Uwe Strauss

True Green Laser Diodes at 524 nm with 50 mW Continuous Wave Output Power on c-Plane GaN

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.

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.

Spectral dynamics of 405 nm (Al,In)GaN laser diodes grown on GaN and SiC substrate

We investigate the spectral properties of violet 405 nm (Al,In)GaN laser diodes (LDs). Depending on the substrate the LDs are grown on, the lasing spectra show significant differences. LDs grown on low dislocation GaN substrate have a broad spectrum with several longitudinal modes, while LDs grown on SiC substrate are lasing on a single longitudinal mode.With increasing current, the laser emission of LDs grown on SiC substrate jumps from one longitudinal mode to another (mode hopping), whereas GaN substrate LDs show a smooth but asymmetric mode comb. The different envelopes of these spectra can be understood by assuming a variation of the gain for each individual longitudinal mode. With a high spectral resolution setup, we measure the gain of each longitudinal mode, employing the Hakki-Paoli method. Measurements show a slightly fluctuating gain for the modes of GaN substrate LDs, but much larger fluctuations for LDs on SiC substrate. We carry out simulations of the longitudinal mode spectrum of (Al,In)GaN laser diodes using a rate equation model with nonlinear gain (self saturation, symmetric and asymmetric cross saturation) and including gain fluctuations. With a set of parameters which is largely identical for LDs on either substrate, the simulated spectra truly resemble those typical for LDs on GaN or SiC substrate.

Gain of blue and cyan InGaN laser diodes

Experimental gain spectra of 450 and 490 nm laser diodes on c-plane GaN are analyzed by detailed comparison with the results of a fully microscopic theory. The gain calculation shows the importance of electron LO-phonon coupling. The whole spectral gain shape, not only the low energy tail, is strongly influenced by the LO-phonon contribution. The inhomogeneous broadening parameter increases by a factor of about two for the cyan laser diode in comparison with the blue laser structure. This indicates an increase in alloy and thickness fluctuations for the longer wavelength material.

Measurement and simulation of filamentation in (Al,In)GaN laser diodes.

(Al,In)GaN-based laser diodes with ridge widths broader than a few micrometer tend to show filamentation effects in the lateral direction. By time-resolved scanning near-field optical microscopy, we find different kinds of filaments depending on ridge width and lateral position. We investigate these effects systematically and compare them to the results of corresponding simulations, which are based on a simple rate equation model including the lateral dimension. By this comparison we find a consistent and reasonable set of material parameters that can describe the laser diode. Furthermore, we discuss several reasons for filamentation dynamics like ridge asymmetry or spatial hole-burning, as well as critical temperatures that induce filamentation.

Beyond blue pico laser: development of high power blue and low power direct green

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.

Antiguiding factor of GaN-based laser diodes from UV to green

We measure the antiguiding factor of (Al,In)GaN laser diodes emitting in the violet, blue and green spectral range by combining optical gain-spectroscopy with measurements of the charge-carrier induced refractive index change. A precise determination of the thermal resistance of the laser diodes allows us to keep the temperature of the active region constant during the whole measurement and thus to exclude any thermal effect on the refractive index. For laser diodes emitting in the range from 409 to 511 nm, the antiguiding factor at the laser wavelength is 4.1±0.5.

Longitudinal mode competition and mode clustering in (Al,In)GaN laser diodes.

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.

Advances in performance and beam quality of 9xx-nm laser diodes tailored for efficient fiber coupling

The impact of new direct-diode and fiber laser systems on industrial manufacturing drives the demand for highbrightness diode laser pump sources suitable for simple fiber coupling with high efficiency. Within the German funded project HEMILAS laser mini-bars with different bar geometries and small fill factors were investigated. We present results on 9xx nm bars with tailored beam parameter products for simplified coupling to fibers with core diameters of 200μm and 300μm with a numerical aperture of 0.22 and compare beam quality parameters, brightness, conversion efficiency, and thermal performance of different bar designs. Optimized epitaxy structures yield conversion efficiency maxima above 66%. The slow axis divergence angle of mini-bars with a fill factor of 10% featuring five 100μm wide and 4mm long emitters based on this epitaxy structure stays below 7°, which corresponds to a beam parameter product of 15mm mrad, up to very high output power of over 45W. This result was achieved for mounting on actively cooled submounts using hard solder. A similar bar with 5mm cavity length and using soft soldering reached an output power of 60W at the same beam parameter product. At 4mm cavity length, no COMD failures were observed up to currents exceeding the thermal rollover and the maximum output cw power was 95W.

Eye-Safe Data Transmission of 1.25 Gbit/s Over 100-m SI-POF Using Green Laser Diode

We report on 1.25-Gbit/s transmission over 100-m step-index polymer optical fiber (SI-POF) with 6.5-dB margin. Nonreturn-to-zero modulation with offline decision feedback equalization technique has been applied to an intensity modulated direct-detection optical channel. An edge emitting direct injection laser diode with 515 nm is used, which allows us to improve the system power budget due to a lower attenuation of the POF at this wavelength. The fiber-coupled optical power has been adjusted to 0 dBm which corresponds to eye-safety requirements.