Juergen Jahns

Orbital angular momentum experiments with broadband few cycle pulses

Light distributions of Bessel-Gauss and Laguerre-Gauss type carry an orbital angular momentum and thus can be regarded as particular types of optical vortex beams. Optical vortices in highly intense femtosecond laser pulses are expected to lead to a variety of specific applications like momentum selective spectroscopy, nonlinear laser-material interaction or quantum information processing. Here we report on experiments with a Ti:sapphire laser oscillator at wavelengths around 800 nm. To compare the pulsed and cw case, the system was driven with and without mode-locking. At the minimum pulse duration of about 10 fs, a FWHM spectral bandwidth of 120 nm was available. By applying diffractive spiral phase elements, beams with topological charges of m = 1 and m = 2 were formed. The specific propagation behavior was studied by detecting spatially resolved intensity and spectral maps. In addition to the helical beam generation with fixed phase patterns, adaptive approaches based on liquid-crystal microdisplays are considered.

Optoelectronic multichip module based on planar-integrated free-space optics

We present an architectural approach to overcome the interconnection problem of modern VLSI-circuits and demonstrate it experimentally in form of a multi-chip-module (MCM) in which four optoelectronic VLSI-chips communicate optically via a planar-integrated free-space optical system. The MCM implements a distributed parallel computing model and is compact and robust. The optical system has been integrated on the surfaces of a slab of quartz glass by means of lithographic microfabrication techniques. The quartz substrate also serves as circuit board for the opto- electronic VLSI-chips. Our approach allows dense packaging (> 100 per mm2) of large numbers of optical interconnects.

Thermal management in planar optical systems with active components

Planar integrated free-space optics has been suggested and demonstrated as a micro-optical systems technology for optical interconnection and processing. It is based on the integration of micro-optical elements on a single glass substrate. Active optoelectronic components are mounted onto the substrate using hybrid integration techniques. An important problem related to the packaging is the heat removal from these active device arrays. A high interconnection density -- which is desirable from an architectural point of view -- can cause a dissipated power on the order of 100 W/cm2. This may compromise the performance of individual devices and the system as a whole. As cooling mechanisms, we consider convection which requires sufficiently large surfaces and conduction in an intermediate layer of high thermal conductivity between the passive optics and the optoelectronics. Both, structured silicon coolers and diamond layers are of interest for a practical realization. Here, we discuss material and design aspects of heat spreaders. Both, theoretical modeling and experimental results are presented. A test setup including an array of vertical cavity surface emitting lasers (VCSELs) is analyzed. The temperature distribution on the array is determined experimentally by the shift of the optical wavelength. Computer simulations are used to evaluate the experimental data.

Low-latency optoelectronic processor-memory interconnection demonstrator

A principal performance limitation of current computers is memory access latency. The random access time of DRAM can be as low as 20 ns but the overhead imposed by communication latency can increase the retrieval time to 150 ns in single processor systems or 1 ms in large multiprocessor systems. Optically interconnected VLSI offers the possibility of reductions in the communication component of memory latency of an order of magnitude. The improvement arises from the potential of direct high bandwidth low-latency links between any one chip and each one of a set of others. This potential principally arises from the ease of an optical implementation of fan-out and fan-in operations, together with the intrinsically high bandwidth of optical links. We have designed a scaleable system of processor-memory interconnections to explore this technology. Optical fan-out and fan-in modules will link a single processor to a bank of memory chips. The approach allows for multiple processors to be connected to multiple memory banks in an analogous fashion. The demonstrator will use 1-D VCSEL and photodiode arrays to provide optical i/o for the CPU and memory chips. The optical fan-out, fan-in and image relay can be implemented using an integrated planar optical system.

Confocal imaging with diffractive optics and broadband light sources

Confocal imaging is a technique used in microscopy and sensing. The use of microoptic technology allows one to build compact integrated confocal systems. Diffractive elements are used to introduce chromatic dispersion into the imaging setup. This allows one to gain depth or spectral information. In this poster, we discuss the performance of a diffractive confocal imaging systems and its potential applications. In particular, we will focus on an application in spectroscopy whose impulse response is matched to the spectrum of a specific light source. Matched spectroscopy may be of use in environmental sensing and process control. For the realization of the optics we consider integrated free-space optics using a planar 2D layout.

