Patent Publication Number: US-2017366256-A1

Title: Optical component

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2016-121367, filed on Jun. 20, 2016, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to an optical component. 
     BACKGROUND 
     In the related art, in a supercomputer or the like, a technique for performing optical communication using an optical module is known. In addition, in a case where an abnormality is detected in the optical communication by the optical module, a technique for specifying a failure occurrence place which causes the abnormality is known (for example, refer to Japanese Laid-open Patent Publication No. 2011-211565 and Japanese Laid-open Patent Publication No. 5-199192). In such a technique, for example, an optical loopback in which a transmitted signal is returned in an optical processing section is used. 
     However, in the techniques in the related art, there is a problem that it is difficult to reduce the size of an optical component in which an optical loopback can be realized. For example, when an optical path switch including a movable portion is used to realize an optical loopback, the size of an optical component is increased due to the optical path switch. 
     SUMMARY 
     According to an aspect of the embodiments, an apparatus includes includes a light emitter; an optical receiver; first and second electro-optical crystal layers configured to intersect with each other; and a lead wire configured to supply a signal for changing refractive indexes of the first and second electro-optical crystal layers, wherein the first and second electro-optical crystal layers are switched according to the signal between a first state where light from the light emitter is transmitted through the first electro-optical crystal layer and a second state where the light is reflected by the first and second electro-optical crystal layers and the reflected light is incident on the optical receiver. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating an example of an optical path during communication in an optical component according to a first embodiment; 
         FIG. 2  is a diagram illustrating an example of an optical path during optical loopback in the optical component according to the first embodiment; 
         FIG. 3  is a diagram illustrating an example of an optical transmission system to which the optical component according to the first embodiment is applied; 
         FIG. 4  is a diagram illustrating an example of an optical path during signal transmission in the optical transmission system according to the first embodiment; 
         FIG. 5  is a diagram illustrating an example of an optical path in a first state of the electrical loopback in the optical transmission system according to the first embodiment; 
         FIG. 6  is a diagram illustrating an example of an optical path in a second state of the electrical loopback in the optical transmission system according to the first embodiment; 
         FIG. 7  is a diagram illustrating an example of an optical path in a first state of the optical loopback in the optical transmission system according to the first embodiment; 
         FIG. 8  is a diagram illustrating an example of an optical path in a second state of the optical loopback in the optical transmission system according to the first embodiment; 
         FIG. 9  is a flowchart illustrating an example of processing by the central controller of the optical transmission system according to the first embodiment; 
         FIG. 10  is a diagram illustrating an example of an optical path during communication in an optical component according to a second embodiment; 
         FIG. 11  is a diagram illustrating an example of an optical path during optical loopback in the optical component according to the second embodiment; 
         FIG. 12  is a diagram illustrating an example of an optical transmission system to which the optical component according to the second embodiment is applied; 
         FIG. 13  is a diagram illustrating an example of an optical path during communication in an optical component according to a third embodiment; and 
         FIG. 14  is a diagram illustrating an example of an optical path during optical loopback in the optical component according to the third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of an optical component according to the present disclosure will be described in detail with reference to the drawings. 
     Optical Path during communication in Optical Component according to First Embodiment 
       FIG. 1  is a diagram illustrating an example of an optical path during communication in an optical component according to a first embodiment. As illustrated in  FIG. 1 , an optical component  100  according to the first embodiment is, for example, an optical component with a built-in loop-back function that includes a block  110 , a block  120 , a voltage control circuit  130 , and a control line  131 . The optical component  100  is provided in an optical module (for example, refer to  FIG. 3 ) including a light emitter (Tx: transmitter) and an optical receiver (Rx). 
     A transmission path  101  is a path through which light incident from the light emitter (Tx) of the optical module provided with the optical component  100  is emitted to an optical transmission line. A reception path  102  is a path through which light incident from an optical module opposite to the optical module provided with the optical component  100  via the optical transmission line is emitted to the optical receiver (Rx: receiver) of the optical module provided with the optical component  100 . 
     The block  110  is a block that is formed by providing a reflection layer  111 , for example, in a cubic block through which light is transmitted. The reflection layer  111  is provided at an angle of 45 degrees with respect to one set of adjacent surfaces (the bottom surface and the right surface in  FIG. 1 ) in the cubic block. The reflection layer  111  reflects light emitted from the light emitter (Tx) of the optical module provided with the optical component  100  at an incident angle of 45 degrees (changes the angle of the light by 90 degrees) to emit the light to the block  120 . In addition, the reflection layer  111  reflects light which is incident from the optical transmission line and is emitted from the block  120  at an incident angle of 45 degrees to emit the light to the optical receiver (Rx) of the optical module provided with the optical component  100 . 
     For example, in a case where a VCSEL (Vertical Cavity Surface Emitting LASER) is used for the light emitter (Tx) of the optical module, light is emitted from the VCSEL provided on the base in a direction perpendicular to the base. The VCSEL is a semiconductor laser. On the other hand, the optical transmission line such as an optical fiber is provided in a direction parallel to the base. The traveling direction of the light is changed by the reflection layer  111  using the block  110 , and thus the light emitted from the VCSEL can be incident on the optical fiber. 
     The block  120  is formed, for example, by providing electro-optical crystal layers  121  and  122  in a cubic block through which light is transmitted. The electro-optical crystal layers  121  and  122  are formed to be intersected with each other on diagonal lines of the cubic block. For example, the electro-optical crystal layer  121  is provided at an angle of 45 degrees with respect to one set of adjacent surfaces (the left surface and the rear surface in  FIG. 1 ) in the cubic block. The electro-optical crystal layer  122  is provided at an angle of 45 degrees with respect to one set of adjacent surfaces (the left surface and the front surface in  FIG. 1 ) in the cubic block. The electro-optical crystal layers  121  and  122  are perpendicularly intersected with each other. 
     The electro-optical crystal layers  121  and  122  are transmission plates or mirrors. Each of the electro-optical crystal layers  121  and  122  is switched according to the voltage of the control signal applied from the voltage control circuit  130  via the control line  131 . For example, the refractive indexes of the electro-optical crystal layers  121  and  122  are switched according to the voltage applied from the voltage control circuit  130 . Therefore, the refractive indexes are switched, and thus switching is achieved between a state where the incident light is totally reflected and a state where the incident light is transmitted. 
     As an example, the electro-optical crystal layers  121  and  122  can be realized by using a thin film which is made of kalium tantalum-niobate (KTN) crystals having a large change in the refractive index with respect to the applied voltage due to a large electro-optical coefficient (for example, an electro-optical coefficient of 600 pm/V or more). Here, the electro-optical crystal layers  121  and  122  can be made by various electro-optical crystals each of which the transmittance changes according to the applied voltage. For example, the electro-optical crystal layers  121  and  122  be made by using lithium niobate. 
