Common-path Interferometer Research Articles

All-fiber orthogonal-polarized white-noise-modulated laser for short-coherence dynamic interferometry.

An all-fiber orthogonal-polarized white-noise-modulated laser (AOWL) for short-coherence dynamic interferometry is proposed. Short-coherence laser is achieved by current modulating of a laser diode with the band-limited white noise. A pair of orthogonal-polarized lights with adjustable delay for short-coherence dynamic interferometry are output by the all-fiber structure. In the non-common-path interferometry, the AOWL can significantly suppress the interference signal clutter with 73% side lobe suppression ratio, that improves the positioning accuracy of zero optical path difference. The wavefront aberrations of a parallel plate are measured with the AOWL in the common-path dynamic interferometers, avoiding the fringe crosstalk.

High-Resolution, Broad-Range Detection Setup for Polarimetric Optical Fiber Sensors

A common-path polarization interferometer using a Wollaston prism and an area detector for the measurement of retardation or optical path difference is presented. Employing a moderate-resolution 1280 by 1024 pixel monochrome camera, it offers a measurement range of approximately 780 radians at 830 nm and 1350 radians at 515 nm while maintaining a high measurement resolution. Retardation introduced by a zero-order waveplate or a Soleil–Babinet compensator was measured to evaluate the performance of the interferometer. Based on the presented measurement results, the resolution of the measurement is estimated to be better than 0.002 rad.

Common-path tandem interferometer for thin-film thickness measurements

Conventional optical interferometers utilized for gauging thin-film thickness can offer non-contact, high-resolution, and full-field measurements but have to endure the noise caused by environmental perturbations. Possessing not only the superior performance of conventional interferometers but also the advantages of high measurement repeatability and high tolerance to environmental disturbances, a common-path tandem interferometer was proposed. In this paper, the interferometer’s measurement theory and experimental setup are demonstrated, and the experimental results from the uses of the experimental setup are then presented and discussed. The experimental results validate the interferometer and show a measurement repeatability of up to 3 nm, curve fitting inaccuracy is one reason causing the measurement error; in addition, the results agree with the interferometer’s high resistance to environmental perturbations.

Wavefront Sensing by a Common-Path Interferometer for Wavefront Correction in Phase and Amplitude by a Liquid Crystal Spatial Light Modulator Aiming the Exoplanet Direct Imaging

We implemented the common-path achromatic interfero-coronagraph both for the wavefront sensing and the on-axis image component suppression, aiming for the stellar coronagraphy. A common-path achromatic interfero-coronagraph has its optical scheme based on a nulling rotational-shear interferometer. The angle of rotational shear can be chosen at a small angular extent of about 10 deg. Such a small angular shear maintains the coronagraphic contrast degradation known as the stellar leakage effect, caused by a finite stellar size. We study the phase and amplitude wavefront control by a liquid crystal spatial light modulator of reflection type which is used as the pixilated active adaptive optics unit. Therefore, adaptive optics perform a wavefront-correcting input toward a stellar interfero-coronagraph aiming at the direct exoplanet imaging. Presented here are both the numeric evaluations and the lab experiment stand to prove the declared functionality output.

Retrieving the phase of diffraction orders generated with tailored gratings.

A technique is performed to quantitatively evaluate the intensity and phase of the diffraction orders generated by tailored phase gratings displayed onto a liquid-crystal spatial light modulator (LC-SLM). The SLM displays the grating together with a lens to obtain the Fourier transform. The setup is converted into a polarization common-path interferometer by simply rotating a polarizer. This configuration allows a phase-shifting interferometry algorithm to be applied to retrieve the phase of the diffraction orders. The quadratic phase arising in the system, which must be subtracted, is calibrated using triplicator gratings of varying periods. Various tailored designs with controlled phase shift between diffraction orders are experimentally tested to prove the advantage and simplicity of the technique.

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.

Phase-resolved all-fiber reflection-based s-NSOM for on-chip characterization.

