Path Length Difference Research Articles

Assessing effect of ground reflections on gunshot noise exposure

Investigation of weapons noise exposure outdoors necessitates including the impact of ground reflections. When analyzing the reflections’ impact on A-weighted equivalent level (LAeq), the contribution of the reflected waveform to the total level can often be acceptably modeled using a Friedlander equation-based source model and an ideal reflection based on the measurement geometry. However, the Friedlander model does not adequately model the negative phase of a gunshot waveform, making it inaccurate for locations where the waveform of the direct blast and the waveform of the reflection begin to overlap temporally. This merging of the waveforms for locations where the direct and ground-reflected path length difference is relatively small also makes it difficult to separate the waveforms and calculate the reflected wave’s contribution to noise exposure. As such, this paper investigates different approaches to estimating the contribution of the ground reflected wave to the LAeq, using direct and reflected waveforms in regions where they are well separated and then adjusting timing and amplitude for different geometries where merging takes place. Results reveal regions where the ground reflection contributes negligibly or even negatively to the total LAeq, which is not predicted by the Friedlander model nor by simple energy-addition approaches. [Work supported by Air Force Research Laboratory through Ball Aerospace Technology Corporation.]

Ranging with a frequency-shifted feedback laser using frequency-comb driven phase modulation of injected radiation

Theoretical and experimental data is presented for the application of an injection-seeded frequency-shifted feedback (FSF) laser for high accuracy ranging. Previous work discussed such a ranging scheme with a phase-modulated single-frequency laser where the phase modulation is done by an electro-optical modulator driven by a single frequency Ω which is swept over a certain bandwidth depending on the given experimental situation. In the present theoretical and experimental work, the phase modulation of the injection laser is done by a frequency comb with temporally fixed frequency components at intervals Ω d spanning a bandwidth adapted to the geometry of the object to be measured. It is shown that the superposition of such FSF radiation returning from the object and a reference surface on a detector leads to a train of sinusoidal pulses with an instantaneous frequency Ωinst in the radio-frequency range. The repetition rate of these pulses is Ω d and their duration is . The central result of this work is the observation that the path length difference between reference and object surface can be deduced from Ωinst, e.g. by frequency counting. The benefit of this approach lies in the fact that active frequency variation is not needed; all features of the entire system (FSF laser plus phase-modulated injected radiation) are constant in time. Proof-of-concept results using an FSF-laser ranging scheme based on a semiconductor laser are presented.

A Wideband Microwave Holography Methodology for Reflector Surface Measurement of Large Radio Telescopes

Most reflector surface holographic measurements of a large radio telescope utilize a geostationary satellite as the signal source. The shortcoming is that those measurements could only be done at a limited elevation angle due to the satellite’s relatively stationary state. This paper proposed a new wideband microwave holographic measurement method based on radio sources to achieve full-elevation-angle measurement with small size reference antenna. In theoretical derivation, the time delay and phase change due to path length and device difference between the antenna under test and reference antenna are compensated first. Then the correct method of wideband holography effect, which is because of antenna pattern differing under different wavelengths when receiving a wideband signal, is presented. To verify the proposed methodology, a wideband microwave holographic measurement system is established, the data processing procedure is illustrated, and the reflector surface measurement experiments on a 40 m radio telescope at different elevation angles are conducted. The result shows that the primary reflector surface root-mean-square at around elevation angles of 28°, 44°, 49°, and 75° are respectively 0.213 mm, 0.170 mm, 0.188 mm, and 0.199 mm. It is basically consistent with the real data, indicating that the proposed wideband microwave holography methodology is feasible.

Non-invasive probing of dynamic ion migration in light-emitting electrochemical cells by an advanced nanoscale confocal microscope

In this study, we firstly propose an optical approach to investigate the ion profile of organic films in light-emitting electrochemical cells (LECs) without any invasive sputtering processes. In contrast to previous literatures, this pure optical strategy allows us to record clear and non-destructive ion profile images in the (Ru(dtb-bpy)3(PF6)2) consisted organic layer without interferences of complex collisions from the bombardment of secondary sputter induced ions in a conventional time-of-flight secondary ion mass spectrometry. By using the advanced position sensitive detector (PSD)-based Nanoscale Confocal Microscope, ion distribution profiles were successfully acquired based on the observation of nanoscale optical path length difference by measuring the refractive-index variation while the thickness of the LEC layer was fixed. Dynamic time-dependent ion profile displayed clear ion migration process under a 100 V applied bias at two ends of the LEC. This technique opens up a new avenue towards the future investigations of ion distributions inside organic/inorganic materials, Li-ion batteries, or micro-fluid channels without damaging the materials or disturbing the device operation.

