Liquid-core Optical Fiber Research Articles

Cascaded four-wave mixing in liquid-core optical fibers

Ultrafast nonlinear interactions in optical fibers are commonly employed for generating light with tailored properties, with four-wave mixing (FWM) being a widely used mechanism. Existing systems mainly rely on fibers with solid glass cores, facing limitations due to a lack of tunability and susceptibility to noise. Here, fibers with fluidic cores emerge as a promising alternative for efficient FWM, offering novel functionalities and expanded parameter ranges. In this study, we investigate single and cascaded FWM in liquid-core fibers regarding spectral tunability and interplay with the Raman effect. The study relies on binary liquids used as core materials in combination with ultrashort ps-pulses and seeding. Strong side bands were observed whose spectral position could be adjusted by the liquid composition and the seed wavelength. Seeding additionally leads to higher-order side bands, which we assign to cascaded FWM. Furthermore, we explore the interaction between FWM and stimulated Raman scattering by adjusting the FWM peaks to overlap or deviate from the Raman bands through variations of the core liquid and the seed wavelength. The presented results shed light on the unique characteristics of the liquid-core fiber platform in the context of parametric nonlinear interactions, particularly regarding tunability and interaction with Raman scattering. These findings offer new possibilities for the development of light sources capable of Raman-free photon pair generation for quantum technology or for creating tunable narrowband spectra for imaging applications in life sciences.

Stimulated emission in a CuInS2/ZnS core-shell quantum-dot-doped liquid-core optical fiber.

We fabricated QD liquid-core optical fibers by doping C u I n S 2/Z n S (CIS/ZnS) core/shell QDs with cladding times of 90 and 60min, respectively, and compared and analyzed their emission properties with those of bare core C u I n S 2 QDs. For CIS/ZnS core/shell QDs (with cladding time of 90min) doped fibers, their emission transmits the longest distance in the fiber, and the emission intensity is approximately 4.73 times that of bare-core QD-doped fibers. Additionally, the fact that the full-width at half-maximum is narrowing and the spectral intensity is rapidly increasing superlinearly with excitation power indicates that stimulated emission happens in the fiber. The optical performance was compared and showed good agreement with a theoretical two-level system model for the QDs confined in an optical waveguide.

Hybridization of solitons in realistic non-instantaneous fibers

Non-local nonlinearities are an integral part of numerous physical systems. Yet, their dissipative nature makes it hard to identify and verify stable eigenstates rigorously. This work sheds new light onto the existence of temporally non-local solitons – a class of dissipative self-maintaining states in time theoretically predicted to emerge in media with a time-delayed (i.e., non-instantaneous) nonlinear response. Revisiting the non-instantaneous nonlinear Schrödinger equation introduced by Conti et al., I introduce a semi-analytical approach to reveal such non-instantaneous solitons in liquid-type media which exhibit realistic non-exponential response functions featuring both rise and decay times. I investigate the susceptibility of such states to typical perturbations, such as third-order dispersion, instantaneous Kerr nonlinearity, and causality of the material response. Empirical numeric experiments illustrate how non-instantaneous solitons may compete against and even fuse with instantaneous (Kerr-type) solitons to an inseparable hybrid solitary waves. While finding a numerical solution to define the nature of this mixed state remains a challenge, I introduce a phase balance condition and other benchmarks which will allow to identify the occurrence of such states in realistic liquid-core fiber systems in future work. The findings of this study give reason to hypothetize a new type of dissipative solitary state and provide new means to understanding information transport and system dynamics of highly non-instantaneous nonlinear systems, particularly liquid-core optical fibers and waveguides.

