Multiple Interferometer Research Articles

Affordable Highly Sensitive Fiber Optical Sensor Based on Coupled Fibers with Harmonically Varying Refractive Index

Optical fiber sensors consist of multiple Mach–Zehnder (MZ) interferometers and are common in the protection of different compounds. These sensors are very sensitive to any intrusion or threat. However, the spatial resolution is proportional to the number of MZ interferometers along the sensor. Consequently, a long sensor with a high resolution can be costly. In this paper, we suggest replacing the cascaded MZ interferometers with a couple of adjacent fibers, each of which have a harmonically varying refractive index. In this theoretical study, it is shown that two fibers with varying refractive indices demonstrate a sensitivity equivalent to that of multiple MZ interferometers. Furthermore, when the coupling coefficient between the fibers is weak, an analytical expression can be derived for the transmission between the fibers. This transmission reveals a quantization rule for which the light coupling between the two fibers vanishes.

Development of a multifunctional real-time data processing system for interferometers on EAST

In the latest campaign of EAST experiments, two new interferometers were installed, a dispersion interferometer (DI) based on a Carbon Dioxide laser and a solid source interferometer (SSI) based on microwave multiplier sources. To make them available for the Plasma Control System (PCS) system, each of them needs to be provided with a real-time processing system to extract the detector output signal and afterward obtain the electron density information through signal processing. To obtain interferometer data quickly and reliably, a unified hardware template was applied to both interferometers. Three main parts are included in this hardware template — digitization, digital signal processing, and output modules. In particular, the digitization section uses a multi-channel Analog-to-Digital Convertor (ADC) to acquire the signals that need to be calculated. The digital signal processing part is implemented by an FPGA, which is the hardware basis of this multifunctional template that can be used for multiple interferometers. This section includes two main parts, wrapped density signal extraction and signal unwrapping. For DI it is a phase calculation based on the intensity ratio, while for SSI it is a phase calculation by means of FFT or phase demodulation. Both computational approaches form independent IP cores to build systems quickly, while these IP cores provide parameter interfaces to adapt to different application scenarios. The output module provides a variety of data transmission methods, including fiber optic transmission and Digital-Analog-Convertor (DAC) based analog transmission, to interface with PCS or other subsequent systems.This data processing system template has been applied to each of the above two interferometers and valid data were obtained in a recent EAST experiment campaign. It demonstrates the usefulness of the template and provides a reference for the design of data processing systems on future devices.

Modelling, experimental validation and analysis of a multiple stage PC-PMF Sagnac loop interferometer with spectral shape versatility using an optimized fitting algorithm

Here we experimentally demonstrate a multi-stage Sagnac loop with high spectral shape versatility by employing a serial sequence of different polarization controllers (PC) and Polarization-Maintaining Fibres (PMF), and we develop a general mathematical model to simulate it for N PMF stages inside of its loop using the Jones formalism. Our model is then validated for the cases of 2 and, for the first time, 3 stages by comparing the numerical simulation results with the experimental data, achieving a very small Root Mean Squared Error (RMSE) of around 10−4. Additionally, we propose a heuristic optimization capable of retrieving physical information from the measured spectra about the polarization states inside the loop, opening up the possibility of the use of such modelling as an interrogation tool for sensing applications. Finally, we perform a brief analysis of the interferometric pattern curves obtained in order to further understand its versatility. This spectral flexibility makes such interferometers interesting candidates for use as tunable optical filters and quasi-distributed sensors.

Ultrahigh-resolution and ultra-simple fiber-optic sensor with resonant Sagnac interferometer.

The resonant fiber-optic sensor (RFOS) is well known for its high sensing resolution but usually suffers from high cost and system complexity. In this Letter, we propose an ultra-simple white-light-driven RFOS with a resonant Sagnac interferometer. By superimposing the output of multiple equivalent Sagnac interferometers, the strain signal is amplified during the resonance. A 3 × 3 coupler is employed for demodulation, by which the signal under test can be read out directly without any modulation. With 1 km delay fiber and ultra-simple configuration, a strain resolution of 28f ε/Hz at 5 kHz is demonstrated in the experiment, which is among the highest, to the best of our knowledge, resolution optical fiber strain sensors.

