Abstract:
A wavelength converter is disclosed. The converter comprises a broadband light source producing light having a plurality of wavelengths. Further, a semiconductor optical amplifier is provided that receives the light from the light source. The semiconductor optical amplifier amplifies the light under the control of a control signal related to an optical signal of a first wavelength. Next, a demultiplexer receives the output of the semiconductor optical amplifier and extracts from the amplified optical signal at least one of the plurality of wavelengths.

Description:
RELATED APPLICATIONS  
       [0001]    Priority is hereby claimed under 35 U.S.C. §120 to U.S. Provisional Patent Application Ser. No. 60/338,927 filed Oct. 22, 2001, U.S. Provisional Patent Application Ser. No. 60/373,803 filed Apr. 19, 2002, U.S. patent application Ser. No. 10/104,273 filed Mar. 22, 2002, U.S. patent application Ser. No. 10/177,632 filed Jun. 19, 2002, U.S. patent application Ser. No. 10/188,955, filed Jul. 3, 2002, U.S. patent application Ser. No. 10/190,018, filed Jul. 5, 2002, and U.S. patent application Ser. No. 10/202,054, filed Jul. 23, 2002, each of which is incorporated by reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates to wavelength converters, and more particularly, to a wavelength converter that utilizes a Bragg-grating.  
         BACKGROUND  
         [0003]    Wavelength converters are often used in wavelength division multiplex (WDM) optical communications systems. A wavelength converter is a device that can convert data carried on a first wavelength of light into the same data carried onto a second wavelength of light. Early wavelength converters operated by extracting the data from the first wavelength by demodulation techniques and then re-modulating the data onto a second wavelength of light. This opto-electro-opto conversion process required relatively complex circuitry. More recent wavelength converters are all optical, i.e., the data is converted all in the optical domain using optical components. An example of this is shown in U.S. Pat. No. 6,356,382 to Nakano et al. Thus, there are various methods to perform the wavelength conversion function. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]    [0004]FIG. 1 is a schematic illustration of an embodiment of the present invention.  
         [0005]    [0005]FIGS. 2A to  2 F are schematic diagrams showing the on/off switching functions of a wavelength selective bridge waveguide of this invention.  
         [0006]    [0006]FIGS. 3A to  3 B are cross sectional views for showing coupling configurations of a wavelength-selective bridge waveguide coupled between a waveguide and an outbound waveguide.  
         [0007]    [0007]FIGS. 4A and 4B are functional diagrams for showing wavelength selective bridge waveguides acting as a switch that is coupled between the intersecting waveguides for switching and re-directing optical transmission of a selected wavelength.  
         [0008]    [0008]FIG. 5A illustrates a bridge-beam type switch with integrated Bragg grating element.  
         [0009]    [0009]FIG. 5B illustrates the cross-sectional structure of a bridge-beam type switch in which the grating coupling is normally off.  
         [0010]    [0010]FIG. 5C shows the grating element of a bridge-beam type switch in the “on” position.  
         [0011]    [0011]FIG. 6A illustrates a cantilever-beam type switch with integrated Bragg grating element.  
         [0012]    [0012]FIG. 6B illustrates the cross-sectional structure of a cantilever-beam type switch in which the grating coupling is normally off.  
         [0013]    [0013]FIG. 6C shows the grating element of a cantilever-beam type switch in the “on” position.  
         [0014]    [0014]FIG. 7A illustrates a dual cantilever-beam type switch with integrated Bragg grating element.  
         [0015]    [0015]FIG. 7B illustrates the cross-sectional structure of a dual cantilever-beam type switch in which the grating coupling is normally off.  
         [0016]    [0016]FIG. 7C shows the grating element of a dual cantilever-beam type switch in the “on” position.  
         [0017]    [0017]FIG. 8 illustrates the cross-sectional structure of another embodiment of the grating element.  
         [0018]    [0018]FIG. 9 illustrates an embodiment where the grating elements are fabricated on both the substrate and the movable beam.  
         [0019]    [0019]FIG. 10 illustrates an embodiment where the grating elements are fabricated on the horizontal sides of the movable beam.  
