Optics: measuring and testing – By light interference – Using fiber or waveguide interferometer
Reexamination Certificate
1999-05-17
2002-09-10
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
Using fiber or waveguide interferometer
C385S012000
Reexamination Certificate
active
06449046
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a fiber optic sensor, and more particularly to a fiber optic interferometric sensor (FOIS).
2. Description of Related Art
An important feature of fiber optic interferometric sensors (FOIS) is its multi-plexing capability. Among different interferometric sensor multiplexing techniques, time-division multiplexing (TDM) has been shown to have low crosstalk and high sen-sitivity (U.S. Pat. No. 4,770,535). A polarization-induced signal fading effect severely occurs in a two-arm fiber optic interferometric sensor. A traditional way to overcome the polarization-induced signal fading effect is instead using polarization maintaining fibers to form sensors, but resulting in need of very expensive fibers and relevant components. It is also very difficult to configure the sensing arrays. It even more difficult to precisely align the axial direction of polarization maintaining fiber. As a result, the traditional way using polarization maintaining fiber in sensing arrays is not a practical solution. In order to reduce the polarization-induced signal fading effect, polarization-insensitive fiber optic Michelson interferometer was proposed (U.S. Pat. No. 5,206,924). This sensor includes a “Faraday rotator mirror (FRM)” that can eliminate the polarization fading effect by compensating it with a birefringence effect in a retraced fiber path. The reference paper reports a TDM polarization-insensitive fiber optic Michelson interferometric sensor (TDM-PIFOMIS) to overcome the polarization-induced signal fading effect by combining FRM with unbalanced Michelson interferometers and generating the interference signals by an optical path-matching compensating interferometer (CI). Details are also referred to U.S. patents application Ser. No. 08/806,671, “OPT. Lett., 20, pp.1244-1246, 1995”, “J. Lightwave Technol., 14, pp.1488-1500, 1996”, and “Appl. Phys. Lett., 39, pp. 530-532, 1981”. The TDM-PIFOMIS system with the optical path-matching CI can also significantly reduce the phase-induced intensity noise (PIIN).
The TDM-PIFOMIS system needs a suitable demodulation circuit to demodulate signals from various sensors. The demodulation circuit usually includes phase-generated carrier (PGC) demodulation or passive symmetric demodulation using 3×3 fiber coupler (3×3 demodulation). A carrier phase signal of the PGC demodulation can be easily generated by a PZT phase modulator of a compensating interferometer in the TDM-PIFOMIS system so as to achieve high sensitivity with a larger dynamic range. For the 3×3 demodulation, there is no need of carrier phase signal. This yields a significant advantage to increase the bandwidth of interferometric sensors.
For most fiber-optic sensor multiplexing schemes, the optical power budget limits both the lead fiber length, over which fiber-sensor can be driven remotely, and the number of sensors.
SUMMARY OF THE INVENTION
It is at least an objective of the present invention to provide a polarization-insensitive fiber-optic interferometric sensor system using an erbium-doped fiber amplifier (EDFA). A transmission distance is effectively improved.
It is at least another objective of the present invention to provide a polarization-insensitive fiber-optic interferometric sensor system using an erbium-doped fiber amplifier (EDFA) so that a sensing array can even tolerate an optical loss of about 47 dB or higher. As a result, the system can includes more sensors.
It is at least still another objective of the present invention to provide a polarization-insensitive fiber-optic interferometric sensor system using wavelength-division multiplexing technologies so as to include a larger number of sensors by employing only one input and one output lead optical fibers. Fabrication cost and power consuming rate are effectively reduced. The structure of the sensor system is also simplified so as to have more useful applications in various environments to be detected.
In accordance with the foregoing and other objectives of the present invention, a polarization-insensitive fiber-optic interferometric sensor system is provided. The sensor system includes an optical pulse generator, a post erbium-doped fiber amplifier (EDFA), a first optical bandpass filter (OBPF), a 3-port optical circulator (3POC), a sensing array, an in-line EDFA, a second OBPF, and a receiver. The pulse generator is used to generate a low-repetition-rate optical pulse. The post EDFA is used to receive the optical pulse and amplify the optical pulse for an output. The first OBPF is used to receive and filter the optical pulse that is amplified by the post EDFA. The 3POC, having a first port, a second port, and a third port, is used to receive an output from the first OBPF at the first port. The sensor array coupled to the second port of the 3POC so as to also receive the out from the first OBPF through the 3POC and return an output to the 3POC. The in-line EFDA coupled to the third port of the 3POC so as to receive and amplify the output optical pulse of the array. A second OBPF is used to receive and filter the output from the in-line EDFA. The receiver receives the output from the second OBPF, in which the receiver preferably includes a time-division multiplexing 3×3 (TDM-3×3) receiver or a TDM phase-generated carrier (TDM-PGC) receiver.
In accordance with the foregoing and other objectives of the present invention, an another polarization-insensitive fiber-optic interferometric sensor system is provided. The sensor system includes several optical pulse generator, a first dense wavelength-division multiplexer (DWDM), several post EDFAs, a second DWDM, several first OBPF, several 3POCs, several sensing arrays, a third DWDM, several in-line EDFAs, a fourth DWDM, several second OBPFs, and several receivers. Each of the receivers includes a TDM-3×3 receiver or a TDM-PGC receiver. Each of the optical pulse generators generates an optical pulse with specific wavelength. The first DWDM receives each output optical pulse of the optical pulse generators and exports a first combined optical pulse. The post EFDAs are coupled in series, in which a first one of the post EFDAs receives the first combined optical pulse and the last one of the post EFDAs exports the first combined optical pulse that is amplified by the post EFDAs. The second DWDM receives the amplified first combined optical pulse and exports several first amplified optical pulses with respect to each optical pulse with specific wavelength. Each of the 3POCs includes a first port, a second port, and a third port, in which each the first port respectively receives one of the first amplified optical pulses. Each of the sensing arrays respectively coupled to one of the 3POCs at the second port and returns an output to the coupled one of the 3POC. Each the third port of the 3POCs is coupled to the third DWDM so that the third DWDM receives each output of the sensing arrays through the 3POCs and exports a second combined optical pulse. The in-line EDFAs are coupled in series, in which the first one of the in-line EFDAs receives the second combined optical pulse and the last one of the in-line EFDAs exports the second combined optical pulse that is amplified by the in-line EFDAs. The fourth DWDM receives the amplified second combined optical pulse and exports several optical pulses with differently specific wavelengths. The second OBPFs respectively receive the second amplified optical pulses. The receivers respectively receive output signals from the second OBPFs, in which each of the receivers includes a TDM-3×3 receiver or a TDM-PGC receiver.
REFERENCES:
patent: 5946429 (1999-08-01), Huang et al.
patent: 6211964 (2001-04-01), Luscombe et al.
Chao Hung-Lung
Huang Shih-Chu
Hung Shoren-Chien
Lin Wuu-Wen
Chung-Shan Institute of Science and Technology
Font Frank G.
J.C. Patents
Natividad Phil
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