CN106353898B - Optical vortex generation system - Google Patents

Abstract

The invention is suitable for light beam regulation and control, and provides an optical vortex generating system, which comprises: an excitation light source unit for generating a laser beam; a polarization modulation unit for modulating the laser beam into a circularly polarized laser beam; the optical modulation unit comprises a plurality of modulation groups, and each modulation group comprises a vortex wave plate and a half wave plate and is used for modulating the circularly polarized laser beam into optical vortex of a preset topological charge. The embodiment of the invention can modulate the laser beam into the circularly polarized laser beam, flexibly modulate the circularly polarized laser beam into the optical vortex of any topological charge, has high conversion efficiency, and simultaneously solves the problems of frequency spectrum broadening and other chromatic dispersion brought by the existing generation mode in the pulse optical vortex generation process.

Description

Optical vortex generation system

technical field

The invention belongs to the technical field of light beam regulation, and in particular relates to an optical vortex generation system.

Background technique

The optical vortex is a kind of beam with a spiral phase distribution, and its expression has a phase factor, and each photon in the beam carries The orbital angular momentum of , where l is called the topological charge, /> is the reduced Planck constant. During the transmission process, the center of the beam produces a singularity due to phase uncertainty or mutation, and the light intensity at the singularity is zero, without heating effect, and without diffraction effect. Due to the characteristics, complexity and diversity of optical vortices and optical vortices, as well as their potential applications, they have attracted great attention. The research topic based on optical vortex is fundamental and forward-looking. This research topic has a profound impact on the understanding of the nature of light. Therefore, the research on optical vortex and its characteristics has very important scientific significance, and it is worthy of more extensive research. and in-depth research.

Optical vortices have been widely and practically used due to their particularity in optical angular momentum and dynamical behavior. Optical vortices can not only be used to increase the mode volume of laser cavity, guide light, shift frequency, change angular momentum, but also act as dark solitons in self-focusing media. The orbital angular momentum possessed by the optical vortex can be used for information decoding in free space optical communication. In addition, the most prominent application of optical vortices is in the field of optical micro-manipulation, such as light trapping of particles and atoms, trapping and guiding particles, and rotating absorbed particles.

There are many methods for generating optical vortices. The helical phase plate is the most obvious method to generate helical wavefronts. A plane wave passing through a phase plate can generate an optical vortex with an orbital angular momentum of lh. Although the concept of the helical phase photogram is simple, it requires the processing of nanoscale ultra-precise helicoids in the optical band. Therefore, other ways to generate helical wavefronts have also been proposed, such as the mode converter using cylindrical mirrors and Computational holograms, etc. Due to the increasing complexity of refractive optics, diffractive optical elements are another option for generating optical vortices. Fork-shaped gratings can generate optical vortices in the first-order diffraction direction, and this diffractive optical element is actually a hologram of the desired optical element, or called a computational hologram. The hologram that generates the helical beam is a fork grating or a helical Fresnel mirror, and this technology is widely implemented on commercial SLM (Spatial Light Modulator, spatial light modulator). Compared with traditional hologram processing technology, SLM can produce hologram simply and quickly through dynamic modulation. In addition, there are some other methods, such as non-uniform anisotropic elements, sub-wavelength dielectric gratings and metal nano-antennas.

However, the above methods of generating optical vortexes face several common problems: first, the conversion efficiency is low, and the energy utilization rate of optical vortex generation does not exceed 50%; second, the method based on device adjustment is not flexible enough, and it is usually difficult to achieve The conversion between optical vortices; the third is that an important characteristic of pulsed lasers is the high time resolution of pulses. In the process of modulating femtosecond lasers in the above way, multiple optical devices must be passed through repeatedly during the generation process, resulting in broadening of the beam spectrum. The problem of equal dispersion seriously affects the peak value and time resolution characteristics of femtosecond laser pulses.

Contents of the invention

The technical problem to be solved by the present invention is to provide an optical vortex generation system, which aims to solve the problems of low conversion efficiency and dispersion that cause beam spectrum broadening in the existing optical vortex generation methods.

The present invention is achieved in this way, a system for generating an optical vortex, comprising:

an exciting light source unit for generating a laser beam;

a polarization modulation unit, configured to modulate the laser beam into a circularly polarized laser beam;

The optical modulation unit includes several modulation groups, each of which includes a vortex wave plate and a half-wave plate, for modulating the circularly polarized laser beam into an optical vortex with a preset topological charge.

