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We present data on the polarization degree of the fluorescence, excited in erbium-doped fibers (EDF) by linearly polarized light at three essentially different wavelengths in the spectral range of the fundamental absorption of Er 3+ (1492, 1526, and 1568 nm). Experiments were performed in otherwise similar single-mode fibers with a broad range of erbium concentration (≈640–5600 wt ppm). The polarization degree of the fluorescence demonstrated expected decay with the incident light power and proved to be significantly weaker for the central excitation wavelength of 1526 nm. The maximum value of the experimentally observed effect (≤0.3%) is significantly lower than it is predicted by model of the randomly oriented non-interacting dipoles. This discrepancy is explained by the reduced anisotropy (to the value 0.7–0.8) of the erbium ions absorption/emission, the effect which was utilized to explain the experimentally observed weak polarization hole burning in EDF earlier. Additional reduction in the fluorescence polarization degree and accelerated depolarization relaxation which were experimentally observed in fibers with high erbium concentration are attributed to the spatial migration of the excited state.

Experimental results on observation of slow and fast light propagation via two-wave mixing (TWM) in ytterbium doped fibers (YDF) at the wavelength 1064 nm are reported. For ≈10 mW of the incident cw laser power the effective population dynamic gratings with sub-ms formation time τ g and dominating phase (refractive index) components are formed in YDF by two counter-propagating coherent waves. The TWM at the recorded not shifted phase gratings is characterized by absence of any energy exchange between two interacting counter-propagating waves if they have the same frequency. For the no-degenerated TWM (i.e. that with the frequency off-set Δ Ω between the interacting waves) the maximal energy exchange is observed when Δ Ω is approximately equal to τ g −1 and changes its sign with the inversion of ΔΩ . In the reported experiments an effective slow and fast light propagation with maximal fractional delay of about 0.04 for 3.5 ms pulses was observed for these two maxima located at the frequency off-set ≈±200 Hz.

We report effective formation of population gratings in ytterbium-doped fibers by coherent light at 976 nm, i.e. at the wavelength that is typically utilized for optical pumping of these fibers. The dynamic gratings need sub-mW cw laser power to be recorded and have comparable amplitude (absorption) and phase (refractive index) components. Given the spontaneous relaxation time of a Yb 3+ meta-stable level of ~ 0.8 ms, the grating formation time proved also to be in the sub-ms region with the phase grating component significantly slower than the amplitude one.

We report linear polarization-insensitive detection of optical phase modulation in an adaptive interferometer based on transient two-wave mixing via dynamic population gratings in saturable Er-doped fiber (EDF). The proposed simple and robust configuration has an all-fiber design based on commercially available elements which makes it promising for applications in optical fiber interferometric sensors and coherent optical communication systems. The experimentally investigated single-mode polarization-maintaining EDF arrangement operated at 1492 nm and demonstrated efficient compensation of the polarization drifts below 100 Hz.

Experimental observation of phase (i.e. refractive index) component in the dynamic gratings recorded in erbium-doped optical fiber with saturation of optical absorption is reported. We utilized configuration of transient two-wave mixing with rectangular phase modulation of one of counterpropagating recording waves, where unshifted phase grating resulted in a transient energy exchange. The reported experiments were performed with heavily doped (≈5400 ppm erbium) fibers in a wide spectral range of Er3+ fundamental absorption at selected laser wavelengths 1492, 1526, 1549, and 1568 nm. Relative contribution of phase grating was especially large (up to ≈100% of the maximal amplitude component) at the opposite sides of the investigated spectral range and was significantly lower in its central part. The amplitude grating was found especially strong at short wavelength side of the spectrum (i.e. at 1492 nm), where the grating amplitude proved to be only two times lower than the theoretically predicted from consideration of two-level model.

