Optical waveguides – Optical waveguide sensor
Reexamination Certificate
2001-02-26
2004-02-03
Lee, John D. (Department: 2874)
Optical waveguides
Optical waveguide sensor
C385S013000, C385S031000, C385S032000, C385S039000
Reexamination Certificate
active
06687424
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a sensing pad assembly for monitoring acoustic activity and/or motion of an object supported on the pad. The invention is more particularly concerned with a sensing pad assembly that utilizes an improved variable coupler fiberoptic sensor as a pressure transducing element. The sensing pad is suitable for use in a variety of monitoring applications and is especially useful in systems for continuously monitoring medical patients, or more generally, human subjects.
It is commonplace in medical practice to continuously monitor a patient's vital signs, such as heart rate and respiration rate, for changes that may indicate deterioration of the patient's condition. Continuous monitoring systems typically require the attachment of electrical and/or physical sensors to the patient's body using adhesive or straps. Such sensors are generally uncomfortable to the patient and often limit patient activity due to the presence of the sensors, straps, and associated sensor leads. Moreover, such monitoring systems are subject to false outputs due to unreliability of skin contact with patient movement.
An alternative form of monitoring system has been proposed in which the patient is supported on a sensing pad having an associated pressure transducer that does not contact the patient. Acoustic activity and motion of the patient generate pressure fluctuations in the material of the pad. These fluctuations, which vary in accordance with the parameter or parameters being monitored, propagate through the pad material to the transducer. The transducer then converts them to electrical signals for processing by a monitoring circuit.
U.S. Pat. No. 5,684,460 to Scanlon (hereinafter referred to as “the '460 patent”) discloses one example of a monitoring system as just described.
FIG. 1
illustrates this system in simplified block diagram form. The system includes a fluid-filled sensing pad
1
adapted to support a patient and to transmit pressure fluctuations due to the patient's acoustic activity or motion to a pressure transducer
2
that converts the pressure fluctuations to an electrical output. The pressure transducer is coupled to the internal fluid medium
3
of the pad via a hose
4
. A monitoring circuit
5
monitors the output from the transducer and provides outputs to activate a patient stimulator
6
and an alarm
7
upon the occurrence of predetermined conditions, such as when the transducer output corresponds to no sound and/or movement (below a predetermined threshold) or indicates abnormal activity of the patient.
The sensing pad
1
may be in the form of a fluid-filled mattress or configured for use in some other suitable support such as a vehicle seat or a stroller. Proposed applications of the system for monitoring human subjects include the monitoring of infants at risk for sudden infant death syndrome (SIDS), controlling sleep apnea and snoring, and sensing the onset of sleep for drivers of motor vehicles. Other proposed applications include monitoring machinery for noises and vibrations indicative of atypical operation.
The '460 patent mentions several classes of sensors as being suitable for use as the pressure transducer. Examples include electrical, mechanical, piezoelectric, and fiberoptic sensors.
One type of fiberoptic sensor not explicitly mentioned in the '460 patent, but known to have performance characteristics that are especially well suited for patient monitoring and a variety of other applications, is the variable coupler fiberoptic sensor.
Variable coupler fiberoptic sensors conventionally employ so-called biconical fused tapered couplers manufactured by a draw and fuse process in which a plurality of optical fibers are stretched (drawn) and fused together at high temperature. The plastic sheathing is first removed from each of the fibers to expose the portions for forming the fusion region. These portions are juxtaposed, usually intertwisted one to several twists, and then stretched while being maintained above their softening temperature in an electric furnace or the like. As the exposed portions of the fibers are stretched, they fuse together to form a narrowed waist region—the fusion region—that is capable of coupling light between the fibers. During the stretching process, light is injected into an input end of one of the fibers and monitored at the output ends of each of the fibers to determine the coupling ratio. The coupling ratio changes with the length of the waist region, and the fibers are stretched until the desired coupling ratio is achieved, typically by a stretching amount at which the respective fiber light outputs are equal. The coupler is drawn to such an extent that, in the waist region, the core of each fiber is effectively lost and the cladding may reach a diameter near that of the former core. The cladding becomes a new “core,” and the evanescent field of the propagating light is forced outside this new core, where it envelops both fibers simultaneously and produces the energy exchange between the fibers. A detailed description and analysis of the biconical fused tapered coupler has been given by J. Bures et al. in an article entitled “Analyse d'un coupleur Bidirectional a Fibres Optiques Monomodes Fusionnes”, Applied Optics (Journal of the Optical Society of America), Vol. 22, No. 12, Jun. 15, 1983, pp. 1918-1922.
Biconical fused tapered couplers have the advantageous property that the output ratio can be changed by bending the fusion region. Because the output ratio changes in accordance with the amount of bending, sensors employing such couplers can be used in virtually any sensing application involving motion that can be coupled to the fusion region. For example, U.S. Pat. No. 5,074,309 to Gerdt discloses the use of such sensors for monitoring cardiovascular sounds including both audible and sub-audible sounds from the heart, pulse, and circulatory system of a patient. Other applications of variable coupler fiberoptic sensors can be found in U.S. Pat. No. 4,634,858 to Gerdt et al. (disclosing application to accelerometers), U.S. Pat. No. 5,671,191 to Gerdt (disclosing application to hydrophones), and elsewhere in the art.
Conventional variable coupler fiberoptic sensors have relied upon designs in which the fiberoptic coupler is pulled straight, secured under tension to a plastic support member and, in the resulting pre-tensioned linear (straight) form, encapsulated in an elastomeric material such as silicone rubber. The encapsulant forms a sensing membrane that can be deflected by external forces to cause bending of the coupler in the fusion region. The bending of the fusion region results in measurable changes in the output ratio of the coupler. The displacement of the membrane can be made sensitive to as little as one micron of movement with a range of several millimeters.
FIG. 2
of the accompanying drawings illustrates the basic principles of a sensing apparatus including a variable coupler fiberoptic sensor
10
as described above. In the form shown, the sensor
10
includes a 2×2 biconical fused tapered coupler
11
produced by drawing and fusing two optical fibers to form the waist or fusion region
13
. Portions of the original fibers merging into one end of the fusion region become input fibers
12
of the sensor, whereas portions of the original fibers emerging from the opposite end of the fusion region become output fibers
14
of the sensor. Reference numbers
18
denote the optical fiber cores. The fusion region
13
is encapsulated in an elastomeric medium
15
, which constitutes the sensing membrane. The support member is not shown in FIG.
1
.
In practice, one of the input fibers
12
is illuminated by a source of optical energy
16
, which may be an LED or a semiconductor laser, for example. The optical energy is divided by the coupler
11
and coupled to output fibers
14
in a ratio that changes in accordance with the amount of bending of the fusion region as a result of external force exerted on the sensing membrane. The
Adkins Charles
Baruch Martin C.
Gerdt David W.
Empirical Technologies Corporation
Kang Juliana K.
Lee John D.
Miles & Stockbridge P.C.
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