Planar waveguides with enhanced support and/or cooling features for high-power laser systems

This disclosure provides planar waveguides with enhanced support and/or cooling. One or more endcaps could be disposed between coating/cladding layers at one or more ends of a core region, where the core region is doped with at least one active ion species and each endcap is not doped with any active ion species that creates substantial absorption at pump and signal wavelengths. A core region could include at least one crystal or crystalline material, and at least one cladding layer could include at least one glass. Different types of coolers could be disposed on or adjacent to different coating/cladding layers. Side claddings could be disposed on opposite sides of a planar waveguide, where the opposite sides represent longer sides of the waveguide. Endcaps and one or more coolers could be sealed to a housing, and coolant can flow through a substantially linear passageway along a length of the waveguide. One side of a planar waveguide could be uncooled.