Method and apparatus for generating a random polarized light beam

Abstract

This invention relates to beam manipulation, providing a method and apparatus for generating randomly polarized beams, and pertains to the field of optical manipulation technology. The method includes generating a highly coherent linearly polarized beam using a continuous excitation light source unit; modulating the linearly polarized beam using a phase modulation unit to obtain a vortex beam with arbitrary topological charge; and modulating the vortex beam into a randomly polarized beam using a polarization modulation unit, wherein the response range of the random polarization can cover any point on the surface of a Poincaré sphere. The beam generation method provided by this invention can modulate a vortex beam carrying arbitrary topological charge into a beam with randomly varying polarization states, exhibiting high modulation efficiency, uniform polarization state distribution, and low cost, thus possessing significant practical value.

Description

Technical Field

[0001] This invention relates to the field of optical control technology, and specifically to a method and apparatus for generating randomly polarized light beams. Background Technology

[0002] Randomly polarized light refers to beams of light with randomly distributed polarization states, typically including three types: spatially random distribution, temporally random distribution, and a mixture of both. The former can be generated by scattering, diffuse reflection, and special polarization modulation devices, while the latter can be generated by special polarization selection devices. Mixed distributions are commonly seen in incoherent light such as sunlight and lamplight. However, due to the limitations of their inherent polarization characteristics, these ubiquitous beams have not attracted the same widespread interest from researchers as polarized beams. But with the deepening research into the interaction between light and matter, the new physical phenomena caused by this special beam have gradually gained attention. For example, in theoretical research involving spin-orbit interactions, the three-dimensional polarized light field described by the Poincaré sphere has revealed new spin information. Research on three-dimensional randomly polarized light fields is fundamental and forward-looking, and this topic has significant value for understanding the nature of light; therefore, research on methods for generating randomly polarized beams is necessary.

[0003] Randomly polarized beams, due to their unique optical spin angular momentum, have found practical applications, primarily in the study of spin phenomena in non-paraxial fields generated by randomly polarized paraxial beams. These non-paraxial fields typically include two types: focusing and evanescent fields. In these two special optical fields, researchers have discovered transverse spin components orthogonal to the propagation direction, which distinguishes them from the spin phenomena in random paraxial beams. Furthermore, researchers have studied the two-dimensional evanescent waves of such random beams under specific configurations, obtaining some findings related to the longitudinal spin component.

[0004] Current methods for generating randomly polarized beams primarily address spatially random polarization, including depolarizers that utilize gradient phase differences for depolarization. When incident linearly polarized monochromatic light of a certain diameter passes through a device with a gradient phase difference, the outgoing beam becomes elliptically polarized light with different ellipticities due to variations in phase difference or rotation angle at different positions; the entire beam is a composite of random states. Another type of spatially random polarization device is the liquid crystal polymer depolarizer, which uses a patterned liquid crystal array to convert linearly polarized light into pseudo-randomly polarized light. However, its drawback is that the resulting beam is not completely randomly polarized. For temporally random polarization, multiple waveplates are combined to achieve temporal depolarization. A typical characteristic of this technique is that the designed system requires continuous rotation of the waveplates, and strictly speaking, it cannot achieve temporal randomness.

[0005] Therefore, a novel technique for generating randomly polarized beams is needed to meet the needs of researchers. Summary of the Invention

[0006] In view of this, the purpose of the present invention is to provide a method and apparatus for generating randomly polarized beams, which aims to provide a randomly polarized beam carrying an arbitrary topological charge, wherein the response range of random polarization can cover any point on the surface of a Poincaré sphere, and a beam with a fixed polarization state can also be generated by adjusting the input voltage.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] In a first aspect, the present invention provides an apparatus for generating a randomly polarized beam, the apparatus comprising:

[0009] Excitation source unit, used to generate a highly coherent linearly polarized beam;

[0010] A phase modulation unit is used to modulate the linearly polarized beam generated by the excitation light source unit to obtain a vortex beam with arbitrary topological charge.

[0011] A polarization modulation unit is used to modulate the vortex beam into a randomly polarized beam, the response range of which can cover any point on the surface of the Poincaré sphere.

[0012] As a further aspect of the present invention, the randomly polarized beam is a randomly polarized state with an arbitrary phase.

[0013] As a further aspect of the present invention, the random polarization state is controlled by two continuously refreshed random voltages, which determine the phase of the liquid crystal phase device within the range of 0-2π and 0-π.

