Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: LASERS



Hi Herb:

I do not know what you mean by "semi-confocal". An optics
text I consulted does refer to a resonant cavity with spherical
mirrors. It states that the distance between the mirrors must
be equal to their common radius. If this the "semi-confocal"
system then why is it called this way? Let me try to answer.

The line between the two centers of curvature is the axis.
A ray parallel to the axis (inside the cavity) is reflected back
at an angle with the axis, crosses the axis in the middle, and
becomes parallel to the axis after the second reflection.
That is what the picture shows. Two parallel rays (located
symmetrically with respect to the axis) traveling in the same
direction go through the common focal point (f=R/2) and
follow each other's path. The f=R/2 is true for paraxial
rays only. Presumably this is good enough.
Ludwik Kowalski

Herb Gottlieb wrote:

In theory the two end mirrors must be highly parallel to each other.
However, this condition is almost impossible to maintain in practice
because expansion and contraction of the mirror supports occur
as the laser is heated and cooled during normal operation.
Instead of using flat mirrors that are parallel to each other, modern
helium-neon lasers use concave mirrors spaced in a semi-confocal
arrangement.

Also it is important to add that the end mirrors are not the common
silver-backed cosmetic mirrors but are made of transparent glass
with thin film quarter-wavelength front-surface dielectric coatings
that reflect photons of the desired wavelength and effectively
absorb those of other wavelengths.