Integrated optics generate 1-THz WDM pulse trains
A waveguide array has generated a high-repetition-rate burst of short pulses at multiple center wavelengths simultaneously, well-suited to WDM applications. Other systems have generated fast pulse trains from lower-rate pulses, but the new device's pulse-repetition rate is determined by its design parameters, not by the modelocked laser that generated the lower-rate pulse.
Dan Leaird and colleagues at Purdue University (West Lafayette, IN) and NTT Photonics Laboratories (Naka-gun, Ibaraki, Japan) used an arrayed waveguide grating (AWG) in a system that can generate spatially separated pulse trains at frequencies of 500 GHz or more.1 AWGs are also used for wavelength demultiplexing and for routing in WDM systems.
In this case, the AWG is closely analogous to a device made from bulk optics: the direct space-to-time pulse shaper (DST).2 Both systems spatially separate the light (via a mask or waveguides), and perform a spatial Fourier transform on the light. Therefore, the output temporal intensity profile is a directly scaled representation of the input spatial profile—in this case a series of temporal pulses. The light is emitted in spatially separated output channels with varying center wavelengths, but all channels share identical temporal intensity profiles.
In the Purdue/NTT device, a pulse from a modelocked laser enters to the AWG via a fiber. The light is guided through the slab waveguide into the waveguide array, which acts like a curved diffraction grating and lens. At the other end of the array, the light passes through a second slab waveguide into multiple output fibers. A major advantage of the AWG device is that it is much more compact than the bulk-optical DST (see Fig. 1).
Both devices provide high-repetition-rate bursts of evenly spaced pulses (see Fig. 2). For demonstration, the AWG was pumped by a 50-MHz train of 200-fs pulses at 1560 nm from a passively modelocked fiber laser and provided output with a 1-THz repetition rate.
The key design constraint for the AWG in this mode, say the researchers, is that the free spectral range must be less than the optical bandwidth of the source laser. (Most AWGs used in DWDM, in contrast, require that the free spectral range be larger than the optical bandwidth of the fiber amplifiers used in the DWDM system.) In theory, the device could be designed to create continuous terahertz repetition-rate trains of short pulses when pumped by a moderate-rate laser.
The Purdue/NTT team is working on a new AWG device designed to equalize the pulses within the output-pulse train. They are also combining the AWG with an actively modelocked fiber laser to generate continuous high-repetition-rate trains of short pulses. Within Purdue there is also work in progress on arrays of high-speed optoelectronic modulators with the goal of generating a compact apparatus for impressing a lower-rate parallel electrical data stream onto a serial ultrafast optical channel.
- D. E. Leaird et al., LEOS Newsletter, 13 (6), 3, (December 2000).
- D. E. Leaird, A. M. Weiner, Opt. Lett., 24, 853 (June 1999).
Yvonne Carts-Powell is a freelance science writer based in Belmont, MA.