Method and device for on-line detection of salinity of seater

Abstract

The present invention provides a method and device for on-line detection of the salinity of seawater. A sweep frequency synchronous signal controls a sweep frequency laser light source such that the wavelength of a frequency modulation light wave output by the sweep frequency laser light source is a periodic saw-tooth wave signal. The frequency modulation light wave is divided into two beams, respectively transmitted to a refractive index probe and a temperature probe in seawater. The refractive index probe is an interference instrument structure, and the frequency value of an interference light intensity signal fed back by the refractive index probe is related to the refractive index of seawater. The refractive index of seawater is calculated by performing discrete Fourier transformation on the interference light intensity signal. The temperature probe is internally provided with a fiber Bragg grating, and the Bragg wavelength of the reflection spectrum of the temperature probe is related to the temperature of the seawater. The sweep frequency synchronous signal and the reflection light intensity signal of the fiber Bragg grating are subjected to synchronous discrete sampling, and the temperature value of the seawater is calculated according to a grating temperature sensor demodulation algorithm. The salinity value of the detected seawater is obtained by solving an empirical equation according to the obtained refractive index, the temperature value and the average wavelength of the frequency modulation light wave, thereby implementing on-line detection of the salinity of seawater.

Claims

1. A method for on-line detection of the salinity of seawater, comprising the following steps: step 1: controlling a sweep frequency synchronous signal source to ensure a sweep frequency synchronous signal V(t) output by a sweep frequency synchronous signal source is a saw-tooth wave voltage signal with a sweep frequency period T, wherein a specific form of the sweep frequency synchronous signal is as follows: $V\left(t\right)=\{\begin{array}{cc}{V}_0+\frac{2\left(V_P-V_0\right)}{T}\left(t-\mathrm{nT}\right)& \mathrm{nT}t\mathrm{nT}+\frac{T}{2}\\ V_P+\frac{2\left(V_P-V_0\right)}{T}\left(\mathrm{nT}+\frac{T}{2}-t\right)& \mathrm{nT}+\frac{T}{2}t\mathrm{nT}+T\end{array}$ wherein, t is a time variable, n is a nonnegative integer, n=0, 1 2, . . . , V.sub.0 is a start or end time of V(t) in each of the sweep frequency period T, namely a voltage corresponding to t=nT or t=(n+1)T, while V.sub.P is the V(t) in the middle of each of the sweep frequency period, namely a voltage corresponding to $t=\mathrm{nT}+\frac{T}{2},$ V.sub.00, and V.sub.P>V.sub.0; step 2: by an effect of the sweep frequency synchronous signal source, controlling the wavelength (t) of a frequency modulation light wave output by a sweep frequency laser light source to linearly vary with the sweep frequency synchronous signal V(t), wherein the wavelength (t) is also a saw-tooth wave voltage signal with the sweep frequency period T, and a specific form of the wavelength (t) is as follows: $\left(t\right)=\{\begin{array}{cc}{}_0+\frac{2}{T}\left(t-\mathrm{nT}\right)& \mathrm{nT}t\mathrm{nT}+\frac{T}{2}\\ {}_0+\frac{2}{T}\left(\mathrm{nT}+T-t\right)& \mathrm{nT}+\frac{T}{2}t\left(n+1\right)+T\end{array}$ wherein, .sub.0 is the start or end time of the wavelength (t) in each of the sweep frequency period T, namely a wavelength corresponding to t=nT or t=(n+1)T, and .sub.0 is the minimum wavelength in the sweep frequency process, >0, is the sweep frequency variation range of the wavelength, .sub.0+ is the (t) in the middle of each of the sweep frequency period, namely a wavelength corresponding to $t=\mathrm{nT}+\frac{T}{2},$ and .sub.0+ is the maximum wavelength in the sweep frequency process; step 3: dividing the frequency modulation light wave output by the sweep frequency laser light source into two beams of light, and transmitting the two beams of light into a refractive index detection probe and a temperature detection probe that are placed in seawater being tested, by a wave guide, wherein the refractive index detection probe is an interferometer, a sensor arm of the interferometer comprises of samples of the seawater and a first reflecting mirror, a reference arm of the interferometer is mainly comprised of a reference medium of which the refractive index is known and a second reflecting mirror, the sensor arm and the reference arm are equal in length, namely length l, the light entering the refractive index detection probe forms a sensing light and a reference light by the effect of the interferometer, the sensing light and the reference light form an interference light by an interference effect, and the interference light is fed back by the refractive index detection probe; wherein the temperature detection probe comprises a fiber Bragg grating temperature sensor internally, the frequency modulation light wave entering a temperature detection sensor forms the reflection light and a reflection spectrum by the fiber Bragg grating temperature sensor, and the Bragg wavelength of the reflection spectrum is related to the temperature of the seawater, so the reflection light is called a temperature reflection light; step 4: measuring an interference light intensity I(t) and a temperature reflection light intensity G(t) with a photoelectric detector, wherein