JP2008046046A - Fbg sensor system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an FBG (Fiber Bragg Grating) sensor system capable of accurately measuring the reflected wavelength of FBG, even when measurement is accelerated, using a wavelength variable light source which can be wavelength swept at a high speed, by correcting a wavelength shift in the measurement of the reflected wavelength of the FBG caused due to the lengths of fibers and the sweep cycle (sweep rate) of the wavelength variable light source. <P>SOLUTION: The reflected wavelengths of the FBG 13a-13c are measured as temporary reflected wavelengths λ<SB>T</SB>from an electrical signal b, output from a light receiver 14 and an electrical signal c with the information on the oscillation wavelength of the wavelength variable light source 10. A wavelength correction value λ<SB>C</SB>, which has previously been stored in a memory 16c, for correcting the wavelength shift in the measurement of the reflected wavelengths of the FBG 13a-13c, due to the wavelengths of the fibers 12a-12c and the sweep rate of the wavelength variable light source 10, is read based on the measured temporary reflected wavelengths. The temporary reflected wavelength λ<SB>T</SB>is corrected by the read wavelength correction value λ<SB>C</SB>and the reflected wavelength λ<SB>F</SB>of the FBG 13a-13c is determined. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention combines a tunable light source capable of high-speed wavelength sweep and a light receiver with an FBG (fiber Bragg grating) provided on a remote measurement object via a fiber to measure strain or temperature of the measurement object. In particular, the wavelength shift at the time of measurement of the reflected wavelength caused by the length of the fiber and the sweep period (sweep speed) of the wavelength tunable light source (shift of the reflection spectrum of the FBG in the wavelength sweep direction) It is related with the FBG sensor system which correct | amended.

  The FBG is obtained by periodically changing the refractive index of the core portion within a predetermined length range of the fiber at regular intervals. When light is incident on one end side of the FBG, a specific wavelength (referred to as a Bragg wavelength) is included in the incident light. ) Is reflected, and light of other wavelengths is transmitted. This Bragg wavelength changes according to axial distortion (compression, expansion) received by a portion whose refractive index periodically changes at regular intervals. Therefore, the strain applied to the FBG can be measured by measuring the wavelength of light that is incident upon and reflected from one end of the FBG (reflection wavelength) or the wavelength of light that is transmitted.

  Conventionally, as an FBG sensor system that performs strain measurement, temperature measurement, and the like using such properties of the FBG, there is one that measures the reflected wavelength of the FBG by combining a wavelength variable light source and a light receiver. (For example, see Patent Document 1). That is, the wavelength tunable light source emits light having changed wavelength (measurement light) incident on the FBG, and the light receiver detects the light intensity of the reflected light from the FBG with respect to the light emitted to the FBG. In this case, assuming a display, the wavelength of the measurement light (oscillation wavelength of the variable wavelength light source) is plotted on the horizontal axis, and the light intensity of the reflected light detected by the light receiver is plotted on the vertical axis, and the spectrum of the reflected light from the FBG is plotted. I'm drawing. Accordingly, the reflection wavelength of the FBG is assumed to be the same as the wavelength of the measurement light emitted from the wavelength tunable light source to the FBG when the light detector detects the peak value of the spectrum (reflection spectrum) of the reflected light from the FBG. Measured.

JP 2005-62138 A

  In such a conventional FBG sensor system, the reflection wavelength of the FBG is measured by measuring the delay time τ between the measurement light and the reflected light due to the fiber length L between the wavelength variable light source and the FBG, that is, the measurement light emitted from the wavelength variable light source is FBG. It is assumed that the time τ from the time when the light is reflected and the time when the light is incident on the light receiver as the reflected light is sufficiently small and negligible with respect to the wavelength variable time (sweep speed) of the wavelength variable light source. The delay time τ is expressed by the equation (1) where n is the refractive index of the fiber and C is the speed of light.

      τ = 2nL / C (1)

  Here, the relationship between the delay time τ and the sweep speed of the wavelength tunable light source will be specifically described. In the case where the sweep of the wavelength tunable light source is almost linear, the oscillation wavelength λ is expressed by the following equation (2), where K is a constant proportional to the sweep speed and t is time.

λ = λ ₀ + K · t (2)

In the equation (2), when t = 0, the wavelength λ _F corresponding to the reflected wavelength of the FBG is oscillated by the wavelength variable light source and emitted to the FBG, and after τ = 2 nL / C time, the reflected light is emitted from the FBG. Assuming that the light is incident on the light receiver, the reflection wavelength of the FBG is apparently expressed as the provisional reflection wavelength λ _T as shown in the equation (3).

λ _T = λ _F + K · 2 nL / C (3)

Therefore, as can be seen from the equation (3), when K is very small (the sweep speed is very slow) and the fiber length L is very short, it can be regarded as the equation (4).

λ _T = λ _F (4)

When using an external resonance type wavelength tunable light source that variably (sweeps) the oscillation wavelength of a semiconductor laser by driving a diffraction grating, mirror, or the like as a wavelength tunable light source, the sweep speed is low. There was no problem even if the reflection wavelength of FBG was measured by such a method. However, in order to increase the sweep speed of the wavelength tunable light source as much as possible in order to increase the measurement speed in the FBG sensor system, the delay time τ = 2 nL / C as shown in the above equation (3). The wavelength of the reflection spectrum is shifted due to the influence. That is, the reflection spectrum shifts in the wavelength sweep direction. As a result, there arises a problem that the original reflection wavelength λ _F of the FBG is apparently measured as a provisional reflection wavelength λ _{T including} a wavelength shift.

