Amplitude-modulated Light Research Articles

An Optically Pumped Magnetometer with Omnidirectional Magnetic Field Sensitivity.

In mobile applications such as geomagnetic surveying, two major effects hamper the use of optically pumped magnetometers: dead zones, sensor orientations where the sensors signal amplitude drops; and heading errors, a dependence of the measured magnetic field value on the sensor orientation. We present a concept for an omnidirectional magnetometer to overcome both of these effects. The sensor uses two cesium vapor cells, interrogated by circularly-polarized amplitude-modulated laser light split into two beams propagating perpendicular to each other. This configuration is experimentally investigated using a setup wherein the laser beam and magnetic field direction can be freely adjusted relative to each other within a magnetically shielded environment. We demonstrate that a dead-zone-free magnetometer can be realized with nearly isotropic magnetic-field sensitivity. While in the current configuration we observe heading errors emerging from light shifts and shifts due to the nonlinear Zeeman effect, we introduce a straightforward approach to suppress these systematic effects in an advanced sensor realization.

Vibration measurement by projection of phase-modulated and amplitude-modulated structured light

Most of the whole-field optical methods for vibration measurement have low sensitivity when the points of the studied surface vibrate with the same amplitude. Those techniques also usually require complex and/or expensive solutions which are difficult to implement in engineering processes when the vibration amplitudes are relatively high. In order to overcome those limitations we propose a method for out-of-plane vibration measurement which uses structured light projection. The vibrating surface is obliquely illuminated by straight and parallel interference fringes produced by a Twyman-Green interferometer with a 532-nm laser as light source. In order to enable fringe visualization two techniques were employed, namely, the phase modulation of the fringe pattern by using a vibrating mirror in the interferometer, and a stroboscopic illumination by using a Fabry-Perot etalon amplitude modulator. We demonstrated the technique by measuring the vibration amplitudes of small objects in the millimeter and submillimeter range.

High dynamic range thickness measurement using a double heterodyne interferometer

The present study theoretically demonstrates a high dynamic range optical interference measurement approach based on phase change of the amplitude modulated (AM) wave envelope obtained by on-axis heterodyne interference, which can measure thicknesses from sub-wavelength to several millimeters. First, the AM light wave generated by on-axis heterodyne interference is split into two AM light waves. The sample thickness is obtained from the position of the interference fringes, produced by interfering with the transmitted AM light wave through the sample and reference AM wave. Furthermore, the proposed measurement approach is simplified via 4-beam optical interference using circularly polarized heterodyne-modulated waves.

Enhancement of bandwidth in spin-exchange relaxation-free (SERF) magnetometers with amplitude-modulated light

We demonstrate the bandwidth enhancement of an all-optical spin-exchange relaxation-free (SERF) magnetometer based on amplitude-modulated (AM) light. Alkali metal atoms are modulated directly by the pump beam instead of the modulation field or radio frequency field. The first harmonic demodulation of an AM SERF magnetometer with a modulation intensity of 15 kHz results in a high bandwidth of over 11 kHz with a sensitivity of 20 fT/Hz1/2 at 30 Hz and 60 fT/Hz1/2 at 10 kHz. Meanwhile, the AM SERF magnetometer with DC demodulation presents the same sensitivity as a traditional DC SERF magnetometer (6 fT/Hz1/2 at 30 Hz). The presented technique for modulating the amplitude of the pump beam allows AM SERF magnetometers to enter the domain of high-bandwidth magnetometers and opens the door to many areas that are inaccessible to conventional magnetometers.

Nanometric axial localization of single fluorescent molecules with modulated excitation

Distance measurements are commonly performed by phase detection based on a lock-in strategy. Super-resolution fluorescence microscopy is still striving to perform axial localization but through entirely different strategies. Here we show that an illumination modulation approach can achieve nanometric axial localization precision without compromising the acquisition time, emitter density or lateral localization precision. The excitation pattern is obtained by shifting tilted interference fringes. The molecular localizations are performed by measuring the relative phase between each fluorophore response and the reference modulated excitation pattern. We designed a fast demodulation scheme compatible with the short emission duration of single emitters. This modulated localization microscopy offers a typical axial localization precision of 6.8 nm over the entire field of view and the axial capture range. Furthermore, the interfering pattern being robust to optical aberrations, a nearly uniform axial localization precision enables imaging of biological samples by up to several micrometres in depth. Adapting the amplitude-modulated light detection and ranging approach to super-resolution microscopy offers a typical axial localization precision of 6.8 nm over the entire field of view and the axial capture range, enabling imaging of biological samples by up to several micrometres in depth.

