Volume holographic grating stabilized 780nm VECSEL

Abstract

We report on an optically pumped vertical-external-cavity surface-emitting laser diode coupled by a high reflectance volume holographic grating (VHG). The major feature of such a system is very narrow linewidth and high degree of power scalability, required for pumping upper energy levels of a gain medium. The cavity design lends itself to scaling to 2D VECSEL arrays suitable for low cost, high brightness pump sources.Keywords: VECSEL, high brightness, high power MOTIVATIONVertical External Cavity Surface Emitting Lasers (VECSELs), also called semiconductor disk lasers, are showing promise as next-generation sources for communications, laser cooling, spectroscopy, and high power direct use. Surface-emitting lasers have several advantages over bar-type diodes, namely a significant improvement in beam quality at high powers due to low-order mode operation [1,2]. This class of laser, however, suffers from relatively broad emission linewidth. For applications such as fiber optic communications, diode pumping, and atomic physics, some method of wavelength locking is typically required [3][4][5]. For example, in pumping applications, materials with narrow gain bandwidths (common in low quantum-defect lasers) require that the diode pump source be spectrally matched to the absorption feature to ensure efficiency pumping. Due to an external output coupler, VECSELs have greater adjustability and a greater possible number of cavity designs than other semiconductor lasers [6]. This allows for larger mode sizes and simple 2D scaling, leading to higher output powers in low-order modes than are possible with other laser types, as well the ability to easily incorporate wavelength-selective feedback to accurately control and lock the laser emission [7]. Furthermore, it is expected that the emission bandwidth could be further reduced to the sub-MHz level (in accordance with the Schawlow-Townes time-averaged linewidth equation) due to a long passive cavity length [8]. Volume holographic gratings (VHGs) have been successfully used to wavelength lock semiconductor lasers [9]. In this work, a VHG is used to stabilize the emission wavelength of an optically-pumped 780 nm VECSEL. EXPERIMENTThe design of the VECSEL epitaxy, as shown in Fig. 1 (a), is based partially on the designs reported in [10] and [11], and consists of a highly reflective bottom DBR and partially reflective top DBR surrounding a multi-quantum well active region. The Design provides high operating efficiency of the quantum wells and helps account for thermal effects within the structure. To maintain low optical loss, none of the layers were intentionally doped. The structure was grown bottom-up with the pump reflector DBR built directly onto the GaAs substrate wafer. A sample of the wafer was thinned and cleaved to a square sample for testing. The chip was mounted directly to a Cu heatsink (substrate side down) for testing.