Abstract:
An optical switch includes an input port into which a multiplexed optical signal is input; a dispersing unit that, according to wavelength, disperses the multiplexed optical signal into a plurality of optical signals that are each dispersed in a unique direction; a converging unit that converges the dispersed optical signals; plural mirrors that are arrayed forming a single row in a plane and reflecting the converged optical signals, respectively; and plural output ports through which the reflected optical signals are output. Each of the mirrors has a concave reflective surface that is in the plane and of a predetermined curvature about an axis parallel to the plane.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2007-085453, filed on Mar. 28, 2007, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a technology of an optical switch. 
         [0004]    2. Description of the Related Art 
         [0005]    Conventionally, a primary object of wavelength division multiplexing (WDM) in an optical transmission system is to achieve higher transmission capacity by increasing the number of channels. Recently, higher added value and lower operational costs of optical transmission systems have been expected from increased service menus using wavelength differences, and flexible band utilization. 
         [0006]    However, conventional methods involving optical-to-electronic conversion of an optical signal followed by electronic switching and electronic-to-optical conversion, do not effect higher transmission capacity and lower operational costs. Hence, there is a need for a device that includes plural optical input and output ports, and that selectively manipulates an input multiplexed optical signal without electronic conversion. Included among such devices is an optical switch that, according to wavelength, selectively outputs optical signals included in the multiplexed optical-signal to different output ports respectively corresponding to the wavelengths, (see, for example, Marom, Dan M., et. al. “Effect of Mirror Curvature in MEMS Micro-mirror Based wavelength-selective Switches”). 
         [0007]      FIG. 17  is a schematic of a conventional optical switch. A conventional optical switch  1700  includes a port unit  1710 , a collimating lens unit  1720 , a lens system  1730 , a dispersing element  1740 , a converging lens  1750 , and a mirror unit  1760 . 
         [0008]    The port unit  1710  includes plural ports  1711  to  1715 . The port  1711  is an input port through which an optical signal is input to the optical switch  1700 . The optical signal exits the port  1711  diverging and is output to a collimating lens  1721  included in the collimating lens unit  1720 . The input optical signal is a multiplexed optical signal including plural wavelengths corresponding to different channels. 
         [0009]    The collimating lens unit  1720  includes plural collimating lenses  1721  to  1725 . The collimating lens  1721  collimates the diverged optical signal output from the port  1711  and outputs the collimated optical signal to the lens system  1730 . The port unit  1710  and the collimating lens unit  1720  are arranged along the Z-axis. 
         [0010]    The lens system  1730  spatially separates channels, i.e., wavelengths, of the collimated optical signal output from the collimating lens  1721  and outputs the resulting optical signal to the dispersing element  1740 . The lens system  1730  includes a concave lens  1731  and a convex lens  1732 . The concave lens  1731  diverges and outputs, to the convex lens  1732 , the collimated optical signal output from the collimating lens  1721 . The convex lens  1732  collimates and outputs, to the dispersing element  1740 , the diverged optical signal output from the concave lens  1731 . 
         [0011]    The dispersing element  1740  angularly disperses the optical signal, output from the lens system  1730 , about the X-axis in different directions corresponding to wavelength while outputting the optical signals in the direction of the Y-axis to the converging lens  1750 . The dispersing element  1740  includes a transmissive diffraction grating. 
         [0012]    The converging lens (converging optical system)  1750  converges, onto corresponding mirrors included in the mirror unit  1760  and according to wavelength, the optical signals output from the dispersing element  1740 . Also, according to wavelength, the optical signals pass through different positions on the converging lens  1750 . 
         [0013]    The mirror unit  1760  includes plural mirrors arrayed along the X-axis. The mirrors correspond to the different wavelengths, respectively. For example, mirrors  1761 ,  1762  and  1763  correspond to wavelengths λ 1 , λ 20 , and λ 40 , respectively. 
         [0014]    Each of the mirrors  1761  to  1763  respectively reflects an optical signal having a wavelength corresponding thereto. The reflected optical signals are among the converged optical signals output from the converging lens  1750  and reflected toward the converging lens  1750 . The mirror unit  1760  includes a control unit that controls the reflection angle of each of the mirrors by rotating each mirror about the X-axis (a first rotation axis). 
