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Optics Letters

Chun-Chieh Chang, Daniel Headland, Derek Abbott, Withawat Withayachumnankul, Hou-Tong Chen
We demonstrate a terahertz flat lens based on tri-layer metasurfaces allowing for broadband linear polarization conversion, where the phase can be tuned through a full 2π range by tailoring the geometry of the subwavelength resonators. The lens functionality is realized by arranging these resonators to create a parabolic spatial phase profile. The fabricated 124-μm-thick device is characterized by scanning the beam profile and cross section, showing diffraction-limited focusing and ∼68% overall efficiency at the operating frequency of 400 GHz...
May 1, 2017: Optics Letters
Aleksey Starobor, Evgeniy Mironov, Ilya Snetkov, Oleg Palashov, Hiroaki Furuse, Shigeki Tokita, Ryo Yasuhara
The thermally induced depolarization and Verdet constant of CeF<sub>3</sub> crystals-their most important characteristics-have been studied in the 79-293 K temperature range. It has been found that thermal effects reduce substantially upon cooling down to 79 K and the Verdet constant grows in inverse proportion to the temperature. It was shown that CeF<sub>3</sub> crystals are not inferior to TGG as a medium for Faraday isolators, including cryogenic ones.
May 1, 2017: Optics Letters
Ashfaqul Anwar Siraji, Yang Zhao
We present a simple analytical effective medium approximation (EMA) that can account for finite-sized and interacting inclusions in the Rayleigh scattering regime. The polarizability of an individual inclusion is calculated using the Mie solution and net polarizability is formed, leading to effective parameters. The Mie coefficients are expanded into polynomials and curtailed to the sixth order, which is accurate in the Rayleigh regime. Results obtained using the developed models are compared with experimental results and existing EMAs...
May 1, 2017: Optics Letters
Nobuhide Yokota, Ryo Igarashi, Hiroshi Yasaka
We demonstrate a simple and efficient optical Nyquist pulse generation by using a dual-electrode Mach-Zehnder modulator (MZM) driven by a combination of fundamental and second-harmonic RF signals. The method is based on a raised-cosine-shaped time filter and time lens created in the MZM under operation conditions given by a simple equation. Optical Nyquist pulses with a roll-off factor of 0.52 and intrinsic conversion efficiency of 37% are experimentally confirmed. Numerical analyses indicate that the roll-off factor can be moderately controlled down to ∼0...
May 1, 2017: Optics Letters
Michal Němec, Jan Šulc, Michal Jelínek, Václav Kubeček, Helena Jelínková, Maxim E Doroshenko, Olimkhon K Alimov, Vasilii A Konyushkin, Andrei N Nakladov, Vyatcheslav V Osiko
The paper reports the first room-temperature operation of the in-band pumped Ho:CaF<sub>2</sub> laser. A set of Ho:CaF<sub>2</sub> crystals varying in holmium concentrations (from 0.1 up to 2.5 mol. % of HoF<sub>2</sub>), synthesized using the Bridgeman technique, was used for lasing and related spectroscopy investigation. A pulsed 1.94 μm thulium fiber laser system was used for Ho:CaF<sub>2</sub> longitudinal pumping. For 1.5 mol. % of Ho:CaF<sub>2</sub>, the maximal wavelength tunability (from 2073 to 2114 nm) and output energy (1...
May 1, 2017: Optics Letters
Ziv Glasser, Gidon Zaychik, Rita Abramov, Daniel Gotliv, Shmuel Sternklar
We describe a new technique for incoherent optical frequency domain spectroscopy (I-OFDS) that does not require measurements of the RF phase spectrum in order to reconstruct the optical spectrum. It is based on the addition of either an optical or electronic reference line to the I-OFDS system. Compared to the spectrum acquired by a regular I-OFDS system, high accuracy (error<1%) is predicted and achieved.
May 1, 2017: Optics Letters
Julius Vengelis, Vygandas Jarutis, Valdas Sirutkaitis
We present a technique for photonic crystal fiber dispersion measurement. We demonstrate that investigating supercontinuum using cross-correlation frequency resolved optical gating (XFROG) technique can be used for quantitatively characterizing dispersion and observing orthogonal polarization modes in polarization maintaining photonic crystal fibers. In addition, an XFROG trace of supercontinuum generated in a polarization maintaining photonic crystal fiber reveals complex behavior of orthogonal polarization modes that is different in normal and anomalous dispersion regions of the photonic crystal fiber...
