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

Abstract

The invention discloses a high-fidelity structured light field generation method based on three-beam nonlinear holography, and relates to the technical field of holographic photography. The method comprises the following steps that two beams of structured light which are incident in the x-axis direction and the y-axis direction and are inclined relative to the z-axis are incident, so that a point-shaped interference structure is formed; further processing the dotted interference structure to form a dot matrix binary amplitude hologram; combining the dot matrix binary amplitude hologram with a nonlinear photonic crystal to form a nonlinear hologram; and by using the nonlinear photonic crystal obtained in the step 3, two beams of high-fidelity frequency doubling structured light are respectively reproduced on the x-axis Fourier surface and the y-axis Fourier surface through vertical incidence of fundamental frequency light into a holographic structure. The nonlinear hologram obtained by the method has good anti-noise capability and is insensitive to domain structure edge diffusion in the nonlinear crystal processing process, and the actually obtained nonlinear structure light field has very high fidelity compared with an ideal result.

Description

Technical Field

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

[0002] Holography is essentially a process of recording the amplitude and phase information of the light wavefront and then reconstructing the wavefront. By recording the interference fringes formed by the interaction of a reference beam with the target light, a hologram can be obtained. By then illuminating the hologram with the reconstructed wave, the target light can be obtained again.

[0003] With the development of computer technology, digital holography has been increasingly used. Computational holography can use simulation software to calculate the required hologram without using actual reference light and target light. Computational holography also plays an important role in the field of nonlinear optics. Based on the hologram, the domain structure of uniformly oriented ferroelectric materials can be modulated by the widely used electric field polarization method to obtain the required nonlinear photonic crystal. When the fundamental frequency light is incident on the nonlinear photonic crystal, the target nonlinear structure light in the double frequency diffraction field can be observed. It is worth noting that the hologram generated in this way usually appears as a striped structure and is therefore susceptible to manufacturing errors.

[0004] During the process of domain inversion in nonlinear photonic crystals, ferroelectric domains that grow longitudinally along patterned electrodes under an applied electric field usually undergo uncontrollable lateral diffusion. In femtosecond laser direct writing, although the processing accuracy can be improved, errors caused by mechanical vibrations are inevitable when modulating continuous stripe domain structures. Therefore, for traditional nonlinear photonic crystals based on stripe structures, these manufacturing errors greatly reduce the accuracy of domain structure regulation, thereby limiting the fidelity of the frequency-doubled structure light field manipulated by nonlinear photonic crystals. Summary of the invention

[0005] The technical problem to be solved by the present invention 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] In order to solve the above technical problems, the technical solution adopted by the present invention is: a high-fidelity structured light field generation method based on three-beam nonlinear holography, comprising the following steps:

[0007] S1: Two beams of structured light are incident along the x-axis and y-axis directions and tilted relative to the z-axis to form a point interference structure;

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

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

[0010] S4: Using the nonlinear hologram obtained in step 3, the fundamental frequency light is vertically incident on the holographic structure to reproduce two beams of high-fidelity frequency-doubled structured light on the x-axis and y-axis Fourier planes respectively.

[0011] A further technical solution is that the method for generating a point-shaped interference structure in S1 comprises the following steps:

[0012] The expressions of selecting two structured light beams are:

[0013]

[0014] Among them, 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. The two beams of structured light are superimposed with a tilted phase:

[0015]

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

[0017] Add the lens phase to the two beams:

[0018]

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

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

[0021] A further technical solution is 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] in and Respectively represent A s1 (x,y) and A s2 The maximum value of (x,y) normalizes the intensity function.

