Showing papers on "Interferometry published in 2012"
TL;DR: In this paper, a review of recent advances in time series SAR interferometry methods that further improve accuracy is presented, including improved algorithms applied to image deformation associated with the 2010 eruption of Eyjafjallajokull volcano, slow slip on the Guerrero subduction zone in Mexico, and tectonic deformation in western Anatolia, Turkey.
636 citations
TL;DR: Each type of interferometric sensor is reviewed in terms of operating principles, fabrication methods, and application fields and some specific examples of recently reported interferometeric sensor technologies are presented in detail to show their large potential in practical applications.
Abstract: Fiber optic interferometers to sense various physical parameters including temperature, strain, pressure, and refractive index have been widely investigated. They can be categorized into four types: Fabry-Perot, Mach-Zehnder, Michelson, and Sagnac. In this paper, each type of interferometric sensor is reviewed in terms of operating principles, fabrication methods, and application fields. Some specific examples of recently reported interferometeric sensor technologies are presented in detail to show their large potential in practical applications. Some of the simple to fabricate but exceedingly effective Fabry-Perot interferometers, implemented in both extrinsic and intrinsic structures, are discussed. Also, a wide variety of Mach-Zehnder and Michelson interferometric sensors based on photonic crystal fibers are introduced along with their remarkable sensing performances. Finally, the simultaneous multi-parameter sensing capability of a pair of long period fiber grating (LPG) is presented in two types of structures; one is the Mach-Zehnder interferometer formed in a double cladding fiber and the other is the highly sensitive Sagnac interferometer cascaded with an LPG pair.
524 citations
TL;DR: In this paper, various noncontact optical sensing techniques that can be used to measure distances to objects, and related parameters such as displacements, surface profiles, velocities and vibrations are discussed and compared.
Abstract: This tutorial reviews various noncontact optical sensing techniques that can be used to measure distances to objects, and related parameters such as displacements, surface profiles, velocities and vibrations. The techniques that are discussed and compared include intensity-based sensing, triangulation, time-of-flight sensing, confocal sensing, Doppler sensing, and various kinds of interferometric sensing with both high- and low-coherence sources.
336 citations
TL;DR: Virgo as discussed by the authors is a very large Michelson interferometer with 3 km-long arms, built at Cascina, near Pisa (Italy), with a detailed description of all its different elements is given.
Abstract: This paper presents a complete description of Virgo, the French-Italian gravitational wave detector. The detector, built at Cascina, near Pisa (Italy), is a very large Michelson interferometer, with 3 km-long arms. In this paper, following a presentation of the physics requirements, leading to the specifications for the construction of the detector, a detailed description of all its different elements is given. These include civil engineering infrastructures, a huge ultra-high vacuum (UHV) chamber (about 6000 cubic metres), all of the optical components, including high quality mirrors and their seismic isolating suspensions, all of the electronics required to control the interferometer and for signal detection. The expected performances of these different elements are given, leading to an overall sensitivity curve as a function of the incoming gravitational wave frequency. This description represents the detector as built and used in the first data-taking runs. Improvements in different parts have been and continue to be performed, leading to better sensitivities. These will be detailed in a forthcoming paper.
321 citations
TL;DR: Self-mixing interferometry (SMI) as discussed by the authors is a new configuration of interferometrics that does not require any optical part external to the laser chip and can be employed in a variety of measurements.
