Abstract:
A polarization-insensitive fiber-optic interferometric sensor system has an erbium-doped fiber amplifier (EDFA) at a location between a light source and a sensing array to serve as a post amplifier. An output end of the sensing array is coupled to another EDFA, and further coupled to a receiver, in which this another EDFA is used as an in-line amplifier. As the sensor system is applied for a light source with multiple wavelengths, a dense wavelength division multiplexers (DWDMs) is used to combine the multiple wavelengths into one for amplification, and another DWDM is used to decouple the multiple wavelengths.

Description:
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. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention can be more fully understood by reading the following detailed description of the preferred embodiment, with reference made to the accompanying drawings as follows: 
     FIG. 1 is a structure diagram, schematically a structure of a TDM-PIFOMIS system using a PGC demodulation, according to the first preferred embodiment of the invention; 
     FIG. 2 is a structure diagram, schematically an experimental arrangement for the EDFA used as a post amplifier, according to the first preferred embodiment of the invention; 
     FIG. 3 is a structure diagram, schematically an experimental arrangement for the EDFA used as in-line amplifier, according to the first preferred embodiment of the invention; 
     FIG. 4 is a waveform drawing, schematically illustrating typical several types of pulse train, according to the first preferred embodiment of the invention; 
     FIG.  5 is a frequency spectrum, schematically illustrating an output spectrum of the demodulated signal at 200 Hz when the ER of the optical pulse is 33 dB without EDFA in the TDM-PIFOMIS system, according to the first preferred embodiment of the invention; 
     FIG.  6 A and FIG. 6B schematically illustrate the output peak power (at point B of FIGS. 2 and 3) and the optical. gain versus the peak power of the input pulse (at point A of FIGS. 2 and 3) for the EDFA as a post and an in-line amplifier, respectively, according to the first preferred embodiment of the invention; 
     FIG.  7 A and FIG. 7B schematically illustrate the ER and the relative phase noise (RPN) versus the peak power the input pulse for the EDFA as a post and an in-line amplifier, respectively, according to the first preferred embodiment of the invention; 
     FIG. 8 is a structure diagram, schematically illustrating a structure of an optically amplified WDM/TDM hybrid PIFOMIS system using the PGC demodulation as a receiver, according to the second preferred embodiment of the invention; 
     FIG. 9 is a structure diagram, schematically illustrating a structure of a TDM-PIFOMIS system using 3×3 demodulation, according to the first preferred embodiment of the invention; and 
     FIG. 10 struture diagram, schematically illustrating a structure of an optically amplified WDM/TDM hybrid PIFOMIS system using the 3×3 demodulation, according to the second preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     EXAMPLE 1 
     FIG. 1 is a structure diagram, schematically a structure of a TDM-PIFOMIS system using a PGC demodulation, according the a first preferred embodiment of the invention. In FIG. 1, a TDM polarization-insensitive fiber optic Michelson interferometric sensor (TDM-PIFOMIS) system using a PGC demodulation includes four main parts: an optical pulse generator  100 , a lead fiber subsystem  120 , a sensing array  140 , and a TDM receiver  160 . The optical pulse generator generates a low-repetition-rate optical pulse with narrow pulse width and high ER. The optical pulse generator  100  further includes an optical-guided-wave (OGW) intensity modulator  104  so as to modulate a continuous lightwave from a laser source  102 . The OGW  104  usually includes a high impedance (&gt;1000 Ω) DC bias electrode and a 50 Ω impedance radio frequency (RF) electrode. The OGW  104  is driven by a pulse generator  106  and a precision DC voltage supply  108  respectively through the RF electrode and the DC bias electrode. An input electrical pulse amplitude generated by the pulse generator  106  is, for example, kept at the half-wave voltage V π  of the OGW  104  so as to obtain the maximum output optical pulse amplitude. The bias voltage V BIAS  generated by the precision DC voltage supply  108  is precisely adjusted to generate the output optical pulse with high ER. The sensing array  140  include N unbalanced sensing interferometer (SI) sets (SI 1 , SI 2 , . . . , SI N ). Each of the SI sets includes one fiber coupler (FC) and two Faraday rotator mirrors (FRMs) forming a pair. Two optical fiber paths of the two FRMs in each pair are different with a difference of ΔL in length. The lead fiber subsystem  120  includes an input lead fiber  122  and an output lead fiber  124 , a 3-port optical circulator (3POC), a post EDFA  126 , and an in-line EDFA  128 . In the specification, EDFA is a preferable amplifier and can be replaced by other type of amplifier, such as an optical amplifier. The TDM receiver  160  includes a compensating interferometer CI, which includes a FRM C  and a FRM C′ , an optical receiver  162 , a time delay generator  164 , a sample/hold circuit  168 , a PGC demodulator  170 , and a carrier signal generator  172 . The TDM receiver  160  includes, for example, an InGaAs PIN optical receiver used to convert interfered pulse trains into electrical signals. The compensating interferometer CI is coupled to the optical receiver  162  and an optical bandpass filter (OBPF)  132  through a 2×2 fiber coupler. The time delay generator  164  is coupled to the pulse generator  106 . The carrier signal generator  172  is coupled between the PGC demodulator  170  and a PZT phase modulator of the compensating interferometer CI. 
