Improving the Performance of High-Power Diode Lasers

NASA Tech Briefs, Apr 2005 by Mondry, Mark, Rudy, Paul, Zhou, Hailong

High-power, 808-nm diode lasers are used to pump solid-state lasers, which are employed in many scientific, microelectronics, materials processing, biomedicine, and metrology applications. Achieving improved performance, extended reliability, and lower cost of ownership for these solidstate lasers often translates into developing diode lasers with higher output powers and longer lifetimes. This article reviews several recent technical innovations that enable improved diode laser performance and reduced cost per watt-hour of pump diodes.

Characterizing Performance

The compact nature of diode lasers is extremely attractive from an applications standpoint, but also presents a number of design and manufacturing challenges in terms of device reliability. While virtually every laser creates thermal, electrical, mechanical, and optical stress, the compact size of diode lasers results in a much higher power/size ratio than any other laser type, making these stress issues more severe. Examples include facet damage due to high photon density and junction degradation from high current density and temperatures, as well as subsequent mechanical stress caused by thermal mismatch of materials.

These issues have largely been dealt with for the lower power diode lasers used in data storage and telecom applications; however, development is ongoing for high-power diodes at 808 nm. Tremendous reliability improvements have already been achieved through the use of aluminum-free active area (AAA) construction, which first enabled the industry-standard, 1-cm room temperature, conduction-cooled diode laser bar to go above the 40-watt, 10,000-hour lifetime mark. Current research and development efforts in 808-nm lasers involve advancing several different aspects of diode design, fabrication, and packaging in order to further reduce the cost per watt-hour of these diode laser bars.

To understand the significance of these efforts, it is first necessary to understand how performance for 808-nm pump diode lasers is typically defined. These lasers are produced as single emitters, linear bars (a monolithic chip with multiple emitters), or two-dimensional arrays (stacks of bars). Output power (in watts) is typically the most common specification for diode laser devices, but specifying power alone is insufficient. Bar power, for example, can be increased simply by adding more emitters and making the device longer. Thus, power is usually specified in conjunction with brightness - namely power as a function of the size and number of emitters. In many applications it is advantageous to maximize brightness, as this enables refocusing of the output to the highest possible power density.

In addition to power and brightness, it is also critical to specify the operating lifetime of the diode laser. This mean time to failure is defined-with a 90% confidence level-as the mean time that passes before the power drops by 20%. Because the major degradation mechanisms (e.g., facet oxidation) that reduce lifetime are power dependent, there is a trade-off between these parameters. Consequently, lowering the output power of a given device extends its lifetime significantly. For this reason many laser manufacturers operate the pump diodes at reduced power levels in order to extend their lifetimes well beyond 20,000 hours.

It is also necessary to specify the operating temperature at which these performance specifications are being measured or certified. Because facet damage and the spread of flaws, such as lattice defects, are all highly temperature dependent, a 10�C rise in operating temperature typically results in a 50% drop in lifetime given the same operating power and brightness.

Damage to the output facet has been an important performance-limiting factor in 808-nm diode lasers. In the diode laser manufacturing process, the facet is produced by cleaving the wafer and then applying a multilayer coating. This coating provides the appropriate level of reflectivity and some oxidation protection for the cleaved surface; however, some oxidation can still occur with conventional multilayer coatings. With high-photon fluxes, this oxidation can spread back from the surface, causing performance degradation and ultimately device failure due to catastrophic optical mirror damage (COMD).

The COMD threshold power is a very useful parameter associated with the power-lifetime product (watt-hours) at a given brightness and operating temperature. COMD can be quickly measured and induced at any time by raising the device current and hence, the output power. It is typically referenced after a specified burn-in period (e.g., 200 hours) because COMD decreases with time. As a rule of thumb, a safe operating power level for long-term operation of diode lasers is some fraction of the COMD threshold value.

Expanded Mode Designs

Figure 1 shows a schematic cross-section of a typical, double hetero-junction diode architecture. The laser light is created and amplified in the thin junction and propagates through the junction and waveguide layers. In traditional 808-nm designs, this action produces a single spatial mode in the vertical direcdon, with a beam cross-section on the order of a micron. With high-power diodes now producing over 2 watts from each emitter, this output corresponds to very high intensity both within the chip and at the output facet.