May 1, 2017: Optics Letters
Yinwen Cao, Morteza Ziyadi, Ahmed Almaiman, Amirhossein Mohajerin-Ariaei, Peicheng Liao, Changjing Bao, Fatemeh Alishahi, Ahmad Fallahpour, Bishara Shamee, Asher J Willner, Youichi Akasaka, Tadashi Ikeuchi, Steven Wilkinson, Carsten Langrock, Martin M Fejer, Joseph Touch, Moshe Tur, Alan E Willner
An all-optical pilot-tone-based self-homodyne detection scheme using nonlinear wave mixing is experimentally demonstrated. Two scenarios are investigated using (1) multiple wavelength-division-multiplexed channels with sufficient power of the pilot tones and (2) a single channel with a low-power pilot tone. The eye diagram and bit error rate of the system are studied by tuning various parameters such as pump power, relative phase, and pilot-to-signal ratio.
May 1, 2017: Optics Letters
Gustavo Funes, Jaime Anguita
By means of an experimental setup, we study the time evolution of orbital angular momentum (OAM) crosstalk after a 5 m propagation path with a turbulence perturbation generated by a flat heater. This convective turbulence could simulate the beam passing over hot concrete or building roofs. We study the consequences of using a wide range of transmission OAM states. We also explore the effect of misalignment, and we propose two new different techniques for analyzing the vortex cancellation. Even though results are inconsistent with theoretical predictions, especially for low turbulence regimes, we state that signal-to-noise ratio is the key factor to understanding this discrepancy...
May 1, 2017: Optics Letters
Sébastien Loranger, Raman Kashyap
Many applications of optical fiber, such as specialized fiber Bragg gratings (FBGs), require high uniformity of a fiber's refractive index (RI) along its length. We show here that the mode effective index of most fibers is not constant, even on a short length scale. To help improve fiber manufacturing and selection, we demonstrate a technique for characterizing a meter length single-mode optical fiber's effective RI over a centimeter scale with a precision of 3×10<sup>-6</sup> RI units (RIUs) and an absolute accuracy of 2×10<sup>-4</sup> RIU...
May 1, 2017: Optics Letters
Junliang Dong, Alexandre Locquet, D S Citrin
This Letter presents a method for enhancing the depth resolution of terahertz deconvolution based on autoregressive (AR) spectral extrapolation. The terahertz frequency components with a high signal-to-noise ratio (SNR) are modeled with an AR process, and the missing frequency components in the regions with low SNRs are extrapolated based on the AR model. In this way, the entire terahertz frequency spectrum of the impulse response function, corresponding to the material structure, is recovered. This method, which is verified numerically and experimentally, is able to provide a "quasi-ideal" impulse response function and, therefore, greatly enhances the depth resolution for characterizing optically thin layers in the terahertz regime...
May 1, 2017: Optics Letters
Alberto Lombardini, Esben Ravn Andresen, Alexandre Kudlinski, Ingo Rimke, Hervé Rigneault
Hollow core fibers are considered as promising candidates to deliver intense temporally overlapping picosecond pulses in applications such as stimulated Raman scattering (SRS) microscopy and endoscopy because of their inherent low nonlinearity compared to solid-core silica fibers. Here we demonstrate that, contrary to prior assumptions, parasitic signals are generated in Kagomé lattice hollow core fibers. We identify the origin of the parasitic signals as an interplay between the Kerr nonlinearity of air and frequency-dependent fiber losses...
May 1, 2017: Optics Letters
Rebecca French, Sylvain Gigan, Otto L Muskens
Encoding of spectral information onto monochrome imaging cameras is of interest for wavelength multiplexing and hyperspectral imaging applications. Here, the complex spatiospectral response of a disordered material is used to demonstrate retrieval of a number of discrete wavelengths over a wide spectral range. Strong, diffuse light scattering in a semiconductor nanowire mat is used to achieve a highly compact spectrometer of micrometer thickness, transforming different wavelengths into distinct speckle patterns with nanometer sensitivity...