a whole measurement time is divided into a plurality of measurement time segments equal in length, the length of each time segment is two sweep frequency periods, namely 2T, the interference light intensity I(t) is measured in a first sweep frequency period T of each time segment, and the temperature reflection light intensity G(t) is measured in the second sweep frequency period T of the same time segment; step 5: first defining n=0, and setting the start time of the current time segment to be t=nT; step 6: from the time t=nT, performing a discrete sampling on the interference light intensity I(t) in the first sweep frequency period T of the current time segment in a sampling period T.sub.1 by using an A/D converter to obtain a sampling signal sequence I(m) in the first sweep frequency period T.sub.1, namely from the time t=nT to the time t=(n+1)T, wherein the length of the sequence is L.sub.1, m is a sequence number and is a nonnegative integer, m=0 1 2 . . . L.sub.11; and saving I(m) in a signal processing unit; step 7: from the time t=(n+1)T, performing the discrete sampling on the sweep frequency synchronous signal V(t) and the temperature reflection light intensity G(t) in the second sweep frequency period T of the current time segment in a sampling period T.sub.2 by using the A/D converter to obtain a sweep frequency synchronous signal sequence V(h) and a temperature reflection light intensity sequence G(h) in the second sweep frequency period T, namely from the time t=(n+1)T to the time t=(n+2)T, wherein the lengths of the sequences V(h) and G(h) are equal, namely L.sub.2, h is the sequence number and is a nonnegative integer, h=0 1 2 . . . L.sub.21; and saving V(h) and G(h) in the signal processing unit; step 8: performing FFT (Fast Fourier Transformation) on an interference light intensity sampling signal sequence I(m) in the first sweep frequency period T of the current time segment by the signal processing unit to obtain a spectrum distribution of the interference light intensity I(t) in the current time, and calculating the frequency .sub.s of an alternating current component I.sub.AC(t) of the interference light intensity I(t) in the above mentioned time according to the frequency spectrum distribution; step 9: in accordance with the relation between the frequency .sub.s of the alternating current component I.sub.AC(t) of the interference light intensity and the reflection index n.sub.S of seawater, calculating the refraction index n.sub.S of the samples of the seawater in the first sweep frequency period T, namely from the time t=nT to the time t=(n+1)T, of the current time segment by using the following equation: $n_S=n_R+\frac{{}_{s}T{}_0^2}{8l}$ wherein l represents the lengths of the sensor arm and the reference arm of the refractive index detection probe, and n.sub.R is a refractive index of the reference medium of the probe; step 10: by using the sweep frequency synchronous signal sequence V(h) and the temperature reflection light intensity signal sequence G(h) in the second sweep frequency period T of the current time segment, calculating and obtaining the temperature T.sub.S of the seawater in the second sweep frequency period T, namely from the time t=(n+1)T to the time (n+2)T, according to a fiber Bragg grating temperature sensor demodulation algorithm; step 11: neglecting changes of the refractive index n.sub.S and the temperature T.sub.S of the seawater in each measurement time segment because the refractive index n.sub.S and the temperature T.sub.S of the seawater change relatively slowly, wherein each measurement time segment includes two sweep frequency periods; the reflective indexes n.sub.S of the seawater obtained in the first sweep frequency period T of the time segment, namely from the time t=nT to the time t=(n+1)T, is used as the refractive index n.sub.S of the seawater in the whole time segment, namely from the time t=nT to the time t=(n+2)T; similarly, the temperature T.sub.S of the seawater obtained in the second sweep frequency period T of the time segment, namely from the time t=(n+1)T to the time t=(n+2)T, is used as the temperature T.sub.S of the seawater in the whole time segment, namely from the time t=nT to the time t=(n+2)T; step 12: solving the following empirical equation according to the obtained reflective index n.sub.S and the temperature T.sub.S of the seawater and the average wavelength $\left(\stackrel{\_}{}={}_0+\frac{}{2}\right)$ output by the sweep frequency laser light source in the current time segment: $n_S=n_0+\left(n_1+n_2{T}_S+n_3{T}_S^2\right)S+n_4{T}_S^2+\frac{n_5+n_{6}S+n_7{T}_S}{\stackrel{\_}{}}+\frac{n_8}{{\stackrel{\_}{}}^2}+\frac{n_9}{{\stackrel{\_}{}}^3}$ calculating the salinity S of the seawater in the current time segment, namely from the time t=nT to the time t=(n+2)T, wherein respective coefficients are as follows: n.sub.0=1.31405, n.sub.1=1.77910.sup.4, n.sub.2=1.0510.sup.6, n.sub.3=1.610.sup.8, n.sub.4=2.0210.sup.6, n.sub.5=15.868, n.sub.6=0.01155, n.sub.7=0.00423, n.sub.8=4382, n.sub.9=1.145510.sup.6; step 13: defining n=n+2, updating the start time of the time segment, pointing at the next time segment; and, step 14: repeating steps 6-13 in a circular way, measuring the salinity S of the seawater in any time segment after the start time t=0, namely from the time t=nT to the time t=(n+2)T, thus implementing real-time detection of the salinity of the seawater, wherein n=0 1 2, . . . .