A 3D Interferometer-Type Lightning Mapping Array for Observation of Winter Lightning in Japan

We have developed and deployed a 3D Interferometer-type Lightning Mapping Array (InLMA) for observing winter lightning in Japan. InLMA consists of three broadband interferometers installed at three stations with a distance from 3 to 5 km. At each interferometer station, three discone antennas were installed, forming a right triangle with a separation of 75 m along their two orthogonal baselines. The output of each InLMA antenna is passed through a 400 MHz low-pass filter and then recorded at 1 GS/s with 16-bit accuracy. A new method has been proposed for finding 3D solutions of a lightning mapping system that consists of multiple interferometers. Using the InLMA, we have succeeded in mapping a positive cloud-to-ground (CG) lightning flash in winter, particularly its preliminary breakdown (PB) process. A study on individual PB pulse processes allows us to infer that each PB pulse process contains many small-scale discharges scattering in a height range of about 150 m. These small-scale discharges in a series of PB pulses appear to be continuous in space, though discontinuous in time. We have also examined the positive return stroke in the CG flash and found a 3D average return stroke speed of 7.5 × 107 m/s.

Variable optical true-time delay line breaking bandwidth-delay constraints.

Continuously variable true-time optical delay lines are typically subject to a constraint of the bandwidth-delay product, limiting their use in several applications. In this Letter, we propose an integrated topology that breaks the bandwidth-delay product limit. The device is based on multiple Mach-Zehnder Interferometers (MZIs) arranged in parallel, providing easier control and a larger bandwidth compared to ring resonator-based solutions. The functionality of this architecture is demonstrated with a 4-stage delay line by performing measurements in both the time and frequency domains. The delay line introduces a delay of 90 ps over a bandwidth of more than 22 GHz with a negligible group delay distortion, operates on a wavelength range of about 60 nm, and is scalable to a higher number of MZI stages.

Integration of Multiple Interferometers in Highly Multiplexed Diagnostic KITs to Evaluate Several Biomarkers of COVID-19 in Serum.

In the present work, highly multiplexed diagnostic KITs based on an Interferometric Optical Detection Method (IODM) were developed to evaluate six Coronavirus Disease 2019 (COVID-19)-related biomarkers. These biomarkers of COVID-19 were evaluated in 74 serum samples from severe, moderate, and mild patients with positive polymerase chain reaction (PCR), collected at the end of March 2020 in the Hospital Clínico San Carlos, in Madrid (Spain). The developed multiplexed diagnostic KITs were biofunctionalized to simultaneously measure different types of specific biomarkers involved in COVID-19. Thus, the serum samples were investigated by measuring the total specific Immunoglobulins (sIgT), specific Immunoglobulins G (sIgG), specific Immunoglobulins M (sIgM), specific Immunoglobulins A (sIgA), all of them against SARS-CoV-2, together with two biomarkers involved in inflammatory disorders, Ferritin (FER) and C Reactive Protein (CRP). To assess the results, a Multiple Linear Regression Model (MLRM) was carried out to study the influence of IgGs, IgMs, IgAs, FER, and CRP against the total sIgTs in these serum samples with a goodness of fit of 73.01% (Adjusted R-Squared).

Single-path electro-optic coefficients measurement approach using multiple reflection interference.

An electro-optic coefficient (EOC) is an important optical property of optical crystals. In this work, a single-path multiple reflection interferometer is proposed to measure the components of signed EOCs with high precision. With the help of the normalization method, the influence of incident light was removed, and repeatability of the experimental results was improved. The signed EOCs of a congruent LiNbO3 crystal were determined under a small external electric field as γ31=+9.6±0.3pm/V and γ33=+29.5±0.5pm/V. This study may contribute to the development of potential anisotropic devices of optical crystals.

