CN102590908B - A Fresnel prism and a method of converting the polarization state of terahertz waves - Google Patents

Abstract

The invention relates to a Fresnel prism and a method for converting a polarization state of a terahertz wave. The Fresnel prism is in a bar-type columnar shape and is made of a material which can not generate a dispersion effect on the terahertz wave. The Fresnel prism comprises four arris and two side surfaces which are opposite and parallel to each other. The two side surfaces are squares with same side lengths. Each arris is perpendicular to one edge of the each side surface. An included angle alpha of the each arris and the each side surface satisfies a formula (1), wherein n is a refractive index of the making material of the Fresnel prism to the terahertz wave. A length of the each arris satisfies that the terahertz wave of incident linear polarization which is perpendicular to one side surface can emit from another side surface after generating even numbers of total reflections in the Fresnel prism. According to the invention, the side length a of the side and the minimum length L of each arris can reach that the polarization states of the multi-color linear polarization terahertz waves can be converted into circular polarization simultaneously.

Description

A Fresnel prism and a method of converting the polarization state of terahertz waves

technical field

The invention relates to the technical field of polarization state conversion of terahertz waves, in particular to a Fresnel prism and a method for converting the polarization state of terahertz waves using the Fresnel prism.

Background technique

Terahertz waves have unique properties such as transient, low energy, and coherence, and have great scientific value and broad application prospects in satellite communications, nondestructive testing, and military radar. The terahertz wave whose polarization state is circular polarization is more widely used, especially it provides a special light source for the study of the internal structure of biological macromolecules with a helical structure, and it can also be used for anisotropic materials ( Such as birefringent crystals, biological tissues with fiber orientation, etc.) for polarization-related terahertz wave ultra-broadband spectral research and structural identification. It can be seen that the use of circularly polarized terahertz waves is expected to achieve new major scientific breakthroughs in the interdisciplinary fields of physics and life sciences. However, the existing technology can only generate single-cycle pulsed terahertz waves whose polarization state is linear, and the polarization state must be converted to obtain circularly polarized single-cycle pulsed terahertz waves.

The existing technology uses optical elements such as wave plates to realize the conversion of the polarization state of the terahertz wave. The thickness of the wave plate is designed according to the frequency of the incident ray-polarized terahertz wave, so that the incident ray-polarized terahertz wave can be emitted from the wave plate. Then it becomes a circularly polarized terahertz wave. It can be seen that the existing technology is only suitable for the transformation of the polarization state of the terahertz wave of a single frequency, but for the polychromatic linearly polarized terahertz wave containing multiple frequency components (such as the linearly polarized single-cycle pulsed terahertz wave), the method is It is impossible to simultaneously convert the polarization states of terahertz waves of all frequencies into circular polarization states.

Contents of the invention

The technical problem to be solved by the present invention is to provide a Fresnel prism and a method for converting the polarization state of a terahertz wave, which can simultaneously convert the polarization state of a polychromatic linearly polarized terahertz wave into a circular polarization.

The technical solution of the present invention to solve the above-mentioned technical problems is as follows: a Fresnel prism, the Fresnel prism is strip-shaped and columnar, and is made of a material that does not produce a dispersion effect on terahertz waves;

The Fresnel prism includes four edges and two opposite and parallel sides; wherein,

The two sides are squares with the same side length;

其中的n为该菲涅尔棱镜的制作材料对太赫兹波的折射率；Each of the ribs is perpendicular to a side of each of the sides, and the angle α between each of the ribs and each of the sides satisfies

Wherein n is the refractive index of the Fresnel prism's material to the terahertz wave;

The length of each of the ribs is sufficient to make the linearly polarized terahertz wave incident perpendicular to one of the sides emerge from the other side after an even number of total reflections in the Fresnel prism; each of the sides The side length a of and the minimum length L of each edge satisfy

