Jukka-Tapani Mäkinen

Embedded optical interconnect on printed wiring board

Integration of high-speed parallel optical interconnects into printed wiring boards (PWB) is studied. The aim is a hybrid optical-electrical board including both electrical wiring and embedded polymer waveguides. Robust optical coupling between the waveguide and the emitter/detector should be achieved by the use of automated pick-and-place assembly. Different coupling schemes were analyzed by combining non-sequential ray tracing with Monte-Carlo tolerance simulation of misalignments. A modular demonstrator was designed based on three different kind of optical coupling schemes: butt-coupling and couplings based on microlens arrays and on micro ball lenses. The optical front-ends were implemented with PIN and flip-chip-VCSEL arrays as well as 10-Gb/s/channel electronics onto LTCC-based (low-temperature co-fired ceramic) transmitter and receiver modules, which were surface mounted on high-speed PWBs. An electrical simulation model was developed for the design of a VCSEL-based transmitter circuit. Polymer waveguides were fabricated on separate FR-4 boards to allow characterization of alignment tolerances with different waveguides. Optical and adhesion properties of several potential waveguide materials were characterized. The simulations and experiments suggest that, with optimized optomechanical structures and with low loss waveguides, it is possible to achieve acceptable total path loss and yield with the accuracy of automated assembly.

Fiber-Optic Transceiver Module for High-Speed Intrasatellite Networks

High-speed intrasatellite networks are needed to interconnect units such as synthetic aperture radars, high-resolution cameras, and fast image-compression processors that produce data beyond gigabits per second. We have developed a fiber-optic link, named SpaceFibre, which operates up to 3.125 Gb/s and is compatible with the existing SpaceWire network. The link provides symmetrical, bidirectional, full-duplex, and point-to-point communication. It employs 850-nm vertical-cavity surface emitting lasers, radiation-hardened laser-optimized 50/125 mum graded-index fibers, and GaAs p-i-n photo diodes. The transceiver electronics is realized using a multilayer-ceramic-substrate technology that enables the passive alignment of optical fibers to active devices. The SpaceFibre link demonstrator was tested to transfer data at 2.5 Gb/s over 100 m with a bit error rate of less than 1.3middot10-14. Fiber-pigtailed modules were stressed with temperature variations from -40degC to +85degC, vibrations up to 30 g, and mechanical shocks up to 3900 g. The test results of 20 modules show that the SpaceFibre link is a promising candidate for the upcoming high-speed intrasatellite networks

Injection moulding integration of a red VCSEL illuminator module for a hologram reader sensor

A red VCSEL illuminator module demonstrator was manufactured by injection moulding integration. A red VCSEL chip was first attached to a simple FR4 substrate, which contains bonding pads and conducting wires for the VCSEL chip attachment and electrical driving. The substrate was then placed as an insert in an injection mould. The VCSEL chip shielding and optics formation was made in a one-step injection moulding process. The used optical thermoplastic in the processing was polycarbonate (PC). The pursued optical function of the single spherical surface attained in the moulding was to collimate the emitted red light (&lgr;=664.5 nm) from the VCSEL chip. The main critical issue related to the manufacturing of the illuminator module in the injection moulding process was the durability of bonding wire contacts. A single 25 &mgr;m diameter gold wire was used in wire bonding in order to create the upper contact to the chip. The lower contact was processed by attaching the chip to the substrate using conductive epoxy. A test series of 20 modules using FR4 substrate materials were produced. The number of fully operative modules was 12 resulting total module yield of 60%. The main reason for a non-operative module was loosening of the bonding wire during the injection moulding process. The bonding wire durability in the moulding process can be improved by using glob-top shielding of the VCSEL device before injection moulding and using a lower holding pressure in the injection moulding process. A diamond turned insert was used in the mould in order to create a high quality lens surface on the top of the VCSEL chip. The tower average length after one iteration round by mould modification was 8.676 &mgr;m, so the measured value was on average 20 &mgr;m larger than nominal value. The measured RMS roughness of the processed lens surface was 5 ... 7 nm and the radius -3.23 ... 3.83 mm. The radius of the lens and the length of the tower varied depending of the used process parameters. The manufactured illumination module can be integrated with a CMOS image matrix sensor in order to form a compact hologram reader system. The injection moulding integration principle seems to be very promising method to manufacture intelligently integrated and cost-effective optoelectronic products according to experience with this demonstrator.

