High-power diode lasers with in-situ-structured lateral current blocking for improved threshold, efficiency and brightness

High-power GaAs-based diode lasers are the most efficient light sources and thus used in many industrial applications. Higher peak output power ( P opt ) and efficiency ( η E ) are sought to reduce operating cost (€/W) [1] , but P opt is limited by a range of interacting saturation mechanisms. Improved experimental diagnosis of the dominant effects with lateral and longitudinal hole-burning effects playing a key role is needed and ways to address these issues need to be found [2] . For a given device construction, improvements to the (vertical) epitaxial layer design can lead to strong improvements in both P opt and η E . Here, we study devices with resonator length L = 4 mm and stripe width W = 90 µm, mounted junction down on CuW carriers, with operating wavelength λ ~ 970 nm. As previously reported, although such L = 4 mm, W = 90 µm devices show high peak η E when using asymmetric large optical cavity (ASLOC) epitaxial designs, both P opt and η E are strongly improved through the use of extreme triple asymmetric (ETAS) designs. These ETAS designs use thin p-side waveguide and asymmetric graded index layers in the vicinity of the quantum well (QW) to reduce series resistance, carrier leakage and optical loss [3] . In a previous study, it is shown that compared to the ASLOC structure P opt and η E increases from 14 W to 19 W and 65% to 67%, respectively, at 25 °C under continuous wave (CW) operation using ETAS structures

The goal of this work is twofold. First we study residual mechanical bonding induced stress in state-of-the-art 980 nm high-power single-mode GaAs-based laser diodes (LD) mounted on either standard plain material (Fig. 1-a), basic bi-material (Fig. 1-b) composite, or optimized bi-material composite submounts with a standard Au-Sn eutectic alloy solder pad. Then we show how to enhance the heat dissipation from the operating LD via its submount. The motivation of this work is the understanding of the physical phenomena playing a critical role on the electro-optical performances, current consumption, reliability, and lifetime of the LD-submount assemblies.

A new approach was proposed and its role in improvement of the beam quality of high-power broad-area diode lasers was demonstrated, in which a composite arrow array and trench microstructure was used to suppress the beam waist and tailor the high order lateral modes. The beam waist shows a special shrinkage with increasing injection current resulting from the combined effect of mode tailoring and the thermal lens effect. A 58% improvement in lateral beam parameter product was realized compared with conventional broad-area diode lasers.

We report the first integrated injection-locked high-power continuous-wave diode laser array with an on-chip independently controlled master laser. This device emits a near-diffraction-limited (0.5° full width at half maximum) single-lobed far-field emission beam at single-facet powers up to 125 mW. Also, by current tuning the emission wavelength of the master laser, we observe steering of the single-lobed emission over an angular range of 0.50° at a rate of −1.2×10−2 deg/mA. Our work demonstrates the feasibility of incorporating active optical injection and control in the structure of high-power diode laser devices.

High-power broad area diode lasers emitting at 980 nm with a maximum output power of 20 W at 15degC and with reliable operation over 2000 h at 15 W and 25degC will be presented.

We demonstrate a compact system for single-pass frequency doubling of high-power GaN diode laser radiation. The deep UV laser light at 222.5 nm is generated in a β-BaB2O4 (BBO) crystal. A high-power GaN external cavity diode laser (ECDL) system in Littrow configuration with narrowband emission at 445 nm is used as pump source. At a pump power of 680 mW, a maximum UV power of 16 μW in continuous-wave operation at 222.5 nm is achieved. This concept enables a compact diode laser-based system emitting in the deep ultraviolet spectral range.

High power diode lasers from a few Watts up to several Kilowatts have entered industrial manufacturing environment for materials processing applications. The technology has proven to show unique features, e.g. high efficiency, small size, low energy consumption and high reliability. In the first part of this paper a short description of state-of- the-art high power diode laser technology and applications is provided and the benefits and restrictions of this laser technology will be evaluated. For large scale penetration into the manufacture market, the restrictions, especially the rather poor beam quality of high power diode lasers compared to conventional lasers have to be overcome. Also, the specialities of the high power diode lasers, i.e. their modular structure and their extremely small size have to be translated into laser manufacturing technology. The further improvement of high power diode lasers as well as the development of new diode laser specific manufacturing technologies are the essential topics of a National German Minister Priority Project entitled 'Modular Diode Laser Beam Tools': 22 Partners from industry and institutions, 4 semiconductor experts, 5 laser manufacturers and 14 applicants are working together in frame of this project to work out and transfer a joint strategy and system technology to the benefits of the future of high power diode laser technology. The goals, the structure and the work of this project will be described in the second part of this paper.

