We report on improvement from 50% to 70% power conversion efficiency on a 5-bar stack with 500 W of CW power at 25C coolant temperature resulting from a multi-pronged optimization approach. We also report on wavelength stabilization (0.07 nm/C) and emission bandwidth narrowing (0.3 nm at FWHM) of diode laser pump sources for precision pumping the upper transition levels of lasers that require narrow and stable pump sources such as Er/Yb co-doped or Yb:YAG lasers. These results have been achieved by integration of a Bragg grating inside a semiconductor laser cavity forming a low-loss, weak distributed feedback (DFB) laser, which results in record 53% wall-plug efficiency at 3 W CW operation and 25°C heatsink temperature from a 100-μm aperture diode laser and 45 W of wavelength-locked CW power from a 20% fill factor bar. This technique can be readily applied to diode laser structures for other strategic pump wavelengths.
There is an increasing need for single-spatial-mode, edge-emitting semiconductor lasers with reliable cw output power of around 1 W for applications such as pumps for rare-earth-doped fiber amplifiers and free-space communications. The design of respective devices is still a challenging task for experimenters, and software can assist very much in doing analyses of potentially perspective designs. We have developed a 3D numerical code supplied with a user-friendly interface for active diode-laser structures, taking into account light diffraction and carrier diffusion. The Watt-Ampere characteristics are calculated by changing the drive current density in the equation for the carrier-number density. To evaluate a single-mode stability limit, a procedure is developed to calculate 3-5 higher order optical modes on a 'frozen background': gain, carrier-induced index variation, as produced by the operated mode at a fixed drive level. Modal gains of these modes are compared to the calculated threshold gains for each mode. Because of non-uniform gain saturation by the operated mode, modal gains for higher-order modes increase with drive current due to beneficial overlap of their fields with the gain. When one of the higher-order modes approaches its threshold, this puts an upper limit for stable single-mode operation. A graphical interface allows for viewing near- and far-field patterns of any mode in the form of 3D surfaces or contour-plots. Scanning of profiles of mode intensity in an arbitrary cross section in the output plane and in far-field zone is available, too. Results of analyses of a number of published designs are reported.
Al-free active diode lasers emitting near 970 nm wavelength have been optimized for high electrical-to-optical power conversion efficiency. There are numerous key contributors such as scattering and absorption losses, band alignment, Joule heating, carrier leakage and below-threshold losses that contribute to power loss mechanisms. We report on improvement from 50% to a record-high 73% power conversion efficiency for a 1 cm bar at 10C, resulting from a multi-pronged approach that has been taken to minimize each of the loss mechanisms as to improve the overall power conversion efficiency.
Long term lifetest data is presented for Al-free active region 980 nm multimode laser diodes configured as chip-on-submount devices, as packaged fiber-coupled devices, and as multi-emitter laser bars on a microchannel cooled heatsink. Single emitter devices have been tested in chip-on-submount form. A first set of 120 of these devices were tested in a five-cell matrix at varying junction temperatures and optical output levels to obtain measured values for both random and wear-out failure model parameters. A second set of 187 packaged lasers were placed on accelerated lifetest to measure FIT data. In both cases, the devices were operated for up to 9,000. Another set of chips were packaged and tested inside a fiber-coupled, TEC cooled, 14 pin butterfly case as part of a Telcordia qualification process. These devices were operated for up to 5,000 hours with no failures and no degradation of either the chip or the package. Bar devices with a 20% fill factor were mounted on microchannel heatsinks and tested for one second on, one second off quasi CW operation for 4,000 hours. This test condition places a thermal expansion cycle stress on the devices, however once past the initial burn-in period very little degradation is seen in the output characteristics of the device.
While high-powered broad area lasers emitting between 915nm and 975nm are required for pumping Er+ and Yb+ doped dual clad fiber lasers and amplifiers, the single mode 980nm lasers are used for pumping EDFAs. We report on the performance and a systematic reliability assessment of Alfalight’s first generation Al-free multimode laser diodes with 100µm aperture and 2mm cavity length emitting between 950nm and 980nm. Data from 120 devices in five different multi-cell conditions show median life due to wear-out failure to be over 75.5 years. In addition, over 1,307,600 device-hours of accelerated lifetest data at 3A and a 70C heatsink temperature have been accumulated demonstrating 55 FIT (60% confidence level) at a 2W and 25C operation condition. We also present results from a packaged multimode diode laser with wavelength stabilized at 972nm with a spectral FWHM of 0.3nm demonstrating the capability to use such a device for pumping Er+ and Yb+ doped fibers near the more efficient 975nm portion of the absorption spectrum. Advances made in anti-resonant reflective optical waveguide (ARROW) type single mode diode lasers and the advantages over the conventional positive index guided ridge waveguide type lasers will be discussed. Single mode operation of ARROW single mode laser up to 450mW (ex-facet) was achieved. Results from the facet passivation studies showing successful implementation of non-absorbing mirror (NAM) due to quantum well intermixing using Si implantation in Al-free diode lasers will also be discussed. We have demonstrated reliable operation in excess of 5500 hours in index-guided Al-free diode lasers at a constant power of 500mW at a heatsink temperature of 25C.