Abstract:
A completely thin-film coupled three-cavity dispersion compensation element enables dispersion compensation over wider bandwidths then similar elements having fewer coupled cavities. By cascading these dispersion compensation elements even greater compensation bandwidths can be obtained, thereby further increasing the merit and usefulness of this device.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to a completely thin-film based optical dispersion compensating element.  
           [0003]    2. Description of Related Art  
           [0004]    It is recognized that the demand for higher bit rates and longer propagation distances in fiber optic light wave communication systems is steadily increasing. In such systems, fiber dispersion will become an increasingly important problem. Various possible dispersion compensation approaches will be tried. Presently, second order dispersion has become a huge problem and with it various compensation approaches have been proposed, whose effects we will soon see.  
           [0005]    However, with respect to light wave transmission, the dispersion tolerances have become very strict. Compensation of only second order dispersion is insufficient, rather third order dispersion must also be compensated for.  
           [0006]    Below, FIGS.  11 (A)- 11 (C) and  12  will be used to explain future second order dispersion compensation methods.  
           [0007]    In FIG. 12 the dispersion characteristics as a function of wavelength of single mode fiber (SMF), dispersion compensation fiber (DSF), and dispersion shifted fiber (DSF) are shown. The label  601  is associated with the SMF dispersion versus wavelength curve, the label  602  is associated with the dispersion compensation fiber dispersion versus wavelength curve, and label  603  is associated with the DSF dispersion versus wavelength curve. In FIG. 12, the y-axis is dispersion and the x-axis is wavelength.  
           [0008]    In FIG. 12, it is clear that light input into SMF fiber between the wavelengths of 1.3 μm and 1.7 μm experience dispersion that increases with wavelength. Light input into dispersion compensating fiber will experience dispersion that decreases with wavelength between the wavelengths of 1.3 μm and 17 μm. Light input into DSF will experience dispersion that decreases with wavelength between the wavelengths of 1.3 μm and the neighborhood of 1.55 μm and dispersion that increases with wavelength between the wavelengths of 1.55 μm and 1.8 μm. A 2.5 Gbps (every second 2.5 giga-bits) bit-rate DSF based fiber communication system operating at a wavelength near 1.55 μm, the zero dispersion point, would not suffer the hindering effects of dispersion.  
           [0009]    With first second order dispersion compensation in mind, FIG. 11(A) shows wavelength versus time characteristics and light intensity versus time characteristics of the effects of second order dispersion, FIG. 11(B) shows a light wave transmission system that uses SMF in combination with dispersion compensation fiber for second order dispersion compensation, and FIG. 11(C) shows a light wave transmission system that uses only SMF.  
           [0010]    In FIG. 11, label  501  and  502  refer to the input signal characteristics before entering the fiber. Label  530  and  531  refer to the SMF only based propagation system. Label  502  and  512  refer to input pulse characteristics after passing through the SMF based system denoted by label  530 . Label  520  refers to a dispersion compensating fiber based propagation system composed of dispersion compensating fiber denoted by label  521  and SMF denoted by label  522 . Label  503  and  513  show the characteristics of the input pulse, denoted by label  501  and  511 , after passing through the system denoted by label  520 . Label  504  or  514  refer to the characteristics of the output pulses after the input pulses denoted by label  501  and  511  have passed through the fiber transmission system denoted by  520  and then the new device discussed in this patent, a dispersion compensation element designed for third order dispersion compensation only. In such a system, the characteristics of the output pulses of  504  and  514  would be almost identical to the original characteristics of the input pulses of  501  and  511 . Again, graphs  501 ,  502 ,  503 , and  504  all have a y-axis representing wavelength and an x-axis representing time. Graphs  511 ,  512 ,  513 , and  514  all have a y-axis representing the light signal intensity and an x-axis representing time. Labels  524  and  534  refer to transmitters, and labels  525  and  535  refer to receivers.  
           [0011]    In long distance high speed light wave communication systems using normal SMF, the amount of dispersion increases going from short wavelengths near 1.3 μm to long wavelengths near 1.7 μm, which means that within this region longer wavelengths experience more delay than shorter wavelengths. An output signal pulse train, composed of wavelengths within this bandwidth (1.3 to 1.7 mm), of the SMF system denoted by label  530 , is depicted by the graphs labeled  502  and  512 . This spreading out of the pulses ultimately interferes with the detection capability of the receiver, as pulses overlap with their neighbors.  
           [0012]    One of the methods for solving the problem of dispersion has been no use dispersion compensation fiber in the manner shown in FIG. 11(B).  
           [0013]    Typical dispersion compensation fiber has a dispersion profile where the dispersion decreases going from short wavelengths to long wavelengths, from 1.3 μm to 1.7 μm, in order to compensate for the dispersion profile of typical SMF where the dispersion increases going from short wavelengths to long wavelengths.  
           [0014]    One can connect dispersion compensation fiber, labeled  521 , to SMF, labeled  522 , in the manner shown by label  520  in FIG. 11(B). In the system labeled  520 , using SMF, labeled  522 , having a delay which increases with increasing wavelength, in combination with dispersion shifted fiber having a delay which decreases with increasing wavelength, one can depict the output of the dispersion compensation fiber as shown in the graphs labeled  503  and  513 , where it is clear that the much of the changes shown in the graphs labeled  502  and  512  has been suppressed.  
           [0015]    However, dispersion compensation fiber will not return the propagating pulse back to the input pulse form shown by the graph labeled  501 . Dispersion compensation fiber, as a second-order dispersion compensation technique, can only compensate a traveling pulse up to the form shown by the graph labeled  503 . At this point, both the longer wavelengths and shorter wavelengths of the signal have a greater delay than the center wavelength of the signal. This delay profile results in a pulse with a characteristic ripple on the fall of the pulse, as shown in the graph labeled  513 , and is called third-order dispersion.  
           [0016]    This phenomenon of third-order dispersion becomes a serious problem with increasing bit-rates and distances, as the required accuracy for detection becomes greater. For example, in systems using bit rates of 10 Gbps (10 gigabits every second) and greater, this phenomenon is a serious problem, and for 40 Gbps and greater systems [over distances of only 80 km], the problem is greater.  
           [0017]    Therefore, for future high-speed optical communication systems, it will become difficult to use today&#39;s normal fiber systems. It may become necessary to change the fiber material being used, for example. System construction, from an economic viewpoint will become of increasing importance.  
           [0018]    Given the difficulties associated with only second-order dispersion compensated systems, it is clear that third-order dispersion compensation is necessary.  
           [0019]    It is clear from FIGS.  11 (A)- 11 (C) and FIG. 12, that DSF has very little second-order dispersion in the vicinity of 1.55 μm, but cannot compensate for third-order dispersion, the subject of this section.  
