Cafer Özkul

Application of wavelet transform to hologram analysis: three-dimensional location of particles

The wavelet analysis provides an efficient tool in numerous signal-processing problems and has been implemented in optical processing techniques, such as in-line holography. When the diffraction pattern recorded on a hologram is analyzed by means of a wavelet transform, the 3-D location of small particles can be determined very accurately. The diffraction process can, in fact, be interpreted as a convolution with a family of wavelet functions, or, merely, as a wavelet transform. The scale parameter of the wavelet family is related to the axial distance z that the wave propagates. The original field is then reconstructed by searching for the optimum value of the scale parameter which produces a maximum of the wavelet transform modulus. The technique proposed is implemented and experimental results are presented.

Dynamics and stabilization of a high power side-pumped Yb- doped double-clad fiber laser

Abstract We present an experimental study of the dynamics of a high power Yb3+-doped double-clad fiber laser in various optical configurations operating in the 1.08 μm wavelength. The fiber is side-pumped with a high power laser diode using the V -groove technique. Different self-pulsing regimes are identified resulting from third-order nonlinear effects. The influence of the cavity losses on the dynamical behavior is also investigated. In addition, we show that the system is efficiently stabilized in a unidirectional ring cavity where Brillouin backscattering is suppressed.

Evidence of Brillouin scattering in an ytterbium-doped double-clad fiber laser.

We have designed and performed an experiment that permitted direct observation of Brillouin backscattering in an Yb-doped double-clad fiber laser. Fifteen Brillouin-shifted frequencies were observed for the first time to our knowledge. We clearly demonstrate that stimulated Brillouin scattering is directly responsible for both fast transient dynamics of the laser and reduction of the lasers pulse width.

Mode-lock, Q-switch and CW operation of an Yb-doped double-clad fiber ring laser

We investigate the dynamics of a high power unidirectional Yb-doped fiber ring laser with an intracavity polarizer placed between two halfwave plates. We show that, depending on the respective orientation of the phase plates, the laser operates in different regimes: CW, Q-switched, mode locked, or unstable. The passive method exposed here allows us to choose the operating regime of the laser by a simple rotation of a halfwave plate.

Application of in-line digital holography to multiple plane velocimetry

We present the preliminary results of a digital holographic system that can determine the two-dimensional velocity vector fields in several slices of a sample volume. A CCD camera directly records the diffraction patterns of small particles illuminated by a double-pulse laser diode. In fact, the diffraction can be interpreted as a convolution with a wavelet family of functions. The scale parameter a is related to the distance z between a particle and the CCD camera. Then, the intensity distributions in a plane located at a distance z are reconstructed by computing the wavelet components for the corresponding scale parameter a. Afterwards, a particle image velocimetry algorithm is applied to the numerically reconstructed pair of images. The feasibility of this technique is demonstrated for two simulated displacements.

Generation of high energy femtosecond pulses from a side-pumped Yb-doped double-clad fiber laser

We present a passively mode-locked side-pumped Yb-doped double-clad fiber laser, emitting around 1.05 μm. Polarization additive-pulse mode-locking technique is used. Mode locking is self-starting and produces 670 fs pulses after compression with a grating pair inserted in the cavity. The energy per pulse is 24 nJ.

Digital in-line holography for three-dimensional?two-components particle tracking velocimetry

We have used a digital in-line holography system with numerical reconstruction to determine 2D velocity fields in several slices of a sample volume. This system records directly on a charge-coupled device (CCD) camera the diffraction patterns of small particles illuminated by a double-pulse laser diode. The numerical reconstruction is based on the wavelet transformation method. A slice is reconstructed by computing the wavelet components for different scale parameters. These parameters are related to the axial distance between a particle and the CCD camera. The particle images are identified and localized by analysing the maximum of the wavelet transform modulus and the width of the particle image (L50). Afterwards, a point-matching algorithm is applied to the set pairs containing the particles. This step is followed by velocity vector extraction. The details of the velocity extraction and the data processing method are presented and the simulations and experimental results are discussed.

90-fs stretched-pulse ytterbium-doped double-clad fiber laser.

We report ultrashort, high-energy pulse generation from a side-pumped Yb-doped double-clad fiber laser. The laser is operating in the stretched-pulse regime. It directly generates 4.8-ps, 5-nJ pulses that are extracavity compressed to 90 fs.

Hologram reconstruction by use of optical wavelet transform

High-speed in-line holography is used to visualize the trajectories of glass fibers being drawn out in a turbulent flame. To improve the signal-to-noise ratio, the images are not observed by a conventional reconstruction setup, but the holographic plate is placed directly on the input plane of a wavelet-transform optical system. This processing system is based on a VanderLugt correlator with inclusion of an electrically addressed spatial light modulator. The shape of the matched filters is deduced by successive rotation and dilatation operations of wavelet functions in the Fourier domain. We estimate the three-dimensional location of a fiber element and its orientation by searching for the daughter wavelet that yields the maximum intensity on the output plane of the correlator, which also contains the reconstructed image. The results are compared with those obtained by conventional optical reconstruction. The signal-to-noise ratios of the images observed on the output plane are improved. Moreover, it is shown that the axis coordinate accuracy is improved to Dz = +/-50 microm, instead of +/-0.5 mm for holographic reconstruction.

All-fiber tunable ytterbium-doped double-clad fiber ring laser

We have designed and realized a cw all-fiber tunable Yb-doped double-clad fiber laser in a unidirectional ring cavity. The side-pumping V-groove technique is used, yielding very good launching efficiency of the high-power pumping laser diode, together with a compact configuration. The fiber laser delivers as much as 800 mW of power and is tunable in the spectral range 1.04-1.10 microm . The laser linewidth is ~0.1 nm .