waveguide type liquid-crystal optical switch  10 D is terminated as a dummy port  2 . 
   The ON/OFF state in the double gate type optical switch system is defined in the same manner as in the waveguide type liquid-crystal optical switch (see FIGS.  3 A and  3 B). That is, a state in which an optical signal given from the first input port IN 1  is output from the first output port OUT 1  while the other optical signal given from the second input port IN 2  is output from the second output port OUT 2  is defined as “ON state”. Conversely, a state in which an optical signal given from the first input port IN 1  is output from the second output port OUT 2  while the other optical signal given from the second input port IN 2  is output from the first output port OUT 1  is defined as “OFF state”. In addition, the ratio of the intensity of an optical signal to be output from each output port in the ON or OFF state to the intensity of an optical signal (stray light) not to be output is defined as “extinction ratio”. 
   To obtain the ON state of the double gate type optical switch system, all the waveguide type liquid-crystal optical switches  10 A to  10 D are switched ON. As a result, an optical signal input into the first input port IN 1  is output from the output end c of the first waveguide type liquid-crystal optical switch  10 A, input into the input end e of the second waveguide type liquid-crystal optical switch  10 B and output from the output end g, that is, the first output port OUT 1 . On this occasion, even if there is stray light, the stray light is not output from the original output ports because stray light in the first waveguide type liquid-crystal optical switch  10 A is output from the output end d and reaches the dummy port  2  while stray light in the second waveguide type liquid-crystal optical switch  10 B reaches the dummy port  1 . Thus, a high extinction ratio can be achieved. 
   On the other hand, to obtain the OFF state, all the waveguide type liquid-crystal optical switches  10 A to  10 D may be switched OFF. Also on this occasion, stray light is output from either dummy port  1  or dummy port  2 . 
   As shown in  FIG. 9 , two waveguide type liquid-crystal optical switches  10  maybe connected to each other to form a single optical switch as a whole. This configuration is as follows. One output end c of a first waveguide type liquid-crystal optical switch  10 A is connected to one input end e of a second waveguide type liquid-crystal optical switch  10 B by an optical waveguide  20 . One input end a of the first waveguide type liquid-crystal optical switch  10 A is used as a first input port IN 1 . The other input end f of the second waveguide type liquid-crystal optical switch  10 B is used as a second input port IN 2 . One output end g of the second waveguide type liquid-crystal optical switch  10 B is used as a first output port OUT 1 . The other output end d of the first waveguide type liquid-crystal optical switch  10 A is used as a second output port OUT 2 . The other output end h of the second waveguide type liquid-crystal optical switch  10 B is used as a dummy port. 
   To obtain the ON state of the double gate type optical switch system, the first and second waveguide type liquid-crystal optical switches  10 A and  10 B are switched ON. As a result, an optical signal input into the first input port IN 1  is output from the output end c of the first waveguide type liquid-crystal optical switch  10 A, input into the input end e of the second waveguide type liquid-crystal optical switch  10 B and output from the output end g, that is, the first output port OUT 1 . On this occasion, even if there is stray light, the stray light reaches the other output end d of the first waveguide type liquid-crystal optical switch  10 A and is output from the second output port OUT 2 . Thus, the extinction ratio becomes the same as in the case where a waveguide type liquid-crystal optical switch is used singly. 
   On the other hand, to obtain the OFF state, the first and second waveguide type liquid-crystal optical switches  10 A and  10 B are switched OFF. As a result, each stray light in optical signals input into the first and second input ports IN 1  and IN 2  is output from the dummy port. 
   As described above, the double gate type optical switch system having the two waveguide type liquid-crystal optical switches  10 A and  10 B is inexpensive because the number of waveguide type liquid-crystal optical switches is small. It is however necessary that the double gate type optical switch system is configured so that the second output port OUT 2  is not used in the ON state because it is impossible to output light from the second input port IN 2  to the second output port OUT 2 . 
   Although it is preferable that the double gate type optical switch system is configured so that all waveguide type liquid-crystal optical switches are integrally connected to one another on a substrate by optical waveguides, the invention may be also applied to the case where waveguide type liquid-crystal optical switches formed separately are connected to one another by optical fibers. 
   As described above, in accordance with the invention, there can be provided a waveguide type liquid-crystal optical switch which is low in insertion loss and high in performance compared with the related-art waveguide type liquid-crystal optical switch.