ABSTRACT
This paper utilizes an analytical model of polarization dependent frequency sideband generation via the Kerr effect in a highly nonlinear fiber to determine the state of polarization (SOP) of a laser by all-optical means. Theoretical analysis shows that the power of the nth order sideband generated by the propagation of two lasers with distinct frequencies in the nonlinear medium is proportional to cos 2n(α/2), where α is the angle between the normalized Stokes vectors representing the SOPs of the lasers on the Poincaré sphere. By tailoring the SOP of one laser acting as a reference and experimentally measuring the power of the first order sideband, the SOP of the laser under test is recovered with an error smaller than 10.22° on the Poincaré sphere corresponding to 0.8% the sphere's total area. Comparing the SOPs of two lasers without referencing them to fixed polarizers enables potential applications in remote environmental sensing, novel polarization division multiplexing schemes for enhanced telecommunication data rates, and scientific instrumentation.
ABSTRACT
The local temperature and longitudinal strain at spatial resolution of 0.5% of the pulse-width equivalent length along a nonuniform As2Se3-PMMA tapered fiber is investigated by a Brillouin gain-profile tracing method. This scheme uses a 20 ns pump pulse with the pulse-width equivalent length longer than the fiber under test (FUT) of 50 cm nonuniform As2Se3-PMMA tapered fiber. The whole interaction process of long pump pulse is investigated including pump pulse entering the FUT, overlapping completely with FUT and leaving the FUT. The evolution of Brillouin gain spectrum (BGS) along the nonuniform fiber is formed by the subtraction of frequency-domain BGS of two adjacent sensing points in the trailing edge (where the pump pulse leaves the FUT) of the BOTDA spectrum. The trailing part is preferred due to the pre-amplified acoustic field by the long pumping pulse. Then the local responses of temperature and wide-range longitudinal strain with high spatial resolution of 1.1 cm along the nonuniform As2Se3-PMMA tapered fibers are investigated. The change of the local temperature and strain shifts the BGS that is different along the nonuniform fibers, which forms the distributed measurement. The spatial resolution, the fiber length of the detected section in the proposed method, is 1.1 cm for the local temperature and longitudinal strain measurement, which is 0.5% of the pulse-width equivalent length and is limited by the sampling rate of data acquisition and the fall-time of the pump pulse.
ABSTRACT
This work presents an analytical model accounting for the impact of optical polarization on the generation of frequency sidebands by the Kerr effect in a highly nonlinear fiber. Theoretical analysis shows that for a relative polarization angle, α, between two input lasers expressed on the Poincaré sphere, the optical power of the nth order sideband is proportional to cos 2n(α/2). This theoretical result enables a novel all-optical technique for interrogating changes in polarization with higher sensitivity than conventional measurement schemes using linear polarizers. The predicted theoretical relationship between the sideband power and the relative polarization angle is verified experimentally and sensitivity enhancement by a factor of 1.45 compared to a conventional polarimetric sensor is demonstrated for the 3rd order sideband. This novel nonlinear approach, which allows dynamic range to be traded for an enhanced ability to detect small polarization variations, has potential applications in fusion reactor monitoring, instrumentation and material characterization.
ABSTRACT
High-frequency ultrasound sensors are essential for high-resolution medical ultrasonic imaging and industrial ultrasonic non-destructive monitoring. In this paper, we propose highly sensitive broadband ultrasound sensors based on fused dual-core chalcogenide-polymethyl methacrylate (As2Se3-PMMA) microfibers. We demonstrate that ultrasound response is determined by the differential slope of transmission spectra in the dual-core microfiber, which is verified by detecting the acoustic response in various microfibers of different tapering parameters. A broadband ultrasound frequency range with a high signal-to-noise ratio (SNR) is achieved in the fused dual-core microfiber (DCM) with a sub-micron core diameter and a close core separation due to the large spectral slope at the quadrature points of the transmission spectrum. In addition, we experimentally demonstrate the sensing of ultrasound waves propagating with and without an aluminum plate in the DCM sensor. An ultrasound sensor with a broadband frequency range from 20 kHz to 80 MHz and an average SNR of 31 dB is achieved in a compact fused dual-core As2Se3-PMMA microfiber when it is directly placed on a piezoelectric transducer (PZT).
