Digital closed-loop fiber optical current sensor

Abstract

This present inversion provides a digital closed-loop fiber optical current sensor. The modulation signal of the optical wave phase modulator of the fiber optical current sensor system is modulation square wave, signal processing system extracts any harmonic wave of the photoelectric converter output modulation square waves, and extracts the measured current information from it. The preamplifier of signal processing system is transimpedance amplifier TIA, the bandwidth is extracted 1/650 instantaneous amplitude square wave directly from the modulation square wave (existing), thus the thermal noise of the preamplifier output and shot noise level is reduced to the existing technology of below 1/650; the current-voltage gain of transimpedance amplifier TIA does not depend on the feedback network resistance, thus it can have high current-voltage conversion gain and use low resistance in the feedback network TIA at the same time. So it can reduce resistance thermal noise to negligible that is accounted for a large proportion of TIA output noise.

Claims

1. A digital closed-loop fiber optical current sensor, comprising: an optical path system configured to generate a modulation signal of the optical wave phase modulator of said fiber optical current sensor, wherein a signal processing system extracts only any harmonic wave of a photoelectric converter output modulation square; wherein the optical path system consists of: a broadband light source, an output light of a source passes through a polarizer and becomes a low coherence length of the linear polarized light; an integrated waveguide type optical phase modulator, its function is to adds positive, negative 90 bias phase to the existing linear polarized light, and the its period is set already, and a feedback phase opposite to existing light wave phase; at least one polarization-maintaining fiber delay line, and its role is to enable the linear polarized light produces required delay after passing through the delay line, to extract current information to be measured from an interferometer output signal; at least one wave plate, which makes two linearly polarized lights that their polarization directions are perpendicular to each other produce 90 phase difference; at least one single turn fiber ring or multi fiber ring surround the conductor that will transmit the measured current, its role is to enable the left, right circularly polarized light through the optical fiber ring in the measured current magnetic field generate phase difference .sub.C in accordance with Faraday effect; and then convert .sub.C into the difference .sub.L between two linearly polarization direction perpendicular polarized lights after the left, the right circularly polarized light back through the /4 fiber wave plate, and the .sub.L=.sub.C; a photoelectric converter, its effect is to convert the light phase by phase modulation type optical signal affected by the .sub.s .sub.b and .sub.f together to amplitude modulation square wave electric signal only affected by .sub.S, in which .sub.S is the signal phase generated by the current to measure, .sub.b is phase modulator generates a bias phase, and .sub.f is feedback phase; a trans impedance amplifier TIA, its effect is covert photo detector output low signal-to-noise ratio of the weak current signal to voltage signal that drives subsequent A/D conversion circuit of high signal-to-noise ratio; a digital feedback phase circuit and phase of light wave feedback loop for square wave amplitude detection, square wave amplitude accumulative, and feedback stepped ramp formation mainly, its role is to offset light wave phase changes induced by the measured current; a modulation gain control loop, which consist of modulation gain accumulation and modulation gain calibration, its role is to adjust the gain of said light wave phase feedback loop when feedback phase flyback error occurs, to ensure the feedback phase flyback error zero; an integrated waveguide type optical phase modulator, its function is to add positive, negative 90 bias phase to the existing linear polarized light, and the feedback phase opposite to existing light wave phase; and a bias square wave forming circuit, and its role is to add positive, negative 90 bias phase period set already to existing light wave phase by the integrated waveguide type optical phase modulator, and convert the phase modulation type optical signal into the amplitude modulation square wave signal after passing through photoelectric conversion.

2. The digital closed-loop fiber optical current sensor said according to claim 1, wherein the center frequency of amplitude sine wave of transimpedance amplifier TIA output is the fundamental frequency of said modulated square wave.

3. The digital closed-loop fiber optical current sensor said according to claim 1, wherein the center frequency of amplitude sine wave of transimpedance amplifier TIA output is the harmonic frequency of said modulated square wave.