Abstract: In this review, self-mixing interferometry (SMI), a new configuration of interferometry, is discussed. SMI has practical advantages compared to standard interferometry, for example SMI does not require any optical part external to the laser chip and can be employed in a variety of measurements. Applications range from the traditional measurements related to optical pathlength – like displacement, small-amplitude vibrations, velocity – to sensing of weak optical echoes – for return loss and isolation factor measurements, CD readout and scroll sensing – and also, a special feature because of the interaction with the medium, measurements of physical parameters, like the laser linewidth, coherence length, and the alfa factor. Because it is also a coherent detection scheme, the SMI works close to the quantum limit of the received field, typically -90 dBm, so that minimum detectable amplitudes of 100 pm/ √Hz are currently achieved upon operation on diffusive targets, whereas a corner cube allows half-wavelength counting mode – or 0.5 μm resolution – on a dynamic range up to 2 m and more. With its compact setup, the SMI is easy to deploy in the field and can interface a variety of experiments – from MEMS testing to rotating machines vibration testing to pickup of biological motility. The illustration shows a double-channel, differential SMI incorporated in a thermomechanical test equipment to trace the mechanical hysteresis cycle of the beads of a motor-engine brake.
311 citations
TL;DR: In this article, the theoretical concepts underlying these experiments and the experimental challenges are discussed, including optimizing interferometer designs as well as understanding the role of decoherence, and the potential for probing the quantum superposition principle in the limit of high particle mass and complexity.
Abstract: Recent progress and future prospects of matter-wave interferometry with complex organic molecules and inorganic clusters are reviewed Three variants of a near-field interference effect, based on diffraction by material nanostructures, at optical phase gratings, and at ionizing laser fields are considered The theoretical concepts underlying these experiments and the experimental challenges are discussed This includes optimizing interferometer designs as well as understanding the role of decoherence The high sensitivity of matter-wave interference experiments to external perturbations is demonstrated to be useful for accurately measuring internal properties of delocalized nanoparticles The prospects for probing the quantum superposition principle are investigated in the limit of high particle mass and complexity
302 citations
TL;DR: This work presents an approach to building interferometric telescopes using ideas of quantum information, allowing in principle interferometers with arbitrarily long baselines.
Abstract: We present an approach to building interferometric telescopes using ideas of quantum information. Current optical interferometers have limited baseline lengths, and thus limited resolution, because of noise and loss of signal due to the transmission of photons between the telescopes. The technology of quantum repeaters has the potential to eliminate this limit, allowing in principle interferometers with arbitrarily long baselines.
294 citations
TL;DR: A fiber in-line Fabry-Perot interferometer cavity sensor for refractive index measurement is demonstrated, simple in configuration, easy for fabrication and reliable in operation due to extremely low temperature cross sensitivity.
Abstract: We demonstrate a fiber in-line Fabry-Perot interferometer cavity sensor for refractive index measurement. The interferometer cavity is formed by drilling a micro-hole at the cleaved fiber end facet, followed by fusion splicing. A micro-channel is inscribed by femtosecond laser micromachining to vertically cross the cavity to allow liquid to flow in. The refractive index sensitivity obtained is ~994 nm/RIU (refractive index unit). Such a device is simple in configuration, easy for fabrication and reliable in operation due to extremely low temperature cross sensitivity of ~4.8 × 10−6 RIU/°C.
226 citations
TL;DR: Measurements with high signal-to-noise ratio, resolution and bandwidth are shown to demonstrate the accuracy of the optical referencing and the processing algorithm with 24 hours of averaging time, reaching a signal to noise ratio of 10,750,000 (>21 bits) in the interferogram and 316,000 in the spectrum at 100 MHz resolution.
Abstract: Interferograms from a dual-comb spectrometer are continuously corrected and averaged in real-time. The algorithm is implemented on a field-programmable gate array (FPGA) development board. The chosen approach and the algorithm are described. Measurements with high signal-to-noise ratio, resolution and bandwidth are shown to demonstrate the accuracy of the optical referencing and the processing algorithm with 24 hours of averaging time, reaching a signal to noise ratio of 10,750,000 (>21 bits) in the interferogram and 316,000 in the spectrum at 100 MHz resolution. An interferogram where signal dominates the noise over the full delay range imposed by the 100 MHz repetition rate is reported for the first time.
210 citations
TL;DR: A method to correct aberrations in a tomogram rather than the beam of a broadband optical interferometry system based on Fourier optics principles, which enables object reconstruction (within the single scattering limit) with ideal focal-plane resolution at all depths.