     The following embodiments use that the electro-optical crystal layers  121  and  122  are made from KTN. 
     The reflection layer  111  in the block  110  and the electro-optical crystal layers  121  and  122  in the block  120  can be formed, for example, by a TSSG method, a LPE method, or the like. The TSSG is an abbreviation for top seeded solution growth. The LPE is an abbreviation for liquid phase epitaxy. Here, the method for forming the reflection layer  111  and the electro-optical crystal layers  121  and  122  is not limited thereto, and various forming methods can be used. 
     In a case where the optical module provided with the optical component  100  performs optical communication with the opposing optical module via the optical component  100 , as illustrated in  FIG. 1 , the voltage applied between a first conductor  125  connected to the voltage control circuit  130  via a control line  131  and a second conductor  124  connected to a ground through a line  132 . The applied voltage to the electro-optical crystal layers (use of KTN)  121  and  122  is controlled to be HIGH (for example, a voltage larger than 0 V). Each of the first conductor  125  and the second conductor  124  may be formed with a circle line or a conductor plate or the like. In this case, the electro-optical crystal layers  121  and  122  have a relatively low first refractive index, and are in a state where the incident light is transmitted. 
     The applied voltage is larger than 0 V, the electro-optical crystal layers  121  and  122  of the block  120  transmit the light on the transmission path  101  that is emitted from the block  110  to emit the light to the optical transmission line. Accordingly, the light transmitted from the optical module provided with the optical component  100  is transmitted to the opposing optical module. The electro-optical crystal layers  121  and  122  of the block  120  transmit the light incident from the optical transmission line to emit the light to the block  110 . Accordingly, the light transmitted from the opposing optical module is received by the optical module provided with the optical component  100 . 
     Optical Path during Optical Loopback in Optical Component according to First Embodiment 
       FIG. 2  is a diagram illustrating an example of an optical path during optical loopback in the optical component according to the first embodiment. In  FIG. 2 , components similar to those illustrated in  FIG. 1  are denoted by the same reference numerals, and description thereof is omitted. In a case where the optical loopback is formed by using the optical component  100 , for example, as illustrated in  FIG. 2 , the voltage applied from the voltage control circuit  130  to the electro-optical crystal layers  121  and  122  between the first conductor  125  and the second conductor  124 . The first conductor  125  is controlled to be LOW (for example, 0 V). The first conductor  125  is connected to voltage control circuit  130  through the line  131  and the second conductor  124  is connected to the ground through the line  132 . 
     In this case, the electro-optical crystal layers  121  and  122  have a second refractive index higher than the first refractive index, and are in a state where the incident light is totally reflected. In other words, the electro-optical crystal layers  121  and  122  return the light on the transmission path  103  that is emitted from the block  110  by respectively reflecting the light at an incident angle of 45 degrees to emit the light to the optical receiver (Rx) of the optical module provided with the optical component  100 . Accordingly, the light transmitted from the optical module provided with the optical component  100  is returned to the optical module provided with the optical component  100 . 
     In addition, the electro-optical crystal layers  121  and  122  return the light which is incident from the optical transmission line by respectively reflecting the light at an incident angle of 45 degrees to emit the light to the optical transmission line. Accordingly, the light transmitted from the optical module opposite to the optical module provided with the optical component  100  is returned to the optical module opposite to the optical module provided with the optical component  100 . 
     The return path  103  is a path through which the light incident from the light emitter (Tx) of the optical module provided with the optical component  100  is returned by the electro-optical crystal layers  121  and  122  and is emitted to the optical receiver (Rx) of the optical module provided with the optical component  100 . The return path  104  is a path through which the light incident from the optical transmission line is returned by the electro-optical crystal layers  121  and  122  and is emitted to the optical transmission line. 
     As illustrated in  FIGS. 1 and 2 , according to the optical component  100 , the optical communication path (refer to  FIG. 1 ) and the optical loopback path can be switched by controlling the voltage applied to the electro-optical crystal layers  121  and  122  via the first conductor  125  and the second conductor  124 . Further, the optical component  100  can switch the optical path by controlling the voltage applied to the electro-optical crystal layers  121  and  122 , and thus a small-sized optical component can be adopted, compared to a configuration in which the optical loopback is implemented, for example, by using an optical path switch including a movable portion. 
     Optical Transmission System to which Optical Component according to First Embodiment is applied 
       FIG. 3  is a diagram illustrating an example of an optical transmission system to which the optical component according to the first embodiment is applied. The optical transmission system  30  illustrated in  FIG. 3  includes a first optical transmission apparatus  300 A, a second optical transmission apparatus  300 B, optical transmission lines  301  and  302 , and a central controller  303 . 
     The first optical transmission apparatus  300 A and the second optical transmission apparatus  300 B are opposite to each other and perform optical communication with each other via the optical transmission lines  301  and  302 . The optical transmission line  301  is an optical transmission line such as an optical fiber that transmits an optical signal from the first optical transmission apparatus  300 A to the second optical transmission apparatus  300 B. The optical transmission line  302  is an optical transmission line such as an optical fiber that transmits an optical signal from the second optical transmission apparatus  300 B to the first optical transmission apparatus  300 A. 
     The central controller  303  is a control circuit that controls the first optical transmission apparatus  300 A and the second optical transmission apparatus  300 B. The control by the central controller  303  includes specifying a failure occurrence place in a case where an abnormality is detected in the link between the first optical transmission apparatus  300 A and the second optical transmission apparatus  300 B. 
     For example, as illustrated in  FIG. 3 , the central controller  303  is provided at the outside of the first optical transmission apparatus  300 A and the second optical transmission apparatus  300 B, and is a device that can communicate with the first optical transmission apparatus  300 A and the second optical transmission apparatus  300 B. In this case, various types of communication such as electrical communication, optical communication, or wireless communication can be used for the communication between the first optical transmission apparatus  300 A and the second optical transmission apparatus  300 B and the central controller  303 . Here, the central controller  303  may be a controller provided in any one of the first optical transmission apparatus  300 A and the second optical transmission apparatus  300 B. 
     The first optical transmission apparatus  300 A includes a first board  310 A, a first CPU  320 A, and a first optical module  330 A. The CPU is an abbreviation for central processing unit. The first board  310 A is a base of the first optical transmission apparatus  300 A. The first CPU  320 A and the first optical module  330 A are connected to the first board  310 A. The first board  310 A supplies power to the first optical module  330 A. Further, the first board  310 A can communicate with the central controller  303 . 
     The first CPU  320 A controls the optical communication by the first optical module  330 A. For example, the first CPU  320 A outputs a signal to be transmitted by using the optical signal, to the first optical module  330 A. Further, the first CPU  320 A acquires a signal that is obtained by converting an optical signal received by the first optical module  330 A into an electrical signal. 
