KR20020000759A - Fibre bragg grating sensors for measuring a physical magnitude - Google Patents

Abstract

The Bragg grating sensors have different Bragg wavelengths λ i from each other and comprise a plurality of Bragg gratings 1 i, i = 1,2,... N arranged in turn in the optical fiber 1. In the fiber, the wavelength lambda i placed near the Bragg wavelength and individually varied by different frequencies ω i is simultaneously transmitted to the grating, and the frequency-coded Bragg wavelengths actually reflected by the grating at the same time are separated by frequency Phase decoded. The present invention provides the advantage that the cost is low, especially when a large number of fibers are used.

Description

Bragg grating sensors for measuring physical quantities {FIBRE BRAGG GRATING SENSORS FOR MEASURING A PHYSICAL MAGNITUDE}

Bragg grating sensors for measuring at least one constant physical quantity are known, such as Yun-Iang Rao: “In-fiber Bragg Grating Sensors”, Mess. Sci. Technol. 8 (1997), pages 335-375, with many examples. The physical quantity to be measured may be, for example, tensile stress, temperature, pressure, ultrasound, acceleration, magnetic field and / or force according to the publication.

In the known embodiment, an optical Bragg grating having a grating-specific Bragg wavelength is disposed in at least one light transmission path for transmitting light rays. The wavelength is reflected off the grating when it is included in the light beam transmitted in the transmission path, while the light beam passes through the grating when a wavelength other than the Bragg wavelength is included in the light beam. The Bragg wavelength of the grating changes with the variation in the physical amount acting on the grating.

In all of the above known embodiments the light transmission path consists of an optical fiber in which the Bragg grating is realized by periodic refractive index variations.

In a particular embodiment, a plurality of optical Bragg gratings disposed side by side on the fiber with different grating-specific Bragg wavelengths are disposed on the optical fiber. The intrinsic Bragg wavelength of each grating varies with the variation in the physical amount acting on the grating and is reflected only by the grating if the wavelength is included in the light beam transmitted to the optical fiber.

In a particular embodiment it has two or more fibers instead of just one, said fibers having a plurality of optical Bragg gratings arranged side by side, each having a different lattice-specific Bragg wavelength, wherein each of the fibers It is provided for the transmission of light rays generated by the common light source.

A particular embodiment includes an adjustable light source for generating narrowband light at a particular wavelength. The source is said lattice-specific Bragg wavelength at least once in a particular time sequence, such that a particular wavelength is temporally changed so that it passes through a wavelength range comprising different lattice-specific Bragg wavelengths of all Bragg gratings in the optical fiber (s). To be placed in, controlled.

In this way specific wavelengths that vary in time are coupled to the optical fiber and transmitted in the fiber. Since the transmitted wavelengths lie at the unique Bragg wavelength of each Bragg grating at different points in time and are reflected by the grating, a temporal sequence of reflected grating-specific Bragg wavelengths results.

An adjustable light source for generating narrowband light at a particular wavelength may include, for example, a light source for generating a broadband light having a wavelength range that simultaneously includes the grating-specific Bragg wavelengths of all Bragg gratings in the optical fiber (s), and It is made up of adjustable wavelength scanners that are connected after the source and generate narrowband light beams with a particular wavelength from the broadband light beams. The wavelength scanner causes a temporal fluctuation of a specific wavelength by the constantly monotonically changing signal of the lamp generator. The lamp generator causes certain wavelengths to change in time by always passing back through the entire wavelength range of the broadband light, allowing the wavelengths to be coupled to and transmitted from the optical fiber.

In contrast to the above, an adjustable light source for generating narrowband light rays at a particular wavelength may, for example, cause the optical fiber (s) to produce wideband light rays having a wavelength range that includes different grating-specific Bragg wavelengths of all Bragg gratings. A light source for the light source, an adjustable Fabry-Perot filter connected after the broadband source, and an adjustable interferometric wavelength scanner connected after the Fabry-Perot filter.

A narrowband wavelength range is set in time by the Fabry-Perot filter from the full wavelength range of the broadband light beam, each subrange comprising a grating-specific Bragg wavelength of one Bragg grating. If the filter is set to such a partial range, the filter emits light that includes only the partial range. In order to set the partial range in sequential time, the filter is controlled by a stepwise monotonically changing signal of the step signal generator.