Laser-based displays have gathered much attention because only the displays can express full color gamut of Ultra-HDTV, ITU-R BT.2020. There are many types of laser-based displays. A projector with a single spatial light modulator is mainstream in the displays so far. This projector uses the laser light sources under pulse with duty of 20% to 50%. We improve a triple-emitter red color 638-nm high-power broad area laser diode (LD) for this application. Countermeasures are adopted to improve the output power characteristics and strengthen the facet to the fatal degradation. The improved triple emitter shows the output of 5.0 W at the injection current of 5 A under pulse with duty of 30%, case temperature of 25°C. The output is increased by 4.1% compared to our former one. The peak wall plug efficiency reaches to 41.6% under 25°C. The long-term aging test is also performed. Mean time to failure of the improved LD is estimated 72,000 h under the output of 3.5 W, pulse condition with duty of 30%. The value is ∼3.3 times longer than that of the former.

Due to the narrow absorption peak of Yb in glass at 975nm, a wavelength stabilisation is necessary for pump laser diodes. Our original approach is to use a Distributed Feedback broad area structure which is made possible since we use an Aluminium free active region laser structure. The target is to obtain high optical power of 10W with both a reduce wavelength evolution with temperature and a low spectral width. On a 3mm x 100 μm BA Fabry Perot laser, we have obtained a high power of 10W. Design, fabrication and results on BA DFB lasers will be presented, showing a low spectral width at 976nm at 3W output power.

We report results of multi-cell life tests performed on nearly (500) laser diodes representing our new generation of very efficient high power broad area 9xx nm lasers. The acceleration model showed a steep power dependence of the failure rate with an exponent of nearly 6. Improvement in the facet passivation process resulted in significantly less power acceleration of failures. Analysis of the life test on upgraded lasers showed median lifetime of 1,500,000 hours at operating conditions of 8W and 350°C. Optical powers as high as 17.8W for thermally limited CW operation and 32W for 20 μs pulsed operation were recorded. The CW life test was complemented by a life test performed at power cycling conditions (1Hz repetition rate, 50% duty cycle).

In this work we report on a novel optical design for beam shaping and focalization of high-power diode laser bars. The goals of our study are: the increase the optical throughput of the beam shaping device with respect to standard solutions and either to enhance the irradiance on a target or to inject the laser beam into a smaller fibre than with respect to beam shaping system based on plane surfaces. The high power diode laser bars pose serious difficulties in their optical handling due to their strong difference between the two transverse axes, which induce a strong astigmatic and asymmetric output radiation. As is well known, the beam quality is very different in the two axes called slow axis and fast axis, and in particular the slow axis is composed by the superposition of several multimodal sources. The beam quality in this axis is very low (its etendue may exceed 2000 mm mrad). On the other hand, the fast axis has a very high beam quality, near diffraction limited, although with very high divergence (30°-50°). The common solution for the application of the laser radiation is a fast axis aspheric micro lens in front of the emitters, in order to achieve its collimation. Typical values of the fast axis collimated beam are 0.7mm and less than 6mrad. However, the so obtained collimated beam is poorly focusable with a standard lens, and a few methods were proposed to overcome the problem. The more relevant solutions include: the stepped mirror technique, the plane parallel mirrors pair, micro prisms array and confocal micro lens array. Each of these techniques is based on the equalization of the beam parameter product by the subdivision of the beam in the slow axis and its reshaping. For all these techniques the efficiency spans from 50% to 70%. The best focalization results allow the coupling in a fibre of 400μm diameter, with NA-0.22. The aim of this work is the design and the realization of a new device, that is considered as target the following aspects: 1) the maximum optical efficiency in the beam shaping process, 2) the optimal equalization of the beam parameter product for the two axes, 3) the use of few optical elements and 4) a very compact size. These goals are addressed by a scheme that splits the collimated beam from the laser diode into different portions while the length of the optical paths of each sub element is kept constant, and by the subsequent use of short focal length aspheric lenses for the focalization of the transformed beam. Each sub-beam is deflected by a couple of plane parallel mirrors, whose normal is directed to equalize the BPP without any mutual shadowing. An optimal solution can be easily envisaged for a laser source of common size of 0.7 x 10 mm. The condition on equal optical path length has the noticeable property of placing the virtual position of the individual portions into which the original beam is split at the same distance with respect to target. Thanks to this, their subsequent focusing is unaffected by the axial displacement of the common solution by the stepped mirrors. In fact, to correct this effect, this latter technique requires the use of a prism pair, involving complexity, size enlargement and higher costs. In this work both an extensive ray tracing and optical analysis is presented as well as the experimental characterization of an experimental model. Moreover, we also report on the technique for the realization of th tilted-face plane mirrors of which is composed our beam shaping device. The scheme of beam shaping here reported can be extended to higher power beam by means of the technique of the beam combination by polarization coupling or that of the optical beam compression. Examples of theses developments are discussed in the paper, and experimental results presented. The most direct applications of the class of optical devices here reported are the high power diode laser direct application in material processing or manufacturing, the coupling into multimode optical fiber of the diode laser radiation as well as the fiber laser end pumping.

