Researchers Measure Laguerre-Gaussian Beam Orbital Angular Momentum with Ultra-precise Laser Speckle Techniques

Light beams carrying orbital angular momentum offer exciting possibilities for advanced imaging and manipulation techniques, but precisely controlling and measuring their properties remains a significant challenge. Christopher Perrella, Aman Anil Punse, and Anastasiia Zalogina, working at the University of Adelaide alongside colleagues, now demonstrate a remarkably precise method for determining the azimuthal index of these light beams, achieving an improvement of three orders of magnitude over previous approaches. The team leverages the subtle patterns created by laser speckle to measure this property, which in turn allows for an ultra-precise determination of the refractive index of surrounding materials. This innovative technique, applied to minuscule volumes of liquid within a microfluidic channel, promises a new way to investigate the composition and dynamics of biological samples, potentially revealing spatio-temporal variations and gradients with unprecedented detail.

Speckle Patterns Reveal Index and Azimuthal Precision

This study demonstrates a highly precise method for measuring the azimuthal index of Laguerre-Gaussian beams and, simultaneously, the refractive index of surrounding media. By leveraging laser speckle patterns generated through microscopic spiral phase plates, the researchers achieve a measurement precision three orders of magnitude better than previously reported, reaching a precision of 1. 8 x 10⁻⁵ for the azimuthal index and 6. 4 x 10⁻⁷ refractive index units. The technique successfully interrogates samples as small as 300 picolitres within a microfluidic channel, using sucrose and haemoglobin as test materials.

The method involves training a principal component analysis model with known refractive indices and azimuthal values, allowing for accurate determination of unknowns through speckle pattern analysis. Measurements were validated using two different spiral phase plates and at multiple wavelengths, demonstrating the robustness of the approach. The current work suggests the technique could be extended to measure spatio-temporal variations and gradients within more complex biological samples. The authors acknowledge that the precision of the method is dependent on the quality of the training data and the stability of the optical setup, factors that require careful consideration for optimal performance.

Speckle Patterns Reveal Light’s Angular Momentum

Researchers engineered a novel approach to precisely measure the azimuthal index of light beams possessing orbital angular momentum, leveraging the properties of light speckle patterns for unprecedented accuracy. The technique converts a Gaussian beam into a Laguerre-Gaussian beam using microfabricated spiral phase plates (SPPs) immersed within a microfluidic channel, then meticulously analyzes the resulting speckle patterns. Scientists harnessed the deterministic nature of these patterns to extract information about the light field and surrounding medium, achieving a measurement precision of 2×10⁻⁵ for the azimuthal index. The core of the method relies on a precise understanding of how the SPP alters the phase of the input light, imparting an azimuthally varying wavefront that transforms the Gaussian beam into a Laguerre-Gaussian beam with a specific azimuthal index.

Researchers directly measure the azimuthal index through speckle pattern analysis, circumventing the need for complex interferometric setups or reference beams. To minimize sample volume, the experiments employ microfluidic channels containing 50 μm diameter SPPs, enabling refractive index sensing of picolitre volumes. This direct measurement approach simplifies the experimental setup and allows for potential multiplexing within the microfluidic chamber for high-throughput analysis. This advancement allows for the characterization of minute changes in refractive index, crucial for biological sensing and material characterization.

Precise Measurement of Light’s Orbital Angular Momentum

Researchers have developed a highly precise method for measuring the azimuthal index of light beams possessing orbital angular momentum (OAM), achieving a precision of 3. 3 x 10⁻⁶ refractive index units. This represents an improvement of three orders of magnitude over previous techniques and opens new possibilities for applications relying on the precise control of light’s twisted properties. The team generates Laguerre-Gaussian beams, which carry OAM, using microscopic spiral phase plates (SPPs) and then leverages laser speckle patterns to determine the beams’ azimuthal index. The method involves carefully measuring the refractive index of the medium surrounding the SPP, with the best achieved precision being 0.

7 x 10⁻⁶ RIU, confirmed to be at the system’s fundamental noise limit. Experiments were conducted on samples of sucrose and haemoglobin, using volumes as small as 300 pL within a microfluidic channel, demonstrating the technique’s compatibility with biological samples. Results demonstrate that the precision of the system scales with the height of the SPP, and improved with shorter wavelengths. The best azimuthal index measurements achieved were 1. 8 x 10⁻⁵, representing the most precise measurements of fractional azimuthal index to date. This innovative approach, combining microfluidics and OAM light fields, promises a new form of refractive index sensing capable of measuring spatio-temporal variations and gradients within biological samples with unprecedented accuracy.

Speckle Precision Limited by Photon Shot Noise

This document details a comprehensive analysis of noise sources affecting the precision of refractive index and azimuthal index measurements using a speckle-based technique. The experiment achieves refractive index precision of (2. 2-7. 4) x 10⁻⁶ RIU and azimuthal index precision of (7. 2-10.

6) x 10⁻⁵. The dominant source limiting precision is photon shot noise, with the measured precision aligning with estimated shot noise contributions. The analysis reveals that shot noise fundamentally limits the measurement precision. Increasing signal strength or integration time could potentially improve precision, although this may introduce other challenges. Precise optical alignment is also critical, requiring sub-nanometer alignment.

A monolithic design or fiber-coupled component could improve stability and reduce alignment sensitivity. In essence, the document thoroughly characterizes the noise landscape of this measurement system, identifying shot noise and optical alignment as the most significant factors limiting precision. The research suggests potential avenues for future improvements to enhance measurement accuracy, particularly by addressing the shot noise limitation and improving optical stability.