Solid-core Fiber Research Articles

Harmonic Generation Via χ3 Intermodal Phase Matching in Microfibers

Intermodally phase-matched up- and down-conversion processes based on third-order non-linearity have been proposed to efficiently generate light in the ultraviolet (UV) and mid-infrared (IR) wavelength regions in solid-core silica optical fibers and optical microfibers. Waveguide parameters and practical considerations required for optimum conversion are studied.

100 Gbit/s WDM transmission at 2 µm: transmission studies in both low-loss hollow core photonic bandgap fiber and solid core fiber.

We show for the first time 100 Gbit/s total capacity at 2 µm waveband, using 4 × 9.3 Gbit/s 4-ASK Fast-OFDM direct modulation and 4 × 15.7 Gbit/s NRZ-OOK external modulation, spanning a 36.3 nm wide wavelength range. WDM transmission was successfully demonstrated over 1.15 km of low-loss hollow core photonic bandgap fiber (HC-PBGF) and over 1 km of solid core fiber (SCF). We conclude that the OSNR penalty associated with the SCF is minimal, while a ~1-2 dB penalty was observed after the HC-PBGF probably due to mode coupling to higher-order modes.

Reflectivity and Bandwidth Modulation of Fiber Bragg Gratings in a Suspended Core Fiber by Tunable Acoustic Waves

The acousto-optic modulation of fiber Bragg gratings in a four-hole suspended core fiber is experimentally demonstrated. Strong modulations with a reflectivity amplitude decrease by up to 67% and a 57% bandwidth increase in the Bragg resonance are obtained for gratings of 0.26- and 1-nm 3-dB bandwidths, respectively. The reduction of the required acoustic power for achieving the acousto-optic modulation compared to conventional solid-core single-mode fibers points to more efficient modulator devices in suspended core fibers.

Splice Loss Optimization of a Photonic Bandgap Fiber via a High V-Number Fiber

We report on a low-loss fusion splice between a hollow-core photonic bandgap fiber (NKT's HC-1550-02) and a conventional single mode fiber (Corning's SMF28) using a high V-number intermediate fiber. Compared to the commonly used fiber postprocessing techniques, the intermediate fiber method offers a new adjusting degree of freedom of V-numbers, enabling the reduction of coupling loss to 0.35 dB. Experimentally, a low fusion splice loss of 0.63 dB has been achieved between Nufern's SM1950 (V =2.836) and HC-1550-02 fiber, resulting in an overall insertion loss of 0.73 dB from SMF28 to HC-1550-02 fiber. We believe this is the lowest splice loss value reported for HC-1550-02 fiber using an arc splicer, on the aspect of both using a step-index solid-core fiber (0.63 dB) and a SMF28 fiber (0.73 dB).

Review of diverse optical fibers used in biomedical research and clinical practice.

Optical fiber technology has significantly bolstered the growth of photonics applications in basic life sciences research and in biomedical diagnosis, therapy, monitoring, and surgery. The unique operational characteristics of diverse fibers have been exploited to realize advanced biomedical functions in areas such as illumination, imaging, minimally invasive surgery, tissue ablation, biological sensing, and tissue diagnosis. This review paper provides the necessary background to understand how optical fibers function, to describe the various categories of available fibers, and to illustrate how specific fibers are used for selected biomedical photonics applications. Research articles and vendor data sheets were consulted to describe the operational characteristics of conventional and specialty multimode and single-mode solid-core fibers, double-clad fibers, hard-clad silica fibers, conventional hollow-core fibers, photonic crystal fibers, polymer optical fibers, side-emitting and side-firing fibers, middle-infrared fibers, and optical fiber bundles. Representative applications from the recent literature illustrate how various fibers can be utilized in a wide range of biomedical disciplines. In addition to helping researchers refine current experimental setups, the material in this review paper will help conceptualize and develop emerging optical fiber-based diagnostic and analysis tools.

Birefringence, dispersion and loss properties for PCFs with rectangular air-holes

In this paper, the solid-core fiber designs constructed by omitting one air-hole at the center of square-lattice silica photonic crystals with rectangular shaped air-holes have been investigated theoretically and their characteristics have been compared with those of square ones. Using commercial simulation software, birefringence, dispersion, numerical aperture and confinement loss of the fundamental modes are analyzed.

Damage-free single-mode transmission of deep-UV light in hollow-core PCF.

