Generation of short amplified pulses of light using an absorbing medium

A system is disclosed in which the phase of coherent light impinging on an absorbing cell is rapidly changed to produce a short amplified pulse of light.

FIELD OF THE INVENTION 
This invention relates to the generation of short light pulses and 
particularly to the generation of short coherent light pulses. 
BACKGROUND OF THE INVENTION 
Coherent light pulses are employed to perform photo chemistry and also in 
laser communications systems. In both of these areas, as well as others, 
short pulses can be advantageous. Over a period of time people have 
developed systems for generating shorter and shorter pulses. Two papers 
published by E. Yablonovich [the first with J. Golbhar, Applied Physics 
Letters, 25, 520 (1970) and the second with H. S. Kwok, Applied Physics 
Letters, 30, 158 (1977)] disclose a system in which a coherent light pulse 
is generated by focussing laser light to break down the air through which 
it travels. The resulting plasma prevents further light from passing 
through, resulting in a coherent light pulse having a relatively slow rise 
time, but a fast fall time. This pulse is applied to an absorbing medium 
which absorbs the coherent light pulse, aligning the molecules in the 
medium with the field of the coherent light pulse. At the time of the 
sharp fall time terminating the coherent light pulse, the absorbing medium 
coherently gives up its stored energy, providing a short pulse having a 
pulse width related to the line width of the absorbing medium. The 
amplitude of the short pulse is approximately equal to the amplitude of 
the initial pulse incident upon the absorbing medium. 
BRIEF DESCRIPTION OF THE INVENTION 
In accordance with the teachings of this invention, it has been realized 
that short coherent light pulses can be generated without resorting to 
abrupt amplitude modulation. It has further been realized that such pulses 
can be generated with an amplitude up to four times as great as the 
incident radiation. It has been discovered that a CW laser beam having an 
abrupt 180.degree. phase shift incident upon an absorbing medium results 
in a short pulse synchronized with the abrupt phase transition having an 
amplitude approximately four times the amplitude of the incident CW laser 
radiation. 
The time duration of the pulse thus generated is related to the line width 
of the absorbing medium if the 180.degree. phase shift occurs in a time 
short compared to the time related to the line width. 
It has been found that the amplitude of the resulting pulse is proportional 
to the square of the sign of half the angle of the phase shift. Thus, the 
maximum amplitude occurs at 180.degree. phase shift, while zero amplitude 
occurs at a 360.degree. phase shift. Thus, a special case exists where a 
rapid 360.degree. phase shift is imposed upon the incident radiation. In 
this case, if the 360.degree. phase shift occurs in a time less than the 
time determined by the line width of the absorbing material, the width of 
the pulse in fact is controlled by the rise time of the 360.degree. phase 
shift since the pulse will achieve a maximum amplitude at the 180.degree. 
phase shift, and will be brought back to zero at 360.degree..

DETAILED DESCRIPTION OF THE INVENTION 
Referring to the sole FIGURE, we see a carbon dioxide (CO.sub.2) laser 10 
operated in its CW mode to shine a CW light beam on a phase modulator 11 
which is a cadmium telluride crystal situated in the path of the CW 
CO.sub.2 laser beam in its phase modulating orientation. An electrical 
pulse generator 12 is connected to the phase modulator 11 by an electrical 
cable 13 which is the 50-ohm transmission line. The rise time of the 
pulses provided by the pulse generator 12 is typically one nanosecond. For 
faster rise times, a spark gap can be employed to increase the rise time 
to a hundred picoseconds. The phase modulator 11 passes the CW CO.sub.2 
laser radiation and imposes an additional predetermined phase shift when 
pulses from the pulse generator 12 are applied thereto. The light output 
of the phase modulator 11 is applied to an absorption cell 14 which is six 
meters long and filled with carbon dioxide (CO.sub.2) at a temperature of 
450.degree. C. at a pressure of 100 torr. 
In operation, the CW laser radiation from the CO.sub.2 laser 10 passes 
through the phase modulator 11 and is absorbed by the CO.sub.2 gas in the 
absorption cell 14, such that only 10.sup.-5 of the radiation incident 
upon the CO.sub.2 absorption cell 14 passes therethrough. When a pulse is 
supplied by the pulse generator 12 via the transmission cable 13 to the 
phase modulator 11 having an amplitude which provides an abrupt 
180.degree. phase shift to the CO.sub.2 laser radiation emitting therefrom 
a light pulse having an amplitude four times the radiation incident upon 
the CO.sub.2 absorption cell 14 is emitted from the CO.sub.2 absorption 
cell 14 as a pulse output. The width of the light pulse is approximately 
150 picoseconds for the conditions described. 
Thus, it is seen that a rejection ratio for the short light pulse is 
approximately 400,000. This is determined by taking the amplitude of the 
CW radiation output of the absorption cell 14 and dividing it into the 
amplitude of the light pulse. Since the amplitude of the light pulse is 
four times the incident radiation and the CW radiation passed by the 
CO.sub.2 absorption cell 14 is 10.sup.-5 of the incident radiation, the 
resulting rejection ratio is 400,000. 
It has been found that since the duration of the light pulse is controlled 
by the line width of the CO.sub.2 gas in the cell 14 it is a function of 
temperature and pressure. Thus, for a cell such as set forth above at 
450.degree. C. with a pressure of 50 torr, a 300 picosecond pulse would be 
provided rather than the 150 picosecond pulse provided at 100 torr 
pressure. 
It has been determined that the amplitude of the pulse emerging from the 
CO.sub.2 absorption cell 14 has an amplitude depending upon the phase 
shift provided by the phase modulator 11. Thus, it has been determined 
that the amplitude is proportional to the square of the sign of half the 
angle of the phase shift. It can be seen from this relationship that the 
maximum amplitude occurs at 180.degree., while zero amplitude occurs at 
0.degree. and 360.degree.. Thus, if a signal were applied to the phase 
modulator 11 to vary the phase from 0.degree. through 180.degree. to 
360.degree. phase shift, the pulse width of the output pulse from the 
absorption cell 14 would have a pulse width determined by the rise time of 
the electrical signal provided by the pulse generator 12 so long as that 
signal completed its excursion from 0.degree. to 360.degree. in a time 
less than the time determined by the line width. Thus, in the example 
given above where a 300 picosecond pulse is provided, a shorter pulse 
could be provided if the phase variation occurred from 0.degree. to 
360.degree. in a time less than 300 picoseconds. As pointed out above, a 
spark gap could be employed to create a transition in that period of time. 
While this invention has been described with respect to a particular 
embodiment thereof, numerous other embodiments will become obvious to 
those of ordinary skill in the art in light thereof.