Abstract:
A gain equalizer comprising an optical element having first and second input ports and an output port wherein the first transmission wavelength characteristics between the first input port and the output port and the second transmission wavelength characteristics between the second input port and the output port vary inversely each other relative to wavelengths, first and second input optical fibers to connect to the first and second input ports respectively, an output optical fiber to connect to the output port, and a housing to store the optical element.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to a gain equalizer, and more specifically relates to a gain equalizer for equalizing gain of the optical signal propagating on an optical fiber.  
         BACKGROUND OF THE INVENTION  
         [0002]    In long haul optical fiber transmission systems, especially in optical submarine cable systems, although a variety of optical apparatuses (such as optical amplifiers, gain equalizers and optical filters) are disposed in the middle, the one with the most suitable input/output characteristics for the condition has to be selected when it comes to a connection with an optical fiber cable. Therefore, when it is unable to determine the necessary input/output characteristics in advance, a plurality of optical apparatuses having the estimated input/output characteristics should be prepared, and the appropriate one to connect with the optical fiber cable system is chosen from them at the field.  
           [0003]    Especially, in submarine cable systems, it is necessary to store the optical apparatuses in a pressure-resistant housing which is resistant to the submarine hydraulic pressure. Since it takes a long term to produce and to inspect a pressure-resistant housing, it is preferable that a plurality of optical apparatuses are stored in a single housing and appropriate one is selectable as the need arises. As such gain equalizers to allow a choice of a plurality of input/output characteristics, the gain equalizers with the following two types of configurations are well known. One has a configuration simply to keep a plurality of gain equalizing filters, each being connected to optical input and output fibers which all lead to the outside, and the other one has a configuration in which only one input fiber and one output fiber are disposed, an optical switch is arranged on both input and output sides of a plurality of gain equalizing filters, each having input/output characteristics different from the others, and a controller which is capable of changing the optical switches through a controller which is capable of changing the optical switches through a control light (or mechanical operation) from the outside is installed to choose one of the plurality of the gain equalizing filters. In optical fiber communications, generally two optical fiber lines are used separately for the up and down streams, and thus a plurality of up stream gain equalizing filters and a plurality of down stream equalizing filters are generally kept in a single pressure-resistant housing.  
           [0004]    In the former configuration, the input and output fibers connected with the desired optical apparatus are connected to optical fibers, and the rest of the input and output fibers connected to the unused optical apparatuses are ignored. In this case, it is necessary to prepare in advance a sufficient number of feed-throughs for the optical fibers in the pressure-resistant housing. The number of the feed-throughs is limited because of the size limitation of the feed-through in terms of the size of the pressure-resistant housing and the need for the sufficient quality control. Accordingly, it is impossible to dispose the gain equalizers exceeding the number of the prepared feed-throughs in the single pressure-resistant housing.  
           [0005]    In the latter configuration, because the optical switch is controlled by a control signal such as an optical signal or an electric signal from the outside, it is necessary to have a circuit to detect the control signal, a control circuit to change the optical switch according to the control signal, and also a power feeding system to feed electric power to those circuits.  
         SUMMARY OF THE INVENTION  
         [0006]    It is therefore an object of the present invention to provide a gain equalizer capable of selecting desirable gain (loss) equalizing characteristics using a fewer number of elements.  
           [0007]    Another object of the present invention is to provide a gain equalizer capable of selecting desirable gain (loss) equalizing characteristics using the minimum number of feed-throughs.  
           [0008]    A gain equalizer according to this invention consists of an optical element having first and second input parts and an output port wherein the first transmission wavelength characteristics between the first input port and the output port and the second transmission wavelength characteristics between the second input port and the output port vary inversely each other relative to wavelengths, first and second input optical fibers connecting to the first and second input ports respectively, an output optical fiber connecting to the output port, and a housing to store the optical element.  
           [0009]    With the above configuration, it is possible to select either the first or the second transmission wavelength characteristics (gain wavelength dependency characteristics or loss wavelength dependency characteristics) by inputting an optical signal into the first input optical fiber or the second input optical fiber. Accordingly, it is possible to select one from the two kinds of the gain (loss) wavelength characteristics using a fewer ports. In addition, since it has no electric circuit and optical circuit for switching, a switching control signal and a power feeding system are naturally unnecessary. For example, the transmission wavelength characteristics mean so called gain profile or loss profile.  