Psychophysical Study of Human Visual Perception of Flicker Artifacts in Automotive Digital Mirror Replacement Systems

Aliasing effects due to time-discrete capturing of amplitude-modulated light with a digital image sensor are perceived as flicker by humans. Especially when observing these artifacts in digital mirror replacement systems, they are annoying and can pose a risk. Therefore, ISO 16505 requires flicker-free reproduction for 90% of people in these systems. Various psychophysical studies investigate the influence of large-area flickering of displays, environmental light, or flickering in television applications on perception and concentration. However, no detailed knowledge of subjective annoyance/irritation due to flicker from camera-monitor systems as a mirror replacement in vehicles exist so far, but the number of these systems is constantly increasing. This psychophysical study used a novel data set from real-world driving scenes and synthetic simulation with synthetic flicker. More than 25 test persons were asked to quantify the subjective annoyance level of different flicker frequencies, amplitudes, mean values, sizes, and positions. The results show that for digital mirror replacement systems, human subjective annoyance due to flicker is greatest in the 15 Hz range with increasing amplitude and magnitude. Additionally, the sensitivity to flicker artifacts increases with the duration of observation.

Nonparaxial phasor-field propagation.

Growing interest in non-line-of-sight (NLoS) imaging, colloquially referred to as "seeing around corners", has led to the development of phasor-field (P-field) imaging, wherein the field envelope of amplitude-modulated spatially-incoherent light is manipulated like an optical wave to directly probe a space that is otherwise shielded from view by diffuse scattering. Recently, we have established a paraxial theory for P-field imaging in a transmissive geometry that is a proxy for three-bounce NLoS imaging [J. Dove and J. H. Shapiro, Opt. Express27(13) 18016 (2019)]. Our theory, which relies on the Fresnel diffraction integral, introduces the two-frequency spatial Wigner distribution (TFSWD) to efficiently account for specularities and occlusions that may be present in the hidden space and cannot be characterized with P-field formalism alone. However, because the paraxial assumption is likely violated in many, if not most, NLoS scenarios, in the present paper we overcome that limitation by deriving a nonparaxial propagation formula for the P field using the Rayleigh-Sommerfeld diffraction integral. We also propose a Rayleigh-Sommerfeld propagation formula for the TFSWD and provide a derivation that is valid under specific partial-coherence conditions. Finally, we report a pair of differential equations that characterize free-space TFSWD propagation without restriction.

Study on inertial characteristics of photoresistors in a physical workshop

The article describes a measuring complex based on the 32-bit STM32F103VET6 microcontroller for studying the lux-ampere (light), frequency and inertial characteristics of a photoresistor with different laws of recombination of nonequilibrium charge carriers arising under the influence of light. The developed complex is connected to a personal computer (smartphone) and allows to analyze the rise and fall edges of the photoresistor current, as well as to determine the lifetime of excess charge carriers at different illumination levels. Unlike traditional methods for measuring the parameters of photoresistors operating in a dynamic mode, the proposed measuring complex does not use an oscilloscope and a separate modulator of the luminous flux (generator or light interrupter), this made it possible to significantly reduce the size and cost of the complex, as well as automate the measurement process. To determine the frequency response of the photoresistor, it is proposed to use a frequency synthesizer, which allows, together with a digital-to-analog converter of the microcontroller, to form an amplitude-modulated light flux of the required frequency and modulation depth. The complex described in the work can be connected to the Internet using a Wi-Fi module based on an ESP8266 microcontroller, which allows conducting research in a remote mode. It is also possible to determine the parameters of the photoresistor at different values of the load resistance.