         [0015]    The reflected optical signals pass through the converging lens  1750 , the dispersing element  1740 , and the lens system  1730  to be output to the collimating lens unit  1720 . The collimating lenses  1722  to  1725  correspond to the reflection angles of the mirrors  1761  to  1763  of the mirror unit  1760 , and are arrayed along the Z-axis. The collimating lenses  1722  to  1725  output the received optical signals to ports  1712  to  1715  included in the port unit  1710 . 
         [0016]    The ports  1712  to  1715  are output ports corresponding respectively to the collimating lenses  1722  to  1725 . The ports  1712  to  1715  receive the collimated optical signals output from the collimating lenses  1722  to  1725  and output the optical signals from the optical switch  1700 . 
         [0017]    According to the above configuration, the control unit changes the reflection angle of the mirrors according to the wavelength corresponding thereto. Thus, according to each optical wavelength included in the input optical signal, the optical switch  1700  selects, from among the ports  1712  to  1715 , a port to output the optical signal. 
         [0018]    Furthermore, the optical switch  1700  slightly changes the reflection angles of the mirrors from the optimal angles for coupling the reflected optical signals to the ports  1712  to  1715  to decrease the coupling rate. Thus, the optical switch  1700  attenuates, by an arbitrary level, the optical signals output from the ports  1712  to  1715  by rotating the mirrors about the X-axis. 
         [0019]    However, the optical switch  1700  above has a problem in that since optical signals of different wavelengths respectively pass through different positions on the converging lens  1750 , an optical signal passing through a position away from the center of the converging lens  1750  is likely to be affected by aberration of the converging lens  1750 . In particular, when many wavelengths are used in WDM transmission, the effective area of the converging lens  1750  becomes larger, increasing the effect of the aberration of the converging lens  1750 . 
         [0020]      FIG. 18A  is a graph indicating a transmission band characteristic of the conventional optical switch.  FIG. 18B  is a graph indicating a short-wavelength-side transmission band characteristic of the conventional optical switch.  FIG. 18C  is a graph indicating a long-wavelength-side transmission band characteristic of the conventional optical switch. The horizontal axis indicates a wavelength (channel), and the vertical axis indicates a transmissivity (decibel (dB)). 
         [0021]    A reference numeral  1801  indicates a range of wavelengths used in the WDM transmission. A reference numeral  1802  indicates a transmission band characteristic of the optical switch  1700  when an attenuation level is set to 0 dB by decreasing the coupling rate of the port unit  1710 . A reference numeral  1803  indicates the transmission band characteristic when the attenuation level is set to 20 dB. An optical signal corresponding to a channel  20  passes a point closest to the center of the converging lens  1750 . 
         [0022]    As shown in the  FIGS. 18A to 18C , the transmissivity becomes highest at the channel  20 . The farther away a channel is from the channel  20 , the lower the transmissivity becomes due to the effect of aberration, causing a slope of the transmission band characteristic for each channel. The slope becomes steeper when the attenuation level is 20 dB than when the attenuation level is 0 dB. 
         [0023]    The slope reduces the communication band. In particular, when the output optical signal is attenuated by decreasing the coupling rate of the port unit  1710 , the slope becomes steeper, causing a large reduction of the communication band. Furthermore, the use of plural compensating lenses to correct the aberration of the converging lens  1750  means increased components, resulting in reduced long-term reliability and increased element costs. 
       SUMMARY OF THE INVENTION 
       [0024]    It is an object of the present invention to at least solve the above problems in the conventional technologies. 
         [0025]    An optical switch according to one aspect of the present invention includes a dispersing unit that disperses an input multiplexed optical signal into a plurality of optical signals according to wavelength, and a plurality of mirrors that reflect and selectively output the optical signals to a plurality of output ports, respectively. Each of the mirrors has a concave reflective surface having a predetermined curvature about an axis parallel to a plane in which the concave reflective surfaces are arrayed. 
         [0026]    An optical switch according to another aspect of the present invention includes an input port to which a multiplexed optical signal is input; a dispersing unit that, according to wavelength, disperses the multiplexed optical signal into a plurality of optical signals that are each dispersed in a unique direction; a converging unit that converges the dispersed optical signals; a plurality of mirrors arrayed forming a single row in a plane and reflecting the converged optical signals, respectively; and a plurality of output ports through which the reflected optical signals are output. Each of the mirrors has a concave reflective surface that is in the plane and of a predetermined curvature about a first axis parallel to the plane. 