         [0020]    [0020]FIGS. 11A and 11B illustrate a grating element where the waveguides are both fabricated on the same surface of the substrate. 
     
    
     DETAILED DESCRIPTION  
       [0021]    The present invention describes a method and apparatus for wavelength conversion in an optical telecommunications system. In the following description, numerous specific details are provided to provide a thorough understanding of the embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, etc. In other instances, well-known structures or operations are not shown or described in detail to avoid obscuring aspects of various embodiments of the invention.  
         [0022]    Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.  
         [0023]    Further, although the present invention is described in terms of a WDM system, the apparatus and method of the present invention can equally be applicable to any optical system that utilizes multiple frequencies. Thus, the description below is but one embodiment of the present invention.  
         [0024]    [0024]FIG. 1 illustrates a wavelength converter  101  formed in accordance with the present invention. The wavelength converter includes a broadband light source (BLS)  103 , a semiconductor optical amplifier (SOA)  105 , a wavelength selective de-multiplexer  107 , and a multiplexer  109 . The broadband light source  103  provides light across a spectrum of wavelengths, and more particularly, wavelengths within the band of interest. In one embodiment, the broadband light source  103  provides light in the range of 1520 to 1570 nanometers, also referred to as the “C-band”.  
         [0025]    The broadband light source  103  provides the broadband light as an input to the SOA  105 . The SOA  105  is operative to receive the broadband light from the broadband light source  103  and amplify that broadband light in accordance with an input control signal. The input control signal may either be electrical or optical in nature. Typically, the input control signal is modulated with data. As will be seen in greater detail below, the input control signal is related to the optical signal having a first wavelength that is to be converted to the second wavelength. The output of the SOA  105  is thus broadband light that is amplified and modulated by the control signal. In other words, the output of the SOA  105  is broadband light modulated by the data carried by the optical signal of the first wavelength.  
         [0026]    In many respects, SOA  105  is similar in construction to a conventional semiconductor laser in that it consists of a layer of semiconductor material (known as the active region), sandwiched in between other layers of semiconductors of a different composition. An electrical current (as the control signal) is passed through the device and serves to excite electrons in the active region. When photons travel through the active region, this will cause these electrons to lose some of their extra energy in the form of more photons that match the wavelength (or wavelengths) of the initial input. Therefore, an optical signal passing through the active region is amplified and is said to have experienced gain. Moreover, by varying the electrical current either in the amplitude for time domain, the optical signal can be modulated. Additionally, the semiconductor layers that sandwich the active region are designed to help guide the light through the device. This is achieved through a difference in refractive index from the active region, in much the same way as the refractive index differs between an optical fiber&#39;s core and its cladding help to guide light. The SOA  105  is commercially available from companies such as Alcatel, Kamelian, Opto Speed, and others.  
         [0027]    In one embodiment, the SOA  105  is controlled by a control signal related to the data carried on the optical signal having the first wavelength. In some embodiments, the optical signal having the first wavelength of light is directly input to the SOA  105  to control the amplification effect. In other embodiments, the optical signal having the first wavelength is converted into an electrical signal or electrical pulses that are input into the SOA  105  to control amplification. In either embodiment, the amplification provided by the SOA  105  to the broadband light input is dependent upon (i.e., modulated by) the control signal.  
         [0028]    The output of the SOA  105  is a modulated and amplified broadband signal (I λ ) that is input into the wavelength selective demultiplexer  107 . The wavelength selective demultiplexer  107  includes an input waveguide  111  and a plurality of intersecting waveguides  113   a - n.  The intersecting waveguides  113   a - n  intersect with the input waveguide  111 . Disposed at the intersections of the intersecting waveguides  113  and the input waveguide  111  are switches  115   a - n.  As seen in further detail below, the switches  115   a - n  are selectively capable (when activated) of redirecting light of a specific wavelength into the associated intersecting waveguide  113   a - n.  The switches  115  are Bragg-grating based switches and are of the type disclosed in our co-pending applications noted above and which are herein incorporated by reference in their entirety. However, a description is provided herein for completeness.  
