Multiplexed chirped volume Bragg gratings (Conference Presentation)

In this work spectroscopic and laser characteristic of photo-thermo-refractive (PTR) glasses doped with different concentrations of rare-earth ions (ytterbium-erbium and Neodymium) were comprehensively carried out. Spectroscopic parameters were obtained using some theoretical techniques like Judd-Ofelt theory and Fuchtbauer-Landenburg (F-L) theory. Results show that the optimal concentration of neodymium oxide in PTR is 0.5 mol%, at which the glass demonstrates the best spectral characteristics. It was found that PTR glass doped with 0.1 mol% of erbium oxide and codoped with 2 mol% of ytterbium oxide also shows good spectral-luminescent properties. Laser action on those two samples was demonstrated. optical losses were found to be 0.34 % for neodymium- doped PTR active element and 0.28 % in the case of erbium and ytterbium co-doped active element. These values are quite low and compared to that obtained in commercial laser glasses. It was concluded that the rare-earth ions doped PTR glass isIn this work spectroscopic and laser characteristic of photo-thermo-refractive (PTR) glasses doped with different concentrations of rare-earth ions (ytterbium-erbium and Neodymium) were comprehensively carried out. Spectroscopic parameters were obtained using some theoretical techniques like Judd-Ofelt theory and Fuchtbauer-Landenburg (F-L) theory. Results show that the optimal concentration of neodymium oxide in PTR is 0.5 mol%, at which the glass demonstrates the best spectral characteristics. It was found that PTR glass doped with 0.1 mol% of erbium oxide and codoped with 2 mol% of ytterbium oxide also shows good spectral-luminescent properties. Laser action on those two samples was demonstrated. optical losses were found to be 0.34 % for neodymium- doped PTR active element and 0.28 % in the case of erbium and ytterbium co-doped active element. These values are quite low and compared to that obtained in commercial laser glasses. It was concluded that the rare-earth ions doped PTR glass is a promising material that can be a good candidate for producing the DFB lasers. a promising material that can be a good candidate for producing the DFB lasers.

Spectral beam combining (SBC) has been extensively used for power scaling of laser systems. SBC is an incoherent technique of combining laser radiation from multiple sources with offset wavelengths into a single near-diffractionlimited beam with increased energy brightness. SBC by means of volume Bragg gratings (VBGs) recorded in photo-thermo- refractive (PTR) glass has been shown to be a simple and robust technique for combining high-power laser radiation. High-efficiency large-aperture VBGs were fabricated in PTR glass wafers. While being photosensitive in the UV, PTR glass offers high transmittance in the near-IR and visible parts of spectrum. Excellent mechanical properties and refractive index independent of temperature enable VBGs in PTR glass to withstand high-power laser radiation, making them ideal elements for high-power SBC. We report spectral combination of five randomly polarized fiber lasers with 0.5 nm spectral separation between channels around 1064 nm using reflecting VBGs in PTR glass. Maximum output power of the system is 773 W, corresponding to 91.7% combining efficiency. It is shown that VBGs introduce no significant beam distortions under high-power operation. Additionally, a common-cavity configuration for SBC with automatic wavelength control of sources by intra-cavity VBGs is suggested. Two fiber lasers are combined using this technique and automatic wavelength control is demonstrated. We show how simple power scaling allows obtaining multi-kW near-diffraction-limited laser radiation via SBC with volume Bragg gratings in PTR glass.

Spatial and temporal shaping of ultra short pulses by chirped volume Bragg gratings

In this Letter, we demonstrate a 360 fold spectral bandwidth reduction of femtosecond laser pulses using the method of sum frequency generation of pulses with opposite chirps. The reduction has been achieved in a compact setup in which a single chirped volume Bragg grating replaces conventional stretcher and compressor units. Starting with 180 fs pulses, we have obtained, with a 30% overall efficiency, pulses longer than 100 ps with the spectral bandwidth of 0.23 cm-1 (7 GHz). We also discuss our method on theoretical grounds.

Role of bromine on the thermal and optical properties of photo-thermo-refractive glass

The novel photo-thermo-refractive (PTR) glass developed in ITMO University is a very promising optical material for photonic and plasmonic applications. In this paper authors represent study of tin influence on photo thermo inducted crystallization process and make a comparison of the optical and holographic properties of the new and classic composition of glass. Also during this work was made overall optimization of chemical composition namely was optimized concentration of halides, fluorides, bromides which are responsible for crystalline phase properties. Ions of antimony, which playing key role in catching and transferring electrons emitted during the UV exposure and subsequent heating. Also was lowered the concentration of stray impurity ions which a capable to catch photo-electrons. Optical spectra show that new composition of PTR glass has no absorption band in visible range caused by metal nano particles of silver. That allows recording of pure phase holograms in wide spectral range. Furthermore new PTR glass allows receiving refractive index modulation up to 1500 ppm. And the UV exposures needed to achieve maximum changes in refraction index are 6-7 times lower than in classic glass.