         [0027]    A method according to still another aspect of the present invention is a method of adjusting an optical switch that includes a dispersing unit that disperses an input multiplexed optical signal into a plurality of optical signals according to wavelength, and a plurality of mirrors that reflect and selectively output the optical signals to a plurality of output ports, respectively, in which each of the mirrors has a concave reflective surface having a predetermined curvature about an axis parallel to a plane in which the concave reflective surfaces are arrayed. The method includes first-adjusting the predetermined curvature and a first reflection angle of each of the mirrors about the axis; and second-adjusting a second reflection angle of each of the mirrors for switching between the output ports. 
         [0028]    The other objects, features, and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]      FIG. 1  is a schematic of a mirror unit of an optical switch according to an embodiment; 
           [0030]      FIG. 2A  is a first schematic for explaining a principle of the optical switch; 
           [0031]      FIG. 2B  is a graph indicating a transmission band characteristic of the optical switch shown in  FIG. 2A ; 
           [0032]      FIG. 3A  is a second schematic for explaining the principle; 
           [0033]      FIG. 3B  is a graph indicating a transmission band characteristic of the optical switch shown in  FIG. 3A ; 
           [0034]      FIG. 4A  is a third schematic for explaining the principle; 
           [0035]      FIG. 4B  is a graph indicating a transmission band characteristic of the optical switch shown in  FIG. 4A ; 
           [0036]      FIG. 5  is a graph indicating a transmission band characteristic of a mirror for each rotation angle; 
           [0037]      FIG. 6  is a schematic illustrating an adjusting procedure  1  of the optical switch; 
           [0038]      FIG. 7  is a first schematic illustrating an adjusting procedure  2  of the optical switch; 
           [0039]      FIG. 8  is a second schematic illustrating the adjusting procedure  2  of the optical switch; 
           [0040]      FIG. 9A  is a schematic illustrating a curvature setting of a mirror of the optical switch when the curvature is zero; 
           [0041]      FIG. 9B  is a graph indicating a transmission band characteristic of the optical switch shown in  FIG. 9A ; 
           [0042]      FIG. 10A  is a schematic illustrating the curvature setting when a curvature radius is large; 
           [0043]      FIG. 10B  is a graph indicating a transmission band characteristic of the optical switch shown in  FIG. 10A ; 
           [0044]      FIG. 11A  is a schematic illustrating the curvature setting when the curvature radius is small; 
           [0045]      FIG. 11B  is a graph indicating a transmission band characteristic of the optical switch shown in  FIG. 11A ; 
           [0046]      FIG. 12A  is a schematic illustrating the curvature setting of the mirror for each channel when the curvature radius is large; 
           [0047]      FIG. 12B  is a graph indicating a transmission band characteristic of the optical switch shown in  FIG. 12A ; 
           [0048]      FIG. 13A  is a schematic illustrating the curvature setting of the mirror for each channel when the curvature radius is small; 
           [0049]      FIG. 13B  is a graph indicating a transmission band characteristic of the optical switch shown in  FIG. 13A ; 
           [0050]      FIG. 14  is a first schematic illustrating effect of variation of a mirror curvature about a Z-axis on the transmission band characteristic; 
           [0051]      FIG. 15  is a second schematic illustrating effect of variation of a mirror curvature about the Z-axis on the transmission band characteristic; 
           [0052]      FIG. 16  is a graph indicating a result of a compensation of the transmission band characteristic performed by the optical switch; 
           [0053]      FIG. 17  is a schematic of a conventional optical switch; 
           [0054]      FIG. 18A  is a graph indicating a transmission band characteristic of the conventional optical switch; 
           [0055]      FIG. 18B  is a graph indicating a shorter-wavelength-side transmission band characteristic of the conventional optical switch; and 
           [0056]      FIG. 18C  is a graph indicating a longer-wavelength-side transmission band characteristic of the conventional optical switch. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0057]    Referring to the accompanying drawings, exemplary embodiments according to the present invention are explained in detail below. 
         [0058]      FIG. 1  is a schematic of a mirror unit of an optical switch according to an embodiment. Since configuration of the optical switch according to the embodiment is the same as that of the conventional optical switch  1700  except for a mirror unit, only the mirror unit is shown and other parts are omitted. The same reference numbers identify identical elements. An optical switch  100  according to the embodiment includes a mirror unit  110  instead of the mirror unit  1760  of the optical switch  1700 . 
         [0059]    The mirror unit  110  includes plural mirrors arrayed and housed in a case (not shown). The case includes a driving unit (not shown) that moves the mirrors, and a control unit (not shown) that controls the driving unit. The mirrors are arrayed along the X-axis and respectively correspond to the wavelengths of the multiplexed optical signal. 
