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.
Application of the Transverse Chirped Bragg Grating (TCBG - a reflecting volume Bragg grating with
continuously variable resonant wavelength across the aperture) for the narrowband tunable ring-cavity fiber laser is
presented. The main advantage of the use of TCBG is that its linear translation allows continuous tuning of emission
wavelength within 5-10 nm band. Yb doped fiber laser operating in the wavelength range of 1050-1055 nm of
narrowband emission up to 2.3 W is demonstrated.
We are reporting on a single frequency pulsed fiber laser based on extremely narrow band volume Bragg gratings
(VBGs) recorded in photo-thermo-refractive (PTR) glass. The performance of Yb-doped fiber laser was studied in both
passive and active Q-switch schemes. It is shown stable operation in both single TEM00 transverse mode and single
longitudinal mode regimes. It generates pulses of 40 - 200 ns duration at a repetition rate of 10 - 100 Hz in active and
17-250 KHz in passive Q-switch configurations with a pulse energy of ~50 μJ, limited by the onset of stimulated
Brillouin scattering that leads to fiber fracture.
Chirped Bragg Gratings (CBGs) recorded in photo-thermo-refractive (PTR) glass have been successfully used as
ultrashort pulse stretchers and compressors in a variety of solid-state and fiber chirped pulse amplification (CPA) laser
systems. Compared to traditional pairs of surface gratings, CBG-based stretchers and compressors offer significant
advantage in compactness and robustness. They are insensitive to polarization, require virtually no alignment and can
handle high average and peak power. At the current technology stage PTR-glass CBGs can provide up to 30 nm spectral
bandwidth and up to 300 ps stretched pulse duration. In this paper we propose a concept of sectional CBGs, where
multiple CBGs with different central wavelengths recorded in separate PTR-glass wafers are stacked and phased to form
a single grating with effective thickness and bandwidth larger than each section. We present results of initial experiment
in which pulses from a femtosecond oscillator centered at 1028 nm are stretched by a 32-mm thick CBG to about 160 ps
and recompressed by a monolithic 32-mm CBG with 11 nm bandwidth and by a sectional CBG with two 16-mm thick
sections each having ~ 5 nm bandwidth and offset central wavelengths: 1025.5 and 1031 nm. In both cases, compressed
pulse duration of 350-400 fs, ~ 1.1 × transform-limit was obtained. These results allow CBG-based pulse stretchers and
compressors with high stretch ratio and wide bandwidth to be constructed from multiple sections.
Photo-thermo-refractive (PTR) glass is a new photosensitive material for phase hologram recording. This sodium-zinc-aluminum-silicate glass doped with silver, cerium and fluorine exhibits a refractive index modulation after exposure to UV radiation followed by a thermal treatment. Holographic volume gratings recorded in this glass show an absolute diffraction efficiency exceeding 97%, a thermal stability up to 400°C, and a high tolerance to laser radiation. These features support its application for laser beam control in optical communications and surveillance in space-born systems. A specific aspect of the space environment is the presence of ionizing radiation, which is known to influence the properties of optical materials. We studied the induced absorption in PTR glass after exposure to gamma radiation with doses exceeding 10 Mrad. The larger part of the induced absorption is found in the visible and UV spectral regions while in the infrared the absorption level remains sufficiently low to allow a long-term use of this material in space. In this paper, we discuss the structure of the induced absorption spectra in PTR glass and glass matrix which does not contain photosensitive agents.
Refractive index decrement was discovered in a fiber made from photo-thermo-refractive (PTR) glass. PTR glass is a fluorosilicate glass doped with cerium and silver which demonstrates refractive index change after UV exposure and thermal development due to precipitation of NaF nanocrystals in the irradiated areas. This glass is widely used for volume holographic optical elements recording. Photosensitivity in PTR optical fibers has been shown after exposure to radiation at 325 nm for about 1 J/cm2 followed by thermal development at 520°C. Refractive index difference between exposed and unexposed areas was about 1000 ppm. A Bragg mirror at 1088 nm was recorded in such fiber which showed narrow band reflection within 1 nm.
We report a new photochromic composite polymer that was evaluated in conjunction with its potential applications for optical holographic recording in the whole visible spectral range. It consists of poly-N-epoxypropylcarbazole (PEPC) polymeric matrix with a nitro-brome-substituted spiropyran (BNSP) photochromic dye. The PEPC+BNSP films can be considered as negative photochromic recording media. They are colored in the initial state and bleached upon irradiation within the whole visible spectra. When we placed the bleached samples to the darkness, they slowly revert to the colored form. The real-time holographic recording procedure in PEPC+BNSP films was studied.