Single-pass compact vector light field frequency conversion device and method

Abstract

A single-pass compact vector light field frequency conversion device and method, comprising: a one-dimensional nonlinear photonic crystal with a dual-periodic structure, the nonlinear photonic crystal simultaneously having two quasi-phase matching modes: oo-o and ee-e quasi-phase matching modes, the dual-periodic structure comprising: a first and a second periodic polarization structure region and a relative spacing region between the two structures. The present invention achieves frequency conversion of two polarization states of a vector light field in a single crystal by designing a periodic nonlinear photonic crystal structure, does not require complex optical path design, has a simple, compact and stable structure, is less affected by the external environment, and can be further applied to integrated devices.

Description

Technical Field

[0001] The present invention relates to a technology in the field of frequency conversion in nonlinear optics, specifically to frequency conversion of vector light field achieved through design of nonlinear photonic crystal periodic structure. Background Art

[0002] Due to its unique spatial polarization distribution characteristics, vector light fields have been widely used in optical micromanipulation, optical micromachining, optical communication, single-molecule imaging and other fields. At present, the generation of vector light fields is mainly achieved through two methods: intracavity and extracavity, which are called active and passive methods respectively. These studies are mainly concentrated in the field of linear optics, and the generation of vector light fields is achieved through the combination of intracavity resonant cavity mode selection characteristics and optical diffraction elements. There are relatively few studies on vector light fields in nonlinearity, especially the highly polarization-sensitive characteristics of the nonlinear frequency conversion process will cause the vector light field to return to linear polarized light after frequency conversion. To solve the above problems, researchers proposed to achieve frequency conversion between vector light fields and vector light fields through cascaded nonlinear crystals, Mach-Zehnder interferometers and Signac interferometers. These devices are all through two nonlinear crystals or two times through nonlinear crystals to simultaneously convert the two polarization states and then synthesize nonlinear vector light fields. Therefore, these devices require various optical elements to cooperate with each other, which is relatively cumbersome; at the same time, the frequency conversion of the two nonlinear crystals is greatly affected by the external environment, which causes system phase jitter and affects the quality of the generated nonlinear vector light field beam. Therefore, studying single-pass, compact vector light field frequency conversion devices is crucial for their subsequent practical applications. Summary of the invention

[0003] In view of the defects of existing vector light field frequency conversion systems such as complex optical paths, numerous optical components and great influence from the external environment, the present invention proposes a single-pass compact vector light field frequency conversion device and method. By designing a periodic nonlinear photonic crystal structure, the frequency conversion of two polarization states of the vector light field is simultaneously realized in a single crystal. No complex optical path design is required, the structure is simple, compact and stable, and is less affected by the external environment, so it can be further applied to integrated devices.

[0004] The present invention is achieved through the following technical solutions:

[0005] The invention relates to a single-pass compact vector light field frequency conversion device, comprising: a one-dimensional nonlinear photonic crystal with a double-periodic structure, wherein the nonlinear photonic crystal has two quasi-phase matching modes: oo-o and ee-e quasi-phase matching modes.

[0006] The double periodic structure comprises: a first and a second periodic polarization structure region and a relative spacing region between the two structures.

[0007] The dual-period design respectively designs periodic structures corresponding to the oo-o and ee-e quasi-phase matching modes, and can be separated and placed front and back in a periodic nonlinear photonic crystal.

[0008] In the dual-period design, the relative positions of the two periods need to be specially designed to ensure that the relative phase of the two polarization components after frequency conversion is 0, and the specific design is flexible according to the adopted method.

[0009] The vector light field frequency conversion means: when the vector light field After passing through the nonlinear photonic crystal, frequency-doubled vector light is generated. like Figure 2 As shown. Among them: m, and In the sum frequency process, a 45° linearly polarized light with a frequency of ω2 is introduced. After the two paths of light ω1 and ω2 pass through the nonlinear photonic crystal and sum frequency, a sum frequency vector light with the same polarization distribution as the incident vector light field is finally generated. Technical Effects

[0010] The present invention is based on a nonlinear photonic crystal double-period design, which completes the frequency conversion of transverse polarized light and vertical polarized light respectively, and finally realizes a single-pass, compact vector light field frequency conversion device. Compared with the previous method of completing the vector light field frequency conversion through complex optical path design, the present invention does not require complex optical path design; since the vector light field can achieve frequency conversion in one pass, the structure is simple, compact and stable, and is less affected by the external environment, and can be further applied to integrated devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a side view schematic diagram of a single-pass, compact vector light field frequency conversion device of the present invention.

[0012] Figure 2 It is a schematic diagram of the experimental test and result prediction of the device of the present invention during the frequency doubling process;

[0013] Figure 3 It is a schematic diagram of experimental testing and result prediction of the device of the present invention in the frequency summing process. DETAILED DESCRIPTION

[0014] like Figure 1 As shown, this embodiment relates to a single-pass compact vector light field frequency conversion device, including: a first periodic polarization structure region 1, a second periodic polarization structure region 2 and a relative spacing region 3 between the two structures.