           [0010]    The gain equalizer according to the invention further consists of an optical element having an input port and first and second output ports wherein the first transmission wavelength characteristics between the input port and the first output port and the second transmission wavelength characteristics between the input port and the second output port vary inversely each other relative to wavelengths, an input optical fiber to connect to the input port, first and second output optical fibers connect to the first and second output ports respectively, and a housing to store the optical element.  
           [0011]    With the above configuration, it is possible to select either the first or second transmission wavelength characteristics (gain wavelength characteristics or loss wavelength characteristics) depending on from which one of the first and second output optical fibers an optical signal is output. Accordingly, it is possible to select one from the two kinds of the gain (loss) wavelength characteristics using a fewer ports. In addition, since it has no electric circuit and optical circuit for switching, a switching control signal and a power feeding system are naturally unnecessary.  
           [0012]    When the housing is made to be pressure-resistant, this invention is applicable to submarine cable systems.  
           [0013]    For example, the optical element consists of a WDM optical coupler or a dielectric multilayer optical coupler. This makes it easier to obtain desirable gain (loss) characteristics.  
           [0014]    The gain equalizer according to the invention consists of first and second optical elements, each element having first and second input ports and an output port wherein the first transmission wavelength characteristics between the first input port and the output port and the second transmission wavelength characteristics between the second input port and the output port vary inversely each other relative to wavelengths, first and second input optical fibers to connect to the first and second input ports of the first optical element respectively, a first output optical fiber to connect to the output port of the first optical element, third and fourth input optical fibers to connect to the first and second input ports of the second optical element respectively, a second output optical fiber to connect to the output port of the second optical element, and a housing to store the first and second optical elements.  
           [0015]    The gain equalizer according to the invention consists of first and second optical elements, each element having an input port and first and second output ports wherein the first transmission wavelength characteristics between the input port and the first output port and the second transmission wavelength characteristics between the input port and the second output port vary inversely each other relative to wavelengths, a first input optical fiber to connect to the input port of the first optical element, first and second output optical fibers to connect to the first and second output ports of the first optical element respectively, a second input optical fiber to connect to the input port of the second optical element, third and forth output optical fiber to connect to the first and second output port of the second optical element respectively, and a housing to store the first and second optical elements.  
           [0016]    In those configurations, besides the above-described operation effects, when separate optical fiber systems of up and down streams are used it is possible to select the input/output characteristics of each system separately.  
           [0017]    A gain equalizer according to the invention consists of a first optical element having first and second input ports and a first output port wherein the first transmission wavelength characteristics between the first input port and the first output port and the second transmission wavelength characteristics between the second input port and the first output port vary inversely each other relative to wavelengths, a second optical element having a third input port and second and third output ports wherein the third transmission wavelength characteristics between the third input port and the second output port and the fourth transmission wavelength characteristics between the third input port and the third output port vary each other relative to wavelengths, first and second input light transmitting mediums to connect to the first and second input ports respectively, first and second output light transmitting mediums to connect to the second and third output ports respectively, and an optical connector to connect the first output port with the third input port.  
           [0018]    The gain equalizer according to the invention consists of a first optical element having a pair of first and second ports and a pair of third and forth ports wherein the first transmission wavelength characteristics between the through-ports and the second transmission wavelength characteristics between the split-ports vary inversely each other relative to wavelengths, a second optical element having two ports, one of the ports being connected to the fourth port of the first optical element, wherein the transmission wavelength characteristics between the two ports are practically equal to either of the first and second transmission wavelength characteristics, first and second light transmitting mediums to connect to the first and second ports respectively, a third light transmitting medium to connect to the third port, and a fourth light transmitting medium to connect to the other port of the second optical element.  
           [0019]    With the above configurations, it is possible to choose from even greater numbers of the input/output characteristics. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0020]    The above and other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings, in which:  
         [0021]    [0021]FIG. 1 shows a schematic block diagram of a first embodiment according to the invention;  
         [0022]    [0022]FIG. 2 shows an example of input/output characteristics of WDM optical couplers  12  and  14 ;  
         [0023]    [0023]FIG. 3 shows a schematic block diagram of a second embodiment according to the invention;  
         [0024]    [0024]FIG. 4 shows a first example of a configuration to connect two WDM optical couplers in serial;  
         [0025]    [0025]FIG. 5 shows a second example of a configuration to connect two WDM optical couplers in serial;  
         [0026]    [0026]FIG. 6 shows a third example of a configuration to connect two WDM optical couplers in serial; and  
         [0027]    [0027]FIG. 7 shows a fourth example of a configuration to connect two WDM optical couplers in serial. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]    Embodiments of the invention are explained below in detail with reference to the drawings.  
