Patent Publication Number: US-2002006257-A1

Title: Method and system for compensating for chromatic dispersion

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     [0001] The present application claims the benefit of the filing date of copending U.S. Provisional Application, Ser. No. 60/206,037 filed May 22, 2000, entitled “Method and System for Actively Compensating for Variations in Chromatic Dispersion”. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] The invention relates generally to optical communication systems and, more specifically, dispersion compensation in optical systems.  
       BACKGROUND OF THE INVENTION  
       [0003] As light travels through a waveguide, such as an optical fiber, it experiences chromatic dispersion, which is caused by the difference in propagation speed experienced by each wavelength. Chromatic dispersion in high bit rate systems, typically those at 10 Gbs and above are typically compensated for by utilizing a dispersion compensating fiber.  
       [0004] Second order dispersion, also known as slope refers to the derivative of the dispersion with regard to wavelength, and reflects the fact that as the wavelength changes the rate of dispersion also changes.  
       [0005] Certain types of chromatic dispersion compensating fibers may also compensate for the slope, most notably those utilizing high order modes, for example co-pending U.S. Patent application Ser. No. 09/249,830 entitled Optical Communication System with Chromatic Dispersion Compensation. New profiles that compensate for dispersion and slope have recently been announced, see for example Nielsen et al, published March 2000, Optical Fiber Communication Conference Technical Digest pages 101/TuG6-1 to 103/TuG6-3.  
       [0006] The chromatic dispersion of an optical fiber is dependent on temperature. See for example Kato et al “Temperature Dependence of Chromatic Dispersion in Various Types of Optical Fibers” published at the Optical Fiber Communications (OFC) conference, March 2000, pages 104/TuG7-1 to 106/TuG7-3. For a typical transmission waveguide having positive dispersion and dispersion slope, such as a single mode fiber, dispersion is reduced as the temperature of the fiber increases. For fibers with negative dispersion slope, such as dispersion compensating fibers, the sign of the variation is reversed, so that dispersion increases (i.e. is less negative) as the temperature increases. This effect is best estimated as a shift in the zero dispersion wavelength. The temperature dependence of the zero dispersion wavelength of transmission lines have been shown to be approximately 0.03 nm/degree C as shown by Ghosh; Temperature Dispersion of Refractive Indexes in Some Silicate Fiber Glasses—IEEE Photonics Technology Letters, Vol. 6 No. 3 March 1994 pp. 431-433.  
       [0007] In land based systems, dispersion compensating devices are often installed in a temperature controlled environment, thus ensuring relative stability. However there is some fluctuation in temperature even in the controlled environment, with typical specifications requiring operation from −5 to 55 degrees C. Transmission lines however, are exposed to changes in environmental temperature, and are typically subject to even wider temperature swings.  
       [0008] In transmitting systems up to 10 Gbs, this temperature dependence is not critical enough to warrant being dealt with. With systems of even higher bit rates, such as a 40 Gbs system it becomes important to compensate for this dispersion. Similarly, extremely long paths at 10 Gbs without reconversion to an electronic signal will require compensation for these temperature fluctuations. Furthermore, at high bit rates any temperature fluctuation of the dispersion compensating fiber must also be controlled.  
       [0009] Thus, there is a need for a system of variable chromatic dispersion compensation of optical fibers.  
       SUMMARY OF THE INVENTION  
       [0010] Accordingly, it is a principal object of the present invention to overcome the disadvantages of prior art methods of optical fiber dispersion compensation. This is provided in the present invention by use of variable chromatic dispersion, which is accomplished by controlling the temperature in a dispersion compensation fiber. Raising or lowering the temperature of the dispersion compensating fiber changes the dispersion characteristics of the fiber, thus changing the dispersion compensating characteristics of the fiber.  
       [0011] The present invention relates to an apparatus for variable dispersion compensation, comprising a dispersion compensating fiber whose characteristics are modified by a temperature controlling element.  
       [0012] In accordance with a preferred embodiment of the present invention, there is provided an apparatus for variable dispersion compensation of an optical signal comprising:  
       [0013] a dispersion compensating fiber; and  
       [0014] a first temperature controlling element,  
       [0015] wherein the temperature of said dispersion compensating fiber is modified by said temperature controlling element so as to effect a variable dispersion.  
