Abstract:
A variable optical attenuation system has at least one variable optical attenuation device that receives a light signal, attenuates the light signal, and transmits an attenuated light signal. The variable optical attenuator system further has a controller circuit in communication with the variable optical attenuation device to provide an attenuation control signal to the variable optical attenuation device to cause the variable optical attenuation device to adjust an attenuation factor of the variable optical attenuation device. The variable optical attenuator system also has a data retaining device such as an EEPROM in communication with the controller circuit. The data retaining device has a listing of attenuation factors and corresponding attenuation control signals. A communication interface such as a serial data link, within the variable optical attenuator system provides communication between the controller circuit and an external command system. The external command system indicates a desired attenuation factor and the control circuit accesses the data retaining device to retrieve the corresponding attenuation control signal. The control circuit transmits the corresponding attenuation control signal to the variable optical attenuation device, which then adjusts to assume the desired attenuation factor. A calibration method for the variable optical attenuation system corresponds the attenuation factors to the attenuation control signals and stores the attenuation factors and the corresponding control signals to the data retaining device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to optical transmission systems and more particularly to variable optical attenuators (VOAs) within optical transmission systems. Even more particularly this invention relates to circuits and methods for automatically adjusting variable optical attenuators and to methods and systems for calibrating variable optical attenuators to allow for the automatic adjustment of the variable optical attenuators. 
     2. Description of Related Art 
     Variable optical attenuators (VOA) are well known in the art and permit the attenuation of optical or light signals as they are transferred in an optical transmission system. Variable optical attenuators are of two fundamental types, mechanical and non-mechanical. The mechanical variable optical attenuators have moving parts such as stepper motors to adjust an optical filter to vary the attenuation. In non-mechanical variable optical attenuators, the mechanism employed to adjust the attenuation is either a magneto-optic effect or thermo-optic effect that modifies the light waveguide. The attenuation settings of a non-mechanical variable optical attenuator are generally wavelength dependent. Mechanical variable optical attenuators on the other hand provide adjust the optical attenuation in a manner that provides relative independence of wavelength. 
     Mechanical variable optical attenuators such as described in U.S. Pat. No. 6,149,278 (Mao, et al.) have a pair of substantially parallel mirrors that attenuate an optical signal based, at least in part, on the rotation angle of the mirrors. When the pair or mirrors is in a predetermined position, an input optical signal is directed from an input port to an output port with a minimum insertion loss. As the pair of mirrors is rotated, the optical signal is shifted in a parallel fashion. This provides increased insertion loss and an attenuated signal at the output port. The pair of mirrors are rotated a stepper motor or similarly controlled mechanism. 
     An alternate mechanical variable optical attenuator is described in U.S. Pat. No. 4,398,806 (Bennett, et al.). Bennett et al. describes a variable optical attenuator that has wedge shaped plates that are adjusted for form Fresnel lens structures. Each of plates has two surfaces defined by an angle of convergence, a pair plates are supported with two of the surfaces of each plate being spaced apart and in parallel alignment and with the angle of convergence of the two plates being in opposite directions. A second pair of plates are supported with two of the surfaces of each plate being spaced apart and in parallel alignment and with the angle of convergence the plates. One plate of each pair of plates is adjusted to modify the attenuation of the variable optical attenuator. 
     As the demand for communication networks has increased, wavelength division multiplexing (WDM) is becoming the technique used for increasing the amount of information that can be carried on-fiberoptic cables. Variable optical attenuators are employed within the network to allow the equalization of the gain of the bands of light frequencies transmitted on the fiberoptic cables. Further, the variable optical attenuators allow the addition and removal of selected bands or channels at various terminal points of the communication network. 
     Refer now to FIG. 1 for an overview of an application of a variable optical attenuator. FIG. 1 illustrates an amplification and balancing device for a fiberoptic communication channel. The light  10  from a fiberoptic cable in a wavelength division multiplexed communication system is composed of multiple wavelengths λ 1 , λ 2 , λ 3 , . . . , λ n . Prior to equalization and balancing each of the wavelengths λ 1 , λ 2 , λ 3 , . . . , λ n  have non-equal amplitudes or gains. For proper operation these amplitudes or gains must be equal or balanced. 
