We investigate the angularly, temporally, and spectrally resolved far-field dynamics of a single lateral mode green (Al,In)GaN laser diodes. For applications as directly modulated light source in laser projection, for AR/VR/MR, etc., a stable beam pointing angle and width of the far-field is required. Combing an angle-resolved measurement with a spectrometer and streak camera, we characterize optical intensity as function of far-field angle, wavelength, and time. Beam pointing angle and width are then calculated from the moments of the angular intensity distributions. We observe a stable far-field behavior for the narrow ridge. This is in contrast to strong variations in beam pointing direction and far-field profile during short pulses for earlier (Al,In)GaN laser diodes, where the dynamics could be tracked to heating of the waveguide. Therefore we attribute the observed stable dynamics of state-of-the-art narrow ridge laser diodes to their low internal losses, low forward voltage, and consequently low heating.

Blue high-power semiconductor lasers have increased greatly in performance over the recent decade enabling new application fields from high brightness projection up to materials processing beyond 1000W output power systems. Base for best system performance is optimal chip design and reliability of the semiconductor device. In this paper chip design optimization of blue high-power semiconductor laser bars will be shown: In contrast to IR laser bars with high lateral emitter fill factors beyond 50%, optimum design with maximum output power and efficiency for GaN laser bars is currently at very low fill factors in the range of 10%. Laser bar designs ranging from 5% fill factor up to 12.5% fill factor were fabricated and investigated. Additionally, two different emitter pitches with 200μm and 400μm were compared. The design with an emitter width of 30μm and a pitch of 400μm resulted in overall best performance. Additionally, lifetime investigations of single emitters in TO-packages will be discussed. The laser diodes were tested up to 5000h duration at different conditions in operating temperatures ranging from 64°C to 96°C and output power up to 3.5W. Dominating degradation mechanism is wear-out which is accelerated by optical output power and additional thermal activation. Extrapolation of the test results in combination with an acceleration model points towards a median lifetime of up to 65.000h for 25°C operation.

More and more applications are using GaN laser diodes. Visible blue laser devices are well established light sources for converter based business projection of several thousand Lumens. Additional laser-based concepts like near-to-eye projection push device requirements above heretofore limits. In 2017, threshold currents of 10 and 20mA were reported for single mode blue and green laser, respectively. We will present a drastic reduction of laser threshold of green R&D laser samples by more than a factor of 2 down to 10mA. We also will discuss turn-on delay as a limiting factor for modulation speed and spatial resolution of flying spot projection. On the other side, new applications may occur in the near future. We will present research data on blue laser bars as a possible component for industrial applications like for materials processing. LIV characteristics are measured up to power levels of 107W. We observe power conversion efficiencies of 44% at 60W output power for our best samples.

We present the improvements in power conversion efficiency of high power multi-mode blue laser diodes. The improved device architecture shows a peak wall plug efficiency of ~44.2 % at room temperature. Furthermore, the device delivers an optical output power reaching 3.7 W at the peak efficiency. The signs of thermal roll-over at room temperature start appearing near 5 A of injection current for this particular device design. The device generates an optical output power of more than 7.5 W with overstressed current injection at room temperature. We have also evaluated the high temperature electro-optical performance. The roll-over point is shifted by approximately 1 A when the device is operating at 85°C. The optical output power reaches 4.8 W at thermal roll-over.

The range of applications of blue and green lasers is increasing from year to year. Driving factors are costs and performance. On one hand we study the characteristics of low power R&D c-plane laser structures with improved Gaussian vertical and horizontal beam profile: We present new best values for efficiencies of single mode green lasers of 10.8% at 517nm and new long wavelength data at 532nm with efficiency of 6.5%. Furthermore, we present a new R&D design of a blue single mode laser diode with a very low threshold of 8.5mA. On the other hand, recent R&D results on broad area multi-mode power designs are shown: Efficiencies of 43% at 4W optical output power are achieved. Lifetime tests as long as 10000h are presented. High reliability is reached by a new facet design.

Blue and green InGaN-based R& D laser structures on c-plane GaN substrates are investigated. We analyzed carrier injection efficiencies as well as internal quantum efficiencies up to laser threshold. The injection efficiency of the blue laser structure is measured to be 78%. The internal quantum efficiency of spontaneous emission reaches 50% at 30A/cm(2) and 32% at laser threshold. For the green laser structure we found an injection efficiency of 71%, a maximum of internal efficiency of 36% and, at laser threshold. a value of 28%. Both, recombination on defects as well as Auger effect are identified as relevant loss processes up to the laser threshold. An improved 515nm R& D single mode laser in TO56 can is presented. The optical output power of the green single mode laser reaches 250mW in continuous wave operation underneath thermal roll-over. Wall plug efficiency is as high as 9%. In the next step we investigate high power multimode lasers. The new power green R& D laser reaches maximum power of 1.25W at thermal roll-over. The current-output characteristic is nearly linear up to 0.9A and 0.6W. At higher currents thermal bending is observed. We measured a maximum wall plug efficiency of the green multimode laser of 13%. The power blue R& D laser in TO90 metal can reaches 5.5W prior to roll-over having the wall plug efficiency of 32% at 3.5W.