The beneficial effects of the present invention are: in the present invention, the angle α between each edge and each side of the Fresnel prism satisfies The linearly polarized terahertz wave incident perpendicular to one side can be totally reflected in the Fresnel prism. The length of each edge is such that the linearly polarized terahertz wave incident perpendicular to one side will emerge from the other side after an even number of total reflections in the Fresnel prism. In this way, the two mutual The sum of the additional phases of the vertical electric field components in the Fresnel prism is an odd multiple of (π/2), so the polarization state of the outgoing terahertz wave is converted to circular polarization. Moreover, since the sum of the additional phases is a frequency-independent quantity, the incident linearly polarized terahertz waves of all frequencies are simultaneously converted into circularly polarized states, thereby achieving the purpose of the invention. In addition, the Fresnel prism is made of a material that does not produce a dispersion effect on the terahertz wave, and its two sides are opposite and parallel, and each edge is perpendicular to one side of each side, which ensures that the linear polarization perpendicular to one side is incident The total reflection position of the terahertz wave in the Fresnel prism is on two parallel and opposite interfaces, not on two adjacent interfaces, so the outgoing direction of the terahertz wave can be determined to be perpendicular to On the other hand, this effectively prevents the terahertz wave from changing direction when it exits, so that the total reflection of the additional phase sum of the two mutually perpendicular electric field components contained in the terahertz wave of all frequencies in the Fresnel prism is exactly the same .

On the basis of above-mentioned technical scheme, the present invention can also be improved as follows:

Further, it is made of high-density polyethylene or high-resistance silicon.

Further, the linearly polarized terahertz wave incident perpendicular to one side is a parallel beam, and its beam cross section is circular;

Then the side length of the side is not less than the beam diameter of the terahertz wave.

Further, the linearly polarized terahertz wave incident perpendicularly to one side is a single-period pulsed terahertz wave.

Further, the length of each of the ribs is such that the linearly polarized terahertz wave incident perpendicularly to one of the sides is totally reflected twice in the Fresnel prism and emerges from the other side.

其中的n为该菲涅尔棱镜的制作材料对太赫兹波的折射率；每个所述侧面的边长a与每条所述棱的最小长度L满足

该方法包括：In addition, the present invention also provides a method for converting the polarization state of terahertz waves. The realization of the method is based on the above-mentioned Fresnel prism. The two sides are squares with the same side length, each of the edges is perpendicular to one side of each of the sides, and the angle α between each of the edges and each of the sides satisfies

Where n is the refractive index of the Fresnel prism's material for terahertz waves; the side length a of each side and the minimum length L of each edge satisfy

The method includes:

Step 1: Use any one of the side surfaces of the Fresnel prism as an incident interface, and use the other side as an exit interface; inject a linearly polarized terahertz wave into the Fresnel prism perpendicular to the incident interface Err Prism;

Step 2: The terahertz wave undergoes an even number of total reflections in the Fresnel prism, and the reflection interface of each total reflection is the interface between the medium and the air;

Step 3: The terahertz waves emitted from the exit interface are circularly polarized terahertz waves.

Description of drawings

Fig. 1 is the structural diagram of the Fresnel prism that the present invention proposes;

Fig. 2 is the side view of the Fresnel prism proposed by the present invention;

Fig. 3 is the top view of the Fresnel prism proposed by the present invention;

Fig. 4 is a schematic diagram of an embodiment of terahertz waves propagating in the Fresnel prism proposed by the present invention.

Detailed ways

The principles and features of the present invention are described below in conjunction with the accompanying drawings, and the examples given are only used to explain the present invention, and are not intended to limit the scope of the present invention.

Fig. 1, Fig. 2 and Fig. 3 are respectively the structural diagram, side view and top view of the Fresnel prism proposed by the present invention. A prism is an object surrounded by two intersecting planes, and a Fresnel prism is a kind of prism. In a Fresnel prism, the planes for light incident and exit are called the sides of the Fresnel prism.

As shown in Figure 1-3, the Fresnel prism is a bar-shaped column, which is made of a material that does not produce dispersion for the terahertz wave, preventing the emitted terahertz wave from changing due to the dispersion of the terahertz wave inside it. Towards, the preferred embodiment of the material for the Fresnel prism is high-density polyethylene or high-resistance silicon. These two materials not only have no dispersion effect on the terahertz wave, but also cause very little loss to the terahertz wave. Therefore, the energy density of the outgoing terahertz wave is guaranteed.

The Fresnel prism shown in Figures 1-3 includes four ribs and two opposite and parallel sides. The length of each edge is equal, both are L, and the two sides are squares with the same side length, marked with M and N respectively.

In order to distinguish the two adjacent sides on the side, Figure 1-3 uses a and b to mark the lengths of the two adjacent sides on the side respectively. Since both sides are square and have the same side length, a and b here are in the numerical value above are equal.

