LASER EMISSION IN QUASAR SPECTRA
Most commercial lasers have their active lasing medium contained within an
equivalent to a Fabry-Perot interferometer. The spectral
response of this type of cavity combined with its ability to select a
mode within the lasing medium leads to an extremely sharp emission line.
When there is no cavity you get Amplified Spontaneous Emission (ASE) and
the width of the laser line depends of the plasma parameters and motions:
When the speed of expansion is low, the expansion will be closer to being
isothermal than adiabatic. As the speed of expansion increases, the expansion
will become increasingly adiabatic (rapid cooling), and certain spectral
lines can be expected to display laser action. WC8 stars have relatively
sharp lines; since the widths of the lines in an expanding shell arise from
the Doppler effect, the speed of expansion of the plasma must be low and the
degree of laser action is also expected to be low. The lines become wider
in WC7 stars, indicating that the speed of ejection is greater than in WC8
stars; correspondingly, the degree of laser action is also expected to be
greater. The lines become still wider in WC6 stars; we thus expect the degree
of laser action to further increase. The correlation between expansion
velocity and laser gain has been observed in the spectra of HD 164270,
HD 119078 and HD 115473 in the 6g to 5f laser transition 4658 A of the
C IV ion (Carbon+3). As laser gain increases this line eventually dominates the
spectrum and closely resembles a quasar spectrum. Many quasars show P-Cygni
profiles in their laser lines indicating rapid expansion.
- In astrophysical MASER emissions, the resulting widths of the MASER
lines are about 1 km/s and for large gains it becomes smaller than
the thermal Doppler profile.
- In astrophysical LASERs the line narrowing mechanisms are very
weak because the plasma is optically thin and the gain is small.
ASE leads to very bright laser emission lines due to the exponential
amplification by stimulated emission over large distances.
Laser action has also been shown to occur in He I (Nasser,1987) and many other
ions, quasars ...
In this thesis we have explained the difference between LTE and non-LTE
plasmas and why we must use the rate coefficients for basic physical plasma
processes, instead of the simpler statistical model. We used these rates
in a collisional radiative model of a recombining C IV plasma expanding
adiabatically. From the solution of the level population we show how gain per
unit length is computed, and why large gains occur only in limited regions of
parameter space (ne, Te). We then compared our results to stellar spectra
of Wolf-Rayet stars, and quasars. Closer to earth, many experimental research
labs have succeeded in producing laser action in decaying plasmas, and the
future looks 'bright' for high power short wavelength UV and X-Ray lasers.
(Matthews et al. 1988)
Next Section : Appendix