The adjustable interferometric wavelength scanner generates a narrowband light beam with a specific wavelength from the light set by the Fabry-Perot filter to the narrowband wavelength subrange. The wavelength scanner has a specific wavelength at each subrange in that subrange. It changes in time and is controlled to pass through the partial range. For this purpose, the wavelength scanner is controlled by the signal of the ramp generator which continuously changes monotonically in time in each set partial range.

Since the narrow-band wavelength range is set in succession in time and the specific wavelength meets the lattice-specific Bragg wavelength of the Bragg grating included in the sub-range once once through each of the sub-ranges, the specific wavelengths here respectively It lies at the lattice-specific Bragg wavelength of the Bragg lattice, thus forming a specific wavelength that changes in time. The wavelength is coupled to and transmitted from the optical fiber.

The receiver receives and detects a temporal sequence of grating-specific Bragg wavelengths reflected by the Bragg grating, formed in the optical fiber upon transmission of a particular temporally varying wavelength. Since the grating-specific Bragg wavelength of each Bragg grating varies with the variation in the physical quantity acting on the grating, the Bragg wavelength of each reflected reflected grating is a measure of the actual value of the physical quantity at the grating site. .

The receiver may have a wideband light input, the bandwidth of which includes the grating-specific Bragg wavelengths of all Bragg gratings. Such a wideband input may be implemented by, for example, a photodiode.

Another particular embodiment known includes a light source for generating a wideband light beam having a wavelength range that includes at least one optical fiber grating-specific Bragg wavelength of all Bragg gratings.

The broadband light is coupled to and transmitted from the optical fiber. Since the transmitted light beams simultaneously include the grating-specific Bragg wavelengths of all Bragg gratings, the light rays simultaneously containing the grating-unique Bragg wavelengths reflected from all Bragg gratings of the optical fiber are formed, unlike the particular embodiment.

In another known embodiment the receiver also receives and detects all reflected grating-specific Bragg wavelengths. Again, since the grating-unique Bragg wavelength of all Bragg gratings is changed by the variation in the physical quantity acting on the grating, the Bragg wavelength of each reflected reflected grating is a measure of the actual value of the physical quantity at the grating site. to be.

Since the receiver must separate the reflected grating-specific Bragg wavelengths received at the same time from each other, it requires a narrowband light input to detect only one wavelength. For example, the receiver may include a spectrometer that sequentially measures the reflected grating-specific Bragg wavelengths given in time.

The present invention relates to a Bragg grating sensor for measuring at least one physical quantity.

1 is a schematic block circuit diagram of one embodiment of an apparatus according to the invention.

FIG. 2 is a wavelength spectrum of light rays generated from the light source of the embodiment according to FIG. 1; FIG.

3 is a schematic block circuit diagram of an embodiment of a light source of a device according to the invention.

It is an object of the present invention to include at least one optical transmission path for transmitting light rays and at least two optical Bragg gratings arranged in turn on the transmission path with different grating-specific Bragg wavelengths, in particular two or more optical transmissions. It is to provide a Bragg grating sensor for measuring at least one constant physical quantity, which can be implemented particularly inexpensively when using a path.

This object is achieved by the features set forth in claim 1.

The Bragg grating sensor according to the invention is an important feature of the invention, for each Bragg grating lies at least near the grating-specific Bragg wavelength of the grating and varies individually in time within the wavelength range assigned only to the grating, A light source for generating light rays having a specific light wavelength assigned only to the grating and for simultaneously combining all of the time varying constant wavelengths in the transmission path for the purpose of transmitting simultaneously in the transmission path.

The light source is similar to the known light source described above, in which different time varying specific light wavelengths are coupled into the transmission path for transmission to the path in turn instead of simultaneously, but are constructed differently than the known light source. It works like the known light source.

In particular, each light source of the sensor according to the present invention is included in the light beam generated by the source and each specific light wavelength assigned only to one of the Bragg gratings includes all wavelength ranges simultaneously comprising the particular wavelength during its temporal variation. Must pass only a narrow range of wavelengths, and include only the grating-specific Bragg wavelengths of the Bragg grating to which only that particular wavelength is assigned.

