Wavelength multiplexed optical communications system and method

An optical fiber communications system includes a closed loop birefringent optical fiber bus for supporting optical energy propagated in a first and a second orthogonal polarization state. An optical energy source introduces a fixed frequency system wide carrier onto the optical fiber bus in a first polarization state for propagation in a first direction. Terminal devices coupled to the optical fiber bus include a bus tap resonator for removing a portion of the system wide carrier light and, in addition to modulating the removed light, effecting a frequency shift to a side frequency and effecting a state shift from the first to the other polarization state, and returning the information bearing optical energy to the optical fiber bus for propagation in a direction opposite to the first direction. A receiving device includes a bus tap resonator for removing a portion of the light from the bus and effecting demodulation to recover the information bearing signal.

BACKGROUND OF THE INVENTION 
The present invention relates to communication systems. More particularly, 
it concerns optical communications systems in which a plurality of 
terminal devices communicate with one another over a common optical bus. 
In many types of communications networks, it is common for the terminal 
devices to introduce information bearing signals onto a network bus by 
modulating a locally available carrier source. In the electrical domain, 
stable and highly accurate carrier sources are available that can be tuned 
from one frequency to another to allow frequency division multiplexed 
networks. In optical systems, however, the traditional carrier sources, 
viz., light emitting diodes and laser diodes, have inherent limitations 
which mitigate against high density frequency division multiplexed 
networks. As is known, laser diodes are difficult to tune from one 
wavelength to another and to wavelength stabilize, in part, because of 
their temperature sensitivity, and light emitting diodes are generally 
considered low power devices having a relatively wide spectral output. 
While frequency stabilization schemes are known, the many variables 
involved make the tuning of the local light source from one frequency to 
another difficult. While relatively stable light sources, such as gas 
lasers, are available, their comparatively high cost, physical size, power 
supply requirements, and the attendant difficulty of modulating their 
output makes them unattractive for use in each terminal device in the 
network. 
As can be appreciated, any optical communications system which provides 
high density frequency division multiplexing without using a local carrier 
source in each terminal device possesses significant cost and performance 
advantages over prior systems. 
SUMMARY OF THE INVENTION 
An optical fiber communications system in accordance with the present 
invention includes an optical fiber pathway defined by an optical fiber 
bus which supports the propagation of light in first and second 
characteristic states. A central system wide carrier source injects light 
in one of the two characteristic states into the optical fiber bus at a 
master frequency for propagation throughout the system. A plurality of 
terminal devices are coupled to the optical fiber bus for communication 
with one another by removing a portion of the light provided by the 
central carrier source, effecting a shift to the other characteristic 
state, and modulating the light for return to the optical fiber bus as 
state converted information bearing light for propagation to other 
terminal devices. Receiving devices remove a portion of the information 
bearing light propagated in the other characteristic state and effect 
demodulation to recover the original signal content. 
In the preferred embodiment, the optical pathway is defined by a 
birefringent optical fiber bus formed into a closed loop. The system wide 
carrier source injects light into the optical fiber bus in a first 
polarization mode or state for propagation in a first direction about the 
closed loop. The terminal devices remove a portion of the light propagated 
in the first polarization state and effect a shift to the other 
polarization state and modulate the light for return to the optical bus 
for propagation in a direction opposite to the direction of the light 
propagated in the first polarization state. Additionally, the light 
returned to the system bus is desirably frequency shifted from the 
frequency of the light provided by the system wide carrier source. 
The system wide carrier source includes a frequency stable source of light, 
such as a gas laser, which introduces light into a resonant cavity 
structure that preferentially supports wavelengths of the system wide 
master frequency in the polarization state selected for the system wide 
carrier. A frequency controller coupled to the resonant cavity structure 
functions to provide a frequency deviation error to an amplifier for 
driving a thermal energy generator which introduces thermal energy into 
the resonant cavity structure to control its resonant characteristics to 
lock the resonant cavity to the frequency of the system wide carrier to 
maximize energy transfer into the system bus. 