May 1, 2017: Optics Letters
Yong Sing You, Mengxi Wu, Yanchun Yin, Andrew Chew, Xiaoming Ren, Shima Gholam-Mirzaei, Dana A Browne, Michael Chini, Zenghu Chang, Kenneth J Schafer, Mette B Gaarde, Shambhu Ghimire
Solid-state high-harmonic sources offer the possibility of compact, high-repetition-rate attosecond light emitters. However, the time structure of high harmonics must be characterized at the sub-cycle level. We use strong two-cycle laser pulses to directly control the time-dependent nonlinear current in single-crystal MgO, leading to the generation of extreme ultraviolet harmonics. We find that harmonics are delayed with respect to each other, yielding an atto-chirp, the value of which depends on the laser field strength...
May 1, 2017: Optics Letters
Stephen C Warren-Smith, Mario Chemnitz, Henrik Schneidewind, Roman Kostecki, Heike Ebendorff-Heidepriem, Tanya M Monro, Markus A Schmidt
Intermodal third-harmonic generation using waveguides is an effective frequency conversion process due to the combination of long interaction lengths and strong modal confinement. Here we introduce the concept of tuning the third harmonic phase-matching condition via the use of dielectric nanofilms located on an open waveguide core. We experimentally demonstrate that tantalum oxide nanofilms coated onto the core of an exposed core fiber allow tuning the third harmonic wavelength over 30 nm, as confirmed by qualitative simulations...
May 1, 2017: Optics Letters
Hoang Nguyen, Md Masud Parvez Arnob, Aaron T Becker, John C Wolfe, Matthew K Hogan, Philip J Horner, Wei-Chuan Shih
A multipoint, side-firing design enables an optical fiber to output light at multiple desired locations along the fiber body. This provides advantages over traditional end-to-end fibers, especially in applications requiring fiber bundles such as brain stimulation or remote sensing. This Letter demonstrates that continuous wave (CW) laser micro-ablation can controllably create conical-shaped cavities, or side windows, for outputting light. The dimensions of these cavities determine the amount of firing light and their firing angle...
May 1, 2017: Optics Letters
Jingdan Liu, Daniel Claus, Tingfa Xu, Thorben Keßner, Alois Herkommer, Wolfgang Osten
In this Letter, we demonstrate the application of light field imaging to endoscopy. By introducing a microlens array into the image plane of a conventional endoscope, the 4D light field can be captured in one snapshot. This information can be used to obtain perspective images and to digitally refocus to different planes. These features allow for the recovery of 3D information in minimally invasive surgery. Important optical setup and performance parameters are derived to enable task specific engineering of the light field imaging system...
May 1, 2017: Optics Letters
Song Tang, Lianping Hou, Xiangfei Chen, John H Marsh
Special designs of sampled Bragg gratings have been used to fabricate distributed-feedback semiconductor laser arrays with very precise wavelength spacing and strong coupling coefficients. By dividing one sampling period into two equal sections, each with a grating but with a π-phase shift between the sections, the ±1st-order channels are enhanced while eliminating the zeroth-order reflection. By using multiple phase-shifted sections, only the -1st-order channel is enhanced. Using a single electron beam lithography step and two π-phase-shifted sections, we have fabricated an eight-channel laser array with a channel spacing of 100 GHz...
May 1, 2017: Optics Letters
Xiangyu Quan, Osamu Matoba, Yasuhiro Awatsuji
A technique of three-dimensional (3D) intensity retrieval from defocused, two-dimensional (2D) fluorescent images in the multimodal digital holographic microscopy (DHM) is proposed. In the multimodal DHM, 3D phase and 2D fluorescence distributions are obtained simultaneously by an integrated system of an off-axis DHM and a conventional epifluorescence microscopy, respectively. This gives us more information of the target; however, defocused fluorescent images are observed due to the short depth of field. In this Letter, we propose a method to recover the defocused images based on the phase compensation and backpropagation from the defocused plane to the focused plane using the distance information that is obtained from a 3D phase distribution...
May 1, 2017: Optics Letters
Olga Korotkova, Lutful Ahad, Tero Setälä
We introduce three-dimensional (3D) stationary, electromagnetic Gaussian Schell-model (GSM) sources in the space-frequency domain and derive the realizability conditions that the source parameters must obey. In particular, we extend to 3D GSM sources two approaches to the realizability conditions presented previously for beam-like GSM sources. The results are of interest in the analysis and synthesis of tunable non-paraxial, partially coherent, partially polarized light fields.
May 1, 2017: Optics Letters
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