2. The method for on-line detection of the salinity of the seawater according to claim 1, wherein in step 3, the Bragg wavelength of the reflection spectrum of the temperature detection probe varies along with a temperature change of the seawater; and within a range of the temperature change of the seawater, a variation interval of the Bragg wavelength of the temperature detection probe does not exceed the sweep frequency wavelength range of the sweep frequency laser light source, which means that the Bragg wavelength varies between .sub.0 and .sub.0+.

3. The method for on-line detection of the salinity of the seawater according to claim 1, wherein in step 4 the interference light intensity I(t) is: $I\left(t\right)=\{\begin{array}{cc}{I}_S+I_R+2\sqrt{I_S{I}_R}\cos \left[\begin{array}{c}\frac{4l\left(n_S-n_R\right)}{{}_0}\\ \left(1-\frac{2\left(t-\mathrm{nT}\right)}{T{}_0}\right)\end{array}\right]& \mathrm{nT}t\mathrm{nT}+\frac{T}{2}\\ I_S+I_R+2\sqrt{I_S{I}_R}\cos \left[\begin{array}{c}\frac{4l\left(n_S-n_R\right)}{{}_0}\\ \left(1-\frac{2\left(\mathrm{nT}-t\right)}{T{}_0}\right)\end{array}\right]& \mathrm{nT}+\frac{T}{2}t\left(n+1\right)T\end{array}$ wherein, I.sub.S and I.sub.R are respectively a sensing light intensity and a reference light intensity of the interferometer; if the changes of the I.sub.S and I.sub.R in one sweep frequency period T are neglected, I.sub.S and I.sub.R within one sweep frequency period T are regarded as constants, then the interference light intensity I(t) is the sum of a DC component I.sub.DC and an AC component I.sub.AC(t), namely
I(t)=I.sub.DC+I.sub.AC(t), wherein, the DC component I.sub.DC of the interference light intensity I(t) is:
I.sub.DC=I.sub.S+I.sub.R, and the AC component I.sub.AC(t) of the interference light intensity I(t) is: $I_{\mathrm{AC}}\left(t\right)=\{\begin{array}{cc}2\sqrt{I_S{I}_R}\cos \left[\frac{4l\left(n_S-n_R\right)}{{}_0}\left(1-\frac{2\left(t-\mathrm{nT}\right)}{T{}_0}\right)\right]& \mathrm{nT}t\mathrm{nT}+\frac{T}{2}\\ 2\sqrt{I_S{I}_R}\cos \left[\frac{4l\left(n_S-n_R\right)}{{}_0}\left(1-\frac{2\left(\mathrm{nT}-t\right)}{T{}_0}\right)\right]& \mathrm{nT}+\frac{T}{2}t\left(n+1\right)T\end{array};$ in the above equation, .sub.0, , l, and n.sub.R are constants; the refractive index n.sub.S of the seawater usually changes relatively slowly, so n.sub.S is also regarded as a constant within one sweep frequency period T, and then the AC component I.sub.AC(t) is a single-frequency signal, with a frequency .sub.s ${}_s=\frac{8\left(n_S-n_R\right)l}{{}_0}\frac{}{{}_{0}T}.$

4. The method for on-line detection of the salinity of the seawater according to claim 1, wherein in step 6 the sampling period T.sub.1 is required to meet the requirements of a sampling theorem, namely the following condition: $T_1<\frac{2}{2{}_s}=\frac{{}_0^{2}T}{8\left(n_S-n_R\right)l},$ and the length L.sub.1 of the sampling signal sequence I(m) is: $L_1=\frac{T}{T_1}.$

5. The method for on-line detection of the salinity of the seawater according to claim 1, wherein in step 7 the sampling period T.sub.2 is $T_2=\frac{T}{1024}.$

6. The method for on-line detection of the salinity of the seawater according to claim 5, wherein in step 7 the lengths of the sweep frequency synchronous signal sequence V(h) and the temperature reflection light intensity signal sequence G(h) are both L.sub.2, and L.sub.2 is $L_2=\frac{T}{T_2}=1024.$

7. The method for on-line detection of the salinity of the seawater according to claim 1, wherein in step 10 the fiber Bragg grating temperature sensor demodulation algorithm comprises the following steps: first, finding a maximum temperature reflection light intensity G(h_M) and a corresponding sequence number h_M according to the temperature reflection light intensity signal sequence G(h); second, finding a sweep frequency synchronous signal voltage V(h_M) at this moment according to the sequence number h_M corresponding to the maximum temperature reflection light intensity G(h_M); third, finding a Bragg wavelength of the fiber grating temperature sensor corresponding to the maximum temperature reflection light intensity G(h_M) according to the sweep frequency synchronous signal voltage V(h_M) corresponding to the sequence number h_M; and, fourth, according to the characteristic parameters of the fiber Bragg grating temperature sensor, calculating the current seawater temperature T.sub.S on the basis of the maximum temperature reflection light intensity G(h_M).