Dual-Polarization Bandwidth-Bridged Bandpass Sampling Fourier Transform Spectrometer from Visible to Near-Infrared on a Silicon Nitride Platform.

On-chip broadband optical spectrometers that cover the entire tissue transparency window (λ = 650–1050 nm) with high resolution are highly demanded for miniaturized biosensing and bioimaging applications. The standard spatial heterodyne Fourier transform spectrometer (SHFTS) requires a large number of Mach–Zehnder interferometer (MZI) arrays to obtain a broad spectral bandwidth while maintaining high resolution. Here, we propose a novel type of SHFTS integrated with a subwavelength grating coupler (SWGC) for the dual-polarization bandpass sampling on the Si3N4 platform to solve the intrinsic trade-off limitation between the bandwidth and resolution of the SHFTS without having an outrageous number of MZI arrays or adding additional active photonic components. By applying the bandpass sampling theorem, the continuous broadband input spectrum is divided into multiple narrow-band channels through tuning the phase-matching condition of the SWGC with different polarization and coupling angles. Thereby, it is able to reconstruct each band separately far beyond the Nyquist criterion without aliasing error or degrading the resolution. We experimentally demonstrated the broadband spectrum retrieval results with the overall bandwidth coverage of 400 nm, bridging the wavelengths from 650 to 1050 nm, with a resolution of 2–5 nm. The bandpass sampling SHFTS is designed to have 32 linearly unbalanced MZIs with the maximum optical path length difference of 93 μm within an overall footprint size of 4.7 mm × 0.65 mm, and the coupling angles of SWGC are varied from 0° to 32° to cover the entire tissue transparency window.

High-stability PGC demodulation technique with an additional sinusoidal modulation based on an auxiliary reference interferometer and EFA.

In the reference interferometer demodulation scheme, it's difficult to guarantee in practice that both interferometers have the same optical path length difference (OPD), which makes the phase modulation depth different in different interferometers with the same laser modulation. The random shift of phase modulation depth also affects the demodulation results. An improved phase-generated carrier (PGC) technique is proposed based on an auxiliary reference interferometer and the ellipse fitting algorithm (EFA). The technique ensures the correct fitting of the EFA for small amplitude signals by introducing a sinusoidal signal as an additional phase modulation. The combination of the reference interferometer and EFA can eliminate the effect of different phase modulation depths of the two interferometers caused by different OPDs, the non-linear distortion caused by phase modulation depth shifts, and improve the accuracy of the demodulation results. The experiment results are consistent with the theoretical analysis, and the method extends the application of the EFA in the reference interferometer phase demodulation technique.

The Lighting of the BECONs


 
 
 The imposition of lockdowns in response to the COVID-19 outbreak has underscored the importance of human behavior in mitigating virus transmission. The scientific study of interventions designed to change behavior (e.g., to promote physical distancing) requires measures of effectiveness that are fast, that can be assessed through experiments, and that can be investigated without actual virus transmission. This paper presents a methodological approach designed to deliver such indicators. We show how behavioral data, obtainable through wearable assessment devices or camera footage, can be used to assess the effect of interventions in experimental research; in addition, the approach can be extended to longitudinal data involving contact tracing apps. Our methodology operates by constructing a contact network: a representation that encodes which individuals have been in physical proximity long enough to transmit the virus. Because behavioral interventions alter the contact network, a comparison of contact networks before and after the intervention can provide information on the effectiveness of the intervention. We coin indicators based on this idea Behavioral Contact Network (BECON) indicators. We examine the performance of three indicators: the Density BECON, based on differences in network density; the Spectral BECON, based on differences in the eigenvector of the adjacency matrix; and the ASPL BECON, based on differences in average shortest path lengths. Using simulations, we show that all three indicators can effectively track the effect of behavioral interventions. Even in conditions with significant amounts of noise, BECON indicators can reliably identify and order effect sizes of interventions. The present paper invites further study of the method as well as practical implementations to test the validity of BECON indicators in real data.
 
 

Pulsed laser source digital holography efficiency measurements.

