Abstract:
The invention pertains to optical fiber transmission systems, and is particularly relevant to transmission of large volumes of data over long distances at high rates. An improved apparatus achieving precise dispersion compensation in a fiber span is disclosed. In particular, the invention teaches a configurable dispersion compensation trimmer with automatic detection of configuration.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority to Provisional Application Serial No. 60/385,948 entitled “Configurable Dispersion Compensation Trimmer with Automatic Detection of Configuration”, by Guo, et al., filed Jun. 4, 2002. 
     
    
     
       TECHNICAL FIELD OF THE INVENTION  
         [0002]    The invention pertains to optical fiber transmission systems, and is particularly relevant to transmission of large volumes of data over long distances at high rates. An improved apparatus achieving precise dispersion compensation in a fiber span is disclosed. In particular, the invention teaches a configurable dispersion compensation trimmer with automatic detection of configuration.  
         BACKGROUND OF THE INVENTION  
         [0003]    A goal of many modem long haul optical transport systems is to provide for the efficient transmission of large volumes of voice traffic and data traffic over trans-continental distances at low costs. Various methods of achieving these goals include time division multiplexing (TDM) and wavelength division multiplexing (WDM). In time division multiplexed systems, data streams comprised of short pulses of light are interleaved in the time domain to achieve high spectral efficiency, high data rate transport. In wavelength division multiplexed systems, data streams comprised of short pulses of light of different carrier frequencies, or equivalently wavelength, are co-propagate in the same fiber to achieve high spectral efficiency, high data rate transport.  
           [0004]    The transmission medium of these systems is typically optical fiber. In addition there is a transmitter and a receiver. The transmitter typically includes a semiconductor diode laser, and supporting electronics. The laser may be directly modulated with a data train with an advantage of low cost, and a disadvantage of low reach and capacity performance. After binary modulation, a high bit may be transmitted as an optical signal level with more power than the optical signal level in a low bit. Often, the optical signal level in a low bit is engineered to be equal to, or approximately equal to zero. In addition to binary modulation, the data can be transmitted with multiple levels, although in current optical transport systems, a two level binary modulation scheme is predominantly employed.  
           [0005]    Consequently the data propagates through the optical fiber as a short pulse. One of the impairments that this pulse can suffer is its spreading, or dispersion, in time. Excessive pulse spreading resulting from dispersion will cause interference between adjacent bits at the receiver. Dispersion can occur for a variety of reasons both linear and nonlinear. In multimode fiber, different transverse modes propagate different effective distances, to cause modal dispersion. Consequently optical transport over any appreciable distance is accomplished using single mode fiber. Chromatic dispersion of the pulse occurs because the index of refraction of the glass fiber varies with frequency. Since a short data pulse is comprised of a band of frequencies, chromatic dispersion causes pulse shape distortion and spreading as the different spectral components of the data pulse propagate at different velocities in the fiber. In modem optical transport systems this dispersion, or pulse spreading must be periodically corrected, while comprehending the effect of pulsewidth on the nonlinear impairments in the fiber.  
           [0006]    Correcting for chromatic dispersion is therefore an important engineering challenge in optical transport systems. As the reach or capacity of a long haul optical transport system increases, so do the requirements on dispersion compensation. Dispersion compensation is accomplished by adding lengths of fiber to positively or negatively correct for dispersion. For ultra long haul optical transport systems, dispersion compensation must be done quite often, and must be done with great precision. This precision creates a logistical challenge to ensure the correct dispersion compensation is available at time of installation. Currently dispersion compensators are highly customized, and are not designed to alleviate this logistical challenge. There is a need for flexible dispersion compensators that are settable to a precise dispersion compensation value upon installation.  
           [0007]    A second challenge that arises with ultra long haul transport systems is that there physical plant extends over thousands of kilometers. In current optical transport systems inventory and configuration data is recorded manually. There is a need for the automated recording of dispersion configuration data in particular in optical transport systems.  
           [0008]    It is an object of this invention to teach an improved method and apparatus for measuring dispersion that does not suffer from these limitations in accuracy and precision. It is a further object of this invention to provide a compact apparatus that makes a chromatic dispersion measurement in only a few seconds.  
         SUMMARY OF THE INVENTION  
         [0009]    In the present invention, an improved apparatus achieving precise dispersion compensation in a fiber span is taught as required by ultra long haul optical transport systems capable of transcontinental reach.  
