Showing papers on "Interferometry published in 2021"

Overview of KAGRA: Calibration, detector characterization, physical environmental monitors, and the geophysics interferometer

Abstract: KAGRA is a newly built gravitational wave observatory, a laser interferometer with a 3 km arm length, located at Kamioka, Gifu, Japan. In this series of articles we present an overview of the baseline KAGRA, for which we finished installing the designed configuration in 2019. This article describes the method of calibration (CAL) used for reconstructing gravitational wave signals from the detector outputs, as well as the characterization of the detector (DET). We also review the physical environmental monitoring (PEM) system and the geophysics interferometer (GIF). Both are used for characterizing and evaluating the data quality of the gravitational wave channel. They play important roles in utilizing the detector output for gravitational wave searches. These characterization investigations will be even more important in the near future, once gravitational wave detection has been achieved, and in using KAGRA in the gravitational wave astronomy era.

Using an Atom Interferometer to Infer Gravitational Entanglement Generation

Abstract: If gravitational perturbations are quantized into gravitons in analogy with the electromagnetic field and photons, the resulting graviton interactions should lead to an entangling interaction between massive objects. We suggest a test of this prediction. To do this, we introduce the concept of interactive quantum information sensing. This novel sensing protocol is tailored to provable verification of weak dynamical entanglement generation between a pair of systems. We show that this protocol is highly robust to typical thermal noise sources. Moreover, the sensitivity can be increased both using an initial thermal state and/or an initial phase of entangling via a nongravitational interaction. We outline a concrete implementation testing the ability of the gravitational field to generate entanglement between an atomic interferometer and a mechanical oscillator. Preliminary numerical estimates suggest that near-term devices could feasibly be used to perform the experiment.

Measuring the spectrum of primordial gravitational waves with CMB, PTA and laser interferometers

Abstract: We investigate the possibility of measuring the primordial gravitational wave (GW) signal across 21 decades in frequencies, using the cosmic microwave background (CMB), pulsar timing arrays (PTA), and laser and atomic interferometers. For the CMB and PTA experiments we consider the LiteBIRD mission and the Square Kilometer Array (SKA), respectively. For the interferometers we consider space mission proposals including the Laser Interferometer Space Antenna (LISA), the Big Bang Observer (BBO), the Deci-hertz Interferometer Gravitational wave Observatory (DECIGO), the $\mu$Ares experiment, the Decihertz Observatory (DO), and the Atomic Experiment for Dark Matter and Gravity Exploration in Space (AEDGE), as well as the ground-based Einstein Telescope (ET) proposal. We implement the mathematics needed to compute sensitivities for both CMB and interferometers, and derive the response functions for the latter from the first principles. We also evaluate the effect of the astrophysical foreground contamination in each experiment. We present binned sensitivity curves and error bars on the energy density parameter, $\Omega_{GW}h^2$, as a function of frequency for two representative classes of models for the stochastic background of primordial GW: the quantum vacuum fluctuation in the metric from single-field slow-roll inflation, and the source-induced tensor perturbation from the spectator axion-SU(2) inflation models. We find excellent prospects for joint measurements of the GW spectrum by CMB and space-borne interferometers mission proposals.

First demonstration of 6 dB quantum noise reduction in a kilometer scale gravitational wave observatory

Abstract: Photon shot noise, arising from the quantum-mechanical nature of the light, currently limits the sensitivity of all the gravitational wave observatories at frequencies above one kilohertz. We report a successful application of squeezed vacuum states of light at the GEO 600 observatory and demonstrate for the first time a reduction of quantum noise up to 6.03±0.02 dB in a kilometer scale interferometer. This is equivalent at high frequencies to increasing the laser power circulating in the interferometer by a factor of 4. Achieving this milestone, a key goal for the upgrades of the advanced detectors required a better understanding of the noise sources and losses and implementation of robust control schemes to mitigate their contributions. In particular, we address the optical losses from beam propagation, phase noise from the squeezing ellipse, and backscattered light from the squeezed light source. The expertise gained from this work carried out at GEO 600 provides insight toward the implementation of 10 dB of squeezing envisioned for third-generation gravitational wave detectors.

