Harmonic Distortion Characteristics of a Directly-Modulated Semiconductor Laser Followed by an FM-to-IM Interferometer

Abstract

Recent research activities have been directed to the study of analog lightwave communication systems as CATV systems or phased array radars. Most of these analog systems use laser direct modulation and direct detection. However, analog intensity-modulation (IM) systems place rigid requirements on the linearity of the optical sources [1]. Meeting these requirements, but with a modulation depth that provides an acceptable CNR, is not an easy task, despite the many investigated alternatives, as the use of external modulators and feedforward or predistortion techniques [2]. Another way for transmitting information is offered by the direct frequency modulation (FM) ability of semiconductor lasers. The phenomenon of lasing frequency dynamic shift — frequency chirp — by injection current, is rather a limiting factor for some applications but may be also used for information transmission. FM is used in coherent transmission systems or by IM techniques by proceeding to an FM-to-IM conversion. The prediction of the FM behavior of the device is therefore of great interest for channel location choice and design. The small-signal analysis of direct FM of semiconductor lasers has been the subject of many investigations. Usually, however, only the FM response function has been considered in detail while the distortions in the optical FM signal have been often neglected. In the present study the second- and third-order harmonics are also reported. The study focuses on the small-signal IM and FM behavior of the laser diode. Both components of the optical signal are compared, in particular the harmonic distortion levels. It is found that systems using a frequency-modulated laser and an interferometer would exhibit lower distortion levels than those using direct EM of the laser diode.