Thus, the light source does not require a wavelength scanner or Fabry-Perot filter with a large variable range extending over a large wavelength range that includes all the grating-specific Bragg wavelengths of all Bragg gratings.

The light source preferably only requires a light source that simultaneously generates all the specific light wavelengths assigned to each Bragg grating, and a device for temporal fluctuation of the wavelength in the narrow wavelength range, next to the source. The light source may preferably consist of a conventional, inexpensive broadband source, with adjustable possibilities for each specific wavelength to be fluctuated, each with one adjustable wavelength scanner, the wavelength scanner being adjusted only in a narrow wavelength range. It may be cheap without a high demand for. For example, each adjustable wavelength scanner may be an interferometric wavelength scanner controlled by the signal of the lamp generator as known in the above-mentioned publication.

In a particularly preferred embodiment of the sensor according to the invention, the light source is a broadband source for generating a light beam with a continuous bandwidth, which is generated by the light source and which includes all of the constant light wavelengths which vary individually in time. And a individually adjustable Bragg grating having a grating-specific Bragg wavelength that defines that particular wavelength per particular wavelength. The broadband light beam is supplied simultaneously to all the Bragg gratings and all gratings reflect their grating-specific Bragg wavelengths and are simultaneously delivered as a specific wavelength to be generated by the light source. Each of the adjustable Bragg gratings may be adjusted by one piezoelectric actuator.

In the sensor according to the invention all grating-specific Bragg wavelengths are simultaneously reflected in each Bragg grating due to simultaneous coupling and transmission of specific light wavelengths that change in time. The sensor of the present invention thus comprises a receiver for receiving and detecting all grating-specific Bragg wavelengths that are substantially simultaneously reflected by the Bragg grating when simultaneously transmitting all time varying specific wavelengths in the transmission path. Said "actually" means that the light propagation time difference and the temporal variation of the particular light wavelength assigned only to the grating in the wavelength range assigned to only one grating can be neglected.

The receiver of the device according to the invention may comprise a spectrometer with a narrowband optical input. The spectrometer sequentially measures the reflected grating-specific Bragg wavelengths given in time.

A particular advantage of the sensor according to the invention is shown in the new principle of operation of the light source of the sensor. According to the operating principle, specific wavelengths simultaneously included in the light rays generated by the source are individually varied in time. Such individual variation offers the possibility of temporally varying light rays of different frequencies, at least two different specific light wavelengths being generated by the light source and combined into the at least one light transmission path.

The grating-specific Bragg wavelength reflected by the Bragg grating preferably has a frequency at which the particular light wavelength assigned to the grating and coupled into the light transmission path varies in time within the relevant narrow wavelength range. If two specific wavelengths assigned to two different Bragg gratings vary at different frequencies from each other, the grating-specific Bragg wavelengths reflected by the gratings can be distinguished from one another because they have different frequencies.

This distinguishability has the advantage that the receiver of the sensor according to the invention can have a wideband light input with a bandwidth comprising all grating-specific Bragg wavelengths appearing at different frequencies. Such a wideband input is preferably inexpensive since it can be implemented with a conventional photodetector. This is particularly desirable for sensors with two or more light transmission paths, because only one low cost wideband optical input is provided for each transmission path and no expensive spectrometer is required.

In order to distinguish the reflected grating-specific Bragg wavelengths having different frequencies from each other, the receiver comprises a device for separating the reflected grating-specific Bragg wavelengths having different frequencies from one another according to frequency. The device can be implemented by conventional inexpensive electronic means.

Another advantage of the sensor according to the invention is its high yield compared to conventional sensors which include an adjustable light source for generating specific light wavelengths which are in turn coupled in time in the light transmission path.

Hereinafter, with reference to the accompanying drawings of the present invention will be described in detail.

A typical Bragg grating sensor for measuring at least one particular physical quantity (X), each for transmitting light, eg M (M = 1, 2, 3, ...) optical transmission paths (1), all made of optical fiber Has

In each transmission path 1 each N (N = 2, 3, ...) optical Bragg gratings 11, 12, having different grating-specific Bragg wavelengths λ1, λ2 ... N) ... 1N) are arranged one after the other.

