Large-Spot Laser Diodes with Stable Carrier Frequency by an External Fiber Grating

Abstract

Low-cost, wavelength stabilized laser modules are an important prerequisite for large-scale applications of passive optical transmission systems in the access network. One way to achieve this is to use a laser diode coupled to a lensed fiber incorporating a UV written grating [1]. Here, the wavelength stability is mainly determined by the low temperature dependence of fiber Bragg gratings. In order to reduce costs further, low-loss chip fibre coupling with large alignment tolerances is required. Spot-size transformation by a separate intermediate waveguide chip using tapers has been shown [2] to be a convenient way for the necessary spot-size adaption. However to reduce the problem of all alignment tolerances significantly, monolithic integration of the spot-size adaption on a semiconductor chip itself has been demonstrated using a vertical [3] or lateral [4] taper. Replacing the upper passive waveguide by an active one leads to a Fabry-Pérot laser with integrated spot-size transformation [5]. The integration of a DBR laser with a bulk active region using a simple taper structure was also published [6]. We recently reported on the successful integration of a DFB laser and a spot-size transformer, combining the good lasing characteristics and low loss chip fiber coupling [7], Here we present a modification of this design, using a fiber grating as external wavelength reference, suitable for operation at bitrates of at least 2.5 Gbit/s.