Franz X. Daiminger

High-power laser diodes, laser diode modules, and their applications

High power laser diodes and especially high power laser diode modules made enormous progress in the last few years. Different aspects of high power laser diodes are treated starting from general description of high power laser diodes and their mounting techniques, characterizing the electro- optical behavior of single laser bars and finally presenting beamshaping optics for the collimation of large modules. The later technique allows for symmetrical focal spots in the kilowatt range with a beam quality of about 170 mm*mrad. Different aspects of current applications of high power laser diodes are presented.

2 kW cw Fiber-coupled Diode Laser System

We have developed a high-power laser system that is based on actively cooled GaAs diode laser stacks. Fast axis collimation and subsequent beam rearrangement generates a symmetric laser beam in respect to the beam parameter product along the two main axes. By polarization and wavelength coupling 100 diode laser elements can be coupled into one fiber at a beam parameter product of less than 200 mm*mrad in both directions and more than 2 kW cw output power at the workpiece. At a spot diameter of less than 1 mm the power density exceeds 250 kW/cm2. First material processing experiments show that deep welding at working speeds that meet industrial requirements in steel can be observed. High-power diode lasers show that they become suitable for industrial work.

Performance and lifetime of high-power diode lasers and diode laser systems

High-power diode lasers have reached output power and reliability to meet requirements for industrial applications. Stacking of laser elements to modules increases the output power, beam shaping techniques allow to focus the radiation of a module to a single spot. An integrated diode laser systems with totally 50 laser bars is shown, that includes cooling, power supply and control unit. The laser radiation is transmitted by an optical fiber and an objective focuses the radiation onto the workpiece with a round spot of less than 1 mm diameter and cw power of more than 1 kW.

Aging properties of AlGaAs/GaAs high-power diode lasers

AlGaAs/GaAs high power diode lasers with a nominal output power of 15W were aged at different conditions. At a heatsink temperature of 25 degrees C aging at constant current (CC) and constant power (CP) mode is compared for aging times of 6000 hours. We derived an end-of-life criteria that results in the same lifetime for CC and CP operation assuming identical degradation mechanisms in both cases. The degradation observed differs only significantly beyond 3000-4000 hours of aging with increasing degradation for CP operation. In constant current mode the heatsink temperature is increased resulting in a junction temperature of about 80 degrees C. Assuming an Arrhenius relation the activation energy is estimated. It turns out that different activation energies can be derived either by taking the degradation of the output power at the elevated temperature or at the reference temperature respectively.

Laser-based facet inspection system

We developed a laser based inspection system which was used to monitor defect distributions in optoelectronic devices such as diode lasers. Basically the system works as the well-known laser beam induced current (LBIC) technique. Various lasers emitting in the 633-1300 nm wavelength range were employed as excitation source. A number of high power laser diode arrays (LDA) aged under different aging conditions (parameters: injection current, heat sink temperature, time) were inspected with the system. The scans obtained revealed significant differences for different aging levels of arrays. This finding allows us to determine the aging status of LDA and contributes to find methods for giving failure predictions for individual devices.

Controlling diode laser bar temperature by micro channel liquid cooling

In this paper investigations of temperature control of micro channel cooled high power diode lasers are presented. After a short motivation the theoretical background of a complete liquid cooling system is introduced and the simulation results are compared to the behavior of a real-world system. Because of the nonlinear dependence of the thermal resistance on the water flux modern control algorithms are required to achieve a sufficient control quality and robustness. Their design and characterization are presented as well. Based on the results of diode laser temperature control the potentials to realize a highly efficient wavelength control are discussed. To support future system integration and to realize the controller with minimal hardware expenditure an application specific integrated circuit (ASIC) was developed.

Aging behavior of high-power laser arrays monitored by photocurrent spectroscopy

The well-known method of photocurrent spectroscopy, i.e. the measurement of the spectral sensitivity of a laser diode like for a detector, was found to monitor aging properties of (In)AlGaAs/GaAs high power laser diode arrays (LDA) in a convenient way. Photocurrent spectra of LDAs emitting at 808 nm (1.53 eV) were measured in the 0.8 - 3.0 eV photon energy range. Aging induced changes in different spectral regions reveal the influence of different mechanisms affecting the structure. Conclusions on the microscopic nature of the changes are drawn and applications are discussed.

Monitoring the aging of high-power laser diode arrays

Different diagnostic methods were investigated in order to evaluate their potential as aging sensitive tool for the analysis of high-power laser diode arrays. The well-known method of photocurrent spectroscopy, i.e. the measurement of the spectral sensitivity of a laser diode acting as a detector, was found to monitor aging properties of high power laser diode arrays in a convenient way. Photocurrent spectra of high power laser diode arrays emitting at 808 nm (1.53 eV) were investigated in the 0.8 - 3.0 eV photon energy range. Aging induced changes in different spectral regions reveal the influence of difference mechanisms affecting the structure. One aging effect--the growth of the defect concentration within the optically active layers of the devices--is monitored. Conclusions on the microscopic nature of the changes are drawn and several applications are discussed.