CN202177573U - Optical fiber refractive index sensor based on Michelson interferometer - Google Patents

Abstract

The utility model discloses an optical fiber refractive index sensor based on a Michelson interferometer. The optical fiber refractive index sensor comprises a wideband light source (1), an optical fiber coupler (2), a measurement sensing head (3), a reference sensing head (4), a spectrometer (5) and a computer (6), wherein the wideband light source (1) is connected to a first input port of the optical fiber coupler (2); a first output port of the optical fiber coupler is connected to the measurement sensing head (3) through an optical fiber; a second output port is connected to the reference sensing head (4) through another optical fiber; a second input port is connected with the spectrometer (5); light reflected by both the reference sensing head and the measurement sensing head is interfered when passing through the coupler and then transmitted to the spectrometer (5); the spectrometer (5) is in communication connection with a computer (6); and the measurement sensing head (3) and the reference sensing head (4) are ordinary tail optical fibers of which the end surfaces are perpendicular to the axes of the optical fibers. The optical fiber refractive index sensor can achieve high-precision and wide-range refractive index measurement, has a simple structure and is convenient to operate.

Description

Optical fibre refractivity sensor based on Michelson interferometer

Technical field

The utility model relates to a kind of index sensor, relates in particular to a kind of optical fibre refractivity sensor based on Michelson interferometer.

Background technology

Refractive index is a very important parameters at ambits such as physics, biology, chemistry, and it is accurately measured in significant and purposes of related industries department such as chemical industry, medicine, food.Therefore, the method for many refractive Index Measurement is arisen at the historic moment.Conventional measurement has glancing incidence method, diffraction grating method, laser irradiation and CCD mensuration, also has measuring methods such as wide band absorption spectroscopic methodology, titrimetry and photoluminescence quenching, but these methods of great majority all are confined to visible-range.Subsequently, some new technologies such as optoacoustic, spr sensor and Raman spectrum occur in succession, but these methods cost an arm and a leg and be not easy to operate.Index meter based on LPFG has higher sensitivity, but because LPFG is influenced greatly by the bending of self, reliability receives influence greatly.Based on the index meter of Fabry-Perot-type cavity, though have the little advantage of volume, make complicated and cost an arm and a leg, be subject to the influence of light source stability.Based on the refractive index sensing method of Fiber Bragg Grating FBG, generally to remove fibre cladding, to increase the interaction of fast travelling waves of optical fibre (evanescent field) and refractive index substance to be measured; Cause that the fiber grating bragg wavelength moves the measurement that realizes refractive index; The shortcoming of this method is that stability reduces because the covering of optical fiber is removed, and the intensity that can bear weakens; Range of application is limited, and cost rises simultaneously.

The utility model content

The purpose of the utility model is to overcome the above-mentioned deficiency that prior art exists, and the optical fibre refractivity sensor based on Michelson interferometer is provided, and concrete technical scheme is following.

A kind of optical fibre refractivity sensor based on Michelson interferometer comprises wideband light source, fiber coupler, measurement sensing head, with reference to sensing head, spectrometer and computing machine; The first input end mouth of said fiber coupler is connected through optical fiber with wideband light source; First output port is connected through optical fiber with the measurement sensing head; Second output port be connected through optical fiber with reference to sensing head, second input port is connected through optical fiber with the spectrometer input end; Interfere the time with reference to sensing head and the light of measuring the sensing head reflection, be transferred to spectrometer then through fiber coupler.

In the above-mentioned optical fibre refractivity sensor based on Michelson interferometer, also comprise output data that is used for the receiving spectrum appearance and the computing machine that calculates refractive index, the spectrometer output terminal is connected with said compunication.

The above-mentioned optical fibre refractivity sensor based on Michelson interferometer is measured the sensing head general single mode fiber of not removing covering vertical with shaft axis of optic fibre be end face with reference to sensing head.

In the above-mentioned optical fibre refractivity sensor based on Michelson interferometer, the splitting ratio of fiber coupler is 50%～50%.

In the above-mentioned optical fibre refractivity sensor based on Michelson interferometer, described wideband light source is the fiber broadband light source of C-band (1520nm-1570nm), and said Transmission Fibers is general single mode fiber.

In the above-mentioned optical fibre refractivity sensor based on Michelson interferometer, computing machine with the rule that the test substance variations in refractive index changes, calculates the test substance refractive index according to the contrast of interference fringe.

