Modal Interferometer Research Articles

Switchable Dual-Wavelength Tm-Doped All-Fiber Laser Integrating a Fiber Tip Modal Interferometer

A switchable dual-wavelength thulium-doped all-fiber laser incorporating a fiber tip modal interferometer (FTMI) is reported. The FTMI relies on the splice joint between a standard single-mode optical fiber and a tapered microfiber with 60- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> uniform waist diameter, providing a spectral reflection response that allows the laser to emit beyond 2000 nm, specifically at 2009.20 nm and 2013.65 nm, under a dual-wavelength operation regime. Besides, switching between both laser lines is also observed and controlled by adjustments of the intracavity polarization state, obtaining the ability to operate in a single-wavelength regime at either 2009 nm or 2013 nm. Laser emission lines exhibit linewidths of around 50 pm, side-mode suppression ratios higher than 50 dB and maximum output power of 13.4 mW.

Single-shot spatial light interference microscopy by demultiplexing based on polarization gratings

Off-axis interferometric modules built on an ordinary bright field microscope make it possible to achieve single-shot quantitative phase imaging (QPI) by adding sufficient spatial carrier into the interferograms. However, compared with its on-axis counterparts, imaging configurations for off-axis interferometric modules have several disadvantages regarding optical aberration, stability, and space-bandwidth utilization of the lenses system. Herein, by demultiplexing technology based on polarization gratings, we propose a single-shot spatial light interference microscopy named as polarization-multiplexing light interference microscopy (PLIM) with on-axis imaging configuration to realize single-shot QPI. Although the imaging system is on-axis, the PLIM system still can generate sufficient spatial carrier, so we can adjust the orientation and absolute value of the spatial carrier independently of the magnification ratio of the imaging system. The experimental results prove that the PLIM system has better temporal phase stability compared with conventional grating-based QPI technologies and is suitable for high resolution QPI.

Increasing Measurements Range of Electro-Optic Sensors for Electric Field by Using a Mach–Zehnder Interferometer Modulators Array

Increasing Measurements Range of Electro-Optic Sensors for Electric Field by Using a Mach–Zehnder Interferometer Modulators Array

A comparative study of the surface topography in dry and wet turning using the confocal and interferometric modes

This article is concerned with dry and wet turning of C45 steel using sintered carbide tools. The study focused on the surface topography of workpieces obtained by means of a 3D surface measurement system operating in two modes: confocal and interferometric. The main purpose was to compare the corresponding data measured in the two modes, confocal and interferometric. The study also aimed to analyse the tool wear depending on the turning conditions. The use of a scanning electron microscope and a stereoscopic microscope was essential to comprehensively characterise the workpiece surfaces and determine the cutting tool wear. The results obtained in the confocal and interferometric modes differed. The confocal module proved to be better for characterising the surface topography of C45 steel machined by turning. The surface quality after wet turning was higher than after dry turning; hence the lower values of the 2D parameters (Ra, Rq, Rp and Rv) and of the 3D parameters (Sa, Sq, Sp, Sv). The wear visible on the cutting tool after wet turning was half as much as that after dry turning (e.g. surface abrasion indicator VBBmax equal to 0.21 mm for wet turning and 0.39 mm for dry turning). The results prove that the choice of methods for measuring the surface topography (by determining the relevant parameters and functions) and analysing the surface features can play an important role in the characterisation of the machined surface. A well-thought-out research methodology is thus essential. From the experimental data, it is also clear that the use of mineral oil-based cutting fluid for turning with sintered carbide tools improved the surface quality of workpieces and reduced the tool wear.

Modal dispersion characteristics of LP0m modes in a step-index multimode fiber.

A modal interferometer method (MIM) is applied to measure the differential mode delay (DMD) between the L P 0m modes traversing a step-index multimode fiber (SI-MMF). Only linearly polarized radial modes, i.e.,L P 0m modes, are excited and transmitted in the SI-MMF by using a single-mode-multimode-single-mode (SMS) fiber structure. The measurement principle is based on investigating a transmitted spectrum through temporal decomposition by means of a Fourier transform. The Fourier-transform-based MIM provides simultaneous measurements of the DMD between the L P 0m modes. The wavelength dependence of the DMD is estimated experimentally in both the 1260-1360nm and 1450-1625nm telecommunication bands. The normalized frequency dependence of the DMD is also investigated theoretically. The result suggests that the 1260-1360nm band is preferable to the 1450-1625nm band for a mode-division multiplexing (MDM) transmission employing an SI-MMF in terms of realizing a smaller DMD.

