Abstract:
An optical channel monitor includes: a first optical device to include first, second and third optical ports, light input through the first optical port being led to the second optical port, light input through the second optical port being led at least to the third optical port; a second optical device to include fourth, fifth and sixth optical ports, light input through the fourth optical port being led to the fifth optical port, light input through the fifth optical port being led at least to the sixth optical port; an optical filter to include seventh and eighth optical ports optically connected to the second and fifth optical ports, respectively, a specified wavelength being transmitted between the seventh and eighth optical ports; a first photo detector to detect light output from the sixth optical port; and a second photo detector to detect light output from the third optical port.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2012-061886, filed on Mar. 19, 2012, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to an optical channel monitor that monitors each channel of a WDM signal. 
     BACKGROUND 
     Wavelength division multiplexing (WDM) is put into practical use as one of technologies to realize bulk data transmission in optical communication systems. Multiple channels having different wavelengths are provided in the WDM. In other words, multiple optical signals having different wavelengths are multiplexed in a WDM signal. 
     Optical add-drop multiplexing (OADM) apparatuses are provided in nodes of WDM transmission systems that transmit WDM signals. The optical add-drop multiplexing apparatuses are capable of splitting optical signals having specified wavelengths from the WDM signals to lead the optical signals to clients. In addition, the optical add-drop multiplexing apparatuses are capable of inserting client signals having arbitrary wavelengths into the WDM signals. Furthermore, many optical add-drop multiplexing apparatuses include optical channel monitors (OCMs) for monitoring wavelength channels of the WDM signals. 
       FIG. 1  illustrates an example of the configuration of optical add-drop multiplexing apparatuses each including optical channel monitors. Two optical add-drop multiplexing apparatuses  100 A and  100 B provided on optical transmission lines are illustrated in  FIG. 1 . The optical add-drop multiplexing apparatuses  100 A and  100 B are hereinafter collectively referred to as an optical add-drop multiplexing apparatus  100 . 
     The optical add-drop multiplexing apparatus  100  includes an OADM device  101 , an optical beam splitter (BS)  102 , an optical amplifier  103 , an optical multiplexer  104 , an optical channel monitor (OCM)  105 , a controller  106 , an optical demultiplexer  107 , an optical amplifier  108 , an optical beam splitter (BS)  109 , an optical channel monitor (OCM)  110 , a controller  111 , a monitoring signal transmitter  112 , and a monitoring signal receiver  113 . The configuration and the operation of the optical add-drop multiplexing apparatus  100 A are substantially the same as those of the optical add-drop multiplexing apparatus  100 B. The operation of the optical add-drop multiplexing apparatus  100 A will now be described. 
     The OADM device  101  may insert a client signal into a WDM signal. In addition, the OADM device  101  adjusts the power of each channel of the WDM signal in accordance with an instruction from the controller  106 . The OADM device  101  includes a wavelength selective switch (WSS). 
     The optical beam splitter  102  splits the WDM signal supplied from the OADM device  101  to lead the WDM signal to the optical amplifier  103  and the optical channel monitor  105 . The WDM signal output from the OADM device  101  in the optical add-drop multiplexing apparatus  100 A is transmitted to the optical add-drop multiplexing apparatus  100 B. 
     The optical channel monitor  105  monitors the power of each channel of the WDM signal supplied from the OADM device  101 . The optical amplifier  103  amplifies the WDM signal supplied from the OADM device  101 . The optical multiplexer  104  multiplexes a monitoring signal supplied from the monitoring signal transmitter  112  on the WDM signal amplified by the optical amplifier  103 . 
     Accordingly, the WDM signal and the monitoring signal are transmitted from the optical add-drop multiplexing apparatus  100 A to the optical add-drop multiplexing apparatus  100 B through an optical transmission line  120 A. Similarly, the WDM signal and the monitoring signal are transmitted from the optical add-drop multiplexing apparatus  100 B to the optical add-drop multiplexing apparatus  100 A through an optical transmission line  120 B. 
     The optical add-drop multiplexing apparatus  100 A receives the WDM signal and the monitoring signal transmitted from the optical add-drop multiplexing apparatus  100 B. The optical demultiplexer  107  in the optical add-drop multiplexing apparatus  100 A leads the WDM signal that is demultiplexed to the optical amplifier  108  and leads the monitoring signal that is demultiplexed to the monitoring signal receiver  113 . The optical amplifier  108  amplifies the WDM signal. The optical beam splitter  109  splits the WDM signal amplified by the optical amplifier  108  to lead the WDM signal to the optical channel monitor  110 . An optical splitter  115  splits the WDM signal output from the optical beam splitter  109 . A specified wavelength is selected from the WDM signal that is split and the WDM signal having the selected wavelength is transmitted to a client. 
     The optical channel monitor  110  monitors the power of each channel of the WDM signal received from the optical add-drop multiplexing apparatus  1006 . The controller  111  generates a monitoring signal including control information to be transmitted to the optical add-drop multiplexing apparatus  100 B on the basis of the result of the monitoring by the optical channel monitor  110 . The monitoring signal transmitter  112  leads the monitoring signal generated by the controller  111  to the optical multiplexer  104 . 
     The monitoring signal receiver  113  in the optical add-drop multiplexing apparatus  100 A receives a monitoring signal transmitted from the optical add-drop multiplexing apparatus  1006 . The controller  106  controls the OADM device  101  on the basis of the result of the monitoring by the optical channel monitor  105  and the monitoring signal received from the optical add-drop multiplexing apparatus  1006 . Specifically, the controller  106  controls the OADM device  101  so as to compensate or suppress the power deviation between the channels of the WDM signal. 
     As described above, the optical add-drop multiplexing apparatus  100 A adjusts the power of each channel of the WDM signal to be transmitted to the optical add-drop multiplexing apparatus  100 B on the basis of the power of each channel of the WDM signal to be transmitted to the optical add-drop multiplexing apparatus  100 B and the monitoring signal received from the optical add-drop multiplexing apparatus  1006 . In addition, the optical add-drop multiplexing apparatus  100 A monitors the power of each channel of the WDM signal received from the optical add-drop multiplexing apparatus  100 B and notifies the optical add-drop multiplexing apparatus  100 B of the result of the monitoring by using the monitoring signal. 
