LASER EMITTING PULSES OF VARIABLE PERIOD AND STABILIZED ENERGY
The domain of the invention is that of pulsed lasers of which the
pulse repetition frequency is variable, their energy being stabilized, and
which are pumped by a continuous source.
5
Pulsed lasers currently exist, the pulses of which use intra-cavity
switches, known as Q-switches, where Q represents the quality factor of the
resonant cavity. They are known as Q-switched lasers.
There are two phases in a Q-switched laser. The pumping phase
10 enables the storage of the pumping energy in the laser material. The closed
cavity switch prevents resonance. The switch is opened to generate a pulse.
Resonance is possible. The light present in the modes of the cavity is
_ amplified by the laser material.. An intense pulse is formed. The energy of this ^
pulse is proportional to the energy stored in the laser material during
15 pumping. The switching must be fast in order to ensure effective control of
the energy and temporal profile of the pulse. Conventionally, only the very
fast opening of the Q-switch is controlled. The controlled closure shortly after
the opening may allow the energy per pulse to be reduced.
Each laser material transition has a lifetime. This is the time,
20 excluding pumping, required for half of the population in an excited state of
the laser transition to disappear. It is also the time required in order to attain
half of the population in an excited state for a very long pumping duration in '
the absence of any parasitic effect that may reduce pumping efficiency.
If the period between the pulses is long compared with the lifetime
25 of the excited state of the laser transition used, the energy per pulse is the
maximum. Any increase in the period does not modify the energy per pulse.
For a~*given laser, this energy per pulse is controlled by the power of the
pumping and the lifetime of the excited state.
This is illustrated by Figure 1 which shows two examples of gain
30 curves as a function of time, for a lifetime of the excited state of 250 us, this
gain expressed in arbitrary units being accumulated in the amplifying medium
of a Q-switched laser, by a continuous pumping A for the curve "a" and A/5
for the curve "b". For periods greater than 830 us, the available gain varies
by less than 10%; the energy per pulse varies in similar proportions.
2
At the other end of the curve, when the period is reduced, the
energy per pulse is no longer controlled by the lifetime of the excited state.
The laser operates at medium power. With a fixed period, the energy per
pulse is proportional to the period separating each pulse. Any change in the
5 period modifies the energy per pulse; the latter depends on the energy of the
preceding pulse and the period separating them. In the examples shown in
Figure 1, this is the domain of periods less than 250 us.
Finally, since the energy per pulse varies in proportions similar to
the gain, for pulse periods varying between 150 and 1000 us highly disparate
10 energies per pulse are obtained, since they vary between 150 and 500
arbitrary units.
Moreover, the pulses must meet a minimum energy requirement
for performance, but must not exceed a given energy threshold in order to
. avoid irreversible degradation of the laser.
15
A plurality of methods are used to obtain similar pulses with a
variable repetition frequency.
The first solution carries out a sorting at the output of the Q-
20 switched laser. The laser has a fixed repetition period referred to as the base
period. The output pulses are either rejected or transmitted. The periods
obtained are therefore limited to multiples of the base period; the starting
position of each pulse train is imposed.
25 A different solution consists in modulating the continuous pumping
source power as a function of the required time for the emission of each
pulse. The modulation of the pumping power compensates for the effect of
the increase in the energy of the pulse with the pumping duration; but this
modulation is possible only insofar as the response time constant of the
30 pumping source allows it, notably if the pumping source is a laser. It is not
always possible to modulate the pump quickly enough, or it is not possible to
predict sufficiently in advance when the following pulse will have to be
emitted. The variable power of the pumping modifies the thermal equilibrium
point of the resonant cavity when the repetition frequency changes,
35 generating a thermal instability within the Q-switched laser.
3
A different solution involves the control of the time and duration of
the opening of the Q-switch. The duration of opening depends on the energy
of the preceding pulse and the elapsed time. The duration of opening is
5 therefore controlled as a function of the time that has elapsed since the
preceding pulse and of its energy. The opening and closing switch durations
must also be controlled. A sophisticated electronic control system is
necessary to open and close the Q-switch precisely. This double control is
unusual and complex and its adjustment is difficult. For periods changing with
10 each pulse, the control laws are difficult to adjust and readjustments are
necessary during the life of the laser. Complexity reduces operating safety
and reliability.