       [0016] In another embodiment, multiple fibers are utilized, with the temperature of each separately modified and controlled to compensate for temperature dependent chromatic dispersion.  
       [0017] The present invention also relates to a method of variable dispersion compensation comprising the steps of receiving an optical signal, transmitting the optical signal through a dispersion compensating fiber and varying the temperature of the fiber such that the characteristics of the fiber are changed.  
       [0018] In accordance with the present invention, there is provided a method for ensuring stable dispersion compensation comprising the steps of:  
       [0019] supplying a dispersion compensating fiber, and  
       [0020] controlling the temperature of said dispersion compensating fiber so as to maintain said fiber at a specified temperature.  
       [0021] Additional features and advantages of the invention will become apparent from the following drawings and description. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0022] The above and further advantages of this invention may be better understood by referring to the following description taken in conjunction with the accompanying drawings in which like numerals designate corresponding elements or sections throughout, and in which:  
     [0023]FIG. 1 illustrates a prior art optical communication system;.  
     [0024]FIG. 2 a  illustrates a block diagram of an embodiment of a prior art high order mode dispersion compensation device;  
     [0025]FIG. 2 b  illustrates a block diagram of another prior art dispersion compensation device;  
     [0026]FIG. 2 c  illustrates a block diagram of another prior art dispersion compensation device;  
     [0027]FIG. 3 a  illustrates an embodiment of an optical communication system in accordance with the present invention;  
     [0028]FIG. 3 b  illustrates another embodiment of an optical communication system in accordance with the present invention;  
     [0029]FIG. 4 a  illustrates a block diagram of an embodiment of a dispersion compensating device in accordance with the present invention;  
     [0030]FIG. 4 b  illustrates a block diagram of another embodiment of a dispersion compensating device in accordance with the present invention;  
     [0031]FIG. 4 c  illustrates a block diagram of another embodiment of a dispersion compensating device in accordance with the present invention;  
     [0032]FIG. 5 a  illustrates a block diagram of a program utilized to control the temperature of a dispersion compensating fiber as shown in FIG. 4 a  and FIG. 4 b  in accordance with the present invention;  
     [0033]FIG. 5 b  illustrates a block diagram of a program utilized to control the temperature of a dispersion compensating fiber as shown in FIG. 4 c  in accordance with the present invention;  
     [0034]FIG. 6 illustrates an embodiment of a high level block diagram of the program utilized to control the temperature in the dispersion compensating fiber shown in FIG. 5, and  
     [0035]FIG. 7 illustrates an optical communication system according to another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0036]FIG. 1 shows a prior art optical communications system  10  operable at bit rates of 10 Gbs and above. The system  10  comprises a transmitter  20 , span of transmission fiber  30 , optical amplifier  40  containing a dispersion compensating device (DCD)  50 , and a receiver  60 . The output of transmitter  20  comprising optical signal  25  is connected to the input of a span of transmission fiber  30 , which is meant to include any fiber designed and utilized for long distance transmission, including but not limited to single mode fiber, dispersion shifted fiber and non-zero dispersion shifter fiber. The output of transmission fiber  30  is connected to the input of optical amplifier  40 , which contains a DCD  50 . Optical amplifier  40  comprises in one embodiment a multiple stage optical amplifier such as an Erbium Doped Fiber Amplifier (EDFA).  
     [0037] The DCD in an exemplary embodiment is positioned between stages of the EDFA. Other configurations are also possible. The DCD  50  in one embodiment comprises a dispersion compensating fiber (DCF), or a combination of mode transformers and a high order mode dispersion compensating fiber, such as one described in co-pending U.S. application Ser. No. 09/249,830 whose contents are incorporated herein by reference. It may also contain multiple fibers for compensating for a combination of dispersion orders, such as one described in co-pending U.S. patent application Ser. No. 09/249,920 whose contents are incorporated herein by reference.  
     [0038] Optical amplifier  40  operates to amplify signal  25 , which has been attenuated during transmission through transmission fiber  30 . DCD  50  operates to compensate for the chromatic dispersion experienced by signal  25  during transmission through transmission fiber  30 , and optical amplifier  40 . The output of amplifier  40  is similarly connected to the input of second span of transmission fiber  30 , whose output is connected to a second amplifier  40 . A number of such spans  30  and amplifiers  40  are connected serially until the optical signal  25  requires electrical regeneration. The output of final span  30  is thus connected to DCD  50  whose output is connected to receiver  60 , which converts the optical signal  25  to an electrical signal.  
