Waveguide Bragg gratings are integrated photonic devices with high mechanical stability. In recent years, the femtosecond laser direct writing technology has provided a flexible method for the fabrication of complex optical components in the field of photonics, which has the advantages of being non-contact, non-destructive, maskless and “cold processing”, and has been utilized to fabricate integrated waveguide Bragg grating structures in a variety of bulk materials. This optical sensor can be applied to a wide range of environmental measurements such as temperature, strain, and refractive index. Sapphire crystals are widely used to fabricate sensors that can be used in extreme environments because of their excellent optical properties, ultra-high melting point, and electrical insulation. However, there are fewer reports on the fabrication of waveguide Bragg grating structures on sapphire crystals. In this paper, single mode transmission in sapphire crystals is achieved by optimizing the sizes of multilayer depressed cladding waveguides. And a uniform Bragg grating structure is integrated in the single mode waveguide. Our goal is to realize the fabrication of single mode waveguide Bragg grating integrated structures in sapphire bulk material with temperature sensing using the femtosecond laser direct writing technology.

Firstly, the repetition rate of the femtosecond laser is set to 200 kHz, and the effect of pulse energy on sapphire scribing is investigated for the same scanning speed (2 mm/s). After optimization of the processing parameters, a 10 mm long five-layer single mode depressed cladding waveguide is fabricated along the c-axis of the optical axis using a pulse energy of 25 nJ at a depth of 110 μm below the surface of the undoped sapphire bulk material. The cladding of the waveguide is composed of femtosecond laser-induced negative refractive index modified regions. And the waveguide properties are observed using an end-coupled system. Using the same repetition rate, the scanning speed and pulse energy are varied to integrate a second-order uniform Bragg grating structure with a period of 888.25 nm in the waveguide core. The morphology of the machined waveguide Bragg grating structure is observed using a microscope. After that, waveguide Bragg grating spectral characterization is carried out to observe the spectral variation of gratings with different lengths. Finally, the sensing performance of waveguide Bragg gratings at different temperatures is analyzed.

After fabricating a five-layer depressed cladding waveguide by the femtosecond laser direct writing technique, the waveguide is characterized using an end-face coupling system. The mode profile of the waveguide can be observed by a beam profiler and the mode profiles of waveguides with different sizes are compared (Fig. 5). By reconstructing the refractive index distribution from the obtained optical measurements, the normalized cutoff frequency of the waveguide is calculated to be lower than the normalized cutoff frequency required for single mode, thus confirming the single mode characteristics of the waveguide. The single mode waveguide transmission loss of 0.78 dB/cm is obtained based on the cutback method. Later on, a second-order uniform Bragg grating structure is integrated in a sapphire single-mode waveguide, and two Bragg resonance reflection peaks with different central wavelengths are found by characterizing its reflection spectrum. It is analyzed as a birefringent effect caused by transverse electric (TE) and transverse magnetic (TM) mode excitations, and this polarization phenomenon may be caused by the asymmetry of the refractive index change. The two reflection peaks correspond to 10.76 dB and 13.1 dB side lobe suppression ratios and 3 dB bandwidths of 0.327 nm and 0.347 nm, respectively (Fig. 9). Finally, the temperature sensing performance is tested by increasing the sensor temperature from room temperature to 650 °C. After three repeated experiments, the results show that the central wavelengths of the reflection peaks of the two modes shift toward longer wavelengths when the temperature increases, and the average temperature sensitivities of the TE and TM modes reach 24.8 pm/°C and 24.5 pm/°C, respectively (Fig. 11).

In this paper, single mode waveguide Bragg grating integrated structures are prepared in undoped sapphire bulk crystals using the femtosecond laser direct writing technique. This integrated structure consists of two structures, a single mode waveguide and a waveguide Bragg grating. The single mode properties of the waveguide are demonstrated based on mode profile observation and reconstruction of the refractive index distribution. And the transmission loss of the single mode waveguide is 0.78 dB/cm at 1550 nm. After that, a point-by-point method is used to integrate a second-order uniform Bragg grating in a single mode waveguide, which has a narrow 3 dB bandwidth (<0.35 nm) and a high reflectivity (>90% for TE mode and >95% for TM mode). When the temperature around the sensor is in the range of 100 ℃ to 600 ℃, the average temperature sensitivity reaches 24.8 pm/°C for TE mode and 24.5 pm/°C for TM mode. The fabricated sensor has promising applications in the field of temperature sensing.