         [0029]    [0029]FIG. 1 shows a schematic block diagram of a first embodiment according to the invention. Two WDM optical couplers (e.g. WDM fiber couplers)  12 ,  14  having two terminal pairs are disposed in a pressure-resistant housing  10 . Input optical fibers  16  and  18  which extend to the outside of the pressure-resistant housing  10  connect to input ports X 0  and X 1  of the WDM optical coupler  12  respectively. An output optical fiber  20  which extends to the outside of the pressure-resistant housing  10  connects to an output port Y 0  of the WDM optical coupler  12 , and an output port Y 1  of the WDM optical coupler  12  is terminated by a nonreflecting element (or a absorbing element)  22 . Similarly, input optical fibers  24 ,  26  which extend to the outside of the pressure-resistant housing  10  connect to input ports X 0 , X 1  of the WDM optical coupler  14 . An output optical fiber  28  which extends to the outside of the pressure-resistant housing  10  connects to an output port Y 0  of the WDM optical coupler  14 , and an output port Y 1  of the WDM optical coupler  14  is terminated by a nonreflecting element (or an absorbing element)  30 .  
         [0030]    The nonreflecting elements  22 ,  30  are disposed to prevent unnecessary reflected light. If such reflected light does not exist, those nonreflecting elements  22 ,  30  can be omitted.  
         [0031]    [0031]FIG. 2 shows the input/output characteristics, i.e. the through-port characteristics, between the port X 0  and the port Y 0  (and between the port X 1  and the port Y 1 ) of the WDM optical couplers  12 ,  14 , and the input/output characteristics, i.e. the split-port characteristics between the port X 0  and the port Y 1  (and between the port X 1  and the port Y 0 ) of the WDM optical couplers  12 ,  14 . The horizontal axis expresses wavelength, and the vertical axis expresses transmission factor, respectively. A characteristic curve  32  of a solid line shows through-port characteristics, a characteristic curve  34  of a broken line shows split-port characteristics. As understandable from the characteristic curves  32 ,  34 , the split-port characteristics are reciprocals of the through-port characteristics. When the WDM optical couplers  12 ,  14  are used in their original use, they combine/split wavelengths λ a  and λ b . Since a dielectric multilayer optical coupler also has the identical characteristics, it is possible to use the dielectric multilayer couplers instead of the WDM optical couplers  12 ,  14 . For example, the wavelengths λ a  and λ b  are set so that the signal wavelength band can locate between the wavelengths λ a  and λ b.    
         [0032]    In the embodiment shown in FIG. 1, the wavelength-to-loss (gain) characteristics of the WDM optical coupler  12  are different according to which one of the input optical fibers  16  and  18  is selected. That is, when the input optical fiber  16  is selected, the input/output characteristics of the WDM optical coupler  12  indicate the through-port characteristics shown as the characteristic curve  32  in FIG. 2. On the other hand, when the input optical fiber  18  is selected, the input/output characteristics of the WDM optical coupler  12  indicate the split-port characteristics shown as the characteristic curve  34  in FIG. 2. The relations between the input optical fibers  24 ,  26  and the WDM optical coupler  14  are also identical.  
         [0033]    In the embodiment shown in FIG. 1, although two input optical fibers  16 ,  18  ( 24 ,  26 ) and one output optical fiber  20  ( 28 ) per one line are extended to the outside of the pressure-resistant housing  10 , it is also applicable to reverse the number. FIG. 3 shows a schematic block diagram of such a modified embodiment.  