Solid–liquid-core optical fiber biosensor for highly sensitive and selective detection of 4-chlorophenol in water

A novel solid–liquid-core fiber-optic biosensor was fabricated for highly sensitive and selective detection of 4-chlorophenol in water. The sensor comprised horseradish peroxidase (HRP)-coated U-shaped liquid-core optical fiber (LCOF) and 4-chlorophenol permselective polymer membrane. The U-shaped LCOF was filled with ethanol suspension of SiO2 particles and the polymer membrane was composed of molecularly imprinted polymer, sulfonated polyethersulfone, and polysulfone. The morphology, composition, and surface luminous properties of the sensing region were examined. The effects of the diameter and content of SiO2 particles and temperature of 4-chlorophenol solutions on the sensitivity of the biosensors were investigated. Further, the sensitivity, selectivity, response time, and limit of detection (LOD) of the biosensors was investigated. In addition, the effects of fiber core materials on the light transmission in sensing region were investigated and a biosensor sensing model was established. The proposed sensor exhibited high selectivity for 4-chlorophenol with satisfactory sensitivity, LOD, and response time: -1.18 (µg/L)−1, 30 µg/L, and 400 s, respectively. The results are expected to aid in the development of methods for enhancing sensitivity of fiber-optic sensors and surface luminous intensity of optical fibers.

Adaptive liquid-core optical fibers for advanced soliton control

Adaptive liquid-core optical fibers for advanced soliton control

Investigation of dispersion and nonlinear characteristics of liquid core optical fiber filled with olive oil

This paper introduces a prospective material for photonic laser applications. Different high purity samples of olive oil were subjected to a spectrophotometer to determine the absorption spectra. The transmission behavior of olive oil is about (44.28) % at a wavelength of 532 nm, while it is (62.94) % at 1064 nm. Then, the nonlinear optical properties represented by the nonlinear refractive index and nonlinear absorption coefficient are determined using a highly sensitive method known as the Z-scan technique. Z-Scan experiment was performed using 1064 nm CW Nd:YAG and 532 nm SHG Nd:YVO4 lasers. n2 = 3.99×10-6W/cm2 and beta = -0.0017 m/W for 1064nm wavelength and n2 = 2.45×10-7W/cm2 and beta =-7.26×10-4 m/W for 532nm wavelength. This paper gives simulation results for a liquid-core optical fiber (LCOF) filled with olive oil. The nonlinear propagation constant and group velocity dispersion (GVD) properties are estimated. The simulation of the generation of LCOF is also obtained. The calculations show that LOCF can provide huge nonlinear parameters and a large span of slow varying GVD characteristics in the visible and infrared region, which have potential applications in optical communications and nonlinear optics. The material is olive oil which is classified as an organic compound, having good nonlinear optical properties making it a potential candidate to be for photonic applications.

Highly coherent supercontinuum generation in CS2-infiltrated single-core optical fiber

As is generally known, the broadband near-infrared supercontinuum (SC) is significant for applications such as pump-probe spectroscopy, nonlinear spectroscopy, frequency combs and nonlinear optical parametric amplification. We present the first detailed demonstration of approximate near-infrared, octave-spanning SC generation in an all-normal dispersion CS2 single-hole liquid core optical fiber (LCOF). By solving the generalized nonlinear Schrödinger equation, the SC can be calculated at different parameters. The influence of important parameters—including but not limited to the fiber core diameter, pump wavelength, pump power and pulse width—on the supercontinuum generation (SCG) of a CS2-infiltrated LCOF can be analyzed for the optimization of near-infrared SCG. The results show that at low pump power (such as P0 = 4 kW), the SC generated by using two pump wavelengths (1.55 μm and 1.95 μm) has spectral flatness, good coherence and is octave-spanning. However, the bandwidth is severely affected by the large pulse duration T0, and the coherence is seriously influenced by the pulse duration T0 > 1200 fs at the pump wavelength of 1.55 μm. The effect of pulse duration on the first-order coherence is greater at short pump wavelengths (1.55 μm) than long wavelengths (1.95 μm). Under high power conditions (such as P0 = 20 kW), the coherence of the SC generated remains good at the two-pump wavelength. This study explores an optimized scheme for obtaining a broadband SC with high coherence and fine spectral flatness suitable for specific applications.