Specifically, for example, K = 10 ⁵ nm / s (when 1520 to 1570 nm is swept in 0.5 ms), L = 10 km, n = 1.5, C = 3 × 10 ⁵ km / s. In this case, K · 2nL / C in the above equation (3) is 10 nm, and an apparent wavelength shift of 10 nm occurs with respect to the reflection wavelength λ _F of the original FBG. That is, as shown in FIG. 6B, when the FBG reflection wavelength is measured during a period in which the wavelength variable light source is swept from the short wave to the long wave, the FBG is shifted to the long wave side by 10 nm. On the other hand, in the case where measurement is performed during a period in which the wavelength variable light source is swept from a long wave to a short wave, the wavelength shifts to the short wave side by 10 nm.

The present invention is to measure the reflected wavelength of the FBG as reflection wavelength lambda _T provisional reflection wavelength lambda _T of the measured the temporary, are previously determined store data length and the wavelength tunable light source of the fiber sweep cycle By correcting with the wavelength correction value λ _C for correcting the wavelength shift at the time of measurement of the reflected wavelength generated due to the (sweep speed), these problems can be solved, and the wavelength variable capable of high-speed wavelength sweep. An object of the present invention is to provide an FBG sensor system that can accurately measure the reflected wavelength of the FBG even when the measurement speed is increased using a light source.

In order to solve the above problems, in the FBG sensor system according to claim 1 of the present invention, light in a predetermined wavelength range is measured on the FBG (13a to 13c) provided in the measurement object via the fiber (12a to 12c). In the FBG sensor system that enters as light and measures the reflected wavelength of the FBG from the reflected or transmitted light of the measurement light, the light that has been swept in a predetermined sweep cycle including the light in the predetermined wavelength range is oscillated. Then, a wavelength variable light source (10) that makes the light incident on the FBG as the measurement light and a reflected or transmitted light from the FBG of the measurement light incident on the FBG are received as a first electric signal. The reflection wavelength of the FBG is set as a temporary reflection wavelength λ _T from the first light receiver (14) to be converted, the first electric signal output from the first light receiver, and the oscillation wavelength of the wavelength tunable light source. Measurement , Based on the reflected wavelength lambda _T of the measured tentative, advance in the memory stored in (16c), of the FBG generated due to said predetermined sweep period of the length of the fiber and the wavelength tunable light source reflection of the FBG which reads the wavelength correction value lambda _C for correcting the wavelength shift of the measurement of the reflection wavelength, read in said wavelength correction value lambda _C by correcting the reflection wavelength lambda _T of the temporary contains no wavelength shift And processing means (20) for determining the wavelength λ _F.

Further, in the FBG sensor system according to claim 2 of the present invention, in the FBG sensor system according to claim 1 described above, the processing means includes the first electric signal output from the first light receiver. The peak value corresponding to the reflected wavelength of the FBG is detected, and the oscillation wavelength of the wavelength tunable light source when light corresponding to the detected peak value is incident on the first light receiver is obtained, and the obtained oscillation The wavelength was set to the provisional reflection wavelength λ _T of the FBG.

  In the FBG sensor system according to claim 3 of the present invention, in the FBG sensor system according to claim 1 or 2, the wavelength tunable light source is a semiconductor laser (1) in which one laser light emitting end face is AR-coated. A collimating lens (2) for collimating the light emitted from the AR-coated end face of the semiconductor laser, and a diffraction grating for receiving the collimated light emitted from the collimating lens and diffracting it at an angle corresponding to the wavelength. 3), a reflector (35) and a reflector driving means (50), and diffracted light with respect to the collimated light incident from the diffraction grating is reflected on the reflection surface of the reflector to the diffraction grating. When it is reflected and diffracted again by the diffraction grating, and the diffracted light obtained thereby is incident on the semiconductor laser through the collimator lens, The angle of the reflecting surface of the reflector is set so that the diffracted light incident on the conductor laser becomes light having a desired wavelength and the desired wavelength is swept back and forth including the predetermined wavelength range. And a MEMS scanner (60) that is repeatedly changed at the predetermined sweep cycle by a reflector driving means.

  Further, in the FBG sensor system according to claim 4 of the present invention, in the FBG sensor system according to claim 3 described above, the reflector of the MEMS scanner has a fixed substrate (36, 37) and predetermined edges from the fixed substrate. A shaft portion (38, 39) that is extended by a predetermined length in width and can be twisted and deformed along the length direction, and is connected to the tip of the shaft portion by its own edge portion, A reflector (40) provided with the reflecting surface for reflecting the diffracted light from the diffraction grating, and the reflector driving means of the MEMS scanner includes a shaft portion of the reflector and a reflector. A force is applied to the reflecting plate by a drive signal having a frequency corresponding to the natural frequency of the portion composed of the plate, and the reflecting plate is reciprocally rotated at the predetermined sweep cycle at the natural frequency or a frequency close thereto. Configured.

  In the FBG sensor system according to claim 5 of the present invention, in the FBG sensor system according to claim 3 or 4, the wavelength tunable light source is further provided on an optical path through which the zero-order light of the diffraction grating is emitted. And an optical resonator (4) that transmits light of a predetermined wavelength, and a second light receiver (5) that receives the transmitted light emitted from the optical resonator and converts it into a second electrical signal. The oscillation wavelength of the wavelength tunable light source can be obtained from the second electric signal output from the second light receiver.

