A beam combiner (24) may include source elements (100), each configured to output a beam of light locked at a center wavelength different from center wavelengths of other source elements. The beam combiner may include a dispersive element like a transmission grating (102) configured to combine the beams of light into a combined beam, and a beam separator (103) configured to separate the combined beam into an output beam (109) and a locking beam (112) in the common branch (104) of the external resonator. The beam combiner includes a spatial filter configured to prevent crosstalk within the locking beam, and to redirect the locking beam to the source elements (100). The dispersive element may be configured to disperse the locking beam into constituent wavelength beams. Each constituent wavelength beam may be directed to a respective one of the source elements for locking that source element at its center wavelength, and may correspond in wavelength to the center wavelength of the respective source element. The spatial filter may comprise two lenses (105,107) building a telescope with an aperture (106) in the focal plane and a retro-reflector (108) for reflecting the laser beams back to the source elements (100).