Fiber Numerical Aperture Research Articles

High numerical aperture large-core photonic crystal fiber for a broadband infrared transmission

In this paper we present a large mode area photonic crystal fiber made of the heavy metal oxide glass CS-740, dedicated for a broadband light guidance in the visible, near- and mid-infrared regions of wavelengths from 0.4 to 4.7μm. The fiber is effectively multi-mode in the considered wavelength range. It is composed of a ring of air-holes surrounding the core, with a high linear filling factor of 0.97. The fiber was made using a standard stack-and-draw technique. Each hole has a size of approx. 2.5×3.0μm and diameter of core is 80μm. Fiber attenuation is below 3dB/m in the 0.9–1.7μm wavelength range, while at 4.4μm (mid-IR) it is approx. 5dB/cm. Bending loss at the 1.55μm wavelength is 0.45dB per loop of 8mm radius. Fiber numerical aperture is 0.53 at 1.55μm. The effective mode area of the fundamental mode is approx. 2400μm2 in the wavelength range of 0.8–1.7μm. We present a proof-of-concept demonstration that our large core photonic crystal fiber is able to efficiently collect light directly from a mid-IR quantum cascade laser without use of additional optics and can be used for pigtailing mid-IR sources and detectors.

Optimization of beam transformation system for laser-diode bars.

An optimized beam transformation system (BTS) is proposed to improve the beam quality of laser-diode bars. Through this optimized design, the deterioration of beam quality after the BTS can be significantly reduced. Both the simulation and experimental results demonstrate that the optimized system enables the beam quality of a mini-bar (9 emitters) approximately equal to 5.0 mm × 3.6 mrad in the fast-axis and slow-axis. After beam shaping by the optimized BTS, the laser-diode beam can be coupled into a 100 μm core, 0.15 numerical aperture (NA) fiber with an output power of over 100 W and an electric-optical efficiency of 46.8%.

Theoretical modeling and investigations of AZO coated LMR based fiber optic tapered tip sensor utilizing an additional TiO2 layer for sensitivity enhancement

This article presents a theoretical model for lossy mode resonance (LMR) based fiber optic (FO) tapered tip sensor coated with aluminum doped zinc oxide (AZO) and titanium dioxide (TiO2) layer. In LMR based FO tapered tip sensor, sensing signal is measured in terms of retro-reflecting spectra whereas in conventional LMR sensors it is measured in terms of the transmittance. A major advantage of the proposed sensor is that it requires small sample volume. Also its compact tapered tip design makes it more suitable for point sensing. In this work, for the first time, a detailed analysis of AZO coated LMR based FO tapered tip sensor has been carried out. Additionally, the combined effect of AZO and TiO2 thickness ratio along with the fiber parameters such as fiber tip radius, tip length and numerical aperture on the reflectance spectrum has been investigated. The proposed geometry along with appropriate combination of various fiber parameters and the thin films thickness ratio provides approximately sixfold increase in the sensitivity compared to the conventional tapered LMR sensors. Sensitivity of the proposed sensor has also been compared with straight fiber geometry. Refractive index (RI) sensitivity of the proposed sensor is in the range of 1500–9000nm/RIU.

Direct and Efficient Optical Coupling Into Plasmonic Integrated Circuits From Optical Fibers

We demonstrate direct and efficient optical coupling from an optical fiber into plasmonic integrated circuits (PICs) at 1.55- $\mu \text{m}$ wavelength using ultra-compact plasmonic dipole nanoantennas. The PICs consist of slot waveguides with single-chip integrated Yagi–Uda antennas. To improve the optical coupling efficiency from optical fibers, we add 500-nm-thick Su-8 film with a high refractive index as the top cladding. We experimentally achieve 8.6% couple-in efficiency from a high numerical aperture fiber to the plasmonic slot waveguide and 46% total couple-out efficiency from the plasmonic slot waveguide. We also quantitatively characterize the dependence of the couple-in efficiency to the spot size of the incident light.

