METHOD OF MEASUREMENT AND APPARATUS FOR MEASUREMENT OF AMPLITUDE RATIO OF TWO FIRST HARMONICS OF THE SIGNAL OBTAINED FROM SAGNAC SYSTEM

Abstract

The object of the present invention is a measurement method and a system for measurement of an amplitudes ratio of two first harmonics of a signal obtained from the Sagnac fibre-optic inter-ferometer system as well as the Sagnac fibre-optic system, and an application of the said system to measure the amplitude ratio of two first harmonics obtained from the Sagnac fibre-optic system and the Sagnac fibre-optic interferometer system for detection of rotational movements, in particular seismic rotational movements and rotational movements of structures.

Claims

1. A method to measure the amplitude ratio of two first harmonics of a signal obtained from the Sagnac system, characterised in that it comprises steps wherein: the electrical signal from an optical detector is transmitted to a transimpedance amplifier adjusting the electrical parameters of a detection element; the amplified signal is transmitted to an input of a programmed amplifier determining the initial amplification for the whole measurement signal; next, the signal is separated into two paths, one of them leading the signal directly to the input of a fast analogue-digital converter ADC which processes the whole signal of the second harmonic, and the second part of the signal being transmitted to a group of programmed low- and band-pass filters of fixed phase characteristics, where the signal first harmonic is extracted; after that, the extracted first harmonic signal is transmitted to the programmed amplifier input, where it is amplified to the level appropriate for the second analogue-digital converter ADC, where the conversion of the signal into the digital form synchronously with the second harmonic signal takes place; the obtained digital signals of the first and the second harmonics are transmitted to a signal processor in order to obtain the amplitude and phase values of both signals, that the rotational speed of the fibre-optic loop is calculated from.

2. The method of claim 1, characterised in that the obtained digital signals of the first and the second harmonics are transmitted through a fast logic system implemented on a programmable gate array FPGA to the signal processor where the conversions are carried out simultaneously by means of fast Fourier transform (FFT) in order to obtain the amplitude and phase values for both signals.

3. The system to measure the amplitude ratio of the first two harmonics of the signal obtained from the Sagnac fibre-optic interferometer system, characterised in that it comprises the transimpedance amplifier connected with a program amplifier that is coupled with the first analogue-digital converter used for processing of the second harmonic signal and with the second analogue-digital converter used for processing of the first and the second harmonic signals through a group of programmed low- and band-pass filters and the programmed amplifier, both analogue-digital converters being coupled with the logic system transmitting the signals to the signal processor.

4. The system of claim 3, characterised in that the logic system is implemented on programmable gate array FPGA.

5. A Sagnac fibre-optic interferometer system comprising a light source in the form of a superluminescent diode connected to an isolator that is connected to a depolariser and then with a fibre-optic coupler of 22 type, and a pair of fibre-optic polarisers and, wherein the polariser is connected to the fibre-optic coupler of 22 type that is further connected to a fibre-optic phase modulator and a fibre-optic loop, wherein a detector is connected to the first coupler, characterised in that it further comprises an electronic processing system of claim 3.

6. An application of the system of claim 3 for the detection of rotational movements, in particular seismic rotational movements and rotational movements of structures.

7. The application of the system of claim 5 for the detection of rotational movements, in particular seismic rotational movements and rotational movements of structures.

8. A Sagnac fibre-optic interferometer system comprising a light source in the form of a superluminescent diode connected to an isolator that is connected to a depolariser and then with a fibre-optic coupler of 22 type, and a pair of fibre-optic polarisers and , wherein the polariser is connected to the fibre-optic coupler of 22 type that is further connected to a fibre-optic phase modulator and a fibre-optic loop, wherein a detector is connected to the first coupler , characterised in that it further comprises an electronic processing system of claim 4.

9. An application of the system of claim 4 for the detection of rotational movements, in particular seismic rotational movements and rotational movements of structures.

Description

[0031] The solutions known from the state of the art, and embodiments of the present invention are presented in the figures, where:

[0032] FIG. 1 illustrates the basic configuration of the fibre-optic system enabling interferential measurement of the Sagnac effect, i.e. the angular velocity of system rotation in the axis perpendicular to the plane of the fibre-optic loop;

[0033] FIG. 2 illustrates the minimum configuration for a fibre-optic gyroscope, that provides reversibility;

[0034] FIG. 3 illustrates the minimum configuration for a Sagnac fibre-optic interferometer with the so called open feedback loop. The presented configuration was used as the system to generate Sagnac phase shift that is detected by the present invention;

[0035] FIG. 4 is a block diagram of the electronic measurement system unit that illustrates the essence of the described invention;

[0036] FIG. 5 illustrates the system according to the present invention in the basic variant described as the embodiment of the invention.