           [0020]    The phenomenon of third-order dispersion in high-speed long distance light communication systems, and the necessity of compensating for it, is gradually becoming recognized as being important. There have been many attempts at compensating for third-order dispersion, but none of them have been successful enough to be realized.  
           [0021]    One example of a third-order dispersion compensation device, that is proposed by the inventors, a dielectric thin-film device, can successfully compensate for pure third-order dispersion, and as such has the potential for greatly advancing light wave communication systems.  
           [0022]    In high bit-rate optical fiber communications, for example 40 Gbps and 80 Gbps, both second and third-order dispersion compensation is necessary. For a many channel light wave system, sufficient broad bandwidth third-order dispersion compensation or narrow bandwidth (only the channel portions of the band) second-order dispersion is necessary.  
           [0023]    In order to compensate the dispersion in each channel, the inventors propose a dispersion compensation element that is adjustable in wavelength. In addition they propose a dispersion compensation element that is adjustable in both wavelength bandwidth and amount of group delay (amount of dispersion compensation adjustable).  
           [0024]    Using simply one dispersion compensation unit, it is extremely difficult to obtain a sufficiently wide bandwidth group delay characteristics, a sufficient amount of dispersion compensation, as well as complex group delay shapes.  
           [0025]    The proposed dispersion compensation elements can be cascaded in series to produce excellent group delay versus wavelength characteristics or good dispersion compensation. These elements can be connected together, for example via a collimator type lens assembly, to produce much larger size dispersion characteristics. However, an important question is how small can the loss be made, as the total loss is proportional to the number of elements, since the loss is additive.  
           [0026]    If the dispersion seen by the light signal changes, the amount of dispersion compensation provided by the dispersion compensation element has to change correspondingly. However, for bandwidths as wide as 15 nm and 30 nm, changing the amount of dispersion compensation is difficult.  
           [0027]    When connecting these dispersion compensation elements in series, to make broad bandwidth dispersion characteristics, for example at 15 nm and 30 nm, it is critical to be able to connect these elements in a simple, low loss manner.  
           [0028]    It is important to realize a single element that can compensate for dispersion over a wide bandwidth on the order of 15 nm and 30 nm, without the need to cascade multiple dispersion compensation elements together.  
           [0029]    However, single thin-film based dispersion compensation elements give reasonable dispersion compensation over bandwidths from 1 to 5 nm.  
         SUMMARY OF THE INVENTION  
         [0030]    In consideration of the points discussed below, the purpose of the invention is the realization of a device with sufficient dispersion compensation over a broad bandwidth. Specifically, being able to produce the required group delay versus wavelength characteristics necessary for the required amount of third-order dispersion compensation, using a small device, that is easy to use, has low loss, has high reliability, is suitable for production, and low-cost. In other words, using a thin-film unit as the base, being able to provide adjustable group delay bandwidth with adjustable group delay for the purposes of third-order dispersion compensation, or second and third-order dispersion compensation together.  
           [0031]    In order to realize a compact and efficient dispersion compensator, a completely thin-film based wavelength dispersion (or simply dispersion) compensation device or dispersion compensation element that can be easily assimilated into an optical fiber transmission system, is proposed. This dispersion compensator is composed of at least five fundamental layers, each possessing unique optical properties. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]    The foregoing and other objects, features and advantages of the present invention will be better understood from the following description taken in connection with the accompanying drawings, in which:  
         [0033]    [0033]FIG. 1 is a diagram for explaining dispersion compensation provided by this invention;  
         [0034]    [0034]FIG. 2 is a schematical cross section of the thin-film layers used by this invention;  
         [0035]    [0035]FIG. 3 is a schematical oblique view of the thin-film layers used by this invention;  
         [0036]    [0036]FIG. 4 is diagrams showing some group delay versus wavelength curves characteristic of this invention;  
         [0037]    FIGS.  5 (A) to  5 (D) is a figure used to explain a method based on connecting many units for improving the group delay versus wavelength characteristics of this invention;  
         [0038]    FIGS.  6 (A) to  6 (D) is a figure schematically showing some of the possible connections between dispersion compensation units;  
         [0039]    FIGS.  7 (A) and  7 (B) is a schematical view for explaining an example of a composite dispersion compensation structure;  
         [0040]    [0040]FIG. 8 is a schematical view for explaining an example of a composite dispersion compensation structure;  
         [0041]    [0041]FIG. 9 is a diagram showing the group delay versus wavelength characteristics of the composite dispersion compensation structure shown in FIG. 7;  
         [0042]    [0042]FIG. 10 is a diagram showing the group delay versus wavelength characteristics of the thin-film three-cavity dispersion compensation element.  
         [0043]    FIGS.  11 (A) to  11 (C) is a schematical view for explaining a method for compensating for both second and third-order dispersion.  
         [0044]    [0044]FIG. 12 is a schematical view showing the dispersion characteristics of standard types of available fibers. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0045]    The drawings regarding the practical realization of the form of this invention will be referred to below. In order to understand this invention, a general outline of the components making up the device, the general shape, and arrangement of the sub-components will explained with respect to drawings. Concerning the circumstances of the explanation of this invention, some drawings will show magnified versions of the structures showed in other drawings. Not all the realizable forms of this invention, described in this patent will have similar drawings. In each drawing structural parts that are the same will be labeled with the same number. Overlapping explanations may be abbreviated.  
         [0046]    Concerning the discussion of the invention below, light dispersion compensation or simply dispersion compensation, light dispersion compensation element or simply dispersion compensation element, light dispersion compensation method or simply dispersion compensation method.  
         [0047]    In a fiber propagation or communication system, for example with a light signal propagating in the vicinity of 1.55 um, second order and above dispersion (to be explained later) occurs due to the structure of the fiber. We propose a low loss dispersion compensation unit that can compensate for second order and above dispersion, in both a fixed and changeable manner. Two of these elements, when placed in an opposing arrangement, constitute a composite dispersion compensation element or construction.  
         [0048]    This invention, in a low loss manner can compensate for second and third order dispersion and above in a highly effective manner.  
         [0049]    With respect to the discovered composite dispersion compensation, it can compensate for many types of dispersion depending upon the arrangement of the two dispersion compensation units relative to each other. For example, it can compensate for only third-order dispersion, only second-order dispersion, both second and third-order dispersion, and greater than third-order dispersion.  
         [0050]    There are various forms that this invention, a dispersion compensation element can take, for the purposes of sales or other uses.  
         [0051]    The meaning of second and third order dispersion is shown graphically in FIG. 11 a , in a graph of wavelength versus time, with first second and then third order dispersion compensated for. Second order dispersion causes the wavelength versus time curve to stretch and elongate. Third order dispersion causes the wavelength versus time curve to have a quadratic dependence.  
