High-fidelity structured light field generation method based on three-beam nonlinear holography

Abstract

The application discloses a kind of high-fidelity structured light field generation methods based on three-beam nonlinear hologram, it is related to holography technical field.The method includes the following steps: by along x-axis and y-axis direction incidence, two beams of structured light incidence relative to z-axis tilt, form point interference structure;Point lattice binary amplitude hologram is formed by further processing to the point interference structure;Using the point lattice binary amplitude hologram, it is combined with nonlinear photonic crystal, and nonlinear hologram is formed;Using the nonlinear photonic crystal obtained in step three, by base frequency light vertical incidence holographic structure, two beams of high-fidelity frequency-doubled structured light are respectively reproduced on x and y axis fourier surface.The nonlinear hologram obtained by the method has good noise immunity, and is not sensitive to domain structure edge diffusion in the process of nonlinear crystal processing, and the actual obtained nonlinear structured light field has very high fidelity compared with ideal result.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of holography, in particular to a high-fidelity structured light field generation method based on three-beam nonlinear holography. BACKGROUND

[0002] Holography is essentially a process of recording the amplitude and phase information of the light wave front and reconstructing the wave front. By forming interference fringes through the interaction of a reference light and a target light and recording them, a hologram can be obtained, and by using a reconstruction wave to irradiate the hologram, the target light can be reobtained.

[0003] With the development of computer technology, digital holography technology has been increasingly applied. Among them, computational holography can calculate the required hologram by means of simulation software without using actual reference light and target light. Computational holography technology also plays an important role in the field of nonlinear optics. Based on the hologram, the domain structure of a uniformly oriented ferroelectric material can be modulated by the widely used electric field polarization method, and the required nonlinear photonic crystal can be obtained. When the fundamental frequency light is incident on the nonlinear photonic crystal, the target nonlinear structured light in the frequency-doubled diffraction field can be observed. It is worth noting that the hologram generated in this way usually exhibits a fringe-like structure, and is therefore susceptible to manufacturing errors.

[0004] During the domain inversion of the nonlinear photonic crystal, the ferroelectric domains growing longitudinally along the patterned electrode under an applied electric field usually undergo uncontrollable lateral diffusion. In the femtosecond laser direct writing method, although the processing precision can be improved, errors caused by mechanical vibration are inevitable when modulating continuous fringe domain structures. Therefore, for traditional nonlinear photonic crystals based on fringe structures, these manufacturing errors greatly reduce the precision of domain structure modulation, thereby limiting the fidelity of the frequency-doubled structured light field manipulated by the nonlinear photonic crystal. SUMMARY

[0005] The technical problem to be solved by the present application is how to provide a high-fidelity structured light field generation method based on three-beam nonlinear holography with high fidelity of frequency-doubled structured light field.

[0006] To solve the above technical problems, the technical solution adopted by the present application is: a high-fidelity structured light field generation method based on three-beam nonlinear holography, comprising the following steps:

[0007] S1: forming a point-like interference structure by incidenting two beams of structured light inclined relative to the z-axis along the x-axis and y-axis directions;

[0008] S2: further processing the point-like interference structure to form a dot matrix binary amplitude hologram;

[0009] S3: using the dot matrix binary amplitude hologram, combining it with a nonlinear photonic crystal to form a nonlinear hologram;

[0010] S4: using the nonlinear hologram obtained in step three, by vertically incident holographic structure with base frequency light, two high-fidelity frequency doubling structure lights are respectively reproduced on x and y axis Fourier plane.

[0011] Further technical solutions are that the method for generating point interference structure in S1 comprises the following steps:

[0012] The expressions of the two beams of structured light are selected as:

[0013]

[0014] Wherein, A s1 (x,y),A s2 (x,y) and φ s1 (x,y),φ s2 (x,y) represent the intensity and phase distribution of the two beams of light respectively, and the above two beams of structured light are superimposed with a tilt phase:

[0015]

[0016] Wherein λ is the wavelength of incident light;

[0017] The two beams of light are superimposed with a lens phase:

[0018]

[0019] Preferably, the tilt angle is φ1=φ2=1 / 180*p.

[0020] Preferably, the focal length is selected as f1=0.1m, f2=0.2m.

[0021] Further technical solutions are that the structure function of the hologram in step S3 is:

[0022]

[0023] p1(x,y)=φ s1 (x,y)+φ s1_tilt (x,y)+φ s1_lens (x,y,f1)

[0024] p2(x,y)=φ s2 (x,y)+φ s2_tilt (x,y)+φ s2_lens (x,y,f2)

[0025]

[0026] wherein with respectively represent A s1 (x,y) and A s2 (x,y) are the maximum values, so that the intensity function is normalized.

