CN105157875A - Temperature sensor based on Michelson interferometer having optical fiber and air ring cavity structure - Google Patents

Abstract

The invention provides a temperature sensor based on an optical fiber air ring cavity Michelson interferometer (Michelson? interferometer), characterized in that: incident optical fiber (1), air ring cavity structure (2), graphene film (3) and the gold film (4); the two ends of the air ring cavity structure (2) are respectively connected with the incident optical fiber (1) and the graphene film (3); the two ends of the graphene film (3) are respectively connected with the air ring cavity structure ( 2) Connected to the gold film (4); the incident optical fiber (1) and the air ring cavity structure (2) together with the graphene film (3) and the gold film (4) constitute a Michelson interferometer; the invention has high sensitivity and small structure , good flexibility and low cost, and can be applied to various practical projects of high temperature measurement.

Description

A Temperature Sensor Based on Michelson Interferometer of Fiber-optic Air Ring Cavity

technical field

The invention provides a Michelson interferometer temperature sensor based on an optical fiber air ring cavity, which belongs to the technical field of optical fiber sensing.

Background technique

Optical fiber Michelson interferometers are widely used in the field of sensing. The traditional fiber Michelson interferometer makes the He-Ne laser enter the single-mode fiber through the coupling lens, and then splits it into two beams of equal intensity through the fiber coupler. Two beams of light enter the reference arm and the sensing arm respectively for propagation. The light transmitted in the two interference arms is reflected by the mirrors on the end faces of the respective optical fibers and returns to the optical fiber. During this process, the sensing arm is affected by the measured physical quantity, so the light transmitted in the two arms produces an optical path difference. Interference will occur at the other output of the fiber coupler. The output interference signal interferogram is detected by a photodetector. Compared with sensors of other structures such as fiber optic sensors based on Mach-Zehnder interferometers, the fiber optic sensors based on Michelson interferometers have a particularly significant advantage in that the sensors measure reflection spectra. This feature makes it more convenient to apply to real life. In recent years, a new type of all-fiber Michelson interferometer has been proposed. By introducing some fiber processing technologies, the interferometer structure can be realized on one fiber. Compared with the traditional interferometer, the new all-fiber interferometer is simple in structure and easy to use. A femtosecond laser is used in the manufacture of a temperature sensor based on a fiber-optic air ring cavity Michelson interferometer proposed in this patent. Femtosecond lasers are mainly used for precision micro-nano processing, which can achieve ultra-high resolution and ultra-high precision, thereby achieving nanoscale processing and manufacturing. This all-fiber Michelson interferometer has the advantages of high sensitivity, small structure, good flexibility, and low cost.

The air ring cavity structure in the temperature sensor of the Michelson interferometer based on the fiber-optic air ring cavity makes the light transmitted only in the fiber core be transmitted in the form of the core mode and the air ring cavity mode respectively, and the gold film makes the light in the fiber The light transmitted in the core and the air ring cavity is reflected at the end face of the gold film and interferes at the starting end face of the air ring cavity structure. The interference spectrum is displayed on the spectrometer. When the sensor is used to measure the temperature of the external environment, the interference fringes will drift every time the temperature is changed. This is because the air ring cavity mode and the fiber core mode respond differently to temperature changes, which causes the original optical path difference to change. As the optical path difference changes, the interference fringes will change. After a series of temperature changes, a series of corresponding interference fringes can be observed on the spectrometer. In some characteristic positions of the interference fringes, such as interference peaks or interference valleys, it can be seen that the wavelength where the extreme value is located has shifted, and temperature measurement can be realized by monitoring the drift of the interference spectrum.

Contents of the invention

The purpose of the present invention is to provide a temperature sensor based on Michelson interferometer of optical fiber air ring cavity. The device can convert the variation of the temperature to be measured into the wavelength drift of the detection signal. It has the advantages of high sensitivity, small structure, good flexibility and low cost.