[0027] The beneficial effects of adopting the above technical scheme are: the present invention can simultaneously regulate the diffraction light field in the x-axis and y-axis directions, and can control different focal lengths to distinguish the structured light fields in the two directions; the nonlinear hologram obtained by the method of the present invention has good noise resistance, is insensitive to the edge diffusion of the domain structure in the nonlinear crystal processing process, and the nonlinear structured light field actually obtained has a high fidelity compared with the ideal result; the hologram structure obtained by the present invention has good continuity, and can be well combined with the electric polarization domain inversion technology to prepare high-quality nonlinear photonic crystals. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0029] Figure 1a It 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 an embodiment of the present invention;

[0031] Figure 2a It 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 an embodiment of the present invention;

[0033] Figure 3 is a flow chart of the method described in an embodiment of the present invention;

[0034] Figure 4 This is the preparation process of the nonlinear hologram in the present invention;

[0035] Figure 5 It is a structural schematic diagram of the experimental device in the present invention;

[0036] Figure 6 It is the experimental result curve taking Airy beam as an example in the present invention. DETAILED DESCRIPTION

[0037] The following is a clear and complete description of the technical solutions in the embodiments of the present invention in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0038] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0039] Classical nonlinear holography involves only modulation along the x-axis, lacking modulation along the y-axis, which can carry independent information due to its orthogonality to the x-axis, such as Figure 1a The reconstruction process is shown in Figure 2a As shown. Usually, the FW (fundamental wave) beam is incident perpendicular to the nonlinear crystal along the z-axis, that is, 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 it excites can be expressed as E p =A p exp(-ik2z), where k2=2k1 due to the conservation of momentum in the frequency doubling process. For the frequency doubling structured light ( Figure 2b ), 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] Among them, A s1 and A s2 Represent the complex amplitudes of the two incident structured lights. Therefore, the interference between FW (fundamental wave) and the doubled frequency 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 cross modulation term 2A s1 A s2 cos[ik2sin(φ1)x-ik2sin(φ2)y] can be ignored; due to the second-order nonlinear coefficient χ (2) There are only two possible values ​​d ij or -d ij Therefore, 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]. Through 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) beam is incident vertically on the modulated nonlinear photonic crystal, and the nonlinear diffraction field E r It can be expressed as:

[0047]

[0048] It can be seen that the nonlinear diffraction field reconstructs two independent target structured lights, whose propagation directions are respectively along the x-axis and y-axis with inclination angles φ1 and φ2 relative to the z-axis, as shown in Figure 2b More importantly, due to the angle information between the FW (fundamental wave) beam and the SH (target second harmonic) beam, the resulting nonlinear photonic crystal appears as a lattice structure in the xy plane. The irregular lattice in the resulting hologram can be further replaced with circles with the same center and area to obtain a nonlinear hologram. Since the nonlinear structured light in the x-axis and y-axis directions can be controlled separately, the phase of lenses with different focal lengths can be superimposed in the 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 structured light in two different directions.

[0049] Based on the above analysis, Figure 3 As shown, the present application discloses a high-fidelity structured light field generation method based on three-beam nonlinear holography, comprising the following steps:

[0050] S1: Two beams of structured light are incident along the x-axis and y-axis directions and tilted relative to the z-axis to form a point interference structure;

[0051] S2: further processing the point-shaped interference structure to form a dot matrix binary amplitude hologram;

[0052] S3: using the lattice binary amplitude hologram and combining it with a nonlinear photonic crystal to prepare a nonlinear hologram;

[0053] S4: Using the nonlinear hologram obtained in step 3, the fundamental frequency light is vertically incident on the holographic structure to reproduce two beams of high-fidelity frequency-doubled structured light on the x-axis and y-axis Fourier planes respectively.

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

[0055] The expressions of selecting two structured light beams are:

[0056]

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

[0058]

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

[0060] In order to make the two diffracted structured light fields imaged on different Fourier planes respectively, the lens phase is superimposed on the two beams of light:

[0061]

[0062] In order to facilitate the observation of the diffraction light field, the focal lengths are selected as f1=0.1m and f2=0.2m.

[0063] Using the calculation method of computational 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] in, and Respectively represent A s1 (x,y) and A s2 The maximum value of (x,y) normalizes the intensity function.

