Showing papers on "High harmonic generation published in 1970"

An experimental and theoretical study of high repetition rate Q-switched Nd: YA1G lasers

Abstract: An acoustooptic loss modulator made by bonding an X-cut quartz transducer to a fused silica scattering medium offers a practical means for repetitively Q-switching continuously pumped Nd : YAlG lasers at repetition rates up to 50 kHz. The peak power output of a multitransverse mode laser is typically enhanced by a factor of about 500 relative to CW operation at low (≪5 kHz) repetition rates and by larger factors when the transverse mode structure is suitably restricted. At high repetition rates, the peak power becomes smaller and the average power output approaches the CW level. The theoretical Q-switching behavior derived from the rate equations governing an ideal four-level laser is presented in graphical form, and is found to describe the observed behavior well when the laser is restricted to oscillation in the TEM 00 transverse mode. Similar calculations for Q-switched intracavity second harmonic generation indicate that the peak output power available at the second harmonic slightly exceeds that available at the fundamental, and that the harmonic coupling necessary to optimally couple the Q-switched laser is several orders of magnitude smaller than that required for CW intracavity conversion. Experiments using Ba 2 NaNb 5 O 15 For intracavity Q-switched harmonic generation roughly verified these predictions. A peak power output of 220 watts at 0.532 µ in the TEM 00 mode was achieved. Surface pitting of the nonlinear crystal after a few hours of operation prevented a thorough comparison with the calculated results. Preliminary experiments using the doubled Q-switched Nd : YAlG laser and an ammonia dihydrogen phosphate crystal to generate ultraviolet (the 0.266-µ harmonic) yielded a peak output power of 40 watts.

Nonlinear Effects in Surface Acoustic Waves

Abstract: Experimental results on harmonic generation and parametric mixing of surface acoustic waves are reported. Harmonics of signals at 615 MHz and 1.69 GHz propagating on YZ LiNbO3 substrates were observed: the experiments on parametric mixing used two surface acoustic waves at f1=490 MHz and f2=1.09 GHz. Optical probing technique was used to observe directly the spatial variations of the harmonics, the sum, and difference frequency signals. It will be shown that the nonlinear interactions can be phenomenologically described by coupled amplitude equations. The nonlinear coupling constants are determined by comparing the experimental results with the calculations.

Harmonic generation in octave bandwidth traveling-wave tubes

Abstract: This paper describes the investigation of harmonic generation in traveling-wave tubes (TWT's) via large-signal analysis and digital computer techniques. Efficiency degradation and harmonic power content are shown to be importantly related to such TWT design considerations as circuit dispersion, harmonic coupling impedance ratio, and gain level. Also described is the phenomenon of second harmonic interference in the beam bunching process and how it leads to substantial efficiency reduction in TWT's employing relatively nondispersive structures.

Optical probing of acoustic surface‐wave harmonic generation

Abstract: The growth of nonlinearly generated harmonics of acoustic surface waves in LiNbO3 substrates has been observed directly by an optical probe. The attenuation of acoustic surface waves as a function of the level of input power and loss of energy to the nonlinearly generated harmonics is measured. The large nonlinear coupling to the high harmonics of surface acoustic waves, which is an encouraging sign for parametric amplification, may impose some limitations on linear devices at high power levels.

Observation of Acoustically Induced Phase-Matched Optical Harmonic Generation in GaAs

Abstract: Radiation near 5.3 \ensuremath{\mu}m arising from the collinear phase-matched mixing of a 10.6-\ensuremath{\mu}m $Q$-switched laser beam and a pulsed ultrasonic shear wave has been observed in GaAs. Phase matching occurs when the crystal momentum of the acoustic phonon is adjusted to equal the familiar momentum mismatch observed in conventional second-harmonic generation in cubic GaAs. Three components of the susceptibility governing the four-wave interaction process were measured.

Second-harmonic generation with the GaAs laser

Abstract: The second harmonic of pulsed GaAs injection lasers was generated with the nonlinear crystal α-iodic acid (HIO 3 ) phase matched by angular tuning. The injection laser was operated with an external cavity, and the HIO 3 crystal was placed inside the cavity to increase the conversion efficiency. The maximum harmonic power observed was 15 μW with a fundamental power of 4.5 watts inside the cavity. Since the lasing bandwidth was typically 30 A and the calculated bandwidth for collinear harmonic generation was only 6.3 A, a grating was used to narrow the lasing bandwidth to 3 A. The expected large increase in harmonic power did not occur. It is shown that this is due to sum-frequency generation by laser modes lying outside the harmonic bandwidth. It is also found that the spectral width of the harmonic becomes bigger than the collinear bandwidth when a divergent laser beam is used.