           [0010]    In one embodiment of the invention, a flexible dispersion compensator that is settable to a precise dispersion compensation value is disclosed.  
           [0011]    In another embodiment of the invention a configurable dispersion compensation trimmer is disclosed.  
           [0012]    In another embodiment of the invention, a configurable dispersion compensation trimmer with automatic detection of configuration is disclosed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which:  
         [0014]    [0014]FIG. 1 is a schematic illustration of a prior art multiplexed optical transport system.  
         [0015]    [0015]FIG. 2 is a schematic illustration of a dispersion compensator including a configurable dispersion compensation trimmer in accordance with the invention.  
         [0016]    [0016]FIG. 3 is a connection table that illustrating the connections of the configurable dispersion compensation trimmer to achieve different dispersion compensation trims, in accordance with a preferred embodiment of the invention.  
         [0017]    [0017]FIG. 4 is a schematic illustration of a dispersion compensation trimmer section with automatic detection in accordance with one aspect of the invention.  
         [0018]    [0018]FIG. 5 is a schematic illustration of an automated optomechanical switch configured to achieve a dispersion compensation trimmer section with switchable trim in accordance with one aspect of the invention.  
         [0019]    [0019]FIG. 6 is a drawing of the physical implementation of the multi-layer and fiber routing guide of a preferred embodiment of the dispersion compensation trimmer.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments described herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention.  
         [0021]    [0021]FIG. 1 is an illustrative block diagram of an optical transport system  110  for data and/or voice transmission used to support the present invention. Typical long haul optical transport dense wavelength division multiplexed (DWDM) systems transmit 40 to 80 10 Gbps (gigabit per second) channels across distances of 1500 to 6000 km in a single 30 nm spectral band. Shown in the figure is a duplex system in which traffic is both transmitted and received between parties at opposite end of the link. The optical carrier is generated using transmitters  120 . In current DWDM long haul transport systems transmitters  120  are DFB lasers stabilized to specified frequencies on the ITU frequency grid and externally modulated.  
         [0022]    In a DWDM system, different channels operating at distinct carrier frequencies are multiplexed using a multiplexer  121 . Such multiplexers may be implemented using array waveguide (AWG) technology or thin film technology, or a variety of other technologies. After multiplexing, the optical signals are coupled into the transport fiber for transmission to the receiving end of the link. The total link distance may in today&#39;s optical transport systems be two different cities separated by continental distances, from 1000 km to 6000 km, for example. To successfully bridge these distances with sufficient optical signal power relative to noise, the total fiber distance is separated into fiber spans  122 , and the optical signal is periodically amplified using an in line optical amplifier  123  after each fiber span  122 . Typical fiber span distances between optical amplifiers  123  is 50-100 km. Thus, for example, 30 100 km spans would be used to transmit optical signals between points 3000 km apart. Examples of inline optical amplifers  123  include erbium doped fiber amplifiers (EDFAs) and semiconductor optical amplifiers (SOAs).  
         [0023]    Often, there is also included dispersion compensation modules  124  with the in line amplifiers  123 . These dispersion compensator modules  124  adjust the phase information of the optical pulses in order to compensate for the chromatic dispersion in the optical fiber while counteracting the role of optical nonlinearities in the optical fiber.  
         [0024]    At the receiving end of the link, the optical channels are de-multiplexed using a demultiplexer  125 . Such de-multiplexers may be implemented using array waveguide (AWG) technology or thin film technology, or a variety of other technologies. Each channel is then optically coupled to separate optical receivers  126 . The optical receiver  126  is typically comprised of a semiconductor photodetector and accompanying electronics.  
         [0025]    It is a purpose of this invention to teach improved dispersion compensators. An improved apparatus achieving precise dispersion compensation in a fiber span is disclosed. In particular, the invention teaches a configurable dispersion compensation trimmer with automatic detection of configuration.  
         [0026]    It should be noted that FIG. 1 depicts an optical transport system  110  supporting duplex operation wherein each endpoint can both send and receive voice and data traffic. This is important to achieve a typical conversation or data transaction. In FIG. 1, duplex operation is shown to use two distinct fibers, the both together often referred to as a fiber pair. For example, optical transport systems are sometimes deployed with bidirectional traffic providing duplex service on a single fiber.  