         [0060]    For example, mirrors  111 ,  112 ,  113 , and  114  respectively correspond to wavelengths λ 1  (channel  1 ), λ 20  (channel  2 ), λ 30  (channel  30 ), and λ 40  (channel  40 ). Each reflective surface of the mirrors is concave and has a predetermined curvature about an axis parallel to the plane in which the reflecting surface lies (a Z-axis). 
         [0061]    The control unit controls the driving unit to rotate the each mirror about the X-axis to switch the output port to which a received optical signal is reflected, and about the Z-axis to change the reflection angle in the angular dispersion direction (the X-axis direction). 
         [0062]    The reflection angle, in the X-axis direction, of each mirror varies depending on the position on the converging lens  1750  through which a corresponding optical signal passes. For example, the reflection angle becomes larger as the distance between the position and the center of the converging lens  1750  becomes larger. 
         [0063]      FIG. 2A  is a first schematic for explaining a principle of the optical switch  100 .  FIG. 2B  is a graph indicating a transmission band characteristic of the optical switch  100  shown in  FIG. 2A . Only one of the channels (channel  20  herein) is shown in  FIGS. 2A and 2B  (and also in  FIGS. 3A to 4B  and  9 A to  11 B). 
         [0064]    It is assumed that the converging lens  1750  shown in  FIG. 2A  (also in  FIGS. 3A ,  4 A,  9 A,  10 A,  11 A,  12 A, and  13 A) is an ideal lens having no aberration and that the reflected optical signal is output to the port  1715 . A reference numeral  206  shown in  FIG. 2B  (also in  FIGS. 3B and 4B ) indicates a range of wavelengths of the channel  20 . 
         [0065]    A reference numeral  204  shown in  FIG. 2B  indicates a transmission band characteristic when the reflective surface of a mirror  112  has no curvature and the mirror  112  is rotated about the Z-axis. A reference numeral  205  indicates a transmission band characteristic when the reflective surface is flat and the mirror  112  is not rotated about the Z-axis. 
         [0066]      FIG. 2A  shows a configuration of the optical switch  100  when the reflective surface has no curvature and the mirror  112  is rotated about the Z-axis. In this case, an optical signal  201  having a short wavelength, an optical signal  202  having a center wavelength, and an optical signal  203  having a long wavelength are reflected in parallel with one another and converged to one point on the dispersing unit  1740  by the converging lens  1750 . 
         [0067]    As a result, the optical signals  201 ,  202 , and  203  are input to the port  1715  by the collimating lens  1725  at an identical angle. In this case as shown in  FIG. 2B , a transmissivity of each wavelength included in the channel  20  is equally decreased when comparing the transmission band characteristics  204  and  205 . 
         [0068]      FIG. 3A  is a second schematic for explaining the principle.  FIG. 3B  is a graph indicating a transmission band characteristic of the optical switch  100  shown in  FIG. 3A .  FIG. 3A  shows a configuration of the optical switch  100  when the reflective surface of the mirror  112  is curved and the mirror  112  is rotated about the Z-axis. 
         [0069]    In this case, the optical signals  201 ,  202 , and  203  are reflected by the mirror  112  at respectively different angles. After being reflected by the mirror  112 , the optical signal  201  is converged, by the converging lens  1750 , to a point away from the center of the dispersing unit  1740 . 
         [0070]    After being reflected by the mirror  112 , the optical signal  203  is converged, by the converging lens  1750 , to a point close to the center of the dispersing unit  1740 . As a result, the optical signal  201  is input to the port  1715  at a large angle by the collimating lens  1725 . On the other hand, the optical signal  203  is input to the port  1715  at a small angle by the collimating lens  1725 . 
         [0071]    In this case as shown in  FIG. 3B , a transmissivity of the optical signal  201  greatly decreases in the transmission band characteristic  204  compared with the transmission band characteristic  205 . On the other hand, a transmissivity of the optical signal  203  decreases less than that of the optical signal  201 . As a result, the transmission band characteristic  205  can be compensated using the compensation level of the transmission band characteristic  204  in which the transmissivity increases as the wavelength of the channel  20  becomes longer. 