Terminal devices which include a transmitter remove a portion of the system 
wide carrier light, effect a frequency shift to an adjacent side frequency 
of the master frequency, effect a shift to the orthogonal polarization 
state, and modulate the light for return as a modulated, frequency shifted 
and state switched signal into the optical bus for propagation in a 
direction opposite to the direction of the system wide carrier. Each 
transmitter includes a resonant cavity structure which removes a portion 
of the system wide carrier from the optical bus and passes the removed 
light through an electro-optical modulator for modulation and return to 
the optical bus as a frequency shifted, direction reversed, and state 
switched modulated signal. The electro-optical modulator can be defined by 
an oriented electro-optical crystal excited by an electrical signal which 
is effective to modulate the light passed through the crystal. The 
resonant cavity structure is locked to the master frequency by a control 
loop which introduces thermal energy into the resonant cavity structure to 
alter its resonant characteristics and tunes the resonant cavity to the 
master frequency of the system wide carrier. 
In terminal devices which include a receive function, a resonant cavity 
structure removes a portion of the light propagated in the orthogonal 
state from the optical fiber bus and presents the removed signal to a 
demodulator for recovery of the information content. Receiving devices, as 
in the case of the transmitting devices, are locked to the frequency of 
the system wide carrier by a control loop which introduces thermal energy 
into the resonant cavity structure to alter its resonant characteristics. 
The resonant cavity structures used in the devices which are coupled to the 
optical fiber bus can take the form of integrated tap resonators defined 
by an anisotropic material, such as lithium niobate, in which light 
guiding channels are created by appropriate doping with a titanium or 
equivalent dopant. 
An optical fiber communication system in accordance with the present 
invention provides for a system in which a highly accurate frequency 
source provides a system wide carrier sufficient to accommodate the needs 
of the entire communication system to thereby dispense with a requirement 
for local frequency sources at each terminal device. 
A principal objective of the present invention is, therefore, the provision 
of an improved optical fiber communications system by which various 
terminal devices within the system may readily communicate with one 
another. Other objects and further scope of applicability of the present 
invention will become apparent from the detailed description to follow, 
taken in conjunction with the accompanying drawings, in which like parts 
are designated by like reference characters.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
An optical fiber communications system in accordance with the present 
invention is illustrated in schematic form in FIG. 1 and designated 
generally therein by the reference character 10. As shown, the system 10 
is preferably configured as a closed loop or ring defined by a 
birefringent single mode optical fiber bus 12, which is designed to 
propagate optical energy in two mutually exclusive and orthogonal 
polarization states, designated herein as the TE and TM states. Other 
suitable system configurations include the star system in which all 
communication channels return to a common node for retransmission to an 
intended destination. Single mode optical fibers having birefringent 
characteristics can be obtained by using nonsymmetrical core geometries 
and by core stressing arrangements in which stresses of different 
quantitative magnitudes are provided along orthogonal major and minor axes 
to induce orthogonal propagation velocity differences. Such stressed core 
fibers can be obtained, for example, through the use of noncircular core 
configurations with optical materials having different coefficients of 
thermal expansion. The system 10 includes a system wide carrier source 14 
and a plurality of terminal devices TD.sub.0, TD.sub.1, . . . , 
TD.sub.n-1, TD.sub.n which are coupled to the optical fiber bus 12 for 
communication with one another. The power output of the system wide 
carrier source 14 is such that it will accommodate the power requirements 
of all terminal devices TD.sub.n to be connected to the bus 12 as well as 
fiber attenuation and coupling losses. The system wide carrier source 14, 
as described more fully below, is effective to generate a stable source 
light at a preferred master frequency in one of the two polarization 
states or modes, i.e., the TE state in the context of the preferred 
embodiment, and inject or otherwise introduce the TE state light into the 
optical bus 12 for propagation in one direction, for example, the 
counterclockwise direction as shown in FIG. 1. Each of the terminal 
devices TD.sub.n is designed to remove a portion of the system wide TE 
state carrier light and modulate, state shift, and frequency shift the 
light for return to the bus 12 as TM state light travelling in the 
opposite direction, that is, the clockwise direction in the case of FIG. 
1. The information bearing TM state light is supported by the birefringent 
optical bus and propagated in the clockwise direction with each terminal 
device TD.sub.n connected to the bus sensing the TM state information 
bearing light and responding when appropriately addressed. The addressed 
terminal device TD.sub.n removes a portion of the TM state information 
bearing light and demodulates the light to recover the information content 
for utilization. 
