The Steve Cundiff group is working on developing ultrafast-modelocked fiber lasers because this technology would be significantly less expensive than ultrashort-pulse titanium:sapphire (Ti:S) lasers for routine laboratory use. Fiber lasers rely on fiber technology developed for the telecommunications industry, but they present some challenges. For instance, these lasers cannot use the Kerr-lens modelocking used in Ti:S laser to generate a frequency comb. Instead, they use nonlinear polarization rotation or require a saturable absorber such as a semiconductor or a waveguide array.
The Cundiff group has developed a new way to modelock fibers based on soliton formation in aluminum-gallium-arsenide (AlGaAs) waveguide arrays, using a method that is analogous to Kerr-lens modelocking. The AlGaAs waveguide arrays are capable of reshaping and shortening femtosecond pulses in time. Self-focusing is needed to create a laser cavity that can operate as a pulsed laser. The performance of the AlGaAs waveguide has verified predictions made by theorist collaborators at the Universities of Washington, Central Florida, and Quebec (Canada).
The Cundiff group recently demonstrated a new erbium-doped fiber laser incorporating an AlGaAs waveguide array as a passive fast saturable absorber. The laser produces a regular train of pulses, which indicates that the laser is modelocking. However, an analysis of these pulses showed that the pulses were essentially a train of noise bursts, indicating that the modelocking is incomplete. The performance of the laser was good enough, however, to suggest that waveguide arrays could be used for modelocking other types of lasers such as diode lasers. The group is currently working on perfecting the design of the new fiber laser and optimizing its modelocking performance.