Abstract:
An optical apparatus comprising, converting units converting electrical signals into signal lights with different wavelength, polarization control units controlling polarizing states of the signal lights into polarization controlled lights respectively, an optical multiplexer multiplexing the polarization controlled lights into a multiplexed light, an optical branching unit branching the multiplexed light and outputting a branched light, a polarizing unit extracting only signal lights of the specified polarizing state from the branched light into an extracted light, and a control unit detecting intensity of the extracted light. Pilot signals are applied to modulate the electrical signals or the polarization controls. The polarization control units controls the polarizing states of the signal lights based on the pilot signal frequencies of the detection result by the control unit.

Description:
[0001]    The present invention claims foreign priority to Japanese application 2006-328447, filed on Dec. 5, 2006, which is incorporated herein by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    A wavelength division multiplex (WDM) transmission system is known to be one of the high-speed and large capacity optical transmission systems. Recently, a technology for transmitting WDM light of 1000 wavelengths or more has been researched. In order to increase the number of multiplexed wavelengths, that is, the number of channels, a wavelength interval of WDM light must be narrowed. 
         [0003]    However, when a wavelength interval of WDM light becomes narrow, crosstalk between adjacent channels becomes highly possible. In order to eliminate such crosstalk, a polarized orthogonal control technology that suppresses interference between adjacent channels has been known. 
         [0004]    For example, one of the known WDM transmitting terminals has five or more optical transmitters and wavelength interval of output signal lights from the optical transmitters have no periodicity and when re-using a specific wavelength interval, pair of signals using the specific wavelength interval are separated more than two channels. Also, the known terminal has a polarized state control unit controlling the polarized states of the signal lights in order to make the polarized states of the adjacent signal light crossing in orthogonal at an output end of the terminal. 
         [0005]    In another one of the known terminal, in order to transmit output signal light with multiplexed signal lights where adjacent signal lights are in the orthogonal polarizing direction, two steps of multiplexing are processed. 
         [0006]    At the first step, signal lights with orders of odd number are combined to a first multiplexed light and signal lights with orders of even number are combined to a second multiplexed light. Then, the polarization of the first multiplexed light and the second multiplexed light are controlled so that the polarization direction of the second multiplexed light becomes orthogonal to the polarization direction of the first multiplexed light. 
         [0007]    At the second step, the first multiplexed light and the second multiplexed light are combined. In the two steps, output signal light with multiplexed signal lights that adjacent signal lights are in the orthogonal polarizing direction, with each polarized state of the multiplexed signal lights is maintained. 
         [0008]    In the WDM transmission system utilizing the polarized orthogonal control technology, in order to eliminate crosstalk, it is required to realize perfect orthogonal cross of polarizing directions of adjacent channels. However, in the polarized orthogonal control apparatus of related arts, it is not easy to detect whether adjacent channels are orthogonal in the polarizing direction, or to realize orthogonal polarizing directions of adjacent channels on the basis of the result of detection. 
       SUMMARY 
       [0009]    According to an aspect of an embodiment, an optical apparatus comprises a plurality of converting units converting a plurality of electrical signals into a plurality of signal lights, wavelength of the plurality of signal lights being different from each other, a plurality of polarization control units controlling polarizing states of the plurality of signal lights and outputting a plurality of polarization controlled lights respectively, an optical multiplexer multiplexing the plurality of polarization controlled lights into a multiplexed light, an optical branching unit branching the multiplexed light and outputting a branched light, a polarizing unit extracting only signal lights of the specified polarizing state from the branched light and outputting an extracted light, and a control unit detecting intensity of the extracted light and controlling the plurality of polarization control units on the basis of the detection result, wherein the plurality of electrical signals are modulated by a plurality of pilot signals respectively and the detection of the control unit is based on the plurality of pilot signals. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a block diagram showing a structure of a polarized orthogonal control apparatus in a first embodiment. 
           [0011]      FIG. 2  is a diagram for explaining the polarization control principle in the polarized orthogonal control apparatus in the first embodiment. 
           [0012]      FIG. 3  is a diagram for explaining a polarization control principle of the polarized orthogonal control apparatus in the first embodiment. 
           [0013]      FIG. 4  is a diagram for explaining the polarized control principle of the polarized orthogonal control apparatus in the first embodiment. 
           [0014]      FIG. 5  is a block diagram showing a structure of the polarized orthogonal control apparatus in a second embodiment. 