     The first CPU  320 A controls switching between enabling and disabling of the electrical loopback in the electrical loopback control circuit  332 A via the first board  310 A. Further, the first CPU  320 A transmits the detection result of the link abnormality in the optical communication by the first optical module  330 A, or the detection result of the signal in the electrical loopback and the optical loopback to be described later, to the central controller  303  via the first board  310 A. 
     The first optical module  330 A is an optical module that performs optical communication with the second optical transmission apparatus  300 B under the control of the first CPU  320 A. The first optical module  330 A includes a first optical component  100 A, a driver  331 A, an electrical loopback control circuit  332 A, a CDR  333 A, a VCSEL  334 A, a PD  335 A, a CDR  336 A, and a voltage control circuit  130 A. The CDR is an abbreviation for clock data recovery. The PD is an abbreviation for photo detector. 
     The driver  331 A supplies a drive voltage based on the power supplied from the first board  310 A, to the CDR  333 A, the VCSEL  334 A, the PD  335 A, the CDR  336 A, and the voltage control circuit  130 A. 
     The electrical loopback control circuit  332 A can switch enabling and disabling of the electrical loopback in own circuit under the control of the first CPU  320 A. For example, in a case where the electrical loopback in the electrical loopback control circuit  332 A is disabled, the electrical loopback control circuit  332 A outputs the signal that is output from the first CPU  320 A to the CDR  336 A as it is. In a case where the electrical loopback in the electrical loopback control circuit  332 A is disabled, the electrical loopback control circuit  332 A outputs the signal that is output from the CDR  336 A to the first CPU  320 A as it is. 
     Further, in a case where the electrical loopback in the electrical loopback control circuit  332 A is enabled, the electrical loopback control circuit  332 A returns the signal that is output from the first CPU  320 A to own circuit, and outputs the returned signal to the first CPU  320 A. In a case where the electrical loopback in the electrical loopback control circuit  332 A is enabled, the electrical loopback control circuit  332 A returns the signal that is output from the CDR  336 A to own circuit, and outputs the returned signal to the CDR  333 A. 
     The CDR  333 A performs clock data recovery processing at the transmission side for the signal that is output from the electrical loopback control circuit  332 A, and outputs the signal that is subjected to the clock data recovery processing to the VCSEL  334 A. The clock data recovery processing includes, for example, processing of extracting a clock from an input signal and shaping the signal. The VCSEL  334 A is a light emitter that converts a signal output from the CDR  333 A into an optical signal and emits the converted optical signal to the first optical component  100 A. 
     The first optical component  100 A has a configuration corresponding to the optical component  100  illustrated in  FIG. 1 . The reference numerals that are obtained by adding A to the end of the reference numerals of the components of the optical component  100  are given to the components of the first optical component  100 A. Further, lenses  337 A,  338 A,  339 A, and  340 A are provided in the first optical component  100 A. 
     The lens  337 A is provided on the surface of the block  110 A on the VCSEL  334 A side (the bottom surface in  FIG. 3 ), collimates light emitted from the VCSEL  334 A, and emits the light to the reflection layer  111 A. The lens  338 A is provided on the surface of the block  110 A on the block  120 A side (the right surface in  FIG. 3 ), condenses light that is emitted from the lens  337 A and reflected by the reflection layer  111 A, and emits the light to the block  120 A. 
     The lens  339 A is provided on the surface of the block  110 A on the block  120 A side (the right surface in  FIG. 3 ), collimates light that is emitted from the block  120 A, and emits the light to the reflection layer  111 A. The lens  340 A is provided on the surface of the block  110 A on the PD  335 A side (the bottom surface in  FIG. 3 ), condenses light that is emitted from the lens  339 A and reflected by the reflection layer  111 A, and emits the light to the PD  335 A. 
     The PD  335 A is an optical receiver that converts light emitted from the first optical component  100 A into an electrical signal and outputs the converted electrical signal to the CDR  336 A. The CDR  336 A performs clock data recovery processing at the receiving side for the signal that is output from the PD  335 A, and outputs the signal that is subjected to the clock data recovery processing to the electrical loopback control circuit  332 A. 
     The voltage control circuit  130 A has a configuration corresponding to the voltage control circuit  130  illustrated in  FIG. 1 . The voltage control circuit  130 A applies a voltage to the electro-optical crystal layers  121 A and  122 A between the first conductor  125 A and the second conductor  124 B. The first conductor  125 A is provided from the drive voltage supplied from the driver  331 A. The first conductor  125 A is connected to voltage control circuit  130 A through the line  131 A and the second conductor  124 A is connected to the ground through the line  132 A. Further, the voltage control circuit  130 A switches the voltage applied to the electro-optical crystal layers  121 A and  122 A via the control line  131 A under the control of the central controller  303  via the first board  310 A. Here, the voltage control circuit  130 A may control the voltage under the control of the central controller  303  via the first board  310 A and the first CPU  320 A. 
     In a case where the voltage that is applied to the electro-optical crystal layers  121 A and  122 A by the voltage control circuit  130 A is HIGH, as illustrated in  FIG. 3 , the light emitted from the VCSEL  334 A is transmitted to the second optical transmission apparatus  300 B via the optical transmission line  301 . Further, the light transmitted from the second optical transmission apparatus  300 B via the optical transmission line  302  is incident on the PD  335 A. 
     In a case where the voltage that is applied to the electro-optical crystal layers  121 A and  122 A by the voltage control circuit  130 A is LOW, the light emitted from the VCSEL  334 A is returned by the block  120 A and is incident on the PD  335 A. Further, the light transmitted from the second optical transmission apparatus  300 B via the optical transmission line  302  is returned by the block  120 A, and is transmitted to the second optical transmission apparatus  300 B via the optical transmission line  301 . 
     The configuration of the second optical transmission apparatus  300 B is the same as that of the first optical transmission apparatus  300 A. The reference numerals that are obtained by replacing A in the end of the reference numerals of the components of the first optical transmission apparatus  300 A with B are given to the components of the second optical transmission apparatus  300 B. 
     In a case where the voltage that is applied to the electro-optical crystal layers  121 B and  122 B between the first conductor  125 B and the second conductor  124 B. the drive voltage to the first conductor  125 B is provided by the voltage control circuit  130 B of the second optical transmission apparatus  300 B is HIGH, as illustrated in  FIG. 3 , the light emitted from the VCSEL  334 B is transmitted to the first optical transmission apparatus  300 A via the optical transmission line  302 . The first conductor  125 B is connected to voltage control circuit  130 B through the control line  131 B and the second conductor  124 B is connected to the ground through the line  132 . Further, the light transmitted from the first optical transmission apparatus  300 A via the optical transmission line  301  is incident on the PD  335 B. 
