Non-reciprocal light field regulation and control device and method based on non-linear spatial filtering

Abstract

The invention discloses a non-reciprocal light field regulation and control device and method based on non-linear spatial filtering, the device adopts a sandwich type integrated structure, and the core comprises a non-linear medium, and a first linear modulation module and a second linear modulation module which are integrated through femtosecond laser direct writing on the two sides of the non-linear medium; a strict collinear phase matching condition in a nonlinear frequency conversion process is used as a natural spatial domain filter. When light is transmitted in the forward direction, high-frequency space phase information introduced by the first module is physically filtered out in nonlinear conversion due to non-collinear phase mismatch, and an output end only presents modulation information of the second module; and reverse transmission is opposite. Through anisotropic selective transfer of a spatial frequency spectrum, the dependence of a traditional nonreciprocal device on a magneto-optical material is overcome, high-fidelity and one-way controlled transmission of complex spatial wavefront is realized, and the device has important application value in the fields of integrated photonics, optical information encryption and the like.

Description

Technical Field

[0001] This invention relates to nonlinear optics and optical information processing technology, specifically to a non-reciprocal optical field manipulation device and method based on nonlinear spatial filtering. Background Technology

[0002] Non-reciprocal manipulation of optical fields is a crucial foundation for achieving optical isolation, unidirectional information transmission, and secure optical information processing. Existing technologies focus on synthesizing non-reciprocal wavefronts using 3D nonlinear holograms or nonlinear detour phases. Such schemes rely on the manipulation of nonlinear polarizability (…). The complex spatial modification of nonlinear optical fields is challenging to fabricate, and its non-reciprocity stems from the asymmetric structure of the nonlinear source itself, limiting its compatibility with complex spatial optical fields carrying continuous spectra. Furthermore, phase mismatch in nonlinear processes is widely considered a negative factor suppressing energy conversion efficiency. How to transform the angular sensitivity of phase matching into a proactive spatial filtering method and combine it with femtosecond laser direct writing technology to achieve high-fidelity integrated non-reciprocal devices is a pressing technical challenge.

[0003] In recent years, nonmagnetic nonreciprocal schemes based on time-varying modulation, acousto-optic effects, and nonlinear optical effects have attracted widespread attention. However, existing nonmagnetic nonreciprocal methods mostly focus on nonreciprocal transmission in the intensity or frequency dimensions, and direct, programmable nonreciprocal manipulation of spatial optical fields (such as phase, amplitude, and holographic encoded information) remains challenging. Furthermore, some schemes rely on complex multi-wave coupling or strong pumping conditions, resulting in high system complexity and limited stability. In nonlinear optical processes, the phase-matching condition determines the efficiency of different nonlinear processes. Especially in cases involving spatial frequency modulation, collinear and non-collinear nonlinear processes exhibit significant differences in phase-matching conditions, potentially introducing selective transfer or suppression of spatial information. However, current technologies have not fully utilized this "nonlinear spatial filtering" effect to achieve structurally simple, direction-dependent nonreciprocal manipulation of optical fields. Summary of the Invention

[0004] Purpose of the invention: The purpose of this invention is to provide a highly integrated non-reciprocal optical field control device and method based on nonlinear spatial filtering, overcoming the shortcomings of existing non-magnetic non-reciprocal devices, such as complex structure and low spatial information fidelity.

[0005] Technical Solution: The non-reciprocal optical field modulation device based on nonlinear spatial filtering of the present invention includes a first linear modulation module, a nonlinear medium, and a second linear modulation module arranged sequentially along the optical path propagation direction. The first and second linear modulation modules are integrated into both sides or near the surface of the nonlinear medium via femtosecond laser direct writing. The nonlinear medium is configured to satisfy the collinear phase matching condition of the fundamental frequency light and the frequency harmonic light. The overall structure is a sandwich structure.

[0006] Preferably, the first linear modulation module, the second linear modulation module and the nonlinear medium are closely coupled in the near field, thereby eliminating the diffraction diffusion of spatial information before entering the nonlinear action region and improving the extinction ratio of non-reciprocal modulation.

[0007] Preferably, the linear modulation module is a refractive index modulation structure, a phase grating, an amplitude grating, or a complex amplitude modulation structure. More preferably, the linear modulation module is a refractive index modulation structure, a phase grating, an amplitude grating, or a complex amplitude modulation structure formed by femtosecond laser direct writing.