We report on detailed investigation of the photo-electromotive-force (photo-EMF) effect in semiconductor with bipolar photoconductivity of impurity type in a spectral region close to the fundamental absorption edge, where both the photoconductivity and the photo-EMF response reach their maxima. The experiments performed with CdTe:V crystal (Δ E ≈ 1.51 eV) in the spectral range of 826–853 nm show that dynamics of the photo-EMF signal formation in this crystal is governed by relaxation of slow majority photocarriers–holes with the life-time which goes down from τ ≈ 9 μs at 850 nm to ≈6.5 μs at 826 nm. The minority photocarriers (electrons), which contribute approximately 10% of the total photoconductivity, also demonstrate significant wavelength dependence of their diffusion length with L D ≈ 8.5 μm at 850 nm and ≈6.3 μm at 830 nm. The reported results show significant improvement in responsivity of the bipolar CdTe:V photo-EMF detector when it is operated at λ = 840 nm (by factor ≈2.5 as compared with that at 852 nm).

Theoretical analysis of the transient two-wave mixing (TWM) of two continuous waves with rectangular phase modulation counter-propagating through the Er-doped optical fiber is presented. Approximate analytic solutions obtained for undepleted interacting waves of low power (neglecting large-scale saturation of the fiber optical absorption) and of high power (with significant spatially uniform large-scale absorption saturation) are complemented by numerical simulation for a broad region of the recording light power. The experimental dependence of the TWM efficiency on the recording power obtained for a single-mode fiber with ≃640 ppm erbium concentration and with initial optical density ≈8.4 at λ = 1526 nm demonstrated the initial, growing and decaying parts predicted theoretically. The maximum experimentally observed relative TWM signal amplitude proved to be approximately 3 times lower than expected.

Theoretical analysis of two-wave mixing (TWM) via reflection population grating, aiming to support current experimental investigations of dynamic gratings in Erbium (Er)-doped fibers, is presented. It is shown that for low recording intensities the stationary TWM via reflection saturated absorption gratings is well described in approximation of a spatially uniform grating amplitude. The analysis of the transient TWM is performed both for rectangular and for sinusoidal phase modulation of one of the recording waves and it is shown how the recorded grating amplitude and characteristic recording time can be obtained from the detected signals. It is pointed out that the majority of the existing experimental results on TWM in Er-doped fibers are qualitatively explained within the framework of the developed model.

We report on experimental observation of transient mixing of two mutually coherent waves of orthogonal polarizations collinearly propagating through birefringent Er-doped fiber. The dynamic grating responsible for this two-wave mixing (TWM) process is formed via effect of polarization hole burning (PHB) and has spatial period equal to the beat length of the fiber birefringence. Experiments were performed at the laser wavelength 1549 nm in 2-m long Er-doped fiber with 2-cm birefringence beat length without optical pumping. We observed that the transient collinear TWM has amplitude and characteristic relaxation time close to those of transient PHB effect measured for linearly polarized light and similar to corresponding characteristics of transient TWM observed for counter-propagating orthogonally polarized waves in the same fiber.

Summary Dynamic two-wave mixing of periodically phase modulated beams in liquids with thermal mechanism of optical nonlinearity for measurements of thermo-optical characteristics and, in particular, of optical absorption is proposed. Unlike conventional “pump-and-probe” technique, the configuration under consideration does not need an additional probe beam. This reduces problems with alignment and ensures an increased output signal, which proves to be proportional to the recorded photo-thermal grating amplitude. The method is illustrated with experiments performed at λ=633 nm using a model sample of acetone dyed with “methylene blue”. Evaluation of the limit sensitivity of the configuration for detection of low optical absorption is also given.

A single-detector adaptive configuration for polarization-insensitive homodyne detection of optical phase modulation is proposed. Experiments were performed at a HeNe laser wavelength ( λ =633 nm) and employed GaAs photo-EMF adaptive detectors. It has been shown that residual fluctuations in the output signal are basically due to nonlinearity of the adaptive photodetector response to the intensity of the polarization-modulated signal wave.

Measurements of non-steady-state photocurrents and of time-modulated conductivities for homogeneous illumination of varying intensity are performed with a photorefractive iron-doped barium–calcium titanate crystal of the congruently melting composition (Ba 0.77 Ca 0.23 TiO 3 , BCT). The diffraction efficiency and the two-beam coupling gain are determined from conventional holographic experiments. Debye screening and diffusion lengths are found to be of the order of 0.1 μm. The response-time of the material is 2 s (light intensity 0.2 W/cm 2 , wavelength 514 nm) and the dark storage time is 6 s. The measured refractive index changes are only one third of the values, which are expected from the known electrooptic coefficients and space-charge fields.