[0037] The application of the Sagnac effect for construction of a fibre-optic gyroscope was proposed in mid 70s of the previous century. In mid eighties, the system assumed the form of a commercial apparatus, used for navigation of aircraft and vehicles, guiding missiles, and for inertial navigation of spacecraft. During the research carried out by many research grups, a few different configurations of the fibre-optic gyroscope were developed. As mentioned before, the present invention uses the minimum configuration for the Sagnac fibre-optic interferometer described in the work by R. Ulrich, Fibre-Optic Rotation Sensing with Low Drift, Optics Letters, Vol. 5, 1980, pp. 173-175 wherein a light source with a broad spectrum was used, additionally equipped with an isolator and fibre-optic depolariser in order to reduce the reverse signal and the polarisation noise.

[0038] The main factor that distinguishes the proposed Sagnac fibre-optic interferometer system from classical fibre-optic gyroscope structures is that fact that it measures the angular velocity and not the angle. Therefore, the problem of drift appearing in optical gyroscopes can be practically disregarded here. The effected optimisation of parameters like the loop radius, optical power of the light source, and the length of the fibre used, allowed to guarantee high sensitivity of the device of 1.Math.10.sup.9 rad/s/Hz.sup.1/2. The determination of the angular velocity is done by measuring the amplitudes of the first (A.sub.1) and the second (A.sub.2) harmonics of the ouput signal according to the fomula (1).

[0039] In order to determine the harmonic components of the outupu signal, synchronous detection is used. The modulator working frequency was determined experimentally, assuming the criteria of the interferometer response linearity on lack of rotational excitation as well as maximisation of that response.

[0040] Due to the large difference between the first and the second harmonics values, providing a large dynamic range is required on their concurrent measurement.

[0041] Therefore, signal filtration was implemented, that results in appropriate separation of the signal first harmonic from much stronger second harmonic signal. In real world execution it consists in splitting the signal into two independent paths of the first and the second harmonics. Next, the signal is fed to the analogue-digital converter. The digital form of both signals is multiplied by the reference signal with the phase modulator frequency. Further digital processing enables determination of the component first and second harmonics of the recorded signal, which provides information on angular velocity.

[0042] Additionally, apart from detection properties, the system is equipped with advanced functions of data recording and transmitting and the possibility to change the measurement path remotely. The recorded data is sent, by means of the communication module, to a remote server archiving the data. The server provides access to the data and to remote control of device parameters. This is provided by a GSM/GPRS module enabling wireless communication with the remote server on a network.

[0043] The solution propsed in the present patent application is aimed at its application for measuring rotational effects in rotational seismology area. The measurement of rotational effects requires application of high sensitivity sensors operating in a broad frequency range. Currently, the problems concerning such measurements are connected with the lack of appropriate apparatus. Classical seismometers are linear velocity sensors, which definitely eliminates their application for examination of rotational movement. The recording of rotational movements is provided by gyroscopic sensors or a system of laser gyroscopes. Their presence in a seismometric array system can also be indirectly inferred.

[0044] In Polish Patent Application No. P.344540, 2000, we can find a sensor enabling the measurement of torsional vibrations. The described rotational pendulum seismometerTAPS (Two Antiparaller Pendulum Seismometer) consists of two seismometers situated anti-parallel on a common vertical axis. However, this solution enables detection of linear velocities and then determination of the rotation and translation components from their values by means of a special mathematical procedure. This method, however, is an indirect method and can demonstrate irregularities in certain conditions caused by unevenness of attenuation factors for individual seismometer components.

[0045] Another sensor type for measuring rotational movement, the angle of rotation in principle, are optical gyroscopes based on the application of the Sagnac effect. These systems are used in aircraft navigation and in vehicle mounted systems. However, their sensitivity is generally insufficient for applications in rotational seismology, mainly due to their fitness to measure the rotation angle and not rotational speed. Furthermore, these systems, directly applied for seismic measurements, are solutions with a limited sensitivity.

[0046] It has to be stressed out that the main factor distinguishing the proposed measurement method intended for the Sagnac fibre-optic interferometer system from classical fibre-optic gyroscope structures is that fact that it measures the angular velocity and not the angle values. Such solution enables obtaining a system that eliminates the problem of drift ocurring in optical gyroscopes. Furthermore, fibre-optic gyroscopes are characterised by low measurement dynamics range and by electronic systems specialized in measuring the angle of rotation.

[0047] The proposed method enables obtaining a sensor of high dynamics and measurement accuracy. Furthermore, the presented method guarantees obtaining information on the angular velocity in a fully direct way, which contributes to the minimization of measurement uncertainty.

[0048] The present invention solves the problem of a detection system used in the fibre-optic rotational seismometer in a minimum configuration of the gyroscopic system with an open feedback loop. The system employs the measurement of the signal first and second harmonic amplitudes whose ratio provides information on the angular velocity value. The essence of the present invention is the implementation of a group of filters in order to separate the first and the second harmonics whose values differ largely, into two separate paths. So far, an analogue-digital converter of high dynamics was used, which definitely increased the cost of the detection system.