ABSTRACT
We provide a correction to a figure in our published paper [Opt. Express28, 3789 (2020)10.1364/OE.384004].
ABSTRACT
We present a novel optical signal processing scheme for enhancing the minimum detectable environmental perturbation of intensity-based fiber sensors. The light intensity is first stabilized by inducing a sinusoidal intensity modulation and extracting the first-order sideband generated by self-phase modulation (SPM) in a nonlinear medium. The light with stabilized intensity is then sent through a sensor and the sensor induced power variation is magnified by first inducing a sinusoidal intensity modulation, then undergoing SPM, and finally extracting a higher-order sideband. The advantage of the proposed stabilization-magnification (SM) sensing scheme is experimentally demonstrated by applying a damped vibration on an intensity-based fiber sensor and comparing the minimum detectable strain value of the proposed scheme with that of a conventional sensing scheme. Experimental results demonstrate minimum detectable strain improvement by a factor of 3.93. This new SM sensing scheme allows for the detection of perturbations originally too weak to be detected by a given intensity-based fiber sensor, which will be beneficial for a variety of applications such as high frequency ultra-sound detection.
ABSTRACT
In this Letter, we design and fabricate elliptical-core (ECORE) chalcogenide-polymethyl methacrylate (As2Se3-PMMA) microfibers to explore the birefringence impact on stimulated Brillouin scattering. Numerical simulations based on the finite-element method and elastodynamic equation are utilized to calculate the phase and group birefringence and Brillouin gain spectra of the fundamental mode in three ECORE As2Se3-PMMA microfibers at different core diameters. Experimentally measured and numerically calculated results show that as the core diameter of the minor axis of an ECORE microfiber with a ratio of 1.108 is reduced from 1.50 µm to 0.87 µm, a high group birefringence of â¼10-3 to â¼10-2 and a large Brillouin frequency shift difference of â¼6MHz to â¼30MHz are achieved, while the Brillouin gain spectra are broadened significantly from â¼70MHz to â¼140MHz. The high-birefringence ECORE As2Se3-PMMA microfiber is important for Brillouin sensing due to the tailorable high birefringence and ultrahigh nonlinearity.
ABSTRACT
We present the characterization of high extinction ratio (ε) square optical pulses using a photon counting technique, as other techniques only offer a limited range of measurement up to 60 dB. High-ε pulses are generated by applying a square pulse modulation on sinusoidally modulated optical signals, then inducing self-phase modulation (SPM) using the nonlinear Kerr effect and extracting an SPM-generated sideband. We measured a 10 ns Kerr-generated optical pulse exhibiting a 120.1 dB extinction ratio, originating from a conventional electro-optic modulator delivering a pulse with a 20-dB extinction ratio, by counting the number of photons at the peak and the pedestal of the generated pulse. These proven high-ε pulses allow for long-range distributed vibration sensing in optical time-domain reflectometry systems and open new horizons in high-Q microring sensors.
ABSTRACT
We demonstrate the enhancement of the resolution of a fiber optical sensor using all-optical signal processing. By sweeping the frequency of a tunable laser across a fiber Bragg grating, a signal corresponding to the reflection spectrum of the FBG is generated. If another laser with fixed power and frequency is launched into a highly nonlinear fiber along with the FBG-shaped signal, the Kerr effect gives rise to a number of frequency sidebands, where the power in each of the sidebands is proportional an integer exponent of the signal and pump powers. By filtering out particular sidebands, this potentiation effect reduces the width of the FBG-shaped signal, making shifts in its central wavelength easier to distinguish. We report a maximum resolution enhancement factor of 3.35 obtained by extracting the n = -4 order sideband, and apply resolution enhancement to improve the resolution of an FBG based temperature sensor. The method described in this paper can be applied to existing fiber based sensors and optical systems to enhance their resolution.
