Oct 15 2007
Researchers from Singapore, China and Japan have unveiled what they believe to be the first high-power self-mode-locked ceramic laser. The team says its simple approach can be applied to other high-power lasers, particularly those emitting at wavelengths where mode-locking elements such as semiconductor saturable absorber mirrors (SESAMs) are not available. (Optics Letters 32 2741)
"Our self-mode-locked ceramic laser is simple and compact," researcher Guoqiang Xie from Nanyang Technological University in Singapore told optics.org. "Our laser cavity uses just three mirrors and a ceramic rod. Compared to Kerr-lens mode-locking, the self-mode-locking is not so sensitive to cavity alignment and the mode-locking region is relatively long."
Self-mode-locking means that no additional active or passive mode-locking elements (such as SESAMs) are used in the laser cavity. The mode-locking is purely driven by optical and thermal effects in the laser material itself.
Xie says that his team's Yb:Y2O3 ceramic laser has an average output power of 2.7 W and emits 1.1 picosecond pulses at a repetition rate of 126 MHz. "The pulse energy reaches 21 nJ and a peak power of 19 kW," he said. "The pulses will be very useful for frequency doubling, frequency mixing and other nonlinear processes, or as seeding pulses for further amplifying large-scale ceramics."
The first step in the process is to pump the ceramic material with a 30 W fiber-coupled laser diode bar emitting at 937 nm. Xie explains that the diffraction loss induced by thermal lens aberration, in combination with the Kerr-lens self-focusing effect in the gain medium, results in the self-mode-locking.
"It was important to understand the thermal lens and its aberration value as well as having a suitable laser mode size when we designed the laser cavity," commented Xie. "We estimate the thermal lens aberration value and the diffraction loss induced by the aberration to obtain the nonlinear loss modulation required for mode-locking. In our laser, a laser mode radius of approximately 120 microns in the ceramic is used, which generates a nonlinear loss modulation of 2 x 10-4."
The team is now investigating nonlinear processes such as multiple pulsing in the ceramic and hopes to extend the self-mode-locking technique to other high-power lasers. Xie added that there are no plans to commercialize the cavity design at the moment.