Frequency Conversion

Abstract—A hybrid fiber–radio access network architecture for simultaneous wireline and wireless transmissions of Data-Over-Cable Service Interface Specification (DOCSIS) signals is pre-sented. An all-optical harmonic up-conversion technique using a dual-drive ...

In the non-stationary process of doubling the frequency of high-power laser radiation in metamaterials, the effects of self-and cross-phase modulation, related to the cubic nonlinearity of the medium, are considered. It is assumed that the pump wave propagates in a region with negative refraction. It is shown that in the case of a nonlinear optical process of frequency doubling of an intense light wave in a metamaterial, it is important to take into account the influence of a powerful pump wave. It is shown that the frequency shift of the maximum of the harmonic spectrum in the metamaterial is related to the cubic nonlinearity of the medium. It is shown how the width of the spectrum of the second-harmonic depends on the pump intensity, linear and nonlinear phase detuning. In addition, analytical expressions are obtained for the optimal values of frequency detuning and group velocities on which the maximum of the harmonic spectrum is obtained for non-stationary generation in a metamaterial. Estimations of the quasistatic interaction lengths and the frequency shift of the second-harmonic spectrum are given. By varying the linear phase detuning or the pump intensity, it is possible to manipulate the frequency shift of the maximum of the harmonic spectrum, and from here one can realize a smooth rearrangement of the central frequency of the harmonic spectrum.

High harmonic generation (HHG) of intense infrared laser radiation (Ferray et al., J. Phys. B: At. Mol. Opt. Phys. 21:L31, 1988; McPherson et al., J. Opt. Soc. Am. B 4:595, 1987) enables coherent vacuum-UV (VUV) to soft-X-ray sources. In the usual setup, energetic femtosecond laser pulses are strongly focused into a gas jet, restricting the interaction length to the Rayleigh range of the focus. The average photon flux is limited by the low conversion efficiency and the low average power of the complex laser amplifier systems (Keller, Nature 424:831, 2003; Südmeyer et al., Nat. Photonics 2:599, 2008; Röser et al., Opt. Lett. 30:2754, 2005; Eidam et al., IEEE J. Sel. Top. Quantum Electron. 15:187, 2009) which typically operate at kilohertz repetition rates. This represents a severe limitation for many experiments using the harmonic radiation in fields such as metrology or high-resolution imaging. Driving HHG with novel high-power diode-pumped multi-megahertz laser systems has the potential to significantly increase the average photon flux. However, the higher average power comes at the expense of lower pulse energies because the repetition rate is increased by more than a thousand times, and efficient HHG is not possible in the usual geometry. So far, two promising techniques for HHG at lower pulse energies were developed: external build-up cavities (Gohle et al., Nature 436:234, 2005; Jones et al., Phys. Rev. Lett. 94:193, 2005) and resonant field enhancement in nanostructured targets (Kim et al., Nature 453:757, 2008). Here we present a third technique, which has advantages in terms of ease of HHG light extraction, transverse beam quality, and the possibility to substantially increase conversion efficiency by phase-matching (Paul et al., Nature 421:51, 2003; Ren et al., Opt. Express 16:17052, 2008; Serebryannikov et al., Phys. Rev. E (Stat. Nonlinear Soft Matter Phys.) 70:66611, 2004; Serebryannikov et al., Opt. Lett. 33:977, 2008; Zhang et al., Nat. Phys. 3:270, 2007). The interaction between the laser pulses and the gas occurs in a Kagome-type Hollow-Core Photonic Crystal Fiber (HC-PCF) (Benabid et al., Science 298:399, 2002), which reduces the detection threshold for HHG to only 200 nJ. This novel type of fiber guides nearly all of the light in the hollow core (Couny et al., Science 318:1118, 2007), preventing damage even at intensities required for HHG. Our fiber guided 30-fs pulses with a pulse energy of more than 10 μJ, which is more than five times higher than for any other photonic crystal fiber (Hensley et al., Conference on Lasers and Electro-Optics (CLEO), IEEE Press, New York, 2008).