• • " . . - . - .. Y' - The object of the invention is to obtain a pulse laser with
15 continuous pumping, the emission of the pulses having a variable period and
stable energy per pulse, as shown in figure 2.
The invention is based on the addition of an injector of which the
emitted beam with the same wavelength as that of the laser is injected into
20 the laser material in order to be amplified. The effect of this amplification
which consumes gain is to simulate a reduction in the time constant of the
laser material; the adjustment of the power of the injector modifies the
apparent time constant of the laser material. The apparent time constant is
that which is required in order to obtain a population half of that which would
25 be obtained for a long pumping duration (typically in the order of 3 to 4 times
greater than the time constant) and a constant injector power.
- - More precisely, the subject matter of the invention is a laser device
suitable for emitting pulses with a variable period and with stabilized energy
which includes:
30 - a resonant cavity including
- an amplifying medium presenting a stabilized gain G and
suitable for emitting laser pulses at a wavelength X, and
- a Q-switch,
- and a source of continuous pumping of the amplifying medium.
4
It is mainly characterized in that it furthermore includes an injector
positioned outside the resonant cavity, suitable for emitting a beam of
wavelength X into the amplifying material for the duration of the pumping, and
which includes means for adjusting the power of this beam in order to reduce
5 the gain of the amplifying medium to G/k, where k is a real number greater
than 1.7.
Thanks to the injector and with stable continuous pumping, each pulse
can be temporally precisely controlled with a stabilized energy per pulse.
10 The thermal load of the pulsed laser is stable and independent from
the pulse demands, since the pumping is continuous at a constant level.
The Q-switch has few constraints.
: .. T The. injector preferably shares the same pumping means as the laser |
15 itself. The injector is thus active, from the start of the pumping of the laser,
providing the energy stabilization per pulse with no external controls. j
According to one variant, the injector is disposed outside the pumping
source-amplifying medium axis.
The injector may be a laser diode or a microchip laser or an
20 electroluminescent diode.
Other characteristics and advantages of the invention will become
apparent from reading of the detailed description which follows, given by way
of a non-limiting example, with reference to the attached drawings, in which:
25 Figure 1 shows schematically two examples of gain curves
expressed in arbitrary units as a function of time, for a continuous pumping of
power A for the curve "a" and A/5 for the curve "b",
~~ Figure 2 shows schematically variable-period, energy-stable
pulses,
30 Figure 3 shows schematically an example of a pulsed laser device
according to the invention,
Figure 4 shows schematically the gain curves obtained with a
laser device according to the invention under conditions equivalent to those
of example 1 in Figure 1.
V
5
The same elements are identified by the same references from
one figure to another.
The curves in Figure 1 are explained in the following manner:
5 - the power of the pumping is in competition with the
fluorescence of the laser material (amplifying medium). The
pumping increases the population of excited states of the laser
material. The gain increases with the proportion of the
population in an excited state. The fluorescence is proportional
10 to the gain. The fluorescence is an incoherent emission at the
same wavelength as the laser. The fluorescence consumes the
gain. At the start of the pumping, there is no significant number
of excited states, there is no gain, there is no fluorescence.
- >. •"' ^ The gain increases with the pumping. While it remains low, the
15 fluorescence consumes little gain. With the increase in the
gain, the fluorescence limits this increase. Finally, the gain
provided by the pumping is entirely consumed by the
fluorescence. The gain level is maximum. This level is a
function of the pumping power.
20 - N.B.: for high pumping powers, the fluorescence is not the first i
characteristic limiting the gain. This process of increasing the
gain can also be limited by the total quantity of excited states
that can be created in the laser material.
- the time constant is defined without pumping, it is the duration
25 required for half of the excited states to disappear through
fluorescence. It is an intrinsic characteristic of the laser
material,
- well within the time constant, the energy per pulse is
proportional to the pumping duration,
30 - well beyond the time constant, the energy per pulse is fixed,
- the energy per pulse is proportional to the pumping power (the
curve "a" is "homothetic" of the curve "b").
