Our study examines the transmission characteristics of bi-tapered optical fibers, i.e. fibers that have a tapered down and up span with a waist length separating them. The applications to aqueous and vapor phase biomolecular sensing demand high sensitivity. A bi-tapered optical fiber platform is suited for label-free biomolecular detection and can be optimized by modification of the length, diameter and surface properties of the tapered region. We have developed a phase sensitive method based on interference of two or more modes of the fiber and we demonstrate that our fiber sensitivity is of order 10-4 refractive index units. Higher sensitivity can be achieved, as needed, by enhancing the fiber design characteristics.

An electromagnetic transmitter typically consists of individual components such as a waveguide, antenna, power supply, and an oscillator. In this communication we circumvent complications associated with connecting these individual components and instead combine them into a non-traditional, photonic enabled, compact transmitter device for tunable, ultrawide band (UWB) radiation. This device is a centimeter scale, continuous, thin film superconducting ring supporting a persistent super-current. An ultrafast laser pulse (required) illuminates the ring (either at a point or uniformly around the ring) and perturbs the super-current by the de-pairing and recombination of Cooper pairs. This generates a microwave pulse where both ring and laser pulse geometry dictates the radiated spectrum's shape. The transmitting device is self contained and completely isolated from conductive components that are observed to interfere with the generated signal. A rich spectrum is observed that extends beyond 30 GHz (equipment limited) and illustrates the complex super-current dynamics bridging optical, THz, and microwave wavelengths.

This study focuses on the characterization of bi-tapered optical fibers, specifically signal analysis for sensing applications. A bi-tapered optical fiber platform is ideal for the label-free detection of biomolecules since the signal can be optimized by modification of the structure of the tapered region of the fiber. Molecular binding to the fiber surface changes the refractive index and thickness of the biolayer, which interacts with propagating light, causing a measureable phase shift in the output [1]. Optical fibers have a silica core surrounded by a cladding; the refractive index allows for total internal reflection of light through the core. Depending on the thickness of the cladding, a portion of the electric field permeates the core/cladding interface and decays as a function of distance from the interface resulting in an evanescent wave. Such waves are the basis for the surface plasmon resonance biosensing technique. Bi-tapering of single mode fibers results in a fiber with a much thinned cladding and allows the the field to exist outside the tapered region. Coating the tapered region with biomolecular recognition elements provides a biosensing capability on the fibers: binding of analyte to the recognition layer results in a molecular conformational change that is detected as a change in the light propagation pattern, that is, the refractive index (RI) [2-4]. In this study we focus on the characterization of the fiber signal as influenced by the refractive index of the surrounding aqueous solution. Single mode fibers were tapered biconically to two different waist lengths (L; 5 and 10 mm), and two different waist diameters (d; 10 and 14 microns). Previous experiments found that these taper dimensions result in total phase changes of less than 2Π for the entire range of refractive indices tested. Optical fibers with 9/125µm core/cladding diameter and pigtail connectors were used. The ends of the fibers were cleaved at a 90° angle with the Vytran LDC-200-G optical fiber cleaving system. These cleaved fibers were spliced with the Vytran GPX-3000 graphite filament, fusion fire-polishing system. Once spliced, the GPX-3000 was used to taper the fiber to the appropriate waist length and diameter, as mentioned above. Fibers were mounted in in a custom Teflon flow cell and exposed to varying concentrations of aqueous glycerol solutions. Refractive index of glycerol solutions was measured using an ABBE-3L refractometer; the solution refractive indices ranged from 1.3330 to 1.3405. The signal was generated using tunable laser that scans across the wavelength spectrum from 1480 to 1550 nm. The light enters into the SMF fiber, passes down-taper to the fiber waist region, re-enters the up-taper region, and is eventually received at the photodetector. We applied a novel signal analysis of our transmission data to extract a phase shift related to detection of solution refractive index changes around the bi-tapered fiber. The tapered fibers work by the interaction of (at least) two modes in the taper region: a high order mode (HOM) associated with the cladding and the fundamental core mode. These modes generate an oscillatory response of the optical fiber sensor as the wavelength is scanned. Here we analyzed smaller refractive index variations (~0.0005 RIU) and found that a nearly linear wavelength shift of the interference patterns occurs. The main new element is that the data analysis can resolve extremely small variations in the refractive index. In our analysis we decompose the signal data into Fourier components and use the dominant oscillation frequency for further analysis. The signal analysis enables improved high-resolution detection and this protocol will now be applied to bio-functionalized bi-tapered fibers for the detection of real-world analytes in aqueous solutions. References [1] B.J. King, I. Idehenre, P.E. Powers, A.M. Sarangan, J.W. Haus and K.M. Hansen, "Tapered optical fibers for aqueous and gaseous phase biosensing applications," Proc. of the SPIE 2013, 85700G-85700G-85710. [2] J. Wen Bin, L. Huan Huan, T. Swee Chuan, C. Kin Kee and A. Lim, "Ultrahigh Sensitivity Refractive Index Sensor Based on Optical Microfiber," Photonics Technology Letters, IEEE, 24(2012) 1872-1874. [3] S. Lacroix, F. Gonthier, R.J. Black and J. Bures, "Tapered-fiber interferometric wavelength response: the achromatic fringe," Opt. Lett., 13(1988) 395-397. [4] G. Salceda-Delgado, D. Monzon-Hernandez, A. Martinez-Rios, G.A. Cardenas-Sevilla and J. Villatoro, "Optical microfiber mode interferometer for temperature-independent refractometric sensing," Opt. Lett., 37 (2012) 1974-1976.