8. A device for on-line detection of the salinity of seawater for the method for on-line detection of the salinity of the seawater according to claim 1, comprising a water platform, the refractive index detection probe and the temperature detection probe, wherein the water platform is respectively connected with the refractive index detection probe and the temperature detection probe through two fibers; the water platform outputs the frequency modulation light wave of which the wavelength is a periodic saw-tooth wave voltage signal, the frequency modulation light wave is divided into two beams which are respectively transmitted to the refractive index detection probe and the temperature detection probe in seawater; the refractive index detection probe is an interferometer, and the temperature detection probe is internally provided with a fiber Bragg grating.

9. The device for on-line detection of the salinity of the seawater according to claim 8, wherein the water platform comprises a sweep frequency laser light source, a sweep frequency synchronous signal source, a first photoelectric detector, a second photoelectric detector, an A/D converter, a signal processing unit, a control unit, a first 12 fiber coupler, a first fiber circulator and a second fiber circulator; the temperature detection probe comprises a fiber Bragg grating temperature sensor; the sweep frequency laser light source is provided with a fiber interface and an electrical interface; the sweep frequency laser light source is connected with the sweep frequency synchronous signal source through the electrical interface; the sweep frequency laser light source is connected with an arm 1 of the first 12 fiber coupler through the fiber interface; an arm 2 and an arm 3 of the first 12 fiber coupler are respectively connected with an arm 1 of the first fiber circulator and an arm 1 of the second fiber circulator; an arm 2 and an arm 3 of the first fiber circulator are respectively connected with the refractive index detection probe and the first photoelectrical detector; an arm 2 and an arm 3 of the second fiber circulator are respectively connected with the temperature detection probe and the second photoelectric detector; the sweep frequency synchronous signal source, the first photoelectric detector and the second photoelectric detector all are connected with the A/D converter; the A/D converter is connected with the signal processing unit; the signal processing unit is connected with a control unit; and the control unit is also connected with a sweep frequency synchronous signal source.

10. The device for on-line detection of the salinity of the seawater according to claim 8, wherein the refractive index detection probe comprises a second 12 fiber coupler, a first fiber self-focusing lens, a second fiber self-focusing lens, a seawater sample cavity, a reference medium, a first reflecting mirror and a second reflecting mirror, wherein an arm 1 of the second 12 fiber coupler is connected with the arm 2 of the first fiber circulator; an arm 2 and an arm 3 of the second 12 fiber coupler are respectively connected with the first fiber self-focusing lens and the second fiber self-focusing lens; the seawater sample cavity is positioned between the first fiber self-focusing lens and the first reflecting mirror, and the reference medium is positioned between the second fiber self-focusing lens and the second reflecting mirror.

11. A device for on-line detection of the salinity of seawater for the method for on-line detection of the salinity of the seawater according to claim 2, comprising a water platform, the refractive index detection probe and the temperature detection probe, wherein the water platform is respectively connected with the refractive index detection probe and the temperature detection probe through two fibers; the water platform outputs the frequency modulation light wave of which the wavelength is a periodic saw-tooth wave voltage signal, the frequency modulation light wave is divided into two beams which are respectively transmitted to the refractive index detection probe and the temperature detection probe in seawater; the refractive index detection probe is an interferometer, and the temperature detection probe is internally provided with a fiber Bragg grating.

12. A device for on-line detection of the salinity of seawater for the method for on-line detection of the salinity of the seawater according to claim 3, comprising a water platform, the refractive index detection probe and the temperature detection probe, wherein the water platform is respectively connected with the refractive index detection probe and the temperature detection probe through two fibers; the water platform outputs the frequency modulation light wave of which the wavelength is a periodic saw-tooth wave voltage signal, the frequency modulation light wave is divided into two beams which are respectively transmitted to the refractive index detection probe and the temperature detection probe in seawater; the refractive index detection probe is an interferometer, and the temperature detection probe is internally provided with a fiber Bragg grating.