[0049] The proposed solution enables signal recording with very high accuracy and in a broad frequency range.

DETAILED TECHNICAL DESCRIPTION OF THE INVENTION

[0050] The block diagram of the electronic measuring system is illustrated in FIG. 4.

[0051] The electrical signal from the optical detector 10 is transmitted to a transimpedance amplifier 11 adjusting the electrical parameters of the detection element. The amplified signal is transmitted to the input of a programmed amplifier 12 determining the initial amplification for the whole measurement signal. Then the signal is separated into two paths. One of them leads the signal directly to the input of a fast analogue-digital converter ADC 13 which processes the whole signal of the second harmonic. The second part of the signal is transmitted to a group of programmed low- and band-pass filters 14 of fixed phase characteristics, where the signal first harmonic is extracted. The extracted first harmonic signal is transmitted to the programmed amplifier 15 input, where it is amplified to the level appropriate for the second analogue-digital converter ADC 16, where the conversion of the signal into the digital form synchronously with the second harmonic signal takes place.

[0052] The obtained signals of the first and the second harmonics are transmitted through a fast logic system made on a programmable gate array FPGA 17 to the signal processor 18 where the conversions are carried out simultaneously by means of fast Fourier transform (FFT) in order to obtain the amplitude and phase values for both signals. These values are then converted according to the formulas (reference to the formulas above) into a numerical value corresponding to the rotational velocity of the fibre-optic loop.

[0053] These values are transmitted to a microcomputer 19 that collects the data in an internal FLASH memory of high storage capacity, analyses the signal in order to detect the required signal changes, and enables control of all elements of the measurement system.

EMODIMENTS OF THE INVENTION

Example

[0054] The optical component (20) of the fibre-optic rotation sensor consists of the following elements: [0055] a source (1) in the form of a superluminescent diode (from Exalos, of the following characteristics: centre wavelength .sub.0=1305.7 nm, bandwidth B=31.2 nm, optical power P=9.43 mW), [0056] an isolator (9) (from FCA, optical loss =0.34 dB, centre wavelength .sub.0=1310 nm, isolation level45 dB), [0057] a depolariser (10) (from Phoenix Photonics, degree of polarization DOP <5%, optical loss =0.20 dB), [0058] a pair of fibre-optic couplers (2 and 5) of 22 type (from Phoenix Photonics, power division factors of 50:50%, optical loss =0.20 dB), [0059] a pair of fibre-optic polarisers (6 and 7) (from Phoenix Photonics, extinction level =43 dB, optical loss =0.45 dB), [0060] fibre-optic phase modulator (8) in the form of a piezo ceramic shape with resonance frequency of f=21 kHz (from Piezomechanik GmbH), [0061] a fibre-optic loop (3) of a single-mode fibre (from Corning, SMF28e type) coiled to the diameter of about 240 mm and about 5000 mm in length, [0062] a pair of detectors (4 and 4) (from Optoway Technology, Inc., sensitivity S=0.9 A/W). [0063] The important elements of the electronic processing system (21) conist of the following elements: [0064] a transimpedance amplifier (11) (MTI04CQ from Mazet), [0065] a programmed amplifier (12) (LTC1564 from Linear Technology), [0066] fast analogue-digital converter (13 and 16) (AD7986 from Analog Devices), [0067] a group of programmed low- and band-pass filters (14), [0068] a programmed amplifier (15), [0069] a programmed gate array FPGA (17) (XC7Z010-1CLG400C from Xilinx), [0070] a signal processor (18) (XC7Z010-1CLG400C from Xilinx), [0071] a microcomputer (19) (AES-Z7MB-7Z010-G from MicroZend).

[0072] The proposed solution offers broad application possibilities in rotational seismology, solving the ever growing problem of lack of experimental data concerning rotational efects, caused by the lack of appropriate detection apparatus. The field requires devices providing an extreme sensitivity of the magnitude of 10.sup.9 rad/s/Hz.sup.1/2. The presented method enables construction of a sensor fully satisfying the above conditions. The sensivity state above enables recording of rotational movements occurring during earthquakes. The research in this field can significantly contribute to the explanation of the nature of those phenomena and their physics.

[0073] Furthermore, the presented measurement method enables measuring high amplitude rotational movements of engineering structures, of the magnitude of 10 rad/s in the frequency range of 0.1-10 Hz. Continuous monitoring of rotational movements of structures is extremely important for safety. Therefore, the presented method enables obtaining a system with a broad operation range, both in terms of the amplitude and frequency.

[0074] The application of the proposed method in a three-axial layout shall enable monitoring of rotational movements simultaneously in three directions.