Spectral Compression of a Dispersion-Managed Mode-Locked Tm:Fiber Laser at $1.9~\mu \text{m}$

We demonstrate self-phase modulation (SPM)-based spectral compression of the pulse from an all-fiber dispersion-managed Thulium (Tm)-doped fiber laser at 1.9 $\mu \text{m}$ . The laser consists of ultrahigh numerical aperture fiber for the dispersion management to avoid soliton mode-locking and is mode-locked by nonlinear amplified loop mirror. Spectrum of the pulse is compressed from 7 to 3 nm by SPM in an anomalous dispersion core-pumped Tm-doped fiber, together with a passive fiber. Numerical simulations agree well with the experimental results. Limitation of high-efficiency spectral compression due to soliton evolution in the anomalous dispersion fibers is also discussed.

OptogenSIM: a 3D Monte Carlo simulation platform for light delivery design in optogenetics.

Optimizing light delivery for optogenetics is critical in order to accurately stimulate the neurons of interest while reducing nonspecific effects such as tissue heating or photodamage. Light distribution is typically predicted using the assumption of tissue homogeneity, which oversimplifies light transport in heterogeneous brain. Here, we present an open-source 3D simulation platform, OptogenSIM, which eliminates this assumption. This platform integrates a voxel-based 3D Monte Carlo model, generic optical property models of brain tissues, and a well-defined 3D mouse brain tissue atlas. The application of this platform in brain data models demonstrates that brain heterogeneity has moderate to significant impact depending on application conditions. Estimated light density contours can show the region of any specified power density in the 3D brain space and thus can help optimize the light delivery settings, such as the optical fiber position, fiber diameter, fiber numerical aperture, light wavelength and power. OptogenSIM is freely available and can be easily adapted to incorporate additional brain atlases.

Digital confocal microscopy through a multimode fiber.

Acquiring high-contrast optical images deep inside biological tissues is still a challenging problem. Confocal microscopy is an important tool for biomedical imaging since it improves image quality by rejecting background signals. However, it suffers from low sensitivity in deep tissues due to light scattering. Recently, multimode fibers have provided a new paradigm for minimally invasive endoscopic imaging by controlling light propagation through them. Here we introduce a combined imaging technique where confocal images are acquired through a multimode fiber. We achieve this by digitally engineering the excitation wavefront and then applying a virtual digital pinhole on the collected signal. In this way, we are able to acquire images through the fiber with significantly increased contrast. With a fiber of numerical aperture 0.22, we achieve a lateral resolution of 1.5µm, and an axial resolution of 12.7µm. The point-scanning rate is currently limited by our spatial light modulator (20Hz).

Dissipative Soliton Resonance in a Wavelength-Tunable Thulium-Doped Fiber Laser With Net-Normal Dispersion

We demonstrate an all-fiber figure-eight mode-locked thulium-doped fiber laser with a wide tunable range in both pulsewidth and wavelength. A 45-m-long ultrahigh numerical aperture fiber is used to manage the cavity dispersion, and the net cavity dispersion is calculated to be 0.8585 ps 2 at 1900 nm. With net-normal dispersion, the experimental laser, operating in a dissipative soliton resonance region, generates stable rectangular pulses. The pulsewidth varies from 480 ps to 6.19 ns with the increasing pump power, and its center wavelength has a 28.95-nm tunable range (from 1940.22 to 1969.17 nm) by properly adjusting the polarization controllers. The maximum of average output power and pulse energy is 60.73 mW and 19.51 nJ, respectively. The rectangular pulses have a clamped peak power of about 3.16 W. The wavelength-tunable fiber laser with high-energy output operating at 2 μm has great potential in various application fields.

High brightness beam shaping and fiber coupling of laser-diode bars.