     In the foregoing, all the unbalanced SI sets have equal light path difference ΔL to the CI of the TDM receiver  160 . Generally, each sensor unit of the sensing array  140  preferably has an equal output power. Power splitting ratios of all fiber couplers (FC 1 , FC 2 , . . . , FC N ) are 1:1 in order to obtain the maximum visibility of the output intensity of each sensor unit. The power splitting ratios X k  of several fiber couplers (C N , C N−1 , . . . , C 2 ), which are separately coupled to the SI sets, are different in order to obtain equaled output power of each sensor. The X k  (here, 2≦k≦N) has been derived and calculated in a paper published in J. Lightwave Technol.,14,pp.1488-1500, 1996. The TDM-PIFOMIS system with a property of equal optical path difference ΔL between the CI and all sensors can significantly reduce fundamental phase-induced intensity noise (PIIN). 
     In the TDM-PIFOMIS system, there are three locations can be equipped with EDFA. One serving as the post EDFA  126  is coupled between the OGW  104  and the sensing array  140 . One serving as the in-line EDFA  128  is coupled between the sensing array  140  and the CI of the TDM receiver  160 . One serving as a pre-EDFA is coupled between the CI and a photodetector receiver  162  of the TDM receiver  160 , in which the pre-EDFA located before the photodetector receiver  162  is not useful owing to the unstable amplification for the optical pulse trains with interference signals of the sensing array  140 . For example, the unstable optical gain of the pre-EDFA is induced by the fluctuated optical pulse trains. For the in-line EDFA  128 , since the amplitudes of the optical pulse trains propagating in the output lead fiber  124  of the TDM-PIFOMIS system are stable, the amplitudes of the optical pulse trains are stable after being amplified by the in-line EDFA  128 . A preferred coupling architecture includes only the post EDFA  126  and the in-line EDFA  128  even though the pre-EDFA maybe also included. 
     An experiment is performed to investigate how the sensing capability is improved by the post EDFA  126  and the in-line EDFA  128 , both of which are introduced in the invention. FIG. 2 is a structure diagram, schematically an experimental arrangement for the EDFA used as a post amplifier, according to the first preferred embodiment of the invention. FIG. 3 is a structure diagram, schematically an experimental arrangement for the EDFA used as an in-line amplifier, according to the first preferred embodiment of the invention. In FIG.  2  and FIG. 3, for more convenient descriptions, only one SI 1 , set in the sensing array  140  is shown. A structure with multiple SI sets is described later. Moreover, in the experiment, the sensing array  140  and the CI of the TDM receiver  160  (FIG. 1) are put in a vibration-isolated, acoustic-shield box so as to prevent any ambient perturbations so that more precise experimental data can be obtained. 
     In FIG. 2, there are two 1:99 (1%) monitoring couplers  134   a ,  134   b . The monitoring coupler  134   a  is located before the post EDFA  126  and the monitoring coupler  134   b  is located after an optical bandpass filter (OBPF)  130 . Both the monitoring couplers  134   a ,  134   b  are respectively used to tap off an input signal power and an amplified output signal power of the post EDFA  126 . The OBPF  130  is used to filter out an amplified spontaneous emission (ASE) noise from the post EDFA  126  so as to maintain high ER and to reduce system phase noise level. The OBPF  130  has bandwidth of 1 nm at 0.5 dB and an insertion loss of about 1.5 dB. A variable optical attenuator (VOA 1 ) coupled between the OGW  104  and the monitoring coupler  134   a  is used to control the input signal power level of the post EDFA  126 . A variable optical attenuator (VOA 2 ) coupled after the monitoring coupler  134   b  is used to control the amplified output signal power level into the sensor SI 1 . 