13. A device for on-line detection of the salinity of seawater for the method for on-line detection of the salinity of the seawater according to claim 4, comprising a water platform, the refractive index detection probe and the temperature detection probe, wherein the water platform is respectively connected with the refractive index detection probe and the temperature detection probe through two fibers; the water platform outputs the frequency modulation light wave of which the wavelength is a periodic saw-tooth wave voltage signal, the frequency modulation light wave is divided into two beams which are respectively transmitted to the refractive index detection probe and the temperature detection probe in seawater; the refractive index detection probe is an interferometer, and the temperature detection probe is internally provided with a fiber Bragg grating.

14. A device for on-line detection of the salinity of seawater for the method for on-line detection of the salinity of the seawater according to claim 5, comprising a water platform, the refractive index detection probe and the temperature detection probe, wherein the water platform is respectively connected with the refractive index detection probe and the temperature detection probe through two fibers; the water platform outputs the frequency modulation light wave of which the wavelength is a periodic saw-tooth wave voltage signal, the frequency modulation light wave is divided into two beams which are respectively transmitted to the refractive index detection probe and the temperature detection probe in seawater; the refractive index detection probe is an interferometer, and the temperature detection probe is internally provided with a fiber Bragg grating.

15. A device for on-line detection of the salinity of seawater for the method for on-line detection of the salinity of the seawater according to claim 6, comprising a water platform, the refractive index detection probe and the temperature detection probe, wherein the water platform is respectively connected with the refractive index detection probe and the temperature detection probe through two fibers; the water platform outputs the frequency modulation light wave of which the wavelength is a periodic saw-tooth wave voltage signal, the frequency modulation light wave is divided into two beams which are respectively transmitted to the refractive index detection probe and the temperature detection probe in seawater; the refractive index detection probe is an interferometer, and the temperature detection probe is internally provided with a fiber Bragg grating.

16. A device for on-line detection of the salinity of seawater for the method for on-line detection of the salinity of the seawater according to claim 7, comprising a water platform, the refractive index detection probe and the temperature detection probe, wherein the water platform is respectively connected with the refractive index detection probe and the temperature detection probe through two fibers; the water platform outputs the frequency modulation light wave of which the wavelength is a periodic saw-tooth wave voltage signal, the frequency modulation light wave is divided into two beams which are respectively transmitted to the refractive index detection probe and the temperature detection probe in seawater; the refractive index detection probe is an interferometer, and the temperature detection probe is internally provided with a fiber Bragg grating.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) FIG. 1 is a structural view of a device for on-line detection of the salinity of seawater of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(2) The present invention is described in detail below in conjunction with the attached drawings.

Embodiment 1

(3) A method for on-line detection of the salinity of seawater includes the following steps:

(4) Step 1: Controlling a sweep frequency synchronous signal source to ensure a sweep frequency synchronous signal V(t) output by the sweep frequency synchronous signal source is a saw-tooth wave voltage signal with a sweep frequency period T, where the specific form is as follows:

(5) $V\left(t\right)=\{\begin{array}{cc}{V}_0+\frac{2\left(V_P-V_0\right)}{T}\left(t-\mathrm{nT}\right)& \mathrm{nT}t\mathrm{nT}+\frac{T}{2}\\ V_P+\frac{2\left(V_P-V_0\right)}{T}\left(\mathrm{nT}+\frac{T}{2}-t\right)& \mathrm{nT}+\frac{T}{2}t\mathrm{nT}+T\end{array}$
where t is a time variable, n is a nonnegative integer, n=0 1 2, . . . , V.sub.0 is the start or end time of V(t) in each sweep frequency period T, namely a voltage value corresponding to t=nT or t=(n+1)T, while V.sub.P is the middle time of V(t) in each sweep frequency period, namely a voltage value corresponding to

(6) $t=\mathrm{nT}+\frac{T}{2},$
V.sub.00, and V.sub.P>V.sub.0.

(7) Step 2: By the effect of the sweep frequency synchronous signal source, the wavelength (t) of the frequency modulation light wave output by the sweep frequency laser light source linearly varies with the sweep frequency synchronous signal V(t), where the wavelength (t) is a saw-tooth wave signal with a sweep frequency period T, and the specific form is as follows:

(8) $\left(t\right)=\{\begin{array}{cc}{}_0+\frac{2}{T}\left(t-\mathrm{nT}\right)& \mathrm{nT}t\mathrm{nT}+\frac{T}{2}\\ {}_0+\frac{2}{T}\left(\mathrm{nT}+T-t\right)& \mathrm{nT}+\frac{T}{2}t\left(n+1\right)T\end{array}$
where .sub.0 is the start or end time of the wavelength (t) in each sweep frequency period T, namely a wavelength value corresponding to t=nT or t=(n+1)T, while .sub.0 is the minimum wavelength in the sweep frequency process, >0, is the sweep frequency variation range of the wavelength, .sub.0+ is the middle time of (t) in each sweep frequency period, namely a wavelength value corresponding to

(9) $t=\mathrm{nT}+\frac{T}{2},$
and .sub.0+ is the maximum wavelength in the sweep frequency process.