In this paper, a 1064 nm pulsed laser source and a short-wave IR (SWIR) camera are used to measure the total system efficiency associated with a digital holography system in the off-axis image plane recording geometry. At a zero path-length difference between the signal and reference pulses, the measured total system efficiency (15.9%) is consistent with that previously obtained with a 532 nm continuous-wave laser source and a visible camera [Appl. Opt.58, G19 (2019)APOPAI0003-693510.1364/AO.58.000G19]. In addition, as a function of the temporal delay between the signal and reference pulses, the total system efficiency is accurately characterized by a component efficiency, which is formulated from the ambiguity function. Even with multimode behavior from the pulsed laser source and substantial dark current noise from the SWIR camera, the system performance is accurately characterized by the resulting ambiguity efficiency.

Visualizing Double-Slit Interference on a Shoestring

Student misconceptions of the double-slit experiment (Fig. 1) are abundant. The most common ones that we observe include: (1) belief that constructive interference requires both pathlengths to be integer multiples of the wavelength (“L1 = n1λ” and “L2 = n2λ”) rather than only the pathlength difference (ΔL ≡ |L1 – L2| = nλ); (2) failure to understand that the justification for d sin θ = nλ is that d sin θ is a good approximation to ΔL; (3) confusion about the limits in which the approximation d sin θ = nλ is valid; and (4) confusion about the limits in which the approximation sin θ = tan θ = y/D is valid. Most of these misconceptions have been observed in the past. We consider the first of these to be the most damaging, since it will hinder students from understanding interference in other contexts in the future. To address this misconception, we have designed an exercise that strongly reinforces the correct understanding (ΔL = nλ) while also helping overcome the other misconceptions.

Michelson interferometry with a diode laser

A typical Fabry–Pérot diode laser is used to study the relationship between the laser spectrum and the interference signal obtained from a Michelson interferometer. The interference signal is measured over a range of path length differences of a few centimeters, and the laser spectrum is obtained with a spectrometer of a high resolution of 0.005 nm. Contrary to the laser datasheet, the laser is found to have several longitudinal modes even at high injection currents well above the threshold current. A simple model based on treating the laser's longitudinal modes as pure sinusoidal waves is derived and successfully used in modeling the complex and repetitive interference signal. The spectral spacing and the laser cavity length can be found from the interference signal without the need to use a spectrometer, provided that the interference signal is measured over a long enough range of path length differences.

Optical Linewidth Tolerant mmW Generation Employing a Dual-Stage Active Demultiplexer

In this letter, we propose a novel linewidth tolerant millimetre-wave (mmW) generation scheme employing a dual-stage active demultiplexer and an optical frequency comb (OFC). A unique feature of this architecture is that the two heterodyned tones travel the same path, alleviating the challenges associated with mismatches in the path lengths. We analyse the influence of the path length difference and source linewidth on the purity of the generated mmW signal and verify the tolerance, of the proposed scheme, to the optical linewidth. In addition, an experimental investigation of the performance of a 64 QAM discrete multitone (DMT) analog-radio over fibre (A-RoF) distribution system operating at 37.5 GHz using the proposed architecture, is presented. A BER below the HD-FEC limit is achieved, after 25 km fibre transmission, when employing an OFC with linewidth of ~30 kHz and ~3.1 MHz. Moreover, no significant performance penalty (at the HD-FEC limit) is observed due to an increase in the OFC linewidth, highlighting the linewidth tolerance of the proposed system.

Optical analogue of the Thomas effect in special relativity using Michelson-Gires-Tournois interferometer: Comprehensive error analysis and recovery of the direction of the Thomas rotation angle

Previously, we reported a simple, low-cost, thin–film–based optical analogue of the Thomas Effect in Special Relativity (SR) using an ideal Michelson-Gires-Tournois interferometer (MGTI). This opens the door for other SR related phenomena to be studied analogously. Toward this goal, there are two important tasks. First, it is imperative to conduct a comprehensive evaluation of the technical limitations of this platform under two non-ideal (oftentimes unavoidable) operating conditions, namely: (i) an imperfect Gires-Tournois resonator (GTR), and (ii) the presence of interferometric error due to mismatch in the path-length arm difference of the MGTI. Second, it is also important to develop a technique to recover the direction information of the Thomas angle which is lost during the intensity measurement. Here, we report that (i) the GTR's imperfect back mirror must be fabricated with a minimum reflectance r0 value of greater than 0.9534 to limit the maximum phase deviation error by only less than 1%, (ii) the extracted Thomas angle is more sensitive to changes in interferometric error δl than the GTR imperfection. However, perfect extraction of the Thomas angle can be achieved for special range of the interferometric error δl, and (iii) the mimicked Thomas angle is distorted when the front reflectance coefficient r1 is greater than 0.7. Lastly, we introduce a method to recover the directional information of the Thomas angle by using a counter-intuitive additional positive/negative interferometric error. This method introduces no new optical components, and has high tolerance to fluctuation of the interferometric error.

