Experimental verification of longitudinal spatial hole burning in high-power diode lasers

For high-power diode lasers, asymmetric reflectivities of facets are employed in order to improve slope efficiency. In recent years, the cavity lengths of these laser diodes have been increased to better distribute heat in order to improve output power. However, these two methods result in an inhomogeneous longitudinal profile of photon density, which leads to a non-uniform gain profile and is typically referred to as longitudinal spatial hole burning (LSHB). LSHB is believed to one of the limiting factors in scaling the output power of diode lasers. In this work, the LSHB effect is confirmed experimentally. The longitudinal photon density distribution, carrier density distribution, and gain distribution were calculated using a finite difference method to solve the spatially-varying rate equations in an 808 nm high-power diode laser. The experimental work was carried out by direct observation of spontaneous emission from a window patterned into the top contact of a 1.5 mm cavity length 808 nm diode laser. Because the spontaneous emission rate is proportional to the square of carrier density, the carrier density distribution could be measured for the device. The non-uniformity observed in this device agrees with the calculated carrier density profile, strongly supporting the existence of the LSHB effect in the device.