Patent Publication Number: US-2010129079-A1

Title: Optical wdm transmission apparatus and optical wdm transmission method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2008-301979, filed on Nov. 27, 2008, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to an optical wavelength division multiplexer (WDM) transmission apparatus and optical WDM transmission method. 
     BACKGROUND 
       FIG. 8  is a schematic depicting an example of a typical wavelength division multiplexer (WDM) transmission apparatus. The configuration depicted in  FIG. 8  is of an optical WDM transmission apparatus that selects from among light separated according to wavelength and light input from an external apparatus. An optical WDM transmission apparatus  800  includes a preamplifier  801  provided upstream at the input of an optical signal and having an amplifier  810  that amplifies the optical signal; a wavelength separating unit  802  that includes a demultiplexer (DMUX)  820 , which demultiplexes a multiplexed optical signal according to wavelength (channels ch 1  to chn); an optical power varying unit  803  that includes a optical attenuator (VOA)  830  for each wavelength and changes the power of the optical signal by adjusting the level of attenuation; a wavelength multiplexing unit  804  that includes a multiplexer (MUX)  840  and multiplexes the optical signals of differing wavelengths into a multiplexed signal; and an optical amplifier  805  that amplifies the multiplexed signal using a post amplifier  850  and transmits the amplified multiplexed signal to a subsequent station such as another optical WDM transmission apparatus (see, for example, Japanese Laid-Open Patent Publication No. 2001-197010). 
     The preamplifier  801  further includes a detector (PD)  811  that is upstream from the amplifier  810  and detects the power of the optical signal input to the amplifier  810  and a detector (PD)  812  that is downstream from the amplifier  810  and detects the power of the optical signal amplified and output by the amplifier  810 . The optical amplifier  805  further includes a detector (PD)  851  that is upstream from the post amplifier  850  and detects the power of the optical signal input to the post amplifier  850  and a detector (PD)  852  that is downstream from the post amplifier  850  and detects the power of the optical signal amplified and output by the post amplifier  850 . 
     The optical power varying unit  803  further includes optical receivers (PD 1 )  831  that are downstream from the VOAs  830  and detect the power of the optical signals transmitted through the VOAs  830 . 
     According to the configuration depicted in  FIG. 8 , ADD light of each channel may be input from external sources through ports  860 . Further, light separated according to wavelength by the wavelength separating unit  802  may be transmitted as is and output to the optical power varying unit  803 . Another configuration example may output to an external destination through the ports  860 , light separated according to wavelength by the wavelength separating unit  802 , or output to the optical power varying unit  803 , light input through the ports  860  and separated according to wavelength. 
       FIG. 9  is a schematic depicting components of the optical WDM transmission apparatus related to optical power control. The example depicted in  FIG. 9  includes the optical power varying unit  803  and the wavelength multiplexing unit  804  depicted in  FIG. 8 . The optical WDM transmission apparatus  800  further includes a monitoring control unit  870  that monitors the power of the optical signal. The monitoring control unit  870  includes a monitoring unit  871  that monitors detection values of the optical receivers (PD 1 )  831  respectively provided for each wavelength of the optical power varying unit  803 ; a storage unit  872  storing therein power control parameters; a calculating unit  873  that, based on the parameters stored in the storage unit  872  and the detection values monitored by the monitoring unit  871 , calculates a control value for the power of the optical signal; and a VOA control unit  874  that variable controls the attenuation levels of the VOAs  830  respectively provided for each wavelength. An external terminal  882  outputs a target input power value for the post amplifier  850  per channel and an amplified spontaneous emission (ASE) correction value, the storage unit  872  storing both as parameters. 
     When the power of the optical signals is adjusted, an external variable wavelength optical power source  880  is connected to the ports  860  and optical signals of each wavelength (ch 1  to n) are input. A power meter  881  is connected to an output port  845  of the wavelength multiplexing unit  804  and the power of the optical signal passing through the optical power varying unit  803  and the wavelength multiplexing unit  804  is measured. 
