Programmable Phase Optics
GPC implemented by planar-integrated micro-optics
In most applications, the GPC method has been implemented in macro-optical systems. In these systems, however, the optical components are well defined and the effect of the phase contrast filter as well as the imaging response could be efficiently analysed based on the paraxial approximation. To realise the applications of the GPC method to current technologies in electro-optical data transport, it is important to carry out the method in a scaled-down optical system such as its implementation using planar-integrated micro-optical components. Integration of free-space micro-optics in a surface of a thick transparent substrate has attracted much attention because its concept mutually complies with the requirements in the development of integrated electronic circuits. Its implementation thereby expands to "real world" applications such as in the fields of micro-optical-electro-mechanical systems (MOEMs), opto-electronics, optical computing, optical communications, and more.
The folded version of a 4-f lens configuration is
implemented in a planar integrated micro-optics platform as shown in Figure 1
(see further reading). Using multimask
lithography, the multiple-phase level diffractive micro-optical elements were
fabricated on the top-side of the substrate while the base contains
reflection-coated surfaces. An incident wavefront, which is normal to the planar
setup, is coupled into the substrate through a binary diffraction grating with
period 2 micron. The beam is deflected inside the 12 mm-thick fused silica
substrate with a propagation angle of 12 degrees. After reflection from the
base, the beam is focused by the 5 mm-diameter diffractive micro-lens (L1). The
micro-lenses are reflection coated and are fabricated using two binary
lithographic steps that make up a 4-phase level element. The focal length and
f-number of the micro-lenses are f=25 mm and f/#~5 respectively. The distance
from the coupling grating to L1 is equivalent to the focal length, which
indicates that the object plane is located at the surface of the input grating.
L1 focuses the beam to the Fourier plane where a reflection coated phase
contrast filter (PCF) is fabricated on the substrate to perform a pi-phase shift
of the on-axis region of the focused light. The PCF is designed for operation at
wavelength=633 nm and is etched as a 5-micron-diameter hole on the substrate.
Anisotropic etching process is used to form a steep-edged cylindrical hole. The
reverse Fourier transform is performed in the second half of the symmetric
system. Both micro-lenses are slightly elliptical in order to compensate for the
astigmatism due to the oblique optical axis.