     In a case where the voltage that is applied to first conductor  125 B and the electro-optical crystal layers  121 B and  122 B via the first and second conductors  125 B and  124 B by the voltage control circuit  130 B is LOW, the light emitted from the VCSEL  334 B is returned by the block  120 B and is incident on the PD  335 B. Further, the light transmitted from the first optical transmission apparatus  300 A via the optical transmission line  301  is returned by the block  120 B, and is transmitted to the first optical transmission apparatus  300 A via the optical transmission line  302 . 
     Optical Path During Signal Transmission in Optical Transmission System According to First Embodiment 
       FIG. 4  is a diagram illustrating an example of an optical path during signal transmission in the optical transmission system according to the first embodiment. In  FIG. 4 , components similar to those illustrated in  FIG. 3  are denoted by the same reference numerals, and description thereof is omitted. 
     As illustrated in  FIG. 4 , the first optical module  330 A can be divided into an electrical transmission section  411 , an optical transmission section  412 , an optical reception section  413 , and an electrical reception section  414 . The electrical transmission section  411  includes, for example, the electrical loopback control circuit  332 A and the CDR  333 A illustrated in  FIG. 3 . The optical transmission section  412  includes, for example, the VCSEL  334 A and the first optical component  100 A illustrated in  FIG. 3 . The optical reception section  413  includes, for example, the first optical component  100 A and the PD  335 A illustrated in  FIG. 3 . The electrical reception section  414  includes, for example, the CDR  336 A and the electrical loopback control circuit  332 A illustrated in  FIG. 3 . 
     Similarly, the second optical module  330 B illustrated in  FIG. 3  can be divided into an electrical transmission section  421 , an optical transmission section  422 , an optical reception section  423 , and an electrical reception section  424 . The electrical transmission section  421  includes, for example, the electrical loopback control circuit  332 B and the CDR  333 B illustrated in  FIG. 3 . The optical transmission section  422  includes, for example, the VCSEL  334 B and the second optical component  100 B illustrated in  FIG. 3 . The optical reception section  423  includes, for example, the second optical component  100 B and the PD  335 B illustrated in  FIG. 3 . The electrical reception section  424  includes, for example, the CDR  336 B and the electrical loopback control circuit  332 B illustrated in  FIG. 3 . 
     In a case where the link abnormality is detected, the central controller  303  specifies a failure occurrence place among the electrical transmission sections  411  and  421 , the optical transmission sections  412  and  422 , the optical reception sections  413  and  423 , the electrical reception sections  414  and  424 , and the optical transmission lines  301  and  302  (refer to  FIG. 3 ). 
     The path  401  is a path of the signal that is output from the first CPU  320 A to the first optical module  330 A. The path  402  is a path of the signal that is output from the second CPU  320 B to the second optical module  330 B. In a case where actual data transmission is performed between the first optical transmission apparatus  300 A and the second optical transmission apparatus  300 B, the paths  401  and  402  are as illustrated in  FIG. 4 . 
     The path  401  illustrated in  FIG. 4  includes the first CPU  320 A, the electrical transmission section  411 , the optical transmission section  412 , the optical reception section  423 , the electrical reception section  424 , and the second CPU  320 B. The path  402  illustrated in  FIG. 4  includes the second CPU  320 B, the electrical transmission section  421 , the optical transmission section  422 , the optical reception section  413 , the electrical reception section  414 , and the first CPU  320 A. 
     As an example, it is assumed that a failure such as a fault occurs in the optical transmission section  412  (shaded area). In this case, since a failure does not occur in the path  402 , the first CPU  320 A can normally receive the signal from the second CPU  320 B. Accordingly, it can be determined that the electrical transmission section  421 , the optical transmission section  422 , the optical transmission line  302 , the optical reception section  413 , and the electrical reception section  414  in the path  402  are “OK” (no failure). 
     On the other hand, due to the failure of the optical transmission section  412  in the path  401 , the second CPU  320 B is unable to normally receive the signal from the first CPU  320 A. Accordingly, it can be determined that a failure occurs in any one of the electrical transmission section  411 , the optical transmission section  412 , the optical transmission line  301 , the optical reception section  423 , and the electrical reception section  424  in the path  401 . 
     The second CPU  320 B notifies the central controller  303  of the fact that the signal from the first board  310 A is not normally received, by using the control signal. In response to the notification, the central controller  303  starts to specify a failure occurrence place by using the electrical loopback and the optical loopback (for example, refer to  FIGS. 5 to 8 ). 
     Optical Path in First State of Electrical Loopback in Optical Transmission System According to First Embodiment 
       FIG. 5  is a diagram illustrating an example of an optical path in a first state of the electrical loopback in the optical transmission system according to the first embodiment. In  FIG. 5 , components similar to those illustrated in  FIG. 4  are denoted by the same reference numerals, and description thereof is omitted. As illustrated in  FIG. 4 , in a case where the second CPU  320 B is unable to normally receive the signal from the first CPU  320 A, the central controller  303  first enables the electrical loopback of the second optical transmission apparatus  300 B, for example. Accordingly, the paths  401  and  402  are as illustrated in  FIG. 5 . 
     The path  401  illustrated in  FIG. 5  includes the first CPU  320 A, the electrical transmission section  411 , the optical transmission section  412 , the optical transmission line  301 , the optical reception section  423 , the electrical reception section  424 , the electrical transmission section  421 , the optical transmission section  422 , the optical transmission line  302 , the optical reception section  413 , the electrical reception section  414 , and the first CPU  320 A. The path  402  illustrated in  FIG. 5  includes the second CPU  320 B, the electrical transmission section  421 , the electrical reception section  424 , and the second CPU  320 B. 
     In this case, since a failure does not occur in the path  402 , the second CPU  320 B can normally receive the signal from the second CPU  320 B. Accordingly, in the path  402 , it can be newly determined that the electrical reception section  424  is “OK”, excluding the components determined as “OK”. On the other hand, due to the failure of the optical transmission section  412  in the path  401 , the first CPU  320 A is unable to normally receive the signal from the first CPU  320 A. Accordingly, in the path  401 , it can be determined that a failure occurs in any one of the electrical transmission section  411 , the optical transmission section  412 , the optical transmission line  301 , and the optical reception section  423  excluding the components determined as “OK”. 