[0014] As a further embodiment of the present invention, the polarization modulation unit includes a voltage generation unit and a random phase generation unit;

[0015] The voltage generating unit is used to generate two constantly refreshed random voltages within two different ranges.

[0016] The random phase generation unit is used to independently control the deflection angle of liquid crystal molecules in the two liquid crystal phase plates using the random voltage, thereby realizing the control of the modulation phase of the two liquid crystal phase plates within 0-2π and 0-π respectively; and then the phase compensation of the emitted beam of the two liquid crystal phase plates is performed by a quarter-wave plate, thereby realizing the independent control of the polarization state on the equatorial plane and the meridional plane of the Poincaré sphere.

[0017] Secondly, the present invention also provides a method for generating a randomly polarized beam, wherein the randomly polarized beam is generated based on the above-mentioned random polarization beam generating apparatus, and the method for generating the random polarization beam includes the following steps:

[0018] A highly coherent linearly polarized beam is generated using a continuous excitation light source unit.

[0019] The linearly polarized beam generated by the excitation source unit is modulated using a phase modulation unit to obtain a vortex beam with arbitrary topological charge.

[0020] The vortex beam is modulated into a randomly polarized beam using a polarization modulation unit, and the response range of the random polarization can cover any point on the surface of the Poincaré sphere.

[0021] As a further aspect of the present invention, the random polarization state is controlled by two continuously refreshed random voltages.

[0022] As a further aspect of the present invention, the random polarization state is controlled by two continuously refreshed random voltages.

[0023] As a further aspect of the present invention, the polarization modulation unit is suitable for laser beams of corresponding wavelengths, and the polarization state of the beam can be independently controlled on the equatorial and meridional planes of the Poincaré sphere, including:

[0024] A random voltage generator is used to generate two constantly refreshed random voltages within two different ranges.

[0025] The deflection angle of the liquid crystal molecules in the two liquid crystal phase plates is independently controlled by the random voltage, thereby realizing the control of the modulation phase of the two liquid crystal phase plates within the range of 0-2π and 0-π respectively.

[0026] A quarter-wave plate is used to compensate the phase of the emitted beam from two liquid crystal phase plates, thereby enabling independent control of the polarization state on the equatorial and meridional planes of the Poincaré sphere.

[0027] Compared with existing technologies, the present invention proposes a method and apparatus for generating randomly polarized beams, which has the following beneficial technical effects:

[0028] This invention provides a method and apparatus for generating a randomly polarized beam. First, a linearly polarized beam with high coherence is generated using a continuous excitation light source unit. Then, a phase modulation unit is used to modulate an ordinary laser beam to obtain a vortex beam with arbitrary topological charge. Finally, a polarization modulation unit is used to modulate the vortex beam into a randomly polarized beam, the response range of which can cover any point on the surface of a Poincaré sphere.

[0029] Compared with the prior art, this invention modulates a regular laser beam into a randomly polarized beam with arbitrary topological charge by utilizing a phase modulation unit and a polarization modulation unit. This results in high modulation efficiency, uniform polarization state distribution, low cost, and great practical value.

[0030] These or other aspects of this application will become more apparent from the following description of embodiments. It should be understood that the foregoing general description and the following detailed description are exemplary and explanatory only, and are not intended to limit the application. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments or prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. In the drawings:

[0032] Figure 1 This is a flowchart illustrating a method for generating a randomly polarized beam according to an embodiment of the present invention.

[0033] Figure 2 This is a detailed structural schematic diagram of a randomly polarized beam generating device provided in an embodiment of the present invention;

[0034] Figure 3 This is a schematic diagram of a Poincaré sphere demonstrating the polarization state change in the random polarization beam generating device provided in this embodiment of the invention.

[0035] Figure 4 This is a schematic diagram of the fast axis direction of the first liquid crystal phase plate in the random polarization beam generating device provided in the embodiment of the present invention;

[0036] Figure 5 This is a schematic diagram of the fast axis direction of the second liquid crystal phase plate in the random polarization beam generating device provided in the embodiment of the present invention;

[0037] Figure 6 This is a schematic diagram of the fast axis direction of the quarter-wave plate in the random polarization beam generating device provided in this embodiment of the invention.

[0038] The purpose, features, and advantages of this application will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0039] The present application will now be further described in conjunction with the accompanying drawings and specific embodiments. It should be noted that, without conflict, the various embodiments or technical features described below can be arbitrarily combined to form new embodiments.

[0040] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be further described in detail below with reference to specific examples and the accompanying drawings. It should be understood that the specific embodiments described herein are merely illustrative and are not intended to limit the scope of this application.

