Soenke Tautz

Recent results of blue and green InGaN laser diodes for laser projection

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°C to 80°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°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 nonradiative 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 E0=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 ~60-80mA, high slope efficiency ~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°C.

Highly reliable and efficient laser bars and cost efficient packaging

High power diode lasers convince by their very efficient conversion of electrical into optical energy. Besides high efficiencies and record absolute power levels, reliability in all possible operation modes and cost become increasingly important. We present diode laser bars in the 940nm range with wall plug efficiencies of about 65% at an emission power of 100W and with excellent reliability. The test had been performed on a stack with 5 bars at an output power of 100W per 1cm bar and after about 4000hrs test time, lifetimes of more than 40 000hour were estimated. The efficiency of these bars was at the beginning and at the end of this test about 65%. Operation modes between cw operation and q-cw (200μsec pulses) were evaluated and it will be shown, that pulses in the range of 1Hz are the hardest conditions, which can cause catastrophic failures. Using submounts with matched thermal expansion coefficient, this failure was prevented and lifetimes similar to cw-operation were reached. In order to reduce costs of laser power, we developed a laser package that offers high power at good reliability and provides a collimated beam for about 5

GaInN laser diodes from 440 to 530nm: a performance study on single-mode and multi-mode R&D designs

/W, as a cost target in mass production conditions. This was achieved by using packaging concepts that were developed for high power semiconductor devices. These results will further enhance the applicability of diode lasers in industrial application.

InGaN power laser chips in a novel 50W multi-die package

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 nitride based laser diodes for projection

In this paper we report recent developments on high power blue laser chips. Reduction of internal losses as well as optimized thermal management had been essential to increase optical output power. R and D samples with average performance of 3W optical output at junction temperatures of 130°C are demonstrated. The chips are suitable for use in a novel multi chip housing: For the first time up to 20 blue laser chips have been packaged into one compact housing resulting in the first InGaN laser device with optical output > 50W. The highly integrated package offers a unique small size. The outer dimensions of the package are 25.5mm x 35mm with an emitting surface of 16mm x 16.5mm. Therefore the complexity of optical alignment is dramatically reduced and only a single sheet multi lens array is required for beam collimation. Besides the unique technical performance the multi-die package offers significantly lower assembly costs because of the reduced complexity and assembly time. The butterfly package contains 4 bars with up to 5 multimode laser chips in series connection on each bar operating at 2.3A. The typical module wavelength is 450nm +/- 10nm. At a case temperature of 50°C the R and D samples achieve efficiencies of typ. 30% and an optical output power of 50W corresponding to an electrical power consumption of ~165W. This new technology can be used for high performance light engines of high brightness projectors.

Laser light source and method for producing a laser light source

Summary form only given. Two fields for laser projection have attended big interest since several years due to the potential of high volume markets. One is the mobile projection; ideally a projection unit embedded in a mobile phone. For customer acceptance a reasonable price and high brightness of such a device are crucial. Mobile pico projectors need a brightness of 15 to 20 lumens. While red and blue laser with sufficient output power are available, direct green single mode laser with the necessary power are still under development. We report R&D results of single mode green laser with output power up to 200mW. This demonstrates that long term operation of green laser at 80mW to 100mW output power is realistic. The second field is the business projection with brightness up to several thousands of lumens. The concept here is to use blue high power laser as blue light source and also for the green light by conversion technology. The challenge is to reduce the cost of the light engine which is strongly dominated by the output power of the laser. We report R&D results of blue high power laser with an output power up to 3 watt.