An exemplary system wide carrier source 14 in accordance with the present 
invention is illustrated in FIG. 2 and includes a light source in the form 
of a frequency and amplitude stabilized laser 20, a bus tap resonator 22, 
and a frequency controller 24 including a detector 26, a drive amplifier 
28, and a thermal energy source 30. 
The laser source 20 has an output which defines the master frequency for 
the system 10 and a power output sufficiently high to support all terminal 
devices TD.sub.n to be coupled into the system 10. The laser source 20 can 
take the form of a helium-neon laser operating characteristically at 
1152.3 nm. As can be appreciated, other gas or gas mixture lasers or 
suitably stabilized semiconductor lasers operating at a preferred 
frequency are suitable, in general, wavelengths of between 0.8 and 1.5 
microns being preferred. The output of the laser source 20 is butt coupled 
into a transfer link 32 which presents the optical energy to both the bus 
tap resonator 22 and the detector 26. 
The birefringent bus tap resonator 22 includes a resonant cavity loop 34 
which is laterally coupled to the optical bus 12 through a lateral 
coupling 36 and also laterally coupled to the transfer link 32 through 
another lateral coupling 38. As indicated by the directional arrows 
associated with each lateral couple, 36 and 38, light energy introduced 
into the transfer link 32 from the laser source 20 is laterally 
transferred into the resonant cavity loop 34 through the lateral coupling 
38 and from the resonant cavity loop 34 through the lateral coupling 36 
into the optical bus 12 for propagation in the direction indicated, that 
is, to the left in FIG. 2 or counterclockwise in the context of the system 
illustrated in FIG. 1. The lateral couplings 36 and 38 are of the type 
that preserve the polarization state of the transferred light, that is, 
the TE state light in the transfer link 32 will excite the corresponding 
TE state in the resonant cavity loop 34. The bus tap resonator 22 
functions, in part, to buffer or decouple the output of the laser source 
20 so changes in optical loading on the optical bus 12 will not be 
reflected back to or otherwise presented to the laser source 20 to 
adversely affect the frequency of the laser source 20. If desired, 
additional isolation in the form of a magnetic isolator or a functional 
equivalent can be provided at the output of the laser source 20. The 
resonant cavity loop 34 has an effective optical length which will 
resonantly support those wavelengths that are an integral number of its 
effective optical length, including the master frequency provided by the 
laser source 20. 
The light introduced into the transfer link 32 by the laser source 20 is 
also provided to the detector 26. When the resonant characteristics of the 
bus tap resonator 22 are coincident with the frequency of the laser source 
20, the maximum amount of light will be transferred through the lateral 
coupling 38 into the resonant cavity loop 34, leaving a minimum of light 
in the transfer link 32. Conversely, when the resonant characteristics of 
the bus tap resonator 22 are not coincident with the frequency of the 
laser source 20, less energy will be transferred through the lateral 
coupling 38 to the resonant cavity loop 34, leaving more energy in the 
transfer link 32. The detector 26 measures the light exiting the transfer 
link 32 and provides a frequency deviation output, graphically illustrated 
in FIG. 2a, in which the minima represents an optimally tuned situation 
with maximum energy transfer from the transfer link 32 into the resonant 
cavity loop 34 and the adjacent inclining slopes and maximas represents 
out of tune conditions. The frequency deviation error signal is provided 
to the drive amplifier 28 which, in turn, provides its output to the 
temperature controller 30 which is in a heat conducting relationship, as 
symbolically represented by the arrow Q, with the bus tap resonator 22. 
Depending upon the magnitude of the frequency deviation error signal, 
greater or lesser quantities of thermal energy are introduced into the bus 
tap resonator 22 to alter the resonant characteristics of the resonant 
cavity loop 34 by changing its physical characteristics. As can be 
appreciated, the operation of the detector 26, the amplifier 28, and the 
temperature controller 30 define a control loop that functions to cause 
the bus tap resonator 22 to lock to and preferentially support light 
energy at the preselected system wide carrier frequency. In the preferred 
embodiment, the control loop functions as a D.C. feedback loop to lock a 
resonant mode of the bus tap resonator 22 to the side of the system wide 
carrier frequency with the offset being a function of the loop gain. While 
a D.C. feedback loop is preferred, it is possible to use A.C. feedback 
techniques for dithering the frequency of the system wide carrier about 
its average value as described in applicant's commonly assigned U.S. 
patent application Ser. No. 783,436, filed Nov. 3, 1985, and entitled 
OPTICAL COMMUNICATION SYSTEM EMPLOYING FREQUENCY REFERENCE. The response 
time of the frequency controller 24 should be commensurate with the heat 
transfer rates to provide accurate and stable control. The system wide 
carrier source 14 thus provides a stabilized, fixed frequency, system wide 
carrier for utilization by the various terminal devices TD.sub.n within 
the system 10. 