In the Fresnel prism provided by the present invention, each rib is perpendicular to one side of each side, as shown in Figures 1-3, any rib is perpendicular to the side with side length b on any side.

其中的n为该菲涅尔棱镜的制作材料对太赫兹波的折射率，如图1-3所示，由于每条棱均与边长为b的边垂直，因而各棱与各侧面的夹角α即为相应棱与边长为a的边的夹角。棱与侧面的夹角α满足

可使垂直于任一侧面入射的太赫兹波在该菲涅尔棱镜内部发生全反射，从而使该太赫兹波所包含的两个相互垂直的电场分量产生附加位相。根据该数量关系，在该菲涅尔棱镜的制作材料选用高密聚乙烯的情况下，这里的n为1.525,则α等于55.7°。In addition, the angle α between each edge and each side satisfies

Among them, n is the refractive index of the Fresnel prism’s material for terahertz waves. As shown in Figure 1-3, since each edge is perpendicular to the side with side length b, the distance between each edge and each side is The angle α is the angle between the corresponding edge and the side whose side length is a. The angle α between the edge and the side satisfies

The terahertz wave incident perpendicular to any side can be totally reflected inside the Fresnel prism, so that the two mutually perpendicular electric field components contained in the terahertz wave can generate an additional phase. According to the quantitative relationship, when the Fresnel prism is made of high-density polyethylene, n here is 1.525, and α is equal to 55.7°.

In the Fresnel prism provided by the present invention, the length L of each edge must meet the requirement that the linearly polarized terahertz wave incident perpendicular to one side undergo an even number of total reflections in the Fresnel prism before being transmitted from the other side. Side shot. The condition satisfied by L means that the sum of the additional phases obtained by the two mutually perpendicular electric field components contained in the terahertz wave in the Fresnel prism is an odd multiple of (π/2), so that the polarization of the terahertz wave The state is converted from the linear polarization of the incident to the circular polarization of the exit. In addition, since the sum of the additional phases is a constant after L is determined, it has nothing to do with the frequency of the terahertz wave, so even if the incident linearly polarized terahertz wave is polychromatic (that is, the frequency is not single), from the The terahertz wave emitted from the other side of the Nell prism can also be converted into circular polarization state. Therefore, the present invention achieves the invention purpose of simultaneously converting the polarization state of the incident polychromatic linearly polarized terahertz wave into circular polarization state.

In order to reduce the loss of the Fresnel prism to the terahertz wave as much as possible and ensure the energy density of the outgoing terahertz wave, in the present invention, the preferred embodiment of the length L of each edge is as follows: L satisfies the requirement that the incidence perpendicular to one side The linearly polarized terahertz wave emerges from the other side after two total reflections in the Fresnel prism. Figure 4 is a schematic diagram of the terahertz wave propagating and undergoing total reflection in this embodiment. The viewing angle of the figure is the same as Figure 2 is the same, both are side view angles.

可使垂直于一个侧面入射的线偏振太赫兹波在该菲涅尔棱镜内发生全反射。每条棱的长度均满足使垂直于一个侧面入射的线偏振的太赫兹波在该菲涅尔棱镜中发生偶数次全反射后从另一个侧面出射，这样，太赫兹波所包含的两个相互垂直的电场分量在该菲涅尔棱镜内的附加位相总和为（π/2）的奇数倍，因而出射太赫兹波的偏振态即转换为圆偏振。而且，由于该附加位相总和是与频率无关的量，因此入射的所有频率的线偏振太赫兹波均同时转换成了圆偏振态，从而实现了发明目的。另外，该菲涅尔棱镜由不对太赫兹波产生色散作用的材料制成，其两侧面相对且平行，每条棱垂直于每个侧面的一条边，这保证了垂直于一个侧面入射的线偏振太赫兹波在菲涅尔棱镜内发生全反射的位置是在两个平行相对的界面上，而不会在两个相邻的界面上，因而太赫兹波的出射方向也就可以确定为垂直于另一个侧面，这有效防止了太赫兹波在出射时发生变向，使所有频率的太赫兹波所包含的两个相互垂直的电场分量在菲涅尔棱镜中发生全反射的附加位相总和完全一致。It can be seen that, in the present invention, the angle α between each edge and each side of the Fresnel prism satisfies

