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UK Research Center Aims To Exploit Famed History

The renowned BT Photonics Technology Research Centre in the UK is now occupied by R&D outfit The Centre for Integrated Photonics. Richard Stevenson finds out what the start-up plans to do with its intellectual-property and wafer-processing-equipment inheritance from BT and Corning.
In late 2004 The Centre for Integrated Photonics (CIP), an independent research and development firm with a rich heritage in InP-based optical components, released its first products. Electro-absorption modulators (EAMs), which operate at either 1300 or 1550 nm and at 10 or 40 Gbit/s, are the result of a development program that began when British Telecom (BT) owned the premises, and continued when Corning took over the site.

Today, however, CIP s facility is owned by the East of England Development Agency. Fitted out with £40 million ($74 million) of inherited high-tech equipment, these facilities are used by the not-for-profit research organization to develop products for both industry and academia. The firm has released its EAMs partly in a bid to raise its profile and stimulate collaboration with industrial partners. The components themselves have useful characteristics, including low-voltage operation, high optical throughput and high-power handling.

The key attribute of EAMs - devices which are analogous with camera shutters - is that they generate short optical pulses at frequencies unattainable by laser modulation. Since they do not emit light, they are used in conjunction with a continuous-wave laser. Applying an electric field across the device enables rapid switching, thereby circumventing the slower electron-hole recombination process. Dave Moodie, CIP s EAM specialist, said: "The speed is fundamentally limited by the uncertainty principle", implying that the devices could operate in the terahertz regime.

CIP s EAMs can be used in either singlemode or dense-wavelength division multiplexing communications. The devices consist of an intrinsically doped absorber region of less than 1 μm thick, sandwiched between p- and n-doped InP layers (figure 1). The "camera shutter" is triggered by applying only about 3 V across the device.

Within the absorbing region is a multiquantum well structure. An electric field alters the wavefunctions of electrons and holes within the wells, thereby shifting the absorption spectrum of the device and blocking the light output. Moodie says that a benefit of using a quantum well structure - which creates a well-resolved exciton spectral absorption edge that can be seen at room temperature - is a greater change in absorption with voltage and therefore a lower operating voltage.

To transmit high-frequency optical signals, light is coupled into and out of the EAM. High optical throughput is essential for commercial devices, and so waveguiding is implemented in two dimensions. The difference in refractive index between the absorber region and the n- and p-doped InP layers introduces optical confinement in the growth direction, while confinement perpendicular to this axis is achieved by mesa etching followed by regrowth of high-index material. CIP s design involves fabricating a relatively tall, narrow mesa, followed by planarization of a regrown, current-blocking, Fe-doped, InP layer using PCl3. This approach differs from most other EAM manufacturers, which use shorter mesas and need to regrow dielectric layers.

Moodie says that CIP s design has the advantage of distributing optical absorption more gradually along the EAM s length, which improves optical power handling. The optical mode between the EAM and the optical fiber is also better matched, thereby reducing coupling losses and resulting in low device insertion losses (typically

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