     [0039] In one embodiment additional system elements such as demultiplexers (not shown) are often utilized prior to the final DCD  50  so that each sub-band may be fully compensated. In an exemplary embodiment transmitter  20  comprises multiple transmitters each operating at a lower frequency being multiplexed together in a wavelength division multiplexed (WDM) configuration, and receiver  60  comprises a demultiplexer and multiple receivers being operated at a lower clock rate, or a single receiver operating at the full clock rate.  
     [0040]FIG. 2 a  illustrates an exemplary DCD  50 . The device comprises a mode transformer  70 , high order mode DCF  80 , and a second mode transformer  70 . The incoming signal from optical waveguide  20 , which is typically in the fundamental mode, also known as the LP 01  mode, is converted in the mode transformer  70  substantially to a single high order mode, such as the LP 02  mode. DCF  80 , compensates for both the dispersion and dispersion slope, following which the signal is reconverted by mode transformer  70  to the LP 01  mode.  
     [0041]FIG. 2 b  illustrates another embodiment of DCD  50 , and comprises a spool of single mode DCF  90  connected to a section of conventional single mode fiber  85  by a splice  95 . Fiber  90  in one embodiment compensates both dispersion and at least some of the dispersion slope.  
     [0042]FIG. 2 c  illustrates another embodiment of DCD  50  comprising mode transformer  70 , high order mode DCF  80 , second mode transformer  70  and single mode fiber  100 . Single mode fiber  100  acts to complete the compensation, and has a slope and dispersion whose sign is opposite that exhibited by high order mode DCF  80 . The lengths of single mode fiber  100  and DCF  80  are in one embodiment chosen to substantially compensate for the dispersion and slope exhibited by transmission fiber  30 .  
     [0043]FIG. 3 a  illustrates system  110  designed according to the teaching of the present invention to compensate for the variability in the chromatic dispersion of the transmitting fiber  30 . System  110  comprises a transmitter  20 , span of transmission fiber  30 , optical amplifier  40  containing a DCD  50 ′ designed according to the teaching of the present invention, optical coupler  120 , dispersion measurement device  130 , controller  140 , control bus  150 , and receiver  60 .  
     [0044] Transmitter  20  outputs optical signal  25  into the input of a span of transmission fiber  30 , whose output is connected to the input of optical amplifier  40 , which contains DCD  50 ′. Amplifier  40  acts to amplify signal  25  which has been attenuated during transmission through transmission fiber  30 . DCD  50 ′ is controlled via bi-directional bus  150  from controller  140 , and operates to compensate in a fixed manner for dispersion and dispersion slope and to actively compensate, as will be explained hereinbelow, for variability in chromatic dispersion experienced by signal  25 .  
     [0045] The output of amplifier  40  is similarly connected to the input of a second span of transmission fiber  30 , and the output of the second span of transmission fiber  30  is connected to the input of the next amplifier  40 . A number of such spans  30  and amplifiers  40  are connected serially until the signal  25  requires electrical regeneration. The final span  20  is thus connected to DCD  50 ′ whose output is through coupler  120  to receiver  60  which converts the optical signal to an electrical signal. Coupler  120  acts to tap a portion of the signal, and to connect this portion of the signal to dispersion measurement device  130 , which acts to measure the overall dispersion of system  110 , in a method well known to those versed in the art.  
     [0046] The output of dispersion measurement device  130  is fed as an input to controller  140 , which operates in a manner to be explained further below by controlling the temperature of each of the connected DCD  50 ′ to compensate for any variability in dispersion.  
     [0047] Bus  150  is a bi-directional bus that in one embodiment comprises a control channel transmitted over transmission fiber  30 . Controller  110  receives as an input over the bi-directional bus  150  the temperature measurement of each of the attached DCD  50 ′.  