The strong beam quality mismatch in the fast and slow axes of laser-diode bars requires a significant beam shaping method to reach the parameters needed for fiber coupling. An effective solution to this problem is proposed that is based on a right-angle prism array and a distributed cylinder-lens stack. Coupling 12 mini-bars into a standard 100μm core diameter and 0.15 numerical aperture fiber is achieved, and the output power can reach 400W. Using this technique, production of compact and high brightness fiber-coupled laser-diode modules is possible.

Properties of tellurite-based high numerical aperture fibers with diamagnetic core and cladding glasses

The fabrication and characterization of a tellurite based high numerical aperture magneto-optical glass fiber are presented. Different glass groups were investigated in terms of their structural, thermal, mechanical, optical and magnetic performance as a core/cladding pair and their refractive index contributions to the numerical aperture. A well matched core/cladding pair was selected: 75%TeO2–20%ZnO–5%Na2O (mol.%) for core glass, 42%TeO2–18%ZnO–15%K2O–20%Ge2O–5SiO2 (mol.%) for cladding glass. The fibers have a good cross section and a numerical aperture as high as 0.9. The Verdet constant of this high numerical aperture fiber was close to that of bulk glass.

Stretched-pulse and solitonic operation of an all-fiber thulium/holmium-doped fiber laser

Stretched-pulse operation of a mode-locked thulium/holmium-doped fiber laser has been demonstrated using a high numerical aperture (NA) fiber inside the laser cavity for intracavity dispersion compensation. The high NA fiber exhibits normal group-velocity dispersion allowing for the net-cavity dispersion to be positive. We experimentally investigate the laser dynamics as a function of the net-cavity dispersion, observing the transition from stretched-pulse to solitonic operation as the length of high NA fiber was reduced. In the stretched-pulse regime the laser produced pulses with a bandwidth of 30nm and duration of 450fs. Methods for compensating the third-order dispersion using high NA fibers are proposed.

Geometrical parameter analysis of a high-sensitivity fiber optic angular displacement sensor.

In this work, we present an analysis of the influence of geometrical parameters on the sensitivity and linear range of a fiber optic angular displacement sensor, through computational simulations and experiments. The geometrical parameters analyzed are the lens focal length, the gap between fibers, the fiber cladding radii, the emitting fiber critical angle (or, equivalently, the emitting fiber numerical aperture), and the standoff distance (distance between the lens and the reflective surface). Besides, we analyze the sensor sensitivity regarding any spurious linear displacement. The simulation and experimental results show that the parameters that play the most important roles are the emitting fiber core radius, the lens focal length, and the light coupling efficiency, whereas the remaining parameters have little influence on the sensor characteristics.

Phosphate Yb3+ doped air-cladding photonic crystal fibers for laser applications

We present two single-mode photonic crystal fibers with air-cladding for laser applications. We developed PCF structures created by replacing respectively one and seven air-holes with 6%mol ytterbium doped glass. With increasing mode area you can increase power level and shorten fiber length in laser construction. We achieved 53 and 19 cm optimum length fiber lasers with 4.7 W and 12.4 W respectively maximum output power. The slope efficiency of the lasers were 15.6% and 42.4%. The use of air-cladding brought very high numerical aperture of pump waveguides in both laser fibers resulting with the value of 0.72 and 0.91 respectively. Full Text: PDF References J. Limpert, O. Schmidt, J. Rothhartd, F. Röser, T. Schreiber, A. Tunnermann, "Extended single-mode photonic crystal fiber lasers", Opt. Express 14, 2715 (2006), CrossRef G. Bouwmans, R. M. Percival, W. J. Wadsworth, J. C. Knight, P. St. J. Russell, "High-power Er:Yb fiber laser with very high numerical aperture pump-cladding waveguide", Appl. Phys. Lett. 83, 817 (2003) CrossRef Y. Yeong, J. K. Sahu, D. N. Payne, J. Nilsson, "Ytterbium-doped large-core fiber laser with 1.36 kW continuous-wave output power", Opt. Express 12, 6088 (2004), CrossRef H.Yin, P. Deng, Y. Zhang, F. Gan, "Determination of emission cross section of Yb 3+ in glasses by the reciprocity method", Mat.Lett. 30, 29 (1997) CrossRef D.E. McCumber, "Theory of Phonon-Terminated Optical Masers", Phys. Rev. 134, 299 (1964) CrossRef D.E. McCumber, "Einstein Relations Connecting Broadband Emission and Absorption Spectra", Phys. Rev. 136, 954 (1964) CrossRef L. Li et al., "Investigation of modal properties of microstructured optical fibers with large depressed-index cores", Opt. Lett. 30, 3275 (2005) CrossRef W. J. Wadsworth et al.,"Very high numerical aperture fibers", Photonics Techn. Lett. 16, 843 (2004) CrossRef L. Zenteno, J.Lightwave Techn. 11, 1435 (1993) Y. Lee, M. J. F. Digonnet, S. Sinha, KE. Urbanek, R. L. Byer, S. Jiang, JSQE 1, 93 (2009)

