A fiber grating based distributed light source

Line scan cameras are used for rapidly monitoring a moving web or sheet of material. Lighting for line scan inspection should illuminate a long narrow rectangle, which is imaged onto the linear array of pixels in a line scan camera. This distributed light source should provide a uniform power density at the desired wavelengths. Tungsten halogen lamps and LED arrays can meet many of these objectives, but not in a highly directional beam with minimal thermal issues. We have developed a new distributed light source that is based on diffracting light from a highly blazed grating written in the core of a single mode fiber. The grating is blazed such that out-coupling is 90 degrees to the fiber axis. The fiber is bonded to a cylindrical optic that collimates the azimuthal power distribution. Connecting a single laser diode to the fiber can generate 1 milliwatt per square centimeter over a 10 cm by 0.5 cm rectangular region. Longer gratings and/or multiple segments can be connected to illuminate longer regions. The distributed power density, spatial uniformity, degree of collimation, and spectral bandwidth of these illuminated rectangles are reported. This highly directional distributed source will enhance the utility of line scan cameras in multiple applications.