4. The digital closed-loop fiber optical current sensor according to claim 1, wherein transimpedance amplifier TIA includes a high gain amplifier A, a photoelectric converter as the main component of the input circuit, and a bandstop filter feedback network BSF; Which connects the A reverse input end and output end of aid BSF; The output signal of said TIA is removed from the output of the A, the signal can be directly used for the subsequent signal processing, also be further amplified and then carry out signal processing.

5. The closed-loop fiber optical current sensor according to claim 1, wherein transimpedance amplifier TIA band-stop filter of the feedback network is composed of the first branch 1, and the second branch 2 in parallel.

6. The digital closed-loop fiber optical current sensor said according to claim 5, wherein the first branch 1 of transimpedance amplifier TIA band-stop filter of the feedback network is composed of the resistor RLand inductance L in series, and the second branch 2 is composed of the resistor RC and capacitor C in series.

7. The digital closed-loop fiber optical current sensor said according to claim 5, wherein the first branch 1 of transimpedance amplifier TIA band-stop filter of the feedback network is composed of the resistor RLand inductance L in series, and the second branch 2 is composed of capacitor C.

8. The digital closed-loop fiber optical current sensor said according to claim 5, wherein the first branch 1 of transimpedance amplifier TIA band-stop filter of the feedback network is composed of the inductance L, and the second branch 2 is composed of capacitor C in series.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is the simplified circuit and optical path diagram of the existing square wave modulation-square detection model digital closed-loop fiber optical current sensor.

(2) FIG. 2(a) is the implementation schematic diagram of the present invention square-sine wave combined reflective digital closed-loop fiber optical current sensor, and FIG. 2(b) is the implementation schematic diagram of the present invention square-sine wave combined loop type digital closed-loop fiber optical current sensor.

(3) FIG. 3 is the basic structural diagram of the tansimpedance amplifier (hereinafter referred to as TIA) employed in the existing technique.

(4) FIG. 4 is optimum structure schematic of the existing technique feedback capacitance type TIA, and its aim is to make TIA work stably, did not cause oscillation due to noise voltage.

(5) FIG. 5 is structure schematic of the existing technique R-C compensation feedback network TIA, and its aim is to reduce the feedback capacitance effect for the TIA bandwidth and noise performance.

(6) FIG. 6 is structure schematic of the existing technique bootstrap input TIA, and its purpose is to reduce the PIN capacitance effect for the TIA bandwidth and the noise performance, in which the voltage gain of the buffer is 1.

(7) FIG. 7 is the basic structure schematic of the present invention TIA.

(8) FIG. 8 is the relationship diagram between the TIA band-stop filter feedback network input current and the output voltage of the present invention, in which the ZBSF is the current-voltage conversion gain of TIA, and the gain value is written Z0 when it is the center the frequency of the feedback network.

(9) FIG. 9 is characteristic diagram of the ideal impedance (i.e. the current-voltage conversion gain) of the present invention TIA feedback network.

(10) FIG. 10 is the input current waveform (above diagram) and the output voltage wave (below diagram) of the TIA which is gain through element simulation circuit of the simulation software simulink of the Matlab.

SUMMARY OF THE INVENTION

(11) The main objective of this present invention is: to reduce the noise power level of the digital closed-loop fiber optical current sensor, to improve the bandwidth, sensitivity and the dynamic range of the digital closed-loop fiber optical current sensor.

(12) This present invention provides a digital closed-loop fiber optical current sensor. The modulation signal of the optical wave phase modulator of said the fiber optical current sensor system is modulation square wave, signal processing system extracts only any harmonic wave of the photoelectric converter output modulation square waves, and extracts the measured current information from it.