Optical harmonic generation and laser light frequency mixing processes in nonlinear media

Abstract: The essential results achieved during the ten years since the coming of lasers, in the fields of second, third and higher harmonic generation in reflected and transmitted, continuous and pulsed laser light, frequency mixing processes in various materials, and nonlinear optical properties of liquids, gases, crystals and other materials are reported concisely from the available literature. Stress is laid on the newer and more stimulating experimental results, and on their relation with other nonlinear phenomena.

Optical harmonic generation in liquid crystals

Abstract: The non-linear optical properties of several cholesteric and nematic liquid crystals are studied in laser experiments on generation of secondand third-harmonic radiation. No secondharmonic is observed in any of the liquid crystal mesomorphic phases, indicating that the local molecular ordering in these phases is centrosymmetric. The generality of this result to datediminishes the potential of liquid crystals as nonlinear materials. Large intensity changes ofthird harmonic are observed at several phase transitions, and their possible origins are discussed. Tentative results are presented on experiments to utilize the high optical rotatory power of the cholesteric phase to obtain phase matching in third-harmonic generation.

Avalanche diode circuits

Abstract: An avalanche diode of the type capable of generating Trapatt mode oscillations is used for generating extremely sharp output voltage spikes. An externally applied input pulse has a current density of JT> qvsN, where vs is the saturated drift velocity and N is the impurity concentration of majority carriers in the highresistance layer of the diode. The output spikes can be used for high speed gating, pulse regeneration, or harmonic generation.

Contribution to the Quantum Theory of the Seond-Harmonic Generation in a Molecular Crystal†

Abstract: The quantum theory of the second-harmonic generation is developed for diamagnetic molecules through a variational perturbation method. The advantage of this method is that it requires only to know the wave function Ψ of the ground state of the non-perturbed molecule; thus, the coefficient characterizing the second-harmonic generation can be expressed as a function of the wave function Ψ and of vectors determined by a variation calculus; so this coefficient can be computed numerically. This calculation is achieved for the hexamethylene-tetramine crystal.

Small-signal mixing and harmonic generation in thin magnetic films

Abstract: A method is developed for calculating the response of a thin magnetic film subjected to static magnetic fields and small time‐varying fields. An applied field is considered small if it produces rotations, φ1 of Ms such that sin2φ1≈2φ1 and cos2φ1≈1. The measured response is the time derivative of that component of flux which arises from Ms. A component of this flux change generates a voltage in a conductor held close to the film. It is shown that this sensed voltage contains not only the frequency of the applied field but also its harmonics. Independent variables are chosen to be the magnitude and direction of the static field and the direction of the small time‐varying fields. General expressions for the output voltage generated by the film at fundamental and second‐haramonic frequencies are developed. Theoretically predicted signal behavior is illustrated by fixing two of the independent variables and plotting the dependence of output voltage on the remaining variable.

Measurements of photon correlations of second harmonic generated light

Abstract: Results of measurements of the second‐order intensity correlation function of the optical field produced by cw optical second harmonic generation in a LiNbO3 crystal pumped by a He–Ne laser operating in two independent axial modes are reported. The observed correlation function is shown to be in agreement with theoretical descriptions of second harmonic generation and with coherence theory.

Optimum harmonic generation in plasmas

Abstract: Using the expressions for the second and third harmonic components of the current density in a plasma, subjected to an external alternating field and a dc electric field, the authors have investigated the conditions for maximum efficiency of harmonic generation in a plasma It is concluded that (i) the intensity of harmonics is higher in the transmitted component than in the reflected component from a plasma slab, and that (ii) the efficiency of second-harmonic conversion is maximum when 0?80 <(?p/?)2 < 0?90 and dimensionless plasma thickness (?0=?z/c) in the range 3?20

Nonlinear optics and the defect solid state

Abstract: The condition for the generation of second harmonics in the dipole approximation is that the nonlinear crystal medium lack a center of inversion. Conversely, a center of inversion precludes optical second harmonic generation. It is well known, however, that lattice defects introduced into a crystal may alter the local point symmetry in the medium, and in fact may convert local inversion symmetry to that which corresponds to a noncentrosymmetric point group. It is thus clear that selected defects in nonlinear media may be of interest for studying nonlinear optical processes which depend on the square of the electric field strength. The specific case of second harmonic generation employing spatially ordered FA-centers in alkali halide crystals is considered.