Each of the terminal devices TD.sub.n preferably includes both transmitting 
and receiving functions although single function transmitting and 
receiving terminal devices can be utilized. An exemplary architecture for 
a transmitting terminal device TD.sub.n is illustrated in FIG. 3 and is 
designated generally therein by the reference character 40. The 
transmitter 40 includes a bus tap resonator structure 42, a resonant 
frequency controller, generally designated as 44, and an electro-optic 
modulator 46 which functions to frequency shift, modulate and state shift 
a portion of the TE state light removed from the optical bus 12. 
The bus tap resonator 42 includes a resonant cavity loop 48 having an 
effective optical length that supports optical energy at various 
frequencies or modes including frequencies which are adjacent to and 
displaced relative to the system wide carrier master frequency. The 
resonant cavity loop 48 is laterally coupled to the optical bus 12 through 
a lateral coupling 50 with a portion of the TE state light circulated in 
the bus 12 transferred through the lateral coupling 50 into the resonant 
cavity loop 48. The removed light is resonantly supported and laterally 
transferred through a lateral coupling 55 into a transfer link 52. A 
portion of the light from the transfer link 52 is presented through a lens 
54a to a beam splitter 54 which passes a portion of the TE state light 
into the electrooptic modulator 46, which functions as described below, 
and reflects another portion into the frequency controller 44. The TE 
state light presented to the frequency controller 44 is passed through a 
TE state discriminator 56 which discriminates against any TM state 
communication components that may be present. The desired TE state light 
is presented to a detector 58 which produces an output that increases in 
proportion to the difference between the frequency of the sensed TE state 
optical energy and the resonant characteristics of the bus tap resonator 
42 in a manner analagous to that described above. The output of the 
detector 58 is presented to a drive amplifier 60 which drives a thermal 
controller 62 to generate and transfer thermal energy to the bus tap 
resonator 42 to alter the physical characteristics of the bus tap 
resonator and the effective optical length of the resonant cavity loop 48. 
The frequency controller 44 is thus effective to control the resonant 
characteristics of the bus tap resonator 42 and lock the bus tap resonator 
to the system wide carrier frequency at one of its many resonant 
frequencies or modes. 
The TE state light is also passed from the transfer link 52 through the 
lens 54a and the beam splitter 54, a one quarter wave plate 66, and 
through the electro-optic modulator 46 to a mirror 68 or a functionally 
equivalent device which returns the light through the electro-optic 
modulator 46, the quarter wave plate 66, the beam splitter 54, and the 
lens 54a into the transfer link 52. As described more fully below in 
relation to FIG. 6, the electro-optic modulator 46, in addition to 
modulating the light energy, also effects a shift to the orthogonal TM 
state and a frequency shift to a communications frequency. 
The information to be introduced into the optical bus 12 as modulated TM 
state light is initially ported through a signal input 68a into the 
electro-optic modulator 46, which is effective to modulate, state shift, 
and frequency shift the TE state energy removed from the optical bus 12 
and return the energy as modulated TM state and frequency shifted light 
into the transfer link 52, through the lateral coupling 55 the resonant 
cavity loop 48, and the lateral coupling 50 into the optical bus 12 for 
propagation in a direction opposite to the direction of the system wide 
carrier, that is, to the left in FIG. 3 and clockwise in the context of 
FIG. 1. 
A receiver in accordance with the present invention is illustrated in 
schematic form in FIG. 4 and designated generally therein by the reference 
character 70. The receiver includes a bus tap resonator 72, a frequency 
controller 74, and a demodulator 76. The bus tap resonator 72 includes a 
resonant cavity loop 78 that has an effective optical length that 
resonates with the TM state information bearing optical energy as well as 
the TE state system wide carrier frequency. A portion of the information 
bearing optical energy is transferred from the optical communications bus 
12 through a lateral coupling 80 into the resonant cavity loop 78 and 
transferred through the lateral coupling 82 into a transfer link 84 which 
includes an output port 86, on the left in FIG. 4, for directing system 
wide source TE state light through a lens 86a into the frequency 
controller 74 and, on the right, an output port 88 for directing the 
freqency shifted, modulated TM state light through a lens 88a into the 
demodulator 76. 
