CN114690318B - Mode wave plate based on few-mode optical fiber and mode conversion method - Google Patents

Abstract

The invention discloses a mode wave plate based on few-mode optical fibers and a mode conversion method, which belong to the field of optical communication and optical field regulation and control, and mainly utilize TE in the few-mode optical fibers ₀₁ And TM ₀₁ Mode walk-off phenomenon due to the non-degenerate nature of the mode. TE (TE) ₀₁ And TM ₀₁ The mode is propagated in a section of optical fiber to bring about accumulation of a specific phase difference, the phase difference is proportional to the length of the optical fiber, the operation of mode rotation and the like can be realized, and the mode regulation based on the few-mode optical fiber is named as a mode wave plate by analogy with the regulation of the half wave plate and the quarter wave plate in the traditional polarized optics on the polarization of the light beam. Compared with the traditional wave plate, which can only change the polarization state of the light field, the mode wave plate can act on the space mode, can change the space distribution of polarization and phase at the same time, is a high-order mode wave plate, is hopefully applied to the fields of sensing, measuring, optical tweezers, optical communication and the like, and fills the blank of the related technology.

Description

A mode wave plate and mode conversion method based on few-mode optical fiber

Technical Field

The invention belongs to the field of optical communication and light field regulation, and relates to a mode wave plate based on few-mode optical fiber and a mode conversion method.

Background Art

In 1941, Jones introduced a two-column element array to describe polarized light in detail, called the Jones matrix. The Jones matrix has also become a convenient means to study polarization optics and analyze polarization devices. There are many types of polarization devices, the most common of which are wave plates or phase retarders. Polarization wave plates use the anisotropy of certain special materials. From polarization optics, we know that the amplitude ratio and phase difference of two orthogonal polarized lights can determine the polarization state of the synthetic light wave. Parallel plates made of anisotropic crystal materials can control the amplitude ratio and phase difference of two orthogonal polarized lights, thereby achieving the purpose of changing the polarization state of the incident light, which is called a wave plate or phase retarder. Typical wave plates include half-wave plates and quarter-wave plates. If the optical path difference between the o-light and the e-light acting on the wave plate in the incident light field is λ/2, the wave plate is called a half-wave plate. Circularly polarized light is still circularly polarized after passing through a half-wave plate, but the rotation direction changes. Linear polarized light remains linearly polarized after passing through a half-wave plate, but the direction of the light vector rotates, and the rotation angle is twice the angle between the incident linear polarized light and the fast (slow) axis of the wave plate. If the optical path difference between the o-light and the e-light acting on the incident light field by the wave plate is λ/4, the wave plate is called a quarter-wave plate. When the angle between the light vector of the incident linear polarized light and the fast (slow) axis of the wave plate is ±45°, the outgoing light is circularly polarized light; conversely, when circularly polarized light is incident, linearly polarized light is emitted from the quarter-wave plate. The quarter-wave plate can also convert linearly polarized incident light into elliptically polarized light.

The above wave plate only acts on the polarization state of the light field, and does not involve the change of the spatial phase of the light field. In other words, except for polarization, it is impossible to arbitrarily transform the high-order mode. However, mode control is currently widely used in the fields of optical tweezers, mode division multiplexing optical communications, sensing and imaging. In view of this, it is extremely necessary to design a mode wave plate based on few-mode fiber as a supplement and extension of the existing wave plate, which is convenient for convenient control and change of the mode and has broad application prospects.

Summary of the invention

In view of the shortcomings of existing wave plates in controlling light fields, the present invention provides a mode wave plate and a mode conversion method based on few-mode optical fiber, aiming to provide a new mode control method to achieve simultaneous control of mode polarization and phase distribution, which is expected to be applied in the fields of sensing, measurement, optical tweezers and optical communication, filling the gap in related technologies.

To achieve the above-mentioned purpose, according to one aspect of the present invention, a mode wave plate based on few-mode fiber is provided, comprising a mode excitation module, a few-mode fiber and a mode output module connected in sequence; the mode excitation module is used to input a first-order mode group mode containing TM ₀₁ and TE ₀₁ components into the few-mode fiber, and a certain phase difference is accumulated in the mode output module after passing through a preset length of the few-mode fiber, and the phase difference causes a change in the synthetic mode, thereby realizing a change in the mode.

