Coherent light generators – Particular beam control device – Q-switch
Reexamination Certificate
2001-03-30
2003-11-25
Lee, Eddie (Department: 2815)
Coherent light generators
Particular beam control device
Q-switch
C372S012000, C372S013000, C372S025000, C372S029021, C372S030000, C372S070000
Reexamination Certificate
active
06654391
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to Q-switched lasers, and in particular to Q-switched lasers with frequency conversion in the laser cavity, and more particularly to methods for operating such lasers.
BACKGROUND OF THE INVENTION
The application of Q-switching techniques to lasers has made it possible to produce short pulses with high peak-powers. Many standard Q-switched lasers are capable of producing pulses with a duration on the order of a few cavity decay times (i.e., from a few nanoseconds to many tens of nanoseconds) and peak powers from the kilowatt to the megawatt range.
In lasers without Q-switches and in which the lasing medium is continuously pumped, the population inversion (i.e., the proportion of lasant atoms or molecules in the high energy state and ready to participate in stimulated emission) is fixed at a threshold value when oscillation is steady. Even under pulsed operating conditions, particularly at high repetition rates, the population inversion exceeds the threshold value by only a relatively small amount due to the onset of stimulated emission. Q-switching techniques employ a Q-switch positioned inside the laser cavity to modulate the laser cavity loss. When the Q-switch is on cavity loss is very high and laser action is prevented. Consequently, as the lasing medium is pumped the population inversion builds up to levels far exceeding the threshold population holding when the Q-switch is absent. Now, when the Q-switch is suddenly turned off, the cavity loss decreases rapidly and the laser suddenly has a gain that greatly exceeds loss. As a result, the energy stored in the lasing medium is released in the form of a short and intense pulse.
Various types of Q-switches employing different principles have been described in the prior art. In general, these Q-switches fall into two groups: active Q-switches and passive Q-switches. Active Q-switches require external control to turn them on and off. For the most part, active Q-switches either employ mechanical elements (e.g., mechanical shutters, rotating prisms, etc.) or elements relying on the electro-optic or acousto-optic effects. Passive Q-switches typically rely on an optical nonlinearity of the element used (e.g., a saturable absorber). For more information on Q-switches the reader is referred to Orazio Svelto, “Principles of Laser Optics”, Plenum Press, (translated by David C. Hanna), 1998, pp. 313-319.
FIG. 1
shows a typical prior art Q-switched laser
10
with an active Q-switch
12
controlled by a Q-switch control
14
. A lasing medium
16
of laser
10
is pumped by a pump source
18
such as a bank of laser diodes, a source of pump light or any other suitable pumping mechanism. Pump source
18
is controlled by a pump control
20
to pump lasing medium
16
continuously or nearly-continuously and to thus achieve a population inversion among atoms
30
of medium
16
. In other words, pump source
18
ensures that there is a large number of “pumped” atoms
30
A indicated by full circles (i.e., atoms
30
A are in an upper energy state). Atoms
30
A are ready to emit light
28
when stimulated. Laser
10
has a cavity
22
defined between a high reflector
24
and an output coupler
26
.
When Q-switch
12
is in the on state it prevents light
28
emitted by atoms
30
A of lasing medium
16
from setting up resonant modes between mirrors
24
and
26
of cavity
22
(e.g., by deflecting light
28
out of cavity
22
). Hence, loss in cavity
22
is high and no output light
28
is coupled out through output coupler
26
. As Q-switch
12
is turned off, the loss in cavity
22
decreases and once it equals the gain (first intersection), stimulated emission takes place, as shown in FIG.
2
A. More specifically, as loss &ggr;(t) drops below gain g(t) laser
10
starts to build up and light
28
is out-coupled through output coupler
26
(see
FIG. 1
) in the form of a pulse
32
. The peak of pulse
32
generally coincides with the time at which gain g(t) and loss &ggr;(t) are once again equal (second intersection). After that, pulse
32
decays along with decreasing gain g(t).
