Optical systems, such as microscopes, telescopes, and lasers, often suffer from aberrations that distort the quality and information of the light field. These aberrations can be caused by various factors, such as imperfect optical elements, atmospheric turbulence, or scattering media. Traditionally, adaptive optics (AO) is used to correct phase aberrations by using feedback mechanisms that adjust the shape or position of a deformable mirror or a spatial light modulator. However, phase aberrations are not the only source of error in optical systems. Polarization aberrations, which affect the state of polarization (SOP) of the light field, can also degrade the performance and resolution of optical systems, especially those that are sensitive to the vectorial properties of light.
What are polarization aberrations and why are they important?
Polarization aberrations are deviations from the ideal SOP of the light field that can be introduced by various sources, such as:
- Stressed optical elements, such as lenses, mirrors, or prisms, that induce birefringence or diattenuation in the light field.
- Fresnel’s effects, such as reflection, refraction, or transmission, that alter the SOP of the light field depending on the angle of incidence and the refractive index of the media.
- Polarizing effects in materials or biological tissues, such as dichroism, optical activity, or scattering, that change the SOP of the light field depending on the wavelength, orientation, or structure of the molecules.
Polarization aberrations can have significant consequences for optical systems, such as:
- Reducing the contrast, intensity, or fidelity of the images or signals obtained by the optical system.
- Introducing extra phase distortion or geometric aberrations that compound the effects of phase aberrations.
- Affecting the vectorial information analysis, such as the degree of polarization, the polarization orientation, or the ellipticity, that can reveal important features or properties of the target object or scene.
Therefore, correcting polarization aberrations is crucial for improving the performance and functionality of optical systems, especially those that rely on the vectorial nature of light, such as polarization microscopy, polarimetry, or quantum optics.
What is vectorial adaptive optics and how does it work?
Vectorial adaptive optics (V-AO) is an innovative technique designed to correct both polarization and phase aberrations in optical systems. It extends the concept of traditional AO, which only corrects phase aberrations, to the vectorial domain, which encompasses higher-dimensional feedback correction of both polarization and phase. V-AO can be implemented using different methods, such as:
- Sensor-based V-AO: A vectorial sensor, such as a Mueller matrix (MM) sensor, is used to measure the polarization and phase aberrations of the light field, and then provide feedback to a V-AO module, which consists of a polarization module and a phase module, for pre-correction. The V-AO module can use various devices, such as liquid crystal devices, metasurfaces, or digital micromirror devices, to manipulate the SOP and phase of the light field. The goal is to achieve the desired light field output, such as a uniform SOP or a specific phase profile.
- Sensorless V-AO: A V-AO module is used to generate a series of trial light fields with different SOP and phase profiles, and then evaluate their performance based on a predefined metric, such as the image quality, the signal-to-noise ratio, or the vectorial information content. The V-AO module then adjusts its parameters based on an optimization algorithm, such as genetic algorithm, particle swarm optimization, or gradient descent, to find the optimal light field output that maximizes the metric.
- Hybrid V-AO: A combination of sensor-based and sensorless V-AO, where a vectorial sensor is used to provide initial or partial feedback to the V-AO module, and then a sensorless optimization is performed to fine-tune the light field output.
What are the applications and benefits of vectorial adaptive optics?
V-AO is a powerful tool capable of improving the performance and functionality of various optical systems, such as:
- Microscopy: V-AO can enhance the resolution, contrast, and information content of microscopic images by correcting the polarization and phase aberrations introduced by the objective lens, the sample, or the imaging medium. V-AO can also enable the manipulation and analysis of the vectorial properties of the light field, such as the SOP, the orbital angular momentum, or the spin angular momentum, which can reveal important features or properties of the sample, such as the birefringence, the optical activity, or the chirality.
- Telescopes: V-AO can improve the quality and fidelity of astronomical images by correcting the polarization and phase aberrations induced by the atmospheric turbulence, the telescope optics, or the interstellar medium. V-AO can also enable the measurement and analysis of the vectorial properties of the light field, such as the polarization, the phase, or the coherence, which can provide valuable information about the source, such as the magnetic field, the temperature, or the distance.
- Lasers: V-AO can enhance the intensity, stability, and efficiency of laser beams by correcting the polarization and phase aberrations caused by the laser cavity, the amplification medium, or the propagation medium. V-AO can also enable the generation and control of the vectorial properties of the laser beams, such as the SOP, the phase, or the mode, which can enable various applications, such as optical communication, optical trapping, or optical encryption.
V-AO is a novel and versatile technique that pushes the boundaries of traditional scalar beam shaping by providing feedback control of extra vectorial degrees of freedom. It paves the way for next generation AO functionality by manipulating the complex vectorial field.