     Optical Path in Second State of Electrical Loopback in Optical Transmission System according to First Embodiment 
       FIG. 6  is a diagram illustrating an example of an optical path in a second state of the electrical loopback in the optical transmission system according to the first embodiment. In  FIG. 6 , components similar to those illustrated in  FIG. 5  are denoted by the same reference numerals, and description thereof is omitted. After enabling the electrical loopback of the second optical transmission apparatus  300 B as illustrated in  FIG. 5 , the central controller  303  disables the electrical loopback of the second optical transmission apparatus  300 B, and enables the electrical loopback of the first optical transmission apparatus  300 A. Accordingly, the paths  401  and  402  are as illustrated in  FIG. 6 . 
     The path  401  illustrated in  FIG. 6  includes the first CPU  320 A, the electrical transmission section  411 , the electrical reception section  414 , and the first CPU  320 A. The path  402  illustrated in  FIG. 6  includes the second CPU  320 B, the electrical transmission section  421 , the optical transmission section  422 , the optical transmission line  302 , the optical reception section  413 , the electrical reception section  414 , the electrical transmission section  411 , the optical transmission section  412 , the optical transmission line  301 , the optical reception section  423 , the electrical reception section  424 , and the second CPU  320 B. 
     In this case, since a failure does not occur in the path  401 , the first CPU  320 A can normally receive the signal from the first CPU  320 A. Accordingly, in the path  401 , it can be newly determined that the electrical transmission section  411  is “OK”, excluding the components determined as “OK”. 
     On the other hand, due to the failure of the optical transmission section  412  in the path  402 , the second CPU  320 B is unable to normally receive the signal from the second CPU  320 B. Accordingly, in the path  402 , it can be determined that a failure occurs in any one of the optical transmission section  412 , the optical transmission line  301 , and the optical reception section  423  excluding the components determined as “OK”. 
     As illustrated in  FIGS. 5 and 6 , in the electrical loopback, the signal output from each CPU is returned in the electrical module (electrical loopback control circuits  332 A and  332 B). Accordingly, it can be determined that a failure does not occur in the electrical path portion (the electrical transmission section  411  or the electrical reception section  414 ) of the first optical module  330 A, or in the electrical path portion (the electrical transmission section  421  or the electrical reception section  424 ) of the second optical module  330 B. 
     Although a case where a failure occurs in the optical transmission section  412  is described, in contrast, in a case where a failure occurs in the electrical path portion of the first optical module  330 A or the electrical path portion of the second optical module  330 B, it is possible to determine the failure occurrence place at this point. 
     Optical Path in First State of Optical Loopback in Optical Transmission System according to First Embodiment 
       FIG. 7  is a diagram illustrating an example of an optical path in a first state of the optical loopback in the optical transmission system according to the first embodiment. In  FIG. 7 , components similar to those illustrated in  FIG. 6  are denoted by the same reference numerals, and description thereof is omitted. After enabling the electrical loopback of the first optical transmission apparatus  300 A as illustrated in  FIG. 6 , the central controller  303  disables the electrical loopback of the first optical transmission apparatus  300 A, and enables the optical loopback of the second optical transmission apparatus  300 B. Accordingly, the paths  401  and  402  are as illustrated in  FIG. 7 . 
     The path  401  illustrated in  FIG. 7  includes the first CPU  320 A, the electrical transmission section  411 , the optical transmission section  412 , the optical transmission lines  301  and  302 , the optical reception section  413 , the electrical reception section  414 , and the first CPU  320 A. The path  401  is returned by the block  120 B included in the optical transmission section  422  and the optical reception section  423 . Here, it is assumed that a failure does not occur in the return portion of the block  120 B and the optical transmission section  422  and the optical reception section  423  are excluded from the path  401 . The path  402  illustrated in  FIG. 7  includes the second CPU  320 B, the electrical transmission section  421 , the optical transmission section  422 , the optical reception section  423 , the electrical reception section  424 , and the second CPU  320 B. 
     In this case, since a failure does not occur in the path  402 , the second CPU  320 B can normally receive the signal from the second CPU  320 B. Accordingly, in the path  402 , it can be newly determined that the optical reception section  423  is “OK”, excluding the components determined as “OK”. 
     On the other hand, due to the failure of the optical transmission section  412  in the path  401 , the first CPU  320 A is unable to normally receive the signal from the first CPU  320 A. Accordingly, in the path  401 , it can be determined that a failure occurs in any one of the optical transmission section  412  and the optical transmission line  301  excluding the components determined as “OK”. 
     Optical Path in Second State of Optical Loopback in Optical Transmission System according to First Embodiment 
       FIG. 8  is a diagram illustrating an example of an optical path in a second state of the optical loopback in the optical transmission system according to the first embodiment. In  FIG. 8 , components similar to those illustrated in  FIG. 7  are denoted by the same reference numerals, and description thereof is omitted. After enabling the optical loopback of the second optical transmission apparatus  300 B as illustrated in  FIG. 7 , the central controller  303  disables the optical loopback of the second optical transmission apparatus  300 B, and enables the optical loopback of the first optical transmission apparatus  300 A. Accordingly, the paths  401  and  402  are as illustrated in  FIG. 8 . 
     The path  401  illustrated in  FIG. 8  includes the first CPU  320 A, the electrical transmission section  411 , the optical transmission section  412 , the optical reception section  413 , the electrical reception section  414 , and the first CPU  320 A. The path  402  illustrated in  FIG. 8  includes the second CPU  320 B, the electrical transmission section  421 , the optical transmission section  422 , the optical transmission line  302 , the optical transmission line  301 , the optical reception section  423 , the electrical reception section  424 , and the second CPU  320 B. The path  402  is returned by the block  120 A included in the optical reception section  413  and the optical transmission section  412 . Here, it is assumed that a failure does not occur in the return portion of the block  120 A and the optical transmission section  412  and the optical reception section  413  are excluded from the path  402 . 
     In this case, since a failure does not occur in the path  402 , the second CPU  320 B can normally receive the signal from the second CPU  320 B. Accordingly, in the path  402 , it can be newly determined that the optical transmission line  301  is “OK”, excluding the components determined as “OK”. 
     On the other hand, due to the failure of the optical transmission section  412  in the path  401 , the first CPU  320 A is unable to normally receive the signal from the first CPU  320 A. Accordingly, in the path  401 , it can be determined that a failure occurs in the optical transmission section  412  excluding the components determined as “OK”. In this way, it can be determined that a failure occurs in the optical transmission section  412  (the optical transmission section  412  is “NG”). 
     As illustrated in  FIGS. 7 and 8 , in the optical loopback, the signal output from each CPU is returned in the optical module (the first optical component  100 A and the second optical component  100 B). Accordingly, it can be determined that the optical transmission section  412  among the optical transmission section  412 , the optical reception section  413 , and the optical transmission line  301  is a failure occurrence place. 
