Engineers at Purdue University (West Lafayette, IN) have demonstrated that an arrayed waveguide grating (AWG) is capable of producing a high-repetition-rate burst of short pulses at multiple, spatially separated, output wavelengths from a lower-repetition-rate femtosecond pulse laser. This finding has potentially significant implications, because the same device might help to dramatically increase the transmission speed and amount of data that can be sent over a single channel.
"We realized that rather than just being used to separate wavelengths, AWG technology could be modified for another application," explains Andrew Weiner, "If you send a pulse of light into one of these devices, you can get a burst of pulses coming out."
Weiner and colleagues earlier pioneered optical systems that shape and manipulate pulses of laser light—a technology used to study and control ultrafast processes inside molecules. Weiner and research engineer Daniel Leaird modified this technology to create a new type of "pulse shaper" that generates ultrafast packets of data sent over optical fibers. They then modified commercially available arrayed waveguide gratings to perform a function similar to their pulse-shaping system.
The key to making the arrayed waveguide gratings generate rapid-fire pulses, the researchers said, is to tailor the free spectral range of the device—the inverse of the delay increment per guide in the waveguide array—so that multiple filter passbands fit within the input laser bandwidth. In traditional arrayed waveguide gratings, the FSR is typically large to ensure that a unique output within the dense wavelength-division-multiplexing (DWDM) system is present at each AWG output. For the generation of high-repetition-rate trains of pulses, the researchers worked in the opposite regime, in which the optical bandwidth of the source laser exceeds the free spectral range. If the input pulses are bandwidth limited, the input pulse width is less than the delay increment per guide.
The researchers discovered that the device can turn a single pulse of light from a modelocked erbium fiber laser into a rapid-fire burst of 21 pulses, each separated by only picoseconds—at least 10 times faster than the transmission speed of each channel in state-of-the-art commercial optical communication systems.
Properties of the device allow generation of identical, wavelength-shifted, very high-rate-pulse trains for hybrid time-division-multiplexing/WDM communications and photonics signal processing. Initial work has been performed on tailoring the output temporal profile to generate a flat-topped pulse train burst. The researches believe that, when applied to a high-repetition-rate modelocked source (>10 GHz), these devices could lead to continuous very high-repetition-rate output trains (>500 GHz).
- D. E. Leiard, A. M. Weiner, S. Shen, CLEO 2001 Tech. Digest, CtuM45.