Alexei L. Glebov

Optical interconnect modules with fully integrated reflector mirrors

A robust and cost-effective technology for integration of 45/spl deg/ reflector mirrors and polymer waveguides (WGs) into optical interconnect (OI) substrates is developed. The planar WGs are formed from photopatternable polymers with propagation losses as low as 0.05 dB/cm. The mirrors with losses of 0.5-0.8 dB are fabricated by the microdicing technique allowing lateral and vertical positioning of the mirror plane within several microns. A prototype OI module with integrated channel WGs, mirrors, and assembled connectors is fabricated and successfully tested at 10-Gb/s transmission rate.

Integration technologies for pluggable backplane optical interconnect systems

Integration technologies for board-to-board optical interconnect systems are presented. In the module architecture, optical transmitters and receivers are placed on the line cards and the signals are routed to the optically passive backplane through optical jumpers. The backplane contains a light guiding layer with embedded polymer waveguides (WGs) and 45-deg reflector micromirrors. The WGs are fabricated by direct lithographic patterning and have propagation losses as low as 0.05 dB/cm. The wedge dicing technology is developed for fabrication of the 45-deg micromirrors with 0.5-dB excess losses. The pluggable optical connectors with microlens adaptors couple the light from the optical jumpers into the backplane WGs. Evaluation of the connector alignment tolerances demonstrates a very weak dependence of the coupling efficiency on the axial displacement and a more significant effect of the radial shifts. The presented results show that the displacement tolerances can be substantially improved with auxiliary lenses formed on the substrate. Prototype optical interconnect modules with integrated channel WGs, mirrors, and assembled connectors are fabricated with insertion losses of 5 to 6 dB. The modules successfully pass the high-speed transmission tests at data rates up to 11 Gbits/s.

Volume Bragg Gratings as Ultra-Narrow and Multiband Optical Filters

High efficiency volume Bragg gratings (VBGs) in photo-thermo-refractive (PTR) glass provide unmatched optical filtering capabilities with optical densities as high as 50 dB and linewidths as narrow as 1 cm-1. In this work we review recent advances in VBG technologies that enabled key improvements of high efficiency grating properties and led to development of unique VBG based optical filters for Raman spectroscopy and other applications. Such narrow band notch and bandpass filters make ultra-low frequency Raman measurements possible with single stage spectrometers, therefore, largely improving optical throughput of high end Raman instruments while reducing complexity of the measurements. In this work we also present novel volume multiplexed ultra-narrow band VBG filters with high reflection at multiple wavelengths. Such multiband holographic optical elements are formed by overlapping of several high efficiency VBGs in a single glass plate. Raman spectra obtained with multiband VBG filters and single stage spectrometers, show unmatched capability of the filters to provide simultaneous access to Stokes and anti-Stokes Raman modes with frequencies as low as 5 cm-1 at different wavelengths.

Electrooptic planar deflector switches with thin-film PLZT active elements

First prototypes of electrooptic (EO) planar deflector switches (PDSs) are fabricated with hybrid integration on Si substrates. Planar optical modules, made in silica-on-silicon technology, consist of input and output (I/O) waveguide microlenses facing each other and slab waveguides in between. The modules interconnect the I/O fibers with laterally collimated light beams less than 400 /spl mu/m in width at distances up to 100 mm with losses lower than 3 dB. Thin lead lanthanum zirconium titanate (PLZT) films with prism-shaped electrodes grown on SrTiO/sub 3/ substrates form the deflector elements. The PLZT films are more than 10 /spl mu/m thick with EO coefficients about 40 pm/V. The deflector assembly technology provides chip vertical positioning accuracy better than 1 /spl mu/m. The deflector chips are attached to the optical substrates with thermo-compression flip-chip bonding. The optical power losses of the modules with test silica chips can be as low as 3.6 dB. However, the lowest module losses achieved with PLZT are about 10 dB. The channel-to-channel switching operations are demonstrated at about 40 V and switching times less than 500 ns.

Volume-chirped Bragg gratings: monolithic components for stretching and compression of ultrashort laser pulses

Abstract. An innovative type of optical component—a volume Bragg grating—has recently become available commercially and has found wide applications in optics and photonics due to its unusually fine spectral and angular filtering capability. Reflecting volume Bragg gratings, with the grating period gradually changing along the beam propagation direction (chirped Bragg gratings—CBGs) provide stretching and recompression of ultrashort laser pulses. CBGs, being monolithic, are robust devices that have a footprint three orders of magnitude smaller than that of a conventional Treacy compressor. CBGs recorded in photo-thermo-refractive glass can be used in the spectral range from 0.8 to 2.5 μm with the diffraction efficiency exceeding 90%, and provide stretching up to 1 ns and compression down to 200 fs for pulses with energies and average powers exceeding 1 mJ and 250 W, respectively, while keeping the recompressed beam quality M2<1.4, and possibly as low as 1.1. This paper discusses fundamentals of stretching and compression by CBGs, the main parameters of the gratings including the CBG effects on the laser beam quality, and currently achievable CBG specifications.

