Abstract
An innovative type of optical component—a volume Bragg grating—has recently become available commercially and has found wide applications in optics and photonics due to its unusually fine spectral and angular filtering capability. Reflecting volume Bragg gratings, with the grating period gradually changing along the beam propagation direction (chirped Bragg gratings—CBGs) provide stretching and recompression of ultrashort laser pulses. CBGs, being monolithic, are robust devices that have a footprint three orders of magnitude smaller than that of a conventional Treacy compressor. CBGs recorded in photo-thermo-refractive glass can be used in the spectral range from 0.8 to 2.5 μm with the diffraction efficiency exceeding 90%, and provide stretching up to 1 ns and compression down to 200 fs for pulses with energies and average powers exceeding 1 mJ and 250 W, respectively, while keeping the recompressed beam quality M 2 <1.4 , and possibly as low as 1.1. This paper discusses fundamentals of stretching and compression by CBGs, the main parameters of the gratings including the CBG effects on the laser beam quality, and currently achievable CBG specifications.
Highlights
A number of applications of ultrashort laser pulses in medicine, industry, and defense require high average power and high pulse energy
Reflecting volume Bragg gratings, with the grating period gradually changing along the beam propagation direction provide stretching and recompression of ultrashort laser pulses
CBGs recorded in photo-thermo-refractive glass can be used in the spectral range from 0.8 to 2.5 μm with the diffraction efficiency exceeding 90%, and provide stretching up to 1 ns and compression down to 200 fs for pulses with energies and average powers exceeding 1 mJ and 250 W, respectively, while keeping the recompressed beam quality M2 < 1.4, and possibly as low as 1.1
Summary
A number of applications of ultrashort laser pulses in medicine, industry, and defense require high average power and high pulse energy. This additional propagation length causes additional losses (about 8% in Fig. 5) but it enables a doubling of the stretching time and, almost double the pulse energy with the use of the same CPA device It was shown in Ref. 27 that placing a sequence of several CBGs having adjacent reflection spectra and with proper spacing between them provides the same stretching and compression as a monolithic volume Bragg grating (VBG) with the same reflection spectrum and thickness. Fine tuning of the spacing between sections of a multisectional compressor [Fig. 6(b)] provides a temporal shaping of the compressed pulse.[27] As it was mentioned above, practical CBGs that can be installed in CPA ultrashort pulse laser systems became a reality when the technology of extremely high optical homogeneity PTR glass and the technology of extremely uniform large aperture hologram recording was demonstrated and made commercially available by the OptiGrate Corp, (Oviedo, Florida) (www.optigrate.com). For gratings having a spectral bandwidth from 2 to 20 nm, the achievable stretching time is 500 ps
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