Blue and green laser diodes are gaining momentum in applications requiring visible sources with high density of light, spectral purity or quick modulation rates. Challenges and advances toward better efficiency and performance will be presented.

Longitudinal mode competition in (Al,In)GaN laser diodes at λ = 445nm and 515 nm with mode competition frequencies from 10 MHz to 150 MHz is observed. Up to two dozen lasing modes oscillate with the lasing mode rolling from the short wavelength edge to the long wavelength edge of the gain profile. The experimental results can be described very well with a set of multi-mode rate equations including self-, symmetric and asymmetric cross gain saturation. By tuning essential parameters of the gain saturation terms, mode competition disappears and single mode operation as well as mode clustering is found. This proves that the mechanisms of gain saturation have not only a profound impact on the complex temporal-spectral behavior but also explains mode clustering in (Al,In)GaN laser diodes, both in pulsed and continuous wave (cw) operation as a natural nonlinear effect without the necessity to add noise.

InGaN quantum wells for direct green lasers with indium concentrations of about 30% are studied in respect to defect structure and influence of defects on device lifetime. A new test structure enables us to determine strain relaxation in In-rich layers by X-ray diffraction even at typical well thicknesses of few nanometers. We show that there is a high risk to generate screw and edge dislocations in the In-rich quantum wells in non-optimized structures as verified by transition electron microscopy. Such defects strongly influence the device lifetimes of green lasers. Non-optimized devices have a drop of output power on a time scale of 100 h. The threshold current increases proportional to the square root of time. Additionally, a linear correlation of slope efficiency versus one over laser threshold current is observed. The degradation process is explained as reduced carrier injection efficiencies related to the generation of additional charged defects in the active layers. Optimized active layers already enable stable operation over a period of more than 1000 h at 50 mW constant power at 517 nm emission wavelength with an extrapolated increase of operation current by <30% within this time. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

There is a big need on R&D concerning visible lasers for projection applications. The pico-size mobile projection on the one hand awaits the direct green lasers with sufficiently long lifetimes at optical powers above 50mW. In this paper we demonstrate R&D-samples emitting at 519nm with lifetimes up to 10.000 hours. The business projection on the other hand requires high power operation and already uses blue lasers and phosphor conversion, but there is a strong demand for higher power levels. We investigate the power limits of R&D laser structures. In continuous wave operation, the power is limited by thermal roll-over. With an excellent power conversion efficiency of up to 29% the thermal roll-over is as high as 2.5W for a single emitter in TO56 can. We do not observe significant leakage at high currents. Driven in short pulse operation to prevent the laser from self heating, linear laser characteristics of optical power versus electrical current are observed up to almost 8W of optical power.

Mobile laser projection is of great commercial interest. Today, a key parameter in embedded mobile applications is the optical output power and the wall plug efficiency of blue and green lasers. We report on improvements of the performance of true blue riedge waveguide InGaN lasers at 452nm with cw-output power up to 800mW in overstress and mono mode operation up to 500mW in a temperatures range of 20 degrees C to 80 degrees C. We succeeded in high and almost temperature independent wall plug efficiencies >20% at stable output power levels from 200 to 500mW in cw-operation. Due to several improvements of our blue laser diodes we now estimate life times is in the order of 40khrs for 80mW output power in cw-operation at 40 degrees C. Additional overstress degradation tests at power levels up to 200mW show a strong dependency of lifetime with output power. Furthermore, we present pioneering results on true green InGaN laser diodes on c-plane GaN-substrates. The technological challenge is to achieve In-rich InGaN-quantum wells with sufficiently high material quality for lasing. We investigated the competing recombination processes below laser threshold like non-radiative defect recombination by electro-optical measurements, such confirming that low defect densities are essential for stimulated emission. A model for alloy fluctuations in In-rich InGaN-MQWs based on spectral and time resolved photoluminescence measurements yields potential fluctuations in the order of E-0=57meV for our blue laser diodes. To get a closer insight into the physics of direct green InGaN-Laser we investigated the inhomogeneous broadening of experimentally measured gain curves via Hakki-Paoli-measurements in comparison to calculated gain spectra based on microscopic theory showing the importance of strong LO-phonon coupling in this material system. Investigations of current dependent gain measurements and calculations yield a factor of 2 higher inhomogeneous broadening for our green lasers than for our blue laser diodes on c-plane GaN. Based on the improvements of the material quality and design we demonstrate true green InGaN-Laser in cw-operation at 522nm with more than 80mW output power on c-plane GaN. The combination of low laser threshold similar to 60-80mA, high slope efficiency similar to 0.65W/A and low operating voltage 6.9-6.4V of our green monomode RWG-Laser results in a high wall plug efficiency of 5-6% in a temperature range of 20-60 degrees C.