         [0027]    Other common variations include the presence of post-amplifiers and pre-amplifers just before and after the multiplexer  121  and de-multiplexer  125 . Another variation that may be employed is the optical dropping and adding of channels at cities located in between the two end cities. The invention disclosed herein, would find application in any of these variations, as well as others. For example, the improved dispersion compensator module taught herein would benefit short reach, or metro applications which may not include an inline optical amplifier  123 .  
         [0028]    In FIG. 2 is shown elements of the invention in relation to dispersion compensator  124 . In accordance with the invention, dispersion compensator  124  comprises a standard dispersion compensator module  202  and dispersion compensator trimmer  210 . Dispersion compensator trimmer  210  comprises short dispersion trim section  212 , intermediate dispersion trim section  214  and long dispersion trim section  216 . In the preferred embodiment each of the trim sections is a length of fiber capable of compensating for a certain amount of dispersion. Dispersion compensator  124  further comprises main input  201  and main output  203 . Standard dispersion compensator module  202  further comprises internal output  205  and internal input  207 . In a preferred embodiment internal output  205  is comprised of a short length of connectorized fiber. In a preferred embodiment internal input  207  is comprised of a short length of connectorized fiber. Dispersion compensator trimmer  210  further comprises short dispersion trim section input  221 , short dispersion trim section output  223 , intermediate dispersion trim section input  225 , intermediate dispersion trim section output  227 , long dispersion trim section input  229  and long dispersion trim section output  231 . In a preferred embodiment short dispersion trim section input  221  is comprised of a length of connectorized fiber. In a preferred embodiment short dispersion trim section output  223  is comprised of a length of connectorized fiber. In a preferred embodiment intermediate dispersion trim section input  225  is comprised of a length of connectorized fiber. In a preferred embodiment intermediate dispersion trim section output  227  is comprised of a length of connectorized fiber. In a preferred embodiment long dispersion trim section input  229  is comprised of a length of connectorized fiber. In a preferred embodiment long dispersion trim section output  231  is comprised of a short length of connectorized fiber.  
         [0029]    The connectors on the connectorized inputs and outputs may further be specified to enable a large degree of interconnectivity as will be taught below, in reference to FIG. 3. Therefore, in a preferred embodiment internal output  205  is comprised of a short length of connectorized fiber. Internal input  207  is comprised of a short length of connectorized fiber. Short dispersion trim section input  221  is comprised of a length of connectorized fiber and short dispersion trim section output  223  is comprised of a length of connectorized fiber. Intermediate dispersion trim section input  225  is comprised of a length of connectorized fiber and intermediate dispersion trim section output  227  is comprised of a length of connectorized fiber. Long dispersion trim section input  229  is comprised of a length of connectorized fiber and long dispersion trim section output  231  is comprised of a short length of connectorized fiber. Output  205  and inputs  223 ,  227  and  231  are coupled to female connectors. Inputs  207  output  223 ,  227  and  231  are coupled to male connectors. In this embodiment it will be understood that any of the male connectors may be coupled with any of the female connectors. It should be understood that the orientation of the connectors is not critical and may be reversed, so long as a secure connection between the lengths of fiber are achieved.  
         [0030]    In a preferred embodiment standard dispersion compensator  202  is comprised of a dispersion element that will correct for approximately 70-100% of the required dispersion in an average fiber span  122 . In a preferred embodiment this dispersion element comprises a length of dispersion compensating fiber. This dispersion element is positioned between main input  201  and internal output  205 , or this dispersion element is positioned between internal input  207  and main output  203 . In this embodiment short dispersion trim section  212 , intermediate dispersion trim section  214  and long dispersion trim section  216  are comprised of additional lengths of dispersion compensating fiber.  
         [0031]    In an alternate embodiment standard dispersion compensator  202  is comprised of a dispersion element that will correct for approximately 100-130% of the required dispersion in an average fiber span  122 . In a preferred embodiment this dispersion element comprises a length of dispersion compensating fiber. This dispersion element is positioned between main input  201  and internal output  205 , or this dispersion element is positioned between internal input  207  and main output  203 . In this alternate embodiment short dispersion trim section  212 , intermediate dispersion trim section  214  and long dispersion trim section  216  are comprised of lengths of SMF-28 fiber.  