  In the FBG sensor system according to claim 6 of the present invention, in the FBG sensor system according to claim 3 or 4 described above, the wavelength variable light source uses the 0th-order light of the diffraction grating as output light.

  In the FBG sensor system according to claim 7 of the present invention, in the FBG sensor system according to claim 6 described above, the wavelength tunable light source is further provided on an optical path from which the 0th-order light of the diffraction grating is emitted, A light branching means (6) for branching the zero-order light into two and emitting one zero-order light as the output light; and receiving the other zero-order light emitted from the light branching means and receiving a predetermined wavelength And an optical resonator (4) that transmits the light of the second and a second light receiver (5) that receives the transmitted light emitted from the optical resonator and converts it into a second electrical signal, The oscillation wavelength of the wavelength tunable light source can be obtained from the second electric signal output from the light receiver.

In the FBG sensor system according to claim 8 of the present invention, in the FBG sensor system according to claim 5 or 7, the processing means includes the first electrical signal from the first light receiver and the second signal. An A / D converter (15) that receives the second electric signal from the photoreceiver and converts it into a digital value, and a digital value of the first electric signal output from the A / D converter. A first memory (16a) for sequentially storing at a predetermined address, and a second memory (16b) for sequentially storing the digital value of the second electric signal output from the A / D converter at a predetermined address A peak value detecting means (17) for reading a digital value stored at the predetermined address of the first memory and detecting a peak value corresponding to the reflection wavelength of the FBG; and the second mem- ory. The digital value for obtaining the oscillation wavelength of the wavelength tunable light source stored at the predetermined address of the optical resonator is read, the address where the predetermined wavelength of the optical resonator is stored is detected, and the detected Based on the address and the address where the peak value output from the peak value detecting means is stored, the wavelength tunable light source when the light corresponding to the peak value is incident on the first light receiver. A peak value wavelength calculating means (18) for obtaining an oscillation wavelength, setting the obtained oscillation wavelength as the provisional reflection wavelength λ _T of the FBG, and the wavelength correction for correcting the wavelength shift at the time of measuring the reflection wavelength of the FBG. a third memory for storing and holding the wavelength correction value lambda _C (16c), on the basis of the reflection wavelength lambda _T of the temporary output from the peak value wavelength calculating unit, the wavelength correction from said third memory It reads the lambda _C, and a read-out wavelength correction at value lambda _C by correcting the reflection wavelength lambda _T of the temporary seek reflection wavelength lambda _F of the FBG reflection wavelength calculating means (19).

In the FBG sensor system according to the first, second, and eighth aspects of the present invention, the reflection wavelength of the FBG is set as the provisional reflection wavelength λ _T from the first electric signal output from the first light receiver and the oscillation wavelength of the wavelength variable light source. Based on the measured provisional reflection wavelength λ _T , the measurement of the reflection wavelength of the FBG caused by the length of the fiber and the predetermined sweep period of the wavelength variable light source stored in advance in the memory The wavelength correction value λ _C for correcting the wavelength shift of the FBG is read out, and the provisional reflection wavelength λ _T is corrected by the read wavelength correction value λ _C to obtain the reflection wavelength λ _{F of the} FBG. Even when the measurement speed is increased using a wavelength variable light source capable of wavelength sweeping, the reflected wavelength of the FBG can be measured accurately.

  In the FBG sensor system according to claims 3 and 4 of the present invention, the reciprocal sweep of the wavelength tunable light source is performed by the MEMS scanner, so that the high-speed wavelength sweep can be performed and the measurement speed can be increased.

  In the FBG sensor system according to claims 5 and 7 of the present invention, the light of a predetermined wavelength of the zero-order light of the diffraction grating in the wavelength tunable light source is converted into the second electric signal. Thus, the oscillation wavelength of the wavelength tunable light source can be obtained.

  In the FBG sensor system according to the sixth aspect of the present invention, since the 0th order light of the diffraction grating in the wavelength tunable light source is used as the output light, the intensity fluctuation of the output light caused by the influence of the internal resonance mode of the semiconductor laser can be reduced. Specifically, it can be about 1/10 of the intensity fluctuation (about 1 dB) when the light emitted from the end surface of the semiconductor laser not coated with AR is used as the output light.

  The configuration of the FBG sensor system according to the embodiment of the present invention is shown in FIGS. First, as shown in FIG. 2, the wavelength tunable light source 10 capable of high-speed wavelength sweep converts the light emitted from the AR-coated end surface of the semiconductor laser (LD) 1 into collimated light by the collimating lens 2. The diffracted light that enters the diffraction grating 3 and is emitted from the diffraction grating 3 with respect to the incident light enters the MEMS scanner 60. The MEMS scanner 60 includes a reflector 35 and reflector drive means 50, and the diffracted light with respect to the collimated light incident from the diffraction grating 3 is reflected by the reflection surface of the reflector 35 to the diffraction grating 3, and again the diffraction grating. When the diffracted light diffracted by the light beam 3 is incident on the LD 1 via the collimator lens 2, the diffracted light incident on the LD 1 becomes light having a desired wavelength, and the desired wavelength. Is reciprocally changed (reciprocatingly rotated) at a predetermined sweep cycle by the reflector driving means 50 so that the reciprocating sweep is performed including a predetermined wavelength range. With such a configuration, the wavelength-swept light is oscillated and output from the end surface of the LD 1 which is not AR-coated, and becomes output light (measurement light a).