[0027] The beneficial effects produced by the above technical solutions are that the application can simultaneously control the diffraction light field in the x-axis and y-axis directions, and can control different focal lengths to distinguish the structural light field in two directions; the nonlinear hologram obtained by the method has good anti-noise ability, is not sensitive to the domain structure edge diffusion in the nonlinear crystal processing process, and has high fidelity compared with the ideal result; the hologram structure obtained by the application has good continuity, can be well combined with the electrode polarization domain inversion technology, and can prepare a high-quality nonlinear photonic crystal. BRIEF DESCRIPTION OF DRAWINGS

[0028] The application will be further described in detail below in combination with the drawings and specific embodiments.

[0029] Figure 1a is a schematic diagram of the storage process of a traditional nonlinear hologram;

[0030] Figure 1b is a schematic diagram of the storage process of a three-beam hologram in the embodiment of the application;

[0031] Figure 2a is a schematic diagram of the reconstruction process of a traditional nonlinear hologram;

[0032] Figure 2b is a schematic diagram of the reconstruction process of a three-beam hologram in the embodiment of the application;

[0033] Figure 3 is a flowchart of the method described in the embodiment of the application;

[0034] Figure 4 is a preparation flowchart of a nonlinear hologram in the application;

[0035] Figure 5 is a structural schematic diagram of an experimental device in the application;

[0036] Figure 6 is a curve of experimental results of an Airy beam in the application. DETAILED DESCRIPTION

[0037] With reference to the drawings of the embodiments of the present application, the technical solutions in the embodiments of the present application are clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments of the present application, all the other embodiments obtained by those skilled in the art without creative work fall within the scope of the present application.

[0038] In the following description, many specific details are set forth in order to provide a thorough understanding of the present application. However, the present application can be practiced without the specific details, other than those described herein, and it is understood that the present application is not limited to the embodiments described herein and can be practiced with or without other apparatuses, systems, structures, methodologies, procedures, components, materials, and so on. Therefore, the present application is not limited to the specific embodiments disclosed below, but includes any alternatives falling within the scope of the present application.

[0039] The classical nonlinear holography only involves the modulation along the x-axis direction, lacking the modulation along the y-axis direction, while the latter can carry independent information due to its orthogonality with the x-axis, such as Figure 1a The reconstruction process is shown in FIG. 2B, which is as follows: Figure 2a Generally, the FW (fundamental wave) beam is incident along the z-axis perpendicular to the nonlinear crystal, i.e. E FW = A FW exp(-ik1z), where A FW represents the complex amplitude of the FW wave, represents the wave number, and λ1 represents the wavelength. The NPW (nonlinear polarization wave) wave excited thereby can be expressed as E p = A p exp(-ik2z), where k2 = 2k1 due to the momentum conservation in the frequency doubling process. For the frequency-doubled structure light (E Figure 2b ) with an inclination angle φ1 and φ2 with the FW (fundamental wave) beam along the x-axis and y-axis directions, respectively, can be expressed as:

[0040] E s1 = A s1 exp[-ik2z-ik2 sin(φ1)x]

[0041] E s2 = A s2 exp[-ik2z-ik2 sin(φ2)y]

[0042] where A s1 and A s2 represent the complex amplitudes of the two incident structure lights, respectively. The interference between the FW (fundamental wave) and the frequency-doubled light can be expressed as:

[0043] I(x, y) = |E p +E s1 +E s2 | 2

[0044] = |E p | 2 + |E s1 | 2 + |E s2 | 2 + 2A p A s1 cos[ik2sin(φ1)x] + 2A p A s2 cos[ik2sin(φ2)y] + 2A s1 A s2 cos[ik2sin(φ1)x - ik2sin(φ2)y]

[0045] where the background signal I0= |E p | 2 + |E s1 | 2 + |E s2 | 2 and the cross modulation term 2A s1 A s2 cos[ik2sin(φ1)x - ik2sin(φ2)y] can be neglected; since the second order nonlinear coefficient χ (2) only has two possible values d ij or -d ij , the functional expression of the domain structure of the required nonlinear photonic crystal can be expressed as f(x, y) = sgn{I1}, where I1= 2A p A s1 cos[ik2sin(φ1)x] + 2A p A s2 cos[ik2sin(φ2)y]. By Fourier-Taylor expansion, it can be seen that the above nonlinear modulation function f(x, y) exhibits a dominant first order component f1(x, y) = A p A s1 exp(-ik2sin(φ1)x) + A p A s2 exp(-ik2sin(φ2)y).