         [0032]    In a preferred embodiment, the length of fiber in short dispersion trim section  212  provides amount of dispersion equal to δ, intermediate dispersion trim section  214  provides amount of dispersion equal to 2δ, and long dispersion trim section  216  provides amount of dispersion equal to 4δ. It should be noted that more than three trim sections can be included in the dispersion compensation trimmer. If so, the lengths of fiber in the trimmers can be dictated by the series:  
         2 0 δ,2 1 δ,2 2 δ . . . 2 n-1 δ 
         [0033]    where “n” is the number of trimmers. The result is that through correct permutation an offset dispersion of δ, 2δ, 3δ . . . 2 n  δ can be achieved in general. Of course, no dispersion trimming is achieved when the trimmers are bypassed.  
         [0034]    In an alternate preferred embodiment, short dispersion trim section  212 , intermediate dispersion trim section  214  and long dispersion trim section  216  are disposed vertically on top of each other in order to conserve space. As shown in FIG. 6, in an additional improvement, the front surface  605  of the case  600  of the dispersion compensation trimmer  210  is curved to guide the routing of pigtails  610  and relieve stress on the fiber.  
         [0035]    The use of the standard dispersion compensator module  202  and dispersion compensator trimmer  210  may now be understood in reference to FIG. 2. To achieve a dispersion compensator  124  with only the amount of dispersion afforded by standard dispersion compensator module  202 , internal output  205  is connected to internal input  207 . In this configuration, an optical signal flows into main input  201 , through the dispersion element in standard dispersion compensator module  202 , and out main output  203 . To achieve a dispersion compensator  124  with the amount of dispersion afforded by standard dispersion compensator module  202  plus the amount of dispersion in dispersion compensator trimmer  210 , internal output  205  is connected to one of short dispersion trim section input  221 , intermediate dispersion section input  225  or long dispersion trim section input  229 , while internal input  207  is connected to one of short dispersion trim section output  221 , intermediate dispersion section output  225  or long dispersion trim section output  229 . In this configuration, an optical signal flows into main input  201 , through the dispersion element in standard dispersion compensator module  202 , through dispersion compensator trimmer  210  and out main output  203 .  
         [0036]    In FIG. 3 is a table showing the connections for achieving eight different levels of dispersion using dispersion compensator trimmer  210  shown in FIG. 2. For example, to achieve a trim value of 5δ internal output  205  is connected to short dispersion trim section input  221 , short dispersion trim section output  223  is connected to long dispersion trim section input  229 , and long dispersion trim section output  331  is connected to internal input  207 . In this manner, the 1δ of dispersion trim in short dispersion trim section  212  is added to the 4δ of dispersion trim in long dispersion trim section  216  to achieve an additive total dispersion trim of 5δ. Total dispersion trim values ranging incrementally from 0δ to 7δ are obtained by following the connections laid forth in FIG. 3.  
         [0037]    In FIG. 4 is shown in schematic illustration a dispersion compensation trimmer section with automatic detection  400  in accordance with one aspect of the invention. The dispersion compensation trimmer section with automatic detection  400  comprises a DC power supply  402  and a ground reference  404 . The dispersion compensation trimmer section with automatic detection  400  further comprises resistor  410 , resistor  412 , resistor  414 , resistor  416 , and resistor  418 . As shown in FIG. 4, DC power supply  402  is electrically connected to one lead of resistor  410 , the other lead of resistor  410  is connected to one lead of resistor  412 , the other lead of resistor  412  is connected to one lead of resistor  414 , the other lead of resistor  414  is connected to one lead of resistor  416 , the other lead of resistor  416  is connected to resistor  418 , the other lead of resistor  418  is connected to ground reference  404 . In this manner, DC power supply  402  ground reference  404 , resistor  410 , resistor  412 , resistor  414 , resistor  416 , and resistor  418  comprise a voltage dividing resistor ladder as is well known in the art.  
         [0038]    As shown in FIG. 4, the dispersion compensation trimmer section with automatic detection  400  further comprises long dispersion trim section electronic output connection  431 , long dispersion trim section electronic input connection  429 , intermediate dispersion trim section electronic output connection  427 , intermediate dispersion trim section electronic input connection  425 , short dispersion trim section electronic output connection  423  and short dispersion trim section electronic input connection  421 . In a preferred embodiment, long dispersion trim section electronic output connection  431 , long dispersion trim section electronic input connection  429 , intermediate dispersion trim section electronic output connection  427 , intermediate dispersion trim section electronic input connection  425 , short dispersion trim section electronic output connection  423  and short dispersion trim section electronic input connection  421  are realized by electrical wiring with connectors that are embedded in the optical connectors in such a manner as to electrically connect when an optical connection is made. As shown in FIG. 4, the other end of the electrical wiring is connected to the resistors in the voltage dividing resistor ladder.  