The effect of the method according to the invention is to reduce
35 the equivalent time constant of the laser material. This is obtained by
6
injecting into the laser material a beam of the same wavelength X as that of
the beam emitted by the laser material. In this material, the beam emitted by
the injector will be amplified in proportion to the amount of the gain. Since the
pumping is continuous, there is competition between the gain provided by the
5 pumping and the consumption of this gain - by the amplification of the
injection. This injection will limit the gain G available in the cavity beyond a
certain level. For a given pumping level, this is equivalent to a reduction in
the lifetime of the excited state of the laser material. The equivalent lifetime is
adjusted by the power level of the injection. The amplified injection must be
10 rejected by the Q-switch to avoid the beginning of the construction of a pulse.
The beam is emitted by the injector when the Q-switch is in the closed
position. The role of the injector is effective for the duration of the pumping; it
can be emitted or not when the Q-switch is in the open position.
. - . I' - The energy stored* in the. laser material is limited by this gain
15 consumption of the injector. This limitation of the gain prevents any creation
of pulses with excessive energy, independently of the elapsed period before
the pulse.
This injected beam is amplified by the laser medium of which the
gain and therefore the energy are then limited. This injector thus provides a
20 static protection against pulses with excessive energy.
An injector is normally used to increase the emission quality of the
laser: purity and spectral position, transverse mode quality or temporal pulse
precision. To do this, the injector emits in the emission mode and direction of
the laser. A very low power of the injector in the laser is sufficient. The beam
25 emitted by the injector must be in resonance in the laser cavity. The injector
emits when the switch is open to facilitate the required emission. An injector
Gan also be used in a continuous laser. The use according to the invention
therefore differs from normal uses.
30 Figure 3 shows an example of a Q-switched laser according to the
invention. It includes a resonant cavity 1 which comprises a first output mirror
11 and a second mirror 12; it is shown as bent in an L-shape, but could also
be linear or annular.
V'
7
• -
It includes a laser material 10 suitable for emitting a beam of
wavelength X; an example of such a material is Nd:YAG where X= 1.064 urn
or Ho:YAG where X= 2.1 urn.
The cavity 1 also includes a controlled Q-switch 15 enabling the
5 emission of pulses on demand. This Q-switch may, for example, be an
acousto-optical switch which diverts the beam from a resonant (or tuned)
cavity path towards a non-resonant (or detuned) cavity path where the
second mirror 12 then acts as a partial output mirror, as shown in the figure,
and vice versa. When the cavity is detuned, the pumping power is stored in
10 the laser material. When the cavity is tuned, a pulse can be formed if the gain
is sufficient. An electro-optical switch, causing the polarization to turn on
demand, can also be used in an architecture selecting the polarization of the
light; a polarizer is then added in the resonant cavity.
15 The cavity 1 is coupled to a continuous pumping source 2, for
example by a coupling mirror 22, transparent for the wavelength of the
pumping source and reflecting for the beam emitted by the laser material 10,
but which remains weakly transparent at the wavelength of the laser.
An emitter 3 of a beam 31 of the same wavelength X is coupled
20 with the laser material 10 in such a way as to inject said beam into this
material. This emitter 3 may more generally be any continuously emitting light
source of which the emission spectrum covers the spectral emission band in
which the Q-switched laser must emit. The beam 31 is in continuous
emission for the entire duration of the pumping of the laser. This emitter is
25 also referred to as an injector. It is located outside the resonant cavity 1, so
as not to be disturbed by the pulses emitted by the resonant cavity 1.
This injector 3 is, for example, passive; it is then located on the
path of the pumping beam 21 in order to be controlled by this pumping beam.
The injector thus uses a fraction of the pumping beam 21. This then involves
30 a passive protection. The assembly is static without active control to provide
protection. The emission of the injector is not necessarily in a resonant mode
of the laser cavity.
