A novel photonic bandgap engineering technology promises lower-cost manufacturing of high-performance components for metro and access applications.
James O'Gorman Eblana Photonics
In the past, the engine for growth in communication services was the long-haul sector, often referred to as the "information suerhighway". In contrast, the current drivers are the end business and private consumers, which have led increasing demands for scalable, flexible, high-speed and customised bandwidth services in metropolitan, local-area and wide-area networks (MANs, LANs and WANs), and in the access loop. Metro and access systems are essential features of the evolving communications environment as they represent the "interchanges" and "on/off ramps" without which the information superhighway cannot function effectively.
Metro and access applications represent a significant evolutionary challenge for optoelectronics and have implications for technologies and products and their successful deployment. The diverse and vibrant nature of the broadband market represents an enormous challenge to the systems and components to be deployed. Indeed, as penetration spreads to the end-user, this will ultimately lead to optoelectronics/ photonics becoming a consumer technology.
The most important criterion for products deployed in this area is cost, particularly in metro access applications. Like all communications segments, access is moving to broadband, with a consequent transformation from voice- to packet-based data orientation. Access broadband provisioning has not yet spread significantly outside DSL and cable modem service deployments - primarily due to lack of a "killer" application (as well as cost). But that killer application is now nearing, in the form of down-streamed video. As was the case for cell phones, it is expected that - from broadband penetration rates of about 10% - the fibre residential and access broadband market will explode.
The key laser components for the metro and access segments are directly modulated lasers (both discrete devices and arrays), Coarse Wavelength Division Multiplexing (CWDM) specified lasers for high-speed datacoms, and devices targeted at optical supervisory channels. In addition (and primarily for the metro core) other components are also desirable, including high-speed electro-absorption lasers and tunable filters and lasers. In the metro environment optical amplifiers will be located mainly at the link end-points, particularly at nodes with high connectivity. In such regimes the primary requirement will be for low-noise modest amplification by so-called mini-amps or amplets pumped by economic, uncooled lasers of modest power output.
Component integration will become another key trend, but only if it leads to higher reliability and a better price/performance ratio. Clear winners will be technologies with improved yields where increases in volumes lead to improved automation opportunities.
The key point is that the metro and access environment is not just a scaled-down long-haul look-alike. Also, components are more than just smaller, cheaper, lower-specification versions of long-haul components: they need additional functionality at more economic cost. Nonetheless affordability is a key driver (more so in the current climate). Clearly many suppliers cannot leverage their capability in long-haul components due to high cost constraints and the unsuitability of products to the peculiarities of the metro and access environment.
Market surveys indicate that the major active component market opportunity for the next few years is in the metro environment. Eblana Photonics has therefore developed a generic technology platform for manufacturing affordable high-performance components for metro and access consumer and close-to-consumer applications. The technologies allow the fabrication of:
- economic high-performance single- mode transmitter sources (currently delivering in excess of 50dB SMSR);
- economic pump lasers for amplifiers and mini-amplifiers (cooled and uncooled);
- economic single-mode laser arrays (both for transmitter and pump lasers).
The same technologies also provide a path to more complex integrated structures such as:
- tunable laser diodes;
- integration of optics with electronics.
This diverse capability arises from the platform's removal of complex and inherently difficult device fabrication processes such as re-growth as well as the introduction of manufacturing and packaging simplicity.
The technical complexity of repeated growth, re-growth and etching involved in making conventional high-performance laser diodes cannot be scaled up efficiently for the volumes required by the metro and access markets.
In contrast, Eblana's technology platform uses photon mode engineering, where topology and geometrical effects control laser modes.
Subsequent to growth of the laser layer structure, topological and geometric structures (Figure 1) are formed to create photonic bandgap structures which control the selected optical spatial and longitudinal modes
propagating within the laser. Figure 3 shows how a photonic bandgap topology can improve the emission spectrum.
Electronic band structure results from the interaction between electron Schrödinger waves and the spatially varying potential arising from the semiconductor lattice. In photonic band structures, it is the interaction of an optical wave (rather than an electron wave) with dielectric constant variations of the optical material that selectively disallows particular photon energy levels or wavelengths while allowing others to propagate more freely. Programming device performance using photonic bandgap topologies has the advantage of not requiring re-growth of the laser material. This impacts device reliability, given that failure mechanisms associated with re-grown semiconductor hetero-interfaces do not now arise. Consequently the manufacturing approach ensures that advanced mode-controlled components are volume manufacturable, since advanced manufacturing technologies and production methodologies widely used in volume silicon device manufacturing can be used for improved manufacturing efficiency.
The key advantages of the photon mode engineering approach are:
- ease of production and cost;
- intrinsic reliability and absence of re-grown semiconductor heterointerfaces (which can have long-term reliability issues);
- genericity and efficiency of manufacturing high-specification photonic devices at all wavelength ranges and in all materials;
- application of a single platform to deliver lasers with high SMSR single-wavelength emission and/or high power emission;
- scalability to arrays of laser sources (single- or multi-wavelength) on a single substrate;
- simpler manufacturing processes allow a unique development path for integration of high-performance photonic components with integrated circuits (PICs).
Eblana has already developed two product groups from this platform:
- high-performance single-wavelength transmitter sources; and
- amplifier pump laser diodes, with large emission apertures for high-output-power pump lasers with high device reliability.
The transmitter sources exhibit enhanced performance in emission spectral purity. Fabricated devices show side-mode suppression ratios (SMSRs) of 54dB (Figure 4). Furthermore, this single-mode characteristic is retained when directly modulated at high bit rates: error-free operation (<10-12 bit error rates) has been demonstrated in data transmission trials at data rates of 2.5Gbit/s over 25km of single-mode fibre.
A large-aperture pump laser diode design ensures reliable devices with higher emitted power before catastrophic failure (or, when operated at low powers, longer device lifetime) by emitting light in a stable single spatial mode through a large emitting aperture. Figure 2 shows light versus current curves for two laser structures: one optimised for high-power and the other for low-threshold/high-temperature operation. Also, the manufacturing platform is generic and applies for all laser pump wavelengths of interest (980, 1480 and 14XXnm).
In conclusion, the diverse nature of the metro and access environment leads to exacting requirements on technologies. Affordability is a driving issue, while the large volume of components needed will entail the transformation of optoelectronics into a consumer technology. The transition of optoelectronics into a highly dynamic industry targeted at a broad-based consumer market will require component suppliers to deploy technologies based on generic manufacturing platforms (as for CMOS silicon ICs).
For active photonic components, these manufacturing platforms must apply to all laser diode wavelengths and applications, and must be based on intrinsically mass-manufacturable, high-yield processes. Photon mode engineering fulfils this need for a generic manufacturing platform, on which multiple product sets can be based. Initial products such as high-specification, single-wavelength transmitter lasers and economic pump lasers have been unveiled and are under test with selected users.