Highly discriminative simultaneous measurement of level, refractive index and temperature of liquid by dual-wavelength fiber ring-cavity laser sensor

In this paper, a novel dual-wavelength fiber laser sensor is proposed and completed by the parallel configuration of modified fiber Bragg grating and modal interferometer. The stable lasing emissions are realized and the comprehensive tests are performed in terms of level, refractive index and temperature of liquid. By the demodulation of joint wavelength and intensity matrix, high-discriminative simultaneous measurement is achieved, and the obtained sensitivities are 19.88 pm/°C, 159.32 dB/RIU and 568 pm/mm, respectively, with the errors below 2.52 %. Additionally, the dynamic field tests are performed, and the limits of detection of 0.15 mm, 0.42°C and 9 × 10-4 RIU are experimentally verified.

Highly linear lithium niobate Michelson interferometer modulators assisted by spiral Bragg grating reflectors.

Highly linear electro-optic modulators are key components in analog microwave photonic links, offering on-chip direct mixing of optical and RF fields. In this work, we demonstrate a monolithic integrated Michelson interferometer modulator on thin-film lithium niobate (LN), that achieves linearized performance by modulating Bragg grating reflectors placed at the end of Michelson arms. The modulator utilizes spiral-shaped waveguide Bragg gratings on Z-cut LN with top and bottom electrodes to realize extensive reflectors, essential for linearized performance, in a highly integrated form. Optical waveguides are realized using rib etching of LN with precisely engineered bottom and top cladding layers made of silicon dioxide and SU-8 polymer, respectively. The compact design fits a 3 mm long grating in an 80 µm × 80 µm area, achieving a broad operating bandwidth up to 18 GHz. A spurious free dynamic range (SFDR) of 101.2 dB·Hz2/3 is demonstrated at 1 GHz, compared to 91.5 dB·Hz2/3 for a reference Mach-Zehnder modulator fabricated on the same chip. Further enhancement in SFDR could be achieved by reducing fiber-to-chip coupling loss. The proposed demonstration could significantly improve the linearity of analog modulator-based integrated optical links.

Highly sensitive temperature and strain sensor based on an antiresonant hollow core fiber probe with the Vernier effect.

A highly sensitive temperature and strain sensor based on an antiresonant hollow core fiber (ARHCF) probe with the Vernier effect is proposed and experimentally demonstrated. The ARHCF probe is used as a reference interferometer by sandwiching an ARHCF, which is insensitive to temperature, strain, and refractive index, between a single-mode fiber (SMF) and a polarization-maintaining fiber (PMF). The polarization mode interferometer (PMI), fabricated by splicing a section of PMF with a fiber polarizer at a 45-degree angle, works as a sensing interferometer. The Vernier effect is introduced by connecting the reference interferometer and the PMI in parallel. The experimental results show that by introducing the Vernier effect, the temperature sensitivity is improved from -1.68 to -15.7nm/∘C and the strain sensitivity is improved from 5.09 to 47.65 pm/µε. The magnification is consistent with the theoretical results. The reference segment of the proposed sensor is not affected by ambient factors, which provides a new strategy and idea for the development of multiparameter sensors based on the Vernier effect.

Robustness enhancement of dynamic spectroscopic ellipsometry by compensating temperature dependency of the monolithic polarizing interferometer.

This paper describes a robust dynamic spectroscopic ellipsometer that can provide a highly accurate and reliable real-time spectroscopic polarization measurement capability for various in-line nanoscale measurement applications. The robustness of dynamic spectroscopic ellipsometry is enhanced significantly by employing a compensation channel that removes the temperature dependency of the monolithic polarizing interferometric module, and it results in highly accurate dynamic spectral ellipsometric measurements. We present how the monolithic interferometer is affected by external disturbances and show experimentally that the proposed scheme can provide a few hundreds of times long-term stability enhancement compared with a single-channel-based dynamic spectroscopic ellipsometer scheme.