         [0052]    [0052]FIG. 1 is used to explain the concept of dispersion compensation in a fiber transmission system. The curve labeled  1101  is the remaining dispersion of the fiber, after the second order dispersion of the fiber has been compensated. This remaining dispersion is referred to as third-order dispersion. This remaining third-order dispersion can be compensated using a third-order dispersion compensation device with the group delay versus wavelength characteristics labeled  1102 . The group delay versus wavelength characteristics of the combination of the third-order dispersion compensation device plus the fiber is described by the curve labeled  1103 . In FIG. 1, compensation is shown as occurring between wavelength λ 1  and λ 2 , resulting in the flat curve labeled  1103 . In FIG. 1, the vertical-axis is the group delay and the horizontal-axis is wavelength.  
         [0053]    FIGS.  2  through show the structure of the dispersion compensation elements (the dispersion compensation elements make up the composite dispersion compensation device in a manner where each dispersion compensation element has an opposing dispersion compensation element making up a set of opposing surfaces. As each dispersion compensation element can act alone as a dispersion compensator, it will be referred to as dispersion compensation element or dispersion compensation unit to distinguish it from the composite dispersion compensation device) that are the subject of this invention. FIG. 2, to be discussed later, shows the cross section of the thin-film layers making up a dispersion compensation element, FIG. 3 shows how the thin-film layer thickness values can vary with distance, and FIG. 4 shows the group delay versus wavelength characteristics of the thin-film layer structures.  
         [0054]    An example of the structure of the dispersion compensation unit of this invention is shown in FIG. 2. In FIG. 2, the cross section of the thin-film layers is shown. Label  100  refers to the thin-film structure of the dispersion compensation unit. The arrow of label  101  refers to the direction of the input light. The arrow of label  102  refers to the direction of the output light. Labels  103  and  104  refer to the mirror layers (referred to as reflection layer or light reflection layer) where the reflection is below 100%. Label  105  refers to the mirror layer having the highest reflection value, between 98 and 100%. Labels  108  and  109  refer to the light transmission layers (or simply transmission layers) and layers  111  and  112  refer to the cavities. Label  107  refers to the substrate, for example BK-7 glass.  
         [0055]    The relation between the reflectance values, R( 103 ), R( 104 ), and R( 105 ), of each of the mirror layers, labeled  103 ,  104 , and  105 , in FIG. 2 is that R( 103 )         R( 104 )         R( 105 ). If the above condition is changed so that R( 103 )&lt;R( 104 )&lt;R( 105 ) then it becomes easier to produce these devices. The closer the reflectance of R( 105 ) is to 100% the better the performance of the device. That is to say, the center wavelength of the input light sees reflection layers whose reflection values gradually increase with distance into the filter, finally ending in a reflection value as close to 100% as possible. It is desirable to have reflection layers with reflection values that lie within the following ranges, where 60%         R( 103 )         77%, 96%         R( 104 )         99.8%, 98%         R( 105 ). Various group delay versus wavelength characteristics can be realized when R( 103 ), R( 104 ), and R( 105 ) are allowed to vary within the stated constraints. One can increase the performance of these dispersion elements by ensuring that the reflectance of R( 105 ) is as close to 100%.  
         [0056]    For ease of production of the dispersion compensation elements, the cavity layer optical path lengths are allowed to be different. Allowing the cavity lengths to be different gives more freedom in the design conditions associated with the allowable range of reflection values of the reflection layers. The thin-film structure is entirely composed of quarter wavelength layers, the basic structural unit of these devices, and so the optical thickness is an integer multiple of a quarter wavelength. The realization of a third-order dispersion compensator using such a structure simplifies production, and results in a product that has high reliability as well as low cost.  
         [0057]    In reality, when considering the production of these thin-film structures, the basic unit of the thin-film dispersion compensation unit, a quarter wavelength, has an allowable tolerance region. For the purposes of this device, it is sufficient that the layer unit tolerances fall with λ/4±10.0% (where all the layer optical thickness errors are not the same, rather the maximum optical layer thickness error is ±10% with other layer thickness errors less than this value. It is possible that a set of optical thickness errors that fall within this bound can give both results within the specification as well as results that are not within the specification depending on the exact distribution of errors). However, if the layer unit accuracy becomes higher, for example λ/4±1.0%, then the production yield will improve. If the layer unit accuracy is further increased to λ/4±0.5% then the production yield increases still more, as for example, the deviations of the device center wavelength from the desired center wavelength decreases with increasing layer accuracy. Units produced within this tolerance will have a high reliability yield resulting in an overall production cost that is less.  
         [0058]    Concerning the formation of the quarter wavelength layers that make up the structure of the dispersion compensation units. Each quarter wavelength layer, the basic unit of these devices, is formed on top of the next one, is a continuous process. The resultant filter is entirely composed of quarter wavelength layers, in other words a multiple of an integer number of quarter wavelengths. This means that the reflection layer and transmission layer are also in turn composed of quarter wavelength layers that were deposited in a continuous process.  
         [0059]    The thin-film structure of FIG. 3 is the same as the thin-film structure labeled  100  in FIG. 2 except that the width of the thin film layers change with distance.  
         [0060]    [0060]FIG. 3 shows an example of a thin-film dispersion compensation unit, labeled  200 , that is the basic building block used in our discovery. The first, second, and third dispersion compensator using such a structure simplifies production, and results in a product that has high reliability as well as low cost.  
         [0061]    In reality, when considering the production of these thin-film structures, the basic unit of the thin-film dispersion compensation unit, a quarter wavelength, has an allowable tolerance region. For the purposes of this device, it is sufficient that the layer unit tolerances fall with λ/4±10.0% (where all the layer optical thickness errors are not the same, rather the maximum optical layer thickness error is ±10% with other layer thickness errors less than this value. It is possible that a set of optical thickness errors that fall within this bound can give both results within the specifications as well as results that are not within the specifications depending on the exact distribution of errors). However, if the layer unit accuracy becomes higher, for example λ/4±1.0%, then the production yield will improve. If the layer unit accuracy is further increased to λ/4±0.5% then the production yield increases still more, as for example, the deviations of the device center wavelength from the desired center wavelength decreases with increasing layer accuracy. Units produced within this tolerance will have a high reliability yield resulting in an overall production cost that is less.  
         [0062]    Concerning the formation of the quarter wavelength layers that make up the structure of the dispersion compensation units. Each quarter wavelength layer, the basic unit of these devices, is formed on top of the next one, is a continuous process. The resultant filter is entirely composed of quarter wavelength layers, in other words a multiple of an integer number of quarter wavelengths. This means that the reflection layer and transmission layer are also in turn composed of quarter wavelength layers that were deposited in a continuous process.  