     Control of the power adjustment executed by the monitoring control unit  870  includes: 
     1) The VOA control unit  874  fully opening (setting to the minimum attenuation level) all of the VOAs (ch 1  to n) and measuring, per channel, the optical loss (Ltotal_w, where w is each channel) occurring in the paths from the ports  860  (optical input terminal) to the output port  845  (output terminal of the wavelength multiplexing unit  804 ). At this time, since measurement is simplified, the power of the optical signal input from the variable wavelength optical power source  880  is 0 dBm/ch.
 
2) The optical signal power (OP_w) detected by the optical receivers (PD 1 )  831  respectively provided for each channel being recorded and stored in the storage unit  872 .
 
3) Setting a target power (Otgt_w) per channel to suppress optical power differences between channels in the multiplexed optical signal input to the post amplifier  805 . The set values are recorded to the storage unit  872 .
 
4) The calculating unit  873  calculating optical loss (Lmux_w) for each channel of the wavelength multiplexing unit  804 . The calculation equation being Lmux_w=Ltotal_w−OP_w.
 
5) By adding optical loss to the target powers per channel, the calculating unit  873  calculating a range in which the optical receivers (PD 1 )  831  preferably detect the power of the optical signals. The calculation equation being Opd 1 _w=Otgt_w+Lmux_w.
 
6) Subsequently, the attenuation levels of the VOAs  830  being adjusted through the VOA control unit  874  so that the optical signal powers are detected by the optical receivers (PD 1 )  831  within the range Opd 1 . Thus, based on the power of each optical signal before multiplexing and preliminarily measured information of the control unit, the power of the optical signal at each wavelength is calculated and control is performed (see, for example, Japanese Laid-Open Patent Publication No. 2007-067759).
 
     However, with the conventional technology above, optical signal power is not appropriately controlled when wideband noise (ASE) is included in the input optical signal.  FIG. 10  is a schematic depicting the state of an optical signal with respect to the MUX provided in the wavelength multiplexing unit. Wavelength is indicated along the horizontal axis, while power is indicated along the vertical axis. In this example, the optical receiver (PD 1 )  831  provided in the optical power varying unit  803  is able to measure only the total power of an optical signal that includes ASE. As depicted in (a) of  FIG. 10 , the optical receivers (PD 1 )  831  provided upstream of the MUX  840  detect an optical signal λ 1  that includes wideband ASE (for convenience in the explanation hereinafter, among the optical signal wavelengths λ 1  to n, explanation is given for only λ 1 ). 
     According to the nature of wavelength multiplexing, the MUX  840  transmits the optical signal λ 1  as depicted in (b) of  FIG. 10  and thus, the wavelength multiplexing unit  804  provided downstream from the optical power varying unit  803  has a filter property. As a result of the filter property, the optical signal output from the MUX  840  includes a component of the optical signal and only ASE near the wavelength of the optical signal transmitted through the MUX  840 , as depicted in (c) of  FIG. 10 . Thus, by adjusting the VOAs  830  according to only the power detected by the optical receivers (PD 1 )  831 , the power of the optical signal input for each channel to the post amplifier  805  is below the desirable set value because the power detected by the optical receivers (PD 1 )  831  includes that of ASE components, which cover a wideband and are removed during transmission through the MUX  840 , as depicted in (d) of  FIG. 10 . 
     According to the control of the VOAs  830  based on power detection at such optical receivers (PD 1 )  831 , the difference between target values set for the post amplifier  805  and the actual optical power output by the wavelength multiplexing unit  804  becomes significant. Thus, due to the size of ASE components of the input optical signal and bandwidth of the optical signal, optical signal power is not adjusted to a proper value, for each wavelength after multiplexing. Specifically, the total power of the multiplexed wavelengths is not adjusted to a target value and wavelength deviation in the power of the optical signals of each wavelength occurs inhibiting improvement in the quality of the optical signals. 
