Kimmo Paivasaari

Adaptive interferometer using self-induced electro-optic modulation

We demonstrate the linear transformation of small phase excursions of a speckled wavefront into modulation of the transmitted intensity using the polarization self-modulation effect in photoconductive electro-optic crystals. The technique is more advantageous than the widely used photorefractive two-wave mixing technique.

Polarization self-modulation of the nonstationary speckle field in a photorefractive crystal.

We have observed the nonstationary polarization modulation of a speckle pattern transmitted through a photorefractive crystal under the alternating electric field of a square-wave form. A simple model based on Gaussian-beam propagation in the crystal with the diffusion type of nonlinearity is proposed to explain the main features of the experiment. Application of the phenomenon to noncontact and highly sensitive measurements of a rough surface movement is suggested.

Grating translation technique for vectorial beam coupling and its applications to linear signal detection

We develop the theory of vectorial beam coupling in cubic photorefractive crystals to describe the effect of fast phase modulation on output intensities and polarizations. Special emphasis is given to new features of the grating translation technique compared with its scalar variant. We show, in particular, that a strong nonlocal ac response in crystals of the sillenite family can be effectively used for the linear detection of fast signals. Theoretical results are supported by polarization ac experiments with Bi12TiO20 crystals.

Linear phase demodulation in photorefractive crystals with nonlocal response

On the basis of the vectorial theory of light diffraction in cubic photorefractive crystals, we derive analytical expressions to describe the output intensities and polarization states of two coherent beams coupled via a dynamic index grating and subjected at the input to a fast phase modulation. It is shown that the linear transformation of fast phase excursions into intensity modulation can be achieved in the case of a nonlocal ac response by means of proper polarization filtering. Theoretical results are supported by the data obtained in experiments with Bi12TiO20 crystals of different orientations. The polarization technique is used for the evaluation of the space-charge fields created in different crystals.

Photorefractive correlation filtering of time-varying laser speckles for vibration monitoring

We proposed and experimentally studied a technique of vibration object monitoring. The technique is based on self-diffraction of the speckle pattern on the adaptive correlation filter recorded in photorefractive Bi12TiO20 crystal. This filter is recorded owing to the light-induced scattering known as fanning effect. The proposed technique is as sensitive as interferometric systems and it is very simple for implementation: neither reference nor readout beam is needed for operation. It is shown that the system has a linear response on the lateral displacement of the object surface and its dynamic range can be easily varied depending on experimental conditions.

Reduction of the space-charge field in photorefractive crystals

Abstract The space-charge-field amplitude inside photorefractive crystals was evaluated using three different experimental techniques: (i) two-wave mixing gain measurements, (ii) direct phase demodulation under external dc-electric field, and (iii) phase demodulation under ac-field with proper polarization filtering. The experimental data could be simultaneously fitted with the theory in the frame of the traditional one-level photorefractive model only if an additional reduction of the space-charge field is assumed. The traditional model fails to explain appearance of this reduction factor. Measurements were carried out with Bi 12 TiO 20 and GaP photorefractive crystals.

Adaptive sensors of rough-surface ultrasonic vibrations based on the polarization self-modulation effect

New interferometric technique for the linear detection of small out-of-plane vibrations of a rough surface caused by ultrasound is described. The technique is based on the polarization self-modulation effect in photorefractive crystals and does not involve light diffraction. The signal- to-noise ratio of the measured system is approaching to that of the classical homodyne interferometer. The system possesses the large field of view providing the measurements of small phase shift of a speckled wavefront. Moreover, the proposed interferometer is adaptive to slow environment changes with response time about 1 ms keeping ability to measure ultrasonic vibrations of as small amplitude as 0.1 nm. Comparison of the proposed technique with the photorefractive two-wave mixing is given. It is shown that the proposed technique has number of advantages providing higher signal-to-noise ratio and higher sensitivity.

High sensitivity detection of laser ultrasound by photorefractive CdTe:Ti under an applied ac field

The performance of a laser ultrasound receiver based on two-wave mixing in titanium doped cadmium telluride crystals under an applied ac field is described. High sensitivity and fast response time is achieved using a low-power laser.

Polarization self-modulation in photorefractive crystals – a new approach for adaptive detection of ultrasonic surface displacement

We demonstrate that the polarization self-modulation (PSM) in photoconductive electrooptic crystals can be effectively used for detection of small ultrasonic vibrations of rough surfaces.

Photorefractive crystals for efficient linear sensing of speckle-pattern displacements

Novel technique for the efficient remote sensing of speckle- pattern displacements is proposed. It is based on the polarization self-modulation effect recently discovered in photorefractive crystals. The technique provides linear responses on the amplitude of the lateral displacement and high sensitivity which is comparable with the interferometric one. Both sensitivity and dynamic range of measurements can be adjusted varying the average speckle size on the input face of crystal. The simple and inexpensive experimental set-up does not include any reference or read-out beam. Proposed technique can be applied for optical measurements of the transient motion of inspected surface such as objects vibration monitoring.