Highly efficient mirrorless optical parametric oscillator pumped by nanosecond pulses.

A highly efficient mirrorless optical parametric oscillator (MOPO), pumped by narrowband nanosecond pulses at 1064 nm, is demonstrated. The MOPO is based on quasi-phase-matched parametric interaction of counter-propagating photons in 1-mm-thick periodically poled Rb-doped KTiOPO4 crystal with a period of 755 nm. It generates a co-propagating signal at 1740 nm and a counter-propagating idler at 2741 nm, achieving mJ-level output with a total signal-and-idler conversion efficiency of 47%. Both generated waves present narrow spectral bandwidths, thanks to the unique properties of the counter-propagating nonlinear interaction. The high conversion efficiency, inherently narrow spectral width, and simplicity of the optical setup make the MOPO an attractive alternative to conventional co-propagating optical parametric oscillators.

Cascaded counter-propagating nonlinear interactions in highly-efficient sub-µm periodically poled crystals.

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.

Periodic poling of Rb-doped KTiOPO4 by coercive field engineering.

We demonstrate fine-pitch periodic poling of bulk Rb-doped KTiOPO4 using a coercive field grating induced by ion-exchange. These samples were used for quasi phase matched (QPM) second harmonic generation at 795 nm with a normalized conversion efficiency of 1.7% W-1cm-1. Additionally, the ion-exchange introduced a refractive index change over the sample thickness resulting in a phasematching wavelength shift of 0.21 nm, adding extra tunability to the QPM device.

Parametric down-conversion with nonideal and random quasi-phase-matching.

Quasi-phase-matching (QPM) has enriched the capacity of parametric down-conversion (PDC) in generating biphotons for many fundamental tests and advanced applications. However, it is not clear how the nonidealities and randomness in the QPM grating of a parametric down-converter may affect the quantum properties of the biphotons. This paper intends to provide insights into the interplay between PDC and nonideal or random QPM structures. Using a periodically poled nonlinear crystal with short periodicity, we conduct experimental and theoretical studies of PDC subject to nonideal duty cycle and random errors in domain lengths. We report the observation of biphotons emerging through noncritical birefringent-phasematching, which is impossible to occur in PDC with an ideal QPM grating, and a biphoton spectrum determined by the details of nonidealities and randomness. We also observed QPM biphotons with a diminished strength. These features are both confirmed by our theory. Our work provides new perspectives for biphoton engineering with QPM.

Highly efficient continuous wave blue second-harmonic generation in fs-laser written periodically poled Rb:KTiOPO4 waveguides.

Waveguides were inscribed into a 9.5 mm long periodically poled KTiOPO4 crystal with a Ti:sapphire femtosecond-laser. Waveguiding was achieved between two parallel written tracks with spacings of 17-19 µm. The fundamental power of 1.6 W for frequency doubling in the waveguide was delivered by a continuous wave Ti:sapphire laser at a wavelength of 943.18 nm. A maximum output power of 76 mW in the blue spectral region was achieved. This corresponds to a single pass normalized conversion efficiency of 4.6% W(-1) cm(-2).