Integrated Silicon Fourier Transform Spectrometer with Broad Bandwidth and Ultra‐High Resolution

AbstractAn ultra‐high resolution Fourier transform spectrometer (FTS) realized in silicon photonic platform that can operate with broad band, narrow band as well as a combination of broad band and narrow band signals is reported. The ultra‐high resolution of the spectrometer is achieved by exploiting multiple techniques: a Michelson interferometer (MI) structure to increase the optical path delay (OPD), a hybrid waveguide design to reduce insertion loss, an optimized heater and air trenches to achieve higher thermal efficiency. Moreover, to further increase the OPD of the spectrometer to increase its resolution, a novel multiple interferometers approach is employed which combines balanced MI with N statically imbalanced MIs, thereby increasing the OPD of a single MI by factor of N + 1. An FTS spectrometer consisting of N = 2 such MIs is fabricated and experimentally characterized using unknown broad bandwidth input signal spectra of about 180 nm centered around 1550 nm, a narrow line laser input signal, and a combination of broad and narrow band signals demonstrating spectral resolution of about 0.16 nm.

Quantum-Enhanced Fiber-Optic Gyroscopes Using Quadrature Squeezing and Continuous-Variable Entanglement

We evaluate the fundamental performance of a fiber-optic gyroscope (FOG) design that is enhanced by the injection of a quantum-optical squeezed vacuum. In the presence of fiber loss, we compute the optimum attainable enhancement below the standard quantum limit in terms of the angular velocity estimator variance from a homodyne measurement. We find that currently realizable amounts of single-mode squeezing are sufficient to access the maximum quantitative improvement, but that this gain in maximum rotation sensitivity is limited to a marginal constant factor. We then propose an entanglement-enhanced FOG design that segments a fixed amount of available fiber into multiple fiber interferometers and feeds this sensor array with a multimode-entangled squeezed vacuum resource. Our design raises the fundamental improvement in sensitivity to an appreciable factor of $e\ensuremath{\approx}2.718$.

Three-dimensional interferometric stage encoder using a single access port

We present a novel multi-dimensional positional encoder approach that uses range-resolved interferometry (RRI) to simultaneously interrogate multiple interferometers differing in optical path length, using only a single photo detector and a single diode laser. Employing only stage-mounted non-polarising beamsplitters and reflections from three orthogonal, externally mounted, planar mirrors, three Cartesian measurement directions are formed, that when combined with measurements from a stage-mounted reference reflection, allow for three-dimensional measurements of stage motion obtained via a single access port. Through a variety of movements of a three-dimensional Piezo-electric stage over its nominal stage working range of ± 50 μm, simultaneous measurements of displacements from three orthogonal dimensions are acquired with a 10 kHz quadrature bandwidth, achieving typical displacement noise densities of below 0.4 nm/Hz, with the noise floor dropping to below 0.1 nm/Hz above 50 Hz, as well as high linearity with cyclic errors amplitudes well below 1 nm in all three directions.

Optimization of quantum noise by completing the square of multiple interferometer outputs in quantum locking for gravitational wave detectors

The quantum locking technique, which uses additional short low-loss sub-cavities, is effective in reducing quantum noise in space gravitational wave antenna DECIGO. However, the quantum noise of the main interferometer depends on the control systems in the sub-cavities. Here we demonstrate a new method to optimize the quantum noise independently of the feedback gain by completing the square of multiple interferometer outputs in the quantum locking system. We successfully demonstrate in simulations that this method is effective in optimizing the homodyne angle to the best quantum-noise-limited sensitivity.

