Jens Kiessling

Continuous-wave optical parametric terahertz source

Here, we present a continuous-wave optical parametric terahertz light source that does not require cooling. It coherently emits a diffraction-limited terahertz beam that is tunable from 1.3 to 1.7 THz with power levels exceeding 1 microW. Simultaneous phase matching of two nonlinear processes within one periodically-poled lithium niobate crystal, situated in an optical resonator, is employed: The signal wave of a primary parametric process is enhanced in this resonator. Therefore, its power is sufficient for starting a second process, generating a backwards traveling terahertz wave. Such a scheme of cascaded processes increases the output power of a terahertz system by more than one order of magnitude compared with non-resonant difference frequency generation due to high intracavity powers. The existence of linearly polarized terahertz radiation at 1.35 THz is confirmed by analyzing the terahertz light with metal grid polarizers and a Fabry-Pérot interferometer.

Cascaded optical parametric oscillations generating tunable terahertz waves in periodically poled lithium niobate crystals

We present a continuous-wave (cw) singly-resonant optical parametric oscillator (SROPO) based on MgO-doped periodically poled lithium niobate (PPLN) delivering single-frequency idler output from 2.33 to 5.32 microm. In this system, we observe additional spectral components that have been attributed to stimulated Raman lines in other studies. However, we are able to assign them unambiguously to cascaded optical parametric processes. The tunable forward and backward idler waves generated by these additional phase-matched oscillations have frequencies that are tunable around 3.5 and 1.5 THz, respectively.

Pump-enhanced optical parametric oscillator generating continuous wave tunable terahertz radiation

We demonstrate a tunable cw terahertz (THz) parametric oscillator based on periodically poled MgO-doped lithium niobate, directly converting the 1030 nm pump wave into the THz regime. The tunability ranges from 1.2 to 2.9 THz at output power levels between 0.3 and 3.9 μW. To overcome the high pump threshold caused by THz absorption in the nonlinear crystal, we employ an enhancement cavity with a finesse of 500 at the pump wavelength. The intracavity pump threshold at 1.4 THz is measured to be 350 W for a crystal length of 2.5 cm.

High power and spectral purity continuous-wave photonic THz source tunable from 1 to 4.5THz for nonlinear molecular spectroscopy

We report a diffraction-limited photonic terahertz (THz) source with linewidth <10MHz that can be used for nonlinear THz studies in the continuous wave (CW) regime with uninterrupted tunability in a broad range of THz frequencies. THz output is produced in orientation-patterned (OP) gallium arsenide (GaAs) via intracavity frequency mixing between the two closely spaced resonating signal and idler waves of an optical parametric oscillator (OPO) operating near = 2µm. The doubly resonant type II OPO is based on a periodically poled lithium niobate (PPLN) pumped by a single-frequency Yb:YAG disc laser at 1030nm. We take advantage of the enhancement of both optical fields inside a high-finesse OPO cavity: with 10W of 1030nm pump, 100W of intracavity power near 2µm was attained with GaAs inside cavity.

Generation of tunable continuous-wave terahertz radiation by photomixing the signal waves of a dual-crystal optical parametric oscillator

We present a continuous-wave dual-crystal optical parametric oscillator that generates signal waves of about 1.4??m wavelength with a tunable difference frequency ?? between 0.64 and 5.3?THz. A model of the parametric gain in such a system indicates that the lowest reachable value for ?? depends on the refractive index dispersion of the nonlinear material used. As a proof of principle, we demonstrate the generation of terahertz radiation by photomixing the signal waves. Frequencies between 0.64 and 0.85?THz are created and detected coherently with ion-irradiated InGaAs interdigitated photomixers.

Hybridly-pumped continuous-wave optical parametric oscillator

We present the first to our knowledge continuous-wave singly-resonant optical parametric oscillator (SROPO) generating tunable signal and idler waves with less than 100 mW single-frequency pump power. This low threshold is achieved by an additional intracavity gain medium that is pumped incoherently. The idler power with respect to the single-frequency pump power shows a bistable behavior which depends strongly on the pumping of the additional amplifier. Furthermore, we demonstrate that such a setup allows a SROPO to be completely diode pumped.

Note: Coherent detection of terahertz radiation employing a continuous wave optical parametric source.

The combination of an all-optical terahertz source with a photoconductive antenna to achieve coherent detection is presented. This approach aims to overcome the frequency limits introduced by optoelectronic terahertz sources commonly used. Here the Gaussian-shaped and linearly polarized terahertz waves are generated by a continuous wave optical parametric oscillator with a power of 3 μW at 1.4 THz. The infrared signal light of the optical parametric oscillator can be used to coherently detect the generated terahertz wave with a photoconductive antenna. As a proof-of-principle experiment we determine the thickness profile of a plastic lens using phase shifting interferometry.

Self-gated mid-infrared short pulse upconversion detection for gas sensing

Pulsed nonlinear-optical upconversion is used for mid-infrared signal detection. A setup for both mid-infrared generation and upconversion based on a single pump laser enables sensitive light detection and is utilized for gas spectroscopy. With the demonstrated pulsed setup, quantum efficiencies above 80 % for the upconversion of a Gaussian beam signal and 25 % for the upconversion of backscattered radiation are achieved and in agreement with theoretical predictions. Combined with efficient background suppression due to spectral and temporal gating, this results in highly sensitive detection of the infrared signals. As a demonstration of application, the presented system is used for methane sensing in an open path geometry, highlighting the potential for stand-off leak detection with a concentration resolution better than 1.5 ppm·m.

Lithium niobate: wavelength and temperature dependence of the thermo-optic coefficient in the visible and near infrared

The thermo-optic coefficient of lithium niobate (LiNbO3) has been measured in the temperature range from 10 to 160 °C using an interferometric setup. Undoped and magnesium-doped congruently melting LiNbO3 and undoped stoichiometric LiNbO3 were studied over a wide wavelength range in the visible and near infrared (450 – 600 nm and 900 – 1130 nm) using a frequency-doubled cw optical parametric oscillator. Experimental results for congruently grown lithium niobate were aggregated using a Schott equation to describe the wavelength and temperature dependence of the thermo-optic coefficient.

Sensitive infrared detection through efficient pulsed upconversion for remote sensing applications

Infrared spectroscopic measurements are a key tool for species identification and substance detection. Growing importance falls to the mid-infrared 3–12 μm range, where strong, fundamental spectroscopic features are found. One issue especially in stand-off remote sensing scenarios with low-intensity backscattered signals is the limited sensitivity of available detectors for the (MIR) wavelength range. Upconversion of MIR radiation by nonlinear-optical sum frequency generation towards detection in the near near-infrared (NIR) range has been demonstrated as an approach to overcome this limitation [1].