         [0029]    [0029]FIGS. 2A and 2B are schematic diagrams for showing the principles of operation of the switches  115 . A multiplexed optical signal is transmitted in an optical waveguide  110  over N multiplexed wavelengths λ 1 , λ 2 , λ 3 , . . . , λ N  where N is a positive integer. This is a general characterization of a plurality of wavelengths carried by the waveguide  110 . In the embodiment of FIG. 1, the waveguide  110  is equivalent to the input waveguide  111  and the optical signals λ 1 , λ 2 , λ 3 , . . . , λ N  are equivalent to I λ .  
         [0030]    In FIG. 2A, a wavelength selective bridge waveguide  120  is moved to an on-position and coupled to the waveguide  110 . An optical signal with a central wavelength λi particular to the Bragg gratings  125  disposed on the bridge waveguide  120  is guided into the wavelength selective bridge waveguide  120 . The remaining wavelengths λ 1 , λ 2 , . . . , λ i−1 , . . . , λ i+1 , . . . , λ N  are not affected and continues to propagate over the waveguide  110 . The Bragg gratings  125  have a specific pitch for reflecting the optical signal of the selected wavelength λ i  onto the wavelength selective bridge waveguide  120 .  
         [0031]    In FIG. 2B, the wavelength selective bridge waveguide  120  is moved away from the waveguide  110  to a “bridge-off” position. There is no coupling between to the waveguide  110  and therefore no “detoured signal” entering into the bridge waveguide  120 . The entire multiplexed signal over wavelengths ∥ 1 , ∥ 2 , ∥ 3 , . . . , λ N  continue to propagate on the waveguide  110 .  
         [0032]    [0032]FIGS. 2C and 2D illustrate a detailed configuration of the Bragg-gratings formed on the wavelength selective bridge waveguide  120 . The pitch between the gratings  125  defines a selected wavelength that will be reflected onto the bridge waveguide  120  when the wavelength selective bridge waveguide is at an on-position coupled to the waveguide  110  as that shown in FIG. 2A. Furthermore, as shown in FIGS. 2E and 2F, the Bragg-gratings  125  may be formed on a surface of the bridge waveguide  120  opposite the waveguide  110 . Again, as the bridge waveguide  120  is moved to an “on” position coupled to the waveguide  110  in FIGS. 2C and 2E, an optical signal of a selected wavelength defined by the pitch between the Bragg gratings is coupled into the bridge waveguide  120 . When the bridge waveguide  120  is moved to an “off” position in FIGS. 2D and 2F, the bridge waveguide  120  is completely decoupled and there is no “detoured signal” into the bridge waveguide  120 .  
         [0033]    [0033]FIG. 3A shows a wavelength selective bridge waveguide  220  coupled between a bus waveguide  210  and a second waveguide  230 . A multiplexed optical signal is transmitted in a bus waveguide  210  over N multiplexed wavelengths λ 1 , λ 2 , λ 3 , . . . , λ N  where N is a positive integer. The wavelength selective bridge waveguide  220  has a first set of Bragg gratings disposed on a first “bridge on-ramp segment”  225 - 1  for coupling to the bus waveguide  210 . An optical signal with a central wavelength λ i  particular to the Bragg gratings  225  disposed on the bridge waveguide  220  is guided through the first bridge ramp segment  225 - 1  to be reflected into the wavelength selective bridge waveguide  220 .  
         [0034]    The remainder optical signals of the wavelengths λ 1 , λ 2 , λ 3 , λ i−1 , . . . , λ i+1 , . . . , λ N are not affected and continues to transmit over the waveguide  210 . The Bragg grating  225  has a specific pitch for reflecting the optical signal of the selected wavelength λ i  onto the wavelength selective bridge waveguide  220 . The wavelength selective bridge waveguide  220  further has a second set of Bragg gratings as a bridge off-ramp segment  225 - 2  coupled to an outbound waveguide  230 . The second set of Bragg gratings has a same pitch as the first set of Bragg gratings. The selected wavelength λ i  is guided through the bridge off-ramp segment  225 - 2  to be reflected and coupled into the outbound waveguide  230 .  
         [0035]    The bridge waveguide  220  can be an optical fiber, waveguide or other optical transmission medium connected between the bridge on-ramp segment  225 - 1  and the bridge off-ramp segment  225 - 2 .  