High-resolution nonfluorescent imaging with structured illumination for patterned surface measurement

Surface patterning is widely performed in various applications, and hence high-resolution, high-throughput, and nondestructive measurement of patterned surfaces is required. The current optical methods used for such measurements are constrained, in terms of spatial resolution, by the diffraction limit. In this study, structured illumination was employed for high-resolution wide-field imaging measurements. Structured illumination microscopy is well known as a fluorescent bio-imaging technique. For nonfluorescent imaging and industrial measurement applications, however, speckle noise, generated by the structured illumination pattern, environmental vibrations, and thermal drift, causes problems for the high-precision positioning and control of the illumination pattern. To solve these problems, we developed a structured illumination microscope that detects the phase of the structured illumination as a fringe signal by using an additional interferometric measurement system. In this system, a low-coherence light source is employed for illumination pattern generation and high-resolution imaging with low speckle noise. The constructed optical system consists of multiple Michelson interferometers that allow the manipulation of low-coherence interference fringes at the sample position and the detection of the phase via another sensor. Robust imaging can be implemented by feeding back the structured illumination phase through super-resolution image processing. We experimentally achieved high-resolution nonfluorescent imaging with low speckle noise and resolved patterned surfaces beyond the diffraction limit without high-precision positioning stages. Furthermore, the image reconstruction algorithm typically used in structured illumination microscopy was improved by adopting a Fourier ptychography approach. Thus, artifact reduction, as well as decreases in luminance and pitch errors, was achieved, and this result represents a step forward in the development of practical measurement methods for patterned surfaces.

A metal-ceramic coaxial cable with multipoint Fabry-Pérot interferometers for monitoring distributed high temperature

Abstract A metal ceramic coaxial cable (MCCC) sensor containing multiple Fabry-Perot interferometers (FPIs) has been designed, fabricated, and evaluated for measuring distributed high temperatures. The MCCC sensor demonstrated here consists of stainless steel tube and wire conductors, alumina tube insulator, and three pairs of air gap reflectors distributed along the cable. The multipoint sensor operation is realized by the joint time-frequency domain technique, which combines the functions of time-domain reflectometry and frequency division multiplexing to enable the measurement of spatially distributed temperature. The 3-point MCCC-FPI sensor has been demonstrated for distributed temperature measurements between 250 and 550 °C and operability up to 1000 °C. The sensor offers high sensitivity of >0.18 MHz/°C with a measurement resolution of 1 Hz and reasonably small high temperature measurement error range of ±3 °C.

Offset simultaneous conjugate atom interferometers

Correlating the signals from simultaneous atom interferometers has enabled some of the most precise determinations of fundamental constants. Here, we show that multiple interferometers with strategically chosen initial conditions ("offset simultaneous conjugate interferometers" or OSCIs) can provide multi-channel readouts that amplify or suppress specific effects. This allows us to measure the photon recoil, and thus the fine structure constant, while being insensitive to gravity gradients, general acceleration gradients, and unwanted diffraction phases - these effects can be simultaneously monitored in other channels. An expected 4-fold reduction of sensitivity to spatial variations of gravity (due to higher-order gradients) and a 6-fold suppression of diffraction phases paves the way to measurements of the fine structure constant below the 0.1-ppb level, or to simultaneous sensing of gravity, the gravity gradient, and rotations.

On-chip parallel Fourier transform spectrometer for broadband selective infrared spectral sensing

Optical spectrometers enable contactless chemical analysis. However, decreasing both their size and cost appears to be a prerequisite to their widespread deployment. Chip-scale implementation of optical spectrometers still requires tackling two main challenges. First, operation over a broad spectral range extending to the infrared is required to enable covering the molecular absorption spectrum of a broad variety of materials. This is addressed in our work with an Micro-Electro Mechanical Systems (MEMS)-based Fourier transform infrared spectrometer with an embedded movable micro-mirror on a silicon chip. Second, fine spectral resolution Δλ is also required to facilitate screening over several chemicals. A fundamental limit states that Δλ is inversely proportional to the mirror motion range, which cannot exceed the chip size. To boost the spectral resolution beyond this limit, we propose the concept of parallel (or multi-core) FTIR, where multiple interferometers provide complementary optical paths using the same actuator and within the same chip. The concept scalability is validated with 4 interferometers, leading to approximately 3 times better spectral resolution. After the atmospheric contents of a greenhouse gas are monitored, the methane absorption bands are successfully measured and discriminated using the presented device.