Abstract: Aberrations in optical microscopy reduce image resolution and contrast, and can limit imaging depth when focusing into biological samples. Static correction of aberrations may be achieved through appropriate lens design, but this approach does not offer the flexibility of simultaneously correcting aberrations for all imaging depths, nor the adaptability to correct for sample-specific aberrations for high-quality tomographic optical imaging. Incorporation of adaptive optics (AO) methods have demonstrated considerable improvement in optical image contrast and resolution in noninterferometric microscopy techniques, as well as in optical coherence tomography. Here we present a method to correct aberrations in a tomogram rather than the beam of a broadband optical interferometry system. Based on Fourier optics principles, we correct aberrations of a virtual pupil using Zernike polynomials. When used in conjunction with the computed imaging method interferometric synthetic aperture microscopy, this computational AO enables object reconstruction (within the single scattering limit) with ideal focal-plane resolution at all depths. Tomographic reconstructions of tissue phantoms containing subresolution titanium-dioxide particles and of ex vivo rat lung tissue demonstrate aberration correction in datasets acquired with a highly astigmatic illumination beam. These results also demonstrate that imaging with an aberrated astigmatic beam provides the advantage of a more uniform depth-dependent signal compared to imaging with a standard Gaussian beam. With further work, computational AO could enable the replacement of complicated and expensive optical hardware components with algorithms implemented on a standard desktop computer, making high-resolution 3D interferometric tomography accessible to a wider group of users and nonspecialists.
196 citations
TL;DR: In this paper, the output beam splitter of a Mach-Zehnder interferometer is inserted or removed after a photon has already entered the device, which illustrates the inadequacy of a naive wave or particle description of light.
Abstract: Wave-particle complementarity is one of the most intriguing features of quantum physics. To emphasize this measurement apparatus-dependent nature, experiments have been performed in which the output beam splitter of a Mach-Zehnder interferometer is inserted or removed after a photon has already entered the device. A recent extension suggested using a quantum beam splitter at the interferometer's output; we achieve this using pairs of polarization-entangled photons. One photon is tested in the interferometer and is detected, whereas the other allows us to determine whether wave, particle, or intermediate behaviors have been observed. Furthermore, this experiment allows us to continuously morph the tested photon's behavior from wavelike to particle-like, which illustrates the inadequacy of a naive wave or particle description of light.
TL;DR: The τ interferometer is a portable and inexpensive device for obtaining spatial interferograms of microscopic biological samples without the strict stability and the highly coherent illumination that are usually required for interferometric microscopy setups.
Abstract: This Letter presents the τ interferometer, a portable and inexpensive device for obtaining spatial interferograms of microscopic biological samples without the strict stability and the highly coherent illumination that are usually required for interferometric microscopy setups The device is built using off-the-shelf optical elements and can easily operate with low-coherence illumination, while being positioned in the output of a conventional inverted microscope The interferograms are processed into the quantitative amplitude and phase profiles of the sample Based on the phase profile, the optical-path-delay profile is obtained with temporal stability of 018 nm and spatial stability of 042 nm Further experimental demonstration of using the τ interferometer for imaging the quantitative thickness profile of a live red blood cell is provided
TL;DR: A simple and robust refractive index (RI) sensor based on a Mach-Zehnder interferometer has been demonstrated and a tunable optical delay line (ODL) is inserted into the other arm to compensate for the variation of the optical length difference.
Abstract: A simple and robust refractive index (RI) sensor based on a Mach–Zehnder interferometer has been demonstrated. A section of optical microfiber drawn from silica fiber is employed as the sensing arm. Because of the evanescent field, a slight change of the ambient RI will lead to the variation of the microfiber propagation constant, which will further change the optical length. In order to compensate the variation of the optical length difference, a tunable optical delay line (ODL) is inserted into the other arm. By measuring the delay of the ODL, the ambient RI can be simply demodulated. A high RI sensitivity of about 7159 μm/refractive index unit is achieved at microfiber diameter of 2.0 μm.