     [0048]FIG. 3 b  illustrates another embodiment of an optical communication system utilizing the DCD  50 ′ of FIG. 4 a ,  4   b  or  4   c , in which the system comprises a transmitter  20 , span of transmission fiber  30 , optical amplifier  40  containing DCD  50 ′ and receiver  60 . Temperature elements  160  and  180  are utilized in a fixed mode to maintain the temperature of DCF  80  and SMF  100  at a fixed temperature. In one embodiment, the fixed temperature is a temperature above the ambient temperature. In this embodiment, temperature elements  160 ,  180  may be heating elements, in which cooling is accomplished by reducing their activation level. By maintaining a fixed temperature, the characteristics of DCF  80  and SMF  100  are kept constant with no variability due to temperature variations.  
     [0049] During initial installation an operating temperature above the maximum expected ambient temperature is selected, and the temperature is maintained by a feedback loop utilizing thermo-coupler  170  and  190  respectively. Maintaining a fixed temperature utilizing a heating element and a thermo-coupler and a simple controller (not shown) is well known to those skilled in the art.  
     [0050]FIG. 4 a  illustrates a block diagram of one embodiment of DCD  50 ′, which is a modification of DCD  50  of FIG. 2 a  according to an embodiment of the present invention and comprises first mode transformer  70 , high order mode DCF  80  placed on heat conducting spool  115 , temperature element  160 , thermo coupler  170  and second mode transformer  70 . Temperature element  160  is operated so as to control the temperature of DCF  80 . Preferably DCF  80  is a length of fiber placed on a heat conducting spool  115 , and temperature element  160  is connected to the spool  115 . Thermo coupler  170  is utilized to measure the temperature of the spool, and the temperature is fed over bi-directional bus  150  to controller  140  (FIG. 3 a ) so as to create a feedback loop.  
     [0051] In one embodiment, temperature element  160  comprises a heating element, which in operation raises the temperature of DCF  80  by heating spool  115 . If a lower temperature is desired element  160  is shut off and both spool  115  and DCF  80  lose heat to the surrounding environment until the desired temperature is reached. It is necessary in this embodiment to maintain spool  115  at a temperature above the expected ambient temperature so as to enable passive cooling.  
     [0052] In another embodiment temperature element  160  comprises a thermo electric heating and cooling element, which allows for a wide range of temperature control without regard to the ambient temperature.  
     [0053]FIG. 4 b  illustrates another embodiment of DCD  50 ′, which is a modification of the DCD  50  of FIG. 2 b  according to an embodiment of the present invention, comprising a spool of single mode DCF  90  on a heat-conducting spool  115 , temperature element  160  and thermo coupler  170 . Element  160  is operated so as to control the temperature of DCF  90 . Preferably DCF  90  is a length of fiber placed on a heat-conducting spool  115 , and element  160  is connected to the metal spool. Thermo coupler  170  is utilized to measure the temperature of the spool, and the temperature is fed over bi-directional bus  150  to controller  140  (FIG. 3 a ).  
     [0054] In one embodiment, temperature element  160  comprises a heating element, which in operation raises the temperature of DCF  90  by heating spool  115 . If a lower temperature is desired heating element  160  is shut off and the spool  115  and DCF  90  lose heat to the surrounding environment until the desired temperature is reached. It is necessary in this embodiment to maintain spool  115  at a temperature above the expected ambient temperature so as to enable passive cooling. In another embodiment temperature element  160  comprises a thermo electric heating and cooling element, which allows for a wide range of temperature control without regard to the ambient temperature.  
     [0055]FIG. 4 c  illustrates another embodiment of DCD  50 ′, which is a modification of the DCD  50  of FIG. 2 c  according to an embodiment of the present invention, comprising mode transformer  70 , high order mode DCF  80  on optional heat conducting spool  115 , temperature element  160 , thermo coupler  170 , second mode transformer  70 , single mode fiber  100  on optional heat conducting spool  105 , temperature element  180  and thermo coupler  190 . Temperature element  160  is operated so as to control the temperature of DCF  80 . Preferably DCF  80  is a length of fiber placed on a heat conducting spool  115 , and temperature element  160  is connected to the metal spool. Thermo coupler  170  is utilized to measure the temperature of the spool, and the temperature is fed over bi-directional bus  150  to controller  140  (FIG. 3 a ).  