     The light  10  from a fiberoptic cable is the input to an optical demultiplexer  15 . The optical demultiplexer  15  separates the wavelengths λ 1 , λ 2 , λ 3 , . . . , λ n  of the multiplexed light signal  10  into the individual light signals  20   a ,  20   b , . . . ,  20   n . The individual light signals  20   a ,  20   b , . . . ,  20   n  are each applied respectively to variable optical attenuators  25   a ,  25   b , . . . ,  25   n . The outputs of the variable optical attenuators  25   a ,  25   b , . . . ,  25   n  are then the inputs to the multiplexer  30 . The individual light signals  20   a ,  20   b , . . . ,  20   n  are then recombined to form the input light signal input to the erbium-doped fiber amplifier (EDFA)  35 . The erbium-doped fiber amplifier  35  amplifies the light signal to form the output light signal  40  that is transferred to a subsequent fiberoptic cable. 
     The variable optical attenuators  25   a ,  25   b , . . . ,  25   n  are each calibrated to adjust the gain of the wavelengths λ 1 , λ 2 , λ 3 , . . . , λ n  of the individual light signals  20   a ,  20   b , . . . ,  20   n  such that outputs of the variable optical attenuators  25   a ,  25   b , . . . ,  25   n  are approximately equal. The recombined and amplified light signal  40  now has wavelengths λ 1 , λ 2 , λ 3 , . . . , λ n  that have equal gain and are balanced. 
     An alternate application for variable optical attenuators is shown in FIG.  2 . This application illustrates a switching application when separate wavelength channels within different light signals  110 ,  125 ,  130 , and  175  are added and removed for redirection to receiving and transmitting nodes (not shown) of the communication network. Generally the light signals  110  and  130  are demultiplexed to their individual wavelength channels. Those individual wavelength channels that are to be removed from the light signals  110  and  130  are separated and transferred to the receiving nodes. The individual wavelength channels that are to be added are combined with the remaining wavelength channels to form the light signals  125  and  175 . In order for the removed wavelength channels and the newly formed light signals to have appropriate balance and gain across all the wavelength channels, a variable optical attenuator is added to each channel to perform the attenuation to equalize the gain of each channel. 
     Referring now to FIG. 2 for a more detailed discussion. The light signals  110  and  130  are respectively the inputs to the demultiplexers  115  and  135 . The demultiplexers  115  and  135  decompose the light signals  115  and  135 -into their component wavelength channels. Those wavelength channels  116  that are to be transferred to the light signal  125  become a first set of inputs to the multiplexer  120 . Those wavelength channels  117  that are to be removed from the decomposed light signal  110  are transferred to the set of variable optical attenuators  150 . Similarly, those wavelength channels  136  that are to be transferred to the light signal  175  are transferred to the multiplexer  170  and those wavelength channels  137  that are to be removed from the decomposed light signal  130  are the inputs to the set of variable optical attenuators  150 . 
     The variable optical attenuators  150  attenuate the wavelength channels  117  to balance and equalize the light signals to form the wavelength channels  119 . The wavelength channels  119  are the inputs to the demultiplexer  140 , which decomposes the wavelength channels into the individual light signals  142  that are transferred to the receiving nodes of the communication system. Similarly, the variable optical attenuators  150  attenuate the wavelength channels  137  to balance and equalize the light signals to form the wavelength channels  139 . The wavelength channels  139  are the inputs to the demultiplexer  160 , which decomposes the wavelength channels into the individual light signals  162  that are transferred to other receiving nodes of the communication system. 