Intensity-based holographic imaging via space-domain Kramers–Kronig relations

Abstract: Holography is a powerful tool to record waves without loss of information that has benefited optical, X-ray and electronic imaging applications by quantifying phase delays induced by light–matter interactions However, holographic imaging is technically demanding in that it generally requires an interferometric setup, a coherent source and long-term stability Here, we present holographic imaging in which a phase image is obtained directly from a single intensity measurement in oblique illumination Our approach is based on space-domain Kramers–Kronig relations that transform the spatial variation in intensity to the spatial variation in phase We demonstrate two-dimensional holographic imaging and three-dimensional refractive index tomography of microscopic objects and biological specimens from intensity images measured with an optical microscope and illumination control The proposed method does not require iterative processes nor strict constraints and opens up a new approach to non-interferometric holographic imaging in various spectral regimes An intensity-based holographic imaging via space-domain Kramers–Kronig relations is presented, allowing the phase image of an object to be obtained directly from a single intensity measurement with oblique illumination

Realization of a complete Stern-Gerlach interferometer: Toward a test of quantum gravity.

Abstract: The Stern-Gerlach effect, found a century ago, has become a paradigm of quantum mechanics. Unexpectedly, until recently, there has been little evidence that the original scheme with freely propagating atoms exposed to gradients from macroscopic magnets is a fully coherent quantum process. Several theoretical studies have explained why a Stern-Gerlach interferometer is a formidable challenge. Here, we provide a detailed account of the realization of a full-loop Stern-Gerlach interferometer for single atoms and use the acquired understanding to show how this setup may be used to realize an interferometer for macroscopic objects doped with a single spin. Such a realization would open the door to a new era of fundamental probes, including the realization of previously inaccessible tests at the interface of quantum mechanics and gravity.

A review of specialty fiber biosensors based on interferometer configuration.

Abstract: Optical fiber biosensors have attracted extensive research attention in fields such as public health research, environmental science, bioengineering, disease diagnosis and drug research. Accurate detection of biomolecules is essential to limit the extent of disease outbreaks and provide valuable guidance for regulatory agencies to take timely measures. Among many optical fiber sensors, optical fiber biosensors based on specialty fibers have the advantages of biocompatibility, small size, high measurement resolution, high stability, and immunity to electromagnetic interference. In this paper, four types interferometer biosensors based on specialty fiber, namely Mach-Zehnder interferometer (MZI), Michelson interferometer (MI), Fabry - Perot interferometer (FPI) and Sagnac interferometer (SI), are reviewed in terms of operating principles, sensing structure, and application fields. The fiber types are further divided into micro-nano optical fiber, thin core fiber, polarization maintaining fiber, polymer fiber, microstructure optical fiber. Furthermore, this paper evaluates the advantages and disadvantages of these interferometer biosensors. Finally, main challenging problems and expectational development direction of specialty fiber interferometer biosensors are summarized. This text clearly shows the huge development potential of optical fiber biosensors in biomedical. This article is protected by copyright. All rights reserved.

A universal fully reconfigurable 12-mode quantum photonic processor

Abstract: Photonic processors are pivotal for both quantum and classical information processing tasks using light. In particular, linear optical quantum information processing requires both large-scale and low-loss programmable photonic processors. In this paper, we report the demonstration of the largest universal quantum photonic processor to date: a low-loss 12-mode fully tunable linear interferometer with all-to-all mode coupling based on stoichiometric silicon nitride waveguides.

A High Sensitivity Optical Fiber Interferometer Sensor for Acoustic Emission Detection of Partial Discharge in Power Transformer

Abstract: We propose a Michelson ultrasonic sensing system for detecting the acoustic emission generated by the partial discharge in power transformer. In order to guide the sensor head design in the sensing system for high sensitivity, a theoretical model is established to investigate the effects of the sensor head dimensions on the response sensitivity. After that, an optimized sensor head is designed. In the frequency range from 80 kHz to 200 kHz, whether the PZT sensor is installed in the oil or on the tank, the average response sensitivity of the proposed sensing system is higher than that of the conventional PZT system. When the distance between the sensor head and ultrasonic source is 300 mm in oil, the average detection limit of the Michelson ultrasonic sensing system is about 0.26 Pa, which is about 18.6% of that of the PZT system. Moreover, experiment results show that the detectable partial discharge initial voltage for the proposed optical system is 21.5% lower than that for the PZT system. The enhanced sensitivity makes the Michelson ultrasonic sensing system a potential method to detect the small defects in power transformer.

