Noncritical Phase Matching Research Articles

Generation of high-peak-power tunable infrared femtosecond pulses in an organic crystal: application to time resolution of weak infrared signals

High-intensity femtosecond pulses tunable in the 0.8–1.6-μm range have been generated by parametric amplification of a continuum white light in a new organic crystal, N-(4-nitrophenyl)-L-prolinol (NPP). The traditional concept of noncritical phase matching was revised in view of requirements linked to the observation of ultrafast subpicosecond nonlinear phenomena. The notions of θ (noncritical) and λ (noncritical) phase matching are introduced together with their applications. An experimental determination of phase-matching curves for both second-harmonic generation and three-wave mixing has been carried out. A θ noncritical phase-matching configuration for second-harmonic generation at 1.15 μm and a quasi-λ noncritical phase-matching configuration in the near IR for three-wave mixing were evidenced. Frequency and pump-intensity dependences of the gain have also been studied. Parametric emission at degeneracy was observed, with the emitted bandwidth extending from 1.0 to 1.4 μm. Time resolution of the amplified signal has been carried out by cross correlating the pump with the incoming signal, evidencing a reduced time broadening of the interacting pulses; a new spectroscopic method with subpicosecond time resolution is derived from the previous nonlinear optical characterization experiments by replacing the IR continuum from the water cell by any sample emitting in the same frequency range. This method, termed parametric amplification and sampling spectroscopy, was used for temporal analysis of amplified and emitted infrared signals generated in an NPP crystal.

Generation of 22 mW of 532-nm radiation by frequency doubling in Ti:MgO:LiNbO_3 waveguides

We report temperature phase-matched second-harmonic generation of 1.064-μm radiation in Ti-indiffused waveguides in MgO-doped LiNbO3 substrates. Noncritical phase matching of the TE2 mode was achieved at 102–106°C. Owing to the high-temperature operation and the low photorefractivity of the waveguide, it was possible to generate 22-mW average-power 532-nm output. No evidence of photorefractive scattering was observed during 5 h of operation at 3-mW output.

Influence of thermal self-interaction on second harmonic generation in beams with an elliptic cross section

An analysis is made of continuous generation of the second harmonic as a result of thermal self-interaction in beams with an elliptic cross section. It is shown that cylindrical focusing of the fundamental-frequency beam in the direction of noncritical phase matching makes it possible to compensate partly thermal self-interaction only when the beam power is much higher than the critical value. The efficiency of conversion to the second harmonic can also be increased by some misalignment of the wave vectors on the beam axis. In the case of generation in the direction of critical phase matching the efficiency of such optimization methods depends on the ratio of the temperature gradients of the refractive index at both (fundamental and second harmonic) frequencies. Dependences of the harmonic power on these parameters are investigated for different birefringence angles and recommendations are put forward on optimization of the process in a wide range of crystals.

A proposed tunable coherent 16 mu m source in noncritically phase-matched CdSe and AgGaSe2

The generation of tunable coherent infrared radiation in nonlinear crystals by difference frequency mixing of a Nd:YAG laser with the same pumped dye laser is considered. Noncritical phase matching is achieved in AgGaSe2 throughout the available collinear tuning range, including 16 mu m, through introduction of noncollinearity between the pump lasers, while in CdSe temperature tunability is also of use for this purpose. The system parameters are evaluated, including the acceptance angle, spectral bandwidth and tolerance in noncollinear angle, and found to be promising for efficient tunable generation of infrared radiation.

Near-infrared noncritically phase-matched second-harmonic generation in KNbO3

Noncritical type-I phase matching at room temperature has been observed in KNbO3 for λpm=860 nm using the nonlinear optical coefficient d32=−40‖d11 (SiO2) ‖. By increasing the crystal temperature up to 180 °C the phase-matching wavelength is increased to λpm=950 nm. At room temperature the efficiency for second-harmonic generation was 15% for a power density Iω =0.5 MW/cm2 of the fundamental and for a crystal length l=5.74 mm.

