Patent Publication Number: US-2005135733-A1

Title: Integrated optical loop mirror

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims priority from U.S. Provisional Patent application Ser. No. 60/530,658 filed Dec. 19, 2003. 
    
    
     MICROFICHE APPENDIX  
      Not Applicable.  
     TECHNICAL FIELD  
      The present invention relates to optical communications systems and particularly to integrated optical loop mirrors.  
     BACKGROUND OF THE INVENTION  
      Semiconductor lasers typically use reflectors or mirrors to define a lasing region where photons can be reflected back and forth so that they can readily stimulate emissions from the gain medium. Various reflection techniques are known in the prior art such as cleaved facets, grating reflectors and etched facets.  
      Cleaved facets are typically formed by scribing lines or nicks in the semiconductor wafer to generate fracture locations. Mechanical force is then applied to fracture the wafer along the scribe lines. This is typically a labour intensive manual process performed by a skilled operator. Placement accuracy is limited to about ±10 μm, which is problematic for phase sensitive applications.  
      Coatings can be added to the cleaved facet to control reflections. In order to control reflections at multiple wavelengths, multiple coatings are required which can be both expensive and impractical. An important disadvantage of this technique is that these cleaved facets must be located at the edges of the resulting semiconductor chip, which restricts the possibility of integrating devices having such cleaved facets on the same substrate as other opto-electronic devices.  
      Gratings in waveguides such as, for example Bragg gratings, can be used for reflecting optical signals and are well known in the art. The reflection characteristics of gratings are inherently wavelength dependent and meticulous design and calibration are required to enable a wide tuning range which can increase costs. An extra growth step is typically required to produce the gratings on the waveguides and therefore adds to manufacturing costs. Optical gratings, especially those designed to provide a wide tuning range are bulky and when implemented on a semiconductor substrate, use up valuable chip real estate.  
      Another technique for producing optical reflections is by etched mirrors. The fabrication is relatively straight forward but etched mirrors are quite lossy due to the rough edges of the etched surface. Etched mirrors have poor reflection control and a low reflectance at perpendicular incidence. Applying coatings to the etched mirrors in order to control reflections is also difficult. Etched mirrors have wavelength independent reflection characteristics which is very difficult to overcome when wavelength dependent characteristics are desired.  
      Non-linear optical fiber loop mirrors are known in the art. A non-linear optical fiber loop mirror  100  is illustrated in  FIG. 1  and consists of a directional optical coupler  102  having a first input port  104  and a second input port  106  and two output ports  108 ,  110  and a fiber loop  112  connecting both output ports  108 ,  110  of the coupler  102  and a non-linear element  114  located asymmetrically in the fiber loop  112 . An optical signal  116  entering the optical coupler  102  at input port  104  is split in two and each half  118 ,  120  travels around the loop  112  in opposite directions. The non-linear element  114  introduces a phase shift in each of the counter-rotating optical signals at different times due to its asymmetrical location in the fiber loop  112 . The result is that the counter-rotating signals reach the coupler  102  with different phase shifts and their interaction can cause a portion of the signals to be directed to the second input port  106 . Such optical loop mirrors rely on characteristics of non-linear elements in the fiber loop  112  to affect the interaction of the counter-rotating signals. Some embodiments use especially long fiber loops in order to obtain the desired results. Optical fiber loop mirrors are not well suited for reflection purposes as a mirror as part of an active region of a laser for example because of the bulk of the optical fiber and the cost of interfacing the optical fiber to the laser.  
      Optical waveguide ring resonators are also known in the art. An optical waveguide ring resonator  200  is illustrated in  FIG. 2  and consists of a directional optical coupler  202  having a first input port  204  and a second input port  206  and two output ports  208 ,  210  and an optical waveguide loop  212  connecting output port  210  of the coupler  202  back to input port  206  to create an optical ring in which an optical signal can pass repeatedly until wavelength dependent characteristics induce resonance. No reflection function is exploited in such a configuration.  
      Accordingly, a method and system for providing a cost effective and compact optical reflection function, which lends itself to integration with other optical elements on a substrate, remains highly desirable.  
     SUMMARY OF THE INVENTION  
      It is therefore an object of the present invention to provide an integrated optical loop mirror which can be constructed on a substrate.  
