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JP2000111361

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DESCRIPTION JP2000111361
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
optical fiber sensor for measuring depth, temperature and the like, and more particularly to an
optical fiber sensor for measuring a direct current signal.
[0002]
2. Description of the Related Art Conventionally, piezoelectric sensors have been used to measure
depth and temperature. On the other hand, an optical fiber sensor is used for a hydrophone
(acoustic sensor) or a seismometer (acceleration sensor). As a modulation / demodulation method
of these optical fiber sensors, as disclosed in Japanese Patent Laid-Open No. 7-140044, laser
light frequency-modulated into a sine waveform is input to an optical fiber interferometer
(hereinafter referred to as PGC homodyne method), and interference The atan method or the like
is used in which the amplitude ratio of the first and second harmonic components of the laser
modulation frequency is determined from light and the phase difference is calculated from the
inverse tangent of the amplitude ratio.
[0003]
However, since the conventional optical fiber sensor measures the maximum amount of change
in the optical path difference of the interferometer when the interferometer receives the
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measurement signal, that is, the amplitude of the optical path difference vibration. Although the
relative change amount could be determined, the absolute value could not be determined. For
this reason, although there is a demand for using an optical fiber sensor also in measurement
which obtains absolute values such as depth and temperature, it has not been realized.
[0004]
A light source for outputting frequency-modulated laser light, an optical fiber interferometer to
which laser light output from the light source is input, and O / E conversion of the output of the
optical fiber interferometer O / E conversion means for converting into electrical signals,
harmonic component extraction means for extracting two or more harmonic components from
the electrical signal output from the O / E conversion means, harmonics output from the
harmonic component extraction means An amplitude ratio calculating unit that calculates an
amplitude ratio of wave components, and an optical path difference calculating unit that
calculates an optical path difference of the optical fiber interferometer from the amplitude ratio
output from the amplitude ratio calculating unit.
[0005]
DESCRIPTION OF THE PREFERRED EMBODIMENTS << First Embodiment >> <Configuration> FIG.
2 is a block diagram of a first embodiment of the present invention.
A light source 11-1 that outputs laser light to which frequency modulation of the PGC homodyne
system is added, a sensor unit 12 to which an optical signal is input from the light source 11-1,
and an O / E conversion to which an optical signal is input from the sensor unit 12 , Harmonic
component extraction means 15 for extracting the m (m: positive integer) order harmonic
component of the laser modulation frequency to which an electric signal is inputted from the O /
E converter 13, harmonic component extraction means Amplitude ratio calculating means 17 for
calculating the amplitude ratio of the harmonic components output from 15; modulation index
calculating means 18 for calculating the modulation index C from the amplitude ratio output
from the amplitude ratio calculating means 17; modulation index calculation The optical path
difference calculating means 19 calculates the optical path difference DL from the modulation
index C output from the means 18.
[0006]
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<Operation> The light source 11-1 outputs laser light frequency-modulated to the modulation
frequency f0 = w0 / 2p and the maximum frequency deviation fc. When the laser beam output
from the light source 11-1 is input to the sensor unit 12 having the optical path difference ΔL,
the interference light I represented by the equation (1) is output.
[0007]
I = A + B cos (Ccosw0t + f) (1) C = 2 p fcnDL / c (2) where n: effective refractive index of the core
of the optical fiber, c: light velocity in vacuum, f: inside the sensor unit 12 The phase difference
between the existing sensing arm and the reference arm, and A and B are constants depending
on the amplitude of the laser light and the like. When the first Bessel function is used for the
cosine part of the second term, the equation (1) can be expressed by the equation (3).
[0008]
I = A + BJ 0 (C) cosf + S {2 BJ 2 m -1 (C) sinf · cos (2 m -1) w 0 t + 2 BJ 2 m (C) cos f · cos 2 m w 0
t} At this time, the sensor unit is a Mach-Zehnder type, Michelson type, etc. It is assumed that it is
composed of a two-beam interferometer.
[0009]
The O / E converter 13 converts the interference light I output from the sensor unit 12 into an
electrical signal.