Light-ellipticity and polarization-angle dependence of magnetic resonances in rubidium vapor using amplitude-modulated light: Theoretical and experimental investigations

We report on experimental and theoretical investigations of the polarization dependence of magnetic resonance generated by synchronous optical pumping. Magnetic resonances with narrow linewidth are generated experimentally using a rubidium vapor cell with octade-cyltrichlorosilane (OTS) antirelaxation coating on inner walls. We studied the effect of light ellipticity on the amplitudes and widths of magnetic resonances by matching the light modulation frequency with 2 {\Omega}_L (alignment) and {\Omega}_L(orientation) in a Bell-Bloom interaction geometry, where {\Omega}_L corresponds to the Larmor frequency. Both 2 {\Omega}_L and \OmegaL resonance amplitudes showed a strong dependence on the light ellipticity. In addition, we showed that the duty cycle of light modulation changes the slope of amplitude variations in 2{\Omega}_L and {\Omega}_L resonances with light ellipticity. As a potential application, we showed that the difference between 2{\Omega}_Land {\Omega}_Lresonance amplitudes can be used for in situ measurement of light ellipticity. We also studied the dependence of 2{\Omega}_L and {\Omega}_L resonance amplitudes on the polarization angle of linearly polarized light. These amplitudes oscillate periodically with the polarization angle. We found this oscillatory behavior to be sensitive to the tilt in magnetic field direction from the polarization plane. Such a property could be used to realize a vector magnetometer. A density matrix based theoretical model is developed to simulate the magnetic resonance spectrum for different light polarizations. Our theoretical model accurately reproduces the above mentioned experimental observations.

All-optical isotropic scalar 4He magnetometer based on atomic alignment.

We propose a helium scalar magnetometer based on a triple resonance setup, showing no dead angles, and which can be implemented in an all-optical way. This triple-resonance scheme involves optical pumping with amplitude-modulated light, complemented by a modulated light-shift. Both light beams propagate parallel so that a single optical access to the atomic cell is needed. Experimental results are in good agreement with our theoretical model. The main error sources affecting the magnetometer accuracy are discussed.

Three-dimensional/two-dimensional convertible display based on computer-generated holograms and an amplitude-modulated spatial light modulator

A three-dimensional/two-dimensional (3-D/2-D) convertible display was proposed. The display system offers 2-D images via the traditional display technology and offers 3-D images via the computer-generated hologram (CGH) technology. In this system, 2-D and 3-D images are generated from the same amplitude-modulated spatial light modulator (SLM). In the 2-D display mode, the panel displays an amplitude-modulated image and the 2-D image is projected on a polarization-dependent diffuser (PD) screen. In the 3-D display mode, the SLM panel displays CGHs to generate 3-D images. The zero-order diffraction of the amplitude-type CGH will be blocked by a spatial filter (SF), which is designed by a polarizer. Furthermore, a liquid crystal element was employed to modulate the polarization state of the reconstructed light to let the 3-D image quality not to be reduced by the PD. Finally, the proposed system can offer 2-D and 3-D images both clearly. The system suppresses the zero-order diffraction noise of the amplitude-type CGH system successfully.

Гетеродинный светодальномер

In this paper, we consider the possibilities and ways to improve the accuracy of linear measurements and reduce modulation power, as well as their implementation in the construction of a new high-precision laser rangefinder. To do this, it is necessary to implement the transition to a new method of the modulation method that occurs when the operating point of the light modem goes over to the average linear portion of the modulation characteristic of light (MHS). In this case, the dependence of the amplitude-modulated reception light on the distance is characterized by a first-order Bessel function J1 (x) instead of J0 (x), which takes place at the compensation method of the extremum. In addition, such a transition leads not only to a decrease in modulation power, but also to nonlinear distortions and spreads of the modulation phase. All this leads to the fact that there is an increase in the accuracy of linear measurements by almost an order of magnitude. The phase error m? of linear measurements is estimated at hundredths of a millimeter, 0,015–0,02 mm. However, for the implementation of this method, there are difficulties associated with the process of phase detection of the receiving modulated light. There are no photoelectric multipliers operating at a frequency of 600–1200 MHz. The proposed work provides a schematic diagram of the heterodyne laser range finder, which makes it possible to realize the operation of a light-range meter with circular polarization of light by using an auxiliary generator-heterodyne.