[0008] Preferably, the nonlinear medium is a crystal material with second-order nonlinear effects, including but not limited to periodically polarized lithium niobate, lithium niobate, potassium titanyl phosphate, or barium β-borate crystals.

[0009] Preferably, the first linear modulation module and the second linear modulation module are loaded with different spatial phase and / or amplitude encoding information.

[0010] Preferably, the collinear phase matching condition is adjusted by crystal orientation, temperature tuning, or periodic polarization structure, so that only the collinear nonlinear process occurs efficiently, while the nonlinear nonlinear process carrying spatial frequency modulation produces significant phase mismatch.

[0011] When light propagates in the forward direction and passes through the first linear modulation module, the nonlinear medium and the second linear modulation module in sequence, the nonlinear nonlinear process corresponding to the spatial phase or amplitude information introduced by the first linear modulation module in the nonlinear medium is effectively suppressed due to phase mismatch. Its spatial information cannot be effectively transferred to the output light field, and the output light field mainly carries the spatial modulation information loaded by the second linear modulation module. When light propagates in the reverse direction and passes through the second linear modulation module, the nonlinear medium, and the first linear modulation module in sequence, the spatial modulation information introduced by the second linear modulation module is also filtered out in the nonlinear medium due to non-collinear phase mismatch. The output light field mainly carries the spatial modulation information loaded by the first linear modulation module, thereby realizing the non-reciprocity of light field control.

[0012] The non-reciprocal optical field modulation method based on nonlinear spatial filtering described in this invention includes: forming an integrated linear modulation module on both sides of a nonlinear medium by direct writing with a femtosecond laser; adjusting environmental parameters to bring the nonlinear medium into a collinear phase-matched state; performing a first linear spatial modulation on the incident light; utilizing the narrow-band characteristics of the nonlinear collinear phase-matching bandwidth to filter out high-frequency spatial components generated by modulation; performing nonlinear interaction in the nonlinear medium that satisfies the collinear phase-matching condition; and performing a second linear spatial modulation so that the forward and reverse propagating optical fields carry different spatial modulation information.

[0013] Preferably, the linear modulation module is a refractive index modulation structure, a phase grating, an amplitude grating, or a complex amplitude modulation structure formed by direct writing with a femtosecond laser.

[0014] Preferably, the collinear phase matching condition is adjusted by crystal orientation, temperature tuning, or periodic polarization structure, so that only the collinear nonlinear process occurs efficiently, while the nonlinear nonlinear process carrying spatial frequency modulation produces significant phase mismatch.

[0015] Preferably, the environmental parameters include laser wavelength, crystal temperature, polarization direction of incident light, and numerical aperture. These parameters are set to ensure that the nonlinear medium satisfies the collinear phase matching condition and that the nonlinear spatial frequency components generated by the first linear modulation module are in a phase mismatch state.

[0016] Preferably, the environmental parameters include: the laser being at a specific phase-matching wavelength, which for lithium niobate material is 1030 nm under collinear phase-matching conditions; meeting the normal incidence condition, i.e., the laser is incident perpendicularly to the crystal and passes through the pre-processed holographic structure; the crystal temperature is at room temperature and cannot be too high; meeting the polarization matching condition: the polarization direction of the incident fundamental frequency light must be parallel to the specific crystal axis direction of the nonlinear medium; meeting the spatial spectrum matching condition: the diffraction angle corresponding to the spatial frequency component introduced by the holographic structure must be greater than the phase-matching angle bandwidth of the nonlinear medium at the current wavelength, thereby ensuring that the high-frequency information generated by the first linear modulation module can be physically filtered out in the phase mismatch region; limiting the optical field collimation and numerical aperture: the incident fundamental frequency light must meet the paraxial approximation condition, and its beam divergence angle should be much smaller than the phase-matching angle bandwidth of the nonlinear medium, so as to ensure that the unmodulated background signal can be efficiently frequency converted; the peak power density of the fundamental frequency light must be below the damage threshold of the nonlinear medium, and greater than the effective excitation threshold of the nonlinear conversion, so as to ensure that the output optical field after spatial filtering has sufficient signal-to-noise ratio and contrast.

[0017] The non-reciprocal optical field modulation device based on nonlinear spatial filtering described in this invention has applications in optical information encryption, one-way holography and AR display, integrated photonics, and precision optical anti-counterfeiting.