         [0063]    The thin-film structure of FIG. 3 is the same as the thin-film structure labeled  100  in FIG. 2 except that the width of the thin film layers change with distance.  
         [0064]    [0064]FIG. 3 shows an example of a thin-film dispersion compensation unit, labeled  200 , that is the basic building block used in our discovery. The first, second, and third reflection layers are labeled  201 ,  202 , and  203  respectively. The substrate is labeled  205 , and the first and second transmission layers are labeled  206  and  207  respectively. The first and second cavities are labeled  211  and  212  respectively. Label  220  indicates the surface where the light is incident on and label  230  shows the direction of the incident light. Label  240  shows the direction of the output light. Label  250  shows the direction of the first taper or change of layer thickness. Label  260  shows the direction of the second taper or change of layer thickness. Labels  270  and  271  show two possible directions or paths that the light takes in a multi-reflection configuration.  
         [0065]    The order of the layers from the substrate, labeled  205 , for example BK-7 glass, is the third reflection layer  203 , the second transmission layer  207 , the second reflection layer  202 , the first transmission layer  206 , and the first reflection layer  201 .  
         [0066]    The thickness of the first transmission layer,  206 , varies in the direction indicated by the arrow,  250 , in FIG. 3. The thickness of the second transmission layer,  207 , varies in the direction indicated by the arrow,  260 , in FIG. 3. When the center wavelength of the first and second cavity are the same, the relation or condition mentioned before between R( 103 ), R( 104 ), and R( 105 ) must be satisfied. This is equivalent to the reflectance of layers  201 ,  202 , and  203 , denoted by R( 201 ), R( 202 ), R( 203 ), satisfy the condition that R( 201 )         R( 202 )         R( 203 ).  
         [0067]    The reverse order of the thin-film layers is also valid. In other words, referring to FIG. 3, the light can be incident first upon a suitable substrate, followed by the first reflection layer,  201 , followed by the first cavity layer,  206 , followed by the second reflection layer,  202 , followed by the second cavity layer  207 , followed by the third reflection layer,  203 . In this order, the condition that R( 103 )         R( 104 )         R( 105 ) must still be maintained.  
         [0068]    The group delay versus wavelength characteristics of the thin-film dispersion compensation element labeled  200  in FIG. 3, are shown in FIG. 4, when the light is incident upon surface  220  in the direction of label  230  and the output light is labeled  240 , under two possible multiple reflection paths labeled  270  and  271 .  
         [0069]    The group delay versus wavelength characteristics when the incident beam of center wavelength λ 0  is incident on three different places,  280 ,  281 , and  282  in FIG. 3, is shown in FIG. 4. The vertical axis is group delay and the horizontal axis is wavelength.  
         [0070]    In FIG. 4, the group delay versus wavelength curve labeled  2801  results whenever light is incident upon any of the points along the path labeled  270  in FIG. 3. The group delay versus wavelength characteristics hardly change, but the center wavelength, λ 0 , does change. The center wavelength is the point on the group delay versus wavelength curve where the slope is zero. When the light is incident upon any of the points along the path labeled  271 , except for the intersection between  271  and  270 , in FIG. 3, then either one of two possible group delay versus wavelength curves, labeled  2811  and  2812 , can result. Along this path, the center wavelength changes very little, but the group delay characteristics change significantly. Simply, a filter possessing cavity layers that monotonically increase in opposite directions, as labeled  250  and  260  in FIG. 3, can have group delay versus wavelength characteristics as shown by the curves in FIG. 4.  
         [0071]    Depending on the dispersion compensation application, the center wavelength, λ 0 , of the graphs  2801 ,  2811 , and  2812  in FIG. 4, can be adjusted suitably, as well as the particular group delay characteristics can be set. For example, though not shown here, between the graphs  2801 , and  2812 ,  2801  and  2811 , and  2811  and  2812 , there exist many possible group delay shapes.  
         [0072]    In order to match the dispersion compensation element wavelength to the desired wavelength in the optical signal, the optical signal can be moved along the line labeled  270  in FIG. 3. In order to adjust the group delay versus wavelength characteristics of the filter to match the desired characteristics, the optical signal can be moved along the line labeled  271  in FIG. 3. The point of intersection where lines  270  and  271  cross is the optimal point where the input optical signal should enter the dispersion compensation element.  
         [0073]    Looking at the group delay versus wavelength characteristics in FIG. 4, it is clear that the just the dispersion compensation element labeled  200  in FIG. 3 can be used for both pure third-order dispersion compensation, as evidenced by the graph labeled  2801 , and second-order dispersion compensation, as evidenced by the graphs labeled  2811  and  2812 .  
         [0074]    It is clear from the above explanations regarding FIGS. 2 through 4 concerning a dispersion compensation element, that given the graphs of FIG. 1 and FIG. 4, that these elements are capable of third-order dispersion compensation. Furthermore, with respect to using these devices in a composite dispersion compensation device, the invention referred to in this patent, it is clear that dispersion compensation will occur.  
         [0075]    Individually, the thin-film based dispersion compensation elements discussed before have group delay versus wavelength characteristics that offer dispersion compensation over bandwidths up to 3 nm with a group delay peak greater than 2 ps. For example, at center wavelengths in the vicinity of 1.55 mm, thin-film compensators with a compensation bandwidth of 1.5 nm and group delay peak values between 3 and 6 ps have been constructed. While these bandwidths and group delay peaks are sufficient for single channel compensation in a light wave communication system, it is not sufficient for multiple channels. Multiple channel systems can typically require bandwidths between 10 and 30 nm as well as much larger group delay peak values. Therefore, it is necessary to improve on the dispersion characteristics of the thin-film based compensation elements discussed so far in order to be able to compensate for the dispersion of many channels. FIGS. 5 through 10 are used in the explanation that follows concerning the improvement of the dispersion compensation element.  