[0010]
The harmonic component extraction unit 15 extracts the first harmonic component and the third
harmonic component of the laser modulation frequency from the electric signal output from the
O / E converter 13.
The harmonic component extraction means 15 is constituted by known means such as analog
processing by synchronous detection used in DFT or atan method.
By extracting the harmonic component, the amplitude Zm of the m-th harmonic component
represented by the equation (4) is obtained.
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[0011]
Z2m-1 = BJ2m-1 (C) sinfZ2m = BJ2m (C) cosf (4) In the amplitude ratio calculating means 17, the
amplitude Z1 of the first harmonic component output from the harmonic component extracting
means 15 and the third harmonic The amplitude ratio X1, 3 of the component to the amplitude
Z3 is calculated based on the equation (5).
[0012]
Xm, m + 2 = Jm (C) / Jm + 2 (C) (5) The amplitude ratio X is not limited to only 1st and 3rd, and
is even between odd harmonics including sinf or even harmonics including cosf The same result
can be obtained by calculating the amplitude ratio of.
However, if a higher order Bessel function is used, the sensitivity, the dynamic range, etc. will be
degraded.
[0013]
The modulation index calculation means 18 calculates the modulation index C from the
amplitude ratio X1, 3 output from the amplitude ratio calculation means 17. The modulation
index C is calculated by a known method. For example, the modulation index C is calculated by a
conversion function or a conversion table corresponding to the inverse function of the equation
(5).
[0014]
The optical path difference calculating means 19 calculates the optical path difference DL = Cc /
2pfcn from the modulation index C output from the modulation index calculating means 18 using
the equation (2).
[0015]
When applied to a depth sensor using the determined optical path difference DL, the optical path
difference DL is a function of the displacement Y according to P (sensor weighted pressure
resulting from water pressure) = s (proportional coefficient) × Y (displacement) The load
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pressure P can be calculated from the known proportionality factor (stiffness of the sensing
mechanism part) using the above to obtain the depth.
[0016]
<Effect> According to the optical fiber sensor shown in the first embodiment, the optical path
difference DL independent of the phase difference φ which is a temperature drift term can be
obtained, so that a DC signal such as temperature or depth not affected by the temperature drift
can be obtained. It can measure accurately.
[0017]
Since the measurement of the DC signal can be performed by the optical fiber sensor, the system
can be configured without a power supply, and it is possible to have the effect unique to the
optical fiber sensor that the installation place is not limited.
In addition, the sensor is also excellent in multiplexing.
[0018]
Example 2 <Configuration> FIG. 3 shows a system configuration of Example 2 of the present
invention.
The same reference numerals are given to the same components as in the first example and the
description is omitted.
In addition to the frequency modulation for PGC, the frequency is a pilot signal for envelope
detection (D = 2 p fp-c n DL / c> p / 2) (modulation frequency fp-0 <DFT frequency resolution,
frequency deviation fp-c) A light source 11-2 for outputting modulated laser light, and a
harmonic component output from the harmonic component extraction means 15 is accumulated
for a period of 1/2 fp-0, and a drop is output for the maximum value of the harmonic component
It comprises line detection means 16.
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[0019]
<Operation> The electric field vector e (t) of the laser beam output from the light source 11-2 is
expressed by the equation (6) because the frequency modulation for the pilot signal is further
added.
[0020]
e (t) = e0 cos (2 pnt + fc / f0 cos 2 pf 0 t + fp-c / fp-0 cos 2 p fp-0 t + y) (6) The laser light output
from the light source 11-2 is sent to the sensor unit 12 having the optical path difference ΔL
The interference light I obtained upon input is expressed by equation (7).
[0021]
I = A + Bcos (Ccosw0t + Dcoswp-0t + f) (7) When the first Bessel function is used for the cosine
part of the second term of the equation (7), the interference light I can be expressed by the
equation (8) it can.
[0022]
I = A + BJ 0 (C) cosf + S {2 BJ 2 n -1 (C) sin (D cos wp-0 t + f) cos (2 n -1) w 0 t + 2 BJ 2 n (C) cos
(D co ws p-0 t + f) cos 2 nw 0 t} (8 From the equation (8), the amplitude Zm of the m-th harmonic
component of the laser modulation frequency f0 is expressed by the equation (9).
