Berndt Kuhlow

Maximum directivity beam-former at 60 GHz with optical feeder

We present an optically controlled 60 GHz array antenna which may be used as a smart antenna in an envisaged broadband mobile communication system. The desired field patterns of the antenna were synthesized using the maximum directivity beam-forming algorithm which enables an optimum radio link to a selected mobile terminal to be created while the signals of other terminals are suppressed by the nulls of the antennas field distribution. The 60 GHz signals were generated by optically heterodyning the signals of two laser diodes. The field distribution of the antenna was formed by a silica based photonic beam-forming network. The experimentally obtained data confirmed the calculated field patterns.

Antireflection-coated diffractive optical elements fabricated by thin-film deposition

The use of thin-film deposition in the fabrication of antireflection-coated diffractive optical elements is discussed. The antireflection coatings for these diffractive elements are optimized on the basis of an angular spectrum approach and the method of characteristic matrices. A minimum reflectivity as low as 1 x 10 -4 is realized using in situ controlled multilayers of TiO 2 and SiO 2 . The blazed profile of the diftractive optical elements is approximated by a stepped profile with up to 32 phase levels. The highest measured diffraction efficiency for 32-level Fresnel zone lenses was 97%.

Effect of fabrication errors on multilevel Fresnel zone lenses

Blazed Fresnel zone lenses for the 1.5-μm wavelength were fabricated in quartz glass by means of microstructuring technology. The blazed profile in each zone of the lenses was approximated by two, four, and eight discrete levels. The effects of fabrication errors, such as depth and alignment errors, on the diffraction efficiency of the different Fresnel zone lenses were investigated. Further the location and intensity of the parasitic foci appearing due to the discrete level approximation are calculated. Theoretical results along with experimental measurements are presented.

Diffractive microlenses with antireflection coatings fabricated by thin film deposition

Two-dimensional arrays of Fresnel zone microlenses were fabricated and coated with antireflection layers by an ion beam sputter deposition technique. The reflection of these lenses was analyzed on the basis of an angular spectrum approach for different substrate materials. A minimum reflectivity as low as 2 x 10-4 was realized by means of in situ controlled multilayers of TiO2 and SiO2. The lenses have a circular aperture of 2 mm and different focal lengths for the wavelengths of 1.52 and 0.63 μm, respectively. The kinoform profile in each zone of the Fresnel zone lenses was approximated by an eight-level profile. Such stepped profiles were realized with several masks written with an electron beam and transferred by photolithographic technology. Our measurements reveal that the spot sizes of the fabricated microlenses are close to the diffraction-limited values, and the highest measured diffraction efficiencies for the eight-level structures are greater than 80%.

Optical waveguide components with high refractive index difference in silicon-oxynitride for application in integrated optoelectronics

Planar optical waveguide structures in communication network applications require a number of indispensable features, e.g., low insertion loss, efficient fiber-to-chip coupling, polarization independent operation, and high integration density, as well as reliable and cost- efficient fabrication. The presented concept of integrated optical waveguide structures is based on the high refractive index material system of silicon-oxynitride on silicon. This system has the potential to meet all mentioned requirements. We describe the design and fabrication of such waveguide structures and demonstrate basic integrated optical components with good performance.

Fabrication of a multichannel wavelength-division multiplexing–passive optical net demultiplexer with arrayed-waveguide gratings and diffractive optical elements

A novel, to our knowledge, integrated wavelength-division multiplexing-passive optical net demultiplexer that uses an arrayed-waveguide grating and diffractive optical elements is presented. The demultiplexer is used to distribute 1.3-microm wavelength signals and to multiplex an eight-channel wavelength-division multiplexer spectrum at a 1.55-microm wavelength. The device shows high functionality and good optical performance. The measured cross talk was less than -21 dB, and the 3-dB bandwidth was determined to be 97 GHz, which is close to the theoretical value of 93 GHz. Average losses of 4.5 and 8 dB were measured for the 1.3- and the 1.55-microm signals, respectively.

Spatial frequency filtering in an optical imaging system with an extended incoherent light source

A single-lens imaging system is described that provides spatial frequency filtering to an extended incoherent light source. Two ways to calculate the modified image are considered: First, the image is derived from the filtered diffraction pattern by Fourier synthesis. Second, the convolution of the object function with a Fourier transform of the filter function has to be evaluated with regard to a phase function that corresponds to the light source position.

Holographic elements for optical interconnects at 1.5 μm

Holographic optical elements for interconnecting electronic switching stages with light of 1.5 micrometers wavelength are presented. These elements include deflection holograms recorded in dichromated gelatin for deflecting the light and diffractive spherical gratings fabricated by microstructuring for focusing and collimating the light. The diffraction efficiency of these elements can reach 90% and focused spot sizes can be within the diffraction limit.

Blazed Fresnel zone lenses approximated by discrete step profiles: effects of fabrication errors

Blazed Fresnel zone lenses for 1.5 micrometers wavelength were fabricated in quartz glass by means of microstructuring technology. The blazed profile in each zone of the lenses was approximated by 2, 4, and 8 discrete levels. The effects of fabrication errors, such as depth and alignment errors, on the diffraction efficiency of the different Fresnel zone lenses were investigated. Further the location and intensity of the parasitic foci appearing due to the discrete level approximation are calculated. Theoretical results along with experimental measurements are presented.

Multilevel Fresnel zone lenses capable of being fabricated with only one binary mask

Multi-level approximated Fresnel zone lenses with reduced level numbers in the outer zones are investigated and compared with Fresnel zone lenses of unique level numbers over the whole lens. Calculations of the fabrication error effects on diffraction efficiency for both lens types are performed. Measurements of focussing efficiency show that. especially for Gaussian beam illumination, segmented Fresnel zone lenses can reach nearly as high focussing efficiencies as normal Fresnel zone lenses. Arrays of segmented lenses can be fabricated using only one binary Fresnel zone lens mask with the aid of a modified optical stepper with fixed scaling down factors in the ratio of 1: 1/(root)2 from one to the next step of pattern transfer in the lithographic process. The fabrication of such lenses could be advantageous because the generation of precise e-beam written masks with a large number of binary ring zones is time consuming and expensive.