         [0076]    [0076]FIG. 5 shows the group delay versus wavelength characteristics and hence the dispersion compensation characteristics can be improved by cascading many dispersion compensation elements. FIG. 5(A) shows the group delay versus wavelength characteristics of only one dispersion compensation element. FIG. 5(B) shows the result of either cascading two dispersion compensation elements possessing similar group delay versus wavelength characteristics but at different center wavelengths or using two reflections along a line in a composite dispersion compensation structure made up of two dispersion compensation elements possessing similar dispersion characteristics but at different center wavelengths. In a similar manner the number of cascaded dispersion elements can be increased to three and four or equivalently the number of reflections in a composite structure can be increased to three and four. FIG. 5(C) shows the results of cascading three dispersion compensation elements possessing similar group delay versus wavelength characteristics but different center wavelengths. FIG. 5(D) shows the results of cascading three dispersion compensation elements, two possessing similar group delay versus wavelength characteristics and one possessing different group delay versus wavelength characteristics, all having different center wavelengths. In all the graphs in FIG. 5, the vertical axis is group delay and the horizontal axis is wavelength. The realization of a device capable of realizing the dispersion characteristics shown in the graphs of FIG. 5 is the discovery written about in this patent. For example, such a device, to be discussed later, is shown in FIGS.  7 (A) and (B) and  8 , a composite dispersion compensation structure. Such a device can be placed at suitable positions along the path of a light wave fiber communication system. For example, directly to fiber, at a receiver, before or after an amplifier, for each channel after a demultiplexer (DMUX), after a transmitter, and after or before a regeneration point.  
         [0077]    In FIG. 5, labels  301  through  309  refer to the group delay versus wavelength characteristics of single dispersion compensation elements. Label  310  refers to the resultant group delay versus wavelength curve when two dispersion compensation elements with similar group delay versus wavelength characteristics but different center wavelengths are connected together. Label  311  refers to the resultant group delay versus wavelength curve when three dispersion compensation elements with similar group delay versus wavelength characteristics but different center wavelengths are connected together. Label  312  refers to the resultant group delay versus wavelength curve when three dispersion compensation elements, two of which have similar group delay versus wavelength characteristics but all having different center wavelengths are connected together. In FIG. 5(A), the label (a) refers to the dispersion compensation bandwidth (here in units of wavelength), and the label (b) refers the peak value of the group delay curve (here in units of time). In FIG. 5, the group delay versus wavelength curves labeled  302  through  307  and  309  all have about the same group delay peak value and dispersion compensation bandwidth. However the curve labeled  308  has a dispersion compensation bandwidth that is smaller but a group delay peak value that is larger than the curves labeled  302  through  307  and  309 . The center wavelengths of the curves labeled  301  through  309  are all different.  
         [0078]    In FIG. 5(B), comparing the group delay versus wavelength characteristics of the resultant curve labeled  310  to the individual curves labeled  302  and  303 , the group delay peak is 1.6 times as large and the dispersion compensation bandwidth is 1.3 times as wide. In FIG. 5(C), comparing the group delay versus wavelength characteristics of the resultant curve labeled  311  to the individual curves labeled  304 ,  305 , and  306 , the group delay peak is 2.3 times as large and the dispersion compensation bandwidth is 2.5 times as wide. In FIG. 5(D), comparing the group delay versus wavelength characteristics of the resultant curve labeled  312  to the individual curves labeled  307 , and  309 , the group delay peak is 3 times as large and the dispersion compensation bandwidth is 2.3 times as wide.  
         [0079]    The group delay versus wavelength characteristics of the thin-film dispersion compensation elements explained in FIGS. 2 through 4 can be described by two parameters, the group delay peak value and the dispersion compensation bandwidth. By changing the design conditions of the reflection layers and the transmission layers these group delay versus wavelength parameters can be changed. This is illustrated in FIG. 5(D), where the group delay versus wavelength characteristics of the curve labeled  307  are different from the group delay versus wavelength characteristics of the curve labeled  308 . Curve  307  had a lower group delay peak value but wider dispersion compensation bandwidth than curve  308 . Such curves can be combined to produce all kinds of group delay versus wavelength characteristics.  
         [0080]    These kinds of thin-film dispersion compensation elements can be realized, for example using the thin-film designs defined in claim  4  and claim  5 . Actual dispersion compensation elements have been realized using these designs, for example having center wavelengths at 1.55 mm, group delay peak values on the order of 700 fs, and dispersion compensation bandwidths between 16 and 18 nm.  
         [0081]    The thin-film designs, A through H, possess two transmission layers or cavities sandwiched between reflection layers. However, this is not the limit of the invention discussed in this patent. Structures with one, three, and four cavities are possible and have been realized.  
         [0082]    By combining group delay versus wavelength characteristics, like those shown in FIG. 4 and FIG. 5(D), in the appropriate manner, not only can third-order dispersion be compensated for, but also residual second-order fiber dispersion.  
         [0083]    One way to achieve effective dispersion compensation, dispersion compensation that is suitable for many situations, is to be able to adjust the group delay versus wavelength characteristics of the dispersion compensation element.  
         [0084]    [0084]FIGS. 2 and 3 illustrate a form of thin-film adjustable dispersion compensation element, as the thickness of the two transmission layers vary with distance in opposite directions. By changing the position where the input light is incident on the surface of the element labeled  200 , the group delay versus wavelength characteristics can be changed as well as the center wavelength. The method chosen to move the light across the surface of the dispersion compensation element is dependent upon the dispersion compensation situation. For example, a low cost solution would be to use a screw type of arrangement where the input beam could be moved by hand. However, if better adjustment accuracy was required, an electromagnetic step or continuous motor, or a voltage controlled PZT motor could be used. This method of adjustment can be combined with a prism, dual fiber ferule assembly, or optical waveguide type of element to produce an accurate, easy to use method of adjusting the position of the input beam on the surface of the dispersion compensation element. If, instead of a thin-film layer, one of the cavities is an air gap then the group delay characteristics of the device can be adjusted by adjusting the length of the air gap.  
         [0085]    With regards to the thin-film layers used to build this invention, a dispersion compensation element, it is necessary to define some terms and conventions. Here, quarter wavelength layers are made up of relatively high refractive index material layers (called H) and relatively low refractive index material layers (called L). The thin-film structures are defined using quarter wavelength layers of SiO 2  and Ta 2 O 5 , labeled L and H respectively. These layers are deposited using an IAD (ion assisted deposition) process. When an H layer is deposited over an L layer, the resultant structure is considered one set, labeled LH. Thus 5 sets of LH, Labeled (LH) 5 , would consist of ten layers in the order of LHLHLHLHLH.  
         [0086]    In the same manner, when an L layer is deposited over another L layer, the resultant structure is considered one set, labeled LL. Thus 3 sets of LL, labeled (LL) 3 , would consist of six layers in the order LLLLLL. This same convention applies to the term HH.  
         [0087]    In the explanation of this invention, the label H was connected with one example of a dielectric material, Ta 2 O 5 . However, other dielectric materials, such as TiO 2  and Nb 2 O 5  as well as Si and Ge based materials are allowable. Similarly, the label L was connected with one example of a dielectric material, SiO 2 , as it is both cheap and has a high reliability. However, other dielectric materials can be used, as long as their dielectric constant is less than the dielectric constant of the material that is associated with the symbol H.  
