Stephen Marcus

Large‐spot thermal coupling of CO2 laser radiation to metallic surfaces

Thermal coupling of pulsed 10.6‐μm laser radiation to aluminum and titanium targets was measured as a function of incident fluence, focal‐spot size, and ambient pressure. Thermal coupling coefficients were measured with both calorimetric and fast‐response surface‐thermocouple techniques. Thermal coupling coefficients of over 0.3 were observed with the onset of a well‐developed plasma at the target surface. The thermal coupling was observed to increase slightly with increasing irradiated spot size and to decrease monotonically with increasing laser fluence. Under conditions of low ambient pressure (∼0.5 Torr) the breakdown threshold was increased by a factor of 5 and at high incident fluences the thermal coupling was roughly a factor of 2 higher than at atmospheric pressure.

Solid-state laser synthetic aperture radar

Here we report the operation of an optical synthetic aperture radar employing, for the first time to our knowledge, a solid-state laser as the source. The experimental data-acquisition and digital processing techniques are described, by which spatial resolution superior to that limited by diffraction is demonstrated.

Synthetic-aperture-radar imaging with a solid-state laser

We report the operation of an imaging Nd:YAG microchip-laser synthetic-aperture radar, with which we imaged two-dimensional (2-D) models of military targets. The images obtained showed spatial resolution significantly better than the diffraction limit of the real aperture in the along-track dimension. The signal processing is described, and the measurement sensitivity is both predicted and verified. In addition, 2-D images with high resolution in both dimensions were generated by using an asymmetric aperture to match the along-track synthetic-aperture resolution with the across-track diffraction-limited resolution.

Laser radar component technology

The development of the laser soon led to recognition of its potential in radar applications. As radar systems were designed and implemented, however, it became clear that major refinement of the laser sources was needed to meet specific radar transmitter requirements in terms of waveforms, spectral purity, stability, beam quality, and power. The sensitivity and bandwidth of receivers were also in some cases insufficient, and new enabling detector technologies had to be created. This article provides a framework for much of the component development by describing the wideband high-power carbon dioxide (CO/sub 2/) imaging radar system at the Firepond Laser Radar Research Facility. The paper also details the critical components developed for the Firepond CO/sub 2/ range-Doppler imaging laser radar transmitter as well as transmitters for short-range CO/sub 2/ radars.

Performance of a transversely excited pulsed HF laser

Operation of a pulsed HF laser with unique spectral and temporal behavior is reported. R -branch oscillation in the 1-0 band and peak multiline powers of 700 kW in 200-ns pulses have been observed. The gain of the 75-cm laser was measured to be 235 dB.

Electrooptically Q-switched CO 2 waveguide laser

A flowing-gas CO(2) waveguide laser whose cw output is 13 W has been electrooptically Q-switched repetitively at rates up to 108 kHz and, with a 20% duty cycle, up to 345 kHz. To extract power efficiently in the Q-switched mode, it is necessary to operate the laser at far higher pulse repetition frequencies than for low-pressure lasers. A simplified theory was used to describe this behavior accurately. The short-term (10-msec) frequency stability was measured to be less, similar200 kHz.

Cavity dumping and coupling modulation of an etalon‐coupled CO2 laser

A novel technique for cavity dumping or coupling modulation of a CO2 laser, has been implemented. This technique, which employs an electro‐optically tuned etalon as an output coupler, offers improved efficiency as compared to conventional Brewster‐angle coupling, and it provides convenient end‐mirror outcoupling.

Gain and relaxation studies in transversely excited HF lasers

The results of measurements of the spatial and temporal dependence of the gain of a pulsed SF 6 -H 2 hydrogen fluoride laser are reported and are compared with those for CO 2 lasers. While the propagation in space of a locally excited gain medium was observed to be comparable for two lasers, the time developments were vastly different. The spatial dependence of the gain for each of these lasers was measured by probing a pin laser amplifier with a small oscillator beam, while varying the distance of the probe beam from the plane of the amplifier discharge. The spatial FWHM of the gain for both HF and CO 2 was typically observed to be 6 mm, considerably larger than the visible glow region. Using delay techniques, the oscillator-amplifier configuration was also employed to study the gain of these lasers as a function of time. While the CO 2 gain typically decays after tens of microseconds, the HF gain typically does so after 1 to 2 μs. In addition, by varying the formation rate of HF, it was shown that HF itself is most likely the principal agent of this very rapid gain relaxation. This is a severe basic limitation to the efficient power output of all CW and long-pulse HF lasers. Short pulse HF devices, however, can be operated at relatively high energy. We have demonstrated this by obtaining 350-mJ pulses from a 75-cm pin laser using a 50-ns excitation pulse from a Marx bank.

Laser heating of metallic surfaces

Abstract : Thermal coupling of pulsed 10.6 micrometer laser radiation to aluminum, copper, and titanium targets has been measured as a function of incident fluence focal spot size, and ambient pressure, using both calorimetric and fast-response surface-thermocouple techniques. A peak enhancement in thermal coupling of approximately a factor of ten was observed to occur at the onset of a well-developed plasma at the surfaces of the copper and aluminum targets. After passing through a maximum, the enhanced coupling decreased with increasing fluence and approached CW values at high incident laser fluences. For small spot sizes (area approximately equal to, or less than 0.03 sq cm), most of the enhanced absorption occurred outside the focal spot. The fraction of energy coupled to the target within the focal spot increased with increasing spot size. Under conditions of low ambient pressure (approximately 0.5 torr), the breakdown threshold was increased by a factor of 5, and at high incident fluences the thermal coupling for aluminum was roughly a factor of 2 higher than at atmospheric pressure.

Performance of a sealed-off CO2-isotope laser amplifier for high resolution optical radar/lidar applications

The essential features and performance characteristics of a narrowband isotopic CO2 laser amplifier system are described. Sealed-off operation with over 30 dB net power gain, negligible frequency chirp and up to 70 microsec pulse duration, 300 mJ pulse energy, and 10 Hz pulse repetition rate have been achieved.