Birefringence Axes Research Articles

Nano-achiral complex composites for extreme polarization optics.

Composites from 2D nanomaterials show uniquely high electrical, thermal and mechanical properties1,2. Pairing their robustness with polarization rotation is needed for hyperspectral optics in extreme conditions3,4. However, the rigid nanoplatelets have randomized achiral shapes, which scramble the circular polarization of photons with comparable wavelengths. Here we show that multilayer nanocomposites from 2D nanomaterials with complex textured surfaces strongly and controllably rotate light polarization, despite being nano-achiral and partially disordered. The intense circular dichroism (CD) in nanocomposite films originates from the diagonal patterns of wrinkles, grooves or ridges, leading to an angular offset between axes of linear birefringence (LB) and linear dichroism (LD). Stratification of the layer-by-layer (LBL) assemblednanocomposites affords precise engineering of the polarization-active materials from imprecise nanoplatelets with an optical asymmetry g-factor of 1.0, exceeding those of typical nanomaterials by about 500 times. High thermal resilience of the composite optics enables operating temperature as high as 250 °C and imaging of hot emitters in the near-infrared (NIR) part of the spectrum. Combining LBL engineered nanocomposites with achiral dyes results in anisotropic factors for circularly polarized emission approaching the theoretical limit. The generality of the observed phenomena is demonstrated by nanocomposite polarizers from molybdenum sulfide (MoS2), MXene and graphene oxide (GO) and by two manufacturing methods. A large family of LBL optical nanocomponents can be computationally designed and additively engineered for ruggedized optics.

Polarization-Sensitive OCT Imaging of Scleral Abnormalities in Eyes With High Myopia and Dome-Shaped Macula

The relevance of visualizing scleral fiber orientation may offer insights into the pathogenesis of pathologic myopia, including dome-shaped maculopathy (DSM). To investigate the orientation and density of scleral collagen fibers in highly myopic eyes with and without DSM by polarization-sensitive optical coherence tomography (PS-OCT). This case series included patients with highly myopic eyes (defined as a refractive error ≥6 diopters or an axial length ≥26.5 mm) with and without a DSM examined at a single site in May and June 2019. Analysis was performed from September 2019 to October 2023. The PS-OCT was used to study the birefringence and optic axis of the scleral collagen fibers. The orientation and optic axis of scleral fibers in inner and outer layers of highly myopic eyes were assessed, and the results were compared between eyes with and without a DSM. A total of 72 patients (51 [70.8%] female; mean [SD] age, 61.5 [12.8] years) were included, and 89 highly myopic eyes were examined (mean [SD] axial length, 30.4 [1.7] mm); 52 (58.4%) did not have a DSM and 37 (41.6%) had a DSM (10 bidirectional [27.0%] and 27 horizontal [73.0%]). Among the 52 eyes without DSM, the 13 eyes with simple high myopia had primarily inner sclera visible, displaying radially oriented fibers in optic axis images. In contrast, the entire thickness of the sclera was visible in 39 eyes with pathologic myopia. In these eyes, the optic axis images showed vertically oriented fibers within the outer sclera. Eyes presenting with both horizontal and bidirectional DSMs had clusters of fibers with low birefringence at the site of the DSM. In the optic axis images, horizontally or obliquely oriented scleral fibers were aggregated in the inner layer at the DSM. The vertical fibers located posterior to the inner fiber aggregation were not thickened and appeared thin compared with the surrounding areas. This study using PS-OCT revealed inner scleral fiber aggregation without outer scleral thickening at the site of the DSM in highly myopic eyes. Given the common occurrence of scleral pathologies, such as DSM, and staphylomas in eyes with pathologic myopia, recognizing these fiber patterns could be important. These insights may be relevant to developing targeted therapies to address scleral abnormalities early and, thus, mitigate potential damage to the overlying neural tissue.

