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We present experimental results on polarization ellipse self-rotation (PSR) and polarization hole burning (PHB) in the acetylene-filled hole-core photonic crystal fiber for the P5, P7, and P9 vibrational-rotational Doppler broadened (~500MHz) lines at the gas pressures 0.4-4Torr and W-scale power. The experimental values of PHB and PSR effects are in good agreement between them and with predictions of the developed semi-classic model for the acetylene molecules as the saturable rotating dipoles (gyrators) with random orientations. The maximum experimental PSR value normalized to the gas initial absorbance proved to be significantly lower than that observed in alkali-metal vapors earlier..

We present results of the experimental investigations of noise characteristics of the self-referencing optical phase demodulator based on a bulk low-pressure acetylene cell at the wavelength 1530.37nm of P9 acetylene absorption line. The configuration is simple, does not need phase locking, and can be operated with high-etendue waves. While the intensity noise of the utilized semiconductor laser at frequency >250MHz was below the photonic noise, the demodulator resolution (i.e., the minimum detectable phase modulation amplitude) was significantly reduced by an excessive laser phase noise. To reduce the output noise, we propose and analyze different balanced detection configurations that increase the resolution to the standard quantum level.

Experimental results on the transient optical nutation effect inside an acetylene-filled hollow-core photonic crystal fiber (HC-PCF) are reported. The experiments used 15 ns optical pulses with peak powers up to 5 W. The light wavelength was centered at 1530.37 nm, which corresponds to the P9 acetylene ( 12 C 2 H 2 ) vibrational-rotational absorption line. The gas pressure inside the PCF, with hollow core diameter of ∼10.3 μm, was kept around 0.12 Torr. Comparison of the experimental data with numerical simulations using the Maxwell-Bloch equations allowed us to evaluate the characteristic longitudinal and transverse relaxation times around 10 ns, as well as the transition dipole moment (1.36 × 10 −32 Cm).

We present a new concept of the homodyne interferometric adaptive detection of optical phase modulation. To ensure adaptivity, i.e. stabilization of the interferometer operation point, we utilize the phase memory of a two-level quantum system, resonantly illuminated with the information bearing signal wave. Phase modulation of the transmitted signal wave transforms into the intensity modulation via interference with the collinearly propagating dipole radiation of the excited two-level system. The latter acts like a reference wave since it has a phase corresponding to that of the signal wave but averaged over the transverse relaxation time T 2 of the quantum system. Experimental demonstration with the acetylene-filled hollow-core micro-structured optical fiber at the communication wavelength of 1530nm of the acetylene P9 absorption line is presented. It is shown that the response to the introduced phase modulation is quadratic when the acetylene inhomogeneously broadened absorption line is excited in its center and is a linear one if it is excited at one of the absorption line sides.

Dynamic population Bragg gratings can be recorded in the rare-earth-doped (e.g. doped with erbium or ytterbium) optical fibers with mWatt-scale cw laser power. Two-wave mixing (TWM) via such gratings is utilized in single-frequency fiber lasers and in adaptive interferometric fiber sensors with automatic stabilization of the operation point. Slow and fast light propagation can also be observed in the vicinity of narrow (~20-200Hz) spectral profile of stationary no-degenerate TWM. In particular, slow light propagation is observed for the purely amplitude grating, recorded in the erbium-doped fiber in spectral range 1510-1550nm. In its turn, in ytterbium-doped fibers at 1064nm (or in erbium-doped fiber at the wavelength below 1500nm) the dynamic grating has significant contribution of the phase component, the TWM profile has essentially asymmetric form, and both slow and fast (superluminal) light propagation is possible at different frequency off-sets between the counter-propagating interacting waves.

Low-pressure acetylene in the hollow-core photonic crystal structure fibers is an excellent medium for the room-temperature investigation of the coherent quantum effects in communication wavelength region. Pulsed excitation enables observation of new coherent phenomena like optical nutation or photon echo and evaluation of important temporal characteristics of the light-molecule interactions. We also report original experimental results on the pulsed excitation of the electromagnetically induced transparency in co- and counter-propagation configurations.

