We present a dense wavelength division multiplexer based on volume Bragg gratings (VBGs) with a channel spacing of Δλ = 1.5 nm. Multiplexing efficiencies of ηSM = 97% have been demonstrated with single-mode, frequency-stabilized diode laser radiation. By use of VBGs in an external-cavity laser we constrict the spectral bandwidth of passively cooled multimode diode laser bars with 19 broad-area emitters to δλ95% = 120 pm. When the multimode high-power diode laser radiation with a beam propagation factor of M(2) ≈ 45 is overlaid, the multiplexing efficiency decreases to ηMM = 85%. Temperature control of the VBGs expands the high-efficiency operation range.