  In FIG. 2, the light emitted from the end surface of the LD 1 that is AR-coated is converted into collimated light by the collimating lens 2 so as to be incident on the diffraction grating 3, but between the LD 1 and the collimating lens 2. A condensing lens and a fiber are provided in the lens, and the light emitted from the end surface of the LD 1 that is AR-coated is condensed by the condensing lens and incident on the fiber. The light passing through the fiber is converted into collimated light by the collimating lens 2. Then, the light may enter the diffraction grating 3.

  The reflector 35 and the reflector driving means 50 constituting the MEMS scanner 60 will be described in detail later. Note that a MEMS (Micro Electro Mechanical Systems) scanner means a scanner formed by a micro electro mechanical structure (a structure that operates mechanically under the control of an electrical signal).

  Here, wavelength sweeping by the MEMS scanner 60 will be described. When the drive signals Da and Db (details will be described later) shown in FIG. 5 are applied to sweep the MEMS scanner 60 back and forth at a predetermined sweep period (the above-described reflector 35 is rotated back and forth), FIG. As shown, the reciprocating rotation angle of the MEMS scanner 60 changes substantially sinusoidally, and as a result, the swept wavelength also changes sinusoidally. Accordingly, as shown in FIG. 6B, the reciprocal sweep of the MEMS scanner 60 is performed so that the predetermined wavelength range (measurement wavelength range) of the measurement light a is close to the straight line of the waveform that changes in a sine wave shape. Is set (range of reciprocating rotation angle). That is, it is set by adjusting the amplitude of the drive signals Da and Db. As a specific example, for example, when the measurement wavelength range is 1520 to 1570 nm, the reciprocal sweep wavelength range (sweep wavelength range) of the MEMS scanner 60 is set to about 1470 to 1620 nm that is sufficiently wide with respect to this measurement wavelength range. Is done. One of the drive signals Da and Db is output to the processing means 20 as a trigger signal Tr.

  In FIG. 2, the zero-order light of the diffraction grating 3 is incident on an optical resonator 4 such as an etalon, and only light of a predetermined wavelength is transmitted. Then, the transmitted light is converted into a second electrical signal c by the light receiver (PD) 5 and output to the processing means 20 shown in FIG. That is, transmitted light having a predetermined wavelength interval, for example, a frequency of 15 GHz corresponding to the wavelength sweep of the output light (measurement light a) is generated and converted into the second electric signal c by the light receiver 5. The wavelength (frequency) of this transmitted light is known. Therefore, the oscillation wavelength of the wavelength tunable light source 10 (the wavelength of the measurement light a swept) can be obtained using the second electric signal c obtained by photoelectrically converting the transmitted light.

  The tunable light source 10 is not limited to the light output from the end surface of the LD 1 that is not AR-coated as shown in FIG. May be output light. In that case, as shown in FIG. 3, the 0th-order light from the diffraction grating 3 is branched by an optical branching means 6 such as an optical coupler, and one is output and the other is incident on the optical resonator 4. When the 0th-order light is output light in this way, the intensity fluctuation of the output light caused by the influence of the internal resonance mode of the LD1 is compared with the case where the light emitted from the end surface of the LD1 that is not AR-coated is output light. Can be small. Specifically, it can be reduced to about 1/10 of the intensity fluctuation (about 1 dB) when LD1 is used as the output light. Further, by applying HR coating (HR: High-Reflection) to the end surface of the LD 1 which is not AR-coated, the light intensity of the output light when the 0th-order light is output light can be increased.

  Next, in FIG. 1, the optical circulator 11 makes the measurement light a from the wavelength tunable light source 10 incident on a plurality of FBGs 13a to 13c having different reflection wavelengths connected in series via the fibers 12a to 12c, and The reflected light of the measurement light “a” reflected and returned from each of the FBGs 13 a to 13 c is received and emitted to the light receiver (PD) 14. The light receiver 14 converts the reflected light into a first electric signal b and outputs it to the processing means 20.

  Based on the first electric signal b from the light receiver 14 and the second electric signal c from the wavelength tunable light source 10, the processing means 20 has no wavelength shift caused by the fiber length of the fibers 12a to 12c. The reflection wavelength of each of the FBGs 13a to 13c in the state is calculated, and is configured by the A / D converter 15, the memories 16a to 16c, the peak value detection means 17, the peak value wavelength calculation means 18, and the reflection wavelength calculation means 19. ing.

  That is, the A / D converter 15 is, for example, a 2ch A / D converter, and starts from the above-described trigger signal Tr input from the wavelength tunable light source 10, and uses an internal clock (for example, 10 MHz) with high frequency accuracy. The first electric signal b and the second electric signal c are sequentially converted into digital values at the same timing.

  The memories 16a and 16b respectively receive the digital value of the first electric signal b and the second electric signal c that are sequentially output from the A / D converter 15 based on an instruction from a control unit (not shown). Among the digital values, at least the wavelength shift generated due to the above-described measurement wavelength range and the above-described fiber length and sweep speed (sweep cycle) shown in FIG. 6B (hereinafter simply referred to as wavelength shift as appropriate). The digital value included in the minute is sequentially stored at a predetermined address. As a specific example, for example, the digital value of the first electric signal b and the digital value of the second electric signal c are both stored in addresses 0 to 4000 of the memories 16a and 16b.