     To add wavelength channels to the light signals  125 , and  175  the multiplexers  145  and  155  from the transmitting nodes of the communication system receive the individually separated wavelength channels  147  and  157  respectively. The multiplexers  145  and  155  combine the wavelength channels  147  and  157  to form the light signals  149  and  159  that are then inputs to the sets variable optical attenuators  150 . The variable optical attenuators  150  balance and equalize the gain of the light signals  149  and  159  to respectively form the light signals  118  and  138 . The light signals  118  and  138  are then respectively inputs to the multiplexers  120  and  170 . The multiplexer  120  combines the light signals  116  and  118  to form the light signal  125 , which is transferred, to a fiberoptic cable for transmission to a next node of the communication network. Likewise, the multiplexer  170  combines the light signals  136  and  138  to form the light signal  175 , which is transferred, to a fiberoptic cable for transmission to a next node of the communication network. 
     The variable optical attenuators  150  balance and equalize the gain of the light signals  149  and  159  to compensate for wavelength dependent insertion loss of the optical components of the system. Further the variable optical attenuators  150  can be placed to receive the light signals  110 ,  175 ,  125 , and  130  to compensate for insertion loss in the transmission media of any of the light signals  110 ,  175 ,  125 , and  130 . For example if the Light signals  110  and  175  have optical components inserted into the path such as another receiver, the variable optical attenuator  150  can be applied to the light signals  110 ,  175 ,  125 , and  130  to provide stable signals when the receiver is placed in the path. 
     FIG. 3 illustrates a variable optical attenuator control system as illustrated by U.S. Pat. No. 6,061,171 (Taylor et al.). Light signals  200   a , . . . ,  200   d  are inputs to the variable optical attenuator devices  205   a , . . . ,  205   d . The adjustments of the attenuation of the variable optical attenuator devices  205   a , . . . ,  205   d  are communicated from the variable optical attenuator controllers  225   a , . . . ,  225   d  by the control signals  235   a , . . . ,  235   d . Taylor et al. describes a feedback structure that allows the variable optical attenuator controllers  225   a , . . . ,  225   d  to adjust the variable optical attenuator devices  205   a , . . . ,  205   d  based on the relative power or intensity of the attenuated light signals  207   a , . . . ,  207   d . The variable optical attenuator devices  205   a , . . . ,  205   d  attenuate the light signals  200   a , . . . ,  200   d  to form the attenuated light signals  207   a , . . . ,  207   d , which are the input signals to the couplers  210   a , . . . ,  210   d.    
     The couplers  210   a , . . . ,  210   d  sample a portion of the attenuated light signals  207   a , . . . ,  207   d  and split the attenuated light signals  207   a , . . . ,  207   d  to light signals  218   a , . . . ,  218   d . The light signals  218   a , . . . ,  218   d  are inputs to the optical monitor control system  220  that measures the relative magnitudes of the light signals  218   a , . . . ,  218   d  and thus the intensity of the light signals  207   a , . . . ,  207   d . The couplers  210   a , . . . ,  210   d  transfer the remaining light signals to the fiberoptic cable  215   a , . . . ,  215   d.    
     The optical monitor control system  220  transfers the measured values of the light signals  218   a , . . . ,  218   d  as the electrical signals  223   a , . . . ,  223   d  to the variable optical attenuator controllers  225   a , . . . ,  225   d . Each of the variable optical attenuator controllers  225   a , . . . ,  225   d  determine from the power of the light signals  218   a , . . . ,  218   d  the number of steps or amount of movement is required to adjust the attenuation factor of the variable optical attenuators  205   a , . . . ,  205   d.    
     “Backlash” denotes the attenuation setting accuracy when adjusting the variable optical attenuator devices  205   a , . . . ,  205   d  including adjustment made by reversing the direction of motor operation. It can be shown that the accuracy or “backlash” of the system as shown can cause the variable optical attenuator devices  205   a , . . . ,  205   d  to have error in their adjustment. 
     The feedback from the couplers  210   a , . . . ,  210   d , the optical monitor control system  220  and the variable optical attenuator controllers  225   a ,. . . ,  225   d  can cause excessive amounts of the “backlash.” 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide a variable optical adjustment system to provide adjustment to a variable optical attenuator based on its desired attenuation factor. 
     Another object of this invention is to provide a communication system for transmitting a desired attenuation factor for a variable optical attenuator to a variable optical attenuator controller. 
     Further, another object of this invention is to provide a method for calibrating a variable optical attenuator to determine and record the adjustment steps versus the attenuation factor for a variable optical attenuator. 