         [0034]    In the modified embodiment shown in FIG. 3, two WDM optical couplers  42 ,  44  having two terminal pairs are disposed in a pressure-resistant housing  40 . An input optical fiber  46  which extends to the outside of the pressure-resistant housing  40  connects to an input port X 0  of the WDM optical coupler  42 . The other input port X 1  of the WDM optical coupler  42  is terminated by a nonreflecting element (or an absorbing element)  48 . Output optical fibers  50 ,  52  which extend to the outside of the pressure-resistant housing  40  connect to output ports Y 0 , Y 1  of the WDM optical coupler  42 . Similarly, An input optical fiber  54  which extends to the outside of the pressure-resistant housing  40  connects to the input port X 0  of the WDM optical coupler  54 . The input port X 1  of the WDM optical coupler  44  is terminated by a nonreflecting element (or an absorbing element)  56 . Output optical fibers  58 ,  60  which extend to the outside of the pressure-resistant housing  40  connect the output ports Y 0 , Y 1  of the WDM optical coupler  44 .  
         [0035]    In the embodiment shown in FIG. 3, the wavelength-to-loss (gain) characteristics of the WDM optical coupler  42  are different according to which one of the output optical fibers  50 ,  52  is selected. That is, when the output optical fiber  50  is selected, the input/output characteristics of the WDM optical coupler  42  indicate the through-port characteristics shown as the characteristic curve  32  in FIG. 2, and when the output optical fiber  52  is selected, the input/output characteristics of the WDM optical coupler  42  indicate the split-port characteristics shown as the characteristic curve  34  in FIG. 2. The relations between the WDM optical coupler  44  and the output optical fibers  58 ,  60  are practically identical.  
         [0036]    When two WDM optical couplers or dielectric multilayer optical couplers having the characteristics shown in FIG. 2 are connected in serial, it is possible to select one from more than two wavelength-to-gain (loss) characteristics. FIGS. 4 through 7 show connection examples of the above connecting configuration. In the below, the through-port characteristics are evaluated as +1, the split-port characteristics are evaluated as −1, and the practically flat characteristics relative to the wavelength are evaluated as 0. The connection examples shown in FIGS. 4 through 7 are prepared for every line and inserted into a pressure-resistant housing.  
         [0037]    In the connection example shown in FIG. 4, an input port X 0  of a WDM optical coupler  72  connects to an output port Y 0  of a WDM optical coupler  70 . The WDM optical couplers  70  and  72  both have the input/output characteristics shown in FIG. 2. Input optical fibers  74 ,  76  connect to the input ports X 0 , X 1  of the WDM optical coupler  70  respectively, and output optical fibers  78 ,  80  connect to the output ports Y 0 , Y 1  of the WDM optical coupler  72  respectively.  
         [0038]    The WDM optical couplers  70 ,  72  are stored in a pressure-resistant housing  82 , and the input optical fibers  74 ,  76  and the output optical fibers  78 ,  80  are extended to the outside of the pressure-resistant housing  82 . When a plurality of lines are contained in a single cable, gain equalizers with the similar configuration are disposed in the housing  82  for the remaining one or more lines.  
         [0039]    When an optical signal inputs to the input optical fiber  74  and outputs from the output optical fiber  78 , the wavelength-to-gain (loss) characteristics are evaluated as +2 since they are obtained through multiplying the through-port characteristics of the WDM optical coupler  70  by the through-port characteristics of the WDM optical coupler  72 . When the optical signal inputs to the input optical fiber  74  and outputs from the output optical fiber  80 , the wavelength-to-gain (loss) characteristics are evaluated as 0 since they are obtained through multiplying the through-port characteristics of the WDM optical coupler  70  by the sprit-port characteristics of the WDM optical coupler  72  and indicate practically flat characteristics relative to wavelengths.  
         [0040]    When the optical signal inputs to the input optical fiber  76  and outputs from the output optical fiber  78 , the wavelength-to-gain (loss) characteristics are evaluated as 0 since they are obtained through multiplying the split-port characteristics of the WDM optical coupler  70  by the through-port characteristics of the WDM optical coupler  72  and indicate practically flat characteristics relative to wavelengths. When the optical signal inputs to the input optical fiber  76  and outputs from the output optical fiber  80 , the wavelength-to-gain (loss) characteristics are evaluated as −2, since they can be obtained through multiplying the split-port characteristics of the WDM optical coupler  70  by the split-port characteristics of the WDM optical coupler  72 .  