Typically, Q-switched laser
10
is operated to produce a number of pulses
32
at a certain repetition rate, as shown in FIG.
2
B. This repetition rate is shown as fixed, but it may also vary with time. For that purpose, pump source
18
is set up to continuously pump medium
16
at a constant pump rate R
p
. Meanwhile, loss &ggr;(t) is periodically modulated by Q-switch control
14
, which opens and closes Q-switch
12
very rapidly. Thus, loss &ggr;(t) changes between a low level (Q-switch
12
off) and a high level (Q-switch
12
on). In response, lasing medium
16
generates photons &phgr;(t) of light
28
in pulses
32
, as shown. The population of atoms
30
A in the upper state is at a high or initial level N
i
before each pulse
32
. A number of photons &phgr;(t) of light
28
are emitted as a function of time from atoms
30
A during pulse
32
. The population of atoms
30
A in the upper state reaches a low or final level N
f
after each pulse
32
. Once pulse
32
is completely out-coupled from cavity
22
, Q-switch control
14
waits and then turns Q-switch
12
back on to build up the population of atoms
30
A to the initial level N
i
in preparation for subsequent pulse
32
.
After each pulse
32
gain g(t) is depleted well below the lasing threshold and remains there for a substantial amount of time even while being pumped by pump source
18
in preparation for subsequent pulse
32
. In fact, when laser
10
is continuously pumped at rate R
p
, as shown in
FIG. 2B
, gain g(t) is below threshold without the aid of Q-switch
12
being turned on for a duration after pulse
32
that is significant. At high repetition rates this duration is a substantial percentage (5 to 50%) of the interpulse time &tgr;
p
. When laser
10
operates at low repetition rates this duration is a substantial percentage (5 to 50%) of the lasing medium's
16
fluorescence lifetime (&tgr;) (the lifetime of atoms
30
A in the upper state).
Given this situation, the prior art teaches that Q-switch
12
should be turned on after all the useful energy of pulse
32
is extracted from cavity
22
, which sets a minimum time, but before laser
10
reaches the lasing threshold and again emits, which sets a maximum time. Avoiding this later emission ensures that no energy is taken away from the desired subsequent pulse
32
. Consequently, the exact time when Q-switch
12
is turned back on after pulse
32
can be any time before laser
10
reaches the lasing threshold. In practice, it does not matter if this time is longer or shorter, as long as it is neither too short, so it does not interfere with the out-coupling of pulse
32
, nor too long, so it does not fail to store energy for next pulse
32
. Thus, Q-switch
12
is set to turn on after a “safe” intermediate time to ensure stable operation. The prior art also notes, that setting Q-switch
12
to be turned on right after pulse
32
produces instabilities in power levels of subsequent pulses
32
, fluctuations in build-up times, as well as artifacts (e.g., secondary emissions). For further theory of operating Q-switched lasers the reader is referred to William G. Wagner et al., “Evolution of a Giant Laser Pulse”, Journal of Applied Physics, Vol. 34, No. 7, 1963, pp. 2040-5 as well as Walter Koechner, “Laser Engineering”, Springer Series in Optical Sciences, Vol. 1, Springer-Verlag, Berlin Heidelberg, 4
th
edition (1996), Chapter 8, and Orazio Svelto, op. cit.
Due to the above-mentioned intricacies as well as other considerations, most Q-switched lasers are operated at their fundamental frequency within the “safe” regime. Thus, for reasons that will be explained by the invention, most Q-switched lasers are not fully optimized for practical applications where intracavity frequency conversion is required. In other words, most Q-switched lasers are not well-adapted to have frequency conversion elements (e.g., nonlinear optical materials for frequency doubling) positione
Landau Matthew C
Lee Eddie
Lightwave Electronics
Lumen Intellectual Property Services Inc.
LandOfFree
Method for operating Q-switched lasers with intracavity... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for operating Q-switched lasers with intracavity..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for operating Q-switched lasers with intracavity... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3184835