     To accomplish at least one of these and other objects, a variable optical attenuation system has at least one variable optical attenuation device that receives a light signal, attenuates the light signal, and transmits an attenuated light signal. The variable optical attenuator system further has a controller circuit in communication with the variable optical attenuation device to provide an attenuation control signal to the variable optical attenuation device to cause the variable optical attenuation device to adjust an attenuation factor of the variable optical attenuation device. The variable optical attenuator system also has a data retaining device such as an EEPROM in communication with the controller circuit. The data retaining device has a listing of attenuation factors and corresponding attenuation control signals. A communication interface such as a serial data link, within the variable optical attenuator system provides communication between the controller circuit and an external command system. The external command system indicates a desired attenuation factor and the control circuit accesses the data retaining device to retrieve the corresponding attenuation control signal. The control circuit transmits the corresponding attenuation control signal to the variable optical attenuation device, which then adjusts to assume the desired attenuation factor. 
     The variable optical attenuation system is calibrated to correspond the attenuation factors to the attenuation control signals and store the attenuation factors and the corresponding control signals to the data retaining device. During calibration, the communication interface is used by an external calibration system to command the control circuit to send attenuation control signals to the variable optical attenuator device to adjust the attenuation factor. The external calibration system then measures the attenuation factor that corresponds to an adjustment position set by the attenuation control signal. The measurement is then transmitted from the external calibration system to the control circuit for placement within the data retaining device. 
     To facilitate communication during the calibration, the communication interface operates with a set of protocols. This protocol includes a send control signal magnitude code. The send control signal magnitude code include a magnitude value that is used by the control circuit to transmit the magnitude of the control signal to cause the selected variable optical attenuator device to advance to a specified adjustment. The protocol further includes a save attenuation factor code. The external calibration system transmits the save attenuation factor to the control circuit indicating that the present setting of the variable optical attenuator device has the appended attenuation and that the control signal magnitude and the attenuation should be placed in the data retaining device. At the completion of the calibration process, the external calibration system verifies the contents of the data retaining device by sending a read attenuation factor code. The read attenuation factor code instructs the control circuit to transmit the attenuation factor for the denoted address location within the data retaining device. 
     The variable optical attenuation system communicates with the external command system through the communication interface using a communication protocol. The communication protocol includes a current attenuation query code that is transmitted from the external command system requesting a current attenuation factor at which the variable optical attenuation device is set. A current attenuation response code is transmitted from the controller circuit to the external command system indicating the current attenuation factor of the variable optical attenuation device. The external command system sends a select active device code to the controller circuit indicating which variable optical attenuation device is to be active. A set active device attenuation code is transmitted from the external command system to the controller circuit indicating the desired attenuation factor at which the active variable optical attenuation device is to be set. Once the variable optical attenuator device is set to a desired attenuation factor for the particular fiber communication link connected to the variable optical attenuator device, the external command system sends a store active device attenuation code to the controller circuit commanding the controller circuit to record a current attenuation factor for the active device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a balancing and equalization application of a variable optical attenuator of the prior art. 
     FIG. 2 is a schematic diagram of an adding and removing wavelength channel application for a variable optical attenuator of the prior art. 
     FIG. 3 is a schematic diagram of a variable optical attenuator adjustment system of the prior art. 
     FIG. 4 is a schematic diagram of a variable optical attenuator adjustment system of this invention. 
     FIG. 5 is block diagram of a variable optical attenuator calibration system of this invention. 
     FIG. 6 is table defining the communication protocol for calibrating and adjusting variable optical attenuators of this invention. 
     FIG. 7 is a flow diagram for the method for calibrating variable optical attenuators of this invention. 