和

主要依赖于支持TM₀₁和TE₀₁模式的少模光纤。首先依据光纤结构参数计算或测量出的TM₀₁和TE₀₁有效折射率，计算出两个模式累积的相位差

其中

和

分别为TM₀₁和TE₀₁的传播常数，L为光纤长度。入射光场为含有或只含有TM₀₁和TE₀₁的叠加光场，叠加光场的TM₀₁和TE₀₁分量具有微弱的传播常数差，在少模光纤传输中会累积相位差，累积的相位差会造成输出叠加光场的改变，达到对模式偏振和空间相位的调控，发挥类似于波片的模式波片作用。The first-order mode group includes weakly degenerate TM ₀₁ and TE ₀₁ as well as degenerate modes

and

It mainly relies on the few-mode optical fiber that supports TM ₀₁ and TE ₀₁ modes. First, the TM ₀₁ and TE ₀₁ effective refractive indices calculated or measured based on the optical fiber structural parameters are used to calculate the accumulated phase difference between the two modes.

in

and

are the propagation constants of TM ₀₁ and TE ₀₁ , respectively, and L is the fiber length. The incident light field is a superimposed light field containing or only containing TM ₀₁ and TE _01. The TM ₀₁ and TE ₀₁ components of the superimposed light field have a slight propagation constant difference, which will accumulate phase differences in few-mode fiber transmission. The accumulated phase difference will cause changes in the output superimposed light field, achieving the regulation of mode polarization and spatial phase, and playing a mode wave plate role similar to that of a wave plate.

If the constructed mode wave plate is similar to a half-wave plate, the phase difference satisfies δ = (2n + 1)π, where n is an integer. The corresponding few-mode fiber length is calculated based on this formula. A typical scenario is when the input light field is a circularly polarized ±1-order OAM mode, the circular polarization direction and OAM order of the output light field will be reversed, specifically manifested as left-handed circularly polarized input, right-handed circularly polarized output, right-handed circularly polarized input, left-handed circularly polarized output, and the order of the OAM mode is reversed. This scenario in which the circular polarization direction and the OAM mode order change simultaneously can be compared to the effect of a half-wave plate rotating the linear polarization direction.

If the constructed mode wave plate is similar to a quarter wave plate, the phase difference satisfies δ=(2n+1/2)π, where n is an integer. The corresponding few-mode fiber length is calculated based on this formula. A typical scenario is when the input light field is a superposition field of TM ₀₁ and TE ₀₁ , and the input light field is a hybrid vector mode. After the mode wave plate acts on the input light field, the output light field is a circularly polarized first-order OAM mode. Every point in the hybrid vector mode space is linearly polarized. This scenario of converting the polarization direction of spatially distributed linear polarization into a circularly polarized light field can be compared to the role of a quarter wave plate in converting linearly polarized light into circularly polarized light.

Preferably, the few-mode optical fiber can adopt a refractive index distribution type such as a step refractive index distribution or a gradient refractive index distribution. When the few-mode optical fiber is a step refractive index distribution, the optical fiber structure is simple and easy to manufacture. It is a mature and common few-mode optical fiber type. The effective refractive index of the supported mode can be quickly calculated by analytical and numerical methods. According to the effective refractive index difference between TM ₀₁ and TE ₀₁ , the change amount of the mode transmitted through the optical fiber is calculated. When the few-mode optical fiber is a gradient refractive index distribution, the optical fiber structure is simple, easy to manufacture, and the process is mature. The effective refractive index of the supported mode can be calculated by numerical method using simulation software. According to the effective refractive index difference between TM ₀₁ and TE ₀₁ , the change amount of the mode transmitted through the optical fiber is calculated.

Preferably, the few-mode optical fiber can be made of various other optical fibers that also support TM ₀₁ and TE ₀₁ , including multimode optical fiber and ring optical fiber, among which ring optical fiber is preferred. When the optical fiber adopts multimode optical fiber, radial high-order modes are not suppressed, and the input light field needs to be accurately excited. Specifically, the mode of the input optical fiber needs to limit the radial light field, and a spatial light modulator can be used to load a radial distribution function for limitation. When the optical fiber adopts a ring optical fiber, radial high-order modes are suppressed, mode crosstalk is small, and the output mode field is simplified. Due to the restriction of the radial light field by the ring optical fiber, the phenomenon of the mode wave plate is better.

