Vasile Rotar

Spectral Combining and Coherent Coupling of Lasers by Volume Bragg Gratings

The use of volume Bragg gratings (VBGs) recorded in photo-thermo-refractive (PTR) glass for laser beam control is described. These new optical elements provide extremely narrow spectral and angular selectivity and have a high level of resistance to high-power pulsed and continuous-wave laser radiation. These features of PTR volume gratings are used for transverse and longitudinal mode selection, passive coherent coupling, and spectral beam combining (SBC) of semiconductor, solid state, and fiber lasers.

Polarization-controlled switching between diffraction orders in transverse-periodically aligned nematic liquid crystals

Transverse-periodic-oriented nematic liquid crystals (LCs) are a special type of optical axis grating that are capable of very high efficiency diffraction (theoretically, 100%) in thin layers of materials with thickness comparable to the radiation wavelength. In particular, they fully diffract linearly polarized input beam into circularly polarized +1st and -1st diffraction orders. We experimentally demonstrate switching between diffraction orders of such gratings when the polarization of the incident beam changes from right-circular to left-circular and vice versa with the aid of an electrically controlled LC phase retarder. Such a setup in which the diffraction efficiency and direction are controlled externally, without application of an electric field to the transverse-periodic grating, provides additional control opportunities and does not compromise the quality of the grating. The grating used in the experiment was 1.5 microm thick and had a period of 4 microm. The contrast ratio of switching between the +1st and -1st orders was as high as 267:1 for a He-Ne laser beam with a switching time of 6.6 ms.

Volume Bragg semiconductor lasers with near diffraction limited divergence

The problem of high-brightness, narrow line semiconductor lasers sources is important for different kinds of applications. The proposed solution of the problem is the use of an external cavity with volume Bragg grating for effective angular and spectral selection. High-efficient volume Bragg gratings provide complete selection directly in space of wave vectors and serve as a diaphragm in angular space. The condition of effective selection is the provision of a substantial difference in losses for a selected mode by matching angular selectivity of a Bragg grating with divergence of the selected mode. It was proposed off-axis construction of an external cavity with a transmitting volume Bragg grating as an angular selective element and a reflecting volume Bragg grating as a spectral selective feedback. In such external cavity broad area laser diodes have shown stable near-diffraction limited generation in the wide range of pumping current. For LD with 0.5% AR-coated mirror and 150 μm stripe it was achieved 1.7 W output power with divergence of 0.62° at current exceeding six thresholds. Total LD slope efficiency in the considered external cavity is less then slope efficiency of free running diodes by 3-5% only. Spectral width of such locked LD emission was narrowed down to 250 pm in the whole range of pumping current.

Binary volume phase masks in photo-thermo-refractive glass

Permanent binary phase masks with planar surfaces and high tolerance to laser radiation are recorded in the volume of photo-thermo-refractive glass using the contact copying technique and binary amplitude master masks. Conversion of a Gaussian beam to higher order modes is shown.

Angle Selective Enhancement of Beam Deflection in High-Speed Electrooptic Switches

Fast electrooptic (EO) deflector switches (DSs) have high potential for applications in optical burst transport networks. EO properties of active materials used in the DSs can impose some limitations on their beam deflection efficiencies. Using a test setup with planar silica waveguide microlens arrays, thin-film ferroelec- tric oxide beam deflectors, and glass volume Bragg gratings, we demonstrate that the beam deflection angle can be increased by more than a factor of 5 for the same switching voltages. The tech- nology enables significant performance improvement of the fast EO DSs.

Semiconductor 1.7 W volume Bragg laser with divergence close to a diffraction limit

Novel volume Bragg laser (VOBLA) with two photo-thermo-refractive volume Bragg gratings feedback is studied on the basis of 150-mum-stripe LD. High-efficient, near diffraction-limited operation at power of 1.7 W and spectral width of 250 pm are demonstrated.

Multiplexed Volume Bragg Gratings for Spectral Beam Combining of High Power Fiber Lasers

The recent development of kW fiber laser sources makes the concept of laser systems operating at power levels from tens of kilowatts up to 100-kilowatt levels a reality. The use of volume Bragg gratings for spectral beam combining is one approach to achieve that goal. To make such systems compact, lower the complexity and minimize the induced thermal distortions we propose and demonstrate the use of special volume Bragg elements which have several Bragg gratings written inside as combining optical components. The multiplexed volume Bragg gratings (MVBGs) were recorded in photo-thermo refractive glass and three beams with total power of 420 W were successfully combined using one MVBG. The combining efficiency was 97% and there was no significant beam quality degradation. The results demonstrated that the approach of using multiplexed volume Bragg gratings for spectral beam combining is an excellent extension to the current state of the art combining techniques. Especially valuable is the capability to reduce the number of optical elements in the system and while being able to manage the expected thermal load when kilowatt level sources are used for beam combining.

Longitudinal mode selection in laser cavity by moiré volume Bragg grating

A Fabry-Perot etalon, consisting of two π phase shifted reflecting volume Bragg gratings, is presented. These gratings are obtained as a moiré pattern resulting from sequential recording of interference patterns with different periods in photo-thermo-refractive glass and called moiré volume Bragg gratings (MVBGs). A detailed investigation of the fundamental operating principles and measurement techniques for phase shifted gratings is shown. Experimental results demonstrating a MVBG with a 15 pm bandwidth and 90% transmission at resonance are presented. The use of the MVBG for longitudinal mode selection in a laser resonator is shown.

Direct Recording of Phase Plates in Holographic Material with Using of Probabilistic Amplitude Masks

Recording of robust permanent phase plate in photosensitive glass is proposed via contact method with probabilistic computer-generated amplitude mask with varying random binary transmission grid of micron size. Spatial light modulation is a possible application.

Holographic Screens in Photo-Thermo-Refractive Glass

Holographic screens were recorded in Photo-Thermo-Refractive glass. Those screens were recorded in two beams and reference-free configurations. High efficiency screens were used for imaging, low efficiency - for measurement of laser beam parameters.