Observation of Temperature-Independent Longitudinal-Mode Patterns in Violet-Blue InGaN-Based Laser Diodes Christoph Eichler, Sven-Silvius Schad, Ferdinand Scholz, Daniel Hofstetter, Stephan Miller, Andreas Weimar, Alfred Lell, and Volker Härle Abstract—We present measurements on an aperiodic device- specific longitudinal-mode pattern in InGaN laser diodes. The characteristic shape of this pattern occurs only if the laser is driven slightly above threshold; in addition, it tunes with temperature at exactly the same rate as the cavity modes. By careful selection of the collection optics and averaging ten rapid scans over 15 min in a high-resolution Fourier transform spectrometer, we could exclude possible explanations like beating of mode families, self-pulsation, or external reflections. A naive simulation of the longitudinal modes profiting from their individual “gain profile” along the cavity suggests that we see the signature of quantum-well thickness fluctuations. Index Terms—Emission spectrum, InGaN laser diodes, irregular spectral envelope. I. INTRODUCTION O PTOELECTRONIC devices based on InGaN have seen a decade of rapid progress and are now ready for applica- tions in large-area displays, lighting, and high-resolution optical storage systems [1]. Nevertheless, many of the fundamental properties of the nitride material system are not well known and a further understanding is required for future progress in light-emitting diode efficiency or laser reliability [2]. Especially material- and growth-related problems and their influence on device performance are not yet understood very well [3]–[5]. The emission spectra of laser diodes offer a particularly simple yet very rich pool of information, which has not been taken advantage of fully. Nevertheless, different methods like, for instance, Hakki–Paoli gain measurements, or Fourier trans- forms have been successfully applied and interpreted [6], [7]. II. EXPERIMENT Here, we present spectral measurements on two different InGaN diode lasers as a function of current and temperature. While the first of these devices was a commercial diode from C. Eichler, S. Miller, A. Weimar, A. Lell, and V. Härle are with Osram Opto Semiconductors, Regensburg 93049, Germany. S.-S. Schad and F. Scholz are with the University of Ulm, Ulm 89069, Germany. D. Hofstetter is with the University of Neuchatel, Neuchatel CH-2000, Switzerland. Fig. 1. Spectral envelopes of the Osram laser diode as a function of device temperature and at a constant power level of 1 mW. The spectra were first normalized to one and then displaced vertically for clarity. The inset shows part of a high resolution spectrum along with its spectral envelope. The cavity length was 600 m and the facet reflectivities 98% and 70%. Nichia, Inc, the second laser came from Osram Opto Semicon- ductors. The Nichia device was grown on sapphire, whereas the Osram laser was fabricated on a SiC substrate. The exact device structure of the Osram laser is described elsewhere [8]. Emission spectra were acquired using antireflection-coated collimation optics and subsequent coupling of the collimated beam into a Fourier transform spectrometer (NEXUS 870). In order to exclude short time scale spectral fluctuations, we aver- aged each spectrum ten times, resulting in a measurement time of roughly 15 min per spectrum. During the measurements, the lasers were driven continuously at a constant power/current level just above threshold. An internal Si photodiode of the spectrometer and the highest possible resolution of 0.1 cm were used for the experiments. Due to the short wavelength, a typical emission spectrum of such a laser consists of roughly 200 Fabry–Pérot modes. In order to highlight the important information, we will, therefore, show the spectral envelopes of these spectra only. A spectral envelope is the envelope function of the Fabry–Pérot mode spectrum. In the inset of Fig. 1, we present a small section (roughly 20 Fabry–Pérot modes) of a longitudinal mode spectrum along with its spectral envelope. Fig. 2 (top and bottom) shows several envelope functions of both the Nichia and the Osram laser as a function of injection current. Especially for the Osram device, we observe that the en- velopes are very smooth for low injection currents, and become increasingly structured when approaching lasing threshold. Far from threshold, the laser shows also a pronounced band-filling. This band-filling and the concomitant wavelength shift stop Published in Photonics Technology Letters (IEEE) 17, issue 9, 1782-1784, 2005 which should be used for any reference to this work 1