CN102944918B - Faraday rotation mirror structure - Google Patents

Abstract

The invention discloses a structure of a Faraday rotating mirror, which comprises a coaxially arranged optical fiber, a collimating lens, a Faraday rotator, a reflective lens and a magnetic ring. The focal plane coincides with the front focal plane of the reflective lens, the rear end surface of the reflective lens is coated with a high reflection film, and the reflective surface is arranged on the back focal plane of the reflective lens, the magnetic ring is sleeved on the outside of the Faraday rotator, and It can generate a fixed electromagnetic field after electrification at both ends. The optical fiber is used to receive light from the outside and emit it to the slope of the light collimating lens through its end face. The collimating lens is used to collimate the light from the optical fiber. The Faraday rotator is used to deflect the parallel light from the collimating lens under the action of the electromagnetic field generated by the magnetic ring to achieve the polarization state of the light. The invention can eliminate the debugging of the reflected light path, and directly carry out mechanical assembly.

Description

A Faraday Rotating Mirror Structure

technical field

The invention belongs to the field of optical fiber sensing, and more specifically relates to a Faraday rotating mirror structure.

Background technique

In recent years, sensors are developing in the direction of sensitivity, precision, adaptability, compactness and intelligence. In this process, fiber optic sensors are favored. Optical fiber sensors are mainly composed of optical fibers and light detectors. Its basic working principle is that the light from the light source passes through the optical fiber, and during the transmission process, it is subjected to external disturbances (such as vibration, pressure, temperature, electric field, magnetic field or acoustic vibration, etc.), resulting in the optical properties of the light (such as intensity, wavelength, frequency, etc.) , phase, polarization state, etc.) changes, called the modulated signal light, these modulated signal light is sent to the photodetector, and after demodulation, the measured parameters are obtained. However, when an optical signal propagates in a single-mode fiber, birefringence is very likely to occur, which will cause a change in the polarization state of the optical signal and cause errors. In order to avoid the above problems, a Faraday rotator is introduced to eliminate the change of the polarization state of the optical signal due to birefringence and keep the polarization state of the optical signal unchanged.

1 is a cross-sectional view of a Faraday rotating mirror used in the prior art. The Faraday rotating mirror 11 includes an optical fiber 12, a C-Lens 13, a Faraday rotator 14, a mirror 15 and a magnet 16. Optical fiber 12 is a single-mode optical fiber, and the end face of the optical fiber (generally inclined at 8° to increase the return loss) is located at the focal point of C-Lens 13. The signal light is input from optical fiber 12, and is collimated and converted into parallel light by C-Lens 13. The parallel light deviates from the main optical axis and has a certain deflection angle. After passing through the Faraday rotator 14, under a certain magnetic field, the polarization state of the light rotates by a certain angle ψ, and is perpendicular to the reflector 15. The reflected light returns to the original path and passes through the Faraday again. The rotator 14 and the C-Lens 13 are output from the optical fiber 12, and the polarization state of the light is rotated by 2ψ at this time. Therefore, the deviation of the polarization state in the optical fiber sensing system can be compensated by using the Faraday rotating mirror 11, so as to realize stable operation of the system. Based on the above working principle, it is necessary to ensure that the mirror surface of the reflector is perpendicular to the collimated parallel light rays, which requires precise optical adjustment, which greatly increases the complexity of the manufacturing process and thus requires a long assembly time.

Contents of the invention

Aiming at the defects of the prior art, the purpose of the present invention is to provide a Faraday rotating mirror structure, which can avoid the debugging of the reflected light path and directly carry out mechanical assembly.

To achieve the above object, the present invention provides a Faraday rotator structure, comprising coaxially arranged optical fiber, collimating lens, Faraday rotator, reflective lens and magnetic ring, the end face of the optical fiber is arranged at the focal point of the collimating lens, The rear focal plane of the collimator lens coincides with the front focal plane of the reflective lens. The rear end surface of the reflective lens is coated with a high reflection film, and the reflective surface is set on the back focal plane of the reflective lens. The magnetic ring is set on the Faraday rotation The outside of the device, and can generate a fixed electromagnetic field after the two ends are energized, the optical fiber is used to receive the light from the outside, and emit it to the slope of the light collimating lens through its end face, and the collimating lens is used to collimate the light from the optical fiber The light is collimated, and the collimated parallel light is sent to the Faraday rotator. The Faraday rotator is used to make the parallel light from the collimating lens realize the light polarization state deflection angle ψ under the action of the electromagnetic field generated by the magnetic ring, and polarize The light after the state deflection is sent to the reflective lens. The reflective lens is used to return the light from the Faraday rotator to the Faraday rotator. The Faraday rotator is also used to make the electromagnetic field generated by the light from the reflective lens Under the action, the light polarization state is deflected by an angle ψ again, and the polarized light is emitted to the collimator lens. On the rear focal plane of the collimator lens, the height r and angle of the parallel light rays collimated by the collimator lens θ is as follows:

其中R为准直透镜的后端面的球面半径，反射式透镜接收到来自法拉第旋转器的光线与反射式透镜返回到法拉第旋转器的光线的关系如下：Where α is the angle between the fiber and the vertical axis, is the angle between the collimating lens and the vertical axis, n _f is the core refractive index of the optical fiber, n _c is the refractive index of the collimating lens, f is the focal length of the collimating lens and

Where R is the spherical radius of the rear end surface of the collimating lens, the relationship between the light received by the reflective lens from the Faraday rotator and the light returned by the reflective lens to the Faraday rotator is as follows:

r ₂ =-r ₁

θ ₂ = -θ ₁

where _r1 and _θ1 are the height and angle, respectively, of the light rays incident on the front focal plane of the reflective lens, and _r2 and _θ2 are the height and angle, respectively, of the light rays reflected back to the front focal plane of the reflective lens.