         [0072]      FIG. 4A  is a third schematic for explaining the principle.  FIG. 4B  is a graph indicating a transmission band characteristic of the optical switch  100  shown in  FIG. 4A .  FIG. 4A  indicates a configuration of the optical switch  100  when the reflective surface of the mirror  112  is curved and the mirror  112  is rotated about the Z-axis in a direction opposite to that in the case of  FIG. 3A . 
         [0073]    In this case, the optical signals  201 ,  202 , and  203  are reflected by the mirror  112  at respectively different angles. After being reflected by the mirror  112 , the optical signal  201  is collected, by the converging lens  1750 , to a point close to the center of the dispersing unit  1740 . 
         [0074]    After being reflected by the mirror  112 , the optical signal  203  is collected, by the converging lens  1750 , to a point far from the center of the dispersing unit  1740 . As a result, the optical signal  201  is input to the port  1715  at a small angle by the collimating lens  1725 . On the other hand, the optical signal  203  is input to the port  1715  at a large angle by the collimating lens  1725 . 
         [0075]    In this case as shown in  FIG. 4B , the transmissivity of the optical signal  203  greatly decreases in the transmission band characteristic  204  compared with the transmission band characteristic  205 . On the other hand, a transmissivity of the optical signal  201  decreases less than that of the optical signal  203 . As a result, the transmission band characteristic  205  can be compensated using a compensation level of the transmission band characteristic  204  in which the transmissivity decreases as the wavelength of the channel  20  becomes longer. 
         [0076]    As shown in  FIGS. 2A to 4B , a concave reflective surface of the mirror  112  and rotation of the mirror  112  about the Z-axis enables the optical switch  100  to compensate the transmission band characteristic of each channel using a compensation level corresponding to each channel. 
         [0077]    Thus, the optical switch  100  compensates the slope of the transmission band characteristic of each channel caused by the aberration of the converging lens  1750 . When beam radii in both directions of the X-axis and the Z-axis of an optical signal passing through the optical switch  100  are identical, a coupling rate η of the output port unit  1710  can be calculated as follows. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0078]    λ 0  indicates each center wavelength of an optical signal reflected by each mirror. Δλ indicates a difference between a wavelength of an optical signal and the center wavelength. ω 0  indicates a beam spot size of an optical signal when reflected by each mirror. θ indicates a rotation angle when the mirror is rotated about the X-axis. R indicates a curvature radius of each mirror. α indicates a constant of the optical system forming the optical switch  100 . 
         [0079]      FIG. 5  is a graph indicating a transmission band characteristic according to the rotation angle of a mirror. The transmission band characteristics of channels of which wavelengths are within the range of 1555.55 nanometer (nm) to 1555.90 nm are shown. Reference numerals  501  to  511  indicate transmission band characteristics respectively corresponding to the rotation angles of 0 degrees, 0.024 degrees, 0.048 degrees, 0.072 degrees, 0.096 degrees, 0.12 degrees, 0.144 degrees, 0.168 degrees, 0.192 degrees, 0.216 degrees, and 0.24 degrees. 
         [0080]    As shown in  FIG. 5 , transmissivity of each wavelength included in the channel  20  becomes substantially constant when the slope of the transmission band characteristics for each channel caused by the aberration of the converging lens  1750  is compensated by the optical switch  100 . Further, the transmissivity of each channel overall decreases as the rotation angle increases. 
         [0081]      FIG. 6  is a schematic illustrating an adjusting procedure  1  of setting the compensation level for a Ch.  1  of the optical switch  100 . A reference numeral  601  indicates a transmission band characteristic when the reflective surface of the mirror  111  is flat, and the mirror  111  is not rotated about the Z-axis. A reference numeral  602  indicates a transmission band characteristic indicative of a compensation level when the mirror  111  is curved and rotated about the Z-axis. 
         [0082]    The transmission band characteristic  601  of the channel  1  illustrates that transmissivity increases as wavelength increases. On the other hand, when the mirror  111  is rotated about the Z-axis as shown in  FIG. 4A , the transmission band characteristic of the channel  1  can be compensated using a compensation level of a transmission band characteristic in which a transmissivity decreases as a wavelength increases as shown in  FIG. 4B . As a result, a transmission band characteristic of the channel  1  becomes constant such as a transmission band characteristic  603 , and the slope of the transmission band characteristic can be compensated. 
         [0083]    For example, the transmission band characteristic of a channel  40  shown in  FIG. 18A  can be compensated using a compensation level of the transmission band characteristic in which transmissivity increases as wavelength increases as shown in  FIG. 3B  since the transmissivity of the channel  40  decreases as wavelength increases. As a result, a transmission band characteristic of the channel  40  becomes constant such as the transmission band characteristic  603 , and the slope of the transmission band characteristic can be compensated. 