     Signal light monitors that monitor the power of each signal light of wavelength division multiplexing signal light are proposed as a related technology. Such a signal light monitor includes an optical splitter provided on an optical fiber transmission line; a wavelength tunable filter that transmits a light component having a specified wavelength included in the light split by the optical splitter; a sweeper that sweeps the wavelength transmitted through the wavelength tunable filter in a specified wavelength region; a photo detector that receives the light transmitted through the wavelength tunable filter; a sampling unit that samples the output from the photo detector; a storage unit that stores the variation in time in the sampling unit; and an arithmetic processor that performs arithmetic processing to the wavelength output from the photo detector on the basis of data stored in the storage unit. For example, such a technology is disclosed in Japanese Laid-open Patent Publication No. 10-173266. An optical multiplexer/demultiplexer described in Japanese Laid-open Patent Publication No. 2006-310963 is known as another related technology. 
     SUMMARY 
     According to an aspect of the invention, an optical channel monitor includes: a first optical device configured to include a first optical port, a second optical port, and a third optical port, light input through the first optical port being led to the second optical port, light input through the second optical port being led at least to the third optical port; a second optical device configured to include a fourth optical port, a fifth optical port, and a sixth optical port, light input through the fourth optical port being led to the fifth optical port, light input through the fifth optical port being led at least to the sixth optical port; an optical filter configured to include a seventh optical port optically connected to the second optical port of the first optical device and an eighth optical port optically connected to the fifth optical port of the second optical device, a specified wavelength being transmitted between the seventh optical port and the eighth optical port; a first photo detector configured to detect light output from the sixth optical port of the second optical device; and a second photo detector configured to detect light output from the third optical port of the first optical device. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates an example of the configuration of optical add-drop multiplexing apparatuses each including optical channel monitors in related art; 
         FIG. 2  illustrates an example of the configuration of the optical channel monitor; 
         FIG. 3  illustrates an example of the configuration of optical transmission apparatuses each including an optical channel monitor according to an embodiment; 
         FIG. 4  illustrates an example of the configuration of the optical channel monitor according to the embodiment; 
         FIG. 5  is a diagram for describing how to monitor channels of a WDM signal; 
         FIG. 6  illustrates a first example of the optical channel monitor; 
         FIG. 7  illustrates a second example of the optical channel monitor; 
         FIG. 8  illustrates a third example of the optical channel monitor; 
         FIG. 9  illustrates a fourth example of the optical channel monitor; 
         FIG. 10  is a flowchart illustrating an exemplary method of controlling the WDM signal by using a result of monitoring by the optical channel monitor; 
         FIG. 11  illustrates an embodiment of the configuration of an optical add-drop multiplexing apparatus including the optical channel monitors according to the embodiment; 
         FIG. 12  illustrates an example of the configuration of a multiplexing-demultiplexing module; and 
         FIG. 13  illustrates another embodiment of the configuration of an optical add-drop multiplexing apparatus including the optical channel monitors according to the embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The optical add-drop multiplexing apparatus  100  illustrated in  FIG. 1  includes the two optical channel monitors. For example, the optical add-drop multiplexing apparatus  100 A includes the optical channel monitor  105  that monitors the power of each channel of the WDM signal to be transmitted to the optical add-drop multiplexing apparatus  100 B and the optical channel monitor  110  that monitors the power of each channel of the WDM signal received from the optical add-drop multiplexing apparatus  100 B. 
     For example, the optical channel monitor includes a wavelength tunable filter and a photo detector (PD), as illustrated in  FIG. 2 . In this case, the wavelength tunable filter is controlled so as to transmit the wavelength of a channel on which the optical power is to be detected. Accordingly, sweeping the wavelength transmitted through the wavelength tunable filter allows the power of each channel of the WDM signal to be monitored. 
     However, the wavelength tunable filter that is capable of precisely controlling the transmitted wavelength is expensive. Accordingly, when an optical transmission apparatus (for example, the optical add-drop multiplexing apparatus  100  illustrated in  FIG. 1 ) includes multiple optical channel monitors, the optical transmission apparatus is increased in cost. In other words, the cost of the optical transmission apparatus is increased when the optical transmission apparatus has a function or a circuit to monitor multiple WDM signals. 
     An optical channel monitor capable of reducing the cost of a device or a circuit to monitor multiple WDM signals will be described below. 
       FIG. 3  illustrates an example of the configuration of optical transmission apparatuses each including an optical channel monitor according to an embodiment. Two optical add-drop multiplexing apparatuses  1 A and  1 B provided on optical transmission lines are illustrated in  FIG. 3 . The optical add-drop multiplexing apparatuses  1 A and  1 B are hereinafter collectively referred to as an optical add-drop multiplexing apparatus  1 . The optical add-drop multiplexing apparatus is an example of the optical transmission apparatus including the optical channel monitor. 
     The optical add-drop multiplexing apparatus  1  includes an OADM device  11 , an optical beam splitter (BS)  12 , an optical amplifier  13 , an optical multiplexer  14 , an optical channel monitor (OCM)  15 , a controller  16 , an optical demultiplexer  17 , an optical amplifier  18 , an optical beam splitter (BS)  19 , a monitoring signal transmitter  20 , and a monitoring signal receiver  21 . The configuration and the operation of the optical add-drop multiplexing apparatus  1 A are substantially the same as those of the optical add-drop multiplexing apparatus  1 B. The operation of the optical add-drop multiplexing apparatus  1 A will now be described. 
     The OADM device  11  may insert a client signal into a WDM signal (here, a WDM signal  1 ). In addition, the OADM device  11  adjusts the power of each channel of the WDM signal  1  in accordance with an instruction from the controller  16 . The OADM device  11  includes a wavelength selective switch (WSS). 
     The optical beam splitter  12  splits the WDM signal  1  supplied from the OADM device  11  to lead the WDM signal  1  to the optical amplifier  13  and the optical channel monitor  15 . The WDM signal  1  output from the OADM device  11  in the optical add-drop multiplexing apparatus  1 A is transmitted to the optical add-drop multiplexing apparatus  1 B through an optical transmission line  120 A. 