Further, the polarization modulation unit includes a first polarizer and a quarter-wave plate, and the included angle between the long axis direction of the quarter-wave plate and the polarization direction of the first polarizer is π/4 .

Further, the generation system also includes a light splitting unit, an optical path compensation unit, a reflection unit, a polarization unit, and a beam combining unit;

The light splitting unit is used to divide the laser beam generated by the exciting light source unit into a modulation beam and a detection beam;

The reflection unit is configured to adjust the path of the modulated light beam, so that the modulated light beam is incident on the polarization modulation unit according to a preset path;

The polarization modulation unit is configured to modulate the modulated beam into a circularly polarized laser beam;

The optical modulation unit is configured to modulate the circularly polarized laser beam into an optical vortex of a preset topological charge incident on the polarization unit;

The polarization unit is configured to perform polarization modulation on the optical vortex to enter the beam combining unit;

The optical path compensation unit is configured to perform optical path compensation on the detection beam, so that the optical path compensated detection beam and the optical vortex are synchronously incident on the beam combining unit;

The beam combining unit is configured to combine the detection beam after optical path compensation and the optical vortex to obtain a combined beam.

Further, the optical path compensation unit includes a first reflector, a second reflector, a third reflector, a fourth reflector and a second polarizer;

The detection light beam is incident on the second polarizer after the optical path is adjusted by the first reflector, the second reflector, the third reflector and the fourth reflector in sequence;

The detection light beam after the optical path adjustment is subjected to polarization modulation by the second polarizer, and then enters the beam combining unit.

Further, the fast axis direction of the vortex wave plate rotates continuously along the circumference of the vortex wave plate, and the change angle of the fast axis direction around the circumference is π.

Further, the optical axis of the vortex wave plate coincides with the optical axis of the circularly polarized laser beam.

Further, the vortex wave plate is a liquid crystal half wave plate.

Further, when it is necessary to generate an optical vortex whose phase changes clockwise, the half-wave plate of the last group of modulation groups in the optical modulation unit is reserved.

Further, when it is necessary to generate a femtosecond cylindrical vector beam whose phase changes counterclockwise, the half-wave plate of the last group of modulation groups in the optical modulation unit is removed.

Further, the light splitting unit is a polarization-insensitive device.

Compared with the prior art, the present invention has the beneficial effect that: the embodiment of the present invention modulates the laser beam generated by the excitation light source unit into a circularly polarized laser beam, and modulates the circularly polarized laser beam into a preset topological charge through the optical modulation unit. Optical vortex. The embodiment of the present invention can modulate the laser beam into a circularly polarized laser beam, and flexibly modulate the circularly polarized laser beam into an optical vortex of any topological charge, which has high conversion efficiency and solves the problem of pulsed optical vortex generation in the existing generation method. Dispersion problems such as spectral broadening brought about in the

Description of drawings

Fig. 1 is a schematic structural diagram of an optical vortex generation system provided by the first embodiment of the present invention;

Fig. 2 is a detailed structural schematic diagram of an optical vortex generation system provided by the first embodiment of the present invention;

Fig. 3 is a schematic structural diagram of an optical vortex generation system provided by the second embodiment of the present invention;

Fig. 4 is a detailed structural schematic diagram of an optical vortex generation system provided by the second embodiment of the present invention;

Figure 5a is a schematic diagram of the fast axis direction of the vortex wave plate;

Figure 5b is a schematic diagram of the fast axis direction of the half-wave plate;

Figure 6a is a schematic diagram of the phase distribution of an optical vortex with a topological charge of 1;

Figure 6b is a schematic diagram of the phase distribution of an optical vortex with a topological charge of 3;

Figure 7a is the fork-shaped grating fringes produced by the interference of linearly polarized optical vortex with a topological charge of 1 and linearly polarized light;

Figure 7b shows the fork-shaped grating fringes produced by the interference of linearly polarized optical vortex with a topological charge of 3 and linearly polarized light.

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.

The principles involved in the embodiments of the present invention are as follows:

The Jones matrix of a circularly polarized laser beam can be expressed as:

The Jones matrix _M1 of a clockwise-varying vortex wave plate is expressed as:

Taking a right-handed circularly polarized laser beam as an example, the Jones matrix of the beam generated after passing through a modulation group including a vortex wave plate and a half-wave plate can be expressed as:

That is to say, the topological charge that causes the phase plane to change in the counterclockwise direction is -1 optical vortex. The Jones matrix after inserting the half-wave plate after the above modulation group can be modulated as:

At this point, the incident right-handed circularly polarized laser beam is modulated as:

That is, the topological charge that causes the phase plane to change clockwise is 1 optical vortex.