[0015] The substrate of the vector light field frequency conversion device is a ferroelectric nonlinear crystal, so as to realize periodic polarization to meet quasi-phase matching.

[0016] The ferroelectric nonlinear crystal must have a nonlinear coefficient that satisfies both oo-o and ee-e quasi-phase matching.

[0017] The length of the first periodic polarization structure region 1 is L1, and it is composed of a number of periodic structure a cycles, and the duty cycle of the periodic structure a is D1=a1 / a.

[0018] The length of the second periodic polarization structure region 2 is L2, and it is composed of a number of periodic structures b cycles, and the duty cycle of the structure period b is D2=b1 / b.

[0019] The length of the relative spacing region 3 is L3, and its structural size needs to ensure that the relative phase of the nonlinear harmonics generated by the two nonlinear processes is 0.

[0020] In the process of quasi-phase-matched second harmonic frequency generation, the harmonic intensity Where: ω is the incident fundamental frequency, n ω and n 2ω is the refractive index of fundamental frequency light and double frequency light in lithium niobate crystal, c is the speed of light, ε0 is the dielectric constant of vacuum, d ij is the effective nonlinear coefficient, I ω is the incident fundamental frequency light intensity; L is the periodic structure length, and the periodic structure corresponds to the Fourier expansion coefficient C m =[2sin(mπD)] / mπ, where m and D represent the order of the reciprocal lattice vector used and the duty cycle of the structure, respectively.

[0021] The length of the first periodic polarization structure region 1 and the length of the second periodic polarization structure region 2 satisfy Where: In the periodically poled lithium niobate structure, the nonlinear coefficient used to satisfy the oo-o phase matching form is d 16 , quasi-phase matching corresponds to the mth level; the nonlinear coefficient used to satisfy the ee-e phase matching form is d 33 , quasi-phase matching corresponds to the nth level.

[0022] By properly adjusting the length L3 of the relative spacing region 3, the relative phase of the second harmonics generated by the two phase matching is made to be 0.

[0023] In this embodiment, specifically for the frequency conversion from 1064nm to 532nm, m=n=3-level quasi-phase matching is adopted, and the duty cycle is selected as D1=D2=0.5. The calculation results show that the period a=18μm, b=21μm, the length L1=1mm, and L2=9.7mm.

[0024] After specific actual experiments, a nonlinear photonic crystal was made according to the above parameters. When a vector light field with a wavelength of 1064nm was incident on the nonlinear photonic crystal, a frequency-doubled vector light field with a wavelength of 532nm was generated. The polarization state was measured using a polarizer to verify its vector characteristics. Figure 2 The figure shows the experimental test device and result prediction schematic diagram of the present invention.

[0025] Compared with the prior art, the device does not require the construction of a complex optical path, and has the characteristics of simple and compact structure, good stability, high efficiency, and integration.

[0026] The above-mentioned specific implementation can be partially adjusted in different ways by those skilled in the art without departing from the principle and purpose of the present invention. The protection scope of the present invention shall be based on the claims and shall not be limited by the above-mentioned specific implementation. Each implementation scheme within its scope shall be subject to the constraints of the present invention.

Claims

1. A single-pass compact vector light field frequency conversion device, characterized in that: It includes: a one-dimensional nonlinear photonic crystal with a double-periodic structure, wherein the nonlinear photonic crystal has two quasi-phase matching modes at the same time: oo-o and ee-e quasi-phase matching modes; The double periodic structure comprises: a first and a second periodic polarization structure region and a relative spacing region between the two structures; The lengths of the first and second periodic polarization structure regions satisfy Wherein: the length of the first periodic polarization structure region is L1, the length of the second periodic polarization structure region is L2, in the periodic polarization lithium niobate structure, the first periodic polarization structure region satisfies the oo-o phase matching form and the nonlinear coefficient used is d 16 , quasi-phase matching corresponds to the mth level; the second periodic polarization structure region satisfies the ee-e phase matching form and the nonlinear coefficient used is d 33 , quasi-phase matching corresponds to the nth level, D1 is the duty cycle of the first periodic polarization structure region, and D2 is the duty cycle of the second periodic polarization structure region; The length of the relative spacing region is L3, and its structural size needs to ensure that the relative phase of the nonlinear harmonics generated by the two nonlinear processes is 0.

2. The single-pass compact vector light field frequency conversion device according to claim 1, characterized in that: The vector light field frequency conversion means: when the vector light field After passing through the nonlinear photonic crystal, frequency-doubled vector light is generated. Among them: m, and They represent the topological charge, azimuth and initial phase of the vector light field respectively. In the sum frequency process, a 45° linearly polarized light with a frequency of ω2 is introduced, and finally a sum frequency vector light with the same polarization distribution as the incident vector light field is generated.

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