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External cavity quantum dots see yellow

13 Oct 2008

Researchers in Germany fabricate quantum dot lasers that produce second harmonics in the yellow-red spectral range.

Quantum dot (QD) lasers engineered to emit a second harmonic in the yellow-red range could be ideal for high-resolution spectroscopy, say researchers at the University of Dusseldorf and German firm Innolume. Placed in an external cavity, the laser's linewidth fell to just several tens of kilohertz and powers of several milliwatts in the yellow were observed (Applied Physics B 92 501).

"By placing these lasers in an external Littrow configuration, we have shown that they are suitable for high-precision experiments, where the instantaneous linewidth of the laser is crucial," Alexander Nevsky of Dusseldorf's Institute for Experimental Physics told optics.org. "We also demonstrated direct nonlinear frequency conversion and generated high-power coherent tunable radiation in a previously unreachable range."

External cavity diode lasers are commonplace on today's market and boast properties such as a small footprint, low cost and large tuning range. However, some spectral ranges are difficult to access due to material limitations. InGaAs quantum wells for example are limited to emissions around 1100 nm. This is where InGaAs QD lasers come in, as they fill the gap between 1100 and 1300 nm and provide access to the yellow-orange-red region of the spectrum.

The semiconductor structure for the gain chip was grown by molecular-beam epitaxy on a GaAs substrate. The active layer consisted of 10 layers of InAs QDs covered by an InGaAs layer. The chip also had a ridge waveguide curved stripe.

"The facet to which the waveguide is perpendicular is anti-reflection-coated to 1% reflectivity, whereas facet to which the waveguide is 5 degrees slanted is anti-reflection-coated to 0.1%," commented Nevsky. "The length of the chip is 4 mm."

To form the external cavity, Nevsky and colleagues placed their QD laser between two aspheric lenses and varied the angle of incidence on a diffraction grating to tune the emission wavelength. They observed a tuning range in excess of 200 nm as well as an output power of more than 500 mW at a central wavelength of 1180 nm.

"Our set-up for generating the second harmonic at 578 nm consists of an external ring optical resonator with a periodically poled LiNbO3 (PPLN) crystal," explained Nevsky. "To fulfill the stable resonant conditions, the frequency of the resonator is actively stabilized to the laser using the Pound-Drever-Hall method."

According to Nevsky, the second harmonic at 578 nm has already been used for high-resolution spectroscopy of ytterbium atoms stored in a magneto-optical trap. The team has also replaced the resonator containing the PPLN crystal with a single-pass PPLN waveguide and is expecting new results in the yellow imminently.

"The next step is frequency stabilization of the QD laser to a high-finesse ULE resonator in order to achieve a linewidth of around 1 Hz," said Nevsky. "This is necessary for high-resolution spectroscopy of the clock transition in cold ytterbium atoms, which can be used as an extremely accurate optical frequency reference."

Jacqueline Hewett is editor of Optics & Laser Europe magazine.

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