     The optical channel monitor  15  monitors the power of each channel of the WDM signal  1  supplied from the OADM device  11 . In addition, the optical channel monitor  15  monitors the power of each channel of a WDM signal  2  received from the optical add-drop multiplexing apparatus  1 B. The optical channel monitor  15  simultaneously monitors the power of each channel of the WDM signal  1  and the power of each channel of the WDM signal  2 , as described in detail below. 
     The optical amplifier  13  amplifies the WDM signal  1  supplied from the OADM device  11 . The optical multiplexer  14  multiplexes a monitoring signal  1  supplied from the monitoring signal transmitter  20  on the WDM signal  1  amplified by the optical amplifier  13 . Accordingly, the WDM signal  1  and the monitoring signal  1  are transmitted from the optical add-drop multiplexing apparatus  1 A to the optical add-drop multiplexing apparatus  1 B through the optical transmission line  120 A. Similarly, the WDM signal  2  and a monitoring signal  2  are transmitted from the optical add-drop multiplexing apparatus  1 B to the optical add-drop multiplexing apparatus  1 A through an optical transmission line  120 B. 
     The optical add-drop multiplexing apparatus  1 A receives the WDM signal  2  and the monitoring signal  2  transmitted from the optical add-drop multiplexing apparatus  1 B. The optical demultiplexer  17  in the optical add-drop multiplexing apparatus  1 A leads the WDM signal  2  to the optical amplifier  18  and leads the monitoring signal  2  to the monitoring signal receiver  21 . The optical amplifier  18  amplifies the WDM signal  2 . The optical beam splitter  19  splits the WDM signal  2  amplified by the optical amplifier  18  to lead the WDM signal  2  to the optical channel monitor  15 . An optical splitter  25  splits the WDM signal output from the optical beam splitter  19  (that is, the WDM signal  2 ). A specified wavelength is selected from the WDM signal that is split and the WDM signal having the selected wavelength is transmitted to a client. 
     The monitoring signal transmitter  20  generates the monitoring signal  1  to be transmitted to the optical add-drop multiplexing apparatus  1 B on the basis of the result of the monitoring by the optical channel monitor  15 . The monitoring signal  1  includes information indicating the power of each channel of the WDM signal  2 . The monitoring signal  1  is realized by an optical signal having a wavelength different from that of the WDM signal  1 . The monitoring signal  1  generated by the monitoring signal transmitter  20  is multiplexed on the WDM signal  1  by the optical multiplexer  14 . The monitoring signal receiver  21  in the optical add-drop multiplexing apparatus  1 A receives the monitoring signal  2  transmitted from the optical add-drop multiplexing apparatus  1 B. 
     The controller  16  controls the OADM device  11  on the basis of the result of the monitoring of the WDM signal  1  by the optical channel monitor  15  and the monitoring signal  2  received from the optical add-drop multiplexing apparatus  1 B. Specifically, the controller  16  controls the OADM device  11 , for example, so as to compensate or suppress the power deviation between the channels of the WDM signal  1 . 
     As described above, the optical add-drop multiplexing apparatus  1 A adjusts the power of each channel of the WDM signal  1  to be transmitted to the optical add-drop multiplexing apparatus  1 B on the basis of the power of each channel of the WDM signal  1  to be transmitted to the optical add-drop multiplexing apparatus  1 B and the monitoring signal  2  received from the optical add-drop multiplexing apparatus  1 B. In addition, the optical add-drop multiplexing apparatus  1 A monitors the power of each channel of the WDM signal  2  received from the optical add-drop multiplexing apparatus  1 B to notify the optical add-drop multiplexing apparatus  1 B of the result of the monitoring by using the monitoring signal  1 . 
     The configuration and the operation of the optical add-drop multiplexing apparatus  1 B are substantially the same as those of the optical add-drop multiplexing apparatus  1 A. Specifically, the optical add-drop multiplexing apparatus  1 B adjusts the power of each channel of the WDM signal  2  to be transmitted to the optical add-drop multiplexing apparatus  1 A on the basis of the power of each channel of the WDM signal  2  to be transmitted to the optical add-drop multiplexing apparatus  1 A and the monitoring signal  1  received from the optical add-drop multiplexing apparatus  1 A. In addition, the optical add-drop multiplexing apparatus  1 B monitors the power of each channel of the WDM signal  1  received from the optical add-drop multiplexing apparatus  1 A to notify the optical add-drop multiplexing apparatus  1 A of the result of the monitoring by using the monitoring signal  2 . 
       FIG. 4  illustrates an example of the configuration of the optical channel monitor according to the embodiment. Referring to  FIG. 4 , the optical channel monitor  15  includes a first optical device  31 , a second optical device  32 , a tunable filter  33 , and photo detectors  34  and  35 . The optical channel monitor  15  is mounted in the optical add-drop multiplexing apparatus  1  in the example illustrated in  FIG. 3 . 
     The optical channel monitor  15  simultaneously monitors the power of each channel of the two WDM signals. It is assumed in the following description that the optical channel monitor  15  monitors the power of each channel of the WDM signal  1  and the power of each channel of the WDM signal  2 . In other words, the WDM signal  1  and the WDM signal  2  are input into the optical channel monitor  15 . 
     The first optical device  31  has an optical port P 1 , an optical port P 2 , and an optical port P 3 . The first optical device  31  leads light input through the optical port P 1  to the optical port P 2  and leads light input through the optical port P 2  at least to the optical port P 3 . 
     The second optical device  32  has an optical port P 4 , an optical port P 5 , and an optical port P 6 . The second optical device  32  leads light input through the optical port P 4  to the optical port P 5  and leads light input through the optical port P 5  at least to the optical port P 6 . 
     The tunable filter  33  has an optical port P 7  and an optical port P 8 . The optical port P 7  is optically connected to the optical port P 2  of the first optical device  31 . The optical port P 8  is optically connected to the optical port P 5  of the second optical device  32 . The tunable filter  33  transmits a specified wavelength between the optical port P 7  and the optical port P 8 . The tunable filter  33  does not have directivity. Accordingly, the tunable filter  33  transmits light of the specified wavelength from the optical port P 7  to the optical port P 8  and transmits light of the specified wavelength from the optical port P 8  to the optical port P 7 . 
     The configuration of the respective optical ports P 1  to P 8  and the method of mounting them are not specifically restricted. In other words, each optical port may be realized in an arbitrary configuration and by an arbitrary method allowing input and output of light. 