Thick Bragg gratings recorded in the volume of photo-thermo-refractive (PTR) glass have found wide application in high-power laser beam control. They have been successfully used for spectral beam combining, angular magnifying for beam scanning, selection of transverse and longitudinal modes in laser resonators etc. Laser damage was studied in PTR glass blanks and gratings fabricated under different conditions. No threshold reduction was found after grating recording in PTR glass. The laser damage threshold of a volume Bragg grating for a single mode 1 ns pulse at 1054 nm focused on inclusions was 7 J/cm 2 , while irradiation of inclusion-free sites resulted in surface damage at 20 J/cm 2 . N-on-1 laser damage tests 1 for 10 Hz repetition rate 1064 nm multimode 8 ns pulses focused to a 78 µm diameter spot have shown a 32 J/cm 2 threshold for a Pt-melted sample with detected inclusions, and a surface damage threshold greater than 42 J/cm 2 for a sample with no detected Pt-inclusions.

Using a stretched hollow core fiber and an acousto-optic modulator based pulse shaper, we demonstrate compression, characterization and shaping of ultrashort pulses from a high average power laser system.

We have developed chirped volume Bragg grating compact pulse compressors and robustly single-mode large-core fibers using chirally-coupled-core (CCC) structures, recently demonstrating high average power and 200 fs FCPA based on these new components. A combination of these two technologies is becoming an enabler for finally achieving trully compact high power ultrashort-pulse lasers.

Photo-thermo-refractive (PTR) glass is an optical Na–Al–Zn–K–O–F–Br silicate glass doped with Ag, Ce, Sb, and Sn that undergoes photo-thermo-induced volume crystallization of nanosized NaF responsible for localized refractive index changes. PTR glass has found numerous commercial applications, but the intricate mechanism of photo-thermo crystallization is far from being understood. In this paper, we demonstrate that, additional to crystalline phase precipitation, liquid–liquid phase separation (LLPS), i.e. amorphous droplets embedded in the matrix glass, appears concurrently over a wide range of temperatures. The immiscibility temperature is 925°C. The droplet phase is richer in SiO2, rendering the alkali-rich remaining matrix glass a lower glass transition temperature and a higher electrical conductivity than the original glass. The droplet's surface does not catalyze NaF nucleation. Although the effects of LLPS on optical properties of PTR glass are still to be explored, it could contribute to unwanted scattering losses and/or uncontrolled refractive index change. The substantial change in the original glass composition resulting from LLPS should play an important role on NaF crystallization kinetics, and therefore must be considered for an overall understanding of the crystallization mechanism underpinning the refractive index change in PTR glass.

Specific absorption spectra of cerium in multicomponent silicate glasses

With the development of augmented reality displays and integrated photonics devices, researchers are faced with the task of developing new methods for recording the waveguide holograms and creating the diffractive waveguides. Recording of such type diffraction elements is associated with increasing the complexity level since large convergence angles of the recording beams must be provided. The object wave must be inside the substrate during recording. A manuscript demonstrates a stable and simple method for multiplexed recording of the Bragg diffraction gratings for AR displays using phase masks. This recording technique is accompanied with no need in strong vibration isolation because of interferometric branches absence in the optical scheme. Presented research is distinguished by conical illumination of the phase mask with a single recording laser beam to manufacture the slant volume gratings for AR waveguide displays. An important result for the research is in experimental confirmation of the beneficial application of non-selective surface phase mask in comparison with volume selective one. Diffraction waveguides in this experiment made of photo-thermo-refractive (PTR) glass — unique material for integration of phase diffraction elements into waveguide platform. Creation of substrate-mode multiplexed Bragg gratings in planar waveguide made from PTR glass is the challenge of the research.

First step of Er-Yb laser based on PTR glass creation was made. Complex study of luminescent characteristics of PTR glass doped with Er-Yb with recorded holograms was made. Direct measurements of population inversion of main Erbium level ⁴I_13/2 for different Erbium concentrations and pumping power was made. Was shown that spectral and luminescent characteristics of poly-functional PTR glass doped with Er-Yb and recorded holograms are comparable to the traditional laser barium phosphate glass.

Method to assess the homogeneity of partially crystallized glasses: Application to a photo-thermo-refractive glass

Results of a long-term research in spectral narrowing and transverse mode selection in semiconductor lasers by means of volume Bragg gratings recorded in a photo-thermo-refractive (PTR) glass are described. PTR glass is a multicomponent silicate optical glass which changes its refractive index after UV exposure followed by thermal development. This feature enables recording of volume holograms with efficiency exceeding 97% in visible and near IR spectral regions which tolerate high temperatures up to 400°C, high power laser radiation. Transmitting and reflecting volume Bragg gratings recorded in such manner have spectral and angular selectivity down to 0.01 nm and 0.1 mrad, respectively. These spectral and angular selectors were used as transmitting and reflecting elements of external resonators for high-power semiconductor laser diodes (LDs). Transmitting Bragg gratings provide tunability of LDs in the range up to 60 nm, spectral narrowing down to 200 pm, stabilization of wavelength within 500 pm. Reflecting Bragg gratings allow spectral narrowing down to 20 pm, stabilization of wavelength below 100 pm at temperature variations up to 75 K. A single transverse mode emission for wide stripe LDs is observed at pumping currents exceeding 10 thresholds. Narrowing and stabilization of emission spectra of LD bars is demonstrated. It is important that all these features are achieved by passive elements with efficiency exceeding 97% and unlimited lifetime while actual brightness increase exceeded two orders of magnitude.