           [0015]      FIG. 6  is a block diagram showing a structure of the polarized orthogonal control apparatus in a third embodiment. 
           [0016]      FIG. 7  is a block diagram showing a structure of the polarized orthogonal control apparatus in a fourth embodiment. 
           [0017]      FIG. 8  is a diagram for explaining a modulation principle of the polarized orthogonal control apparatus in a fourth embodiment. 
           [0018]      FIG. 9  is a diagram for explaining the modulation principle of the polarized orthogonal control apparatus in the fourth embodiment. 
           [0019]      FIG. 10  is a block diagram showing a structure of the polarized orthogonal control apparatus in a fifth embodiment. 
           [0020]      FIG. 11  is a block diagram showing a structure of the polarized orthogonal control apparatus in a six embodiment. 
           [0021]      FIG. 12  is a block diagram sowing a structure of the polarized orthogonal control apparatus in a seventh embodiment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
         [0023]    In the following description, n is an integer of 2 or larger and k is an integer ranging from 1 to n, and the (k)th channel and the (k+1)th channels are defined as adjacent channels. In each embodiment explained below, the like structures are designated with identical reference numerals for simplifying the description. 
       FIRST EMBODIMENT 
       [0024]      FIG. 1  is a block diagram showing a structure of a polarized orthogonal control apparatus in a first embodiment. As shown in  FIG. 1 , the polarized orthogonal control apparatus of the first embodiment includes an optical transmitter  100  having 2k of electro-optical converters ( 1   a ,  1   b ,  1   c , and  1   d ), 2k of polarization controllers ( 2   a ,  2   b ,  2   c , and  2   d ), an optical multiplexer  3 , an optical branching device, a polarizer  5 , and a control unit  6 . 
         [0025]    The electro-optical converters  1   a ,  1   b ,  1   c  and  1   d  are provided for every channel and are respectively equipped with a light emitting element, such as a laser diode and convert an electrical signal into a signal light. The polarization controllers  2   a ,  2   b ,  2   c ,  2   d  are provided for every channel. The (2k−1)th polarization controller  2   a ,  2   c  and the (2k)th polarization controller  2   b ,  2   d  respectively control the polarization state of the signal lights output respectively from the (2k−1)th electro-optical converter  1   a ,  1   c  and the (2k)th electro-optical converter  1   b ,  1   d.    
         [0026]    The optical multiplexer  3  multiplexes signal lights respectively output from polarization controllers  2   a ,  2   b ,  2   c , and  2   d . The optical branching device  4  branches the output light from the optical multiplexer  3  to optical receiver  200  and the controller  6 . The polarizer  5  extracts only signal light in the particular polarization state from the signal lights output from the optical branching device  4 . The controller  6  controls the polarization controllers  2   a ,  2   b ,  2   c ,  2   d  to satisfy orthogonal condition in the polarizing directions of the (2k−1)th channel and the (2k)th channel.  
         [0027]    The controller  6  is provided with k of pilot signal generators ( 11   a ,  11   b ), k of inverters ( 12   a ,  12   b ), an optical-electro converter  13 , k of multipliers ( 15   a ,  15   b ), and k of discriminators ( 16   a ,  16   b ). The pilot signal generators  11   a ,  11   b  generate low-frequency signals of different frequencies as pilot signals. The pilot signal output from the (2k−1)th pilot signal generator  11   a ,  11   b  is supplied to the (2k−1)th electric-optical converter  1   a ,  1   c.    
         [0028]    The inverters  12   a ,  12   b  invert phase of the pilot signal output from the pilot signal generators  11   a ,  11   b . The pilot signal output from the inverters  12 ,  12   b  is supplied to the (2k )th electro-optical converters  1   b ,  1   d . The electro-optical converters  1   a ,  1   b ,  1   c , and  1   d  respectively conduct intensity modulation by superposing the pilot signal on the electric signal. 
         [0029]    The optical-electro converter  13  is equipped with a light receiving element such as a photodiode and converts a signal light output from the polarizer  5  into an electric signal. Filters  14   a ,  14   b  are respectively equipped with a band-pass filter for transmitting only the signal of the frequency identical to the pilot signal of each channel among the signal lights output from the optical-electro converter  13 . That is, the (2k−1)th filter  14   a ,  14   b  allows the signals corresponding to the signal lights output from the (2k−1)th and (2k )th electro-optical converters  1   a ,  1   b ,  1   c , and  1   d.    