Wavelength continuously tunable and wavelength spacing variable comb filter based on a High-Birefringence fiber loop mirror

We demonstrated an all-fiber comb filter that can continuously tune the wavelength for four switchable wavelength spacings. The comb filter consists of a fiber loop mirror (FLM) that is inserted with three polarization maintaining fibers (PMFs) and three polarization controllers (PCs). The transmission characteristics of the comb filter was theoretically analyzed. The output characteristics of the comb filters shows a dependence of the orientation angles of the PCs. With the angles being optimally set, the comb filter with four different wavelength spacings can be achieved. The wavelength continuously tunable and wavelength spacing variable mechanisms are elaborated. The three PCs are employed to enable the tunable and switchable functions. The two PCs between the PMFs serve as a polarization rotator to align the birefringent axes of the PMFs for varying the wavelength spacing while the PC adjacent to the optical coupler can be used for wavelength tuning function. When the two quarter-waveplates (QWPs) of the PC are optimally set at π/4 and - π/4, the half-waveplate (HWP) acts as a wavelength tuning element, and when the QWPs are set at zero degrees, the HWP serves as an amplitude tuning element. The center wavelength of the comb filter could be linearly tuned with the rotation angle of the HWP, and a π/2 change in the rotation angle leads to a wavelength change in one free-spectral-range (FSR). A π/4 change in the rotation angle could bring the normalized peak transmission from its maximum at 1 to minimum at 0. The proposed tunable technique has been verified by our experimental results. We experimentally demonstrated the comb filters can switch among FSRs of 0.75 nm, 1.01 nm, 1.51 nm, and 3.1 nm with tunable functions.

Unsupervised topological analysis of polarized light microscopy: application to quantitative birefringence imaging.

The determination of birefringence (magnitude and axis orientation) of optical materials is of significant interest in various fields. In the case of composite samples, this task becomes complicated and time-consuming; therefore, a partially automated procedure for reconstructing birefringence spatial distribution becomes valuable. Herein, we propose a procedure to reconstruct the spatial distributions of the retardance and optical axis orientation in a geological thin section from sparse quantitative birefringence measurements, using automatic boundary detection on cross-polarized light microscopy images. We examine two particular areas on the selected geological thin section: one that presents a uniaxial crystal with a circular cross-section of its refractive index ellipsoid and the other with grains of varying orientations. The measurement gives the orientation of the grain's optical axis both in and out of the plane of the thin section, which explains the qualitative observations with the cross-polarized light microscope. Future work will connect the measured orientation of the rock thin section with its 3D geological orientation in the field.

Single-input polarization-sensitive optical coherence tomography through a catheter.

Intravascular polarimetry with catheter-based polarization-sensitive optical coherence tomography (PS-OCT) complements the high-resolution structural tomograms of OCT with morphological contrast available through polarimetry. Its clinical translation has been complicated by the need for modification of conventional OCT hardware to enable polarimetric measurements. Here, we present a signal processing method to reconstruct the polarization properties of tissue from measurements with a single input polarization state, bypassing the need for modulation or multiplexing of input states. Our method relies on a polarization symmetry intrinsic to round-trip measurements and uses the residual spectral variation of the polarization states incident on the tissue to avoid measurement ambiguities. We demonstrate depth-resolved birefringence and optic axis orientation maps reconstructed from in-vivo data of human coronary arteries. We validate our method through comparison with conventional dual-input state measurements and find a mean cumulative retardance error of 13.2deg without observable bias. The 95% limit of agreement between depth-resolved birefringence is 2.80 · 10-4, which is less than the agreement between two repeat pullbacks of conventional PS-OCT (3.14 · 10-4), indicating that the two methods can be used interchangeably. The hardware simplification arising from using a single input state may be decisive in realizing the potential of polarimetric measurements for assessing coronary atherosclerosis in clinical practice.

Photothermally Induced Birefringence in an Optical Nanofiber for All‐Optical Polarization Manipulation

AbstractThis study reports the experimental observation of photothermally induced birefringence (PIB) in an optical nanofiber (NF). The PIB originates from the interaction between the azimuthally asymmetric evanescent field of a guided pump beam in mode and the gas molecules surrounding the NF. Light absorption of gas molecules results in azimuthal inhomogeneity in gas density, which induces birefringence of the NF waveguide. The swift heat transfer and molecular transportation in sub‐micrometer scale accounts for the fast response of the PIB with a measured rising and falling time within 9 and 70 ns, respectively. With a 12‐mm‐long NF, phase retardance as large as 3π is achieved. A theoretical model based on the rate equation is also developed for studying the photothermal dynamics and gas kinetics in PIB generation. This novel model is well consolidated by the numerical simulations, with the results agreeing well with the experiments. In consideration of the extremely low insertion loss, fast response, and reconfigurable principal axes of birefringence, the PIB can be used for all‐optical polarization manipulation. In addition, NF with PIB can also be an interesting platform for studies of gas–surface interaction, light–sound interaction controlling, sub‐micrometer scale heat and mass transfer, and molecular spectroscopy.