Similarly, to generate an optical vortex whose topological charge changes counterclockwise is l (here, the topological charge whose phase changes clockwise is defined as positive, and the counterclockwise change is negative), the Jones matrix M of the modulation group needs to be expressed as:

where l represents the topological charge that generates the optical vortex. In order to obtain the topological charge l optical vortex whose phase surface changes in the clockwise direction, the Jones matrix M _l' of the modulation group needs to be expressed as:

where _H0 represents a half-wave plate with the fast axis along the horizontal direction, and the Jones matrix is expressed as:

In order to generate optical vortices with higher topological charges, the following relationship exists between the Jones matrices of the modulation group:

M _l =M _l-1 H ₀ M ₁ (9)

From the above formula, it can be found that for the optical vortex that needs to generate any high topological charge, it can be generated by combining the optical vortex with the bottom topological charge with modulation group modulation; on the other hand, for the optical vortex with negative topological charge can be modulated by a half-wave plate corresponding to Optical vortices of positive topological charges are produced.

Based on the above principles, the embodiment of the present invention generates an optical vortex of any topological charge by using a modulation group combining a vortex wave plate and a half-wave plate. As shown in Figure 1, an optical vortex generation system includes:

Exciting light source unit 101 for generating laser beams;

a polarization modulation unit 102, configured to modulate the laser beam into a circularly polarized laser beam;

The optical modulation unit 103 is configured to modulate the circularly polarized laser beam into an optical vortex with a preset topological charge.

Specifically, as shown in FIG. 2 , the polarization modulation unit 102 includes a first polarizer 1021 and a quarter-wave plate 1022, the long-axis direction of the quarter-wave plate 1022 is clamped by the polarization direction of the first polarizer 1021 The angle is π/4. The laser beam generated by the excitation light source unit 101 is generally similar to elliptically polarized light, so the polarization modulation unit 102 is required to modulate it into circularly polarized light.

The optical modulation unit 103 includes several modulation groups 1031, and each modulation group 1031 includes a vortex wave plate and a half-wave plate;

The modulation group 1031 is used to modulate the incident circularly polarized laser beam into an optical vortex with a preset topological charge.

In the embodiment of the present invention, the laser beam generated by the excitation light source unit 101 is firstly modulated into a circularly polarized laser beam by the polarization modulation unit 102, and then the circularly polarized laser beam passes through a modulation group composed of a vortex wave plate and a half-wave plate, and is Optical vortices modulated into preset topological charges. In this embodiment, what the vortex wave plate adopts is a liquid crystal half-wave plate that rotates continuously along the wave circle in the direction of the fast axis. The laser transmittance of this liquid crystal half-wave plate is as high as 99%. Polarized laser beams are modulated into optical vortices with high efficiency and high purity.

The optical vortex generating system provided in this embodiment can be used to generate optical vortices of any topological charge, and is also applicable to the field of pulsed lasers.

As shown in Figure 3, another optical vortex generation system provided by the present invention includes:

Exciting light source unit 201 for generating laser beams;

The light splitting unit 202 is used to divide the laser beam generated by the excitation light source unit 201 into a modulation beam and a detection beam;

a reflection unit 203, configured to adjust the path of the modulated light beam, so that the modulated light beam enters the polarization modulation unit 204 according to a preset path;

The polarization modulation unit 204 is configured to modulate the modulated beam into a circularly polarized laser beam. Specifically, as shown in FIG. 4 , the polarization modulation unit 204 includes a first polarizer 2041 and a quarter-wave plate 2042, the long axis direction of the quarter-wave plate 2041 and the polarization direction of the first polarizer 2041 are clamped The angle is π/4.

The optical modulation unit 205 is configured to modulate the circularly polarized laser beam into an optical vortex of a preset topological charge to enter the polarization unit 206 . Specifically, the optical modulation unit 205 includes several modulation groups 2051, and each modulation group 2051 includes a vortex wave plate and a half-wave plate. In practical applications, the initial directions of the fast axes of the vortex wave plates in all modulation groups are the same unanimous.

A polarizing unit 206, configured to polarize the optical vortex and then enter the beam combining unit 207;

The optical path compensation unit 208 is configured to perform optical path compensation on the detection beam, so that the optical path compensated detection beam and the optical vortex are synchronously incident on the beam combining unit 207 . As shown in Figure 4, the optical path compensation unit 208 includes a first reflective mirror 2081, a second reflective mirror 2082, a third reflective mirror 2083, a fourth reflective mirror 2084 and a second polarizer 2085, and the detection beam passes through the first reflective mirror 2081 in turn , the second reflector 2082, the third reflector 2083 and the fourth reflector 2084 are adjusted for the optical path, and then incident to the second polarizer 2085, and the detection beam after the optical path adjustment is adjusted for polarization by the second polarizer 2085, and then incident to the Beam combining unit 207.