         [0030]    The multipliers  15   a ,  15   b  synchronously detect the signal having passed the filters  14   a ,  14   b . For this purpose, the (2k−1)th multiplier  15   a ,  15   b  multiplies the signal having passed the (2k−1)th filter  14   a ,  14   b  and the pilot signal output from the (2k−1)th pilot signal generator  11   a ,  11   b.    
         [0031]    The (2k−1)th discriminator  16   a ,  16   b  control the polarization state of the signal light having passed the (2k−1)th and (2k)th polarization controllers  2   a ,  2   b ,  2   c , and  2   d  to obtain the maximum output from the (2k−1)th multiplier  15   a ,  15   b . As will be described later, when an output value of the multiplier  15   a ,  15   b  becomes the maximum value, the polarizing direction of the (2k−1)th channel becomes orthogonal to the polarizing direction of the (2k )th channel. 
         [0032]    In the first embodiment, the condition for achieving orthogonal polarizing direction of the (2k−1)th channel and the (2k)th channel is that the output values of the multipliers  15   a ,  15   b  become the maximum values. 
         [0033]    The optical receiver  200  is provided with an optical demultiplexer  7  demultiplexing an signal light output from the optical branching device  4  into 2k light signals of different wavelengths in every channel and 2k optical-electro converters  8   a ,  8   b ,  8   c , and  8   d  for converting 2k signal lights output from this optical demultiplexer  7 . Here, the polarized orthogonal control apparatus in the first embodiment may be equipped only with the optical transmitter  100  or may be equipped with both optical transmitter  100  and optical receiver  200 , 
         [0034]    Next, a reason why the polarizing directions become orthogonal in the (2k−1)th channel and the (2k )th channel when both multipliers  15   a ,  15   b  provide the maximum outputs will be explained. When an optical power of the pilot signal output from the pilot signal generators  11   a ,  11   b  is expressed with the following equation (1) optical powers of the pilot signals of the (2k−1)th channel and the (2k )th channel when the optical power is intensity-modulated with the pilot signal can be expressed respectively with the equations (2) and (3). However, light intensity is indicated as A, angular velocity as ω, and time as t, respectively. 
         [0000]        f (2 k− 1)= A× sin ω t   (1) 
         [0000]        f (2 k− 1)= A× sin ω t   (2) 
         [0000]        −f 2 k=−A× sin ω t   (3) 
         [0035]    A polarizing plane of the polarizer  5  is defined as the standard optical phase angle. For the (2k−1)th channel, the control target is set to set the polarizing angle of this channel is set to the identical phase with the standard optical phase angle. Moreover, for the (2k ) channel, the control target is set the polarizing angle of this channel orthogonal to the standard optical phase angle (namely, the polarizing angle is deviated by 90° from the standard optical phase angle). 
         [0036]      FIG. 2  to  FIG. 4  are diagrams for explaining the polarization control principle in the polarized orthogonal control apparatus of the present embodiment. As shown in  FIG. 3 , if the polarizing angle of each channel is deviated from the standard optical phase angle, an optical power intensity modulation degree for such deviation changes. An optical power intensity modulation degree of the (2k−1)th channel is expressed with the following equation (4) when the polarizing angle of the relevant channel and the reference optical phase angle are in the in-phase state and if the in-phase state is deviated by only θ 1 radian, above modulation degree is expressed with the following equation (5). 
         [0000]        A× sin ω t ×| cos0 |=A ×sin ω t   (4) 
         [0000]        A ×sin ω t ×| cos θ1|  (5) 
         [0037]    The pilot signal of the (2K)th channel is inverted, as explained above, with inverters  12   a ,  12   b . Therefore, an optical power intensity modulation degree of the (2k )th channel is expressed with the following equation (6) when the polarizing angle of the relevant channel and the reference optical phase angle are in the orthogonal relationship and if the orthogonal state is deviated only by θ 2 radian, such modulation degree is expressed with the equation (7). 
         [0000]        A ×sin ω t ×(−| sin 0|)=0  (6) 
         [0000]        A ×sin ω t ×(−| sin θ2|)  (7) 
         [0038]    Since the pilot signals of the identical frequency are superposed in the (2k−1)th channel and the (2k )th channel, an optical power of the pilot signal detected with the (2k−1)th multiplier  15   a ,  15   b  is equal to the power attained by adding the powers of the equations (5) and (7). Accordingly, a voltage of the output signal of the (2k−1)th multiplier  15   a ,  15   b  is determined with the following equation (8). 