     In FIG. 3, the in-line EDFA  128  is included. Similarly, there are two 1:99 (1%) monitoring couplers  134   c ,  134   d . The monitoring coupler  134   c  is located before the in-line EDFA  128  and the monitoring coupler  134   d  is located after the optical bandpass filter (OBPF)  132 . Both the monitoring couplers  134   c ,  134   d  are respectively used to tap off an input signal power and an amplified output power of the post EDFA  128 . The OBPF  132  is used to filter out an amplified spontaneous emission (ASE) noise from the in-line EDFA  128  so as to maintain high ER and to reduce system phase noise level. The sensor SI 1    140  is coupled after the OGW  104 . The VOA 1  coupled between the sensor SI 1    140  and the monitoring coupler  134   c  is used to control the input signal power level of the in-line EDFA  128 . The VOA 2  coupled after the monitoring coupler  134   d  is used to control the launched input signal power into the CI. 
     A test signal at 200 Hz is generated by a signal generator for test in the experiment. The test signal is applied upon one of the sensor&#39;s fiber arm through a PZT phase modulator to provide an effective phase signal of 3.4×10 −2  rad/(Hz) ½ . A carrier signal with 20 kHz, generated by other signal generator, is applied upon one of the Cl&#39;s fiber arm through another PZT phase modulator to provide an optimum phase of 2.37 rad for the PGC demodulator. The interference pulse trains are detected by the receiver and the sensing signal is demodulated by the PGC demodulator. In FIG. 4, the upper signal train is an output waveform from the OGW  104 , the middle signal train is an output waveform from the SI 1    140 , and the lowest signal train is an output waveform from the Cl&#39;s fiber arm. After demodulation of the PGC demodulator  170 , a test signal with 200 Hz is shown in FIG.  5 . FIG. 5 is a frequency spectrum, schematically illustrating an output spectrum of the demodulated signal at 200 Hz when the ER of the optical pulse is 33 dB without EDFA in the TDM-PIFOMIS system, according to the first preferred embodiment of the invention. 
     In FIG.  2  and FIG. 3, the input signal power of the post EDFA  126  or the in-line EDFA  128  is adjusted by adjusting the VOA 1 . The amplified output signal power of the post EDFA  126  or the in-line EDFA  128  is adjusted by adjusting the VOA 2 . A received optical peak power of about −14 dBm is therefore obtained so as to keep a peak-to-peak voltage of the output interference signal of the sample/hold circuit  160  to be about 10 Volts in all measurements. The waveforms of the input signal pulse and the amplified output signal pulse of the post EDFA  126  are respectively detected by a monitoring receiver  136   a  and a monitoring receiver  136   b  in FIG. 2, in which the waveforms are schematically shown in FIG. 4 at the top train. FIG. 4 is a waveform drawing, schematically illustrating typical several types of pulse train, according to the first preferred embodiment of the invention. Similarly, the waveforms of the input signal pulse and the amplified output signal pulse of the in-line EDFA  128  are respectively detected by a monitoring receiver  136   c  and a monitoring receiver  136   d  in FIG. 3, in which the waveforms are schematically shown in FIG.4 at the middle train. 
     FIG.  6 A and FIG. 6B schematically illustrate the output peak power (at point B of FIGS. 2 and 3) and the optical gain versus the peak power of the input pulse (at point A of FIGS. 2 and 3) for the EDFA as a post and an in-line amplifier, respectively, according to the first preferred embodiment of the invention. In FIGS. 6A and 6B, an altemative current (AC) gain, such as the optical gain for the EDFAs  126 ,  128  with modulated pulses, is higher than a DC gain, such as the optical gain for the EDFAs  126 ,  128  with un-modulated continuous-wave(CW) light). This is because the low-duty-cycle optical pulse is less saturating than the CW light. For example, for the input light power level at −5 dBm, the AC gain and output power are 27 dB and 22 dBm, respectively, as the DC gain and the output are 17.3 dB and 12.8 dBm, respectively. 