Preferably, the selection of the length of the few-mode fiber has multiple values. Taking the mode wave plate analogous to a half-wave plate as an example, the phase difference satisfies δ=(2n+1)π, n is an integer, and the corresponding few-mode fiber length is calculated according to this formula. n can be an integer such as 0, 1, 2, but the fiber length corresponding to n=0 is preferably selected. Defects are inevitable in the optical fiber manufacturing process, and the optical fiber will also be affected by environmental disturbances, including factors such as temperature and pressure. Therefore, the shorter few-mode optical fiber accumulates less random phase disturbances, and the mode wave plate can more accurately and stably control the mode.

Preferably, the mode wave plate can act on a synthetic light field of TM ₀₁ and TE ₀₁ with any amplitude ratio, so as to achieve a mode control effect similar to that of a wave plate on a specific complex light field.

The second aspect of the present invention provides a mode conversion method based on few-mode optical fiber, in which a first-order mode group mode containing TM ₀₁ and TE ₀₁ components is input to the few-mode optical fiber. After passing through a preset length of the few-mode optical fiber, the two modes of the first-order mode group component accumulate a certain phase difference, and the phase difference causes a change in the synthetic mode, thereby achieving a mode change.

和

当一阶模群模式为TM₀₁和TE₀₁时，两个模式累积的相位差为：The first-order mode group includes weakly degenerate TM ₀₁ and TE ₀₁ as well as degenerate modes

and

When the first-order mode group modes are TM ₀₁ and TE ₀₁ , the accumulated phase difference between the two modes is:

和

分别为TM₀₁和TE₀₁的传播常数，L为少模光纤的长度。in

and

are the propagation constants of TM ₀₁ and TE ₀₁ respectively, and L is the length of the few-mode fiber.

Through the above technical solutions conceived by the present invention, the present invention has the following beneficial effects:

1. The present invention is based on the inherent weak degeneracy of TM ₀₁ and TE ₀₁ in few-mode optical fiber. The phase difference between the two modes will accumulate with the propagation of the optical fiber, which is similar to the phase difference effect of the two polarization components of the traditional wave plate. However, compared with the traditional wave plate that can only change the polarization state of the light field, the mode wave plate can act on the spatial mode and can simultaneously change the spatial distribution of polarization and phase. It is a high-order mode wave plate.

2. The present invention has a wide range of applications and can be widely used in the fields of fiber optic tweezers, fiber optic imaging, optical communications, etc.

3. The present invention is based on the inherent weak degeneracy effect of TM ₀₁ and TE ₀₁ , which is widely present in optical fibers and other types of waveguides and has a large room for improvement.

4. The present invention uses all-fiber components, which are lower in cost, smaller in size, more flexible, and easier to manufacture than wave plates. It is also compatible with existing fiber optic communication systems and is easy to connect and integrate.

5. The mode wave plates provided by the present invention have a wide range of types. The phase difference between TM ₀₁ and TE ₀₁ in few-mode optical fiber transmission is proportional to the optical fiber length. Different optical fiber lengths correspond to different phase delays, which facilitates the manufacture of mode wave plates similar to half-wave plates, quarter-wave plates and other types of wave plates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of an application of a mode wave plate based on few-mode optical fiber provided by the present invention;

FIG2 is a schematic diagram of an application similar to a half-wave plate provided by an embodiment of the present invention;

FIG3 is a schematic diagram of an application similar to a quarter-wave plate provided by an embodiment of the present invention.

DETAILED DESCRIPTION

In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

其中

和

分别为TM₀₁和TE₀₁的传播常数，L为光纤长度。入射光场为含有或只含有TM₀₁和TE₀₁的叠加光场，叠加光场的TM₀₁和TE₀₁分量具有微弱的传播常数差，在少模光纤传输中会累积相位差，累积的相位差会造成输出叠加光场的改变，达到对模式偏振和空间相位的调控，发挥类似于波片的模式波片作用。The present invention provides a mode wave plate based on few-mode fiber, which mainly includes a few-mode fiber of a specific length. The mode wave plate mainly relies on the few-mode fiber supporting TM ₀₁ and TE ₀₁ modes. First, the TM ₀₁ and TE ₀₁ effective refractive indices calculated or measured based on the fiber structure parameters are used to calculate the accumulated phase difference between the two modes.

in

and

are the propagation constants of TM ₀₁ and TE ₀₁ , respectively, and L is the fiber length. The incident light field is a superimposed light field containing or only containing TM ₀₁ and TE _01. The TM ₀₁ and TE ₀₁ components of the superimposed light field have a slight propagation constant difference, which will accumulate phase differences in few-mode fiber transmission. The accumulated phase difference will cause changes in the output superimposed light field, achieving the regulation of mode polarization and spatial phase, and playing a mode wave plate role similar to that of a wave plate.