  In the above description, storage in the memories 16a and 16b is performed in accordance with an instruction from a control unit (not shown). However, if the phase of the trigger signal Tr is appropriate in relation to the measurement wavelength range and the wavelength deviation, For example, the digital value of the first electric signal b and the digital value of the second electric signal c output from the A / D converter 15 may be stored as they are.

  The peak value detection means 17 reads the digital value stored in a predetermined address (for example, 0 to 4000 described above) of the first memory 16a by the first read signal M1, and reflects the reflected wavelengths of the FBGs 13a to 13c. Three peak values corresponding to are detected. Then, the information of each address storing the detected three peak values is output to the peak value wavelength calculating means 18.

  The peak value wavelength calculation means 18 uses the second read signal M2 to store the oscillation wavelength (measurement light a of the measurement light a) of the wavelength variable light source 10 stored at a predetermined address (for example, 0 to 4000 described above) in the second memory 16b. A digital value for obtaining (wavelength) is read out, and an address where a predetermined wavelength (known wavelength) of the optical resonator 4 (see FIG. 2) is stored is detected. For example, each address where the peak value of the transmitted light of the known wavelength (frequency) generated at the 15 GHz interval is stored is detected.

Then, the addresses where the three peak values corresponding to the reflection wavelengths of the FBGs 13a to 13c, which are output from the peak value detection means 17, are stored, and the respective addresses where the known wavelengths are stored are compared. Then, the oscillation wavelength of the wavelength tunable light source 10 when light corresponding to each of the three peak values is incident on the first light receiver 14 is obtained, and for each of the obtained three oscillation wavelengths, these are temporarily calculated. a reflection wavelength λ _T.

The memory 16c indicates the relationship between the temporary reflection wavelength λ _T and the wavelength correction value λ _C for correcting the wavelength shift caused by the fiber length and the sweep speed described above for each of the FBGs 13a to 13c. Is obtained and stored in advance. The wavelength correction value λ _C is K · 2nL / C corresponding to the wavelength shift shown in the above equation (3), and the respective delay times from the FBGs 13a to 13c measured in advance with an OTDR (optical pulse tester). It can be obtained by multiplying τ (= 2 nL / C) by a constant K proportional to the sweep speed (which can be calculated from the wavelength sweep characteristic as shown in FIG. 6B of the wavelength tunable light source 10). Or it can obtain | require also using the fiber length L and refractive index n of each fiber 12a-12c as a design value at the time of installation of an FBG sensor system.

Note that the allowable change range of the provisional reflection wavelength λ _T of each of the FBGs 13a to 13c is determined so as not to overlap each other in consideration of the measurement value, the fiber length, the environmental condition change, and the like. Each of the FBGs 13a to 13c can be specified based on the reflection wavelength [lambda] _T , whereby the respective wavelength correction values [lambda] _C for the respective FBGs 13a to 13c can be read from the third memory 16c.

The reflection wavelength calculation unit 19 reads out the wavelength correction value λ _C for each of the FBGs 13 a to 13 c from the third memory 16 c based on the temporary reflection wavelength λ _T output from the peak value wavelength calculation unit 18, and 3) Based on the equation (5) derived from the equation (repost), the provisional reflection wavelength λ _T is corrected with the read wavelength correction value λ _C to obtain the reflection wavelength λ _F of each of the FBGs 13a to 13c.

λ _T = λ _F + K · 2 nL / C (3)
λ _F = λ _T −K · 2 nL / C = λ _T −λ _C (5)

Here, FIG. 7 shows the relationship among the reflection wavelength λ _F , the provisional reflection wavelength λ _T and the wavelength correction value λ _C shown in the above equation (5). It is assumed that the reflection wavelength λ _{F of} each of the FBGs 13a to 13c has a wavelength like the reflection spectrum shown in FIG. When the reflection wavelength λ _F of each of the FBGs 13a to 13c is measured, the reflection wavelength λ _F is apparently shown in FIG. 7B due to the influence of the delay time τ due to the fiber length L of each of the fibers 12a to 12c. In this way, the wavelength is shifted toward the longer wavelength and becomes the provisional reflection wavelength λ _T. Accordingly, the reflection wavelength λ _F can be obtained by subtracting the wavelength correction value λ _C for each of the FBGs 13 a to 13 c shown in FIG. 7B from the provisional reflection wavelength λ _T. Since the fiber length L of each of the fibers 12a to 12c is the same, the wavelength shift of the reflection wavelength λ _F of each of the FBGs 13a to 13c is twice that of the FBG 13b with respect to the wavelength shift of the FBG 13a. The wavelength shift is three times that.

In the reflected wavelength calculation means 19, the wavelength shift in the case where the reflected wavelength of the FBG is measured during the period in which the wavelength tunable light source 10 is swept from the short wave to the long wave (see FIG. 6B). The correction method is shown by the equation (5). However, in the case where the measurement is performed during the period in which the oscillation wavelength λ is swept from the long wave to the short wave as shown by the equation (6), the correction of the wavelength shift is performed. The method is expressed by equation (7). In this case, the relationship between the reflection wavelength λ _F , the provisional reflection wavelength λ _T and the wavelength correction value λ _C is as shown in FIG.

λ = λ ₀ −K · t (6)
λ _F = λ _T + K · 2 nL / C = λ _T + λ _C (7)

Further, in the reflection wavelength calculating means 19, the sweep characteristic in the vicinity of the measurement wavelength range (see FIG. 6B) of the wavelength tunable light source 10 is approximated by a linear function represented by the general formula λ = A ₀ + A ₁ · x. Although the correction method of the wavelength shift in the case where it is possible is shown by the equation (5), the same applies to the case of approximation by a multi-order function represented by the general formula λ = A ₀ + A ₁ · x + A ₂ · x ² +. It goes without saying that the reflection wavelength λ _F can be obtained by applying the above-described method.