     [0056] Preferably, single mode fiber  100  is a length of fiber placed on a heat-conducting spool  105 , and heating element  180  is connected to the metal spool. Thermo coupler  190  is utilized to measure the temperature of the spool  105 , and the temperature is fed over bi-directional bus  150  to controller  140  (FIG. 3 a ). It is to be understood that temperature elements  160  and  180  can be operated independently of each other.  
     [0057] In one embodiment, temperature element  160  comprises a heating element, which in operation raises the temperature of DCF  80  by heating spool  115 . If a lower temperature is desired embodiment element  160  is shut off and the spool  115  and DCF  90  lose heat to the surrounding environment until the desired temperature is reached. It is necessary in this embodiment to maintain spool  115  at a temperature above the expected ambient temperature so as to enable passive cooling, and to insulate the effect of the temperature of spool  115  from spool  105 .  
     [0058] In another embodiment temperature element  160  comprises a thermo electric heating and cooling element, which allows for a wide range of temperature control without regard to the ambient temperature or the temperature of spool  105 . Proper insulation of the spools  105  and  115  will reduce energy requirements, and prevent undesired temperature swings caused by heat leakage.  
     [0059] In one embodiment, temperature element  180  comprises a heating element, which in operation raises the temperature of single mode fiber  100  by heating spool  105 . If a lower temperature is desired heating element  180  is shut off and the spool  105  and single mode fiber  100  lose heat to the surrounding environment until the desired temperature is reached. It is necessary in this embodiment to maintain spool  105  at a temperature above the expected ambient temperature so as to enable passive cooling. In another embodiment temperature element  160  comprises a thermo electric heating and cooling element, which allows for a wide range of temperature control without regard to the ambient temperature.  
     [0060] While the system is described herein in connection with a temperature controlling element and a thermo coupler, this is not meant to be limiting in any way. The element may be replaced with an external heating and cooling system and the thermo coupler may be replaced by another feedback mechanism to control the temperature.  
     [0061]FIG. 5 a  shows a high level flow chart of one embodiment of the operation of controller  110  of FIG. 3 a . The embodiment illustrated will be described in connection with a DCD  50 ′ as shown in FIG. 4 a  and FIG. 4 b . In step  1000  the system is initialized, including timing constraints for steps. It is to be understood that dispersion changes that are to be compensated in the system are relatively slow, and the temperature change of the DCF  80  and  90  must be stabilized prior to evaluating the effect.  
     [0062] During initialization, the temperature element  160  is set to the middle of the operating temperature range and any trimming of the dispersion compensation is accomplished. This may be done with a variable dispersion element or by replacing a dispersion compensating element with one of a more appropriate value. A small amount of dispersion may be supplied by adjusting the initial temperature of DCF  80  and  90  from the midpoint of the range.  
     [0063] Also during initialization, other factors such as expected temperature swing of transmission fiber  30  from the current ambient temperature must be taken into account by changing the initial set point.  
     [0064] For example, if initialization is being performed on an extremely hot day, transmission fiber  30  is at its hottest temperature and thus at its minimum dispersion as regards any temperature dependent effect. The initial set point thus must allow for maximum additional negative compensation as the single mode fiber will typically experience higher amounts of dispersion as it experiences cooler temperatures. A calculated value may be utilized.  
     [0065] In the embodiment where a heating element is utilized, the environment of the DCD  50 ′, as well as the external environmental temperature of transmission fiber  30  must be taken into account, with the temperature range selected to be always above that of the environment of DCD  50 ′. The current dispersion value as shown as indicated by dispersion measurement device  130  is stored as a baseline.  
     [0066] In step  1010 , the differential dispersion of the system is calculated based in the input from dispersion measurement  130 . Step  1010  sets a timer, so that it may not operate more than once an hour. While once an hour is an exemplary time, it is to be understood that other timing factors may be utilized, with the major factor being the need for the system to stabilize.  
     [0067] In step  1020 , the differential dispersion is examined. If the differential dispersion is less than zero, the dispersion has decreased which may be partially caused by an increase in the temperature of the transmitting fiber  30 . The controller than proceeds to step  1040 , which operates to increase the temperature of DCD  80  of FIG. 4 a  or DCD  90  of FIG. 4 b  respectively, by ten degrees C, through the operation of temperature element  160 , as monitored by thermo coupler  170 . The system then proceeds to step  1010 , which will calculate the differential dispersion again after 1 hour, which is the preset time to allow for the system to stabilize.  