Attenuation Coefficients of Tubular Conduits for Liquid Phase Absorbance Measurement: Shot Noise Limited Optimum Path Length.

We trace the history of liquid core waveguides (LCWs, also called liquid core optical fibers) and the role Teflon AF (TAF) has played in their development. We show that, in any shot noise limited situation, the optimum signal-to-noise ratio (S/N) occurs at a path length of 1/αa{ln[1 + 2(αa/αb)]}, approximately 2/αb under most conditions, αa and αb being the light attenuation coefficient due to the analyte and the background, respectively. The analysis shows that LCW length should be selected depending on the applicable αb value. An overly long LCW may exhibit a lower signal-to-noise ratio. Water-filled TAF-clad fused-silica (FS) tubes show the lowest attenuation across the wavelength range. Nevertheless, except at λ ≥ 600 nm, the observed αb values far exceed those reported for pure water: it appears that both impurities in the water and waveguide losses are involved. In examining the attenuation in various water-filled tubes, we find that the transmission of air-surrounded FS tubes is second only to TAF-clad FS tubes and is better than that of TAF tubes or externally mirrored FS tubes. Surprisingly, except for a window centered at ∼250 nm, light transmission in a water-filled poly(tetrafluoroethylene) (PTFE) tube is worse than in poly(ether ether ketone) (PEEK) tubing. Light transmission in PTFE tubes improves with increasing wall thickness.

A Vertical Flow Method for Sensitive Raman Protein Measurement in Aqueous Solutions.

Raman scattering is intrinsically faint. Raman spectroscopy would be more valuable with improvements in the signal detection efficiency. To improve the signal detection efficiency, we propose a vertical flow method, which is a derivative of the liquid core optical fiber technique employed for sensitive Raman signal detection. In the vertical flow method, the sample solution flows from a pinhole to prepare a liquid column wrapped with air and uses total reflection at the sample-air interface to confine the excitation beam and Raman signal. The Raman signal emerges from the pinhole for efficient collection. This method enhanced Raman signal intensity by up to 90 times. When measuring a bovine serum albumin aqueous solution, the limit of detection (LOD) was 0.029 ± 0.003 mg mL-1 (0.44 ± 0.05 μM). We discuss the signal enhancement factor dependence on several parameters in the vertical flow method. Our method provides a simple technique to improve Raman spectroscopy sensitivity using universal materials.

Spontaneous forward Brillouin scattering in carbon disulfide

In recent years, guided acoustic wave Brillouin scattering has become an important tool in photonics, serving as the basis for everything from new forms of information processing to silicon lasers. Due to low losses and long interaction lengths, fiber optic systems offer an intriguing platform to harness these guided-wave light-sound interactions. However, within typical fiber optic systems these interactions are exceedingly weak---requiring complex microstucturing to yield appreciable light-sound coupling. Here, we enhance this light-sound coupling by using a ${\mathrm{CS}}_{2}$-filled liquid core optical fiber. Owing to tight confinement of the optical and acoustic modes within the fiber core, as well as the large electrostrictive response of ${\mathrm{CS}}_{2}$, this system yields an unprecedented forward Brillouin gain for a fiber optic system. To demonstrate this physics, we measure multipeaked spontaneous forward Brillouin scattering power spectra, yielding information about the fiber geometry, material properties, and acousto-optic coupling strength. To interpret these data, we simulate the spontaneous Brillouin scattering power spectrum for this fiber system. These results reveal that hybridized acoustic excitations within the fiber core and cladding produce this characteristic multipeaked power spectrum. In the future, the large forward Brillouin coupling, long interaction lengths, and low losses of liquid-core fibers may enable new forms of distributed sensing, lasers with customizable emission, and physics including continuum optomechanical cooling.

Long-term stable supercontinuum generation and watt-level transmission in liquid-core optical fibers.