It may be located on the path of the pumping beam 21 in front of
the resonant cavity 1 as shown in the figure: the injection is effected via the
35 same path as that of the pumping beam. According to one variant, the
i
r
5-
8
pumping beam is divided into two, one part being directly steered towards the
laser material 10, the other being steered towards the injector 3 located
outside the pump-material path.
In the case of a passive injector, the power of the emitted beam is
5 fixed in the design.
According to a different operating mode, the injector 3 is active
and includes its own control means. It can be positioned in any location that
allows a distributed illumination of the laser material, to have a homogeneous
effect, notably outside the pumping beam 21.
10 The power of the beam 31 emitted by the injector is adjusted
taking into account the transmission of the coupling mirror, in such a way as
to reduce the gain G of the laser material by a factor k, where k is a real
number greater than 1.7.
. . . . T - This injector may be an electroluminescent diode, or may itself be
15 a laser such as a laser diode or a microchip laser composed of the same
material as the laser itself, of which the parallel surfaces are reflecting on one
side and partially reflecting on the side of the output at the wavelength of the
laser. It is not necessary for the emitted wavelength to be resonant in the
cavity of the laser.
20 In the case, for example, of an electroluminescent diode or laser
diode, the means for controlling and the means for adjusting the power of the
emitted beam are the electrical current.
In addition to the advantages already mentioned, it can also be
25 noted that, since the period of the pulses and the energy of the preceding
pulse no longer need to be taken into account, the control of the Q-switch is
simple.
^ Furthermore, thermal stability is retained even in the absence of
emission of pulses, since the continuous pumping can be maintained without
30 risk.
Figure 4 shows two examples of gain curves obtained with a
continuous pumping A, the one "a" with a Q-switched laser as described in
Figure 1 with a stabilized gain G, the other "a" with a Q-switched laser
provided with an injector according to the invention adjusted to a power level
35 allowing a stabilized gain level of G/5 (k=5).
•
9
Since the energy per pulse varies in proportions similar to the
gain, the energy per pulse is stable at better than 10% (from 90 to 100
arbitrary units) for any period greater than 150 us. Whereas, without the
injector, the energy per pulse would vary by 300% (from 170 to 500).
5
Emissions of pulses with a variable period and controlled energy
per pulse have thus been obtained.

Claims
1. A method for using a laser device suitable for emitting variable-period
pulses, which Includes:
- a resonant cavity (1) including an amplifying medium (10)
presenting a stabilized gain G and suitable for emitting laser
pulses at a wavelength X, and a Q-switch (15),
- a source of continuous pumping (2) of the amplifying medium,
- an injector (3) positioned outside the resonant cavity (1),
suitable for emitting a beam of wavelength X into the amplifying
material (10),
- means for adjusting the power of the injected beam,
; : V t characterized in that it Includes: a step of.emission by the injector of a
beam for the duration of the pumping and when the Q-switch is in the closed j;;
position, and in that the power of the injected beam is adjusted to reduce the
gain of the amplifying medium to G/k, where k is a real number greater than
1.7.
2. The method for using a laser device as clairiied in the preceding
claim, characterized in that the injector is controlled by the purnpihg
source. ,
3. A laser device suitable for emitting variable-period pulses, which
includes:
- a resonant cavity (1) including an amplifying medium (10)
r ^ presenting a stabilized gain G and suitable for emitting laser
^ pulsesat a wavelength A,, and a Q-switch (15),
- a source of continuous pumping (2) of the amplifying medium,
- an injector (3) positioned outside the resonant cavity (1),
suitable for emitting a beam of wavelength X into the amplifying
material (10),
characterized in that it includes means for adjusting the power of
the injected beam configured to reduce the gain of the amplifying
medium to G/k, where k is a real number greater than 1.7.
AMENDED CLAIMS - ARTICLE 19
4. The laser device as claimed In the preceding claim, characterized In
that the injector (3) Is disposed outside the axis between the pumping
source (2) and the amplifying medium (10).
.5. The laser device as claimed in either of claims 3 and 4, characterized
In that the injector (3) is a laser diode or microchip laser or an
electroluminescent diode.