Development and Applications of Glass-clad Ti:Al2O3 Crystal Fiber

Titanium (Ti):sapphire is one of the most commonly used materials in broadband fluorescence and lasers. This material has various applications such as in the construction of tunable lasers, mode-locked lasers and biological measurement systems (e.g., optical coherence tomography and Stimulated Raman spectroscopy). However, Ti:sapphire possesses two inherent drawbacks: a low absorption cross-section and a short lifetime. Therefore, the pump saturation intensity of Ti:sapphire is high, and lasing at a low threshold is difficult. Thus, Ti:sapphire bulks do not exhibit strong broadband emission below the laser threshold, and can hardly be directly used as an amplified spontaneous emission light source. A Ti:sapphire CF with a core diameter of less than 20 μm was fabricated using a laser-heated pedestal growth method. Both SEM and TEM images indicate that the lowest propagation loss of Ti:sapphire waveguides, which can reduce the Rayleigh scattering loss, could be attributed to a high quality crystalline core, smooth perimeter, and uniform diameter. Glass-clad structures provide non-sensitive for environmental variations and useful overlapping of pump and signal lights, which improves the optical efficiency compared to a bulk structure. Broadband and high-brightness ASE light sources were generated using a glass-clad Ti:sapphire CF. Utilizing a double-pass setup, backward ASE power increased substantially. Up to 42 mW of ASE power was generated using two pieces of 1-W 520-nm LDs as the pump source. The 163.8-nm near-Gaussian spectrum and 77.59-W∙mm-2sr-1 radiance rendered the light source eminently suitable for OCT applications in which high spatial resolution and low-image-pixel crosstalk are necessary. In applications such as an intra-cavity Ti:sapphire laser, the highly efficient structure in present research was pumped by both a solid-state 532-nm laser and a 520-nm LD. Because of the gain guiding effect, the pattern of the lasing mode was close to the fundamental mode. The threshold and slope efficiency with a LD pump with respect to the absorbed pump power were 118.2 mW and 29.6%, respectively. At 1-W pump power, the maximum output power was 215.8 mW. To our knowledge, this is the first time that such a high slope efficiency has been achieved with a Ti:sapphire laser with a LD pumps at such a low threshold. Using an external cavity Ti:sapphire CF laser, the lowest threshold achieved for this laser was 37.3 mW. With a 17.8% output coupling, the threshold power and slope efficiency were 123.2 mW and 18.6%, respectively. Broad wavelength tuning was achieved with a birefringent filter and a blazed grating. With the birefringent filter as a wavelength-tuning element, the wavelength associated with output power levels exceeding 50 mW was 125 nm. With the blazed grating as a wavelength-tuning element, the strong dispersion characteristics of the grating facilitates spectral mode selection and continuous wavelength tuning throughout the 180-nm coverage. With widespread applications of Ti:sapphire in the area of broadband light sources, tunable and mode-locked lasers, the glass-clad crystal fibers should have a good chance to play an important role in the future.