S. E. Schwarz

PASSIVE Q‐SWITCHING OF A CO2 LASER

Passive Q‐switching of a CO2–N2–He laser has been obtained, using SF6 gas as the saturable absorber. Peak power is 1 kW, in what appears to be a single transverse mode. This is 200 times the CW level for the same configuration and one‐fifth that obtained with a mechanical Q switch. Pulse rates are in the range 103 to 104 pulses per sec. Operation is on a single vibrational‐rotational line, unlike the case of CW operation.

Microbolometers for infrared detection

We describe a novel room-temperature detector for the wavelength range 10–1000 µm. This detector consists of a thin bismuth bolometer film with dimensions much smaller than a wavelength. The small size of the detector results in reduced NEP and faster response. A video NEP of 1.6×10–10 W/Hz1/2 is obtained at 119 µm, remaining within a factor of 10 of this value for modulation frequencies up to 25 MHz. When used as a mixer, the device is predicted to have an NEP of 3.5×10–18 W/Hz. It is easily fabricated with conventional planar processing techniques and can be replicated in arrays. The device is expected to be most useful when the radiation to be detected is spatially coherent.

SINGLE‐MODE OPERATION AND MODE LOCKING OF HIGH‐PRESSURE CO2 LASERS BY MEANS OF SATURABLE ABSORBERS

Single‐longitudinal‐mode operation of pulsed, high‐pressure, transversely excited CO2 lasers has been obtained by means of a saturable absorber (SF6) cell placed inside the laser cavity. Eighty percent of the multimode ouput power is obtained in the single‐mode output. Single‐mode operation has been verified by means of a scanning Fabry‐Perot interferometer. Pressure broadening of the saturable absorber by a buffer gas has been found to cause multimode mode‐locked operation of the laser.

Design and performance of a noncontacting probe for measurements on high-frequency planar circuits

Optimal design of a noncontacting magnetic probe for measurements on the interior of planar high-frequency circuits has been studied, and performance of the probe has been determined. The design was studied using enlarged models tested at frequencies 100 times lower than those of the actual intended use. The nature of its errors has been investigated, and some techniques for error reduction have been found. The accuracy of measurements on circuits with SWR >

Mechanism and properties of point‐contact metal‐insulator‐metal diode detectors at 10.6 μ

Detection mechanisms of point‐contact MIM diodes at 10.6 μ wavelength have been studied. Detailed measurements of the static current‐voltage characteristic and its first and second derivatives as functions of bias voltage were made simultaneously with infrared sensitivity measurements. The results indicate that the static current‐voltage characteristics extend to infrared frequencies. We find that thermal effects do not contribute significantly to detection at this wavelength, and that the tungsten whisker acts as a rather efficient receiving antenna. The antenna and its shunting capacitance apply a 3×1013‐Hz ac voltage to the diode which in our experiment has amplitude 124 mV, through an impedance which is much less than the junction impedance. The diode nonlinearity d2I/dV2 varies from 6.5×10−5 to 6.7×10−3 A/V2 as dc bias increases from 0 to 300 mV. None of these conclusions apply to detection at 6328 A; it is found that detection in the visible is dominated by thermal or photoconductive effects.

PASSIVE MODE LOCKING OF A CO2 LASER

Self‐locking of longitudinal cavity modes has been observed in a CO2–N2–He laser with an SF6 absorber inside the cavity. Pulses as short as 20 nsec, with peak power in the range 104–105W, have been obtained. Pulse trains longer than 100 μsec have been observed. Operation is repetitive at millisecond intervals.

Saturation of infrared absorption in gaseous molecular systems

Saturable absorbers for use in the infrared, consisting of molecular gases, are considered. A basic objective of the study is the understanding and prediction of saturation behavior. A model for the absorber is developed. Features considered include 1) strong interaction between the absorbing pair of vibrational-rotational levels and numerous neighboring levels; 2) spatial diffusion of molecules; 3) temperature gradients; 4) distortion of optical beams by the saturable absorber; 5) inhomogeneous broadening of absorbing transitions; 6) effects of geometry of the absorption cell. The model is intended to be quite general in its applicability, but is particularly considered in relation to absorption of CO 2 laser radiation by sulfur hexafluoride. The equations arising from the absorber model are complex, but under suitable assumptions can be simplified. Experiments have been performed to determine the values of the relevant parameters for SF 6 absorbers. These values are inserted into the general absorber model and the resulting equations solved by digital computation. The theoretical saturation behavior thus obtained is in reasonably good agreement with saturation behavior observed in experiments with SF 6 .

Antennas and waveguides for far-infrared integrated circuits

Antennas and waveguides for the wavelength range 0.1-3 mm are considered. Emphasis is placed on those designs which lend themselves to integration with each other and with other components such as diodes. The general properties of FIR antennas are reviewed. A novel silicon waveguide antenna is discussed, and its design, simulation, fabrication, and performance at 119 μm are described. This antenna has a highly symmetrical, single-lobed beam with 3 dB beamwidths of 35 and 38° in the E - and H -planes, respectively. The gain (measured in microwave simulation) is 12.8 dB. This antenna is well suited for integration with Schottky diodes. The related subject of FIR waveguides is discussed. Experiments with metal transmission lines at 119 μm are described and dielectric guides related to the waveguide antenna are also considered. Using components such as these it may soon be possible to construct receiver front ends for this wavelength range in integrated-circuit form.

Properties of infrared cat‐whisker antennas near 10.6 μ

Radiation patterns of 10.6‐μ infrared antennas have been obtained experimentally and interpreted by means of a simple theory. It is found that the effective antenna length is equal to the length of the etched portion of the sharpened cat‐whisker antenna. This effective antenna length appears to be caused by decoupling of the electromagnetic field across a shape discontinuity of the antenna wire. The results suggest a simple means for defining shape and size of infrared antenna structures.

Millimeter wave monolithic schottky diode imaging arrays

Planar Schottky diodes are integrated with bow-tie antennas to form a one-dimensional array. The energy is focused onto the antennas through a silicon lens placed on the back of the gallium-arsenide substrate. A polystyrene cap on the silicon lens reduces the reflection loss. A self-aligning process with proton isolation has been developed to make the planar Schottky diodes with a 1.1-THz zero-bias cutoff frequency. The antenna coupling efficiency and imaging properties of the system are studied by video detection measurements at 94 GHz. As a heterodyne receiver, a double-sideband mixer conversion loss of 11.2 dB and noise temperature of 3770°K have been achieved at a local oscillator frequency of 91 GHz. Of this loss, 6.2 dB is attributed to the optical system and the antenna.