21 May 2007
Post-deadline papers at this year's CLEO conference revealed what's happening at the cutting edge of optics research. Here is our selection.
• Cascaded injection-locked 1.55 µm VCSELs for high-speed transmission.
Xiaoxue Zhao and colleagues at the University of California, Berkeley, have used a cascaded injection-locking scheme to develop high-speed laser transmitters with low-frequency components. The team achieved tailorable RF response with bandwidth of about 52 GHz by adjusting the locking parameters of two slave lasers independently. Such high-speed transmitters are critical components for 100 Gbit/s Ethernet and local area networks.
In the scheme, the VCSEL cavity acts as a red-shifted high-Q optical amplifier under high-power optical injection, providing single-sideband amplification of the modulation signal up to frequencies nearly one order of magnitude higher than that of free-running VCSELs. This technique may allow the cascading of additional VCSELs in a daisy chain.
• Diode-pumped 22 W average power UV laser with user-selectable pulse width and >50% conversion efficiency.
A diode-pumped Nd:YAG laser producing 39 W at 1064 nm has been frequency tripled with a conversion efficiency of greater than 50% to produce 22 W at 355 nm. Developed by researchers at the Lawrence Livermore National Laboratory, it operates at 300 Hz with pulse energies of more than 73 mJ.
The laser was specially designed to meet stringent requirements for conditioning KDP frequency conversion crystals. This was achieved using a diode-pumped Nd:YAG laser with a master oscillator power amplifier (MOPA) design.
• High energy, long pulse filaments in air.
Oliver Chalus and a team at the University of New Mexico have made the first observations of 300 µm diameter filaments of 100 to 200 mJ energy at 266 nm. The work confirms the theory that the filamentation seen in femtosecond lasers can be scaled up for longer pulses above 1 nanosecond in the ultraviolet range.
The experiment indicated an abrupt threshold for beam collapse at 100 mJ. The laser filament had a cylindrically symmetrical profile, which was found to be independent of the initial beam profile.
• Counter-propagating QPM parametric interactions: a mirrorless OPO in sub-µm periodically poled KTiOPO4.
Mirrorless optical parametric oscillation (OPO) has been demonstrated for the first time, taking advantage of the quasi-phasematching (QPM) approach. Using a sub-µm periodically poled KTiOPO4 crystal, researchers at Stockholm's Royal Institute of Technology achieved a conversion efficiency of 16.5%, which equates to a quantum efficiency of 33%.
The experiment found that the spectral shape of the signal replicated that of the pump, removing the need for external mechanisms to manipulate the spectrum required in conventional co-propagating OPO. The counter-propagatory idler, in the mid-IR, had a bandwidth of 3 GHz.