[0018] The non-reciprocal optical field manipulation device based on nonlinear spatial filtering described in this invention serves as a direction-sensitive physical key in optical information encryption, significantly improving communication security through physical-level spectral filtering. In unidirectional holography and AR displays, it effectively suppresses backscattering and achieves high-fidelity unidirectional image reconstruction using spatial spectral filtering. In the field of integrated photonics, it provides a miniaturized, non-magnetic on-chip optical isolation solution, achieving efficient mode selection and logic operations through the "spatial sieve" effect. Furthermore, combined with the high-threshold process of femtosecond laser direct writing and its stringent phase-matching dependence, this device constructs extremely high technical barriers in the field of precision optical anti-counterfeiting, realizing comprehensive technological empowerment applications from information security and new displays to photonic integration and high-end anti-counterfeiting.

[0019] The control principle of the control device is as follows: by utilizing the selective transfer characteristics of spatial frequency by nonlinear interaction, the non-collinear spatial frequency components introduced by the input modulation module are phase mismatched and filtered out, so that the forward and reverse propagating light fields exhibit spatial distribution characteristics determined by different modulation modules.

[0020] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: 1. This invention overcomes the traditional technical prejudice that phase mismatch is harmful, cleverly utilizing the narrow bandwidth of phase-matched angles as a natural spatial filter. Compared to traditional wavefront synthesis mechanisms, this scheme represents a novel approach based on spectral selective suppression.

[0021] 2. This invention is not a simple functional stacking, but rather a clever transformation of the angular sensitivity of the phase-matching condition into a spatial domain filter. This mechanism enables the device to automatically distinguish between pre-modulation and post-modulation information, achieving extremely high non-reciprocal contrast.

[0022] 3. The integrated structure achieved through femtosecond laser direct writing ensures that the modulation module and the nonlinear region are in a near-field coupled state. This significantly suppresses free-space diffraction diffusion of the spatial spectrum before filtering, resulting in a significant improvement in non-reciprocal contrast compared to traditional discrete schemes.

[0023] 4. Traditional views hold that achieving non-reciprocity of spatial information requires the introduction of asymmetric nonlinear gains. This invention demonstrates that efficient non-reciprocal logic can be constructed using phase mismatch, a physical effect often considered negative, enabling the direct direction-selective transmission of phase, amplitude, and holographic encoded information in a spatial optical field.

[0024] 5. No complex 3D nonlinear structure design is required; it can be achieved simply by linearly modifying the surface of a standard nonlinear crystal. This device is small in size and highly stable, and can be widely used in fields such as holographic displays and optical information encryption. It is suitable for applications in holographic displays, beam shaping, optical information encryption, photodiodes, and integrated photonics. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the overall structure of the non-reciprocal optical field modulation device of the present invention.

[0026] Figure 2 This is a schematic diagram of the integration of a femtosecond laser direct-write linear modulation module with a nonlinear medium.

[0027] Figure 3 This is a schematic diagram illustrating the phase matching relationship between collinear and non-collinear interactions in nonlinear interactions. Detailed Implementation

[0028] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0029] against Figure 1 (Overall structure): such as Figure 1 As shown, the device of this invention employs a compact "sandwich" integrated structure. This structure consists sequentially of a first linear modulation module, a nonlinear medium, and a second linear modulation module along the optical path propagation direction. This layout ensures that the spatial optical field undergoes spectral expansion by the input module before entering the nonlinear region, and completes final wavefront loading at the output module.

[0030] against Figure 2 (Femtosecond direct writing integrated system): such as Figure 2 As shown, the first and second linear modulation modules are directly fabricated on the two sides or near the surface of the nonlinear medium using femtosecond laser direct writing technology to form an integrated device. This fabrication method enables micron-level near-field coupling between the modulation structure and the nonlinear interaction region, effectively suppressing the free diffraction and diffusion of spatial frequency components during transmission, thereby ensuring a high extinction ratio for spatial filtering.

[0031] against Figure 3 (Spatial filtering physical mechanism): such as Figure 3 As shown, the non-reciprocal control mechanism of this invention is manifested as spatial frequency selective transfer in a nonlinear process. The figure illustrates collinear phase matching (…). Δk =0) and non-collinear phase matching ( Δ k≠The difference between 0 and 0 is that only the fundamental frequency optical component that satisfies the paraxial collinearity condition can be efficiently converted into a new frequency optical field, while the high-frequency spatial component introduced by the input modulation module is physically filtered out because it is outside the phase matching bandwidth, thereby realizing non-reciprocal control of spatial information in different propagation directions.