(13) Wherein the optical path consists of:

(14) A broadband light source, the output light of the source passes through the polarizer and becomes a low coherence length of the linear polarized light;

(15) An integrated waveguide type optical phase modulator, its function is to adds positive, negative 90 bias phase to the existing linear polarized light, and the its period is set already, and the feedback phase opposite to existing light wave phase;

(16) At least one polarization-maintaining fiber delay line, and its role is to enable the linear polarized light produces required delay after passing through the delay line, to extract current information to be measured from the interferometer output signal;

(17) At least one wave plate, which makes two linearly polarized lights that their polarization directions are perpendicular to each other produce the /4 wave plate with 90 phase difference. The angle between fast axis directions of the wave plate and fast axis directions of polarization-maintaining optical fiber delay line is 45, and its role is to make the linearly polarized beams from the polarization-maintaining fiber delay lines perpendicular to each other converts into a left circularly polarized light and right circularly polarized light through the wave plate, and make the left, right circularly polarized light in the opposite direction pass through the plate and then restore into the two linear polarization direction perpendicular polarized lights;

(18) At least one single turn fiber ring or multi fiber ring surround the conductor that will transmit the measured current, its role is to enable the left, right circularly polarized light through the optical fiber ring in the measured current magnetic field generate phase difference C in accordance with Faraday effect. And then convert C into the difference L between two linearly polarization direction perpendicular polarized lights after the left, the right circularly polarized light back through the /4 fiber wave plate, and the L=C;

(19) Wherein the circuitry consists of:

(20) A photoelectric converter, its effect is to convert the light phase by phase modulation type optical signal affected by the s, b and f together to amplitude modulation square wave electric signal only affected by S, in which S is the signal phase generated by the current to measure, b is phase modulator generates a bias phase, and f is feedback phase;

(21) A transimpedance amplifier TIA, its effect is covert photodetector output low signal-to-noise ratio of the weak current signal to voltage signal that drives subsequent A/D conversion circuit of high signal-to-noise ratio. Which is characterized in that the feedback network is not low pass filter network in the prior art, but the band-stop filter network; the output voltage waveform is not the low SNR amplitude square wave in the prior art, but high SNR sine wave of a harmonic on behalf of amplitude modulation square wave. Therefore, can extent the small signal region linear of digital closed-loop fiber optic current sensor.

(22) A digital feedback phase circuit and phase of light wave feedback loop for square wave amplitude detection, square wave amplitude accumulative, and feedback stepped ramp formation mainly, its role is to offset light wave phase changes induced by the measured current. Which is characterized in that square wave amplitude is determined by the amplitude of the digital sine wave modulation with high signal-to-noise ratio, rather than the amplitude of the digital sine wave modulation with low signal-to-noise ratio like the existing technologies. It can avoid the existing a lot of noise reduction operation technology required, improve the response speed of the closed-loop fiber optical current sensor;

(23) A modulation gain control loop, which consist of modulation gain accumulation and modulation gain calibration, its role is to adjust the gain of said light wave phase feedback loop when feedback phase flyback error occurs, to ensure the feedback phase flyback error zero;

(24) An integrated waveguide type optical phase modulator, its function is to add positive, negative 90 bias phase to the existing linear polarized light, and the its period is set already, and the feedback phase opposite to existing light wave phase;

(25) A bias square wave forming circuit, and its role is to add positive, negative 90 bias phase period set already to existing light wave phase by the integrated waveguide type optical phase modulator, and convert the phase modulation type optical signal into the amplitude modulation square wave signal after passing through photoelectric conversion.

(26) Wherein the center frequency of amplitude sine wave of transimpedance amplifier TIA output is the fundamental frequency of said modulated square wave.

(27) Wherein the center frequency of amplitude sine wave of transimpedance amplifier TIA output is the harmonic frequency of said modulated square wave.

(28) Wherein transimpedance amplifier TIA comprises a high gain amplifier A, a photoelectric converter as the main component of the input circuit, and a bandstop filter feedback network BSF. Which connects the A reverse input end and output end of aid BSF. The output signal of said TIA is removed from the output of the A, the signal can be directly used for the subsequent signal processing, also be further amplified and then carry out signal processing.