High angular resolution gravitational wave astronomy

Abstract: Since the very beginning of astronomy the location of objects on the sky has been a fundamental observational quantity that has been taken for granted. While precise two dimensional positional information is easy to obtain for observations in the electromagnetic spectrum, the positional accuracy of current and near future gravitational wave detectors is limited to between tens and hundreds of square degrees, which makes it extremely challenging to identify the host galaxies of gravitational wave events or to detect any electromagnetic counterparts. Gravitational wave observations provide information on source properties that is complementary to the information in any associated electromagnetic emission. Observing systems with multiple messengers thus has scientific potential much greater than the sum of its parts. A gravitational wave detector with higher angular resolution would significantly increase the prospects for finding the hosts of gravitational wave sources and triggering a multi-messenger follow-up campaign. An observatory with arcminute precision or better could be realised within the Voyage 2050 programme by creating a large baseline interferometer array in space and would have transformative scientific potential. Precise positional information of standard sirens would enable precision measurements of cosmological parameters and offer new insights on structure formation; a high angular resolution gravitational wave observatory would allow the detection of a stochastic background and resolution of the anisotropies within it; it would also allow the study of accretion processes around black holes; and it would have tremendous potential for tests of modified gravity and the discovery of physics beyond the Standard Model.

Optical Coherence Transfer Mediated by Free Electrons

Abstract: We investigate the optical coherence properties carried by the quantum state of a single free-flying charged and massive p article – the electron. We propose feasible Mach-Zehnder-like linear and nonlinear optical interferometric detection, incorporating electron-microscope beams.

Six-wavelength-switchable narrow-linewidth thulium-doped fiber laser with polarization-maintaining sampled fiber Bragg grating

Abstract: A compound-cavity-based six-wavelength-switchable single-longitudinal-mode (SLM) thulium-doped fiber laser (TDFL) with a homemade polarization-maintaining sampled fiber Bragg grating (PM-SFBG) is proposed and demonstrated. The PM-SFBG in the 2 μm band is studied theoretically and experimentally, and is utilized as a polarization-dependent multi-channel reflecting filter in a multi-wavelength-switchable fiber laser, both for the first time. The SLM lasing in each channel is guaranteed by using a tri-ring sub-cavity. 6 single-wavelength operations are easily obtained and switched each other, and the maximum power and wavelength fluctuations are as low as 0.71 dB and 0.02 nm, respectively. The frequency noise of each lasing wavelength is measured through an unbalanced Michelson interferometry system, and the linewidths for all six lasing wavelengths are among 0.60–1.08 kHz, calculated by the β-separation line method on the basis of the measured frequency noise spectra. Benefiting from the enhanced polarization hole burning effect formed in the gain fiber, 9 switchable dual-wavelength operations with different wavelength intervals and orthogonal polarizations at two lasing wavelengths are obtained with the maximum power and wavelength fluctuations of only 0.95 dB and 0.03 nm, respectively. We believe the proposed TDFL will find great applications as an ideal light source in free space optical communication and optical sensing.

Linear array focused-laser differential interferometry for single-shot multi-point flow disturbance measurements

Abstract: In this Letter, a modification of the well-known focused-laser differential interferometer (FLDI) is demonstrated, with the primary focus being increasing the number of probed locations efficiently. To generate multiple beams in the FLDI system, a diffractive optical element is used. This approach is significantly more cost-effective and easier to implement than the current approach of generating multiple FLDI beam pairs using a series of Wollaston prisms. The measurements shown here utilize a 1D linear array of points, and the ability to generate a 2D array is demonstrated using two linear diffractive optical elements in tandem. Therefore, this technique, referred to as linear array FLDI (LA-FLDI), is able to provide measurements of fluid disturbances at multiple discrete locations while allowing for high data acquisition rates (>1MHz). This technique provides a much simpler approach to multipoint FLDI measurements and can increase the throughput of FLDI measurements in impulse aerospace testing facilities.