         [0000]      | cos θ1|−| sin θ2|  (8) 
         [0039]      FIG. 3  shows a relationship of | cos θ| for the angle θ. In addition,  FIG. 4  shows a relationship of −| sin θ| for the angle θ. It is obvious from  FIG. 3  and  FIG. 4  that the maximum value can be obtained from the equation (8) within the range equal to −180° or more but 180° or less when both θ 1 and θ 2 are equal to −180°, 0°, or 180°. 
         [0040]    Thus, when the polarizing angle of the (2k−1)th channel is matched with (or inverted from)the phase of the reference optical phase angle and the polarizing angle of the (2k )th channel is orthogonal to the reference optical phase angle, that is when the polarizing angle of the (2k−1)th channel is orthogonal to the polarizing angle of the (2k )th channel, an output voltage of the (2k−1)th multiplier  15   a ,  15   b  has the maximum value. 
         [0041]    Accordingly, the orthogonal polarizing directions may be attained in the adjacent channels by controlling each polarization controller  2   a ,  2   b , and  2   c  to provide the maximum output voltage of each multiplier  15   a , l 5   b , that is, the maximum input voltage of each discriminator  16   a ,  16   b.    
       SECOND EMBODIMENT 
       [0042]      FIG. 5  is a block diagram showing a structure of the polarized orthogonal control apparatus as a second embodiment. As shown in  FIG. 5 , the second embodiment has a structure that the control unit  106  of the optical transmitter  101  includes a pilot signal generator  11   c,  a filter  14   c , a multiplier  15   c , and a discriminator  16   c  and these elements are used in common for all channels. 
         [0043]    In the second embodiment, the pilot signal output from the pilot signal generator  11   c  is supplied to the (2k−1)th electro-optical converter  1   a ,  1   c . To the (2k )th electro-optical converter  1   b ,  1   d , the pilot signal output from the pilot signal generator  11   c  and is then inverted with the inverter  12   a ,  12   b.    
         [0044]    The filter  14   c  is formed of a band-pass filter sending only the signal of the frequency equal to that of the pilot signal output from the pilot signal generator  11   c  among the signal lights output from the optical-electro converter  13 . The multiplier  15   c  implements synchronous detection of the signal having passed the filter  14   c  by multiplying the signal having passed the filter  14   c  and the pilot signal output from the pilot signal generator  11   c.    
         [0045]    The discriminator  16   c  controls polarizing state of the signal light passing through the (2k−1)th and (2k )th polarization controllers  2   a ,  2   b ,  2   c , and  2   d  to satisfy the condition assuring orthogonal relationship of the polarizing directions in both (2k−1)th channel and the (2k )th channel. This condition ensures the maximum output value of the multiplier  15   c . The reason is identical to that explained with reference to  FIG. 2  to  FIG. 4  in the first embodiment. However, the multiplier  15   c  detects a total sum of optical power of the (2k−1)th channel and a total sum of optical power of the (2k )th channel. The other structure is identical to that in the first embodiment. 
         [0046]    In the second embodiment, adjustment is necessary in the following procedures when the optical transmitter  101  is activated. First, the third (omitted in  FIG. 5 ) to the (2n)th channels are in the extinct state and the first and second polarization controllers  2   a ,  2   b  are manipulated to provide the maximum output voltage of the multiplier  15   c.    
         [0047]    Next, while the first and the second channels are kept in the light emitting state, the fifth (omitted in  FIG. 5 ) to the (2n)th channels are in the extinct state and moreover the third to fourth polarization controller (omitted in  FIG. 5 ) is manipulated to provide the maximum output voltage of the multiplier  15   c . Such manipulations are continued up to the (2k−1)th and (2k )th channels. 
       THIRD EMBODIMENT 
       [0048]      FIG. 6  is a block diagram showing a structure of the polarized orthogonal control apparatus as a third embodiment. As shown in  FIG. 6 , the third embodiment has a structure that pilot signal generators  11   a ,  11   b ,  11   d , and  113 , filters  14   a ,  14   b ,  14   d , and  14   e , and discriminators  16   a ,  16   b ,  16   d , and i 6   e  are provided for each channel in the control unit  116  of the optical transmitter  111  and the pilot signals of different frequencies are superposed in each channel. 
