Introduction With the continuous development of laser technology, ultra-narrow linewidth lasers show important application prospects in many fields. However, the output linewidth of conventional semiconductor laser chips is usually large, which cannot meet the practical application requirements. Therefore, it is of great significance to study the key technologies for preparing ultra-narrow linewidth lasers. As a new type of optical component, volume Bragg grating, with the advantages of tunable diffraction wavelength, stable laser wavelength, and narrow linewidth, has become an effective way to solve this problem. This work tried to prepare 1.5 μm volume Bragg grating (VBG) based on chlorine-containing PTR glass to meet the demand for ultra-narrow linewidth lasers in the fields of new generation of coherent laser communication, LIDAR, and high-precision fiber sensing.
Methods The glass samples were prepared as long strips with a diameter of 5 mm and a length of 28 mm. The thermal expansion was recorded by warming the glass samples from room temperature to 800 ℃ at a rate of 10 ℃/min using a DIL 402PC/4 high-temperature thermal expansion meter from NETZSCH, Germany. The samples of chlorine-containing PTR glass before and after exposure were also tested by differential thermal analysis using a Q600SDT integrated thermal analyser from TA Instruments,USA. This is an important parameter to understand the dimensional stability of the glass during temperature change. A D2 PHASER type X-ray diffractometer from Bruker, Germany was used to analyze the changes in crystal structure of chlorine containing PTR glass before and after exposure, nucleation and precipitation heat treatment to understand the structural evolution and property changes of the material.
A Lambda 750S UV?Vis?NIR spectrophotometer from Perkin Elmer, USA was used to study the optical absorption properties of chlorine-containing PTR glass before and after exposure, nucleation and decrystallisation heat treatments, and to understand its absorption capacity at different wavelengths.
The glass powder was mixed with high purity KBr and pressed into tablets, and then scanned by infrared spectroscopy with a scanning speed of 0.158 cm/s. A Nicolet iS50 Fourier infrared spectrometer from Thermo Fisher Scientific, USA was used. The samples were then analyzed for their infrared transmittance properties. The infrared transmission properties of the samples were analyzed to understand the transmission properties of the materials in the infrared band. An in Via-Peflex type Raman spectrometer from Renishaw, UK was used. The molecular vibration and vibrational information of the samples were further analyzed through Raman spectroscopy to further understand the structure and properties of the materials.
Results and discussion Firstly, the thermal properties of the new PTR glass were investigated. Its transition temperature (Tg) and softening temperature (Tf) were 446.2 ℃ and 499.4 ℃, respectively. These two parameters are important for glass processing and applications. The coefficient of thermal expansion was similar to that of bromine-containing PTR glass in the temperature range of 30-400 ℃, indicating that the substitution of Br? by Cl? has a small effect on the thermal expansion properties of PTR glass.Secondly, it was found that NaF nanocrystals were successfully precipitated from chlorine-containing PTR glass after the precipitation heat treatment by XRD (X-ray diffraction) and TEM (transmission electron microscopy) inspection. These nanocrystals were distributed as dark grey elliptical particles with a particle size of 5 nm in the glass. high-resolution transmission electron microscopy (HRTEM) further revealed the lattice structure of these nanocrystals, and the spacing of adjacent crystal planes corresponded to the (111) crystal planes of NaF crystals. In addition, the detection of absorption spectra of chlorine-containing PTR glasses showed a new broad and weak absorption peak at 413 nm, which was attributed to the absorption of plasma AgCl.
Compared with the plasma AgBr absorption peak precipitated in the conventional PTR glass, the plasma AgCl absorption peak of the chlorine-containing PTR glass blueshifted in the UV sepctrum, which supplied the possibility of developing short-wavelength VBG products. In terms of glass structure, Raman and Fourier infrared spectroscopy analyses showed that the structure of chlorine-containing PTR glasses did not change significantly after the precipitation heat treatment. Finally, the article explores the performance of chlorine-containing PTR glasses for VBG applications. The diffraction efficiency of VBG was found to be positively correlated with the refractive index modulation and grating thickness. Under the same conditions of exposure dose and heat treatment process, increasing the thickness of VBG can significantly improve its diffraction efficiency. The experimental results show that the centre wavelengths of VBGs with thicknesses of 10 mm and 16 mm are 1 549.44 nm and 1 549.48 nm, respectively, and the half-height full widths are 0.0611 nm and 0.0570 nm, respectively.
Conclusions Novel chlorine-containing PTR glass components were designed and prepared using a two-step melting process to produce photothermally sensitive refractive glasses with a homogeneity of 10?6. 325 nm UV irradiation led to a decrease in the transition temperature and the onset of precipitation of the PTR glass, and an increase in the precipitation properties, which is very favourable for the preparation of high-quality VBGs. XRD and TEM results confirmed that NaF nanocrystals of an average size of 5?nm were precipitated from the chlorine-containing PTR glass. NaF nanocrystals with an average particle size of 5 nm. Raman and FTIR spectra showed that holographic exposure and heat treatment further enhanced the glass network structure. A reflective volume Bragg grating with a centre wavelength of 1.5 μm was prepared, with a diffraction efficiency of 50% and a half-height full width of 0.0570 nm, which sets the foundation for the development of 1.5 μm ultra-narrow linewidth semiconductor lasers.