Birefringent Interferometer Research Articles

Achromatic correction for birefringent interferometers that improve Fourier transform spectrometers and hyperspectral imaging

Spectroscopy and hyperspectral imaging are widely used tools for identifying compounds and materials. One optical design is a polarization interferometer that uses birefringent wedges, like a Babinet-Soleil compensator, to create the interferograms that are Fourier transformed to give the spectra. Such designs have lateral spatial offset between the no and ne optical beams, which reduces the interferogram intensity and creates a spatially dependent phase that is problematic for hyperspectral imaging. The lateral separation between the beams is wavelength dependent, created by the achromatic nature of Babinet-Soleil compensators. We introduce a birefringent wedge design for Fourier transform spectroscopy that creates collinear no and ne optical beams for optimal interference and no spatial dependent phase. Our 3-wedge design, which we call a Wisconsin interferometer, improves the signal strength of polarization spectrometers, and eliminates phase shifts in hyperspectral imaging. We anticipate that it will find use in analytical, remote sensing, and ultrafast spectroscopy applications.

Enhancing hyperspectral imaging through macro and multi-modal capabilities

Hyperspectral imaging (HSI) has emerged as an effective tool to obtain spatially resolved spectral information of artworks by combining optical imaging with spectroscopy. This technique has proven its efficacy in providing valuable information both at the large and microscopic scale. Interestingly, the macro scale has yet to be thoroughly investigated using this technology. While standard HSI methods include the use of spatial or spectral filters, alternative methods based on Fourier-transform interferometry have also been utilised. Among these, a hyperspectral camera employing a birefringent common-path interferometer, named TWINS, has been developed, showing a high robustness and versatility. In this paper, we propose the combination of TWINS with a macro imaging system for the study of cultural heritage (CH). We will show how the macro-HSI system was designed, and we will demonstrate its efficient capabilities to collect interferometric images with high visibility and good signal of both reflectance and fluorescence on the same field of view, even on non-flat samples. Our hyperspectral camera for macro studies of both reflectance and fluorescence data is a completely new asset in the CH panorama and beyond. The relevance of the macro technology is demonstrated in two case studies, aiding in the analysis of biofilms on stone samples and of the degradation of dyed textiles.

A bolometric hyperspectral camera based on a birefringent interferometer for remote sensing in the thermal infrared

Remote sensing in thermal infrared bands (TIR) is largely dominated by cumbersome dispersive-type hyperspectral imagers, which usually require expensive and cryo-cooled quantum detectors to make up for their low optical throughput. Here, we present a compact and low-cost TIR hyperspectral camera based on the Fourier-transform approach. It combines an uncooled bolometer detector and a common-path birefringent interferometer made of calomel (Hg2Cl2). It features high optical throughput, an interferometric contrast greater than 90% even for incoherent radiation, spectral resolution tunable up to 4.5 cm-1, robust and long-term interferometric stability. Retrieving in a few minutes the infrared spectrum in all pixels of the TIR image, it could constitute a valuable tool for evaluating radiative cooling materials’ spatial and spectral properties over extended areas. We test the capabilities of the instrument by measuring the emissivity map of different butterfly wings, which provide a natural example of radiative cooling.

Use of hyperspectral imaging to monitor the effectiveness of plasma-generated atomic oxygen for non-contact cleaning of indigo dyed silk

This study explores the use of hyperspectral imaging (HSI) to monitor the effectiveness of plasma-generated atomic oxygen (AO) treatment for non-invasive cleaning of cultural heritage object. Silk samples dyed with indigo blue, including those soiled with soot to mimic historical artifacts, were treated with plasma-generated atomic oxygen for varying durations. Using HSI with a TWINS [1,2] birefringent interferometer, diffuse reflectance and light-induced fluorescence are observed. That allowed a precise evaluation of sample degradation avoiding any invasive sample extraction. This research not only contributes to the field of cultural heritage conservation but also enhances understanding of indigo colour degradation processes and the evaluation of non-invasive cleaning techniques on sensitive materials.

Quantum-enhanced strain and curvature sensing based on a birefringent fiber Sagnac interferometer

Quantum-enhanced strain and curvature sensing based on a birefringent fiber Sagnac interferometer

Field Extended Birefringent Static Doppler Wind Imaging Interferometer

According to the requirements for wide field of view and high resolution in wind imaging detection, the principle of a novel field extended birefringent wind imaging interferometer is proposed. Based on the principle of birefringent polarization wind imaging interferometer, the new optical system scheme of the field extended birefringent static Doppler wind imaging interferometer is designed, which overcomes the limitation of the field of view in the original birefringent polarization wind imaging interferometer. Compared with the original scheme, the new scheme eliminates the cross-field phase change between the top and bottom fields, and the available field of view of the instrument is no longer limited. The Jones matrix model are used to simulate to prove the correctness of the principal scheme of the field extended birefringent wind imaging interferometer.

