April 06, 2012

New ultra-small laser opens up a world of possibilities

University of Sydney - Computing and medicine are among the many fields which could be revolutionised by a new form of ultra-small laser. It is the first laser to be mode-locked making it highly precise, ultra-fast and ultra-small. It will have applications in computing, measuring and diagnosing diseases, and processing materials - all areas where lasers are already used. It will also open up entirely new areas such as precision optical clocks for applications in metrology, ultra-high speed telecommunications, microchip-computing and many other areas."

"It's the first time we've been able to use a micro-cavity resonator to lock the modes of a laser, which is how ultra-short pulsed lasers are created. Lasers that have their modes locked generate the shortest optical pulses of light," explained Dr Moss.

Making lasers that can pulse at very high and flexible repetition rates - much higher than those achieved with electronics - is a field that has been pursued by scientists around the world. Different research groups have proposed a variety of solutions to creating these lasers, but this is the first success.

"Our new laser achieves extremely stable operation at unprecedentedly high repetition rates of 200 Gigahertz, while maintaining very narrow line widths, which leads to an extremely high quality pulsed emission," said Dr Moss.

Nature Communications - Demonstration of a stable ultrafast laser based on a nonlinear microcavity

Ultrashort pulsed lasers, operating through the phenomenon of mode-locking, have had a significant role in many facets of our society for 50 years, for example, in the way we exchange information, measure and diagnose diseases, process materials, and in many other applications. Recently, high-quality resonators have been exploited to demonstrate optical combs. The ability to phase-lock their modes would allow mode-locked lasers to benefit from their high optical spectral quality, helping to realize novel sources such as precision optical clocks for applications in metrology, telecommunication, microchip-computing, and many other areas. Here we demonstrate the first mode-locked laser based on a microcavity resonator. It operates via a new mode-locking method, which we term filter-driven four-wave mixing, and is based on a CMOS-compatible high quality factor microring resonator. It achieves stable self-starting oscillation with negligible amplitude noise at ultrahigh repetition rates, and spectral linewidths well below 130 kHz.

Filter-driven four-wave mixing design. (a) Schematic of the central component—a monolithically integrated 4-port high-Q (Q=1.2 million) microring resonator (fibre pigtails not shown) (b) High repetition rate laser based on filter-driven FWM: the microring resonator

To better quantify the pulse-to-pulse amplitude stability of the two lasers we recorded the electrical radio-frequency (RF) spectrum of the envelope signal, collected at the output, using a fast photodetector. Unstable oscillation (in the pulse amplitude) was always observed for the long cavity design for the EYDFA for both continuous wave (CW) and pulsed regimes, owing to the presence of a large number of cavity modes oscillating in the ring resonance. In complete contrast to this, the short-cavity configuration for the EDFA could easily be stabilized to give the very clean result of Fig. 3e,f, by simply adjusting the main cavity length to centre an, ideally, single cavity mode with respect to the ring resonance, thereby completely eliminating any main cavity low-frequency beating. Several stable oscillation conditions were found through tuning the delay by over 2 cm. For the same gain, the optical bandwidth (insets in Fig. 3c,d) for unstable laser operation was wider (that is, leading to shorter output pulses) because the instability resulted in a strong amplitude modulation of the optical pulse train in the main cavity, thus increasing the statistical peak power and enhancing the nonlinear interactions. This is a common occurrence in mode-locked lasers

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