Integrated polarization mode interferometer in 220-nm silicon-on-insulator technology.

A compact integrated and high-efficiency polarization mode interferometer in the 220-nm silicon-on-insulator platform is presented. Due to the operation with two polarization modes in a single waveguide, low propagation losses and high sensitivities combined with a small footprint are achieved. The designed and fabricated system with a 5-mm-long sensing region shows a measured excess loss of only 1.5 dB with an extinction ratio up to 30 dB, while its simulated homogeneous bulk sensitivity can exceed 8000 rad/RIU. The combination with a 90° hybrid readout system offers single wavelength operation with unambiguousness for phase shifts up to 2π and constant sensitivity.

Design of a compact and highly sensitive modal interferometer using hybrid modes of a dielectric loaded plasmonic waveguide.

We show the presence of hybridization between fundamental TE and first higher-order TM modes in a dielectric loaded plasmonic waveguide of appropriately chosen core dimensions. Furthermore, a critical hybridization point is achieved at which both modes have nearly equal fraction of the TE and TM polarizations. Exploiting the interference among such modes, we propose the design of a compact and highly sensitive modal interferometer. The bulk and surface sensitivities of the proposed sensor are found to be ∼3-10µm/RIU for refractive index (RI) ∼1.33-1.36 and ∼0.7nm/nm for an adsorbed layer of RI 1.45, respectively. The proposed sensor gives robust performance against fabrication imperfections and is stable against temperature fluctuations due to extremely low temperature cross-sensitivity (∼10-15pm/∘C for a temperature change up to ∼100∘C).

Sensitivity-enhanced temperature sensor by cascaded configuration of polarization mode interferometer and Lyot filter based on Vernier effect

We proposed and experimentally demonstrated a novel high-sensitivity optical fiber temperature sensor (OFTS) which is composed of the Lyot filter cascading a polarization mode interferometer (PMI). The Lyot filter is fabricated by inserting a polarization-maintaining fiber (PMF) between two linear fiber polarizers at two 45-degree angles. By simply cleaving the end fiber of the PMI, the free spectral range (FSR) can be maximized to approach the FSR of the Lyot filter, which introduces the Vernier effect. Experimental results show that the cascaded sensor can provide high-temperature sensitivity of 10.666 nm/°C and 26.562 nm/°C, which are 6.71 and 12.168 times that of single PMI, respectively. The method is demonstrated to achieve a stable and controlled Vernier effect obtaining ultra-high temperature sensitivity, which agrees well with the theoretical simulation results. The proposed sensor has the advantages of high sensitivity, good stability, and easy fabrication.

Simultaneous measurement of strain, curvature, and temperature using a hollow square core fiber

The hollow core photonic crystal fibers have attracted a significant interest due to their abilities not only in managing the optical properties, but also to guide light with low loss. In this work, a hollow square core fiber (HSCF), whose guidance is based on the antiresonance (AR), is implemented as an optical fiber sensor to simultaneously measure the strain, curvature, and temperature. The sensor was designed in a transmission layout where the HSCF was spliced between two single mode fibers (SMF), enabling the appearance of two interferometers, the Mach-Zehnder interferometer (MZI) and the cladding modal interferometer (CMI). Both interferometers, allied to the AR guidance, show different responses to the physical measurands. The sensor revealed a maximum strain sensitivity of −2.72 pm/με, for the CMI component within a strain range up to 600με. As for the curvature response, two curvature ranges were analyzed. For the first range (from 0 m−1 to 0.7 m−1) a higher sensitivity of −2.21 nm/m−1 was attained for the CMI, while for the second range (between 0.7 m−1 and 1.5 m−1) a maximum sensitivity of −0.63 nm/m−1 was achieved by the resonance. On the other hand, for the temperature measurements a sensitivity of 32.8 pm/°C was attained by the MZI within a maximum temperature variation of 60°C. The proposed sensor reveals promising to be employed in a wide range of applications where the measurement of several physical parameters is required.