         [0036]    [0036]FIG. 3B shows another wavelength selective bridge waveguide  220 ′ is coupled between a bus waveguide  210  and a second waveguide  230 ′. A multiplexed optical signal is transmitted in a bus waveguide  210  over N multiplexed wavelengths λ 1 , λ 2 , λ 3 , . . . , λ N  where N is a positive integer. The wavelength selective bridge waveguide  220 ′ has a first set of Bragg gratings disposed on a first “bridge on-ramp segment”  225 - 1  for coupling to the bus waveguide  210 . An optical signal with a central wavelength λ i  particular to the Bragg gratings  225 - 1  disposed on the bridge waveguide  220 ′ is guided through the first bridge ramp segment  225 - 1  to be reflected into the wavelength selective bridge waveguide  220 ′.  
         [0037]    The remainder optical signals of the wavelengths λ 1 , λ 2 , λ 3 , λ i−1 , λ i+1 , . . . , λ N  are not affected and continues to transmit over the waveguide  210 . The Bragg gratings  225 - 1  have a specific pitch for reflecting the optical signal of the selected wavelength λ i  into the wavelength selective bridge waveguide  220 ′. The wavelength selective bridge waveguide  220 ′ further has a bridge off-ramp segment  225 - 2 ′ coupled to an outbound waveguide  230 ′ near a section  235  of the outbound waveguide  230 . The section  235  on the outbound waveguide  230 ′ has a second set of Bragg gratings having a same pitch as the first set of Bragg gratings. The bridge waveguide  220  can be an optical fiber, waveguide or other optical transmission medium connected between the bridge on-ramp segment  225 - 1  and the bridge off-ramp segment  225 - 2 ′.  
         [0038]    [0038]FIG. 4A shows a wavelength selective bridge waveguide  320  is coupled between a bus waveguide  310  and an intersecting waveguide  330 . Indeed, the following description shows the operation of the switches  115   a - n  at the intersection of the input waveguide  111  and the intersecting waveguides  113   a - n.  A multiplexed optical signal is transmitted in a bus waveguide  310  over N multiplexed wavelengths λ 1 , λ 2 , λ 3 , . . . , λ N  where N is a positive integer. The wavelength selective bridge waveguide  320  (also referred to as the switch  115  of FIG. 1) has a first set of Bragg gratings disposed on a first “bridge on-ramp segment”  325 - 1  for coupling to the bus waveguide  310 . An optical signal with a central wavelength λ i  particular to the Bragg gratings  325  disposed on the bridge waveguide  320  is guided through the first bridge ramp segment  325 - 1  to be reflected into the wavelength selective bridge waveguide  320 . The remainder optical signals of the wavelengths λ 1 , λ 2 , λ 3 , . . . , λ i−1 , λ i+1 , . . . , λ N  are not affected and continues to propagate over the waveguide  310 .  
         [0039]    The Bragg gratings  325  have a specific pitch for reflecting the optical signal of the selected wavelength λ i  into the wavelength selective bridge waveguide  320 . The wavelength selective bridge waveguide  320  further has a second set of Bragg gratings  325  as a bridge off-ramp segment  325 - 2  coupled to an outbound waveguide  330 . The bridge waveguide  320  can be an optical fiber, waveguide or other optical transmission medium connected between the bridge on-ramp segment and the bridge off-ramp segment  325 - 2 .  
         [0040]    [0040]FIG. 4B is another embodiment with the bus waveguide  310  disposed in a vertical direction and an interesting outbound waveguide  330  disposed along a horizontal direction. As will be seen below, this embodiment of the switch is used in the non-movable bridge waveguide  109 .  
         [0041]    The structures shown in FIGS.  2 - 4  can be implemented as MEMS devices. For example, FIG. 5A depicts an illustrative embodiment of bridge-beam type switchable grating structure with integrated Bragg grating elements. The structure is fabricated using MEMS technology and semiconductor processing described below. On the substrate  701 , a cladding layer  702  is formed first. Then the core layer  703  is deposited and patterned to form waveguide core that is shown more clearly in the cross-sectional view FIG. 5B. The bridge beam  501  is a waveguide consisting of integrated Bragg gratings  520  and an embedded electrode. When this waveguide, called a bridge waveguide, is electrostatically bent close enough to a waveguide  510 , the wavelength that meets the Bragg phase-matching condition is coupled into the bridge waveguide. Through the bridge waveguide, the selected wavelength can then be directed into a desired output waveguide.  