     The photo detector  34  detects light output from the optical port P 6  of the second optical device  32 . The photo detector  35  detects light output from the optical port P 3  of the first optical device  31 . 
     The WDM signal  1  is input into the optical port P 1  of the first optical device  31 . Then, the WDM signal  1  is output from the optical port P 2  and is led to the optical port P 7  of the tunable filter  33 . The tunable filter  33  transmits a wavelength component specified by an instruction to select a wavelength (wavelength selection instruction). A transmitted wavelength specified by the wavelength selection instruction is called λs. In this case, the tunable filter  33  extracts a λs component from the WDM signal  1  and outputs the extracted λs component through the optical port P 8 . 
     The λs component of the WDM signal  1  output from the optical port P 8  of the tunable filter  33  is led to the optical port P 5  of the second optical device  32 . Then, the λs component of the WDM signal  1  is output from the optical port P 6 . Accordingly, the photo detector  34  detects the λs component of the WDM signal  1 . Specifically, the photo detector  34  generates current corresponding to the λs component of the WDM signal  1 . 
     The WDM signal  2  is input into the optical port P 4  of the second optical device  32 . Then, the WDM signal  2  is output from the optical port P 5  and is led to the optical port P 8  of the tunable filter  33 . The tunable filter  33  extracts a λs component from the WDM signal  2  and outputs the extracted λs component through the optical port P 7 . 
     The λs component of the WDM signal  2  output from the optical port P 7  of the tunable filter  33  is led to the optical port P 2  of the first optical device  31 . Then, the λs component of the WDM signal  2  is output from the optical port P 3 . Accordingly, the photo detector  35  detects the λs component of the WDM signal  2 . Specifically, the photo detector  35  generates current corresponding to the λs component of the WDM signal  2 . 
     A wavelength sweeper  41  instructs the tunable filter  33  to select a wavelength so that the transmitted wavelength λs of the tunable filter  33  is swept over the wavelength range of the WDM signal. For example, as illustrated in  FIG. 5 , it is assumed that the WDM signal includes waveform channels ch  1  to ch n. In this case, the wavelength sweeper  41  instructs the tunable filter  33  to select a wavelength so that the transmitted wavelength λs is swept over all the waveform channels ch  1  to ch n. Although the transmitted wavelength λs is swept from a short wavelength side to a long wavelength side in the example illustrated in  FIG. 5 , the transmitted wavelength λs may be swept from the long wavelength side to the short wavelength side. 
     When the transmitted wavelength λs of the tunable filter  33  is swept in response to the wavelength selection instruction, the signals output from the photo detectors  34  and  35  are varied with the transmitted wavelength λs. For example, in the example illustrated in  FIG. 5 , when the transmitted wavelength λs=λ 1 , current corresponding to the optical signal on the waveform channel ch  1  is generated by the photo detector. When the transmitted wavelength λs=λ 3 , current corresponding to the optical signal on the waveform channel ch  3  is generated by the photo detector. 
     A sampling device  42  samples the signal output from the photo detector  34 . Similarly, a sampling device  43  samples the signal output from the photo detector  35 . A device that converts a current signal into a voltage signal (for example, a trans-impedance amplifier (TIA)) may be provided between the photo detector  34  and the sampling device  42  and between the photo detector  35  and the sampling device  43 . The sampling devices  42  and  43  are each realized by, for example, an analog-to-digital (A/D) converter. 
     A channel detector  44  detects the powers of multiple channels of the WDM signal  1  on the basis of the signal output from the sampling device  42 . Similarly, a channel detector  45  detects the powers of multiple channels of the WDM signal  2  on the basis of the signal output from the sampling device  43 . The channel detectors  44  and  45  may detect the power of each channel in synchronization with a waveform selection signal generated by the wavelength sweeper  41 . 
     As described above, upon sweeping of the transmitted wavelength λs of the tunable filter  33 , the power of each channel of the WDM signal  1  is detected by the channel detector  44  and the power of each channel of the WDM signal  2  is detected by the channel detector  45 . In other words, the optical channel monitor  15  of the present embodiment is capable of simultaneously monitoring the respective channels of the two WDM signals with one tunable filter. 
     The wavelength sweeper  41  and the channel detectors  44  and  45  are each realized by, for example, a digital signal processor (DSP). In this case, the sampling devices  42  and  43  may be included in the digital signal processors or may be provided outside the digital signal processors. The wavelength sweeper  41 , the sampling devices  42  and  43 , and the channel detectors  44  and  45  may be part of the optical channel monitor  15 . Alternatively, the wavelength sweeper  41 , the sampling devices  42  and  43 , and the channel detectors  44  and  45  may be provided outside the optical channel monitor  15 . 
     Although the tunable filter  33  is not specifically restricted, the tunable filter  33  is realized by, for example, an acoustic-optic tunable filter (AOTF). In this case, the control signal (that is, the wavelength selection instruction) for specifying the transmitted wavelength of the tunable filter  33  is an acoustic wave signal (for example, a surface acoustic wave (SAW)) having a frequency corresponding to the transmitted wavelength. Accordingly, the wavelength sweeper  41  may control the frequency of the acoustic wave signal to sweep the transmitted wavelength of the tunable filter  33 . 
     The tunable filter  33  may be realized by using another device. For example, the tunable filter  33  may be realized by a thin film filter. Alternatively, the tunable filter  33  may be realized by an optical filter the transmitted wavelength of which is varied with the temperature. In this case, the transmitted wavelength is controlled with, for example, current to be supplied to a heater provided near the optical filter. Alternatively, the tunable filter  33  may be realized by an optical device that disperses input light. In this case, the angle of the optical device is controlled so that the light of a desired wavelength is led to the corresponding optical port. Such an optical device is realized by, for example, a dielectric multilayer film. 
     Exemplary examples of the optical channel monitor  15  will now be described with reference to  FIG. 6  to  FIG. 9 . The wavelength sweeper  41 , the sampling devices  42  and  43 , and the channel detectors  44  and  45  illustrated in  FIG. 4  are omitted in  FIG. 6  to  FIG. 9 . 
     In the example illustrated in  FIG. 6 , the first optical device  31  and the second optical device  32  are each realized by an optical coupler. Specifically, an optical coupler  31   a  is an example of the first optical device  31  and an optical coupler  32   a  is an example of the second optical device  32 . 