      Accordingly, an aspect of the present invention provides an optical loop mirror comprising an optical coupler and an optical waveguide formed on a substrate. The optical coupler has at least one nominal input port and at least a first nominal output port and a second nominal output port. The optical waveguide has a first end and a second end, wherein the first end is optically coupled to the first nominal output port and the second end is optically coupled to the second nominal output port.  
      In some embodiments, the substrate is a semiconductor substrate.  
      In some embodiments, the substrate is comprised of glass.  
      In some embodiments, the substrate is comprised of lithium niobate.  
      In some embodiments, the optical coupler is a multimode interference (MMI) coupler.  
      In some embodiments, the optical coupler has only the one nominal input port and the first and second nominal output ports.  
      In some embodiments, the optical coupler has a second nominal input port.  
      In some embodiments, the optical coupler and said optical waveguide are monolithically formed on the semiconductor substrate.  
      In some embodiments, the optical coupler and said optical waveguide are formed of photonic crystals.  
      In some embodiments, the optical loop mirror comprises a whispering gallery type waveguide.  
      In some embodiments, the optical waveguide is coupled to said optical coupler such that light energy can flow through said waveguide in only one pass in each direction.  
      In some embodiments, the optical loop mirror is integrated on said semiconductor substrate with other optoelectronic devices.  
      In some embodiments, the optical loop mirror is incorporated in a distributed feedback (DFB) laser.  
      In some embodiments, the optical loop mirror is incorporated in a semiconductor optical amplifier (SOA).  
      In some embodiments, the optical loop mirror is incorporated in a Mach-Zehnder interferometer.  
      In some embodiments, the optical loop mirror is incorporated in a dual-pass semiconductor optical amplifier (SOA), wherein a first SOA is connected to a first nominal input port of the optical loop mirror and a second SOA is connected to a second nominal input port of the optical loop mirror.  
      In some embodiments, the waveguide has a wavelength filter between the first end and second end.  
      In some embodiments, the wavelength filter has a coupled ring resonator.  
      In other embodiments, the waveguide has a transmission tap between the first end and second end.  
      A further aspect of the present invention provides an optical loop mirror for reflecting an optical signal. The optical loop mirror has an optical coupler and an optical waveguide formed on a substrate. The optical coupler has at least one nominal input and at least a first nominal output and a second nominal output. The optical waveguide has a first end and a second end, wherein the first end and the second end are connected to the optical coupler such that the optical signal can flow through the waveguide in only one pass in each direction.  
      Yet another aspect of the present invention provides a method for manufacturing an optical loop mirror. The method has steps of forming an optical coupler on a semiconductor substrate and forming an optical waveguide on the semiconductor substrate. The optical coupler has at least one nominal input port and at least a first nominal output port and a second nominal output port. The optical waveguide has a first end and a second end, wherein the first end is optically coupled to the first nominal output port and the second end is optically coupled to the second nominal output port. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:  
       FIG. 1  is a schematic illustration of a nonlinear optical fiber loop mirror of the prior art;  
       FIG. 2  is a schematic illustration of an optical waveguide ring resonator of the prior art;  
       FIG. 3  is a schematic illustration of a 1×2 optical loop mirror according to an embodiment of the present invention;  
       FIG. 4  is a schematic illustration of a 2×2 optical loop mirror according to an embodiment of the present invention;  
       FIG. 5  is a schematic illustration of a 2×3 optical loop mirror according to an embodiment of the present invention;  
       FIG. 6  is a schematic illustration of a 1×2 optical loop mirror with a filtered loopback according to an embodiment of the present invention;  
       FIG. 7  is a schematic illustration of a 1×2 optical loop mirror with a transmission tap according to an embodiment of the present invention;  
       FIG. 8  is a schematic illustration of a 2×2 optical loop mirror with a transmission tap according to an embodiment of the present invention;  
       FIG. 9  is a schematic illustration of a pseudo two-port circulator using a 2×2 optical loop mirror according to an embodiment of the present invention;  
       FIG. 10  is a schematic illustration of a distributed feedback laser with controlled facet phase according to an embodiment of the present invention;  
       FIG. 11  is a schematic illustration of a dual-pass semiconductor optical amplifier according to an embodiment of the present invention;  
       FIG. 12  is a schematic illustration of multi-parallel semiconductor optical amplifier (SOA) according to an embodiment of the present invention;  
       FIG. 13  illustrates a physical representation of the 1×2 optical loop mirror of  FIG. 3 ; and  
       FIG. 14  illustrates a physical representation of the 1×2 optical loop mirror of  FIG. 3 , implemented using a whispering gallery type waveguide loop. 