Non-centrosymmetric solid core photonic crystal fiber for suppressing parasitic twist in fiber sensing coil

During the fabrication of fiber sensing coils, the parasitic twist of fibers would lead to the undesired elliptical birefringence in the fiber, which is the main cause of magnetically induced bias error for fiber optic gyroscope(FOG). For the purpose of suppressing the parasitic twist in the fiber coil, a polarization-maintaining solid core photonic crystal fiber (PCF) with non-centrosymmetric distribution of bending stiffness is proposed and investigated. The proposed PCF would bend towards a particular direction under the external forces, resulting in the consistency of birefringent axes arrangement, thus suppressing the nonreciprocal errors induced by both radial and axial components of the magentic field simultaneously. Dependence study of the fiber bending stiffness distribution on several key parameters is provided. The optical properties are also explored and compared with the previously studied PCF. Fiber coils fabricated with the proposed PCF are promising in reducing the magnetical sensitivity of FOG, thus making it possible for FOG to get rid of the thick magnetic shield, which is significant for the miniaturization of FOG.

Optical Alignments and Error Analysis of the Stress Measurement Technique Based on Photoelastic Modulator (PEM)

A sensitive method for measuring the magnitude and angle of the fast axis of low level linear birefringence in optical materials has been developed over past 20 years. However, different optical alignments have been determined using the same data processing method. All possible optical alignments and their corresponding data processing are investigated in this paper to explore the mechanism of data processing. Proper applications of the system with different optical alignments

Birefringent Axes Aligning System for Electro-optic Probe Fabrication Using Polarization Maintaining Fiber

Fabrication method of a fiber optic electric field sensing probe using polarization maintaining fiber (PMF) is described including a birefringent axes aligning system. The sensing probe is basically a fiber-coupled Fabry-Perot resonator where a thin birefringent wafer of LiTaO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> with an external cavity is mounted onto the end of PMF. An optical wave from a superluminescent laser diode after a linear polarizer parallel to the slow axis of the PMF is made to enter the LiTaO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> crystal with polarization parallel to its extraordinary axis and the reflected optical wave is detected after passing through a polarization analyzer. By measuring a minimum value of a ratio of the reflected optical power at 90° to that of 0° of analyzer, we successfully demonstrated that the angular accuracy of the slow axis of the PMF with respect to the extraordinary axis of the LiTaO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> can be evaluated. Aligning both axes precisely, the electro-optic effect associated with LiTaO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and polarization is optimized with greater stability. Long term stability of the fabricated PMF sensing probe was enhanced by about 3 times compared to that of a conventional single mode fiber one. As a result of uncertainty analysis, the method we proposed has an overall angular uncertainty of 1° when the used fiber optic components have polarization angle tolerance of 1°.

Identification and quantification of fibrotic areas in the human retina using polarization-sensitive OCT.

Subretinal fibrosis is one of the most prevalent causes of blindness in the elderly population, but a true gold standard to objectively diagnose fibrosis is still lacking. Since fibrotic tissue is birefringent, it can be detected by polarization-sensitive optical coherence tomography (PS-OCT). We present a new algorithm to automatically detect, segment, and quantify fibrotic lesions within 3D data sets recorded by PS-OCT. The algorithm first compensates for the birefringence of anterior ocular tissues and then uses the uniformity of the birefringent optic axis as an indicator to identify fibrotic tissue, which is then segmented and quantified. The algorithm was applied to 3D volumes recorded in 57 eyes of 57 patients with neovascular age-related macular degeneration using a spectral domain PS-OCT system. The results of fibrosis detection were compared to the clinical diagnosis based on color fundus photography (CFP), and the precision of fibrotic area measurement was assessed by three repeated measurements in a sub-set of 15 eyes. The average standard deviation of the fibrotic area obtained in eyes with a lesion area > 0.7 mm2 was 15%. Fibrosis detection by CFP and PS-OCT agreed in 48 cases, discrepancies were only observed in cases of lesion area < 0.7 mm2. These remaining discrepancies are discussed, and a new method to treat ambiguous cases is presented.

