Eugeniu Rotari

Volume-chirped Bragg gratings: monolithic components for stretching and compression of ultrashort laser pulses

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 M2<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.

Incoherent combining of 100-W Yb-fiber laser beams by PTR Bragg grating

Volume diffractive gratings (Bragg gratings) in photo-thermo-refractive (PTR) inorganic glass are proposed for incoherent laser beam combining because they have narrow spectral selectivity and diffraction efficiency greater than 95% from visible to near IR regions. They showed no laser-induced damage, no thermal lens, and no Bragg angle shift under CW Yb-fiber laser (1096 nm) irradiation at 100 kW/cm2. It opens the way to rugged, low-cost, efficient optics for high-power laser systems. Based on theoretical modeling of PTR Bragg gratings, we have designed a high-efficient technology for incoherent combining of two or several laser beams with certain wavelength shift. Two 100 W beams of Yb-fiber lasers in the range of 1080-1100 nm with the wavelength separation of 11 nm were combined with efficiency exceeding 75% while material losses did not exceed 2-4%. No fading or parameter change of PTR Bragg grating working in two 100 W beams were found. It was found that the process limiting efficiency of incoherent beam combining is the spectral widening of radiation of Yb-doped fiber lasers. At high power, their spectral width exceeds spectral selectivity of Bragg grating and causes a decrease of diffraction efficiency.

Tunable narrowband optical parametric oscillator using a transversely chirped Bragg grating

We demonstrate a novel technique for locking and tuning of a near-degenerate nanosecond optical parametric oscillator (OPO) using a transversely chirped volume Bragg grating. When the grating was translated, the OPO signal wavelength could be tuned from 1011 to 1023 nm (3.5 THz). The OPO was based on a periodically poled KTiOPO(4) as a nonlinear crystal and was pumped at 532 nm with up to 1.9 mJ of energy. The generated signal at an energy of 0.37 mJ had a bandwidth of 0.50 nm and a suppression of broadband background of >30 dB. The demonstrated technique is widely applicable for the construction of narrowband tunable sources.

Radiation-induced absorption in a photo-thermo-refractive glass

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.

Cross-correlation technique for dispersion characterization of chirped volume Bragg gratings

We propose using cross-correlation frequency-resolved optical gating for dispersion characterization of optical elements with high dispersion, such as ultrashort pulse stretchers and compressors. The technique is based on spectrally resolved second-order cross correlation (sum frequency generation) of a stretched pulse with a reference short pulse. Dispersion of optical elements with a high pulse stretching ratio can be completely characterized using this method, even with moderate resolution of spectral measurements of the cross-correlation signal. The proposed technique is used to measure dispersion of a chirped Bragg grating recorded in photo-thermo-refractive glass. It was found that dispersion of these gratings is almost linear with wavelength and has approximately the same slope with opposite signs for two orientations of the grating with opposite faces of the grating used as the input face. Analysis of higher-order dispersion of the grating shows some variations of dispersion across the aperture of the grating, mostly in the amount of third-order dispersion contribution.

Sectional Chirped Volume Bragg Grating Compressors for High-Power Chirped-Pulse Amplification

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.

Ring cavity tunable fiber laser with external transversely chirped Bragg grating

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.

200W, 350fs fiber CPA system enabled by chirped-volume-Bragg-gratings and chirally-coupled-core fiber technology (Withdrawal Notice)

This paper was presented at the SPIE conference indicated above and has been withdrawn from publication at the request of the authors.

Study of a narrowband optical parametric oscillator at degeneracy with a transversely chirped Bragg grating

We investigate the energy and spectral properties of a narrowband (0.9 nm) optical parametric oscillator, tuneable around the degeneracy point from 1055 to 1066 nm, by usage of a transversely chirped volume Bragg grating.

Development and characterization of negative photochromic compounds

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.