TL;DR: In this paper, a practical nonlinear interferometry for precision phase measurement with parametric amplifiers as the nonlinear beam splitters is analyzed, and it is found that the signal due to phase shift is enhanced by a factor of the amplification gain as compared to a linear interferometer with the same phase-sensing light intensity while the quantum noise is kept at the vacuum level.
Abstract: A nonlinear interferometer uses nonlinear elements as beam splitters to split and to recombine optical waves for interference. As a result, the interference fringe size has a nonlinear dependence on the intensity of the field for phase sensing and leads to an enhanced phase signal. In this paper, a practical scheme of nonlinear interferometry for precision phase measurement is analyzed with parametric amplifiers as the nonlinear beam splitters. It is found that the signal due to phase shift is enhanced by a factor of the amplification gain as compared to a linear interferometer with the same phase-sensing light intensity while the quantum noise is kept at the vacuum level, thus, effectively increasing the signal-to-noise ratio (SNR) beyond the standard quantum limit. Furthermore, the scheme is not as sensitive to the detection loss as the linear scheme with a squeezed state for noise reduction. However, losses inside the interferometer limit the enhancement factor in SNR. We apply the concept to a Michelson interferometer but with parametric amplifiers involved for gravitational-wave detection. We find that effective power is increased by the gain of the amplifiers without actually increasing the cycling power inside the interferometer. Furthermore, the full benefits with squeezed input and variational output or the combination of a quantum nondemolition interferometer for sensitivity beyond the standard quantum limit apply here with even better results. Such a nonlinear interferometer will find wide applications in precision measurements.
TL;DR: An ultra-stable, high-power cw Nd:YAG laser system, developed for the ground-based gravitational wave detector Advanced LIGO (Laser Interferometer Gravitational-Wave Observatory), was comprehensively characterized and found to fulfill most design requirements.
Abstract: An ultra-stable, high-power cw Nd:YAG laser system, developed for the ground-based gravitational wave detector Advanced LIGO (Laser Interferometer Gravitational-Wave Observatory), was comprehensively characterized Laser power, frequency, beam pointing and beam quality were simultaneously stabilized using different active and passive schemes The output beam, the performance of the stabilization, and the cross-coupling between different stabilization feedback control loops were characterized and found to fulfill most design requirements The employed stabilization schemes and the achieved performance are of relevance to many high-precision optical experiments
TL;DR: By integrating thousands of plasmonic interferometers per square millimeter with a microfluidic system, this work demonstrates a sensor able to detect physiological concentrations of glucose in water over a broad wavelength range (400-800 nm).
Abstract: In this work, we report the design, fabrication, and characterization of novel biochemical sensors consisting of nanoscale grooves and slits milled in a metal film to form two-arm, three-beam, planar plasmonic interferometers. By integrating thousands of plasmonic interferometers per square millimeter with a microfluidic system, we demonstrate a sensor able to detect physiological concentrations of glucose in water over a broad wavelength range (400–800 nm). A wavelength sensitivity between 370 and 630 nm/RIU (RIU, refractive index units), a relative intensity change between ∼103 and 106 %/RIU, and a resolution of ∼3 × 10–7 in refractive index change were experimentally measured using typical sensing volumes as low as 20 fL. These results show that multispectral plasmonic interferometry is a promising approach for the development of high-throughput, real-time, and extremely compact biochemical sensors.
TL;DR: In this paper, the effect of losses on the phase sensitivity of the SU(1,1) interferometer for different configurations was studied. And they showed that this type of interferometers is robust against losses that result from an inefficient detection system.