The linearly polarized terahertz wave incident perpendicular to one side can be totally reflected in the Fresnel prism. The length of each edge is such that the linearly polarized terahertz wave incident perpendicular to one side will emerge from the other side after an even number of total reflections in the Fresnel prism. In this way, the two mutual The sum of the additional phases of the vertical electric field components in the Fresnel prism is an odd multiple of (π/2), so the polarization state of the outgoing terahertz wave is converted to circular polarization. Moreover, since the sum of the additional phases is a frequency-independent quantity, the incident linearly polarized terahertz waves of all frequencies are simultaneously converted into circularly polarized states, thereby achieving the purpose of the invention. In addition, the Fresnel prism is made of a material that does not produce a dispersion effect on the terahertz wave, and its two sides are opposite and parallel, and each edge is perpendicular to one side of each side, which ensures that the linear polarization perpendicular to one side is incident The total reflection position of the terahertz wave in the Fresnel prism is on two parallel and opposite interfaces, not on two adjacent interfaces, so the outgoing direction of the terahertz wave can be determined to be perpendicular to On the other hand, this effectively prevents the terahertz wave from changing direction when it exits, so that the total reflection of the additional phase sum of the two mutually perpendicular electric field components contained in the terahertz wave of all frequencies in the Fresnel prism is exactly the same .

The existing linearly polarized terahertz waves are generated based on mechanisms such as optical rectification, photoconductive antennas, or laser-excited air plasmas to radiate parallel beams. The beam cross section is circular, that is, the above-mentioned line perpendicular to a side The polarized terahertz wave is a parallel beam, and its beam section is circular, so in the Fresnel prism provided by the present invention, the side lengths of each side (a and b in Fig. 1-3) are not less than the terahertz wave The beam diameter can ensure that the polarization states of all incident terahertz waves can be simultaneously converted by the Fresnel prism without energy loss, thus effectively ensuring that the energy of the outgoing circularly polarized terahertz waves meets the usage requirements. The minimum value of the side length is equal to the beam diameter of the incident terahertz wave, so the side is the circumscribed square of the circular terahertz beam.

The present invention can simultaneously convert the polarization state of the incident polychromatic terahertz wave from linear polarization to circular polarization. An embodiment of the incident polychromatic linearly polarized terahertz wave is a single-period pulsed terahertz wave, that is, the above-mentioned vertical The linearly polarized terahertz wave incident on one side is a single-period pulsed terahertz wave.

的关系，这样，既实现了上述的发明目的，使出射的圆偏振太赫兹波的能量密度满足了使用需求，又节省了菲涅尔棱镜制作材料的使用，达到了节约资源、降低成本的目的。In the Fresnel prism provided by the present invention, as long as the length L of the edge satisfies the requirement that the linearly polarized terahertz wave incident perpendicular to one side occurs in the Fresnel prism an even number of times (two times in a preferred embodiment) The object of the invention can be achieved after being reflected and emitted from the other side; and in the case of a parallel beam with a circular beam cross-section, as long as the side lengths (a and b) of the side are not less than the beam diameter, it can be guaranteed The energy density of the outgoing terahertz wave meets the usage requirements. In order to make L and a (and b) coordinate with each other and reduce the use of materials, in the present invention, the side length a (and b) of each side and the minimum length L of each edge can be further set so that the two meet

In this way, the above-mentioned purpose of the invention is achieved, the energy density of the outgoing circularly polarized terahertz wave meets the use requirements, and the use of Fresnel prism materials is saved, and the purpose of saving resources and reducing costs is achieved. .

The present invention also proposes a method for converting the polarization state of a terahertz wave, which is implemented based on the above-mentioned Fresnel prism. The state is converted from linear polarization to circular polarization at the same time, that is, after the linearly polarized polychromatic terahertz waves enter the Fresnel prism, the polarization states of the outgoing polychromatic terahertz waves are all converted to circular polarization.

Taking Figure 4 as an example for illustration, the method includes:

Step 1: Take any side of the Fresnel prism (such as side M) as the incident interface, and use the other side (such as side N) as the exit interface; inject the linearly polarized terahertz wave perpendicular to the incident interface M into the Niel Prism.