Optical parametric oscillators (OPOs) producing longwave output from a much shorter pump wavelength suffer from low conversion efficiency into the idler due to the large quantum defect compared with similar devices operating in the 3 – 5 μm regime. One method to increase pump to idler conversion efficiency is to recycle the undesired and higher energy signal photons into additional idler photons in a second nonlinear stage. We present numerical simulation results showing the improvement in efficiency that can be obtained in a linear, two stage, cascaded orientation patterned gallium arsenide (OPGaAs) nanosecond OPO. It includes diffraction, crystal loss, phase mismatch, pump depletion, and back conversion; and it assumes monochromatic waves but it neglects group velocity dispersion. For a singly resonant oscillator (SRO) pumped by a 2.054 μm Tm:Ho,YLF laser with 45 ns pulse widths, the addition of the second crystal in the cavity increases idler generation by overall factor of two and exceeds the quantum defect limit. The model has been validated by comparison with SNLO for the case of a single-stage OPO, and suggests crystal and resonator parameters that will lead to an optimized cascaded OPO.

We report a widely tunable narrowband terahertz (THz) source via difference frequency generation (DFG). A narrowband THz source uses the output of dual seeded periodically poled lithium niobate (PPLN) optical parametric generators (OPG) combined in the nonlinear crystal 4-dimthylamino-N-methyl-4-stilbazolium-tosylate (DAST). We demonstrate a seamlessly tunable THZ output that tunes from 1.5 THz to 27 THz with a minimum bandwidth of 3.1 GHz. The effects of dispersive phase matching, two-photon absorption, and polarization were examined and compared to a power emission model that consisted of the current accepted parameters of DAST.

The discovery of high temperature superconductors (HTS) and the expected applications in the field of ultrafast opto-electronics has created a unique opportunity where the technology has the potential to bridge the frequency gap from infrared to microwave. A pulsed ultrafast laser impinging on a HTS thin film grown using yttrium barium copper oxide (YBCO) excites transient electron dynamics to generate radiation that spans from the terahertz to the microwave regime. The radiation phenomena were demonstrated by making transient photo-excitation measurements using an ultrafast laser to induce non-equilibrium quasi-particle dynamics. The photo-response from a laser of an average power of 1 W and a pulse duration greater than 120 fs (808 nm wavelength) incident on charged YBa2Cu2O7-δ (YBCO) thin film at superconductive temperatures was measured using a series of microwave antennas. From the observed nanosecond response time of the transient pulse, we were able to extract frequency band structure in the GHz regime that was dependent on the incident beam diameter, pulse duration, power, and the physical structure of the YBCO thin film. The electron-phonon energy relaxation time is known to be on the order of a picosecond. However, by manipulating the resistive and kinetic inductive response of the material we demonstrate the ability to generate wideband microwave frequencies with a transient response on the order of the nanosecond time scale. Quasi-particle dynamics and the temporal response were analyzed using the Rothwarf-Taylor rate equations.

