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Laboratory for Ultrafast Nonlinear Optics

Research

Structured light

Light orbital angular momentum and structured light

List of Researchers

Youngbin Na

국문요약

빛이 스핀뿐만 아니라 궤도 각운동량도 가질 수 있다는 것이 발견된 이래로, 빛의 궤도 각운동량에 관한 연구는 물리, 공학, 생물학 등 다양한 분야에서 널리 연구되고 있다. 궤도 각운동량을 수반하는 빛은 광 보텍스라 불리며, 이것은 위상 특이점, 궤도 각운동량, 그리고 도넛 형태의 세기 분포라는 대표적인 3가지 특성을 갖는다. 특히, 빛의 궤도 각운동량은 2가지 상태만 가능한 스핀과 다르게 이론적으로 무한한 임의의 모든 정수 상태를 가질 수 있으며, 정수 상태들의 결맞은 결합을 통해 분수 값에 해당하는 궤도 각운동량 상태를 만드는 것이 가능하다. 이러한 점에서 빛의 궤도 각운동량은 정보 전달을 위한 추가 자유도로써 고전 및 양자 통신 분야에서 다가오는 통신 용량의 한계를 극복할 수 있는 해결책으로 주목받고 있다.


FIG. 1. Fundamental characteristics of optical vortex beams

본 연구실에서는 공간광변조기라는 장치를 통해 다양한 광원에서 궤도 각운동량을 수반하는 구조화된 빛을 생성하고 이들의 특성을 분석하였다 [1-3]. 또한, 구조화된 빛들의 구조적 특성을 이용한 다양한 응용 기술에 관해 연구하였다 [4,5]. 최근에는, 딥러닝 기술을 적용하여 궤도 각운동량 모드의 고분해능 측정 기법을 제안하였으며 [6], 신호 노이즈, 대기 난류와 같은 외부 교란에 의해 왜곡된 OAM 모드를 인식하는 적응형 모델을 설계하여 높은 분해능과 안정성을 겸비한 측정 시스템을 구축하였다 [7].


FIG. 2. Experimental generation of fractional Bessel-Gaussian (BG) beams. (a) Some examples of the measured intensity profiles and (b) phase shift modulation of the fractional BG beam with radial mode kr=7.10 and OAM mode l=1.45.


FIG. 3. Structure of the designed CNN: B, block; FC, fully connected layer. A series of blocks which consists of a convolutional layer and a max pooling layer extract features describing the input object. The extracted features are classified through two FC layers, also referred to as dense layer. Finally, a spatial mode of the input is determined by a softmax activation function included in the last FC layer.

현재는 구조화된 빛을 이용한 고차원 양자 얽힘 상태의 임의 제어 연구를 수행하고 있다. 원하는 스펙트럼을 갖도록 재단된 구조화된 빛을 펌프 빔으로 자발매개하향변환을 발생시키고, 이때 생성된 얽힘 상태의 나선형 스펙트럼을 측정함으로써 입력 OAM 모드와 출력 나선 스펙트럼 간의 상관관계를 기술하는 맵핑 규칙을 조사하고자 한다. 향후, 이를 다광자 및 다차원 시스템으로 확장하여 보안성과 통신 용량이 향상된 차세대 양자 네트워크 구현을 위한 연구 기반을 제공하는 것을 목표로 하고 있다.


FIG. 4. Schematic diagram for arbitrary control of orbital angular momentum entanglement. The process is divided into two parts, prediction of pump beam shape and structured light-induced entanglement.

Our research

Since the discovery that light can possess orbital angular momentum (OAM) as well as spin angular momentum, the study on the OAM has been widely used in various fields, such as physics, engineering, and biology. Light carrying an OAM is called an optical vortex, and it has three representative characteristics, phase singularity, orbital angular momentum, and doughnut shape intensity pattern. While the spin of light has only two possible states as its eigentate, left- and right-circular polarization, the OAM possesses an arbitrary integer state, whose mode index is unlimited. In addition, it is possible to generate fractional OAM states by coherent superposition of integer OAM modes. In this respect, light OAM is attracting attention as a solution that can overcome the limitation on communication capacity in the field of classical and quantum communications as an additional degree of freedom for information transfer.


FIG. 1. Fundamental characteristics of optical vortex beams

1) Generation of arbitrary OAM modes and its high-resolution measurement

We generated structured light carrying OAM from various light sources using a spatial light modulator (SLM) and analyzed its characteristis [1-3]. In addition, various application techniques using the structural characteristics of such beams were studied [4,5]. Recently, we proposed a high-resolution scheme for measuring OAM modes by applying a deep-learning technique [6]. Then, we designed an adaptive model for recognizing OAM modes distorted by external disturbances, such as signal noise and atmospheric turbulence, to contruct a stable measurement system with high resolution [7].


FIG. 2. Experimental generation of fractional Bessel-Gaussian (BG) beams. (a) Some examples of the measured intensity profiles and (b) phase shift modulation of the fractional BG beam with radial mode kr=7.10 and OAM mode l=1.45.


FIG. 3. Structure of the designed CNN: B, block; FC, fully connected layer. A series of blocks which consists of a convolutional layer and a max pooling layer extract features describing the input object. The extracted features are classified through two FC layers, also referred to as dense layer. Finally, a spatial mode of the input is determined by a softmax activation function included in the last FC layer.

2) Arbitrary control of high-dimensional entanglement states by structured light

Currently, we are studying arbitrary control of high-dimensional quantum entanglement using structured light. Structured ligth tailored to possess the desired OAM spectrum is used as a pump beam to produce spontaneous parametric down-conversion (SPDC), and spiral spectrum of the generated entangled state is measured. Then, we investigate the mapping rule to describe the correlation between an input OAM mode and an output spiral spectrum. In the future, we aime to extend this to multi-photon and multi-dimensional systems to provide a research basis for the implementation of next-generation quantum networks with improved security and communication capacity.


FIG. 4. Schematic diagram for arbitrary control of orbital angular momentum entanglement. The process is divided into two parts, prediction of pump beam shape and structured light-induced entanglement.

Publication

[1] S. I. Hwang, D. H. Song, and D.-K. Ko, Tailoring light structures using an off-axis double axicon holographic pattern. Opt. Commun. 283, 4209-4213 (2010).
[2] S. I. Hwang, D. H. Song, and D.-K. Ko, Dynamic control and Stabilization of Laguerre Gaussian Beam by Shifted Zone Plate Method. J. Korean Phys. Soc. 56, 42-46 (2010).
[3] S. I. Hwang, D. H. Song, and D.-K. Ko, Composition and characterization of femtosecond optical lattices using double axicon holographic pattern. Jpn. J. Appl. Phys. 50, 042701 (2011).
[4] Y. Na and D.-K. Ko, High-resolution refractometry using phase shifting interferometry based on spatial light modulator and vortex probe. Opt. Laser Technol. 112, 479-484 (2019).
[5] Y. Na and D.-K. Ko, Amplitude-modulated log-polar coordinate mapping for generating top-hat line-shaped beams with steep edges and a high aspect ratio. 134, 106587 (2021).
[6] Y. Na and D.-K. Ko, Deep-learning-based high-resolution recognition of fractional-spatial-mode-encoded data for free-space optical communications. Sci. Rep. 11, 2678 (2021).
[7] Y. Na and D.-K. Ko, Adaptive demodulation by deep-learning-based identification of fractional orbital angular momentum modes with structural distortion due to atmospheric turbulence. Sci. Rep. 11, 23505 (2021).

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