         [0088]    The design of this invention is not limited to only two kinds of materials. Many different kinds of materials can be used, labeled L 1 , L 2 , L 3 , etc . . . and H 1 , H 2 , H 3 , etc . . .  
         [0089]    Similarly, the process used to construct the thin-film structure or deposit the thin-film layers, L and H, was an IAD process. However, the construction of this invention is not limited to the use of this process. Other processes, such as sputtering and ion plating, can be used to produce effective dispersion compensation elements.  
         [0090]    The dispersion compensation element, labeled  200  in FIG. 3, is in the form of a wafer. A desired section of the wafer can be cut out, including all the layers and substrate, in the vertical direction from input surface,  220  through substrate  205 . This sub-section or small chip can then be placed in combination with a collimator lens in a cylindrical case or tube to make a compact, dispersion compensation element.  
         [0091]    [0091]FIG. 6 shows the packaging structure and series connection of such structures necessary to achieve dispersion compensation devices possessing the group delay versus wavelength characteristics shown in FIG. 5. FIG. 6(A) shows two dispersion compensation elements directly connected in series where the light signal travels through both of them. FIG. 6(B) shows three dispersion compensation elements directly connected in series. FIG. 6(C) shows two separate positions on one thin-film structure, possessing transmission layers with tapers, being connected in series to form a net dispersion compensating structure. FIG. 6(D) shows the structure of FIG. 6(A) packaged in one case.  
         [0092]    In FIG. 6, labels  410 ,  420 ,  430 , and  440  refer to dispersion compensation structures based on the direct connection of dispersion compensation elements. Labels  411 ,  412 ,  421 - 423 ,  431 ,  442 , and  443  refer to individual dispersion compensation elements. Label  416  is the thin-film portion of a dispersion compensation element. Labels  415 ,  4151 - 4154 ,  426 ,  4261 ,  4262 ,  436 ,  4361 ,  4362 ,  446 ,  4461 ,  4462  refer to fiber. Labels  413 ,  4131 ,  414 ,  4141 ,  424 ,  425 ,  434 ,  435 ,  444 ,  445  are arrows that show the direction the light signal is traveling. Label  418  refers to a DFFA (dual fiber ferule assembly) made up of a lens, labeled  417 , and fiber, labeled by  4151  and  4152 . Label  441  is a case. Label  431  refers to a thin-film wafer made up of thin-film layers deposited on a substrate where the width of the transmission layers change with distance. Labels  432  and  433  refer to two points on the surface of  431  where there is the desired dispersion compensation. Labels  415 ,  4152 ,  426 ,  436 , and  446  refer to connecting fiber, inside the package labels  4151 ,  4153 ,  4154 ,  4261 ,  4262 ,  4361 ,  4362 ,  4461 , and  4462  refer to input and output fiber external to the package.  
         [0093]    In FIG. 6(A) the path of the light signal is as follows. The light enters the dispersion compensation structure in the direction shown by label  413 , into the fiber labeled  4153 . From  4153 , the light enters the first dispersion compensation element labeled  411 , where the light undergoes dispersion compensation. Next the light exits  411 , and travels through fiber  415 , entering the second dispersion compensation element labeled  412 . After undergoing dispersion compensation, the light exits  412 , entering fiber  4154  in the direction indicated by label  414 .  
         [0094]    Label  4112  refers to a blow up of the area bounded by the dotted line labeled  4111 , showing the internal details of this area. This area is made up of two pieces of fiber, labeled  4151  and  4152 , and a lens labeled  417 , which make up the DFFA. Light enters fiber  4151  in the direction indicated by the label  4131 , passing through the lens  417 , and entering the thin-film chip labeled by  416 .  
         [0095]    The thin-film chip labeled  416  possesses the group delay versus wavelength characteristics shown in FIG. 5(A). Light that enters  416 , first going through fiber  4151  and passing through lens  417 , experiences third-order dispersion compensation. The light that exits  416 , passes through lens  417  again, then goes through fiber  4152  in the direction labeled  4141  to enter the dispersion compensation element labeled  412 . Fiber  4152  and fiber  415  are essentially the same. Fiber  4151  and fiber  4153  are also essentially the same. The dispersion compensated light signal, after passing through  412 , goes through the output fiber  4154  in the direction labeled  414 .  
         [0096]    Light passing through the structure labeled  510  in FIG. 6(A) will experience dispersion compensation according to the group delay versus wavelength characteristics shown in FIG. 5(B).  
         [0097]    The light passing through fiber  4151  in the direction of  4131 , entering the DFFA  418 , reflecting off the thin-film dispersion compensating chip,  416 , entering fiber  4152  in the direction of  4141  will experience from 0.3 to 0.5 dB loss, referred to as the coupling loss. This loss is quite small, for example in comparison to the loss of a fiber bragg grating. However, in order to achieve dispersion compensation over wider bandwidths, like 15 and 30 nm, the method described in FIG. 5 was introduced. In such a method, where the individual dispersion compensation elements are cascaded, the coupling loss can rapidly increase to where it becomes a serious problem. For example, just connecting 10 dispersion compensation units would result in coupling loss between 3 to 5 dB.  
         [0098]    With the goal of making a dispersion compensation device or developing a dispersion compensation method that is valid for wider bandwidths, but without suffering a large coupling loss, FIGS. 7 through 10 are presented along with their explanation in the following discourse.  
         [0099]    Before going into this discussion, a more detailed explanation concerning dispersion compensation is presented for a deeper understanding.  
         [0100]    In FIG. 6(B), the light signal proceeds through device  420  in the following manner. Light enters fiber  4261  in the direction of  424 , entering the dispersion compensation element  421 . Dispersion compensated light outputs  421  to enter fiber  426 . From this point on, the light experiences further dispersion compensation as it travels through dispersion compensation elements  422  and  423 . The dispersion compensation experienced by the light that is output of device  420 , traveling through fiber  4252  in the direction of  425 , is according to the curve shown in FIG. 5(C).  
         [0101]    The structure labeled  430  in FIG. 6(C), achieves the same dispersion compensation characteristics as the device shown in FIG. 6(A). In the structure shown in FIG. 6(C), fiber  436  is used to connect two points on the same wafer, labeled  432  and  433 , whose dispersion characteristics are the same as the dispersion characteristics of the dispersion compensation elements  411  and  412 .  
         [0102]    Can compensate for dispersion in the manner depicted in FIG. 6.  
         [0103]    The structure depicted in FIG. 6(D) can compensate for dispersion in the same manner as the structure of FIG. 6(A). Two DFFAs,  442  and  443  can be connected via fiber,  446 , and locked in case  441 . Light is input into fiber  4461  and output fiber  4462 , the output of structure  440 , after passing through  442  and  443 . Not shown in this figure is that this structure,  440 , is above a thin-film wafer of the form shown in FIG. 3. The structure,  440 , could be moved via some electronic circuit, adjusting the positions of  442  and  443  over the wafer surface, and thereby changing the group delay versus wavelength curve.  
