to produce a standing wave in the non linear medium application_number US 54039909 A description according to a preferred embodiment a pump fiber is disposed at the center of a fiber bundle array such fiber bundle arrays may be fabricated by introducing the fibers into a fused silica capillary tube which is then drawn down in a tapering machine which is available commercially the pump fiber core has a generally smaller aperture or tighter focal spot than the apertures of the other fibers in the bundle the smaller core of the pump fiber makes the pump fiber beam expand more rapidly as shown in fig1 the first element of the device is a fiber bundle array 100 the fiber bundle array 100 consists of an array of close packed fibers 101 the largest fill factor in such an array may be obtained by using a triangular shaped lattice array as shown in fig1 a generally triangular shaped lattice geometry is used other suitable geometries for the fiber bundle array 100 include generally rectangular or square shaped arrays shown in fig3 however persons skilled in the art will appreciate that other lattice geometries may be used for the fiber bundle array 100 for example the fiber bundle array 200 in fig3 may be configured of more close packed fibers 201 that may overlap not shown the fiber bundle array 100 includes a plurality of outer fibers 101 the outer fibers 101 are referred to as probe fibers 101 each probe fiber 101 has a core 102 and a cladding layer 103 a central pump fiber 105 is disposed at the center of the.