Terahertz wave is a kind of electromagnetic wave, and its propagation law follows the propagation law of electromagnetic wave, that is, the terahertz wave incident perpendicular to the interface of two substances, when it is injected into the second substance from the first substance (air in the present invention), (Fresnel prism in the present invention), its propagation law remains unchanged. Therefore, after the linearly polarized terahertz wave enters the Fresnel prism perpendicular to the incident interface M, the propagation direction remains unchanged, and then reaches the interface between the Fresnel prism and air for the first time.

Step 2: The terahertz wave undergoes an even number of total reflections in the Fresnel prism, and the reflection interface of each total reflection is the interface between the medium and the air.

The medium in this step refers to the material for making the Fresnel prism.

After the terahertz wave reaches the interface between the Fresnel prism and the air in step 1, since the above-mentioned angle α between each edge and each side satisfies Therefore, the total reflection of the terahertz wave will occur at the interface, which is determined by the propagation law of the electromagnetic wave (terahertz wave in the present invention).

In the Fresnel prism proposed by the present invention, the length of each edge is sufficient to make the linearly polarized terahertz wave incident perpendicular to a side (such as the side M in Figure 4) generate an even number of waves in the Fresnel prism. times (the embodiment in Figure 4 is 2 times) total reflection before exiting from the other side (Side N in Figure 4), so the terahertz wave will have an even number of total reflections in the Fresnel prism, each Subtotal reflection occurs at the interface between the Fresnel prism and the air, and both occur inside the Fresnel prism.

The terahertz wave includes two electric field components perpendicular to each other. After the terahertz wave undergoes an even number of total reflections, the sum of the additional phases obtained by the two electric field components is an odd multiple of (π/2), and the sum of the additional phases is equal to The frequency is independent and is a constant, which means that after the linearly polarized polychromatic terahertz wave undergoes an even number of total reflections, the sum of the additional phases obtained by the two mutually perpendicular electric field components contained in the terahertz wave of all frequencies is equal to is an odd multiple of (π/2), so the polarization states of terahertz waves at all frequencies become circularly polarized.

Step 3: The terahertz wave emitted from the exit interface is the circularly polarized terahertz wave.

After the linearly polarized incident terahertz wave undergoes an even number of total reflections in step 2, the terahertz wave emitted from the exit interface of the Fresnel prism (such as side N in Figure 4) is a circularly polarized terahertz wave. The circular polarization state of the outgoing terahertz wave can be verified by the existing electro-optic sampling detection method and the existing method of combining the terahertz polarizer with the terahertz energy receiver.

This shows that the present invention has the following advantages:

可使垂直于一个侧面入射的线偏振太赫兹波在该菲涅尔棱镜内发生全反射。每条棱的长度均满足使垂直于一个侧面入射的线偏振的太赫兹波在该菲涅尔棱镜中发生偶数次全反射后从另一个侧面出射，这样，太赫兹波所包含的两个相互垂直的电场分量在该菲涅尔棱镜内的附加位相总和为（π/2）的奇数倍，因而出射太赫兹波的偏振态即转换为圆偏振。而且，由于该附加位相总和是与频率无关的量，因此入射的所有频率的线偏振太赫兹波均同时转换成了圆偏振态，从而实现了发明目的。另外，该菲涅尔棱镜由不对太赫兹波产生色散作用的材料制成，其两侧面相对且平行，每条棱垂直于每个侧面的一条边，这保证了垂直于一个侧面入射的线偏振太赫兹波在菲涅尔棱镜内发生全反射的位置是在两个平行相对的界面上，而不会在两个相邻的界面上，因而太赫兹波的出射方向也就可以确定为垂直于另一个侧面，这有效防止了太赫兹波在出射时发生变向，使所有频率的太赫兹波所包含的两个相互垂直的电场分量在菲涅尔棱镜中发生全反射的附加位相总和完全一致。(1) In the present invention, the angle α between each edge and each side of the Fresnel prism satisfies

