Nonlinear Optical Crystals: A Complete Survey

https://www.ulp.ethz.ch/content/dam/ethz/special-interest/phys/quantum-electronics/ultrafast-laser-physics-dam/publications_awards/publications/2006/248__PhysRep_429__67__2006_.pdf

Passively modelocked surface-emitting semiconductor lasers

We present an experimental study on the drilling of metal targets with ultrashort laser pulses at high repetition rates (from 50 kHz up to 975 kHz) and high average powers (up to 68 Watts), using an ytterbium-doped fiber CPA system. The number of pulses to drill through steel and copper sheets with thicknesses up to 1 mm have been measured as a function of the repetition rate and the pulse energy. Two distinctive effects, influencing the drilling efficiency at high repetition rates, have been experimentally found and studied: particle shielding and heat accumulation. While the shielding of subsequent pulses due to the ejected particles leads to a reduced ablation efficiency, this effect is counteracted by heat accumulation. The experimental data are in good qualitative agreement with simulations of the heat accumulation effect and previous studies on the particle emission. However, for materials with a high thermal conductivity as copper, both effects are negligible for the investigated processing parameters. Therefore, the full power of the fiber CPA system can be exploited, which allows to trepan high-quality holes in 0.5mm-thick copper samples with breakthrough times as low as 75 ms.

High speed laser drilling of metals using a high repetition rate, high average power ultrafast fiber CPA system.

Surface-emitting semiconductor lasers can make use of external cavities and optical pumping techniques to achieve a combination of high continuous-wave output power and near-diffraction-limited beam quality that is not matched by any other type of semiconductor source. The ready access to the laser mode that the external cavity provides has been exploited for applications such as intra-cavity frequency doubling and passive mode-locking. The purpose of this Topical Review is to outline the operating principles of these versatile lasers and summarize the capabilities of devices that have been demonstrated so far. Particular attention is paid to the generation of near-transform-limited sub-picosecond pulses in passively mode-locked surface-emitting lasers, which are potentially of interest as compact sources of ultrashort pulses at high average power that can be operated readily at repetition rates of many gigahertz.

https://eprints.soton.ac.uk/15016/1/TropperJPD04.pdf

Vertical-external-cavity semiconductor lasers

Tm(3+)-Yb(3+) codoped transparent oxyfluoride glass ceramics containing LaF(3) nanocrystals were obtained by thermal treatment on the as-made glasses. The formation of LaF(3) nanocrystals and the incorporation of Tm(3+) and Yb(3+) into LaF(3) nanocrystal lattice were confirmed by X-ray diffraction and high resolution transmission electron microscopy. Infrared quantum cutting involving Yb(3+) 950-1100 nm ((2)F(5/2)--> (2)F(7/2)) emission was achieved upon the excitation of the (1)G(4) energy level of Tm(3+) at 468 nm. We measured the photoluminescence properties of these glass ceramics. We also investigated the thermal treatment duration dependent quantum efficiency, and found that the quantum efficiency is 13% increased for the 0.5Tm(3+)-4Yb(3+) doped glass ceramic with a maximum value of 144%, and 16% increased for the 0.5Tm3+-8Yb3+ doped glass ceramic with a maximum value of 162%, respectively.

Enhanced cooperative quantum cutting in Tm3+- Yb3+ codoped glass ceramics containing LaF3 nanocrystals.

We demonstrate 1.5-W continuous-wave output power from a vertical external-cavity surface-emitting laser (VECSEL) based on InGaAs quantum wells as the gain medium. The VECSEL is pumped by the output of a single-bar diode-laser array at 814 nm and produces an optical-to-optical efficiency of 19%. The high output power is made possible by the use of a sapphire window optically contacted to the intracavity semiconductor surface for heat removal. We demonstrate the good beam quality of the VECSEL output by obtaining 1-W output from a single-mode fiber for 1.5 W launched with simple lenses. Pulsed operation produces a maximum peak power of ∼4.4 W and maximum average power of ∼2 W.

High power and good beam quality at 980 nm from a vertical external-cavity surface-emitting laser

We have constructed a numerical model of seeded optical parametric oscillators that is appropriate for nanosecond or longer pulsed operation. We have also experimentally characterized the performance of a KTP ring optical parametric oscillator. We present a description of the model and show that its predictions agree well with the observed oscillator performance. We compare spatial beam quality, spectra, efficiency, and full-beam and spatially resolved temporal profiles. Backconversion of signal and idler light to pump is found to affect all the aspects of performance.

Comparison of a numerical model with measured performance of a seeded, nanosecond KTP optical parametric oscillator

We measure and model parametric gain and oscillation for two crystals arranged for walkoff compensation. We show how the orientation of the crystals determines the relative sign of the nonlinear mixing coefficient in the two crystals. This sign dramatically influences small signal gain and oscillator performance, and we show how to determine the correct crystal orientation from parametric-gain measurements. The performance of two-crystal oscillators is examined with particular attention to beam tilts, conversion efficiency, and beam quality. We find reduced efficiency and increased oscillation threshold when the coefficients have opposite signs in a two-crystal ring oscillator. Sign reversal seems to have little influence on spectral purity or far-field beam profiles when the oscillator is seeded.

Parametric amplification and oscillation with walkoff-compensating crystals

We show by experiment and mathematical model that angular and frequency acceptance bandwidths for frequency mixing in a nonlinear crystal can often be improved by segmenting the crystal and reversing the spatial or temporal walk-off in alternating segments. We analyze nonlinear mixing primarily in real space, (x, t), rather than Fourier space, (k, ω), and show that acceptance bands for sum- and difference-frequency mixing can be increased by up to a factor equal to the number of crystal segments. We consider both high- and low-efficiency mixing as well as parametric gain, and show that in many cases of practical interest the increased bandwidth substantially improves conversion efficiency. We also attempt to clarify the role of acceptance bandwidths in frequency mixing.

Increased acceptance bandwidths in optical frequency conversion by use of multiple walk-off-compensating nonlinear crystals

The authors present a compact, robust, solid-state blue light (490 nm) source capable of greater than 5 mW of output in a TEM{sub 00} mode. This device is an optically pumped, vertical external-cavity surface-emitting laser (VECSEL) with an intracavity frequency doubling crystal.

/pdf/intracavity-frequency-doubling-of-a-diode-pumped-external-4woswdvi3t.pdf

W. J. Alford

Papers

High power and good beam quality at 980 nm from a vertical external-cavity surface-emitting laser

Comparison of a numerical model with measured performance of a seeded, nanosecond KTP optical parametric oscillator

Parametric amplification and oscillation with walkoff-compensating crystals

Increased acceptance bandwidths in optical frequency conversion by use of multiple walk-off-compensating nonlinear crystals

Intracavity Frequency Doubling of a Diode-Pumped, External Cavity, Surface Emitting Semiconductor Laser