Chapter 3 - Single-Mode and Tunable Laser Diodes

Abstract

The chapter reviews the state of the art of single-mode and electronically tunable laser diodes as required in numerous applications in optical fiber communications, sensing, and measurement. The chapter is divided into three main parts: treating the single-mode laser diodes, the continuously tunable laser diodes, and the widely but discontinuously tunable devices. For most applications, the continuous tuning mode and the simple and unambiguous wavelength control are most essential, so the recent development has focused mainly on the achievement of large continuous tuning range and simple device handling, i.e., only one control current for the wavelength setting. For tuning ranges below about 10 nm, various types of high-performance (single-mode, continuous tuning, narrow linewidth, easy to handle) devices already exist, based on the technologically well developed DFB (distributed feedback) and DBR (distributed Bragg reflector) laser structures. For physical reasons, the maximum continuous tuning range of electronically tunable monolithic diode lasers is limited to about 15 nm, corresponding to a relative range of 1% at 1.55 μm wavelength. In applications where one can accept the discontinuous tuning mode, novel device concepts have been presented offering discontinuous tuning up to about 100 nm, which almost equals the gain bandwidth of InGaAsP at 1.55 μm. However, the wavelength access is still an issue for these new structures, because typically only about 10 to 30 channels with different spacing can individually be addressed. The essential device parameter influencing the wavelength access is the bandwidth of the filtering curve, so that the present development concentrates particularly on the improvement of the wavelength selectivity.