J. R. Whinnery

Long‐Transient Effects in Lasers with Inserted Liquid Samples

Buildup and decay transients were observed when polar or nonpolar liquid cells were placed within the resonator of a helium—neon laser operating in the red at 6328 A. Similar but smaller effects were also observed with two solids. Time constants were the order of a few seconds for all materials, which suggests a thermal phenomenon, but general heating effects were ruled out by the strong localization of the phenomenon. Transverse motion of the cell by about one beam width caused new transients similar to the initial ones.It is believed that the effects are caused by absorption of the red light in the material, producing a local heating in the vicinity of the beam and a lens effect arising from the transverse gradient of refractive index. Absorptions of 10−3 to 10−4 parts per centimeter are sufficient to produce the effects, and are believed to be reasonable values for the materials studied. One of the most important applications may in fact be for the measurement of small absorbancies.The experiments are ...

New Thermooptical Measurement Method and a Comparison with Other Methods

A simple and sensitive method of measuring the thermally induced index changes arising from absorption of a laser beam in a low-loss material is described. The sample is placed outside the laser cavity, but at the position of minimum radius of curvature of the wavefront, which is a confocal distance behind the waist. It is estimated that the method is sensitive enough to measure absorption coefficients of the order of 5 x 10(_6) cm(_1) and it is shown experimentally to have good accuracy on low-loss materials. A detailed comparison of sensitivity and accuracy estimates is given for the various published thermal-lens methods for measuring low absorption coefficients.Although the method is illustrated for the thermal effect, it is applicable to other nonlinear index changes induced by laser beams.

Coaxial-Line Discontinuities

The equivalent circuits representing the effects from certain step-type discontinuities are obtained for coaxial transmission lines. It is shown that for the cases studied the effects of the local waves in the vicinity of the discontinuity can be accounted for exactly for calculation of relation between input- and output-end quantities by placing an admittance at the plane of the discontinuity in a transmission-line or principal-wave equivalent circuit. If frequency is low enough so that transverse dimensions are a small fraction of a wavelength, the discontinuity admittance is found to be a pure capacitance for these cases. Curves are presented giving the discontinuity capacitances in terms of the coaxial-line radii. Curves of a frequency factor F are calculated showing the amount the discontinuity capacitance must be corrected as transverse dimensions become comparable to wavelength. Curves of a proximity factor P are also given showing corrections to be applied to the discontinuity capacitance if a short-circuiting disk termination is placed near the plane of the discontinuity. The analysis of the case of a simple-step discontinuity on the inner cylinder is carried through in detail, and analyses of other cases are outlined.

Equivalent Circuits for Discontinuities in Transmission Lines

Exact equivalent circuits for several types of discontinuities in parallel-plane transmission lines are obtained by Hahns method of matching electromagnetic-wave solutions. Values of lumped elements to be used in these are given in curve form, with rules for using results in the corresponding coaxial-line problems. Experimental checks are reported, which verify results of the calculations and stress the importance of the discontinuity capacitances appearing in the equivalent circuits.

Dispersion of Picosecond Pulses in Coplanar Transmission Lines

The dispersion of coplanar-type transmission lines has been extended to the terahertz regime to examine the distortion of picosecond electrical pulses. Dispersion of coplanar waveguides is compared to equivalent microstrip lines. Agreement with available experimental data is demonstrated for coplanar strips, An approximate dispersion formula for coplanar waveguides is also reported for CAD applications.

Focusing of a light beam of Gaussian field distribution in continuous and periodic lens-like media

The behavior of a light beam traveling in continuous or periodic lens-like media is considered. The light beam is assumed to be launched with a Hermite-Gaussian or Laguerre-Gaussian field distribution. Under suitable conditions and for most focusing media we find that the beam center follows a paraxial ray, and an independent equation determines the motion of the beam radius. The form of the field distribution does not change. In a continuous inhomogeneous focusing medium, the beam cross section varies periodically in distance with a period half of that of the oscillatory motion of the beam center. In periodic media, a particular solution is found for the case when the beam propagates with a nearly constant cross section. Discussions include an acoustic lens-guide and a lens-guide using heat diffusion in a gas.

THERMALLY SELF‐INDUCED PHASE MODULATION OF LASER BEAMS

Self‐phase modulation of laser beams by media which possess a temperature‐dependent refractive index gives rise to far‐field aberrational rings. Steady‐state aberration profiles were measured and quantitative agreement with theory was obtained. A simple qualitative explanation of the ring structure is given. A self‐induced frequency modulation of 1.5‐W argon laser pulses was also observed and chirping detected.

High power with high efficiency in a narrow single‐lobed beam from a diode laser array in an external cavity

High output power (700 mW) in a nearly diffraction‐limited (0.7°) single‐lobed nonsteering output beam is obtained from a gain‐guided diode laser array in a novel, easily fabricated external cavity configuration. The laser output beam is collimated in both lateral and vertical directions. Differential quantum efficiency as high as 70% has been measured along with excellent linearity of the optical power versus current characteristic. The power and efficiency are the highest reported for a coupled‐stripe laser array emitting a narrow single‐lobed beam.

THERMAL SELF‐FOCUSING OF LASER BEAMS IN LEAD GLASSES

Strong thermal self‐focusing of an argon laser beam has been observed when the beam passes through lead glass. Focal lengths of less than 20 cm were obtained. The beams showed the effects of spherical aberration and at certain power levels beam trapping occurred. A simple theory is proposed to explain the observed trapping. Computer solutions have been obtained which reasonably agree with experimental determination of focal planes and spot radius.

Thermal convention and spherical aberration distortion of laser beams in low-loss liquids

Experiments and approximate analysis are given for spherical aberration interference rings and for convection distortion arising in the thermal self-defocusing effects produced by laser beams passing through materials of finite loss. The importance of these thermal effects for moderate-power beams in materials of relatively low loss is stressed.