Improved phase sensitivity in a quantum optical interferometer based on multiphoton catalytic two-mode squeezed vacuum states

Abstract: The usage of non-Gaussian states to improve the phase sensitivity of interferometers has been studied before. In this paper, we introduce the multiphoton catalysis two-mode squeezed vacuum (MC-TMSV) state as an input of the Mach-Zehnder interferometer (MZI) and study its phase sensitivity with photon-number parity measurement and the influence of photon losses. We also study the statistical properties of the MC-TMSV state in terms of the average photon number, the Wigner function, and the Mandel-$Q$ parameter. Our results show that the MC-TMSV state can exhibit stronger nonclassical characteristics, thereby making the phase sensitivity more precise with either the increase of the photon-catalyzed number or the decrease of the transmissivity. Furthermore, we also consider the effects of photon losses involving external- and internal-loss processes of the MZI, and we find that in both cases the sensitivity with the MC-TMSV state can be significantly better than that with the TMSV state under the same parameters especially in the serious photon losses and small initial squeezing regimes. We also find that the multiphoton catalysis is more sensitive to the external photon losses compared with the internal ones. Our results here can find important applications in quantum metrology.

Nanometric Precision Distance Metrology via Hybrid Spectrally Resolved and Homodyne Interferometry in a Single Soliton Frequency Microcomb.

Abstract: Laser interferometry serves a fundamental role in science and technology, assisting precision metrology and dimensional length measurement. During the past decade, laser frequency combs---a coherent optical-microwave frequency ruler over a broad spectral range with traceability to time-frequency standards---have contributed pivotal roles in laser dimensional metrology with ever-growing demands in measurement precision. Here we report spectrally resolved laser dimensional metrology via a free-running soliton frequency microcomb, with nanometric-scale precision. Spectral interferometry provides information on the optical time-of-flight signature, and the large free-spectral range and high coherence of the microcomb enable tooth-resolved and high-visibility interferograms that can be directly read out with optical spectrum instrumentation. We employ a hybrid timing signal from comb-line homodyne, microcomb, and background amplified spontaneous emission spectrally resolved interferometry---all from the same spectral interferogram. Our combined soliton and homodyne architecture demonstrates a 3-nm repeatability over a 23-mm nonambiguity range achieved via homodyne interferometry and over 1000-s stability in the long-term precision metrology at the white noise limits.

Dual-comb hyperspectral digital holography

Abstract: Holography1 has always held special appeal as it is able to record and display spatial information in three dimensions2–10. Here we show how to augment the capabilities of digital holography11,12 by using a large number of narrow laser lines at precisely defined optical frequencies simultaneously. Using an interferometer based on two frequency combs13–15 of slightly different repetition frequencies and a lensless camera sensor, we record time-varying spatial interference patterns that generate spectral hypercubes of complex holograms, revealing the amplitudes and phases of scattered wave-fields for each comb line frequency. Advancing beyond multicolour holography and low-coherence holography (including with a frequency comb16), the synergy of broad spectral bandwidth and high temporal coherence in dual-comb holography opens up novel optical diagnostics, such as precise dimensional metrology over large distances without interferometric phase ambiguity, or hyperspectral three-dimensional imaging with high spectral resolving power, as we demonstrate with molecule-selective imaging of an absorbing gas. Dual-comb digital holography based on an interferometer composed of two frequency combs of slightly different repetition frequencies and a lensless camera sensor allows highly frequency-multiplexed holography with high temporal coherence.

A tunable Fabry-Pérot quantum Hall interferometer in graphene

Abstract: Electron interferometry with quantum Hall (QH) edge channels in semiconductor heterostructures can probe and harness the exchange statistics of anyonic excitations. However, the charging effects present in semiconductors often obscure the Aharonov–Bohm interference in QH interferometers and make advanced charge-screening strategies necessary. Here we show that high-mobility monolayer graphene constitutes an alternative material system, not affected by charging effects, for performing Fabry–Perot QH interferometry in the integer QH regime. In devices equipped with gate-tunable quantum point contacts acting on the edge channels of the zeroth Landau level, we observe—in agreement with theory—high-visibility Aharonov–Bohm interference widely tunable through electrostatic gating or magnetic fields. A coherence length of 10 μm at a temperature of 0.02 K allows us to further achieve coherently coupled double Fabry–Perot interferometry. In future, QH interferometry with graphene devices may enable investigations of anyonic excitations in fractional QH states. Similar to optical waves, electrons can also interfere, but they require high-quality devices with minimal scattering for an experimental observation of this effect. An interferometer based on a single sheet of graphene provides an alternative to the more standard semiconductor devices and may in future enable access to exotic quantum effects, such as anyon braiding.

