High-speed optical characterization of intensity and phase dynamics of a 1.55 /spl mu/m VCSEL for short-reach applications

Abstract

Using sonograms and phase retrieval we experimentally investigate the pattern dependence of the amplitude and phase dynamics, the linewidth enhancement factor, and the chirp of a data-modulated 1.55ptm vertical-cavity surface-emitting laser (VCSEL). OCIS codes: (250.7260) Vertical cavity surface emitting lasers; (120.5050) Phase measurement 1 IntroductionVertical-cavity surface-emitting lasers (VCSELs) operating at a wavelength of 1.55ptm are new and potentially important light sources for optical access applications, such as central-office interconnects, parallel-optical data links or metro-feeders, where they could allow for the low-cost deployment of short reach links [1,2]. At this wavelength standard-single-mode fibers have minimum attenuation and erbium-doped fiber amplifiers (EDFAs) can be used for optical amplification. Due to the lasers' low threshold current and power consumption they are also potential transmitter sources for innovative technologies like integrated optical interconnects on electrical circuit boards, enabling enhanced data rates in backplanes, computers, or server systems [3]. For all these applications it is important to temporally resolve the output of the VCSEL to identify optimal driving conditions and to predict and mitigate degradation from optical fiber transmission impairments. In this paper we report on the measurement of the intensity and phase dynamics of a directly data-modulated 1.55ptm-VCSEL and their pattern dependence using sonograms and phase retrieval. We also calculate the linewidth enhancement factor oc and the frequency chirp for different driving conditions and modulation patterns. In contrast to earlier measurements [4], which only quantify the chirp-parameter for sinusoidal modulation at certain output powers, the results presented here perfectly describe the behaviour of the VCSEL under typical operating conditions. The linear approximation of the chirp parameter as a small-signal parameter depends highly on the driving conditions of the laser, thus our complete dynamic characterization leads to a better understanding of the properties of the device.2Vertical-cavity surface-emitting laser (VCSEL)We used a commercially available, pigtailed, uncooled VCSEL with buried tunnel junction (Vertilas) at the wavelength of 1.55ptm, rated for 2.5Gb/s modulation but driven at a data rate of 1OGb/s. As shown in the inset of Fig. la, the coaxial packaged VCSEL was mounted at the end of a 50 Q microstrip line, in series with a 12 Q surface mount chip resistor. Such careful laser mounting significantly improved the VCSEL's modulation response at low frequencies (< 5GHz). However, the mounting had no impact on the high-frequency behaviour. Fig. lb presents the small-signal modulation characteristic at 25°C. The insets give back-to-back eye diagrams at data rates of 2.5Gb/s and 1OGb/s. The DC-characteristics are given in Fig. la, revealing a low threshold current of IthO0.9mA, a maximum laser output power around 0.8mW, and a slope efficiency of 0.135mW/mA. The linewidth of the VC...

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