    
    
      It will be noted that, throughout the appended drawings, like features are identified by like reference numerals.  
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      The present invention provides an integrated optical loop mirror which can be constructed on a semiconductor, glass or lithium niobate (LiNbO 3 ) substrate. It provides great improvement over prior art integrated mirror solutions such as cleaved facets, Bragg gratings, etched mirrors and Mach-Zehnder interferometers. Optical loop mirrors can be placed on a wafer to within photo-lithographic accuracy (currently, about 0.1 microns). This is particularly advantageous for the terminating facet phase of a distributed feedback (DFB) laser, for example. The optical loop mirror also offers better performance than current etched mirrors.  
      The placement of an optical loop mirror on a semiconductor wafer is not restricted to the edge of the wafer as is the case with cleaved facet mirrors. This allows for much higher integration of optoelectronic elements or optical systems on a single wafer. The magnitude and phase of the reflection can be accurately controlled over wavelength (including making it wavelength independent), either by design or dynamically, which offers more flexibility than a Bragg reflector.  
      The present invention is thus well suited as a building block for constructing highly integrated optical circuits and especially monolithical photonic integrated circuits.  
       FIG. 3  illustrates a first embodiment  300  of the optical loop mirror of the present invention. The optical loop mirror is monolithically formed on a semiconductor substrate. An input signal waveguide  303  connects to a 1×2 optical coupler  302 , formed on a semiconductor substrate, via a nominal input port  304  and two nominal output ports  306 ,  308 . An optical waveguide  310 , formed on the same substrate, is connected to the optical coupler  302  such that a first end of the optical waveguide  310  is connected to output port  306  and a second end of the optical waveguide  310  is connected to output port  308 , such that optical signals exiting output port  306  are looped back into output  308 .  
      In use, an optical signal  312  enters input port  304 , is split by the optical coupler  302  and half the signal  312   a  exits port  306  and the other half of the signal  312   b  exits port  308 . Each half signal,  312   a ,  312   b  loop back into the other output port  308  and  306  respectively. The optical coupler  302  recombines the signals into signal  312   c  which exits the optical coupler  302  by nominal input port  304 .  
      For all practical purposes, optical waveguide  310  has wavelength independent transmission characteristics and since the split optical signals symmetrically counter-propagate in the waveguide loop, the optical loop mirror  300  behaves as an optical mirror, reflecting the signal  312  back from port  304 . Thus an integrated high performance mirror can be provided which is easy to manufacture using current technologies and can be located anywhere on a semiconductor wafer.  
      In a preferred embodiment, the optical coupler  302  is a 3-dB multimode interference (MMI) coupler. Other split ratios and other types of couplers such as directional couplers or Y-junctions, can be used as well.  
      A recent technique for waveguiding, reported extensively in the technical literature, uses materials having photonic band gaps, otherwise known as photonic crystals. Photonic crystals are well suited for constructing optical loop mirrors of the present invention, and can yield especially compact implementations. There is also a renewed interest in Y-junction type couplers when constructed using photonic crystals.  
