Dense wavelength multiplexing for a high power diode laser
At present methods of polarization and wavelength multiplexing with dielectric coatings are used to increase the brightness of diode lasers. The number of suitable diode laser wavelengths is limited by the temperature- and current-dependent spectral characteristics of the diode laser and the slope of dielectric edge-filters. By use of external volume diffraction gratings it is possible to constrict the emission spectra of diode lasers and to reduce the wavelength shift related to temperature or current injection. Due to the stabilization of the center wavelengths and the reduced bandwidth of the diode emission multiplexing of diode laser beams at small distance of the centre wavelengths can be realized. The performance of wavelength stabilization by use of volume diffraction gratings adapted to AR-coated diode laser arrays in an external cavity as well as to standard high power diode laser arrays is discussed. Furthermore modules of two diode laser beams are combined by wavelength dependent diffraction of an adapted volume diffraction grating. The spectra of the diode lasers of the same wafer are stabilized with a centre wavelength spacing of about 3 nm and more than 95% of the optical output power of each beam within a spectral width less than 0.7 nm. An efficiency of more than 80% for multiplexing of two diode laser bars with good beam quality in fast axis of M(exp 2) < 2 is achieved.