The utility model compared with prior art has following advantage:

(1) sensor of the utility model can be eliminated different losses cause in the inner light path of instability and the sensor-based system branch of light source measuring error well, reduce external environment and change the influence that measurement is brought, and has improved measuring accuracy.

(2) to have solution concentration low more for the sensor of the utility model, the advantage that resolution is high more.

(3) sensor construction of the utility model is simple, and cost is low, need not do special processings such as removing covering to optical fiber, and is easy to operate.

(4) sensor of the utility model also can be used for trace, dangerous liquid detection except being used for general liquid detection.In addition, the utility model also can be used for the industrial processes that refractive index changes and monitors in real time.

This sensor can be realized high precision, refractometry on a large scale, and is simple in structure, easy to operate.

Description of drawings

Fig. 1 is based on the optical fibre refractivity sensor construction synoptic diagram of Michelson interferometer.

Fig. 2 a and Fig. 2 b measure sensing head and with reference to the synoptic diagram at sensing head and material interface place.

Fig. 3 is respectively 1%, 10% and at 30% o'clock, the sensor-based system reflectance spectrum of simulation when sugar juice concentration.

Fig. 4 is for the solution of different refractivity, the interference fringe contrast of simulation.

The sensor-based system reflectance spectrum that Fig. 5 a records for experiment when sugar juice concentration is respectively 25.93% and 48.72%; Fig. 5 b is for increasing to 48.72% process from 4.76% when sugar juice concentration, and the interference fringe contrast is along with the experiment and the analog result of variations in refractive index.

Fig. 6 a representes that when sugar juice concentration was respectively 25.93% and 41.18%, the interference fringe contrast over time.Fig. 6 b representes that when sugar juice concentration is respectively 25.93% and 41.18% the interference fringe peak wavelength over time near 1550nm.

Embodiment

Below in conjunction with accompanying drawing further detailed explanation is done in the practical implementation of the utility model, but enforcement of the utility model and protection domain be not limited thereto, to the utility model do essence identical be equal to the protection domain that replacement all belongs to the utility model.

Referring to Fig. 1, based on the optical fibre refractivity sensor of Michelson interferometer comprise wideband light source 1, fiber coupler 2, measure sensing head 3, with reference to sensing head 4, spectrometer 5 and computing machine 6.Wherein, Wideband light source 1 is connected to the first input end mouth of fiber coupler 2; First output port of fiber coupler 2 connects the connection of measurement sensing head 3, the second output ports with reference to sensing head 4, through coupling mechanism the time, interferes and pass through second input port entering spectrometer 5 of coupling mechanism 2 by measurement sensing head 3 with reference to the light of sensing head 4 reflections; 5 communications are connected computing machine 6 with spectrometer, carry out data processing.The concrete measurement is the interference spectrum when being gone out to measure sensing head and being inserted measured matter and reference material respectively with reference to sensing head by spectrometer measurement; Its corresponding data is input to computing machine; Through calculating the contrast that obtains interference fringe, obtain the refractive index of detected solution according to formula (6).

Fig. 2 a and Fig. 2 b Fig. 2 a and Fig. 2 b measure sensing head and with reference to the synoptic diagram at sensing head and material interface place.Measure sensing head and form by the end face general single mode fiber vertical with shaft axis of optic fibre with reference to sensing head; 3,4 is respectively the measurement sensing head shown in Fig. 1 and with reference to sensing head; 7,9 respectively expression measure sensing head and with reference to the interface of sensing head and material, 8,10 represent test substance and reference material respectively.

In utility model, described wideband light source 1 is the C-band (wideband light source of 1520nm～1570nm).The splitting ratio of fiber coupler 2 is 50%～50%.Transmission Fibers is a single-mode fiber.

When measuring, in (like the solvent of solution to be measured) in the sensing head insertion reference material, measure sensing head and insert in the test substance (like solution).The interference fringe contrast is following with the principle that the residing test substance variations in refractive index of measurement sensing head changes:

According to the Fresnel reflection law, ignore loss, then measure sensing head and be respectively with reference to the reflective light intensity of sensing head:

$I_{3R}\left(\mathrm{\λ}\right)=\frac{1}{2}{\left(\frac{n_f-n_x}{n_f+n_x}\right)}^2{I}_0\left(\mathrm{\λ}\right)---\left(1\right)$

$I_{4R}\left(\mathrm{\λ}\right)=\frac{1}{2}{\left(\frac{n_f-n_{x0}}{n_f+n_{x0}}\right)}^2{I}_0\left(\mathrm{\λ}\right)---\left(2\right)$

I wherein ₀(λ) be the output intensity of wideband light source, n _fBe the effective refractive index of fiber core, n _xAnd n _X0It is respectively the refractive index of test substance and reference material.