         [0084]      FIG. 7  is a first schematic illustrating an adjusting procedure  2  of the optical switch  100 .  FIG. 8  is a second schematic illustrating the adjusting procedure  2 . When the optical switch  100  attenuates an optical signal output from the port  1715  by slightly changing the reflection angle of the mirror  111  from the optimal angle and decreasing a coupling rate of the port  1715 , the optical switch  100  adjusts overall transmissivity of the channel  1  by changing the attenuation level after making the transmissivity of the channel  1  constant as shown in  FIG. 6 . 
         [0085]    More specifically, the coupling rate of the port  1715  is increased by rotating the mirror  111  about the X-axis and slightly changing the reflection angle of the mirror  111  as shown in  FIG. 7 . As a result, the transmissivity of the transmission band characteristic  603  that has decreased overall by the adjusting procedure  1  shown in  FIG. 6  can be increased overall as shown in  FIG. 7 . 
         [0086]    The transmissivity decreased by the adjusting procedure  1  varies depending on the curvature of the mirror  111  about the Z-axis, the reflection angle of the mirror  111  in the X-axis direction, or a combination thereof. Therefore, the rotation angle of the mirror  111  to increase the transmissivity differs according to the curvature, the reflection angle in the X-axis direction, or the combination thereof. 
         [0087]    According to the embodiment, the optical switch  100  can compensate, using the adjusting procedure  1 , a slope of a transmission band characteristic caused by the aberration of the converging lens  1750  without a compensating lens. 
         [0088]    Furthermore, the transmissivity for each channel that has been decreased by the adjusting procedure is adjusted to become a desired transmissivity using the adjusting procedure  2  shown in  FIGS. 7 and 8 . Therefore, the slope can be compensated for each channel, and the required transmissivity can be maintained at the same time. 
         [0089]      FIG. 9A  is a schematic illustrating curvature setting of the mirror  112  of the optical switch  100  when the curvature is zero.  FIG. 9B  is a graph indicating a transmission band characteristic of the optical switch  100  shown in  FIG. 9A .  FIG. 9A  shows a configuration of the optical switch  100  when the reflective surface of the mirror  112  is flat and the mirror  112  is not rotated about the Z-axis. A reference numeral  901  shown in  FIG. 9B  (and also  FIGS. 10B and 11B ) indicates a range of wavelengths within which transmissivity of the channel  20  becomes equal to or greater than 0.5 dB. 
         [0090]    In this case, the optical signals  201 ,  202 , and  203  of the channel  20  are reflected by the mirror  112  in parallel to one another, and collected to one point on the dispersing unit  1740  by the converging lens  1750 . As a result, the optical signals  201 ,  202 , and  203  are input, by the collimating lens  1725 , to the port  1715  at an angle for an optimal coupling state. 
         [0091]    In this case, coupling loss does not occur for all of the wavelengths in the channel  20 . Therefore, a transmission band characteristic  902  of the channel  20  indicates that a transmissivity of each wavelength in the channel  20  becomes constant. 
         [0092]      FIG. 10A  is a schematic illustrating curvature setting when the curvature radius is large.  FIG. 10B  is a graph indicating a transmission band characteristic of the optical switch  100  shown in  FIG. 10A .  FIG. 10A  shows a configuration of the optical switch  100  when the curvature radius of the reflective surface of the mirror  112  is large, and the mirror  112  is not rotated. 
         [0093]    In this case, as shown in  FIG. 10A , the optical signals  201 ,  202 , and  203  of the channel  20  are reflected by the mirror  112  at different angles, and collected to different points on the dispersing unit  1740  by the converging lens  1750 , respectively. As a result, the optical signals  201 ,  202 , and  203  are input to the port  1715  at different angles by the collimating lens  1725 , respectively. 
         [0094]    Since coupling loss occurs for the optical signals  201  and  203 , the range  901  within which transmissivity is equal to or greater than 0.5 dB becomes narrower than the range  901  shown in  FIG. 9B . 
         [0095]      FIG. 11A  is a schematic illustrating curvature setting when the curvature radius is small.  FIG. 11B  is a graph indicating a transmission band characteristic of the optical switch  100  shown in  FIG. 11A .  FIG. 11A  shows a configuration of the optical switch  100  when the curvature radius of the reflective surface of the mirror  112  is small, and the mirror  112  is not rotated. 