ABSTRACT
We propose a wide-range strain sensor based on Brillouin frequency and linewidth in a 50 cm-long As2Se3-polymethyl methacrylate (As2Se3-PMMA) hybrid microfiber with a core diameter of 2.5 µm. The distributed information over the hybrid microfiber is measured by a Brillouin optical time-domain analysis (BOTDA) system. The wide dynamic range strain from 0 to 15000â µÉ is enabled by measuring the Brillouin frequency and linewidth due to the low Young's modulus of As2Se3 core and the high mechanical strength of PMMA cladding. The deformation of the As2Se3-PMMA hybrid microfiber is observed when the strain is greater than 1500â µÉ by measuring the distributed Brillouin frequency and Brillouin linewidth over the 50 cm-long hybrid microfiber. The measured errors based on the Brillouin frequency in the range of 0-1500â µÉ and 1500-15000â µÉ are 42â µÉ and 105â µÉ, respectively. The measured error based on the Brillouin linewidth is 65â µÉ at 0-1500â µÉ and the maximum error is 353â µÉ when the tensile strain is 15000â µÉ. No strain memory effect is observed compared with the polymer optical fiber due to Young's modulus in As2Se3 is larger than that in polymer. Numerical simulations are developed to accurately predict the strain dependence of Brillouin frequency in the As2Se3-PMMA hybrid microfiber.
ABSTRACT
Chalcogenide fibers are currently being used widely in nonlinear optical signal processing, as they exhibit ultrahigh nonlinearity. Here, we propose a sensor based on stimulated Brillouin scattering for simultaneous temperature and strain measurement in a dual-core tapered As2Se3-polymethyl methacrylate fiber using a Brillouin optical time-domain analysis system. Different Brillouin frequency responses under temperature and strain variations and the separation of Brillouin frequency shifts (BFSs) in two principal polarization axes are demonstrated experimentally over a 50-cm-long tapered dual-core hybrid microfiber. The temperature coefficients are -3.8272MHz/∘C and -3.3302MHz/∘C, and the strain coefficients are -0.06143MHz/µÎµ and -0.03463MHz/µÎµ. Due to the different temperature and strain dependences of Brillouin frequency peaks in two polarizations, temperature and strain resolutions of 1°C and 33µÎµ are realized, respectively. Numerical simulations are also reported to account for the BFS difference in two polarization axes.
ABSTRACT
We present a new all-optical method for the magnification of small-intensity fluctuations using the nonlinear Kerr effect. A fluctuation of interest is impressed onto a sinusoidally modulated optical signals (SMOS) and spectral sidebands are generated as the SMOS experiences self-phase modulation in a nonlinear medium. Magnification of these temporal variation is obtained by filtering one of the sidebands. For small fluctuations, the amount of magnification obtained is proportional to (2m + 1), with m being the sideband order. This technique enhances fiber-based point sensor capabilities by bringing signals originally too small to be detected into the detection range of photodetectors.
ABSTRACT
We report high Brillouin frequency shifts of 0.08±0.02 MHz/N in a 60 cm As2Se3-Poly(methyl methacrylate) (As2Se3-PMMA) hybrid microtaper with 2 µm As2Se3 core and 100 µm PMMA cladding diameters under a transverse load. Through finite element analysis of stimulated Brillouin scattering under the influence of contact stress from the loading fixture, such shifts are in accordance with a numerical value of 0.06±0.01 MHz/N. Further numerical analysis shows that uncertainties in the Brillouin frequency shift are the result of birefringence that is 27 times stronger than what is found in standard silica fibers.
ABSTRACT
We would like to list the following errata that was found in the manuscript.
ABSTRACT
We demonstrate simultaneous generation of Stimulated Brillouin Scattering (SBS) and guided-acoustic-wave Brillouin scattering (GAWBS) from electrostriction of optical waves in a 60cm As 2Se 3-PMMA microtaper waveguide. The GAWBS in the microtaper couples with SBS through a complex energy transfer between weak Stokes and Anti-Stokes (AS) continuous waves in the presence of a high power pulsed pump wave. This results in an amplification of Stokes wave at 7.4 GHz due to modulation of the optical fiber by GAWBS at 211 MHz generated by the pump in addition to a strong Stokes peak at 7.62 GHz and a secondary peak at 7.8 GHz that are contributed by SBS for a 2µm As 2Se 3 core radius. Such strong coupling of forward and backward Brillouin scattering due to large acoustic impedance between the core and cladding in such compact, highly nonlinear fibers plays a vital role in simultaneously sensing longitudinal and trasnverse strain within the core as well as its surroundings.
