Martin Traub

Homogenization of high power diode laser beams for pumping and direct applications

High power diode lasers have become an established source for numerous direct applications like metal hardening and polymer welding due to their high efficiency, small size, low cost and high reliability. These laser sources are also used for efficient pumping of solid state lasers as Nd:YAG lasers. To increase the output power of diode lasers up to several kilowatts, the emitters are scaled laterally by forming a diode laser bar and vertically by forming a diode laser stack. For most applications like hardening and illumination, though, the undefined far field distribution of most commercially available high power diode laser stacks states a major drawback of these devices. As single emitters and bars can fail during their lifetime, the near field distribution does not remain constant. To overcome these problems, the intensity distribution can be homogenized by a waveguide or by microoptic devices. The waveguide segments the far field distribution by several total internal reflections, and these segments are overlaid at the waveguides exit surface. By the microoptic device, the near field is divided into beamlets which are overlaid by a field lens. Both approaches are presented, and realized systems are described.

Novel Design of a Gain-Switched Diode-Pumped Fiber Laser

To realize a completely monolithic, pulsed, fiber laser without free space elements we describe a gain-switched fiber laser pumped with a pulsed diode laser at about 965 nm with more than 30 μJ in a 200 ns pulse. For best beam quality we use a single mode fiber with a 6 μm core diameter. We report lasing of an Yb-doped double-clad fiber at 1080 nm and a pulse energy of about 8 μJ with a variable repetition rate from 1 - 50 kHz. The experimental results are compared with the data of a time resolved simulation and basic analytically derived formulas.

kW-class direct diode laser for sheet metal cutting based on DWDM of pump modules by use of ultra-steep dielectric filters

A direct diode laser was built with > 800 W output power at 940 nm to 980 nm. The radiation is coupled into a 100 µm fiber and the NA ex fiber is 0.17. The laser system is based on pump modules that are wavelength stabilized by VBGs. Dense and coarse wavelength multiplexing are realized with commercially available ultra-steep dielectric filters. The electro-optical efficiency is above 30%. Based on a detailed analysis of losses, an improved e-o-efficiency in the range of 40% to 45% is expected in the near future. System performance and reliability were demonstrated with sheet metal cutting tests on stainless steel with a thickness of 4.2 mm.

High Power Three-Section Integrated Master Oscillator Power Amplifier at 1.5 μm

We present the design and the performance of a monolithically integrated master oscillator power amplifier at 1.5 μm. The three-section device includes a distributed feedback laser, a modulation section, and a high power tapered amplifier. In order to mitigate the coupling effects of the light reflected at the facets, the device has been designed with a bent longitudinal axis and a tilted front facet. The device delivers >400 mW mode-hopping free output power. In static regime, the modulation section allows an extinction ratio of 35 dB.

High-power dense wavelength division multiplexing of multimode diode laser radiation based on volume Bragg gratings

We present a dense wavelength division multiplexer based on volume Bragg gratings (VBGs) with a channel spacing of Δλ = 1.5 nm. Multiplexing efficiencies of ηSM = 97% have been demonstrated with single-mode, frequency-stabilized diode laser radiation. By use of VBGs in an external-cavity laser we constrict the spectral bandwidth of passively cooled multimode diode laser bars with 19 broad-area emitters to δλ95% = 120 pm. When the multimode high-power diode laser radiation with a beam propagation factor of M(2) ≈ 45 is overlaid, the multiplexing efficiency decreases to ηMM = 85%. Temperature control of the VBGs expands the high-efficiency operation range.

Simulation of spectral stabilization of high-power broad-area edge emitting semiconductor lasers.

The simulation of spectral stabilization of broad-area edge-emitting semiconductor diode lasers is presented in this paper. In the reported model light-, temperature- and charge carrier-distributions are solved iteratively in frequency domain for transverse slices along the semiconductor heterostructure using wide-angle finite-difference beam propagation. Depending on the operating current the laser characteristics are evaluated numerically, including near- and far-field patterns of the astigmatic laser beam, optical output power and the emission spectra, with central wavelength and spectral width. The focus of the model lies on the prediction of influences on the spectrum and power characteristics by frequency selective feedback from external optical resonators. Results for the free running and the spectrally stabilized diode are presented.

