Effects of detuned loading on the modulation performance of widely tunable MG-Y lasers Marek G. Chaciński* a , Richard Schatz a , Mats Isaksson b , Olle Kjebon a , Qin Wang c , a Royal Institute of Technology KTH, Isafjordsgatan 22,164-40 Kista, Stockholm, Sweden *Tel: (46) 87 904 054, Fax: (46) 87 904 090, e-mail: marekch@kth.se b Syntune AB, 164 40 Kista, Sweden c Acreo AB, Electrum 236, 164 40 Kista, Sweden ABSTRACT The Detuning Loading Effect, i.e., the effects of the modulation performance on the position of the lasing mode relative to the Bragg reflection peak, is investigated in a Modulated Grating Y-branch laser. By proper adjustment of the lasing mode position, simultaneous chirp reduction and modulation bandwidth enhancement can be obtained. The lasing mode position is also crucial for side mode suppression ratio and output power. Keywords: Detuned Loading, Modulated Grating Laser, Widely Tunable Laser, High Speed Modulation, Chirp 1. INTRODUCTION Wavelength tunable laser are attractive alternatives to fixed wavelength laser in Wavelength Division Multiplex systems since each device can address any wavelength [1-5]. This leads to simpler backup management and the possibility of wavelength routing of the network traffic. Monolithically integrated tunable lasers are especially interesting due small size, low power consumption and production cost. Modulated Grating Y-branch lasers utilizes the Vernier effect of two reflectance combs with slightly different periodicity in order to select a DBR-like reflection peak within a wide wavelength range. An additional phase section further enables a continuous fine tuning of the wavelength by adjusting the position of the lasing mode relative to the selected reflection peak. In this way quasi-continous tuning within the entire C-band can be achieved. In this paper we investigate the dependence on the static and dynamic performance due to the detuning of the lasing mode from the reflection peak, so called detuned loading [6-9]. Increasing the current in the phase section of a DBR laser will decrease the refractive index and cause a continuous movement of the mode towards the shorter wavelength side. At a certain point the reflectivity for the main mode has decreased below the reflectivity of a sidemode and a mode jump to the long wavelength side of the reflectivity peak will occur. This process will repeat with higher bias leading to a periodic saw tooth shaped variation of the wavelength as a function of phase current. However, at high bias the tuning efficiency decreases since the carrier density is saturated by increased nonlinear recombination. Rapid current modulation of the gain section of a laser will induce carrier density variations in the laser. This gives rise not only to optical gain fluctuations but also index fluctuations due to the so called alpha-parameter of the material. These gain and index-fluctuations give in turn rise to intensity and frequency fluctuations of the laser light, respectively. The relative amount of frequency modulation compared to the intensity modulation of the laser is described by the chirp factor, also called the alpha-parameter or linewidth enhancement factor of the structure. In a laser where the gain and index-fluctuations are homogeneous throughout the laser the alpha-parameter of the structure equals the alpha-parameter of the material. However, in DBR-lasers this is not true since only a part of the cavity, the gain section is modulated. If the laser is detuned so that the lasing mode is located on the flank of the reflectivity peak, the index modulation will give rise to modulation of the cavity losses which decreases or increases the effective (netgain) modulation of the laser. This detuned loading effect leads to a chirp factor of the laser structure that is larger or smaller than the material alpha parameter of the material depending on if the laser is lasing on the short wavelength or the long wavelength side of the Bragg reflectivity peak. This detuned loading effect will also affect the effective differential gain of the laser and hence the resonance frequency and bandwidth [6-8]. Semiconductor Lasers and Laser Dynamics III, edited by Krassimir P. Panajotov, Marc Sciamanna, Angel A. Valle, Rainer Michalzik, Proc. of SPIE Vol. 6997, 699709, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.783242 Proc. of SPIE Vol. 6997 699709-1 2008 SPIE Digital Library -- Subscriber Archive Copy