In-Fiber Mach–Zehnder Interferometer Sensor Based on Er Doped Fiber Peanut Structure in Fiber Ring Laser

Abstract: A novel temperature sensor based on rare earth fiber peanut (REFP) shape structure in fiber ring laser (FRL) is proposed and demonstrated. A single-mode rare earth doped fiber is used to fabricate a peanut structure to produce Mach–Zehnder interferometer (MZI). Benefited from the strong thermo-optic effect of rare earth ions in fiber, high sensitivity measurement of temperature is exploited to realize. More importantly, there is no need for additional filters compared to traditional fiber laser sensors. The sensitivity of refractive index (RI) was measured to further verify the sensing performance of MZI. The experimental results show that compared with the external sensing unit, the Er-doped fiber peanut (EDFP) structure has higher thermo optic effect coefficient and superior temperature sensitivity. The temperature and RI sensitivity of the EDFP is 0.158 nm/°C and −19.55 nm/RIU, respectively. The optical fiber MZI sensor based on the EDFP has the advantages of high sensitivity, good repeatability, simple fabrication, and compact structure. It has good application prospects in biomedicine, aerospace, and marine development.

Time-delay interferometry without delays

Abstract: The space-based gravitational-wave observatory LISA relies on a form of synthetic interferometry (time-delay interferometry, or TDI) where the otherwise overwhelming laser phase noise is canceled by linear combinations of appropriately delayed phase measurements. These observables grow in length and complexity as the realistic features of the LISA orbits are taken into account. In this paper we outline an implicit formulation of TDI where we write the LISA likelihood directly in terms of the basic phase measurements, and we marginalize over the laser phase noises in the limit of infinite laser-noise variance. Equivalently, we rely on TDI observables that are defined numerically (rather than algebraically) from a discrete-filter representation of the laser propagation delays. Our method generalizes to any time dependence of the armlengths; it simplifies the modeling of gravitational-wave signals; and it allows a straightforward treatment of data gaps and missing measurements.

Slow light bimodal interferometry in one-dimensional photonic crystal waveguides

Abstract: Strongly influenced by the advances in the semiconductor industry, the miniaturization and integration of optical circuits into smaller devices has stimulated considerable research efforts in recent decades. Among other structures, integrated interferometers play a prominent role in the development of photonic devices for on-chip applications ranging from optical communication networks to point-of-care analysis instruments. However, it has been a long-standing challenge to design extremely short interferometer schemes, as long interaction lengths are typically required for a complete modulation transition. Several approaches, including novel materials or sophisticated configurations, have been proposed to overcome some of these size limitations but at the expense of increasing fabrication complexity and cost. Here, we demonstrate for the first time slow light bimodal interferometric behaviour in an integrated single-channel one-dimensional photonic crystal. The proposed structure supports two electromagnetic modes of the same polarization that exhibit a large group velocity difference. Specifically, an over 20-fold reduction in the higher-order-mode group velocity is experimentally shown on a straightforward all-dielectric bimodal structure, leading to a remarkable optical path reduction compared to other conventional interferometers. Moreover, we experimentally demonstrate the significant performance improvement provided by the proposed bimodal photonic crystal interferometer in the creation of an ultra-compact optical modulator and a highly sensitive photonic sensor.

Simultaneous measurement of temperature and curvature using ring-core fiber-based Mach-Zehnder interferometer.

Abstract: In this paper, the Mach-Zehnder interferometer (MZI) based on ring-core fiber was proposed and manufactured. Benefiting from the identical diameters of ring-core fiber, no-core fiber, and single-mode fiber, the MZI fiber sensor can be prototyped by sandwiching the ring-core fiber between the no-core fiber and the single-mode fiber (SMF). With the proposed specific structure of the ring-core fiber, the simultaneous measurement of temperature and curvature was achieved with the MZI sensor by means of monitoring the wavelength shift of interference dips. Experimental results have shown that the sensitivity of curvature sensing could reach up to -3.68 nm/m-1 in the range from 1.3856 m-1 to 3.6661 m-1 with high linearity of 0.9959. Meanwhile, the maximum temperature sensitivity is measured to be 72 pm/°C with a fairly good linearity response of 0.9975. In addition, by utilizing the 2×2 matrix algorithm, the dual demodulation of temperature and curvature can be readily realized for the purpose of direct sensing. It is believed that the proposed special structure-based MZI sensor may show great potential applications in the field of fiber-optics sensing and structural health monitoring (SHM).