(10) Step 3: The frequency modulation light wave output by the sweep frequency laser light source is divided into two beams of light, and transmitted the two beams of light into a refractive index detection probe and a temperature detection probe that are placed in the detected seawater, by a wave guide, where the refractive index detection probe is an interference instrument structure, a sensor arm of the interference instrument is mainly comprised of samples of the detected seawater and a first reflecting mirror, a reference arm of the interference instrument is mainly comprised of a reference medium of which the refractive index is known and a second reflecting mirror, the sensor arm and the reference arm are equal in length, namely length l, the light entering the refractive index probe forms sensing light and reference light by the effect of the interference instrument, the sensing light and the reference light form interference light by the interference effect, and the interference light is fed back by the refractive index probe; where the temperature detection probe is internally provided with a fiber Bragg grating temperature sensor, the frequency modulation light wave entering the temperature sensor forms reflection light and a reflection spectrum by the effect of the fiber Bragg grating temperature sensor, and the Bragg wavelength of the reflection spectrum is related to the temperature value of seawater, so the reflection light is called temperature reflection light; where the temperature detection probe is internally provided with a fiber Bragg grating temperature sensor; the Bragg wavelength of the reflection spectrum of the temperature detection probe varies along with the temperature change of the seawater; and within the whole temperature change range of the seawater, the variation range of the Bragg wavelength of the temperature detection probe does not exceed the sweep frequency wavelength range of the sweep frequency laser light source, which means that the Bragg wavelength varies between .sub.0 and .sub.0+.

(11) Step 4: The interference light intensity signal I(t) and a temperature reflection light intensity signal G(t) are measured with a photoelectric detector, where the whole measurement time is divided into a plurality of measurement time segments equal in length, the length of each time segment is two sweep frequency periods, namely 2T, the interference light intensity I(t) is measured in the first sweep frequency period T of each time segment, and the temperature reflection light intensity G(t) is measured in the second sweep frequency period T of the same time segment; where the interference light intensity I(t) is:

(12) $I\left(t\right)=\{\begin{array}{cc}{I}_S+I_R+2\sqrt{I_S{I}_R}\cos \left[\begin{array}{c}\frac{4l\left(n_S-n_R\right)}{{}_0}\\ \left(1-\frac{2\left(t-\mathrm{nT}\right)}{T{}_0}\right)\end{array}\right]& \mathrm{nT}t\mathrm{nT}+\frac{T}{2}\\ I_S+I_R+2\sqrt{I_S{I}_R}\cos \left[\begin{array}{c}\frac{4l\left(n_S-n_R\right)}{{}_0}\\ \left(1-\frac{2\left(\mathrm{nT}-t\right)}{T{}_0}\right)\end{array}\right]& \mathrm{nT}+\frac{T}{2}t\left(n+1\right)T\end{array}$
where I.sub.S and I.sub.R are respectively the sensing light intensity and reference light intensity of the interference instrument; if the changes of the I.sub.S and I.sub.R in one sweep frequency period T are neglected, I.sub.S and I.sub.R within one sweep frequency period T are regarded as constants, and then the interference light intensity I(t) is the sum of a DC component I.sub.DC and an AC component I.sub.AC(t):
I(t)=I.sub.DC+I.sub.AC(t)
where the DC component I.sub.DC of the interference light intensity I(t) is:
I.sub.DC=I.sub.S+I.sub.R
the AC component I.sub.AC(t) of the I.sub.AC(t) is:

(13) $I_{\mathrm{AC}}\left(t\right)=\{\begin{array}{cc}2\sqrt{I_S{I}_R}\cos \left[\frac{4l\left(n_S-n_R\right)}{{}_0}\left(1-\frac{2\left(t-\mathrm{nT}\right)}{T{}_0}\right)\right]& \mathrm{nT}t\mathrm{nT}+\frac{T}{2}\\ 2\sqrt{I_S{I}_R}\cos \left[\frac{4l\left(n_S-n_R\right)}{{}_0}\left(1-\frac{2\left(\mathrm{nT}-t\right)}{T{}_0}\right)\right]& \mathrm{nT}+\frac{T}{2}t\left(n+1\right)T\end{array}$
in the above equation, .sub.0, , l and n.sub.R are constants; the refractive index n.sub.S of the seawater usually changes relatively slowly, so n.sub.S may also be regarded as a constant within one sweep frequency period T, and then the AC component I.sub.AC(t) within one sweep frequency period T is a single-frequency signal, with a frequency value .sub.s:

(14) ${}_s=\frac{8\left(n_S-n_R\right)l}{{}_0}\frac{}{{}_{0}T}.$

(15) Step 5: First, n=0 is defined, and the start time of the current time segment is set to be t=nT.