     The optical coupler  31   a  is configured so that light input through the optical port P 1  is led to the optical port P 2 , light input through the optical port P 2  is led to the optical port P 1  and the optical port P 3 , and light input through the optical port P 3  is led to the optical port P 2 . Similarly, the optical coupler  32   a  is configured so that light input through the optical port P 4  is led to the optical port P 5 , light input through the optical port P 5  is led to the optical port P 4  and the optical port P 6 , and light input through the optical port P 6  is led to the optical port P 5 . 
     The optical couplers  31   a  and  32   a  are, for example, waveguide optical couplers or optical fiber couplers. The optical couplers  31   a  and  32   a  may be, for example, 2×1 optical couplers. However, the optical couplers  31   a  and  32   a  are not limited to the 2×1 optical couplers and may be realized by m×n optical couplers (m and n are arbitrary integers). In this case, in the optical coupler  31   a , two optical ports, among the m-number optical ports, are used as the optical ports P 1  and P 3  and one optical port, among the n-number optical ports, is used as the optical port P 2 . Similarly, in the optical coupler  32   a , two optical ports, among the m-number optical ports, are used as the optical ports P 4  and P 6  and one optical port, among the n-number optical ports, is used as the optical port P 5 . 
     As described above, the use of the optical coupler  31   a  and the optical coupler  32   a  as the first optical device  31  and the second optical device  32 , respectively, allows the channel monitoring operation described above with reference to  FIG. 4  and  FIG. 5  to be realized. However, in the configuration illustrated in  FIG. 6 , the wavelength component of the WDM signal  1  passing through the tunable filter  33  is led to not only the optical port P 6  but also the optical port P 4 . This wavelength component may have an effect on a circuit optically connected to the optical port P 4 . Similarly, the wavelength component of the WDM signal  2  passing through the tunable filter  33  is led to not only the optical port P 3  but also the optical port P 1 . This wavelength component may have an effect on a circuit optically connected to the optical port P 1 . 
     Accordingly, the optical channel monitor  15  preferably has a function to remove or suppress the effect on the circuits optically connected to the optical ports P 1  and P 4 . This function is realized by, for example, configurations illustrated in  FIG. 7  to  FIG. 9 . 
     In the example illustrated in  FIG. 7 , the optical channel monitor  15  includes an optical isolator  36  and an optical isolator  37 , in addition to the optical couplers  31   a  and  32   a , the tunable filter  33 , and the photo detectors  34  and  35 . The optical isolator  36  is optically connected to the optical port P 1  of the optical coupler  31   a . The optical isolator  36  is mounted so as to transmit light to be input into the optical port P 1  and block light output from the optical port P 1 . The optical isolator  37  is optically connected to the optical port P 4  of the optical coupler  32   a . The optical isolator  37  is mounted so as to transmit light to be input into the optical port P 4  and block light output from the optical port P 4 . 
     With the above configuration, the wavelength component of the WDM signal  1  passing through the tunable filter  33 , which is output from the optical port P 4 , is blocked by the optical isolator  37 . Similarly, the wavelength component of the WDM signal  2  passing through the tunable filter  33 , which is output from the optical port P 1 , is blocked by the optical isolator  36 . Accordingly, unnecessary optical signal components have no effect on the circuits optically connected to the optical ports P 1  and P 4 . 
     In the example illustrated in  FIG. 8 , an optical coupler  31   b  is used as the first optical device  31  and an optical coupler  32   b  is used as the second optical device  32 . The optical coupler  31   b  is configured so that the split ratio of the optical port P 3  is higher than the split ratio of the optical port P 1 . In other words, the optical coupler  31   b  splits the light input through the optical port P 2  so that the output power from the optical port P 3  is higher than the output power from the optical port P 1 . Similarly, the optical coupler  32   b  is configured so that the split ratio of the optical port P 6  is higher than the split ratio of the optical port P 4 . In other words, the optical coupler  32   b  splits the light input through the optical port P 5  so that the output power from the optical port P 6  is higher than the output power from the optical port P 4 . 
     With the above configuration, most of the wavelength component of the WDM signal  1  passing through the tunable filter  33  is led to the photo detector  34  through the optical port P 6  by the optical coupler  32   b . At this time, although part of the wavelength component of the WDM signal  1  passing through the tunable filter  33  is output from the optical port P 4 , the power of the light output from the optical port P 4  is sufficiently low. Similarly, most of the wavelength component of the WDM signal  2  passing through the tunable filter  33  is led to the photo detector  35  through the optical port P 3  by the optical coupler  31   b . At this time, although part of the wavelength component of the WDM signal  2  passing through the tunable filter  33  is output from the optical port P 1 , the power of the light output from the optical port P 1  is sufficiently low. 
     As described above, unnecessary optical signal components output from the optical ports P 1  and P 4  are suppressed in the example illustrated in  FIG. 8 . The split ratio of each of the optical couplers  31   b  and  32   b  is, for example, 10:1. However, the split ratio of each of the optical couplers  31   b  and  32   b  is not limited to 10:1. 
     In the example illustrated in  FIG. 9 , the first optical device  31  and the second optical device  32  are each realized by an optical circulator. Specifically, an optical circulator  31   c  is an example of the first optical device  31  and an optical circulator  32   c  is an example of the second optical device  32 . 
     The optical circulator  31   c  leads light input through the optical port P 1  to the optical port P 2  and light input through the optical port P 2  to the optical port P 3 . Light input through the optical port P 2  is not led to the optical port P 1 . Similarly, the optical circulator  32   c  leads light input through the optical port P 4  to the optical port P 5  and light input through the optical port P 5  to the optical port P 6 . Light input through the optical port P 5  is not led to the optical port P 4 . 
     With the above configuration, the wavelength component of the WDM signal  1  passing through the tunable filter  33  is led to the photo detector  34  by the optical circulator  32   c . The wavelength component of the WDM signal  1  passing through the tunable filter  33  is not led to the optical port P 4 . Similarly, the wavelength component of the WDM signal  2  passing through the tunable filter  33  is led to the photo detector  35  by the optical circulator  31   c . The wavelength component of the WDM signal  2  passing through the tunable filter  33  is not led to the optical port P 1 . Accordingly, unnecessary optical signal components have no effect on the circuits optically connected to the optical ports P 1  and P 4 . 
