JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 17, SEPTEMBER 1, 2010 2611 Narrow Linewidth DFB Lasers Emitting Near a Wavelength of 1064 nm Stefan Spießberger, Max Schiemangk, Andreas Wicht, Hans Wenzel, Olaf Brox, and Götz Erbert Abstract—We report on the realization of narrow linewidth high power DFB diode lasers emitting near 1064 nm in stable longitu- dinal and lateral single mode. The linewidth is analyzed in depen- dence of the output power for lasers with cavity lengths of 1 and 2 mm by means of a heterodyne beat note technique. The minimum intrinsic linewidth is 22 kHz FWHM (full width at half maximum, at 100 s time scale) for an output power of 150 mW and a cavity length of 2 mm. The minimum total linewidth is mainly determined by technical noise and corresponds to 234 kHz FWHM at an output power of 70 mW. The influence of current noise on the linewidth is investigated and compared for different cavity lengths. Re-broad- ening at high output power is only observed for the contribution of technical noise to the linewidth. The intrinsic linewidth shows the theoretically expected 1/P -dependence at all power levels. Index Terms—Current noise, distributed feedback lasers (DFB), linewidth, semiconductor laser. I. INTRODUCTION D UE to their excellent compactness, reliability, and effi- ciency narrow linewidth semiconductor lasers emitting near a wavelength of 1064 nm are interesting candidates to re- place existing optically pumped solid state lasers used for co- herent optical free space communication (inter-satellite-links). Coherent detection can be performed at significantly lower in- tensities than direct detection [1]. This is beneficial whenever optical repeaters cannot be placed along the transmission path as in case of long distance free space optical communication. However, for coherent detection systems a significantly reduced laser linewidth compared to direct systems is inevitable. Spontaneous emission is the dominant contribution to the intrinsic linewidth for most semiconductor lasers [2]. For in- creasing output power, the predicted linewidth caused by spon- taneous emission events decreases with the inverse of the op- tical output power (1/P -dependence). Additional contribu- tions like spatial hole burning [3], [4] and side mode partition noise [5], [6] may also play a significant role if the photon den- sity is spatially varying significantly within the cavity or if the Manuscript received January 25, 2010; revised April 27, 2010, June 24, 2010; accepted June 27, 2010. Date of publication July 08, 2010; date of current ver- sion September 01, 2010. This work is supported by the German Space Agency DLR with funds provided by the Federal Ministry of Economics and Technology (BMWi) under Grant Number 50YB0810. S. Spießberger, A.Wicht, H. Wenzel, O. Brox, and G. Erbert are with the Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, 12489 Berlin, Germany (e-mail: stefan.spiessberger@fbh-berlin.de). M. Schiemangk is with the Institut für Physik, Humboldt-Universität zu Berlin, 10117 Berlin, Germany. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2010.2056913 side mode suppression ratio is not sufficiently high. At higher optical output power a re-broadening of the linewidth is often reported, which is explained by longitudinal [4], [7] and lateral spatial hole burning [8], carrier fluctuations and spontaneous emission in confinement layers [9], [10], and mode partition be- tween the main mode and weak side modes [5]. However, in an experimental setup the measured spectral linewidth is further broadened by technical noise (“technical linewidth”). Although this contribution is usually dominating the FWHM linewidth for low noise semiconductor lasers, it is often overlooked in publications. Technical noise is usually assumed to be power independent. The only exception is [11], where the technical noise decreases with increasing output power at small power levels and then re-broadens at higher power levels. As mentioned before, the intrinsic linewidth decreases for higher output power. Eventually, it becomes comparable to the linewidth determined by technical noise. Hence, the ex- perimentally determined linewidth approaches the technical linewidth rather than the intrinsic linewidth for high output power [12], [13]. Depending on the application either the tech- nical frequency noise or the intrinsic frequency noise is relevant for the assessment of the laser performance. For coherent op- tical communication the intrinsic noise is the important factor [14] whereas for applications like spectroscopy the FWHM linewidth might be relevant. The smallest spectral linewidth of distributed feedback (DFB) lasers in the high power range reported so far is 300 kHz at 500 mW [15]. However, no details of the experimental setup or the analysis were presented. In this paper we present narrow linewidth DFB lasers which can meet the requirements given by coherent optical transmission. This paper is organized as follows: First, the design of the DFB lasers is briefly reported and the method we apply to mea- sure and analyze the linewidth of the diode lasers is described. We then show the dependence of output power and emission spectrum on the injection current and operating temperature. Fi- nally, the results of the linewidth measurements are presented and discussed. II. DESIGN AND FABRICATION The epitaxial layer structure and the ridge waveguide (RW) structure providing vertical and lateral optical confine- ment, respectively, are optimized for high-power operation of Fabry–Perot lasers emitting at the same wavelength [16]. Details of the DFB-laser structure are described in [17]. The lasers were not particularly optimized to obtain a small spectral linewidth. However, we believe that due to the small overlap 0733-8724/$26.00 © 2010 IEEE