An OPO Without Resonator Mirrors

Posted on 2007-08-23 as a part of the Photonics Spotlight.

Permanent link: http://www.rp-photonics.com/spotlight_2007_08_23.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

Ref.: C. Canalias and V. Pasiskevicius, “Mirrorless optical parametric oscillator”, Nature Photonics 1, 459 (2007); encyclopedia articles on optical parametric oscillators

The idea has been around for quite a while, but now it has been realized in an experiment at KTH in Stockholm: an optical parametric oscillator which does not have a resonator. This is not just a high-gain device, acting as an optical parametric generator, but something more subtle: quasi-phase matching in a periodically poled nonlinear crystal with a very small poling period allowed to phase-match an interaction where the idler propagates in a direction opposite to that of signal and pump beams. The signal wave then needs to propagate only in a single direction, so that a resonator is not required. The same holds for the idler.

The great technical difficulty behind that experiment was to fabricate a KTP crystal with a poling period of ≈0.8 μm. A photolithographic technique has been developed for that remarkable result; note that the poling periods in QPM crystals are usually at least a few microns, often tens of microns, making the fabrication much easier (but still not very easy!). Making a fine electrode structure with photolithography is only the first issue to overcome; it is not obvious that the crystal material would develop a high quality domain structure.

It is not yet clear how useful such mirrorless OPOs will be in any application, but at least one can realize remarkable phase-matching and tuning properties (resulting e.g. in very narrow-band idler pulses) as well as the practical advantage of not requiring resonator mirrors and their alignment. For sure, it is nice physics!