Frequency-related parameters derived from the uterine electromyogram (EMG) signals are widely used in many pregnancy monitoring and preterm delivery prediction studies. Although they are classical parameters, they are well suited for quantifying uterine EMG signals and have many advantages over amplitude-related parameters. The present work aims to compare various frequency-related parameters according to their classification performances (pregnancy vs. labor) using the receiver operating characteristic (ROC) curve analysis. The comparison between the parameters indicates that median frequency is the best frequency-related parameter that can be used for distinguishing between pregnancy and labor contractions. We conclude that median frequency can be the representative frequency-related parameter for classification problems of uterine EMG.

ABSTRACT The second harmonic spectrum of Cr2-O3 is studied in the 1.7-2.9 eV spectral range as a function of temperature. Below the Néel temperature TN=307.5 K a nonlinear electric susceptibility chieijk(c) appears, which changes sign under the time-reversal operation. The interference of this susceptibility with the time-invariant magnetic susceptibility chimijk(i) leads to a pronounced polarization dependence for circularly polarized light propagating along the optical axis. This gives a novel tool to study antiferromagnetic domains with opposite orientation of the order parameter.

Two-photon absorption measurement has been carried out in a single 80 nm×10 μm ZnO nanowire using femtosecond laser pulses in the wavelength range of 700-800 nm. In addition to the deep-level green emission around 530 nm due to surface defects and the near band-edge ultraviolet emission around 360 nm due to the exciton, a second harmonic peak has been observed. The strength of the frequency-doubled component is found to enhance while the two-photon absorption wavelength is tuned towards the exciton wavelength of the nanowire. This behavior can be ascribed to the resonant exciton absorption in ZnO nanowires.

A new method for efficient, broadband sum and difference frequency generation of ultrafast pulses is demonstrated. The principles of the method follow from an analogy between frequency conversion and coherent optical excitation of a two-level system. For conversion of ultrafast pulses, the concepts of adiabatic conversion are developed further in order to account for dispersion and group velocity mismatch. The scheme was implemented using aperiodically poled nonlinear crystals and a single step nonlinear mixing process, leading to conversion of near-IR (∼790 nm) ultrafast pulses into the blue (∼450 nm) and mid-IR (∼3.15 μm) spectral regions. Conversion bandwidths up to 15 THz FWHM and efficiencies up to 50% are reported.

We derive new temperature dependent Sellmeier equations for the extra-ordinary and ordinary refractive indices of 0.5% MgO-doped stoichiometric LiTaO3 crystal. The equations are based on quasi phase-matched frequency conversion measurements, as well as on interferometric measurements of the thermal expansion and thermal dispersion. These equations fit experimental data over wide spectral ranges: 0.35–6 μm for the extra-ordinary wave and 0.375–3.75 μm for

We show that quadratic solitons are equivalent to solitons of a nonlocal Kerr medium. This provides new physical insight into the properties of quadratic solitons, often believed to be equivalent to solitons of an effective saturable Kerr medium. The nonlocal analogy also allows for novel analytical solutions and the prediction of novel bound states of quadratic solitons.

We report a high-power, widely tunable, cw singly resonant optical parametric oscillator (OPO) based on MgO:sPPLT. The OPO is pumped in the green by a cw diode-pumped Nd:YVO4 laser at 532 nm and can provide continuously tunable output across 848-1430 nm. Using a 30 mm crystal and double-pass pumping, an oscillation threshold of 2.88 W has been obtained, and single-pass idler powers in excess of 1.51 W have been generated over 1104-1430 nm for 6W of pump power at an extraction efficiency of 25.2% and photon conversion efficiency of 56.7%.

A diode-pumped Nd:YAG laser was frequency doubled to 0.532 microm with an intracavity KTiOPO(4) crystal in a V-cavity arrangement, achieving an output power of 140 W. Acousto-optic Q switching was employed at repetition rates of 10-30 kHz, and the beam quality was assessed at M(2) approximately 50. It was deduced on the basis of our model that the strength of the nonlinear frequency conversion is the main parameter determining the pulse width.