[0023]
Z2 n-1 = BJ 2 n-1 (C) sin (D cos wp-0 t + f) Z 2 n = BJ 2 n (C) cos (D cos wp-0 t + f) (9) Similar to
the first example, output from O / E converter 13 From the electric signal, the harmonic
component extraction means 15 extracts the first harmonic component and the second harmonic
component of the laser modulation frequency.
[0024]
The envelope detection means 16 respectively accumulates the first harmonic component and
the second harmonic component outputted from the harmonic component extraction means 15
for one period 1/2 fp-0 of the pilot signal, and the maximum value of each is accumulated.
Output.
As a result, since the terms of sin (D cosw p−0t + f) and cos (D cos wp−0t + f) of the equation
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(9) become 1, they can be expressed by the equation (10).
[0025]
(Zn) max = BJn (C) (10) The amplitude ratio calculating means 17 calculates the amplitude ratio
X between the maximum values of the first harmonic component and the second harmonic
component.
[0026]
Therefore, the amplitude ratio X12 to the first harmonic component and the second harmonic
component is X12 = J1 (C) / J2 (C) according to the equation (5).
[0027]
Hereinafter, the optical path difference DL is determined in the same manner as in the first
example.
[0028]
<Effect> According to the optical fiber sensor shown in the second embodiment, the optical path
difference DL independent of the phase difference φ which is the temperature drift term can be
obtained as in the first embodiment, so that the temperature and temperature DC signals such as
depth can be measured with high accuracy.
[0029]
Furthermore, since it has an advantage that it is theoretically not affected by the phase difference
f, it is possible to perform measurement with a stable sensitivity at all times without performing
special processing.
[0030]
EXAMPLE 3 <Configuration> FIG. 4 shows a system configuration of Example 3 of the present
invention.
The same components as those in the specific example 1 or the specific example 2 will be
assigned the same reference numerals and explanation thereof will be omitted.
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[0031]
<Operation> If the second half term of the equation (9) is further expanded by the first Bessel
function, the harmonic component I of the interference light I can be expressed by the equation
(12).
[0032]
I (kf0 ± lfp-0) = I (k, l) = BJk (C) Jl (D) · sinf (k + 1: odd) · cosf (k + 1: even) (12)
Here, k and l are harmonic orders with respect to modulation frequency f0 and fp-0, respectively.
Therefore, the amplitude ratio of the harmonic components when k + 1 is an odd number or the
harmonic components when k + 1 is an even number is obtained.
For example, if the amplitude ratio is taken between the modulation frequency f0 of (k, l) = (1, 0)
and the modulation frequency 2f0-fp-0 of (k, l) = (2, 1), then Z (1, 1) 0) / Z (2, 1) = J1 (C) / J2 (C) ·
J0 (D) / J1 (D) (13).
Here, since there is a known relationship of D = fp-c / fc × C = aC (0 <a <1), the modulation index
C is calculated from this amplitude ratio using an appropriate conversion function or conversion
table, According to the equation (2), it is converted to a DC optical path difference DL.
[0033]
<Effect> According to the optical fiber sensor shown in the third embodiment, it is possible to
adjust the response by the modulation index C of each harmonic component by selecting fp-c / fc
= a (0 <a <1). it can.
Since the response by the modulation index C can be adjusted, there is an effect that the degree
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of freedom in setting the sensor sensitivity and the optical path difference of the interferometer
is increased.
[0034]
EXAMPLE 4 <Configuration> FIG. 5 shows a system configuration of Example 4 of the present
invention.
The same reference numerals are given to the same components as in the specific example 1 or
the specific example 2 or the specific example 3, and the description will be omitted.
