Yu. O. Barmenkov

Time-domain fiber laser hydrogen sensor

We report a novel scheme for a fiber-optic hydrogen sensor based on an erbium-doped fiber laser with a palladium-coated tapered fiber within the laser cavity. The tapered fiber acts as a hydrogen-sensing element. When the sensing element is exposed to a hydrogen atmosphere, its attenuation decreases, changing the cavity losses and leading to a modification of the laser transient. The hydrogen concentration is obtained by simple measurement of the buildup time of the laser. This technique translates the measurement of hydrogen concentration into the time domain, and it can be extended to many intensity-based fiber sensors. Relative variations in the buildup time of up to 55% at an increase of the hydrogen concentration from 0 to 10% are achieved with a resolution of better than 0.1%.

Resonant and thermal changes of refractive index in a heavily doped erbium fiber pumped at wavelength 980 nm

We report a theoretical and experimental study of the refractive index variation in a heavily doped erbium silica fiber within the spectral range 1500–1580nm under the pumping at the wavelength 980nm. The two main contributions in the refractive index change are addressed—the resonant part determined by the saturation effect in the fiber and the thermal part stemming from the fiber heating due to the excited-state absorption and Stokes losses. We demonstrate that the thermal contribution in the resultant refractive index change is a notable value, which is the feature of erbium fibers with a high concentration of erbium ions.

Blue and infrared up-conversion in Tm3+-doped fluorozirconate fiber pumped at 1.06, 1.117, and 1.18 μm

We report up-conversion in a fluorozirconate Tm3+-doped fiber pumped by the first and second Stokes components of a Raman-shifted Nd3+:YAG laser. Relative fluorescence in the ultraviolet, blue, red, and near-infrared regions is analyzed as a function of the intensity ratio between the Stokes components. It is demonstrated that side fluorescence in the blue at 480 nm strongly dominates over other transitions when pumping with the two Stokes components. This suggests that high-efficiency blue up-conversion lasing is possible by using this pumping scheme with double-clad, Nd3+- or Yb3+-doped fiber laser pumps.

Excited-state absorption in erbium-doped silica fiber with simultaneous excitation at 977 and 1531 nm

We report a study of the excited-state absorption (ESA) in erbium-doped silica fiber (EDF) pumped at 977 nm, when the fiber is simultaneously excited by signal radiation at 1531 nm. We show, both experimentally and theoretically, that ESA efficiency at 977 nm gets strongly enhanced only in the presence of signal power. Experimentally, this conclusion is supported through the detection of upconversion emission, a “fingerprint” of the ESA process, and through the measurements of the EDF nonlinear transmission coefficient for the pump wavelength, which is sensitive to the ESA value. It is shown that the experimental data are precisely modeled with an advanced five-level Er3+ model developed for the EDF.

Spectral dependence of the excited-state absorption of erbium in silica fiber within the 1.48–1.59μm range

We report the study of the excited-state absorption (ESA) spectrum of Er3+ in erbium-doped silica fiber (EF) within the telecom band, 1.48–1.59μm. Two experimental techniques, the detection of up-conversion emission following the ESA process and the measurement of the EF nonlinear transmission coefficient, both supplied with a theoretical background, allow obtaining a mutually consistent spectral behavior of the ESA parameter, which is the ratio of ESA to ground-state absorption cross section, at 1.48–1.59μm excitation.

Experimental demonstration of a Raman effect–based optical transistor

We demonstrate optical pulse switching and amplification based on simultaneous Raman and Kerr effects in a fiber-loop mirror. Stimulated Raman scattering is shown to play a dominant role in the transfer function of the loop with highly germania-doped fibers, in agreement with theoretical predictions. Optical switching of a 100-ps signal pulse with an amplification factor of 10 was demonstrated in a 20-mol.% GeO(2)-doped fiber.

Transform-limited pulses generated by an actively Q-switched distributed fiber laser

A single-mode, transform-limited, actively Q-switched distributed-feedback fiber laser is presented, based on a new in-line acoustic pulse generator. Our technique permits a continuous adjustment of the repetition rate that modulates the Q factor of the cavity. Optical pulses of 800 mW peak power, 32 ns temporal width, and up to 20 kHz repetition rates were obtained. The measured linewidth demonstrates that these pulses are transform limited: 6 MHz for a train of pulses of 10 kHz repetition rate, 80 ns temporal width, and 60 mW peak power. Efficient excitation of spontaneous Brillouin scattering is demonstrated.

Continuous-wave measurement of the fiber nonlinear refractive index

We propose a technique for measuring the nonlinear refractive index of fiber based on transmission-coefficient measurements in a fiber Sagnac interferometer. In contrast with traditional methods, the proposed method uses a single optical source operating in cw mode and direct intensity measurement, enabling one to avoid the errors caused by fiber dispersion and uncertainty of spectral peak difference measurements that occur with pulse-based methods. The nonlinear refractive index in 20-mol. % GeO(2) fiber was measured to be (3.1 +/- 0.2) x 10(-16) cm(2) W(-1) at 1064 nm.

Application of the Z-scan technique to a saturable photorefractive medium with the overlapped ground and excited state absorption

Abstract Z-scan technique is applied to a saturable medium described by a generalized three-level scheme of photoexcitation in the condition of overlapping spectra of the ground and excited states. Correctness of the Z-scan data interpretation is analyzed in the framework of the model. The approach is applied for measuring the refractive indices Δn0 and n2 in polymer films containing bacteriorhodopsin.

Kinetics of dynamic hologram recording in polymer films with immobilized bacteriorhodopsin

Abstract