The collimating lens is a C-Lens lens.

The reflective lens is a C-Lens lens coated with a high reflective film on the back end.

The collimating lens is also used to send the light from the Faraday rotator back to the optical fiber, and the optical fiber is also used to send the light from the collimating lens to the outside.

Through the above technical solutions conceived by the present invention, compared with the prior art, the present invention has the following beneficial effects: the Faraday rotating mirror structure of the present invention ensures collimated parallel light rays by setting the angle between the end face of the optical fiber and the end face of the collimating lens It is coaxial with the collimating lens, and the reflective lens coated with a high-reflection film on the rear end is used instead of the reflector. Assembly, so as to improve the assembly efficiency of devices, is more suitable for mass production, and can be widely used in optical fiber communication and optical fiber sensing systems.

Description of drawings

FIG. 1 is a cross-sectional view of a Faraday rotating mirror structure in the prior art.

Fig. 2 is a cross-sectional view of the Faraday rotating mirror structure of the present invention.

Fig. 3 is a schematic diagram of the change of the polarization state of the light after the light passes through each element of the Faraday rotating mirror.

Fig. 4 is a schematic diagram of the optical path of the collimator lens in the present invention.

Fig. 5 is a schematic diagram of the optical path of the reflective lens in the present invention.

Detailed ways

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention.

As shown in FIG. 2 , the Faraday rotator structure of the present invention includes a coaxially arranged optical fiber 111 , a collimating lens 112 , a Faraday rotator 113 , a reflective lens 114 and a magnetic ring 115 .

The end face of the optical fiber 111 is located at the focal point of the collimator lens 112 .

The rear focal plane of the collimator lens 112 coincides with the front focal plane of the reflective lens 114 .

The back surface of the reflective lens 114 is coated with a high reflection film, and the reflective surface is disposed on the back focal plane of the reflective lens 114 . In this embodiment, the collimating lens 112 is a C-Lens lens, and the reflective lens 114 is a C-Lens lens with a rear end surface coated with a high reflection film.

The magnetic ring 115 is sheathed on the outside of the Faraday rotator 113 and can generate a constant electromagnetic field after the two ends are energized.

The optical fiber 111 is used to receive light from the outside, and transmit it to the slope of the light collimating lens 112 through its end face.

The collimating lens 112 is used to collimate the light from the optical fiber 111 and send the collimated parallel light to the Faraday rotator 113 .

The Faraday rotator 113 is used to make the parallel light from the collimating lens 112 realize the light polarization state deflection angle ψ (in the illustration, 45° is taken as an example) under the action of the electromagnetic field generated by the magnetic ring 115, and deflect the polarization state The light rays are emitted to the reflective lens 114.

The reflective lens 114 is used to return the light from the Faraday rotator 113 back to the Faraday rotator 113 .

The Faraday rotator 113 is also used to make the polarization state of the light from the reflective lens 114 be deflected by an angle ψ again under the action of the electromagnetic field generated by the magnetic ring 115 , and transmit the polarized light to the collimator lens 112 . Due to the Faraday effect, the direction of optical rotation depends on the direction of the applied magnetic field and has nothing to do with the direction of light propagation. At this time, the polarization state of the light is rotated by 2ψ (that is, 90° in this example)

The collimator lens 112 is also used to send the light from the Faraday rotator 113 back to the optical fiber 111 through the original path.

The optical fiber 111 is also used to emit light from the collimator lens 112 to the outside.

The structure of the invention ensures that the collimated parallel light rays are coaxial with the collimating lens, avoids the debugging of the reflected light path, and realizes direct mechanical assembly. The change of the polarization state of the light after passing through the elements of the Faraday rotating mirror of the present invention refers to FIG. 3 . It should be noted that the arrows in this figure only indicate the propagation direction of the optical signal.

As shown in Figure 4, on the rear focal plane of the collimating lens, the height r and angle θ of the collimated parallel rays are as follows:

其中R为准直透镜的后端面的球面半径。Where α is the angle between the fiber and the vertical axis, is the angle between the collimating lens and the vertical axis, n _f is the core refractive index of the optical fiber, n _c is the refractive index of the collimating lens, f is the focal length of the collimating lens and

Where R is the spherical radius of the rear end surface of the collimating lens.