Abstract: We study the effect of losses on the phase sensitivity of the SU(1,1) interferometer for different configurations. We find that this type of interferometer is robust against losses that result from an inefficient detection system. This type of loss only introduces an overall prefactor to the sensitivity but does not change the $1/n$ scaling, where $n$ is the average number of particles inside the interferometer, characteristic of the Heisenberg limit. In addition, we show that under some conditions the SU(1,1) interferometer with coherent state inputs is also robust against internal losses. These results show that the SU(1,1) interferometer is a viable candidate for experimentally reaching the Heisenberg limit with current technology. Possible implementations of this interferometer using four-wave mixing in atomic vapors or an atom interferometer in a spinor Bose-Einstein condensate are compared.
TL;DR: A novel tilted fiber Bragg grating-based magnetic field sensor by incorporating magnetic fluid is proposed and experimentally demonstrated, based on the refractive index change of magnetic fluid with external magnetic field.
Abstract: A novel magnetic field fiber sensor based on magnetic fluid is proposed. The sensor is configured as a Sagnac interferometer structure with a magnetic fluid film and a section of polarization maintaining fiber inserted into the fiber loop to produce a sinusoidal interference spectrum for measurement. The output interference spectrum is shifted as the change of the applied magnetic field strength with a sensitivity of 16.7 pm/Oe and a resolution of 0.60 Oe. The output optical power is varied with the change of the applied magnetic field strength with a sensitivity of 0.3998 dB/Oe.
TL;DR: In this paper, the concentration of a blood protein in an aqueous buffer solution was measured using an opto-fluidic device that couples a waveguide interferometer with a micro-fluideic channel.
Abstract: Optical interferometry is amongst the most sensitive techniques for precision measurement. By increasing the light intensity, a more precise measurement can usually be made. However, if the sample is light sensitive entangled states can achieve the same precision with less exposure. This concept has been demonstrated in measurements of known optical components. Here, we use two-photon entangled states to measure the concentration of a blood protein in an aqueous buffer solution. We use an opto-fluidic device that couples a waveguide interferometer with a microfluidic channel. These results point the way to practical applications of quantum metrology to light-sensitive samples.
TL;DR: A simple, compact, and highly sensitive optical fiber directional bend sensor is presented that defines a pair of directions along which the bending response of the Mach-Zehnder interferometer transmission spectrum is different and thus could be used for bending vector measurement.
Abstract: A simple, compact, and highly sensitive optical fiber directional bend sensor is presented. This device consists of a lateral-offset splicing joint and an up-taper formed through excessive fusion splicing method. The lateral-offset splicing breaks the cylindrical symmetry of the fiber and defines a pair of directions along which the bending response of the Mach-Zehnder interferometer transmission spectrum is different and thus could be used for bending vector measurement. For a curvature range from -3 to 3 m(-1), the bending sensitivities at 1463.86 nm and 1548.41 nm reach 11.987 nm/m(-1) and 8.697 nm/m(-1), respectively.
TL;DR: This paper reviews two kinds of typical in-line fiber optic interferometers formed in single-mode fibers fabricated with different post-processing techniques and some recently reported specific technologies for fabricating such fiber opticinterferometers are presented.
Abstract: In-line fiber optic interferometers have attracted intensive attention for their potential sensing applications in refractive index, temperature, pressure and strain measurement, etc. Typical in-line fiber-optic interferometers are of two types: Fabry-Perot interferometers and core-cladding-mode interferometers. It's known that the in-line fiber optic interferometers based on single-mode fibers can exhibit compact structures, easy fabrication and low cost. In this paper, we review two kinds of typical in-line fiber optic interferometers formed in single-mode fibers fabricated with different post-processing techniques. Also, some recently reported specific technologies for fabricating such fiber optic interferometers are presented.
TL;DR: The two 4 km long LIGO detectors operated by the Laser Interferometer Gravitational-wave Observatory (LIGO) were modified in 2008 to read out the gravitational wave channel using the DC readout form of homodyne detection and to include an optical filter cavity at the output of the detector as mentioned in this paper.