[0046] In the reconstruction process, a FW (fundamental wave) light beam is vertically incident on the modulated nonlinear photonic crystal, and the nonlinear diffraction field E r can be expressed as:

[0047]

[0048] It can be seen that the nonlinear diffraction field reconstructs two independent target structure lights, whose propagation directions have an inclination angle φ1 and φ2 along the x-axis and y-axis directions relative to the z-axis, respectively, as shown inFigure 2b More importantly, due to the angular information between the FW (fundamental wave) beam and the SH (target second harmonic) beam, the resulting nonlinear photonic crystal exhibits a lattice structure in the x-y plane. The resulting hologram can be further replaced with irregular lattices in circles with the same center and area, and a nonlinear hologram can be obtained. Since the nonlinear structure light in the x-axis and y-axis directions can be regulated respectively, different focal length lens phases can be superimposed in two directions By changing the focal length f lens The target light field can be reconstructed on the Fourier plane at a specified distance, thereby better distinguishing the nonlinear structure light in two different directions.

[0049] In summary, as Figure 3 The application discloses a high-fidelity structured light field generation method based on three-beam nonlinear holography, including the following steps:

[0050] S1: Form a point-like interference structure by two beams of structured light incident along the x-axis and y-axis directions and inclined relative to the z-axis;

[0051] S2: Further process the point-like interference structure to form a dot matrix binary amplitude hologram;

[0052] S3: Use the dot matrix binary amplitude hologram to combine it with a nonlinear photonic crystal to prepare a nonlinear hologram;

[0053] S4: Use the nonlinear hologram obtained in step three to reproduce two beams of high-fidelity frequency-doubled structured light on the x and y axis Fourier planes by vertically incident fundamental light on the holographic structure.

[0054] The application obtains a nonlinear hologram based on simulation calculation software Matlab, and the first-order component f1(x,y) = A p A s1 exp(-ik2sin(φ1)x)+A p A s2 exp(-ik2sin(φ2)y) in the above nonlinear modulation function f(x,y) does not contain the z-axis component, so the three-beam interference process between the FW (fundamental wave) and the two target structured light beams can be simplified to the interference of two beams of structured light respectively inclined from the x-axis and y-axis directions in the calculation process.

[0055] The expressions of the two beams of structured light are selected as:

[0056]

[0057] Wherein, A s1 (x,y), A s2 (x,y) and φs1 (x,y), φ s2 (x,y) respectively represent the intensity and phase distribution of the two beams of light. In order to show that the two beams of structured light are respectively inclined to be incident in the x-axis and y-axis directions, a tilt phase can be superimposed:

[0058]

[0059] where λ is the wavelength of the incident light; in order to obtain a hologram with simple structure and an easily distinguishable diffracted light field, the tilt angle is selected as φ1=φ2=1 / 180*p.

[0060] In order to make the two diffracted structured light fields be able to be imaged on different Fourier planes respectively, continue to superimpose a lens phase on the two beams of light:

[0061]

[0062] where in order to facilitate observation of the diffracted light field, the focal length is selected as f1=0.1m, f2=0.2m.

[0063] Using the calculation method of computer holography, the structure function of the hologram can be obtained:

[0064]

[0065] p1(x,y)=φ s1 (x,y)+φ s1_tilt (x,y)+φ s1_lens (x,y,f1)

[0066] p2(x,y)=φ s2 (x,y)+φ s2_tilt (x,y)+φ s2_lens (x,y,f2)

[0067]

[0068] where, and respectively represent the maximum value of A s1 (x,y) and A s2 (x,y), so that the intensity function is normalized.

[0069] Experimental verification:

