S. Sinzinger, J. Jahns
Optische Nachrichtentechnik
Free-space optical imaging systems can provide a large number of parallel optical interconnections refered to as large space-bandwidth product. For optoelectronic applications the sources and emitters are generally not densely packed but rather form "dilute" arrays. Examples are the optical windows of smart pixel arrays or the cores of 2D fiber bundles. Hybrid imaging has been demonstrated as a useful concept for the imaging of such dilute arrays. It combines conventional 4F imaging with microchannel imaging to provide high resolution at the locations of the optical windows [1][2]. Imaging lenses with large apertures can be avoided even if many parallel channels are implemented. This makes the hybrid approach especially interesting for application in planar optics [3].
The flexibility and design freedom in a hybrid imaging system can be increased by integrating deflecting gratings with the microlenses (Fig.1). In this case the microlenses are used to image the output windows of the sources into the center of the imaging lens. This lens in the center is necessary to image the planes of the microlenses onto each other in order to avoid the diffractive spreading of the microchannels. Here a low aperture lens is sufficient to resolve the apertures of the microlenses. The diameter of the imaging lens is determined by the diameter of the point spread function of the microlenses at the location of the imaging lens. Additional design freedom in such a system is provided through the focal lengths of the microlenses. Ray tracing simulations show that this can be utilized to avoid vignetting and geometrical aberrations such as distorsion, field curvature, astigmatism, and coma (Fig.2) [4].
Fig. 1: Modified hybrid imaging setup;
A: Array of deflecting microlenses; L: Imaging lens
Fig.2: Raytracing simulation: spot diagram for an off-axis point located at (16 mm,16 mm) for a) non-optimized microlenses; b) optimized microlenses (imaging lens: NA = 0.05; microlenses: NA = 0.34).
References:[1] A. W. Lohmann, "Image Formation of Dilute Arrays For Optical Information Processing", Opt..Comm.86 (1991), 365-370
[2] F. B. McCormick, "FreeSpace optical interconnection techniques" in "Photonics in Switching", J.E. Midwinter (ed), Academic Press, Boston , vol. 2 (1993), 169-239
[3] J. Jahns et al., "Parallel optical interconnections using surface-emitting microlasers and a hybrid imaging system", Opt.Comm. 109 (1994), 328-337
[4] S. Sinzinger, J. Jahns, "Variations of the hybrid imaging concept for optical computing applications", OSA Techn.Digest 10 (1994), paper OTuE9