The present invention is realized through the following technical solutions:

A temperature sensor based on a Michelson interferometer of an optical fiber air ring cavity, characterized in that: it is composed of an incident optical fiber (1), an air ring cavity structure (2), a graphene film (3) and a gold film (4); the air The two ends of the ring cavity structure (2) are respectively connected with the incident optical fiber (1) and the graphene film (3); the two ends of the graphene film (3) are respectively connected with the air ring cavity structure (2) and the gold film (4) ; The incident optical fiber (1), the air ring cavity structure (2), the graphene film (3) and the gold film (4) together constitute a Michelson interferometer.

The temperature sensor based on the Michelson interferometer of the fiber-optic air ring cavity is characterized in that: the incident optical fiber (1) can be a G.652 single-mode optical fiber, and the length of the incident optical fiber (1) is 20-40cm.

The temperature sensor of a Michelson interferometer based on an optical fiber air ring cavity is characterized in that: the optical fiber used in the air ring cavity structure (2) can be a G.652 single-mode fiber with a length of 50um to 100um. The outer circle radius of the annular cavity structure (2) is 4um-7um, the inner circle radius is 2um-5um, and the radius difference of the concentric circles of the air annular cavity structure (2) is 2um.

The temperature sensor based on the Michelson interferometer of the optical fiber air ring cavity is characterized in that the thickness of the gold film (3) is 100nm.

The working principle of the present invention is: when the light emitted by the broadband light source reaches the air ring cavity through the incident fiber, the core diameter of the incident fiber is thicker than that of the section of fiber with the air ring cavity structure. In this way, the light originally transmitted in the fiber core is divided into two parts, one part of the light will continue to transmit along the fiber core, and the other part of the light will enter the air ring cavity for transmission. The light transmitted in the medium is reflected at the end face of the gold film and interferes at the initial end face of the air ring cavity structure, and the reflection spectrum is displayed by a spectrometer.

The light transmitted between the fiber core and the air ring cavity interferes, which can be obtained by using the known formula:

Where I is the intensity of the interference signal, I _core and I _cavity are the light intensity of the light reflected from the gold film surface to the fiber core and the air ring cavity structure respectively, λ is the wavelength of the incident light, and OPD is the round-trip of the Michelson interferometer optical path difference, is the initial phase of the interference. The initial OPD can be expressed by formula (2):

OPD＝2n _core L(2)

Among them, n _core is the refractive index of the fiber core, and L is the length of the air ring cavity.

Due to the existence of thermo-optic effect and fiber thermal expansion, n _core and L will change with the change of temperature. The formula for the change of optical path difference OPD with temperature change ΔT of Michelson interferometer:

ΔOPD＝OPD(a _TO +a _TE )ΔT(3)

where a _TO and a _TE are the thermo-optic coefficient and the thermal expansion coefficient of the optical fiber silicon medium, respectively. From formula (1) and formula (3), it can be concluded that the temperature sensitivity is

$\frac{{\mathrm{<span\; class="notranslate">\; \&Delta;\&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{\mathrm{<span\; class="notranslate">\; \&Delta;T</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; TE</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

Where λ ₀ is the wavelength at which the intensity is maximum or minimum, and Δλ ₀ is the wavelength shift at λ ₀ . The temperature sensitivity is mainly determined by the thermo-optic effect of the sensor.

When the sensor is used to measure the external temperature, every time the external temperature is changed, the optical path difference between the air ring cavity mode and the fiber core mode changes due to the different response degrees of the air ring cavity mode and the fiber core mode to the temperature, thus The phase difference between the air ring cavity mode and the fiber core mode is changed, and the interference fringe drifts, and the signal to be measured can be restored by monitoring the wavelength drift of the interference spectrum.

The beneficial effects of the present invention are: the temperature sensor is manufactured based on the Michelson interferometer of the air ring cavity structure. The light transmitted in the core is due to the mismatch between the size of the incoming fiber and the core of the air ring cavity, so that part of the light originally transmitted in the incoming fiber core will continue to be transmitted along the burnt core, and the other part of the light will enter the air ring cavity. When reaching the gold membrane, the light transmitted in the air annulus and in the fiber core will reflect at the end face of the gold membrane and interfere at the beginning end face of the air annulus. When changing the ambient temperature of the sensor, the corresponding interference spectrum will drift. The sensor uses a Michelson interferometer, which has better stability, higher sensitivity, and is not easily affected by changes in the external refractive index.

