Tunable Y-branch dual-wavelength diode lasers in the VIS and NIR range for sensor applications

Abstract

Some spectroscopic applications require excitation light sources with dual-wavelength laser emission. For example, in absorption spectroscopy one on-resonance and one off-resonance wavelength are needed for concentration measurements. For shifted excitation Raman difference spectroscopy (SERDS), two excitation wavelengths are used to distinguish between disturbing light and Raman signals. In both cases an adjustable wavelength spacing allows optimizing the measurements according to the spectral width of the target. In addition, a tuning of the spectral distance also allows generating a tunable sum or difference frequency signal, enabling further applications. In this paper, diode lasers with customized designs according to the spectral requirements of the applications will be presented. As basis for all devices, Y-branch diode lasers with an integrated grating for wavelength stabilization are realized. The emission of the two branches is combined in an implemented Y-shaped coupler. The bent waveguides are sine shaped S-bends. The spectral tuning is performed via implemented heater elements next to the Distributed Bragg Reflector (DBR) gratings or via the injection current when using a Distributed Feedback (DFB) grating. Powervoltage current characteristics, spectral and tuning properties will be shown. The devices emitting at 671 nm and 785 nm are used for SERDS, whereas devices at 965 nm were tested as seed sources for pulsed master oscillator power amplifiers (MOPA) suitable for the detection of water vapor. Devices at 785 nm are also suitable for the generation of THz radiation using difference frequency generation. A widely tunable Y-branch diode laser near 972 nm is used for the sum frequency generation in an up-conversion system.