By formula (1) and (2),, can obtain measuring sensing head and be with reference to the light intensity of reflected light when coupling mechanism interferes of sensing head according to interference theory:

$I_2\left(\mathrm{\λ}\right)=\frac{1}{2}{I}_{3R}\left(\mathrm{\λ}\right)+\frac{1}{2}{I}_{4R}\left(\mathrm{\λ}\right)+2\sqrt{\frac{1}{2}{I}_{3R}\left(\mathrm{\λ}\right)\frac{1}{2}{I}_{4R}\left(\mathrm{\λ}\right)}\cos \mathrm{\Δ\φ}---\left(3\right)$

Wherein Δ φ is the phase differential of light beam (measure sensing head folded light beam and with reference to the sensing head folded light beam), has

$\mathrm{\Δ\φ}=2\frac{2\mathrm{\π}}{\mathrm{\λ}}{n}_f\mathrm{\Δl}---\left(4\right)$

Δ l is the arm length difference of Michelson interferometer.Normalized interference spectum R (λ) can be expressed as:

$R\left(\mathrm{\λ}\right)=I_2\left(\mathrm{\λ}\right)/I_0\left(\mathrm{\λ}\right)=$

$\frac{1}{4}[{\left(\frac{n_f-n_{x0}}{n_f+n_{x0}}\right)}^2+{\left(\frac{n_f-n_x}{n_f+n_x}\right)}^2+2\sqrt{{\left(\frac{n_f-n_{x0}}{n_f+n_{x0}}\right)}^2{\left(\frac{n_f-n_x}{n_f+n_x}\right)}^2}\cos \mathrm{\Δ\φ}]---\left(5\right)$

According to following formula, the contrast V that can obtain interference fringe does

$V=10\log 10\left[\frac{I_{2\max }}{I_{2\min }}\right]=10\log 10{\left[\frac{n_f^2-n_{x0}{n}_x}{n_f(n_{x0}-n_x)}\right]}^2---\left(6\right)$

Formula (6) shows; The contrast of interference fringe and fibre core, relevant with reference to the refractive index of solvent and solution; Because fibre core and can be through consulting acquisition, so, promptly can obtain the refractive index of required measurement solution through measuring the contrast of interference fringe with reference to the refractive index of solvent.

According to equation (5), we have calculated the reflectance spectrum of variable concentrations sugar juice, and are as shown in Figure 3.Wherein, Michelson interferometer arm length difference Δ l and fiber core refractive index n _f1.8mm and 1.44961 respectively is 1.3119 with reference to the refractive index of solvent pure water, and the corresponding respectively sugar juice mass percent of curve A, B and C is 1%, 10% and 30%.As can beappreciated from fig. 3, each reflectance spectrum all has good interference fringe, and fringe contrast increases (promptly increasing along with the decline of solution refractive index) along with the reduction of solution concentration.

Fig. 4 has represented that interference fringe contrast V is along with the solution refractive index n _xVariation.As shown in the figure, work as n _x＞n _X0The time interference fringe contrast V along with the solution refractive index n _xIncrease and reduce, work as n _x＜n _X0The time V along with refractive index n _xIncrease and increase.Work as n _x=n _X0The time, this moment, fringe contrast was maximum, that is to say with solvent material as a reference, test substance refractive index n _xMore near the reference material refractive index n _X0, fringe contrast is big more, and promptly the resolution of sensor increases with the reduction of solution concentration.

For further checking the feasibility of the utility model, the spy carries out following experiment:

Experiment 1:

In experiment; Use the reflected light spectrogram of the fiber sensor measuring different refractivity solution of the utility model; Shown in Fig. 5 a, the corresponding respectively optical fiber sensor head of curve A and B is positioned over the reflectance spectrum of 25.93% mass percent sucrose solution and 48.72% mass percent sucrose solution among the figure.Wherein, the length of Michelson interferometer two arms is about 40cm, and arm length difference is 1.8mm.Can find out that from Fig. 5 a the increase that the interference fringe contrast is put the solution refractive index with optical fiber sensor head reduces (ie in solution concentration is high more, and contrast is more little).

Table 1 is same liquid (sucrose solution) the change of refractive relation of interference fringe contrast and different quality percent concentration.

