Abstract
Mirrorless optical parametric oscillators (MOPOs) are very attractive parametric devices that rely on the nonlinear interaction of counter-propagating photons to inherently establish distributed feedback, without the use of external mirrors or surface coatings. These devices offer unique spectral and coherence properties that will benefit a large variety of applications ranging from spectroscopy to quantum communications. The major obstacle in exploiting their full potential is ascribed to the difficulty in engineering a nonlinear material in which the generation of counter-propagating waves can be phase matched. Here we present a reliable and consistent technique for fabrication of highly-efficient sub-micrometer periodically poled Rb-doped KTiOPO4. We experimentally demonstrate the first cascaded counter-propagating interactions in which the generated forward signal serves as a pump for a secondary MOPO process, reaching pump depletion larger than 60%. The cascaded process exemplifies the high efficiency of our nonlinear photonic structures. Our domain-engineering technique paves the way to realize counter-propagating schemes and devices that have been deemed unfeasible until now.
Highlights
Mirrorless optical parametric oscillators (MOPOs) are a special class of optical parametric oscillators based on the three-wave mixing process in which the pump photon is split into two photons, signal and idler, propagating in opposite directions[1]
Sub-μm-periodicity quasi-phase matching (QPM) crystals required for low threshold MOPO, which explains the long delay until the first experimental demonstration
One of the salient achievements presented in this work is the development of the novel technique which allows consistent fabrication of high quality sub-micrometer-periodicity QPM structures in bulk Rb-doped KTiOPO4 (RKTP) crystals
Summary
A PPRKTP with the QPM periodicity of 755 nm fabricated as described above was used to demonstrate cascaded counter-propagating nonlinear interactions. From the spectral bandwidth of the corresponding signals, we estimate the expected i1 and i2 bandwidths to be 33 GHz and 4.3 GHz, respectively It should be noted that the pulse-length of the s2-i2 pair of the second cascade is of the order of 40 ps, which would give the transform-limited bandwidth of about 10 GHz, in accordance with the measured value. The wavelength of the s3 is on the other hand within the bandwidth of i1 so we can expect that the third MOPO cascade is helped by cross-seeding from the idler of the first MOPO cascade Another strong indication of the presence of the third cascade can be gleaned from the discrepancy of the measured pump depletion and the efficiency of the two first MOPO cascades at the pump energies above 140 μJ (see Fig. 4). The idler spectrum below this pump energy does not contain the peak attributable to the third MOPO process
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