Our research demonstrates the design and fabrication of a biosensor based on the tapered optical fiber. The fiber is tapered biconically to a diameter of approximately 7 μm, which allows the evanescent field of propagating light to interact with molecules attached to the tapered surface. This sensing platform is capable of fast, continuous, specific, sensitive, and label-free molecular detection in the aqueous phase. Detection is demonstrated across multiple fibers, and the individual fibers are reusable. The system described previously has been modified for detection of volatile organic compounds. The fabrication of the modified design is also shown with preliminary results.

The output of seeded, dual periodically poled lithium niobate (PPLN) optical parametric generators (OPG) are combined in the nonlinear crystal 4-dimthylamino-N-methyl-4-stilbazolium-tosylate (DAST) to produce a widely tunable narrowband THz source via difference frequency generation (DFG). We have demonstrated that by employing this type of configuration we are able to tune our system seamlessly, without mode-hops, from 1.5 THz to 21THz with a minimum bandwidth of 3.1 GHz. The bandwidth of the source was measured by using the THz transmission spectrum of water vapor lines over a 3-meter path length. By selecting of the DFG pump wavelength to be at 1380 nm and the signal wavelength to tune over a range from 1380 nm to 1570 nm, we produced several maxima in the output THz spectrum that was dependent on the phase matching ability of the DAST crystal and the efficiency of our pyro-electric detector. Due to the effects of dispersive phase matching, filter absorption of the THz waves, and two-photon absorption multiple band gaps in the overall spectrum occur and are discussed. Employing the dual generator scheme, we have obtained THz images at several locations in the spectrum using an infrared camera that runs at a rate of 35 frames per second. We have demonstrated the ability to image 13 THz to 20 THz under static conditions. We will present images of carbon fibers illuminated at different THz frequencies.

We report experiments on a multi-mode fiber-based device that reimages the input pattern after specific propagation distances. The reimaging has two propagation length scales related to the Talbot self-imaging in a periodic grating and image revival effects. We use a beam propagation method to simulate diffraction and refraction of light in the optical fiber. The details of the fiber preparation and optical experiments are described. We study the optical imaging properties using a close-packed array of sub-apertures placed at regular positions on a triangular lattice. We numerically analyze the propagation, diffraction and coupling characteristics of the beam oscillating inside the fiber. Our simulations identify the optimal reimaging length of the multi-mode (MM) fiber to get high fidelity image revival. Experiments are performed to validate the simulation results.

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We examine through simulation and experiments the operation of multi-mode-optical fiber that reimages a periodic input optical pattern. Besides the observation of a partial Talbot self-imaging effect we also find a recurrence effect due to the core/cladding boundary of the optical fiber. We use the beam propagation method to simulate diffraction and refraction of light in the optical fiber device. The details of the device are described and its optical properties using a close-packed hexagonally-shaped array of apertures placed at regular positions on a triangular lattice are examined. Our simulations identify the optimal length of the Multi-mode fiber to get high fidelity self-image. Experiments are performed to validate the simulation results.

We report the efficient and stable operation at 1549nm of an actively Q-switched fiber laser based on an Er3+/Yb3+ doped double-clad single mode fiber. It operates with a repetition rate from 45 to 120kHz and pulse duration from 34 to 80ns; the output pulse shape and peak pulse intensity are stable over hours of operation. For a repetition rate of 120kHz and a maximum pump power of 8.1W we obtained an average output power of 4.0W with an overall efficiency of 50% and with a minimum pulse duration of 34ns.

We present our recent investigations on time-resolved measurements of alterations in the temporal luminescence decay of upconversion phosphors induced by electron beam treatment. The latter is a promising alternative to low-temperature and dry sterilization of surfaces for sensitive packaging materials. Especially in the food and medical sector regulations concerning sterility are increasingly tightened. For this, a secure proof for electron-beam-assisted sterilization is required. However, no non-destructive and in situ method exists up to now.Our approach to provide a secure proof of sterilization is to place a suitable marker material based on rare-earth-doped phosphors inside or on top of the packaging material of the respective product. Upon electron irradiation the marker material changes its luminescent properties as a function of applied energy dose. We verified the energy dependence by means of time-resolved measurements of the luminescent decay of different upconversion materials.In our experimental realization short laser pulses in the near-infrared range excite the marker material. The emitted light is spectrally resolved in a monochromator, collected via a silicon photo diode, and analyzed with an oscilloscope. As the main results we observe a reduction of luminescence lifetime due to electron beam treatment dependent on the emission wavelength. Hence, the electron beam induces changes in the particles' up-and down-conversion properties from which the applied energy dose can be derived.