Sergiy Kaim

Stretching and compressing of short laser pulses by chirped volume Bragg gratings: analytic and numerical modeling

Abstract. The theory of stretching and compression of short light pulses by chirped volume Bragg gratings (CBGs) is reviewed based on spectral decomposition of short pulses and on wavelength-dependent coupled wave equations. Analytic theory of diffraction efficiency of CBG with constant chirp and approximate theory time-delay dispersion is presented. Comparison of approximate analytic results with exact numeric coupled-wave modeling shows excellent agreement for smooth heterogeneities of CBGs.

Forced air cooling of volume Bragg gratings for spectral beam combination

Volume Bragg gratings have been successfully used in spectral beam combining of high power fiber lasers with narrow channel separation and in four channel passive coherent beam combining of fiber lasers. Future application of beam combining with kilowatt level lasers requires a more detailed understanding of how to cool the gratings without hurting beam quality. Forced air cooling blown across both surfaces of the grating is both easy and cheap, but has been avoided in the past due to concerns of how the air density fluctuations will hurt beam quality. It is now shown that forced air cooling has no adverse effect on the M2 parameter due to density fluctuations in the air, and can efficiently cool VBG’s such that no degradation in beam quality is seen due to thermal distortions.

Saturation of multiplexed volume Bragg grating recording

Recording of volume Bragg gratings (VBGs) in photo-thermo-refractive glass is limited to a maximum refractive index change about 0.002. We discuss various saturation curves and their influence on the amplitudes of recorded gratings. Special attention is given to multiplexed VBGs aimed at recording several gratings in the same volume. The best shape of the saturation curve for production of the strongest gratings is the threshold-type curve. Two-photon absorption as a mechanism of recording also allows increasing the strength of multiplexed VBGs.

Characterization of Beam Quality by the “Power-in-the-Bucket”

Beam quality is described by “bucket-f” radius, r_f, and angular radius θ_f, both encapsulating fraction f of total beam power. Dimensionless product r_f*θ_f/λ depends on f and on beam shape.

Self-Fourier-Transform Beams and Their Quality Characterization

We introduced new axially-symmetric optical beams, which are Self-Fourier-Transformed and are close to waveguide modes. Quality characteristics for those and other beam types were found: Mx2, Power-in-the-Bucket product r·θ/λ etc.