Beam-correcting holographic doublet for focusing multimode laser diodes

We design and fabricate a beam-correcting, holographic focusing doublet for laser diodes in the presence of a recording-to-readout wavelength shift. This two-hologram assembly compensates for the strong chromatic variation of the spot size and its lateral position, which are typical of a single diffractive element. The doublet, recorded at 488 nm, successfully corrects the astigmatism of a multimode laser diode beam at 820 nm and focuses it to a near-diffraction-limited 1/e-squared spot width with a significantly extended depth of field. (Author)