SOA speed-up uses optical assist

Sunny Bains

At the Ecole Polytechnique Federale de Lausanne (EPFL; Switzerland), researchers have proposed a new way of implementing a semiconductor optical amplifier (SOA) that uses the injection of a continuous-wave light beam to make it several times faster without reducing gain or increasing current. The device makes use of a change in the point of optical transparency that occurs when the signal pulse interacts with the SOA. Researchers say the new configuration, potentially useful for both high-speed WDM and OTDM applications, may be integrable on a single chip.¹

A transparency point of a photonic device is one at which a wavelength is absorbed and emitted equally. The new SOA configuration, which the Swiss team calls an OSAT (for optical speed-up at transparency) uses an incoming light beam at the material-gain transparency point to boost the production of carriers when they are most needed. These are essential because when the incoming signal-at λ₁-is amplified by the SOA, there is a decrease in the number of charge carriers available to produce gain. Normally, such high gain would not be available again until these carriers had been replenished-a relatively slow process if it`s allowed to happen naturally (electronically).

However, the decrease in charge carriers has a secondary effect: it shifts the transparency point of the material. The continuous-wave beam-at λ₂-is at this transparency point, having almost no effect on the device before the signal arrives. While the signal is present and amplification takes place, however, the resulting drop in charge carriers pushes the injection wavelength into the device`s absorbing region, and the absorption, in turn, produces charge carriers (see figure). Thus, λ₂ quickly returns what λ₁ has taken away.

The EPFL group says that as well as allowing optical gain up to 30 dB and recovery times as low as 19 ps, the device has advantages in terms of saturation power, current required, design flexibility, and bandwidth. In addition, the device should be both integrable and cascadable. The need for an additional component (to supply the injection wavelength), say researchers, should not be a serious drawback. For more information, contact Marc-Andre Dupertuis at [email protected].

REFERENCE

1. M. A. Dupertuis et al., IEEE Phot.Tech. Lett. 12 (11), 1453 (November 2000).

As the injected optical power increases, the recovery time of the OSAT SOA drops dramatically without any significant change in the device gain.