The huge potential of tunable optical parametric oscillators (OPOs) derives from their exceptional wavelength versatility, as they are in principle not limited by the wavelength coverage dictated by the energy levels and transitions in a laser gain medium. However, while the OPO concept has been experimentally demonstrated already more than half a century ago, the progress in development of practicable and reliable turn-key devices that operate in continuous-wave (cw) mode has been stalled by several technical obstacles. This applies particularly for systems that sought to deliver tunable output across the visible spectral range (VIS), where only relatively recent advances have spurred the development of operationally stable benchtop devices. We discuss the principles and design challenges of such technically practicable cw OPOs, focusing on singly resonant OPO cavity designs that are linked with frequency conversion of the primary OPO output into different ranges of the visible spectrum. In this context, suitable choices and combinations of (quasi-phase-matched) nonlinear crystals are examined. We further discuss the overall performance highlights as well as current limitations of state-of-the art tunable cw OPO designs, and present first measurement results from conceptual approaches to shift and/or extend the wavelength coverage in future design layouts that eventually target commercialization. Last no least, after presenting real-world applications in an illustrative manner, we critically discuss how OPO technology, on the long run, can be expected to perform in the competition with alternatives based on common tunable laser designs.
There is recently an increasing interest in holographic techniques and holographic optical elements (HOEs) related to virtual reality and augmented reality applications which demand new laser technologies capable of delivering new wavelengths, higher output powers and in some cases improved control of these parameters. The choice of light sources for optical recording of holograms or production of HOEs for image displays is typically made between fixed RGB wavelengths from individual lasers (457 nm, 473 nm, 491 nm, 515 nm, 532 nm, 561 nm, 640 nm, 660 nm) or tunable laser systems covering broad wavelength ranges with a single source (450 nm – 650 nm, 510 nm – 750 nm) or a combination. Lasers for holographic applications need to have long coherence length (>10 m), excellent wavelength stability and accuracy as well as very good power stability. As new applications for holographic techniques and HOEs often require high volume manufacturing in industrial environments there is additionally a growing demand for laser sources with excellent long-term stability, reliability and long operational lifetimes. We discuss what performance specifications should be considered when looking at using high average power, single frequency (SF) or single longitudinal mode (SLM) lasers to produce holograms and HOEs, as well as describe some of the laser technologies that are capable of delivering these performance specifications.
Bulk and surface absorption in lithium triborate (LBO) and lithium niobate (LiNbO<sub>3</sub>) are measured using two sensitive measurement techniques, a photoacoustic spectrometer (PAS) and a photothermal common-path interferometer (PCI). As pump light sources, optical parametric oscillators are employed, covering the wavelength ranges 212 − 2500 nm (PAS) and 1460 − 1900 nm and 2460 − 3900 nm (PCI). The spectrometers are used to measure absorption spectra of optical materials across this wide spectral range and to compare the methods in the shared wavelength regime.
Nonlinear optical materials are important to extend the spectral coverage of existing lasers, mainly via frequency
doubling and with optical parametric oscillators. The quality of the materials and components, e.g. in terms of
residual absorption, is pivotal for the performance of the devices. The paper presents high-sensitivity absorption
measurements of nonlinear optical materials. They were performed using a photoacoustic spectrometer which
combines high sensitivity with broad spectral coverage. This allows one not only to quantify the level of residual
absorption but also to assist in the characterization of the materials in terms of optically relevant impurities and
imperfections. The spectrometer covers the wavelength range between 407 and 2600 nm using a pulsed optical
parametric oscillator as excitation source. Pulse energies up to 100 mJ allow one to record absorption spectra
with a sensitivity down to 10 ppm/cm.
The paper presents spectra of lithium niobate and lithium triborate crystals which are important for highpower
nonlinear optical applications. The results are discussed with respect to material impurities and the
suitability of individual samples for frequency conversion.