Due to their unique properties such as transparency, tunability, nonlinearity, and dispersion flexibility, liquid-core fibers represent an important approach for future coherent mid-infrared light sources. However, the damage thresholds of these fibers are largely unexplored. Here we report on the generation of soliton-based supercontinua in carbon disulfide (CS2) liquid-core fibers at average power levels as high as 0.5W operating stably for a long term (>70 h) without any kind of degradation or damage. Additionally, we also show stable high-power pulse transmission through liquid-core fibers exceeding 1W of output average power for both CS2 and tetrachloroethylene as core materials.

Effect of Temperature on Supercontinuum Generation in CS2-Core Optical Fiber

We investigate the effect of temperature on the supercontinuum generation of CS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> single-hole liquid core optical fiber (LCOF). By solving the generalized nonlinear Schrödinger equation, the effect of the temperature on spectral broadening, coherence, soliton effects, and modulation instability was analyzed. The results turn out that the supercontinuum generation by pumping the LCOP with 1.95-μm laser is robust over a broad temperature range (15-45 °C). The optimal temperature to generate the broadest spectrum of SC via LCOF is 15 °C, which is independent on the pump power. The coherence of the generated SC is independent on the temperature at low pump power (such as P0 = 4 kW), while at a high pump power (such as P 0 = 40 kW) the coherence of the long wave (around 2600 nm) tends to be deteriorated at low temperature (e.g., 10 °C). The underlying reason for such phenomenon has been discussed as well.

Soliton-like pulse propagation in a normal dispersive liquid-core optical fiber.

Soliton-like pulse propagation, where the pulse intensity profile is conserved, except for its trailing edge, is found in a normal dispersive CS2-core optical fiber by a computer simulation. During propagation, it is found that the chirp of a pulse arising from the delayed nonlinear response compensates for the chirp arising from the second-order dispersion near the pulse peak position and the pulse experiences a negative frequency shift and a negative temporal shift. The effects of the instantaneous nonlinear response on the soliton-like pulse propagation are also evaluated, and the calculated spectrum agreed well with the experimental spectrum.

Supercontinuum generation in organic liquid-liquid core-cladding photonic crystal fiber in visible and near-infrared regions

In this paper, we propose a liquid core-cladding photonic crystal fiber (PCF), which is engineered with different available organic optofluidics, to generate supercontinuum in the visible and near-infrared regimes by using the symmetrized split-step Fourier method. Simulations reveal that in response to launching 50 fs input pulses of 10 kW peak power, centered about λ0=1032 and 1560 nm, into a 10 mm long liquid core-cladding PCF, maximum 2 μm supercontinua from 500 to 2500 nm can be achieved. Our numerical study is important for the new field of visible and near-IR supercontinuum generation in liquid-core optical fibers.

Supercontinuum generation by stimulated Raman–Kerr scattering in a liquid-core optical fiber

We investigate nonlinear picosecond pulse propagation at 532 nm in an optical fiber capillary filled with toluene liquid and demonstrate supercontinuum generation from 550 nm up to 870 nm. Although it operates in the strong normal dispersion regime, we show that supercontinuum light is generated through the combined effects of multiple cascaded Raman scattering and stimulated Raman–Kerr scattering, which enables a large spectral broadening of the Raman lines owing to the strongly anisotropic orientational relaxation of liquid molecules. We further present numerical simulations of the nonlinear Schrodinger equation, including the Raman scattering, to show the spectro-temporal dynamics.

Programmable long-period grating in a liquid core optical fiber.

A programmable fiber long-period grating (LPG) is experimentally demonstrated in a liquid core optical fiber with a low insertion loss. The LPG is dynamically formed by a temperature gradient in real time through a micro-heater array. The transmission spectrum of the LPG can be completely reconfigured by digitally changing the grating period, index contrast, length, and design. The phase shift inside the LPG can also be readily defined to enable advanced spectrum shaping. Owing to the high thermo-optic coefficient of the liquid core, it is possible to achieve high coupling efficiencies with driving powers as low as a few tens of milliwatts. The proposed thermo-programmable device provides a potential design solution for dynamic all-fiber optics components.