     Processing by Central Controller of Optical Transmission System according to First Embodiment 
       FIG. 9  is a flowchart illustrating an example of processing by the central controller of the optical transmission system according to the first embodiment. The central controller  303  executes the steps illustrated in  FIG. 9 , for example. First, the central controller  303  determines whether or not an abnormality in the link between the first optical component  100 A and the second optical component  100 B is detected (step S 901 ), and waits until the link abnormality is detected (No loop in S 901 ). 
     In step S 901 , for example, the central controller  303  waits until a signal indicating a link abnormality between the first optical component  100 A and the second optical component  100 B is received from the first CPU  320 A, or the second CPU  320 B, or any combination thereof. The link abnormality includes, for example, an abnormality that occurs at the time of link up when the first optical component  100 A and the second optical component  100 B are activated, and an abnormality that occurs during signal transmission after link up between the first optical component  100 A and the second optical component  100 B. 
     In step S 901 , when the link abnormality is detected (Yes in step S 901 ), the central controller  303  enables the electrical loopback of the second optical module  330 B (step S 902 ). For example, the central controller  303  enables the electrical loopback of the second optical module  330 B by transmitting a signal for instructing the second CPU  320 B to enable the electrical loopback of the electrical loopback control circuit  332 B to the second CPU  320 B. Accordingly, the signals output from the first CPU  320 A and the second CPU  320 B are respectively returned (for example, refer to  FIG. 5 ) in the electrical module of the second optical module  330 B. 
     Next, the central controller  303  acquires the signal detection result from the first CPU  320 A and the second CPU  320 B (each CPU) (step S 903 ). The signal detection result acquired from the first CPU  320 A by the central controller  303  is information indicating whether or not the first CPU  320 A can normally receive the signal which is output from the first CPU  320 A and returned. The signal detection result acquired from the second CPU  320 B by the central controller  303  is information indicating whether or not the second CPU  320 B can normally receive the signal which is output from the second CPU  320 B and returned. 
     Next, the central controller  303  disables the electrical loopback of the second optical module  330 B (step S 904 ). For example, the central controller  303  disables the electrical loopback of the second optical module  330 B by transmitting a signal for instructing the second CPU  320 B to disable the electrical loopback of the electrical loopback control circuit  332 B to the second CPU  320 B. 
     Next, the central controller  303  enables the electrical loopback of the first optical module  330 A (step S 905 ). For example, the central controller  303  enables the electrical loopback of the first optical module  330 A by transmitting a signal for instructing the first CPU  320 A to enable the electrical loopback of the electrical loopback control circuit  332 A to the first CPU  320 A. Accordingly, the signals output from the first CPU  320 A and the second CPU  320 B are respectively returned (for example, refer to  FIG. 6 ) in the electrical module of the first optical module  330 A. 
     Next, the central controller  303  acquires the signal detection result from the first CPU  320 A and the second CPU  320 B (each CPU) (step S 906 ). Next, the central controller  303  disables the electrical loopback of the first optical module  330 A (step S 907 ). For example, the central controller  303  disables the electrical loopback of the first optical module  330 A by transmitting a signal for instructing the first CPU  320 A to disable the electrical loopback of the electrical loopback control circuit  332 A to the first CPU  320 A. 
     Next, the central controller  303  enables the optical loopback of the second optical module  330 B (step S 908 ). For example, the central controller  303  enables the optical loopback of the second optical module  330 B by transmitting a signal for instructing the voltage control circuit  130 B to switch the voltage applied to the electro-optical crystal layers  121 B and  1226  from HIGH to LOW to the voltage control circuit  130 B. Accordingly, the signals output from the first CPU  320 A and the second CPU  320 B are respectively returned (for example, refer to  FIG. 7 ) in the optical module of the second optical module  330 B. 
     Next, the central controller  303  acquires the signal detection result from the first CPU  320 A and the second CPU  320 B (each CPU) (step S 909 ). Next, the central controller  303  disables the optical loopback of the second optical module  330 B (step S 910 ). For example, the central controller  303  disables the optical loopback of the second optical module  330 B by transmitting a signal for instructing the voltage control circuit  1306  to switch the voltage applied to the electro-optical crystal layers  121 B and  122 B from LOW to HIGH to the voltage control circuit  130 B. 
     Next, the central controller  303  enables the optical loopback of the first optical module  330 A (step S 911 ). For example, the central controller  303  enables the optical loopback of the first optical module  330 A by transmitting a signal for instructing the voltage control circuit  130 A to switch the voltage applied to the electro-optical crystal layers  121 A and  122 A from HIGH to LOW to the voltage control circuit  130 A. Accordingly, the signals output from the first CPU  320 A and the second CPU  3206  are respectively returned (for example, refer to  FIG. 8 ) in the optical module of the first optical module  330 A. 
     Next, the central controller  303  acquires the signal detection result from the first CPU  320 A and the second CPU  3206  (each CPU) (step S 912 ). Next, the central controller  303  disables the optical loopback of the first optical module  330 A (step S 913 ). For example, the central controller  303  disables the optical loopback of the second optical module  3306  by transmitting a signal for instructing the voltage control circuit  130 A to switch the voltage applied to the electro-optical crystal layers  121 A and  122 A from LOW to HIGH to the voltage control circuit  130 A. 
     Next, the central controller  303  specifies a failure occurrence place based on the signal detection results acquired in steps S 903 , S 906 , S 909 , and S 912  (step S 914 ). Next, the central controller  303  registers information indicating the failure occurrence place specified in step S 914  in a predetermined log (step S 915 ), and ends a series of processing. The predetermined log is, for example, a log stored in a memory of the central controller  303 . Further, in step S 915 , the central controller  303  may control link down between the first optical component  100 A and the second optical component  100 B. 
     As described above, the optical component  100  according to the first embodiment includes the electro-optical crystal layers  121  and  122  on the transmission path and the reception path. The electro-optical crystal layers  121  and  122  can be switched between a first state where the light on the transmission path and the light on the reception path are respectively transmitted, and a second state where the light from the light emitter is reflected and is incident on the optical receiver and the light from the second optical transmission line is reflected and emitted to the first optical transmission line. 
     Further, switching between the first state and the second state in the electro-optical crystal layers  121  and  122  is performed according to the control signal applied via the control line  131 . Accordingly, the optical loopback can be implemented without using, for example, an optical path switch including a movable portion, and thus it is possible to reduce the size of the optical component in which the optical loopback can be implemented. 
     A second embodiment will be described focusing on the differences from the first embodiment. In the first embodiment, the configuration in which the reflection layer  111  and the electro-optical crystal layers  121  and  122  are respectively provided in the blocks  110  and  120  is described. In contrast, in the second embodiment, a configuration in which the reflection layer and the electro-optical crystal layers are provided in one block will be described. 
