17 Sep 2007
A tunable, mirror-free infrared source that is easy to align has been experimentally demonstrated for the first time.
The first mirrorless optical parametric oscillator (MOPO) has been unveiled by researchers in Sweden. The simplicity of the device with its ability to tune with high precision in the near- and mid-infrared makes the MOPO an interesting option for a number of applications (Nature Photonics 1 459).
"Optical parametric oscillators (OPOs) are useful and versatile sources of coherent light. They allow extension of wavelength range, functionality and thus the application area of laser sources," Carlota Canalias and Valdas Pasiskevicius, researchers at AlbaNova University Center, told optics.org. "Conventional OPOs are hard to align and miniaturize. A mirror-free OPO requires nothing more than a pump laser and a properly engineered nonlinear medium."
Potential applications include spectroscopy; a source of entangled counter-propagating photons in quantum optics experiments; or as a very broadband parametric amplifier of ultra-short pulses. "Eventually compact, efficient coherent light sources could be built where wavelength, tunability range, power and beam properties could be tailored for specific applications," commented Canalias and Pasiskevicius.
The team's device uses a nonlinear second-order interaction in which a 821.4 nm pump photon is split into two counter-propagating signal and idler photons of 1139.7 nm and 2940.8 nm respectively. One photon travels along the original direction of the pump photon, while the other is propagated in the opposite direction. The wavelength of the signal photon can be changed by tuning the pump wavelength.
"The counter-propagating nature of the two generated waves automatically establishes a distributed feedback and thus there is no need for an external cavity or complicated antireflection coatings," explained Canalias and Pasiskevicius. "The result is an extremely simple parametric oscillator - just pump the structure and it starts oscillating without any alignment or adjustment."
The technique, known as quasi-phase-matching, involves designing a ferroelectric material (in this case PPKTP) to maximize the efficiency of the second-order nonlinear interaction. "A sub-micrometer periodicity is maintained within the ferroelectric material over the order of a centimeter," explained Canalias and Pasiskevicius. "It is not an overstatement to say that quasi-phase-matched counter-propagating second order interactions in engineered structures can give rise to a whole new class of devices."
Canalias and Pasiskevicius are looking to further reduce the periodicity of the ferroelectric structure. "This will lead to new wavelength ranges and new types of interactions," they concluded. "At the same time, investigating alternative materials such as semiconductor structures and waveguides is a very promising direction."