Comprehensive size effect on PbSe quantum dot-doped liquid-core optical fiber

We have theoretically studied a comprehensive size effect on the spectra of PbSe quantum dot (QD)-doped liquid-core optical fiber, including PbSe QD's particle size and particle number effect, and fiber length and fiber diameter effect. The doping concentration, pump intensity and wavelength were fixed at proper values for comparison. The red shift of emission spectral peak increased with QD diameter, QD number, fiber length and fiber diameter, and reached up to saturation with increasingly QD number which was explained in detail. The evolutions of spectral intensity with the four size parameters were gained, and the related “optimal” (under the certain other parameters) fiber length, diameter and QD number were observed for PbSe QDs of different size as the dopant. Furthermore, each kind of the “optimal” value changed with the other three size parameters. These four size parameters restricted each other, and affected the spectral features together. The calculating results fitted well to the experimental data. This research might be a theoretical basis in the design of optical fiber-based device.

Reduced reabsorption and enhanced propagation induced by large Stokes shift in quantum dot-filled optical fiber

With tunable emission wavelength, high photoluminescence quantum yield, and broad absorption, colloidal quantum dots are attractive for the application in optical fiber as dopants. However, most of the quantum dots have a large overlap between their absorption and photoluminescence spectra, resulting in reabsorption loss which hinders the realization of long-distance waveguides. Therefore, ZnCuInS/ZnSe/ZnS quantum dots with large Stokes shift were proposed to fabricate a liquid-core optical fiber in this work. In this work, ZnCuInS/ZnSe/ZnS QDs with an average size of 3.3 nm were synthesized and the optical properties of the QD-filled fiber were also investigated as a function of fiber length and doping concentration. Compared to the control sample filled with CdSe/CdS/ZnS quantum dots, the ZnCuInS/ZnSe/ZnS quantum dot-based waveguides showed reduced reabsorption and enhanced signal propagation, which demonstrates great potential of large Stokes-shift quantum dots in optical waveguide devices. A reduced reabsorption and enhanced propagation of ZnCuInS/ZnSe/ZnS QDs-doped liquid-core optical fiber was achieved due to the large Stokes shift.

Stimulated emission and optical gain in PbSe quantum dot-doped liquid-core optical fiber based on multi-exciton state

We studied the properties of stimulated emission and optical gain of 4.4nm PbSe quantum dot (QD)-doped liquid-core optical fiber based on multi-exciton state under strong pumping condition as a function of QD solution concentration, fiber length and pump power in order to establish the conditions to maximize stimulated emission intensities and optical gain. Auger recombination lifetime and internal quantum efficiency (IQE) were introduced in the multi-exciton model for calculation. Shifts of the spectral peak position were observed and explained in detail. The narrowing spectral half band width and the super linear intensity dependence of the output power with the pump accounted for the generation of stimulated emission. The maximal optical gain of ~30dB was obtained which was larger than that of the single-exciton model under the same fiber parameters. Our interesting results might be useful in the design of optical fiber-based amplifiers and lasers.

High-quality and controllable hollow beam generation by a liquid-core optical fiber

We present the generation of high-quality and beam parameter controllable hollow beams with a liquid-core optical fiber (LCOF) filled with CS2 by adjusting off-axis orientation and LCOF structure. The dependences of the dark spot size (DSS), ring beam width (RBW) and transverse field intensity distribution of the hollow beam on the incidence angle of laser beam, the length and core diameter of the LCOF are investigated experimentally, and analyzed theoretically by using Zemax simulation. The results show that the quality of the hollow beam is related to the times of total internal reflection of skew rays in the fiber core; the DSS and RBW are closely controlled by the incidence angle and core diameter. High-quality thin-ring hollow beam is achieved experimentally by the LCOF of the core diameter of 100μm and the length of 1m with the incidence angle of less than 10°. The hollow beam generated has high stability, and is weakly dependent on the bending of the LCOF.