Ultrafast Hole Deformation Revealed by Molecular Attosecond Interferometry

Abstract: Understanding the evolution of molecular electronic structures is the key to explore and control photochemical reactions and photobiological processes. Subjected to strong laser fields, electronic holes are formed upon ionization and evolve in the attosecond timescale. It is crucial to probe the electronic dynamics in real time with attosecond-temporal and atomic-spatial precision. Here, we present molecular attosecond interferometry that enables the in situ manipulation of holes in carbon dioxide molecules via the interferometry of the phase-locked electrons (propagating in opposite directions) of a laser-triggered rotational wave packet. The joint measurement on high-harmonic and terahertz spectroscopy (HATS) provides a unique tool for understanding electron dynamics from picoseconds to attoseconds. The optimum phases of two-color pulses for controlling the electron wave packet are precisely determined owing to the robust reference provided with the terahertz pulse generation. It is noteworthy that the contribution of HOMO-1 and HOMO-2 increases reflecting the deformation of the hole as the harmonic order increases. Our method can be applied to study hole dynamics of complex molecules and electron correlations during the strong-field process. The threefold control through molecular alignment, laser polarization, and the two-color pulse phase delay allows the precise manipulation of the transient hole paving the way for new advances in attochemistry.

Laser Interference Lithography for Fabrication of Planar Scale Gratings for Optical Metrology

Abstract: Laser interference lithography is an attractive method for the fabrication of a large-area two-dimensional planar scale grating, which can be employed as a scale for multi-axis optical encoders or a diffractive optical element in many types of optical sensors. Especially, optical configurations such as Lloyd’s mirror interferometer based on the division of wavefront method can generate interference fringe fields for the patterning of grating pattern structures at a single exposure in a stable manner. For the fabrication of a two-dimensional scale grating to be used in a planar/surface encoder, an orthogonal two-axis Lloyd’s mirror interferometer, which has been realized through innovation to Lloyd’s mirror interferometer, has been developed. In addition, the concept of the patterning of the two-dimensional orthogonal pattern structure at a single exposure has been extended to the non-orthogonal two-axis Lloyd’s mirror interferometer. Furthermore, the optical setup for the non-orthogonal two-axis Lloyd’s mirror interferometer has been optimized for the fabrication of a large-area scale grating. In this review article, principles of generating interference fringe fields for the fabrication of a scale grating based on the interference lithography are reviewed, while focusing on the fabrication of a two-dimensional scale grating for planar/surface encoders. Verification of the pitch of the fabricated pattern structures, whose accuracy strongly affects the performance of planar/surface encoders, is also an important task to be addressed. In this paper, major methods for the evaluation of a grating pitch are also reviewed.

Detection of second-mode instabilities on a flared cone in Mach 6 quiet flow with linear array focused laser differential interferometry

Abstract: In this work, an enhanced version of the focused laser differential interferometer (FLDI) is used to measure second-mode instabilities in the boundary layer on a flared cone in Mach 6 quiet flow A diffractive optical element added to the FLDI system provides six beams that pass through a single Wollaston prism, allowing for multiple points of measurements without the need for additional Wollaston prisms This technique, linear array-FLDI (LA-FLDI), is used for six simultaneous measurements of second-mode instabilities in a hypersonic boundary layer This is the first time that this variation of FLDI has been used for second-mode instability measurements and demonstrates that the inclusion of a diffractive optical element into the traditional FLDI and splitting of beam power does not preclude such usage Thus, LA-FLDI appears to have significant potential for increasing the efficiency of measurements of high-speed boundary layers by enabling multiple measurements at different locations within a single facility run The velocity of the second-mode wavepacket is estimated and a bispectral analysis is presented showing that harmonics are associated with quadratically phased coupled interactions

Large Displacement Motion Interferometry With Modified Differentiate and Cross-Multiply Technique