Description of drawings

Fig. 1 is the temperature sensor schematic diagram of the Michelson interferometer based on fiber optic air ring cavity of the present invention;

Fig. 2 is the optical fiber end face schematic diagram before and after the femtosecond laser punching of the present invention;

Detailed ways

Below in conjunction with accompanying drawing and embodiment example, the present invention will be further described:

Referring to accompanying drawing 1, a kind of temperature sensor based on the Michelson interferometer of optical fiber air ring cavity is made up of incident optical fiber (1), air ring cavity structure (2), graphene film (3) and gold film (4); The two ends of the air ring structure (2) are respectively connected with the incident optical fiber (1) and the graphene film (3); the two ends of the graphene film (3) are respectively connected with the air ring structure (2) and the gold film (4) connected; the incident optical fiber (1), the air ring cavity structure (2), the graphene film (3) and the gold film (4) together constitute a Michelson interferometer. The manufacturing process of the air ring cavity structure (2) is as follows: use a femtosecond laser to drill holes on the end face of the optical fiber, adjust the laser spot diameter to about 2um, and drill a circle of small holes along the fiber core in the cladding, each small hole There is a cross in between. At the same time, the intensity of the femtosecond laser and the distance of the focal point are adjusted to control the depth of the small hole. After being processed by a femtosecond laser, an annular air cavity can be formed at the junction of the core and the cladding. The end face of the fiber before and after drilling is shown in Figure 2, where the black area in the center represents the fiber core, and it can be found that the diameter of the fiber core decreases before and after drilling. After the hole is drilled, because the inner surface of the annular air cavity is not smooth enough, HF is used to corrode the optical fiber to make it smooth. The process of plating gold film (3) on one end of the air ring cavity is as follows: because the gold film is coated on the end face of the cavity, it is easy to spray gold into the cavity by spraying gold, so a layer of graphite should be coated on the surface of the gold-plated optical fiber first olefin film, thereby preventing the sprayed gold from entering the cavity.

The working mode of the device of the invention is as follows: one end of the temperature sensor is connected to a broadband light source, and the other end is connected to a spectrometer. When the light emitted by the broadband light source reaches the air ring cavity structure through the incident fiber, the light originally transmitted in the fiber core is divided into two parts, one part enters the air ring cavity, and the other part will continue to transmit along the fiber core. The gold film makes the light transmitted in the fiber core and the air ring cavity reflect at the end face of the gold film and interfere at the starting end face of the air ring cavity structure. When the external temperature of the sensor is changed, the corresponding interference spectrum will drift, and the change of the reflection spectrum of the Michelson interferometer is monitored by a spectrometer, so that the sensitivity of the sensor can be obtained.

Claims (4)

1. the invention provides the temperature sensor of a kind of Michelson interferometer based on optical fiber air ring cavity (Michelsoninterferometer), it is characterized in that: be made up of incident optical (1), air ring cavity configuration (2), graphene film (3) and golden film (4); The two ends of air ring cavity configuration (2) are connected with graphene film (3) with incident optical (1) respectively; Graphene film (3) two ends are connected with golden film (4) with air ring cavity configuration (2) respectively; Incident optical (1) forms Michelson interferometer jointly with air ring cavity configuration (2) and graphene film (3) and golden film (4).

2. the temperature sensor of a kind of Michelson interferometer based on optical fiber air ring cavity according to claim 1, it is characterized in that: incident optical (1) can adopt G.652 single-mode fiber, incident optical (1) length is 20 ~ 40cm.

3. the temperature sensor of a kind of Michelson interferometer based on optical fiber air ring cavity according to claim 1, it is characterized in that: the optical fiber that air ring cavity configuration (2) uses can adopt G.652 single-mode fiber, length is 50um ~ 100um, the ring cavity exradius of air ring cavity configuration (2) is 4um ~ 7um, inner circle radius is 2um ~ 5um, and the concentric circles semidiameter of air ring cavity configuration (2) is 2um.

4. the temperature sensor of a kind of Michelson interferometer based on optical fiber air ring cavity according to claim 1, is characterized in that: golden film (4) thickness is 100nm.