Specifically, it can act on the synthetic light field of TM ₀₁ and TE ₀₁ with any amplitude ratio, so as to achieve a mode control effect similar to that of a wave plate on a specific complex light field.

Specifically, when implementing the mode wave plate to manipulate the mode, a suitable mode excitation method is required, and the mode excitation can be achieved by using a phase plate, a Q plate, a spatial light modulator, a mode directional coupler, etc. The phase plate has a free-space Gaussian light as an input end and a high-order mode as an output end. The free-space Gaussian light is output from a single-mode optical fiber and collimated to free space by a collimator. The phase plate may include a spiral phase distribution for generating an OAM mode.

Specifically, for the Q-plate, the input end is free-space Gaussian light, and the output end is high-order mode vector light. The free-space Gaussian light is output by a single-mode optical fiber and collimated to free space by a collimator. When the input light is linearly polarized, a vector mode can be generated. When the input light is converted into circularly polarized Gaussian input light by a quarter-wave plate, the Q-plate can generate a circularly polarized OAM mode.

Specifically, for the spatial light modulator, the input end is free-space Gaussian light, and the output end is a high-order mode. The free-space Gaussian light is output by a single-mode optical fiber and collimated to free space by a collimator. The spatial light modulator includes a transmission type and a reflection type. The reflection type spatial light modulator includes two usage modes: normal incidence and oblique incidence, and both can modulate X-polarized light into a high-order mode such as an OAM mode. At the same time, vector light can be generated by superposition.

Specifically, for the mode directional coupler, the input end is a single-mode optical fiber, and the output end is a few-mode optical fiber, a multi-mode optical fiber or a ring optical fiber. Through optical fiber pre-tapering matching and simultaneous tapering and fusion splicing of two optical fibers, the Gaussian base mode at the input end in the cone region is coupled to the high-order linear polarization mode or circular polarization OAM mode at the output end. The working states of the input end and the output end are adjusted by a polarization controller to output the high-order linear polarization mode or circular polarization OAM mode of the optical fiber.

Specifically, the few-mode optical fiber needs to support TM ₀₁ and TE ₀₁ modes. The selection of the few-mode optical fiber is not limited to the few-mode optical fiber, and multimode optical fiber and ring optical fiber may also be selected.

Specifically, when the selected optical fiber is a multimode optical fiber or a few-mode optical fiber that supports radial high-order modes, when generating the input mode, the radial high-order mode needs to be restricted to suppress the excitation of the radial high-order mode in the optical fiber and make the output mode purer. For example, when using a spatial light modulator excitation mode, the spiral phase pattern needs to be superimposed with the radial distribution restriction of the light field.

Specifically, when the selected optical fiber is a ring optical fiber, the effective refractive index difference between the mode groups is increased by increasing the core-clad refractive index difference, and the crosstalk between the mode groups is smaller; the ring core layer design improves the restriction of radial high-order modes, and the mode excitation does not need to consider the influence of radial high-order modes. The above two characteristics jointly promote the stable transmission of TM ₀₁ and TE ₀₁ modes, and the mode wave plate based on the ring optical fiber is more stable and more adaptable.

The following is a description with reference to specific embodiments and drawings.

As shown in Fig. 1, the present invention provides a schematic diagram of the application of a mode wave plate based on few-mode fiber, including: mode excitation 1, few-mode fiber 2, and mode output 3. The input light field contains TM ₀₁ and TE ₀₁ mode components. When the input field is transmitted in the optical fiber, due to the slight effective refractive index difference between TM ₀₁ and TE ₀₁ , the two modes at the output end will accumulate a certain phase difference, which will cause the change of the synthetic mode, such as the change of the spatial polarization distribution of the mode. Analogous to the traditional polarization wave plate, the device can achieve the change of the mode, which is called a mode wave plate.