Ultra-precision fabrication of high density micro-optical backbone interconnections for data center and mobile application

A microoptical 3D interconnection scheme and fabricated samples of this fiberoptical multi-channel interconnec- tion with an actual capacity of 144 channels were shown. Additionally the aspects of micrometer-fabrication of such microoptical interconnection modules in the view of alignment-tolerances were considered. For the realiza- tion of the interconnection schemes, the approach of planar-integrated free space optics (PIFSO) is used with its well known advantages. This approach offers the potential for complex interconnectivity, and yet compact size.

Planar-integrated free-space optics for interchip interconnects

In this paper, we present the use of three-dimensional planar-integrated free-space optics as an interconnection technique. This 3-D optics offers the potential to enter or remove many signals in parallel to/from a chip. This can solve one of the fundamental problems of electronic (actually, of any 2-D) interconnects. Furthermore, using specific components, signal fan-out can be rather easily implemented in contrast to other approaches.

Current crowding and optical saturation effects in GaInN / GaN LEDs grown on insulating substrates

Current crowding in mesa-structure GaInN/GaN light-emitting diodes (LEDs) grown on insulating substrates is analyzed. A model developed reveals an exponential decrease of the current density with distance from the mesa edge. Devices with stripe- shaped mesa geometry display current crowding and a saturation of the optical output power at high injection currents. It is shown that the optical power saturation depends on the device geometry. It is also shown that saturation is less pronounced in LEDs employing a ring-shaped mesa geometry, which reduces current crowding, as compared to the conventional square- shaped mesa geometry.

J. Turunen and F. Wyrowski (eds.), Diffractive Optics for Industrial and Commercial Applications , Akademie Verlag, Berlin, Germany, 1997. 426 pp.

The field of diffractive optics is at the same time old and young. Diffraction gratings have been known for two centuries and used extensively in spectroscopy, for example. Also the theoretical understanding of the basic properties of grating diffraction has been well developed since that time. Until the 1960s, the technological foundation of grating manufacture used to be precision mechanics. It was then, when with the advent of the laser, things gradually started to change. On the one hand, laser interferometry became an additional tool to fabricate grating structures with very small periods. On the other hand, many new applications for optics started to develop based on the use of different types of laser sources. Some examples that may be mentioned are—besides modern spectroscopic techniques—areas like material processing, optical communications and information processing, optical data storage, etc. Consequently, the term “diffractive optics” has obtained a different flavor during the past 20–30 years. New types of diffractive elements were being developed with new technologies. This started in the mid-1960s with the invention of computer-generated holography which allowed to create “arbitrary” wavefronts (this means, within practical limits) by diffracting a light wave at an irregular binary structure. The computation and fabrication of computer-generated holograms was made possible by then newly available digital computers and plotting equipment. Since the early 1970s, people started to make diffractive elements using microfabrication techniques (lithography, etching, etc.) adapted from the processing of electronic circuits. These ideas were initially demonstrated in industrial research laboratories like Philips and Thomson-CSF.

Light efficient parallel interconnect using integrated planar free-space optics and vertical-cavity surface-emitting laser diodes

Planar optics is an approach to monolithically integrate free-space optical system. Diffractive microoptical elements are etched into a transparent substrate which serves as a medium for light propagation as well as a board for optoelectronic or electronic components. Mirrors and imaging optics are sued to keep the light traveling within the substrate along a zigzag path. Arrays of VCSEL devices can be integrated on the substrate by means of flip-chip bonding. Among the interesting applications of that technology are optical interconnections for VLSI systems where the optics can provide a large number of parallel channels. A critical issue of the practical realization is the light efficiency of the optical interconnect. Here, we propose a planar-optical implementation of the interconnect using analog grey-scale lithography resulting in a light- efficiency optical system.