We report on a phase-resolved, reflection-based, scattering-type near-field scanning optical microscope technique with a convenient all-fiber configuration. Exploiting the flexible positioning of the near-field probe, our technique renders a heterodyne detection for phase measurement and point-to-point frequency-domain reflectometry for group index and loss measurement of waveguides on a chip. The important issue of mitigating the measurement errors due to environmental fluctuations along fiber-optic links has been addressed. We perform systematic measurements on different types of silicon waveguides which demonstrate the accuracy and precision of the technique. With a phase compensation approach on the basis of a common-path interferometer, the phase drift error is suppressed to ∼ 0.013°/s. In addition, characterizations of group index, group velocity dispersion, propagation loss, insertion loss, and return loss of component waveguides on a chip are all demonstrated. The measurement accuracy of the propagation loss of a ∼ 0.2 cm long nano-waveguide reaches ±1 dB/cm. Our convenient and versatile near-field characterization technique paves the way for in-detail study of complex photonic circuits on a chip.

Carrier frequency by axial displacements of lens and grating in a double-aperture common-path interferometer

Double-aperture common-path interferometer (DACPI) is built on a 4f optical imaging system. It consists of two lenses, two apertures at input plane, and a grating at Fourier plane. As known, a carrier frequency is introduced when first lens or grating is displaced axially. Nevertheless, the effect when both displacements are present is not described yet. In this manuscript, we describe the interrelation existing among the axial displacements of the first lens and, grating. Additionally, with least squares method, we find with high accurate the initial position of the first lens and grating, for calibrating the 4f optical system. It is important to note that no tilt between the two beams is requested to produce this effect, and it turns out to be simpler, and more accurate than other existing methods. This proposal will be discussed and illustrated in detail with experimental results.

Phase noise elimination in frequency sweeping interferometry based on a common-path interferometer.

We propose a common-path interferometer (CPI) method to suppress the ambient and laser phase noise in frequency sweeping interferometry. The CPI realizes the multiplexing of the main and auxiliary interferometer to ensure the common-mode characteristic of the interference phase noise, which can be eliminated by signal mixing. In experiments, we obtain a dynamic range of up to 110 dB. Compared with the compensation method using a separated auxiliary interferometer, the CPI method improves the dynamic range by 10 dB and is immune to ambient noise. The proposed method enables high-precision distributed polarization measurements of optical fibers and devices.

Continuous wave Bessel beams with a strong longitudinal acceleration in free space

A variety of transversely accelerating optical beams, such as Airy, Mathieu, and Weber beams, have been proposed and intensively studied in the past few decades, while longitudinal acceleration of optical beams in free space has been considered much less and mostly for ultrashort optical pulses. In this work, we create two-component continuous wave Bessel beams that exhibit extremely high longitudinal acceleration in free space, with the group velocity changing by a factor of 10 in just a few centimeters of propagation. The beam components are co-propagating interfering optical beams that can have different frequencies and angular spectra. We also demonstrate large-magnitude negative group velocities and zero-group-velocity modes for a two-component beam. The group velocities are measured interferometrically, using a common-path optical interferometer. The measurement results agree well with the theoretical predictions. The presented methods to control and measure the group velocity of light in free space are expected to attract the attention of researchers working on optical interferometry, ultrafast optics, nonlinear optics, and optical tweezers.

Label free optical transmission tomography for biosystems: intracellular structures and dynamics.

There is an increasing need for label free methods that could reveal intracellular structures and dynamics. In this context, we develop a new optical tomography method working in transmission - full-field optical transmission tomography (FF-OTT). The method can measure the forward scattering signals and reveals the time-dependent metabolic signals in living cells. FF-OTT is a common path interferometer taking advantage of the Gouy phase shift - a π phase shift that the light wave experiences around the focus. By modulating the position of the focus one can alter the phase of the scattered light. Demodulation of images with different phases rejects the background and enhances the light from the depth-of-field, thus producing an optical section. We test FF-OTT by imaging single-cell diatoms and ex vivo biological samples. In fresh samples, we show that the intracellular motions create visible intensity fluctuations in FF-OTT so that the method is able to reveal a metabolic dynamic contrast. FF-OTT was found to be an efficient label free technique that can be readily implemented thanks to a robust common-path speckle-free interferometer design using an incoherent light source.

