Yurij N. Parkhomenko

Stable operating region in a harmonically actively mode-locked fiber laser

We theoretically study the recovery of a harmonically actively mode-locked soliton fiber laser from pulse dropout. In such lasers, a large number of pulses propagate simultaneously in the cavity. In order to obtain stable operation, pulses that are dropped due to changes in environmental conditions should recover, while other pulses that propagate in the cavity should remain stable. Soliton perturbation theory is used to find stability conditions for the noise in a time slot where a steady state pulse exists and in a time slot where a pulse is dropped. In the stable operating region of the laser, noise is stable in the presence of a pulse while noise becomes unstable in time slots where a pulse is dropped. Such a stability condition ensures that the laser can recover from accidental pulse dropout. A good agreement between the results of a reduced model and the results of a comprehensive numerical simulation was obtained. The results of this paper may enable to improve the stability of actively mode-locked fiber lasers.

Mode Selection in Resonators With Conical Reflectors

The characteristics of laser oscillators can be modified toward specific requirements by inserting suitable optical elements in the resonator. We show that replacing one of the resonator mirrors by a conically shaped reflector can lead to a significant increase in the volume of the fundamental mode resulting in improved power efficiency. Alternatively, the first azimuth mode, which is important for various applications, such as high-resolution microscopy, can be selected by inverting the reflector head angle.

Two regions of mode selection in resonators with biprismlike elements

A resonator structure in which one reflector is replaced by a biprismlike reflecting surface is investigated theoretically. It is shown that such a modification leads to two regions of parameters, each with different regimes of mode selection. The first region has an improved laser power output because of the nearly flat-top mode shape. In the second region the biprism is inverted, with the result that the main oscillating mode can be the first odd mode. The line singularity contained in such a mode is one example of singular beams that are employed in various fields, such as micromanipulators and advanced high-resolution metrology.

Laser-mode selection by a combination of biprism-like reflectors with narrow amplitude masks

In recent work the laser mode selectivity induced separately by a biprism-like reflector and by an absorbing strip was investigated by numerical analysis. It was shown that each of these elements in an otherwise conventional resonator was suitable to cause the laser to oscillate preferentially on the first odd mode that contains a line singularity, which is a useful dark beam (i.e., a laser beam with a dark central region) for high-resolution metrological applications. We study the combined effect of these two mode-selecting elements and show that the unified analysis leads to much better performance than could be expected from a simple superposition of the performance with each element alone. The results indicate that the mode selectivity can be enhanced by at least a factor of 3 compared with that of laser resonators with biprism-like reflectors alone. Thus a laser equipped with such a combined element will oscillate on a pure first-order mode with high power efficiency. Moreover, calculations show that the resultant dark beam, focused for metrological applications, has a significantly improved shape compared with the dark beam obtained by external modulation of a fundamental Gaussian beam.

Intracavity beam shaping for nanoscale surface metrology

Interest in beams containing phase singularities (dark beams) is evolving as applications are being realized, in particular for high-resolution microscopy and measurements. While these dark beams are usually generated with the help of diffractive elements, in this paper two new approaches for the generation of dark beams within the laser cavity are proposed and investigated. Simulations demonstrate that distributions similar to dark beams can be obtained by various laser modes.

Mode Selection in Resonators With Bi-Lens Mirror

Modifications of laser resonators enable the intracavity generation of beams with singularities suitable for a novel approach to high resolution microscopy. The characteristics of a resonator with a bi-lens mirror replacing one laser mirror are investigated in this paper. It is shown that the highest selectivity of an oscillating mode with a linear singularity can be achieved with convex bi-lens reflectors while fairly good selectivity is still possible with concave mirror possessing minimum diffraction losses.

Mode selection in diffraction-coupled semiconcentric resonators by means of a biprismlike element

Laser beams containing singularities are gaining increased interest for applications in metrology, particle manipulation and other technological paradigms. While these beams are usually generated by external modulation of a fundamental Gaussian laser beam, it is advantageous to generate them directly within the laser resonator whenever possible. In this paper we propose and investigate the mechanism of selecting a mode possessing a linear singularity using strong diffraction coupling of two adjacent resonators using a bi-prism-like element.

Theoretical study of an actively mode-locked fiber laser stabilized by an intracavity Fabry-Perot etalon: linear regime

We study theoretically the effect of an intracavity etalon on actively mode-locked fiber lasers by solving the master equation for the laser when nonlinearity in the laser is negligible. The first-order dispersion of the material inside the etalon can increase the pulse duration by a factor of 10. The minimum pulse duration is obtained when the relative frequency offset between the free spectral range of the etalon and the modulation frequency of the active mode locking is of the order of 10−2. The group-velocity dispersion of the material inside the etalon as well as the finesse of the etalon affect the total cavity dispersion. The etalon helps to suppress both a simultaneous lasing in several supermodes and lasing in higher-order pulse modes of the master equation. The etalon also helps lock the central wavelength of the laser to the etalon comb.

Selection of dark modes in resonators with conical reflectors

Selection of modes containing different dark regions was studied in resonators with conical reflectors. The possibility of selecting whole subgroups of such modes was shown in circularly symmetric resonators. To handle single-mode selection employing extra intracavity spatial filters, modified integral equations and a numerical method of their analysis are proposed. Usage of the filter symmetry reduces the size of the four-dimensional matrices corresponding to the equation kernels, and they are analyzed by algorithms for two-dimensional matrices with the best convergence. The optimum resonator parameters for effective selection of different dark modes are found.

Shaping of dark beams in resonators with a bi-lens reflector

A laser resonator configuration is proposed in which the fundamental oscillating mode is an odd mode with a line singularity. The resonator is based on the replacement of one reflector by a bi-lens split mirror. The beam emitted by such a laser is an ideal source for the recently introduced singular beam microscopic procedure. We numerically investigate resonators with convex and concave faces of the reflector. It is shown that resonators of the latter type of reflectors have higher effectiveness in shaping modes with linear singularity. Their selectivity is also higher, than resonators with roof-mirrors that were investigated earlier.