[0041] It should be noted that all uses of the terms "first" and "second" in the embodiments of the present invention are for the purpose of distinguishing two different entities or different parameters with the same name. Therefore, "first" and "second" are merely for convenience of expression and should not be construed as limiting the embodiments of the present invention. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion, such as other steps or units inherent in a process, method, apparatus, product, or device that includes a series of steps or units.

[0042] Because existing technologies for generating time-domain depolarized light cannot truly achieve complete randomness of the beam polarization state in the time domain, nor can they customize the topological charge.

[0043] To address the aforementioned technical problems, this invention proposes a method and apparatus for generating a randomly polarized beam. By using a phase modulation unit to modulate an ordinary laser beam, a vortex beam with arbitrary topological charge is obtained, and then the vortex beam is modulated into a randomly polarized beam using a polarization modulation unit.

[0044] The following section details a method for generating a randomly polarized beam proposed in this invention. This is a novel method for generating a time-domain randomly polarized beam, which can achieve true time randomness.

[0045] Traditional methods for generating randomly polarized beams in the time domain rely on the continuous rotation of waveplates to accumulate random polarization states over time. Since the polarization state of the beam generated by this method changes continuously over time, it does not strictly fall under the category of random polarization. In contrast, using electrically controllable liquid crystal phase devices to apply random voltages allows for random control of the beam's polarization state.

[0046] This invention derives an optical matrix expression for controlling the polarization state of the Poincaré sphere surface through theoretical analysis, and designs an electrically controllable liquid crystal phase device based on this. The principles involved in the embodiments of this invention are as follows:

[0047] The Jones matrix of a linearly polarized beam can be expressed as:

[0048]

[0049] The Jones matrix J of the first liquid crystal phase plate, whose fast axis makes an angle of 45° with the linear polarization direction, is represented as:

[0050]

[0051] In the formula, δ represents the phase difference between the fast and slow axes of the first liquid crystal phase plate; i represents the imaginary unit, and i is a positive number.

[0052] The Jones matrix J′ of the second liquid crystal phase plate, where the fast axis makes a 0° angle with the linear polarization direction, is expressed as:

[0053]

[0054] In the formula, δ′ represents the phase difference between the fast axis and the slow axis of the second liquid crystal phase plate.

[0055] The Jones matrix M of a quarter-wave plate with a fast axis at an angle of -45° to the linear polarization direction is expressed as:

[0056]

[0057] The Jones matrix E of the outgoing beam after the incident ray-polarized beam passes through the aforementioned optical elements is expressed as:

[0058]

[0059] The Jones matrix of the outgoing beam is decomposed to establish the parametric expression of the Poincaré sphere;

[0060] S1=|E x | 2 -|E y | 2 =2(E RCP * E ICP +E ICP * E RCP )=cosδ′sinδ

[0061] S2 = E x * E y +E y * E x =2i(E RCP * E ICP -E ICP * E RCP )=sinδ′sinδ

[0062] S3=i(E y * E x -E x * E y )=2(|E ICP | 2 -|ERCP | 2 )=cosδ

[0063] In the formula, E x E represents the x-component of the electric field vector. y E represents the y-component of the electric field vector. RCP E represents the right-hand component of the electric field. ICP This represents the left-hand component of the electric field.

[0064] A diagram of the Poincaré sphere is shown below. Figure 3 As shown, where δ=θ, δ′=φ, therefore, to achieve a response covering all polarization states on the surface of the Poincaré sphere, the values ​​of θ and φ should be in the ranges of 0-π and 0-2π, respectively.

[0065] Analysis of the above formulas reveals that for a beam whose polarization response range covers any point on the surface of a Poincaré sphere, the phase delay of the liquid crystal phase device can be controlled by generating a random voltage.

[0066] Based on the above principles, embodiments of the present invention generate a randomly polarized beam whose polarization response range covers any point on the surface of a Poincaré sphere by combining an electrically controllable liquid crystal phase device with a quarter-wave plate, and can customize complex beams with arbitrary topological charges by combining an SLM (spatial light modulator).

[0067] See Figure 1 As shown, an embodiment of the present invention provides a method for generating a randomly polarized beam, wherein the method includes the following steps S1-S3;

[0068] Step S1: Generate a highly coherent linearly polarized beam using the continuous excitation light source unit 101;

[0069] Step S2: Modulate the linearly polarized beam generated by the excitation source unit 101 using the phase modulation unit 201 to obtain a vortex beam with arbitrary topological charge.