     FIG. 8 is a plot of the number of pulses of the control signal from a reference point versus the attenuation for each setting of the control signal. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The variable optical attenuator system of this invention, as shown in FIG. 4, has light signals  300   a , . . . ,  300   d  as inputs to the variable optical attenuator devices  305   a , . . . ,  305   d . The attenuation of each of the variable optical attenuator devices  305   a , . . . ,  305   d  is controlled by the variable optical attenuator controller  325  from settings stored in the EEPROM  330 . The setting or the amount of steps or pulses necessary to cause each variable optical attenuator device  305   a , . . . ,  305   d  to be adjusted is stored with a corresponding attenuation. The variable optical attenuator controller  325  has a serial link  322  that contains a communication protocol indicating the attenuation factor to which an activated variable optical attenuator device  305   a , . . . ,  305   d  is to be set. The variable optical attenuator controller  325  receives the coding designating the amount of attenuation that is desired for a desired variable optical attenuator device  305   a , . . . ,  305   d . The variable optical attenuator controller  325  determines the location within the EEPROM the location of the attenuation factor and the corresponding number of steps or pulses required to adjust the activated variable optical attenuator device  305   a , . . . ,  305   d  from a reference setting to the setting for the desired attenuation factor. 
     If the activated variable optical attenuator device  305   a , . . . ,  305   d  is at an attenuation factor other than the reference setting, the variable optical attenuator controller  325  must calculate the amount of adjustment the variable optical attenuator device  305   a , . . . ,  305   d  must make in order to achieve the desired attenuation factor. The control signal  335  is transferred from the variable optical attenuator controller  325  to the activated variable optical attenuator device  305   a , . . . ,  305   d.    
     The outputs of each of the variable optical attenuator devices  305   a ,  305   d  are the attenuated light signals  300   a , . . . ,  300   d  applied to the couplers  310   a , . . . ,  310   d . The couplers  310   a , . . . ,  310   d  sample a portion of the attenuated light signals  307   a , . . . ,  307   d  and split the attenuated light signals  307   a , . . . ,  307   d  to light signals  318   a , . . . ,  318   d  and  315   a , . . . ,  315   d . The light signals  318   a , . . . ,  318   d  are inputs to the optical monitor control system  320  that measures the relative magnitudes of the electrical  318   a , . . . ,  318   d  and thus the intensity of the light signals  307   a , . . . ,  307   d . The couplers  310   a , . . . ,  310   d  transfer the remaining light signals to the fiberoptic cable  315   a , . . . ,  315   d.    
     The optical monitor control system  320  compares the magnitude of the light signals  318  and thus the intensity of the light  307   a , . . . ,  307   d  to the intensity of the light signals  300   a , . . . ,  300   d  to determine the attenuation factor required for the application of the variable optical attenuators. 
     The variable optical attenuator controller  325  has a communication interface that is the serial link  322  such as the well known Electronic Industries Alliance (EIA) standard RS-232 that in this embodiment is connected to the optical monitor control system  320 . The serial link  322  provides the physical circuitry and transmission medium to transfer the communication protocol that allows the variable optical attenuator controller  325  to activate a desired variable optical attenuator device  305   a , . . . ,  305   d , determine control signal required by the transmitted attenuation factor. Refer now to FIG. 6 for a discussion of the protocol of the communication interface. The first command ( 1 ) is a query from the external system such as the optical monitor control system  320  to request that the variable optical attenuator controller  325  provide the attenuation setting of a designated variable optical attenuator device  305   a , . . . ,  305   d . The response from the variable optical attenuator controller  325  is the attenuation setting for the requested variable optical attenuator device  305   a , . . . ,  305   d.    
     The second command ( 2 ) is transmitted from the external system to the variable optical attenuator controller  325  designated which of the variable optical attenuators  305   a , . . . ,  305   d  are to be activated. The third command ( 3 ) is transmitted from the external system to the variable optical attenuator  325  designating the attenuation factor at which the selected variable optical attenuator device  305   a , . . . ,  305   d  is to be set. The variable optical attenuator controller  325  then extracts the number of steps or pulsed required to set the active variable optical attenuator device  305   a , . . . ,  305   d  is adjusted from a reference setting to the required attenuation and then determines the number of the steps or pulses required from the present setting. 