       FIG. 4  illustrates a 2×2 optical loop mirror embodiment  400  of the present invention. In this variation, coupler  402  is a 2×2 multimode interference (MMI) optical coupler having a first nominal input  404  and a second nominal input port  406  and nominal output ports  408 ,  410 . The MMI coupler introduces a differential phase shift between the output ports  408 ,  410  such that optical signals passing from port  404  to port  410 , and signals passing from port  406  to  408 , encounter a 90° phase delay. Optical signals passing from port  404  to port  408 , and signals passing from port  406  to  410 , encounter no phase delay. In a useful situation, optical signal  414   a  enters port  406 , and optical signal  414   b  which is identical to optical signal  414   a  but with a 90° phase delay, enters input port  404 . In this situation, due to the phase interactions in the coupler  402 , optical signals  414   a  and  414   b  arrive at port  410 , 180° out of phase and effectively cancel each other out. Optical signals  414   a  and  414   b  are combined at port  408  and since signal  414   a  encounters 90° phase delay and optical signal  414   b  already has a 90° phase delay, the two signals combine to form optical signal  414  which propagates through waveguide  412  in a counter-clockwise direction only, from port  408  to port  410 . The optical coupler  402  splits signal  416  and part of the signal exits port  406  with no additional phase delay as signal  414   a ′ and the other part of the signal exits port  404  as signal  414   b ′ with an additional phase delay of 90°. Thus signal  414   a  is effectively reflected back from port  406  with a 90° phase shift and signal  414   b  is effectively reflected back from port  404  with a 90° phase shift. The unique ability of this embodiment to direct optical signals in one direction around the mirror loop can have many useful applications, especially where it is desired to perform specific processing of the optical signal before being “reflected” back. It would be impractical to design such a device using optical fiber because it is very difficult to define phase using optical fiber. By contrast, because the present invention can be manufactured a chip using photolithography, it is easy to achieve the manufacturing tolerances required to control phase easily.  
       FIG. 5  illustrates a third embodiment  500  of the optical loop mirror of the present invention. In this embodiment, a 2×3 optical coupler  502  has two nominal input ports  504 ,  506  and three nominal output ports  508 ,  510 ,  512 . Two of the output ports  508 ,  510  are connected by waveguide loop  514 . Its operation is similar to that of the previous embodiments with the added feature of having one output port  512 , free for transmission. Thus, a portion of the input signals can be reflected back to the input ports by the waveguide  514  and a portion can be transmitted out of output port  512 .  
      As can be seen from the embodiments described above, many variations of the basic loop mirror are possible. To generalize the above examples, the basic optical loop mirror of the present invention is a semiconductor substrate having formed on it, an M×N-port optical coupler having M input ports and N output points, wherein at least two of the output ports are looped back to each other via an optical waveguide loop, also formed on the same substrate, in order to provide a reflection function. Many variations of the design are possible, including multiple waveguide loops, waveguide loops with different lengths to generate reflections with different delays, using various types of couplers or couplers with different splitting ratios.  
      In other embodiments of the present invention, the waveguide loops can be interrupted by a number of different devices.  FIG. 6  illustrates a fourth embodiment  600  of the optical loop mirror of the present invention. In this embodiment, a first optical coupler  602  has one nominal input port  604  to accept optical signal  626  and two nominal output ports  606 ,  608  which are looped back to each other by waveguide loop  610 . Waveguide  610  contains a coupled ring resonator  612  which acts as a wavelength filter. Coupled ring resonator  612  is composed of a second optical coupler  614  and an optical waveguide loop  624 . The arrow through the symbol for optical coupler  614  in  FIG. 6 , denotes that the optical coupler  614  can be controlled either by design or dynamically in order to affect coupling to the resonator ring  624 . Optical coupler  614  has input/output ports  616 ,  618 ,  620  and  622 . Port  622  is looped back to port  618  by optical waveguide loop  624  to provide the resonance ring. Other types of filters can also be used in this configuration. A filter network could be used in place of coupled ring resonator  612 . Such a filter network could include auto-regressive elements, moving average elements as well as active elements to dynamically control the wavelength and Q of the filter.  
       FIG. 7  illustrates a fifth embodiment  700  of the optical loop mirror of the present invention. In this embodiment, the optical loop mirror  700  has a transmission tap in the loop. This not only allows partial transmission through the optical loop mirror but can also be used to control the effective reflectance of the optical loop mirror by allowing precise control over the percentage of light reflected back to the input. In this embodiment, a 1×2 optical coupler  702  has a nominal input port  704  to accept optical signal  734  and two nominal output ports  706 ,  708  which are looped back to each other by waveguide loop  710 . The waveguide loop  710  has a transmission tap  711  to tap off a portion of the optical signal flowing through waveguide  710 . Transmission tap  711  consists of a 2×2 controllable optical coupler  712  having input/output ports  714 ,  716 ,  718  and  720 ; waveguides  722 ,  724 ; and a 1×2 optical coupler  726  acting as a signal combiner. In operation, a portion of the optical signal entering port  718  continues through to port  714  and a portion is diverted to port  716 . The coupling ratio can be controlled by controllable coupler  712 . Likewise, a portion of the optical signal entering port  714  continues through to port  718  and a portion is diverted to port  720 . The diverted signal exiting port  716  flows through waveguide  724  to port  730  of coupler  726  where it combines with the diverted signal exiting port  720  and flowing through waveguide  722  to port  728 . The combined diverted signals then exit port  732 . Thus the signals diverted by controllable coupler are tapped off and made available at port  732 ; and the signals not diverted are reflected back out of input port  704 .  