Is light narrowing possible with dense-vapor paraffin coated cells for atomic magnetometers?

We investigated the operation of an all-optical rubidium-87 atomic magnetometer with amplitude-modulated light. To study the suppression of spin-exchange relaxation, three schemes of pumping were implemented with room-temperature and heated paraffin coated vacuum cells. Efficient pumping and accumulation of atoms in the F=2 ground state were obtained. However, the sought-for narrowing of the resonance lines has not been achieved. A theoretical analysis of the polarization degree is presented to illustrate the absence of light narrowing due to radiation trapping at high temperature.

Efficient and Fast Implementation of Embedded Time-of-Flight Ranging System Based on FPGAs

Time-of-flight cameras perceive depth information about the surrounding environment with an amplitude-modulated near-infrared light source. The distance between the sensor and objects is calculated through measuring the time the light needs to travel. To be used in fast and embedded applications, such as 3-D reconstruction, visual SLAM, human-robot interactions, and object detection, the 3-D imaging must be performed at high frame rates and accuracy. Thus, this paper presents a real-time field programmable gate arrays platform that calculates the phase shift and then the distance. Experimental results shown that the platform can acquire ranging images at the maximum frame rate of 131fps with a fine measurement precision (appropriately 5.1mm range error at 1.2m distance with the proper integration time). Low resource utilization and power consumption of the proposed system make it very suitable for embedded applications.

Financial states of world financial and commodities markets around sovereign debt crisis

Measurement of reflected optical light from the surface of microscopic structures can offer a precise tool to investigate the oscillatory motion of micro- and nano-mechanical resonators. We demonstrate an optical investigation of multiple flexural modes of a nanomechanical resonator in thermal motion based on amplitude-modulated laser light, with higher-order resonant modes up to ~ 17 MHz measured with this technique. Scanning focused laser light reproduces the mode shape of each flexural mode, and theoretical estimation of measurement sensitivity shows a displacement sensitivity of ~ 10 −15 m/Hz1/2. For nanomechanical devices incompatible with interferometric systems, this technique can exhibit sufficient sensitivity to explore small motional states.

A photophone-based remote nondestructive testing approach to interfacial defect detection in fiber-reinforced polymer-bonded systems

Externally bonded fiber-reinforced polymer is an increasingly popular material to be used in strengthening and retrofitting aging structures. In such structures, debonding defects may occur at or near the interface between fiber-reinforced polymer and concrete. As such debonding in fiber-reinforced polymer-bonded systems is generally brittle in nature, there is a need of a reliable inspection technique that can provide early warning of interfacial defects such that premature failure of fiber-reinforced polymer-strengthened structures can be avoided. A remote nondestructive testing approach based on the working principle of a photophone is presented here as an economical alternative to laser Doppler vibrometry for detecting interfacial defects. Concrete specimens retrofitted with fiber-reinforced polymer are excited acoustically by white noise, while the surface of the structure is illuminated by a light source. If an interfacial defect exists beneath the surface, the surface will exhibit a frequency response different from an intact surface. The surface of the fiber-reinforced polymer portrays the role of flexible mirror in a photophone, which encodes information about surface vibration into amplitude-modulated light signal. A light detector then captures the irradiance of the reflected beam, and the amplitude modulation is converted into frequency domain in post-processing. With this technique, defect dimensions and thus damage extent can be inferred from the frequency spectrum obtained. The obtained results correspond well with the theoretical calculation, demonstrating the robustness and the applicability of the proposed technique in civil infrastructure.

Universal anyons at the irradiated surface of topological insulator.