         [0096]    In this case, as shown in  FIG. 11A , the optical signals  201 ,  202 , and  203  of the channel  20  are reflected by the mirror  112  at largely different angles, and converged to different points on the dispersing unit  1740  by the converging lens  1750 , respectively. As a result, the optical signals  201 ,  202 , and  203  are input to the port  1715  at largely different angles by the collimating lens  1725 , respectively. 
         [0097]    Since large coupling loss occurs for the optical signals  201  and  203 , the range  901  within which transmissivity is equal to or greater than 0.5 dB becomes far narrower than the range  901  shown in  FIG. 9B . 
         [0098]    Thus, although a slope of the transmission band characteristic can be compensated by a curvature of the reflective surface, this comes at the expense of a narrowing of the range  901 . Therefore, as attenuation is a function of rotation of the mirrors  111 ,  112 ,  113 , and  114  about the Z-axis, the range of attenuation corresponds with the range of rotation and hence, the curvature radius of the reflective surface of each mirror  111 ,  112 ,  113 , and  114  is set such that the smallest range  901  occurring within the attenuation range becomes as wide as possible. 
         [0099]    The curvature of the reflective surface can be obtained by using a difference of linear expansion coefficient between the reflective surface material and the substrate material. In this case, a desired curvature at an operating temperature can be obtained by adjusting a deposit temperature of the reflective surface material or a deposit film pressure. 
         [0100]      FIG. 12A  is a schematic illustrating curvature setting of the mirror for each channel when the curvature radius is large.  FIG. 12B  is a graph indicating a transmission band characteristic of the optical switch  100  shown in  FIG. 12A . Only a channel  30  is shown in  FIG. 12A . A reference numeral  1201  shown in  FIG. 12B  indicates a range of wavelengths included in the channel  30 . 
         [0101]    When the rotation angle of the mirror  110  about the Z-axis is limited, curvature about the X-axis of the reflective surface of each mirror can be different depending on the position on the converging lens  1750  through which each optical signal passes. For example, the curvature is set smaller as the each optical signal passes through a point farther from the center of the converging lens  1750 . 
         [0102]    Since the slope of the transmission band characteristic  1202  of the channel  30  is gradually downwards as shown in  FIG. 12B , the curvature radius of the reflective surface of the mirror  113  corresponding to the channel  30  is set large as shown in  FIG. 12A . As a result, the gradual upward slope of the transmission band characteristic  1202  of the channel  30  can be compensated by a compensation level of the transmission band characteristic  1203  having a gradual upward slope. For example, the curvature of each mirror can be changed by changing the deposition amount of the reflective surface material. 
         [0103]      FIG. 13A  is a schematic illustrating curvature setting of the mirror for each channel when the curvature radius is small.  FIG. 13B  is a graph indicating a transmission band characteristic of the optical switch  100  shown in  FIG. 13A . Only a channel  40  is shown in  FIG. 13A . A reference numeral  1301  shown in  FIG. 13B  indicates a range of wavelengths included in the channel  40 . 
         [0104]    Since the slope of the transmission band characteristic  1302  of the channel  40  is steeply downwards as shown in  FIG. 13B , the curvature radius of the reflective surface of the mirror  114  corresponding to the channel  40  is set small. As a result, the steep downwards slope of the transmission band characteristic  1302  of the channel  40  can be compensated by a compensation level of the transmission band characteristic  1303  having a steep upward slope. 
         [0105]      FIG. 14  is a first schematic illustrating the effect of variation of the mirror curvature about the Z-axis on the transmission band characteristic. Only the mirror  111  is shown in  FIG. 14 . A reference numeral  1401  indicates a curvature radius of the mirror  111 . A reference numeral  1402  indicates a point to which optical signals reflected between A-A′ of the mirror  111  are converged. A reference numeral  1403  indicates a point to which optical signals reflected between B-B′ of the mirror  111  are converged. When the curvature of the mirror  111  about the Z-axis differs between the A-A′ and the B-B′, a difference between the point  1402  and the point  1403  occurs. 
         [0106]      FIG. 15  is a second schematic illustrating the effect of variation of the mirror curvature about the Z-axis on the transmission band characteristic. A horizontal axis indicates the degree of rotation of the mirror  111  about the X-axis. The vertical axis indicates a transmissivity of the optical switch  100 . Reference numerals  1501  and  1502  indicate a transmission band characteristic of optical signals reflected at the A-A′ and the B-B′ of the mirror  111 , respectively. 
