26 Oct 2005
A look at some of this month's innovations including a photonic crystal fiber amplifier with a peak output power of 1.1 MW.
Photonic crystal fiber
Researchers at US firm Aculight have developed a microchip-laser seeded photonic crystal fiber (PCF) amplifier that generates diffraction-limited 0.45 ns duration pulses with a peak power of 1.1 MW. Based on a large-core, Yb-doped PCF, the amplifier is also said to have a peak spectral brightness of greater than 10&nbsolkW/(cm2sr Hz). (Optics Letters 30 2694)
"To our knowledge, the obtained peak spectral brightness represents a record for fiber-based sources," say the authors. "The reported source provides a good example of how fiber amplifiers can overcome the limitations of traditional bulk solid-state technology. It also shows the benefits of combining bulk oscillators and fiber amplifiers for pulsed operation."
Placing different sized current-confinement apertures at different locations in a VCSEL's DBR mirrors allows the source to emit high-powers single-mode, according to scientists from Arizona State University, US. Led by Yong-Hang Zhang, the team says it has fabricated a VCSEL that operates at room temperature, emits 7.5 mW continuous wave and has a side mode suppression ratio of 20 dB. (Applied Physics Letters 87 161108)
Charles Lieber's group at Harvard University, US, has grown gallium nitride (GaN) nanowires that have a lasing threshold power density of 22 kW/cm2. "To our knowledge, this is the lowest lasing threshold at room temperature reported for GaN materials," say Lieber and colleagues. "It is also comparable to that of the CdS nanowires used to realize the first single-nanowire electrically-driven laser." (Applied Physics Letters 87 173111)
The group grows its nanowires by MOCVD, which results in natural free-standing Fabry-Perot cavities with triangular cross-sections and uniform diameters. Along with the structural properties, the team lists a non-polar growth direction and silicon doping as essential to lowering the threshold power density.
Scientists in the UK and Australia say that the use of a custom-designed taper can give multi-mode fiber (MMF) devices the performance of single-mode fiber (SMF) components. In a proof of principle experiment, the team from the University of Bath, UK, and the Anglo Australian Observatory and Redfern Optical Components, both of Australia, made a MMF Bragg grating device with the narrow spectral width of an SMF grating. (Optics Letters 30 2545)
"If the number of single-mode fibers matches the number of spatial modes in the multimode fiber, the transition can have low loss in both directions," say the team. "This enables the high performance of single-mode fiber devices to be attained in multimode fibers."