     Optical Path during communication in Optical Component according to Second Embodiment 
       FIG. 10  is a diagram illustrating an example of an optical path during communication in an optical component according to the second embodiment. In  FIG. 10 , components similar to those illustrated in  FIG. 1  are denoted by the same reference numerals, and description thereof is omitted. As illustrated in  FIG. 10 , an optical component  100  according to the second embodiment includes, for example, a block  110 , a voltage control circuit  130 , and a control line  131 . 
     In the optical component  100  according to the second embodiment, the electro-optical crystal layer  1001  is further provided in the block  110  in which the reflection layer  111  is provided. The electro-optical crystal layer  1001  is provided at an angle of 45 degrees with respect to one set of adjacent surfaces (the bottom surface and the right surface in  FIG. 10 ) in the cubic block and in a direction perpendicular to the reflection layer  111 . 
     The electro-optical crystal layer  1001  is a half mirror of which the transmittance is switched according to the voltage applied from the voltage control circuit  130  via the control line  131 , similar to the electro-optical crystal layers  121  and  122  via the first and second conductors  125  and  124  illustrated in  FIG. 1 . In addition, similar to the electro-optical crystal layers  121  and  122  illustrated in  FIG. 1 , the electro-optical crystal layer  1001  can be made by using an electro-optical crystal layer such as KTN or lithium niobate. Further, the electro-optical crystal layer  1001  can be formed, for example, by a TSSG method, a LPE method, or the like. 
     The following embodiments use that the electro-optical crystal layers  121  and  122  are made from KTN. 
     In a case where the optical module provided with the optical component  100  performs optical communication with the opposing optical module via the optical component  100 , as illustrated in  FIG. 1 , the voltage applied from the voltage control circuit  130  to the electro-optical crystal layer  1001  is controlled to be HIGH (for example, a voltage larger than 0 V). In this case, the electro-optical crystal layer  1001  has a low refractive index, and is in a state where the incident light is transmitted. 
     In this case, the electro-optical crystal layer  1001  transmits the light on the transmission path  101  that is emitted from the light emitter (Tx) of the optical module provided with the optical component  100  to emit the light to the reflection layer  111 . The reflection layer  111  reflects the light emitted from the electro-optical crystal layer  1001  to emit the light to the optical transmission line. Accordingly, the light transmitted from the optical module provided with the optical component  100  is transmitted to the opposing optical module. 
     Further, the electro-optical crystal layer  1001  transmits the light which is incident from the optical transmission line to emit the light to the reflection layer  111 . The reflection layer  111  reflects the light emitted from the electro-optical crystal layer  1001  to emit the light to the optical receiver (Rx) of the optical module provided with the optical component  100 . Accordingly, the light transmitted from the opposing optical module is received by the optical module provided with the optical component  100 . 
     Optical Path During Optical Loopback in Optical Component According to Second Embodiment 
       FIG. 11  is a diagram illustrating an example of an optical path during optical loopback in the optical component according to the second embodiment. In  FIG. 11 , components similar to those illustrated in  FIG. 10  are denoted by the same reference numerals, and description thereof is omitted. 
     In a case where the optical loopback is formed by using the optical component  100 , for example, as illustrated in  FIG. 11 , the voltage applied from the voltage control circuit  130  to the electro-optical crystal layer  1001  via the first conductor  125  and the second conductor  124  is controlled to be LOW (for example, 0 V). In this case, the electro-optical crystal layer  1001  has a higher refractive index than a lower refractive index. The lower refractive index that is the voltage applied to electro-optical crystal layer  1001  via the first and second conductor  125  and  124  is HIGH and the higher refractive index (LOW) is in a state where the incident light is totally reflected. 
     In other words, the electro-optical crystal layer  1001  reflects the light on the transmission path  101  that is emitted from the light emitter (Tx) of the optical module provided with the optical component  100  at an incident angle of 45 degrees to emit the light to the reflection layer  111 . The light that is emitted from the light emitter (Tx) and is emitted from the electro-optical crystal layer  1001  to the reflection layer  111  is reflected by the reflection layer  111  at an incident angle of 45 degrees, and is emitted to the optical receiver (Rx) of the optical module provided with the optical component  100 . Accordingly, the light transmitted from the optical module provided with the optical component  100  is returned to the optical module provided with the optical component  100 . 
     Further, the electro-optical crystal layer  1001  reflects the light which is incident from the optical transmission line at an incident angle of 45 degrees to emit the light to the reflection layer  111 . The light which is incident from the optical transmission line and is emitted from the electro-optical crystal layer  1001  to the reflection layer  111  is reflected by the reflection layer  111  at an incident angle of 45 degrees, and is emitted to the optical transmission line. Accordingly, the light transmitted from the optical module opposite to the optical module provided with the optical component  100  is returned to the optical module opposite to the optical module provided with the optical component  100 . 
     As illustrated in  FIGS. 10 and 11 , in the optical component  100  according to the second embodiment, the electro-optical crystal layer  1001  that switches the optical path according to the voltage applied from the voltage control circuit  130  is provided in the block  110  including the reflection layer  111 . Accordingly, as the optical component  100 , an optical component smaller than, for example, the optical component  100  illustrated in  FIGS. 1 and 2  can be adopted. 
     For example, in the optical module using the VCSEL as described above, the block  110  that includes the reflection layer  111  for changing the traveling direction of the light is used. In contrast, in the second embodiment, the electro-optical crystal layer  1001  can be provided in the block  110 . Accordingly, even without increasing the size of the optical component  100 , the optical loopback for switching the optical path according to the voltage applied from the voltage control circuit  130  can be implemented. 
     Optical Transmission System to which Optical Component According to Second Embodiment is Applied 
       FIG. 12  is a diagram illustrating an example of an optical transmission system to which the optical component according to the second embodiment is applied. In  FIG. 12 , components similar to those illustrated in  FIG. 3  are denoted by the same reference numerals, and description thereof is omitted. However, to avoid the complicated Figure, the first conductor  125  and the second conductor  124  are omitted from  FIG. 12 . When the optical component  100  illustrated in  FIGS. 10 and 11  is applied to the first optical component  100 A and the second optical component  100 B illustrated in  FIG. 3 , the optical transmission system is configured as illustrated in  FIG. 12 . 
     For example, the first optical component  100 A includes a block  110 A including a reflection layer  111 A and an electro-optical crystal layer  1001 A, instead of the block  110  and the block  120  illustrated in  FIG. 1 . The lens  338 A condenses light that is emitted from the lens  337 A and reflected by the reflection layer  111 A, and emits the light to the optical transmission line  301 . The lens  339 A collimates light emitted from the optical transmission line  302 , and emits the light to the reflection layer  111 A. The voltage control circuit  130 A controls the voltage applied to the electro-optical crystal layer  1001 A via the first conductor  125  and second conductor  124 . 
