Monday, August 24, 2009

Technical Challenges During the Emergence of the Laser - 1960’s

Q-Switched Ruby Laser with "Rat’s Nest" Calorimeter - 1962
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In the late 1950’s and early 1960’s, scientists accomplished the extraordinary feats of predicting, discovering and making the first lasers operational. Throughout the 1960s, scientists continued to lead in discovering new solid, gas and liquid materials that could be used as the active medium in lasers, providing new output wavelengths, higher energy and/or pulsed power outputs and greater efficiencies.

By 1961, electrical and mechanical engineers also joined laser R&D staffs in the development and refinement of laser systems and related equipment. We were faced with technical challenges for which we were not prepared in our education and/or prior experience. Some of the challenges we faced were:

  • Engineers and physicists did not usually work together or even speak the same technical language. We learned to work in teams and to develop mutual respect for each other - because we needed each other’s unique experience and expertise.
  • There were no textbooks and few journal articles about lasers; we had to learn about them as we worked on them. We were discovering new phenomena and revising existing theories.
  • In the 1960’s, most engineers’ knowledge of optics was limited to what they learned in a few weeks of study in sophomore physics. Many of us had to learn more depth in geometrical optics from a book by Jenkins & White; wave (or physical) optics from a book by Strong.
  • Light was traditionally measured in photometric units (lumens, foot candles, angstroms etc). We had to transition to radiometric units (joules, watts, nanometers etc).
  • Safety aspects of laser beams was neither known nor respected. Laser safety became an R&D field of its own. Laser safety goggles had not been invented.
  • There was no instrument used to measure the energy in an optical pulse (i.e. output of a pulsed laser.) Robert M. Baker, a Fellow Electronics Engineer at the Westinghouse Defense Center, devised and tested a “rats nest” calorimeter, composed of tens of meters of coated, fine copper wire, tangled and placed in a small beaker. The pulsed laser beam was directed into the “rats nest”; the change in electrical resistance, due to the heat rise in the copper, was measured; the temperature rise in the wire was calculated and related to the laser pulse energy absorbed by the “rats nest”.
  • The physics of “negative absorption” or “optical gain” could only be understood through an understanding of modern physics and quantum mechanics. Some of us had “lightly” learned these fields in graduate studies; others had to struggle through these topics in other ways.
  • Operation of solid lasers, like ruby, required fluent knowledge and facility in cryogenics and high voltage power supplies and capacitor banks. Most engineers had to learn these practices “on the job”.
  • As new applications of lasers were proposed in fields such as defense, materials processing, medical therapeutics, communications, remote sensing and others, engineers were required to devise, revise and adapt equipment to accommodate laser and optical components, devices and systems.
  • We learned, by mistakes, that a high power, pulsed ruby laser cannot be focused with an achromat lens without destroying the cement that joins the components of the lens together. Achromat lenses were not needed for monochromatic laser light.
  • We also learned that most anti-reflective coatings, needed on gas laser tubes and the ends of solid laser rods, were also vulnerable to damage by the laser radiation. We solved this problem by positioning the end of the laser rods and the windows at Brewster’s angle to minimize reflections; thereby eliminating the need for AR coatings.

This list is far from comprehensive, but it’s what first came to mind and it’s long enough for this blog posting. Perhaps you were also working on lasers in the 1960’s. I would invite you to comment on other challenges that you faced.

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1 comment:

Gordon F Snyder Jr said...

Dan - I'm a little too young to have been working on this stuff in the 60's.
That said - these posts are excellent. Really enjoying reading them!