(16) Step 6: From the time t=nT, discrete sampling is carried out on the interference light intensity I(t) signal in the first sweep frequency period T of the current time segment in a sampling period T.sub.1 by using an A/D converter to obtain a sampling signal sequence I(m) in the first sweep frequency period T.sub.1, namely from the time t=nT to the time t=(n+1)T, wherein the length of the sequence is L.sub.1, m is the sequence number and is a nonnegative integer, m=0, 1, 2 . . . L.sub.11; and I(m) is saved in a signal processing unit; where the sampling period T is required to meet the requirements of a sampling theorem, namely the following condition:

(17) $T_1<\frac{2}{2{}_s}=\frac{{}_0^{2}T}{8\left(n_S-n_R\right)l},$
and the length L.sub.1 of the sampling signal sequence I(m) is:

(18) 0$L_1=\frac{T}{T_1}.$

(19) Step 7: From the time t=(n+1)T, discrete sampling is carried out on the sweep frequency synchronous signal V(t) and temperature reflection light intensity G(t) in the second sweep frequency period T of the current time segment in a sampling period T.sub.2 by using the A/D converter to obtain a sweep frequency synchronous signal sequence V(h) and a temperature reflection light intensity sequence G(h) in the second sweep frequency period T, namely from the time t=(n+1)T to the time t=(n+2)T, wherein the lengths of the sequences V(h) and G(h) are equal, namely L.sub.2, h is the sequence number and is a nonnegative integer, h=0 1 2 . . . L.sub.21; and V(h) and G(h) are saved in the signal processing unit;

(20) where the sampling period T.sub.2 is

(21) $T_2=\frac{T}{1024};$
the lengths of the sweep frequency synchronous signal sequence V(h) and the temperature reflection light intensity signal sequence G(h) are both L.sub.2, and L.sub.2 is

(22) $L_2=\frac{T}{T_2}=1024.$

(23) Step 8: FFT (Fast Fourier Transformation) is carried out on the interference light intensity sampling signal sequence I(m) in the first sweep frequency period T of the current time segment by the signal processing unit to obtain a spectrum distribution of the interference light intensity I(t) in the current time, and the frequency value .sub.s of an alternating current component I.sub.AC(t) of the interference light intensity I(t) in the above mentioned time is calculated according to the frequency spectrum distribution.

(24) Step 9: In accordance with the relation between the frequency value .sub.s of the alternating current component I.sub.AC(t) of the interference light intensity and the index of reflection n.sub.S of seawater, the index of refraction n.sub.S of the samples of the detected seawater in the first sweep frequency period T, namely from the time t=nT to the time t=(n+1)T, of the current time segment is calculated by using the following equation:

(25) $n_S=n_R+\frac{{}_{s}T{}_0^2}{8l}$
where l represents the lengths of the sensor arm and the reference arm of the refractive index detection probe, and n.sub.R is the known refractive index of the reference medium of the probe.

(26) Step 10: By using the sweep frequency synchronous signal sequence V(h) and the temperature reflection light intensity signal sequence G(h) in the second sweep frequency period T of the current time segment, the temperature value T.sub.S of the detected seawater in the second sweep frequency period T, namely from the time t=(n+1)T to the time t=(n+2)T is calculated and obtained according to a fiber Bragg grating temperature sensor demodulation algorithm;

(27) where the fiber Bragg grating temperature sensor demodulation algorithm includes the following steps:

(28) First, finding the maximum temperature reflection light intensity value G(h_M) and the corresponding sequence number h_M according to the temperature reflection light intensity signal sequence G(h);

(29) second, finding the sweep frequency synchronous signal voltage value V(h_M) at this moment according to the sequence number h_M corresponding to the maximum temperature reflection light intensity value G(h_M).

(30) third, finding the Bragg wavelength of the fiber grating temperature sensor corresponding to the maximum temperature reflection light intensity value G(h_M) according to the sweep frequency synchronous signal voltage value V(h_M) corresponding to the sequence number h_M; and,

(31) fourth, according to the characteristic parameters of the fiber Bragg grating temperature sensor, calculating the current seawater temperature value T.sub.S on the basis of the maximum temperature reflection light intensity value G(h_M).

(32) Step 11: Changes of the refractive index n.sub.S and the temperature T.sub.S of the seawater in each measurement time segment can be neglected because the refractive index n.sub.S and the temperature T.sub.S of the seawater change relatively slowly, wherein each measurement time segment includes two sweep frequency periods; the reflective indexes n.sub.S of the seawater samples obtained in the first sweep frequency period T of the time segment, namely from the time t=nT to the time t=(n+1)T, are approximated as the refractive index n.sub.S of the seawater samples in the whole time segment, namely from the time t=nT to the time t=(n+2)T; similarly, the temperature values T.sub.S of the seawater obtained in the second sweep frequency period T of the time segment, namely from the time t=(n+1)T to the time t=(n+2)T, are approximated as the temperature value T.sub.S of the seawater in the whole time segment, namely from the time t=nT to the time t=(n+2)T.