Transmission of UV light with high beam quality and pointing stability is desirable for many experiments in atomic, molecular and optical physics. In particular, laser cooling and coherent manipulation of trapped ions with transitions in the UV require stable, single-mode light delivery. Transmitting even ~2 mW CW light at 280 nm through silica solid-core fibers has previously been found to cause transmission degradation after just a few hours due to optical damage. We show that photonic crystal fiber of the kagomé type can be used for effectively single-mode transmission with acceptable loss and bending sensitivity. No transmission degradation was observed even after >100 hours of operation with 15 mW CW input power. In addition it is shown that implementation of the fiber in a trapped ion experiment increases the coherence time of the internal state transfer due to an increase in beam pointing stability.

Thermal Sensitivity of the Birefringence of Air-Core Fibers and Implications for the RFOG

We present an analytical model of the drift in the resonant fiber optic gyroscope (RFOG) caused by thermally induced changes in the fiber beat length ${\rm L_{B}(T)}$ and find that the relevant metric for thermal-drift performance is not ${\rm dL_{B}/dT}$ but rather ${\rm(1/L_{B}^{2})dL_{B}/dT}$ . This disfavors conventional solid-core PM fibers for which ${\rm L_{B}}$ is very short and ${\rm dL_{B}/dT}$ is large. Because the air core of hollow-core fibers does not support the strains associated with differential thermal expansion in conventional PM fibers, the birefringence of hollow-core fibers should be more thermally stable than conventional fibers with a correspondingly smaller ${\rm (1/L_{B}^{2})dL_{B}/dT}$ . To identify the relative merits of different types of fibers in the RFOG, we measure ${\rm (1/L_{B}^{2})dL_{B}/dT}$ for a variety of air-core and solid-core fibers by recording the shift in the resonance frequencies of an all-fiber ring resonator when its temperature is changed. We find that the PM air-core fiber offers a 270-fold improvement in ${\rm (1/L_{B}^{2})dL_{B}/dT}$ coefficient compared to a Panda fiber.

First Demonstration of a 2-<inline-formula> <tex-math notation="TeX">$\mu{\rm m}$ </tex-math> </inline-formula> OTDR and Its Use in Photonic Bandgap <inline-formula> <tex-math notation="TeX">${\rm CO}_{2}$ </tex-math> </inline-formula> Sensing Fiber

We report the first demonstration of an optical time domain reflectometer (OTDR) designed for the emerging wavelength band of 2 μm. A dynamic range of 30 dB with a spatial resolution of 5 m is achieved. The characterization of three different fibers using the 2-μm OTDR is described. Two solid core fibers (SMF with a cutoff at 1.7 μm and SMF 28e) have been probed to ascertain their transmission loss values, while measurements in a 19-cell hollow core photonic bandgap fiber show the presence of carbon dioxide gas in the fiber core and allow the contribution that it makes to the fiber loss to be quantified.

Fiber-Optic Temperature Sensor Using a Fabry–Pérot Cavity Filled With Gas of Variable Pressure

We report a high-temperature fiber-optic sensor based on measuring the spectral fringes of a Fabry-Perot (FP) cavity on a microstructure fiber (MF) when the gas pressure in the cavity is varied through the holes in the MF. Theoretical analysis shows that the absolute temperature can be deduced from the slope of the spectral shift versus pressure curve, which requires no calibration and is insensitive to the FP cavity length variations. For demonstration, we fabricated a miniature sensor whose FP cavity is formed by sandwiching a fuse-silica tube between a side-hole MF and a solid-core fiber. Using the holes in the MF as gas channels, the pressure in the FP cavity is controlled. The sensor was tested for operation above 1000°C. Strain-insensitive temperature measurement was demonstrated at ambient temperature for a strain range up to 3600 μe.

Divided-pulse nonlinear compression

We report on the nonlinear pulse compression of temporally divided pulses, which is presented in a proof-of-principle experiment. A single 320 fs pulse is divided into four replicas, spectrally broadened in a solid-core fiber, and subsequently recombined. This approach makes it possible to reduce the nonlinearities in the fiber and therefore to use total input peak power of about 13.3 MW, which is more than three times higher than the self-focusing threshold. Finally, the combined output pulse could be compressed to sub-100 fs pulse duration. This general and universal approach holds promise for overcoming fundamental limitations of the pulse peak power that lead to destruction of the fiber or ionization limitations in high-energy hollow-core compression.