     SUMMARY 
     According to an aspect of an embodiment, an optical WDM transmission apparatus includes plural optical attenuators that respectively attenuate the power of optical signals separated according to wavelength; plural first optical receivers that respectively detect the power of the attenuated optical signals; a multiplexer that multiplexes the optical signals; a second optical receiver that detects the power of the multiplexed optical signal; and a monitoring control unit that includes a first control system that controls the optical attenuators so that the powers detected at the first optical receivers respectively become target values, and a second control system that, based on the power detected by the second optical receiver and information concerning the number of wavelengths corresponding to the optical signals input, controls the optical attenuators so that the powers of the optical signals respectively become the target values. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic depicting a configuration of optical power adjustment before operation of an optical WDM transmission apparatus according to a first embodiment; 
         FIG. 2  is a flowchart of pre-operation processing; 
         FIG. 3  is a schematic of parameters stored to the storage unit during the processing depicted in FIG.; 
         FIG. 4  is a schematic depicting a configuration of optical power adjustment during operation of the optical WDM transmission apparatus according to the first embodiment; 
         FIG. 5  is a flowchart of control processing during operation; 
         FIG. 6  is a schematic depicting a configuration of optical power adjustment before operation of an optical WDM transmission apparatus according to a second embodiment; 
         FIG. 7  is a flowchart of control processing during operation; 
         FIG. 8  is a schematic depicting an example of a typical WDM transmission apparatus; 
         FIG. 9  is a schematic depicting components of the optical WDM transmission apparatus related to optical power control; and 
         FIG. 10  is a schematic depicting the state of an optical signal with respect to the MUX provided in the wavelength multiplexing unit. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Preferred embodiments of the present invention will be explained with reference to the accompanying drawings. An optical WDM transmission apparatus according the present invention includes a control system that, based on the power of each optical signal separated according to wavelength, adjusts the power of each optical signal, and a control system that adjusts the power of each optical signal by detecting the optical signal power after multiplexing and performing a given calculation. The optical power of a multiplexed optical signal id detected using an optical receiver provided in a wavelength mulitplexing unit. Thus, the power of each optical signal separated according to wavelength is adjusted by VOAs to adjust the post-multiplexing optical power. 
     A first embodiment assumes a configuration that receives input of optical signals separated according to wavelength.  FIG. 1  is a schematic depicting a configuration of optical power adjustment before operation of an optical WDM transmission apparatus according to the first embodiment. A portion of the configuration of an optical WDM transmission apparatus  100  is depicted in  FIG. 1 . The optical WDM transmission apparatus  100 , on an input side, includes ports  160  to which a variable wavelength optical power source  180  is connected. The variable wavelength optical power source  180  inputs optical signals of wavelengths λ 1  to n to the optical WDM transmission apparatus  100 . 
     An optical power varying unit  103  includes an optical attenuator (VOA)  130  for each wavelength and adjusts (attenuates) the power of the optical signals of each wavelength. Optical receivers (PD 1 )  131  downstream from the VOAs  130  detect the power of the optical signals after attenuation. A wavelength multiplexing unit  104  includes a MUX  140  and multiplexes the optical signals output from the optical power varying unit  103 . An optical receiver (PD 2 )  143  detects the total power of the multiplexed signal. 
     Thus, the attenuation level of each of the VOAs  130  is controlled by the detection by the optical receivers (PD 1 )  131  respectively provided immediately downstream from the VOAs  130  and for each wavelength. Additionally, the detection value of the optical receiver (PD 2 )  143  provided downstream from the MUX  140  is used. Specifically, the power of the optical signal at each wavelength is obtained. Consequently, loss at the MUX  140  and the ASE of each wavelength is considered and the attenuation level of each of the VOAs  130  is appropriately controlled. 
     A monitoring control unit  170  acquires the detection values of the optical receivers (PD 1 )  131  that detect the power of the optical signals at each wavelength of the optical power varying unit  103 , and the detection value of the optical receiver (PD 2 )  143  that detects the power of the multiplexed optical signal. The monitoring control unit  170  includes a monitoring unit  171 ; a storage unit  172  storing therein power control parameters; a calculating unit  173  that, based on the parameters stored in the storage unit  172  and a monitoring value of the monitoring unit  171 , calculates a control value for optical signal power; and a VOA control unit  174  that, based on the calculated control value, variably controls the attenuation level of the VOAs  130 . An external terminal  182  outputs a per-channel, target input power value for a post amplifier; the storage unit  172  stores target input power value as a parameter. 
