Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2000-06-29
2002-04-02
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S227190
Reexamination Certificate
active
06365891
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to arrays of fiber optic interferometric sensors and mechanisms for maximizing the signal to noise ratio in amplified sensor arrays that are time domain multiplexed.
BACKGROUND OF THE INVENTION
Arrays of fiber optic interferometric sensors show promise in applications where size, electrical interference, and electromagnetic detection make electronic sensors impractical. Such interferometric sensors are capable of measuring a parameter (i.e., a measurand) with a very high dynamic range (e.g., 120 dB). Optical sensor arrays are formed by connecting a series of sensors using fiber optic lines. If each sensor in an array requires a dedicated fiber to carry the detection signal, the large number of fibers required quickly becomes unwieldy as the number of sensors increases. Thus, as the number of sensors in an optical array increases, time domain multiplexing (TDM) becomes necessary to maintain a low fiber count. Electrical and optical frequency domain multiplexing have been attempted, but they are unmanageable for arrays comprising hundreds of sensors. As a result, large sensor arrays are organized into long strings of sensors which perform TDM by returning information from sensors placed at discrete intervals. A typical passive sensor array using TDM is constructed in a ladder type configuration. This design has only a few fiber lines and permits a small deployment size. It is desirable to provide a multiplexing scheme which includes a large number of interferometric sensors in an array while preserving the high dynamic range of the sensors and maintains a high signal to noise ratio (SNR).
As shown in
FIG. 1
, a conventional passive optical array
10
using TDM is formed by using a splitter coupler
140
to couple a distribution bus
100
to a first end of an optical sensor
110
. A second splitter coupler
142
couples a return bus
120
to a second end of the optical sensor
110
. A detection signal is sent from a source (not shown) which is then partially coupled into the first sensor
110
in an array of n sensors. The remainder of the detection signal continues along the distribution bus to subsequent couplers, each coupling a fraction of the detection signal into successive sensors.
Each sensor modifies the optical signal coupled into it from the distribution bus
100
based on external (e.g., acoustic) perturbations to be detected. The perturbed signal is then coupled onto the return bus
120
by coupler
142
. The return bus then transmits the perturbed signals out of the array for processing.
The basic principle of TDM is as follows. The length of the path that the optical signal takes from the source, along the distribution bus
100
, through the coupler
140
, the sensor
110
, the coupler
142
and back along the return bus
120
is different for each sensor. Therefore, the return signals arrive at the detector at different time intervals depending on the path length. Sensors closer to the signal source have a shorter path than sensors near the end of the array. Thus, sensors near the source place the return signals on the return bus slightly earlier than sensors farther down the array. This assumes that the time delay through each of the sensors is relatively equal. The signals are then transmitted outside the array to be sequentially processed by other hardware to extract the sensed information. Because each of the return signals has different time delay based upon differing distances between the sensor and the source, it is possible to use optical signals in a pulsed form. Based on the foregoing, each sensor
110
returns a signal pulse which is slightly delayed from the signal pulse returned by the previous sensor, and therefore enables the various signal pulses to be temporally separated at the detector. To avoid overlap of the returned signals on the return bus
120
and at the detector, the pulse length and frequency of the optical signals are selected so that the return signals do not overlap on the return bus.
FIG. 8
illustrates a timing diagram for a sensor array employing TDM to multiplex the return signals onto the return bus for detection and processing. In time period
1
, the signal source outputs a detection pulse of length &tgr;. The signal source then waits a period of T
System
before resetting itself and repeating the detection pulse (shown as time period
1
′). Once the detection pulse has been issued from the signal source, it is split into each sensor. The signal from each sensor returns at a different time depending on each sensor's respective distance from the signal source. The path lengths are chosen carefully so that the return signals are placed on the return bus at successive intervals with only a short intervening guard band (T
Guardband
) between the return signals to prevent signal overlap. Once the last sensor has returned a signal N to the detector, the system waits a reset period (T
Reset
) and then restarts the process. The period T
Reset
is selected to assure that the return pulse N from the last sensor arrives at the detector before the return pulse
1
′ from the first sensor arrives in response to the second detection pulse. An exemplary period for T
Reset
is approximately equal to T
Guardband
. Thus, the repetition period for T
System
is approximately N×(&tgr;+T
Guardband
). For example, for a system having a path difference of approximately 8.2 meters between adjacent sensors, &tgr; is selected to be approximately 40 nanoseconds and T
Guardband
is selected to be approximately 1 nanosecond. When the array is configured to include 300 sensors (i.e., N=300), then T
System
is approximately 12.3 microseconds. For this exemplary configuration, a repetition rate of approximately 80 kHz assures that the last return signal in response to a detection pulse does not overlap with the first return signal in response to the next detection pulse. Note that in
FIG. 8
the time offset between the detection pulse and the first return pulse is not shown because the offset varies in accordance with the optical path length from the source to the first sensor, through the first sensor and back to the detector.
The advantage of TDM is that it allows simple interrogation techniques. No switching hardware is necessary, allowing a reduction in the cost and the size of the array. However, one of the problems with TDM is that it reduces the time each sensor is available for detection. If each sensor were given a dedicated fiber to report the result of its detections, it could provide a continuous stream of information. However, when TDM is implemented to reduce the number of fibers, no such continuous reporting is possible. The amount of time any one sensor is sampled is reduced to 1/N of a continuously sampled sensor. As the number of sensors grows, the amount of time and the frequency that any one sensor is sampled is further reduced.
The limited sampling time increases the significance of the signal to noise ratio (SNR). Since under TDM, a short sample is extrapolated to represent a much longer period (N times longer than its actual sample time), it is much more essential that each sample be interpreted correctly by the detector. Noise is a significant source of interpretation errors and therefore the SNR must be kept as high as possible with as little degradation of the SNR along the sensor array as possible. A high SNR reduces the number of interpretation errors by the detection system.
The detection signal experiences a significant loss as it propagates through the passive array. The sources of loss include, for example, (1) fiber loss, splice losses, and coupler insertion loss, (2) sensor loss, and (3) power splitting at each coupler on the distribution and return busses.
Simple splitting (loss item (3)), which is the method used to couple the optical sensor to the distribution and return buses, results in large losses and a severe degradation in the SNR. The amount of light in the detection signal coupled from the distribution bus into the sensor depe
Digonnet Michel J. F.
Hodgson Craig W.
Shaw H. John
Wagener Jefferson L.
Board of Trustees of the Leland Stanford Junior University
Knobbe Martens Olson & Bear LLP
Le Que T.
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