[0070] Through the external electric field polarization method, the designed hologram can be combined with the nonlinear crystal lithium niobate to modulate the domain structure in lithium niobate. The process of preparing a nonlinear hologram is as follows: Figure 4The thickness of the z-cut lithium niobate crystal used in the experiment is 50 nm, and the size is 100 mm*80 mm. First, the holographic pattern electrode is prepared, following the steps: first, on the heating table, the cleaned lithium niobate crystal is fixed on a glass with paraffin, so as to prevent the lithium niobate crystal from displacement or fragmentation in the subsequent process. Then, the photoresist S1813 is used to coat the surface of the lithium niobate crystal, forming a photoresist layer with a thickness of about 1 nm. Then, using SF100 maskless lithography machine, the designed hologram can be lithographed on the surface of the lithium niobate crystal. After development, the thickness of the chromium metal layer on the lithium niobate can be prepared by electron number evaporation. The photoresist covered by the blank part of the lithography pattern can be dissolved by acetone and fall off, so as to leave the chromium metal layer based on the hologram on the surface of the lithium niobate. Heating again, the lithium niobate crystal can be taken off from the glass, and we can get the lithium niobate crystal with pattern electrode. Then, the lithium niobate crystal is adhered to the ITO glass with lithium chloride solution, and the high-voltage power supply is used to apply a voltage of 1200V with the chromium metal layer as the positive electrode and the ITO glass as the negative electrode for pattern polarization. Then, using the acid reagent to etch the chromium metal layer, the nonlinear hologram based on the lithium niobate crystal can be obtained.

[0071] Figure 5 and Figure 6 The experimental device is shown in the schematic diagram, and the experimental results of the Airy beam are shown as an example. A titanium: sapphire femtosecond laser (Revolution, Coherent, USA) and a pump light optical parametric amplifier (TOPAS-Prime, Light Conversion, Lithuania) are used as light sources, with a pulse duration of 50 fs and a repetition rate of 1 kHz. We first select the wavelength of the fundamental light beam to be 1300 nm. Because the laser intensity directly emitted by the amplifier is too strong, in order to avoid damage to the elements in the optical path, an attenuator is added in front of the optical path to adjust the energy of the incident fundamental light. A 4f optical system composed of two spherical lenses (f=500 mm and 200 mm) is used to expand and collimate the incident light beam to match the size of the lithium niobate nonlinear hologram. Long-pass and short-pass filters are used to block the remaining part of the fundamental light and filter the unwanted wavelengths.

[0072] Taking the Airy light as an example, when the fundamental light is incident on the nonlinear hologram, the frequency-doubled Airy beam can be seen at the Fourier plane (d=200 mm), at which time the light wavelength changes from invisible 1300 nm to red 650 nm, as shown in Figure 2b The domain structure in the lithium niobate crystal can be observed by using a lens (f=45 mm), in which the black circles distinguish the domain walls of opposite polarization directions.

[0073] In summary, the method can simultaneously control the diffraction light field in the x-axis and y-axis directions compared with the classical holographic method, and can control different focal lengths to distinguish the two-directional structured light field; the obtained nonlinear hologram has good anti-noise ability, is not sensitive to domain structure edge diffusion in the nonlinear crystal processing process, and the actually obtained nonlinear structured light field has very high fidelity compared with the ideal result; the obtained hologram structure has good continuity, can be well combined with the electrode polarization domain inversion technology, and high-quality nonlinear photonic crystals can be prepared.

Claims

1. A high-fidelity structured light field generation method based on three-beam nonlinear holography, characterized by The steps include: S1: Two beams of structured light incident along the x-axis and y-axis, tilted relative to the z-axis, form a point-like interference structure; S2: further processing the point-shaped interference structure to form a dot matrix binary amplitude hologram; S3: using the lattice binary amplitude hologram and combining it with a nonlinear photonic crystal to form a nonlinear hologram; S4: Using the nonlinear hologram obtained in step 3, two high-fidelity frequency-doubled structured lights are reproduced on the x-axis and y-axis Fourier planes respectively by vertically incident fundamental frequency light on the holographic structure; The method for generating a point-like interference structure in S1 comprises the following steps: The expressions of two structured light beams are selected as follows: Among them, A s1 (x,y),A s2 (x,y) and φ s1 (x,y),φ s2 (x, y) represents the intensity and phase distribution of the two beams of light, respectively. The two beams of structured light are superimposed with a tilted phase: Where λ is the wavelength of the incident light; Add the lens phase to the two beams: The structure function of the hologram in step S3 is: p1(x,y)=φ s1 (x,y)+φ s1_tilt (x,y)+φ s1_lens (x,y,f1) p2(x,y)=φ s2 (x,y)+φ s2_tilt (x,y)+φ s2_lens (x,y,f2) in and Respectively represent A s1 (x,y) and A s2 The maximum value of (x,y) normalizes the intensity function.

2. The high-fidelity structured light field generation method based on three-beam nonlinear holography according to claim 1, characterized in that: The tilt angle is φ1 = φ2 = 1 / 180*π.

3. The high-fidelity structured light field generation method based on three-beam nonlinear holography according to claim 1, characterized in that: The focal length is selected as f1=0.1m, f2=0.2m.

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