       FIG. 10  is a flowchart illustrating an exemplary method of controlling the WDM signal by using the result of monitoring by the optical channel monitor. A process of controlling the power of each channel of the WDM signal  1 , performed by the optical add-drop multiplexing apparatus  1 A illustrated in  FIG. 3 , will now be described. The optical add-drop multiplexing apparatus  1 A uses the result of monitoring of the WDM signal  1  by the optical add-drop multiplexing apparatus  1 B to control the WDM signal  1 . Accordingly, the operations of the optical add-drop multiplexing apparatuses  1 A and  1 B are illustrated in the flowchart illustrated in  FIG. 10 . 
     Referring to  FIG. 10 , in S 1 , the optical channel monitor  15  in the optical add-drop multiplexing apparatus  1 A detects optical power P 1 (λ) of each channel of the WDM signal  1  output from the OADM device  11 . The WDM signal  1  output from the OADM device  11  is split by the optical beam splitter  12  and is led to the optical channel monitor  15 . For example, when the WDM signal  1  is led to the optical port P 1  of the optical channel monitor  15  illustrated in  FIG. 4 , the optical power P 1 (λ) of each channel of the WDM signal  1  is detected by the photo detector  34  (or the channel detector  44 ). Transmission power information representing the optical power P 1 (λ) detected by the optical channel monitor  15  is supplied to the controller  16 . 
     In S 11 , the optical channel monitor  15  in the optical add-drop multiplexing apparatus  1 B detects optical power P 2 (λ) of each channel of the WDM signal  1  received from the optical add-drop multiplexing apparatus  1 A. In the optical add-drop multiplexing apparatus  1 B, the WDM signal  1  is amplified by the optical amplifier  18 , the WDM signal  1  that is amplified is split by the optical beam splitter  19 , and the WDM signal  1  that is split is led to the optical channel monitor  15 . 
     In S 12 , the monitoring signal transmitter  20  in the optical add-drop multiplexing apparatus  1 B generates the monitoring signal  2  including reception power information representing the optical power P 2 (λ) detected by the optical channel monitor  15 . The monitoring signal transmitter  20  transmits the monitoring signal  2  to the optical add-drop multiplexing apparatus  1 A. The monitoring signal  2  is transmitted to the optical add-drop multiplexing apparatus  1 A through the optical transmission line  120 B, along with the WDM signal  2 . 
     The monitoring signal receiver  21  in the optical add-drop multiplexing apparatus  1 A receives the monitoring signal  2  transmitted from the optical add-drop multiplexing apparatus  1 B. The monitoring signal receiver  21  supplies the received monitoring signal  2  to the controller  16 . In other words, the reception power information representing the optical power P 2 (λ) detected by the optical add-drop multiplexing apparatus  1 B is supplied to the controller  16 . 
     In S 2 , the controller  16  in the optical add-drop multiplexing apparatus  1 A calculates optical power deviation ΔP(λ) on the basis of the optical power P 1 (λ) detected by the optical channel monitor  15  in the optical add-drop multiplexing apparatus  1 A and the optical power P 2 (λ) detected by the optical channel monitor  15  in the optical add-drop multiplexing apparatus  1 B. Here, the controller  16  calculates the optical power deviation ΔP(λ) for each channel of the WDM signal  1 .
 
Δ P (λ) [dB]= P 1(λ) [dBm]− P 2(λ) [dBm]
 
     The optical power deviation ΔP(λ) occurs in the transmission from the optical add-drop multiplexing apparatus  1 A to the optical add-drop multiplexing apparatus  1 B. In other words, the optical power deviation ΔP(λ) is affected by the optical amplifier  13  in the optical add-drop multiplexing apparatus  1 A, the optical transmission line  120 A, and the optical amplifier  18  in the optical add-drop multiplexing apparatus  1 B. For example, gain deviation may occur in the optical amplifiers  13  and  18 . The optical transmission line has a wavelength dependent loss (WDL). In addition, a stimulated Raman scattering (SRS) tilt may occur on the optical transmission line. 
     In S 3 , the controller  16  in the optical add-drop multiplexing apparatus  1 A generates a control signal for controlling the optical power of each channel of the WDM signal  1  on the basis of the optical power deviation ΔP(λ). The OADM device  11  controls the power of each channel of the WDM signal  1  in accordance with the control signal. Here, the controller  16  controls the OADM device  11 , for example, so that the respective channels of the WDM signal  1  have the same optical power or substantially the same optical power in the optical add-drop multiplexing apparatus  1 B. It is assumed that the OADM device  11  has a function to control the levels of the individual channels of the WDM signal. In this case, for example, the OADM device  11  controls the amounts of attenuation of the individual channels of the WDM signal. 
     The flowchart in  FIG. 10  illustrates the method of controlling the WDM signal  1  to be transmitted from the optical add-drop multiplexing apparatus  1 A to the optical add-drop multiplexing apparatus  1 B. A method of controlling the WDM signal  2  to be transmitted from the optical add-drop multiplexing apparatus  1 B to the optical add-drop multiplexing apparatus  1 A is substantially the same as the method illustrated in  FIG. 10 . However, the control of the WDM signal  2  is performed by the controller  16  in the optical add-drop multiplexing apparatus  1 B. The controller  16  in the optical add-drop multiplexing apparatus  1 B uses the transmission power information about the WDM signal  2  detected by the optical channel monitor  15  in the optical add-drop multiplexing apparatus  1 B and the reception power information about the WDM signal  2  detected by the optical channel monitor  15  in the optical add-drop multiplexing apparatus  1 A to control the power of each channel of the WDM signal  2 . 
     The optical channel monitor  15  monitors each channel of the WDM signal  1  in the flowchart illustrated in  FIG. 10 . However, the optical channel monitor  15  monitors each channel of the WDM signal  1  and each channel of the WDM signal  2 . For example, in S 1  in the flowchart illustrated in  FIG. 10 , the optical channel monitor  15  in the optical add-drop multiplexing apparatus  1 A detects the optical power of each channel of the WDM signal  1 . The optical channel monitor  15  also detects the optical power of each channel of the WDM signal  2  received from the optical add-drop multiplexing apparatus  1 B, although not illustrated in  FIG. 10 . The optical add-drop multiplexing apparatus  1 A notifies the optical add-drop multiplexing apparatus  1 B of information representing the optical power of each channel of the WDM signal  2  by using the monitoring signal  1 . 