Substantial Improvement of Color-Rendering Properties of Conventional White LEDs Using Remote-Type Red Quantum-Dot Caps.

A new type of remote red quantum-dot (QD) component was designed and fabricated to improve the color-rendering properties of conventional white LED (light-emitting diode) lightings. Based on an optical simulation, the rectangular cavity-type QD cap was designed with an opening window on the top surface. Red QD caps were fabricated using a typical injection molding technique and CdSe/ZnS QDs with a core/shell structure whose average size was ~6 nm. Red QD caps were applied to conventional 6-inch, 15-W white LED downlighting consisting of 72 LEDs arrayed concentrically. The red QD caps placed over white LEDs enhanced the red components in the long-wavelength range resulting in the increase of the color rendering index (CRI) from 82.9 to 94.5. The correlated color temperature was tuned easily in a wide range by adopting various configurations consisting of different QD caps. The spatial and angular homogeneities were secured on the emitting area because QD caps placed over the white LEDs did not exhibit any substantial optical path length difference. The present study demonstrates that adopting QD caps in conventional LED lightings provides a flexible and efficient method to realize a high color-rendering property and to adjust correlated color temperature appropriately for a specific application.

Light-adapted flicker optoretinograms captured with a spatio-temporal optical coherence-tomography (STOC-T) system.

For many years electroretinography (ERG) has been used for obtaining information about the retinal physiological function. More recently, a new technique called optoretinography (ORG) has been developed. In one form of this technique, the physiological response of retinal photoreceptors to visible light, resulting in a nanometric photoreceptor optical path length change, is measured by phase-sensitive optical coherence tomography (OCT). To date, a limited number of studies with phase-based ORG measured the retinal response to a flickering light stimulation. In this work, we use a spatio-temporal optical coherence tomography (STOC-T) system to capture optoretinograms with a flickering stimulus over a 1.7 × 0.85 mm2 area of a light-adapted retina located between the fovea and the optic nerve. We show that we can detect statistically-significant differences in the photoreceptor optical path length (OPL) modulation amplitudes in response to different flicker frequencies and with better signal to noise ratios (SNRs) than for a dark-adapted eye. We also demonstrate the ability to spatially map such response to a patterned stimulus with light stripes flickering at different frequencies, highlighting the prospect of characterizing the spatially-resolved temporal-frequency response of the retina with ORG.

Modelling, characterization, and applications of silicon on insulator loop terminated asymmetric Mach Zehnder interferometer

This work presents a loop terminated asymmetric Mach–Zehnder interferometer (LT-aMZI) structure based on the widespread silicon-on-insulator (SOI) platform. Four different path length differences of the LT-aMZI, which correspond to free spectral ranges (FSR) from 0.8 to 6.4 nm, are designed. These designs are compared to the common asymmetric Mach–Zehnder interferometer (C-aMZI) and are shown to be more compact. These devices are suitable for optical filtering as well as wavelength demultiplexing (WDM) applications. A compact analytical model is derived that accurately describe the operation of the LT-MZI devices. The designs are then fabricated using Electron Beam Lithography (EBL) and characterized. The experimental data show good agreement when compared to the simulation results. To our knowledge, this is the first time LT-aMZI fabrication and characterization. Moreover, the LT-MZI spectrum can be tuned not only by the interferometer arms phase difference like C-MZI, but also by using its directional couplers coefficients, forming a spectral tunable filter. Finally, we determine the performance parameters of optical sensors and modulators and show that our proposed LT-MZI structure will enhance the sensor figure of merit (FOM) and modulator speed, power consumption and Vπ × L compared to C-MZI. A comparison between symmetric and asymmetric MZI sensors and the advantage of the latter is also mentioned.

Gbit/s secure key generation and distribution based on the phase noise of an amplified spontaneous emission source.

