Cube Beam Splitter Research Articles

Parallel two-step spatial carrier polarized phase-shifting common-path digital holography

A zero-order term causes bad effect on the phase reconstruction of off-axis holography, which can be solved by two-step carrier phase-shifting. Therefore, a common-path digital holography is proposed by using parallel two-step spatial carrier polarized phase-shifting in a 4f optical system for quantitative phase imaging. This holography is established based on a 4f optical system by inserting a 45° tilted cube polarized beamsplitter to split the horizontal and vertical polarization components in one beam. Via intentional polarization modulation, two off-axis holograms with π/2 phase-shift can be captured in one shot, so the subtraction between two holograms would cancel out the zero-order term. The phase distribution of a thin specimen can be retrieved with enhanced quality. There are two apertures at the input plane of the 4f optical system, one for object and the other for nothing, while only one polarized cube beamsplitter is inserted into a 4f optical system. Thus, this system is common-path with strong anti-noise ability and high stability, while it also has simple optical configuration and easy optical alignment. Several experimental results would be presented to demonstrate the validity of the proposed holography.

Compact and High-Efficiency Liquid-Crystal-on-Silicon for Augmented Reality Displays

Compact and high efficiency microdisplays are essential for lightweight augmented reality (AR) glasses to ensure longtime wearing comfort. Liquid-crystal-on-silicon (LCoS) is a promising candidate because of its high-resolution density, high brightness, and low cost. However, its bulky illumination system with a polarizing beam splitter (PBS) cube remains an urgent issue to be overcome. To reduce the volume of the LCoS illumination system, here, we propose a compact structure with four thin PBS cuboids. Through simulations, the optical efficiency of 36.7% for an unpolarized input light can be achieved while maintaining reasonably good spatial uniformity. Such a novel design is expected to have a significant impact on future compact and lightweight AR glasses.

Single-shot characterization of special multi-soliton and noise-like pulse emissions in a mode-locked fiber laser

Single-shot characterization of three categories of multi-pulse emissions in a passively mode-locked fiber laser based on nonlinear polarization rotation is experimentally reported. The mechanism to control the intracavity polarization state relies on a polarizer beam splitter cube together with a half-wave and a quarter-wave retarders. With a precise adjustment of the retarders, the fiber laser can reach different mode locking regimes involving noise-like pulses (NLP) and solitons. These regimes include (i) NLP emission exhibiting broad-bandwidth supercontinuum spectrum, (ii) multiple soliton emission in the form of soliton bunches with rectangular-shaped envelope, and (iii) NLP emission with complex pulse patterns, such as the one obtained from the breakup of a rectangular NLP, or the harmonic mode locking regime exhibiting multiple, uniformly spaced NLP wave packets. The experimental results presented in this work provide useful information for a better understanding of different types of multi-pulse emission from the same fiber laser system, complementing the studies focused on the nature and physical mechanisms underlying NLP emission.

Double field-of-view single-shot common-path off-axis reflective digital holographic microscope

Digital holography is a versatile three-dimensional imaging technique that has the ability to record the complex wave-front of an imaged object in two-dimensions and retrieve it in three-dimensions. Several technical challenges of digital holographic systems have been overcome by proposing single-shot acquisition and common-path configurations. However, the limited fiel-of-view (FOV) of digital holography is the most fundamental and technically challenging aspect of this technology. With this in mind, we have developed a digital holographic microscope (DHM) with a doubled FOV together with it leverages single-shot acquisition, common-path, and off-axis configuration and operates in the reflection mode. The double FOV is achieved by spatial frequency multiplexing of two different areas of the object beam by the use of a cube beam splitter. The common-path and off-axis configuration are obtained by employing a plate beam splitter just before the microscope objective. Several experiments are carried out, and the results are presented to demonstrate the validity and effectiveness of the proposed DHM for quantitative phase imaging of (semi) transparent and reflective objects. Based on the experimental results, the proposed microscope shows advanced performance in biomedical imaging as well as inspection of engineered surfaces with its simplicity, higher stability (temporal and mechanical), compactness, low cost, and most importantly double FOV capabilities.

