NMRC: Research Highlights - Photonics

NMRC has built a world class reputation in the key optoelectronics technologies of epitaxial growth, optoelectronic device fabrication, micro-optics, and optical sub-systems for photonics communications and metrology. Its mainstream activities in device fabrication (both active optoelectronics and micro-optics) are directed toward applications in high added-value optoelectronic sub-systems to meet the optical signal switching and multiplexing functions demanded by advanced ICT photonics communications for data and telecommunications. It is also developing systems for photonic metrology. The highlights of photonics research in 1999 at NMRC are presented below.

Red VCSELs Herald a Bright Future for Plastic Optical Fibre (POF) Communications

Figure 7:- The BREDSELS' VCSEL-to-POF ribbon module. A single VCSEL channel is operating and thereby illuminates one of the 8 POF channels.

NMRC is developing bright red vertical cavity surface emitting lasers (VCSELs) specifically designed for use with POF. VCSELs are particularly attractive optical sources for POF applications in that they have circular output beam profiles, operate at very low drive currents (a few mA) and can be modulated to high frequencies (several GigaHertz), allowing the highest data transfer rates. The VCSELs fabricated at NMRC within this project lead world research in this field.

NMRC and its research partners have also achieved considerable success in the development of a VCSEL-to-POF module consisting of a linear array of 8 VCSELs coupled to an 8-channel POF ribbon as shown in Figure 7. Each element of the VCSEL array, operating at 665 nanometres, can produce a room temperature, continuous-wave output power of 2 mW at around 10 mA. Each individual channel is capable of transferring data at 150 Mbit/s over 50 m, and hence this module has a total carrying capacity of 1.2 Gbit/s, a performance more associated with silica fibre data rates.

NMRC's Photonics Research goes full Spectrum with Gallium Nitride Blue LED Sources

In 1999, NMRC established a gallium nitride device fabrication and materials growth capability. Already, blue LED emitters (see Figure 8) fabricated at NMRC have been demonstrated. Figure 9 shows wavelength spectra as a function of bias for these blue LEDs. Further development will address the optimisation of the light extraction from the LED into the air. This is a source of losses because of the large discontinuity in refractive index. In the coming year, research will be focused on the production of green and amber LEDs, at the 510 nanometres and 570 nanometres POF absorption minima, exhibiting high temperature stability, for high bit rate POF IEEE1394B datacom applications.

	Figure 8:- Blue emitting GaN LED fabricated at NMRC.
	Figure 9:- Wavelength spectrum as a function of bias. Enlarge

Resonant Cavity LEDs (RCLEDs) Offer Cost Effective Alternative to VCSELs for Datacom Applications Resonant-cavity LEDs (RCLEDs) offer a cost effective alternative to VCSELs for datacommunication applications especially at data rates less than 500 Mbit/s. NMRC's RCLED devices at 665 nanometres (reflectance and photoluminescence spectra Figure 10), which use selective oxidation technology along with current spreading techniques, have achieved an external efficiency of 4%. These features, allied to a more tolerant wafer growth, make these devices competitive with VCSEL technology in POF links at lower data rates.


Figure 10:- Reflectance and photoluminescence spectra of red (665 nm) resonant cavity LED.

Frequency-Stabilised Laser Diodes for Sensors and Telecommunications

NMRC has continued its development of frequency-stabilised lasers using the Centre's proprietary technology. These lasers operate in the 1550 nanometre range where a number of gas lines provide the basis for optical sensors as well as being the wavelength of absorption minimum single mode optical fibre for telecommunications. Over 40dB side-mode suppression ratio has been achieved, a performance level that is normally associated with more complex and costly distributed feedback lasers.

A cost effective process technology to microfabricate a silicon platform (as shown) suitable for the manufacture of transceivers for telecommunications applications has been developed. The modules are manufactured by potassium hydroxide (KOH) etch micromachining to mount optoelectronic and OE chips to optical interconnects such as fibres. Electrophoretic resist deposition for metal interconnection patterning together with compensation layout for protection of convex corner integrity during etch have been successfully applied. A recent advance has been the development of a high coupling efficiency technology to passively align OE chips to their interconnects using solder bump flip-chip technologies.

Mach Zehnder Interferometers have been fabricated on silicon (Figure shows a Silicon nitride Mach-Zehnder interferometer on silicon substrate). These interferometers are intended for single mode transmission, and use silicon nitride as the transmission layer. There is a waveguide rib etched in the nitride with critical control over the rib etch depth. These optical interferometers find generic chemical applications in fields from telecommunications and datacommunications to sensor applications. In a specific sensor application, the silicon nitride is encased between two silicon dioxide layers with a window over one arm of the interferometer for application of Langmuir Blodgett calixarene films.

Diffractive optical elements (DOEs) are important in several industrial sectors including opto-electronics. Their ability to generate patterns and spot-arrays means that they can be used for applications such as interconnect and computer backplane clock distribution. Figure shows the pattern generated by an 8 level DOE designed to split a single laser beam into four. The pattern was transferred to a glass substrate using an excimer-laser- ablation, rapid-prototyping system specifically for this purpose. Collaborative research is also currently underway to develop a hot-embossing process for inexpensive mass production.

Semiconductor lasers are more efficient and compact than other solid state and gas lasers, however, they have poorer beam quality. The semiconductor laser beam is highly asymmetric and divergent requiring costly corrective optics to use the beam in high power applications. NMRC scientists have developed a laser structure that reduces the beam divergence in the fast direction from a typical 35-50 degrees to 19-21 degrees. This allows the use of cheaper optics as well as efficient (>75%), direct butt-coupling of the semiconductor light to optical fibres. 1.3 W per element has been demonstrated and these elements can be combined in arrays. A direct application will be in seamless welding for the medical device industry.

NMRC's Single Photon Avalanche Diodes (SPADs) (see figure) are designed for fluorescence and luminescence measurements. The 20 micron diameter SPADs have recently exhibited a world-class ultralow dark count performance of less than 50 counts per second at room temperature, and just 3 counts per second when Peltier cooled to -10 degrees Celsius, at 5 Volts above reverse breakdown voltage. A quantum efficiency of 66% at 632.8 nanometres was measured for these devices.