Coherent mid-infrared supercontinuum generation in tapered suspended-core As39Se61 fibers pumped by a few-optical-cycle Cr:ZnSe laser.

We report on efficient supercontinuum generation in tapered suspended-core $ {{\rm As}_{39}}{{\rm Se}_{61}} $As39Se61 fibers pumped by a femtosecond mode-locked Cr:ZnSe laser. The supercontinuum spectrum spans the mid-infrared spectral region from 1.4 to 4.2 µm, and its spectral coherence is proved by heterodyning with a single-frequency narrow-linewidth Er-fiber laser at 1.55 µm, measuring a beat note with 27-dB signal-to-noise ratio in a resolution bandwidth of 100 kHz. The intensity stability of the supercontinuum radiation is also characterized by relative intensity noise measurements.

Refractive index spectral dependence, Raman spectra, and transmission spectra of high-purity 72Ge, 73Ge, 74Ge, 76Ge, and natGe single crystals.

The results of the precise measurement of the refractive index of stable germanium isotopes Ge72, Ge73, Ge74, and Ge76 single crystals with high enrichment and a germanium single-crystal Genat of natural isotopic composition with the Fourier-transform interference refractometry method from 1.94 to 20 µm with 0.1 cm-1 resolution and accuracy of 2×10-5 to 1×10-4 are shown. The content of 72 impurities measured by the mass spectrometric method was below 10-5-10-6 wt. %. Oxygen and carbon concentrations measured by IR spectroscopy in all crystals are within 5×1015 cm-3. All germanium crystals have a hole conductivity with specific electric resistivity of 45-50 Ohm×cm. The coefficients of the generalized Cauchy dispersion function approximating the experimental refractive index values all over the measuring range are given. The transmission and Raman spectra are also given.

Influence of absorption on the refractive index determination accuracy by the minimum deviation method.

By means of computer simulation, the influence of absorption index, κ, on the accuracy of determining the refractive index, n, of optical materials by the minimum deviation method (prism method) is studied at different measuring angular accuracies from 5'' to 0.1''. The n values within 1.0-4.0 and κ values within 0-0.1 are considered. It is shown that (1) except for the far-infrared region, the error in determining n by formula (1) commonly used in the minimum deviation method, i.e., neglecting absorption, is less than the error due to the accuracy of measuring the prism and minimum deviation angles; (2) the difference between n values calculated with and without absorption taken into consideration is, in most cases, negligible throughout the optical range; (3) however, the total error in determination of n is higher when absorption is taken into account than that in the case when absorption is neglected. The further the increase in accuracy of measuring the angles and sensitivity of radiation detectors, the more the proposed calculation technique will allow one to optimize the prism angle value and to calculate more correctly the values of n and error in its determination.

Specifics of spectral loss measurement in infrared fibers.

Main error sources and their contribution to the total error of measuring the optical loss spectrum by the cut-back method are analyzed in the mid-IR range (2-20 µm) for multimode optical fibers made from materials with high (2-4) refractive index values. It is shown that in case of insufficient fiber length, neglecting the refractive index value leads to a systematic overestimation of the measured optical losses: the higher the refractive index of a fiber core material, the greater an overestimation. The main errors are typically introduced by a bad repeatability of preparation quality of fiber ends and a lower signal-to-noise ratio of mid-IR range experimental setups in comparison with the near-IR range.

Light transmission in negative curvature hollow core fiber in extremely high material loss region.

In this paper we demonstrate the light transmission in a spectral range of 2.5 to 7.9 µm through a silica negative curvature hollow core fiber (NCHCF) with a cladding consisting of eight capillaries. A separation between the cladding capillaries was introduced to remove the additional resonances in the transmission bands. The measured optical loss at 3.39 µm was about 50 dB/km under a few modes waveguide regime.

Mid-infrared supercontinuum generation in tapered chalcogenide fiber for producing octave-spanning frequency comb around 3 µm.

We demonstrate mid-infrared (mid-IR) supercontinuum generation (SCG) with instantaneous bandwidth from 2.2 to 5 µm at 40 dB below the peak, covering the wavelength range desirable for molecular spectroscopy and numerous other applications. The SCG occurs in a tapered As(2)S(3) fiber prepared by in-situ tapering and is pumped by femtosecond pulses from the subharmonic of a mode-locked Er-doped fiber laser. Interference with a narrow linewidth c.w. laser verifies that the coherence properties of the near-IR frequency comb have been preserved through these cascaded nonlinear processes. With this approach stable broad mid-IR frequency combs can be derived from commercially available near-IR frequency combs without an extra stabilization mechanism.

Refractive index spectral dependence, Raman spectra, and transmission spectra of high-purity 28Si, 29Si, 30Si, and natSi single crystals.

We performed precise measurement of the refractive index of stable silicon isotopes 28Si, 29Si, and 30Si single crystals with enrichments above 99.9 at.% and a silicon single-crystal natSi of natural isotopic composition with the Fourier-transform interference refractometry method from 1.06 to more than 80 µm with 0.1 cm(-1) resolution and accuracy of 2×10(-5)…1×10(-4). The oxygen and carbon concentrations in all crystals are within 5×10(15) cm(-3), and the content of the metal impurities is 10(-5)…10(-6) at.%. The peculiar changes of the refractive index in the phonon absorption region of all silicon single crystals are shown. The coefficients of the generalized Cauchy dispersion function approximating the experimental refractive index values all over the measuring range are given. The transmission and Raman spectra are also studied.

Demonstration of a waveguide regime for a silica hollow--core microstructured optical fiber with a negative curvature of the core boundary in the spectral region > 3.5 µm.

We present a numerical and experimental demonstration of a waveguide regime in a broad band spectral range for the hollow core microstructured optical fibers (HC MOFs) made of silica with a negative curvature of the core boundary. It is shown that HC MOFs with the cladding consisting only of one row of silica capillaries allows to guide light from the near to mid infrared despite of high material losses of silica in this spectral region. Such result can be obtained by a special arrangement of cladding capillaries which leads to a change in the sign of the core boundary curvature. The change in the sign of the core boundary curvature leads to a loss of simplicity of boundary conditions for core modes and to "localization" and limitation of their interaction with the cladding material in space. Such HC MOFs made of different materials can be potential candidates for solving problem of ultra high power transmission including transmission of CO and CO2 laser radiation.

Demonstration of CO2-laser power delivery through chalcogenide-glass fiber with negative-curvature hollow core.

A technologically simple optical fiber cross-section structure with a negative-curvature hollow-core has been proposed for the delivery of the CO2 laser radiation. The structure was optimized numerically and then realized using Te20As30Se50 (TAS) chalcogenide glass. Guidance of the 10.6 µm СО2-laser radiation through this TAS-glass hollow-core fiber has been demonstrated. The loss at λ=10.6 µm was amounted ~11 dB/m. A resonance behavior of the fiber bend loss as a function of the bend radius has been revealed.