     FIG.  7 A and FIG. 7B schematically illustrate ER and relative phase noise (RPN) versus the peak power of the input pulse for the EDFA as a post and an in-line amplifier, respectively, according to the first preferred embodiment of the invention. In FIGS. 7A and 7B, the larger the input light power, the higher the ER for both post EDFA  126  and in-line EDFA  128 . A minimum phase detection sensitivity (MPDS) can be calculated from the RPN quantity, in which MPDS=3.4×10 −2 ×10 PRN/20  rad/(Hz) ½ . 
     The MPDS of an un-amplified TDM-PIFOMIS system with ER of 33 dB is about 2.4×10 −5  rad/(Hz) ½  at about 1 kHz, as shown in FIG.  5 . In order to assure the optically amplified TDM-PIFOMIS system to be operated with low phase noise, it is necessary that the RPN level is maintained to be less than 3 dB, which is the quantity for the un-amplified system. Thus, the allowable maximum RPN level is about −60 dB for the TDM-PIFOMIS system. This means that the MPDS preferably is better than 3.4×10 −5  rad/(Hz) ½  at about 1 kHz. To satisfy these requirements, the worst ERs for the post EDFA  126  and the in-line EDFA  128  respectively are, for example, about 20 dB and 17.8 dB, as shown in FIGS. 7A and 7B. Thus, the minimum required peak power level of the input optical pulse for the post and the in-line amplifier cases respectively are −20 dBm and −25 dBm, as shown in FIGS. 6A and 6B. The corresponding output peak power levels are 9.5 dBm and 6.9 dBm, respectively. In this condition, the optical gain for the post EDFA  126  and the in-line EDFA  128  respectively are 29.5 dB and 31.9 dB. In this experiment, the maximum launched peak power levels of the input optical signal pulses into the post EDFA  126  and the in-line EDFA  128  respectively are −5.0 dBm and −8.6 dBm. 
     In the invention, experimental results shows the feasibility of using an EDFA in the PGC-demodulated TDM-PIFOMIS system as a post and an in-line amplifier. The EDFA, particularly for the in-line EDFA  128  coupled after the output lead fiber  124  shown in FIG. 1, does not degrade the interference optical phase sensing signal from the sensing array  140 . The amplitudes of the optical pulse trains, for example, in FIG. 4 are very stable after being amplified by the in-line EDFA  128 . This is one significant advantage of the optically amplified TDM-PIFOMIS system. 
     Moreover, the improved power budget can effectively increase the number of sensors and transmission distance for a field application. In the TDM-PIFOMIS system, the number of sensors included in the invention is determined by operating conditions of the post EDFA  126  and the in-line EDFA  128 . In FIG. 1, the EDFAs  126 ,  128  are respectively coupled to a port  1  and a port  3  of the 3POC, which is located in front of the sensing array  140 . The 3POC has a property to forwardly pass signals to the next port so that when the optical signal pulses returns from the sensing array  140  is forwardly passed to the in-line EDFA  128  without affecting the post EDFA  126 . According to the current operating conditions in the TDM-PIFOMIS system, for example, the minimum required peak power level of the input signal pulse for the in-line EDFA  128  is −25 dBm. In order to obtain a desired high output power of about 22 dBm, the peak power of the input signal pulse for the post EDFA  126  is necessary to have −5dBm. As a result, the sensing array of the system can tolerate a power loss budget of about 47 dB. 
     The FRM, 3POC, and 3 dB-FC respectively have single-pass insertion loss of 1 dB, 1 dB, and 3 dB. An excess loss of the 3 dB-FC is 0.2 dB. The upper limit of the allowable number of the sensing arrays for the system with post and in-line EDFAs is  32 . Further considering the settling time of the high-resolution sample-and-hold circuit, it is a challenging achievement to demodulate the pulse trains of a 32-sensors array. So, utilization of sensor subarrays can solve this problem. For example, a 16-sensor sub-array system may have loss of about 36 dB. As a result, the 47-dB loss budget can be used for a typical four-sub-array (about 42.6 dB) TDM-PIFOMIS system with 16 sensors per sub-array. Hence, a 1×4 fiber coupler (FC) with equal splitting ratio and four separate sets of 3POC, in-line EDFA  128 , output lead fiber  124 , and TDM optical receiver are allowable for such a four-sub-array system. In this situation, the system complexity and cost may increase. In order to further improve this TDM-PIFOMIS system with multiple-sub-array-system, a second preferred embodiment is proposed. 