         [0039]    For example, one end of long dispersion trim section electronic output connection  431  is mechanically attached to long dispersion trim section output  231  and the other end of long dispersion trim section electronic output connection  431  is electrically connected to resistor  410  and voltage source  402 . Similarly, one end of long dispersion trim section electronic input connection  429  is mechanically attached to long dispersion trim section input  229  and the other end of long dispersion trim section electronic input connection  429  is electrically connected to resistor  412  and resistor  410 . Similarly, one end of intermediate dispersion trim section electronic output connection  427  is mechanically attached to intermediate dispersion trim section output  227  and the other end of intermediate dispersion trim section electronic output connection  427  is electrically connected to resistor  414  and resistor  412 . Similarly, one end of intermediate dispersion trim section electronic input connection  425  is mechanically attached to intermediate dispersion trim section input  225  and the other end of intermediate dispersion trim section electronic input connection  425  is electrically connected to resistor  416  and resistor  414 . Similarly, one end of short dispersion trim section electronic output connection  423  is mechanically attached to short dispersion trim section output  223  and the other end of short dispersion trim section electronic output connection  423  is electrically connected to resistor  418  and resistor  416 . Similarly, one end of short dispersion trim section electronic input connection  421  is mechanically attached to short dispersion trim section input  221  and the other end of short dispersion trim section electronic input connection  421  is electrically connected to resistor  418  and ground reference  404 .  
         [0040]    The dispersion compensation trimmer section with automatic detection  400  further comprises voltage readout position  441 , voltage readout position  443 , voltage readout position  445 , voltage readout position  447 , voltage readout position  449 , and voltage readout position  451 . Voltage readout position  441  is situated between ground reference  404  and resistor  418 . Voltage readout position  443  is situated between resistor  418  and resistor  416 . Voltage readout position  445  is situated between resistor  416  and resistor  414 . Voltage readout position  447  is situated between resistor  414  and resistor  412 . Voltage readout position  449  is situated between resistor  412  and resistor  410 . Voltage readout position  451  is situated between resistor  410  and DC power supply  402 .  
         [0041]    The operation of the dispersion compensation trimmer section with automatic detection may now be described in reference to FIG. 3 and FIG. 4. Upon assembly, short dispersion trim section input  221 , is connected to short dispersion trim section output  223 , intermediate dispersion trim section input  225 , is connected to intermediate dispersion trim section output  227 , and long dispersion trim section input  229  is connected to long dispersion trim section output  231 . These connections have the advantage of protecting the optical connectors from dirt. Because long dispersion trim section electronic output connection  431 , long dispersion trim section electronic input connection  429 , intermediate dispersion trim section electronic output connection  427 , intermediate dispersion trim section electronic input connection  425 , short dispersion trim section electronic output connection  423  and short dispersion trim section electronic input connection  421  are realized by electrical wiring with ends that are embedded in the optical connectors in such a manner as to electrically connect when an optical connection is made, resistor  410 , resistor  414  and resistor  418  are electrically shorted, and the voltage at voltage readout position  441 , the voltage at voltage readout position  443 , the voltage at voltage readout position  445 , the voltage at voltage readout position  447 , the voltage at voltage readout position  449 , and the voltage at voltage readout position  451  reflect this electrical configuration. Upon installation, the correct amount of dispersion trim is chosen and configured using FIG. 4 as reference. The voltage at voltage readout position  441 , the voltage at voltage readout position  443 , the voltage at voltage readout position  445 , the voltage at voltage readout position  447 , the voltage at voltage readout position  449 , and the voltage at voltage readout position  451  reflect a new electrical configuration after the optical connections have been made. The new electrical configuration can be determined by monitoring the voltage readout positions.  