[0070] Step S3: The vortex beam is modulated into a randomly polarized beam using the polarization modulation unit 301. The response range of the random polarization can cover any point on the surface of the Poincaré sphere.

[0071] The random polarization state is controlled by two constantly refreshed random voltages.

[0072] In this embodiment, the polarization modulation unit 301 is suitable for laser beams of corresponding wavelengths, and the polarization state of the beam can be independently controlled on the equatorial and meridional planes of the Poincaré sphere, including:

[0073] A random voltage generator is used to generate two constantly refreshed random voltages within two different ranges.

[0074] The deflection angle of the liquid crystal molecules in the two liquid crystal phase plates is independently controlled by the random voltage, thereby realizing the control of the modulation phase of the two liquid crystal phase plates within 0-2π and 0-π respectively;

[0075] A quarter-wave plate is used to compensate the phase of the emitted beam from two liquid crystal phase plates, thereby enabling independent control of the polarization state on the equatorial and meridional planes of the Poincaré sphere.

[0076] In embodiments of the present invention, a device for generating randomly polarized beams is also provided, such as... Figure 2 The detailed structural diagram of the generating device is shown. The device includes an excitation light source unit 101, a phase modulation unit 201, and a polarization modulation unit 301.

[0077] The excitation light source unit 101 is used to generate a linearly polarized beam with high coherence.

[0078] The excitation light source unit 101 includes a laser light source 1011 and a first polarizer 1012. The transmission polarization direction of the first polarizer 1012 is consistent with the polarization direction of the outgoing light from the laser light source 1011, which is used to further improve the polarization degree of the incident beam.

[0079] The phase modulation unit 201 is used to modulate the linearly polarized beam generated by the excitation light source unit 101 to obtain a vortex beam with arbitrary topological charge.

[0080] The phase modulation unit 201 includes a half-wave plate 2011 and an SLM 2012. The half-wave plate 2011 is used to rotate the polarization direction of the incident beam to enhance the intensity of the first diffraction order in the beam output by the SLM 2012. The SLM 2012 is used to control the phase that generates an arbitrary topological charge.

[0081] The polarization modulation unit 301 is used to modulate the vortex beam into a randomly polarized beam, and the response range of the random polarization can cover any point on the surface of the Poincaré sphere.

[0082] In this embodiment, the randomly polarized beam has a randomly polarized state with an arbitrary phase. This random polarization state is controlled by two continuously refreshed random voltages, which determine the phase variation of the liquid crystal phase device within the ranges of 0-2π and 0-π.

[0083] The polarization modulation unit 301 includes a voltage generation unit 3011, a first liquid crystal phase plate 3012, a second liquid crystal phase plate 3013, and a quarter-wave plate 3014. The voltage generation unit 3011 generates two continuously refreshed random voltages within corresponding phase ranges. The first liquid crystal phase plate 3012 and the second liquid crystal phase plate 3013 are controlled by the two random voltages to generate phase delays of 0-π and 0-2π, respectively. The quarter-wave plate 3014 is used for phase compensation of the system. Figure 4 , Figure 5 and Figure 6 Schematic diagrams of the fast axis directions of the first liquid crystal phase plate 3012, the second liquid crystal phase plate 3013, and the quarter-wave plate 3014 are given respectively.

[0084] In this embodiment of the invention, the polarization modulation unit further includes a random phase generation unit; the random phase generation unit is used to independently control the deflection angle of the liquid crystal molecules in the two liquid crystal phase plates using the random voltage, thereby realizing the change control of the modulation phase of the two liquid crystal phase plates within 0-2π and 0-π respectively, and performing phase compensation on the outgoing beam of the two liquid crystal phase plates through a quarter-wave plate, thereby realizing independent control of the polarization state on the equatorial plane and meridional plane of the Poincaré sphere.

[0085] In this embodiment of the invention, a linearly polarized incident beam is first generated by an excitation light source unit, and after passing through a phase modulation unit, it is customized into a vortex beam that can carry arbitrary topological charge. The vortex beam is further transformed into a complex beam with random polarization state after passing through a polarization modulation unit.

[0086] The present invention provides a device for generating randomly polarized beams, which can modulate vortex beams carrying arbitrary topological charges into beams with randomly changing polarization states. It has high modulation efficiency, uniform polarization state distribution, low cost, and good practical value.