The beam combining unit 207 is configured to combine the detection beam after optical path compensation and the optical vortex to obtain a combined beam. In this embodiment, in order to detect the topological charge of the optical vortex, the beam combining unit 207 combines the optical vortex with the optical vortex after optical path compensation, and then injects it into an external detection system. Because the optical vortex and the detection beam combine to produce a fork-shaped grating after interference, the fork number of the generated fork-shaped grating represents the topological charge of the optical vortex. The combined beams obtained by the combining unit 207 are incident on the external detection system, and the external detection system compares the number of forks of the obtained fork-shaped grating with the expected designed topological charge. If the two values are the same, it proves that the optical The vortex is the expected optical vortex, if not consistent, it can be judged that the optical vortex is not the expected optical vortex.

The laser light source unit 201 is used to generate a laser beam, and the polarization modulation unit 204 is used to modulate the incident modulated beam into a circularly polarized laser beam. The excitation light source unit 201 uses a laser, which can be a continuous laser or a pulsed laser. The polarization modulation unit 204 includes a first polarizer 2041 and a quarter-wave plate 2042 .

In the specific implementation process, the circularly polarized laser beam is modulated into an optical vortex with a topological charge of 1 after passing through the first modulation group of the optical adjustment unit 205, and the optical vortex with a topological charge of 1 passes through the (m-1) group After the modulation group, it is modulated into an optical vortex with a topological charge of m; by removing the last half-wave plate in the optical path generated by the optical vortex with a topological charge of m, it can be modulated into an optical vortex with a topological charge of -m.

Taking the right-handed circularly polarized laser beam as an example, the right-handed circularly polarized laser beam enters the optical modulation unit 205, and after being modulated by different numbers of (l=m) modulation groups, it is modulated so that the polarization direction is opposite to that of the incident circularly polarized laser beam, and the topological charge is The optical vortex of m; in the case of removing the half-wave plate of the last group of modulation groups, the modulation produces an optical vortex with topological charge -m whose polarization is consistent with the incident circularly polarized light. The modulation result of the left-handed circularly polarized laser beam is opposite; by changing the angle (π/2) of the quarter-wave plate, the conversion of the left-handed circularly polarized laser beam can be realized. Specifically, the positive and negative of the topological charge of the optical vortex is only related to the last half-wave plate of the last modulation group, and the half-wave plate in the last modulation group is kept, and the optical vortex whose phase change is clockwise with a topological charge of l is obtained. Vortex: Remove the last half-wave plate of the last group of modulation groups to obtain an optical vortex whose phase changes counterclockwise and whose topological charge is -1.

The optical path compensation unit 208 in this embodiment is used to supplement the optical path of the detection beam, and the detection beam after optical path compensation will be incident on the beam combining unit 207 synchronously with the generated optical vortex, because in practical applications, the optical vortex Interference with linearly polarized light produces a fork-shaped grating, and the number of forks generated represents the topology and number of the optical vortex, so that the topological charge of the optical vortex can be detected conveniently.

In this embodiment, the devices used in each unit are shown in Figure 4, wherein the reflection unit 203 uses a mirror, the optical path compensation unit 208 uses four mirrors and a polarizer for optical compensation, and the beam combining unit 207 is a beam splitter. The polarizing unit 206 is a polarizer. Specifically, when the fast axis of the vortex wave plate rotates once along the circumference of the wave plate, the angle in the direction of the fast axis changes by π, and the topological charge that can be used to modulate the optical vortex changes by 1. The basic beam modulation structure of this embodiment can be determined according to the topological charge of the optical vortex to be generated, that is, how many modulation groups are required to form the optical modulation unit 205 . In this embodiment, the topological charge of the optical vortex can also be modulated to a reduced order. For example, l (l=m-n) order femtosecond cylindrical vector beam can be generated by sequential modulation of n vortex wave plates by optical vortex with topological charge m.

