Fabrication and characterization of indium fluoride multimode fused fiber couplers for the mid-infrared.

Results of the fabrication and characterization of optical fiber couplers made of multimode step-index fluoroindate (InF3) fibers are presented. The fabrication setup was customized for this type of glass with a constant source of controlled nitrogen flow heated to a target temperature with an accuracy ±1°C. Combined with a novel fast fusion approach and with excellent control of the viscosity throughout the process, the clean gas flow and well-controlled temperature enable the fabrication of fused fiber couplers absent of any noticeable crystallization. A coupling ratio of 45/55 was achieved, with an excess loss of 0.35 dB, at 1.7 µm. To the best of our knowledge, this represents the first low excess loss (<1 dB), multimode, InF3 fiber couplers.

Viscosity of fluoride glass fibers for fused component fabrication.

Fluoride glasses show great promise for mid-IR fiber-based applications. Their brittleness and low glass transition temperature have thus far been obstacles towards obtaining low-loss fused components. Here, we suggest a simple method to measure glass viscosity over a range of process temperatures of interest for fused coupler fabrication. We achieved tapers of inverse taper ratio (ITR) 0.12 in multimode fluoroindate fibers. Tapers with loss <0.1dB at ITR 0.3 and no visible defects were fabricated with high repeatability. This work paves the way towards low-loss fused optical couplers in fluoride glass fiber.

Enhanced Electro-optic Sampling with Quantum Probes.

Employing electro-optic sampling (EOS) with ultrashort probe pulses, recent experiments showed direct measurements of quantum vacuum fields and their correlations on subcycle timescales. Here, we propose a quantum-enhanced EOS where bright photon-number entangled twin beams are used to derive conditioned nonclassical probes. In the case of the quantum vacuum, this leads to a sixfold improvement in the signal-to-noise ratio over the classically probed EOS. In addition, engineering of the conditioning protocol yields a reliable way to extract higher-order moments of the quantum noise distribution and robust discrimination of the input quantum states, for instance, a vacuum and a few-photon cat state. These improvements open a viable route toward robust tomography of quantum fields in space-time, an equivalent of homodyne detection in energy-momentum space, and the possibility of precise experiments in real-space quantum electrodynamics.

Z-scan measurement of the nonlinear refractive index of graphene.

Under strong laser illumination, few-layer graphene exhibits both a transmittance increase due to saturable absorption and a nonlinear phase shift. Here, we unambiguously distinguish these two nonlinear optical effects and identify both real and imaginary parts of the complex nonlinear refractive index of graphene. We show that graphene possesses a giant nonlinear refractive index n(2)≃10(-7) cm(2) W(-1), almost 9 orders of magnitude larger than bulk dielectrics. We find that the nonlinear refractive index decreases with increasing excitation flux but slower than the absorption. This suggests that graphene may be a very promising nonlinear medium, paving the way for graphene-based nonlinear photonics.

Two-fold symmetric geometries for tailored phase-matching in birefringent solid-core air-silica microstructured fibers.

The effect of birefringence in 2-fold-symmetric microstructured optical fibers on the phase matching conditions for four-wave mixing is analyzed. The three general types of four-wave mixing are considered. General features are obtained through analytic expansions of phase-matching formulas. Three commonly used designs of fibers are analyzed numerically. Particular designs allow the generation of specified wavelengths, supercontinuum or entangled photons.

Microstructured fiber source of photon pairs at widely separated wavelengths.

We demonstrate a source of photon pairs with widely separated wavelengths, 810 and 1548 nm, generated through spontaneous four-wave mixing in a microstructured fiber. The second-order autocorrelation function g((2))(0) was measured to confirm the nonclassical nature of a heralded single-photon source constructed from the fiber. The microstructured fiber presented herein has the interesting property of generating photon pairs with wavelengths suitable for a quantum repeater able to link free-space channels with fiber channels, as well as for a high-quality telecommunication wavelength heralded single photon source. It also has the advantage of potentially low-loss coupling into standard optical fiber. These reasons make this photon pair source particularly interesting for long-distance quantum communication.