Direct generation of spatially entangled qudits using quantum nonlinear optical holography.

Nonlinear holography shapes the amplitude and phase of generated new harmonics using nonlinear processes. Classical nonlinear holography influenced many fields in optics, from information storage, demultiplexing of spatial information, and all-optical control of accelerating beams. Here, we extend the concept of nonlinear holography to the quantum regime. We directly shape the spatial quantum correlations of entangled photon pairs in two-dimensional patterned nonlinear photonic crystals using spontaneous parametric down conversion, without any pump shaping. The generated signal-idler pair obeys a parity conservation law that is governed by the nonlinear crystal. Furthermore, the quantum states exhibit quantum correlations and violate the Clauser-Horne-Shimony-Holt inequality, thus enabling entanglement-based quantum key distribution. Our demonstration paves the way for controllable on-chip quantum optics schemes using the high-dimensional spatial degree of freedom.

Direct generation of high brightness path entangled N00N states using structured crystals and shaped pump beams.

Optical N00N states are N-photon path entangled states with important applications in quantum metrology. However, their use was limited till now owing to the difficulties of generating them in an efficient and robust manner. Here we propose and experimentally demonstrate two new simple, compact and robust schemes to generate path entangled N00N states with N = 2 that emerge directly from the nonlinear interaction. The first scheme is based on shaping the pump beam, and the second scheme is based on modulating the nonlinear coefficient of the crystal. These new methods exhibit high coincidence count rates for the detection of a N00N state, reaching record value of 2 × 105 coincidences per second. We observe super-resolution by measuring the second order correlation on the generated N = 2 state in an interferometric setup, showing the distinct fringe periodicity at half of the optical wavelength. Our findings may pave the way towards scalable and efficient sources for super-resolved quantum metrology applications and for the generation of bright squeezed vacuum states.

Two-wavelength Tm:YLF/KGW external-cavity Raman laser at 2197 nm and 2263 nm.

This paper presents a KGW Raman laser with an external-cavity configuration at the 2 µm region. The Raman laser is pumped by an actively Q-switched Tm:YLF laser, especially designed for this purpose emitting at 1880 nm. Due to the KGW bi-axial properties, the Raman laser is able to lase separately at two different output lines, 2197 nm and 2263 nm. The output energies and pulse durations that were achieved for these two lines are 0.15 mJ/pulse at 21 ns and 0.4 mJ/pulse at 5.4 ns, respectively. To the best of our knowledge, this is the first time that the KGW crystal, which is well known for its wide use in shorter wavelengths, is demonstrated in a Raman laser in the 2 µm region. According to the achieved results and due to the KGW properties, it appears to be a suitable crystal for energy scaling and efficient Raman conversion in this spectral range. An estimation of the Raman gain coefficient for this wavelength is provided as well.

1.2 kW clad pumped Raman all-passive-fiber laser with brightness enhancement.

An all-fiber clad pumped Raman fiber laser (FL) oscillator with a CW power of 1.2 kW and an efficiency of 85% is presented. To the best of our knowledge, this laser is the highest power and the highest efficiency Raman FL demonstrated in any configuration with brightness enhancement (BE). To the best of our knowledge, it is also the first greater than kilowatt FL of any kind that does not utilize rare-earth doping in the oscillator fiber (all passive fiber). The beam quality (BQ) of the Raman laser was M2=2.75 at 1 kW, and the pump-Stokes BE factor was approximately 7. The laser consists of a specially designed triple-clad fiber which confines the low BQ pump power into the multi-mode inner clad, while generating the Raman signal in the large-mode-area core. The second Stokes is inhibited by selecting the appropriate inner clad-to-core area ratio and by the oscillator's selective fiber Bragg grating reflectors.

Highly efficient all-fiber continuous-wave Raman graded-index fiber laser pumped by a fiber laser.

We report for the first time, to the best of our knowledge, an all-fiber Raman graded-index (GRIN) fiber laser pumped by a fiber laser. This configuration points to potential future power and brightness increases. Continuous-wave power of 135 W with an M2 value of 2.5 was obtained at a wavelength of 1081 nm with an optical-to-optical efficiency of 68%. A commercial GRIN core fiber acts as the Raman fiber in a power oscillator configuration that includes fiber Bragg gratings (FBGs) written onto the GRIN fiber. The efficiency and brightness demonstrated here are, to the best of our knowledge, the highest reported in any Raman GRIN fiber laser. A brightness enhancement of the pump beam by a factor of 5.6 is attained due to the transverse profiles of Raman gain and FBG reflection in the GRIN fiber.

250 W clad pumped Raman all-fiber laser with brightness enhancement.

We report a strictly all-fiber clad pumped Raman fiber laser with a CW power of 250 W. To the best of our knowledge, this is the highest power Raman fiber laser demonstrated in any configuration allowing brightness enhancement. In addition, this is the first report of a Raman clad pumped all-fiber laser. The brightness of the pump source was enhanced by a factor of ∼3.8. This result was achieved by the design of a novel triple-clad fiber, with tight pump power inner confining clad that both maximized the Raman gain and inhibited the second Stokes radiation. We discuss power-increase effects on the beam quality, efficiency, and brightness enhancement.

Polarization-mixing optical parametric oscillator.

We report the experimental realization of a new type of optical parametric oscillator in which oscillation is achieved by polarization rotation in a linear retarder, followed by nonlinear polarization mixing. The mixing is performed by a type II degenerate parametric downconversion in a periodically poled KTP crystal pumped by a 1064 nm pulsed Nd:YAG pump. A single, linearly polarized beam, precisely at the degenerate wavelength is generated. The output spectrum has a narrow linewidth (below the instrumentation bandwidth of 1 nm) and is highly stable with respect to variations in the crystal temperature.

Nearly diffraction-limited signal generated by a lower beam-quality pump in an optical parametric oscillator.

The beam quality of an optical parametric oscillator (OPO) within a singly resonant, confocal-positive branch unstable resonator is investigated. Resonator configurations have been found in which the beam quality of the outgoing signal exceeds the beam quality of the pump. Cavity magnification and pump-pulse duration are found to determine the signal beam quality. It is shown that signal M2 decreases with increasing pump-pulse duration for a given cavity magnification. In an experimental demonstration of a LiNbO3 OPO within an unstable resonator, pumped by a multitransversal mode beam, a signal beam with an almost-single transversal mode has been generated, whereas the multitransversal mode behavior of the pump beam is projected on the idler beam.