MICROWAVE LASER USED TO DISCOVER COSMIC BACKGROUND RADIATION

Ruby Microwave Laser

Microwave laser (Tabor and Sibilia, 1963) used by Penzias and Wilson (1965) to discover the 3 Kelvin microwave black body cosmic background radiation.

Quantum Description

A couple of years after the first successful construction of the gas microwave laser, Bloembergen (1956) suggested an alternative approach with paramagnetic materials by using three levels instead of two. The solid state microwave ruby laser is based on power saturation of the first and third levels of a three level paramagnetic ion (triply ionized chromium) by the use of strong pumping radiation. The populations of level 1 and 3 tend to become equal in these circumstances, thus establishing a population inversion between level 3 and 2.

The very small energy separations typical of microwave laser transitions requires that the crystal be cooled to cryogenic temperatures to reduce the thermal contributions to the excited states, especially the lower laser level, thus allowing for rapid radiative de-population of level 2. Low temperatures are also required in order to reduce non-radiative processes such as phonon collisions (lattice vibrations) which compete with the radiative processes and can reduce the lifetime of the upper laser level 3 ( it must have a longer lifetime than that of the lower laser level, if the decay or relaxation rate of level 2 to 1 is more rapid than the decay from level 3 to 2 then continuous operation is possible ).

Mechanical Description

Since the microwave laser was to be used as an amplifier and not as an oscillator, potential feedback caused by impedance mismatching at various frequencies was eliminated by using a travelling-wave approach which afforded high gain in one direction and little or no gain in the reverse direction. It consisted (Tabor and Sibilia, 1963) of a slow-wave structure built of a conductive comb sandwiched by a pair of ruby crystals all contained within a dual mode waveguide cavity. The cavity must transmit the pump power while simultaneously allowing the signal frequency to pass as well. The population inversion is pumped by a microwave klystron whose frequency closely matches the resonant frequency of the chromium transition from level 1 to level 3. The ruby is cooked by a microwave oven while simultaneously being chilled by liquid helium at 4.2 degrees Kelvin from absolute zero.

Microwave energy falling within the bandpass is exponentially amplified as a function of distance traveled through the active medium up to a factor of 10,000 or more. Its noise temperature was too low to be measured and operated at 4.17 GHz but could be tuned by varying the strength of the external magnetic field.

NOTE:The modern incarnation of this type of laser is the fiber optic laser amplifier.

Practical Use

This very-precise microwave laser preamplifier was used by Arno Penzias and Robert Wilson (1965) for astronomical research but was originally built by Bell Laboratories for the Telstar satellite communication experiments. After having eliminated all possible sources of noise ( including pigeon droppings they nicknamed white-dielectric !) they discovered the 3 Kelvin black body cosmic background radiation. They later won the 1978 Nobel Prize in Physics for this discovery.

Penzias (right) and Wilson (left) near the horn antenna used to make their discovery (Courtesy of AT&T archives)

The ruby microwave laser is still in use in the NASA Deep Space Network and as a compact amplifier aboard spacecraft.

VLBI

Very high resolution radio images of laser stars (quasars) rely on the hydrogen microwave laser as an ultra precise clock to synchronize data measured by widely separated radio antennae of a VLBI network (Very Long Baseline Interferometry).

REFERENCES

  1. Bloembergen,N.: 1956, Phys.Rev., 104, 324.
  2. Penzias,A.A., Wilson,R.W.: 1965, Astrophys.J., 142, 419, 1149.
  3. Tabor,W.J., Sibilia,J.T.: 1963, Bell Syst.Tech.J., 42, 1863.
  4. Wilson,R.W.: 1990, in Modern Cosmology in Retrospect, eds. Bertotto,B. et al., Cambridge Univ. Press.
  5. COsmic Background Explorer WWW ( COBE )
  6. The Cosmic Microwave Background Radiation
  7. VLBI; Very Long Baseline Interferometry
Wilson : "Philosophically, I liked the steady-state cosmology. So I thought that we should report our results as a simple measurement; the measurement might be true after the cosmology was no longer true !"
-R.W. Wilson (1990)
Laser History