     When the power of the optical signal is adjusted, an external variable wavelength optical power source  180  is connected to the ports  160  and optical signals of each wavelength (ch 1  to n) are input. The optical signals are separated according to wavelength, i.e., the power of each of the optical signals before multiplexing is detected by the optical receivers (PD 1 )  131  and the power of the multiplexed signal is detected by the optical receiver (PD 2 )  143 . 
     Control of the optical signal power according to the configuration above is roughly separated into pre-operation preparation and control during operation, and is executed by the monitoring control unit  170 . 
       FIG. 2  is a flowchart of pre-operation processing. The VOA control unit  174  of the monitoring control unit  170  fully opens (sets to the minimum attenuation level) all of the VOAs  130  (step S 201 ). The variable wavelength optical power source  180  inputs an optical signal for each wavelength to the ports  160  (step S 202 ). For convenience in the explanation hereinafter, the input power here is 1 mW. 
     The optical receivers (PD 1 , PD 2 )  131 ,  143  respectively detect the power of the light received. The optical receiver (PD 2 )  143  detects an optical power Opd 2 _w (step S 203 ). The optical receivers (PD 1 )  131  detect an optical power Opd 1 _w (step S 204 ). w is a given wavelength. The optical power Opd 1 _w and Opd 2 _w are stored in the storage unit  172 . 
     The calculating unit  173 , based on the difference between the optical power Opd 1 _w and the optical power Opd 2 _w, calculates, for each wavelength, optical loss (Lomux_w) occurring in the wavelength multiplexing unit  104  (step S 205 ). Here, optical loss occurring in paths from the ports  160  to the optical receiver (PD 2 )  143  is calculated, for each wavelength, using the difference of the input power and the optical power Opd 2 _w; optical loss occurring in paths from the ports  160  to the optical receivers (PD 1 )  131  is calculated, for each wavelength, using the difference of the input power and the optical power Opd 1 _w; and based on the differences between the above calculation results, optical loss Lomux_w at the MUX  140  is obtained for each wavelength. 
     The calculating unit  173  calculates loss VOALoss at the VOAs  130  (step S 206 ). The calculation is performed using the difference of the power of the optical signal input to one of the ports  160  and the optical power Opd 1 _w detected by the optical receivers (PD 1 )  131 ; i.e., 1 mW-Opd 1 _w. The value obtained indicates the minimum loss (min) at the corresponding VOA  130 . The loss at the VOA VOALoss (min) is stored in the storage unit  172 . 
     The target input power Otgt for the post amplifier provided downstream from the wavelength multiplexing unit  104  is set (step S 207 ). The target input power Otgt is stored in the storage unit  172 .  FIG. 3  is a schematic of parameters stored to the storage unit during the processing depicted in  FIG. 2 . Although explanation of the above processing has been given with respect to one wavelength, the processing is executed with respect to each of the wavelengths λ 1  to n, optical loss Lomux_w at the MUX  140  is recorded to the storage unit  172  for each wavelength. 
       FIG. 4  is a schematic depicting a configuration of optical power adjustment during operation of the optical WDM transmission apparatus according to the first embodiment. As depicted in  FIG. 4 , during operation, the variable wavelength optical power source  180  is disconnected from the ports  160  and optical signals of wavelengths λ 1  to n are input through the ports  160 . Further, an optical amplifier  105  is connected to the downstream side of the wavelength multiplexing unit  104 , and outputs the optical signal amplified by the post amplifier  150  to another optical WDM transmission apparatus downstream. The external terminal  182  outputs, to the monitoring control unit  107 , the number of wavelengths of the optical signals input to the ports  160  (effective wavelengths). 
       FIG. 5  is a flowchart of control processing during operation. Among the wavelengths λ 1  to n, optical signals of the effective wavelengths are input through the ports  160 , which are input terminals (step S 501 ). Here, the input power is unknown (arbitrary) and the following processing is executed with respect to each wavelength in ascending order of wavelength. 
     The optical receivers (PD 1 )  131  detect the optical power for each wavelength input (step S 502 ). The calculating unit  172  reads the loss VOALoss at the VOAs  130  from the storage unit  172  (step S 503 - 1 ), and calculates the input optical power Opin_w for each wavelength (step S 503 - 2 ). Here, processing is executed with respect to each wavelength in ascending order of wavelength. 
       input optical power for each wavelength  Opin   —   w=Opd 1 +VOA Loss(min)+ VOAλ   
     Where, VOAλ is the VOA attenuation level. 