Parametric up-conversion of infrared radiation and its applications

The results of theoretical and experimental investigations of conversion of infrared radiation to the visible (or ultraviolet and near infrared) range are summarized. An analysis is made of the physical basis of the process, which involves generation of the sum or difference frequency in media with a quadratic nonlinear polarizability. In particular, the critical and noncritical phase matching, the angular and spectral widths of the phase-matching curve, the energy and quantum efficiency of the conversion process, and similar topics are considered. The applications of the conversion process in detection of infrared signals, infrared vision, and nonlinear infrared spectroscopy are dealt with. Typical results obtained so far are given briefly. The following topics are discussed: the criteria for selection of nonlinear media useful for the near and middle infrared range; detection of thermal infrared signals; response time and other parameters of nonlinear media; methods for the calculation of the spatial (angular) structure of the output radiation and its relationship to the structure of the initial infrared field; spatial resolution and number of resolved elements in parametric infrared viewers; comparison of systems based on tangential (noncritical) and critical vector phase matching; and so on. Spectral resolution and response time of nonlinear infrared spectroscopic methods are discussed. It is shown that various parametric detector analyzers for the infrared range are in many respects not inferior or considerably better than conventional infrared devices. This applies particularly to the response time because parametric converters have no competitors in, for example, picosecond infrared spectroscopy. Extensive use of parametric infrared converters in science and technology will represent a new stage in the mastering of the infrared range.

Second-harmonic generation in KB<inf>5</inf>O<inf>8</inf>· 4H<inf>2</inf>O from 217.1 to 315.0 nm

Second-harmonic generation (SHG) has been observed in KB <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</inf> ċ 4H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O (KB5) between 217.1 and 315.0 nm by angle tuning in a single crystal using a single cut. A conversion efficiency of 9.2 percent was observed for type I noncritical phase matching at 217.1 nm for a peak power of 15 kW at 434.2 nm. The nonlinear coefficients d <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">31</inf> and d <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">32</inf> are estimated to be approximately <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.1 \times 10^{-10}</tex> ESU ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4.0 \times 10^{-25}</tex> m/V) and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.08 \times 10^{-10}</tex> ESU ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.29 \times 10^{-25}</tex> m/V), respectively.

Observation of noncritically phase-matched second-harmonic generation in an optical waveguide

Experimental observation of noncritically phase-matched second-harmonic generation in a liquid/TiO2/quartz waveguide is reported. The phase-match condition, approximately independent of waveguide thickness, was achieved by tuning the liquid refractive index. Nonlinear interaction occurs in the quartz between the fundamental (1.06 μm) TE0 mode and the harmonic (0.53 μm) TM0 mode. Mode effective indices were measured versus liquid index to determine the phase-match point. Coherence lengths greater than 3.5 mm were achieved, but propagation lengths were limited to 4 mm by losses due to surface scattering. These losses arise from waveguide design requirements of noncritical phase matching, not experimental film quality. Conversion efficiency was approximately 0.01%.

Internal thermal noise in optical frequency converters

We present a detailed analysis of noise due to the frequency-converted internal thermal radiation in a proustite crystal pumped by a ruby laser and adjusted for noncritical phase matching with a 10.6-μm signal. A three-dimensional expression is developed for the wave-number mismatch Δk, and phase-matching surfaces are plotted, which make clear the relative importance of phase matching and system filters in determining the spectral and spatial characteristics of the frequency-converted noise. Comparison of theory and experiment establishes that a crystal absorption coefficient of 0.22 cm−1 at 10.6 μm will account for the observed noise level. This compares favorably with a direct measurement of 0.4 cm−1 for the crystal loss coefficient. Good agreement is found between experiment and predictions of the noise temperature dependence.

Wide angular aperture image up-conversion

Infrared images at 1.06 μ are up-converted in KDP with an angular aperture of more than 20°. The images are up-converted by scanning the orientation of the KDP. The technique is applicable when noncritical phase matching does not allow sufficient angular aperture.