Holographic optical coupling and beamsplitting elements for optoelectronic interconnects and sensors

Abstract

Holographic optical elements (HOEs) are compact and flexible in their function: they can integrate focussing deflection and beamsplitting into a single element. Optical systems consisting of several elements can be built by recording planar HOEs side by side onto a common planar substrate -- a light guiding plate. By folding the light path, i.e., using multiple reflections within this plate, the light is guided from one HOE to the other. This concept is investigated for two applications: (1) An optoelectronic board to board interconnection based on holographic coupling elements on a light guiding plate: Diode lasers and pin diodes were used as optoelectronic transmitters at a data rate of 500 MBits/s. The coupling elements were volume gratings, recorded in dichromated gelatine (DCG) and coated onto the surface of the light guiding plate. Inside the plate the light was guided by total internal reflection. An overall loss of this transmission system of -2.6 dB over a distance of 11 cm was observed. (2) A miniaturized sensor head for the optical measurement of velocities of fluids based on laser Doppler velocimetry (LDV): HOEs mounted onto a glass substrate are used for beamsplitting and deflection. Volume holograms in DCG exhibit good optical efficiency (75% transmission of a cascade of two HOEs). In contrast to conventional sensor heads with diffractive devices one can achieve achromatic behavior which makes the sensor insensitive against wavelength drifts or mode hopping of a semiconductor laser.