Direction-Discriminating torsion sensor based on optical fiber Mach-Zehnder interferometer

Optical fiber torsion sensors play a pivotal role in artificial intelligence, construction engineering, structural health monitoring and other fields. Research on low cost, simple structures and measurement methods has become a priority for researchers. Here, we propose an intensity-modulated and direction-recognizing torsion sensor based on two mode fiber with a sandwich structure, that serves as a mode interferometer formed by fundamental modes LP01 and second order core modes LP11 excited by means of offset fusion splicing. The capacity of intensity modulation and torsional direction recognition comes from the antisymmetric phase of fiber mode, which makes the sensor work like Marius’ law. The antisymmetric phase of the LP11 mode leads to an asymmetric transmission spectrum, whose direction enables the sensor to distinguish the torsion direction. Peaks and troughs in the measured spectrum interchange when the torsion direction is reversed. The initial angle between two offset directions enables control of the zero-crossing point within in the twisting range. The torsion sensitivity reaches 364%/(rad/cm) with a measurement accuracy of ± 0.22 rad/m.

Multiparametric sea state fields from synthetic aperture radar for maritime situational awareness

This paper introduces a method for estimating a series of sea state parameters from satellite-borne synthetic aperture radar (SAR). The method was realized in a near real time (NRT) application which allows for the processing of data from different satellites and modes. The algorithm estimates the total significant wave height Hs, dominant and secondary swell and windsea wave heights, first, and second moment wave periods, the mean wave period and the period of wind sea. The algorithm was applied to the Sentinel-1 (S1) C-band Interferometric Wide Swath Mode (IW), Extra Wide (EW) and Wave Mode (WV) Level-1 (L1) products and also extended to X-band TerraSAR-X (TS-X) StripMap (SM) products. The scenes are processed in raster and result in continuous sea state fields, with the exception of S1 WV, where averaged values for sea state parameters for along-orbit imagettes of 20 km × 20 km are presented.The developed empirical algorithm consists of two parts: a first CWAVE_EX (extended CWAVE) part, based on a linear regression approach, and a subsequent machine learning part using the support vector machine (SVM) technique. A series of new data preparation steps (i.e. filtering, denoising) and new features estimated from SAR images are also introduced. The algorithm was tuned and validated using two independent global wave model hindcasts, WaveWatch-3 and MFWAM as well as National Data Buoy Center (NDBC) measurements. The achieved root mean squared errors (RMSE) for CWAVE_EX for the total Hs are 0.60 m for low-resolution modes S1 IW (10 m pixel) and EW (40 m pixel) and 0.35 m for S1 WV and TS-X SM (pixel spacing ca.1–4 m) in comparison to model predictions. The RMSEs of the retrieved wave periods are in the range of 0.45–0.90 s for all of the satellites and models considered. Similarly, the dominant and secondary swell, and wind sea wave height RMSEs are in the range of 0.35–0.80 m. The SVM postprocessing improves the accuracy of the initial results of CWAVE_EX for Hs and reaches an RMSE of 0.25 m for S1 WV. Comparisons to 64 NDBC buoys, collocated at distances shorter than 50 km to S1 WV imagettes worldwide, result in an RMSE of 0.41 m. All results and the methods presented are novel in terms of the accuracy achieved, combining the classical approach with machine learning techniques, and performing an automatic NRT processing of multiparametric sea state fields from L1 data with automatic switching for different satellites and modes.The complete archive of S1 WV L1 Single Look Complex products from December 2014 until February 2021 was processed to create a sea state parameter database and validated using model hindcast and buoy measurements. The derived parameters are available to the public within the scope of the European Space Agency's Climate Change Initiative.