         [0104]    In order to increase the dispersion compensation bandwidth and group delay peak, one can connect dispersion compensation elements in series to produce resultant group delay versus wavelength characteristics like the ones shown in FIG. 5.  
         [0105]    However, using the method shown in FIG. 6, which involves connecting many collimator based dispersion compensating elements together, results in a large amount of loss. The inventors propose a dispersion compensation method or device to reduce this loss, as shown in FIGS. 7 and 8.  
         [0106]    [0106]FIG. 7 is used to explain the details of the composite dispersion compensation structure. FIG. 7(A) shows a side view and FIG. 7(B) shows a view from the top. The dotted lines in FIG. 7(B) refer to the parts that cannot be seen from the top, but are explained about anyway.  
         [0107]    In FIG. 7, label  701  refers to the composite dispersion compensation structure proposed by the inventors. Labels  703  and  704  are dispersion compensation elements, to be explained below, that can be connected in series as discussed previously. Labels  710  and  720  refer to substrates. Labels  711  and  721  refer to thin-film structures that are deposited above the substrates and that possess the group delay versus wavelength characteristics that are necessary for dispersion compensation. Label  730  outlines the path that the light single takes, to be discussed later, which is described by the labels  741  to  747 ,  750 , and  760  to  767 . Labels  781  and  782  refer to fiber. Labels  783  and  784  are lenses. Labels  708  and  709  describe the direction along which the thickness of the transmission layers change. D 1  and d 2  are the separation distances of  703  and  704  at the edges.  
         [0108]    Label  701  shows the details of the composite dispersion compensation device, made up of two opposing dispersion compensation elements,  703  and  704 .  
         [0109]    The path of the light signal going through  701  in FIG. 7(A) is described as follows. The light signal enters through fiber  781 , passes through lens  783 , follows the light path  741  before reflecting off dispersion compensation element  703  and experiencing the dispersion compensation provided by the thin-film layers  711 . The light then follows path  742  and reflects off dispersion compensating element  704 , where it experiences dispersion compensation provided by the thin-film layers  721 . In a similar manner, the light continues to reflect off surfaces  711  and  721 , in an alternating fashion, following the path  743  through  747 , then returning back by following path  750 ,  760  through  766 ,  767 , entering lens  784 , and finally entering fiber  782 , the output of the composite dispersion compensation structure  701 .  
         [0110]    It is evident that at each reflection point on the dispersion compensation unit surfaces,  703  and  704 , there is dispersion compensation in the same manner as if separate dispersion compensation units had been connected in series, as in FIG. 6.  
         [0111]    The dispersion compensation elements,  703  and  704  are separated by d 1  at the top of FIG. 7(A) and separated by d 2  at the bottom of FIG. 7(A) in the composite dispersion compensation structure,  701 . The distance d 1  is shorter than the distance d 2 , such that when the input light, incident along path  741 , reaches path  750 , the reflection direction changes, and the light signal returns by way of path  760  through  766 , exiting the device via path  767 . As an example of typical parameter values associated with the composite dispersion compensation structure  701  would be an input angle (the angle between the input light and the normal to surface  711 ) of 5 degrees, a distance d 1  of 10 mm, and an input beam width along path  741  of 1 mm.  
         [0112]    The dispersion compensation elements  703  and  704 , consists of thin-film structures  711  and  721  deposited on substrates  710  and  720 . The thickness of the layers of the layers, running from the bottom of the figure to the top of the figure can vary in the manner shown in FIG. 3. That is to say, the layer thickness is a function of position.  
         [0113]    As one example, the transmission layers of the thin-film structures  711  and  721  could change in the directions indicated by the arrows  708  and  709  in a manner following the explanation of FIG. 3. In this way, the group delay versus wavelength characteristics of every point would have different peak group delay values and different dispersion compensation bandwidths.  
         [0114]    The resultant group delay characteristics of the composite dispersion compensation device  701 , made up dispersion compensation elements  703  and  704 , with input signal path  741 , and output signal path  767  can be explained using an explanation to that given previously for FIG. 5. However, as there are many more reflections, one could imagine a resultant group delay versus wavelength characteristic curve as shown in FIG. 9, along with all the individual group delay versus wavelength characteristics that sum to it.  
         [0115]    The coupling loss is associated with the loss due to the input coupling element, like a collimator, both when the light is input into it and returns to it. The reflection loss is the loss due to the reflection body.  
         [0116]    In general the coupling loss is much greater than the reflection loss. At each point along a dispersion compensation elements surface, there is a maximum reflection loss at the wavelength where the group delay is at a peak value. Typically, this is on the order of 1 dB. For wavelengths outside the compensation bandwidth the reflection is so small that it can be ignored.  
         [0117]    The loss associated with this invention, a composite dispersion compensation device like the one in  701 , is the sum of the losses of each reflection point along the signal light path, plus the one time coupling loss. This total loss is much less than the loss associated with directly connecting dispersion compensation elements in series, that is due to coupling loss summing over every element, as depicted in FIG. 6.  
         [0118]    In FIG. 8 is shown another version of the composite dispersion compensation structure that is labeled  702 . In this case, thin-film layers are deposited on both sides of the substrate  705 . The thin-film layer structures on both sides are labeled  706  and  707  respectively, and are both able to provide dispersion compensation. The input light enters this device in the direction labeled  785 , and exits this device in the direction labeled  786 . The substrate thickness of the upper side is less than the bottom side in the same manner as thickness differences, d 1  and d 2 , discussed in FIG. 7(A).  
         [0119]    The thin-film structures,  706  and  707  in FIG. 8 possess tapers similar to the tapers possessed by the thin-film structures of the dispersion compensation elements of FIG. 7(A).  
         [0120]    In the composite dispersion structure  702  of FIG. 8, light enters in the direction of arrow  785  and follows a path of multiple reflections within substrate  705  in a similar manner to the device in FIG. 7(A). At each reflection there is dispersion compensation provided by the thin-film dispersion compensation elements  706  and  707 . Finally, the light exits  702  in the direction of the arrow  786 .  
         [0121]    The thin-film structure of the dispersion compensation elements  706  and  707  can be described in a similar manner to the thin-film structures  711  and  721 , which was done using FIGS. 2 through 4.  
         [0122]    In FIG. 7(A) the thin-film structures,  711  and  721 , deposited on substrates  710  and  720 , must have at least two reflection layers and one transmission layer. The reflection layer farthest from the input light, or last reflection layer, has the highest reflection value. The reflection layer nearest to the input light, or first reflection layer, has the least reflection value. The reflection values going from the first reflection layer to the last reflection layer are in between the highest and lowest reflection values, but in increasing value with increasing distance from the first reflection layer. Each transmission layer must be sandwiched between two reflection layers.  