(29) Wherein transimpedance amplifier TIA band-stop filter of the feedback network is composed of the first branch 1, and the second branch 2 in parallel.

(30) Wherein the first branch 1 of transimpedance amplifier TIA band-stop filter of the feedback network is composed of the resistor RLand inductance L in series, and the second branch 2 is composed of the resistor RC and capacitor C in series.

(31) Wherein the first branch 1 of transimpedance amplifier TIA band-stop filter of the feedback network is composed of the resistor RLand inductance L in series, and the second branch 2 is composed of capacitor C.

(32) Wherein the first branch 1 of transimpedance amplifier TIA band-stop filter of the feedback network is composed of the inductance L, and the second branch 2 is composed of the resister Rc and capacitor C in series.

(33) Wherein transimpedance amplifier TIA band-stop filter of the feedback network is composed of n difference center frequency band-stop filters in series.

(34) Wherein the highest frequency of TIA has nothing to do with the TIA current-voltage conversion gain, which can improve the currentthe TIA voltage conversion gain without reducing its highest working frequency.

(35) Wherein TIA current-voltage conversion gain does not depend on the resistance of the TIA feedback network, thus it can have high current-voltage conversion gain and use low resistance in the feedback network TIA at the same time. It can reduce resistance thermal noise that had a large proportion of TIA output noise to negligible degree.

(36) This present invention provides a method to adjust a transimpedance amplifier TIA bandwidth. Adjust said feedback resistance value in network can adjust the corresponding transimpedance amplifier TIA bandwidth, and can keep the requirements of the current-voltage conversion gain.

(37) This present invention provides a method to adjust a transimpedance amplifier TIA bandwidth. By changing are described said transimpedance amplifier TIA bandwidth by changing the center frequency differences of band-stop filter in seriesparallel of multi-RLC string connected, and to maintain the current-voltage conversion gain.

(38) The advantages of this present invention are described as below:

(39) (1) The main noise power level of the preamplifier TIA for optical receiver of the digital closed-loop fiber optical current sensor, namely noise power is proportional to the thermal noise power level of the bandwidth and of the shot noise. The noise power level is reduced to 1/650 below of existing technology required TIA, which can greatly improve the sensitivity, dynamic range, time domain response speed or bandwidth of the digital closed-loop fiber optical current sensor.

(40) (2) The current-voltage conversion gain of the preamplifier TIA for optical receiver of the digital closed-loop fiber optical current sensor does not depend on its resistance of the TIA feedback network. Therefore, it cannot adopt resistance, or low resistance, and the resistor thermal noise is negligible. In contrast, the existing techniques required TIA must increase the resistance of the feedback net work to increase the current-voltage conversion gain. So the thermal noise of the resistor have a large proportion of the output noise in the existing techniques required TIA.

(41) (3) The operation frequency of the signal processing system of the digital closed-loop fiber optical current sensor is not affected by the current-voltage conversion gain. Therefore, improving current-voltage conversion gain of the system will not affect the highest work frequency of the system. In contrast, improving the current-voltage conversion gain of the existing techniques will decrease its highest work frequency of the system, proportionally.

(42) (4) The bandwidth of the digital closed-loop fiber optical current sensor is not affected by the current-voltage conversion gain. Therefore, improving current-voltage conversion gain of the system will not affect the system bandwidth. In contrast, improving the current-voltage conversion gain of the existing techniques will decrease its system bandwidth, proportionally.

(43) (5) The bandwidth of the digital closed-loop fiber optical current sensor can be changed by adjusting the resistor of the TIA feedback network, but it will not change the current-voltage conversion gain of the digital closed-loop fiber optical current sensor. In contrast, the current-voltage conversion gain of the existing digital closed-loop fiber optical current sensor will increase or decrease with the resistor of the TIA feedback network.

DETAILED DESCRIPTION OF EMBODIMENTS

(44) The detail of the embodiments is described as below incorporated with the figures by way of cross-reference.

