Abstract

External modulation of a continuous-wave laser source is an alternative to the directly modulated laser. External modulator links can be designed to virtually eliminate frequency chirping and can be combined with high-power shot-noise-limited solid-state lasers. However, systems that require high dynamic range are limited by the inherent nonlinearity of conventional external modulators, such as the Mach-Zehnder. Much work has been done with both electrical predistortion and feedforward¹ techniques as a potential solution. Recent designs rely on complicated electronics or electro-optics to achieve a 20-25-dB suppression of distortion products. The linearized directional coupler offers an advantageous method for achieving large nonlinear suppression. A description of the device, along with experimental verification at low frequencies, has been previously reported.² As can be seen in Fig. 1, this all-optical feedforward approach is quite simple. It is this simplicity that has permitted large improvements in linearity to be achieved in the laboratory, whereas other such linearized modulators (mainly using dual parallel schemes) have shown good results only on paper (see also Ref. 3). Besides permitting tuning to a very linear response, the passive sections provide for a large fabrication tolerance by giving operational control to the response curve.

© 1993 Optical Society of America