This arrangement provides a long path for pump absorption and signal amplification.  In the thin direction of the PWG (parallel to the temperature gradient), the signal and pump beams are guided by total internal reflection as in a fiber.  In the wider direction the beams are collimated by the injection optics. The pump may be injected into one or both PWG ends and aligned with the signal.  The large faces of the ribbon provide an ample surface area for efficient cooling.  Due to the small (200 to 400 µm) thickness of the ribbon, the LGM operates essentially at the temperature of the heat sink to which it is attached, thus avoiding significant thermo-optical effects and thermo-mechanical stresses. PWG has a small volume, which translates to modest (~ 1 Joule-level) stored energy. Key advantages of PWG include high-average power operation with good beam quality and compact packaging.