[0035]
Two light sources 11-3 and 11-4 outputting laser beams different in frequency shift, and a
multiplexing coupler 41 multiplexing optical signals output from the light sources 11-3 and 11-4
The sensor unit 12 to which the optical signal output from the wave coupler 41 is input, the
demultiplexing coupler 42 that demultiplexes the optical signal output from the sensor unit 12,
and the optical signal output from the demultiplexing coupler 42 O / E converter 13 which
performs E conversion, and harmonic component extraction means 15 which extracts m (m:
positive integer) order harmonic component of the laser modulation frequency to which an
electric signal is input from O / E converter 13 And amplitude ratio calculating means 17 for
calculating the amplitude ratio of the harmonic components output from the harmonic
component extracting means 15, and modulation index calculation for calculating the modulation
index C from the amplitude ratio output from the amplitude ratio calculating means 17 Means 18
and modulation index And an optical path difference calculation means 19 for calculating the
optical path difference DL from the modulation index C output from the detection means 18.
Here, since the optical signal output from the demultiplexing coupler 42 is divided into two, the
O / E converter 13 is provided with one set of optical path difference calculation means 19 for
each optical signal, and the optical path difference is provided. Calculate DL respectively.
[0036]
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<Operation> The two light sources 11-3 and 11-4 output laser beams having different frequency
shifts.
The optical signals output from the light source 11-3 and the light source 11-4 are multiplexed
by the multiplexing coupler 41 and input to the sensor unit 12, and are demultiplexed into
optical signals of different frequency shifts by the demultiplexing coupler 42.
The demultiplexed optical signals are input to the O / E converter 13 to calculate an optical path
difference.
[0037]
As apparent from the equation (4), when f = kp / 2 (k is an integer), either the sinf component or
the cosf component becomes zero, which causes a fading phenomenon that can not be measured.
Since each following first type Bessel function behaves as shown in FIG. 6, if a light source with a
frequency shift (∝ modulation index C) multiplied by a fixed factor is multiplexed in advance,
either sinf component or cosf component Demodulation is always possible. The harmonic
component extraction unit 15 extracts the first to sixth harmonic components. In the amplitude
ratio calculating means 17, among the first to sixth harmonic components output from the
harmonic component extracting means 15, the amplitudes of odd-order harmonic components
having a large signal amount or even-order harmonic components are selected. Calculate the
ratio. As described above, the amplitudes of the respective harmonic components are respectively
given by Bessel functions, and the Bessel functions of the first kind of each order behave as
shown in FIG. Where the value of the Bessel function J is small, the S / N ratio is bad and
sufficient accuracy can not be obtained. A large range of the signal amount, that is, a range where
the value of the Bessel function J is sufficiently large is shown in FIG. 6 in the modulation index C
≒ 3 in the first and third order, the modulation index C ≒ 4 in the second and fourth order, and
the third order / In the fifth order, the modulation index C is around 5.5, and the modulation
index C is in the measurement range of about one. Therefore, by performing processing of
extracting the one with a large value of the Bessel function J among these harmonic components
and calculating the modulation index C, the modulation index C can be determined continuously
and accurately over a wide range, Therefore, the optical path difference DL can be measured with
high accuracy.
[0038]
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In the fourth example, the multiplexing system of two light sources having different frequency
shifts has been described, but the same effect can be obtained by increasing the number to three
or more light sources. At this time, when the crosstalk between the light sources and the
difference in the drift term are sufficiently small, the effect of the two light source system or
more can be expected.
[0039]
Also, although multiplexing is performed by wavelength division, the same effect can be obtained
by replacing with other multiplexing methods such as time division.
[0040]
Also, the optical path difference calculation means 19 from the O / E converter 13 is provided as
one set, and one set is provided for each optical signal, but one optical signal demultiplexed by
the demultiplexing coupler 42 is delayed By making it possible, one set can be shared.
[0041]
<Effect> According to the optical fiber sensor shown in the fourth embodiment, by multiplexing
the light sources, demodulation can always be performed using either the sinf component or the
cosf component, so that occurrence of fading can be avoided. The effect is that the measurement
range is improved.
In addition, by calculating the amplitude ratio using the odd-order harmonic component with a
large signal amount, it is possible to obtain the modulation index C continuously and accurately
over a wide range, and therefore the optical path difference DL is measured with high accuracy.
The effect of being able to
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