Characterization of Silver Halide Fiber Optics and Hollow Silica Waveguides for Use in the Construction of a Mid-Infrared Attenuated Total Reflection Fourier Transform Infrared (ATR FT-IR) Spectroscopy Probe

Advances in fiber optic materials have allowed for the construction of fibers and waveguides capable of transmitting infrared radiation. An investigation of the transmission characteristics associated with two commonly used types of infrared-transmitting fibers/waveguides for prospective use in a fiber/waveguide-coupled attenuated total internal reflection (ATR) probe was performed. Characterization of silver halide polycrystalline fiber optics and hollow silica waveguides was done on the basis of the transmission of infrared light using a conventional fiber optic coupling accessory and an infrared microscope. Using the fiber optic coupling accessory, the average percent transmission for three silver halide fibers was 18.1 ± 6.1% relative to a benchtop reflection accessory. The average transmission for two hollow waveguides (HWGs) using the coupling accessory was 8.0 ± 0.3%. (Uncertainties in the relative percent transmission represent the standard deviations.) Reduced transmission observed for the HWGs was attributed to the high numerical aperture of the coupling accessory. Characterization of the fibers/waveguides using a zinc selenide lens objective on an infrared microscope indicated 24.1 ± 7.2% of the initial light input into the silver halide fibers was transmitted. Percent transmission obtained for the HWGs was 98.7 ± 0.1%. Increased transmission using the HWGs resulted from the absence or minimization of insertion and scattering losses due to the hollow air core and a better-matched numerical aperture. The effect of bending on the transmission characteristics of the fibers/waveguides was also investigated. Significant deviations in the transmission of infrared light by the solid-core silver halide fibers were observed for various bending angles. Percent transmission greater than 98% was consistently observed for the HWGs at the bending angles. The combined benefits of high percent transmission, reproducible instrument responses, and increased bending tolerance indicated HWGs should be preferred in the construction of a fiber/waveguide-coupled ATR probe.

Method for rapid multidiameter single-fiber reflectance and fluorescence spectroscopy through a fiber bundle

We have recently demonstrated a means for quantifying the absorption and scattering properties of biological tissue through multidiameter single-fiber reflectance (MDSFR) spectroscopy. These measurements can be used to correct single-fiber fluorescence (SFF) spectra for the influence of optical properties, enabling quantification of intrinsic fluorescence. In our previous work, we have used a series of pinholes to show that selective illumination and light collection using a coherent fiber bundle can simulate a single solid-core optical fiber with variable diameter for the purposes of MDSFR spectroscopy. Here, we describe the construction and validation of a clinical MDSFR/SFF spectroscopy system that avoids the limitations encountered with pinholes and free-space optics. During one measurement, the new system acquires reflectance spectra at the effective diameters of 200, 600, and 1000 μm, and a fluorescence spectrum at an effective diameter of 1000 μm. From these spectra, we measure the absolute absorption coefficient, μ(a), reduced scattering coefficient, μ'(s'), phase function parameter, γ, and intrinsic fluorescence, Qμ(a,x)(f), across the measured spectrum. We validate the system using Intralipid- and polystyrene sphere-based scattering phantoms, with and without the addition of the absorber Evans Blue. Finally, we demonstrate the combined MDSFR/SFF of phantoms with varying concentrations of Intralipid and fluorescein, wherein the scattering properties are measured by MDSFR and used to correct the SFF spectrum for accurate quantification of Qμ(a,x)(f).

Lead Chalcogenide Quantum Dot‐Doped Glasses for Photonic Devices

Tunable absorption and photoluminescence (PL) of lead chalcogenide quantum dots (QDs) doped in glasses due to the quantum confinements effect have been actively investigated for application as saturable absorbers, laser sources, and fiber‐optic amplifiers. Optical properties of QDs have been carefully monitored by controlling their sizes through heat treatment and rare‐earth ion doping. Two‐ and three‐dimensional precipitation of lead chalcogenide QDs were also realized using silver ion exchange and femtosecond laser irradiation in combination with thermal treatment. Prototypes of microstructured single‐mode fibers and tapered fiber amplifiers containing QDs proved potentials of these materials for fiber‐optic amplifiers application. Further research works on QD‐doped solid core fibers, surface passivation of quantum dots and their application for the mid‐infrared optical devices are necessary.