The grating-specific Bragg wavelengths lambda 1, lambda 2 ... lambda N of each grating 11, 12, ... 1N are the physical quantities X acting on the gratings 11, 12, ... 1N. It varies according to the variation ΔX.

When the grating-specific Bragg wavelengths lambda 1, lambda 2 ... lambda N of each Bragg grating 11, 12, ... 1N are included in the light beam transmitted in the transmission path 1, the wavelengths lambda 1, lambda 2 ... lambda N is independently reflected from the gratings 11, 12, ... 1N.

The light source 2 generates a light beam P having the wavelength spectrum shown in FIG. 2. In the wavelength spectrum, the intensity of the light beam P is shown above the wavelength λ.

According to FIG. 2, the light beams P simultaneously have any number N of spectral lines, each of which lies at N different specific light wavelengths λ01, λ02, ... λ0N, each of which is narrow. It is represented by the band intensity curve 100.

Each particular wavelength λ01, λ02, ... λ0N simultaneously present in the light beam P is independently assigned to each Bragg grating 11, 12, ... 1N and at least grating-unique of the grating. In the wavelength ranges Δλ 01, Δλ 02, ... Δλ 0N, which lie near the Bragg wavelengths λ 1, λ 2, ... λ N, and are assigned only to the gratings 11, 12, 1. Changes to

All wavelength ranges Δλ 01, Δλ 02,... Δλ 0 N are each Bragg grating 11, varying with the variation ΔX of the physical quantity X in addition to the respective specific wavelengths Δλ 01, Δλ 02, ... Δλ 0N. 12, ... 1N is selected to include lattice-specific Bragg wavelengths lambda 1, lambda 2, lambda N. The wavelength ranges Δλ 01, Δλ 02,... Δλ 0N are assigned to the Bragg gratings 11, 12, ... 1N, whereby the grating-specific Bragg wavelengths λ 1, λ 2,... When varying within the wavelength ranges Δλ 01, Δλ 02,... Δλ 0N, the grating-specific Bragg wavelengths λ 1, λ 2,... Λ N are reflected from the gratings 11, 12, ... 1N. .

Since all N different specific light wavelengths Δλ01, Δλ02, ... Δλ0N of the light beam P vary in time by different frequencies ω1, ω2, ... ωN, different frequencies ω1, ω2, Each of the specific wavelengths Δλ 01, Δλ 02, ... Δλ 0N having ... ω N) changes individually in time.

FIG. 3 shows a very preferred embodiment of the light source 2, for example for the sensor according to FIG. 1. The light source 2 according to FIG. 3 is a light beam having a continuous bandwidth Δλ 0, which includes all of the specific light wavelengths Δλ 01, Δλ 02,... Δλ 0 N, which are generated from the light source 2 and will be changed individually in time. A broadband source 22 for generating P0. In addition, each individually adjustable Bragg has a grating-specific Bragg wavelength that defines the specific wavelengths Δλ01, Δλ02, ... Δλ0N for each of the specific wavelengths Δλ01, Δλ02, ... Δλ0N that are generated. There are gratings 221, 222, ... 22N. The broadband light beam P0 of the source 22 is transmitted simultaneously to all of the Bragg gratings 221, 222, ... 22N, and all the gratings 221, 222, ... 22N have their grating-specific Bragg wavelengths. And these are simultaneously sent out as specific wavelengths Δλ 01, Δλ 02,... Δλ 0N to be generated from the light source 2.

The specific light wavelengths Δλ 01, Δλ 02, Δλ 0N generated from the adjustable Bragg gratings 221, 222,... 22N may be temporally adjusted by adjusting the gratings 221, 222,... 22N. Can be changed to

All adjustable Bragg gratings 221, 222, ... 22N can be adjusted by respective piezoelectric actuators 231, 232, ... 23N, and the piezoelectric actuators 231, 232, ... 23N It can be controlled by respective electrical control signals U1, U2, ... UN, for example by respective voltages. If the control signals U1, U2, ... UN are varied by frequencies ω1, ω2, ... ωN, the adjustable Bragg grating 221 dependent on the signals U1, U2, ... UN The specific light wavelengths λ01, λ02, ... λ0N generated from, 222, ... 22N are changed in time by the frequencies ω1, ω2, ... ωN.