         [0042]    [0042]FIG. 5B shows the cross-sectional view of bridge-beam type switchable grating structure with integrated Bragg grating elements. After the cladding layer  702  and core layer  703  are deposited, a sacrificial layer is deposited after another cladding layer  704  is deposited and patterned. After the sacrificial layer is patterned and the grating grooves are etched on sacrificial layer, another cladding layer  706  is deposited. The electrode layer  708  and the insulation layer  709  are deposited subsequently. The etching process starts from layer  709  through into layer  704  after patterning. Finally the sacrificial layer is etched to form the air gap  705  between waveguide  510  and grating element  520 . In an alternative way, the waveguide and the grating element can be fabricated on its own substrate first. Then they are aligned and bonded together to make the same structure shown in FIG. 7B. Due to the existence of air gap  705 , the grating is off when the grating element is at normal position (no voltages applied). Referring to FIG. 5C, when an appropriate voltage  710  is applied between the electrode  708  and substrate  701 , the grating element  520  is deflected toward waveguide  510  by the electrostatic force. The grating is turned “on” when the grating element  520  moving close enough to input waveguide  510 .  
         [0043]    [0043]FIG. 6A depicts an illustrative embodiment of cantilever-beam type switchable grating structure with integrated Bragg grating elements. The structure is fabricated using similar MEMS technology and semiconductor processing described above. In this arrangement, the stress and strain in the grating segment  520  can be reduced greatly. Therefore, the lifetime of grating element can be improved. FIG. 6B shows the cross-sectional structure of a cantilever-beam type switch. Referring to FIG. 6C, the cantilever beam  501  is deflected by the electrostatic force. Applying voltages  710  between substrate  701  and electrode  708  controls the electrostatic force applied to the cantilever beam  501 . Therefore, by controlling the applying voltages  710  the wavelength-selective optical function can be activated through varying the degree of coupling between Bragg grating  520  and input waveguide  510 .  
         [0044]    An adequate beam length L is required in order to deflect the beam  501  to certain displacement within the elastic range of the material. For example, a 500 um long cantilever Si beam with the section of 12 um×3 um can be easily deformed by 4 um at the tip of the beam. Another major advantage for the cantilever beam structure is that the movable beam  501  can be shorter and therefore reduce the size of the switch.  
         [0045]    [0045]FIG. 7A illustrates another embodiment of the switch. This is a dual cantilever-beam type switch. In this structure the grating element is fabricated on a movable beam  502 , which is supported by two cantilever beams  505 . In this arrangement, the stress and strain in the grating segment can be eliminated almost completely if the electrode pattern is also located appropriately. Another advantage is that the material of cantilever beams  505  does not necessarily have to be the same as the material of grating element  520 . For instance, cantilever beams  505  can be made of metal to improve the elasticity of the beams. In addition, the anchor structure can be in different forms, e.g., MEMS springs or hinges. Therefore, a large displacement and smaller sized grating element is more achievable in this structure. FIGS. 7B and 7C shows the cross-sectional structure of a dual cantilever-beam type switch. Similar to the operations described above, the grating element  520  is moved towards the waveguide  510  by applying voltages  710  to electrode  708  and substrate  701 .  
         [0046]    [0046]FIG. 8 shows an alternate structure of the grating where the grating is located on the bottom side, or the surface side of the substrate. The structure can be fabricated by applying semiconductor processing technology to form the Bragg gratings  530  on the core layer  703  while positioning the movable beam  501  and the Bragg gratings  530  to have a small gap  705  from the waveguide  510 . Similar to the operations described above, an electric conductive layer  708  is formed on the movable beam  501  for applying the voltage to assert an electrostatic force to bend the movable beam  501 . The electrostatic force thus activates the movable switch by coupling a waveguide  706  to waveguide  510 . The Bragg gratings  530  thus carry out a wavelength-selective optical switch function.  