         [0041]    The through-port characteristics and split-port characteristics are inclined in the opposite direction with the wavelengths, and thus in the connection example shown in FIG. 4, it is possible to select one from the three characteristics of +2 (the doubly intensified through-port characteristics), −2 (the doubly intensified split-port characteristics), and 0 (the practically flat characteristics relative to the wavelengths). When there is a significant difference between the characteristics of the WDM optical couplers  70  and  72 , the final inclination of the input/output characteristics relative to the wavelengths differs depending on whether the optical signal receives the through-port characteristics at the WDM optical coupler  70  while it receives the split-port characteristics at the WDM optical coupler  72  or it is the other way around. That is, 0 can be substantially distinguished as −0 and +0. In this case, it is possible to select one from four input/output characteristics.  
         [0042]    In the connection example shown in FIG. 5, an input port X 0  of a WDM optical coupler  92  connects to an output port Y 1  of a WDM optical coupler  90 . The WDM optical couplers  90 ,  92  both have the input/output characteristics shown in FIG. 2. Input optical fibers  94 ,  96  connect to the input ports X 0 , X 1  of the WDM optical coupler  90  respectively, and output optical fibers  98 ,  100  connect to the output ports Y 0 , Y 1  of the WDM optical coupler  92  respectively.  
         [0043]    The WDM optical couplers  90 ,  92  are disposed in a pressure-resistant housing  102 , and the input optical fibers  94 ,  96  and the output optical fibers  98 ,  100  are extended to the outside of the pressure-resistant housing  102 . If a plurality of lines are contained in a single cable, gain equalizers with the similar configuration should be disposed in the housing  102  for the remaining one or more lines.  
         [0044]    When the optical signal inputs to the input optical fiber  94  and outputs from the output optical fiber  98 , the wavelength-to-gain (loss) characteristics are evaluated as 0 since they are obtained through multiplying the split-port characteristics of the WDM optical coupler  90  by the through-port characteristics of the WDM optical coupler  92 . When the optical light inputs to the input optical fiber  94  and outputs from the output optical fiber  100 , the wavelength-to-gain (loss) characteristics are evaluated as −2 since they are obtained through multiplying the split-port characteristics of the WDM optical coupler  90  by the split-port characteristics of the WDM optical coupler  92 .  
         [0045]    When the optical signal inputs to the input optical fiber  96  and outputs from the output optical fiber  98 , the wavelength-to-gain (loss) characteristics are evaluated as +2 since they are obtained through multiplying the through-port characteristics of the WDM optical coupler  90  by the through-port characteristics of the WDM optical coupler  92 . When the optical signal inputs to the input optical fiber  96  and outputs from the output optical fiber  100 , the wavelength-to-gain (loss) characteristics are evaluated as 0 since they are obtained through multiplying the through-port characteristics of the WDM optical coupler  90  by the split-port characteristics of the WDM optical coupler  92 .  
         [0046]    As described above, in the connection example shown in FIG. 5, it is also possible to select one from the three characteristics of +2 (the doubly intensified through-port characteristics), −2 (the doubly intensified split-port characteristics), and 0 (the practically flat characteristics relative to the wavelength). Similarly to the configuration shown in FIG. 4, when there is a significant difference between the characteristics of the WDM optical couplers  90  and  92 , it is possible to distinguish the 0 as −0 and +0. Accordingly, in such a case, it is possible to select one from four input/output characteristics.  
         [0047]    In the connection example shown in FIG. 6, an input port X 0  of a WDM optical coupler  112  connects to an output port Y 1  of a WDM optical coupler  110 . The WDM optical couplers  110  and  112  both have the input/output characteristics shown in FIG. 2. Input optical fibers  114 ,  116  connect to input ports X 0 , X 1  of the WDM optical coupler  110  respectively, and an output optical fiber  118  connects to an output port Y 0  of the WDM optical coupler  110 . An output optical fiber  120  connects to an output port Y 1  of the WDM optical coupler  112 .  
         [0048]    The WDM optical couplers  110  and  112  are disposed in a pressure-resistant housing  122 , and the input optical fibers  114 ,  116  and the output optical fibers  118 ,  120  are extended to the outside of the pressure-resistant housing  122 . When a plurality of lines are contained in a single cable, gain equalizers with the same configuration are inserted in the housing  122  for the remaining one or more lines.  