We have coherently combined a high-power broad-area laser diode array by using a feedback loop closed off-axis external Talbot cavity. The off-axis feedback from two gratings provides transverse-mode control of broad-area lasers. The Talbot configuration of the external cavity implements diffractive coupling among laser diodes. Feedback from two gratings increases external cavity quality factor and spectrum selection capability. As a result, spatial coherence was improved and spectral linewidth was narrowed down. The high visibility of the far-field profile indicates that high spatial coherence was achieved. We also observed symmetric far-field profiles indicating that laser array was phase locked to in-phase and out-of-phase super-modes, respectively. Transition between these super-modes was observed by tuning one grating's tilted angle.

Military, industrial, and medical applications have expressed interest in using ~1550nm laser diodes as efficient laser sources for reduced eye safety concerns, especially where free-space propagation is concerned. In addition, covert military applications, including pointers and illuminators, benefit from the spectral insensitivity of common sensor types to ~1.5um wavelengths. While more efficient than other sources, high-power broad-area laser diodes can sometimes require complex packaging and beam shaping/combining optics in order to meet requirements for brightness, uniformity, stray light, and environmental insensitivity. This paper will highlight and discuss a range of developments exploring laser diode components near 1.5um for free-space military applications, including broadarea high-power illuminators and highly-collimated pointers. Options for tailored beam conditioning and performance results will also be presented.

Welding thin sheet metal in the 1 mm range with high power diode lasers closes the feed rate gap between conventional techniques like TIG- or plasma-welding and CO2- or Nd:YAG-laser welding. This was presented by Fraunhofer IWS at former ICALEO events. Applying sheet metal welding with high power diode lasers for industrial manufacturing raises the question of monitoring full penetration. Two sensor principles have been investigated concerning their suitability at welding nickel base alloy sheets in a controlled shield gas atmosphere. A special shield gas chamber for high power diode laser applications has been applied to avoid any oxidation of the weld. Optical emission spectroscopy and an infrared sensitive camera delivered promising results each. Both sensor principles are an approach for future optimized welding sensors.Welding thin sheet metal in the 1 mm range with high power diode lasers closes the feed rate gap between conventional techniques like TIG- or plasma-welding and CO2- or Nd:YAG-laser welding. This was presented by Fraunhofer IWS at former ICALEO events. Applying sheet metal welding with high power diode lasers for industrial manufacturing raises the question of monitoring full penetration. Two sensor principles have been investigated concerning their suitability at welding nickel base alloy sheets in a controlled shield gas atmosphere. A special shield gas chamber for high power diode laser applications has been applied to avoid any oxidation of the weld. Optical emission spectroscopy and an infrared sensitive camera delivered promising results each. Both sensor principles are an approach for future optimized welding sensors.

Er³⁺:YAG eye-safe laser emitting at 1.6 μm is an interesting source for various applications such as remote sensing, ranging, designation and free-space communications for two main reasons: its emitting wavelength lies in a region of high atmospheric transmission and high sensor sensitivity and the resonant pumping into the ⁴I_13/2 upper laser manifold ensures highly efficient operation. The recent availability of internal grating stabilized narrow linewidth, high-power laser diodes in the 1.53 μm range, makes this laser even more appealing. The only shortcoming to be solved for a really efficient resonantly diode pumped Er³⁺:YAG laser is how to have a good overlap between the pump radiation and the laser cavity mode. Indeed, due to up-conversion processes among the Er³⁺ ions, to achieve efficient lasers it is necessary to use low doped samples. This requires the use of rods with lengths of several cm that are not compatible with the low beam quality of the diode lasers. In this work, we report on a resonantly diode pumped Q-switched Er³⁺:YAG laser with a crystalline fibre-like geometry emitting at 1.64 μm. In this scheme, the pump radiation is confined into a 60 mm long crystal with a diameter of 1.2 mm thanks to the multiple total internal reflections (TIR) that occur on the barrel surface, allowing efficiently pumping of such a long crystal. A maximum output power of more than 14 W in continuous wave mode and pulse energies of 8 mJ in Q-switching mode have been observed, when pumped with - 40 W of absorbed power. Even if these values are still far from the performances reported using hybrid fibre-bulk laser scheme, these results clearly show that TIR-based Er³⁺:YAG fibre-shaped crystalline rod laser is a promising technology for the development of efficient high-power and high-energy eye-safe laser. Finally, the effect of thermal lensing on such crystalline fibre geometry is discussed.