Abstract: The two 4 km long gravitational wave detectors operated by the Laser Interferometer Gravitational-wave Observatory (LIGO) were modified in 2008 to read out the gravitational wave channel using the DC readout form of homodyne detection and to include an optical filter cavity at the output of the detector. As part of the upgrade to Enhanced LIGO, these modifications replaced the radio-frequency (RF) heterodyne system used previously. We describe the motivations for and the implementation of DC readout and the output mode cleaner in Enhanced LIGO. We present characterizations of the system, including measurements and models of the couplings of the noises from the laser source to the gravitational wave readout channel. We show that noise couplings using DC readout are improved over those for RF readout, and we find that the achieved shot-noise-limited sensitivity is consistent with modeled results.
TL;DR: An all-fiber optical Fabry-Perot interferometer (FPI) strain sensor whose cavity is a microscopic air bubble is demonstrated and strain and temperature sensitivities are studied experimentally.
Abstract: We demonstrate an all-fiber optical Fabry–Perot interferometer (FPI) strain sensor whose cavity is a microscopic air bubble. The bubble is formed by fusion splicing together two sections of single-mode fibers (SMFs) with cleaved flat tip and arc fusion induced hemispherical tip, respectively. The fabricated interferometers are with bubble diameters of typically ∼100 μm. Strain and temperature sensitivities of fabricated interferometers are studied experimentally; a strain sensitivity of over 4 Pm/μe and a thermal sensitivity of less than 0.9 Pm/°C is obtained.
TL;DR: A new technique for absolute distance measurement with a femtosecond frequency comb laser is demonstrated, based on unraveling the output of an interferometer to distinct comb modes with 1 GHz spacing, which results in a measurement accuracy far within an optical fringe.
Abstract: We demonstrate a new technique for absolute distance measurement with a femtosecond frequency comb laser, based on unraveling the output of an interferometer to distinct comb modes with 1 GHz spacing. From the fringe patterns that are captured with a camera, a distance is derived by combining spectral and homodyne interferometry, exploiting about 9000 continuous wave lasers. This results in a measurement accuracy far within an optical fringe ($\ensuremath{\lambda}/30$), combined with a large range of nonambiguity (15 cm). Our technique merges multiwavelength interferometry and spectral interferometry, within a single scheme.
TL;DR: In this paper, a review of optical fiber hydrogen sensors with palladium active element is presented, viz. interferometric-, in tensity-, and fiber grating-based sensors.
Abstract: Palladium-based fiber-optic sensors have been one of the most promising configurations for hydrogen sensing. In the latest decade, fiber-optic sensors have indeed earned a strong interest owing to their ability to monitor molecular hydrogen at specific spatial points-either as a sensing tip device or in large areas via multiple sensing regions distributed along the optical fiber. This review focuses on the various types of optical fiber hydrogen sensors, containing specifically palladium as active element. Three distinct working principles are described, viz. interferometric-, in tensity-, and fiber grating-based sensors; their characteristics and sensing performances are critically overviewed.
TL;DR: A new quantum interferometric scheme based on three-dimensional waveguide devices based on Fock states is proposed and theoretically investigated, expected to open new perspectives to quantum enhanced sensing and metrology performed in integrated photonics.
Abstract: Quantum interferometry uses quantum resources to improve phase estimation with respect to classical methods. Here we propose and theoretically investigate a new quantum interferometric scheme based on three-dimensional waveguide devices. These can be implemented by femtosecond laser waveguide writing, recently adopted for quantum applications. In particular, multiarm interferometers include “tritter” and “quarter” as basic elements, corresponding to the generalization of a beam splitter to a 3- and 4-port splitter, respectively. By injecting Fock states in the input ports of such interferometers, fringe patterns characterized by nonclassical visibilities are expected. This enables outperforming the quantum Fisher information obtained with classical fields in phase estimation. We also discuss the possibility of achieving the simultaneous estimation of more than one optical phase. This approach is expected to open new perspectives to quantum enhanced sensing and metrology performed in integrated photonics.