In this paper, a secure key generation and distribution scheme based on the phase noise of an amplified spontaneous emission source is proposed and experimentally verified. A giant key generator that contains two distribution arms is used to generate a specific beat dependent on the path length difference of the arms. Through the method of balanced subtraction of the local signals, the effect of the intensity noise has been mainly excluded, and the two legal users obtain the consistent differential signal that depended on the phase noise. Experiment results show that the correlation coefficient of the key signals reaches to about 0.89, and the bit generation rate of the scheme achieves to 3.06 Gbit/s under a length of 20 km standard single-mode optical fiber route, as the bit error rate stays under 0.02%. Moreover, the converted digital key stream has passed the NIST statistical test suite, which means that the scheme is inherently random in the statistical sense. With the excellent performance mentioned above, the proposed scheme provides a simple and efficient solution for the method of one-time pad.

A perfect X-ray beam splitter and its applications to time-domain interferometry and quantum optics exploiting free-electron lasers

X-ray free-electron lasers (FELs) deliver ultrabright X-ray pulses, but not the sequences of phase-coherent pulses required for time-domain interferometry and control of quantum states. For conventional split-and-delay schemes to produce such sequences, the challenge stems from extreme stability requirements when splitting Ångstrom wavelength beams, where the tiniest path-length differences introduce phase jitter. We describe an FEL mode based on selective electron-bunch degradation and transverse beam shaping in the accelerator, combined with a self-seeded photon emission scheme. Instead of splitting the photon pulses after their generation by the FEL, we split the electron bunch in the accelerator, prior to photon generation, to obtain phase-locked X-ray pulses with subfemtosecond duration. Time-domain interferometry becomes possible, enabling the concomitant program of classical and quantum optics experiments with X-rays. The scheme leads to scientific benefits of cutting-edge FELs with attosecond and/or high-repetition rate capabilities, ranging from the X-ray analog of Fourier transform infrared spectroscopy to damage-free measurements.

Investigation of dynamic influences in tilted-wave interferometry

Aspherical and freeform lenses allow for compact optical systems and have therefore gained high interest in optics. The interferometric measurement of these forms is a challenge, for which the tilted-wave interferometer (TWI) has been developed. To evaluate the measurement uncertainty of the TWI, both the static and the dynamic influence parameters have to be investigated. In this work, we focus on the dynamic influences on the measurement data of the interferometer. To this end, the individual influences as well as their point of insertion into the process chain are identified. Then the measurement of the interferogram data is modelled as a Monte Carlo simulation. The propagation of different influences through the data process chain to the optical path length differences (OPDs) is also simulated, and the resulting variation of the OPDs is estimated. Furthermore, the variation of the OPDs resulting from measured interferogram data is investigated for comparison. The analysis and quantification of variation of the OPDs along with its contributing influence sources are important steps on the way towards a full uncertainty estimation of optical form measurement with the TWI.

Noise limits on two-photon interferometric sensing

When a photon interferes with itself while traversing a Mach-Zehnder interferometer, the output port where it emerges is influenced by the phase difference between the interferometer arms. This allows for highly precise estimation of the path length difference (delay) but is extremely sensitive to phase noise. By contrast, a delay between the arms of the two-photon Hong-Ou-Mandel interferometer directly affects the relative indistinguishability of the photon pair, affecting the rate of recorded coincidences. This likewise allows for delay estimation, notably less precise but with the advantage of being less sensitive to perturbations of the photons' phase. Focusing on two-photon input states, we here investigate to what degree of noise Mach-Zehnder interferometry retains its edge over Hong-Ou-Mandel interferometry. We also explore the competing benefits of different two-photon inputs for a Mach-Zehnder interferometer and under what parameter regimes each input performs best.

Graphene Superconductivity at Room‐Temperature of a Wide Range and Standard Atmosphere, Based on Vacuum Channels and White‐Light Interferometry

AbstractThe thickness of a monolayer graphene is extremely thin, ≈0.34 nm, so exact measurement of the optical path length and optical path length difference of graphene is challenging. This paper uses white‐light interferometry and a horizontal grating to obtain the optical characteristic curve of graphene. Then, the real existence of vacuum channels (VCs) in monolayer graphene is discovered. These VCs naturally eliminate the collision phenomena, and provide zero electrical resistance and superconductivity (SC) states. Multiple SC states (SCSs), SC persistence, magnetic field response, VCs transmitting a SC current with high efficiency, screen characteristics of VCs, etc. are discussed. Experiments show that only when a graphene ribbon is located at the exact position of the VC, does it transmit the SC current and provide a SCS. The duration of the persistence tests is 2 months. All experiments are conducted in a completely open space of a laboratory. Graphene SC at room‐temperatures of a wide range and standard atmosphere is achieved. This paper uses the optical technology to realize graphene SC, points out the basic reason of realizing it, and provides the experimental evidences.