Source: Laser Focus World.

A single quantum-dot (QD) laser diode developed by a group at the University of Dundee (Dundee, Scotland) generates stable dual and/or multiple longitudinal modes in the near-infrared. The device has potential for production of terahertz radiation via optical difference-frequency generation.

Temperature-stabilized at 20¡ãC, the laser diode is situated in an external-cavity setup containing two volume Bragg gratings (VBGs): one that selectively returns 1177 ¡À0.5 nm and 1182 ¡À0.5 nm wavelengths, and the other that selectively returns 1257 ¡À0.5 nm and 1262 ¡À0.5 nm wavelengths. The glass VBGs have an efficiency of about 15% and grating tilts of 1¡ã to prevent backreflections. The difference frequencies for the two gratings are 0.946 ¡À0.019 THz and 1.078 ¡À0.021 THz, respectively.

Quantum-dot laser diode and volume Bragg gratings (VBGs) in an external cavity produce two wavelength pairs for generation of terahertz radiation

When driven at a current of 70 mA, the laser emitted from the ground state (GS; matching VBG2); at 210 mA, excited-state (ES; matching VBG1) emission dominated the output. Intermediate currents produced a mix of GS and ES emission. Because each VBG is multiplexed (containing two gratings), a total of four wavelengths can be produced by the laser, with the relative output of each of two pairs adjustable by varying the current (the figure shows the output for a 150 mA current). In addition to photomixing for terahertz generation (and potentially two-color terahertz imaging), the laser is useful for spectroscopy. Contact Ross Leyman at r.r.leyman@dundee.ac.uk.