Polarization sensitive en face optical coherence tomography using multichannel acousto-optic deflectors

We present an improved method of polarization sensitive optical coherence tomography that enables measurement and imaging of backscattered intensity, birefringence, and fast optic axis orientation simultaneously with only one single A-scan per transverse measurement location. While intensity and birefringence data are obtained in a conventional way, the optic axis orientation is determined from the phase difference recorded in two orthogonal polarization channels. We report on accuracy and precision of the method by measuring birefringence and optic axis orientation of well defined polarization states in a technical object and present maps of birefringence and, what we believe for the first time, of optic axis orientation in biological tissue.

A polarization sensitive optical coherence tomography (PS-OCT) instrument was used to investigate the retinal pigment epithelium (RPE). The instrument uses the polarization properties of light to record backscattered intensity, retardation and fast axis orientation simultaneously and needs only one measurement per sample location to retrieve these parameters. The polarization state of light backscattered from within the RPE was found to be random. This can be observed in PS-OCT images by random retardation and axis orientation values within the RPE layer. In diseased eyes where the normal retinal structure is corrupted (e.g. RPE atrophy, RPE detachment) the localization of the RPE within OCT images which do not provide polarization information (standard OCT) is rather difficult. Since the RPE is the only structure within the retina to cause this polarization scrambling, PS-OCT can be used for contrast enhancement and enables the exact localization of the RPE in these pathologies. Therefore it is possible to determine if the RPE is still preserved in regions of interest. Furthermore, in patients with RPE atrophy an enhanced penetration depth into the choroid and even into the sclera was observed. Because of birefringence introduced by the sclera the border between choroid and sclera could easily be determined.

Polarization-sensitive optical coherence tomography (PS-OCT) extends OCT by analyzing the polarization states of backscattered light to quantify tissue birefringence. However, conventional implementations require polarization-diverse detection and are therefore incompatible with most commercial OCT systems. As a result, PS-OCT has largely remained restricted to specialized research groups, limiting its broader scientific and clinical use. Here, we present a modular PS-OCT framework that integrates with a standard spectral-domain OCT platform through a detachable rotating achromatic half-wave plate in the sample arm. This waveplate modulates both incident and reflected polarization states. Three or more repeated measurements at distinct waveplate orientations enable reconstruction of the sample's round-trip Jones matrix and the corresponding polarization properties. To mitigate random phase variations between repeated measurements, we introduce a retarder-constrained phase optimization strategy. We validate the framework with imaging of birefringent phantoms and the human retina in vivo, demonstrating reliable reconstruction of retardance and optic axis orientation. This approach requires only minimal hardware modification and is readily deployable on mainstream OCT systems. Lowering technical barriers paves the way for rapid and widespread deployment of PS-OCT across diverse biomedical applications in both research and clinical environments.

Polarization sensitive optical coherence tomography (PS-OCT) is a functional extension of OCT. In addition to imaging based on tissue reflectivity, PS-OCT also enables depth-resolved mapping of sample polarization properties such as phase-retardation, birefringent axis orientation, Stokes vectors, and degree of polarization uniformity (DOPU). In this study, PS-OCT was used to investigate the polarization properties of melanin. In-vitro measurements in samples with varying melanin concentrations revealed polarization scrambling, i.e. depolarization of backscattered light. Polarization scrambling in the PS-OCT images was more pronounced for higher melanin concentrations and correlated with the concentration of the melanin granules in the phantoms. Moreover, in-vivo PS-OCT was performed in the retinas of normal subjects and individuals with albinism. Unlike in the normal eye, polarization scrambling in the retinal pigment epithelium (RPE) was less pronounced or even not observable in PS-OCT images of albinos. These results indicate that the depolarizing appearance of pigmented structures like, for instance, the RPE is likely to be caused by the melanin granules contained in these cells.

In this work, a Polarization Sensitive Optical Coherence Tomography (PS-OCT) system is described with the ability of navigation in neurosurgery. Of great importance in neurosurgery is guiding Deep Brain Stimulation (DBS) needle probe which needs precise and detailed navigation within the brain tissue. PS-OCT is a powerful imaging tool that can be used to obtain detailed information within the tissue at different depths based on the low coherent light interferometry and birefringence properties of the tissue by exploiting polarimetric methods. It is noteworthy to mention that PS-OCT exhibits great resolution and sensitivity based on the tissue birefringence related parameters (retardation and optic axis orientation) especially for the precise placement of DBS probe in the subthalamic nucleus (STN), which is entirely surrounded by myelinated fiber bundles that are highly birefringent. In general, images obtained using this system can provide a spatial map of the anisotropic structure of the tissue.

Shear-wave splitting in the Rajasthan craton, India

Polarization-sensitive optical coherence tomography (PS-OCT) is widely used to image fibrous biological tissues, providing additional image contrast compared to intensity-based OCT, including birefringence and optic axis orientation information. Here a theoretical and experimental analysis of the effect of the system fiber on determination of sample optic axis orientation using a previously reported algorithm based on the angle between Stokes vectors on the Poincaré sphere in fiber-based PS-OCT experimentally confirms that the algorithm only works correctly when the parameters of the system fiber are kept constant during the imaging acquisition of each frame. To extend the use of the algorithm to situations in which the parameters of the system fiber are varied among A-scans or image frames, which is true when the OCT probe scanning does move the sample arm fiber, we propose the use of polarization maintaining fiber (PMF) with a special polarization axis orientation as the sample arm fiber for extracting sample optic axis orientation using fiber-based PS-OCT systems. The use of the PMF makes the system insensitive to variations of the sample fiber birefringence for determining optic axis orientation by automatically removing the offset ambiguity from each A-scan. We demonstrated by using a computer evaluation model that the proposed method is capable of extracting the relative optic axis orientation correctly when the PMF birefringence is varied among A-scans due to external disturbances to the system fiber, provided that the polarization axis orientation (slow or fast) of the PMF is kept constant during the measurement.