可将准直光线的离轴消除，保证准直的平行光线与准直透镜同轴。It can be seen from the above formula that when α and angle satisfy

The off-axis of the collimated light can be eliminated to ensure that the collimated parallel light is coaxial with the collimating lens.

为5°，这样不会影响准直器的回波损耗，且保证准直的平行光线与准直透镜同轴，而光线偏角亦减小很多。In this embodiment, the material of the collimating lens is SF11 type material, its refractive index n _c =1.7447, the optical fiber adopts SMF-28 type optical fiber, and its core refractive index n _f =1.4682, usually the optical fiber head of the optical fiber The angle is 8°, then the angle can be obtained by the above formula

It is 5°, which will not affect the return loss of the collimator, and ensure that the collimated parallel light is coaxial with the collimator lens, and the deflection angle of the light is also greatly reduced.

As shown in Figure 5, the light is incident from the front focal plane of the reflective lens, and the reflection plane is located on the back focal plane, that is, the 2f system. According to the matrix optics calculation, the transmission matrix of the reflective lens is: $\left[\begin{array}{cc}-1& 0\\ 0& -1\end{array}\right].$

According to the above transmission matrix, the relationship between the reflected light and the incident light (specifically, the light received by the reflective lens from the Faraday rotator is the incident light, and the light returned to the Faraday rotator by the reflective lens is the reflected light) is as follows:

r ₂ =-r ₁

θ ₂ = -θ ₁

where _r1 and _θ1 are the height and angle, respectively, of the light rays incident on the front focal plane of the reflective lens, and _r2 and _θ2 are the height and angle, respectively, of the light rays reflected back to the front focal plane of the reflective lens.

It can be shown from the above formula that the reflected light must be parallel to the incident light. If the light is incident on the lens axis, the reflected light will completely coincide with the incident light. Therefore, there is no need for optical path adjustment at all, and mechanical assembly can be performed directly.

On the basis of the above theoretical knowledge, the structure of the Faraday rotating mirror proposed by the present invention properly designs the angle between the end face of the optical fiber and the end face of the collimator lens, which can ensure that the collimated parallel light rays are coaxial with the collimator lens, and the rear end face is coated with high The reflective lens of the reflective film replaces the reflector, and the parallel light enters from the optical axis of the reflective lens. No matter how the incident angle changes, the reflected light can return to the original optical path, which can eliminate the adjustment of the reflected optical path and directly carry out mechanical assembly.

It is easy for those skilled in the art to understand that the above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present invention, All should be included within the protection scope of the present invention.

Claims (4)

1. a faraday rotation mirror structure, is characterized in that,

The optical fiber, collimation lens, Faraday rotator, reflective lens and the magnet ring that comprise coaxial setting;

The end face of described optical fiber is arranged at the focus place of described collimation lens;

The back focal plane of described collimation lens and the front focal plane of described reflective lens overlap;

The rear end face of described reflective lens is coated with highly reflecting films, and the reflecting surface of described reflective lens is arranged at the back focal plane of described reflective lens;

Described magnet ring is the outside that is sheathed on described Faraday rotator, and can after the energising of two ends, produce changeless electromagnetic field;

Described optical fiber is used for receiving from outside light, and by its end face, is transmitted on the inclined-plane of described collimation lens;

Described collimation lens is used for the light from optical fiber is collimated, and the parallel rays after collimation is transmitted into to Faraday rotator;

Faraday rotator is realized light polarization state deflection angle ψ from the parallel rays of described collimation lens for making under the electromagnetic field effect of described magnet ring generation, and the light emission after polarization state deflection is arrived to described reflective lens;

Described reflective lens will be for turning back to described Faraday rotator from the former road of the light of described Faraday rotator;

Described Faraday rotator is also realized polarization state deflection angle ψ again from the light of described reflective lens for making under the electromagnetic field effect of described magnet ring generation, and the light emission after the polarization state polarization is arrived to described collimation lens;

Height r and the angle θ of the parallel rays after described collimation lens collimation are as follows:

The angle that wherein α is described optical fiber and vertical axis,

for the angle of described collimation lens and vertical axis, n _ffor the fiber core refractive index of described optical fiber, n _cfor the refractive index of described collimation lens, the focal length that f is collimation lens and

the spherical radius of the rear end face that wherein R is described collimation lens;

Described reflective lens receives that to turn back to the relation of light of described Faraday rotator from the light of described Faraday rotator and described reflective lens as follows:

r ₂=-r ₁

θ ₂=-θ ₁

R wherein ₁and θ ₁respectively height and the angle of the light of described reflective lens front focal plane place's incident, r ₂and θ ₂respectively height and the angle that reflects back into the light at described reflective lens front focal plane place.

2. faraday rotation mirror structure according to claim 1, is characterized in that, described collimation lens is the C-Lens lens.

3. faraday rotation mirror structure according to claim 1, is characterized in that, the C-Lens lens that described reflective lens is rear end face plating highly reflecting films.

4. faraday rotation mirror structure according to claim 1, is characterized in that,

Described collimation lens will be also for launching back described optical fiber from the former road of the light of described Faraday rotator;

Described optical fiber is also outside for arriving from the light emission of described collimation lens.

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