       FIG. 8  illustrates a variation of the two input optical loop mirror of  FIG. 4  having a transmission tap  813  in the mirror loop  812 . The concept of tapping the optical signal is similar to that of the embodiment of  FIG. 7  in that a controllable optical coupler  814  is used divert a portion of the optical signal flowing through the mirror loop but since the optical signal only flows in one direction, only a 2×1 controllable optical coupler  814  is needed and only one waveguide  822  is needed to transport the diverted the signal  824   c  and a combiner is not required.  
      Thus in operation, in a useful situation, optical signal  824   a  enters port  806 , and optical signal  824   b  which is identical to optical signal  824   a  but with a 90° phase delay, enters input port  804 . In this situation, due to the phase interactions in the coupler  802 , optical signals  824   a  and  824   b  arrive at port  810 , 180° out of phase and effectively cancel each other out. Optical signals  824   a  and  824   b  are combined at port  808  and since signal  824   a  encounters 90° phase delay and optical signal  824   b  already has a 90° phase delay, the two signals combine to form optical signal  824  which propagates through waveguide  812  in a counter-clockwise direction only, from port  808 , through controllable coupler  814  to port  810 . A portion of the optical signal  824  entering port  816  of controllable coupler  814 , continues through port  818  to port  810  as signal  824   d  and a portion is diverted through port  820  to waveguide  822  as signal  824   c . The optical coupler  802  splits the returning signal  824   d  and part of the signal  824  exits port  806  with no additional phase delay as signal  824   a ′ and the other part of the signal exits port  804  as signal  824   b ′ with an additional phase delay of 90°. Thus signal  824   a  is effectively reflected back from port  806  with a 90° phase shift and signal  824   b  is effectively reflected back from port  804  with a 90° phase shift. Thus, the effective reflectance of the optical loop mirror  800  can be controlled by controlling the coupling ratio of controllable coupler  814 . Equivalently, the tapped optical power, which is the portion of the optical signal which is not reflected, can be controlled.  
      In many applications it is desirable to have an optical signal reflected back along the input path, as illustrated in the preceding embodiments. In some applications however, it is desirable to have an optical signal reflected back along a portion of the input path but then exit via a different path in the manner of an optical circulator.  FIG. 9  illustrates an integrated pseudo two-port optical circulator embodiment  900  of the present invention. This embodiment is based on the 2×2 optical loop mirror of  FIG. 4 , integrated with a second 2×2 multimode interference (MMI) optical coupler  902  and optical device  908  which performs identical operations to the phase and magnitude of the optical signals flowing through the two parallel waveguides  918 ,  920 . Thus, in operation, optical signal  930  enters MMI coupler  902  via port  910 . Optical coupler  902  splits the signal  930  into signals  930   a  and  930   b , signal  930   b  undergoing a 90° phase delay. Signal  930   a  exits port  914  and flows through waveguide  918  of device  908  and enters port  922  of MMI optical coupler  904 . Signal  930   b  exits port  916  and flows through waveguide  920  of device  908  and enters port  924  of MMI optical coupler  904 . (This embodiment illustrates one method of generating two signals having a 90° phase difference as used in the embodiment of  FIG. 4 .) Coupler  904  causes signals  930   a  and  930   b  to combine into signal  930   c  having a 90° phase delay by virtue of the phase interactions as described with reference to  FIG. 4 . Signal  930   c  flows in a counter-clockwise direction around waveguide  906  and back into port  926  where it is split into signal  930   d  having a 90° phase shift and signal  930   e  having a 180° phase shift. Signal  930   d  flows through waveguide  918  into port  914  of coupler  902  and signal  930   e  flows through waveguide  920  into port  916  of coupler  902 . Phase interactions within coupler  902  cancel the combined signals directed to port  910  and direct the combined signals out of port  912  as signal  930   f  having a phase delay of 180°. As long as device  908  performs identical operations to optical signals flowing through waveguides  918 ,  920 , the signal  930  will be reflected out of port  912 . This embodiment is well suited to implementing a dual-pass Mach-Zehnder interferometer.  