Polarization sensitive optical coherence tomography (PS-OCT) was used to measure and image retardation and birefringent axis orientation of in vitro human cornea. We used a two-channel PS-OCT system employing a phse sensitive recording of the interferometric signals in two orthogonal polarization channels. Using an algorithm based on a Hilbert transform, it is possible to calculate the retardation and the slow axis orientation of the sample with only one A-scan per transversal measurement location. While the retardation information is encoded in the amplitude ratio of the two interferometric signals, the axis orientation is encoded entirely in their phase difference. We present maps of retardation and slow axis distribution of normal and pathologic human cornea in longitudinal cross sections and en face images, obtained at the back sruface of the cornea. The results can be explained by a birefringence model based on stacked collagen fibril lamellae of different orientations.

The laser beam is often steered in different directions in a variety of laser-advanced microprocessing technologies, such as laser micromachining, laser patterning and laser selective-area doping. Scanners are traditionally used to direct the beam to different locations on the workpiece. The mechanical motion of x-y mirrors in such scanners affects the accuracy of beam positioning and repeatability in high precision manufacturing. Beam steering without any moving optical component is necessary to overcome the limitations of current scanning technology. Stationary optical devices such as acousto-optic modulators (AOM), acousto-optic deflectors (AOD) and acousto-optic tunable filters (AOTF) have widespread applications in the field of laser microfabrication for intensity modulation and laser beam steering. AODs are commonly used with continuous-wave lasers in laser direct writing systems to provide flexible and high-speed beam scanning with high precision and accuracy.A pair of AODs is mounted orthogonally to provide dual axis (x-y) laser scanning. The deflection scan angle, however, is small due to the finite distance between the two separate deflectors. This optical configuration can be simplified by realizing 2D AO interaction in a single AO crystal. This paper discusses an integrated AOD to allow compact construction and efficient operation with large deflection scan angle. This type of AODs would be useful for large-area processing with high throughput.The laser beam is often steered in different directions in a variety of laser-advanced microprocessing technologies, such as laser micromachining, laser patterning and laser selective-area doping. Scanners are traditionally used to direct the beam to different locations on the workpiece. The mechanical motion of x-y mirrors in such scanners affects the accuracy of beam positioning and repeatability in high precision manufacturing. Beam steering without any moving optical component is necessary to overcome the limitations of current scanning technology. Stationary optical devices such as acousto-optic modulators (AOM), acousto-optic deflectors (AOD) and acousto-optic tunable filters (AOTF) have widespread applications in the field of laser microfabrication for intensity modulation and laser beam steering. AODs are commonly used with continuous-wave lasers in laser direct writing systems to provide flexible and high-speed beam scanning with high precision and accuracy.A pair of AODs is mounted orthogonally ...

In this work we are using a high-speed swept source PS-OCT system working at 1060 nm, for acquiring intensity, retardation, fast axis and degree of polarization uniformity images of retina, choroid and sclera in vivo.

Using a low-coherence Michelson interferometer, we measure two-dimensional images of optical birefringence in bovine tendon as a function of depth. Polarization-sensitive detection of the signal formed by interference of backscattered light from the sample and a mirror in the reference arm give the optical phase delay between light that is propagating along the fast and slow axes of the birefringent tendon. Images showing the change in birefringence in response to laser irradiation are presented. The technique permits rapid noncontact investigation of tissue structural properties through two-dimensional imaging of birefringence.

We propose novel spectral domain polarization sensitive optical coherence tomography with single camera spectrometer including a multiplexed custom grating, camera lenses and high-speed multi-line CCD camera. Two polarization beams are measured on different lines of high speed CCD camera. Due to slightly different incident angle of two beam collimators, two orthogonal polarization channel beams are separately measured by different lines of CCD camera each other. After the system is implemented, signal acquisition is performed for polarization-sensitive imaging of various samples.

We present, to the best of our knowledge, the first ultrahigh resolution, polarization sensitive OCT images of human tissue in vivo. The system is based on a Mach Zehnder interferometer and a transversal scanning of the sample. A broadband superluminescent diode with a center wavelength of 897nm and a bandwidth (FWHM) of 155nm was used. The depth resolution of the system was measured with ~ 4 micrometer in air. The actual scanning speed of 1000 transversal lines per second enables the acquisition of an image (B-scan) consisting of 1600(x) x 1000(z) pixels in 1 second. The whole signal is recorded by a polarization sensitive detection unit at the interferometer exit which enables a phase resolved measurement. From the recorded data we were able to obtain backscattered intensity, retardation and cumulative fast axis orientation of the sample. Images of these parameters obtained from a technical sample and from a human cornea in vivo are presented.

We demonstrate theoretically and experimentally that the phase retardance and relative optic-axis orientation of a sample can be calculated without prior knowledge of the actual value of the phase modulation amplitude when using a polarization-sensitive optical coherence tomography system based on continuous polarization modulation (CPM-PS-OCT). We also demonstrate that the sample Jones matrix can be calculated at any values of the phase modulation amplitude in a reasonable range depending on the system effective signal-to-noise ratio. This has fundamental importance for the development of clinical systems by simplifying the polarization modulator drive instrumentation and eliminating its calibration procedure. This was validated on measurements of a three-quarter waveplate and an equine tendon sample by a fiber-based swept-source CPM-PS-OCT system.

Shear-wave splitting, lithospheric anisotropy, and mantle deformation beneath the Arabia–Eurasia collision zone in Iran