Anyons have recently received great attention due to their promising application in topological quantum computation. The best validated system that enjoys the anyonic excitations are the Laughlin states. The quasi-particles in Laughlin states are neither fermions nor bosons but possess the discrete statistical angle θ = π/m, with m being an integer. Here we report a possible realization of the universal Abelian anyons, whose statistical angle can be tuned continuously by external parameters and can take any arbitrary values interpolating θ = 0 and θ = π. The proposed setup is the surface state of a three dimensional topological insulator driven by an amplitude-modulated circularly-polarized light. It is found that the external field leads to a particular Floquet phase, which is a two-spatial-dimensional analogy of the Weyl semimetal phase in the Floquet first Brillouin zone. The chiral anomaly of this phase results in a U(1) Chern-Simons gauge theory with a tunable Floquet Chern number. Owing to this underlying gauge field theory, the irradiated surface of topological insulator constitutes a promising platform for the observation of the universal anyons.

Resolving multiple propagation paths in time of flight range cameras using direct and global separation methods

Time of flight (ToF) range cameras illuminate the scene with an amplitude-modulated continuous wave light source and measure the returning modulation envelopes: phase and amplitude. The phase change of the modulation envelope encodes the distance travelled. This technology suffers from measurement errors caused by multiple propagation paths from the light source to the receiving pixel. The multiple paths can be represented as the summation of a direct return, which is the return from the shortest path length, and a global return, which includes all other returns. We develop the use of a sinusoidal pattern from which a closed form solution for the direct and global returns can be computed in nine frames with the constraint that the global return is a spatially lower frequency than the illuminated pattern. In a demonstration on a scene constructed to have strong multipath interference, we find the direct return is not significantly different from the ground truth in 33∕136 pixels tested; where for the full-field measurement, it is significantly different for every pixel tested. The variance in the estimated direct phase and amplitude increases by a factor of eight compared with the standard time of flight range camera technique. © 2015 Society of Photo-Optical Instrumentation Engineers (SPIE) (DOI: 10 .1117/1.OE.54.11.113109)

Application of lidar techniques to time-of-flight range imaging.

Amplitude-modulated continuous wave (AMCW) time-of-flight (ToF) range imaging cameras measure distance by illuminating the scene with amplitude-modulated light and measuring the phase difference between the transmitted and reflected modulation envelope. This method of optical range measurement suffers from errors caused by multiple propagation paths, motion, phase wrapping, and nonideal amplitude modulation. In this paper a ToF camera is modified to operate in modes analogous to continuous wave (CW) and stepped frequency continuous wave (SFCW) lidar. In CW operation the velocity of objects can be measured. CW measurement of velocity was linear with true velocity (R2=0.9969). Qualitative analysis of a complex scene confirms that range measured by SFCW is resilient to errors caused by multiple propagation paths, phase wrapping, and nonideal amplitude modulation which plague AMCW operation. In viewing a complicated scene through a translucent sheet, quantitative comparison of AMCW with SFCW demonstrated a reduction in the median error from -1.3 m to -0.06 m with interquartile range of error reduced from 4.0 m to 0.18 m.

Antirelaxation coatings in coherent spectroscopy: Theoretical investigation and experimental test

We describe a theoretical model, based on a density matrix and the Liouville equation, for the investigation of magneto-optical resonances in alkali-metal atomic vapor, in particular in the case of the electromagnetically induced transparency (EIT) in the presence of antirelaxation coatings. The influence of the coating is parametrized with an empirical coefficient describing its efficiency; the calculations are extended to a broad range of coating quality, contrary to previous works, and to uncoated cells. The model takes into account also different configurations for the EIT formation and different efficiency of optical pumping, as determined by the coating characteristics and the atomic energy structure. The model is validated by investigating the EIT with degenerate Zeeman levels in $^{39}\text{K} {D}_{1}$ and Cs ${D}_{2}$ lines, which exhibit respectively an almost negligible and a relevant impact of hyperfine optical pumping. The results are compared to experimental data, exhibiting good agreement; in particular, for the $^{39}\text{K} {D}_{1}$ line, recent findings are shown here in the case of degenerate and nondegenerate EIT with amplitude-modulated light. Our results demonstrate an effective approach for the investigation of antirelaxation coatings and their contribution in the formation of magneto-optical resonances in alkali-metal atoms, in different regimes and with largely different efficiencies. This sheds new light on well-known but not yet entirely clarified phenomena and their behavior as a function of experimental parameters.