      Mach-Zehnder interferometers (MZI) are commonly fabricated using lithium niobate (LiNbO3) and this material is well suited for implementing MzIs according to the present invention. Lithium niobate provides good optical coupling to optical fiber and it can be easily patterned photo-lithographically. Lithium niobate also exhibits strong linear electro-optical effects which can be used to change the index of refraction dynamically and thus is well suited for building fast efficient modulators.  
       FIG. 10  illustrates a distributed feedback (DFB) laser with controlled facet phase in an embodiment of the present invention. The DFB laser  1000  has an active region  1002  bounded by optical loop mirrors  700   a ,  700   b  with transmission taps to provide controlled reflections for the active region. This design permits photo-lithographically terminated gratings, easy integration with a wavelength locker and/or a back facet monitor. This embodiment also permits the design of dynamically controllable front and back facet reflectivity by using optical couplers with controllable coupling ratios. The operation of the optical loop mirror with transmission tap is described with reference to  FIG. 7 .  
      With reference to  FIG. 11 , a dual-pass semiconductor optical amplifier  1100 , is illustrated. It is based on the configuration of the embodiment of  FIG. 4 , but with a section of active gain material  1104  in the optical waveguide loop  1106 .  
      With reference to  FIG. 12 , a multi-parallel semiconductor optical amplifier (SOA)  1200  is illustrated. It is based on the configuration of the integrated pseudo two-port optical circulator of  FIG. 9 , the operation of which has been previously described. Optical device  1202  comprises two semiconductor optical amplifiers (SOA)  1206  and  1208 , spanning waveguides  1214  and  1216  respectively, optical signals in the waveguides  1214 ,  1216  receive light amplification in both directions through SOAs  1206 ,  1208 , all the while, keeping the output signal separated from the input signal thereby obviating the need for an external circulator.  
      A conventional SOA has a carrier density profile which is symmetric about the center and slightly non-uniform along the length. The degree of non-uniformity is a result of forward and backward propagating amplified-spontaneous emission (ASE), and increases with applied current which generates this emission. When a signal is then coupled into the SOA, the non-uniformity increases, and shifts toward the output facet where the power is the strongest, and the depletion of carrier-density-dependent gain is the largest. By contrast, with the optical loop mirror multi-parallel SOA embodiment of the present invention, the signal is coupled into both facets simultaneously, thus maintaining the symmetry and reducing the degree of non-uniformity. Advantages of the optical loop mirror implementation of include, higher gain due to a more efficient use of carriers and a reduction in noise figure due to a more uniform carrier distribution. For an input power large enough to saturate the gain, the higher efficiency is a result of undepleted gain available to the signal at both facets. At the same time, the uniformity of the depletion is improved, as the split signals are both equally affected by the forward- and backward-propagating ASE. As the noise figure degrades with higher depletion, this may result in a lower noise figure.  
      In some respects, the optical loop mirror SOA principle is similar to the use of counter-propagating pumps in the design of erbium-doped fiber amplifiers (EDFAs) and has similar advantages of gain and noise figures over conventional optical amplifiers. The optical loop mirror SOA, however uses counter-propagating signals instead of counter-propagating pumps.  
       FIG. 13  illustrates a physical representation of the basic optical loop mirror of  FIG. 1 . An input signal waveguide  303  connects to a 1×2 optical coupler  302 , formed on a semiconductor substrate, which connects to optical waveguide  310 .  
       FIG. 14  illustrates a physical representation of the basic optical loop mirror of  FIG. 1 , implemented using a whispering gallery type looped waveguide  310 . An input signal waveguide  303  connects to a 1×2 optical coupler  302 , formed on a semiconductor substrate, which connects to whispering gallery optical waveguide  310 . The whispering gallery waveguide reflects the optical signal off of the outer curved surface to direct the signal.  
      The embodiment(s) of the invention described above is(are) intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.