Analysis and calibration of linear birefringence orientation parameters derived from Mueller matrix for multi-layered tissues

Mueller matrix polarimetry can provide abundant microstructural information of tissues. Mueller matrix polar decomposition (MMPD) is capable of extracting intrinsic polarimetric characteristics of tissue-like media, including the orientation of the linear birefringence axis. However, when measuring a multi-layered sample with linear birefringence of different orientation directions, there is a deviation between the measured and ideal values of the MMPD linear birefringence orientation parameter. Since the multi-layered anisotropic structures widely exist in biomedical samples, a modified MMPD parameter to calculate linear birefringence orientation more precisely for layered tissues is crucial. Here we propose a modified MMPD parameter to calculate linear birefringence orientation for multi-layered media. By measuring different customized phantoms and pathological tissue slices, we demonstrate that the modified parameter can show more accurately the linear birefringence orientation angle of multi-layered media. The proposed parameter can be helpful for the quantitative evaluation of not only the pathological tissue slices with fibers, but also many other material samples with layered birefringent structures.

Influence of multiple reflections on the transmission coefficients of uniaxial plane-parallel plates.

A study on the influence of multiple reflections on the transmission coefficients of uniaxial plane-parallel plates is presented. Two representative models are analyzed: one that considers only the first transmission, and a rigorous one, taking into account the multiple reflections within the plate. Modules, phases, and the interference between $p$ and $s$ transmitted fields are evaluated in a wide range of angles of incidence by means of three emblematic examples that illustrate the effects of thickness, birefringence, and optical axis orientation. For simplicity, whereas the optical axis can form an arbitrary angle with the interface, it is restricted to the plane of incidence. A complete theoretical framework is provided along with general reference guidelines derived from numerical examples.

Femtosecond laser‐induced birefringent microdomains in sodium‐borate glass for highly secure data storage

AbstractIn this paper, the inscription of polarization‐sensitive birefringent structures of two types by femtosecond laser pulses is described. Depending on the laser beam parameters, the orientation of the slow axis of birefringent structures can be either parallel or perpendicular to the polarization of the laser beam. The dependence of retardance and slow axis of birefringence on laser exposure parameters is revealed. The thermal stability of both types of structures is examined. The possibility of highly secure data storage based on the different thermal behavior of the laser‐modified region is demonstrated.

Laser Writing of Crystalline Structures in Lithium-Niobium-Germanate Glasses

It is demonstrated that spatially selective crystallization of lithium-niobium-germanate glasses with the compositions (mol.%) 25Li2O∙30Nb2O5∙45GeO2 and 30Li2O∙25Nb2O5∙45GeO2 can be formed by using a femtosecond laser beam with formation of birefringent structures containing lithium niobate. A correlation was established between the orientation of the fast axis of birefringence of crystallized tracks and the polarization plane of the laser beam.

Laser Writing of Polarization-Dependent Birefringence in Sodium-Borate Glasses

The conditions for the formation of microdomains with polarization-dependent birefringence by a femtosecond laser beam in sodium-borate glasses with different Na2O content were determined. The dependences of the phase shift in the light passing through the birefringent zones on the parameters of laser exposure and on the chemical composition of the glass were determined. It is shown that an increase in the Na2O content leads to a decrease in the minimum number of pulses required to form zones with polarization-dependent birefringence and under certain conditions to the formation of a special type of modification for which the slow axis of birefringence is parallel to the polarization plane of the laser beam.

In vivo imaging of the depth-resolved optic axis of birefringence in human skin.

Recent progress has enabled the reconstruction of the local (i.e., depth-resolved) optic axis (OAx) of biological tissue from measurements made with polarization-sensitive optical coherence tomography (PS-OCT). Here we demonstrate local OAx imaging in healthy human skin in vivo. The images reveal dense, weaving patterns that are imperceptible in OCT intensity tomograms or conventional PS-OCT metrics and that suggest a mesh-like tissue organization, consistent with the morphology of dermal collagen. Using co-registered polarization-sensitive optical coherence microscopy, we furthermore investigated the impact of spatial resolution on the recovered OAx patterns and confirmed their consistency. OAx orientation as a contrast mechanism merits further exploration for applications in dermatology.

PM fiber based sensing tapes with automated 45° birefringence axis alignment for distributed force/pressure sensing.

