Investigation of design parameters of 633 nm diode lasers with internal surface gratings for narrow spectral linewidth

Abstract

Summary form only given. Applications like spectroscopy, interferometry and holography are in the need of long coherence lengths. Helium-neon (HeNe) lasers and semiconductor diode lasers have been used to address these applications, because these devices provide a narrow spectral linewidth. HeNe lasers are bulky and fragile due to their long resonator being made of glass. Small-sized diode lasers have a broad gain spectrum over several nanometers and a high reflective rear facet coating that is spectrally broad as well. Therefore, for diode lasers an external wavelength stabilization, which is usually a grating, needs to be realized on a micro-optical bench in order to preserve a small footprint of the device [1,2].However, the hybrid integration of multiple components on a micro-optical bench requires active alignment which drives the costs. Thus, the monolithical integration of a wavelength selective element into the chip itself is favorable. The monolithical integration of distributed feedback (DFB) or distributed Bragg reflector (DBR) grating structures is a frequently used technology in the near-infrared wavelength range. But DFB or DBR gratings have not established themselves at wavelengths below 760 nm due to the even smaller dimensions and the use of layers containing InP. Recently, we were able to integrate 10th order DBR surface gratings into ridge waveguide lasers (RWL) after a single epitaxy by the use of standard i-line lithography and standard BCl3 reactive ion etching [3]. Therefrom, a straightforward and mass-production compatible process was established. In this work, the fabrication of surface DBR gratings by the use of a temperature-controlled wafer backside during etching will be introduced. Furthermore, we will present an investigation of various design parameters (see Fig. 1) such as grating and gain length, grating etch depth, grating period as well as width of the ridge waveguide stripe that have been varied to achieve optimized laser devices. The grating and gain length as well as the grating etch depth is investigated to optimize output power and reflectivity. The grating period and the effective refractive index define the emission wavelength. The width of the ridge waveguide stripe was varied in order to optimize the beam quality. DBR-RWLs with a front facet reflectivity of 1% exhibit a single longitudinal optical output power of more than 100 mW at a very good beam quality and a high side mode suppression ratio. Additionally, DBR-RWLs with a higher reflectivity at the front facet were fabricated to achieve devices with a large coherence length rather than high optical output power. A self-delayed heterodyne measurement resulted in a linewidth of less than 1 MHz at 14 mW, which corresponds to a coherence length of more than 100 m.