(45) In order to improve the dynamic range and the response speed of the digital closed-loop fiber optical current sensor, the noise power level of the preamplifier output signal of the signal processing system can be reduced. It not only increases the measurement accuracy in the tiny signal range, but also deceases the lag time of signal process. The solution of the present invention provided is described as below: wave phase modulation adopted square wave, signal processing did not us square wave but based on the sine wave (Hereafter referred to as the square wavesine wave scheme). In order to explain why this design can achieve the goal, the application first analyze the characteristics of the signals need to process of the digital closed-loop fiber optical current sensor.

(46) As a example, the FIG. 1 is the simplified circuit and optical path diagram of the existing square wave modulation-square detection model digital closed-loop fiber optical current sensor (see: U.S. Pat. No. 5,914,781 and other publications.). In this figure, the low-pass TIA was a transimpedance amplifier (TIA) technology current-voltage conversion type low-pass preamplifier, the polarizer (analyzer), depolarizer, delay line et cetera did not draw up. The phase offset square wave amplitude Up that produced 7/2 phase shift was an identical The determined integrated waveguide phase modulator produced an identical constant, and it had nothing to do with the measured signal, and the square wave period T was determined by the optical path and had nothing to do with the measured signal. The output current wave of the detector PIN (i.e. a photoelectric converter) was identical to the signal optical. It was modulation square wave of the instantaneous amplitude Um(t)=Up.Math.i(t), in which i(t) was the measured current. The objective adopting closed-loop was to generate and track the adjusted feedback signal according to the change of i(t), and then to force the system output signal to zero. The digital feedback signal was staircase signal corresponding to Serrodyne modulation that was determined by the various moments of the square wave amplitudes. The work of the digital closed-loop was to generate right staircase square wave feedback phase, and its phase back zero when the staircase square wave phase was more than 2, then generate next staircase square wave. When the phase flyback, the PIN, the preamplifier, A/D, square amplitude detector, et cetera may produce phase error, so must adjust staircase amplitude according to the phase error. The function was realized through the modulation gain accnmulation and modulation gain calibration links.

(47) In the existing square wave solutions are that conversing the output amplitude modulation square wave light current of the PIN to the amplitude modulation square voltage by the low-pass TIA, and then extracting the instantaneous amplitude modulation from the amplitude modulation square wave voltage signal.

(48) However, FIG. 1 shows that regardless of the method, as long as the square wave amplitude analyzer in the figure analyze the amplitude of the amplitude modulation square wave correctly, the later signal processing part can work in the same way. The existing technique can realize the same digital closed-loop function that above system carried out. So do not have to use the low-pass TIA.

(49) The following will explain why the digital closed-loop function of the fiber optical current sensor only depended on the amplitude of the amplitude modulation square wave, and why this present invention provided solution can analyzed the amplitude of the amplitude modulation square wave, improve the signal-to-noise of output signal of the preamplifier of the he signal processing system.

(50) For the common fiber interferometer sensor, the photoelectric output current I(t) of the photoelectric converter can be expressed as (T. Giallorenzi, J. Bucaro, A. Dandridge, G. Sigel, et al., Optical Fiber Sensor Technology [J], IEEE Trans. MTT, 1982, 30(4): 472-511):
I(t)=I0[1+.Math.cos (s(t)+f(t)+b(t))](1)

(51) In which the I0 is proportional to the system input optical power, a is a coefficient related to optical decay and optical modulation efficiency, s(t), b(t), f(t) are the phase difference or compensation phase difference of measured signal s, offset modulation signal sb, feedback signal sf by two different propagation path (or patterns) lights. Under the FIG. 1 shown digital closed-loop condition, the offset phase b(t) was positive, negative square wave of /2 amplitude, 1 duty factor, T period. The positive, negative square wave adding on the electro-optic modulator generate square wave voltage. To simplify the analysis, make alpha=1, here the corresponding I(t) were when b(t)=/2, /2 respectively:
I+(t)=I0[1+cos(s(t)+f(t)+/2)]=I0[1sin(s(t)+f(t))](0TT/2)(2)
I(t)=I0[1+cos(s(t)+f(t)/2)]=I0[1+sin(s(t)+f(t))](T/2<tT)(3)