It should be noted that in the modulation process, each vortex wave plate needs to be modulated coaxially, and the initial direction of the fast axis of the vortex wave plate remains the same. It should also be noted that since the half-wave plate of the modulation group in the optical modulation unit 205 is used to modulate the polarization direction of the output beam, when it is necessary to generate an optical vortex whose polarization direction is consistent with the incident circularly polarized light, the last one along the optical path The half-wave plate should be removed; when the polarization direction of the optical vortex generated by the desired modulation is opposite to the incident polarization, the last half-wave plate along the optical path should be retained.

Using this embodiment, as long as the direction of the quarter-wave plate is changed by π/2 through rotation, the direction of the phase change can be changed without changing the topological charge of the optical vortex. Figures 5a and 5b are schematic diagrams of the distribution in the fast axis direction of the vortex wave plate and the half wave plate, respectively. Figures 6a and 6b are schematic diagrams of phase distributions of optical vortices with topological charges of 1 and 3, respectively. Figures 7a and 7b show the fork-shaped grating fringes produced by the interference of the linearly polarized optical vortex with the topological charges 1 and 3, respectively, and the linearly polarized light.

Compared with the prior art, the above-mentioned embodiments provided by the present invention can be modulated by circularly polarized laser beams to generate arbitrary topological charges with high efficiency, high purity and high stability through the combination of vortex wave plates whose fast axis direction changes continuously along the circumference of the wave plate The optical vortex realizes the modulation of the light field. This technology is of great significance in cutting-edge fields such as processing, communication, optical manipulation, and surface-enhanced Raman scattering research.

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 should be included in the protection of the present invention. within range.

Claims (9)

1. A generation system of optical vortex, it is characterized in that, described generation system comprises: an exciting light source unit for generating a laser beam; a polarization modulation unit, configured to modulate the laser beam into a circularly polarized laser beam; The optical modulation unit includes several modulation groups, each of which includes a vortex wave plate and a half-wave plate, and the vortex wave plate is a liquid crystal half-wave plate that rotates continuously along the wave circle in the direction of the fast axis, and is used to convert the The circularly polarized laser beam is modulated into an optical vortex with a preset topological charge. During the modulation process, each vortex wave plate is coaxially modulated, and the initial direction of the fast axis of the vortex wave plate remains consistent. 2. The generating system according to claim 1, wherein the polarization modulation unit comprises a first polarizer and a quarter-wave plate, and the direction of the long axis of the quarter-wave plate is the same as that of the first polarizer. The included angle of the polarization direction of a polarizer is π/4. 3. The generation system according to claim 2, wherein the generation system further comprises a light splitting unit, an optical path compensation unit, a reflection unit, a polarization unit, and a beam combining unit; The light splitting unit is used to divide the laser beam generated by the exciting light source unit into a modulation beam and a detection beam; The reflection unit is configured to adjust the path of the modulated light beam, so that the modulated light beam is incident on the polarization modulation unit according to a preset path; The polarization modulation unit is configured to modulate the modulated beam into a circularly polarized laser beam; The optical modulation unit is configured to modulate the circularly polarized laser beam into an optical vortex of a preset topological charge incident on the polarization unit; The polarization unit is configured to perform polarization modulation on the optical vortex to enter the beam combining unit; The optical path compensation unit is configured to perform optical path compensation on the detection beam, so that the optical path compensated detection beam and the optical vortex are synchronously incident on the beam combining unit; The beam combining unit is configured to combine the detection beam after optical path compensation and the optical vortex to obtain a combined beam. 4. The generating system according to claim 3, wherein the optical path compensation unit comprises a first reflector, a second reflector, a third reflector, a fourth reflector and a second polarizer; The detection light beam is incident on the second polarizer after the optical path is adjusted by the first reflector, the second reflector, the third reflector and the fourth reflector in sequence; The detection light beam after the optical path adjustment is subjected to polarization modulation by the second polarizer, and then enters the beam combining unit. 5. The generation system according to any one of claims 1 to 4, wherein the fast axis direction of the vortex wave plate rotates continuously along the circumference of the vortex wave plate, and the change angle of the fast axis direction around the circumference is for π. 6. The generating system according to any one of claims 1 to 4, wherein the optical axis of the vortex wave plate coincides with the optical axis of the circularly polarized laser beam. 7. The generation system according to claim 1, wherein when it is necessary to generate an optical vortex whose phase changes clockwise, the half-wave plate of the last group of modulation groups in the optical modulation unit is reserved. 8. The generating system according to claim 1, wherein when it is necessary to generate a femtosecond cylindrical vector beam whose phase changes counterclockwise, the half-wave plate of the last group of modulation groups in the optical modulation unit is removed. 9. The generating system according to claim 4, wherein the light splitting unit is a polarization-insensitive device.