Laboratory demonstration of the birefrigent point-diffraction interferometer wavefront sensor

The direct imaging of extrasolar planets requires extreme adaptive optics (ExAO) in the near-infrared wavelengths. The ExAO needs a high-efficient wavefront sensor (WFS) to measure the phase aberration accurately with a small number of photons. In addition, the WFS in the ExAO is required to run at high sampling rates of 1 − 7 kHz. To meet these requirements, we developed the birefringent point-diffraction interferometer (b-PDI) presented in our earlier paper. We tested the b-PDI in the laboratory with a polychromatic light source with wavelengths of 800 ± 100 nm. The b-PDI showed a relatively high efficiency, comparable to that of a fixed pyramid WFS. The b-PDI has a low calculation cost and a small readout region, which are suitable for high-speed sampling at 6.5 kHz.

Wavelength calibration of birefringent interferometers for 2-D measurement of plasma flow.

Imaging birefringent interferometers are used to measure plasma flow in 2-D via the Doppler shift of a spectral emission line. Applications include plasma physics study in fusion energy experiments and in the Earth's upper atmosphere. We present a new, to the best of our knowledge, method for wavelength calibration that does not require measurement at the rest wavelength of the targeted spectral line, nor measurement using a tuneable laser source. This is useful when such light sources are not available. Fringes measured at known wavelengths from the emission lines of gas-discharge lamps are used to constrain an instrument model which can generate the required calibration data. In the process, optical path difference, dispersion and misalignments are characterized. The "2π ambiguity" of interferogram phase data is handled using circular statistics, allowing the wavelength span of the calibration lines to far exceed the unambiguous measurement range of the instrument. The technique is demonstrated to an accuracy of ±1 pm (±0.7 km/s flow-equivalent) over a 40 nm visible wavelength range.

K-Space Hyperspectral Imaging by a Birefringent Common-Path Interferometer.

Fourier-plane microscopy is a powerful tool for measuring the angular optical response of a plethora of materials and photonic devices. Among them, optical microcavities feature distinctive energy-momentum dispersions, crucial for a broad range of fundamental studies and applications. However, measuring the whole momentum space (k-space) with sufficient spectral resolution using standard spectroscopic techniques is challenging, requiring long and alignment-sensitive scans. Here, we introduce a k-space hyperspectral microscope, which uses a common-path birefringent interferometer to image photoluminescent organic microcavities, obtaining an angle- and wavelength-resolved view of the samples in only one measurement. The exceptional combination of angular and spectral resolution of our technique allows us to reconstruct a three-dimensional (3D) map of the cavity dispersion in the energy-momentum space, revealing the polarization-dependent behavior of the resonant cavity modes. Furthermore, we apply our technique for the characterization of a dielectric nanodisk metasurface, evidencing the angular and spectral behavior of its anapole mode. This approach is able to provide a complete optical characterization for materials and devices with nontrivial angle-/wavelength-dependent properties, fundamental for future developments in the fields of topological photonics and optical metamaterials.

Birefringent Interferometer Cascaded With PM-FBG for Multi-Parameter Testing

In this paper, a structure depending on a birefringent interferometer (BFI) cascaded with polarization-maintaining FBG (PM-FBG) is proposed and experimentally demonstrated. Two different kinds of PM optical fiber spliced with a 45° rotation between their polarization axes composed of BFI. The dip wavelength shifts have a function with the surrounding strain and temperature. To detect the surrounding refractive index (RI), the Michelson interferometer (MI) implemented with waist-enlarged technology is also proposed. Though evaluating the quality of the birefringent interferometric, MI, and PM-FBG signals as a function of temperature, strain, and refraction index, it has been found that this compact sensor can detect these three parameters. We expect this novel sensor to be of great value for bright structural applications.

Static full-Stokes Fourier transform imaging spectropolarimeter capturing spectral, polarization, and spatial characteristics.