     [0068] In the event that the differential dispersion has not decreased, step  1030  operates to check whether the differential dispersion is positive, i.e. has the dispersion increased. If the differential dispersion is positive, which may be caused by among other factors the temperature of transmitting fiber  30  having decreased, step  1050  is run, which operates to decrease the temperature of DCD  80  of FIG. 4 a  or DCD  90  of FIG. 4 b  respectively by ten degrees C, through the operation of temperature element  160 , as monitored by thermo coupler  170 .  
     [0069] The system then proceeds to step  1010 , which will compare the differential dispersion again after 1 hour, so as to allow for the system to stabilize. In accordance with the system shown in FIG. 3 a , the temperature elements of all of the DCD&#39;s of the system are operated under the same control, so as to compensate for any drift in the overall system.  
     [0070] In the event that at step  1030  it is determined that the differential dispersion has not increased, the system proceeds to step  1010 , which will compare the differential dispersion again after 1 hour.  
     [0071]FIG. 5 b  illustrates a high level flow chart of another embodiment of the operation of controller  110  of FIG. 3 a , in connection with a DCD  50 ′ as shown in FIG. 4 c . In step  1100  the system is initialized, including timing constraints for steps as well temperature step size. In an exemplary embodiment 1 hour is used as the timing constraint and 10 degrees C is used as the temperature step. Other timing constraints and step sizes may be utilized without exceeding the scope of the invention. It is to be understood that temperature changes in the system are relatively slow, and the temperature must be stabilized prior to evaluating the effect.  
     [0072] In the initialization, the temperature elements  160  and  180  are set to their midpoints, so that the DCD  80  and SMF  100  may reach the midpoints of their operating temperature ranges, and any trimming of the compensation must be accomplished by external means. A small amount of dispersion may be supplied by adjusting the initial temperature of DCF  80  and SMF  100  from the midpoint of the range. During initialization, other factors such as expected temperature swing of transmission fiber  30  from the current ambient temperature must be also be taken into account by changing the initial set point. The current dispersion value as shown as indicated by dispersion measurement device  130  is stored as a baseline.  
     [0073] In step  1110  the differential dispersion of the system is calculated based in the input from dispersion measurement  130 . Step  1110  sets a timer, so that it may not operate more than once an hour. While once an hour is an exemplary time, it is to be understood that other timing factors may be utilized.  
     [0074] In step  1120  the differential dispersion is examined. If the differential dispersion is less than zero, the dispersion has decreased which may be caused by an increase in the temperature of the transmitting fiber  30 . The controller then proceeds to step  1130 , which checks whether DCF  80  is within 10 degrees C, the step size, of its maximum operating temperature. If it is not, the controller proceeds to step  1140 , which operates to increase the temperature of DCF  80  by ten degrees C. The controller then returns to step  1110 , which will operate again after a delay of one hour.  
     [0075] If in step  1130  the DCF is found to be within 10 degrees C of its maximum temperature, step  1150  is operated, which functions to decrease the temperature of SMF  100  by its step size, counterbalancing the temperature dependent dispersion of transmitting fiber  30 . The controller then returns to step  1110 , which will operate again after a delay of one hour. The step size of SMF  100  in an exemplary embodiment is the same as that of DCF  80 , namely 10 degrees C. Other step sizes may be utilized, and there is no requirement that the step size of SMF  100 , or the fixed time delay of SMF  100  be the same as that of DCF  80 .  
     [0076] If in step  1120  the differential dispersion is not found to be less than zero, step  1160  is operated, and the controller checks whether the differential dispersion is greater than zero. If it is not greater than zero, the controller returns to step  1110 , which will operate after a delay of one hour.  
     [0077] If in step  1160  the differential dispersion is found to be greater than zero, the controller proceeds to step  1170 , which checks to see whether DCF  80  is at its minimum operating temperature. If it is not, step  1180  is operated, which functions to decrease the operating temperature of DCF  80  by the step size of ten degrees. The controller then returns to step  1110 , which will operate after the delay time.  
     [0078] If in step  1170 , DCF  80  is found to be at its minimum operating temperature, step  1190  is operated, which functions to increase the temperature of SMF  100 , which functions to counterbalance the variable dispersion of transmitting fiber  30 . The controller then returns to step  1110 , which will operate again after a delay of one hour.  