ABSTRACT
A temperature-insensitive strain sensor is proposed and demonstrated based on a dual-core As2Se3-polymethyl methacrylate (PMMA) taper. Longitudinal and transverse forces on the As2Se3 cores are induced by thermal expansion/contraction of the PMMA cladding due to an order of magnitude difference between the thermal expansion coefficients of As2Se3 and PMMA. At an optimal PMMA layer thickness, the wavelength shift caused by the thermally induced forces on the refractive index of the dual-core fiber cores counterbalances that caused by the thermally induced fiber length variation leading to temperature-insensitive transmission. Temperature-insensitive strain measurement over a temperature range from 30°C to 40°C is demonstrated in a dual-core As2Se3-PMMA fiber with an As2Se3 core diameter of 0.61 µm and a PMMA cladding diameter of 34.4 µm. Thermally induced forces in hybrid fibers open the path towards the realization of novel sensors and devices that are immune to temperature fluctuations.
ABSTRACT
We report for the first time that transmission of optical pulses centered at a wavelength of 1550 nm through a tapered dual-core As2Se3-PMMA fiber inscribes an antisymmetric long-period grating. The pulse power is equally divided between even and odd modes that superpose along the dual-core fiber to form an antisymmetric intensity distribution. A permanent refractive-index change that matches the antisymmetric intensity distribution is inscribed due to photosensitivity at the pulse central wavelength. The evolution of the transmission spectrum of the dual-core fiber is experimentally measured as the accumulated time that the fiber is exposed to the pulse is increased. A theoretical model of an antisymmetric long-period grating in a dual-core fiber computationally reproduces the experimentally observed evolution of the transmission spectrum. Experimental results indicate that antisymmetric long-period gratings induce effective group-velocity matching between the even and odd modes of the dual-core fiber, and reveal for the first time that long-period gratings can lead to slow light propagation velocities.
ABSTRACT
We present a novel approach for the generation of high extinction-ratio square pulses based on self-phase modulation of sinusoidally modulated optical signals (SMOS). A SMOS in a nonlinear medium experiences self-phase modulation induced by the nonlinear Kerr effect leading to the generation of distinct sidebands. A small variation in the peak power of the SMOS leads to a large variation in the power of the sidebands. Impressing a square pulse on the SMOS and filtering a sideband component results in a higher extinction-ratio square pulse. The advantage of high extinction-ratio pulses is demonstrated by a reduced background noise level in the Rayleigh backscattering traces of a phase-OTDR vibration measurement system.
ABSTRACT
In this paper, polarization dependence of the nonlinear interaction between two sinusoidally modulated optical signals (SMOSs) in a randomly birefringent silica optical fiber is investigated analytically and experimentally. Vector analysis is performed on the nonlinear interaction between two orthogonally polarized or co-polarized SMOSs carried by identical or different laser wavelengths in a randomly birefringent silica optical fiber. Dependence of the nonlinear interaction on the polarization states of two SMOSs is investigated by analyzing the power of the first-order sideband as a result of the nonlinear interaction. The presented theoretical study reveals the polarization dependence of the nonlinear interaction between two SMOSs as confirmed by the close agreement between theoretical and experimental results. This study provides insight into the polarization dependence of operation of a Kerr phase interrogator and the tools for optimizing the performance of sensing devices based on a Kerr phase interrogator.
ABSTRACT
We propose and demonstrate sub-micron displacement sensing and sensitivity enhancement using a two-frequency interferometer and a Kerr phase-interrogator. Displacement induces phase variation on a sinusoidally modulated optical signal by changing the length of the path that either of the signal's two spectral components propagates through. A Kerr phase-interrogator converts the resulting phase variation into power variation allowing for sub-micron displacement sensing. The sensitivity of this novel displacement sensor is enhanced beyond the wavelength-limited sensitivity of the widely used Michelson interferometric displacement sensor. The proposed approach for sensitivity enhancement creates a whole new class of sensors with ultra-high sensitivity.
