Compact source of energetic sub-nanosecond red pulses based on cascaded quasi-phase-matching in aperiodically poled lithium niobate.

We present a simple, compact source of sub-nanosecond pulsed red radiation, based on cascaded nonlinear optical processes-degenerate optical parametric generation and sum-frequency generation-performed with a sample of aperiodically poled lithium niobate pumped by passively Q-switched 1.064 µm Nd:YAG laser. This system does not require feedback from an optical cavity; a single pass of the short pump is all that is required to obtain the cascaded processes, which shortens the output pulse. When pumped with a 1.2 ns, 75 µJ pulse, we obtain 670 ps pulses centered around 709 nm with an energy of 2.8 µJ, corresponding to a peak power of over 4 kW. A numerical model that predicts qualitatively the main characteristics of this source is also presented.

Randomly aperiodically poled LiNbO3 crystal design by Monte Carlo-Metropolis with simulated annealing optimization for ultrabroadband photon pair generation.

We report a scheme for generating ultrabroadband two-photon states by spontaneous parametric downconversion (SPDC) using randomly aperiodically poled crystals designed with an optimization algorithm based on the Monte Carlo-Metropolis method with simulated annealing. A particular SPDC source is discussed, showing results of the spectral and temporal properties of the emitted two-photon states, obtaining almost transform-limited SPDC biphoton wave packets. We also analyze the effect of fabrication errors on the SPDC.

Sub-nanosecond terahertz radiation obtained with an aperiodically poled lithium niobate and organic HMQ-TMS.

We report a compact, simple source of terahertz radiation that can be tuned to well-defined frequencies spanning ∼1.4 to 10 THz, based on difference-frequency generation in an HMQ-TMS crystal. The pair of pump pulses required for this process is obtained by optical parametric generation in an aperiodically-poled lithium niobate crystal; the center wavelength of this pair of pulses is around 1.45 µm. We obtained 40 nJ THz pulses using 38 µJ, 0.85 ns pump pulses.

Production of pairs of synchronized pulses by optical parametric generation and oscillation using aperiodically poled lithium niobate.

We report two sources of synchronized pairs of pulses based on optical parametric generation (OPG) and oscillation (OPO) that use aperiodically poled lithium niobate (APLN) as the nonlinear medium. The main purpose of these sources is to obtain terahertz radiation through difference-frequency generation. The APLN crystal was designed to generate two signals with their corresponding idlers from a single pump pulse; the signal wavelengths are around 1450 nm, and the frequency difference between them is tunable between ∼1 and 10 THz. In the OPO configuration, pumped by a Q-switched Nd:YAG laser (12 ns FWHM, 13 mJ), we obtain pairs of synchronized signals (∼5 ns FWHM) with a combined energy of 740 µJ; each signal has a bandwidth of <105 GHz. In the OPG configuration, we use a Q-switched Nd:YLF laser that emits shorter pulses (1.6 ns FWHM, 350 µJ), obtaining synchronized signal pulses with a combined energy of 38 µJ; each signal has a pulse width of 0.8 ns and a bandwidth of <175 GHz. The advantages of these sources for difference frequency generation are discussed.

Synchronization of pairs of nanosecond pulses from a laser with two gain crystals pumped with two different sources.

We report a laser that emits two Q-switched pulses, one at 1.047 µm and the other at 1.064 µm, generated by a Nd:YLF and a Nd:YVO4, respectively. The crystals are pumped by two fiber-coupled diode lasers (808 nm and 880 nm); the delay between the pulses can be controlled by adjusting the power of the pumps. Two kinds of Q-switching techniques are reported, passive (Cr:YAG saturable absorber) and active (electro-optic modulator). We model both the active and passive Q switching and make a comparison between numerical simulations and experiments. We show experimentally and theoretically that in both cases the pulses can be synchronized; however, the stability of the synchronization (sensitivity to pump power fluctuations) is better for active than for passive Q switching. We also report that under certain experimental conditions a third wavelength is obtained, 1156 nm, which corresponds to the first Stokes shift of the 1047 nm pulse produced by stimulated Raman scattering from the Nd:YVO4 crystal.

Switchable phase/intensity sensor made with ring-shaped and hexagonal ferroelectric domains.

We present a device, similar to a Shack-Hartmann sensor, that can detect both the intensity distribution and wavefront of an incident wave. Its operation is based on the use of an array of electrically controllable Fresnel zone plates made in a ferroelectric crystal, lithium niobate. This sensor, which requires only one camera, can be quickly switched between intensity- and phase-detecting modes. Two kinds of arrays are shown: Fresnel zone plates with a few ring-shaped ferroelectric domains and plates made with nested hexagonal domains. Both arrays are suitable for use in a Shack-Hartmann wavefront sensor. However, since in lithium niobate domains naturally tend to form hexagons, it is easier to make hexagonal, rather than ring-shaped, domains and, consequently, smaller zone plates can be produced. This allows an increase in the number of zone plates and a reduction in their focal length, which improves the fidelity of the reconstructed wavefront.

Modified Shack-Hartmann sensor made with electrically controlled ferroelectric zone plates.

A modified Shack-Hartmann wavefront sensor based on an array of electrically controlled zone plates made of ferroelectric domains is presented. The camera used for image acquisition is also used for wavefront sensing. An experimental simulation of the use of this sensor to enhance astronomical images obtained by "Lucky Imaging" is presented.

Development of a new pulsed source for photoacoustic imaging based on aperiodically poled lithium niobate.

We present the development of a source of deep-red radiation for photoacoustic imaging. This source, which is based on two cascaded wavelength conversion processes in aperiodically poled lithium niobate, emits 10 nanosecond pulses of over 500 µJ at 710 nm. Photoacoustic images were obtained from phantoms designed to mimic the optical and acoustic properties of oral tissue. Results indicate this device is a viable source of optical pulses for photoacoustic applications.