High Sensitivity Balloon-Like Sensor Based on Twin-Core and Twin-Hole Fiber

An all-fiber Mach–Zehnder interferometric (MZI) sensor for measuring temperature and refractive index (RI) is proposed and experimentally demonstrated. A high sensitivity balloon-like mode interferometer is fabricated with a twin-core and twin-hole fiber (TCTHF). The variations in ambient temperature and RI cause changes in phase differences between the modes, leading to shifts in the interference spectrum. The two resonance dip wavelength shifts within the spectrum are used to investigate the temperature and RI characteristics of the sensor. Experimental results show that the two dips have different responses to temperature and RI, indicating that the sensor can realize simultaneous measurement of temperature and RI. The obtained maximum sensing sensitivities are 0.051 nm/°C and 423.168 nm/RIU. The proposed sensor has potential applications in physical, biological, and chemical sensing owing to its high sensitivity, low cost, and small size.

PROPERTIES of Al2O3 CERAMIC COATINGS APPLIED BY ALD METHOD ON 100Cr6 STEEL

The purpose of the paper was to assess the morphology, chemical composition, geometric surface structure and contact angle, as well as to analyse the tribological properties of Al2O3 coatings deposited on 100Cr6 steel using the technique of atomic layers ALD. Al2O3 coatings were selected due to their tribological properties, adhesion to the substrate and hardness. Tribological tests were performed in a reciprocating motion in a ballon- disc combination, with a counter-specimen of 100Cr6 steel, under the conditions of technically dry friction. Surface morphology was observed under a scanning microscope, and an EDS analyser enabled the performance of an analysis of the chemical composition of the samples. Wetting of the surface of the Al2O3 coating was determined by means of an optical strain gauge. The geometric surface structure before and after the tribological tests was analysed by means of a confocal microscope with an interferometric mode. The tests proved that during technically dry friction, the average coefficient of friction for the Al2O3 coating is lower by approximately 30% than for 100Cr6 steel. The tested coatings are characterised by hydrophobicity.

Optical Fiber Sensor for Monitoring the Evaporation of Ethanol-Water Mixtures.

An inline optical fiber sensor is proposed to monitor in real time the evaporation process of ethanol–water binary mixtures. The sensor presents two interferometers, a cladding modal interferometer (CMI) and a Mach–Zehnder interferometer (MZI). The CMI is used to acquire the variations in the external medium refractive index, presenting a maximum sensitivity of 387 nm/RIU, and to attain the variation in the sample concentration profile, while the MZI monitors temperature fluctuations. For comparison purposes, an image analysis is also conducted to obtain the droplet profile. The sensor proposed in this work is a promising alternative in applications where a rigorous measurement of volatile organic compound concentrations is required, and in the study of chemical and physical properties related to the evaporation process.

Linear Displacement and Straightness Measurement by Fabry-Perot Interferometer Integrated with an Optoelectronic Module

This research develops a three degrees of freedom (DOF) measurement system by integrating Fabry-Perot interferometer and photoelectronic inspection module to determine linear displacement, horizontal and vertical straightness geometric error parameters simultaneously. The interferometer and the photoelectronic inspection module in a three DOF measurement system share the same light source, and the two structures are used to measure linear displacement and straightness errors. The experimental results are utilized to calculate the relevant error parameters according to ISO standards and numerical analysis. They show that after the machine error compensation, the positioning deviation of the system is reduced from 55 μm to 19 μm, corresponding to the reduction of 65%. The accuracy is promoted from 65 μm to 31 μm, about the improvement of 52%. The horizontal and vertical straightness errors of the machine are 4.30 μm and 5.71 μm respectively.

High-speed compact folded Michelson interferometer modulator.

We propose and experimentally demonstrate a novel compact folded Michelson interferometer (FMI) modulator with high modulation efficiency. By folding the 0.5 mm-long phase shift arms, the length of the modulation area of the FMI modulator is only 0.25 mm. Meanwhile, the traveling wave electrode (TWE) is also shorter, which decreases the propagation loss of the RF signal and contributes to a small footprint. The Vπ-L of the present device is as low as 0.87 V·cm at -8 V bias voltage. The minimum optical insertion loss is 3.7 dB, and the static extinction ratio (ER) is over 25 dB. The measured 3-dB electro-optical (EO) bandwidth is 17.3 GHz at a -6 V bias. The OOK eye diagram up to 40 Gb/s is demonstrated under 2 V driver voltage.