Charlotte Liljestrand

Highly efficient continuous wave blue second-harmonic generation in fs-laser written periodically poled Rb:KTiOPO 4 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).

Periodic poling of Rb-doped KTiOPO 4 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.

Thermal stability of ferroelectric domain gratings in Rb-doped KTP

We study the thermal stability of domain walls in periodically poled Rb-doped KTP crystals. Domain-wall motion is observed after annealing the crystals above 550 °C. This motion is highly anisotropic along the a- and b- crystallographic axes. Along the b-axis domain wall motion is in the order of tens of micrometers. In the a-direction, it results in either three orders of magnitude smaller domain wall displacement or in domain merging, depending on the initial domain configuration. We show that the thermal stability of the domain gratings depends on their periodicity, resulting in complete backswitching for sub-micrometer domains annealed at 730 °C.

Cascaded counter-propagating nonlinear interactions in highly-efficient sub-mu m 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.

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.

Periodically Poled KTP with Sub-wavelength Periodicity: Nonlinear Optical Interactions with Counter Propagating Waves

We present the recent development on periodically poled nonlinear crystals with subwavelength periodicities as short as 500 nm. These devices show conversion efficiencies larger than 45%. Their uni ...

Mirrorless optical parametric oscillator pumped by nanosecond pulses

Fifty years ago, S.E. Harris [1] proposed a special class of optical parametric oscillators (OPOs), the so-called mirrorless optical parametric oscillators (MOpOs), in which the signal and idler waves are generated in opposite directions. The mOpOs do not require reflected waves or a cavity to establish the positive feedback for oscillation. Instead, the feedback is accomplished by the counter-propagating nonlinear interaction itself. Indeed, it is the counter-propagating phase matching scheme that intrinsically provides the MOPOs with unique spectral properties, namely, generation of intrinsically narrowband counter-propagating idler and a co-propagating signal spectrum of which is a frequency-translated version of the spectrum of the pump. [2-3]. These properties cannot be achieved in conventional OPOs without complex set-ups and components. The large phase mismatch of the counter-propagating waves demands an extraordinarily large birefringence of the nonlinear optical material, which makes quasi-phase matching (QPM) the most feasible route to achieve MOPO. However, MOPOs require the QPM period to be in the sub-micrometer range, which is a challenging task. Thus, it is not surprising that the MOPO had to wait 41 years for its first experimental demonstration [2]. Nevertheless, up-to-now, all the MOPO demonstrations have been done with broadband ps-pumping. This can be ascribed to the fact that the efficiency and threshold of the MOPO depend on the quality and length of the sub-μm QPM grating.

Highly efficient mirrorless optical parametric oscillator pumped by nanosecond pulses

Fifty years ago, S.E. Harris [1] proposed a special class of optical parametric oscillators (OPOs), the so-called mirrorless optical parametric oscillators (MOpOs), in which the signal and idler waves are generated in opposite directions. The mOpOs do not require reflected waves or a cavity to establish the positive feedback for oscillation. Instead, the feedback is accomplished by the counter-propagating nonlinear interaction itself. Indeed, it is the counter-propagating phase matching scheme that intrinsically provides the MOPOs with unique spectral properties, namely, generation of intrinsically narrowband counter-propagating idler and a co-propagating signal spectrum of which is a frequency-translated version of the spectrum of the pump. [2-3]. These properties cannot be achieved in conventional OPOs without complex set-ups and components. The large phase mismatch of the counter-propagating waves demands an extraordinarily large birefringence of the nonlinear optical material, which makes quasi-phase matching (QPM) the most feasible route to achieve MOPO. However, MOPOs require the QPM period to be in the sub-micrometer range, which is a challenging task. Thus, it is not surprising that the MOPO had to wait 41 years for its first experimental demonstration [2]. Nevertheless, up-to-now, all the MOPO demonstrations have been done with broadband ps-pumping. This can be ascribed to the fact that the efficiency and threshold of the MOPO depend on the quality and length of the sub-μm QPM grating.

High power single-crystal fiber CW 946 nm laser and blue generation based on rubidium-doped PPKTP

Blue lasers have numerous applications in spectroscopy, underwater communication and material processing. Direct emission in the blue region can be achieved with laser diodes, with powers up to the watt level. Higher powers require efficient laser operating around 950 nm to be frequency doubled to the blue spectrum, which usually suffer from a lower gain than conventional 1μm lasers. Generally, intracavity frequency doubling set-ups and pulsed operation are used to increase the second harmonic generation efficiency, with output powers up to 4 W in cw and 1.3W in pulsed regime. In the past few years, both lasers materials and non linear crystals have progressed enough to allow the very simple extracavity frequency doubling of continuous wave lasers in periodically poled materials to acceptable powers.

Sub-wavelength domain engineering in KTP isomorphs: QPM devices with counter-propagating photons

We review the recent advances in fabrication of ferroelectric-domain gratings of sub-μm periodicity in KTP and Rb-doped KTP for counter-propagating second-order nonlinear optical interactions. Their performance as QPM devices will be discussed.