(52) Without the feedback signal f(t), I+(t), I(t) were:
I+(t)=I0[1sin(s(t))](0tT/2)(4)
I(t)=I0[1+sin(s(t))](T/2<tT)(5)

(53) It is indicated that output interference signal was the positive, negative amplitude sin square wave added to direct current I0. The square amplitude reflected the size of s, i.e. measured signal size because the expression equation of I(t) was derived from cos(s/2)=sin /2.Math.sin s. In which, sin /2 represents the amplitude of 1 flat positive, negative square wave p0(t), sin s(t) represents modulation on the flat positive, negative square wave amplitude. In order to meet the requirement for measurement accuracy, the sin s(t) works in the sin s(t)s(t) linear segments according to the design, i.e. in s(t)<<1 arc range. So the measured signal carried information was in the amplitude of the positive, negative square wave, measuring the instantaneous amplitude of the positive, negative square wave, the measured signal was determined.

(54) For the fiber optical current sensor in FIG. 1, s(t)=4NV.Math.i(t), in which, N is the number of turns of the fiber optical sensor, V is Verdet constant fiber, i(t) is the current to be measured. The amplitude of sin ss(t) positive, negative square wave p(t)=s(t).Math.p0(t) can be expressed as:

(55) $\begin{array}{cc}P\left(t\right)=s\left(t\right).Math.\left[\underset{n=1}{\overset{}{.Math.}}\frac{1}{2n-1}\sin \left(\left(2n-1\right){}_{0}t\right)\right]& \left(6\right)\end{array}$

(56) In which, 0=2 f0, f0=1/T and f0 is the fundamental frequency. It is showed that p(t) consists of a series of (1/(2n1))sin((2n1)0t) as the carriers, s(t) the modulation signal amplitude modulation waves. Each amplitude modulation wave carried P(t) amplitude s(t) information. So making use of any amplitude modulation (such as the fundamental wave) of p(t) only, can correctly detect the amplitude of the amplitude modulation square wave. Since each amplitude modulation bandwidth is that of s(t) (also known as bandwidth of i(t)) two times of Wi, the center frequency was (2n1)f0 respectively. In the existing solution, extract the modulation square wave amplitude using low-pass preamplifier the output square wave waveform. On the condition of f0=500 kHz, Wi=5 kHz, even if only using before 7 modulation square waves to recovery the P(t) approximately, the low-pass preamplifier bandwidth was 6.5 MHz above at least. In contrast, when using one modulated wave extract the amplitude modulation square wave, the preamplifier bandwidth was only 10 kHz. It was the 1/650 less than existing solution. Only by this point, the preamplifier noise power level is proportional to the bandwidth of the thermal noise and the shot noise (shot noise) level, reduce to the existing scheme of 1/650. This is the principle of the present patent application how to use amplitude modulation sine wave to extract the amplitude of the modulation square wave, to greatly improve the preamplifier output signal to noise ratio of the signal processing system, and then improve he dynamic range of the system and the response speed.

(57) FIG. 2 is the implementation schematic diagram of the present invention square-sine wave combined reflective digital closed-loop fiber optical current sensor. In the diagram, preamplifier for optical receiver is no longer a low-pass TIA using in existing digital closed-loop fiber optic current sensor, but the selective amplifier that only amplifying the selected frequency band signal.

(58) FIG. 7 is the general structure of the embodiment of selective amplified TIA, the relationship between the input current and the output voltage of the band stop filter (bandstop filter:BSF) feedback network is shown in FIG. 8, the ideal impedance of the feedback network (i.e. current-voltage conversion gain) characteristics is shown in FIG. 9.