     The calculating unit  173  reads the target input power Otgt from the storage unit  172  (step S 504 - 1 ), and calculates a VOA control target value VOAtgt_w for each wavelength (step S 504 - 2 ). 
         VOA  control target value for a given wavelength  VOAtgt   —   w=Opin   —   w −( Otgt +( Opd 1 —   w−Opd 2 —   w )) 
     The VOA control unit  174  executes VOA control for adjusting the attenuation level of the VOAs  130  (step S 505 ). Here, it is determined whether the difference of the optical power Opd 1 _w detected for a wavelength by an optical receiver (PD 1 )  131  and the target input power Otgt_w is below a predetermined allowable margin of error (step S 506 ). 
       power  Opd 1 —   w−Otgt &gt;allowable margin of error 
     If the result of the comparison at step S 506  exceeds the allowable margin of error (step S 506 : NO), processing returns to step S 502  and VOA control is executed again. If the result is within the allowable margin of error (step S 506 : YES), the following processing is executed. 
     The optical output power Opd 2  after multiplexing is detected by the optical receiver (PD 2 )  143  of the wavelength multiplexing unit  104  (step S 507 ). An average power per wavelength Opd 2   w _ave is calculated (step S 508 ). The optical receiver (PD 2 )  143  is disposed downstream from the MUX  140  and hence, the power for only one wavelength cannot be detected, i.e., the total power for all of the wavelengths in the multiplexed optical signal is detected by the optical receiver (PD 2 )  143 . Thus, by dividing the total power detected Opd 2  by the number of wavelengths n, an average power per wavelength is obtained. 
       average power per wavelength  Opd 2 w   —   ave=Opd 2/ n (mw) 
     Next, it is determined whether the difference of the average power per wavelength Opd 2   w _ave and the target input power Otgt of the post amplifier  150  is below a predetermined allowable margin of error (step S 509 ). 
       average power per wavelength  Opd 2 w   —   ave −target input power  Otgt &gt;allowable margin of error 
     If the result of the comparison at step S 509  indicates the difference to be within the allowable margin of error (step S 506 : YES), a series of the processing ends. On the other hand, if the result indicates the difference to exceed the allowable margin of error (step S 506 : NO), processing proceeds to step S 510 , and reset processing is executed. 
     At step S 510 , the wavelength (ch) to be subject to reset processing is confirmed (step S 510 ). The larger of the average power per wavelength Opd 2   w _ave and the target input power Otgt of the post amplifier  150  is determined (step S 511 ). Reset processing for the wavelength (ch) is executed in ascending order of wavelength. 
       the average power per wavelength  Opd 2 w   —   ave −the target input power  Otgt    
     If the result of the comparison at step S 511  indicates the average power per wavelength Opd 2   w _ave to be less than the target input power Otgt (step S 511 : &lt;0), the VOA control unit  174  reduces the attenuation level of the VOA  130  (step S 512 ); if the average power per wavelength Opd 2   w _ave is greater than the target input power Otgt (step S 511 : &gt;0), the VOA control unit  174  increases the attenuation level of the VOA  130  (step S 513 ). After steps S 512  and S 513 , the number of the wavelength (ch) to be reset is updated by adding 1 (step S 514 ), flow returns to step S 507  and processing from step S 507  is executed. Thus, the processing at step S 509 , i.e., control whereby the difference of the average power per wavelength Opd 2   w _ave and the target input power Otgt is within the predetermined allowable margin of error, is executed. 
     According to the control processing above, irrespective of optical signal bandwidth, or more specifically regardless of whether ASE is included, a wavelength multiplexed optical signal may be controlled to a desirable value for each wavelength included, wavelength deviation is eliminated, and the optical signal power for each wavelength may be made uniform and output. Further, with the configuration above, detection at the optical receiver (PD 2 )  143  after wavelength multiplexing and calculation of optical power per wavelength by the monitoring control unit  170  is used in VOA attenuation control; therefore, connection of an optical spectrum analyzer, optical channel monitor, optical power meter, etc. to the ports  160  is unnecessary. 