Segmented Four-Element Photodiodes in a Three-Dimensional Laser Beam Angle Measurement

Based on the registration of two laser beam projections, a method for measuring the angular deviation of a laser beam from its initial position in three-dimensional space is proposed and experimentally demonstrated. The laser ray is directed into the beam-splitting cube, which distributes the ray into two mutually orthogonal parts, on the path of which, two four-segment photodiodes are located at different distances from the beam-splitting cube. The value of the angular deviation from a certain initial position of the laser beam in three-dimensional space is obtained after the mathematical processing of the measured position of the laser beam projections’ centers in the photodiodes coordinates systems. The innovation of this method is that the resulting angular deviation of the laser beam is in three-dimensional space due to the registration of a pair of projections. The experimental results also demonstrate that the proposed method has a high speed-measuring potential and can be used for solving a wide range of problems.

A motorized rotation mount for the switching of an optical beam path in under 20ms using polarization control.

We present a simple motorized rotation mount for a half-wave plate that can be used to rapidly change the polarization of light. We use the device to switch a high power laser beam between different optical dipole traps in an ultracold atom experiment. The device uses a stepper motor with a hollow shaft, which allows a beam to propagate along the axis of the motor shaft, minimizing inertia and mechanical complexity. A simple machined adapter is used to mount the wave plate. We characterize the performance of the device, focusing on its capability to switch a beam between the output ports of a polarizing beam splitter cube. We demonstrate a switching time of 15.9(3) ms, limited by the torque of the motor. The mount has a reaction time of 0.52(3) ms and a rotational resolution of 0.45(4)°. The rotation is highly reproducible, with the stepper motor not missing a step in 2000 repeated tests over 11h.

Quality inspection of cube beam splitters by awhite light interferometric approach.

Beam splitters have a wide range of applications as a key component in optical systems. Adopting beam splitters with geometric defects in an optical system, e.g., an interferometric measurement system, may cause additional optical path difference and degrade the measurement accuracy. The quality inspection of beam splitters is essential to meet the accuracy requirements for modern optical systems. Most of the current quality inspection methods rely on inefficient and inaccurate manual observation. Therefore, for commonly used cube beam splitters (CBSs), we propose a digital method to quantify the geometric quality based on the white light interferometric principle. A Fourier domain analysis is used to calculate the CBS misalignment error and perpendicularity error. This method is verified by inspecting six different CBS samples. The experimental results show that all samples have varying degrees of misalignment and perpendicularity errors. The maximum perpendicularity error is 0.93°, and three of the six samples have misalignment errors larger than 50µm. Nanometer level precision of the misalignment measurement can be achieved.

Pulse Burst Generation and Diffraction with Spatial Light Modulators for Dynamic Ultrafast Laser Materials Processing.

A pulse burst optical system has been developed, able to alter an energetic, ultrafast 10 ps, 5 kHz output pulse train to 323 MHz intra-burst frequency at the fundamental 5 kHz repetition rate. An optical delay line consisting of a beam-splitting polariser cube, mirrors, and waveplates transforms a high-energy pulse into a pulse burst, circulating around the delay line. Interestingly, the reflected first pulse and subsequent pulses from the delay line have orthogonal linear polarisations. This fact allows independent modulation of these pulses using two-phase-only Spatial Light Modulators (SLM) when their directors are also aligned orthogonally. With hybrid Computer Generated Holograms (CGH) addressed to the SLMs, we demonstrate simultaneous multi-spot periodic surface micro-structuring on stainless steel with orthogonal linear polarisations and cylindrical vector (CV) beams with Radial and Azimuthal polarisations. Burst processing produces a major change in resulting surface texture due to plasma absorption on the nanosecond time scale; hence the ablation rates on stainless steel with pulse bursts are always lower than 5 kHz processing. By synchronising the scan motion and CGH application, we show simultaneous independent multi-beam real-time processing with pulse bursts having orthogonal linear polarisations. This novel technique extends the flexibility of parallel beam surface micro-structuring with adaptive optics.

A two-beam interferometer with the tuning of the interference pattern period by simulating its rotation

Two-beam interferometer with fixed mirrors and fixed recording media, analytical analysis of optical scheme and design of the construction are presented. Tuning of the spatial frequency of the interference pattern on the media is carried out by rotation simulation mechanism. A working sample device of interferometer based on a beam-splitting cube 20 mm in size and implementing the investigated concept has been created. In the device, a half of convergence angle of interfering beams with a diameter of 5 mm is tunable in the range from 11 to 19°, its value is setting using a degree scale with a resolution of 0.1°. The deviation of actual value of the angle through the scale reading is experimentally obtained in the device. It was found that the absolute error does not exceed 1%. Developed interferometer may be useful an instance in holographic (including liquid) materials studies, development a new design of static Fourier spectrometers and making the interference based lithography process.