     As described above, the optical component  100  according to the second embodiment includes the electro-optical crystal layer  1001  on the transmission path and the reception path. The electro-optical crystal layer  1001  can be switched between a first state where the light on the transmission path and the light on the reception path are respectively transmitted, and a second state where the light from the light emitter is reflected and is incident on the optical receiver and where the light from the second optical transmission line is reflected and emitted to the first optical transmission line. 
     In addition, switching between the first state and the second state in the electro-optical crystal layer  1001  is performed according to the control signal applied via the control line  131 . Accordingly, the optical loopback can be implemented without using, for example, an optical path switch including a movable portion, and thus it is possible to reduce the size of the optical component in which the optical loopback can be implemented. 
     The electro-optical crystal layer  1001  is provided in combination with the reflection layer  111  that changes the direction of the light which is perpendicularly emitted from the VCSEL to the direction of the optical transmission line. That is, in the first state, the electro-optical crystal layer  1001  transmits the light from the VCSEL to emit the light to the reflection layer  111 . Also, in the first state, the electro-optical crystal layer  1001  transmits the light which is incident from the second optical transmission line to emit the light to the reflection layer  111 . 
     In addition, in the second state, the electro-optical crystal layer  1001  reflects the light which is incident from the VCSEL to emit the light to the reflection layer  111  before the light reaches the reflection layer  111 , and the light is emitted to the optical receiver. Further, in the second state, the electro-optical crystal layer  1001  reflects the light which is incident from the second optical transmission line to emit the light to the reflection layer  111  before the light reaches the reflection layer  111 , and the light is emitted to the first optical transmission line. 
     Accordingly, it is possible to dispose the reflection layer  111  that changes the direction of the light which is perpendicularly emitted from the VCSEL to the direction of the optical transmission line, and the electro-optical crystal layer  1001  that forms the return path for the optical loopback, in a space-saving manner. Therefore, it is possible to reduce the size of the optical component that is provided on the base using the VCSEL and in which the optical loopback can be implemented. 
     Third Embodiment 
     A third embodiment will be described focusing on the differences from the first and second embodiments. In the first and second embodiments, the configuration in which the VCSEL is used for the optical transmission section is described. In contrast, in the third embodiment, a configuration in which a laser diode (LD) is used instead of the VCSEL for the optical transmission section will be described. 
     Optical Path During Communication in Optical Component According to Third Embodiment 
       FIG. 13  is a diagram illustrating an example of an optical path during communication in an optical component according to the third embodiment. In  FIG. 13 , components similar to those illustrated in  FIG. 1  are denoted by the same reference numerals, and description thereof is omitted. For example, in a case where an LD that emits light parallel to the base is used instead of the VCSEL for the optical transmission section of the optical module provided with the optical component  100 , the reflection layer  111  that changes the direction of the light from the optical transmission section may not be provided. Therefore, as illustrated in  FIG. 13 , for example, the optical component  100  may have a configuration in which the block  110  illustrated in  FIG. 1  is omitted. In a case where the voltage applied to the electro-optical crystal layers  121  and  122  via the first conductor  125  and the second conductor  124  by the voltage control circuit  130  is HIGH, paths of light are as illustrated in  FIG. 13 . 
     That is, the electro-optical crystal layers  121  and  122  transmits the light on the transmission path  101  that is emitted from the light emitter (Tx) of the optical module provided with the optical component  100  to emit the light to the optical transmission line. Accordingly, the light transmitted from the optical module provided with the optical component  100  is transmitted to the opposing optical module. 
     In addition, the electro-optical crystal layers  121  and  122  transmit the light which is incident from the optical transmission line to emit the light to the optical receiver (Rx) of the optical module provided with the optical component  100 . Accordingly, the light transmitted from the opposing optical module is received by the optical module provided with the optical component  100 . 
     Optical Path During Optical Loopback in Optical Component According to Third Embodiment 
       FIG. 14  is a diagram illustrating an example of an optical path during optical loopback in the optical component according to the third embodiment. In  FIG. 14 , components similar to those illustrated in  FIG. 13  are denoted by the same reference numerals, and description thereof is omitted. In a case where the optical loopback is formed by using the optical component  100 , for example, as illustrated in  FIG. 14 , the voltage applied from the voltage control circuit  130  to the electro-optical crystal layers  121  and  122  via the first conductor  125  and the second conductor  124  is controlled to be LOW (for example, 0 V). In this case, the electro-optical crystal layers  121  and  122  have a high refractive index, and are in a state where the incident light is totally reflected. 
     That is, the electro-optical crystal layers  121  and  122  return the light on the transmission path  101  that is emitted from the light emitter (Tx) of the optical module provided with the optical component  100  by respectively reflecting the light at an incident angle of 45 degrees to emit the light to the optical receiver (Rx) of the optical module. Accordingly, the light transmitted from the optical module provided with the optical component  100  is returned to the optical module provided with the optical component  100 . 
     In addition, the electro-optical crystal layers  121  and  122  return the light which is incident from the optical transmission line by respectively reflecting the light at an incident angle of 45 degrees to emit the light to the optical transmission line. Accordingly, the light transmitted from the optical module opposite to the optical module provided with the optical component  100  is returned to the optical module opposite to the optical module provided with the optical component  100 . 
     As described above, according to the optical component  100  of the third embodiment, for example, in a configuration in which an LD that emits light parallel to the base is used, similarly to the first embodiment, it is possible to reduce the size of the optical component in which the optical loopback can be implemented. 
     As described above, according to the optical component, it is possible to reduce the size of the optical component in which the optical loopback can be implemented. 
     For example, in the case of connecting CPUs in a supercomputer or the like by an optical communication path, two opposing optical modules are used. In a case where a transmission abnormality occurs in the optical communication by the two optical modules, from the view point of the maintenance, it is preferable to specify a failure occurrence place among the two optical modules and the optical transmission line. 
     In this regard, for example, a method of specifying a failure occurrence place by using an electrical loopback and an optical loopback is considered. However, when an optical path switch including a movable portion is used to make the optical loopback, the size of the optical component is increased due to the optical path switch. 
     Also, a method of specifying a failure occurrence place by reconnecting each optical module and each optical cable and changing the combination of the optical modules is considered. However, in a supercomputer, for example, there is a case where one optical cable is shared by a plurality of optical modules via a fiber box, or there is a case where the optical path other than the maintenance object is also influenced by reconnecting the cables. 
     In contrast, according to each of the embodiments described above, the electro-optical crystal layer (half mirror) such as KIN is used, and thus the optical path can be changed by the control signal applied to the electro-optical crystal layer. Therefore, it is possible to make the optical loopback without increasing the size of the optical component. Further, it is possible to specify a failure occurrence place without reconnecting the cables. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.