Daniel Haertle

Highly tunable low-threshold optical parametric oscillation in radially poled whispering gallery resonators.

Whispering-gallery resonators (WGRs), based on total internal reflection, possess high quality factors in a broad spectral range. Thus, nonlinear-optical processes in such cavities are ideally suited for the generation of broadband or tunable electromagnetic radiation. Experimentally and theoretically, we investigate the tunability of optical parametric oscillation in a radially structured WGR made of lithium niobate. With a 1.04  μm pump wave, the signal and idler waves are tuned from 1.78 to 2.5  μm--including the point of degeneracy--by varying the temperature between 20 and 62 °C. A weak off centering of the radial domain structure extends considerably the tuning capabilities. The oscillation threshold lies in the mW-power range.

Photoacoustic absorption spectrometer for highly transparent dielectrics with parts-per-million sensitivity.

A sensitive photoacoustic absorption spectrometer for highly transparent solids has been built and tested. As the light source an optical parametrical oscillator pumped by a nanosecond pulse laser with 10 Hz repetition rate is employed, covering the complete wavelength range from 407 to 2600 nm. A second-harmonic-generation unit extends the range of accessible wavelengths down to 212 nm. A lead-zirconate-titanate piezo transducer, directly coupled to the sample, detects the photoacoustically generated sound waves. Absorption spectra of lithium triborate, lithium niobate, and alpha barium borate crystals with absorption coefficients down to 10(-5) cm(-1) are presented.

Fabrication and characterization of whispering-gallery-mode resonators made of polymers

High-quality whispering-gallery-mode resonators made of polymethylmethacrylate (PMMA) are fabricated by simple mechanical turning and polishing according to a technique used by Ilchenko et al. to produce crystalline whispering-gallery-mode resonators with high quality factors (Q-factors). The high-Q PMMA resonators are investigated in two wavelength regimes: in the near infrared between the wavelengths 1470 and 1580 nm and at the wavelength 635 nm. The Q-factor in the infrared regime is limited by material absorption to 3 x 10(5) At 635 nm the Q-factor is limited by surface scattering only and reaches 4 x 10(7), which is a new record for polymers.

Kinetics of photorefractive recording for circular light beams

We show, theoretically and experimentally, that the buildup of the space-charge field in photorefractive crystals is far from monoexponential for circular light beams. This is a general property of the two-dimensional (2D) case, in contrast to the one-dimensional case. The results form a basis for determination of the photoelectric parameters of photorefractive crystals within a wide intensity range, which is important, e.g., for solving of the optical-damage problem in LiNbO3 and LiTaO3 crystals.

Light-induced scattering of femtosecond laser pulses in iron-doped lithium niobate crystals

Self-amplification of weak scattered coherent light waves in photorefractive crystals leads to losses, known as light-induced scattering or holographic scattering. We find with 532 nm light that it is reduced in LiNbO3:Fe for femtosecond laser pulses as compared to cw laser light. Light-induced scattering of pulses is completely absent in samples with sufficiently small Fe2+ content, in contrast to the scattering of cw light. Additional differences include a slower buildup time, a weaker Bragg selectivity, and a narrower angular distribution of the scattered light for pulsed illumination. The differences can be attributed mainly to the smaller temporal coherence of pulses.

Optical cleaning of lithium niobate crystals

Lithium niobate (LiNbO3) is an important optical material due to a unique combination of physical properties, in particular excellent electro-optic and nonlinear-optical characteristics. The main obstacle for the use of LiNbO3 crystals in optics is “optical damage” - the deterioration of light beams because of the formation of unwanted refractive index changes. The reason is the photorefractive effect: charge separation owing to the bulk photovoltaic effect leads to strong refractive index changes via the linear electro-optic effect [1]. It is accepted that remnant Fe centers are responsible for optical damage in undoped LiNbO3 crystals. Our new method for optical damage suppression, the “optical cleaning” [2], directly attacks the cause of the optical damage, namely the presence of photo-excitable electrons. It allows an effective removal of these electrons from an exposed area at sufficiently high temperatures using the bulk photovoltaic effect.

Photoacoustic detection of weak absorption in lithium niobate

Lithium niobate (LiNbO3) is an important material for a wide range of nonlinear optical applications (e.g. optical parametric oscillators). Crystal properties have seen considerable progress in recent years. Residual absorption due to crystal impurities, however, can still lead to limitations in high-power applications. Here, absorption coefficients α < 0.01 cm−1 can have an impact, which are difficult to determine in direct optical measurements. A recent study on absorption in the visible wavelength range used dedicated samples with 20 mm thickness [1]. Techniques applicable for the thickness of commonly used nonlinear optical crystals (0.5 or 1 mm) do require considerable experimental effort (as in [2]).

Intracavity frequency conversion: from bow-ties to whispering galleries

Providing optical feedback by a resonator enhances the efficiency of nonlinear optical effects, e.g. frequency conversion. The bow-tie cavity is known to be a very successful scheme and it has made its way into the commercial world of second harmonic generation and parametric oscillation. We demonstrate a continuouswave optical parametric oscillator based on a bow-tie cavity converting monochromatic pump light at 1.03 μm wavelength to signal light being tunable from 1.25 to 1.85 μm and to corresponding idler light from 2.3 to 5.3 μm. We observe a signal power of up to 7 W, an idler power up to 3 W, and a mode-hop free operation over 10 h without any active stabilization. Furthermore, we have extended the tuning range of the parametric oscillator to the terahertz region: Our system converts near-infrared pump light to a monochromatic wave with a frequency of 1.35 THz and a power of 2 μW. Now, the straightforward next development step is to reduce the footprint of such devices. For this purpose another type of ring cavity is very promising: the whispering gallery resonator. This system offers unequaled opportunities because of its low loss leading to a high finesse. We discuss the challenges for transferring the parametric oscillation scheme to whispering gallery resonators, addressing the preparation of suitable resonators with a quality factor of 107 and a finesse of 500 and locking of the pump laser to a cavity mode for 3 hours.

Photorefraction in LiNbO 3 :Fe crystals with femtosecond laser pulses

The photorefractive effect in lithium niobate crystals (LiNbO3) is investigated using femtosecond (fs) laser pulses. This research is triggered by an increasing interest in using LiNbO3 for nonlinear applications with high light intensities [1]. Such applications are affected by the photorefractive effect, since it can destroy the beam profiles and the phase matching conditions due to “optical damage” and “light-induced scattering”. To make measurements with ultra-high intensities, fs pulses are quite convenient. In addition, interesting photorefractive effects have already been discovered using short pulses and LiNbO3 [2, 3].