Cross-polarized common-path temporal interferometry for high-sensitivity strong-field ionization measurements.

Absolute density measurements of low-ionization-degree or low-density plasmas ionized by lasers are very important for understanding strong-field physics, atmospheric propagation of intense laser pulses, Lidar etc. A cross-polarized common-path temporal interferometer using balanced detection was developed for measuring plasma density with a sensitivity of ∼0.6 mrad, equivalent to a plasma density-length product of ∼2.6 × 1013 cm-2 if using an 800 nm probe laser. By using this interferometer, we have investigated strong-field ionization yield versus intensity for various noble gases (Ar, Kr, and Xe) using 800 nm, 55 fs laser pulses with both linear (LP) and circular (CP) polarization. The experimental results were compared to the theoretical models of Ammosov-Delone-Krainov (ADK) and Perelomov-Popov-Terent'ev (PPT). We find that the measured phase change induced by plasma formation can be explained by the ADK theory in the adiabatic tunneling ionization regime, while PPT model can be applied to all different regimes. We have also measured the photoionization and fractional photodissociation of molecular (MO) hydrogen. By comparing our experimental results with PPT and MO-PPT models, we have determined the likely ionization pathways when using three different pump laser wavelengths of 800 nm, 400 nm, and 267 nm.

Highly-stable generation of vector beams through a common-path interferometer and a DMD

Complex vector modes of light, non-separable in their spatial and polarisation degrees of freedom, are revolutionising a wide variety of research fields. It is therefore not surprising that the generation techniques have evolved quite dramatically since their inception. At present it is common to use computer-controlled devices, among which digital micromirror devices have become popular. Some of the reason for this are their low-cost, their polarisation-insensitive and their high-refresh rates. As such, in this manuscript we put forward a novel technique characterised by its high stability, which is achieved through a common-path interferometer. We demonstrate the capabilities of this technique experimentally, first by generating arbitrary vector modes on a higher-order Poincaré sphere, secondly, by generating vector modes in different coordinates systems and finally, by generating various vector modes simultaneously. Our technique will find applications in fields such as optical manipulations, optical communications, optical metrology, among others.

Optical diffraction tomography of 3D microstructures using a low coherence source.

Optical diffraction tomography (ODT) is a label-free technique for three dimensional imaging of micron-sized objects. Coherence and limited sampling of 3D Fourier space are often responsible for the appearance of artifacts. Here we present an ODT microscope that uses low temporal coherence light and spatial light modulators to retrieve reliable 3D maps of the refractive index. A common-path interferometer, based on a spatial light modulator, measures the complex fields transmitted by a sample. Measured fields, acquired while scanning the illumination direction using a digital micro-mirror device, are fed into a Rytov reconstruction algorithm to obtain refractive index maps whose accuracy is directly evaluated on microfabricated 3D test objects. Even for challenging shapes such as pyramids, bridges, and dumbbells, we obtain volumetric reconstructions that compare very well with electron microscopy images.

Efficient Switchable Common Path Interferometer for Transmission Matrix Characterization of Scattering Medium

Transmission matrix can be used as an elegant tool to represent the light transmission characteristics of a scattering medium. In this paper, we propose a novel common path interferometer technique to measure the transmission matrix utilizing the polarization rotation of incident beam. Specifically, the horizontally polarized and vertically polarized components of the incident beam are used as signal light and reference light, respectively. By rotating the polarization of the incident beam, the ratio between the powers of signal and reference light can be freely adjusted and balanced. The phase shift method is employed to retrieve the transmission matrix of scattering medium. Once the transmission matrix is known, the polarization of the incident light beam can be rotated to switch off the reference and only the signal will remain. Focusing and manipulation of light through a ground glass has been experimentally demonstrated with the measured transmission matrix. An optical focus enhancement reaches 83% of the theoretical maximal value is obtained, which is about 30% improvement compared to the previous co-propagation method. The transmission matrix characterization method has the potential to become a general tool to harness the multiple scattering of light through complex medium.