     In S 11  in the flowchart illustrated in  FIG. 10 , the optical channel monitor  15  in the optical add-drop multiplexing apparatus  1 B detects the optical power of each channel of the WDM signal  1  received from the optical add-drop multiplexing apparatus  1 A. The optical channel monitor  15  also detects the optical power of each channel of the WDM signal  2  to be transmitted to the optical add-drop multiplexing apparatus  1 A, although not illustrated in  FIG. 10 . 
     As described above, in the optical add-drop multiplexing apparatus  1  illustrated in  FIG. 3 , one optical channel monitor  15  monitors the two WDM signals. For example, in the optical add-drop multiplexing apparatus  1 A, the optical channel monitor  15  monitors the WDM signal  1  to be transmitted to the optical add-drop multiplexing apparatus  1 B and the WDM signal  2  received from the optical add-drop multiplexing apparatus  1 B. In contrast, in the optical add-drop multiplexing apparatus  100  illustrated in  FIG. 1 , the WDM signal to be transmitted to the optical add-drop multiplexing apparatus  100 B is monitored by the optical channel monitor  105  and the WDM signal received from the optical add-drop multiplexing apparatus  100 B is monitored by the optical channel monitor  110 . 
     Accordingly, with the configurations according to the embodiment, it is possible to reduce the number of the optical channel monitors mounted in the optical add-drop multiplexing apparatus. In other words, the adoption of the optical channel monitor  15  of the embodiment allows the cost of the optical transmission apparatus (the optical add-drop multiplexing apparatus  1  here) having the function to monitor multiple WDM signals to be reduced. 
     A switch to select one WDM signal from multiple WDM signals may be provided at the input side of the optical channel monitor illustrated in  FIG. 2 . In this case, it is possible to monitor the multiple WDM signals with one optical channel monitor. However, since it is not possible to simultaneously monitor the multiple WDM signals with this configuration, it takes a long time to monitor the WDM signals. In contrast, with the configuration according to the embodiment, since it is possible to simultaneously monitor the two WDM signals, it takes a short time to monitor the WDM signals. 
       FIG. 11  illustrates an embodiment of the configuration of an optical add-drop multiplexing apparatus including the optical channel monitors according to the embodiment. The optical add-drop multiplexing apparatus illustrated in  FIG. 11  has a first route and a second route. The “route” means an optical transmission line that extends in a certain direction in this specification. Each route includes an incoming route and an outgoing route. The number of the routes in the optical add-drop multiplexing apparatus may be represented by a unit of “degree.” For example, the optical add-drop multiplexing apparatus illustrated in  FIG. 11  is a two-degree OADM apparatus. 
     The optical add-drop multiplexing apparatus illustrated in  FIG. 11  includes OADM modules  2 X and  2 Y and multiplexing-demultiplexing modules (MUX/DEMUX)  3 X and  3 Y. The OADM modules  2 X and  2 Y are provided for the corresponding routes. In the embodiment illustrated in  FIG. 11 , the OADM module  2 X is provided for the first route and the OADM module  2 Y is provided for the second route. 
     An OADM device  11   x , an optical beam splitter (BS)  12   x , an optical amplifier  13   x , an optical multiplexer  14   x , an optical channel monitor (OCM)  15   x , a controller  16   x , an optical demultiplexer  17   x , an optical amplifier  18   x , an optical beam splitter (BS)  19   x , a monitoring signal transmitter  20   x , and a monitoring signal receiver  21   x  mounted in the OADM module  2 X are substantially the same as the OADM device  11 , the optical beam splitter (BS)  12 , the optical amplifier  13 , the optical multiplexer  14 , the optical channel monitor (OCM)  15 , the controller  16 , the optical demultiplexer  17 , the optical amplifier  18 , the optical beam splitter (BS)  19 , the monitoring signal transmitter  20 , and the monitoring signal receiver  21 , respectively, illustrated in  FIG. 3 . An OADM device  11   y , an optical beam splitter (BS)  12   y , an optical amplifier  13   y , an optical multiplexer  14   y , an optical channel monitor (OCM)  15   y , a controller  16   y , an optical demultiplexer  17   y , an optical amplifier  18   y , an optical beam splitter (BS)  19   y , a monitoring signal transmitter  20   y , and a monitoring signal receiver  21   y  mounted in the OADM module  2 Y are substantially the same as the OADM device  11 , the optical beam splitter (BS)  12 , the optical amplifier  13 , the optical multiplexer  14 , the optical channel monitor (OCM)  15 , the controller  16 , the optical demultiplexer  17 , the optical amplifier  18 , the optical beam splitter (BS)  19 , the monitoring signal transmitter  20 , and the monitoring signal receiver  21 , respectively, illustrated in  FIG. 3 . 
     However, the OADM module  2 X leads a WDM signal received via the first route to the OADM module  2 Y. Similarly, the OADM module  2 Y leads a WDM signal received via the second route to the OADM module  2 X. 
     The OADM module  2 X outputs the WDM signal received via the second route to the first route. The OADM device  11   x  may insert an add signal generated by the multiplexing-demultiplexing module  3 X into the WDM signal. The OADM device  11   x  may block one or more channels of the WDM signal. Similarly, the OADM module  2 Y outputs the WDM signal received via the first route to the second route. The OADM device  11   y  may insert an add signal generated by the multiplexing-demultiplexing module  3 Y into the WDM signal. The OADM device  11   y  may block one or more channels of the WDM signal. 
     An optical splitter  22   x  splits the WDM signal received via the first route. The WDM signal split by the optical splitter  22   x  is led to the OADM device  11   y  and the multiplexing-demultiplexing module  3 X. Similarly, an optical splitter  22   y  splits the WDM signal received via the second route. The WDM signal split by the optical splitter  22   y  is led to the OADM device  11   x  and the multiplexing-demultiplexing module  3 Y. 
     In the above configuration, the optical channel monitor  15   x  monitors the WDM signal to be transmitted to the first route and the WDM signal received via the first route. The optical channel monitor  15   y  monitors the WDM signal to be transmitted to the second route and the WDM signal received via the second route. 