Cascaded multiple Fabry-Perot interferometers fabricated in no-core fiber with a waveguide for high-temperature sensing.

An optical fiber high-temperature sensor is proposed and demonstrated by use of cascaded multiple Fabry-Perot interferometers in no-core fiber with a waveguide fabricated by femtosecond laser pulse inscription. The device can sustain the high temperature up to 1100°C, and the temperature sensitivity obtained is 8.9 pm/°C within the temperature range 100°C-400°C, and 16.36 pm/°C within the temperature range 400°C-1100°C, respectively. Such a no-core fiber-based device can be fabricated in a simple way and operated reliably, which makes it attractive for extreme environment monitoring.

Wavelength-Division Demultiplexing Enhanced by Silicon-Photonic Tunable Filters in Ultra-Wideband Optical-Path Networks

With the recent growth of network traffic demands, a high-capacity optical-path network based on reconfigurable optical add-drop multiplexers (ROADMs) is highly desirable. Among the various technologies available, extending the useable frequency band is one of the most promising solutions since it can enlarge the network capacity with relatively small signal-quality degradation. However, increasing the number of wavelength signals detracts flexible signal-drop capabilities of the ROADM because the effective dynamic range of the digital coherent receiver is restricted by the number of wavelength signals input to the receiver. To overcome this difficulty, we adopt the ROADM structure that sets an optical wavelength-tunable filter in front of each receiver. This scheme retains the effective receiver dynamic range even if a large number of wavelength signals are multiplexed. In this paper, we demonstrate flexible signal drop in a C+L-band optical-path network by using an optical wavelength-tunable filter implemented on a single silicon-photonic chip. The prototype filter is monolithically implemented in conjunction with an input-port selector so as to reduce the total system loss. The filter comprises multiple asymmetric Mach-Zehnder interferometers for wavelength tuning and symmetric Mach-Zehnder interferometers for input-port selection. Its effectiveness is experimentally confirmed by measuring the bit-error ratios of 32-Gbaud/100-Gbps dual-polarization quadrature-phase-shift-keying signals and 32-Gbaud/200-Gbps dual-polarization 16-ary quadrature-amplitude-modulation signals. A single wavelength signal is extracted out of 285 wavelength signals multiplexed over the C+L band and successfully demodulated with negligible penalty in terms of the optical signal-to-noise ratio. The net fiber capacity reaches 57 Tbps.

Design of a multiple self-mixing interferometer for a fiber ring laser.

In this Letter, a novel multiple self-mixing interferometer for a fiber ring laser (FRL) is designed by introducing a circular feedback cavity. A system model is established based on an injection-seeded erbium-doped FRL proposed by Dragic. Owing to the reflection of the collimating lens, the multiplied fringes have different depths, which shows two Doppler frequencies in the spectrum of the self-mixing signal. A double-peak frequency identification algorithm is proposed to extract the Doppler frequency from the unique signal. This technique has the potential to improve the accuracy of fiber self-mixing measurement systems, particularly in Doppler velocimeters.

Nanoscale tilt measurement using a cyclic interferometer with polarization phase stepping and multiple reflections.

High-accuracy tilt or roll angle measurement is required for a variety of engineering and scientific applications. A cyclic interferometer with multiple reflections has been developed to measure small tilt angles. To accomplish this task, a novel and simple method, phase shift by polarization, was developed. The results of these studies show that the multiple reflection cyclic interferometers can be used to measure object tilts on the order of 0.2 nano-radians. We develop the theory for polarization phase step and show that accurate measurements can be made with the cyclic interferometer.