A saturable absorber is a nonlinear optical material that becomes more transparent as the intensity of light falling upon it increases. Passive saturable absorbers can be integrated into laser systems to provide modelocking and into fiberoptic systems for passive optical regeneration. The traditional saturable absorber is a multiple-quantum-well (MQW) structure that requires expensive equipment for fabrication—cleanroom-housed metal-oxide chemical-vapor deposition or similar approaches to create the structure itself, and ion implantation to reduce the device's saturation recovery time from the nanosecond to the more practical picosecond range.
Researchers at Alnair Labs (Saitama-ken, Japan), the National Institute of Advanced Industrial Science and Technology (Ibaraki, Japan), Tokyo Metropolitan University (Tokyo, Japan), and the Research Center for Advanced Science and Technology Tokyo, Japan) have created a saturable absorber from a layer of single-walled carbon nanotubes sandwiched between two pieces of glass, termed a saturable absorber incorporating nanotubes (SAINT). The fabrication process is simple, consisting of spraying nanotubes onto glass. Because carbon nanotubes are chemically stable, no hermetic sealing is required. The optical damage threshold of the device is higher than that of a MQW saturable absorber; in addition, a SAINT works in transmission—an easier-to-work-with mode of operation than the reflective mode required for a MQW device.
The nanotubes themselves are synthesized by ablating a metal-catalyzed carbon target with a Nd:YAG laser in 500 Torr of argon gas. The resulting tubes, with a mean diameter of 1.1 nm, are dispersed in ethanol and sprayed onto 1-mm-thick substrates with an airbrush (see figure). The researchers used the SAINTs for two purposes: a noise-suppressing saturable absorber for 1550-nm light, and a modelocked fiber laser operating in the same 1550-nm spectral region.
The light source for the saturable-absorber setup was a fiber laser producing 1-ps pulse bunches (of about 120 pulses at a time) at an 80-GHz repetition rate. A fiber collimator and an aspheric lens brought the light to a focus on the SAINT. Varying the spot size by shifting the nanotube sample along the optical axis varied the intensity of light falling on the carbon-nanotube film. At a maximum peak intensity of 5.8 MW/cm2, the device reached a transmission of almost 69%, whereas transmission dropped to about 63% at lower powers. Spectral measurements indicate an inhomogeneously broadened absorption that responds on a 1-ps timescale. The performance of this first SAINT device is still far from its full potential, the researchers note.
Two configurations
Modelocking a fiber laser is ordinarily done with a semiconductor saturable absorber mirror (SESAM). For their modelocking experiment using a SAINT instead of a SESAM, the researchers put together a fiber laser in two different configurations, one with a ring geometry and the other of linear orientation. The erbium-doped ring laser was backward pumped with a 980-nm laser diode, with two optical isolators ensuring operation in one direction only. The SAINT and associated optics were simply inserted in a break in the ring—a geometry impossible with an ordinary reflective SESAM.
The ring laser began to modelock at a pump power of 18 mW, which could then be backed off to 14 mW, with the laser operating at 6.1 MHz in single-pulse mode (higher pump powers resulted in multiple-pulse operation at harmonics of 6.1 MHz). The resulting soliton pulses had a full-width half-maximum (FWHM) width of 1.1 ps, and were somewhat chirped—though the chirping could be reduced with the use of low-dispersion fiber. When the SAINT was removed from the laser cavity, all modelocking stopped, even at high pump powers.
The linear version of the modelocked fiber laser produced nonsoliton pulses at a repetition rate of 9.85 MHz, a FWHM width of 318 fs, a 3-dB spectral width of 13.6 nm, and an average power of almost 1 mW. "We believe this is the first demonstration of using carbon nanotubes for practical applications in the field of applied optics," said Sze Set, general manager of research and development at Alnair Labs, who noted that there was great interest in this material at this year's Optical Fiber Communications Conference (Atlanta, GA; March 23–28).