Patent•
31 May 2012
TL;DR: In this paper, a rotary coupler is used to interface with an optical tomography imaging probe, where the rotary coupler is in optical communication with the sample arm and the first optical fiber is disposed in a common protective sheath.
Abstract: In part, the invention relates to an image data collection system. The system can include an interferometer having a reference arm that includes a first optical fiber of length of LI and a sample arm that includes a second optical fiber of length of L2 and a first rotary coupler configured to interface with an optical tomography imaging probe, wherein the rotary coupler is in optical communication with the sample arm. In one embodiment, L2 is greater than about 5 meters. The first optical fiber and the second optical fiber can both be disposed in a common protective sheath. In one embodiment, the system further includes an optical element configured to adjust the optical path length of the reference arm, wherein the optical element is in optical communication with the reference arm and wherein the optical element is transmissive or reflective.
TL;DR: In this paper, the authors present a qualitative investigation into all of the relevant noise sources and the methods by which they can be identified and mitigated in order to achieve quantum noise limited balanced homodyne detection.
Abstract: The advent of stable, highly squeezed states of light has generated great interest in the gravitational wave community as a means for improving the quantum-noise-limited performance of advanced interferometric detectors. To confidently measure these squeezed states, it is first necessary to measure the shot-noise across the frequency band of interest. Technical noise, such as non-stationary events, beam pointing, and parasitic interference, can corrupt shot-noise measurements at low Fourier frequencies, below tens of kilo-hertz. In this paper we present a qualitative investigation into all of the relevant noise sources and the methods by which they can be identified and mitigated in order to achieve quantum noise limited balanced homodyne detection. Using these techniques, flat shot-noise down to Fourier frequencies below 0.5 Hz is produced. This enables the direct observation of large magnitudes of squeezing across the entire audio-band, of particular interest for ground-based interferometric gravitational wave detectors. 11.6 dB of shot-noise suppression is directly observed, with more than 10 dB down to 10 Hz.
TL;DR: A functional optical microfiber mode interferometer and its applications for absolute, temperature-insensitive refractive index sensing and a sinusoidal pattern due to the beating between modes are reported on.
Abstract: We report on a functional optical microfiber mode interferometer and its applications for absolute, temperature-insensitive refractive index sensing. A standard optical fiber was tapered down to 10 μm. The central part of the taper, i.e., the microfiber, is connected to the untapered regions with two identical abrupt transitions. The transmission spectrum of our device exhibited a sinusoidal pattern due to the beating between modes. In our interferometer the period of the pattern—an absolute parameter—depends strongly on the surrounding refractive index but it is insensitive to temperature changes. The period, hence the external index, can be accurately measured by taking the fast Fourier transform (FFT) of the detected interference pattern. The measuring refractive index range of the device here proposed goes from 1.33 to 1.428 and the maximum resolution is on the order of 3.7×10−6.
TL;DR: Based on the structure, a simple and low-cost Mach-Zehnder interferometer (MZI) formed by cascading two peanut-shape structures in the single-mode fiber is demonstrated in this article.
Abstract: A peanut-shape fiber structure that can realize the coupling and recoupling between the fiber core mode and the cladding modes is proposed in this paper. Based on the structure, a simple and low-cost Mach-Zehnder interferometer (MZI) formed by cascading two peanut-shape structures in the single-mode fiber is demonstrated. The theory and the experimental results show that the first peanut-shape structure can couple the light energy of the core mode into the cladding modes and the second peanut-shape structure can recouple the light in the cladding modes into the core mode. A high-quality interference spectrum with a fringe visibility of about 13 dB is observed. Moreover, it has very good mechanical strength compared with the MZIs based on the tapers or the offset structures. When the interferometer length L = 22 mm, the temperature sensitivity of the device is ~ 46.8 pm/°C and the strain sensitivity is ~ 14 pm/μe. Such kind of interferometer would find potential applications in communication and sensing fields.