Two-dimensional microlens arrays in silica-on-silicon planar lightwave circuit technology

Two-dimensional (2-D) microlens arrays have been fabricated with silica-on-silicon planar lightwave circuit (PLC) technology. Several experimental techniques and computer simulation methods are applied to characterize properties of single and double microlens arrays, with one and two refracting surfaces, respectively. Systematic comparison of the measured and simulated beam propagation profiles enables optimi- zation of the lens and module design resulting in higher input-output coupling efficiency. The insertion losses of the lens-slab-lens optical modules with 90-mm-long slab waveguides are measured to be 2.1 and 3.5 dB for the double and single lens modules, respectively. Comprehen- sive analysis reveals the major loss contributions. Excess losses of the modules caused by variations of the lens curvatures, material refractive indexes, light wavelength, etc., can be controlled within the acceptable limits. Further possibilities for the module loss reduction are discussed. Fairly weak wavelength dependence as well as overall stability of the module properties indicate that the microlens arrays are suitable for dense wavelength division multiplexing (DWDM) photonic networks.

Integrated waveguide micro-optic elements for 3D routing in board-level optical interconnects

Planar waveguides and embedded microelements such as 45o vertical mirrors, lateral mirrors, bends, and microlenses comprise main building blocks of the waveguide-based optical printed circuit boards (PCB) for board-level optical interconnects (OI). These microelements enable a variety of three dimensional (3D) routing architectures which are required to support high density interconnects in optical boards. Optical polymers have proved to be the materials of choice for large-scale OI modules with propagation dimensions exceeding 100 mm. In order to meet the loss budget available for the integrated OI modules, the polymers are expected to have optical losses less than 0.05 dB/cm. Both channel and slab waveguides can be used to transmit the signals between the input and output ports. In the case of channel waveguides, the critical issues are the waveguide core shaping, propagation losses and ability to form various passive elements such as bends, crossings and reflective mirrors. In the case of slab waveguides, two dimensional waveguide microlenses have to be designed to collimate the light beams for propagation at longer distances with the controllable beam divergences. The 45o micromirrors can be used to couple the light signal in and out of the waveguiding layer and enable 3D routing of the optical signal in the waveguiding layers. In this work, we present the experimental and computational results on the development of different waveguide devices and microelements for the board level OI.

Flexible pillars for displacement compensation in optical chip assembly

In chip-to-chip optical interconnect systems with surface mounted light-sources and detectors, thermal and mechanical effects can cause lateral displacements of the assembled devices. These displacements can result in optical signal losses that can critically deteriorate the bit-error-rate of the digital system. We demonstrate that, for a given loss budget of 1 dB, the use of flexible optical pillars with 150-/spl mu/m height and 50-/spl mu/m diameter can double the lateral displacement tolerance from about 15 to 30 /spl mu/m. The pillars fabricated from Avatrel polymer form an air-free path between the light source and the substrate and cause maximum optical power losses less than 0.2 dB.

Mechanically Flexible Chip-to-Substrate Optical Interconnections Using Optical Pillars

We experimentally characterize the benefits of using surface-normal mechanically flexible optical waveguides, or optical pillars, for chip-to-substrate optical interconnection. In order to benchmark the performance of the optical pillars, the optical coupling efficiency from a light source to an optical aperture with and without an optical pillar is measured. For a light source with 12deg beam divergence, a 50times150 mum optical pillar improves the coupling efficiency by 2-4 dB compared to pillar-free (free-space) optical coupling. A 30times150 m optical pillar improves the coupling efficiency by 3-4.5 dB. This demonstrates the importance of using optical pillars when small photodetectors (PDs) and dense optical input/outputs (I/Os) are needed. The optical excess losses of 50times150 mum optical pillars are measured to be less than 0.2 dB. Due to the high mechanical flexibility of the pillars, we also demonstrate that optical pillars enhance the optical coupling efficiency between the chip and substrate when they are misaligned in the lateral direction. This is especially important since the coefficient of thermal expansion of the chip and substrate are often mismatched, and preserving optical alignment and interconnection between them is critical during thermal excursions. The lateral mechanical compliance of the optical pillars is also measured and can be as great as 30 mum/mN. The optical pillars are also shown to be compliant under a compressive force thus allowing the optical I/Os to be assembled on nonplanar surfaces such as low-cost organic substrates.

Planar hybrid polymer-silica microlenses with tunable beamwidth and focal length

Thermally tunable planar microlenses have been fabricated with a hybrid process utilizing silica-on-silicon and polymer planar lightwave circuit technologies. The silica microlens has a double concave gap filled with an optical polymer. The lens collimates the light beam with a beamwidth of less than 450 /spl mu/m at distances up to 100 mm. The experiments presented demonstrate that by varying the lens temperature in a range of 60/spl deg/C the beamwidth can be tuned by /spl plusmn/40% at different distances from the lens. The beam-propagation-method simulations show that adjusting the lens curvature can control the beamwidth tunability range and thermal sensitivity. That allows optimization of the lens designs for different applications.