     [0079] Thus, SMF  100  in FIG. 4 c  allows for a broader operating temperature range than that shown in FIG. 4 a.    
     [0080]FIG. 6 shows another embodiment of a system  200  according to the present invention, utilizing multiple dispersion measurement devices  130 , couplers  120  and controllers  140 . For each DCD  50 ′, a coupler  120 , dispersion measuring device  130  and controller  140  is supplied to control the heating element of the DCD  50 ′. The system is more costly, however it has the advantage being able to compensate each span  30  independently. It is often advantageous to ensure nominal zero dispersion at the amplifier  40 , which can be accomplished by the system  160 .  
     [0081]FIG. 7 shows an embodiment of a dispersion measurement device  130  according to the present invention, comprising SMF  210 , oscilloscope  220 , and output digital data  230 . The signal from coupler  120  is transmitted through a suitable length of SMF fiber, such as a 1 kilometer spool, with a known dispersion. This is utilized as an offset, to enable the controller  140  to identify the direction of resultant dispersion. SMF  210  is connected to the input of oscilloscope  220 , such as an Agilent 86100 wide bandwidth oscilloscope, available from Agilent Technologies, Palo Alto, Calif., which is programmed to output a calculation of the Q factor of the received signal. The Q-factor is output from the oscilloscope  220  over data lines  230 , which may be an RS-232 bus or other suitable communication path.  
     [0082] In the event that the Q-factor has improved from its previous state, this indicates that the positive dispersion added by SMF  210  has been somewhat compensated for by having less dispersion in transmission fiber  30 . Corrective action is called for, and controller  110  will issue a command according the relevant instructions as shown in FIG. 5 a  and FIG. 5 b  respectively. The offset of SMF  210  is important, as the temperature of the DCF  80  or DCF  90  cannot be rapidly swept. The exact direction must be identified in order to take the slow but correct response. Other dispersion measurement devices may be utilized without exceeding the scope of the invention.  
     [0083] A high order mode fiber exhibits high negative dispersion and dispersion slope, which allows for a short length of fiber to be utilized while compensating for a long length of transmission SMF. Dispersion on the order of −200 ps/mn/km is a typical value, with slope on the order of −4 to −5 ps/nm 2 /km being typical. High order mode fibers exhibit shifts in temperature similar to other fibers, i.e. approximately 0.03 nm/° C. A typical span of 80 kilometers of non-zero dispersion shifted fiber exhibits 200-300 ps/nm dispersion and is compensated for by a length of 1-2 kilometers of high order mode fiber with a length of single mode fiber as a trimming fiber to match the specific dispersion and slope characteristics of the transmission span.  
     [0084] As a specific example, shifting the temperature of a 1.5 kilometer span of high order mode fiber, with a slope of −4.5 ps/nm 2 /km at 1550 nm, by +10° C., effects a dispersion shift of:  
     (Temperature dependence of zero dispersion wavelength)×(Slope of fiber)×(Length of fiber)×(Temperature shift)= 
     0.03  nm/° C. ×4.5  ps/nm   2   /km ×1.5 kilometer×10 ° C.=− 2.0  ps/nm.    
     [0085] It is to be understood that other methods of dispersion measurement can be utilized in connection with the invention. All that is required is a means of identifying the direction of dispersion correction required. Various methods are known to those skilled in the art and will not be further described.  
     [0086] In addition, the system has been described in connection with the temperature control of a high order mode dispersion compensating fiber connected in series with a single mode fiber as a trimmer. This is not meant to be limiting in any way, and is meant to include the use of a standard dispersion compensating fiber with a trimming fiber, and to include the use of a second high order mode fiber as a trimming fiber. The single mode fiber being used as a trimming may fiber, is one embodiment a standard single mode fiber such as SMF-28® sold by Corning, Inc. In another embodiment it is dispersion shifted fiber, and in still another embodiment comprises non-zero dispersion shifted fiber.  
     [0087] Having described the invention with regard to certain specific embodiments thereof, it is to be understood that the description is not meant as a limitation, since further modifications may now suggest themselves to those skilled in the art, and it is intended to cover such modifications as fall within the scope of the appended claims.