Next, the reflector 35 and the reflector driving means 50 of the MEMS scanner 60 constituting a part of the wavelength tunable light source 10 shown in FIG. 2 will be described in detail. As shown in FIG. 4, the reflector 35 includes a pair of fixed substrates 36 and 37 arranged in parallel with each other in a horizontally long rectangle, and the fixed substrate from the center of the long side edge of the pair of fixed substrates 36 and 37. A pair of shaft portions 38 and 39 that extend in a direction perpendicular to 36 and 37 with a predetermined width and length and can be twisted and deformed along the length direction, and one of the long side edges of the horizontally long rectangle. The reflector 40 is connected to the tip of the shaft portion 38 at the center and connected to the tip of the shaft 39 at the center of the other long side edge. Since the central portion of the reflector 40 is supported by the shaft portions 38 and 39 that can be torsionally deformed, the reflector 40 can rotate with respect to the fixed substrates 36 and 37 with the line connecting the shaft portions 38 and 39 as the central axis. it can. Further, the natural frequency f ₀ of the portion composed of the shaft portions 38 and 39 and the reflecting plate 40 is determined by the shape and mass of the reflecting plate 40 itself and the spring constant of the shaft portions 38 and 39.

  A reflective surface 41 for reflecting light is formed on one surface side of the reflective plate 40. The reflecting surface 41 may be formed by mirror-finishing the reflecting plate 40 itself, or may be formed by depositing or bonding a highly reflective film (not shown). The reflector 35 is integrally cut out from a thin semiconductor substrate by etching or the like, and has high conductivity by metal film vapor deposition.

  The support substrate 45 is made of an insulating material, and support bases 45a and 45b projecting forward are formed on the upper and lower portions on one side, and the fixed substrates 36 and 37 of the reflector 35 are formed on the upper and lower sides. Are fixed in contact with the support bases 45a and 45b. In addition, electrode plates 46 and 47 that are opposed to both ends of the reflection plate 40 of the reflector 35 are formed in patterns at both ends of the central portion on the one surface side of the support substrate 45. The electrode plates 46 and 47 constitute a reflector driving means 50 (see FIG. 2) together with a drive signal generator 55 which will be described later, and an electrostatic force is alternately and periodically applied to both ends of the reflector plate 40. Then, the reflecting plate 40 is reciprocally rotated around the line connecting the shaft portions 38 and 39. The rotation axis of the reflecting plate 40 is set to be parallel to the diffraction grooves of the diffraction grating 3 (see FIG. 2). The reflector 35 configured as described above receives the diffracted light from the diffraction grating 3 by the reflection surface 41 of the reflection plate 40, makes the reflected light incident on the diffraction grating 3, and diffracts it again.

On the other hand, the drive signal generator 55 constituting a part of the reflector driving means 50 (see FIG. 2) is an electrode plate based on the potential of the reflector 35 as shown in FIGS. 5 (a) and 5 (b). applied to 46 and 47, has a frequency corresponding to the natural frequency _{f 0} (or a frequency corresponding to the frequency in the vicinity of the natural frequency _{f 0),} the drive signal Da whose phases are shifted from each other by 180 °, the Db Then, an electrostatic force (attraction) is alternately and periodically applied between the electrode plate 46 and one end side of the reflection plate 40 and between the electrode plate 47 and the other end side of the reflection plate 40. Is rotated reciprocally within a predetermined angular range at the natural frequency f ₀ or a frequency in the vicinity thereof. Further, the drive signal generator 55 converts one of the two drive signals Da and Db into the first electric signal b from the light receiver 14 and the second electric signal c from the wavelength variable light source 10 as A / D. A trigger signal Tr for conversion is output to the A / D converter 15 (see FIG. 1). FIG. 5 shows the case where the two drive signals Da and Db are rectangular waves with a duty ratio of 50%, but the duty ratio of both signals may be 50% or less, and the waveform is also a rectangular wave. It is not limited to sine waves, triangular waves, and the like.

In the MEMS scanner 60 (see FIG. 2) configured by the reflector 35 and the reflector driving unit 50, the reflector 35 is provided with a predetermined width and a predetermined length from the pair of fixed substrates 36 and 37 and the edge thereof. The shaft portions 38 and 39 that are extended and twisted along the length of the shaft portions 38 and 39 are connected to the tips of the shaft portions 38 and 39 at their edges, and are symmetrical with respect to the shaft portions 38 and 39. The reflection plate 40 is formed with a reflection surface 41 formed on one side, and has a frequency corresponding to the natural frequency f ₀ of the portion formed by the shaft portions 38 and 39 of the reflector 35 and the reflection plate 40. It empowers reflector 40 by a drive signal, thereby reciprocally rotating the reflective plate 40 at the natural frequency f ₀ or frequencies in the vicinity thereof.

For this reason, the reflector 40 can be reciprocally rotated at a high speed with a small amount of electrical energy, and the center of rotation is inside the reflector 40 (in this case, the central portion). The amount of change in the reflection angle of the incident light on can be increased. The spring constants of the shaft portions 38 and 39 are determined by the length, width, thickness, and material of the shaft portions 38 and 39, and the natural frequency f ₀ depends on the spring constant and the shape, thickness, material, and the like of the reflector 40. By selecting these parameters, the natural frequency f ₀ can be set within a range of several hundreds of Hz to several tens of kHz.