         [0107]    A reference numeral  1503  indicates a range of the degree of rotation within which the transmissivity of the optical signal reflected at the B-B′ is equal to or greater than 0.5 dB. As shown in  FIG. 15 , when a difference between the points  1402  and  1403  occurs, a range of rotation angle for obtaining a desired transmissivity differs between the optical signals reflected at the A-A′ and the B-B′. 
         [0108]    In particular, when the curvature of the reflective surface of the mirror  111  is set large to switch the output port and limit the degree of rotation of the mirror  111  about the X-axis at the same time, the difference between the points  1402  and  1403  becomes large. Accordingly, the mirror  111  has to be made such that the curvature about the Z-axis between the A-A′ and the B-B′ does not vary. 
         [0109]    The curvature about the Z-axis of the mirror  111  may be set small to reduce the difference between the points  1402  and  1403 . More specifically, a rear surface of the mirror  111  may have anisotropic strength such as a rib, or anisotropic material may be bonded or vapor-deposited to the reflective surface along the Z-axis. For example, the mirror  111  may have a rib along the Z-axis so that the curvature about the Z-axis becomes smaller than that about the X-axis. 
         [0110]      FIG. 16  is a graph indicating a result of compensation of a transmission band characteristic performed by the optical switch  100 . A reference numeral  1601  indicates a transmission band characteristic before the compensation when the level of attenuation performed by the optical switch  100  is 20 dB. A reference numeral  1602  indicates a transmission band characteristic after the compensation when the level of attenuation performed by the optical switch  100  is 20 dB. 
         [0111]    The mirror unit  110  has been rotated about the Z-axis by 0.2 degrees herein. A diffraction grating having the frequency of 1000 lines/millimeter (mm) as the dispersing unit  1740 , and a lens having a focal length of 100 mm as the converging lens  1750  have been used. The slope of the transmission band characteristic has been reduced from 1.33 db/nm before the compensation to 0.33 dB/nm after the compensation. 
         [0112]    Though it is explained in the above embodiment that the optical switch  100  includes the converging lens  1750  that is convex, the present invention can be applied to a case where the converging lens is concave. 
         [0113]    Furthermore, it is explained in the above embodiment that the curvature of the reflective surface and the reflection angle about the X-axis of the mirror unit  110  are set, for each mirror, according to the point on the converging lens  1750  through which an optical signal having a corresponding wavelength passes. However, the transmission band characteristic varies due to various factors at the time of actual design. Therefore, the curvature and the reflection angle may be adjusted by monitoring the transmission band characteristic to compensate the slope. 
         [0114]    Furthermore, though it is explained in the above embodiment that the port  1711  is an input port and the ports  1712  to  1715  are output ports, each of the ports  1711  to  1715  may be either an input port or an output port. For example, the port  1711  may be used as an output port, and the ports  1712  to  1715  may be used as input ports. 
         [0115]    In this case, the optical switch  100  outputs, from among optical signals input from the ports  1712  to  1715 , an optical signal selected according to a wavelength to the port  1711 . Furthermore, the optical switch  100  can be applied to various optical transmission apparatuses in an optical transmission system. 
         [0116]    Furthermore, a monitor to monitor the optical signals output from the port unit  1710  can be provided. In the case, the monitor monitors a transmissivity of the optical signal output from the port unit  1710 , and the control unit controls the driving unit so that the transmissivity becomes constant for each wavelength. Furthermore, the monitoring may be periodically performed by the monitor to compensate a slope of a transmission band characteristic caused by a change in temperature and deterioration of an optical system that occur over time. 
         [0117]    As explained above, according to the embodiment, a slope of a transmission band characteristic for each channel can be compensated without a compensating lens. Furthermore, the slope can be compensated and a desired transmissivity can be maintained at the same time. 
         [0118]    Though it is explained in the above embodiment that the plural mirrors correspond to wavelengths of optical signals, the mirrors do not have to perfectly correspond to the wavelengths, and may be arrayed in the dispersing direction of the dispersing element. Furthermore, though it is also explained in the above embodiment that the output ports correspond to reflection angles of the mirrors, the output ports do not have to perfectly correspond to the reflection angles, and may be arranged at positions to which the optical signals output from the dispersing element are input. 
         [0119]    Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.