Low-loss hollow-core fibers with improved single-modedness

Hollow-core fibers (HCFs) are a revolution in light guidance with enormous potential. They promise lower loss than any other waveguide, but have not yet achieved this potential because of a tradeoff between loss and single-moded operation. This paper demonstrates progress on a strategy to beat this tradeoff: we measure the first hollow-core fiber employing Perturbed Resonance for Improved Single Modedness (PRISM), where unwanted modes are robustly stripped away. The fiber has fundamental-mode loss of 7.5 dB/km, while other modes of the 19-lattice-cell core see loss >3000 dB/km. This level of single-modedness is far better than previous 19-cell or 7-cell HCFs, and even comparable to some commercial solid-core fibers. Modeling indicates this measured loss can be improved. By breaking the connection between core size and single-modedness, this first PRISM demonstration opens a new path towards achieving the low-loss potential of HCFs.

Suspended hollow core fiber for terahertz wave guiding

A polymer fiber with a suspended hollow core is proposed as a low-loss terahertz (THz) waveguide. THz guiding mode is formed due to the suspended hollow core and the air inside the suspended hollow core fiber (SHCF). The low-loss property of the SHCF is achieved due to the fact that the air part of the suspended hollow core traps a large portion of mode power of the THz wave. By using a finite-element method, both the SHCF and the suspended solid core fiber (SSCF) are comparatively investigated. The effective indices, mode area, power fraction, relative absorption loss, and mode profile of the SHCF and the SSCF are presented. Simulation results show that the proposed SHCF exhibits better loss property than the SSCF.

Highly birefringent polymer terahertz fiber with honeycomb cladding

Two highly birefringent polymer terahertz (THz) fibers were proposed in this paper, which were formed with honeycomb cladding and some elliptical air holes in the fiber core. The losses and mode birefringence of two different fibers are investigated by finite-different time-domain method. The results show that fiber 2 can achieve both high birefringence (larger than 0.022) and low confinement loss (0.01dB/m) in a wide THz frequency range. Moreover, compared with a round solid-core fiber, guiding loss of the THz fiber caused by polymer material absorption can be reduced effectively as a part of the mode power is trapped in the air holes.

A theoretical study of large solid-core square-lattice silica photonic crystal fibers with square air-holes

Large solid-core fiber designs constructed by omitting nine air-holes at the center of square-lattice silica photonic crystals with square shaped air-holes have been investigated theoretically and their characteristics have been compared with those of circular ones. The V-parameter was estimated by using the step-index fiber approximation and the effective mode area calculated for the structures considered. To estimate the effective core radius of the structure with square shaped air-holes an approximation based on the air-filling fraction was used. The dispersion and confinement losses were also obtained and compared for both of the structures.

Use of a coherent fiber bundle for multi-diameter single fiber reflectance spectroscopy

Multi-diameter single fiber reflectance (MDSFR) spectroscopy enables quantitative measurement of tissue optical properties, including the reduced scattering coefficient and the phase function parameter γ. However, the accuracy and speed of the procedure are currently limited by the need for co-localized measurements using multiple fiber optic probes with different fiber diameters. This study demonstrates the use of a coherent fiber bundle acting as a single fiber with a variable diameter for the purposes of MDSFR spectroscopy. Using Intralipid optical phantoms with reduced scattering coefficients between 0.24 and 3 mm−1, we find that the spectral reflectance and effective path lengths measured by the fiber bundle (NA = 0.40) are equivalent to those measured by single solid-core fibers (NA = 0.22) for fiber diameters between 0.4 and 1.0 mm (r ≥ 0.997). This one-to-one correlation may hold for a 0.2 mm fiber diameter as well (r = 0.816); however, the experimental system used in this study suffers from a low signal-to-noise for small dimensionless reduced scattering coefficients due to spurious back reflections within the experimental system. Based on these results, the coherent fiber bundle is suitable for use as a variable-diameter fiber in clinical MDSFR quantification of tissue optical properties.

Higher-order-mode fiber optimized for energetic soliton propagation

We describe the design optimization of a higher-order-mode (HOM) fiber for energetic soliton propagation at wavelengths below 1300 nm. A new HOM fiber is fabricated according to our design criteria. The HOM fiber is pumped at 1045 nm by an energetic femtosecond fiber laser. The soliton self-frequency shift process shifts the center wavelength of the soliton to 1085 nm. The soliton has a temporal duration of 216 fs and a pulse energy of 6.3 nJ. The demonstrated pulse energy is approximately six times higher than the previous record in a solid core fiber at wavelengths below 1300 nm.