For broad ridge (Al,In)GaN laser diodes, which are inevitable for high output power applications in the near-UV to blue spectral region, filaments appear, which influence the far-field beam quality. We present an extensive study of the optical mode profile of conventional c-plane LD test structures with ridge widths from 1.5 to 10 micrometers. The broad ridge samples are optimized to reach several hundred milliwatt of cw output power. Spectral and spatial resolved near- and far-field measurements show, that the characteristic lateral multi-lobed far-field pattern can be interpreted as superposition of interfering phase-locked filaments in the ridge waveguide.

To improve the output power of (Al,In)GaN laser diodes (LDs), the ridge width has to be increased, but beam quality of GaN‐based broad area lasers is still a critical point. We present a multidimensional characterization of the optical laser mode, propagating from the near‐into the far‐field, and show that the formation of the characteristic multi‐lobed lateral far‐field pattern of broad ridge (Al,In)GaN LDs is the result of interfering, phase locked filaments, that build up the laser mode. In this context we show that variation of driving current or temperature may have an significant impact on the spectral and spatial stability of the laser beam. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Broad (Al,In)GaN laser diodes (LD) with ridge widths of some micrometers show up multi mode patterns. We analyze (Al, In) GaN LDs with different ridge widths in the UV and blue spectral regime simultaneously with temporal, spectral and spatial resolution in the near- and far-field. We transform the laser beam of the LD by a lens system and get a magnified image of the near-field. By using a CCD camera we get the near-field intensity information of the image. In combination of an optical fiber with a photomultiplier or a spectrometer we get the temporal and spectral information. Additionally we can image the propagation from near-to far-field. In this paper we describe the properties of our setup and compare a near-field (SNOM) and far-field (imaging with a microscope objective) approach to the measurement of the LDs multi-dimensional characteristics. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

True blue lasers with wavelengths of similar to 450 nm are of great interest for full color laser projection. These kind of applications usually require high output power and, in particular, an excellent wall plug efficiency within a wide temperature range. In this paper we therefore present experimental and theoretical investigations of the temperature behavior of 60mW InGaN lasers in a range of -10 degrees C to 100 degrees C.The laser parameters threshold current density, slope efficiency and operating voltage describe the wall plug efficiency of the device. The slope efficiency does not show any significant temperature dependence which is due to an almost temperature independent injection efficiency in the temperature range that is of interest for most commercial applications. In contrast, the laser threshold current density increases with temperature and we determine a characteristic temperature T-0 of about 141K for our devices emitting at 445nm. This increasing threshold current density can be explained by lower gain of the quantum wells at higher temperature. Furthermore, Auger recombination influences the threshold as verified by simulations. The second electro-optical parameter is the electrical voltage, which is dominated by electrical barriers. The voltage decreases with increasing temperature and compensates the increasing threshold current resulting in a nearly constant high wall plug efficiency of 13% between -10 degrees C and 100 degrees C.

Broad area (Al,In)GaN laser diodes (LDs) are suitable for high optical output power in the near UV to blue spectral range. But for ridge widths larger than a few micrometers, the occurrence of filamentation is well known. We present experimental evidence that the single filaments tend to be phase-locked with defined phase offset and build up a so called supermode. Depending on driving current a coherent or incoherent superposition of different supermodes can be observed, which has a significant impact on the corresponding lateral far-field pattern. By a simulated reconstruction of the lateral mode profile of the laser mode propagating in free space we retrieve the field and phase distribution of the laser mode in the waveguide. In this context the coupling mechanism is discussed and the mode behavior is compared to supermodes in GaAs laser diode arrays.

We present the investigation of an optimized epitaxial layer structure in order to reach high optical output powers of several watts. The influence of an interlayer, which is located between the multiple quantum well layers and the electron blocking layer, is analysed carefully. For high current densities the leakage current density increases with increasing interlayer thickness. Correlating the leakage current density with the injection efficiency it is found that by decreasing the interlayer thickness constant injection efficiency for high cur-rent regions can be reached. This is essential for a linear laser characteristic up to high optical output power levels. Further enhancement can be achieved by enlarging the total wave-guiding layer structure. As a result the optical power density in the resonator was decreased. With these improvements threshold current density and slope efficiency of 2.7 kA/cm(2) and 2.1 W/A can be achieved. The optical output power is increased to 8.7 W for a 20 mu m broad single-emitter ridge-waveguide laser in pulsed operation. (C) 2009 WILEY-VCH Veriag GmbH & Co. KGaA, Weinheim

The blue light sources of choice for laser projection will be long wavelength nitride lasers because of their high wall plug efficiency and their small form factor compared to blue light sources using second harmonic generation. High quality of the far field is required for laser projection. However, optical confinement of GaN/AlGaN structures becomes increasingly difficult when wavelength shifts from 405 nm to the true blue spectrum. We will show excellent far fields by suppression of higher order laser modes and by improved waveguiding, respectively. We present simulations of laser modes as a function of ridge width. The slow axis far field is improved by absorber layers that guarantee in-plane values of M-2 < 1.5. Sufficient vertical confinement of the laser light by thick cladding layers is necessary to suppress laser modes guided within the substrate. We present 440 rim lasers for projection with threshold currents of 20 mA and slope efficiencies more than 0.8 W/A.

Ridge widths of GaN laser diodes (LDs) are typically in the order of few gm. In contrast to GaAs material systems, beam quality of GaN broad area lasers is still a critical point. By time resolved scanning near-field optical. microscopy (SNOM) on pulsed electrically pumped LDs with different ridge widths we observe dynamic features caused by thermal and carrier induced changes of the refractive index like filamentation and lateral mode competition, which strongly influence the far-field of the LDs. Using a high spectral resolution spectrometer we can additionally resolve the individual longitudinal modes of the laser spectrum both time resolved and correlated to the lateral position in the waveguide. Especially on samples with broad ridges we observe complex spectral and spatial dynamics of the laser mode like different filaments lasing on different longitudinal mode combs.

We investigate two types of 405 nm (In, Al)GaN test laser structures (TLSs), one of them grown on SiC substrates, the other grown oil low dislocation density freestanding GaN substrates. Measuring the lasing spectra, of these structures, we observe all individual behavior depending oil the substrate. TLSs oil GaN substrates show a])road longitudinal mode spectrum above threshold, whereas TLSs on SiC are lasing only on one mode with various jumps of the laser ernission at certain currents. Estimating the gain of each longitudinal mode with the Hakki-Paoli method, we find minute variations of the gain for TLSs on GaN substrate. In contrary, TLSs on SiC substrate show much larger fluctuations of the gain for individual longitudinal modes. Using a rate equation model with nonlinear gain effects, we simulate the longitudinal mode spectrum of both types of TLSs. Once we modify the gain of each longitudinal mode as observed in the gain measurements, the simulated spectra resemble the SiC or GaN substrate TLS spectra.

For broad ridge (Al,In)GaN laser diodes, which are inevitable for high output power applications in the UV and blue spectral range, filaments or higher order lateral modes build up, which influence the far-field beam quality. We investigate the lateral profile of the optical laser mode in the waveguide experimentally by temporal and spectral resolved scanning near-field optical microscopy measurements on electrically pulsed driven laser diodes and compare these results with one-dimensional simulations of the lateral laser mode in the waveguide. We present a model that describes the optical mode profile as a superposition of different lateral modes in a, refractive index profile which is modified by carrier- and thermal-induced effects. In this way the mode dynamics on a. nanosecond to microsecond time scale can be explained by thermal effects.

(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.