A new type of laser that emits “dark” pulses could provide better signals for telecommunications, according to physicists in the US who have created the device. The dark pulses, which consist of intensity dips in an otherwise continuous beam of laser light, are effectively the opposite of the bright bursts in a normal pulsed laser.

“The laser emits a brief pulse of darkness, if you will,” explains one of the researchers Richard Mirin, who is at the National Institute of Standards and Technology (NIST) in Boulder, Colorado. “And so you can think of it as a continuous-wave laser that has a really fast shutter in front of it.”

Dark lasers are not entirely new. For some 20 years, physicists have been able to create so-called dark soliton lasers. Solitons are light pulses that propagate without spreading, and are often used in fibre optics. Their dark counterparts are simply gaps in a continuous beam that do not spread either. But dark solitons are difficult to create and, when they are created, it is done outside the laser using a combination of tricky pulse-shaping techniques.

The new dark pulse laser, on the other hand, forms the dark pulses inside the laser itself. “We believe ours is the first example of a direct generation of a dark pulse,” says Mirin.

Quantum-dot diode laser

We believe ours is the first example of a direct generation of a dark pulse Richard Mirin, NIST

Mirin, whose colleagues work at NIST and JILA, a joint institute of NIST and the University of Colorado at Boulder, based his dark pulse laser on a standard quantum-dot diode laser. This type of device contains a tiny junction between a positively doped semiconductor (p-type) region, which has holes in its normal electronic structure, and a negatively doped semiconductor (n-type) region, which has a surplus of electrons.

When an electric current is driven through the junction, electrons and holes recombine inside the quantum dot, releasing energy in the form of light. This light is amplified using an adjacent cavity, thereby generating a laser beam.

Negative solutions

Quantum-dot diode lasers can be made to produce pulsed or “mode-locked” light rather than continuous light by tailoring the cavity, and this is governed by the Haus equation, named after the late Slovene-American physicist Hermann Haus. Pulses are described by solutions to the equation, which includes terms that relate, for example, to current injection and efficiency. In the past, researchers have generated bright pulses by considering the positive solutions to the equation. But now, Mirin and colleagues have looked at the negative solutions to generate dark pulses.

For tests, the NIST/JILA team built a quantum dot from indium gallium arsenide and topped it with a 5 mm long semiconductor waveguide. Measuring the output with a fast photodetector, they recorded a train of dark pulses, each of which was 90 × 10^–12 s (90 ps) wide and just 30% of the normal intensity.

“Mode-locked lasers, – that is, pulses of light on zero background – have been along for quite a while now and have very wide applications, both in science and technology,” says Andy Weiner, a researcher at Purdue University, US, who has done previous work on dark soliton lasers. “So there is some intrinsic interest if you discover a different operation mode for a mode-locked laser, such as dark pulse mode as in this paper.”

Will it catch on?

Mirin suggests that his group’s dark pulse laser could find applications in telecommunications, because the dark pulses are less prone to disperse than regular, bright pulses. But Weiner thinks it is unlikely to catch on. “Current practice and directions in lightwave communications are such that I don’t think it likely there will be practical interest in dark pulse lightwave communication systems.”

The research is published in Optics Express.