     The fourth command ( 4 ) is transmitted from the external system to the variable optical attenuator controller  325  commanding the variable optical attenuator controller  325  to store the present attenuation setting to the EEPROM  330 . This allows the variable optical attenuator controller  325  to determine any future.adjustments as the number of pulses required to vary the attenuation factor of the active variable optical attenuator device  305   a , . . . ,  305   d.    
     The remaining commands ( 5 - 7 ) provide a command protocol for calibrating the variable optical attenuator devices  305   a , . . . ,  305   d  to determine the attenuation of each of the variable optical attenuator devices  305   a , . . . ,  305   d  versus the settings or pulses of the variable optical attenuators. These commands are to be discussed hereinafter. 
     Since the commands use the attenuation factor to determine the setting of the variable optical attenuator devices  305   a , . . . ,  305   d , the EEPROM  330  must contain the attenuation factors and the setting or number of pulses from a reference setting to determine how to adjust the variable optical attenuator devices  305   a , . . . ,  305   d  to desired attenuation. By setting the variable optical attenuator devices  305   a ,. . . ,  305   d  directly rather than using a feedback mechanism such as shown in the prior art, the “backlash” is avoided or at least minimized. However, in order to take advantage of the direct setting of the attenuation factor of this invention, each variable optical attenuator device  305   a , . . . ,  305   d  must be calibrated to correspond the attenuation factor to a setting or number of pulses from a reference setting. To accomplish this, a calibration system, as shown in FIG. 5, has a light source  400  that emits a standardized light signal. The back reflection meter  405  is connected to the output of the light system to monitor the light reflected within the system so as to determine the intensity of the light being applied to the variable optical attenuator system  420 . The polarization-dependent loss controller  470  provides adjustment to the transmission system to control and minimize the polarization-dependent loss of the calibration system. The light source  400 , the back reflection meter  405 , and the polarization-dependent loss controller  470  are connected  460  by the serial link to the computer system  465 . The computer system  465  provides controlling codes to activate and deactivate the light source  400  and to control the polarization-dependent loss controller  470 . The back reflection measurements by the back reflection meter  405  to the computer system  465 . 
     The output of the polarization-dependent loss controller  470  is the input to the optical switch  415 . The optical switch  415  is in communication with the computer system  465  to select one a path through the switch to its output. The variable optical attenuator system  420  has multiple variable optical attenuator devices that are to be calibrated and each path at the output of the optical switch  415  is connected to one of the multiple variable optical attenuator devices. The optical switch  415  steers the light signal to the variable optical attenuator device that is being calibrated. 
     The power supply  425  is connected to the variable optical attenuator system  420  to provide necessary voltage to the variable optical attenuator system  420  being calibrated. 
     The outputs of the variable optical attenuator system  420  are connected to the inputs of the optical switch  450 . The optical switch provides a path to connect a selected input to the output of the optical switch  450 . The optical switch  450  is in communication with the computer  465  through the serial link  460  to provide the control signals that select which of the inputs of the optical switch  450  to its output. The output of the optical switch  450  is connected to the power meter  455 . The power meter  455  determines the intensity of the light output of the variable optical attenuator device being measured. The power meter  455  is in communication with the computer system  465  through the serial link  460  to transfer the measurement of the intensity of the light signal as attenuated by the selected variable optical attenuator device. 
     The computer system  465  determines the attenuation for each setting or set of pulses that adjusts the variable optical attenuator device. The computer system  465  then transfers the measurement to the variable optical attenuator controller of the variable optical attenuator system  420 . The variable optical attenuator system  420  stores the attenuation factor and the corresponding setting in the EEPROM of the variable optical attenuator system  420 . 
