The Mayo SPPDG has a long history of research in the field of high speed electronics, but since the mid-1990's, the group has become active in the field of optoelectronics as well. The following sections describe some of Mayo's measurement capabilities and projects in this area.
In 1997 the SPPDG began researching optoelectronic components for high speed digital communication applications. The early research efforts focused on systems that utilized vertical cavity surface emitting lasers (VCSELs). VCSELs offer many advantages over standard edge-emitting lasers including lower power, lower cost, and better ease of integration with integrated circuits. SPPDG developed several new packaging techniques to demonstrate the technical feasibility of a low-cost replacement for the standard transistor outline packages ("TO" cans) which were commonly used for packaging VCSELs at that time. The SPPDG-designed packages did in fact demonstrate a cost efficient method to "break through" the 1 GHz performance barrier imposed by the TO can packages.
In 1998, under the auspices of a project referred to as “Free-Space Optical Interconnects (FSOIA)”, SPPDG assisted industry collaborators in the design of the electronics packaging for eight parallel optical channels operating in a nonblocking optical crossbar configuration, with each channel operating at rates of 2.5 gigabits per second (Gbps). These parallel channels allowed for aggregate throughput rates exceeding 10 Gbps. Modulation of the VCSELs for the transmit channels and photodetectors on the receiving channels was accomplished with integrated circuits fabricated in cost-effective bulk CMOS technologies. Extreme precision was required in the assembly process of the integrated circuits configured in a three-by-three array, since the integrated circuits had to be placed on a 10 x 10 cm support substrate, with placement accuracy in the 2-3 micron range. Because none of the other collaborating teams had the appropriate assembly equipment to support this level of assembly accuracy but SPPDG did, our staff assembled the demonstration articles for all three of the participating teams.
In 2000, SPPDG designed transmit and receive boards capable of 10 Gbps transmission across a fiber optic link. These boards used components with the highest performance VCSELs and photodiodes available at that time. The combination of the matching transmitter and receiver boards demonstrated the goal of 10 Gbps, with very low bit error rate in a very high shock and vibration environment. Actual tests of the printed circuit boards were performed in a borrowed wing-mounted pod attached to a “hard point” on an F-18 jet fighter based at the China Lake, California Naval Air Center.
Present Work in Optical Device Characterization
The SPPDG's laboratory has established capabilities to measure the optical-to-electrical performance of transducers such as PIN and avalanche photodiodes (APDs), as well as optical transmitters (VCSELs). Our laboratory can perform measurements on discrete and array devices in either bare die form, or as packaged parts. Characterization of the optical-to-electrical and electrical-to-optical conversion performance of the devices can be performed from DC to over 25 GHz pulse rates at optical wavelengths of 500 nm through 1630 nm. Spectral responsivity measurements can be performed from 300 nm to 10 mm. All of these measurements are also performed in a low noise environment.
Recently we have expanded our ability to measure the characteristics of optical sources, specifically VCSELs and photodetectors (both APDs and PIN photodiodes) by dedicating an entire laboratory to this work. We can characterize both the wavelengths and optical output of the laser sources, both singly and in combination, and can modulate various on-off keying pseudo-random sequences (also: PRN sequences) at multi-GHz modulation rates onto the optical outputs from one laser, or from several lasers at the same time. Likewise, we can measure the electrical output of both APDs and PIN photodiodes when they are exposed to either the DC output, or the PRN-modulated output, of the laser sources, again, either singly or in combination. We also have the ability to measure the effects of various scattering media positioned between the sources and the detectors. The following four-panel figure depicts a portion of this new dedicated laboratory, illustrating a portion of the measurement test suite, as well as detailed views of the components being measured (both laser sources and photodetectors).
Spectroscopy laboratory facility configured for the testing of individual solid state lasers (e.g., vertical cavity surface-emitting lasers, [VCSELs]) or simultaneously, groups of such lasers; and independent or concomitant testing of solid state photodetectors (e.g., PIN diodes or avalanche photodiodes [APDs]).
The photo below presents several electronic circuit boards containing packaged-part photodetectors of both types (i.e., PIN photodiodes and APDs) as they are configured for installation into the test suite pictured above. The photodetectors have sufficiently fast electrical response that they can detect PRN modulations of the light sources well into the Gb/second range.
Detector circuit boards configured to receive electrical signals from one or more PIN diodes or avalanche photodiodes (APDs), along with electronics to support the optical sensing devices.
Finally, we are beginning the evaluation of PCBs with one or more embedded optical interconnect paths to function in combination with the types of optical sources and solid-state photodetectors described above.