(33) Step 12: The following empirical equation is solved according to the obtained reflective index n.sub.S and the temperature value T.sub.S of the seawater and the average wavelength

(34) $\left(\stackrel{\_}{}={}_0+\frac{}{2}\right)$
output by the sweep frequency laser light source in the current time segment:

(35) $n_S=n_0+\left(n_1+n_2{T}_S+n_3{T}_S^2\right)S+n_4{T}_S^2+\frac{n_5+n_{6}S+n_7{T}_S}{\stackrel{\_}{}}+\frac{n_8}{{\stackrel{\_}{}}^2}+\frac{n_9}{{\stackrel{\_}{}}^3}$
the salinity S of the seawater in the current time segment, namely from the time t=nT to the time t=(n+2)T is calculated, wherein respective coefficients are as follows:
n.sub.0=1.31405, n.sub.1=1.77910.sup.4, n.sub.2=1.0510.sup.6, n.sub.3=1.610.sup.8, n.sub.4=2.0210.sup.6, n.sub.5=15.868, n.sub.6=0.01155, n.sub.7=0.00423, n.sub.8=4382, n.sub.9=1.145510.sup.6.

(36) Step 13: n=n+2 is defined, the start time of the time segment is updated and pointed at the next time segment.

(37) Step 14: Steps 6-13 are repeated in a circular way; the salinity S of the seawater in any time segment after the start time t=0, namely from the time t=nT to the time t=(n+2)T is measured thus implementing real-time detection of the salinity of seawater, wherein n=0 1 2, . . . .

Embodiment 2

(38) A device for on-line detection of the salinity of seawater for the method for on-line detection of the salinity of seawater in Embodiment 1 includes a water platform, a seawater refractive index detection probe and a seawater temperature detection probe. The water platform is respectively connected with the seawater refractive index detection probe and the seawater temperature detection probe through two fibers. The water platform outputs the frequency modulation light wave of which the wavelength is a periodic saw-tooth wave signal. The frequency modulation light wave is divided into two beams which are respectively transmitted to a refractive index probe and a temperature probe in seawater; the refractive index probe is an interference instrument structure, and the temperature probe is internally provided with a fiber Bragg grating.

(39) The water platform includes a sweep frequency laser light source, a sweep frequency synchronous signal source, a first photoelectric detector, a second photoelectric detector, an A/D converter, a signal processing unit, a control unit, a first 12 fiber coupler, a first fiber circulator and a second fiber circulator. The seawater temperature detection probe includes a fiber Bragg grating temperature sensor. The sweep frequency laser light source is provided with a fiber interface and an electrical interface. The sweep frequency laser light source is connected with the sweep frequency synchronous signal source through the electrical interface. The sweep frequency laser light source is connected with an arm 1 of the first 12 fiber coupler through the fiber interface. An arm 2 and an arm 3 of the first 12 fiber coupler are respectively connected with an arm 1 of the first fiber circulator and an arm 1 of the second fiber circulator. An arm 2 and an arm 3 of the first fiber circulator are respectively connected with the refractive index detection probe and the first photoelectrical detector. An arm 2 and an arm 3 of the second fiber circulator are respectively connected with the seawater temperature detection probe and the second photoelectric detector. The sweep frequency synchronous signal source, the first photoelectric detector and the second photoelectric detector all are connected with the A/D converter. The A/D converter is connected with a signal processing unit. The signal processing unit is connected with a control unit. The control unit is also connected with a sweep frequency synchronous signal source.

(40) The refractive index detection probe includes a second 12 fiber coupler, a first fiber self-focusing lens, a second fiber self-focusing lens, a seawater sample cavity, a reference medium, a first reflecting mirror and a second reflecting mirror. An arm 1 of the second 12 fiber coupler is connected with the arm 2 of the first fiber circulator; an arm 2 and an arm 3 of the second 12 fiber coupler are respectively connected with the first fiber self-focusing lens and the second fiber self-focusing lens. The seawater sample cavity is positioned between the first fiber self-focusing lens and the first reflecting mirror. The reference medium is positioned between the second fiber self-focusing lens and the second reflecting mirror.

(41) The above are detailed descriptions of the present invention in conjunction with specific preferable embodiments, but it cannot be regarded that the specific embodiments of the present invention are limited to the above description. For those ordinarily skilled in the art, various simple modifications or replacements can be made on the basis of the concept of the present invention, which shall all fall within the protective scope of the present invention.