       FIG. 12  illustrates an example of the configuration of a multiplexing-demultiplexing module. A multiplexing-demultiplexing module  3  illustrated in  FIG. 12  is an example of the multiplexing-demultiplexing modules  3 X and  3 Y illustrated in  FIG. 11 . When the multiplexing-demultiplexing module  3  operates as the multiplexing-demultiplexing module  3 X illustrated in  FIG. 11 , a WDM signal A corresponds to an add signal to be led to the OADM device  11   x  and a WDM signal D corresponds to a drop signal received from the optical splitter  22   x . When the multiplexing-demultiplexing module  3  operates as the multiplexing-demultiplexing module  3 Y illustrated in  FIG. 11 , the WDM signal A corresponds to an add signal to be led to the OADM device  11   y  and the WDM signal D corresponds to a drop signal received from the optical splitter  22   y.    
     The multiplexing-demultiplexing module  3  includes an optical beam splitter (BS)  51 , a demultiplexer (Demux)  52 , a multiplexer (Mux)  53 , an optical beam splitter (BS)  54 , and an optical channel monitor (OCM)  55 . The WDM signal D that is input is led to the demultiplexer  52  through the optical beam splitter  51 . The optical beam splitter  51  splits the WDM signal D to lead the WDM signal D to the optical channel monitor  55 . The demultiplexer  52  demultiplexes the WDM signal D for every wavelength to lead the WDM signal D to receivers  56 - 1  to  56 - n . The demultiplexer  52  is realized by, for example, an arrayed waveguide grating (AWG). 
     The multiplexer  53  multiplexes optical signals transmitted from transmitters  57 - 1  to  57 - n  to generate the WDM signal A. The multiplexer  53  is realized by, for example, an AWG. The optical beam splitter  54  splits the WDM signal A supplied from the multiplexer  53  to lead the WDM signal A to the optical channel monitor  55 . 
     The optical channel monitor  55  is realized by the optical channel monitor according to the embodiment. Accordingly, the optical channel monitor  55  monitors each channel of the WDM signal D and monitors each channel of the WDM signal A upon reception of an operation command instructing start of the monitoring operation. Specifically, the optical channel monitor  55  monitors each channel of the WDM signal demultiplexed by the demultiplexer  52  and monitors each channel of the WDM signal multiplexed by the multiplexer  53 . 
       FIG. 13  illustrates another embodiment of the configuration of an optical add-drop multiplexing apparatus including the optical channel monitors according to the embodiment. The optical add-drop multiplexing apparatus illustrated in  FIG. 13  has a first route, a second route, and a third route. In other words, the optical add-drop multiplexing apparatus is a three-degree OADM apparatus. The optical add-drop multiplexing apparatus includes OADM modules  4 X,  4 Y, and  4 Z and a colorless-directionless module  5 . The OADM modules  4 X,  4 Y, and  4 Z are provided for the first route, the second route, and the third route, respectively. 
     Each of the OADM modules  4 X,  4 Y, and  4 Z is substantially the same as the OADM modules  2 X and  2 Y illustrated in  FIG. 11 . However, some components in each of the OADM modules  4 X,  4 Y, and  4 Z are omitted in  FIG. 13  for simplicity. In the configuration illustrated in  FIG. 13 ,  1   x   3  splitters are mounted, instead of the optical splitters  22   x  and  22   y  illustrated in  FIG. 11 . 
     The colorless-directionless module  5  includes an n×1 coupler  61 , an optical amplifier  62 , an optical beam splitter (BS)  63 , a 1×3 splitter  64 , a 3×1 wavelength selective switch (WSS)  65 , an optical amplifier  66 , an optical beam splitter (BS)  67 , a 1×n wavelength selective switch (WSS)  68 , and an optical channel monitor (OCN)  69 . 
     The n×1 coupler  61  optically multiplexes optical signals λ 1  to λn transmitted from the transmitters  57 - 1  to  57 - n  to generate a WDM signal. The optical signals λ 1  to λn have different wavelengths. The optical signals λ 1  to λn may be input into arbitrary ports of the n×1 coupler  61 . In other words, a colorless function is realized. 
     The WDM signal output from the n×1 coupler  61  is amplified by the optical amplifier  62  and is led to the 1×3 splitter  64  and the optical channel monitor  69  by the optical beam splitter  63 . The 1×3 splitter  64  leads the WDM signal to the wavelength selective switch (WSS) in each of the OADM modules  4 X,  4 Y, and  4 Z. The WSS in each of the OADM modules  4 X,  4 Y, and  4 Z selects a desired optical signal from the optical signals λ 1  to λn. The optical add-drop multiplexing apparatus is capable of transmitting the optical signals λ 1  to λn to desired routes. In other words, a directionless function is realized. 
     Each of the OADM modules  4 X,  4 Y, and  4 Z leads the WDM signal received through the corresponding route to the other two OADM modules. The WSS in each of the OADM modules  4 X,  4 Y, and  4 Z selects a desired wavelength in the WDM signals led from the other OADM modules. 
     Each of the OADM modules  4 X,  4 Y, and  4 Z leads the WDM signal received through the corresponding route also to the colorless-directionless module  5 . In other words, the WDM signals through the respective routes are led to the 3×1 wavelength selective switch  65 . The 3×1 wavelength selective switch  65  selects a desired wavelength from each WDM signal. The directionless function is realized in the above manner. 
     The WDM signal output from the 3×1 wavelength selective switch  65  is amplified by the optical amplifier  66  and is led to the 1×n wavelength selective switch  68  and the optical channel monitor  69  by the optical beam splitter  67 . The 1×n wavelength selective switch  68  selects an optical signal of a desired wavelength from the WDM signal that is input. The 1×n wavelength selective switch  68  outputs the selected optical signal through an arbitrary port. In other words, the colorless function is realized. 
     The optical channel monitor  69  is the optical channel monitor according to the embodiment. The optical channel monitor  69  monitors the WDM signal output from the n×1 coupler  61  and the WDM signal output from the 3×1 wavelength selective switch  65 . The optical channel monitor  69  detects the presence of the optical signal and the optical power for each channel of each WDM signal. The optical channel monitor provided in each of the OADM modules  4 X,  4 Y, and  4 Z is also realized by the optical channel monitor according to the embodiment. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.