The linearly polarized terahertz wave incident perpendicular to one side can be totally reflected in the Fresnel prism. The length of each edge is such that the linearly polarized terahertz wave incident perpendicular to one side will emerge from the other side after an even number of total reflections in the Fresnel prism. In this way, the two mutual The sum of the additional phases of the vertical electric field components in the Fresnel prism is an odd multiple of (π/2), so the polarization state of the outgoing terahertz wave is converted to circular polarization. Moreover, since the sum of the additional phases is a frequency-independent quantity, the incident linearly polarized terahertz waves of all frequencies are simultaneously converted into circularly polarized states, thereby achieving the purpose of the invention. In addition, the Fresnel prism is made of a material that does not produce a dispersion effect on the terahertz wave, and its two sides are opposite and parallel, and each edge is perpendicular to one side of each side, which ensures that the linear polarization perpendicular to one side is incident The total reflection position of the terahertz wave in the Fresnel prism is on two parallel and opposite interfaces, not on two adjacent interfaces, so the outgoing direction of the terahertz wave can be determined to be perpendicular to On the other hand, this effectively prevents the terahertz wave from changing direction when it exits, so that the total reflection of the additional phase sum of the two mutually perpendicular electric field components contained in the terahertz wave of all frequencies in the Fresnel prism is exactly the same .

(2) The Fresnel prism in the present invention is made of high-density polyethylene or high-resistance silicon, which can prevent the dispersion of terahertz waves, reduce the loss of terahertz waves in the process of polarization state conversion, and increase the energy of outgoing terahertz waves density.

(3) In the Fresnel prism provided by the present invention, the side length of each side is not less than the beam diameter of the incident terahertz wave, which can ensure that the polarization states of all incident terahertz waves can be simultaneously converted by the Fresnel prism , there will be no energy loss, thus effectively ensuring that the energy of the outgoing circularly polarized terahertz wave meets the usage requirements.

的关系，这样，既实现了上述的发明目的，使出射的圆偏振太赫兹波的能量密度满足了使用需求，又节省了菲涅尔棱镜制作材料的使用，达到了节约资源、降低成本的目的。(4) The present invention sets the side length a (and b) of each side and the length L of each edge, so that the two satisfy

In this way, the above-mentioned purpose of the invention is achieved, the energy density of the outgoing circularly polarized terahertz wave meets the use requirements, and the use of Fresnel prism materials is saved, and the purpose of saving resources and reducing costs is achieved. .

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the protection of the present invention. within range.

Claims (6)

1. A Fresnel prism is characterized in that the Fresnel prism is in a strip column shape and is made of a material which does not generate dispersion effect on terahertz waves;

the Fresnel prism comprises four edges and two opposite and parallel side faces; wherein,

the two side surfaces are squares with the same side length;

each edge is perpendicular to one edge of each side surface, and the included angle alpha between each edge and each side surface meets the requirement

Wherein n is the refractive index of the manufacturing material of the Fresnel prism to the terahertz wave;

the length of each edge is enough to enable linearly polarized terahertz waves incident perpendicular to one side surface to be emitted from the other side surface after even-numbered total reflection in the Fresnel prism; the side length a of each side surface and the minimum length L of each edge meet

2. The fresnel prism according to claim 1, characterized in that it is made of high-density polyethylene or high-resistance silicon.

3. The fresnel prism according to claim 1, wherein the terahertz waves linearly polarized perpendicularly to one of the side faces are parallel beams whose beam cross section is circular;

the side length of the side face is not less than the beam diameter of the terahertz wave.

4. The Fresnel prism according to any one of claims 1 to 3, wherein the terahertz waves that are linearly polarized perpendicularly to one of the side incident surfaces are single-period pulsed terahertz waves.

5. The fresnel prism of claim 1, wherein each of the ribs has a length such that a linearly polarized terahertz wave incident perpendicularly to one of the side surfaces exits from the other side surface after undergoing 2 total reflections in the fresnel prism.

6. A method for converting the polarization state of a terahertz wave, the method being implemented based on the Fresnel prism of claim 1, the Fresnel prism beingThe strip-shaped columnar structure comprises four edges and two opposite and parallel side faces, the two side faces are squares with the same side length, each edge is perpendicular to one side of each side face, and the included angle alpha between each edge and each side face meets the requirement of meeting the requirement of the included angle alpha

Wherein n is the refractive index of the manufacturing material of the Fresnel prism to the terahertz wave; the side length a of each side surface and the minimum length L of each edge meet

The method is characterized by comprising the following steps:

step 1: taking any one side surface of the Fresnel prism as an incident interface and the other side surface of the Fresnel prism as an emergent interface; emitting linearly polarized terahertz waves perpendicular to the incident interface into the Fresnel prism;

step 2: the terahertz waves are subjected to even number of total reflections in the Fresnel prism, and the reflection interface of each total reflection is an interface of a medium and air;

and step 3: the terahertz waves emitted from the emitting interface are circularly polarized terahertz waves.

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