Use of spectroscopy for assessment of color discrimination in animal vision

Animals use color vision for a number of tasks including food localization, object recognition, communication, and mate selection. For these and other specific behaviors involving the use of color cues, models that quantify color discriminability have been developed. These models take as input the photoreceptor spectral sensitivities of the animal and radiance spectra of the surfaces of interest. These spectra are usually acquired using spectroscopic instruments that collect point-by-point data and can easily yield signals contaminated with neighboring colors if not operated carefully. In this paper, I present an equation that relates the optical fiber diameter and numerical aperture to the measurement angle and distance needed to record uncontaminated spectra. I demonstrate its utility by testing the discriminability of two solid colors for the visual systems of a dichromatic ferret and a trichromatic frog in (1) a conspicuous scenario where the colors have little spectral overlap and (2) a perfect camouflage scenario where the spectra are identical. This equation is derived from geometrical optics and is applicable to spectroscopic measurements in all fields.

Disorder-mediated enhancement of fiber numerical aperture

The numerical aperture (NA) of a multimode optical fiber sets the limit of the information transport capacity along the spatial degree of freedom. In this Letter, we report that the application of a highly disordered medium can overcome the capacity limit set by the fiber NA. Specifically, we coated the input surface of a multimode fiber with a disordered medium made of ZnO nanoparticles and transported a wide-field image through the fiber with a spatial resolution beyond the diffraction limit given by the fiber NA. This was made possible because multiple scatterings induced by the disordered medium physically increased the NA of the entire system. Our study will lead to enhancing the spatial resolution of fiber-based endoscopic imaging and also improving the information transport capacity in optical communications.

Effects of the speckle size on non-holographic fiber specklegram sensors

In this work, a technique to improve the dynamical behavior of a non holographic fiber specklegram sensor for measuring high frequency mechanical perturbations is experimentally demonstrated. In these arrangements, the speckle pattern produced by a multimode optical fiber is coupled to an optical fiber of low numerical aperture, which produces a filtering effect that can be used as optical transduction mechanism. In our proposal, the influence of the speckle size on the sensor performance is evaluated by changes in the numerical aperture of the multimode fiber. A strong effect of the near field fiber speckle size on the reproducibility of the mechanical perturbation is found. The behavior of the reproducibility is evaluated by the calculation of the mean of the variance and the correlation between the mechanical and optical normalized signals.

Simulation and experimental studies of an extrinsic fiber optic sensor for liquid refractometry

The working of a fiber optic sensor for sensing of refractive index of liquids based on intensity modulation is studied in detail. Mathematical modeling and simulation for the intensity of light collected by parallel, flat-tipped, dual fiber probe immersed in liquids of different refractive index are carried out. The model consists of transmitting fiber, receiving fiber, reflector and liquid medium of different refractive index. A cone of light emerges out of the transmitting fiber gets reflected in the form of cone of light toward receiving fiber. This cone of angle gets modified in accordance with refractive index of the liquid medium. The output intensity may be determined by considering the overlap area between the receiving fiber core and cross-section of reflected cone emitted by the image of a transmitting fiber. The proposed model takes into account various effects such as numerical aperture of fiber, change of refractive index of liquid medium, and scattering effect. Finally experimental results are compared with simulated ones.