Specific light wavelengths lambda 01, lambda 02, lambda 0N, generated from the adjustable Bragg gratings 221, 222, ... 22N and re-reflected in the direction of the broadband source 22, are generated and transmitted from the source 22. Are separated by light couplers 201, 202, ... 20N from the radiation path of the light beam P0. The optical coupler may preferably be a simple 2 × 2-coupler.

For example, all of the broadband sources 22 are formed in each optical fiber 220 to which the radiating portion of the light beam P0 generated from the broadband source 22 is coupled, and the radiating portion is formed within the optical fiber 220. 222, ... 22N, and the specific wavelengths λ01, λ02, ... λ0N generated from the gratings 221, 222, ... 22N are combined with the combiner 201, 202,... 20N) from the fiber 220.

Irrespective of the specific structure of the light source 2 of the sensor according to the invention, the specific wavelengths λ01, λ02, ... λ0N that occur from the source 22 and vary in time allow simultaneous transmission to the path 1. To all of the transmission paths 1 at the same time.

For example, when M> 1 transmission paths 1 exist as shown in FIG. 1, light beam P is transmitted and the light beam P is transferred to M light beam portions P1, P2, ... PM. The splitting is made by an optical power divider 20, which can be presented as a component of the light source 2. All of the light beam portions P1, P2, ... PM simultaneously include all specific wavelengths λ01, λ02, ... λ0N, each of which varies in time, and are coupled to each of the M transmission paths 1. For example, the light beam P is divided into M light beam portions P1, P2, ... PM each having the same intensity I / M.

The grating-specific Bragg wavelengths λ1, λ2, ... λN reflected from the Bragg gratings 11, 12, ... 1N of one transmission path 1 are in the direction of the light source 2 in the direction of the path 1 Receiver 3 for receiving and detecting all the wavelengths λ1, λ2, ... λN while separating the wavelengths λ1, λ2, ... λN from the path 1 To the optical coupler 10 disposed in the path 1 for transmitting the wavelengths λ 1, λ 2,... The optical coupler 10 can be implemented, for example, by a 2 x 2-coupler in a simple manner.

When there are a plurality of transmission paths 1, one optical coupler 10 is disposed in each path, and each optical input unit 30 of the receiver 3 is assigned to the optical coupler 10. The grating-specific Bragg wavelengths λ 1, λ 2,... Λ N which are reflected from the path 1 and separated from the combiner 10 are simultaneously transmitted to the light input unit 30.

If there are multiple transmission paths 1, the number N of Bragg gratings 11, 12,..., 1N in one transmission path 1 may be equal or different if the paths 1 are different. Or may be different. It is also possible for the grating-specific Bragg wavelengths λ 1, λ 2,..., Λ N for different paths to be the same or different.

The receiver 3 is configured to detect grating-specific Bragg wavelengths λ 1, λ 2,... Λ N received simultaneously from the light input section 30, and the grating-specific Bragg wavelengths are time-varying specific whole. When the wavelengths λ01, λ02, ..., λ0N are transmitted simultaneously in the associated transmission path 1, they are actually simultaneously reflected from the Bragg gratings 11, 12, ... 1N of the path 1.

The receiver 3 also comprises a device 31 in which the grating-specific Bragg wavelengths λ 1, λ 2,... Λ N reflected in the device are separated according to frequency, the Bragg wavelengths being different frequencies. It is represented by (ω1, ω2, ... ωN) and reliably coded by the frequencies (ω1, ω2, ... ωN). The separation according to the frequency of the coded wavelengths [lambda] 1, [lambda] 2, ... [lambda] N can be achieved by conventional means, for example a frequency filter.

Each of the separately coded wavelengths λ 1, λ 2, ... λ N can be phase detected separately in a conventional manner, whereby the value of a particular physical quantity X, to which the coded wavelength is assigned, is the Bragg grating. It can be determined at the position of. By comparing the constant reference value of the quantity X with the value of a particular physical quantity X, the variation ΔX of the variable X can be determined at the position of the grating. The individual phase detection device may for example be an N channel phase detector, which is part of the device 31 in which the wavelengths λ 1, λ 2,...

In order to determine the reference value, the control signal 2 or the light transmission path of the light source 2 may be of different grating-specific Bragg wavelengths, for example one or more of two or more Bragg wavelengths of the M transmission paths 1 of FIG. It may include a grid.