  Therefore, the wavelength tunable light source 10 (see FIG. 2) of the FBG sensor system of the present invention is configured with the MEMS scanner 60 using the reflector 35 and the reflector driving means 50 as described above. The speed can be increased (maximum several 10 kHz).

  In the description of FIG. 4 described above, the reflector 35 is made of a material with high conductivity. However, when the reflector 35 is made of a material with low conductivity, the reflecting surface 41 of the reflector 40 Electrode plates facing the electrode plates 46 and 47 are provided on both sides (or the entire surface) of the opposite surface, and electrode plates are also provided on the back side of the fixed substrates 36 and 37, and the electrode plates are connected by a pattern or the like. . Then, the electrode plate that contacts the electrode plates on the back side of the fixed substrates 36 and 37 is formed on the surface of the support bases 45a and 45b of the support substrate 45, and at least one of them is used as a reference potential line to generate the drive signal described above. What is necessary is just to connect to the device 55.

  Further, the fixed substrates may be formed in a U-shaped frame or a rectangular frame shape by connecting one end side or both ends of the fixed substrates 36 and 37. Moreover, the shape of the reflecting plate 40 is also arbitrary, and may be a circle, an ellipse, an oval, a rhombus, a square, a polygon, or the like in addition to the above-described horizontally long rectangle. Further, in order to reduce air resistance during high-speed reciprocating rotation, a large hole or a large number of small holes may be provided inside the reflector plate 40.

  In the description of FIG. 4 described above, the two electrode plates 46 and 47 facing each other at both ends of the reflection plate 40 of the reflector 35 are provided. However, only one electrode plate (for example, the electrode plate 46) is used for static electricity. Electric power may be applied. Moreover, also about a drive system, you may rotate the reflecting plate 40 reciprocatingly with an electromagnetic force other than the above-mentioned electrostatic force. In this case, for example, a coil is used in place of the electrode plates 46 and 47 described above, a magnetic material or a coil is provided at both ends of the reflection plate 40, and an attractive force due to a magnetic field generated between the coils or between the coil and the magnetic material. And the reflecting plate 40 is reciprocated by the repulsive force.

In addition to the method for providing an electrostatic force or an electromagnetic force described above directly on the reflecting plate 40, the vibration of the natural frequency f ₀ or near the above as well as the entire reflector 35 by the ultrasonic oscillator or the like, the vibration It is also possible to transmit the light to the reflecting plate 40 for reciprocal rotation. In this case, the vibration can be efficiently transmitted to the reflection plate 40 by providing the vibrator on the back side of the support substrate 45 and the support bases 45a and 45b.

  The wavelength tunable light source 10 capable of high-speed wavelength sweep used in the FBG sensor system shown in FIG. 1 is not limited to the wavelength swept using the MEMS scanner shown in FIGS. For example, a wavelength sweep may be performed using a galvanometer and a mirror fixed to the galvanometer instead of the MEMS scanner (for example, “Extended-Cavity Semiconductor Wavelength-Swept Laser for Biomedical Imaging”). (See SHYun, C. Boudoux, MCPierce, JFde Boer, GJTearney, and BEBouma, IEEE Photonic Technology Letters VOL. 16, NO.1, JANUARY 2004). Alternatively, the wavelength may be swept using a polygon mirror instead of the MEMS scanner.

  In the FBG sensor system shown in FIG. 1, the reflected light from each of the FBGs 13 a to 13 c is incident on the light receiver 14, but the transmitted light from each of the FBGs 13 a to 13 c is incident on the light receiver 14. Also good.

The figure which shows the structure of embodiment of this invention Diagram showing the configuration of a wavelength tunable light source The figure which shows another structure of a wavelength variable light source Exploded perspective view for explaining a MEMS scanner Diagram for explaining drive signals The figure for demonstrating the wavelength sweep by a MEMS scanner The figure for demonstrating the correction method of the wavelength shift of a reflected wavelength The figure for demonstrating the correction method of the wavelength shift of a reflected wavelength

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Semiconductor laser (LD), 2 ... Collimating lens, 3 ... Diffraction grating, 4 ... Optical resonator, 5, 14 ... Light receiver (PD), 6 ... Optical branching Means: 10 ... variable wavelength light source, 11 ... optical circulator, 12a-12c ... fiber, 13a-13c ... FBG, 15 ... A / D converter, 16a-16c ... memory , 17... Peak value detection means, 18... Peak value wavelength calculation means, 19... Reflection wavelength calculation means, 20... Processing means, 35. Substrate, 38, 39 ... shaft, 40 ... reflector, 41 ... reflector, 45 ... support substrate, 45a, 45b ... support, 46, 47 ... electrode plate, 50 ... reflector driving means, 55 ... drive signal generator, 60 ... MEMS scanner.