Harmonic Generation and Multisignal Effects in Nonlinear Beam-Plasma Systems

Abstract: Publisher Summary This chapter describes the theory involved in harmonic generation and multisignal effects in nonlinear beam-plasma systems. It is well-known that if the temperature of gas is raised to a sufficiently high value, the thermal agitation of the gas particles might become so large that the electrons could be stripped off by the collisions among the particles. The equivalent circuit representation of a plasma column is found to be an effective and convenient method of modeling the plasma in a nonlinear beam-plasma interaction. One of the chief stipulations in the development of the equivalent circuit concept for a plasma column is that the plasma remains linear under all conditions. Even though the one-dimensional interaction model results in a good qualitative understanding of the interaction process, it is generally optimistic in predicting the output levels from the device. The beam-plasma device is found to be considerably more dispersive than a conventional helix-type traveling-wave amplifier. Nonuniformities in the plasma density are not taken into account in the theoretical development, but they do play a major role in the experimental device.

Theory of Intracavity Optical Second-Harmonic

Abstract: Abslract-An analysis of optical second-harmonic generation internal to the laser cavity is presented. It is shown that the maximum second-harmonic power generated in this way is equal to the maximum fundamental power available from the laser. Further, it is found that there exists a value of nonlinearity that optimally couples the harmonic out for 211 power levels of the laser. The magnitude of the nonlinearity required for optimum coupling is shown to be proportional to the linear losses at the fundamental and inversely proportional to the saturation parameter for the laser transition. For the YA1G:Nd laser at 1.06 p using BagNaNb50ts as the nonlinear material, the required crystal length for optimum coupling is given by 1z(cm)c= 2.7 X lOeL/f where L is the linear round-trip loss and f is the ratio of the fundamental power density in the nonlinear crystal to that in the laser medium. For low-loss cavities, optimum coupling can thus be achieved for crystal lengths of 1 cm or less. The use of a mirror or mirrors within the cavity, reflecting at the harmonic, is considered as a means to couple out the total harmonic in one direction. Considerations of temperature stability and the finite oscillating linewidth of the laser are shown to favor a configuration with a single harmonic mirror located on the same surface as the fundamental mirror.

Self and externally pumped second-harmonic generation with silicon avalanche diodes

Abstract: Self-pumping in a silicon avalanche diode has produced 3% pulsed efficiency at 22.5 GHz, the second harmonic of a ‘trapped’ fundamental. An analogue-computer solution of a simplified Read's equation has shown how the parametric properties of the avalanche process can result in this type of frequency conversion.

Nonlinear Solid-state Specmscopy in the Far Infrared

Abstract: As before in other specnal regions, the development of lasers has greatly stimulated the advance of specmscopy also in the far infrartd which we define for the purpose of this mcering between 10 p~ and 10 mm wavelength. Before 1970 the lasers in the far infrared wm limited in output power to about 10 mW. They employed C.W. elecmc discharges in HCN, 40 etc. to obtain vibrational/”tional inversion. In the few laboraties working at that time on far-infrared laser spectroscopy (Bell Laboratories, ULT., Osaka University) early steps towards far-infrated non-linear spectroscopy wm taken in two ways. With free-space beams the focal intensities were too small to produce, by themselves, measurable hannonic power in nonlinear media, however sum and difference sidebands could be generated by bulk mixing with intense visible light’ The other approach was to use miniature diode smcms as nonlinear media, and apply far-infrand waves in the form of surface plasmons excited on thin wires (“whisker antennae”); indeed the laser power sufficed to generate high harmonics and other mixing products useful for high-resolution specaoscopy which yielded a measmment of light frequency with record accuracy? Saturated absorption -the nonlinearresponse of resonant oscillators- at far-infrartd frequencies also began to be studied around 1970. This type of nonlinear spectroscopy uses optical pumping to balance and eventually overcome the natural relaxation of e.g. two-level systems. With the low powers available only relatively longlived excitations could be probed, mostly vibrations and rotations of low-pressm gas molecules. ’This related closely to the functioning of the gas lasers themselves (Lamb dip) and thus led to the disovery of the optically pumped far-infrared gas lasers.’ Since at the same time powerful TEA-CO, lasers appeared the new lasers were able to provide many orders of magnitude higher far-infrartd power than possible before, to a limit of about 1 MW which still is unsu~passcd.~~ The inmase of far-infrared power available did not immediately lead to an extension of the nonlinear specmscopic studies as in fact wcre possible, namely to investigate harmonic generation and absorption saturation of bulk condensed media. These fields required further technological advances to allow quantitative measurements at very high far-infrared intensity.