The origin of indium fluctuations in indium-rich quantum wells (QWs) is of high interest for direct green laser diodes. We present the correlation of morphological features such as macrosteps investigated by AFM measurements and wavelength fluctuations seen in cathodoluminescence images of InGaN/GaN QWs grown by metal organic vapor phase epitaxy. We observe an opposed wavelength shift of 5 nm in the vicinity of macrosteps for the investigated UV and green InGaN QW samples. We present a growth model taking adsorption, desorption, and migration processes into account to explain this difference via a temperature dependent change in indium incorporation in the vicinity of the observed macrostep edges. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We present analysis of the experimentally determined gain coefficients go for green InGaN lasers with pulsed wavelengths from 495 to 526 nm. Variable facet coating as well as variable cavity length method are used to deduce the dependency of go on wavelength. From the experimental data we found that better laser performance at shorter wavelengths correlates with higher material gain. Therefore, lasers at 504 nm reach wall plug efficiencies (WPE) up to 5.3% at 50 mW cw output power, while lasers with 525 and 529 nm show a WPE up to 3 and 2.3%, respectively. The WPE for longer wavelengths is limited due to thermal rollover in cw operation. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We report on direct green lasers based on nitride semiconductors that are suitable for mobile projection applications. The TO38 packaged lasers achieved in continuous wave operation at 522nm wavelength more than 80mW optical power. The wall plug efficiency reached 5∼6% depending on operating temperature. The good performance of the lasers was possible due to the low threshold currents in range of 60∼80mA and large slope efficiency ∼0.65W/A. The optical beam properties are also shown.

We pushed direct green laser diodes towards longer wavelengths at 524–532 nm based on improvements of epitaxial design and material quality on c-plane GaN substrate. Mounted ridge laser diodes show significant performance improvement in cw operation. For 524 nm laser, wall plug efficiency up to 2.3% at 50 mW optical output power is achieved. In pulse mode operation we demonstrate broad-area test lasers with an emission wavelength of 531.7 nm. Nonpolar and polar substrates are compared with respect to indium content in InGaN quantum wells. The limiting factors for achieving longer wavelengths and better performance of green lasers are discussed from this viewpoint.

The challenges of green InGaN lasers are discussed concerning material quality as a function of InGaN composition, quantum well design and piezoelectrical fields. Investigations of polar quantum well designs and comparison with simulated nonpolar structures demonstrate that the quality of the indium rich layers is more important than the influence of interface charges. A high risk of dark spots at high In concentrations of 26-33% is observed. Small changes of about 2% of In significant reduce or increase the quantity and size of dark luminescence areas. Polar designs are a trade-off between low indium concentrations of 4 nm wide quantum wells and high overlap of electrons and holes in 2 nm narrow designs. Furthermore, our single quantum wells have less non-radiative defects than indium rich multi-quantum well structures. Optimized active layer designs and the material qualities enable us to get green InGaN lasers on c-plane substrates for cw operation at 515-524 nm and wall plug efficiencies of 3.9-2.3%. Slope efficiency of 0.3-0.4 W/A allows up to now highest optical output power of 50 mW. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We present true green InGaN ridge waveguide (RWG) laser diodes (LDs) at 520 nm on c-plane GaN substrates in pulse operation at room temperature. Defect reduction in the In-rich quantum wells by improving growth conditions of the epitaxial layers is the key parameter to demonstrate laser operation at this wavelength. Carrier lifetime measurements in combination with electroluminescence (EL) data and simulations of competing recombination processes below laser threshold confirm that the reduction of defects in the light emitting InGaN quantum wells is essential to realize true green nitride-based LDs with low laser threshold of 125 mA and good slope efficiency of 220 mW/A for optical output power level up to 50 mW. Investigation of the lateral far field of 520 nm RWG LDs shows a perfect beam quality. The measured vertical far field shows a substrate mode due to leakage into the transparent GaN substrate supported by waveguide simulations with an additional internal loss of less than 1 cm(-1). Mounted InGaN-based ridge LDs at 500 nm demonstrate high optical output power of 35 mW in cw operation with good wall plug efficiency of 3.5% at room temperature. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Based on recent improvements of growth of In-rich InGaN quantum wells with low defect density, we demonstrate current driven InGaN laser diodes at wavelengths as long as 500 nm. The laser structures are grown on c-plane GaN substrate and are processed as broad oxide-insulated stripe laser diodes. We discuss the impact of the piezoelectric field on the emission energy of long wavelength laser diodes for this growth orientation. The combination of low threshold current density of 8.2 kA/cm2 with high slope efficiency of 650 mW/A enables high output powers up to several tens of milliwatts.