Off-axis common-path digital holography using a cube beam splitter.

An off-axis common-path digital holography is built up by inserting a 45° tilted cube beam splitter (CBS) into a 4f system that is described in this paper. Two apertures are set as the input of the 4f system, where one supports the object, and the other is vacant. The CBS divides the incoming beam into two copies, which are symmetrical with each other along the semi-reflecting layer. Due to the separation of two beams in a Fourier plane and the flipping of the field of view induced by the CBS, an off-axis hologram can be captured. Moreover, the carrier frequency can be easily modulated by translating the CBS perpendicular to the optical axis. The new proposed scheme has high light utilization, a compact setup, and high temporal stability. The experiments are carried out to demonstrate the validity and stability of the proposed method.

Single-element real-time interferometric system for measuring dynamic temperature field of liquid medium

Temperature plays an important role in biomedicine and industrial production. In this study, we demonstrate a simple and reliable real-time interferometric system based on a single-element for measuring the dynamic temperature field of liquid media. In the proposed system, the reference beam and the object beam enter the beam splitter cube with a splitting ratio of 50:50 to produce interference fringes. The interference light arrives at the observation plane of a CMOS camera after being attenuated, which is capable of monitoring the change process of space carrier phase shift distribution in real time. The common optical path design eliminates the environmental disturbance. The phase shift of the interferograms as the optical path difference between the reference beam passing through liquid and the object beam passing through air is calculated by fast Fourier transform. The temperature changes of 50% polyacrylamide hydrogel (25–65 °C), 4% NaCl (25–65 °C), and ethanol (20–60 °C) were measured by our proposed system. During the experiment, we successfully measured the dynamically changing temperature field of deionized water subjected to a heating and cooling process (25–40 °C). The proposed system offers the advantages of compact structure, high efficiency, long-term stability, and low cost, and it is expected to assist non-contact measurements and biomedical research, making it a viable reference candidate for the field of environmental ecology and medicine.

Simplified dual-channel two-wavelength interferometer using a polarized cube beam splitter.

Two-wavelength interferometers can extend an unambiguous measurement range; however, they suffer from complex optical configurations. To simplify the optical setup for a two-wavelength common-path off-axis interferometer, we propose a dual-channel two-wavelength interferometer using a polarized cube beam splitter. In contrast with the previously presented two-wavelength common-path off-axis interferometer, the proposed method has a simple setup, in which only one polarized cube beam splitter is inserted into the 4f system. With the help of polarization modulation, two single-wavelength interferograms can be captured simultaneously. Several experimental results are presented to demonstrate the advantages and effectiveness of the proposed method.

Optical module for single-shot quantitative phase imaging based on the transport of intensity equation with field of view multiplexing.

We present a cost-effective, simple, and robust method that enables single-shot quantitative phase imaging (QPI) based on the transport of intensity equation (TIE) using an add-on optical module that can be assembled into the exit port of any regular microscope. The module integrates a beamsplitter (BS) cube (placed in a non-conventional way) for duplicating the output image onto the digital sensor (field of view - FOV - multiplexing), a Stokes lens (SL) for astigmatism compensation (introduced by the BS cube), and an optical quality glass plate over one of the FOV halves for defocusing generation (needed for single-shot TIE algorithm). Altogether, the system provides two laterally separated intensity images that are simultaneously recorded and slightly defocused one to each other, thus enabling accurate QPI by conventional TIE-based algorithms in a single snapshot. The proposed optical module is first calibrated for defining the configuration providing best QPI performance and, second, experimentally validated by using different phase samples (static and dynamic ones). The proposed configuration might be integrated in a compact three-dimensional (3D) printed module and coupled to any conventional microscope for QPI of dynamic transparent samples.

Intensity noise characteristics of intracavity contacted VCSELs with rhomboidal oxide current aperture for the magnetometric sensor with Cs133 vapor cell used in magnetoencephalography