     Referring back to FIG. 6 for a discussion of the communication protocol for calibrating the variable optical attenuator devices  305   a , . . . ,  305   d  of FIG.  4 . The communication interface (serial link)  460  of FIG. 5 is in communication with the variable optical attenuator controller  325  to communicate the setting or number of pulses from the reference setting at which the variable optical attenuator devices  305   a , . . . ,  305   d  are set and the measured attenuation for the setting. The first calibration command ( 5 ) is transmitted from the computer system  465  to the variable optical attenuator controller to command the variable optical attenuator controller to send a control signal to the selected variable optical attenuator device  305   a , . . . ,  305   d  to force the selected variable optical attenuator device  305   a , . . . ,  305   d to a number of steps or pulse from a zero (minimum attenuation) or reference setting. The second calibration setting ( 6 ) is a write command sent by the computer system  465  to the variable optical attenuator controller  325  instructing the variable optical attenuator controller  325  to write an attenuation factor with its corresponding setting to a specific address within the EEPROM  330 . The third calibration command ( 7 ) is transmitted by the computer system  465  to instruct the variable optical attenuator controller  325  to read an address location from the EEPROM  330  and transmit the data to the computer system  465 . This allows the computer system to verify that the EEPROM contains the correct attenuation factors for the settings or number of pulses of the control signal transmitted to the selected variable optical attenuator device  305   a , . . . ,  305   d.    
     Refer now to FIG. 7 for a more complete discussion of the method for calibrating a variable optical attenuator system of this invention. The calibration starts (Box  500 ) by first selecting (Box  505 ) the variable optical attenuator device of the variable optical attenuator system that is to be calibrated. In the description of this method it is assumed that the variable optical attenuator system has multiple variable optical attenuator devices, however there in fact may be a single variable optical attenuator device within the system and still be in keeping with the intent of this invention. 
     The calibrating computer system transmits the first calibration command ( 5 ) to force (Box  510 ) the selected variable optical attenuator device to the zero (minimum attenuation) or reference point. The attenuation factor of the selected variable optical attenuator device is then measured (Box  515 ). The computer system then transmits the second calibration command ( 6 ) with the measured attenuation factor to the variable optical attenuator controller indicating the EEPROM location where the attenuation factor and the step or number of pulses from the reference point is to be retained. The variable optical attenuator controller notes the corresponding step or number of pulse from the reference point and stores the attenuation factor and the step or number pulses to the address location requested. 
     The calibrating computer system again transmits the first calibration command ( 5 ) to the variable optical attenuator controller to have the selected variable optical attenuator move to the next increment. The variable optical attenuator controller then forces (Box  530 ) the selected variable optical attenuator device to increment by the next step or block of pulses to force the variable optical attenuator device to the next setting for measurement. The calibrating computer system then examines (Box  535 ) the variable optical attenuator device to determine if the last increment is achieved. If the last increment has not been achieved, the attenuation is measured (Box  515 ) and the attenuation factor transmitted (Box  520 ) to the variable optical attenuator controller for storage (Box  525 ) with the corresponding step in the EEPROM. The variable optical attenuator device is then incremented ( 530 ) and the calibrating computer system (Box  535 ) whether the last increment is achieved. This is repeated until all the increments are measured and then the calibrating computer system determines (Box  540 ) whether all the variable optical attenuator devices of the variable optical attenuator system have been calibrated. If all the variable optical attenuator devices are not calibrated, the next variable optical attenuator device to be calibrated is selected (Box  505 ), forced (Box  510 ) and the steps (Boxes  515 ,  520 ,  525 ,  530 ) to characterize the attenuation versus the step or number of pulses from the minimum step or reference point are repeated until the variable optical attenuator device is calibrated. When all the variable optical attenuator devices for the variable optical attenuator system are calibrated the computer system requests that the variable optical attenuator controller read the contents of the EEPROM and transmit these readings to the calibrating computer system to verify (Box  545 ) that the calibration is recorded properly. Upon successful verification the procedure ends (Box  550 ). 
     FIG. 8 illustrates a plot of characterization data that would be stored in the EEPROM of a single variable optical attenuator. The variable optical attenuator is forced to a setting of zero pulses and the measurement (≈0.1 db) is recorded. The variable optical attenuator is then incremented by 100 pulses or steps in this case and the measurement recorded (≈0.5 db). The variable optical attenuator is incremented by 100 pulses or steps or the total number of pulses from the reference point and each measurement is recorded until the last increment at 1300 pulses or steps is achieved and the measurement (≈22.0 db) is recorded. The measurements maybe made for various environment factors such as temperature to account for the environmental factors as well. 
     While this invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.