Claims (8)

Light in a predetermined wavelength range is incident as measurement light on the FBG (13a to 13c) provided in the measurement target via the fibers (12a to 12c), and the reflected wavelength of the FBG from the reflected light or transmitted light of the measurement light In the FBG sensor system for measuring
A wavelength tunable light source (10) that oscillates light that has been swept in a predetermined sweep period including light in the predetermined wavelength range, and that makes the light incident on the FBG as the measurement light;
A first light receiver (14) that receives reflected light or transmitted light from the FBG of the measurement light incident on the FBG and converts it into a first electrical signal;
The reflected wavelength of the FBG is measured as a temporary reflected wavelength λ _T from the first electric signal output from the first light receiver and the oscillation wavelength of the wavelength variable light source, and the measured temporary reflected wavelength λ _T The wavelength shift at the time of measuring the reflected wavelength of the FBG caused by the length of the fiber and the predetermined sweep period of the wavelength tunable light source, which is stored in advance in the memory (16c), is corrected based on reading the wavelength correction value lambda _C for, read in said wavelength correction value lambda _C by correcting the reflection wavelength lambda _T of the temporary seek reflection wavelength lambda _F of the FBG containing no wavelength shift processing means (20) An FBG sensor system comprising: The processing means detects a peak value corresponding to the reflected wavelength of the FBG from the first electric signal output from the first light receiver, and light corresponding to the detected peak value is The oscillation wavelength of the wavelength tunable light source when incident on the first light receiver is obtained, and the obtained oscillation wavelength is used as the provisional reflection wavelength λ _T of the FBG. FBG sensor system. The wavelength tunable light source is
A semiconductor laser (1) in which one laser light emitting end face is AR coated;
A collimating lens (2) for collimating light emitted from the AR-coated end face of the semiconductor laser;
A diffraction grating (3) that receives collimated light emitted from the collimating lens and diffracts the collimated light at an angle corresponding to the wavelength;
A diffracted light for the collimated light incident from the diffraction grating is reflected to the diffraction grating by the reflection surface of the reflector, and includes a reflector (35) and a reflector driving means (50). When diffracted light is again diffracted by the diffraction grating and incident on the semiconductor laser through the collimating lens, the diffracted light incident on the semiconductor laser becomes light of a desired wavelength. In addition, the MEMS scanner (60) that repeatedly changes the angle of the reflecting surface of the reflector by the reflector driving means at the predetermined sweep period so that the desired wavelength is reciprocally swept including the predetermined wavelength range. The FBG sensor system according to claim 1 or 2, further comprising: The reflector of the MEMS scanner is:
A fixed substrate (36, 37);
Shaft portions (38, 39) that extend from the edge of the fixed substrate with a predetermined width and have a predetermined length and can be twisted and deformed along the length direction;
A reflection plate (40) formed by being connected to the tip of the shaft portion at its edge and provided with the reflection surface for reflecting the diffracted light from the diffraction grating on one surface side; ,And,
The reflector driving means of the MEMS scanner includes:
A force is applied to the reflecting plate by a driving signal having a frequency corresponding to the natural frequency of the portion composed of the shaft portion and the reflecting plate of the reflector, and the reflecting plate is given the predetermined frequency at or near the natural frequency. The FBG sensor system according to claim 3, wherein the FBG sensor system is configured to reciprocate at a sweep cycle of 5. The wavelength tunable light source further includes:
An optical resonator (4) provided on an optical path from which the zero-order light of the diffraction grating is emitted and transmitting light of a predetermined wavelength;
A second light receiver (5) that receives the transmitted light emitted from the optical resonator and converts it into a second electric signal, and from the second electric signal output from the second light receiver. 5. The FBG sensor system according to claim 3, wherein an oscillation wavelength of the wavelength tunable light source can be obtained. The wavelength tunable light source is
5. The FBG sensor system according to claim 3, wherein the 0th-order light of the diffraction grating is output light. The wavelength tunable light source further includes:
A light branching means (6) provided on an optical path from which the zero-order light of the diffraction grating is emitted, branching the zero-order light into two and emitting one zero-order light as the output light;
An optical resonator (4) for receiving the other zero-order light emitted from the light branching means and transmitting light of a predetermined wavelength;
A second light receiver (5) that receives the transmitted light emitted from the optical resonator and converts the transmitted light into a second electric signal, and from the second electric signal output from the second light receiver. 7. The FBG sensor system according to claim 6, wherein the oscillation wavelength of the wavelength tunable light source can be obtained. The processing means includes
An A / D converter (15) that receives the first electric signal from the first light receiver and the second electric signal from the second light receiver and converts them into digital values, respectively;
A first memory (16a) for sequentially storing a digital value of the first electric signal output from the A / D converter at a predetermined address;
A second memory (16b) for sequentially storing the digital value of the second electric signal output from the A / D converter at a predetermined address;
A peak value detecting means (17) for reading a digital value stored in the predetermined address of the first memory and detecting a peak value corresponding to the reflection wavelength of the FBG;
A digital value for obtaining the oscillation wavelength of the wavelength tunable light source stored at the predetermined address of the second memory is read, and an address where the predetermined wavelength of the optical resonator is stored is detected. Based on the detected address and the address at which the peak value output from the peak value detecting means is stored, the light corresponding to the peak value is incident on the first light receiver. A peak value wavelength calculating means (18) for obtaining an oscillation wavelength of the wavelength tunable light source and setting the obtained oscillation wavelength as the provisional reflection wavelength λ _T of the FBG;
A third memory (16c) for storing and holding the wavelength correction value λ _C for correcting a wavelength shift at the time of measuring the reflected wavelength of the FBG;
Based on the provisional reflection wavelength λ _T output from the peak value wavelength calculation means, the wavelength correction value λ _C is read from the third memory, and the provisional reflection is performed with the read wavelength correction value λ _C. The FBG sensor system according to claim 5 or 7, further comprising a reflection wavelength calculation means (19) for correcting the wavelength λ _T to obtain the reflection wavelength λ _F of the FBG.

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