A high-power 975 nm tilted cavity laser with a 0.13 nm K- 1 thermal shift of the lasing wavelength
We address design and performance issues of a 975 nm range tilted cavity laser (TCL) based on GaAs/GaAlAs waveguide and GaInAs quantum wells as an active medium. The all-epitaxial design of a TCL includes a cavity and a multilayer interference reflector (MIR). The cavity and the MIR are constructed as follows. The cavity confines one tilted optical mode, and the wavelength of this mode as a function of the tilt angle follows a first dispersion curve. The reflectivity spectrum of the MIR for tilted incidence of light has a well-pronounced stopband, and the stopband reflectivity maximum follows a second dispersion curve. The two dispersion curves intersect at one optimum tilt angle and one optimum wavelength λ0. At the wavelength λ0 the leakage loss of the optical mode through the MIR to the substrate has a minimum, which favors lasing at this optimum wavelength. The thermal shift of λ0 is governed by the shift of the intersection point. As the photon energy corresponding to the operation wavelength of the laser 975 nm is close to the resonance with the electronic energy gap in GaAs, the refractive index temperature coefficient dn/dT of GaAlAs strongly increases at the GaAs side. This gives a tool to control the thermal shift of λ0 by replacing some layers of the cavity by a superlattice containing narrow insertions of GaAs. Such a superlattice may have the same averaged refractive index at room temperature, but significantly different values of dn/dT, thus affecting the thermal shift of λ0. Within this approach broad-area (100 µm) devices have been fabricated showing a high temperature stability of the lasing wavelength (0.13 nm K−1), low threshold current density (300 A cm−2), a high power operation (>7 W in pulsed mode and >1.5 W in continuous wave mode), a high spectral stability at high output power, and a narrow vertical far-field beam divergence (FWHM ~ 20°).