Concept Exchange Society


Wednesday, January 15, 1997 6:30 pm

At Glenn Johnston's House.

Presentation: Don Farmer

In The Beginning There Were Masers

Pre-Meeting Notice:

Don writes:
In The Beginning There Were Masers.
The laboratory maser first appeared in 1951. The maser's child, the laser, was soon thereafter conceived, and after a lengthy gestation, was born in 1960. Today, masers are nowhere to be found. (Except in the cosmos, where, as we now know, they have been all the while.) Yet lasers have become almost ubiquitous. What happened to the maser? How does it differ from the laser?
And how do either of them work, anyway?
These questions will be addressed. Answers will be interwoven with some of the intriguing historical lore that surrounds the invention and development of these two devices.


Ten in attendance; Phil, Jeff, Meredyth, Roberto, Don, the Muchmores, Glenn, David and myself. Glenn was the gracious host he has always been.

Don came well prepared. He distributed an eleven page Outline of his talk to all of us so we could follow his presentation visually. It had pictures, diagrams and explanations.

Light comes generally from one kind of event. The energy loss of an electron when an atom drops from an excited state to a less excited one.

You pluck a guitar string. That excites it. Its vibration is its excitation. With time the string reverts to its resting (ground) state.

It is analogous to an atom. After having been excited up to a high energy state it will spontaneously decay down the ladder to its ground state. It gives off particles of light - photons - as it does so. The guitar string gives off sound.

The analogy breaks down on the matter of scale. The very small atom has energy levels which are relatively far apart. Effectively the string has a continuum of them.

That the excited atom's energy levels form a characteristic hierarchy is what governs the color of the emmitted light.

Thus the light from a hot fire, from an electrical discharge and from the hot wire in a lamp bulb all come from excited atoms - radiating at random after being excited due to jostling from other atoms or free electrons.

This light is incoherent. The light waves fom different atoms are not synchronized with each other.

Laser light is different. In a laser all the atoms radiate synchronously. The light each atom gives off is in phase with that coming from all the other atoms and is in the same direction.

Waves of the same frequency and direction but incoherent; not synchronized incoherent waves
Synchronous or coherent wavescoherent waves

Don explained why this is so in his talk. The reason is stimulated emission. He brought us through the history of the concept and the physics behind the stimulated emission of radiation. He captivated us with stories about the personalities and the tribulations of those involved in the creation and development of masers and lasers.

Why is it useful to have a coherent source of radiation? How is it lasers are needed for disks or check-out stands or welding or eye surgery or computer printing or ranging (distance measuring) devices? What is it about this light that distinguishes it from ordinary light?

It is these features:

  1. Beam intensity. The beam intensity can be equivalent to that from a thermal light source of enormous temperature. Hence its uses in welding and in mining.
  2. All the energy flows in one direction. It does not spread out over all directions as light from ordinary sources does. The beam can be sharply focussed. Spatial definition can extend down to microns - the size of living cells. Hence its surgical applications. And its uses to read and write with spatial precision.
  3. Coherence. This feature is the essential one for holography.
  4. Monochromatic light. This permits sharp focussing.
  5. Modulatable. Pulse time definition extends down to picoseconds - the time for light to travel a few thousandths of an inch. This feature, together with items 1 and 2, makes it good for ranging.

light travels
1 second 186,000 miles = 300,000 kilometers
1 millisecond 186 miles = 300 kilometers
1microsecond 1000 feet = 300 meters
1 nanosecond 1 foot = 30 centimeters
1 picosecond 12 mils = 300 microns = 500 wavelengths
( 1 mil = 1 thousandth of an inch)

Don's presentation was as witty and charming as it was informative. All left the meeting grateful to him.

January 1997
Marvin Chester

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© m chester 1997 Occidental CA