As shown in FIG. 2 , a specific embodiment of a wave plate similar to a half-wave plate mode provided by the present invention is implemented as follows:

When the wave plate is similar to a half-wave plate, the phase difference accumulated between TM ₀₁ and TE ₀₁ after the action of the mode wave plate is an odd multiple of π, with a typical value of π. A typical scenario is when the input light field is a circularly polarized ±1-order OAM mode, the circular polarization direction and OAM order of the output light field will be reversed, specifically manifested as left-handed circularly polarized input, right-handed circularly polarized output, right-handed circularly polarized input, left-handed circularly polarized output, and the order of the OAM mode is reversed. This scenario in which the circular polarization direction and the OAM mode order change simultaneously can be compared to the effect of a half-wave plate rotating the linear polarization direction, as shown in Figure 2. Unlike traditional wave plates, the fast (slow) axis of the mode half-wave plate is annular.

As shown in FIG3 , a specific embodiment of a wave plate similar to a half-wave plate mode provided by the present invention is implemented as follows:

When the wave plate is similar to a quarter wave plate, the phase difference accumulated between TM ₀₁ and TE ₀₁ after the action of the mode wave plate is (2n+1/2)π, n=0,1,2…, and the typical value is π/2. A typical scenario is when the input light field is a superposition field of TM ₀₁ and TE ₀₁ , and the input light field is a hybrid vector mode. After the mode wave plate acts on the input light field, the output light field is a circularly polarized first-order OAM mode. Every point in the hybrid vector mode space is linearly polarized. This scenario of converting the polarization direction of spatially distributed linear polarization into a circularly polarized light field can be compared to the role of a quarter wave plate in converting linearly polarized light into circularly polarized light, as shown in FIG3. Different from traditional wave plates, the fast (slow) axis of the mode half-wave plate is annular, and the angle between the light vector direction of each point in the input synthetic light field space and the annular fast (slow) axis is ±45°, and the output field is a circularly polarized distribution, which is very similar to the role of a quarter wave plate.

The present invention is not limited to the above-mentioned specific implementation modes. A person skilled in the art can implement the present invention in various other specific implementation modes based on the contents disclosed in the present invention. Therefore, any design that adopts the design structure and concept of the present invention and makes some simple changes or modifications shall fall within the scope of protection of the present invention.

Claims (4)

1. A mode wave plate based on few-mode fiber, characterized in that it comprises a mode excitation module, a few-mode fiber and a mode output module connected in sequence; the mode excitation module is used to input a first-order mode group mode to the few-mode fiber, the first-order mode group mode comprises weakly degenerate TM ₀₁ and TE ₀₁ , and after passing through a preset length of the few-mode fiber, a certain phase difference is accumulated in the mode output module, and the phase difference causes a change in the synthetic mode to achieve a change in the mode; the accumulated phase difference between the two modes is:

in

and

are the propagation constants of TM ₀₁ and TE ₀₁ , respectively, and L is the preset length of the few-mode fiber; The few-mode optical fiber includes a type of step-index profile or a graded-index profile. 2. The mode wave plate based on few-mode fiber according to claim 1, characterized in that the mode wave plate can perform a half-wave plate or a quarter-wave plate; When the mode wave plate behaves as a half-wave plate, the accumulated phase difference between TM ₀₁ and TE ₀₁ after the mode wave plate acts is an odd multiple of π; When the mode wave plate behaves as a quarter wave plate, the accumulated phase difference between TM ₀₁ and TE ₀₁ after the action of the mode wave plate is (2n+1/2)π, n=0, 1, 2…. 3. The mode wave plate based on few-mode fiber according to claim 1 is characterized in that the few-mode fiber is an optical fiber that supports a first-order mode group mode. 4. A mode conversion method based on few-mode fiber, characterized in that a first-order mode group mode is input to the few-mode fiber, the first-order mode group mode includes weakly degenerate TM ₀₁ and TE ₀₁ , and after passing through a preset length of the few-mode fiber, the two modes of the first-order mode group component accumulate a certain phase difference, and the phase difference causes the change of the synthetic mode to achieve the change of the mode; the accumulated phase difference of the two modes is:

in

and

are the propagation constants of TM ₀₁ and TE ₀₁ respectively, and L is the preset length of the few-mode fiber.

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