Hot Laminated Multilayer Polymer Illumination Structure Based on Embedded LED Chips

The dominant technology for manufacturing backlight illumination structure (BLIS) is typically based on the use of individually packaged surface mount device light emitting diodes (LEDs) and special light guide plate (LGP) and diffuser films. The prevailing BLIS package, however, contains several separate diffuser films, which results in a thick and costly structure. In addition, the light coupling from LED to the LGP is sensitive to alignment errors causing nonuniform and inefficient illumination. We have demonstrated a novel hot laminated packaging structure for backlighting solutions, which is based on inorganic LED chips and multilayer polymer structure. The main advantages of the implemented system compared to the traditional light guiding system are easy optical coupling with high efficiency in an integrated and thin package. The performed designs of 3×3, 5×5, and 5×7 LED chip matrices, verified by test structure implementations and characterizations, showed that the final thickness of the BLIS depends on the required uniformity of illumination, allowed LED device pitch and efficiency of the diffuser. The final BLIS demonstrator size was 50×75 mm2 consisting of six 25×25 mm2 modules. Each module consisting 5×5 LED devices resulting in total number of 150 LED devices with 5-mm pitch. The measured key characteristics of the demonstrator were as follows: average brightness 11.600 cd/m2 (ILED = 2 mA), luminous efficiency 22 lm/W, color temperature 5550 K, commission on illumination values (x = 0.331, y = 0.411), Color Rendering Index ≥ 70, and total power conversion efficiency of 6.3%. The combination of the developed Matlab performance simulation tool and cost-of-ownership cost evaluation tool enables us to estimate the manufacturing cost of a specific BLIS element against the required performance, assisting decision-making in different applications and specific individual customer cases.

Hermetic Fiber Pigtailed Laser Module Utilizing Passive Device Alignment on an LTCC Substrate

A hermetic fiber pigtailed laser module utilizing passive device alignment on a low-temperature cofired ceramics (LTCC) substrate is demonstrated. The 3-D shape of the laminated and cofired ceramic substrate provides the necessary alignment structures, including grooves and cavities, for the laser-to-fiber coupling. When the laser diode chip and component tolerances are tight enough, the passive alignment allows high coupling efficiency realizations of multimode fiber pigtailed laser modules. The ceramic substrate is intrinsically hermetic and it opens up the possibility to use the substrate as an integrated part of the hermetic module package. In our concept hermetic sealing is produced by utilizing a Kovar frame, which is soldered to an LTCC substrate. The Kovar frame has a hole for a fiber feed-through and a hermetic glass-metal seal between fiber and frame is processed using a glass preform. The module can be used as a transmitter in a laser pulse time-of-flight distance sensor and in this application it can be overdriven by a factor of 10. This means that the peak optical power in the pulses can be several dozen watts. The laser chip allows this kind of overdriving, due to the fact that the duty factor in the operation is only 0.0% at 2 kHz pulsing frequency, which leads to an average power of several milliwatts. The simulated nominal coupling efficiency between the 210 mum times 1 mum stripe laser and the 200/220 mum step index fiber (NA = 0.22) was 0.65. The measured coupling efficiency of the hermetically sealed prototypes varied from 0.14 to 0.64, where the average was 0.39. A leak rate of 1 times 10-7 . .. 8 times 10-7 [atm times cm3/s] was measured in the helium leak tests of the final operational prototypes, when the modules were tested according to MIL-STD-883F method 1014.9 specification. The rather high leak rate is mainly due to the helium absorbed by the fiber polymer buffer layer and rubber guard tube in the pressurization process. The leak rate for the dummy modules using a buffer stripped fiber without a rubber guard tube was 3 times 10-9 . .. 1 times 10-8 [atm times cm3/s]. The maximum allowed leak rate for this size of hermetic module is 1 times 10-7 [atm times cm3/s]. The background helium level before and after the tests was less than 3 times 10-10 [atm times cm3/s]. Measurements proved that the manufacturing procedure is capable of producing hermetic fiber pigtailed laser modules.