SU-E-J-197: A Novel Optical Interstitial Fiber Spectroscopic System for Real-Time Tissue Micro-Vascular Hemodynamics Monitoring.

To demonstrate a novel interstitial optical fiber spectroscopic system, based on diffuse optical spectroscopies with spectral fitting, for the simultaneous monitoring of tumor blood volume and oxygen tension. The technique provides real-time, minimally-invasive and quantification of tissue micro-vascular hemodynamics. An optical fiber prototype probe characterizesthe optical transport in tissue between two large Numerical Aperture (NA) fibers of 200μm core diameter (BFH37-200, ThorLabs) spaced 3-mm apart. Two 21-Ga medical needles are used to protect fiber ends and to facilitate tissue penetration with minimum local blunt trauma in nude mice with xenografts. A 20W white light source (HL-2000-HP, Ocean Optics) is coupled to one fiber with SMA adapter. The other fiber is used to collect light, which is coupled into the spectrometer (QE65000 with Spectrasuite Operating software and OmniDriver, Ocean Optics). The wavelength response of the probe depends on the wavelength dependence of the light source, and of the light signal collection that includes considerable scatter, modeled with Monte-Carlo techniques (S. Jacques 2010 J. of Innov. Opt. Health Sci. 2 123-9). Measured spectra of tissue are normalized by a measured spectrum of a white standard, yielding the transmission spectrum. A head-and-neck xenograft on the flank of a live mouse is used for development. The optical fiber probe delivers and collects light at an arbitrary depth in the tumor. By spectral fitting of the measured transmission spectrum, an analysis of blood volume and oxygen tension is obtained from the fitting parameters in real time. A newly developed optical fiber spectroscopic system with an optical fiber probe takes spectroscopic techniques to a much deeper level in a tumor, which has potential applications for real-time monitoring hypoxic cell population dynamics for an eventual adaptive therapy metric of particular use in hypofractionated radiotherapy.

Semiellipsoid microlens fabrication method using UV proximity printing

We present a new semiellipsoid microlens fabrication method that controls the printing gap in the UV lithography process without thermal reflow. The UV proximity printing method can precisely control the curvature radius ratio of the semiellipsoid microlens in the fabrication process. The proposed fabrication method facilitates mass production to achieve a high-yield and high-coupling semiellipsoid microlens that is suitable to be used in commercial fiber transmission systems. A semiellipsoid microlens can be tipped on a single-mode fiber end to improve power coupling efficiency from laser diodes. The semiellipsoid microlens allows increasing the fiber spot size and numerical aperture. It is very important to control the geometric parameters in the assembly procedure to increase the optical coupling efficiency between the laser diode and single-mode fiber. Wide misalignment tolerance, low loss, and low manufacturing cost could be achieved by the proposed fabrication method. The theoretical model is first developed to predict the optical coupling efficiency for various microstructure geometries of semiellipsoid microlens and assembly parameters in this study. Then, the Taguchi method is applied to obtain the optimal geometric parameters setting. The results show that optical coupling efficiency could be significantly improved by using the optimal geometric parameters setting.

Fiber based polarization filter for radially and azimuthally polarized light

We demonstrate a new fiber based concept to filter azimuthally or radially polarized light. This concept is based on the lifting of the modal degeneracy that takes place in high numerical aperture fibers. In such fibers, the radially and azimuthally polarized modes can be spectrally separated using a fiber Bragg grating. As a proof of principle, we filter azimuthally polarized light in a commercially available fiber in which a fiber Bragg grating has been written by a femtosecond pulsed laser.