         [0047]    [0047]FIG. 9 is also another alternate structure of switchable gratings. In this structure the grating is located on both top and bottom sides. Similar semiconductor processing technology can be used to form the Bragg gratings  520  on the movable beam  501  and the Bragg gratings  530  on the waveguide  510 . A small gap is formed between waveguides  510  and  706 . An electric conductive layer  708  is also formed on the movable beam  501  for applying the voltage to assert an electrostatic force to bend the movable beam  501 . Similar to the operations described above, the electrostatic force thus activates the switch by coupling the selected wavelength from waveguide  510  to waveguide  706 .  
         [0048]    In the structures described above, the grating element is located faced up or down to the substrate. However, the grating element can also fabricated on the sides of the waveguide, as illustrated in FIG. 10. In this embodiment, the gratings  520  are fabricated on the horizontal sides of the movable beam  501  and the rest of the structure are similar to those structure described above and all the wavelength-selective functions and operations are also similar to those described above. In addition, by rearranging the pattern of the electrode, the grating structure can also be made on the top side of the cantilever or bridge beams. This structure may provide a cost advantage in manufacturing.  
         [0049]    [0049]FIG. 11A shows another structure of switchable gratings. Instead of arranging the coupling waveguides as several vertical layers supported on a semiconductor substrate as shown above, the coupling waveguides  610  and  620  are formed as co-planar on a same cladding layer  802 , supported on a semiconductor substrate  801 . The movable waveguide  610  and coupling waveguide  620  have their own embedded electrodes, similar to those described above. Again, the Bragg gratings  820  can be formed on one or both of the waveguides  610  and  620  as described above. When electrostatic voltages are applied between these electrodes, movable waveguide  610  is moved towards waveguide  620  and thus activate the optical switch. FIG. 11B shows another structure with the gratings  820  facing upward.  
         [0050]    Returning to FIG. 1, thus, each of the switches  115   a - n  extract from the input waveguide  111  one of the frequencies (λ 1 -λ n ) contained in the broadband signal output by the SOA  105 . In FIG. 1, it can be seen that the intersecting waveguide  113   a  contains the optical signal carried by λ 1 . Similarly, the intersecting waveguide  113   b  carries the signal carried by wavelength λ 2 . Intersecting waveguide  113   c  carries the signal carried on wavelength λ 3 . Finally, intersecting waveguide  113   n  carries the signal carried on wavelength λ n . It should be noted that the switches  115   a - n  in the wavelength selective demultiplexer  107  are selectively activated as desired. Thus, the switch  115   a  may be activated to switch the signal carried on wavelength λ 1  to the intersecting waveguide  113   a.  Alternatively, the switch  115   a  may be deactivated such that the intersecting waveguide  113   a  does not carry the signal on wavelength λ 1 . In that situation, the input waveguide  111  continues to carry the signal on wavelength λ 1 . Thus, the wavelength selective demultiplexer  107  can selectively extract one or more wavelengths from the broadband input to one or more intersecting waveguides  113   a - n.    
         [0051]    The intersecting waveguides  113   a - 113   n  are all input into the multiplexer  109 . Once input into the multiplexer  109 , the intersecting waveguides  113   a - 113   n  further intersect an output waveguide  117 . Located at the intersection of the intersecting waveguides  113   a - 113   n  with the output waveguide  117  are switches  119   a - 119   n.  These switches, in one embodiment, are fixed and operate to redirect the signal carried on the intersecting waveguides  113   a - 113   n  into the output waveguide  117 . In an alternative embodiment, the switches  119   a - 119   n  may also be selectively activated to provide another configurable option to the user. The switches  119   a - 119   n  are similar to that of the switches  115   a - 115   n.    
         [0052]    As seen, an input signal having wavelength λ i  can be provided to the wavelength converter  101  of the present invention and be converted into an arbitrary wavelength that is output by the output waveguide  117 . The arbitrary wavelength may be selected from the wavelengths provided by the broadband light source  103 . By selectively controlling the switches  115   a - 115   n,  one or more output wavelengths having the same data carried by the optical signal having the input wavelength λ i  can be output. In this manner, a wavelength converter is implemented.  
         [0053]    From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.