Polarization maintaining (PM) fibers can be used for distributed force/pressure sensing in which the birefringence axis of the PM fiber should preferably be oriented 45° from the direction of the force/pressure for the maximum sensitivity. However, it is a challenge to achieve such 45° axis orientation for a long length of PM fiber in practice. In this paper, we report the development of what we believe the first equipment and process for making PM fiber based sensing tapes, capable of automatically adjusting the fiber axis orientation 45° with respect to the tape surface. In particular, we develop a machine vision system with the ability of continuously determining fiber axis orientation in real time as the fiber passes by and feeding back the orientation information to a fiber rotation apparatus to automatically adjust its orientation before fixing the fiber on a transparent PET tape with UV epoxy. We show the results of a successfully fabricated 70-m-long PM fiber sensing tape achieving an axis orientation accuracy of 45 ± 3° throughout the whole length of the tape, which is further validated with a distributed polarization crosstalk analyzer (DPXA). Finally, we demonstrate distributed transversal load sensing with 14 force applying weights randomly distributed along the sensing tape using the DPXA, with a polarization crosstalk measurement uniformity of 0.62 dB (standard deviation) using the same applied weight of 100 grams. The same sensing tape can also be used for pressure sensing with properly designed fixtures.

Sagnac Interferometer Based on a Highly-Birefringent Two-Core PCF: Theory, Experiment, and Sensing Characteristics

In this article, we present comparative study on the transmission and the sensing characteristics of an all-fiber Sagnac loop interferometer (SLI) made by different kinds of the highly-birefringent (HB) two-core photonic crystal fiber (TCPCF), respectively for strain, torsion, and temperature sensing. The fiber device is analyzed theoretically and experimentally. Transmission spectrum of the SLI using the HB TCPCF shows six spatial frequencies due to superposition of multiple two-beam interferences originating from the intra-core and the inter-core interactions between the fundamental polarization modes propagating in the two birefringent cores. Our results reveal that the transmission characteristics of the fiber device and the sensing features of its multiplexed interferences are strongly dependent on the optical and the structural properties of the two birefringent cores such as their intrinsic birefringences, the effective index contrast between them, and the relative orientation between the (slow) birefringence axes. Finally, the application of the SLI made by the specific HB TCPCF is discussed in the simultaneous measurement of three parameters.

Birefringence properties of anti-resonant octagonal-core and nodeless hollow-core fibers.

We present experimental results of birefringence property measurements for two types of negative curvature anti-resonant hollow-core fibers (AR-HCFs). Both AR-HCFs have a core of 50 µm in diameter, providing an effective area of the fundamental guided mode larger than 1000µm2. Measurements were carried out at wavelengths of 2.25, 2.35, and 2.45 µm. The obtained results demonstrate that both fiber types have birefringence axes that do not correspond to core symmetry axes. The proposed technique for the birefringence axes orientation measurement could be applied to other kinds of HCFs at various wavelengths.

Birefringence Changes of Dendrites in Mouse Hippocampal Slices Revealed with Polarizing Microscopy

Intrinsic optical signal (IOS) imaging has been widely used to map the patterns of brain activity in vivo in a label-free manner. Traditional IOS refers to changes in light transmission, absorption, reflectance, and scattering of the brain tissue. Here, we use polarized light for IOS imaging to monitor structural changes of cellular and subcellular architectures due to their neuronal activity in isolated brain slices. To reveal fast spatiotemporal changes of subcellular structures associated with neuronal activity, we developed the instantaneous polarized light microscope (PolScope), which allows us to observe birefringence changes in neuronal cells and tissues while stimulating neuronal activity. The instantaneous PolScope records changes in transmission, birefringence, and slow axis orientation in tissue at a high spatial and temporal resolution using a single camera exposure. These capabilities enabled us to correlate polarization-sensitive IOS with traditional IOS on the same preparations. We detected reproducible spatiotemporal changes in both IOSs at the stratum radiatum in mouse hippocampal slices evoked by electrical stimulation at Schaffer collaterals. Upon stimulation, changes in traditional IOS signals were broadly uniform across the area, whereas birefringence imaging revealed local variations not seen in traditional IOS. Locations with high resting birefringence produced larger stimulation-evoked birefringence changes than those produced at low resting birefringence. Local application of glutamate to the synaptic region in CA1 induced an increase in both transmittance and birefringence signals. Blocking synaptic transmission with inhibitors CNQX (for AMPA-type glutamate receptor) and D-APV (for NMDA-type glutamate receptor) reduced the peak amplitude of the optical signals. Changes in both IOSs were enhanced by an inhibitor of the membranous glutamate transporter, DL-TBOA. Our results indicate that the detection of activity-induced structural changes of the subcellular architecture in dendrites is possible in a label-free manner.