We demonstrate noise characterization of novel 894.4 nm vertical-cavity surface emitting lasers with intracavity contacts and a rhomboidal oxide current aperture (IC-VCSELs), dedicated to 133Cs D1 line compact optically pumped atomic magnetometers (OPM). The laser relative intensity noise, measured to be -139 dB/Hz at 10 kHz frequency in 1 Hz bandwidth for a laser optical power of 0.8 mW, is decreased with optical power growth. The IC-VCSELs polarization-resolved relative intensity noise is 143 dB/Hz at 10 kHz frequency in 1 Hz bandwidth for 0.8 mW. The emission linewidth of the VCSEL is about 55 MHz. The IC-VCSEL parameters are determined, such as emission linewidth ~ 50-60 MHz, optical power ~ 0.5-1.0 mW, at which the polarization-resolved RIN becomes close to the RIN, which makes it possible to use these lasers in various OPM Mz and Mx schemes. The ultimate sensitivity of OPM was estimated by the ratio of the magnetic resonance to the signal to noise level. It is shown that a OPM based on the IC-VCSELs, assuming magnetic resonance FWHM ~ 1 kHz, can achieve a shot noise-limited sensitivity around 20 fT in 1 Hz bandwidth without any polarization improvements by polarizer or polarization beam splitter cube (PBC). Developed IC-VCSELs is acceptable for use in compact OPM for magnetoencephalography.

Spatially multiplexed interferometric microscopy: principles and applications to biomedical imaging

Digital holographic microscopy (DHM) applied to quantitative phase imaging (QPI) has been successfully demonstrated as a powerful label-free method to analyse the optical properties of cells. Spatially multiplexed interferometric microscopy (SMIM) is a DHM technique that implements a common-path interferometric layout in the embodiment of a standard microscope to achieve QPI. More concretely, SMIM introduces three minimal modifications: (a) replaces the broadband illumination of the microscope by a coherent or partially coherent light source, (b) divides the input plane into two or three regions for transmission in parallel of both imaging and reference beams, and (c) includes a one-dimensional (1D) diffraction grating or a beam splitter cube for holographic recording. Hence, SMIM is a cost-effective, extremely simple, and highly stable manner of converting a standard bright field microscope into a holographic one. The goal of this contribution is to present the SMIM approaches implemented using a 1D diffraction grating, and highlight vast range of capabilities for biomedical imaging including super-resolved, reflective, transflective, noise-reduced and single-shot slightly off-axis amplitude and phase imaging.

Fiber-coupling optical system for high-power and multi-wavelength diode laser bars oriented to integrated biomedical imaging systems

The use of pulsed high-power diode lasers (HPDLs) in the near infrared (NIR) range as multispectral sources has attracted much interest in biomedical applications due to their relatively low cost and small size compared to nanosecond Nd:YAG optical parametric oscillator (OPO) lasers. One of the main limitations of these sources is the availability to combine different wavelengths with high power density in the same beam. Various works have shown that the use of linear arrays of emitters as diode laser bars (DLBs) or stacked arrays of emitters as diode laser stacks (DLSs) allows the combination of multiple wavelengths while maintaining high power densities. Nevertheless, the highly asymmetric beam profile emitted by such laser sources between fast and slow axes implies the need for suitable beam shaping for efficient fiber coupling. In this work, we investigate a novel beam shaping technique to homogenize the beam quality of six DLBs in the wavelength range between 790 nm and 980 nm.We consider fast-axis collimating lenses (FAC) and beam twisters to reduce the beam asymmetry of the individual bars. The beams from the DLBs are then combined into a single multispectral beam using reflective mirrors, dichroic mirrors, and a polarizing beam splitter cube (PBC), and effectively coupled into a 400 µm core-diameter/N.A. = 0.22 optical fiber using a pair of cylindrical lenses. Simulation shows high coupled power densities with ~ 1.8 MW/cm2 at the output of the fiber. The coupling efficiency reaches 89.4%. The use of submillimeter fiber optic probes is particularly promising for photoacoustic endoscopy (PAE) applications requiring minimally invasive examination of internal organs.

Ghost Beam Suppression in Deep Frequency Modulation Interferometry for Compact On-Axis Optical Heads.

We present a compact optical head design for wide-range and low noise displacement sensing using deep frequency modulation interferometry (DFMI). The on-axis beam topology is realised in a quasi-monolithic component and relies on cube beamsplitters and beam transmission through perpendicular surfaces to keep angular alignment constant when operating in air or in a vacuum, which leads to the generation of ghost beams that can limit the phase readout linearity. We investigated the coupling of these beams into the non-linear phase readout scheme of DFMI and implemented adjustments of the phase estimation algorithm to reduce this effect. This was done through a combination of balanced detection and the inherent orthogonality of beat signals with different relative time-delays in deep frequency modulation interferometry, which is a unique feature not available for heterodyne, quadrature or homodyne interferometry.