(59) According to circuit theory, i TIA adopts said band-stop filter feedback network, the TIA will have the following advantages:

(60) (1). The highest work frequency of TIA has nothing to with the TIA voltage conversion gain, thus improving the current-voltage conversion gain will not be limited by the highest working frequency and signal bandwidth required like existing techniques.

(61) (2). TIA current-voltage conversion gain does not depend on the resistance of the TIA feedback network, thus it can have high current-voltage conversion gain and use low resistance in the feedback network TIA at the same time. It can reduce resistance thermal noise that had a large proportion of TIA output noise to negligible degree.

(62) (3). Adjust the feedback resistance value in network can adjust the frequency bandwidth of the transimpedance amplifier TIA, and keep the he current-voltage conversion gain required.

(63) Two above advantages still exist using said multi-RLC series-parallel band-stop filter feedback network. There is no need to apply the method that adjusting the feedback resistor value in network to change TIA frequency bandwidth, but the method that changing the center frequency of each RLC series-parallel bandstop filter to change the TIA frequency bandwidth. And the method that changing TIA bandwidth can also keep the current-voltage conversion gain required.

(64) In the following, setting the feedback network for an example to further explain the advantages of the technology of TIA. The feedback network consists of Z1, Z2 in parallel form, where Z1 is the impedance of the inductive L, Z2 is series impedance of capacitance C and resistance RC. The feedback network bandwidth WB, center frequency f0, center frequency impedance Z0 (i.e. selective frequency TIA current-voltage conversion gain at the center frequency) can be selected singly, the relations between L, C, RC in the network and the three selected parameters are:

(65) $\begin{array}{cc}L=\frac{1}{2.Math.f_0}\frac{W_B}{\sqrt{f_0^2-W_B^2}}{Z}_0& \left(10\right)\\ C=\frac{1}{2.Math.W_B}\frac{f_0}{\sqrt{f_0^2-W_B^2}}\frac{1}{Z_0}& \left(11\right)\\ R_C={\left(\frac{W_B}{f_0}\right)}^2{Z}_0& \left(12\right)\end{array}$

(66) The existing digital closed-loop fiber optical current sensor adopted low pass TIA. In the feedback loop, feedback resistor RF is greater than or equal to TIA current-voltage conversion gain Z0. If the Z0=1 M was required, RF should be greater than or equal to 1 M. For feedback network as an example, WB=10 kHz, f0=500 kHz under the common work condition, the resistor that feedback network required RC=(10/500)2Z0=410.sup.4Z0=400, noise output power level of the feedback network resistor was only (4102/106)1/2=1/50 of existing technology.

(67) It has been proved that the technology of the invention can make the thermal noise of TIA in the output signal and the shot noise level decrease to the existing technique 1/650 below through using the transimpedance amplifier TIA. Here it further proves that, compared with the existing digital closed-loop fiber optical current sensor adopting the low-pass filter type TIA, under the same TIA current-voltage gain condition, the TIA required feedback network resistance of the present invention than the feedback resistance of the existing TIA technology is lower. Therefore, it can greatly reduce noise power level of the feedback network resistance at the TIA output end.

(68) FIG. 10 is the input current waveform (above diagram) and the output voltage wave (below diagram) of the TIA that is gain through element simulation circuit of the simulation software simulink of the Matlab. It used the feedback network in FIG. 10. The period of the modulation square wave is 2 microseconds, signal frequency measured is 5 kHz, but the square wave amplitude is almost no change within 5 cycles in FIG. 10. Therefore, it can determine the instantaneous amplitude of square wave modulation by fundamental wave of the modulation square wave.

(69) At last, in this description of the embodiments, we have detail describe the present invention according to a particular example. The detail embodiment is one example of the invention but not the only one, so the person in this field must be understand that all the alternatives and other equal and/or similar examples are all within the range of the invention and they are all consistent with the spirits of this invention, are all protected by our claims.