Arbitrary spatially variant polarization by unitary transformations in a common path interferometer

We introduce a novel, common path interferometric technique, for generating hybrid spatially variant polarized fields and Poincaré beams. The technique is based on unitary polarization transformations employing a single spatial light modulator in a Sagnac-like common path interferometer. This technique allows generating arbitrary fields as a superposition of orthogonal elliptically polarized vortex basis with opposite handedness. We demonstrate the generation of such fields theoretically and verify it experimentally for azimuthal and spiral polarization. As an example of application, we generated a radially polarized ring-shaped field and characterize it by Stokes polarimetry.

Single-shot terahertz polarization detection based on terahertz time-domain spectroscopy

This paper presents a novel design for single-shot terahertz polarization detection based on terahertz time-domain spectroscopy (THz-TDS). Its validity has been confirmed by comparing its detection results with those of the THz common-path spectral interferometer through two separate measurements for the orthogonal components. Our results also show that its detection signal-to-noise ratios (SNRs) are obviously superior to those of the 45° optical bias THz-TDS by electro-optical sampling due to its operation on common-path spectral interference rather than the polarization-sensitive intensity modulation. The setup works without need of any optical scan, which does not only save time, but also efficiently avoids the disturbances from the fluctuations of the system and environment. Its single-shot mode allows it to work well for the applications with poor or no repeatability.

Improving thermal stability of LiNbO 3 based electro‐optical electric field sensor by depositing a TiO 2 film

Electro-optic electric field sensors based on LiNbO3 are widely used for the measurement of electric fields or transient voltages. However, the working bias of the sensor is influenced by the temperature due to the thermo-optic effect of LiNbO3. An athermal electro-optic electric field sensor was demonstrated by coating a thin layer of TiO2 film which has a negative thermo-optic coefficient, on the common-path interferometer based sensor to compensate LiNbO3's positive thermo-optic coefficient. Both calculation and experimental studies were conducted. The experimental results reasonably agree well with the calculated values, and the results show the sensor coated with a 640 nm thick TiO2 film has a much smaller temperature dependence of 0.011°/°C compared to the initial value of 0.1°/°C.

Statistically Correlating Laser-Induced Damage Performance with Photothermal Absorption for Fused Silica Optics in a High-Power Laser System

Photothermal weak absorption is useful for the diagnosis of absorbing defects on the surface of fused silica optics in high-power lasers. However, how they relate to the laser-induced damage performance remains unclear, especially for a fused silica surface that has been post-treated with different processes (e.g., dynamic chemical etching or magnetorheological finishing). Here, we present a correlation study on the surface defect absorption level and laser-induced damage performance of fused silica optics post-treated with different processes using the photothermal common-path interferometer method. Statistical distribution of the absorbing defects at various absorption levels is obtained. The relationship between the defect density and the laser damage performance was analyzed. We show that the surface absorbing defects of fused silica can be affected by the post-treatment type and material removal amount. Furthermore, we show that the density of the defects with the absorption over 2 ppm is strongly correlated with the damage initiation threshold and damage density. Especially, for high-density defects at this absorption level, the damage density of fused silica optics can be well-predicted. In the low-density range, the density of this kind of defect can reflect the zero-probability damage threshold well. The study exhibits the potential of this methodology to non-destructively detect the key absorbing defects on fused silica surfaces as well as evaluate and optimize the post-treatment level of fused silica optics for high-power laser applications.

Robust dynamic phase-shifting common-path shearography using LCPG and pixelated micropolarizer array

A concise and dependable dynamic phase-shifting shearography system based on common-path interferometer is proposed. Two lateral sheared images with common optical path are generated using a liquid crystal polarization grating (LCPG). The sheared images of the object under test interfere with each other, and a pixelated micropolarizer array is used to introduce pixelated carrier, enabling the measurement of dynamic deformation using dynamic phase-shifting. Compared with the ordinary imaging system, the shearography system only needs to insert LCPG between an imaging lens and a camera with pixelated micropolarizer array. The system is resilient to environmental disturbances benefitting from the compact common-path and dynamic phase-shifting method. Theoretical analysis and experiments have been completed, showing that the system can fully exploit the advantages of shearography and improve the reliability of shearography applications.