     EXAMPLE 2 
     An optically amplified WDM/TDM hybrid PIFOMIS system using the PGC demodulation shown in FIG. 8 is proposed to replace the TDM-PIFOMIS system with a sub-array system. FIG. 8 is a structure diagram, schematically illustrating a structure of an optically amplified WDM/TDM hybrid PIFOMIS system using the PGC demodulation as a receiver, according to the second preferred embodiment of the invention. 
     In FIG. 8, an optically amplified WDM/TDM hybrid PIFOMIS system includes, for example, several optical pulse generators  300  to respectively generate several optical signal pulses, such as m optical signal pulses with respect to wavelengths λ 1 , . . . , and λ m . The optical signal pulses are multiplexed by an m-channel DWDM  302  (1×m DWDM, with ≧0.5 nm passband), which combines all these various optical signal pulses to form a first combined optical pulse. The first combined optical pulse is exported to a first one, EDFA(l) of several post EDFAs  304 . The post EDFAs  304  are coupled in series through an input lead fiber  122  in between (not shown in FIG. 8 but referred to FIG.  1 ). The first combined optical pulse propagates through the post EDFAs  304  for amplification and reach the last one EDFA(n), in which n depends on a practical application but at least one. An output of the post EDFA(N) is demultiplexed by a high isolation 1×m DWDM  306  to output several amplified signal pulses with respect to wavelengths λ 1 , . . . , λ m . The m outputs of the 1×m DWDM  306  are used as input optical signal pulses for m subarrays  308  through several optical bandpass filters (OBPFs)  307  with respect to wavelengths λ 1 , . . . , λ m  to filter out ASE noise from the post EDFAs and 3POCs (3POC 1 , . . . , 3POC m ). Each OBPF  307  is respectively coupled between the DWDM  306  and each 3POC. Each of the m subarrays  308  includes, for example, N sensors. The m subarrays  308  respectively return several optical signals to the 3POCs and are multiplexed by a high isolation 1×m DWDM  310 . Each of the 3POCs has property to forwardly pass signals to the next port so that these returned optical signals are not passed back into the 1×m DWDM  306 . The coupling architecture is, for example, that the outputs of the DWDM  306  are respectively coupled to the first ports of the 3POCs, the sensing arrays  308  are respectively coupled to the second ports of the 3POCs, and the DWDM  310  is coupled to all the third ports of the 3POCs. The DWDM  310  combines the returned optical signals into a second combined optical pulse that is exported to several in-line EDFRAs  312  from the first one of EDFA( 1 )  312  to the last one of EDFA(n)  312  in series. The output lead fiber  124  of FIG. 1 (not shown in FIG. 8) are coupled in between the in-line EDFAs  312  to form a series coupling. The second combined optical pulse is amplified by the in-line EDFAs  312  and is exported to a 1×m DWDM  314 , which demultiplexes the secondly combined optical pulse into m second amplified optical signal pulses with respect to the wavelengths λ 1 , . . . , λ m . The m second amplified optical signal pulses are respectively filtered by m units of OBPFs  316  so as to reduce crosstalk effects between the m processed optical signal pulses and to filter out ASE noise from the in-line EDFAs. Outputting ends of the OBPFs  316  are respectively coupled into m CIs of FIG. 1 (not shown in FIG. 8) that are used to generate interference signals, and then are respectively demodulated by m TDM receivers  318 . 
     For an example with m=4, this optically amplified WDM/TDM hybrid PIFOMIS system can reduce three in-line EDFAs  128  and three output lead fibers  124 . The optically amplified WDM/TDM hybrid PIFOMIS system is effectively simplified, giving several advantages at least including greatly simplified fabrication process and reduced electrical power consumption for field application. 