         [0049]    Accordingly, in the third embodiment, an inverter and a multiplier for synchronous detection are not required. Moreover, the (2k−1)th discriminator  16   a ,  16   b  controls the polarizing state of the signal light passing the (2k−1)th polarization controller  2   a ,  2   c  to provide the maximum output voltage of the (2k−1)th filter  14   a ,  14   b . In this case, the (2k )th discriminator  16   d ,  16   e  controls the polarizing state of the signal light passing the (2k )th polarization controller  2   b ,  2   d  to provide the minimum output voltage of the (2k )th filter  14   d ,  14   e.    
         [0050]    Otherwise, it may be possible to control the polarization controllers  2   a ,  2   b ,  2   c , and  2   d  to provide the minimum output voltage of the (2k−1)th filter  14   a ,  14   b  and to provide the maximum output voltage of the (2k )th filter  14   d ,  14   e . This control realizes orthogonal crossing in the polarizing directions for the (2k−1)th channel and the (2k )th channel. The reason is identical to that in the first embodiment. 
         [0051]    Namely, optical powers of the pilot signals of the (2k−1)th channel and the (2k )th channel when the optical power intensity modulation is implemented using the pilot signals respectively output from the (2k−1)th pilot signal generator  11   a ,  11   b  and the (2k )th pilot signal generator  11   d ,  11   e  can be indicated with the following equations (9) and (10). 
         [0000]        f (2 k− 1)= A ×sin (2 k− 1)ω t   (9) 
         [0000]        f 2 k=−A ×sin2 kωt   (10) 
         [0052]    An optical power intensity modulation degree of the (2k−1)th channel is expressed with the following equation (11) when the polarizing angle of the relevant channel and the reference optical phase angle are in the in-phase state, and if the in-phase state is deviated only by θ 1 radian, such modulation degree is expressed with the following equation (12). Moreover, an optical power intensity modulation degree of the (2k )th channel is expressed with the following equation (13) when the polarizing angle of the relevant channel is orthogonal to the reference optical phase angle and if the orthogonal state is deviated only by θ 1 radian, such modulation degree is expressed with the following equation (14). 
         [0000]        A ×sin (2 k− 1)ω t   (11) 
         [0000]        A ×sin (2 k− 1)ω t ×| cos θ1|  (12) 
         [0000]        A ×sin2 kωt ×| sin0|=0  (13) 
         [0000]        A ×sin2 kωt ×| sin θ2|  (14) 
         [0053]    Accordingly, it may be obvious from the equations (13) and (14)that both θ 1 and θ 2 are set to 0° or to 90° in order to attain the orthogonal state in the polarizing directions of the (2k−1)th channel and the (2k )th channel. The other structure is similar to that of the first embodiment. 
       FOURTH EMBODIMENT 
       [0054]      FIG. 7  is a block diagram showing a structure of the polarized orthogonal control apparatus as a fourth embodiment. As shown in  FIG. 7 , the fourth embodiment includes the optical transmitter  121  equipped to modulate, in the polarizing direction, the signal light passing the (2k−1)th polarization controller  2   a ,  2   c  with the pilot signal output from the (2k−1)th pilot signal generator  11   a ,  11   b.    
         [0055]    Moreover, the pilot signals output from the (2k−1)th pilot signal generator  11   a ,  11   b  and inverted with the inverters  12   a ,  12   b  modulate the signal light passing the (2k )th polarization controller  2   b ,  2   d . When modulation is executed in the polarizing direction, it is identical to implementation of intensity modulation for optical power as in the case of the first embodiment as a result observed from the discriminator. 
         [0056]    Realization of intensity modulation in the polarizing direction will be explained below.  FIG. 8  and  FIG. 9  explain the modulation principle in the polarized orthogonal control apparatus in the forth embodiment. As shown in  FIG. 8 , if it is assumed that the optimum condition may be attained for a certain channel, polarizing angle of an signal light and the reference optical phase angle are in the in-phase state (0°), optical intensity when the polarizing angle is deviated by θ from the optimum point becomes cos θ element of the optical intensity when it is matched with the optimum point, intensity is lowered than that when matching is attained. 
         [0057]    As shown in  FIG. 9 , when an signal light of a certain channel is modulated with the frequency f in the phase direction, amplitude of the optical of such channel changes similar to the intensity modulation, the intensity modulation is realized with the modulation in the polarizing direction. The nearer the polarizing angle of signal light becomes, the more an optical power intensity modulation degree realized by modulation in the polarizing direction becomes nearly zero. This is inverted from the case of the pilot signal having the maximum amplitude at the optimum point. 