Abstract: Millimeter-wave radar interferometry is superior in detecting small displacement motions owing to its short wavelength. However, it is subject to phase ambiguity as the target displacement may often exceed a quarter wavelength. In this article, a modified differentiate and cross-multiply (MDACM) technique is proposed to tackle the phase ambiguity issue for accurate reconstruction of the phase history in millimeter-wave interferometry. In addition, to resolve the imperfections of the interferometric radar system, an MDACM-based integral phase reconstruction approach is presented, which seamlessly integrates alternating current (ac)-coupling-induced distortion correction, $I/Q$ mismatch correction, and direct current (dc) offsets’ calibration. Without causing any phase ambiguity, the proposed technique acts as a black box to take in the raw $I/Q$ signals and correct all the hardware imperfections, and it outputs the desired displacement motions with micrometer accuracy. The simulation results show that the proposed technique can not only linearly recover the displacement motions across a wide range in different noise conditions without any phase ambiguity but also improve the stability by 11 times under ac-coupling-induced distortion. With a custom-designed 120-GHz interferometric radar sensor, experiments were carried out to validate various scenarios including mechanical vibrations and gesture sensing. The experimental results show that the proposed technique can accurately track not only deep subwavelength motion of only $1~\mu \text{m}$ but also multiwavelength displacement of >40 times of the wavelength at 120 GHz.

Wide wavelength-tunable passive mode-locked Erbium-doped fiber laser with a SESAM

Abstract: In this work we present a simple polarization-maintaining wavelength-tunable passive mode-locked Erbium-doped fiber laser with a semiconductor saturable absorber mirror (SESAM) as a mode locker. The cavity includes a Sagnac interferometer-based fiber optical loop mirror (FOLM) as a wide wavelength-tunable filter. Tunable mode-locking was experimentally achieved in the range of 1543.2 nm to 1569.5 nm by thermally adjustment of FOLM wavelength reflection. The output pulses have a repetition rate of 11.16 MHz with pulse duration about 0.9 ps. The experimental results were confirmed by numerical simulations.

High-Performance Graphene-on-Silicon Nitride All-Optical Switch Based on a Mach–Zehnder Interferometer

Abstract: We propose and experimentally demonstrate a high-performance graphene-on-silicon nitride (Si3N4) all-optical switch based on a Mach–Zehnder interferometer (MZI). In our device, the graphene overlaying on a Si3N4 waveguide absorbs part of the pump light power and generates heat. Then, the Si3N4 waveguide underneath can be heated and its refractive index can be changed due to the thermo-optic effect. In this way, the phase of the probe light in the Si3N4 arm with graphene on top is tuned and all optical switching can then be implemented. In the experimental demonstration, an all-optical switch with a chip size of ∼0.36 mm2 is realized with an extinction ratio of 11 dB. The tuning efficiency is measured to be 0.00917 π/mW, which is insensitive to the wavelength of the pump light. All-optical switching is also demonstrated, while the rise and fall time constants are measured to be 571 ns and 1.29 μs, respectively. These results show that our proposed configuration provides a functional integrated component for the development of efficient all-optical control devices with a fast switching speed on the insulator platform. Moreover, by using integrated MZI structure, our design could potentially achieve a broad bandwidth.

Dual-comb ranging with frequency combs from single cavity free-running laser oscillators.

Abstract: Laser ranging (LIDAR) with dual optical frequency combs enables high-resolution distance measurements over long ranges with fast update rates. However, the high complexity of stabilized dual optical frequency comb systems makes it challenging to use this technique in industrial applications. To address this issue, here we demonstrate laser ranging directly from the output of both a free-running dual-comb diode-pumped semiconductor and solid-state laser oscillator. Dual-comb operation from a single cavity is achieved via polarization duplexing with intracavity birefringent crystals. We perform ranging experiments with two implementations of this scheme: a modelocked integrated external cavity surface-emitting laser (MIXSEL) and a Yb:CaF2 solid-state laser. For these proof of principle demonstrations, we measure the distance to a moving mirror mounted on a home-made shaker. The MIXSEL laser has a repetition rate of 2.736 GHz and a repetition rate difference of 52 kHz, and yields a measurement resolution of 1.36 µm. The Yb:CaF2 laser has a repetition rate of 137 MHz and a repetition rate difference of 952 Hz, and yields a measurement resolution of 0.55 µm. In both cases the resolution is inferred by a parallel measurement with a HeNe interferometer. These results represent the first laser ranging with free-running dual-comb solid-state oscillators. With further optimization, resolution well below 1 µm and range well above 1 km are expected with this technique.