[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 the lithium niobate. The process of preparing nonlinear holograms is as follows: Figure 4As shown. The z-cut lithium niobate crystal used in the experiment has a thickness of 50nm and a size of 100mm*80mm. First, prepare the pattern electrode based on the hologram, following the following steps: First, fix the cleaned lithium niobate crystal on a glass slide with paraffin on a heating table to prevent the lithium niobate crystal from displacement or fragmentation in the subsequent process. Then, use photoresist S1813 to coat the surface of the lithium niobate crystal to form a photoresist layer with a thickness of about 1nm. Then, use the SF100 maskless lithography machine to photolithograph the designed hologram on the surface of the lithium niobate crystal. After development, a chromium metal layer with a thickness of 120nm can be prepared on the lithium niobate by electron number evaporation coating. The photoresist covering the blank part of the photolithography pattern can be dissolved and peeled off by acetone, leaving a chromium metal layer based on the hologram on the surface of the lithium niobate. After heating again, the lithium niobate crystal can be removed from the glass slide, and we can get a lithium niobate crystal with a pattern electrode. Then, the lithium niobate crystal is glued to the ITO glass with a lithium chloride solution, and a high-voltage power supply is used to polarize the pattern with a 1200V voltage applied with the chromium metal layer as the positive electrode and the ITO glass as the negative electrode. After that, the chromium metal layer is corroded with an acidic reagent to obtain a nonlinear hologram based on the lithium niobate crystal.

[0071] Figure 5 and Figure 6 The schematic diagram of the experimental setup and the experimental results using the Airy beam as an example are shown. A titanium:sapphire femtosecond laser (Revolution, Coherent, USA) and a pump light optical parametric amplifier (TOPAS-Prime, LightConversion, Lithuania) were used as the light source, with an output pulse duration of 50 fs and a repetition rate of 1 kHz. We first selected the wavelength of the fundamental frequency beam to be 1300 nm, because the intensity of the laser light directly emitted by the amplifier is too strong. In order to avoid damage to the components in the optical path, an attenuator was added in front of the optical path to adjust the energy of the incident fundamental frequency light. A 4f optical system consisting of two spherical lenses (f=500 mm and 200 mm) expanded and collimated the incident beam to match the size of the lithium niobate nonlinear hologram. Long-pass filters and short-pass filters were used to block the rest of the fundamental frequency light and filter out unwanted wavelengths.

[0072] Taking Airy light as an example, when the fundamental frequency light is incident on the nonlinear hologram, the frequency-doubled Airy beam can be seen at the Fourier plane (d = 200mm). At this time, the wavelength of the light changes from the invisible 1300nm to the red 650nm, as shown in Figure 1. Figure 2b The domain structure in lithium niobate crystal can be observed using a lens (f=45 mm), where the black circles distinguish the domain walls with opposite polarization directions.

[0073] In summary, compared with the classical holographic method, the method of the present invention can simultaneously control the diffraction light field in the x-axis and y-axis directions, and can control different focal lengths to distinguish the structured light fields in two directions; the obtained nonlinear hologram has good noise resistance and is insensitive to the edge diffusion of the domain structure during the nonlinear crystal processing process, and the nonlinear structured light field actually obtained has a very high fidelity compared to the ideal result; the obtained hologram structure has good continuity and can be well combined with the electric polarization domain inversion technology to prepare high-quality nonlinear photonic crystals.

Claims

1. A high-fidelity structured light field generation method based on three-beam nonlinear holography, characterized in that The steps include: S1: Two beams of structured light are incident along the x-axis and y-axis directions and tilted relative to the z-axis to form a point 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, the fundamental frequency light is vertically incident on the holographic structure to reproduce two beams of high-fidelity frequency-doubled structured light on the x-axis and y-axis Fourier planes respectively.

2. The high-fidelity structured light field generation method based on three-beam nonlinear holography according to claim 1, characterized in that: The method for generating a point-shaped interference structure in S1 comprises the following steps: The expressions of selecting two structured light beams are: Among them, 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. 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:

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

4. The high-fidelity structured light field generation method based on three-beam nonlinear holography according to claim 2, characterized in that: The focal length is selected as f1=0.1m, f2=0.2m.

5. The high-fidelity structured light field generation method based on three-beam nonlinear holography according to claim 2, characterized in that: 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.

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