         [0042]    In FIG. 5 is shown in schematic illustration an automated optomechanical switch  500  arranged to achieve a dispersion compensation trimmer section with switchable trim in accordance with one aspect of the invention. Optomechanical switch  500  comprises switchable mirror  516 , switchable mirror  514 , switchable mirror  512  and switchable mirror  510 . In a preferred embodiment switchable mirror  516 , switchable mirror  514 , switchable mirror  512  and switchable mirror  510  are comprised of a mirrored surface on both the front surface and back surface of a substrate that is mounted on the armature of a miniature motor that switches the mirror into the optical plane and out of the optical plane as specified by FIG. 3. In FIG. 5 switchable mirror  516 , switchable mirror  514 , switchable mirror  512  and switchable mirror  510  are shown in relation to internal output  205 , internal input  207 , short dispersion trim section input  221 , short dispersion trim section output  223 , intermediate dispersion trim section input  225 , intermediate dispersion trim section output  227 , long dispersion trim section input  229  and long dispersion trim section output  231 . Optomechanical switch  500  further comprises optical output coupler  505  connected to internal output  205 , optical output coupler  523  connected to short dispersion trim section output  223 , optical output coupler  527  connected to intermediate dispersion trim section output  227 , and optical output coupler  531  connected to long dispersion trim section output  231 . In a preferred embodiment optical output coupler  505 , optical output coupler  523 , optical output coupler  527  and optical output coupler  531  are realized by a collimating lens such as a graded index (GRIN) lens and act to collimate the exiting optical signals. Optomechanical switch  500  further comprises optical input coupler  521  connected to short dispersion trim section input  221 , optical input coupler  525  connected to intermediate dispersion trim section input  225 , optical input coupler  527  connected to long dispersion trim section input  229  and optical coupler  507  connected to internal input  207 . In a preferred embodiment optical output coupler  505 , optical output coupler  523 , optical output coupler  527  and optical output coupler  531  are realized by a collimating lens such as a graded index (GRIN) lens and act to efficiently couple the incoming optical signals into optical fiber as is well known in the art.  
         [0043]    Switchable mirror  516  is disposed between internal output  205  and short dispersion trim section input  221 . When switchable mirror  516  is set out of the optical plane the optical signal exiting internal output  205  is coupled into short dispersion trim section input  221 . When switchable mirror  516  is set in the optical plane the optical signal exiting internal output  205  is incident on mirror  516  and is directed away from short dispersion trim section input  221  and towards the in optical plane positions of switchable mirror  514 , switchable mirror  512  and switchable mirror  510 .  
         [0044]    Switchable mirror  514  is disposed between short dispersion trim output  223  and intermediate dispersion trim input  225 . When switchable mirror  514  is set out of the optical plane any optical signal exiting short dispersion trim section output  223  is coupled into intermediate dispersion trim section input  225  and any optical signal propagating from the in optical plane position of switchable mirror  516  will propagate towards switchable mirror  512  and switchable mirror  510 . When switchable mirror  514  is set in the optical plane any optical signal propagating from in optical plane position of mirror  516  will be directed into intermediate dispersion trim section input  225  and any optical signal exiting short dispersion trim section output  223  will be directed toward the in optical plane positions of switchable mirror  512  and switchable mirror  510 .  
         [0045]    Switchable mirror  512  is disposed between intermediate dispersion trim output  227  and long dispersion trim input  229 . When switchable mirror  512  is set out of the optical plane any optical signal exiting intermediate dispersion trim section output  227  is coupled into long dispersion trim section input  229  and any optical signal propagating from the in optical plane position of switchable mirror  516  or switchable mirror  514  will propagate towards switchable mirror  510 . When switchable mirror  512  is set in the optical plane, any optical signal propagating from “in” optical plane position of mirror  516  or  514  will be directed into long dispersion trim section input  229  and any optical signal exiting intermediate dispersion trim section output  227  will be directed toward the in optical plane positions of switchable mirror  510 .  
         [0046]    Switchable mirror  510  is disposed between long dispersion trim output  231  and internal input  207 . When switchable mirror  510  is set out of the optical plane the optical signal exiting long dispersion trim section output  231  is coupled into internal input  207 . When switchable mirror  510  is set in the optical plane the optical signal propagating from any of switchable mirror  516 , switchable mirror  514  or switchable mirror  512  will be coupled into internal input  207 .  
         [0047]    Therefore, by setting the positions of switchable mirror  516 , switchable mirror  514 , switchable mirror  512  and switchable mirror  510  any dispersion trim value of FIG. 3 may be automatically realized. In a preferred embodiment, the position of switchable mirror  516 , switchable mirror  514 , switchable mirror  512  and switchable mirror  510  will be electrically readable, and will therefore telegraph the state of automated optomechanical switch  500  arranged to achieve a dispersion compensation trimmer section with switchable trim.  
         [0048]    While this invention has been described in reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.