         [0049]    When an optical signal inputs to the input optical fiber  114  and outputs from the output optical fiber  118 , the wavelength-to-gain (loss) characteristics are evaluated +1 since they become the through-port characteristics of the WDM optical coupler  110 . When the optical signal inputs the input optical fiber  114  and outputs from the output optical fiber  120 , the wavelength-to-gain (loss) characteristics is evaluated as −2 since they are obtained through multiplying the split-port characteristics of the WDM optical coupler  110  by the split-port characteristics of the WDM optical coupler  118 .  
         [0050]    When the optical signal inputs to the input optical fiber  116  and outputs from the output optical fiber  118 , the wavelength-to-gain (loss) characteristics is evaluated as −1 since they become the split-port characteristics of the WDM optical coupler  110 . When the optical signal inputs to the input optical fiber  116  and outputs from the output optical fiber  120 , the wavelength-to-gain (loss) characteristics are evaluated as 0 since they are obtained through multiplying the through-port characteristics of the WDM optical coupler  110  by the split-port characteristics of the WDM optical coupler  112 .  
         [0051]    In the connection example shown in FIG. 6, it is possible to select one from four characteristics of +1 (the through-port characteristics at a time), −2 (the doubly intensified split-port characteristics), −1 (the split-port characteristics at a time), and 0 (the practically flat characteristics relative to the wavelength).  
         [0052]    In the connection example shown in FIG. 7, an input port X 0  of a WDM optical coupler  132  connects to an output port Y 1  of a WDM optical coupler  130 . The WDM optical couplers  130  and  132  both have the input/output characteristics shown in FIG. 2. Input optical fibers  134 ,  136  connect to input ports X 0 , X 1  of the WDM optical coupler  130  respectively, and an output optical fiber  138  connects to an output port Y 0  of the WDM optical coupler  130 . An output optical fiber  140  connects to an output port Y 0  of the WDM optical coupler  132 .  
         [0053]    The WDM optical couplers  130  and  132  are disposed in a pressure-resistant housing  142 , and the input optical fibers  134 ,  136  and the output optical fibers  138 ,  140  are extended to the outside of the pressure-resistant housing  142 . When a plurality of lines are contained in a single cable, gain equalizers with the same configuration are inserted in the housing  142  for the remaining one or more lines.  
         [0054]    When an optical signal inputs to the input optical fiber  134  and outputs form the output optical fiber  138 , the wavelength-to-gain (loss) characteristics are evaluated as +1 since they become the through-port of the WDM optical coupler  130 . When the optical signal inputs to the input optical fiber  134  and outputs from the output optical fiber  140 , the wavelength-to-gain (loss) characteristics are evaluated as 0 since they are obtained through multiplying the split-port characteristics of the WDM optical coupler  130  by the through-port characteristics of the WDM optical coupler  118  and become practically flat relative to the wavelengths.  
         [0055]    When the optical signal inputs to the input optical fiber  136  and outputs from the output optical fiber  138 , the wavelength-to-gain (loss) characteristics are evaluated as −1 since they become the split-port characteristics of the WDM optical coupler  130 . When the optical signal inputs to the input optical fiber  136  and outputs from the output optical fiber  140 , the wavelength-to-gain (loss) characteristics are evaluated as +2 since they are obtained through multiplying the through-port characteristics of the WDM optical coupler  130  by the through-port characteristics of the WDM optical coupler  132 .  
         [0056]    In the connection example shown in FIG. 7, it is possible to select one from four characteristics of +1 (the through-port characteristics at a time), 0 (the practically flat characteristics relative to the wavelength), −1 (the split-port characteristics at a time), and +2 (the doubly intensified through-port characteristics).  
         [0057]    In the connection examples shown in FIGS. 4 through 7, it is possible to replace the input and the output since the reciprocity theorem is formed.  
         [0058]    As readily understandable from the aforementioned explanation, according to the invention, it is possible to select one from a plurality of gain equalizing characteristics with a fewer elements. In addition, since it is possible to have many options compared to the small number of input ports and output ports, the number of signal ports to be prepared at the outside can be relatively reduced. A control circuit for switching optical function becomes unnecessary, and therefore the configuration can be simplified and a power feeding system is also unnecessary.  
         [0059]    While the invention has been described with reference to the specific embodiment, it will be apparent to those skilled in the art that various changes and modifications can be made to the specific embodiment without departing from the spirit and scope of the invention as defined in the claims.