Computer-generated waveguide holograms developed by Anders Larsson and coworkers at Chalmers University of Technology (Göteborg, Sweden) perform output coupling and beam shaping simultaneously in surface-emitting lasers. The holograms eliminate the need for external beam-shaping optics.
In a standard grating-coupled surface-emitting laser, the eponymous grating converts the guided mode into a near-surface free-space mode, usually through first-order diffraction. With the holographic system, however, the grating area is divided into a number of grating cells or pixels. Spatial phase modulation is accomplished by shifting the position of the grating grooves within one pixel relative to those of neighboring pixels, referred to as grating dislocation. Spatial amplitude also can be controlled by varying the grating duty cycle.
The waveguide holograms are fabricated on an epitaxially grown indium gallium arsenide/aluminum gallium arsenide (InGaAs/AlGaAs) waveguide using electron-beam lithography and reactive ion-beam etching. These techniques allow a minimum step size for grating dislocation of 2.5 nm, which for a grating period of 300 nm results in a phase resolution of 0.05 rad—giving a virtually continuous phase variation. Duty cycle can be varied from 15% to 80%, allowing a large range of amplitude variation.
Initial demonstrations show that effective beam shaping is achieved with the waveguide hologram. Examples include the conversion of a Gaussian waveguide mode into an asymmetric distribution of 20 equal intensity spots, produced as a focused image 10 cm above the surface. The hologram thus acts as an output coupler, beamsplitter, and focusing lens. The phase pattern required to do this is calculated using an iterative Fourier-transform algorithm, and this is converted into grating dislocation. In this example, the diffraction efficiency is 65% and the uniformity error is 8%.
The researchers have also demonstrated image-multiplexing capabilities, for which a patent application has been submitted. Here, images are produced in different directions and focused at different distances above the waveguide surface. Another development is the fabrication of computer-generated waveguide holograms capable of angular multiplexing. This process generates two images that are reconstructed by two guided laser beams.