A static full-Stokes Fourier transform imaging spectropolarimeter incorporating a liquid-crystal polarization modulator (LPM) and birefringent shearing interferometer (BSI) is reported. It can decode the polarization information at each wavelength along the spatial dimension of a two-dimensional data array. The LPM has a high-speed time-division architecture and employs two ferroelectric liquid crystals and two wave plates to produce four polarization states, providing full-Stokes polarimetric information with a high signal-to-noise ratio. The BSI comprises two birefringent crystal plates and generates an optical path difference with good linear distribution for broadband interference, allowing a fast and high-precision spectral recovery. The optimized design of LPM and BSI are introduced in detail. Subsequently, the signal reconstruction is verified through simulations and experiments. The proposed scheme is highly efficient, exhibits a higher spectral resolution, and constitutes a compact technical approach to realize high-dimensional optical measurement.

A compact static birefringent interferometer for the measurement of upper atmospheric winds: concept, design and lab performance

Abstract. A new compact static wind imaging interferometer, called the Birefringent Doppler Wind Imaging Interferometer (BIDWIN), has been developed for the purpose of observing upper atmospheric winds using suitably isolated airglow emissions. The instrument combines a field-widened birefringent delay plate placed between two crossed Wollaston prisms with an imaging system, waveplates and polarizers to produce four fixed 90∘ phase-stepped images of the interference fringes conjugate to the scene of interest. A four-point algorithm is used to extract line-of-sight Doppler wind measurements across the image of the scene. The arrangement provides a similar throughput to that of a field-widened Michelson interferometer; however, the interferometric component of BIDWIN is smaller, simpler to assemble and less complicated to operate. Consequently, the instrument provides a compact, lightweight and robust alternative that can be constructed and operated with lower cost. In this paper, the instrument concept is presented, and the design and optimization of a prototype version of the instrument are discussed. Characterization of the lab prototype is presented, and the performance of the instrument is examined by applying the instrument to measure a low-velocity two-dimensional Doppler wind field with a high precision (5 m s−1) in the lab.

Broadband Optical Activity Spectroscopy with Interferometric Fourier-Transform Balanced Detection.

Spectrally resolved measurements of optical activity, such as circular dichroism (CD) and optical rotatory dispersion (ORD), are powerful tools to study chiroptical properties of (bio)molecular and nanoplasmonic systems. The wider utilization of these techniques, however, has been impeded by the bulky and slow design of conventional spectropolarimeters, which have been limited to a narrowband scanning approach for more than 50 years. In this work, we demonstrate broadband measurements of optical activity by combining a balanced detection scheme with interferometric Fourier-transform spectroscopy. The setup utilizes a linearly polarized light field that creates an orthogonally polarized weak chiral free-induction-decay field, along with a phase-locked achiral transmitted signal, which serves as the local oscillator for heterodyne amplification. By scanning the delay between the two fields with a birefringent common-path interferometer and recording their interferogram with a balanced detector that measures polarization rotation, broadband CD and ORD spectra are retrieved simultaneously with a Fourier transform. Using an incoherent thermal light source, we achieve state-of-the-art sensitivity for CD and ORD across a broad wavelength range in a remarkably simple setup. We further demonstrate the potential of our technique for highly sensitive measurements of glucose concentration and the real-time monitoring of ground-state chemical reactions. The setup also accepts broadband pulses and will be suitable for broadband transient optical activity spectroscopy and broadband optical activity imaging.

ZIF-8/Lipase Coated Tapered Optical Fiber Biosensor for the Detection of Triacylglycerides

A novel method for the detection of triacylglycerides was reported with a pure, stable, label-free enzyme functionalized optical fiber detector. This method has been accomplished by immobilizing Zeolitic imidazolate framework-8 (ZIF-8) packaged lipase on the tapered optical fiber. The evanescent field is generated by tapering the optical fiber birefringent interferometer (BFI). Usage of ZIF-8/lipase as a biosensing film on optical fiber has not previously been reported. 4-nitrophenyl palmitate (4-NP) is used to determine the stability of the lipase encapsulated in ZIF-8. Tapered BFI works at a refractive index (RI), which matches the turning point to provide high sensitivity. The wavelength shift has a quadratic polynomial response relationship with the concentration of triacylglycerol, showing enhanced sensitivity and bio-selectivity. LOD (Limit of detection) of $0.17~\mu \text{M}$ and 0.23 nM is obtained with diameters of $10~\mu \text{m}$ and $7~\mu \text{m}$ , respectively. This sensor can be applied to triacylglycerides detection, requiring a low volume of blood for each sample.

Time-domain photocurrent spectroscopy based on a common-path birefringent interferometer.