         [0058]    Moreover, since slope of the curve (| cos θ| shown in  FIG. 9  is positive in the side where phase is smaller than at the optimum point, while it is negative in the side where phase is larger, phase of intensity modulation at the optimum point is inverted by 180°. Whether phase of the signal light is deviated in the side where it is larger or smaller than the optimum phase can be detected by using the above phenomenon. The other structure is similar to the first embodiment. 
       FIFTH EMBODIMENT 
       [0059]      FIG. 10  is a block diagram showing a structure of the polarized orthogonal control apparatus as a fifth embodiment. As shown in  FIG. 10 , the fifth embodiment applies the fourth embodiment to the second embodiment. That is, the control unit  106  modulates the signal light passing the (2k−1)th polarization controller  2   a ,  2   b  in the polarizing direction with the pilot signal output from the pilot signal generator  11   c.    
         [0060]    Moreover, the control unit  106  modulates the signal light passing the (2k )th polarization controller  2   b ,  2   d  with the pilot signal inverted with the inverters  12   a ,  12   b . The reason of intensity modulation with the modulation in the polarizing direction has already been explained in regard to the fourth embodiment. The other structure is similar to that of the second embodiment. 
       SIXTH EMBODIMENT 
       [0061]      FIG. 11  is a block diagram showing a structure of the polarized orthogonal control apparatus as a sixth embodiment. As shown in  FIG. 11 , the sixth embodiment can be realized by applying the fourth embodiment to the third embodiment. Namely, In the optical transmitter  141 , the control unit  116  modulates the signal light passing the polarization controller  2   a ,  2   b ,  2   c , and  2   d  in the polarizing direction with the pilot signal output from each pilot signal generator  11   a ,  11   d ,  11   b , and  11   e . The reason for realization of the intensity modulation with modulation in the polarizing direction has also ready been explained in the fourth embodiment. The other structure is identical to that of the third embodiment. 
       SEVENTH EMBODIMENT 
       [0062]      FIG. 12  is a block diagram showing a structure of the polarized orthogonal control apparatus as a seventh embodiment. As shown in  FIG. 12 , the control unit  126  of the optical transmitter  151  is equipped with an optical spectrum analyzer  17  in the seventh embodiment. Accordingly, in this embodiment, the electro-optical converter for receiving the signal light output from the pilot signal generator, inverter, and polarizer  5 , filter, multiplier, and discriminator are not required. The other structure is identical to that of the first embodiment. 
         [0063]    The optical spectrum analyzer  17  measures intensity of an signal light output from the polarizer  5  all wavelengths for all wavelengths. The optical spectrum analyzer  17  controls the polarizing state of the signal lights passing the polarization controllers  2   a ,  2   b ,  2   c ,  2   d  to result in the minimum measured value for the (2k−1)th channel and the maximum measured value for the (2k )th channel. Or, the optical spectrum analyzer  17  may control the polarizing state of the signal lights passing the polarization controllers  2   a ,  2   b ,  2   c , and  2   d  to result in the maximum measured value for the (2k−1)th channel and the minimum measured value for the (2k)th channel. 
         [0064]    As explained above, in the embodiments explained above, states of polarizing direction of the adjacent channels can be detected with the multipliers  15   a ,  15   b ,  13   c  and discriminators l 6   a ,  16   b , l 6   c ,  16   d , and  16   e  or optical spectrum analyzer  17 . Moreover, the polarizing direction of signal light of each channel can also be controlled realize orthogonal polarizing directions of the adjacent channels. 
         [0065]    In addition, the first embodiment has an advantage that the number of frequencies of the necessary pilot signal can be reduced to a half in comparison with the third embodiment. Moreover, the second embodiment has an advantage that the apparatus may be simplified in the structure because only one pilot signal frequency is required. Further, the fourth, fifth, and sixth embodiments have advantage that the apparatus can be simplified in comparison with the first, second, and third embodiments, because only the polarizing direction control is required. 
         [0066]    Meanwhile, the first, second, and third embodiments have advantage in comparison with the fourth, fifth, and sixth embodiment that accuracy after control for orthogonal polarizing direction of the signal lights of the adjacent channels is high, because the polarizing direction is not controlled, but the optical intensity is controlled. Moreover, the seventh embodiment has an advantage that the pilot signal is not required. 
         [0067]    Although a few preferred embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claim and their equivalents.