The frequency controller 74 includes a TE state discriminator 90 that 
passes only TE state light while suppressing or substantially attenuating 
TM state optical energy. The TE state optical energy is received by a 
detector 92 which provides an output to a drive amplifier 94 that drives a 
thermal controller 96 for introducing thermal energy into the bus tap 
resonator 72 to affect a change in its physical characteristics to 
correspondingly alter its frequency characteristics in a manner analogous 
to that described above for the bus tap resonators of the system wide 
carrier 14 and the transmitter 40. 
The light presented through the output port 88 and the lens 88a is passed 
through a TM state discriminator 98 with the resulting information bearing 
TM state light being presented to a demodulator 100 which functions to 
demodulate the information bearing TM state light and present the 
recovered signal to an amplifier 102 and an output port 104. 
In the above described embodiment of the system wide carrier source 14, the 
transmitter 40, and the receiver 70, a resonant structure in the form of a 
bus tap resonator preferentially supports light propagated in the 
orthogonal TM and TE states. A suitable birefringent resonant structure is 
illustrated in FIG. 5 and is designated therein by the reference character 
110. The resonant cavity structure 110 is preferably fabricated from a 
lithium niobate substrate (LiNbO.sub.3), which material is optically 
anisotropic, having an ordinary ray index of 2.2 and an extraordinary ray 
index of 2.29, to provide a birefringent velocity difference of 
approximately 4%. As shown in FIG. 5, spaced linear optical guideways or 
channels 114 and 116 are created in the substrate 112 along with a 
continuous resonant cavity loop 118. The light guiding channels 114 and 
116 and the loop 118 may be created by diffusing titanium into the lithium 
niobate substrate. The relationship of the light guiding channels 114 and 
116 to the loop 118 is such that birefringent lateral coupling is 
effected. The initial crystalline orientation of the lithium niobate 
substrate is selected so that both the ordinary and extraordinary rays are 
supported within the light guiding channels to correspondingly support 
both the TE and TM state light. One of the linear channels, for example, 
the channel 114, is connected to or otherwise coupled to the optical bus 
12 while the other of the channels, that is, the channel 116, is used for 
the functions described above in relation to the operation of the system 
wide carrier source 14, the transmitter 40, and the receiver 70. 
The electro-optic modulator 46 suitable for use in the transmitter 70 of 
FIG. 4 is shown in greater detail in FIG. 6 and is designated generally by 
the reference character 130. As shown therein, the electro-optic modulator 
130 is defined by an electro-optical crystal 132, for example, lithium 
niobate, having electrodes 134 and 135 on opposite side faces. Light is 
introduced into the electro-optic crystal 132 from the transfer link 52 
through a collimating lens 136, with the light passing through the 
electro-optic crystal 132 and a quarter wave plate 136a to the mirror 68. 
The direction of the light is reversed by the mirror 68 for return through 
the quarter wave plate 136a, electro-optical crystal 132, and the lens 136 
to the transfer link 52. The electrodes 134 and 135 are coupled to the 
output of a high frequency multiplier 138 which, in turn, accepts a radio 
frequency carrier (e.g., up to 10 GHz) from a radio frequency carrier 
source 140 and the information bearing signal through the input 68a. In 
operation, the information bearing signal, which can take the form of 
digital pulses, is impressed upon the radio frequency carrier via the 
multiplier and presented across the electro-optic crystal 132 with the 
E-field variations exercising the electro-optic crystal 132 in the normal 
manner such that the two characteristic states of the light energy passing 
through the electro-optical crystal 132 are modulating and state shifting 
the energy from the TE to the TM state, and frequency shifting the energy 
upon its return to the transfer link 52. Additional disclosure relating to 
TE/TM state conversion and frequency shifting may be found in Johnson, L., 
Becker, R. et al "Integrated-Optical Channel-Waveguide Frequency Shifter" 
Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, 
Mass., and Risk, W. P., Youngquist, G. S., et al "Acousto-optic 
Birefringent Fiber Frequency Shifters" Edward L. Ginzton Laboratory, 
Stanford Univ., Calif., the disclosures of which are incorporated herein 
by reference to the extent necessary to practice the present invention. 
