Four beams are combined into two, which are then combined into one.

The figure depicts a breadboard version of an optical beam combiner that makes it possible to use the outputs of any or all of four multimode laser diodes to pump a non-planar ring oscillator (NPRO) laser. This apparatus could be an alternative to the one described in the immediately preceding article. Whereas that one utilizes spatial (beam-shaping) beam- combining techniques, this one utilizes a combination of polarization and spatial beam-combining techniques. In both that case and this one, the combined multiple laser-diode pump beams are coupled into an optical fiber for delivery to the NPRO pump optics.

Four Laser-Diode Beams are polarization-combined into two, then narrowed along the fast axis, then combined into one beam incident on an end face of an optical fiber.
As described in more detail in the immediately preceding article, the output of each laser diode has a single- mode profile in the meridional plane containing an axis denoted the “fast” axis and a narrower multimode profile in the orthogonal meridional plane, which contains an axis denoted the “slow” axis. Also as before, one of the purposes served by the beam-combining optics is to reduce the fast-axis numerical aperture (NA) of the laser-diode output to match the NA of the optical fiber. Along the slow axis, the unmodified laser-diode NA is already well matched to the fiber-optic NA, so no further slow-axis beam shaping is needed.

In this beam combiner (see figure), the laser-diode outputs are collimated by aspherical lenses, then half-wave plates and polarizing beam splitters are used to combine the four collimated beams into two beams. Spatial combination of the two beams and coupling into the optical fiber is effected by use of anamorphic prisms, mirrors, and a focusing lens. The anamorphic prisms are critical elements in the NA-matching scheme, in that they reduce the fast-axis beam width to 1/6 of its original value. Inasmuch as no slow- axis beam shaping is needed, the collimating and focusing lenses are matched for 1:1 imaging. Because these lenses are well corrected for infinite conjugates, the combiner offers diffraction-limited performance along both the fast and slow axes.

This work was done by Paul Gelsinger and Duncan Liu of Caltech for NASA’s Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

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Refer to NPO-43783, volume and number of this NASA Tech Briefs issue, and the page number.

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