Thermal Characterization of High-Power Diode Lasers Using Thermoreflectance

Abstract

Optoelectronic devices are pervasive in our daily lives. Temperature is an important parameter which dictates the performance, lifetime and reliability of optoelectronic devices. Thermal considerations are particularly important for modern high-power diode lasers which have found widespread use as pump sources in high energy laser systems.

In this work, we show how CCD camera-based thermoreflectance temperature measurements can be applied to characterize the thermal response of high-power diode lasers. One of the primary routes to thermal failure in diode lasers is through absorption of light at the outcoupling facet. Here we have explored two ways through which light absorption occurs: back-reflection of laser emission onto the facet and the absorption of outgoing emission within the active region at the facet.

Back-reflection (back-irradiance) of laser emission onto the facets of high-power diode lasers is a known consequence of the deployment of these devices in some diode-pumped laser systems and is known to accelerate device failure. We use thermoreflectance imaging to measure the temperature rise near the quantum well at the facet, for diode lasers emitting at several wavelengths, and for a wide range of back-irradiance beam positions. We find that two critical locations exist on the diode laser facet within a few microns of the epitaxial layers, such that when the back-irradiance is positioned at these locations, it leads to a peak in the temperature rise near the quantum well. Moreover, these critical locations are found to be a function of the device emission wavelength and polarization.

Under regular operation, the maximum optical power of high-power diode lasers is primarily limited by catastrophic optical mirror damage which refers to the damage caused to the outcoupling facet triggered by severe surface heating at a high injection current. The surface heating is believed to be caused primarily by non-radiative recombination of carriers generated by partial absorption of outgoing emission at the facet. We devised a technique to quantify the degree of this absorption using a combination of thermoreflectance imaging and a heat transport model of the chip. Using this technique, we carried out device degradation studies by analyzing the change in facet optical absorption with device age across a range of emission wavelengths and facet passivation conditions. We found that facet degradation is rapid over the first few hundred hours of device operation and saturates thereafter. Passivated devices exhibited four times lower facet absorption compared to the non-passivated devices.

In our efforts to better understand the dynamics of diode laser degradation, we measured detailed two-dimensional thermal maps of the active region of these devices at the facet for a wide range of operating currents as the device ages. These maps were found to exhibit high repeatability, fine spatial structure, and large thermal gradients which result from a complex interplay between current distribution, optical emission profile and defect density distribution at the facet and within the cavity. The mean temperature rise at the facet nearly doubles after 200 hours of operation at the rated current. Moreover, the temperature distribution was found to be highly non-uniform along the slow axis with large temperature gradients of ~1.3 K/um and local temperature spikes with characteristic widths of ~5 um. The size and position of local temperature spikes were found to depend strongly on the operating current and correlate to a small degree with the optical intensity profile.