An integrated optic device 150, as shown in FIG. 7, may be used for TE/TM 
state conversion and the frequency shifting function. The integrated optic 
device 150 includes a substrate 152, preferably formed from LiNbO.sub.3, 
having a light guiding channel 154 formed therein with electrodes 156 and 
158 located on adjacent sides of the channel 154. A quarter wave plate 160 
or functionally equivalent device is interposed between the end coupled 
transfer link 162 and the channel 154 and a mirror 160a is located at one 
end of the channel 154 to return light introduced into the channel 154 
from the end coupled transfer link 162. A detector 164 is positioned 
adjacent the end of the substrate opposite the transfer link 162 and 
operates in a manner analagous to the detectors described above to provide 
a frequency deviation error signal to a drive amplifier 166 to drive a 
temperature controller (not shown in FIG. 7). A modulation signal is 
provided from a modulated RF source 168 through appropriate conductors to 
the electrodes 156 and 158. The alignment of the electro-optical crystal 
152 is such to support the orthogonal TE and TM modes to allow 
electrically controllable conversion from the TE to the TM modes. 
In the modulator embodiments described above in relationship to FIGS. 6 and 
7, TE state light supplied by the system wide carrier source 14 is removed 
from the bus tap resonator for modulation, state conversion, and frequency 
shifting prior to return to the bus tap resonator. These functions can be 
achieved in the bus tap resonator by fabricating the bus tap resonator 
from a high frequency electrooptic material and incorporating a frequency 
shift modulator directly into the resonant cavity loop as well as 
inserting a TE/TM state converter into the loop. 
In the embodiments described above, the birefringent bus tap resonators are 
tuned by altering a physical characteristic of the structure that defines 
the resonant cavity to accordingly change the spacing between the orders 
of the TE and TM states resonantly supported within the cavity, and, 
accordingly, the TE state communications frequencies. For example, where 
TE and TM state light is introduced into a birefringent resonator, one of 
the states, e.g., the TE state, will have a velocity greater than the 
other, viz., the TM state. In both cases the resonant cavity loop will 
resonantly support optical energy of both the TE and TM states at 
wavelengths that are an integral number of wavelengths of the effective 
optical length of the cavity, although the spacing between the supported 
TE and TM orders will be different because of velocity differences caused 
by the ordinary and extraordinary indices. Accordingly, unique TE state 
frequencies will be available for the communications functions with only 
minimal cross state modulation effects. For example and as shown in FIG. 
8a, TE and TM state light introduced into a birefringent resonator at a 
common frequency F.sub.x will be resonantly supported, respectively, at 
differing spectral spacings as a function of the difference between the 
ordinary and extraordinary index, as represented by the staggered TE and 
TM lines. As can be appreciated, the TE and TM state or mode lines will 
periodically coincide, e.g., at frequencies F.sub.y and F.sub.z. 
When the characteristics of the resonant structures are altered by 
temperature changes induced by the above-described frequency controllers 
to cause locking to the system master frequency, the spectral line spacing 
changes, as shown in FIG. 8b, with TE and TM state coincidence occurring 
at frequencies F.sub.y' and F.sub.z'. 
While the system has been shown in the context of a ring configuration, as 
can be appreciated, other system structures are possible. For example, all 
communications channels can be returned to a central point or star for 
distribution or retransmission to an intended terminal device. If desired, 
the power output of the central carrier source 14 can be made adaptive, 
that is, the bus tap resonators of those terminal devices TD.sub.n that 
are not active can be intentionally detuned to reduce TE state loading on 
the optical fiber bus 12. 
The present invention provides a communications system in which a single 
source provides a highly stable fixed frequency system wide carrier for 
use by all terminal devices coupled to the system to dispense with the 
need for local frequency sources at each terminal device. 
Thus, it will be appreciated from the above that as a result of the present 
invention a highly effective optical communications system is provided by 
which terminal devices within the system can readily communicate with one 
another. It will be equally apparent and is contemplated that modification 
and/or changes may be made in the illustrated embodiment without departure 
from the invention. Accordingly, it is expressly intended that the 
foregoing description and accompanying drawings are illustrative of the 
preferred embodiment only, not limiting, and that the true spirit and 
scope of the present invention will be determined by reference to the 
appended claims.