     In underwater application, lead fiber length of the WDM/TDM hybrid PIFOMIS system possibly need several hundreds of km. A transmitted power of a long single-mode fiber usually is limited by stimulated Brillouin scattering (SBS) effect. For a 1.55 μm continuous wave laser source, its linewidth is necessary to be less than 16 MHz and propagation length of the single-mode fiber is larger than 22 km, and a threshold power is estimated to be about 3 dBm. In the optically amplified WDM/TDM hybrid PIFOMIS system, a duty cycle of the optical pulse propagated in the input lead fiber  122  is only about 1(2N+1) for each wavelength. For a typical value of N=16, the threshold power of the SBS is about 18 dBm for each wavelength. This is an upper limit of the output power of the post EDFA( 1 ) to be able to efficiently propagate in the input lead fiber  122 . Additionally, the minimum required peak power level of the input optical signal pulse for each post EDFA  126  is about −20 dBm for each wavelength so as to assure the optically amplified WDM/TDM hybrid PIFOMIS system to be operated with low phase noise. Therefore, the allowable transmitted loss between two neighboring post EDFAs  126  can be even as high as about 38 dB if the output power of the post EDFA is enough. The duty cycle of the optical pulse propagated in the output lead fiber is about 1, the threshold power is about 3 dBm for each wavelength. This is an upper limit output power of the in-line EDFA ( 1 ) to be able to efficiently propagate in the output lead fiber  124 . Additionally, the minimum required peak power level of the input optical pulse for each in-line EDFA  128  is about −25 dBm for each wavelength to assure the optically amplified WDM/TDM hybrid PIFOMIS system to be operated with low phase noise. As a result, the allowable transmitted loss between two neighboring in-line amplifiers can be even as high as 28 dB. Multiple post EDFAs  126  and multiple in-line EDFAs  128  respectively used with the input lead fiber  122  and the output lead fiber  124  can increase the transmitted distance. 
     Moreover, the TDM receiver  160  in FIG. 1 or the TDM receivers  318  in FIG. 8 can include a TDM-PGC receiver or a TDM-3×3 receiver. The TDM-3×3 receiver needs no carrier phase signal, and resulting in a further improvement to increase the sensor bandwidth. 
     FIG. 9 is a structure diagram, schematically illustrating a structure of a TDM-PIFOMIS system using 3×3 demodulation, according to the first preferred embodiment of the invention. FIG. 9 is similar to FIG. 1 with a difference of the TDM receiver  160 , which is a TDM-PGC receiver  160  in FIG.  1  and is a TDM-3×3 receiver  160  in FIG.  9 . The receiver type does not affect the characteristics of the invention. If the TDM-3×3 receiver is used as shown in FIG. 9, the structure of the TDM receiver  160  includes another 3POC( 2 ). The TDM-3×3 receiver  160  further includes, for example, a 3×3 fiber coupler, the time delay generator  164 , the sample/hold circuit  168 , several 3×3 demodulators  170  with a proper coupling, and a compensating interferometer CI, which includes a FRM C  and a FRM C′ as shown in FIG.  9 . One arm of the 3×3 fiber coupler is coupled to a second port of the 3POC( 2 ). The first port of the 3POC( 2 ) is coupled to the OBPF  132 . A third port of the 3POC( 2 ) is coupled to an optical receiver( 3 ). An optical receiver( 2 ) and an optical receiver(l) are respectively coupled to these two output arms of the 3×3 fiber coupler. 
     FIG. 10 is a structure diagram, schematically illustrating a structure of an optically amplified WDM /TDM hybrid PIFOMIS system using the 3×3 demodulation, according to the second preferred embodiment of the invention. Again, FIG. 10 is similar to FIG. 8 with a difference between the TDM-PGC receivers  318  and the TDM-3×3 receivers  320 . 
     In conclusion, the invention at least includes several characteristics. By using EDFAs and DWDMs, the optical signal pulses are not degraded in long transmitted distance. This allows the invention to use only one line of the input and the output lead fibers. The number of amplifiers in various uses is greatly reduced, resulting in low fabrication cost, low power consumption, and wide applications. Moreover, due to the use of the post EDFA  126  and the in-line EDFA  128 , the sensor system can tolerate a high power loss as high as 47 dB so that the number of the sensing arrays  308  in the WDM/TDM hybrid PIFOMIS system can be effectively increased, resulting a great increase of detecting ability. Furthermore, the invention uses low cost commercial available single mode optical fiber to reduce fabrication cost of the system. The sensor system is particularly more suitable for a long distance transmission. Moreover, since the amplitude of the optical pulse signal transmitted in the lead fiber is very stable, the optical pulse signal can be amplified by EDFAs with a stable amplification. 
     The invention has been described using exemplary preferred embodiments. However, it is to be understood that the scope of the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements. The scope of the claims, therefore, should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.