830 nm multimode interference coupler-based dual-wavelength master oscillator power amplifier for spectroscopic applications

A monolithically integrated dual-wavelength multimode interference coupler-based master oscillator power amplifier for spectroscopic applications such as Raman spectroscopy is presented. It consists of two laterally separated ridge waveguide laser cavities as master oscillators with individual distributed Bragg reflector gratings as cavity mirrors. The gratings are designed for dual-wavelength laser emission around 830 nm with a spectral distance of 0.5 nm. S-bend shaped waveguides and a multimode interference coupler are used to couple individual laser emissions from the master oscillators into a single waveguide serving as power amplifier. The device provides 0.5 W of dual-wavelength laser emission with measured spectral widths below 20 pm, limited by the spectral resolution of the spectrometer. Along the available power range, both peak wavelengths remain within spectral windows of 50 pm, which enables full flexibility in selecting suitable optical powers for various applications. Applying heater currents to resistors implemented next to the distributed Bragg reflector gratings allows for discrete wavelength tuning by Joule heating towards spectral distances up to 2 nm. Near field widths of 5 µm and far field angles of 17° result in beam propagation ratios of 1.4 (1/e²) and enable easy beam shaping or efficient optical single-mode fiber coupling. In Raman spectroscopy, an alternating dual-wavelength operation of the device allows to separate Raman signals from disturbances such as fluorescence, ambient light or fixed pattern noise by using shifted excitation Raman difference spectroscopy. As an example, Raman measurements on chicken thigh bones are presented. While single Raman spectra show intense background disturbances by sample fluorescence, dual-wavelength excitation and shifted excitation Raman difference spectroscopy reveal organic and mineral signal contributions expected for Raman measurements on bones.