Original experiments on measurements of the coherence length of diffusely scattered CW laser beams using GaAs adaptive photodetectors based on the photo-EMF effect are presented. Simplicity of the configuration, capability to detect aberrated speckle-like wavefronts with high signal-to-noise ratio, and potentially high speed of operation make this configuration potentially useful for Optical Coherence Tomography and Laser Ranging.

We present experimental results which demonstrate that non-steady-state photo-EMF, induced by vibrating interference fringes in Bi12SiO20 photorefractive crystal can be dramatically (by a factor of 103 in our experimental conditions) enhanced in a resonance regime under application of an external DC field. We show that for increasing applied electric field the resonance maximum moves from the dielectric relaxation frequency towards the low frequency end. Its magnitude grows as E02 for low values of the field, for high external field significant broadening of the resonance peak and linear growth of the signal resonance amplitude is observed. This effect and non-trivial dependence of the resonance signal amplitude on the spatial frequency of the pattern is explained by saturation of the space charge grating. Finally, the sensitivity of the biased resonance adaptive photo-EMF detector is estimated.

Measurements of non-steady-state holographic photocurrents and conventional modulated photocurrents are performed with a n-type photorefractive strontium-barium niobate crystal at light wavelength of 532 nm. The effective charge carrier density and the photocarrier diffusion lengths for two orthogonal directions in the crystal are deduced. A ratio of the photoelectron mobilities along the c and a axis of 2.4 is obtained. The dependence of the photocurrent signals on the modulation frequency points to an essential contribution of shallow traps to the charge transport.

We propose utilization of the adaptive photodetectors based on non-steady-state photo-electromotive force for matching of 2D patterns. A simple configuration allows to suppress low spatial frequencies in the patterns and to produce the mismatch signal directly in an electric form. The experimental verification of this technique using a GaAs adaptive photodetector and HeNe lasers is reported.

It is shown that in materials with a special type of photoconductivity temporal response, the running space charge wave is generated under uniform illumination when the external electric field exceeds the threshold value. The experimental evidence of the effect existence in semi-insulating GaAs is presented. The effect can possibly be used to obtain high two-wave mixing gain in photorefractive crystals.

By direct interferometric measurement in Bi 12 TiO 20 under external DC electric field, we demonstrate that the steady-state photorefractive lens produced by the laser beam with the initial circular cross-section is essentially anisotropic: its focusing strength along the direction of the applied electric field is remarkably higher. Experimental data are supported by computer simulations and analytical results.

The generation of (1 + 1)-dimension photorefractive spatial dark solitons by 180° phase or amplitude jumps is reported. Experiments are performed in a cubic photorefractive Bi 12 TiO 20 crystal under an external dc electric field at the wavelength of the cw HeNe laser (633 nm). The waveguiding properties of the generated dark solitons are investigated using another HeNe (or semiconductor) cw laser probe beam.

We present original data on experiments with cubic photorefractive crystal Bi12TiO20 under an external dc electric field in the configuration of Z-scan characterization technique. In particular, behavior of the Z-scan curves versus the external dc field, light polarization, and the (focused beam)/(uniform sample illumination) intensity ratio is reported. The data obtained are interpreted in the framework of an original theoretical analysis based on the standard drift model of the photorefractive effect and assumption of a Gaussian profile of the propagating laser beam.

We report application of adaptive photodetectors based on non-steady-state photoEMF combined with speckle technique for measurement of in- and out-of-plane vibration components for real, diffusely scattering objects. The technique allows real-time measurements of vibrations with amplitudes much lower than the speckle diameter.

A laser profilometer based on adaptive photo-EMF detector that can find application for testing optically rough surfaces is described. Depth resolution of the configuration is shown to depend on not only the coherence length of the wide-spectrum light source used, but also the interelectrode spacing of the detector and crossing angle of interfering beams. Experimental demonstration of the device having an approximately 10 mm resolution and using a GaAs photo-EMF detector and a superluminescent diode was performed in tests of multisurface reflecting object.