Narrow-linewidth coherent population oscillation spectroscopy of room-temperature cesium atomic ensemble

Coherent population oscillations spectroscopy, which is based on the interaction between atoms and the phase locked laser, is a kind of atomic population modulation spectroscopy. When the laser frequency difference is less than natural width of energy level, the coherent oscillation of atomic population will be induced by laser intensity modulation so that the probe laser transmission with narrow bandwidth can be realized. For a closed two-level system (TLS), the spectral line-width is limited mainly by the spontaneous emission lifetime of the upper atomic energy level. As for a three-level atomic system of Λ configuration, the two linearly polarized beams with both σ+ and σ- polarization component, the laser-atom interaction satisfies the selection rule. The spectral line-width mainly depends on the ground-state relaxation time, and the dependence on the line-width of spontaneous radiation is eliminated. In this paper, the laser from a external-cavity diode laser has its frequency locked to Cesium <inline-formula><tex-math id="M1">\begin{document}$6{{\rm{S}}_{1/2}}\left( {F = 3} \right) \to 6{{\rm{P}}_{3/2}}\left( {F' = 3} \right)$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210405_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210405_M1.png"/></alternatives></inline-formula> transition. The frequencies of the two beams are shifted down by two independent double-passed acousto-optic modulators (AOM) to nearly resonate to Cesium <inline-formula><tex-math id="M2">\begin{document}$6{{\rm{S}}_{1/2}}\left( {F = 3} \right) \to 6{{\rm{P}}_{3/2}}\left( {F' = 2} \right)$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210405_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210405_M2.png"/></alternatives></inline-formula> transition. The probe beam and the coupling beam are superposed at polarization beam splitter (PBS) cube and transmitted through the magnetically shielded cesium vapor cell in the same direction. The two beams have approximately the same Gaussian diameter of 6.6 mm. The beams are separated by another PBS behind the vapor cell, and the probe beam is detected by a photodiode. We realize the coherent population oscillation spectroscopy through the Cesium vapor cell at room temperature without buffer gas. The spectral linewidth is typically less than 50 kHz which is far below the spontaneous radiation linewidth(~5.2 MHz). The linewidth of coherent population oscillation spectroscopy of the Λ-type atomic energy level structure depends only on the population associated with the oscillation of multiple degenerate level systems except phase correlations of atomic states. Coherent population oscillation is beneficial to the obtaining of the narrow linewidth spectroscopy through the Rydberg atomic system with long excited state lifetime. Considering the importance of electric field measurement using Rydberg atoms, the method of coherent population oscillation can be used to improve the sensitivity of precise measurements based on Rydberg atoms.

A new experimental apparatus for polarized spectral emissivity measurement in a controlled environment

A new experimental apparatus is designed for polarized spectral emissivity measurements of solid material in a controlled environment such as vacuum, oxidization, reducing atmosphere, etc. The sample heater is placed inside a stainless steel chamber, which allows to heat the sample up to 1073 K. The sample heater can be rotated 180° with an electric rotating platform. The stainless steel chamber is equipped with three flow controllable intake valves to simulate different gas atmosphere. A polarization beam splitting cube is placed in front of the window, and the radiance emitted from the specimen surface is divided into P and S polarized radiance. The polarized radiance can be measured from 1200 to 1600 nm by an indium gallium arsenic detector. The effect of radiation of the spherical chamber is calculated and discussed in detail. In order to validate the capability of the apparatus, the polarized spectral emissivity of silicon wafer at 1.5 μm is measured firstly. The result shows an excellent fit with the simulation. Then, the pure iron and silicon carbide is measured respectively in an atmosphere and vacuum environment. The experimental results verify the functionality and repeatability of the apparatus. The uncertainty assessment shows that the relative combined uncertainty of polarized spectral emissivity is no more than 1.92% for the spectral range considered.

Compact, unit magnification, reflective imaging relay using freeform surfaces

We present a design for a unit magnification imaging relay that is compact. The design uses two mirrors, and one is used by light twice for a three-reflection system. The mirrors are specified with freeform surfaces. There are several tradeoffs of performance depending on field of view, F / #, and wavelength range. One example provides a 30-mm circular field of view at F / 2.8, and another example provides a circular field of view of 40 mm in diameter at F / 4 for a length of ∼400 mm. Adding a third nearly flat mirror allows a system to provide a 50-mm field of view at F / 4 for a length of 420 mm. A last example includes a beam splitter cube to allow accessing the full field of view. These systems are diffraction limited at 550 nm.