     A second embodiment assumes a configuration that multiplexes optical signals demultiplexed by a DMUX and newly inserted (ADD) optical signals.  FIG. 6  is a schematic depicting a configuration of optical power adjustment before operation of an optical WDM transmission apparatus according to the second embodiment. Elements identical to those depicted in  FIG. 1  are given the same reference numerals used in  FIG. 1 . 
     As depicted in  FIG. 6 , on an input side of the optical WDM transmission apparatus  100 , a wavelength separating unit  102  is provided, a multiplexed optical signal input to port  121  is demultiplexed by a DMUX  120  into respective wavelengths λ 1  to n and the resulting optical signals of each wavelength are input to the optical power varying unit  103 . The optical power varying unit  103  includes optical switches  135  upstream to the VOAs  130 . The ports  160  for inserting optical signals (ADD) are connected to the optical switches  135 , and light output from the variable wavelength optical power source  180  is input through the ports  160 . The optical switches  135  may transmit (THROUGH) optical signals of wavelengths λ 1  to n output from the wavelength separating unit  102  through paths (indicated as “a” in  FIG. 6 ). Further, the optical switches  135  may add optical signals in paths (indicated as “b” in  FIG. 6 ) from the ports  160  to the optical signals in the paths “a” by a switching. 
     The external terminal  182  outputs the number of input wavelengths, information concerning the optical signals (“a”) being transmitted in the apparatus, and information concerning the optical signals added (“b”) to the apparatus. The monitoring control unit  170 , based on the above information, determines the input state of the optical signals. Processing for pre-operation preparation is identical to that of the first embodiment depicted in  FIG. 2 . Further, after completion of the pre-operation processing, the variable wavelength optical power source  180  is disconnected from the ports  160  initiating an operation state. In the operation state, through the ports  121  and  160 , optical signals of arbitrary wavelengths and power are input, respectively. 
       FIG. 7  is a flowchart of control processing during operation. As depicted in  FIG. 7 , the processing flow is for the most part, identical to that depicted in  FIG. 5 , and processing steps identical to those in  FIG. 5  are given the same reference numerals used in  FIG. 5 . In the determination at step S 509 , if the difference of the average power per wavelength Opd 2   w _ave and the target input power Otgt of the post amplifier  150  exceeds a predetermined allowable margin of error (step S 509 : NO), the processing proceeds to step S 510 , and when reset processing is executed, it is determined whether the optical signal of focus is an inserted (ADD) optical signal or a transmitted (THROUGH) optical signal (step S 600 ). 
     Thus, when the determination at step S 509  indicates that the allowable margin of error is exceeded, because the ASE power has been removed by a filter property of the DMUX  120  from the optical signals in the paths “a” (THROUGH), the cause of the allowable margin of error being exceeded is determined to be optical signal inserted (path “b”) from the ports  160 . The reset processing from step S 510  may be attenuation control of the VOAs  130  performed with respect to the inserted optical signals and in ascending order of wavelength. 
     According to the processing above, irrespective of optical signal bandwidth, whether optical signals are transmitted through the apparatus or optical signals are inserted, a wavelength multiplexed optical signal may be controlled to a desirable value for each wavelength included, wavelength deviation is eliminated, and the optical signal power at each wavelength may be made uniform and output. The optical WDM transmission apparatus  100  is not limited to transmitting input optical signals, and may output (DROP) optical signals from the ports  160 , receive new optical signals (ADD) through the ports  160 , etc. Further, transmission paths in the network may vary from short distances to long distances. Although power associated with ASE components varies according to such factors, the above configuration enables the optical signal power for each wavelength to be made uniform and output. 
     As described, according to the embodiments, optical signal power may be appropriately adjusted for each wavelength and consequently, wavelength deviation of the wavelengths in a multiplexed optical signal is eliminated and signal quality is improved. Additionally, long distance transmission is enabled. The embodiments disclosed are applicable to optical WDM transmission apparatuses having at least VOAs and an MUX, and improve adjustment precision with respect to the optical power of the multiplexed optical signal output. 
     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.