         [0123]    For the purposes of dispersion compensation, the thin-film structures of FIG. 7(A) must possess any of the following arrangements of reflection layers and transmission layers. If there are two reflection layers then there must be one transmission layer or cavity. If there are three reflection layers then there must be two transmission layers or cavities. If there are four reflection layers then there must be three transmission layers or cavities. If there are five reflection layers then there must be four transmission layers or cavities.  
         [0124]    There must be at least two reflection layers and one transmission layer in the thin-film structures  706  and  707  used in FIG. 8, and at least one reflection layer with a reflectance value greater than or equal to 99.5% as is the same for the thin-film structures in FIG. 7(A). The direction of increasing reflection values in  706  and  707  is opposite to the direction of increasing reflection values in  711  and  721 . The reflection layers having the largest values in  706  and  707  are located farthest from the substrate  705 .  
         [0125]    The separation distances, d 1  and d 2 , between the dispersion compensation elements  703  and  704  in FIG. 7 when chosen to be suitably different result in the input and output signals of FIG. 7(A) to appear on the same side. For the case of FIG. 7(A) d 1 &lt;d 2 .  
         [0126]    If the separation distances, d 1  and d 2 , between the dispersion compensation elements  703  and  704  in FIG. 7 were chosen to be the same then the input and output signals of FIG. 7(A) would appear on opposite sides.  
         [0127]    [0127]FIG. 9 is used to explain the resultant group delay versus wavelength characteristics of the composite dispersion structure displayed in FIG. 7(A). In FIG. 9, label  801  shows the group delay versus wavelength characteristics of each of the reflections that occurs when the light signal reflects off the surfaces of the dispersion compensation elements  703  and  704 . As the arrows  708  and  709 , depicting the change in thin-film layer thickness of  711  and  712 , are in opposite directions, the resultant group delay versus wavelength curves are all symmetric. Label  800  refers to the resultant group delay versus wavelength curve when the group delay versus wavelength curves that result from single reflections are all combined.  
         [0128]    The response of the composite group delay structure  701 , depicted by the resultant group delay versus wavelength curve in FIG. 8, has a wider compensation bandwidth and larger group delay peak value than any of the group delay versus wavelength curves resulting from single reflections in  801 . The loss of  701  is much less than if the same resultant group delay versus wavelength curve had been made using a connection of lens based units like the ones depicted in FIG. 6.  
         [0129]    [0129]FIG. 10 is a graph showing the group delay versus wavelength characteristics, labeled  901  and  902 , of the two seven-layer thin-film dispersion compensation designs presented earlier.  
         [0130]    The curve labeled  901  refers to the group delay versus wavelength characteristics of the thin-film structure defined by the formula F1:  
         F1=(LH)(LL) 9 H(LH) 2 (LL) 11 H(LH) 4 (LL) 9 H(LH) 13 .  
         [0131]    The curve labeled  901  describes a dispersion compensation element with a peak group delay varying between 400 and 700 fs and a compensation bandwidth between 17 and 19 nm centered about 1550 nm. The reflectance values of the mirrors, A, C, E, and G, are on the order of 4%, 65%, 96%, and 100% respectively. In the formula F1, denoted LH as mirror layer A, (LL) 9  as cavity layer B, H(LH) 2  as mirror layer D, (LL) 11  as cavity layer D, H(LH) 4  as mirror layer E, (LL) 9  as cavity layer F and H(LH) 13  as mirror layer G.  
         [0132]    The curve labeled  902  refers to the group delay versus wavelength characteristics of the thin-film structure defined by the formula F2.  
         F2=(LH) [H c ] H(LH) 2 [I] H(LH) 4 [J] H(LH) 13    
         [0133]    where  
         [0134]    H c =(LL) 9 =(LL) 3 (HH) 3 (LL) 2 (HH) 1 (LL) 1    
         [0135]    I=(LL) 11 =(LL) 3 (HH) 3 (LL) 3 (HH) 1 (LL) 2    
         [0136]    J=(LL) 9 =(LL) 3 (HH) 3 (LL) 2 (HH) 1 (LL) 1    
         [0137]    The curve labeled  902  describes a dispersion compensation element with a peak group delay varying between 400 and 700 fs and a compensation bandwidth between 17 and 19 nm centered about 1550 nm.  
         [0138]    The dispersion compensation elements, defined by formulas F1 and F2, whose characteristics are described by curves  901  and  902  have very wide dispersion compensation bandwidths for a single element.  
         [0139]    By using these two dispersion compensation elements in the manner described and explained in FIGS. 5 through 9, the dispersion compensation bandwidth and amount of dispersion compensation can be extended even further.  
         [0140]    In general three-cavity thin-film dispersion compensation devices with group delay versus wavelength characteristics similar to the ones labeled  901  and  902  in FIG. 10 and extremely low loss spectral characteristics must satisfy two important relations found by the inventors. These relations relates the reflectance values of the fundamental layers, denoted R 1 , R 3 , R 5 , and R 7  respectively, and is given by R 1           R 3           R 5           R 7  and R 7           98%.  
         [0141]    The composite dispersion compensation structure can not only be made up of one pair of dispersion compensation elements as discussed previously, but can be made up of many pairs of dispersion compensation elements.  
         [0142]    The subject of this invention, a composite dispersion compensation structure, by effectively using its component parts, i.e. two dispersion compensation elements, can compensate the dispersion over wide bandwidths of 15 nm and 30 nm. Furthermore, narrower bandwidths, for example between 5 to 10 nm, 3 nm and even 1 nm can be compensated for in light wave communication systems.  
         [0143]    This kind of invention, a composite dispersion compensation structure, was used successfully in a 160 Gbit/sec fiber transmission system consisting of over 60 km of DSF. In this experiment, 1.6 ps pulses were pre-compensated by a cascade of two dispersion compensation elements, so that after traveling through 60 km of DSF, there was no distortion due to dispersion.  
         [0144]    In this patent was described a composite dispersion compensation structure made up of dispersion compensation elements and the methods associated with using this structure and its elements for dispersion compensation. The main characteristic of the composite dispersion compensation structure was that many dispersion elements could be combined together, the minimum unit being a pair of opposing structures. A light signal would reflect off the two surfaces many times, with each time resulting in a little more dispersion compensation. The loss occurring between the input and output signal is overwhelmingly due to the individual reflection losses, which are far greater than the coupling loss. Such a device can provide both second and third order dispersion compensation over a wide bandwidth with low loss.