The Photonics Spotlight
An OPO Without Resonator Mirrors
Posted on 2007-08-23 as a part of the Photonics Spotlight (available as e-mail newsletter!)
Permanent link: https://www.rp-photonics.com/spotlight_2007_08_23.html
Abstract: Researchers at KTH in Stockholm have demonstrated the first mirrorless optical parametric oscillator (OPO). This is a device with remarkable physical properties, and its fabrication relied on the feat to periodically pole a KTP crystal with a very fine period.
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!
This article is a posting of the Photonics Spotlight, authored by Dr. Rüdiger Paschotta. You may link to this page and cite it, because its location is permanent. See also the Encyclopedia of Laser Physics and Technology.
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