Inmould integration of a microscope add-on system to a 1.3 Mpix camera phone

A microscope add-on device to a 1.3 Mpix camera phone was selected as a demonstrator system for testing inmould integration of electronic substrates and plastic optics. Optical design of the device was quite challenging due to the fact that illumination system needed to be integrated with a double aspheric singlet lens structure as a single optical piece. The designed imaging lens resolution was adequate to resolve 10 &mgr;m features with a mobile phone camera. In the illumination optics the light from LEDs embedded into the plastic structure was collected and guided to the surface that was imaged. Illumination was designed to be uniform and adequately bright to achieve high resolution images with the camera phone. Lens mould design was tested by using injection moulding simulation software. The critical mould optical surfaces were designed as separate insert parts. Final shapes producing lens surfaces were tooled by diamond turning on nickel coatings. Electronic circuit board inserts with bonded bare LED chips and packaged SMD LEDs were assembled to the mould and then overmoulded with optical grade PMMA. Experiences proved that inmould integration of electronic substrates, bare LED chips and high resolution imaging optics in injection-compression moulding process is feasible. The yield of embedded packaged and also bare chip components was close to 100% after the right injection moulding process parameters were found. Prototype add-on system was characterized by testing the imaging properties of the device with a camera phone.

Add-on laser reading device for a camera phone

A novel add-on device to a mobile camera phone has been developed. The prototype system contains both laser and LED illumination as well as imaging optics. Main idea behind the device is to have a small printable diffractive ROM (Read Only Memory) element, which can be read by illuminating it with a laser-beam and recording the resulting datamatrix pattern with a camera phone. The element contains information in the same manner as a traditional bar-code, but due to the 2D-pattern and diffractive nature of the tag, a much larger amount of information can be packed on a smaller area. Optical and mechanical designs of the prototype device have been made in such a way that the system can be used in three different modes: as a laser reader, as a telescope and as a microscope.

Cost-effective packaging of laser modules using LTCC substrates

The modeling, realization and characterization of photonic module based on the use of Low Temperature Co-fired Ceramics (LTCC) technology is reported. The 3D modeling of the system provides possibility to optimize structures, materials and components in order to achieve optimal performance for the final product and still maintain reasonably low fabrication costs. The cost-effectiveness in the product can be further optimized using an iterative optimization process, in which the effect of module manufacturing tolerances and assembly process tolerances is simulated by a VisVSA Monte-Carlo simulation. The tolerance distributions produced by a VisVSA simulation are used as input parameters together with optical component tolerances in an ASAP Monte-Carlo simulation, in which the final module optical performance distribution in simulated production is obtained. The module cost, performance and optical performance limited yield is possible to define with this iterative process. As an example of this kind of packaging modeling, we present a demonstrator module having a high-power multimode laser diode with a 1μm x 100μm emitting area coupled to a 62.5/125μm graded-index (NA=0.275) multimode fiber. The tolerance modeling results are verified by experimental characterization of the packaged prototypes. Measured coupling efficiencies were in good agreement with simulated ones, when the fiber NA was 0.2 or larger. The measured coupling efficiency, however, was 38% lower than simulated, when the fiber NA was 0.12. This was probably due to the laser mode structure difference between simulation model and reality. Coupling efficiency of 0.46 was obtained in a passively aligned demonstrator module, when the nominal value was 0.48.

Simulation of imaging system's performance

In this paper, an imaging system simulation tool is presented. With the tool, it is possible to simulate the performance (quality) of an imaging system. Furthermore, the system allows optimization of the lens system for a given image sensor. Experiments have shown that the tool is useful in actual lens design.

Parallel optical interconnect between surface-mounted devices on FR4 printed wiring board using embedded waveguides and passive optical alignments

Technologies to design and fabricate high-bit-rate chip-to-chip optical interconnects on printed wiring boards (PWB) are studied. The aim is to interconnect surface-mounted component packages or modules using board-embedded optical waveguides. In order to demonstrate the developed technologies, a parallel optical interconnect was integrated on a standard FR4-based PWB. It consists of 4-channel BGA-mounted transmitter and receiver modules as well as of four polymer multimode waveguides fabricated on top of the PWB using lithographic patterning. The transmitters and receivers built on low-temperature co-fired ceramic (LTCC) substrates include flip-chip mounted VCSEL or photodiode array and 4x10 Gb/s driver or receiver IC. Two microlens arrays and a surface-mounted micro-mirror enable optical coupling between the optoelectronic device and the waveguide array. The optical alignment is based on the marks and structures fabricated in both the LTCC and optical waveguide processes. The structures were optimized and studied by the use of optical tolerance analyses based on ray tracing. The characterized optical alignment tolerances are in the limits of the accuracy of the surface-mount technology.