[0032] Example 1: Non-reciprocal beam shaping device based on femtosecond laser direct writing structure Lithium niobate crystal was selected as the nonlinear medium, and a femtosecond laser with a center wavelength of 800 nm, a pulse width of 290 fs, and a repetition rate of 200 kHz was used as the processing light source. An 80-magnification oil microscope (NA = 0.55) was used to focus the laser onto a near-surface region approximately 20 μm below the crystal surface. The pulse power was set to 300 mW, and the laser scanning speed was 10 μm / s. A local refractive index change was induced through multi-layer parallel scanning, and first and second linear modulation modules were written at the crystal's incident and exit ends, respectively. These modules were designed as helical phase structures or holographic patterns with different topological charges.

[0033] When the fundamental frequency light with a wavelength of 1030nm is incident in the forward direction, the non-collinear nonlinear process corresponding to the spatial phase modulation introduced by the first linear modulation module in the nonlinear crystal is suppressed due to phase mismatch, and the output beam mainly presents the spatial morphology defined by the second linear modulation module; when the light is incident in the reverse direction, only the beam shaping information of the first linear modulation module is retained, thereby realizing non-reciprocal control of beam shaping.

[0034] Example 2: Non-reciprocal holographic display and information encryption method In this embodiment, the first linear modulation module and the second linear modulation module respectively write different holographic phase encoding patterns. By reasonably setting the phase matching conditions of the nonlinear crystal, the forward and reverse propagating light fields form completely different holographic images on the reconstruction plane, realizing holographic display switching and information encryption based on the propagation direction.

Claims

1. A non-reciprocal optical field manipulation device based on nonlinear spatial filtering, characterized in that, The system includes a first linear modulation module, a nonlinear medium, and a second linear modulation module arranged sequentially along the optical path propagation direction. The first and second linear modulation modules are integrated into the two sides or near the surface of the nonlinear medium via femtosecond laser direct writing. The nonlinear medium is configured to satisfy the collinear phase matching condition between the fundamental frequency light and the frequency-doubled light.

2. The apparatus according to claim 1, characterized in that, The first linear modulation module and the second linear modulation module are tightly coupled in the near field to the nonlinear medium. By eliminating the diffraction diffusion of spatial information before entering the nonlinear action region, the extinction ratio of non-reciprocal modulation is improved.

3. The apparatus according to claim 1, characterized in that, The linear modulation module is a refractive index modulation structure, a phase grating, an amplitude grating, or a complex amplitude modulation structure.

4. The apparatus according to claim 1, characterized in that, The nonlinear medium is a crystalline material with second-order nonlinear effects.

5. The apparatus according to claim 4, characterized in that, The crystal material includes lithium niobate, periodically polarized lithium niobate, potassium titanate phosphate, or barium β-borate crystal.

6. The apparatus according to claim 1, characterized in that, The first linear modulation module and the second linear modulation module are loaded with different spatial phase and / or amplitude encoding information.

7. A method for non-reciprocal optical field manipulation using the device described in claim 1, characterized in that, include: An integrated linear modulation module is formed on both sides of a nonlinear medium by direct writing with femtosecond laser; By tuning environmental parameters, the nonlinear medium is brought into a collinear phase-matched state; the incident light is then subjected to first linear spatial modulation. By utilizing the narrow-band characteristics of nonlinear collinear phase-matching bandwidth, high-frequency spatial frequency components generated by modulation are filtered out; nonlinear interaction is performed in a nonlinear medium that satisfies the collinear phase-matching condition; and a second linear spatial modulation is performed, so that the forward and reverse propagating light fields carry different spatial modulation information.

8. The method according to claim 7, characterized in that, The collinear phase matching condition is adjusted by crystal orientation, temperature tuning, or periodic polarization structure, so that only collinear nonlinear processes occur efficiently, while nonlinear nonlinear processes carrying spatial frequency modulation produce significant phase mismatch.

9. The method according to claim 7, characterized in that, The environmental parameters include laser wavelength, crystal temperature, polarization direction of incident light, and numerical aperture. These parameters are set to ensure that the nonlinear medium satisfies the collinear phase matching condition and that the nonlinear spatial frequency components generated by the first linear modulation module are in a phase mismatch state.

10. The application of the non-reciprocal optical field modulation device based on nonlinear spatial filtering as described in claim 1 in optical information encryption, one-way holography and AR display, integrated photonics, and precision optical anti-counterfeiting.

Images