[0087] The above are exemplary embodiments disclosed in this invention. However, it should be noted that various changes and modifications can be made without departing from the scope of the embodiments of this invention as defined by the claims. The functions, steps, and / or actions of the methods according to the disclosed embodiments described herein do not need to be performed in any particular order. Furthermore, although the elements disclosed in the embodiments of this invention may be described or claimed individually, they may be understood as multiple unless explicitly limited to a singular number.

[0088] It should be understood that, as used herein, the singular form "a" is intended to include the plural form as well, unless the context clearly supports an exception. It should also be understood that, as used herein, "and / or" refers to any and all possible combinations of one or more of the associatedly listed items. The embodiment numbers disclosed above are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0089] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention (including the claims) is limited to these examples. Within the framework of the invention, technical features of the above embodiments or different embodiments can be combined, and many other variations of different aspects of the invention exist, which are not provided in the details for the sake of brevity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the invention should be included within the protection scope of the invention.

Claims

1. A device for generating a randomly polarized beam, characterized in that, The device includes: Excitation source unit, used to generate a highly coherent linearly polarized beam; A phase modulation unit is used to modulate the linearly polarized beam generated by the excitation light source unit to obtain a vortex beam with arbitrary topological charge. A polarization modulation unit is used to modulate the vortex beam into a randomly polarized beam, the response range of which covers any point on the surface of the Poincaré sphere; The polarization modulation unit includes a voltage generation unit, a first liquid crystal phase plate, a second liquid crystal phase plate, and a quarter-wave plate. The voltage generation unit is used to generate two continuously refreshed random voltages within corresponding phase ranges. The first liquid crystal phase plate and the second liquid crystal phase plate are controlled by the two random voltages to generate phase delays of 0-π and 0-2π, respectively. The quarter-wave plate is used for phase compensation of the system. The polarization modulation unit includes a random phase generation unit; The random phase generation unit is used to independently control the deflection angle of the liquid crystal molecules in the two liquid crystal phase plates using the random voltage, thereby realizing the control of the modulation phase of the two liquid crystal phase plates within 0-2π and 0-π respectively, and to perform phase compensation on the outgoing beam of the two liquid crystal phase plates through a quarter-wave plate, so as to realize the independent control of the polarization state on the equatorial plane and the meridional plane of the Poincaré sphere. The phase modulation unit includes a half-wave plate and an SLM. The half-wave plate is used to rotate the polarization direction of the incident beam to enhance the intensity of the first diffraction order in the beam output from the SLM. The SLM is used to control the phase that generates arbitrary topological charge.

2. The device for generating a randomly polarized beam according to claim 1, characterized in that, The excitation source unit includes a laser source and a first polarizer. The transmission polarization direction of the first polarizer is consistent with the polarization direction of the emitted light from the laser source, which is used to improve the polarization degree of the incident beam.

3. The apparatus for generating a randomly polarized beam according to claim 2, characterized in that, The randomly polarized beam is a randomly polarized state with an arbitrary phase.

4. The apparatus for generating a randomly polarized beam according to claim 3, characterized in that, The random polarization state is controlled by two constantly refreshed random voltages, which determine the phase of the liquid crystal phase device within the range of 0-2π and 0-π.

5. A method for generating a randomly polarized beam, characterized in that, A random polarization beam is generated based on the random polarization beam generating apparatus according to any one of claims 3-4, and the method for generating the random polarization beam includes the following steps: A highly coherent linearly polarized beam is generated using a continuous excitation light source unit. The linearly polarized beam generated by the excitation source unit is modulated using a phase modulation unit to obtain a vortex beam with arbitrary topological charge. The vortex beam is modulated into a randomly polarized beam using a polarization modulation unit, and the response range of the random polarization is to cover any point on the surface of the Poincaré sphere. The polarization modulation unit is suitable for laser beams of corresponding wavelengths, and the polarization state of the beam can be independently controlled on the equatorial and meridional planes of the Poincaré sphere, including: A random voltage generator is used to generate two constantly refreshed random voltages within two different ranges. The deflection angle of the liquid crystal molecules in the two liquid crystal phase plates is independently controlled by the random voltage, thereby realizing the control of the modulation phase of the two liquid crystal phase plates within 0-2π and 0-π respectively; A quarter-wave plate is used to compensate the phase of the emitted beam from two liquid crystal phase plates, thereby enabling independent control of the polarization state on the equatorial and meridional planes of the Poincaré sphere.

6. The method for generating a randomly polarized beam according to claim 5, characterized in that, The random polarization state is controlled by two constantly refreshed random voltages.

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