In this paper we analyze the influence of a spatial migration of the excited states on suppression of the population gratings recorded in erbium-doped optical fibers (EDF). Such dynamic Bragg gratings are formed via saturation of the optical transition between the fundamental and the meta-stable states of the active ions and are observed experimentally via transient two wave mixing (TWM) of the phase-modulated counter-propagating recording waves. The reported experiments were performed in the spectral range 1492-1568nm, which covers the absorption region of the fundamental transition 4 I 15/2 → 4 I 13/2 of Er 3+ ions. Significantly stronger grating suppression is observed at the central/long-wavelength regions of the above-mentioned spectral region. A set of similar fibers with essentially different Er 3+ ions concentrations with maximum attenuation of 4, 9, 17 and 35 ± 1 dB/m at 1531 nm was investigated and a clear concentration dependence of the grating suppression was observed. The TWM measurements are supported by the original experimental data on the fluorescence depolarization. The observed spectral dependence of the grating reduction factor is explained using the model of the vacant states limited spatial migration of the excitation among the Er 3+ ions.

For the first time, the experimental results on spatially uniform photo-induced refractive index changes Δn induced in ytterbium-doped optical fiber at the wavelength λ= 1064 nm were obtained and directly compared with the efficiency of the transient two-wave mixing (TWM) via phase population grating. It is shown that the TWM efficiency is in a reasonably good accordance with the theoretical evaluation based on the Dn measurements. Similar correspondence was also observed for a significantly weaker spatially uniform saturation of the fiber optical absorption and the TWM efficiency via absorption type population gratings. In contrast to similar data obtained earlier for erbium doped fibers, this allows us to assume that spatial diffusion of the excited state among Yb3+ is significantly less efficient than among Er3+ ions.

Erbium-doper fiber (EDF) is a flexible and promising model medium for investigation of the slow/fast light propagation in saturable optical materials. The experiments are usually performed in the spectral range 1480-1570 nm of the absorption/gain of Er3+ ions using the input power of a sub-mW scale. Conventional experimental configuration allows one to observe, however, the input and the output pulse profiles only. We report an original nondestructive technique for observation of a spatial propagation of the pulses via observation of the transient fluorescence excited by the propagating light-pulses at the fiber side, from which we are able to reconstruct how does the fractional delay and the amplitude of the propagating pulses change along the fiber. Results of a numerical simulation of the nonlinear pulse propagation performed for a saturable two-level medium in low contrast approximation proved to be in a reasonable agreement with the experimental observations.

Preliminary results on utilization of CdTe:V photo-EMF detectors for broad-band ( 10 MHz) adaptive detection of optical phase modulation, which is necessary for laser ultrasonic applications, are reported.Unlike widely used GaAs photo-EMF detectors, devices under consideration demonstrate no remarkable electron-hole competition and ensure sensitivity necessary for detection of 2 mn surface displacement for 0.1 mW of signal beam power in simple transverse configuration without utilization of asymmetric interdigitated surface contacts. For the wavelength used (2 851 nm) dielectric cut-off frequency of typical CdTe:V detector was around 1 MHz, which, in principle, allows us monitoring of as-processed objects moving with in-plane velocity up to 10 rn/s. INTRODUCTION Laser induced ultrasonic [1] is considered as one of the most promising techniques for nondestructive testingwhen remote evaluation of different parameters of solid objects (thickness, presence of defects, rust etc.) is

Photo-thermal effect is in general a well-established technique for remote and nondestructive material and structure evaluation. In particular, the method of transient thermal gratings is typically used for characterization of thermal properties and of low optical absorption in liquids. Conventional experimental configuration implies utilizing two mutually coherent recording beams to produce the thermal grating and one additional probe beam to detect it. We propose to perform recording and simultaneously probing the thermal grating using the same recording beams in configuration of dynamic two-wave-mixing (TWM), which simplifies the detection configuration and, as we hope, increases the sensitivity. In this configuration the sample is irradiated by the interference pattern of two coherent beams, in one of which the periodic phase modulation is additionally introduced. The output signal is detected as an amplitude modulation in one of the transmitted beams using conventional high-sensitivity lock-in amplification technique. The detected output signal is proportional to the amplitude of the thermal grating (but not to the grating diffraction efficiency as in coventional arrangement with additional probe beam), which also potentially increases the sensitivity. While the grating amplitude is evaluated directly from the output signal amplitude detected in this configuration, the photo-thermal grating formation time is obtained from position of the so-called "cut-off" frequency in the signal modulation frequency dependence. Details of experiments with this configuration using slightly dyed acetone sample at the wavelength 633 nm, which resulted in evaluation of the characteristic grating recording time and, finally, of the thermal diffusivity coefficient of the liquid, are presented.

