Scientists have developed a new method of fabricating true crystalline fiber waveguides (CFWs) using yttrium aluminum garnet (YAG) and other materials with similar structures using adhesive-free bond (AFB) technology.
Rare earth (RE)-doped YAG has been widely used as an active medium for solid-state lasers. A number of researchers have been involved in the fabrication of crystalline fibers using RE-doped YAG which have high power scalability over their glass counterparts.
However, low-loss, double-clad, or even single-clad fully crystalline fibers have not been successfully produced through current fiber growth (or pulling) techniques such as laser-heated pedestal growth.
Hence, the scientists have developed this new method to overcome the problems involved in fabrication by exploiting the benefits of crystalline fibers over glass. This approach allows fabrication of all-crystal fiber structure right from core to outer claddings, unlike the crystalline fiber-pulling methods.
A RE-doped YAG crystalline fiber laser principally has greater emission cross section and thermal conductivity, and hence it requires to be just a fraction the length of a conventional glass fiber laser.
Most of the laser applications demand straight crystalline fibers of up to tens of centimeters long. This avoids the concern for the influence of fiber bending loss on glass fiber lasers. It is evident that the fiber core size can be significantly increased while maintaining single-mode laser operation with the precise control of fiber numerical aperture.
Several experiments have proved that a single-mode laser can be achieved in 41mm-long single-clad erbium (Er)-doped YAG CFW with core cross section >60×60μm longitudinally core-pumped at 1.532μm. Further, a nearly quantum defect-limited efficiency (92.8%) was achieved at laser wavelengths of 1.617μm and 1.645μm. The 9.1W maximum output power is limited using the available pump power.
In the current work, scientists have illustrated the first laser diode (LD) cladding-pumped single-mode laser in a double-clad ytterbium (Yb)-doped YAG CFW with a near diffraction-limited beam quality. The 65mm long CFW includes a single-crystal Yb-doped YAG square core with 1% molar doping concentration and a 100 × 100μm undoped YAG inner cladding. The outer cladding is made up of a ceramic spinel.
The CFW was cladding-pumped using a fiber-coupled diode laser at 938nm wavelength. The scientists first illustrated that the laser output at 1.03μm with the uncoated waveguide stops at 2.7W pump power. The small signal gain coefficient in this case was estimated to be over 5 which is larger than any bulk lasers.
The total laser power including forward and backward output powers is 8.5W with a 39.5W input power, corresponding to a 21.5% optical-to-optical efficiency. The laser threshold was reduced to below 0.54W, and the maximum output power was increased to 13.2W following the attachment of an input coupler and output laser coupler at the two CFW ends with fluorinated oil. The low laser threshold shows that both the slope efficiency and laser optical-to-optical efficiency was found to be around 33.4%.
The near-field 2D and 3D laser beam profiles were measured using a pyroelectric camera at the image plane of a 10× microscope objective. Regardless of a square core in the waveguide, the mode appeared to be circular with 94% measured roundness. The beam quality was analyzed through the knife-edge method. The fitted M2 was observed to be 0.2, which denotes a pure single-mode laser in the CFW with 40×40μm core.
It can be concluded that the scientists have realized single-mode CFWs with large-sized cores in a number of RE-doped YAG crystals using AFB technology. They have also illustrated the first LD cladding-pumped single-mode laser in a double-clad Yb:YAG fiber waveguide having near-diffraction-limited beam quality.
They anticipate the suppression of stimulated Brillouin scattering resulting from the large core size, by CFW in addition to low Brillouin gain coefficient. The future work will involve high pulse energy power amplification in CFWs.