We present diffraction-limited photocurrent (PC) microscopy in the visible spectral range based on broadband excitation and an inherently phase-stable common-path interferometer. The excellent path-length stability guarantees high accuracy without the need for active feedback or post-processing of the interferograms. We illustrate the capabilities of the setup by recording PC spectra of a bulk GaAs device and compare the results to optical transmission data.

Efficient framework for the examination of the field of view sensitivity of a field-widened birefringent interferometer.

An efficient approach is presented that allows the field of view sensitivities of a field-widened birefringent interferometer constructed from several stacked birefringent slabs to be examined. The approach utilizes a Jones matrix framework that is valid for birefringent slabs that have their optic axis parallel to the surface of the slab. It neglects Fresnel effects and multiple reflections, but accounts for birefringent splitting and does not neglect higher-order angular effects. The simplified approach allows the angular sensitivity of the optical path difference near the field-widened configuration to be examined in the presence of misalignment and mismatches between the components. Understanding these effects is critical to developing wide-field interferometers that can be utilized for imaging purposes. Here, we present the developed framework and apply it to examine the field of view effects of a three-element field-widened static birefringent interferometer that is being developed for the measurement of upper atmospheric winds. We examine the sensitivity of the device to rotational misalignment, mismatches, and wavelength shifts. Comparisons among the modeled interference fringes, output from Zemax optical design software, and lab observations are used to validate the approach. It is also shown that the approach accurately simulates parasitic fringes associated with unwanted coupling between extraordinary and ordinary waves at the interfaces.

A hyperspectral microscope based on an ultrastable common-path interferometer

We introduce a wide field hyperspectral microscope using the Fourier-transform approach. The interferometer is based on the translating-wedge-based identical pulses encoding system, a common-path birefringent interferometer which combines compactness, intrinsic interferometric delay precision, long-term stability, and insensitivity to vibrations. We describe two different implementations of our system, which maximize fringe visibility and phase invariance over the field of view, respectively. We also demonstrate that our system can be installed as an add-on in a commercial microscope. We show high-quality hyperspectral fluorescence microscopy from stained cells and powders of inorganic pigments in the spectral range from 400 to 1100 nm, proving that our device is suited to biology and materials science. We also introduce an acquisition method that synthesizes a tunable spectral filter, providing band-passed images with the measurement of only two maps.

Practical reflective birefringent fiber interferometer sensor.

This paper proposes a simple and practical reflective birefringent fiber interferometer sensor that consists of a polarization beam splitter, a polarization-maintaining transmission fiber, and a sensor unit comprising two segments of birefringent fibers and a thin-film reflector. This sensor could be used for dual parameter sensing. Experiments with different temperatures and lateral loads applied to the sensor unit demonstrated temperature and load sensitivity of ∼0.0010 nm/°C and 0.298nm/N, respectively. Further study showed that temperature-insensitive lateral load measurement can be achieved by using equal length of birefringent fibers and performing differential wavelength measurement. The sensor is robust against environmental disturbance on the transmission fiber, making it potentially attractive for practical field applications.

Ultracompact focal plane snapshot spectropolarimeter.

We present, to the best of our knowledge, a new focal plane snapshot full Stokes spectropolarimeter. The technique uses a wave plate array in combination with a birefringent interferometer to acquire four independent spectropolarimetric modulated subinterferograms in a single shot. Then, the full wavelength associated Stokes parameters are reconstructed using the inverse Fourier transforms and a simple matrix operation. The principle of the described model is investigated, and demonstration experiments are carried out. The experiments indicate that it not only provides snapshot spectropolarimetric data acquisition but also presents the advantages of being high throughput and ultracompact.

Hyperspectral imaging with a TWINS birefringent interferometer.

We introduce a high-performance hyperspectral camera based on the Fourier-transform approach, where the two delayed images are generated by the Translating-Wedge-Based Identical Pulses eNcoding System (TWINS) [Opt. Lett. 37, 3027 (2012)], a common-path birefringent interferometer that combines compactness, intrinsic interferometric delay precision, long-term stability and insensitivity to vibrations. In our imaging system, TWINS is employed as a time-scanning interferometer and generates high-contrast interferograms at the single-pixel level. The camera exhibits high throughput and provides hyperspectral images with spectral background level of -30dB and resolution of 3 THz in the visible spectral range. We show high-quality spectral measurements of absolute reflectance, fluorescence and transmission of artistic objects with various lateral sizes.