We present an experimental and theoretical study of light scattering and propagation in multimode optical fibers with rough surfaces. In the experiments, performed at λ=655 nm, we used multimode 200 μm diameter silica glass fibers with an etched rough section. As the guided light reaches the rough part of the fiber it is scattered, feeding other modes and leaking out into the surrounding space. After some distance, however, the leakage decreases and the light within the fiber is carried primarily by the modes with low transverse wavenumbers. The light then propagates with a relatively narrow (half-width of about 0.15 rad) and slowly reducing angular spread. To understand this behavior, we develop a diffusion model for the intermode power transfer. The model predicts the formation of a narrow central maximum with a stable propagating angular profile, and relates parameters of the fiber and the surface roughness to the characterstic decay lengths.

We report on simultaneous characterization of space charge gratings in photorefractive PVK-based polymer films by means of photo-EMF and Two-Wave Mixing (TWM) of periodically phase modulated beams. 100 micron thick samples of a polymer DMNPAA:PVK:ECZ:TNF with chromophore (DMNPAA) concentration of 5 wt% were investigated at (lambda) equals 633 nm in reflectance configuration. The amplitudes of the unshifted (i.e. drift induced) and the shifted (i.e. diffusion or saturation induced) components of the photorefractive space- charge field grating were evaluated directly by detection of the fundamental and the second harmonic of the TWM signal and indirectly from the corresponding harmonics of the photo-EMF current. The unshifted grating component exhibited approximately linear dependence on the externally applied dc field E0 and had an amplitude close to E0, which can be interpreted as absence of any remarkable saturation of trapping centers associated with photorefractive recording. Also growing with E0, the amplitude of the shifted component did not depend on the applied field direction, but was nearly as big as the unshifted component for the external fields of about approximately equals 50 V/micrometers . We interpret these facts as well as an experimentally observed double change of sign fo the fundamental harmonic photo-EMF signal with the external field as a result of dramatic growth of the Einstein ratio D/(mu) (relating diffusion coefficient D and mobility (mu) of the photogenerated carriers) - at least up to 1 V for the external dc field mentioned above. This allows us to address the observed shifted component as an external field enhanced diffusion grating, rather than the result of trapping centers saturation. Additionally, the (mu) (tau) product for dominating photocarriers (holes) was evaluated as approximately equals 0.3*10- 10 cm2/V from the photo-EMF measurements.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Adaptive photodetectors, based on non-steady-state photo-EMF, are suitable both for detection of fast phase modulation in one of the beams forming regular interference fringes and of fast transverse displacement of speckle patterns in a reference-less configuration. In particular, they were used for detection of lateral displacements of rough surfaces, including those induced by nanosecond laser pulses (laser ultrasonic). In combination with light sources of low temporal coherence (i.e. superluminiscent diodes) these detectors can be used for micron resolution profilometry of rough surfaces. In this paper we propose and demonstrate utilization of strong (quadratic) dependence of the photo-EMF signal on contrast of the detected light pattern for simple and fast evaluation of a correlation degree between speckle patterns reflected from the same rough surface at different wavelengths. Experimental results obtained with the GaAs photo-EMF detector and two different lines of a cw Ar-ion laser for transmittance and reflectance configuration (with ground glass and metal plates used as light scattering objects) are presented.

The non-steady-state photo-ElectroMotive Force (photo-EMF) effect has been observed in photoreflective (PR) polymer films based on poly(N-vinylcarbazole) (PVK). It was investigated in the typical tilted transmission configuration without and with application of moderate external DC electric fields. The dependencies of the photo- EMF signals on the frequency and the amplitude of the phase modulation, the tilt angle, and the externally applied DC field are qualitatively explained using a simple model of the effect, developed earlier for unipolar photoconductive inorganic crystals without saturation of the trapping centers. Quantitatively, however, the experimental data turned out to be interpreted considering the presence of additional space-charge gratings appearing due to light reflection from the rear interfaces of the thin-film device, as could be proven by numerical simulation. The photo-EMF technique is a powerful experimental tool for the characterization of PR polymers without or at low external DC fields, where conventional holographic techniques give insufficient signal. It can be extended for the evaluation of organic xerographic photoconductors in general.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We present a theoretical and experimental analysis of the Z-scan technique in thick media with thermal nonlinearity in steady-state regime. The theoretical model considers the thick medium as a stack of thin thermal lenses. Numerical solution for the Z-scan technique is obtained. Experimental results for the technique using a sample of methylene blue in a solvent under cw illumination from a 10 mW He-Ne laser beam are obtained. Comparison of the experimental and theoretical results is made. Good agreement between theory and experiment is obtained.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.