Table 1

Sucrose solution concentration (WT%)	Corresponding refractive index	Contrast/dB
			4.76	1.3190	13.30
9.09	1.3254	12.72
			13.04	1.3313	12.31
16.67	1.3367	11.99
			20.00	1.3416	11.71
23.08	1.3462	11.46
			25.93	1.3505	11.20
28.57	1.3543	10.93
			31.03	1.3580	10.63
33.33	1.3614	10.33
			35.48	1.3646	9.99
37.50	1.3676	9.62
			39.39	1.3704	9.24
41.18	1.3730	8.83
			42.86	1.3755	8.41
44.44	1.3779	7.97
			45.95	1.3801	7.51
47.37	1.3822	7.04
			48.72	1.3842	6.56

Fig. 5 b uses data result and the analog result of the Fibre Optical Sensor of the utility model to the refractometry of variable concentrations sucrose solution.Curve A and B are respectively experimental result and theoretical modeling result among the figure.Can find out that from Fig. 5 b although the experiment measuring value is littler than corresponding theoretical modeling value, its variation tendency is consistent.The difference of its numerical values recited mainly is the reason because of two aspects, the one, and only elliptically polarized light rather than linearly polarized light that wideband light source sends, the 2nd, the degeneration of the polarization state that fiber coupler etc. cause.

Experiment 2:

In order further to verify the feasibility of this sensor, we have carried out the stability experiment of this sensor.In this experiment, the fringe contrast of the fiber sensor measuring different refractivity solution reflectance spectrum of application the utility model and reflection peak wavelength are over time, and be as shown in table 2.Wherein, sensor parameters is with experiment 1, and sensing head is positioned over respectively in 25.93% mass percent sucrose solution (solution 1) and the 48.72% mass percent sucrose solution (solution 2).In experiment, observe the reflection peak wavelength change in time through measuring from the nearest peak value of 1550nm.

Table 2

Fig. 6 a concerns for the reflectance spectrum fringe contrast of using this fiber sensor measuring over time.Wherein, The reflectance spectrum fringe contrast that the corresponding respectively optical fiber sensor head of curve A and B is positioned over 25.93% mass percent sucrose solution and 48.72% mass percent sucrose solution over time, the fluctuation in time of interference fringe contrast is respectively ± 1.8% and ± 2.8%.

Fig. 6 b concerns for the reflectance spectrum peak wavelength of using this fiber sensor measuring over time.Wherein, the corresponding respectively optical fiber sensor head of curve A and B be positioned over 25.93% mass percent sucrose solution and 48.72% mass percent sucrose solution reflectance spectrum peak value wavelength over time.Can find out that the interference fringe peak wavelength is random variation in time, its main cause is that vibrations and the temperature fluctuation in the experimental situation interfered the influence than optical path difference to Michelson interferometer.

Can know that from top experiment although reflectance spectrum peak value wavelength in time and random variation, its contrast and peak wavelength have nothing to do, and change very for a short time in time, explain that promptly this sensor stability is feasible.

Claims (6)

1. based on the optical fibre refractivity sensor of Michelson interferometer, it is characterized in that comprising wideband light source (1), fiber coupler (2), measure sensing head (3), with reference to sensing head (4), spectrometer (5) and computing machine (6); The first input end mouth of said fiber coupler (2) is connected through optical fiber with wideband light source (1); First output port is connected through optical fiber with measurement sensing head (3); Second output port be connected through optical fiber with reference to sensing head (4), second input port is connected through optical fiber with spectrometer (5) input end; Reach spectrometer (5) after interfering the time with reference to sensing head and the light of measuring the sensing head reflection through fiber coupler.

2. the optical fibre refractivity sensor based on Michelson interferometer according to claim 1, the output terminal that it is characterized in that spectrometer (5) and the output data that is used for the receiving spectrum appearance and said computing machine (6) communication of calculating refractive index are connected.

3. the optical fibre refractivity sensor based on Michelson interferometer according to claim 1 is characterized in that described measurement sensing head (3) and is end face and the perpendicular ordinary optic fibre tail optical fiber of not removing covering of axis with reference to sensing head (4).

4. the optical fibre refractivity sensor based on Michelson interferometer as claimed in claim 1 is characterized in that: the splitting ratio of fiber coupler is 50%～50%.

5. the optical fibre refractivity sensor based on Michelson interferometer as claimed in claim 1 is characterized in that described wideband light source is the wideband light source of C-band.

6. like each described optical fibre refractivity sensor of claim 1～5, it is characterized in that employed optical fiber is general single mode fiber based on Michelson interferometer.

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