Novel high peak current pulsed diode laser sources for direct material processing

Diode laser systems are well established for applications which demand high continuous wave (cw) power. These applications are material processing like cutting and welding of metals as well as polymers where diode laser systems are less expensive and more compact than solid state lasers. Even though the optical output power and the beam quality of diode lasers are increasing steadily, the use of these sources is generally limited to cw applications. For processes during which ablating of material is demanded, however, conventional diode lasers are inferior compared to pulsed solid state lasers as diode lasers suffer from the absence of optical intracavity q-switching. Some examples of these applications are coating removal and marking. To overcome this drawback, we have developed several diode laser systems that use high peak-current drivers and thereby allow to operate the diode lasers at currents up to 500 A. The pulse source was tested with fiber coupled single emitters, conventional diode lasers and customized AR-coated diode laser bars. With the new diode laser driver, a peak output power of 250 W can be achieved with pulse durations of approx. 100 ns. Polarization coupling of two bars increases the power by a factor of two. Thereby an output power of 500 W can be demonstrated. These systems reach an intensity of 27 MW/cm2 per diode laser bar which is sufficient for ablating processes. We will demonstrate the design of the prototype system as well as results of marking and coating removal experiments with the system.

Fiber coupled diode laser of high spectral and spatial beam quality with kW class output power

High optical output power in the multi-kW range from a fiber coupled diode laser can reach the beam quality of lamp pumped solid state lasers. Direct diode laser application as deep penetration metal welding becomes feasible. Polarization and wavelength multiplexing are established techniques to scale the optical power of diode lasers at almost constant beam quality. By use of volume diffraction gratings in an external cavity laser it is possible to constrict the spectral bandwidth of diode lasers and to reduce the wavelength shift related to temperature or current injection. Due to the stabilization of the wavelength multiplexing of diode laser beams at small distance of the center wavelengths can be realized. The development of a fiber coupled diode laser is presented. The set up consists of twelve modules which serve for an average optical power of 1.5 kW. Each module utilizes dense wavelength multiplexing of two diode laser bars with a center wavelength spacing of 3 nm. The diode laser bars are wavelength stabilized at center wavelengths of 908 nm, 911 nm, 975 nm and 978 nm. The spectral bandwidth of all diode laser bars is within 1 nm in the full power range. Stable operation at an average power of 136 W at 908/911 nm and 115 W at 975/978 nm with a wavelength shift less than 0.1 nm is achieved by the modules. Further coarse wavelength and polarization multiplexing and beam transformation enable fiber coupling to a 600 &mgr;m fiber with a numerical aperture of 0.175 (95% power inclusion).

25-W Monolithic Spectrally Stabilized 975-nm Minibars for Dense Spectral Beam Combining

Low-fill-factor laser bars composed of five narrow-stripe broad-area lasers (30 μm × 6000 μm) with monolithic spectral stabilization are presented. Each laser on the bar emits at a unique wavelength from 970 to 980 nm with a channel spacing of 2.5 nm. Narrow spectral line width c1 nm per emitter is obtained by implementing 40th Bragg order distributed feedback surface gratings with varied grating periods. The design and fabrication of these laser bars is reviewed, with optimal grating etch depth determined using simulations that combine 2D-simulation of the reflectivity and optical losses of a 40th-order surface grating with coupled mode theory. The selected etch depth delivers a grating coupling coefficient of κL ~ 0.2, which is shown to be sufficient to suppress lasing in Fabry-Perot modes for narrow line operation across all wavelength channels. However, additional optical scattering losses of the gratings are found to limit output power and efficiency. The final realized minibars deliver 25-W continuous wave output power with a conversion efficiency >40% and operate with a beam parameter product of ~1.6 mm mrad per single emitter, twofold improved in comparison with typical 90-μm wide broad area laser. Such laser minibars are an attractive source for brilliant dense spectral beam combining.

Simulation and analysis of volume holographic gratings integrated in collimation optics for wavelength stabilization

Integrating volume holographic gratings into micro-optical components such as cylindrical fast-axis collimation lenses (VHG-FAC) for diode lasers constitutes a promising concept for wavelength stabilization by forming an external cavity laser. Compared to standard wavelength stabilization configurations the integrated element reduces the alignment complexity and is furthermore insensitive to the smile-error of diode laser bars. In order to configure and optimize these components the diffraction of the divergent field distribution of a broad area semiconductor laser must be calculated. The present paper presents the extension of the coupled-mode theory in order to calculate the spectral distribution of the diffracted field and the coupling efficiency within the external cavity. The model was extended to three-dimensional space and supplemented to include surface effects, polarization dependency and wave-optical propagation. The asymmetric spectral distribution emitted by an external cavity laser with VBG-FAC is tracked back to the feedback of highly divergent radiation diffracted at the holographic grating. Power losses due to the coupling efficiency within the cavity are also calculated for various field distributions and compared with experimental data. In summary the mathematical model allows to estimate the minimum spectral width and the losses using a VHG-FAC in an external cavity. Thus the injection locking concept using the VHG-FAC can be compared to the spectral characteristics and estimated power losses of standard wavelength stabilization configurations, e.g. the alignment of the grating in the collimated beam.