The control signal for determining the reference value of the light source 2 may be, for example, an N-channel phase reference signal 300, which is for example sent from the N-channel phase detector to the device 31. For example, to the control device 21 for controlling the light source 2.

When the sensor is used for temperature measurement, the light transmission path, including the Bragg grating, used to determine the reference value, can be temperature balanced in the light source. When measuring tensile stress or dilatation force, a light transmission path parallel to the path is arranged, including for example a Bragg grating per light transmission path including the Bragg grating of the sensor, the light transmission path being used to determine a reference value, No mechanical load This enables the detection of the lattice-specific Bragg wavelength shift caused by the separation of the expansion force and temperature and the expansion force.

Each light input 30 may preferably be a wideband input with a bandwidth, which bandwidth is the reflected grating-specific total Bragg wavelength λ 1, represented by a different frequency (ω 1, ω 2,..., Or ω N). λ2, ... λN). The wideband input unit 30 may be a photo detector, for example, a photo diode.

Claims (4)

A Bragg grating sensor for measuring at least one constant physical quantity X, At least one light transmission path 1 for transmitting the light beam P; At least two optical Bragg gratings 11, 12, ... N arranged side by side in the transmission path 1 with different grating-specific Bragg wavelengths λ 1, λ 2,. , The grating-specific Bragg wavelengths λ 1, λ 2, ... or λ N of each of the gratings 11, 12, or 1N act on the gratings 11, 12, or 1N. Fluctuates according to the variation ΔX of the physical quantity X, and if it is included in the light beam transmitted in the transmission path 1, it is reflected only by the grating 11, 12, ... or 1N; A ray source 2, which is a grid of at least the lattice 11, 12, ... or 1N for each Bragg lattice 11, 12, ... or 1N. Within a range of wavelengths Δλ 01, Δλ 02, ... or Δλ 0N which lie near the unique Bragg wavelengths λ 1, λ, ... λN and are assigned only to the gratings 11, 12, ..., or 1N. For generating a light beam P having a specific light wavelength λ01, λ02, ... or λ0N, which is assigned only to the gratings 11, 12, ..., or 1N, which vary individually in time For simultaneously combining all the time varying constant wavelengths λ 01, λ 02,... Or λ 0N into transmission path 1 for the purpose of transmitting simultaneously to (1); A receiver 3 for receiving and detecting all Bragg wavelengths [lambda] 1, [lambda] 2, ... [lambda] N, wherein the wavelengths [lambda] 1, [lambda] 2, ... [lambda] N are all time-varying specific wavelengths [lambda] 01, A sensor characterized in that it is actually simultaneously reflected by Bragg gratings (11, 12, ... 1N) when transmitting [lambda] 02, ... [lambda] 0N simultaneously in the transmission path (1). The method of claim 1, The light source 2 is adapted to generate a light beam P0 having a continuous bandwidth Δλ0 which is generated by it and includes all of the constant light wavelengths λ01, λ02, ... λ0N that vary individually in time. Individually adjustable with wideband source 22 and grating-specific Bragg wavelengths defining the specific wavelengths λ01, λ02, ... λ0N for each particular wavelength (λ01, λ02, ... λ0N) to be generated Bragg gratings 221, 222, ... or 22N, wherein the broadband light beam P0 is simultaneously transmitted to all of the Bragg gratings 221, 222, ... or 22N and all gratings 221, 222, ... or 22N is characterized in that it reflects its grating-specific Bragg wavelength and is simultaneously transmitted as a specific wavelength (λ01, λ02, ... λ0N) to be generated by the light source (2). The method according to claim 1 or 2, At least two different specific light wavelengths λ01, λ02, ... λ0N are generated by the light source 2 and coupled to the at least one light transmission path 1 at different frequencies ω1, ω2,... sensor with a change in time in light beam P of .ωN). The method of claim 3, wherein The receiver 3 is a wideband light having a bandwidth Δλ including the reflected grating-specific Bragg wavelengths λ 1, λ 2, ... λ N represented at different frequencies ω 1, ω 2, ... ω N. And an input (30) and a device (31) for separating the reflected grating-specific Bragg wavelengths (λ1, λ2, ... λN) according to frequency.